Электроактивные материалы на основе хитозана и поливинилового спирта тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Ольвера Берналь Ригель Антонио

• Ольвера Берналь Ригель Антонио
• кандидат наук
• 2023

Оглавление диссертации кандидат наук Ольвера Берналь Ригель Антонио

Реферат

Synopsis

Introduction

CHAPTER 1. Literature review

1.1 Electroactive Polymers

1.1.1 Electronic Polymers

1.1.2 Ionic EAPs

1.2 Biopolymers as potential material for its use as electroactive material

1.3 Chitosan

1.3.1 Physicochemical Properties of Chitosan

1.3.2 Chito san crystalline structure

1.4 Poly (vinyl alcohol)

1.4.1 Poly (vinyl alcohol) crystalline structure

1.5 Electrospinning process

1.6 Implementation of nanofibers as electroresponsive materials

CHAPTER 2. Materials and methods

2.1 Materials

2.2 Methods

2.2.1 Preparation of polymeric solutions with different chitosan content

2.2.2 Electrospinning process

2.2.3 Morphology and diameter determination

2.2.4 Infrared spectroscopy

2.2.45Thermal properties

2.2.6 Mechanical properties

2.2.7 Swelling ratio of nanofiber hydrogels

2.2.8 Electroactive Response

CHAPTER 3. Experimental results

3.1 Formation of chitosan - Poly (vinyl alcohol) based nanofibers by electrospinning

process

3.1.1 Morphology and diameter distribution of Chitosan/PVA nanofibers

3.1.2 FTIR spectroscopy

3.1.3 Thermal Properties

3.1.4 Mechanical Properties

3.1.5 Swelling ratio of chitosan/PVA nanofibers hydrogels

3.1.6 Electroactive response of the nanofiiber hydrogels

3.1.7 Determination of free amine (-NH2) by spectra deconvolution

3.2 Influence of acetic acid on the electrospun of chitosan - Poly (vinyl alcohol) and its electroactive response

3.2.1 Study on the rheological properties of polymeric solutions

3.2.2 Morphology and Diameter Distribution of Chitosan/PVA Nanofibers

3.2.3 Mechanical properties

3.2.4 Thermal Properties

3.2.5 Electroactive Response of the Nanofiber Hydrogels

3.2.6 Determination of free -NH2 by FTIR deconvolution

3.3 Thermal treatment on chitosan - Poly (vinyl alcohol) based micro and nano fibers and its influence on the polymeric crystalline structure

3.3.1 Morphology and Diameter Distribution of Chitosan/PVA Nanofibers

3.3.2 Study on the rheological properties of polymeric solutions

3.3.3 Thermal Properties

3.3.4 Mechanical Properties

3.3.5 Electroactive Response of the Nanofiber Hydrogels

3.3.6 FTIR Spectroscopy

3.3.7 Spectra Deconvolution for The Determination of Intermolecular Hydrogen Bonging and Free Amine (—NH2) Variation Post Thermal Treatment

3.4 Thermal treatment on chitosan - Poly (vinyl alcohol) films and its influence on the polymeric crystalline structure

3.4.1 Thermal properties

3.4.2 Tensile strength test

3.4.3 FTIR Spectroscopy

3.4.4 FTIR Spectra Deconvolution for The Determination of Intermolecular Hydrogen Bonging and Free Amine (—NH2) Variation Post Thermal Treatment

3.4.5 Electroactive response of chitosan/PVA films

3.5 Comparison between the properties of chitosan/PVA-based films and fibers

Conclusions

List of abbreviations

List of Illustrations

List of tables

Index

Index A

Index B

Index C

Index D

Publications

Введение диссертации (часть автореферата) на тему «Электроактивные материалы на основе хитозана и поливинилового спирта»

Реферат Общая характеристика работы

Исследование материалов, способных физически реагировать на внешние раздражители (тепло, электричество, рН, свет и др.), стало потенциальным направлением для создания интеллектуальных систем, применимых в таких областях, как медицина, электроника, робототехника, аэрокосмическая промышленность, бионика и т.д. Особое место занимает исследование материалов на основе полимеров. Полимеры с электрическим приводом, или электроактивные полимеры (ЭАП), продемонстрировали способность к созданию мягких приводов благодаря их механическому отклику на электрическую стимуляцию. ЭАП обладают такими преимуществами полимеров, как эластичность, низкая плотность, регулируемые механические свойства, простота обработки и др.

В последние десятилетия исследователи изучают биополимеры в качестве потенциальных электроактивных материалов. Природные полимеры, такие как хитозан, целлюлоза, крахмал, желатин и другие, обладают подходящими электроактивными характеристиками, обусловленными наличием в них полярных групп, в том числе амино-, гидроксильных и карбоксильных функциональных групп, чувствительных к поляризации.

Хитозан — линейный полисахарид, обладающий свойствами электроактивного полимера. Его электроактивные свойства обусловлены наличием аминогрупп, которые в кислой среде (pH < 7) подвергаются протонированию, в результате чего хитозан ведет себя как катионный полиэлектролит. В результате хитозан имеет высокий потенциал применения в качестве базового компонента при создании электрореактивных мягких приводов.

Среди множества вариантов исполнения мягких приводов волокнистые электроактивные материалы на основе гидрогелей продемонстрировали выдающуюся амплитуду смещения при низком напряжении по сравнению с их объемными аналогами. Это можно объяснить уникальной морфологией микро- и

нановолокон, такой как одномерная форма, высокое отношение поверхности к объему и высокая пористость.

Тем не менее, литература, посвященная изучению электроактивного отклика материалов в форме нановолокон и тому, как различные параметры (например, концентрация полимера, концентрация растворителя, физическая обработка, кристаллизация структуры и т.д.) оказывают непосредственное влияние на элктроактивные свойства материала, ограничена. Кроме того, еще более ограничены возможности использования микро- и нановолокон на основе биополимеров в качестве мягких приводов с электрическим управлением.

Целью исследования было изучение влияния доли свободных групп и степени кристалличности на электроактивные свойства электроформованных волокон и пленок на основе хитозан/ПВС.

Для достижения поставленной цели в рамках диссертационной работы были поставлены следующие задачи:

-Изучить влияние концентрации полимера и растворителя на свойства электроформованных волокон;

-Исследовать зависимость электроактивной реакции от распределения межи внутримолекулярных водородных связей, а также наличия количества свободных аминогрупп в волокнах хитозана/ПВС;

-Изучить влияние уксусной кислоты на изменение количества свободных групп -ЫИг, присутствующих в материале, и его влияние на электроактивные свойства материала;

-Исследовать влияние термической обработки на изменение степени кристалличности и перераспределение водородных связей и возможное формирование микро- и нановолокон хитозана/ПВС с микрогетерогенной структурой и его влияние на элктроактивные свойства материала.

Методы исследования.

Методы исследования, использованные в настоящей работе, были тщательно подобраны для решения поставленных задач и изучения новых аспектов нашего исследования.

Получение нановолокон на основе ПВС и хитозана проводилось на установке электроформования NANON-01A, Япония, с использованием плоского и барабанного коллектора. Электроформование позволило получить нановолокна, которые затем были охарактеризованы с помощью таких методов, как оптическая микроскопия для оценки морфологии и параметров волокон, ИК Фурье-спектроскопия.

Для исследования свойств ПВС-хитозан растворов были использованы: реометр MCR 502, Австрия для определения динамической вязкости полимерных растворов с использованием цилиндрической геометрии; кондуктометр WTW inoLab Cond 720, Германия для измерения электропроводности; Mettler Toledo S213 SevenCompact Duo для определения pH/проводимости.

Для характеристики морфологии волокон ПВС-ХИТОЗАН были получены микрофотографии с использованием оптического микроскопа Olympus STM6 (OLYMPUS Corporation, Токио, Япония). Для исследования молекулярной структуры электроформованных волокон и пленок использовался ИК-Фурье спектрометр Bruker Tensor 37 (Bruker, Германия), на котором были получены инфракрасные спектры поглощения образцов.

Исследование термических свойств электроформованных волокон и пленок на основе хитозан/ПВС проводилось на термогравиметрическом анализаторе (ТГА) TG 209 F1 Libra (Netzsch, Германия). Дифференциальная сканирующая калориметрия (ДСК) проводилась на приборе NETZSCH DSC 204F1 Phoenix.

Измерения на растяжение проводились на машине для испытаний на растяжение Instron 5943 (Instron, Norwood, MA, USA). Толщину измеряли с помощью цифрового микрометра (Techrim T050011, ТЕХРИМ, РФ).

Испытания на электроактивность проводились в электрохимической ячейке при комнатной температуре. Для исследования электроактивности волокон хитозан/ПВС образцы помещались в растворы с различным pH между двумя электродами.

Дополнительно в работе использовались различные компьютерные программы и статистические методы для обработки полученных результатов.

Основные положения, выносимые на защиту:

-Использование состава формовочного раствора для электропрядения, содержащего 4 мас.% хитозана и 5 мас.% ПВС способствует образованию флуктуационной сетки межмолекулярных водородных связей и формированию низкодефектных волокон со средним диаметром 580 ± 10 нм изотропной структуры;

-При повышении концентрации хитозана в формовочном растворе хитозан/ПВС для электропрядения до 4 мас.% происходит увеличение доли свободных аминогрупп, что приводит к повышению электроактивного отклика материала;

-Увеличение доли уксусной кислоты приводит к перераспределению водородных связей в системе ПВС-хитозан в процессе формования полимерных нановолокон. Повышение доли уксусной кислоты до 30 об.% в растворяющей системе формовочного раствора хитозан/ПВС приводит к повышению свободных аминогрупп на поверхности нановолокон на 8,7 %, что приводит к повышению электроактивного отклика материала ввиду перехода от дальнодействующих внутримолекулярных к межмолекулярным водородным связям;

-Разная скорость кристаллизации растворов хитозан/ПВС при их электроформовании и литье раствора на подложку с последующей сушкой приводит к формированию материалов со значимыми различиями в степени кристалличности (пленки - от 57 до 63%, нановолокон от 62,1 до 98%). Дополнительная термическая обработка способна повысить механическую прочность и относительную степень их кристалличности на 5-30 % ввиду уменьшения межмолекулярных расстояний в структуре материалов. Электроактивный отклик нановолокнистых и плёночных материалов хитозан/ПВС в водных средах после термической обработки сильно зависит от их устойчивости к данным средам.

Научная новизна диссертации состоит в следующем:

-Впервые определены условия электроформования нановолокнистых материалов с микрогетерогенной структурой на основе ПВС и хитозана, обладающих заданными электроактивными свойствами;

-Установлено, что электроактивные свойства полученных нановолокон хитозан/ПВС зависят от доли свободных аминогрупп хитозана, изменяются от концентрации полимера и соотношения бинарных растворителей: вода - уксусная кислота;

-Разработаны методы целенаправленного изменения морфологии и электроактивных характеристик полимерных нановолокон и микроволокон, полученных на основе ПВС и хитозана в бинарной системе растворителей, путем варьирования реологических свойств прядильного раствора;

-Установлена зависимость влияния скорости кристаллизации при формировании нановолокон хитозан/ПВС и полимерных пленок на электроактивные свойства материалов.

Научно-техническая задача решаемая в диссертации, заключается в создании нано- и микроволоконной системы, обладающей электроактивными свойствами, являющейся гибкой, устойчивой к набуханию, работающей при малом напряжении, для возможного их применения в качестве искусственных мышц.

Объектом исследования является полимерные прядильные растворы ПВС и хитозана с концентрацией 5,0 - 4,0 (мас.%) и соотношением ПВС:хитозан = 4,0 - 2,0 ^ 5,0 - 4,0 (мас.%) в системе растворителей вода/уксусная кислота, а также электроформованные нано- и микроволокна на их основе при варьировании технологических и рецептурных параметров получения нетканного материала (приложенное напряжение, скорость подачи раствора, расстояние между иглой и коллектором).

Теоретическая значимость результатов диссертационной' работы состоит в установлении закономерностей влияния скорости кристаллизации и структурирования прядильных растворов на особенности формирования нановолокон на основе ПВС и хитозана, обладающих регулируемыми

электроактивными и физико-химическими свойствами. Впервые установлена эмпирическая корреляция между долей свободных аминогрупп хитозана и электрическим откликом волокнистых материалов на основе ПВС и хитозана, позволившая разработать научно-обоснованный подход к получению нановолокнистого полимерного материала с заданными параметрами.

Практическая значимость результатов диссертационной' работы состоит в:

Получении нано- и микроволокон из поливинилового спирта и хитозана из системы растворителей уксусная кислота - вода, обладающих заданными характеристиками и морфологией поверхности, которые перспективны для использования в материалах при создании мягких искусственных мышц различной формы.

Точность работы обеспечивается использованием в исследовании различных методов характеризации, что позволяет всесторонне оценить свойства полученных материалов. Высокая воспроизводимость экспериментальных результатов также способствует точности исследования, так как свидетельствует о согласованности выводов при многократных испытаниях. Кроме того, согласованность данных, полученных разными методами, повышает доверие к результатам исследований, усиливая их обоснованность и надежность. Полученные результаты были тщательно проверены путем сравнения с результатами предыдущих исследований, что дает дополнительную гарантию точности полученных результатов.

Достоверность полученных результатов обеспечивается измерениями с использованием современных методов исследования, высокоточных приборов, а также математических и статистических методов обработки результатов. Результаты исследования успешно публиковались в рецензируемых научных изданиях, как российских, так и международных, и обсуждались на международных и всероссийских конференциях.

Реализация результатов исследований нановолокон хитозан/ПВС дает значительные перспективы, в частности, в области создания мягких манипуляторов. Результаты исследования показывают, что эти материалы

обладают значительным потенциалом для их практического использования. Полученные результаты исследований может привести к созданию быстродействующих и долговечных устройств, которые найдут широкое применение в различных областях, включая робототехнику, биомедицинскую инженерию и другие.

Обращает на себя внимание удивительная долговечность этих образцов: максимальный срок службы при непрерывном использовании составил 5400 циклов. Такая долговечность подчеркивает их пригодность к длительному и многократному использованию, что является важной характеристикой для мягких манипуляторов, работающих в различных областях.

Результаты, полученные в ходе исследования, внедрены в учебный процесс Университета ИТМО.

Апробация результатов исследования. Основные результаты исследования были представлены и обсуждены на следующих конференциях: XLIX научная и учебно-методическая конференция 1111С, Университет ИТМО (29.01.2020 -01.02.2020); Международной научно-практическая online конференция «Интеграция науки, образования и производства - основа реализации плана нации», Карагандинский государственный технический университет (18.06.2020 -

19.06.2020); Материалы XXVII Международной научной конференции студентов, аспирантов и молодых учёных "Ломоносов-2020", Московском государственном университете имени М.В. Ломоносова; L научная и учебно-методическая конференция ППС, Университет ИТМО (01.02.2021 - 04.02.2021); Международной научно-практической online конференции «Интеграция науки, образования и производства - основа реализации Плана нации» (Сагиновские чтения №13), Карагандинский государственный технический университет (17.06.2021 -

18.06.2021); Пятьдесят первой (LI) научной и учебно-методическая конференция Университета ИТМО, Университет ИТМО (02.02.2022 - 05.02.2022); XI Конгресса молодых ученых, Университет ИТМО (04.04.2022 - 08.04.2022); XVIII Международной научно-практической конференции «Новые полимерные композиционные материалы. Микитаевские чтения», Кабардино-Балкарского

государственного университета (ЭУНК КБГУ) (04.06.2022 - 09.06.2022); Всероссийская Конференция МОЛОДЫЕ ПРОФЕССИОНАЛЫ, Университета ИТМО (25 - 27 октября 2022 года); Международная онлайн научно практическая конференция «формирование интеллектуального капитала в условиях цифровой трансформации: опыт, вызовы, перспективы», Карагандинский технический университет имени Абылкаса Сагинова (14.12.2022); Пятьдесят Второй (ЬП) Научной и Учебного - Методической Конференции, Университета ИТМО (31 января - 03 февраля, 2023); XII Конгресс молодых ученых ИТМО, Университет ИТМО (04.04.2023 - 06.04.2023); IX Всероссийская научно-практическая конференция с участием молодых ученых «ИННОВАЦИОННЫЕ МАТЕРИАЛЫ И ТЕХНОЛОГИИ В ДИЗАИНЕ», Государственный Институт Кино и Телевидения, Санкт-Петербург (10.06.2023 - 11.06.2023); III Международная научно-практическая конференция «ФУНДАМЕНТАЛЬНАЯ НАУКА ДЛЯ ПРАКТИЧЕСКОЙ МЕДИЦИНЫ- 2023» АДДИТИВНЫЕ ТЕХНОЛОГИИ, СОВРЕМЕННЫЕ МАТЕРИАЛЫ И ФИЗИЧЕСКИЕ МЕТОДЫ В МЕДИЦИНЕ: ИННОВАЦИИ, Кабардино-Балкарского государственного университета (ЭУНК КБГУ) (06.09.2023 - 09.09.2023).

Личный вклад автора. Автор принимал участие в получении волокон и пленок на основе хитозана и ПВС методами электроспиннинга и полива, для создания электрореактивных волокнистых материалов; постановке и проведении экспериментов, обработке полученных экспериментальных данных; определении и характеризации физико-химических свойств полученных волокон и исходных растворов их прекурсоров; разработке экспериментальной методики определения электроактивного отклика волокнистых материалов, обсуждении полученных результатов и подготовке публикаций.

Структура диссертации и количество страниц. Диссертация состоит из введения, обзора литературы, главы "Методы и материалы", обсуждения результатов, заключения и списка литературы. Работа изложена на 208 страницах, содержит 30 таблиц и 66 рисунка, список литературы включает 198 источников.

Основное содержание работы

Во введении основное внимание уделяется актуальности исследования, обосновывается выбор материалов и методов исследования, а также представлены цели и задачи исследования.

В первой главе представлен обзор литературы, знакомящий с концепциями "умных" материалов на основе полимеров. В ней особое внимание уделено группе полимеров, проявляющих механическую активность в ответ на электрический стимул, известные как электроактивные полимеры (ЭАП), с точки зрения их применения в качестве мягких приводов. Представлена возможность применения гидрогелей в форме нановолокон в качестве потенциальных электроактивных актюаторов и применение биополимеров в качестве электроактивных полимеров.

Во второй главе обоснован выбор объектов исследования и материалов, рассмотрены методики исследования раствора хитозан/ПВС, используемого в процессе электроспиннинга при получении нановолокон, а также представлены методики исследования полученного нетканного волокнистого материала и изучения его электроактивного отклика.

Работа проводилось в четыре этапа. На первом этапе проводилось получение растворов хитозана и ПВС и формирование из них нановолокон с использованием метода электроспиннинга. На втором этапе изучались электроактивные свойства нановолокон хитозан/ПВС. На третьем этапе было исследовано влияние растворителя различной концентрации на количество свободных аминогрупп в волокнах и его влияние на электроактивные свойства. На четвертом этапе изучалось влияние термической обработки на степень кристалличности и ее влияние на термические, механические и электроактивные свойства волокон и пленок.

Исследование свойств растворов полимеров проводилось следующими методами: динамической вязкости растворов полимеров на реометре MCR 502 (Anton Paar, Австрия) с использованием цилиндрической геометрии; измерение

электропроводности и рН растворов проводилось с помощью рН/кондуктометра Mettler Toledo S213 SevenCompact Duo.

Полимерные растворы характеризовали методом ротационной вискозиметрии на реометре MCR 502 (Anton Paar, Грац, Австрия), оснащенном модулем контроля температуры C-PTD 200 и системой измерения "цилиндр-чаша" CC27 (ISO 3219-1:2021). Измерения растворов проводились при постоянной температуре 25 °С и в диапазоне скоростей сдвига 0,1-500 с-1 с логарифмическим профилем изменения скорости сдвига.

Для характеристики морфологии волокон ПВС-Хитозан были получены микрофотографии с использованием оптического микроскопа Olympus STM6 (OLYMPUS Corporation, Токио, Япония).

Для исследования молекулярной структуры электроформованных волокон и пленок использовался ИК-Фурье спектрометр Bruker Tensor 37 (Bruker, Германия), на котором были получены инфракрасные спектры поглощения образцов.

Исследование термических свойств нановолокон и пленок на основе хитозана/ПВС проводилось на термогравиметрическом анализаторе (ТГА) TG 209 F1 Libra (Netzsch, Германия) в атмосфере азота со скоростью потока 40 мл/мин. Образцы исследовались в диапазоне температур от 25 0C до 900 0C со скоростью нагрева 10 К/мин. Данные дифференциальной сканирующей калориметрии (ДСК) были получены на приборе NETZSCH DSC 204F1 Phoenix в атмосфере азота, все образцы были запрессованы в алюминиевые тигли.

Измерения на растяжение проводились на машине для испытаний на растяжение Instron 5943 (Instron, Norwood, MA, USA). Все образцы испытывались в соответствии со стандартом ISO 527-3 при комнатной температуре и скорости испытания 10 мм/мин. Толщину измеряли с помощью цифрового микрометра (Techrim T050011, ТЕХРИМ, РФ). Толщина образца вычисляли как среднее значение толщины, измеренной в трех разных местах испытуемого образца. Было испытано по четыре образца каждой состава и получены средние значения модуля Юнга, предела прочности при растяжении и удлинения при разрыве.

Исследовании электроактивного отклика материала проводили в электрохимической ячейке путем погружения в растворы электролитов при комнатной температуре и приложении напряжения. Для исследования электроактивности волокон хитозан/ПВС образцы помещались в растворы с различным значением pH от 2 до 11. Для измерения скорости смещения волокон использовалась цифровая видеокамера, которая фиксировала электромеханический отклик материала. Полярность напряжения на электродах менялась с интервалом в 1 с. Полученная запись обрабатывалась и анализировалась с использованием программы ImageJ, в которой производилась количественная оценка линейного смещения в зависимости от времени.

Анализ всех экспериментальных результатов проводился с использованием следующих программных продуктов: Microsoft Excel, ImageJ и OriginPro 2019b.

В третьей главе представлены полученные экспериментальные данные и их обсуждение.

В разделе 3.1 представлено изучение влияния концентрации хитозана на получение нановолокон из хитозана и поливинилового спирта (ПВС). Основной задачей являлось систематическое изменение концентрации хитозана в полимерном растворе, используемом для электроспиннинга, и изучение его влияния на морфологию, физические свойства и электроактивное поведение получаемых нановолокон в результате изменения сети межмолекулярных водородных связей.

Электроспиннинг это метод формирования нановолокон путем воздействия электрическим полем на раствор полимера. Технические параметры электроформования и свойства растворов оказывают влияние на морфологию и свойства электроформованных волокон. Вязкость и электропроводность раствора являются важными факторами при получении нановолокон методом электроспиннинга.

Растворы хитозана и хитозана/ПВС проявили неньютоновское поведение. В исследовании растворов наблюдалось сдвиговое разрежение или

псевдопластическое течение, что объясняется наличием в хитозане ионизированных групп —NH2 (Рисунок 1).

9000 -8000 -7000 -

о

я 6000 -

с

2

^ 5000 -

■а

\ 4000 -

в

а 3000 -2000 -1000 -

О I I I I [ I II—I I I I I | I II—I I I I I | I-1 I I I Г I I |-Г—1—I I Г I I I |

0.1 1 10 100 1000

y(s')

Рисунок 1 - Зависимость вязкости от концентрации хитозана Динамическая вязкость хитозана линейно возрастает с увеличением концентрации хитозана в бинарной системе растворителей, состоящей из воды и уксусной кислоты. Кроме того, добавление ПВС к раствору хитозана резко увеличивает кажущуюся вязкость растворов полимерных смесей. Увеличение вязкости можно объяснить водородной связью между —NH2- и —OH-группами хитозана и —OH-группами ПВС, запутанностью полимер-полимерных связей и случайной ориентацией полимерных цепей. При увеличении скорости сдвига вязкость уменьшается, что можно объяснить воздействием групп —NH2 на электростатическое и стерическое отталкивание, а также перестройкой полимерной структуры.

Проводимость растворов хитозана, как и вязкость, имела линейное увеличение при повышении концентрации хитозана. С другой стороны, при введении ПВС в смесь раствор проводимость уменьшалась. Возможно это связано с тем, что при увеличении концентрации хитозана возрастает количество

Хитотан. масс. % Хитотан. ПВС, масс. %

— _ 2 -2/5

--2.5 -2.5/5

-3/5

--3.5 -3.5/5

• 4 -4 5

у = 50 s*1

протонированных аминогрупп. Это приводит к увеличению свободного заряда в полимерной цепи. В результате полимер-полимер взаимодействия при введении ПВС в раствор происходит образование межмолекулярных водородных связей между амино- и ОН-группами хитозана и ОН-группами ПВС, что снижает нейтрализацию свободного заряда в основной цепи хитозана ( Рисунок 2).

Рисунок 2 - Зависимость проводимости от концентрации хитозана

Раствор, содержащий концентрацию хитозана в 4 масс. % в смеси с ПВС с 5

масс. %, показал стабильное формирование волокон в процессе

электроформования (Таблица 1).

Таблица 1 - Морфология и распределение диаметров волокон хитозан/ПВС

Хитозан/ПВС - Диаметр нановолокна (цш) Хитозан/ПВС, масс. %

Уо^е (кУ) 20 22.5 25 27.5 30

2/ 5 2.03 2 1.99 1.90 2.02

(Частицы) (Частицы) (Частицы) (Частицы) (Частицы)

2,5/5 2 1.82 1.56 0.61 0.62

(Частицы) (Частицы) (Частицы)

3/5 1.99 0.63 0.58 0.54 0.54

3.5/5 1.90 0.58 0.54 0.53 0.52

4%/5 2.02 0.55 0.59 0.57 0.58

С целью изучения молекулярного взаимодействия в нановолоконах хитозан/ПВС был проведен анализ FTIR-спектра нановолоконного мата (Рисунок 3). Из анализируемых спектров видно образование водородных связей между ПВС

и хитозаном. Смещение в сторону меньших значений пика растягивающих колебаний -ОН и -ЫН порошка хитозана (3354 см-1) в область около 3300 см-1 для нановолокон хитозан/ПВС Кроме того, пик около 1590 см-1, обусловленный водородной связью между -ЫН-группами хитозана и -ОН ПВС, сместился в сторону меньших значений около 1560 см-1 для волокон хитозан/ПВС. Смещение этих двух характерных пиков указывает на образование межмолекулярной водородной связи между полимерами.

Рисунок 3 - ИК-Фурье спектры образцов нановолокон хитозана, ПВС и

хитозана/ПВС

Результаты исследования на механическую прочность нановолокон хитозан/ПВС при растяжении показали, что концентрация хитозана оказывает непосредственное влияние на механические свойства нановолоконных матов (Рисунок 4). Предел прочности при растяжении увеличился на 67,2% при увеличении концентрации хитозана в нановолокнах с 2,5 до 3,5 масс. Модуль Юнга образцов нановолокон увеличился на 116,4% при увеличении концентрации хитозана в нановолокнах с 2,5 до 4 масс %. Эти результаты объясняются межмолекулярным взаимодействием между хитозаном и ПВС. С другой стороны,

прочность на разрыв волокон, содержащих 4 масс. % хитозана, снизилась почти на 26,6%, что объясняется жесткостью основой цепи хитозана.

к -

Было исследовано влияние концентрации хитозана на электроактивный отклик нановолокон, а также связь этого изменения с долей свободных аминогрупп в структуре волокон.

Тест на электроактивность показал, что скорость смещения сильно зависит от концентрации хитозана, а также от pH раствора (Рисунок 5). С увеличением концентрации хитозана нановолокна становились более электрочувствительными. Волокна с 4 масс. % хитозана демонстрировали перемещение со скоростью 1,86 мм с-1 при pH 3. Образец с концентрацией хитозана 2,5 масс. % при тех же условиях демонстрировал перемещение со скоростью 1,2 мм с-1. При протонировании аминогрупп в структуре хитозана, хитозан ведет себя как катионный полиэлектролит.

Список литературы диссертационного исследования кандидат наук Ольвера Берналь Ригель Антонио, 2023 год

<00 190

Temperature ('CI

Figure 3.23 - DSC thermographs of (a) chitosan, PVA (b) and chitosan/PVA nanofibers, electrospun from polymeric solutions with different acetic acid content

3.2.5 Electroactive Response of the Nanofiber Hydrogels

The electroactive response of chitosan/PVA was studied by measuring the

displacement of the samples as a function of time. The influence of acetic acid concentration, used to dissolve the polymers, and pH (from 2 to 7) on the electroactive response were studied. The samples, made of films based on chitosan/PVA fibers, used for this experiment had a size of 20 x 4 mm (length x width) with a thickness of ~ 0.03 mm in the dry state. The obtained speed displacement for all samples is shown in Index B. Figure 50 demonstrate the electroactive response of the chitosan/PVA fibers when immersed in an acidic solution under an electrical stimulus (10 V) by inversing the polarity each 50 s.

Figure 3.24. Electroactive response of chitosan/PVA fibers under an electrical stimulus. The voltage applied was equal to 0, 10 and -10 V (from left to right)

It was found that the electroactive response of the fibers changed depending on the concentration of acetic acid used in the solution for the fiber formation. These results are shown in Figure 3.25. It is possible to observe that samples obtained from solution with acetic acid content at 10 and 80 vol. % exhibited the lowest speed displacement at all pH solutions, reaching a maximal speed of 0.86- and 1.37-mm s-1 at pH 3. In contrast, the sample obtained from the solution with 30 vol. % of acetic acid demonstrated the fastest speed displacement in all pH, reaching a maximum speed of 2.05 mm s-1 at a pH 3. The electroactive behavior of the material is due to -NH2 groups present in the structure of chitosan. When amino groups enter in contact with an acidic environment it gets protonated, thus chitosan behaves as a cationic polyelectrolyte. Therefore, when chitosan/PVA fibers are immerse in a solution with pH<7 and a voltage is applied, the free ions will be asymmetrically distributed, due to the migration toward their counter electrodes.

pH

Figure 3.25 - Speed displacement of nanofiber electrospun from solutions with

different acetic acid concentration

3.2.6 Determination of free -NH2 by FTIR deconvolution

To determine the hydrogen bonding interaction and the proportion of free amine, deconvolution in Gaussian line shapes was performed on the 3000-3700 cm-1 peak in the FTIR spectrum (Figure 3.26). Hydrogen bond types were analyzed on the -OH region and the -NH region. Absorption peak of free amine group is around 3408 cm-1, absorption peak of intermolecular association is around 3335 cm-1, absorption peak of amide group is around 3240 cm-1, and the characteristic absorption peak of primary ammonium is around 3100 cm-1. For the -OH region the peak of free hydroxyl is around 3580 cm-1, the peak of multimer intermolecular association is around 3462 cm-1. a) b) c)

Wavcnumbcr (cm'1) Wavcnumher (cm-1)

Figure 3.26 - Spectra deconvolution for chitosan/PVA nanofibers samples electrospun different acetic acid content: a) 10 vol. %; b) 30 vol. %; c) 40% vol. %; d)

60 vol. %; e) 80 vol. %

The area under the curve is assigned to each characteristic peak which represents the composition ratio of hydrogen bonds in the -NH and -OH regions, all results are shown in Table 3.15. The spectra deconvolution results show a correlation between the proportion of free amino groups and the electroactive response behavior of the material. As it can be observed fibers obtained with 30 % of acetic acid had a higher proportion of free -NH2 (~8.71 %), meanwhile the fibers obtained from solution containing 10 and 80 vol. % of acetic acid, which exhibited the slowest speed displacement, had the lowest proportion of free -NH2, being 3.3 and 4 %, respectively.

Table 3.15 - Relative strength of the deconvoluted band in the region 3000 - 3700 cm-1 for chitosan (4 wt. %)/PVA (5 wt. %) fibers samples

Sample Hydrogen bond types Abbreviation Wavenumber, cm-1 Relative strength, %

Acetic acid 10% Primary ammonium I -NH+3 ~ 3100 cm-1 7.6

Intermolecular hydrogen bond II OH...ether O ~ 3200 cm-1 20.9

Amide III -CONH- ~ 3240 cm-1 2.3

Intermolecular association IV N2-H1...O5*/ N2-H2...O1* ~ 3335 cm-1 49.6

Free amine V -NH2 ~ 3408 cm-1 3.3

Multimer (Intermolecular association) VI O6H...N2* ~ 3462 cm-1 16.1

Free hydroxyl VI I -OH ~ 3580 cm-1 0.09

Acetic acid 30% Primary ammonium I -NH+3 ~ 3100 cm-1 3.04

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 20

Amide III -CONH- ~ 3240 cm-1 2.1

Intermolecular association IV N2-H1...O5*/ N2-H2...O1* ~ 3335 cm-1 49.6

Free amine V -NH2 ~ 3408 cm-1 8.71

Multimer (Intermolecular association) VI O6H...N2* ~ 3462 cm-1 16.9

Free hydroxyl VI I -OH ~ 3580 cm-1 0.1

Primary ammonium I -NH+3 ~ 3100 cm-1 3.4

Intermolecular hydrogen bond II OH...ether O ~ 3200 cm-1 17.4

Amide III -CONH- ~ 3240 cm-1 7.2

Acetic acid 40% Intermolecular association IV N2-H1...O5*/ N2-H2...O1* ~ 3335 cm-1 46.5

Free amine V -NH2 ~ 3408 cm-1 8.1

Multimer 17

(Intermolecular association) VI O6H...N2* ~ 3462 cm-1

Free hydroxyl VI I -OH ~ 3580 cm-1 ~0

Primary ammonium I -NH+3 ~ 3100 cm-1 3.6

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 23.4

Amide III -CONH- ~ 3240 cm-1 4.8

Acetic acid 60% Intermolecular association IV N2-H1...O5*/ N2-H2...O1* ~ 3335 cm-1 44.7

Free amine V -NH2 ~ 3408 cm-1 4.4

Multimer VI O6H.N2* ~ 3462 cm-1 19.1

(Intermolecular association)

Free hydroxyl VI I -OH ~ 3580 cm-1 0.01

Primary ammonium I -NH+3 ~ 3100 cm-1 4.1

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 20.9

Amide III -CONH- ~ 3240 cm-1 3.8

Acetic acid 80% Intermolecular association IV N2-H1.O5*/ N2-H2.O1* ~ 3335 cm-1 49.6

Free amine V -NH2 ~ 3408 cm-1 4

Multimer VI O6H.N2* ~ 3462 cm-1 17.2

(Intermolecular association)

Free hydroxyl VI I -OH ~ 3580 cm-1 ~0

Chitosan/PVA fibers were successfully electrospun, from solution containing

different acetic acid concentrations. The concentration of acetic acid used for the

dissolution of chitosan and PVA, affected the morphology of the obtained fibers, varying their diameter from 470 nm (solution with 10% acetic acid) to 990 nm (solution with 80% acetic acid). Furthermore, the concentrations of the acid had a further influence on the electroactive properties of the electrospun fibers. The deconvolution of the FTIR spectra shows a difference in the proportions of free -NH2 on the samples. Such variations can be as a result of the acetic acid. This work showed that the concentration of solvent used for the solvation of polymers, when development of electro responsive material, can provide a direct influence on its the electroactive properties.

3.3 Thermal treatment on chitosan - Poly (vinyl alcohol) based micro and nano fibers and its influence on the polymeric crystalline structure

In recent years, the quest for innovative materials with electroactive properties has

driven significant research efforts, fueled by their diverse applications in fields ranging from electronics and sensors to biomedical devices. Among these materials, chitosan and polyvinyl alcohol (PVA) have garnered considerable attention for their promising electroactive characteristics. In this section, it is presented a pioneering investigation that comprehensively explores the influence of pre- and post-fabrication parameters on the electroactive properties of electrospun chitosan/PVA-based micro and nanofibers.

This study marks a crucial milestone as it delves into uncharted territory by exploring the combined effects of pre-fabrication and post-fabrication parameters on the electroactivity of the chitosan/PVA fibers. To achieve this, a meticulous approach was employed, involving the fabrication of the fibers using the electrospinning technique, followed by comprehensive characterization and testing as electroactive materials. Specifically, solutions with varying acetic acid contents (ranging from 50 to 80 vol.%) were utilized, and the rheological properties of these solutions were meticulously analyzed.

The characterization of the resulting fiber mats involved a wide array of techniques, including optical microscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile testing, and FT-IR spectroscopy. Such a multidimensional approach enabled us to gain valuable insights into the structural and chemical properties

of the electrospun chitosan/PVA fibers, paving the way for a profound understanding of their electroactive behavior.

A key aspect of our investigation revolved around subjecting the fiber mats to thermal treatment, which significantly influenced their stability and electroactive response. Notably, fibers electrospun from 80% acetic acid exhibited lower electroactive responses and rapid dissolution, raising intriguing questions about the underlying mechanisms. However, through careful thermal treatment, the stability and electroactive response of all fiber samples were notably improved. Remarkably, the fibers spun with 80% acetic acid witnessed a substantial increase in speed displacement presenting exciting possibilities for their application in specific scenarios.

By shedding light on the intricate interplay between pre- and post-fabrication parameters and their impact on the electroactive properties of chitosan/PVA fibers, this study contributes valuable insights to the development of electroactive materials in various applications. The findings presented in this thesis open up new avenues for further research and innovation in the field of electroactive materials, with potential implications for smart textiles, biomedical devices, and other cutting-edge technologies.

3.3.1 Morphology and Diameter Distribution of Chitosan/PVA Nanofibers

The electrospinning process was carried out using the following technical

parameters: a voltage range between 20 to 30 kV, a distance of 150 mm between the needle and collector, and a feed rate of 0.2 mL/h. This electrospinning process was conducted under environmental conditions with a temperature of 26 °C and a relative humidity of 23%. In index C is shown the micrographs for all electrospun samples. The resulting fibers were obtained from a polymer solution composed of chitosan/PVA dissolved in various concentrations of acetic acid (50, 60, 70, and 80 vol. %) in an aqueous solution. In Table 3.16, the diameter fibers are shown. The obtained micrographs revealed that the concentration of acetic acid used in the various solutions did not have a significant impact on the quality of the fiber formation process. The electrospinning process remained stable for all solutions, resulting in uniform fiber formation without the presence of beads or particles.

Table 3.16 - Morphology and diameter distribution of chitosan/PVA fibers electrospun from solutions with different acetic acid contents

Chitosan (4%)/PVA (5%) N.F. Diameter (^m)

Voltage (kV)

Acetic acid 20 22.5 25 27.5 30

(vol. %)

50 0.62 0.57 0.55 0.51 0.48

60 0.67 0.64 0.61 0.57 0.53

70 0.69 0.67 0.64 0.63 0.62

80 0.97 0.92 0.88 0.81 0.79

As mentioned earlier, the variation in acetic acid concentration used for fiber formation does not seem to affect the overall quality of the electrospun fibers. However, it does impact the diameter of the resulting fibers. The measured diameters reveal that as the concentration of acetic acid in the solution increases, the diameter of the resulting fibers also increases. This variation in fiber diameter in relation to acetic acid concentration allows for the electrospinning of fibers ranging from the nano to the microscale while maintaining a consistent polymer content. The change in fiber diameter can be explained as follows. Chitosan is a cationic polysaccharide with amino groups attached to its backbone. In acidic solutions (pH < 6) amino groups (-NH2) get protonated, forming -NH+3 ions. Thus, generating charge repulsions causing chitosan's chain to expand. As the concentration of acetic acid increases the pH of the solution decreases, hence increasing the number of protonated amino groups in chitosan. Moreover, the protonated amino groups interact by intermolecular hydrogen bonds. Due to the influence of acetic acid the rheological properties of the solutions change. Polymeric solution containing acetic acid at 50% had a lower viscosity and higher conductivity, produced fibers with thinner diameter (~0.482 ^m). On the other hand, fibers obtained from the solution containing acetic acid at 80%, produced thicker fibers (~0.793 ^m), this as a result of its higher viscosity and a lower conductivity.

3.3.2 Study on the rheological properties of polymeric solutions

Rheological properties in polymeric solutions, plays a fundamental roll in the

formation of fibers during electrospinning process. Parameters such as the viscosity and conductivity can change the morphology of the fibers as it was observed in the previous section. All the measured solutions exhibited non-Newtonian behavior characterized by

shear thinning or pseudoplastic flow. This behavior can be attributed to the ionized amine groups in chitosan. The presence of hydrogen bonding between -NH2 and -OH groups of chitosan with the -OH groups of PVA, along with polymer-polymer entanglement and randomly oriented polymeric chains, significantly increases viscosity, leading to the observed pseudoplastic flow. As the shear rate goes higher the viscosity decreases, this can be attributed to the exposed of —NH3 groups affecting the electrostatic and steric repulsion and the realigned polymeric structure.

The viscosity of polymeric solutions obtained by dissolving chitosan in different concentrations of acetic acid (50, 60, 70, and 80 vol. %) was initially examined. As depicted in Figure 3.26, the viscosity of chitosan solutions increases with a rise in acetic acid concentration. This observation becomes evident when measuring the apparent viscosity at a fixed shear rate (y), highlighting the relationship between increased acetic acid concentration and elevated viscosity in chitosan solutions. When measuring the apparent viscosity at y = 50 s-1 is possible to notice that the viscosity increases from 986.93 mPa-s (A.A. 50 vol. %) to 2014.2 mPa-s (A.A. 70 vol. %). The increase in viscosity with higher acetic acid concentration can be explained by the protonation of amino groups in an acidic medium. In an acidic environment, -NH2 groups become protonated (-NH3+), leading to an increase in the charge density along the molecular chain and causing the chain to unfold. As the polymer chain unfolds, the degree of entanglement and intermolecular interactions among the polymer chains also increases. Therefore, with a higher concentration of acetic acid, there are more hydrogen bonds formed between the acetic acid and the polymer, resulting in an increase in the viscosity of the solutions. Nonetheless, it was observed that the viscosity drastically got reduced when the concentration of acetic acid was of 80% (1262.8 mPa-s). The decrease in viscosity is a result of an increase in intramolecular electrostatic repulsion among chitosan molecules with a high positive charge density. Similar results have been reported.

Share rale y (s"')

Figure 3.26 - Scheme of the dependence of viscosity from the concentration of acetic acid (50, 60,70, and 80 vol. %). in polymeric solutions

The viscosity of polymeric solutions containing chitosan/PVA dissolved in acetic acid at various concentrations (50, 60, 70, and 80%) was measured. The results indicate that the addition of poly(vinyl alcohol) significantly increases the viscosity of the polymeric solutions, as summarized in Table 3.17. Additionally, as shown in Figure 53, the viscosity of these solutions increases as the concentration of acetic acid rises. In contrast to what was observed with chitosan solutions (where viscosity exhibited both increases and decreases), chitosan/PVA solutions consistently showed an increase in viscosity with increasing acetic acid concentration. This viscosity increase ranged from 6157 mPa-s (AA50) to 8747.3 mPa-s (AA80). This behavior can be attributed to the entanglement of polymer chains and the high number of hydrogen bonds formed between the ionized amino groups and -OH groups in chitosan, interacting with the -OH groups of PVA.

Table 3.17 - Apparent viscosity measured at y = 50 s 1 of chitosan and chitosan/PVA solutions dissolved in different concentration of acetic acid

Acetic acid (vol. %) 50% 60% 70% 80%

Cs. Cs/PVA Cs. Cs/PVA Cs. Cs/PVA Cs. Cs/PVA

Viscosity (rç) [mPa-s] 986.93 6157 1512.5 6445 2014.2 6847.7 1262.8 8747.3

The changes in conductivity, as depicted in Figure 3.27, depend on two factors: the addition of PVA to chitosan solutions and the concentration of acetic acid. Firstly, the decrease in conductivity with an increase in acetic acid concentration can be attributed to the greater density and strength of hydrogen bonds formed, which in turn reduce the number of free charged groups available.

Figure 3.27 - Scheme of the dependence of conductivity from the concentration of acetic acid in polymeric solutions

Table 3.18 - Conductivity of chitosan and chitosan/PVA solutions dissolved in different concentration of acetic acid

Acetic acid

50%

60%

70%

80%

Cs. Cs/PVA Cs. Cs/PVA Cs. Cs/PVA Cs. Cs/PVA

Conductivity (|S cm-1) 3558.4 2852.6 2695.2 2126.2 2395.4 1682.6 1245.6 999.6

3.3.3 Thermal Properties

As previously mentioned, the focus of this section was on how thermal treatments can affect the electroactive properties of electrospun fibers. However, it's important to note that thermal treatments can also influence the thermal properties of these fibers. Chitosan/PVA fiber mats underwent thermal treatment by being placed in a drying oven for 24 hours at a temperature of 70 °C. Tables 3.19 and 3.20 summarize the TG

(Thermogravimetric) results. The TG thermographs of non-thermally treated chitosan/PVA fibers exhibited a weight loss profile in three temperature stages, as shown in Figure 3.28 (a). Both chitosan and PVA displayed weight losses at two stages. For PVA, the initial reduction in weight was observed within the temperature range of 51134 °C, attributed to the evaporation of moisture content (~4%). Subsequently, a second weight reduction occurred between 157-450 °C, indicating the thermal degradation of PVA (~90.52%). Regarding chitosan, the initial decrease in weight occurred within the temperature range of 35-118 °C, associated with the evaporation of moisture content (~5%). Subsequently, a second weight reduction was observed between 182-400 °C, indicating the thermal degradation and deacetylation of chitosan (~49.68%).

Table 3.19 - Thermogravimetric analysis of chitosan powder, PVA powder, and chitosan/PVA fibers (not thermally threaten)_

Acetic acid, vol. % First Mass Loss (%) Second Mass Loss (%) Third Mass Loss (%) First Stage 1st Peak (°C) Stage 2nd Peak (°C) Second Stage (°C) Third Stage (°C)

PVA 4 90.53 81 _ 304 .......................................................

Chit 5 49.68 67 299 .........................................................

AA50 11.6 65.98 51 93 220 ..........................................................

AA60 11.2 58.18 15.86 56 100 264 428

AA70 10.52 57.98 16.44 56 109 264.8 428

AA80 9.58 59.27 15.81 56 108 266 427

Table 3.20. Thermogravimetric analysis of chitosan/PVA fibers (thermally

threaten). Samples were dried at 70 °C for 24 hours

Acetic acid, vol. % First Mass First Second Mass Third Mass Stage First Stage 2nd Peak (°C) Second Stage (°C) Third

Loss (%) Loss (%) Loss (%) 1st Peak (°C) Stage (°C)

50 2.42 71.07 ^^^^ 52 ...................................................... 231 ......................................................

60 3.1 71.62 ^^^^^ 60 .......................................................... 271 ..........................................................

70 3.26 71.35 ^^^^^ 60 125 272 .........................................................

80 6.47 71.79 ^^^^ 63 125 275 ......................................................

0 200 400 60»

Temperature (°C) M ■ i i ■ i • • ■ •

• M «■ «v >

Figure 3.28 - TG thermographs of (a) chitosan, PVA and chitosan (4 wt.%)/PVA (5 wt. %), electrospun from polymeric solutions with different acetic acid content. (b) 50 vol. %; (c) 60 vol. %; (d) 70 vol. %; (e) 80 vol. %

With the exception of sample AA50, the mass loss in non-thermally treated chitosan/PVA fibers occurred in three stages, as depicted in Figure 3.29. TG (Thermogravimetric) thermographs revealed that the first mass loss takes place within a temperature range of 50-160 °C, which can be attributed to the vaporization of moisture and solvent residues. It's noticeable from the DTG (Derivative Thermogravimetry) curves that in this first stage, two peaks appear. The presence of the second peak in this stage is due to acetic acid residues, which have a boiling point of 118 °C. The second mass loss was observed for F-AA50 in the temperature range of 135-409 °C, while for samples F-AA60, F-AA60, and F-AA80, it occurred in the range of 180-370 °C. This second mass loss is associated with the thermal decomposition of the chitosan/PVA complex. A third mass loss was observed in samples F-AA60, F-AA60, and F-AA80 in the temperature range of 375-500 °C. This can be attributed to the degradation of PVA byproducts and residuals of poly (vinyl acetate), which has a decomposition temperature of approximately 400 °C and is present in the PVA chains.

Figure 3.29 - DTG thermographs of (a) chitosan, PVA and chitosan (4 wt.%)/PVA (5 wt. %), electrospun from polymeric solutions with different acetic acid content. (b) 50 vol. %; (c) 60 vol. %; (d) 70 vol. %; (e) 80 vol. %

In the case of thermally treated samples, notable changes were observed in their DTG (Derivative Thermogravimetry) curves, as depicted in Figure 56(b), 7(c), 7(d), and 7(e) for dried samples F-AA50, F-AA60, F-AA70, and F-AA80, respectively. In contrast to untreated samples, thermally treated samples exhibited two distinct mass loss stages. During the first mass loss stage, thermally treated samples F-AA70 and F-AA80 displayed two peaks at around t = 60 °C and t = 125 °C, which were attributed to the evaporation of water and acetic acid residues. However, thermally treated sample F-AA60 did not exhibit any peaks in this stage, while thermally treated sample F-AA50 showed a single peak related to water evaporation at t = 52 °C. Furthermore, all thermally treated samples exhibited a peak shift to higher temperatures in the second mass loss stage. Notably, sample F-AA50 exhibited the most pronounced shift, transitioning from 220 °C (for the non-thermally treated sample) to 231 °C (for the thermally treated sample). The summarized results can be found in Table 3.20.

The results indicate that the thermal stability of the electrospun fibers is lower in comparison to the thermal stability of pure chitosan and PVA (as shown in Table 3.19). Furthermore, it's noticeable that while the thermal stabilities of samples F-AA60, F-AA70, and F-AA80 are relatively similar, measuring at 264, 264.8, and 266 °C,

respectively, sample F-AA50 exhibits a significant decrease in thermal stability, measuring at 220 °C in comparison to the other samples. This reduced thermal stability of chitosan/PVA fibers can be attributed to the interactions between the polymer components. The presence of amorphous chitosan dispersed within the PVA chains creates defects in the crystalline phase of PVA, hindering the development of crystalline regions. Consequently, the thermal energy required to disrupt hydrogen bonds and melt the free PVA chains becomes lower, leading to a reduction in the melting point of chitosan/PVA fibers.

Furthermore, the TG (Thermogravimetric) results for thermally treated electrospun fiber samples revealed an improvement in their thermal stability when compared to those fiber mat samples that had not undergone thermal treatment. This increase in thermal stability can be attributed to the fact that the heat treatment facilitated greater mobility of the polymer chains. Consequently, the amorphous regions of the chains were able to align and fold, leading to the formation of crystalline regions. This, in turn, contributed to the enhanced thermal stability of the material.

DSC thermographs for chitosan and PVA are shown in Figure 3.30 (a). Chitosan's thermograph showed an endothermic followed by an exothermic peak. The broad endo-thermic peak found at 180 °C can be attributed to the molecular arrangement of chitosan chains. The exothermic peak observed at 286 °C corresponds to the thermal decomposition of chitosan. In the DSC thermograph of PVA, a change in the baseline at 71 °C is indicative of the glass transition temperature (Tg) for PVA. Additionally, an endothermic peak at 170 °C, followed by a sharp endothermic peak at 220 °C, is attributed to the melting point (Tm) and the crystalline polymer fraction of PVA, respectively.

Figure 3.30 - DSC thermographs of chitosan (4 wt.%)/PVA (5 wt. %), electrospun from polymeric solutions with different acetic acid content. (a) 50 vol. %;

(b) 60 vol. %; (c) 70 vol. %; (d) 80 vol. %

The DSC thermographs of both thermally and non-thermally treated chitosan/PVA fibers, electrospun using various acetic acid concentrations, are displayed in Figure 3.30 (b), (c), (d), and (e). In the DSC curves for all samples, whether thermally treated or not, an endothermic peak followed by an exothermic peak is observed. Specifically, the endothermic peak occurs at temperatures of 166, 166, 165, and 164°C for non-thermally treated fibers (F-AA50, F-AA60, F-AA70, and F-AA80, respectively), and at 166, 164, 163.8, and 163°C for thermally treated fibers (F-AA50, F-AA60, F-AA70, and F-AA80, respectively). This endothermic peak is attributed to the melting point (Tm) of the chitosan-PVA blend.The exothermic peak observed at 231, 232, 231 and 231 (F-AA50, F-AA60, F-AA70, and F-AA80, respectively) for non-thermally threaten fibers 227, 231, 229 and 230 (F-AA50, F-AA60, F-AA70, and F-AA80, respectively) for thermally threaten fibers, can be associated to a cross-linking (complex formation between

polymers) reaction on chitosan molecules. Table 3.21 summarize the values of DSC for all fibers samples.

Table 3.21 - Thermogravimetric analysis of chitosan (4 wt. %)/PVA (5 wt. %) fibers (thermally threaten). Samples were dried at 70 °C for 24 hours_

Acetic acid

Glass transition t(g)

Melting point t(m)

Degree of crystallinity

(vol. %) Before After Before After Before After

t-treatment t-treatment t-treatment t-treatment t-treatment t-treatment

50 74 164.2 165.15 72.2 92

60 75 166.0 166.4 68.3 96

70 75 165.5 166.8 65.2 97

80 76 164.45 164.97 62.1 98

The DSC (Differential Scanning Calorimetry) curves of non-thermally treated chitosan/PVA fibers indicate that the baseline shift associated with the glass transition of PVA is no longer visible in any of the nanofiber samples. This absence of a baseline shift can be attributed to the shear stress induced by the electric field during electrospinning, which leads to the rearrangement of polymeric chains. However, in the DSC curves of thermally treated chitosan/PVA fibers, a change in the baseline is observed at temperatures of 74, 75, 75, and 76 °C for F-AA50, F-AA60, F-AA70, and F-AA80, respectively. Indeed, it's important to note that while some reports suggest that electrospun fibers can exhibit increased crystallinity after the electrospinning process, this isn't always the case. In many studies, electrospun fibers have been observed to experience a reduction in their crystalline structures. This phenomenon can be attributed to the high-speed solidification process that stretched polymers undergo during electrospinning. However, thermal treatment, particularly at temperatures above the glass transition temperature of the polymers, can provide the necessary energy for polymer chains to move more freely and rearrange. This relaxation of the stressed molecules that occur during electrospinning is a prerequisite for the development of increased crystallinity in the fibers.

3.3.4 Mechanical Properties

Table 3.22 presents the measured tensile strength, Young's moduli, and elongation at break of chitosan/PVA fiber mats before and after thermal treatment. These results

highlight a significant alteration in the mechanical properties of the electrospun polymeric fibers, influenced not only by the concentration of acetic acid but also by the applied thermal treatment. Rectangular samples were cut to a size of 120 x 10 mm (length x width) and had a thickness of 32, 42, 39, and 34 ^m for F-AA50, F-AA60, F-AA70, and F-AA80 (before heat treatment), respectively, and a thickness of 27, 21, 22, and 31 ^m for F-AA50, F-AA60, F-AA70, and F-AA80 (after heat treatment), respectively.

Interestingly, fiber samples F-AA-50, F-AA-60, and F-AA-70 exhibit distinctive stress-strain curves reminiscent of hard/tough plastic polymers. These curves feature a noticeable peak stress point at the transition from elastic to plastic deformation, a characteristic hallmark of such materials. Conversely, fiber sample F-AA80 demonstrates the characteristic curve of a brittle polymer, undergoing elastic deformation followed by fracture rather than plastic deformation.

Table 3.22 - Young's modulus, tensile strength and elongation at break of chitosan/PVA nanofibers samples before and after heat treatment_

Acetic ^^ Young's Modulus Tensile Strength Elongation at Break

(voL %) (MPa) (MPa) (%)

Not Dry Dry Not Dry Dry Not Dry Dry

50 649.19 ± 52.3 592.3 ± 50.4 13.1 ± 1.22 10.34 ± 1.5 12.13 ± 1.2 7.5 ± 1.21

60 474.8 ± 23.8 462.6 ± 28.25 5.8 ± 1.1 4.11 ± 1.01 13.68 ± 2.9 11.6 ± 3.19

70 594.1 ± 53.6 472.1 ± 23.7 5.26 ± 0.69 4.6 ± 0.8 15.06 ± 0.36 14.3 ± 1.5

80 545.92 ± 52.9 375.415 ± 80.4 11.43 ± 1 9.56 ± 2.6 5 ± 0.86 4.3 ± 0.74

The alteration in the mechanical properties of the electrospun chitosan/PVA fibers can be attributed to the plasticizing effect of acetic acid and water molecules. This plasticizing effect of acetic acid can be explained by the presence of the large acetate ion, which carries a delocalized charge. This ion can act as a plasticizer for the chitosan structure, facilitating a more favorable long-range molecular arrangement, ultimately enhancing the mechanical properties of the fibers.

Figure 3.31 presents the stress-strain diagrams of thermally treated fiber mats, revealing a noticeable decrease in tensile strength and elongation at break for all samples. Notably, sample F-AA50 exhibited a significant reduction in both tensile strength and elongation at break, with values decreasing from 13.1 to 10.34 MPa and from 12.13 to 7.5%, respectively. Conversely, sample F-AA80 displayed a smaller change in both

tensile strength and elongation at break, with values decreasing from 11.43 to 9.56 MPa and from 5 to 4.3%, respectively. These alterations in the mechanical properties of thermally treated fibers could be attributed to the evaporation of residual acetic acid and water molecules, which may induce non-covalent polymer-polymer interactions, primarily through the formation of hydrogen bonds, ultimately affecting the mechanical characteristics of the fiber mats.

Figure 3.31 - Stress-strain diagrams of chitosan/PVA fibers samples before and

after heat treatment

3.3.5 Electroactive Response of the Nanofiber Hydrogels

As observed in previous sections of this manuscript, the physical properties of

electrospun fibers undergo changes based on parameters both before (such as the concentration of acetic acid used to dissolve the polymers) and after (thermal treatment) fiber fabrication. With these observations in mind, the objective of this section was to investigate the impact of thermal treatment on the electroactive response of chitosan/PVA fibers. The measured speed displacement for chitosan/PVA samples before and after thermal treatment, are shown in Index D.

An electrochemical cell was employed to assess the electroactive response of the electrospun fibers, measuring the displacement of the samples over time while subjecting

them to a cyclic potential ranging from -10 to 10 V. The samples utilized in this experiment were derived from both non-thermally and thermally treated electrospun fiber mats, each measuring 20 X 4 mm (length X width).

The obtained results revealed a significant influence of the different concentrations of acetic acid used during fiber fabrication on the material's electroactive response, as illustrated in Figure 3.32 (a). Even though all electrospun fibers had the same polymer concentration, suggesting similar responses to an electric stimulus, two noteworthy behaviors were observed as the concentration of acetic acid used for fabrication increased.

Electrospun samples from solutions with higher concentrations of acetic acid exhibited reduced electrical responsiveness in acidic environments (pH < 7). By this mean, sample F-AA50 exhibited the largest and fastest electrical response of 0.77 mm s-1 in a pH 3. Meanwhile, sample F-AA60 showcase the fastest displacement of 0.546 mm s-1 in a pH4. On the other hand, sample F-AA80, was not suitable for its use in acidic medium below pH 5, due to the fact that all samples dissolved after their immersion to the solutions. The second behavior worth it to be mention, is that all samples exhibited a notable electrical sensitivity between pH 7 - 8, showing a speed displacement peak at this region. In this case fibers that were electrospun from solution with higher concentrations of acetic acid became more electrical responsive in pH 7 - 8. The highest speed displacement peak was found on samples F-AA60 and F-AA70, exhibiting a maximum speed displacement of 1.06 mm s-1 and 1.14 mm s-1 in pH 7, respectively. Meanwhile, samples F-AA50 and F-AA80 had the lowest speed displacement at pH 7, being of 0.75 mm s-1 and 0.51 mm s-1, respectively. From these results it is possible to conclude that the electroactive response sensitivity in acidic and basic mediums, of the fibers changes in dependence to the acetic acid concentration used. It was observed that fibers that were electrospun from solutions with higher acid concentrations tend to have a lower electroactive response in pH < 6, due to high swelling and a fast deterioration of the samples. These behaviors can be attributed to two primary factors. Firstly, the presence of residual acetic acid within the fibers can lead to rapid swelling and degradation, particularly in samples fabricated with higher acetic acid concentrations. Additionally, as discussed in section 3.3.2 (Study on the rheological properties of polymeric solutions),

the quantity of —NH groups that can become ionized (NH+3) in the chitosan chain is directly related to the acid concentration used for solubilization. Consequently, the potential amount of free —NH2 groups and NH+3 cations may be higher in fibers electrospun from highly concentrated acetic acid solutions. This increase in the number of free —NH2 groups and cations likely contributes to the heightened electroactive responsiveness of chitosan/PVA fibers under neutral and basic pH conditions.

Figure 3.32 - Speed displacement of (a) non-thermally treated chitosan/PVA fibers; (b) thermally treated chitosan/PVA fibers

Figure 3.32 (b) displays the speed displacement data obtained from thermally treated samples, revealing the significant impact of thermal treatment on the electroactive response of the fibers. In contrast to non-thermally treated samples, which exhibited rapid deterioration in acidic conditions, the thermally treated samples demonstrated improved stability and greater deformation at acidic pH levels (2 - 3). Notably, F-AA80 stood out as the most remarkable case, transitioning from being inapplicable at pH 3 to exhibiting a speed displacement of 1.37 mm s-1 at the same pH. This enhancement was also observed

across all samples, with speed displacement increasing from 0.77 mm s-1, 0.117 mm s-1, and 0.308 mm s-1 to 2.16 mm s-1, 1.56 mm s-1, and 1.38 mm s-1 at pH 3 for F-AA50, F-AA60, and F-AA70 (thermally treated), respectively. Additionally, it's worth noting that the electroactive response of the fibers in all samples followed a similar pattern. Two distinct speed displacement peaks were observed, one at pH 3 and another around pH 910. Notably, fibers electrospun from higher concentrations of acetic acid exhibited enhanced electrical responsiveness, reaching maximum speed displacements of 1.56 mm s-1, 1.54 mm s-1, and 1.31 mm s-1 at pH 9 for F-AA60, F-AA70, and F-AA80 (thermally treated), respectively. Thermal treatment consistently improved the electroactive response compared to non-thermally treated fibers. This enhancement, including higher stability in acidic environments, could be attributed to the partial removal of residual acetic acid and water, as well as a potential increase in crystallinity within the molecular structure of the fibers.

The obtained results suggest that various factors play a significant role in influencing the electroactive response of materials. Specifically, when it comes to electrospun fiber-based electroactive materials, parameters such as the solvent concentration used before electrospinning have a substantial impact on the material's electroactive properties. Moreover, the application of physical treatments, particularly thermal treatment, has shown the remarkable ability to profoundly modify the material's electroactive response. Interestingly, thermal treatment has also been observed to enhance the stability of fibers in aqueous environments compared to those that haven't undergone thermal treatment. Taking into account the cumulative effects of these findings, it becomes evident that these parameters are crucial in customizing the electroactive properties during the development of responsive materials.

3.3.6 FTIR Spectroscopy

In order to investigate the molecular interactions within the chitosan/PVA nanofibers, we conducted an analysis of the FTIR spectrum for chitosan, PVA powder, and the chitosan/PVA nanofiber mats. Figure 3.33 illustrates the FTIR spectra of chitosan, PVA, and the chitosan/PVA nanofibers (F-AA50, F-AA60, F-AA70, and F-AA80). This Fourier Transform Infrared (FTIR) analysis of chitosan revealed distinct absorption peaks

indicative of its molecular composition. The peak observed at 3354 cm-1 corresponds to the combined stretching vibrations of O—H and N—H groups. The presence of an absorption peak at 2926 cm-1 indicates the stretching vibrations of aliphatic C—H bonds. Furthermore, the peak at 1561 cm-1 is attributed to the stretching vibration of the amino group. Additionally, two characteristic peaks associated with the saccharide structure of chitosan are evident at 892 and 1150 cm-1.

The FTIR spectra of polyvinyl alcohol (PVA) exhibited distinct absorption peaks that provide insights into its molecular characteristics. The absorption peak observed at approximately 3290 cm-1 corresponds to the hydroxyl group (—OH) stretching vibrations. The peak at 2937 cm-1 is attributed to the antisymmetric stretching vibrations of the CH2 groups. Peaks observed at 1709 cm-1 indicate stretching vibrations of the C=O bonds present in the acetate units of PVA. The absorption peak at 1420 cm-1 corresponds to the vibration of the C—H bonds in the methyl group. The absorption peak observed at 1141 cm-1 is assigned to the stretching of C—O associated with the crystalline part of the polymeric chain. The peak around 1087 cm-1 is associated with the asymmetric stretching vibration of the C—O bond in the acetate group. These findings are consistent with the existing literature in the field.

For all chitosan/PVA fibers samples, it is noticeable that the FTIR spectra is very similar to the PVA spectra. It was possible to observe a broad and intense band from 3000 - 3600 cm-1 related to O-H and N-H stretching vibrations. For thermally treated fiber mats this region became narrower and slightly shaper as a result of dehydration process. From the spectra, it is possible to notice the formation of a hydrogen bond between PVA and chitosan, which can be deduced by the shift toward lower values of O-H and N-H stretching vibration peak of chitosan (3354 cm-1) to around 3300 cm-1 for chitosan/PVA nanofibers. Peaks around 1712 and 1640 cm-1 are associated with stretching vibrations of the C=O and C-O bonds of acetate units in PVA molecules. Thermally treated fibers showed a peak shifting from 1640 to 1652 cm-1 (associated to C=O stretching in amide group, amide I vibration) which can be due to the formation of amide group from the reaction of carboxylic with amine groups, as a result from the heat treatment. Additionally, it was possible to notice a shift from the peak from around 1590 cm-1 to

1562 cm-1. This shifting can be related to the hydrogen bonding between -NH of chitosan's group with OH groups of PVA. The peak at around 1410 (non-thermally treated fibers) and 1415 cm-1 (thermally treated fibers) corresponds to the vibrations of the C-H bond of the methyl group (-CH3). The asymmetric stretching vibration of the C-O bond of the acetate group can be observed in the peak at 1075 cm-1. The peak around 842 cm-1 is associated with bending vibrations of C-H bonds in the molecule. These results are in good agreement with previous reports.

C'hitnsan <4 wt.%> PVA (5 wi.%1

Belbr thermal treatment Alter thermal treatment

-A.A. 80 vol. A.A 80 vol. %

■VA. 70 vol A.A. 70 vol. %

-A.A. 60 vol.--A.A. 60 vol. K

-A .A. 50 vol. -A.A. 50 vol. %

I-•-1-'-1-1-1-1-1-•-1-'-1-'-1-«

500 1000 1500 2000 2500 3000 3500 4000

Wavcnumbcr(cm ')

Figure 3.33 - FT-IR spectrum of chitosan, PVA and chitosan/PVA fibers samples

3.3.7 Spectra Deconvolution for The Determination of Intermolecular Hydrogen Bonging and Free Amine (—NH2) Variation Post Thermal Treatment

The mechanical, thermal, and electroactive characteristics of the electrospun

chitosan/PVA fibers were noticeably altered following thermal treatment. As previously discussed, these changes in mechanical and thermal properties may be attributed to two potential effects: an augmentation in crystallinity and/or physical cross-linking between polymer chains. However, from the FT-IR spectra, further insights can be gained. (Figure 56) it is possible to notice that the characteristic peak associated with the crystalline part of the PVA (1141 cm-1) observed in PVA spectrum, was overlapped with the peak at ~1075 cm-1 for all fiber mat samples. Furthermore, it's worth noting that after undergoing

thermal treatment, the fiber samples did not display any significant alterations within this range. Therefore, the shift in mechanical and thermal properties is likely closely associated with a physical cross-linking effect, resulting from the removal of acetic acid residues and water from the microstructure of the electrospun fibers. This removal can lead to an increase in intermolecular hydrogen bonding between the fibers' components —NH2 and —OH groups of chitosan with —OH groups of PVA. Conversely, in the electroactive response test, it was observed that fibers spun from solutions containing high concentrations of acetic acid, with a pH below 5, displayed minimal or no tip displacement. Furthermore, these samples dissolved upon immersion in the acidic medium. Nonetheless, following thermal treatment, not only did their electroactive properties undergo significant improvement, as shown in Figure 55. Moreover, the solubility of the fiber mats decreased significantly, to the extent that all samples exhibited remarkable stability in acidic environments. This enhanced stability of the fibers in aqueous solutions may be attributed to a physical cross-linking effect resulting from the thermal treatment. Simultaneously, the enhancement in their electroactive response could be associated with an augmentation in the quantities of free amino groups integrated into the molecular structure of the fibers.

Due to thermal treatment, the fibers undergo alterations in their physical characteristics. These changes can be ascribed to shifts in intermolecular hydrogen bonding among polymers and variations in the presence of free amino groups. To understand these modifications in the extent of hydrogen bonding and free amine groups following thermal treatment, we conducted a deconvolution analysis using Gaussian line shapes on the peak found in the Fourier Transform Infrared (FTIR) spectrum within the specified range. of 3000-3700 cm-1. Hydrogen bond types were analyzed in the —OH region and the —NH region. For the —NH region, free amine group is around 3408 cm-1, intermolecular association (N2—H1...O5/N2—H2...O1) is around 3335 cm-1, intramolecular association (O3H...O5/ O3H...O6) is around 3366 cm-1, amide group (— CONH-) is around 3240 cm-1, and primary ammonium (—NH+3) is around 3100 cm-1. For the -OH region, free hydroxyl (—OH) is around 3580 cm-1, and multimer

intermolecular association (O6H...N2) is around 3462 cm-1. Table 23 shows the results obtained from the spectra deconvolution.

Table 3.23 - List of peak frequencies and relative strength of the deconvoluted band in the region 3000-3700 cm-1 for thermally and non-thermally treated chitosan/PVA fibers samples _ ____

Sample Hydrogen bond types Abbreviatio n Wavenumber/cm-1 Relative strength/% non-t-treated Relative strength/% t-treated

Acetic acid 50% Primary ammonium I —NH+3 ~ 3100 cm-1 3.93 3.2

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 8.7 5.2

Amide III —CONH— ~ 3240 cm-1 30.1 33

Intermolecular association IV N2— H1...O5/ N2—H2...O1 ~ 3335 cm-1 27.5 28

Intramolecular association V O3H.O5/ O3H...O6 ~ 3366 cm-1 0.8 0.9

Free amine VI —NH2 ~ 3408 cm-1 6.39 6.9

Multimer (Intermolecular association) VII O6H...N2 ~ 3462 cm-1 22.13 22.6

Free hydroxyl VIII —OH ~ 3580 cm-1 0.36 0.43

Acetic acid 60% Primary ammonium I —NH+3 ~ 3100 cm-1 4.4 4.2

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 10.27 8.2

Amide III —CONH— ~ 3240 cm-1 029.3 30.2

Intermolecular association IV N2— H1.O5/ N2—H2...O1 ~ 3335 cm-1 27.46 29.15

Intramolecular association V O3H.O5/ O3H...O6 ~ 3366 cm-1 0.90 0.92

Free amine VI —NH2 ~ 3408 cm-1 6.14 6.45

Multimer (Intermolecular association) VII O6H...N2 ~ 3462 cm-1 20 21.1

Free hydroxyl VIII —OH ~ 3580 cm-1 0.09 0.21

Acetic acid pse-70% Primary ammonium I —NH+3 ~ 3100 cm-1 4.21 3.66

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 9.6 6.69

Amide III —CONH— ~ 3240 cm-1 29.7 32.1

Intermolecular association IV N2— H1.O5/ N2—H2.O1 ~ 3335 cm-1 26.6 27.9

Intramolecular V O3H...O5/ ~ 3366 cm-1 0.88 0.91

association O3H...O6

Free amine VI —NH2 ~ 3408 cm-1 5.89 5.96

Multimer VII O6H...N2 ~ 3462 cm-1 21.32 23.35

(Intermolecular association)

Free hydroxyl VIII —OH ~ 3580 cm-1 0.4 0.64

Primary ammonium I —NH+3 ~ 3100 cm-1 4.5 4.0

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 10.53 6.23

Amide III —CONH— ~ 3240 cm-1 27 32.44

Acetic Intermolecular association IV N2— H1...O5/ ~ 3335 cm-1 27.6 28.9

acid N2—H2...O1

80% Intramolecular V O3H...O5/ ~ 3366 cm-1 0.71 0.79

association O3H...O6

Free amine VI —NH2 ~ 3408 cm-1 6.56 6.85

Multimer VII O6H...N2 ~ 3462 cm-1 21.08 21.77

(Intermolecular association)

Free hydroxyl VIII —OH ~ 3580 cm-1 0.1 0.27

3.4 Thermal treatment on chitosan - Poly (vinyl alcohol) films and its influence on the polymeric crystalline structure

In recent years, the quest for innovative materials with electroactive properties has

driven significant research efforts, fueled by their diverse applications in fields ranging from electronics and sensors to biomedical devices. Our investigation covers a wide range of molecular interactions and structural changes. We explain the complex relationship between these types of treatment, the resulting material properties, and the level of crystallinity.

The films under scrutiny were synthesized through a casting methodology, thereby ensuring meticulous control over compositional homogeneity and structural uniformity. Subsequently, these films underwent a meticulously calibrated thermal regimen, steadfastly maintained at a temperature of 70°C for an uninterrupted duration of 24 hours.

Moreover, our systematic approach encompassed a deliberate gradient of acetic acid concentrations, spanning the spectrum from 50 to 80 vol. %. This deliberate stratagem facilitates the discernment of the nuanced repercussions of distinct treatment intensities on the films' characteristics. As we progressively unravel the intricate interplay between acetic acid exposure, thermal stimuli, and the ensuing alterations in crystalline

morphology, we illuminate the intricate pathways by which these factors collectively orchestrate the films' mechanical, thermal, and functional attributes. Rooted in exacting analysis and scientific rigor, this thesis aspires to unlock the enigmas underpinning this intricate interplay, thereby contributing to a more holistic comprehension of the strategies for tailoring and optimizing chitosan/PVA films across a diverse spectrum of applications.

3.4.1 Thermal properties

The thermal treatments can also influence the thermal properties of the films. Chitosan/PVA films were thermally treated, by placing the samples in a drying oven for 24 hours at a t = 70 °C. Table 3.24 and Table 3.25 summarized TG results. The TG thermographs of not thermally treated chitosan/PVA fibers exhibited a weight loss profile at three temperature stages, as shown in Figure 3.34. Chitosan and PVA had a weight loss at two stages. In the case of PVA, the initial reduction in weight was observed within the temperature range of 51-134 °C, which can be attributed to the evaporation of moisture content (~4%). Subsequently, a second weight reduction took place between 157-450 °C, indicating the thermal degradation of PVA (~90.52%). In the case of chitosan, the initial decrease in weight occurs within the temperature range of 35-118 °C, which is associated with the evaporation of moisture content (~5%). Subsequently, a second weight reduction is observed between 182-400 °C, indicating the thermal degradation and deacetylation of chitosan (~49.68%).

Table 3.24 - Thermogravimetric analysis of chitosan powder, PVA powder, and chitosan/PVA films (not thermally threaten)_

A. Acid First Mass (vol. %) Loss (%)

First

Second Mass Third Mass Stage Loss (%) Loss (%) 1st Peak

(°C)

First Stage Second 2nd Peak Stage

(°C)

(°C)

Third Stage (°C)

PVA Chit 50 60 70 80

4

5

16.59 14.44 14.03 23.14

90.53 49.68 43.65 43.38

43.7

57.08

14.41 13.62 14.01 14.97

81 67 107 98 110 95

185 179 185 184

304 299 295 283 290 290

433 426

431

432

Table 3.25 - Thermogravimetric analysis of chitosan/PVA films (thermally treated). Samples were dried at 70 °C for 24 hours

A. Acid (vol. %) First Mass Loss (%) Second Mass Loss (%) Third Mass Loss (%) First Stage 1st Peak (°C) First Stage 2nd Peak (°C) Second Stage (°C) Third Stage (°C)

50 12.81 43.65 14.62 186 299 435

60 13.57 43.76 14.05 122 199 299 435

70 13.76 45.55 13.35 115 — 298 434

80 14.86 51.95 13.33 — — 300 436

Figure 3.34 - TG thermographs of chitosan (4 wt. %)/PVA (5 wt. %) films dissolved in acetic acid at: (a) 50 vol. %; (b) 60 vol. %; (c) 70 vol. %; (d) 80 vol. %

From the DTG curves it is possible to observe that the mass loss happens in three stages as shown in Figure 3.35. TG thermographs shown that the first mass loss happens in a temperature range of 30 - 180 °C, related to moisture and solvent residue vaporization. It is possible to observe from DTG curves that at the stage for the first mass loss two peaks are formed. The presence of a second peak in this stage is due to acetic acid residues, which has a boiling point of 118 °C. The second mass loss was observed in the range of 180-370 °C. The second mass loss is related to the thermal destruction of the chitosan/PVA complex. A third mass loss was observable in the range of 375-500 °C it can be related to the degradation of PVA byproducts.

Figure 3.35 - DTG thermographs of chitosan (4 wt. %)/PVA (5 wt. %) films dissolved in acetic acid at: (a) 50 vol. %; (b) 60 vol. %; (c) 70 vol. %; (d) 80 vol. %

From the results obtained, it is possible to note that the thermal stability of the thermally treated films increased after being submitted to a thermal treatment. It is possible to notice that the degradation temperature increased from 295, 283, 290, and 290 C to 299, 299, 298, and 300 C, for Fm-AA50, FmAA60, FmAA70, and FmAA80, respectively. This result can be explained due to a crosslinking effect caused by the elimination of water and solvent residues. The elimination of solvent residues promotes the formation of intermolecular interaction due to the release of —OH groups.

DSC thermographs for chitosan/PVA films (Figure 3.36), shows that the variation on the concentration of acetic acid do not show to have any influenced on the thermal properties of the material. The crystalline degree of the films was calculated to assess the variation after thermal treatment. From the evaluation of the crystalline degree by measuring the enthalpy of the melting point, it was possible to notice that thermal treatment increased the degree of crystallinity, as shown in Table 3.26. This can be explained as a result of a relaxation on the polymeric chains.

2.5-

2.01.5100.500-

i ' l-- i-- i-- i-'-1-- i-'

0 50 100 150 200 250 300

Temperature (°C)

Figure 3.36 - DSC thermographs of chitosan (4 wt.%)/PVA (5 wt. %) dissolved in different acetic acid concentrations

Table 3.26 - Values of glass transition temperature (Tg), melting temperature (Tm), degree of crystallinity (%) of chitosan/PVA nanofibers samples before and after heat

treatment

Acetic acid Melting point, Tm Degree of crystallinity, (%)

(vol. %) Before After Before After

t-treatment t-treatment t-treatment t-treatment

50 204.31 206.32 57.69 61.46

60 200.0 207.22 63.19 66.22

70 203.98 207.92 58.13 71.41

80 198.18 200.15 57.13 57.87

3.4.2 Tensile strength test

Table 3.27 shows the obtained tensile strength, Young's moduli, and elongation at break of chitosan/PVA films, measured before and after thermal treatment. Rectangular samples were cut to a size of 50 x 10 mm (length x width) and had a thickness of 64, 59, 53, 77, and 85 ^m for F-AA50, F-AA60, F-AA70, and F-AA80 (before heat treatment), respectively, and a thickness of 59, 49, 65, and 65 ^m for F-AA50, F-AA60, F-AA70, and F-AA80 (after heat treatment),

Based on the obtained results, a notable observation can be made regarding the significant alteration in the mechanical properties of the electrospun polymeric fibers. These changes are evident not only in relation to the concentration of acetic acid but also

as a direct outcome of the applied thermal treatment. Derived from the obtained results, it is noteworthy that films demonstrate distinctive curves that resemble those of hard/tough plastic polymers.

Table 3.27 - Tensile strength and Young's modulus of chitosan/PVA films samples before and after heat treatment

Acetic Young's Modulus Tensile Strength Elongation at Break

acid (MPa) (MPa) (%)

(vol. %) Before After Before After Before After

t-treatment t-treatment t-treatment t-treatment t-treatment t-treatment

50 649.19 ± 52.3 592.3 ± 50.4 13.1 ± 1.22 10.34 ± 1.5 12.13 ± 1.2 7.5 ± 1.21

60 474.8 ± 23.8 462.6 ± 28.25 5.8 ± 1.1 4.11 ± 1.01 13.68 ± 2.9 11.6 ± 3.19

70 594.1 ± 53.6 472.1 ± 23.7 5.26 ± 0.69 4.6 ± 0.8 15.06 ± 0.36 14.3 ± 1.5

80 545.92 ± 52.9 375.415 ± 80.4 11.43 ± 1 9.56 ± 2.6 5 ± 0.86 4.3 ± 0.74

Furthermore, from Figure 3.37 it is possible to notice that thermal treatment

influenced the mechanical properties of the cast films. It is possible to notice that the elongation at break for all samples drastically got reduced from 12.12 to 7.5, 13.68 to 11.6, 15.06 to 14.3, and 5 to 4.3 %, for FmAA50, FmAA60, FmAA70, and FmAA80. This can be explained as a result to the increase in the degree of crystallinity and to the elimination of solvent residues, which could act as a plasticizer.

Before thermal treatment After thermal treatment 1 Acetic acid. vol. % Acctic acid. vol. %

1-'-1-1-1-1-1-'-1-1-1-'-1-*-1

0 20 40 60 80 100 120 140 Tensile strain (%)

Figure 3.37 - Stress-strain diagrams of chitosan/PVA fibers samples before and

after heat treatment.

3.4.3 FTIR Spectroscopy

To explore the molecular interaction within the chitosan/PVA nanofibers, an analysis was conducted on the FTIR spectrum of chitosan, PVA powder, and the chitosan/PVA nanofiber fiber mats. Figure 3.38 shows the FTIR spectra of chitosan, PVA and chitosan/PVA nanofibers (F-AA50, F-AA60, F-AA70, and F-AA80). The Fourier Transform Infrared (FTIR) analysis of chitosan revealed distinctive absorption peaks that signify its molecular composition. The peak observed at 3354 cm-1 corresponds to the combined stretching vibrations of O—H and N—H groups. The presence of an absorption peak at 2926 cm-1 indicates the stretching vibrations of aliphatic C—H bonds. Furthermore, the peak at 1561 cm-1 is attributed to the stretching vibration of the amino group. Additionally, two characteristic peaks associated with the saccharide structure of chitosan are evident at 892 and 1150 cm-1.

The FTIR spectra of polyvinyl alcohol (PVA) exhibited distinct absorption peaks that provide insights into its molecular characteristics. The absorption peak observed at approximately 3290 cm-1 corresponds to the hydroxyl group (—OH) stretching vibrations. The peak at 2937 cm-1 is attributed to the antisymmetric stretching vibrations of the CH2 groups. Peaks observed at 1709 cm-1 indicate stretching vibrations of the C=O bonds present in the acetate units of PVA. The absorption peak at 1420 cm-1 corresponds to the vibration of the C—H bonds in the methyl group. The absorption peak observed at 1141 cm-1 is assigned to the stretching of C—O associated with the crystalline part of the polymeric chain. The peak around 1087 cm-1 is associated with the asymmetric stretching vibration of the C—O bond in the acetate group. These findings are consistent with the existing literature in the field.

- Dry_Chitosan/PVA - AA80% Dry_Chitosan/PVA - AA70% Dry_Chitosan/PVA - AA60% Dry Chitosan/PVA - AA50%

_ ___'

i---1---1---1---i---1---1---1--

500 1000 1500 2000 2500 3000 3500 4000

Wave number (cm')

Figure 3.38 - FT-IR spectrum of chitosan, PVA and chitosan/PVA films

For all chitosan/PVA fibers samples, it is noticeable that the FTIR spectra is very similar to the PVA spectra. It was possible to observe a broad and intense band from 3000 - 3600 cm-1 related to O-H and N-H stretching vibrations. For thermally treated fiber mats this region became narrower and slightly shaper as a result of dehydration process. From the spectra, it is possible to notice the formation of a hydrogen bond between PVA and chitosan, which can be deduced by the shift toward lower values of O-H and N-H stretching vibration peak of chitosan (3354 cm-1) to around 3300 cm-1 for chitosan/PVA nanofibers. Peaks around 1712 and 1640 cm-1 are associated with stretching vibrations of the C=O and C-O bonds of acetate units in PVA molecules. Thermally treated fibers showed a peak shifting from 1640 to 1652 cm-1 (associated to C=O stretching in amide group, amide I vibration) which can be due to the formation of amide group from the reaction of carboxylic with amine groups, as a result from the heat treatment. Additionally, it was possible to notice a shift from the peak from around 1590 cm-1 to 1562 cm-1. This shifting can be related to the hydrogen bonding between -NH of chitosan's group with OH groups of PVA. The peak at around 1410 (non-thermally treated fibers) and 1415 cm-1 (thermally treated fibers) corresponds to the vibrations of the C-H bond of the methyl group (-CH3). The absorption peak at 1141 cm-1 it was observed in all samples which it is assigned to the stretching of C—O associated with the

Chitosan/PVA - AA80% Chitosan/PVA - AA70% — Chitosan/PVA - AA60% -Chitosan/PVA - AA50%

crystalline part of the polymeric chain. This peak confirms the formation of a crystalline structure during films casting process. The asymmetric stretching vibration of the C-O bond of the acetate group can be observed in the peak at 1075 cm-1. The peak around 842 cm-1 is associated with bending vibrations of C-H bonds in the molecule.

3.4.4 FTIR Spectra Deconvolution for The Determination of Intermolecular Hydrogen Bonging and Free Amine (—NH2) Variation Post Thermal Treatment

After subjecting the films to thermal treatment, notable alterations in their

mechanical, thermal, and electroactive properties were observed. These changes, previously attributed to increased crystallinity and potential polymer chain cross-linking, were further investigated. Infrared spectra (FT-IR) analysis revealed that the characteristic peak linked to the crystalline portion of PVA, found at 1141 cm-1, coincided with the peak around ~1075 cm-1. Elimination of acetic acid residues and water likely promoted intermolecular hydrogen bonding between —NH2 and —OH groups of chitosan and —OH groups of PVA. Deconvolution analysis of FTIR spectra in the range of 3000-3700 cm-1 provided insights into hydrogen bond types in both the —OH and — NH regions. In the —NH region. The findings from the deconvolution analysis are summarized in Table 3.28.

Table 3.28 - List of peak frequencies and relative strength of the deconvoluted band in the region 3000-3700 cm-1 for thermally and non-thermally treated chitosan/PVA films samples_ ____

Sample Hydrogen bond types Abbreviation Wavenumber/cm-1 Relative strength/% non t-treated Relative strength/% thermally treated)

Acetic acid 50% Primary ammonium I —NH+3 ~ 3100 cm-1 5.8804 5.80988

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 14.06898 13.71303

Amide III —CONH— ~ 3240 cm-1 27.54422 30.20948

Intermolecular association IV N2— H1...O5/ N2—H2...O1 ~ 3335 cm-1 23.75951 24.59862

Intramolecular association V O3H...O5/ O3H...O6 ~ 3366 cm-1 0.603 0.58848

Free amine VI —NH2 ~ 3408 cm-1 7.39621 9.31912

Multimer (Intermolecular association) VII O6H...N2 ~ 3462 cm-1 20.42862 15.7614

Free hydroxyl VIII —OH ~ 3580 cm-1 0.31905 ~0

Acetic acid 60% Primary ammonium I —NH+3 ~ 3100 cm-1 5.97355 5.6223

Intermolecular hydrogen bond II OH. ether O ~ 3200 cm-1 14.4725 7.18596

Amide III —CONH— ~ 3240 cm-1 27.95289 39.38231

Intermolecular association IV N2— H1...O5/ N2—H2...O1 ~ 3335 cm-1 29.30866 17.69982

Intramolecular association V O3H.O5/ O3H...O6 ~ 3366 cm-1 0.716 0.35828

Free amine VI —NH2 ~ 3408 cm-1 4.9671 10.73758

Multimer (Intermolecular association) VII O6H...N2 ~ 3462 cm-1 16.57115 19.01376

Free hydroxyl VIII —OH ~ 3580 cm-1 0.03815 0

Acetic acid pse-70% Primary ammonium I —NH+3 ~ 3100 cm-1 5.15918 6.95205

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 12.53277 9.09242

Amide III —CONH— ~ 3240 cm-1 31.94073 34.26678

Intermolecular association IV N2— H1...O5/ N2—H2...O1 ~ 3335 cm-1 26.76141 22.80046

Intramolecular association V O3H.O5/ O3H...O6 ~ 3366 cm-1 0.9811 0.77617

Free amine VI —NH2 ~ 3408 cm-1 5.70634 6.67847

Multimer (Intermolecular association) VII O6H...N2 ~ 3462 cm-1 16.88257 19.43365

Free hydroxyl VIII —OH ~ 3580 cm-1 0.0359 ~0

Acetic acid 80% Primary ammonium I —NH+3 ~ 3100 cm-1 5.31276 6.26425

Intermolecular hydrogen bond II OH.ether O ~ 3200 cm-1 13.34493 8.47109

Amide III —CONH— ~ 3240 cm-1 30.66265 33.53292

Intermolecular association IV N2— H1.O5/ N2—H2.O1 ~ 3335 cm-1 27.5163 22.17826

Intramolecular association V O3H.O5/ O3H.O6 ~ 3366 cm-1 0.97206 0.87319

Free amine VI —NH2 ~ 3408 cm-1 5.49881 6.16352

Multimer (Intermolecular association) VII O6H.N2 ~ 3462 cm-1 16.67131 22.15255

Free hydroxyl VIII —OH ~ 3580 cm-1 0.2118 0.036423

3.4.5 Electroactive response of chitosan/PVA films

The electroactive response of chitosan/PVA films was not possible to be measured

due to the fact that the films that where not thermally treated tend to quickly dissolved

when immersed in the electrolytic solution. As the pH was lower the solubility of the films increased, as shown in Figure 66. On the other hand, those films that were thermally treated showed a higher stability when immersed in the electrolytic solution. However, these films tend to react to the low pH by twisting its shape and even if the electric field was applied the samples did not exhibit any mechanical reaction to the electrical stimuli. Furthermore, thermally treated fibers tend to dissolved after few minutes of been immersed in the solution.

Figure 3.39 - Picture of the films after being immersed in distilled water (pH 6.7). (a) non-thermally treated films; (b) thermally treated films

3.5 Comparison between the properties of chitosan/PVA-based films and

fibers

Comparing the results of nanofibers and films with the same polymeric concentration reveals distinct properties. Notably, films exhibit a significantly higher degree of crystallinity at approximately 60%, while electrospun fibers display a much higher degree at 98% after thermal treatment. This discrepancy can be attributed to the processing techniques employed. To evaluate the change in the thermal properties of the films, their degree of crystallinity was calculated. From the evaluation of the degree of crystallinity by measuring the enthalpy of melting point, it can be observed that the heat treatment increases the degree of crystallinity. In addition, as shown in Table 4, the degree of crystallinity is higher in films compared to nanofibers. The difference in the variation of crystallinity degree between chitosan/PVC-based films and nanofibers is due to the manufacturing process of the material. In the molding method, there is a slower solvent release, which gives the polymer chain enough time for ordering and formation of crystalline regions. On the other hand, the increase in the crystallinity after thermal

treatment observed in all nanofibers mat samples, is due to the high porous morphology that this material has, which allow a higher heat convection within the mat, hence fibers that are located inside the porous structure receive the same thermal energy that those that are in the observable surface, allowing the rearrangement of the polymer chain. This can be further support from the FT-IR spectra. Even though the spectra from obtained for chitosan/PVA films and fibers are very similar, the spectra from films showed a characteristic peak at 1141 cm-1 this peak is associated to the crystalline region of PVA. Meanwhile fiber spectra did not show this peak, indicating that electrospinning process reduced the degree of crystallinity of the material.

Furthermore, the variation on the thermal properties between chitosan/PVA films and fibers, can be regarded to the variation on the degree of crystallinity. As shown in Table 3.29, it is possible to notice that films have a higher thermal degradation in comparison to their fiber's counterpart. Another explanation to this behavior is that, due to the higher volume-to-surface ratio present on micro and nanofibers, the heat transfer in much faster in comparison to the bulky structure of films, promoting a faster degradation under lower temperatures.

Table 3.29 - Thermogravimetric analysis of chitosan (wt. 4%)/PVA (wt. 5%) films and fibers (thermally threaten). Samples were dried at 70 °C for 24 hours

A. acid First Mass Second Mass Third Mass First First Second Third

(vol. %) Loss (%) Loss (%) Loss (%) Stage Stage Stage Stage

1st 2nd (°C) (°C)

Peak Peak

(°C) (°C)

Films

50 12.81 43.65 14.62 — 186 299 435

60 13.57 43.76 14.05 122 199 299 435

70 13.76 45.55 13.35 115 — 298 434

80 14.86 51.95 13.33 — — 300 436

Nanofibers

50 2.42 71.07 52 231

60 3.1 71.62 60 271

70 3.26 71.35 60 125 272

80 6.47 71.79 63 125 275

Regarding the tensile properties, as shown in Table 30, the tensile strength and the

elongation at break are higher in chitosan/PVA films in comparison to their film counterpart. This can be attributed that the load distribution in films in greater, this is

because the larger cross-sectional area of films helps to better distribute the mechanical loads over larger volumes. As a result of this stress dispersion films can increase its mechanical strength and toughness. All this can be attributed to the thickness of films which is higher twice higher that the fibers thickness.

Table 3.30 - Young's modulus, tensile strength and elongation at break of chitosan/PVA films and fibers samples before and after heat treatment A acid, Young's modulus Tensile strength Elongation at break

vol. % (MPa) (MPa) (%)

Before After Before After Before After

T-treatment T-treatment T-treatment T-treatment T-treatment T-treatment

Films

50 100.9 ± 30.5 73.7 ± 25.16 16.5 ± 1.89 12.1 ± 2.2 107 ± 1.12 84.6 ± 1.47