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Author ORCID Identifier

https://orcid.org/0000-0003-0904-7369

AccessType

Open Access Dissertation

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2021

Month Degree Awarded

May

First Advisor

E. Bryan Coughlin

Subject Categories

Polymer and Organic Materials | Polymer Chemistry

Abstract

Inspired by nature, this research focuses on designing multifunctional renewable nanocomposites with high toughness and stimuli-responsiveness. In recent years, cellulose nanocrystals (CNCs) have been explored due to their abundance, renewable resource, and unique mechanical strength and structural coloration. CNCs naturally self-assemble into the helicoidal (Bouligand) structure that effectively endure high impacts but is brittle without an attendant soft phase. A thermoresponsive polymer, poly(diethylene glycol methyl ether methacrylate) (PMEO2MA), was incorporated into CNCs via evaporation-induced self-assembly to improve toughness of the resulting nanocomposites and to study responses in polymer dynamics under varying temperature and humidity conditions.

To study microscopic scale responses of PMEO2MA, a water-sensitive dye was implemented to probe the chains dynamics via fluorescence lifetimes. Fluorescence lifetime imaging microscopy (FLIM) generated a fluorescence lifetime distribution within the film that probed the different polymer dynamics. Two-component exponential fitting was needed where τ1 had shorter lifetimes that associated with more hydrated domains while τ2 had longer fluorescence lifetimes likely related to polymer confined to the CNC interfaces. There are three postulated environments the polymer chains were experiencing in the CNCs structure: i) PMEO2MA confined to CNCs via physical absorption, ii) PMEO2MA in-between CNCs structures, iii) PMEO2MA in hydrated regions. We also examined how anchoring PMEO2MARhB to CNCs would affect polymer dynamics and mechanical properties. The grafting-from technique was utilized to covalently bond the polymer chains to CNCs surface to form CNC-g-PMEO2MARhB. Due to one chain-end being restricted to CNCs, the Grafted-CNC films had longer fluorescence lifetimes when compared to their blended PMEO2MARhB-CNC counterpart. The Grafted-CNC films showed improvement in tensile strength and modulus, indicating better stress-transfer in comparison to the blended PMEO2MARhB-CNC. This observation illustrated a correlation between the measured fluorescence lifetime and mechanical properties.

From these studies, CNCs nanocomposites can be tuned by environmental stimuli and different chain confinement resulted in optical responses and mechanical performances. Fluorescence lifetime imaging microscopy was co-opted from biological applications to capture sensitive events in nanoseconds at high resolution to experimentally study polymer dynamics. The technique provided insights at the microscopic scale to improve material design for CNCs nanocomposites.

DOI

https://doi.org/10.7275/22452628.0

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