Stimuli-Responsive Polymer-Based Actuators and Sensors

Abstract

This work covers the general scope of stimuli-responsive polymers, with the special focus on the thermoresponsive poly (N-isopropylacrylamide) (pNIPAm)-based hydrogels and particles (microgels) and their composites, humidity responsive polyelectrolyte-based bilayer devices, and their applications as artificial muscles, sensors and actuators. In Chapters 3-5, a novel humidity responsive bilayer device was generated by combining poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel-based and polyelectrolytes. The two layers were used together as a glue to adhere them to a plastic substrate, which allowed them to lift weights (Chapter 3). The self-folding behavior of the bilayer devices was studied and mathematical models were built up to explain the behavior. The results showed that the model can describe the experimental data well. Accordingly, multiple 3D structures were obtained from the precisely designed 2D planar bilayer sheets (Chapter 4). Subsequently, the shape memory effect (SME) of the bilayer system was studied (in Chapter 5). The semi-crystallization of the polymer in the polyelectrolyte layer enables the SME of the device. Furthermore, the ‘templated device’ was obtained in order to assemble it with a strain sensor, which could serve as humidity sensors. In the second part of the thesis, we describe the development of a semi-interpenetrated (semi-IPN) hydrogel-based bilayer actuator (Chapter 6). The polyelectrolyte is physically trapped in the chemically crosslinked pNIPAm-based hydrogel network, which endows the bilayer with bi-directional deformations in response to pH and temperature in aqueous solution. Furthermore, they are specially designed to function as manipulators (or grippers) and small molecule release devices. In the last part, Chapter 7 describes the synthesis of the N, N’- bis(acryloyl)cystamine (BAC) crosslinked pNIPAm microgels, where the crosslinking density can be altered by exposure to reducing reagents. 1D photonic crystals (etalons) are fabricated from these microgels, which show an approximately linear response to one reducing reagent dithiothreitol (DTT).