Advanced Characterization of Shape Memory Polymers and Electrospun Conducting Polymers for Engineering Applications

Abstract

Smart materials are an attractive solution to low cost, low weight and high performance applications. They have multifunctional properties that can be controlled by external stimuli such as light, heat and electricity. Their properties can be tailored for specific purposes, their fabrication process is cost effective, and their applications do not require an additional electronic control system. Smart materials can be broadly classified into their two major abilities, actuation and sensing. They can be programmed to respond to external stimulus (actuation) or sense the changes in their environment (sensing). Throughout this dissertation, the programming/fabrication of such smart materials and their analytical characterization techniques for both actuation and sensing, are discussed.

In chapter 3, the programming process of a polymer for smart actuation application was evaluated using a coupled thermomechanical finite element framework. The polymer was programmed to store applied strain through a pre-straining process which was later recovered by shape recovery through uniform shrinking of the polymer. The strain was applied in two orthogonal directions by passing the polymer through a set of rollers and its shape recovery was investigated by uniformly heating the polymer. The process parameters were parametrically varied to obtain an optimized shape recovery performance. It was found that process parameters such as feed rate and rate of uniform heating changes the shape recovery performance.

In chapter 4, pre-strained polymer was evaluated for its localized shrinking and shape recovery with an application of an applied electric field. The electric field increases the temperature through resistive Joule heating and generates a temperature gradient through the thickness of the polymer. This temperature gradient creates strain gradient through its thickness resulting in an uneven shrinking of the polymer along its thickness. This results in out-of-plane self-folding of polymers and this is studied for various processing parameters, such as electrical conductivity of the localized region (hinge), width of the hinge and applied voltage. It was found that folding occurs only when the polymer reaches glass transition temperature (Tg) and the speed at which it folds depend on the applied voltage, the width and conductivity of the hinge.

In chapter 5, the structural relaxation of polymers undergoing physical aging is studied for their shape recovery applications. Polymers were aged for several days where they go through structural relaxation by changing their polymer chain conformation. The structural relaxation process was further analyzed by inducing conformational changes to the polymer chains through thermomechanical pre-straining process. The enthalpy lost during the structural relaxation process was quantified for various aging time, pre-straining parameters and their effects on shape recovery performance of the polymer was analyzed.

In chapter 6, polymers under the exposure of space environments, such as UV-C and atomic oxygen were evaluated for their shape recovery performance. The prolonged exposure to these environments causes chain scission in polymer chain backbone, breaks down the polymer chains and introduces foreign functional group to the backbone which degrades the polymer. Polymers were exposed to the UV-C radiation and atomic oxygen for several hours, and the resultant degradation of polymers were analyzed through infrared spectroscopy. The degradation of polymer chains resulted in diminished shape recovery performance in polymers. This chapter lays the experimental foundation for the samples sent to the international space station (ISS) for a 6-months long experiment where they were exposed to space environments of UV-C and atomic oxygen.

In chapter 7, conducting polymer fibers were fabricated through electrospinning process, and characterized for electrical signal sensing applications. Electrospun fiber mat has tremendous potential to be used as an intelligent interface between a human and a prosthetic limb to enable wider range of mobility in patients with spinal cord injuries (SCI). The non-woven electrospun fibers were analyzed for their morphology and their thermal, mechanical and electrical performance were characterized accordingly. Multiple conducting polymers were evaluated for the electrospinning process and the fiber mat’s conductivity was seen to be increasing with increased concentration of conducting polymer solvent.