Preparation and investigation of tailored shape memory polymers

Date of Completion

January 2004

Keywords

Engineering, Chemical|Engineering, Electronics and Electrical

Degree

Ph.D.

Abstract

This dissertation seeks to develop novel shape memory polymers with tailored properties and characterize those materials using diverse techniques. The dissertation is composed of two parts. The first part includes the conception, synthesis, and characterization of three classes of shape memory polymers with excellent shape memory performance and quite distinct thermomechanical responses. The first SMP class is synthesized using a difunctional crosslinker and two monomer species with different homopolymer Tg's, but one single copolymer Tg. In addition to admirable shape fixing abilities, shape recovery abilities with controlled work ability, and tunable transitions, merits for this material include castable processing and optical clarity. The shape fixing, stress-free shape recovery, and constrained shape recovery were quantitatively characterized for this material. A standard approach toward comparing the shape fixing and shape recovery was proposed using a stress controlled shape memory cycle test. The second SMP class developed in this dissertation is also a predominantly glassy material that, derived from an amorphous-semicrystalline polymer blend, is amenable to thermal processing based on a physical crosslinked feature. The effect of composition on crystallization, and therefore work ability, and the Tg were studied. A shape memory cycle test was performed and compared with the first SMP class. The third class, the core of this dissertation, is a semicrystalline rubber that was chemically crosslinked for shape memory application. Besides controlled work ability and a tailorable transition within a limited range, this material is also flexible at room temperature and introducing a second composition can further customize its room temperature flexibility. ^ The second part of this dissertation includes the investigation of the crystallization kinetics and thermal conductivity of the semicrystalline shape memory rubber and the correlation of such properties with the shape memory properties, specifically, the shape fixing and shape recovery speed. Isothermal and non-isothermal crystallization kinetics were first investigated using a conventional thermal method (DSC). A new method, utilizing polarized microscopy with liquid crystal-based compensation and a hot stage, was then used to separately quantify the crystallization kineticsin addition to obtaining morphology information. The results were compared with the conventional thermal method revealing enhanced sensitivity of the optical method. The effects of thermal conductivity on shape fixing and shape recovery speed were analyzed based on conventional transport concepts in Chapter 6. Simple dimensional analysis allowed correlation of thermal diffusion time (thus recovery rate) with the sample geometry and thermal conductivity. The validity of this analysis was verified using a chemically crosslinked PCO with variation in both loading of a thermally conductive filler and sample dimensions. ^

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