Title

The Effect of Polymer Blending On Environmental Stress Cracking Resistance: Role of Polycarbonate Blend Morphology, Miscibility, and Crystallinity

Date of Award

2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

First Advisor

Robert B. Moore

Advisor Department

Polymers and High Performance Materials

Abstract

The environmental stress cracking (ESC) of polymeric materials was investigated in order to elucidate the fundamental polymer properties leading to failure. In particular, the ESC of polymer blends was studied to gain a deeper understanding of the role of phase miscibility, blend morphology, and crystallinity on failure. Initial efforts to develop an ESC test method for polymer blends were based on failure induced by fabricated residual stresses utilizing a modified slow strain rate test. The modified slow strain rate test method was applied to a polycarbonate/polyamide and polycarbonate/polyester blend. Qualitative comparisons between industrially applied bent ESC test methods and the modified slow strain rate test were drawn. Further ESC test method development involved the determination of blend ESC resistance through tensile testing in a fluid environment utilizing an Eyring-type activated process to describe ESC. The validity of the test was confirmed through comparisons of the ESC resistance data to current theories describing the effect of polymer/fluid surface tension and fluid viscosity effects on ESC. It was found that the miscible blend, a polycarbonate/copolyester blend, displayed a rule of mixtures for ESC resistance to all fluids tested, except ether. In contrast, the immiscible blend, a polycarbonate/poly(butylenes terephthalate) blend, displayed a significant negative deviation from the rule of mixtures for ESC resistance. This behavior was attributed to the development of stress sites for craze initiation at the interface between the blend components on the surface of the test sample. The differences in ether resistance compared to the trends found for the fluid ESC resistance in this study were attributed to solvent/stress induced crystallization of the polycarbonate component. DSC traces indicated that significant crystallization of the polycarbonate component was observed for samples with low ether ESC resistance. These data suggest that strongly swelling fluids, e.g. diethyl ether in the presence of polycarbonate, causes local densification from polymer crystallization resulting in voids that facilitate the initiation and growth of crazes. The role of blend morphology, as well as the crystalline component, was studied through variations in polycarbonate molecular weight and injection molding conditions in polycarbonate/poly(butylene terephthalate) blends. Qualitative observations indicated a strong influence of the phase composition at the surface in determining the blend ESC resistance. An increase in polycarbonate component at the surface resulted in an increased ESC resistance to surface active fluids and a decreased resistance to plasticizing fluids. (Abstract shortened by UMI.)