Yu Seung Kim

Surface and wear behavior of bis-(4-hydroxyphenyl) cyclohexane (bis-Z) polycarbonate/polycarbonate–polydimethylsiloxane block copolymer alloys

The utilization of block copolymers containing siloxane segments is well known to be of great interest for surface modifications. This paper focuses on the use of segmented bis-(4-hydroxyphenyl) cyclohexane (bis-Z) polycarbonate – polydimethylsiloxane copolymers as an additive to improve surface and wear properties of this non-crystallizable polycarbonate. The effects of chemical composition, block size, and casting solvent on surface and wear properties of the modified polycarbonate were elucidated by using angle-dependent X-ray photoelectron spectroscopy (XPS), tapping mode atomic force microscopy, and mechanical testing. XPS results showed that the siloxane segments were enriched on the surface even at bulk composition as low as 1 wt% siloxane, depending on siloxane block size and casting solvent. The friction coefficient was strongly dependent not only on the surface enrichment of siloxane but also on the molecular weight of siloxane block segment. The initial friction coefficient had a little influence on the overall wear resistance due to the rapid wear process of the uppermost surface. Instead, stress– strain behavior of the samples played a key role in the observed wear resistance. The latter first increased and then deteriorated with siloxane concentration. A modified Ratner model was successfully employed to interpret the results. q 2002 Published by Elsevier Science Ltd.

Chemical degradation mechanisms of membranes for alkaline membrane fuel cells

Chemical degradation mechanisms of membranes for alkaline membrane fuel cells have been investigated using density functional theory (DFT). We have elucidated that the aryl-ether moiety of membranes is one of the weakest site against attack of hydroxide ions. The results of DFT calculations for hydroxide initiated aryl-ether cleavage indicated that the aryl-ether cleavage occurred prior to degradation of cationic functional group. Such a weak nature of the aryl-ether group arises from the electron deficiency of the aryl group as well as the low bond dissociation energy. The DFT results suggests that removal of the aryl-ether group in the membrane should enhance the stability of membranes under alkaline conditions. In fact, an ether fee poly(phenylene) membrane exhibits excellent stability against the attack from hydroxide ions.