New polyisobutylene stars part XIX - Novel thermoplastic elastomers. II. Properties of star-block copolymers of PSt-b-PIB arms emanating from cyclosiloxane cores

Select mechanical, thermal, and rheological properties of star-blocks consi sting of 5-21 polystyrene-b-polyisobutylene (PSt-b-PIB) arms radiating from cyclosiloxane cores are described. The tensile properties of products cont aining 23-41 wt% of PSt are substantially higher (9.6-23.8 MPa) than those of linear triblocks of comparable arm molecular weights and compositions ov er the 25-85 degrees C temperature range. The mechanical properties of star -blocks seem to be much less sensitive to diblock contamination than linear triblock thermoplastic elastomers of similar hard/soft segment composition . The tensile strength of star-blocks increases by increasing the number of arms (N-w,N-arm) and reaches a plateau in the N-w,N-arm = 5-10 range. Star -blocks exhibit higher strengths with lower PSt segmental M-n than linear t riblocks. Solvent cast triblock copolymers exhibit higher tensile strengths than compression molded products; however, star-blocks show no significant property differences between cast and molded samples. The dynamic melt vis cosities of the star-blocks are substantially lower than those of linear tr iblocks with comparable hard/soft segment compositions, which is consistent with the star's unique microarchitecture and should lead to improved overa ll processibility. (C) 1999 John Wiley & Sons, Inc.