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 consisting of 5–21 polystyrene-b-polyisobutylene (PSt-b-PIB) arms radiating from cyclosiloxane cores are described. The tensile properties of products containing 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 over the 25–85°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 (Nw,arm) and reaches a plateau in the Nw,arm = 5–10 range. Star-blocks exhibit higher strengths with lower PSt segmental Mn than linear triblocks. 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 viscosities of the star-blocks are substantially lower than those of linear triblocks with comparable hard/soft segment compositions, which is consistent with the star's unique microarchitecture and should lead to improved overall processibility.