Effects of Chain Ends and Three-Segment Interactions on Second and Third Virial Coefficients of Four-Arm Star Polystyrenes

Abstract

Second and third virial coefficients (A2 and A3, respectively) for cyclohexane solutions (34.5 °C) of three four-arm star polystyrene (PS) samples with a benzyl group at each arm end (4SPS-Bz) and two four-arm star PS samples with a sec-butyl group at each arm end (4SPS-Bu) were determined as functions of weight-average molecular weight Mw lower than 1:5 × 104. As in the case for linear PS with benzyl ends, A2 for 4SPS-Bz at the theta temperature (34.5 °C), where A2 vanishes for sufficiently high Mw, was negative and decreased with lowering Mw in contrast to the increase of positive A2 for 4SPS-Bu. The negative A2 for 4SPS-Bz was ascribed to the effects of three-segment interactions and binary interactions between middle and junction point segments, while the positive A2 for 4SPS-Bu was explained as due to the effects from arm ends. These findings reveal that the previously observed lowering in apparent theta temperature (with decreasing Mw), where A2 = 0 for a given Mw, for four-arm star polystyrene is attributed to the arm-end effect. The values of A3 for both 4SPS-Bz and 4SPS-Bu were positive and increased with decreasing Mw at the theta temperature, showing significant three-segment interactions among two middle and one junction point or one arm-end segment.