Order-disorder and order-order transitions in mixtures of highly asymmetric triblock copolymer and low molecular weight homopolymers

The order-disorder and order-order transitions (ODT and GOT) of a series of mixtures of a compositionally asymmetric polystyrene-block-polyisoprene-block-polystyrene copolymer (SIS) (f(PS) = 0.131) and low molecular weight PS homopolymers with different molecular weights have been investigated by using transmission electron microscopy (TEM), synchrotron small-angle X-ray scattering (SAXS), and rheology. ODT temperatures determined from the discontinuous change in 1/I-max and sigma(q)(2) vs 1/T plots were found to be in reasonable proximity to the ones determined from the discontinuous change in log G' vs T plots. Various OOT's were also identified for a few of V4113/PS mixtures, as evidenced from both SAXS and rheology. This was further confirmed by TEM for the quenched samples. Spherical microdomains in liquidlike packing (LLP) observed for pure V4113 at low temperature were found not to be an equilibrium phase, which would transform completely to spheres in body-centered-cubic array (BCC) with prolonged annealing time. Our asymmetric SIS/low molecular weight PS mixtures showed a minimum ODT around 10 wt % of PS. This unusual phase behavior has not been observed experimentally in any block copolymer/homopolymer mixture system and is also inconsistent with any existing theoretical predictions. We believe that this is related to the fact that the styrene volume fraction of our SIS triblock copolymer is quite close to the transition boundary between cylinders in hexagonal array (HEX) and BCC. In this case, the chain stretching and/or packing frustration effects, which are usually ignored in normal block copolymer/homopolymer mixtures, can be significant with the addition of low molecular weight homopolymer to the minor phase of the highly asymmetric triblock copolymer.