Unexpected Scaling Behavior of the Order-Disorder Transition of Nearly Symmetric Polystyrene-block-polyisoprene Diblock Copolymers Dilated with a Nominally Neutral Solvent at Very Low Dilution

The scaling behavior of the order-disorder transition (ODT) temperature for several nearly symmetric P(S-b-I) copolymers has been studied over a polymer volume fraction range of 0.9 e φdiblock e 1. To enable precise control of the concentration at high φ and at elevated temperatures as the concentration approaches the neat melt, compressed CO2 is used as the diluent in place of more traditional liquid solvents. Swelling of polymer films byCO2 over the concentration range of interest was determined using high-pressure ellipsometry, while high-pressure static birefringence measurements were used to determine the location of the ODT in dilated bulk samples. Log-log plots of χODT vs φdiblock allowed the determination of the scaling parameter, R, for comparison with the dilution approximation. At polymer volume fractions between 0.88 and 0.98, R varied between-1.2 and-1.3, which is consistent with scaling for neutral liquid diluents. At very low dilution, however, the scaling changes abruptly, and unusually high scaling parameters are observed for all copolymers at nearly identical polymer concentrations. This behavior suggests that solvent is initially segregating at the interface before beginning to distribute throughout the polymer. These data further illustrate the inadequacies of the dilution approximation (mean field) to correctly predict the ODT temperature, especially at very high polymer concentrations.