Self-assembled structures in d8-polystyrene-block-polyisoprene/polystyrene blends in the weak segregation regime: SAXS and TEM study

BACKGROUND: This paper reports an investigation of the microphase-separated morphology and phase behaviour in blends of d-polystyrene-block-polyisoprene with homopolystyrene in the weak segregation regime, using small-angle X-ray, scattering and transmission electron microscopy, as a function of composition, weight-average molecular weight and temperature. The chain length ratio parameter r(M) = M-H/M-C (where M-H and M-C are the weight-average molecular weights of the homopolymer and corresponding block copolymer chain) was selected to encompass all possible types of mutual homopolymer/block. copolymer sizes. RESULTS: In the weak segregation regime the polystyrene block chains behave as a 'wet brush' for r(M) < 1 similarly to the intermediate and strong segregation regimes. For r(M) > 1 a macroscopic phase separation occurs. The domain spacing D increases systematically in the range 0 < r(M) <= 1 with increasing concentration of homopolymer w(P) and increasing r(M) regardless of the implemented specific morphology, but the slope of the periodicity D versus w(P) relation is smaller than in the intermediate and strong segregation regimes. CONCLUSION: The criterion for 'wet and dry brush' morphologies has been applied to explain the changes in microdomain morphology during the self-assembly process. It has been shown that the parameters r(M) and chi N-3/2 (where chi is the Flory-Huggins parameter and N the number of segments per chain) characterize the slope of the D versus w(P) relation in the weak and intermediate segregation regimes. (C) 2009 Society of Chemical Industry