Self‐assembled structures in d 8 ‐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 χ 3/2 N (where χ 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. Copyright © 2009 Society of Chemical Industry