Microphase separation in block copolymer/homopolymer blends: Theory and experiment

The process of microphase separation in diblock copolymer/homopolymer blends is studied both theoretically and experimentally for an asymmetric diblock copolymer and for homopolymer concentration less than 25%. The degree of polymerization of the added homopolymer (N-h) covered all possible cases; N approximate to N-h, N>N-h, and N<N-h. SAXS and rheology are employed and provide the order-disorder transition temperature through the discontinuous changes of the structure factor and the storage modulus, The minority phase can solubilize only a small amount of added homopolymer; addition of a higher amount results in the formation of nonequilibrium structures. Theoretical calculations performed in the strong segregation limit provide the period and the critical value of chi N for the stability of the disordered phase. The theory predicts that (chi N)(c) always increases with the addition of the majority phase. When the minority phase is added, (chi N)(c) can increase (N much greater than N-h and N greater than or equal to N-h) or decrease (N much less than N-h). The experimental results are in good agreement with the theoretical predictions for the two extreme cases; N much greater than N-h and N much less than N-h, but differ when the minority component is added with N greater than or equal to N-h. These results demonstrate that the degree of compatibility between the two blocks of the diblock AB can be effectively controlled by adding a small amount of homopolymer A or B. (C) 1997 American Institute of Physics.