PHASE-EQUILIBRIA IN BLOCK COPOLYMER HOMOPOLYMER MIXTURES

Phase equilibria in block copolymer (AB) + homopolymer (A) blends are considered theoretically in the strong segregation limit. The effect of homopolymer molecular weight is analyzed. In particular, the following are shown: (i) Long enough A-homopolymer chains (N(h) much greater than N(hc)) are incompatible with stretched A-blocks so that homopolymer and block copolymer should be space separated in any morphology. (ii) Interaction between copolymer micelles in the region N(h) much greater than N(hc) crucially depends on homopolymer molecular weight. For N(h) < N(h)* interaction of copolymer micelles of any geometry is purely repulsive (here N(h)* much greater than N(hc)). On the other hand, micelles strongly attract each other (at not too short distances) if N(h) > N(h)*. Separation of the system into two phases (block copolymer and homopolymer) is predicted as a result of this attraction. (iii) Addition of A-homopolymer to an AB-copolymer system with minor A-blocks results in a shifting of the regions of stability of the cylindrical and spherical morphologies toward more symmetric copolymers(smaller fraction of major B-blocks). In particular, the spherical morphology with matrix B-blocks could be the most favorable even if the volume fraction of the B-component is nearly 50%.