DYNAMICS OF PHASE-SEPARATION IN COPOLYMER-HOMOPOLYMER MIXTURES

We carry out computer simulations of phase separation in diblock copolymer-homopolymer mixtures. The homopolymer chains are assumed to be compatible with one of the blocks of the copolymers whereas the two blocks of the copolymer chains are mutually incompatible. One of the characteristic features of this system is that, in certain conditions, it undergoes a double phase separation. That is, by decreasing the temperature, a macrophase separation takes place so that copolymer-rich and homopolymer-rich domains are formed. As the phase separation proceeds, a microphase separation also starts in the copolymer-rich domains. We study the kinetics of this double phase separation and the domain morphology associated with the phase separations in two dimensions. It is shown that a morphological transition occurs by changing the incompatibility strength between the homopolymer and one of the blocks. When the incompatibility is weak, the microphase-separated stripe domains are perpendicular to an interface between the copolymer-rich and the homopolymer-rich phases whereas when it is strong the domains tend to be parallel to the interface. An analytical theory calculating the interfacial energy is presented to understand this transition.