Complex Spherical Micelles in A−B−C-Block Copolymer Melts

Abstract

Spherical micelles with complex inner structure in A−B−C-triblock copolymer melts with NA + NB ≪ NC are considered in strong and superstrong segregation limits. It is assumed that blocks of polymer chain are different in length and strongly incompatible. One of the particular systems which we have in mind is a melt of B−C-block copolymer (NB ≪ NC) with strongly associative A-groups at the end of B-block. In this case the formation of A-aggregates occurs inside the B-micelle, which is assumed in this paper to be spherical in shape. The inner structure of the B-micelle can be complicated. It is possible to distinguish between two principally different cases: (i) all A-blocks form a single aggregate at the center of the B-micelle and (ii) the A-aggregates form a certain structure inside the core of the B-micelle. It has been shown that, depending on the external parameters (such as temperature, monomer interaction parameters, block length, and monomer unit length), the characteristics of the B-micelle (aggregation number, size, position of A-aggregates inside the B-micelle) can be quite different. Thus, the size (aggregation number) of a polymer micelle with a single A-aggregate at the center is smaller than that for the B−C-diblock copolymer case. At the same time, for the polymer micelle with multi-A-aggregate inner structure the size and aggregation number of B-micelle are larger than that for B−C-diblock copolymer case. If the incompatibility of A- and B-blocks is much stronger than that for the B- and C-blocks, the polymer micelle with a single A-aggregate at the center is formed. Otherwise B-micelles have multi-A-aggregate inner structure. The presence of A-aggregates inside the B-micelle leads to stabilization of microphase separation between B and C in the system. For the system under consideration, the superstrong segregation regime for the central A-aggregate may be achieved for relatively small values of surface tension coefficients. In the superstrong segregation regime, the form of the A-aggregates is disklike. The regions of different structures of spherical B-micelles are represented on the diagram of states.