| It is well-established that block copolymers A-b-B, whose segments are incompatible, can function as emulsifiers in blends of homopolymers of A and B. In such blends, the volume fractions of the blend constituents determine the morphology; and the emulsifying power is primarily dictated by the molecular weight ratio of homopolymer to block copolymer. However, when there is exothermic mixing between one of the block copolymer segments and one homopolymer, the role of the molecular weight ratios becomes less significant, since the competition between exothermic and endothermic interactions can strongly influence the morphology.;We have explored the influence of molecular weight ratio, degree of exothermic mixing, incompatibility across an interface, and balance of interfacial swelling on the morphology of solvent-cast films using transmission electron microscopy. Microphase formation in binary A/X-b-B blends is facilitated, when entropic mixing between homopolymer A and block X is insufficient, by exothermic mixing between A and X. Uniform microphase separation can occur with a low molecular weight block copolymer when the homopolymer A has M scA $>$ M scX, and when there is a polydisperse distribution in M scA. However, microphase formation is observed within a window: the upper limit is defined by too high molecular weight of the homopolymer and/or of the block copolymer, coupled with a large repulsion $chi$ scAB, which leads to macrophase segregation; the lower bound is determined by a too small repulsion $chi$ scAB, and/or a too negative $chi$ scAX which results in formation of homogeneous phase.;In ternary A/A-b-B/B or A/X-b-B/B blends, the tendency of the block copolymer to migrate to the interface (interfacial activity) depends on incompatibility of homopolymer A with block B and between homopolymer B and block A or X (N$chi$ scAB and N$chi$ scBA or N$chi$ scBX, respectively). When the block copolymer is interfacially active, modulation of the balance of interfacial swelling of the block copolymer segments can be used to generate a rich variety of morphologies. In addition, when the swelling of a block copolymer segment by the minor phase is sufficiently large relative to that in the major phase, the minor phase becomes the continuous matrix, forming a polygonal network morphology. Conversely, strong interfacial swelling of the coronae by the major phase lowers the emulsifying power of a block copolymer as evidenced by the co-existence of microphase with macrophase even in the presence of a relatively large quantity of block copolymer. |
