ARCHITECTURAL EFFECTS ON THE STABILITY LIMITS OF ABC BLOCK-COPOLYMERS

The random phase approximation has been used to extend the Leibler the ory for the stability limit of a homogeneous melt of A-B diblock copol ymers to examine the onset of microphase and macrophase separation in a variety of ABC block copolymer systems. The stability limit is locat ed by the divergence of the collective structure factor of the melt. W e introduce and analyze three models for ABC block copolymers: linear triblocks, random comb copolymers where a fixed number of A and B teet h are placed randomly along a C backbone, and statistical comb copolym ers, with A or B teeth spaced regularly, but with sequences constructe d using a two parameter Markov process. We compute order-disorder stab ility boundaries for the segregation strength parameter chi(AB)N at th reshold as a function of chi(AC)N, chi(BC)N, composition, and other mo del parameters, and compare the results for the three different archit ectural models. An interesting ''reentrant order-disorder transition'' is located in several model phase diagrams, and is associated with a peculiar situation in which more incompatibility causes less segregati on. In the case of statistical combs, macrophase separation into two l iquid phases can be favored over microphase separation. (C) 1997 John Wiley & Sons, Inc.