CONFINED SINGLE-CHAIN MODEL OF MICROPHASE-SEPARATED MULTIBLOCK COPOLYMERS .1. (AB)N COPOLYMERS

Theoretical models based on confined-chain and mean-field principles have proven particularly useful in elucidating fundamental relationships between microstructural dimensions and molecular characteristics in microphase-separated diblock and triblock copolymers in the strong-segregation limit. However, the predictive capabilities of these formalisms have never been extended to multiblock copolymers, which constitute an important class of high-performance materials. In this work, a confined-chain model is developed for perfectly alternating linear (AB)n multiblock copolymers possessing a lamellar morphology. Predictions clearly indicate that if the copolymer composition and molecular weight (M) are held constant, increasing n increases the homogeneous (residually mixed) interphase volume fraction, thereby decreasing the extent of thermodynamic incompatibility between the A and B blocks and making microphase separation less favorable. If, on the other hand, the block lengths are held constant and M is allowed to vary with n, microphase separation becomes more energetically favored as n is increased from unity. In both cases, the dependence of lambda, f, and D (where lambda is the interphase thickness, f is the interphase volume fraction, and D is the microdomain periodicity) on n and M is explored here and useful scaling relationships are identified. Comparisons are also made with predictions employing the sequential diblock approximation (SDA), in which an (AB)n copolymer is modeled as an AB diblock copolymer of reduced molecular weight (M/n).