MORPHOLOGY AND THERMODYNAMICS OF SYMMETRICAL POLY(A-BLOCK-B-BLOCH-C) TRIBLOCK COPOLYMERS

The morphology of ABC triblock copolymers based on polystyrene-block-p olybutadiene-block-poly(methyl methacrylate) [PS-b-PB-b-PMMA] (SBM ser ies) and their hydrogenated analogues -block-poly(ethylene-co-butylene -block-poly(methyl methacrylate) [PS-b-PEB-b-PMMA] (SEEM series) block copolymers is governed by the relatively weak incompatibility of the end blocks PS and PMMA in comparison to the strong incompatibility of the polybutadiene or poly(ethylene-co-butylene) midblock. The paper de scribes the morphologies of high molecular weight ((M) over bar(n) app roximate to 200 000) block copolymers, which are symmetrical with resp ect to the PS and the PMMA blocks with a varying elastomeric center bl ock (0.06 < w(el) < 0.38). Besides an ABC lamellar morphology (ll) (38 wt % PB or PEB), two other lamellar morphologies are observed for sho rter elastomer chains: At 17 wt % of the elastomeric center block a '' cylinder at the wall'' morphology is observed where PB or PEB cylinder s are located at the lamellar PS/PMMA interface (lc). At 6 wt % of ela stomer, the polybutadiene forms spheres at the PS/PMMA interface (''ba ll at the wall'') (ls). In this case hydrogenation of the butadiene bl ock, which is associated with a further increase in the immiscibility to the end blocks, induces a change of the overall lamellar structure into a cylindrical morphology in which PS cylinders surrounded by PEB rings are dispersed in a PMMA matrix (cr). The transition from the lam ellar (ll) morphology to the (lc) and (ls) morphologies is described b y a simple: extension of the Meier/Alexander/de Gennes/Semenov theorie s of AB block copolymers in the strong segration limit to ABC block co polymers. The theoretical description predicts that morphological tran sitions can be achieved in ABC triblock copolymers at constant composi tion even in the limit of strong segregation.