Novel Factors Influencing Microphase Separation in SB/SBS Block Copolymers

Abstract

Microphase separation in poly(styrene-butadiene) diblock (SB) and triblock (SBS) copolymers is induced by thermodynamic incompatibility between the blocks and is responsible for the formation of microstructures, or domains, which endow the copolymers with thermo-mechanical properties unlike those of either parent homopolymer or those of a random copolymer of equal composition. Microstructural elements, appearing as either co-continuous lamellae or disperse domains of one phase in a continuous matrix of the other, are on the size scale of the domain-forming block. Previous theoretical investigations^1–6 have shown that the microphase-separation transition (MST) and the resultant microstructure depend on temperature and material characteristics such as the polymer composition, molecular weight (M), and molecular architecture (e.g., SB or SBS). In the present work, we demonstrate the significance of two other factors influencing the MST in the solid state: (1) the molecular-weight distribution (MWD), or polydispersity, and (2) a “tapered” block junction. Predictions are made for system energetics and microstructural features in the strong-interaction limit with extended versions of the Leary-Henderson-Williams (LHW) thermodynamic theory.^3–5