Bidisperse Mixed Brushes: Synthesis and Study of Segregation in Selective Solvent

Abstract

We prepare a series of mixed brushes of two incompatible polymers, polystyrene (PS) and poly(2-vinylpyridine) (P2VP) grafted by end groups to Si wafers from carboxyl-terminated P2VP (MW(P2VP) 42 kg/mol) and carboxyl-terminated PS. The molecular mass of PS ranged from 2.98 to 75 kg/mol. The mixed brushes are bidisperse (asymmetric) with respect to the difference of molecular mass of P2VP and PS while the polydispersity of PS and P2VP is narrow. We expose the mixed brushes to selective solvents (toluene for PS and ethanol for P2VP). The morphology and surface chemical composition are investigated using scanning probe microscopy and contact angle measurements. For small chain length asymmetry, the brushes exhibit lateral and perpendicular segregation, and the structure depends on the solvent quality. Upon increasing the molecular weight asymmetry, we encounter the transition from a morphology of laterally segregated domains to a layered (sandwich-like) structure. The location of this transition can be tuned by changing the solvent selectivity. We find qualitative agreement between experiments and self-consistent-field calculations.