Evaluation of the Interaction Parameter for Poly(solketal methacrylate)-block-polystyrene Copolymers

A series of symmetric poly(solketal methacrylate-b-styrene) (PSM-b-PS) copolymers with varying molecular weights that can transform a hydrophobic PSM block to a hydrophilic poly(glycerol monomethacrylate) (PGM) block through an acid hydrolysis were investigated. This simple chemical transformation significantly enhances the segmental interaction parameter (chi), enabling a phase-mixed block copolymer (BCP) to microphase separate without any additives. Temperature-dependent small-angle X-ray scattering (SAXS) measurements as a function of the degree of polymerization (16 <= N <= 316) and PSM hydrolysis conversion were conducted to characterize the order-to-disorder transition (ODT) behavior as well as the lamellar microdomain features. Using a mean-field correlation-hole analysis of the scattering, the chi value for PSM and PS was determined as a function of the conversion of PSM to PGM. For 100% conversion of PSM to PGM, the chi with PS was found to be given by chi = 0.3144 + 36.91/T, with chi = 0.438 at 25 degrees C, which is similar to 13 times larger in magnitude than chi parameter for PSM-b-PS copolymer (similar to 0.035 at 25 degrees C) calculated using a 118 angstrom(3) reference volume. With this large increase in chi, even the smallest synthesized PGM-b-PS copolymers underwent microphase separation, allowing us to achieve a center-to-center lamellar microdomain spacing (commonly referred to as the full pitch) of 5.4 nm, obtained for the lowest molecular weight sample (M-n = 2200 g/mol, N = 16).