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 (χ), 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 χ 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 χ with PS was found to be given by χ = 0.3144 + 36.91/ T, with χ = 0.438 at 25 °C, which is ∼13 times larger in magnitude than χ parameter for PSM- b -PS copolymer (∼0.035 at 25 °C) calculated using a 118 Å 3 reference volume. With this large increase in χ, 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).