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

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journal contribution
posted on 23.01.2018 by Duk Man Yu, Jose Kenneth D. Mapas, Hyeyoung Kim, Jaewon Choi, Alexander E. Ribbe, Javid Rzayev, Thomas P. Russell
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 Å3 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 (Mn = 2200 g/mol, N = 16).