Two-Step Chemical Transformation of Polystyrene-block-poly(solketal acrylate) Copolymers for Increasing χ

We investigated the two-step chemical transformation of a series of symmetric poly­(styrene-b-solketal acrylate) (PS-b-PSA) copolymers with varying molecular weights from 1900 to 27 300 g/mol (13 ≤ N ≤ 194) synthesized by sequential atom transfer radical polymerization. Through acid-catalyzed hydrolysis, the PSA block is converted into poly­(glycerol acrylate) (PGA), which subsequently can be hydrolyzed to a poly­(acrylic acid) (PAA) block. With this two-step conversion, the responsive PSA block becomes increasingly polar as the reaction proceeds, driving the PS-b-PSA copolymer from a phase-mixed state into a strongly segregated state enabled by a significant increase in the segmental interaction parameter (χ). To determine χ for the PS-b-PSA copolymer before and after the conversion of PSA to PGA and then to PAA, a mean-field correlation-hole analysis of the scattering in the phase-mixed state was performed using temperature-dependent small angle X-ray scattering. After the conversion of PSA to PGA, the χ parameter for PS-b-PGA was found to be given by χ = 0.4886 + 20.89/T, with χ = 0.558 at 25 °C, and then it increased further to χ = 0.885 at 25 °C (χ = 0.8122 + 21.55/T) when the PGA block was fully transformed to PAA, which is ∼15 times larger in magnitude than the χ value for PS and PSA (χ = 0.061 at 25 °C) calculated using a 118 Å3 reference volume. As a result of such a large increase in χ, lamellar and cylindrical microdomain spacings of 7.4 and 6.9 nm were achieved from a low-molecular-weight PS-b-PSA (M n = 2500 g/mol, N = 19) after conversion to PS-b-PGA and PS-b-PAA, respectively.