Origin of the Difference in Order-Disorder Transition Temperature between Polystyrene-block-poly(2-vinylpyridine) and Polystyrene-block-poly(4-vinylpyridine) Copolymers

Three polystyrene-block-poly(2-vinylpyridine) (S2VP diblock) and three polystyrene-block-poly(4-vinylpyridine) (S4VP diblock) copolymers with varying molecular weights and block compositions were synthesized via anionic polymerization, and their order-disorder transition temperatures (TODTs) were determined using oscillatory shear rheometry and small-angle X-ray scattering (SAXS). It has been found that for comparable molecular weight and block composition the TODT of S4VP diblock copolymer is exceedingly high compared with that of S2VP diblock copolymer. The experimental observation has been confirmed by theoretical predictions from currently held mean-field theory. For the theoretical predictions, temperature-dependent interaction parameters for the polystyrene (PS)/poly(2-vinylpyridine) (P2VP) pair and the PS/poly(4-vinylpyridine) (P4VP) pair were determined from SAXS profiles obtained at varying temperatures ranging from 125 to 185 °C for a low-molecularweight (LMW) S2VP diblock copolymer and ranging from 160 to 195 °C for an LMW S4VP diblock copolymer and curve fitting to the Leibler theory. The molecular weights of LMW S2VP and LMW S4VP diblock copolymers employed were 10 200 and 2720, respectively, enabling us to obtain SAXS profiles in the disordered state of the respective block copolymers. The temperature-dependent specific volumes of PS, P2VP, and P4VP were determined at temperatures ranging from 25 to 200 °C using spectroscopic ellipsometry. To find the origin of the experimentally observed difference in TODT between S2VP and S4VP diblock copolymers, the thermally stimulated current method and dielectric relaxation spectroscopy were employed to investigate differences in polarizability between S2VP and S4VP diblock copolymers. It is concluded that much higher TODT of S4VP diblock copolymers as compared with the TODT of S2VP diblock copolymers is attributable to the stronger polarizability of P4VP in S4VP diblock copolymer compared with the polarizability of P2VP in S2VP diblock copolymer.