Dissolution kinetics of poly(styrene-co-acrylonitrile) below and above its glass transition temperature

The dissolution behaviour of poly(styrene-co-acrylonitrile) (SAN) was examined across a broad temperature range to identify the governing thermodynamic and kinetic factors. Two high-boiling solvents with contrasting affinities for SAN, namely diethyl succinate (DS, RED = 0.69) and triethyl citrate (TC, RED = 1.37), were selected to study dissolution below and above the SAN glass transition temperature ( T g ≈ 105 °C). Isothermal mixing calorimetry (30–150 °C) showed complete SAN dissolution in DS above 45 °C, with exothermic dissolution enthalpies decreasing from 20 J g⁻¹ at 45 °C to 6 J g⁻¹ at 120 °C, the latter reflecting polymer–solvent interactions once the glassy-rubbery transition no longer contributes. In TC, SAN did not dissolve at 30 °C and was only partially dissolved up to 80 °C; complete dissolution occurred only above T g , with an enthalpy of ∼6 J g⁻¹ at 120 °C. ATR-FTIR monitoring revealed strong effects of solvent quality and temperature on induction time. SAN chains appeared in solution after 480 s in DS but after ∼5000 s in TC at 45 °C, evidencing significant segmental-relaxation limitation in TC. Above T g , induction times sharply decreased to 120 s in DS and 360 s in TC at 120 °C. Isoconversional kinetic analysis showed that dissolution in DS above T g is diffusion-controlled, with low activation energies (6–15 kJ mol⁻¹). Below T g , SAN dissolution transitions from segmental-relaxation-controlled (increasing to 41 kJ mol⁻¹) to diffusion-controlled at higher conversion. Dissolution in TC below T g exhibited even stronger relaxation control, with E a reaching 116 kJ mol⁻¹ . • Dissolution enthalpy shifts from transition-driven below T g to mixing above T g . • Induction time in the poor solvent approaches that of the good solvent above T g . • Below T g , SAN dissolution is governed by segmental relaxations and diffusion. • Above T g , SAN dissolution follows a diffusion-limited, Fickian mechanism.