In situ monitoring of reaction-induced phase separation with modulated temperature DSC: comparison between high-Tg and low-Tg modifiers

A linearly polymerizing and network forming epoxy–amine system will be modified with high-Tg thermoplastics poly(ether sulphone) (PES: Tg=223 °C) and poly(ether imide) (PEI: Tg=210 °C) and with a low-Tg copolymer poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (triblock: Tg=−70 °C). Both PES and the triblock show lower critical solution temperature (LCST)-type demixing behavior, while PEI exhibits upper critical solution temperature (UCST)-type demixing with epoxy resins. Reaction-induced phase separation (RIPS) in these modified systems is studied using Modulated Temperature DSC (MTDSC) as an in situ tool. The thermoplastic-rich phase of the high-Tg modifier will vitrify at some point, while that of the low-Tg modifier stays mobile during curing at the useful curing temperatures, affecting the diffusion rates of the epoxy–amine species in (to) this phase differently. By using the heat capacity signal during quasi-isothermal cure, phase separation can be measured indirectly as a step-wise decrease due to vitrification of the thermoplastic-rich phase or directly as a peak when the heat of phase separation occurs on the time-scale of the modulation. The latter effect has been detected for the first time with MTDSC in the case of RIPS. Temperature-conversion-transformation diagrams unambiguously show the LCST-type demixing behavior of these systems, while information about the in situ developed morphology can be obtained from the heat capacity evolutions in non-isothermal post-cures.