In situ monitoring of reaction-induced phase separation with Modulated Temperature DSC: comparison between high-T_g and low-T_g modifiers

摘要

A linearly polymerizing and network forming epoxy-amine system will be modified with a high-T_g thermoplastic poly(ether sulphone) (PES: T_g=223°C) and with a low-T_g copolymer poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (triblock: T_g=- 70°C). Both PES and the triblock show Lower Critical Solution Temperature (LCST)-type demixing behavior 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- T_g 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 nonisothermal post cures. Thermal properties of materials formed during reaction-induced phase separation (RIPS) of modified epoxy-amines can be obtained from the derivative of the heat capacity signal (dC_p/dT) in non-isothermal conditions following a certain cure schedule. A method is proposed in which the dC_p/dT signal is deconvoluted, resulting in values of T_g and △C_p at T_g, which can be used as probes for the composition and fraction of each phase, respectively. The Couchman relation links the Tg of phase i (T_(g,i)) to the composition by assuming that the phase is a blend of the epoxy-amine species at conversion x and the modifier. The fraction of this phase can be determined from the △DC_p at T_g when additivity is assumed. Evolutions of the composition and fraction as a function of conversion during RIPS in isothermal conditions thus contribute to the understanding of the relation between the cure schedule and the morphological development.