In-situ analysis of the structural formation process of liquid-crystalline epoxy thermosets by simultaneous SAXS/WAXS measurements using synchrotron radiation

We present a mechanistic study of the curing process of liquid-crystalline epoxy resins by the simultaneous measurement of small-angle X-ray scattering/wide-angle X-ray scattering using synchrotron radiation. The systems studied consist of a diglycidyl ether of terephthalydene-bis(4-amino-3-methylphenol) epoxy component and three types of aromatic amine-type curing agents 4,40-diaminodiphenylmethane, 4,4'-diaminodiphenylethane and m-phenylenediamine. The systems studied all exhibit similar thermosetting behaviors; first, the epoxide group reacts with an amine moiety of the curing agent, followed by the formation of the nematic domains in the early stage of the process. The nematic-to-smectic conversion proceeds after an incubation period, implying that a critical amount of nematic volume is necessary for phase transition. We found that the transformation continues beyond the conversion of 80% of the epoxide-amine bond formation and the corresponding entire polymer network formation. The volume fraction of smectic phases after completion of the reaction differs according to the structure of the curing agents. The study clarifies the mechanistic sequence and the preference for smectic phase formation both in terms of time sequence and the dependence of the structure on the curing agents. Polymer Journal (2013) 45, 43-49; doi: 10.1038/pj.2012.196; published online 21 November 2012