Effects of interfacial residual stress on Tg of epoxy resin

Distinct differences in the glass transition temperatures (T_g ) of epoxy resins were observed when their samples were prepared in two different ways. In the first method (A) of preparation, the mixture of epoxy resin and curing agent was cured and cooled directly in an aluminum pan for DSC measurement. There were strong interfacial residual stresses in these samples. In the second method (B), epoxy resin was cured and cooled on a polytetrafluoroethylene (PTFE) plate, cut into small circular pieces and then placed in aluminum pans, and no interfacial residual stresses were found in these samples. The curing and cooling conditions for samples prepared by these two methods were otherwise exactly the same. The samples by method A with strong residual stresses showed lower T_g and broader transition ranges (δT) than those by method B. With an increase in the rate of cooling for epoxy resin, the T_g for samples by method A decreased more than that for samples by method B. The transition range for A samples also increased more than that for B samples. The effects of residual stress on T_g are substantiated by experimental results of the relaxation of interfacial residual stress or the incorporation of freezing stress