Temperature modulated differential scanning calorimetry during isothermal curing of phase separating polymer networks

Temperature modulated differential scanning calorimetry (TMDSC) and conventional differential scanning calorimetry (DSC) have been applied for investigation of isothermal curing of two component polymer networks separating into two phases during network formation. The network component consists of diglycidylether of bisphenol A (DGEBA) cross-linked with diaminodiphenyl methane (DDM) and the linear polymer component is polyethersulfone (PES). Isothermal curing experiments at 373 K and heating experiments after defined isothermal curing times are combined to investigate the evolution of the glass transition temperatures during reaction. While in the initial stages of the reaction the systems are homogeneous mixtures, phase separation takes place in the course of curing into a DGEBA/DDM-rich and a PES-rich phase. Since in the conventional isothermal DSC measurements the changes in the heat capacity, when the system undergoes a transition from a liquid to a glassy solid during network formation, are masked by the heat of reaction, TMDSC was applied to resolve both effects. In the heating experiments after defined isothermal curing periods the changes in the heat capacity due to the glass transition were resolved for both phases by TMDSC whereas in the conventional DSC measurements the glass transition temperature could only be detected for the DGEBA/DDM-rich phase. The phase separation process is indicated by TMDSC for curing times above 50 min for a curing temperature of 373 K. (C) 1998 Elsevier Science B.V. All rights reserved.