A cure kinetics investigation of amine-cured epoxy by Rheo-Raman spectroscopy

In a companion paper, fluorescence lifetime imaging revealed a heterogeneous morphology at macroscopic and molecular length scales in a bis-epoxide blend composed of diglycidyl ether of bisphenol-A (DGEBA) and diglycidyl ether of 1,4-butanediol (DGEBD) cured with meta-phenylenediamine (m-PDA) with a strategically integrated mechanophore. This macroscopic heterogeneity most likely originates during the crosslinking process, which motivates measurements of both the reaction progress and the mechanical properties under controlled cure schedules. Rheo-Raman spectroscopy provides simultaneous measurements of Raman spectra to monitor vibrational modes relevant to the crosslinking process and rheology to track the viscoelastic shear modulus during crosslinking. Here, rheo-Raman measurements confirm that the molecular level heterogeneity is influenced by reaction kinetics differences of the two miscible bis-epoxides with m-PDA. In the companion paper the macroscopic heterogeneity was linked to the self-association of oligomers in DGEBD through interchain hydroxyl-hydroxyl hydrogen bonding. Rheo-Raman studies also revealed that this self-association slows down the overall cure-kinetics of the DGEBD/m-PDA and DGEBA/DGEBD/m-PDA curing reaction, thereby supporting the link between the macroscopic heterogeneity and oligomers in DGEBD.