Influence of tetrahydrofuran on epoxy-amine polymerization

The objective of this work is to understand the influence of the solvent tetrahydrofuran (THF), a hydrogen bond acceptor, on epoxy-amine polymerization reactions. The cure kinetics of the diglycidyl ether of bisphenol A (DGEBA) and 4,4'-methylenebiscyclohexanamine in the presence of THF as a solvent was monitored using Fourier transform infrared (FTIR) spectroscopy in the near-infrared (NIR) region. FTIR analysis showed that the THF remained chemically inert during the epoxy-amine reactions. A mechanistically based two-parameter catalytic model for the intrinsic kinetics was developed. This model adequately described the intrinsic kinetics of this system in the presence and absence of THF. The influence of THF on the intrinsic kinetics was investigated by varying the THF to monomers weight ratio at 60, 50, and 40 degrees C. Results showed that for a given cure temperature the kinetic rate constants decreased with increasing THF content up to a critical composition and remained practically constant above this composition. This retardation effect is due to the formation of a relatively more stable intermediate species caused by the dipole-dipole interaction between THF and amine moieties, where 0.3:1 THF to monomers weight ratio represents a critical composition at which all the N-H bonds are stabilized. A negative substitution effect was observed in these reaction systems, and the presence of THF enhanced this effect up to the critical composition. The kinetic parameters followed Arrhenius behavior in both the presence and absence of THF. The presence of THF reduced the diffusion limitations by increasing the free volume in the reaction medium. At 60 degrees C, diffusion limitations due to vitrification were not apparent for reaction mixtures having a composition greater than 0.2:1 THF to monomers weight ratio, as the intrinsic kinetic model adequately described the entire reaction.