Studies of temperature and time-dependent network formation in commercial epoxy resins using modified imidazole curing agents

Two commercial epoxy resins, MY750 and MY720, were cured over a range of cure temperatures with a series of novel curing agents based on modified imidazoles. A combination of techniques, including dynamic viscometry, proton nuclear magnetic resonance spectroscopy, differential scanning calorimetry, high performance and gel permeation chromatography, were used to analyse the reaction mixture. It appears that for a given formulation and given resin, the viscosity behaviour near gelation is different for different cure temperatures and the mechanism appears to be dependent not only on the nature of the imidazole curing agent, but also on the cure temperature. Higher temperature cure appears to lead to the formation of a less ‘homogeneous’ reaction mixture at the gel point, containing oligomeric chain lengths of higher polydispersity, whereas in the lower temperature cure, oligomers of a smaller range of chain lengths are present.     

A study of the polymerization behaviour of N-(4-phenoxy)-phenylmaleimide using DSC analysis

Summary The thermal cure of a monofunctional maleimide N-(4-phenoxy)-phenylmaleimid (1) was examined using differential scanning calorimetry (DSC). The analogous bis-maleimide bis-(4-maleimidophenyl)ether (2) was also analyzed in the same manner for comparison. Polymerization kinetics derived from scanning DSC measurements showed an order of reaction varying between n=1 and 2 for (1), compared to an order of reaction between n=2 and 3 (by multiple linear regression analysis) for (2). The overall enthalpy of polymerization (H_o) for N-(4-phenoxy)-phenylmaleimide (94kJ/mole) agrees well with that obtained for maleimide itself (89kJ/mole) using conventional calorimetric techniques indicating that polymerization appears complete within the timescale of the experiment. In contrast the analogous BMI displays a markedly lower overall polymerization enthalpy (ca. 66kJ/mole) indicating an apparent conversion of only 37%.     

Microscale Inhomogeneities in Aluminum Solution-Doping of Silica-Based Optical Fibers

The microstructure of aluminum (Al) solution-doped soot layers in modified chemical vapor deposition fabrication of silica-based optical fibers has been studied. It is shown that such Al doping is predominantly determined by deposition temperature. Radial and longitudinal Al doping distributions have been investigated in soot layers, in fully sintered glass layers, and in collapsed preforms. Formation mechanisms are discussed.