CURING KINETICS OF THE SYNTHESIS OF POLY(2-HYDROXYETHYL METHACRYLATE)(PHEMA) WITH ETHYLENE-GLYCOL DIMETHACRYLATE (EGDMA) AS A CROSS-LINKING AGENT

An experimental study was carried out to investigate the effect of eth ylene glycol dimethacrylate (EGDMA, as a crosslinking agent) content o n the curing kinetics of the polymerization of 2-hydroxyethyl methacry late (HEMA), using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). EGDMA may cause a crosslinkin g-facilitated gel effect which reduces the termination rate of living free radicals and enhances the overall reaction rate, but it may also induce a diffusional resistance for the reactants so that some free mo nomers are trapped and pendant vinyl groups are prohibited from reacti on by the crosslinked structure. At higher content of EGDMA, the later effect becomes predominant, and the reaction rate and the final conve rsion are limited. The exothermic peak of the curing reaction tends to carry a shoulder and then split into two peaks as the amount of EGDMA is increased, possibly due to a later reaction of the trapped monomer s and pendant vinyls. The heat of reaction measured by DSC in the scan ning mode is 61.2 kJ/mol C=C. The activation energy (E) of the curing reaction ranges from 56.5 to 78.3 kJ/mol C=C depending on the EGDMA co ntent and the type of operation. The diffusion-limited reaction rate a nd the different thermal history experienced in the nonisothermal and isothermal curing can result in variations of the results in the activ ation energy measurement. (C) 1997 John Wiley & Sons, Inc.