PHOTOPOLYMERS EXHIBITING A LARGE DIFFERENCE BETWEEN GLASS-TRANSITION AND CURING TEMPERATURES

When the glass transition temperature Tg of a completely polymerized system is considerably higher than the temperature of polymerization, the reaction usually stops before completion. With step reactions the difference between Tg and Tcure rarely exceeds 35K. We have found that in the chain crosslinking photopolymerization of 1,6-hexanediol diacrylate a difference of more than 70K can be observed. Photopolymerization continues in the glassy state, albeit at a self-decelerating rate. Possible causes of this continuation are (i) the presence of a significant amount of unreacted monomer up to high conversions and (ii) the persistence of a localized mobility of radical sites in the glassy state through tertiary hydrogen abstraction. It is proposed that the continuation of the reaction is not limited by vitrification but by freezing of secondary relaxations. The initial, fast part of the photopolymerization process was monitored with differential scanning calorimetry (d.s.c.), the slow continuation in the glassy state with Fourier-transform infra-red spectroscopy (FTi.r.). By using thin silicon substrates the same samples could be used with both techniques. In the range of conversions accessible with d.s.c., a perfect correlation with FTi.r. results was obtained. This proves that Beer's law applies to the i.r. data. © 1990.