Influence of pulse-delay curing on softening of polymer structures.

Resin composites may be polymerized according to one of several light-curing modes. These modes include variations in intensity of the curing lights and time delay before final cure. The so-called pulse-delay method has earlier been found to reduce the formation of gaps due to polymerization contraction, without mechanical properties of the resin composite being compromised. It was hypothesized that the slow pre-cure of this method would give rise to a different polymer structure than results when the polymer is cured in one step at high intensity. It was found that although the quantity of remaining double bonds was unaffected, the pulse-delay technique led to polymers of increased susceptibility to softening in ethanol. The softness increased with the intensity of the pre-cure and with the waiting time before final cure. The increased softening may be interpreted as the manifestation of a polymer structure having fewer crosslinks.