Polymerization kinetics and stress development were measured during the UV curing of multifunctional acrylate and methacrylate coatings by photodifferential scanning calorimetry and a cantilever deflection method. Higher light intensity leads to higher double-bond conversion but unfortunately also to higher stress. Higher monomer functionality unfortunately leads both to lower conversion and to higher stress; substituting methacrylate for acrylate does likewise. Longer monomer chain length and more plasticizer, though, lead both to higher conversion and to lower stress. In all cases, it is shown that significant stress starts to develop only late in reactionsat the vitrification conversion. The vitrification conversion falls as more rigid networks are formed (with higher functionality, shorter monomer chain length, lower plasticizer concentration, or methacrylate rather than acrylate), but it is not affected by the light intensity. After vitrificationsin the vitrified statesthe stress rises monotonically with conversion. The rate of stress growth with conversion in the vitrified state rises with higher monomer functionality, shorter monomer chain length, or lower plasticizer concentration. It also rises when methacrylate is substituted for acrylate. These trends of stress growth in the vitrified state are consistent with an increase in the elastic modulus as more rigid networks are formed.

Kinetic gelation models simulate free-radical polymerization on fixed lattices, where propagation and termination reactions are restricted to occur only between nearest neighbors. Here such a model is used with bifunctional sites and with kinetics recast as a Markov process through a stochastic approach. The reaction time is calculated by employing the probability density function and associated Monte Carlo method devised originally by Gillespie. As polymerization proceeds, the evolution of structure is characterized by pair correlation functions of three typesof reacted sites, of doubly reacted sites, and of monomers. These show that as polymerization proceeds, reacted sites and doubly reacted sites come to be distributed more uniformly in space; monomers come to be distributed less uniformly. A higher initiation rate constant, a higher initiator concentration, and a lower propagation rate constant lead to more uniform distribution of reacted sites, of doubly reacted sites, and of monomers. These factors also lead to lower average connectivity between reacted sites. These trends are strongest at low conversions. In contrast, an enhanced primary cyclization leads to less uniform distribution of reacted sites but to more uniform distribution of monomers. It also leads to higher connectivity between reacted sites that are close together but to lower connectivity between reacted sites that are far apart. Finally, at high conversions it leads to a more uniform distribution of doubly reacted sites.

A kinetic gelation model that simulates free-radical network polymerization on a lattice with a stochastic kinetic approach to enable real time calculation was used to assess how initiation rate and primary cyclization affect the overall kinetics of polymerization of difunctional monomers. Changes that cause a more uniform distribution of reacted siteshigher initiation rate or less primary cyclizationincrease the accessibility of free radicals to functional groups, lower the fraction of trapped radicals, and consequently raise the apparent propagation rate constant. On the other hand, the final conversion, determined by kinetic chain length at a given initiator concentration, drops when termination becomes more severe such as under higher initiation rate or when radical trapping worsens such as under enhanced cyclization. In addition, the model simulates the contribution of pendant functional groups to the formation of different structures. The higher the radical concentration brought by higher initiation rate or by less preferred primary cyclization, the lower the fraction of pendant functional groups to form primary cycles and the higher the fractions of pendant functional groups to form cross-links and secondary cycles.

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An improved kinetic model is presented which accounts for radical trapping during the photopolymerization of multifunctional monomers such as diacrylates and dimethacrylates. Following earlier suggestions, the model assumes that trapping of radicals behaves as a unimolecular first-order reaction. The novel feature is that the trapping rate constant is presumed to increase exponentially with the inverse of the free volume; this treatment is qualitatively consistent with the free volume dependence previously proposed for the other rate constants. This improved model predicts the experimental reaction rate trends as well as previous models developed in the literature; more importantly, though, this improved model newly predicts, as no other model has, the following experimental trends in the trapped and active radical concentrations:  (1) that the active radical concentration passes through a maximum while the trapped radical concentration increases monotonically; (2) that a higher light intensity leads to a lower fraction of trapped radicals at a given conversion of functional groups but to a higher trapped radical concentration at the end of the reaction. Moreover, unlike its antecedents, the improved model correctly predicts that the polymerization rate depends more on light intensity the higher the conversion and that higher light intensity can lead to a higher final conversion.