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The effects of varying polymerization conditions on the extent of oxygen inhibition of free-radical photopolymerization were investigated using both experimental and modeling efforts. Specifically, sample thickness, initiation rate, and oxygen concentration were varied, and the resulting photopolymerization kinetics were studied using real-time FTIR and a comprehensive photopolymerization model. As the sample thickness was increased, the overall average double-bond conversion increased due to the greater conversion in the lower depths of the films. Because of the dramatic inhibition at the top of the film, this led to heterogeneity in the film's conversion as shown with the comprehensive model. Increasing the initiation rate decreased the inhibition time and increased the overall average double-bond conversion due to an increase in the overall radical production, allowing better competition of the propagation reaction with the inhibition reaction. The oxygen concentration was varied, and as the oxygen con...

Summary: A comprehensive one-dimensional photopolymerization model was utilized to investigate the effect of oxygen on the free-radical photopolymerization kinetics. The spatial profiling aspect of the model provided insight into the heterogeneity of the cure kinetics due to oxygen inhibition, specifically the variance in the concentration profile of monomer and oxygen. Double bond conversion was negligible for the top ten microns of the film due to continuous oxygen diffusion, and increased with increasing depth. Similarly, the oxygen concentration decreased with increasing depth due to the competition between oxygen diffusion time and the polymerization rate. The effect of initiation rate on the extent of oxygen inhibition was investigated for various oxygen concentrations. As the initiation rate increased, the polymerization rate increased, and eventually approached that of a sample in an inert environment. Similarly, as the oxygen concentration was decreased, the polymerization rate increased. The effect of varying the initiation rate on the cure profile in the oxygen-exposed film was also studied. It was found that the unpolymerized tacky layer decreased from 50 µm to 5 µm with a 3 order of magnitude increase in initiation rate. Using the pseudo steady state approximation, the relationship between polymerization rate and initiation rate was derived for films in an oxygen environment. A direct relationship between the polymerization and initiation rate was found for films in air. The polymerization model supported this derivation and found that as the oxygen concentration was decreased, the dependence on initiation rate, or alpha, decreased, reaching the accepted value of 0.5 for alpha in inert environments. Double bond conversion versus film depth and cure time.

The overall effects of oxygen on thiol–acrylate photopolymerizations were characterized. Specially, the choice of thiol monomer chemistry, functionality, and concentration on the extent of oxygen inhibition were considered. As thiol concentration was increased, the degree of oxygen inhibition was greatly reduced because of chain transfer from the peroxy radical to the thiol. When comparing the copolymerization of 1,6-hexanediol diacrylate with the alkane-based thiol (1,6-hexane dithiol) to the copolymerization with the propionate thiol (glycol dimercaptopropionate), it was found that the propionate system was much more reactive and polymerized to a greater extent in the presence of oxygen. In addition, the functionality was considered where the glycol dimercaptopropionate was compared to a tetrafunctional propionate of similar chemistry (pentaerythritol tetrakis(mercaptopropionate)). Given the same thiol concentration, the higher functionality thiol imparted a faster polymerization rate, due to the increased polymer system viscosity, which limited oxygen diffusion and decreased the extent of overall oxygen inhibition. Thus, preliminary insight is provided into how thiol monomer choice affects the extent of oxygen inhibition in thiol–acrylate photopolymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2007–2014, 2006

The mechanism and kinetics of thiol−ene photopolymerizations utilizing a tetrafunctional thiol monomer copolymerized with acrylate, norbornene, vinyl ether, and vinyl silazane functionalized ene monomers are successfully modeled and experimentally characterized. Modeling predictions demonstrate that the reaction orders in thiol−ene systems are controlled by the ratio of thiyl radical propagation to chain transfer kinetic parameters (kp/kCT). Ratios of kinetic parameters (kp/kCT) were found to vary significantly with the ene functional group chemistry and to have a dramatic impact on polymerization kinetics. For high ratios of kp/kCT, polymerization rates are first order in thiol functional group concentration and nearly independent of ene functional group concentration. For kp/kCT values near unity, polymerization rates are approximately 1/2 order in both thiol and ene functional group concentrations. When kCT is much greater than kp, polymerization rates are first order in ene functional group concentrat...

A comprehensive photopolymerization model that incorporates heat and mass transfer effects, diffusion-controlled propagation and termination, and temporal and spatial variation of species concentration, temperature, and light intensity is applied to systems with varying thermal and optical properties. The absorbance of the polymerizing system is varied by altering either the initiator concentration, sample thickness, or molar absorption coefficient of the initiator. Simulations show that the choice of initiator and sample thickness limits the initiator concentration usable to achieve complete conversion in a sample. Similarly, the initiator and its concentration should independently be chosen since each impact the polymerization differently. Three different thermal boundary conditions and their effects on polymerization are also considered. These boundary conditions include isothermal, perfectly insulating, and perfectly conducting. Simulations show that a higher absorbance sample polymerizes completely w...

Models for predicting multivinyl free radical photopolymerization that incorporate diffusion controlled propagation and termination are discussed. One model focuses on spatial effects by incorporating heat and mass transfer in photopolymerizing films. Temperature, species concentrations, e.g., oxygen and initiator, and light intensity are varied as a function of both time and depth. Specifically, the effect of using polychromatic initiation on oxygen inhibition was investigated. The model predicts that by utilizing initiating radiation of two distinct wavelengths, it is possible to overcome oxygen inhibition and achieve complete cure. The second model incorporates chain length dependent termination (CLDT) and chain transfer to polymer (CTP) in a homogenous, polymerizing system. Specifically, how CTP affects the polymerization rate (Rp), the reaction diffusion coefficient, the scaling relationship between polymerization rate and initiation rate ( Ri), i.e., Rp ∝ R α , and the transition from CLDT to reaction diffusion controlled termination was investigated. The model predicts that, in general, at low double bond conversion, CTP inclusion decreases the reaction diffusion coefficient and increases the polymerization rate. Additionally, increasing the CTP rate decreases the double bond conversion both at which the termination mechanism begins to transition from CLDT to reaction diffusion controlled termination and at which reaction diffusion controlled termination becomes the dominant termination mechanism. The model provides more insight into the termination mechanism and complex polymerization behavior. © 2003 Elsevier Science B.V. All rights reserved.

Thiol−ene photopolymerizations proceed via a sequential radical propagation/chain transfer mechanism that leads to polymer and network formation much like a step growth polymerization. Here, the chain transfer step in this sequential reaction series is shown to be the rate-limiting step. A model has been developed that accurately predicts the observed polymerization kinetic behavior under a variety of circumstances. Chain transfer is modeled as a rate-limiting step with the rate parameter (kp) for the propagation reaction being a factor of 10 greater than that for the chain transfer process (kCT) (kp/kCT = 10). The polymerization rate is first-order overall with first-order dependence on thiol functional group concentration and independent of the ene functional group concentration; Rp ∝ [SH]1[CC]0. Polymerization rate behavior vs functional group concentration change is shown to be only a function of the ratio of propagation to chain transfer kinetic parameters.