Tara M. Lovestead

Models of multivinyl free radical photopolymerization kinetics

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.

Modeling the Effects of Chain Length on the Termination Kinetics in Multivinyl Photopolymerizations

A model is presented that predicts photopolymerization kinetics over several orders of magnitude change in initiation rate. The model incorporates polymerization features that have long been assumed negligible when examining multivinyl photopolymerizations. The assumption that radical termination is chain-length-independent is relaxed by incorporating a chain-length-dependent termination (CLDT) parameter based on Random-walk theory into the kinetic model. Experiments and modeling of multivinyl free-radical photopolymerizations clearly demonstrate that CLDT is important at low conversions, where a deviation from the classical square-root relationship between polymerization rate (R p ) and initiation rate (R i ) is observed (R p R α i , α=1/2, classically). At moderate conversions, when reaction diffusion dominates termination, a transition region is observed from a chain-length-dependent to a chain-length-independent region. During this transition, long chain - long chain termination is reaction diffusion controlled while the short chain -short chain termination event remains translational and segmental diffusion controlled. The scaling exponent, α, gradually increases throughout this region until achieving the classical value, where once attained, a plateau is observed. Chain-length effects were also examined by including chain-transfer (CT) reactions into the kinetic expressions. Upon CT agent addition, a transition region is still observed; however, at low conversion, a adheres more closely to the classical predictions. Most importantly, the model clearly demonstrates a transition from a CLDT region at low conversion to reaction diffusion controlled termination region at high conversion, where chain length is unimportant.