Non-local spatial frequency response of photopolymer materials containing chain transfer agents: II. Experimental results

In part I of this paper the non-local photo-polymerization driven diffusion model was extended to include the kinetics of chain transfer and re-initiation, in order to analyse the effects of chain transfer agents on the system kinetics and to study their use in reducing the average polymer chain length in free-radical based photopolymer materials. Based on these results, it is proposed that one possible way to improve the material response at high spatial frequency is the addition of chain transfer agents. In this paper, the validity of the proposed model is examined by applying it to fit experimental data for an acrylamide/polyvinyl alcohol (AA/PVA) layer containing two different types of chain transfer agent (CTA): sodium formate (HCOONa) and 1-mercapto-2-propanol (CH3CH(OH) CH2SH). The effects on decreasing the average polymer chain length formed, by the addition of chain transfer agent, which in turn reduces the non-local response of the material, are demonstrated. These reductions are shown to be accompanied by improved high spatial frequency response. Key material parameters are extracted by numerically fitting experimentally measured refractive index modulation growth curves using the model. Further independent experimental confirmation of the reduction in the average polymer molecular weight is provided using a diffusion based holographic technique.