Vadim Valerie Krongauz

Morphological changes during anisotropic photopolymerization

Abstract Photosensitive materials that consist of photopolymerizable monomeric components (sensitizer, initiator, monomer, chain-transfer agents, etc.) dissolved in a solid plasticized polymeric matrix are called ‘photopolymers’. We investigated structural changes occurring upon exposure to light in DuPont holographic, proofing and electrostatic photopolymers. In most cases, exposure of photopolymer films and coatings is unidirectional through some type of mask or target. Patterned exposure to light leads to concentration gradients between illuminated regions of photopolymer film and unexposed regions. Similarly, due to the high optical density of these materials, photopolymerization occurs faster near the illuminated surface creating a depthwise gradient in reactant and product concentration. High molecular weight products have lower diffusivity than low molecular weight reactants. Slow product outflow from the reaction zone and a difference in mobility of reactants migrating towards reaction regions in plasticized polymer upon unidirectional exposure lead to a variety of unusual structural changes. We report observation of swelling of the film regions exposed to light, and alignment of the polymeric molecules perpendicular to the illuminated surface. This observation is in agreement with the predictions of our previous model computations. Liquid crystalline mixtures or dichroic dyes added to the formulation align during film exposure to light perpendicular to the surface, parallel to the direction of light incidence. The direction of alignment is parallel to that of the forming polymeric molecules. This directly confirms the migratory mechanism of photopolymerization in films.

Real-time monitoring of diffusion between laminated polymer films

Abstract Diffusion from one layer of a plasticized polymer film to the adjacent laminated layer is examined. Considering the importance of having a non-destructive, real-time method of monitoring interfacial diffusion, we developed a new spectroscopic technique. In the presented method, one of the two layers of a laminated polymer film is doped with electron-acceptor molecules, 2,4,7-trinitro-9-fluorenone, while the other layer contains a polymer with electron-donating side chains, poly(N-vinylcarbazole). As electron-acceptor molecules migrate to the adjacent layer, a charge-transfer complex is formed. The charge-transfer complex has a distinct optical absorption band, which is used in the presented technique to monitor its formation. The kinetics of charge-transfer complex formation is diffusion-controlled, and thus it is used to deduce diffusion coefficients of the mobile electron acceptor in various polymer matrices. The implementation of this technique in monitoring other transport processes is considered as well. Detailed mathematical modelling of the interlayer diffusion process based on the obtained experimental data is presented. Diffusivities found for 2,4,7-trinitro-9-fluorenone tracer in cellulose acetate butyrate, poly(vinyl acetate) and poly(vinyl butyrate) polymer matrices were 5 × 10−9, 2.5 × 10−9 and 2.5 × 10−9 cm2s−1, respectively. Diffusivities increase as diffusion from the laminated layer progresses owing to plasticizing of the matrix by the diffusing molecules.

Imaging in evaluation of polymer coatings adhesion to glass at high humidity

Abstract Adhesion reduction occurring after polymer coated glass was immersed in water was studied in a variety of UV-cured urethane acrylate coatings containing alkyloxysilane adhesion promoting additives. It was observed that water accumulated under the coating surface in drops and formed ‘blisters’ in the glass-polymer interface. A non-destructive imaging technique was developed to measure the average size of the water blisters. The size of the water blisters within the interface was correlated with the wet adhesion force measured by coating resistance to 180° peel. The force of coating resistance to 180° peel off glass surface decreased non-linearly with the increase of the average size of the water blisters. It was concluded that the decrease in adhesion between the coating and glass was a result of stretching and breaking of the silane bridging bonds and polymer fibrils by water condensing on the glass surface within the polymer-glass interface. The mathematical model relating coating wet peel resistance force with the size of the debonding produced by water accumulation was presented.