The formation of densely crosslinked networks by chain crosslinking photopolymerization is discussed in relation to selected applications in the electronics industry. All of these applications make use of the high speed and of the latitude to meet other requirements by variation of the chemical structure of the monomers. The selection comprises: 1. The coating of optical fibers ; 2. The replication of optical discs; 3. The replication of aspherical lenses, used for laser read-out of these discs. Other important processes will only be mentioned briefly for the benefit of a discussion of more fundamental results that have been obtained during the study of the selected examples. These results relate to the crosslink density, the influence of light intensity on polymer structure, the relation between shrinkage and chemical conversion, a parallel with physical aging, kinetics during vitrification, the importance of chain transfer, the build-up of peroxides in photopolymers and computer simulation of network formation by crosslinking polymerization. This serves to illustrate the continuous interaction between development of applications and fundamental research.

A thermodynamic model is given for phase separation induced by the increase of network elasticity during free-radical cross-linking polymerization. The importance of network elasticity in the field of polymer dispersed liquid crystals is stressed. The concept of a conversion-phase diagram is introduced as an attractive way to visualize the onset of phase separation in all situations where the phase separation is induced by polymerization. The results of the model are presented in conversionphase diagrams.

Phase separation in a polymerizing diacrylate/LC mixture is shown to be driven by liquid−gel demixing rather than by liquid−liquid demixing. The structure of the gel strongly influences the initial morphology:  “early” phase separation (at low conversion) produces spherical domains, whereas “late” phase separation (at high conversion) produces nonspherical domains. The higher the conversion at phase separation, the smaller the domains. A new method, simultaneous photo DSC/turbidity measurement, provides the conversion at the appearance of a nematic phase, and optical microscopy shows the development of morphology. A nonspherical droplet shape reflects the inhomogeneous structure of the polymer network. The dependence of the initial morphology on the LC content, temperature of the reaction, and cross-linker content can be explained using conversion−phase diagrams obtained from the Flory−Huggins−Dušek theory. The observable part of the demixing process in a model system composed of 4-n-pentyl-4‘-cyanobiphenyl (K15) and tetraethylene glycol diacrylate (TEGDA) probably proceeds through nucleation and growth rather than through spinodal decomposition. The phase diagram of the unpolymerized monomer/LC mixture is also reported.

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