Photocross-linkable Polymers

Some photocross-linking of polymers can be traced back to ancient days, when pitch was photocross linked for decorative purposes [102]. In modern times, wide varieties of photocross-linkable polymers were developed. The early practice of photo imaging relied mainly upon the photo dimerization reactions. These reactions are common, photo-induced, reactions of organic chemistry, namely intermolecular cyclization. This reaction of cyclization that takes place between two reactive species, with one of them electronically excited, is actually predicted by the Woodward–Hoffmann rule [104]. In contrast, the reactions of thermally excited ground states of molecules proceed by different pathways. Many polymers were synthesized that possess pendant groups capable of photo cyclization intermolecularly to be used in the photo-imaging technology. Photocross-linking tech nology today, however, also uses coupling reactions of radicals, chain growth polymerizations that result in photocross-linking, and some ionic reactions. The light-induced polymerizations of multi-functional monomers that transform liquid resins into solid polymers almost instantly and selectively in the illuminated areas are now versatile processes. They can, however, be achieved in a variety of ways. Thus, for instance, Decker discussed work in the less explored areas of photo-curing, namely laser-induced ultra-fast polymerization and UV curing of binary polymer systems [105]. By using highly sensitive acrylate photoresists, relief images of micronic size can be obtained by fast scanning with a focused laser beam. Also, polymer networks of different architectures can be obtained by UV irradiation of various monomer blends, e.g., acrylate-epoxide, acrylate-vinyl ethers, acrylate-polyene, vinyl ether-maleate, and thiol-polyene [104]. This does not mean, however, that photocross-linking of polymers is now unimportant technologically or scientifically. The fact that considerable research still continues in the field is a direct indication of that. The light cross-linkable reaction, like all cross-linking reactions, results in gelation and the extent of gelation is important in this process. This extent is tied to the quantity of the functional groups in the reaction mixture. Carothers equations relate the critical extent of the gelation, pc, at the gel point to the functionality of the reactants: pc = 2/fave , where fave is the functionality. This equation, however, was written for reaction mixtures that contain two different functional groups in stoichiometric proportions to each other. All the functional groups, however, for various reasons, might not participate in photocross-linking reactions. Even though these reactions may involve identical groups, this equation probably would not apply. A statistical approach was developed by Flory [107, 108] and by Stockmayer [109] to derive an expression for predicting the extent of reaction at the gel point. It is expressed as

where f_wA and f_wB are weight average functionalities of A and B functional groups. Here the functionalities are defined as,

When the cross-linking, however, takes place by cyclization through dimerization of pendant groups, it appears that the gelation should probably be treated instead as a case of multiple dimerizations.

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مواضيع ذات صلة

Polymers with Functional Chalcone Groups

Polymers with Pendant Cinnamoyl Functional Groups

Polymers That Photocross-link by Formation of Cyclobutane Rings

Photosensitizers

The Charge Transfer Processes in Polymers

The Electron Transfer Process

Energy Transfer Process

Interaction of Light with Organic Molecules

The Nature of Light

Electricity-Conducting Polymers

Immobilized Reagents

Immobilized Nonenzymatic Catalysts