Polymer processing photochemical methods

The photochemistry of LC polymers is not only interesting for fundamental reasons (vide supra) but because they can perhaps be formed into useful materials— films, fibers, rods, etc., with specifically tailored mechanical and/or optical properties. Their photochemical reactions may be used to modify these properties in an easily controlled, switchable manner. There is already a considerable body of knowledge on the possible practical applications of a few photochemical reactions of LC polymers. Such possible applications are discussed elsewhere [1-6] and are only briefly touched upon in this chapter. Furthermore, this chapter does not include an extensive compilation of information about the many types of LC polymers, the many methods used to synthesize and process these materials, their detailed properties, and the theoretical basis of their formation and properties. The reader is referred elsewhere [7,8] to capable reviews of these topics. However, a brief introductory review of the main types of LC polymers and their properties that are especially relevant to their photochemistry is given in Section II. 

Polyaniline (137) is one of the most promising conductive polymers and the conductivity could be reversibly controlled by oxidation or protonic doping mechanisms228,229. In addition, polyaniline displays good environmental and thermal stability, and its undoped form is solution processable from both organic and aqueous acid solutions. The polymerization of aniline is usually carried out by a chemical or electrochemical oxidation reaction230,231. However, photochemical methods toward the preparation of poly aniline have recently been reported232-240. 

Synthetic melanins are obtained by biomimetic oxidation reactions using known precursors. So far, four different methods for melanin synthesis have been reported, i.e. in vitro enzymatic, autooxidative, electrochemical, and photochemical methods. Of these, the first two have been generally used, for large scale preparations of the pigment polymers and have been reviewed elsewhere (70, 211). The latter two methods which are discussed here, have been used effectively to understand the mechanism of the melanization process in biological systems. 

There are two basic methods for making polymer materials photochemically degradable (2,3). One method is to chemically incorporate a clu omophore into the polymer chains. Although numerous chromophores have been evaluated, the most commercially successful chromophore is the carbonyl group 2,3,S). Absorption of UV radiation leads to degradation by the Norrish Type I and II processes or by an atom abstraction process (Scheme 1), all of which are typical photoreactions of the carbonyl chromophore. Note diat once radicals are introduced into the system, chain degradation can occur by the autooxidation mechanism (Scheme 2). 

Other chapters in this text will present a detailed evaluation of many of the various ways photophysical and photochemical processes can be used as a tool to examine polymer systems. In order to introduce this material we provide here a brief overview of the capabilities of these methods from a problem-oriented point of view. We have compiled in Table 3 a list of many important phenomena in polymer systems that one would like to examine. Next to each phenomenon we indicate photophysical and photochemical methods that have been, or might be, used to study these phenomena. 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

Photochemical and photophysical processes in cellulose and related compounds have received considerable attention during the last decades, resulting in research work concerned with the improvement of cellulosic materials via physical and chemical modifications. One method was to apply a copolymer between the cellulose and a synthetic polymer which are generally grafted by free radical reactions. 

To relate the wettability changes more firmly to the photooxidation processes and products, a detailed study was carried out with polystyrene. This polymer was selected because the formation of oxidation products in the hydrocarbon surface gave rise to large changes in wettability and because these products would be readily accessible to optical methods of analysis. The ultraviolet absorption spectrum of polystyrene shows a sharp cut-off, and the extinction coefficients for the radiation absorbed are sufficiently high that almost all of the photochemical reaction should be confined to the surface layers. 

Structure and mechanism in photochemical reactions. The reactions of geminal radical pairs created in bulk polymers are presented by Chesta and Weiss in Chapter 13. Of the many possible chemical reactions for such pairs, they are organized here by polymer and reaction type, and the authors provide solid rationalizations for the observed product yields in terms of cage versus escape processes. Chapter 14 contains a summary of the editor s own work on acrylic polymer degradation in solution. Forbes and Lebedeva show TREPR spectra and simulations for many main-chain acrylic polymer radicals that cannot be observed by steady-state EPR methods. A discussion of conformational dynamics and solvent effects is also included. 

In all experiments described in this work only extremely low concentrations of intermediates are considered. This is due to our interest which is primarily focussed on the most important initial steps of the polymerization reaction, which are characteristic of the overall polymerization reaction mechanism. Consequently only low final polymer conversion is exp>ected and, therefore, complications arising from the interaction between the intermediate oligomer states can be neglected. It will be shown that the low temperature conventional optical absorption and ESR spectroscopy are powerful spectroscopic methods which yield a wealth of information concerning structural and dynamical aspects of the intermediate states in the photopolymerization reaction of diacetylene crystals. Therefore, this contribution will center on the photochemical and photophysical primary and secondary processes of this... 

---