Photochromism in bulk polymers

Table 4. Decoloration of photochrome X in bulk polymer. Influence of photochrome incorporation ...

Eisenbach, C. D. (1980). Relation between photochromism of chromophores and free volume theory in bulk polymers. Ben B nse g cs. Phys. Chem. 84, 680-690. 

Because spiropyrans (and especially BIPS) are relatively easy to prepare, a variety of compounds are available from which to deduce correlations (usually semiempirical Hammett-like equations) between structure and photochromic parameters in dilute fluid solutions. The photochromic behavior of a single dye in a variety of solvents can also be correlated with a solvent parameter such as. In applications, however, the dyes are almost always relatively concentrated in a bulk polymer or a polymeric binder film, and various other additives such as antioxidants, surfactants, fire retardants, plasticizers, and other colorants are also present. Under these conditions, the observed photochromic behavior is often very different from that predicted by the dilute fluid solution correlations. 

Inversely, can we expect any change of physical property of a photochromic polymer in bulk under irradiation by light absorption ... 

A similar but much more detailed interpretation was recently given by Eisen-bach who showed the general validity of the WLE-equation in the interpretation of the photochromism of azobenzenes and spirobenzopyrans linked to amorphous bulk polymers. Eisenbach used directly the WLF-equation ... 

In this chapter we will consider to which extent conformational changes of the chromophores and connected chain segments of a photochromic polymer may induce changes of dimension of bulk polymers and thus generate reversible photomechanical effects. 

The fact that in these bulk polymer samples no photoisomerization could be detected has to be attributed only to the severe restrictions of the local chain segmental mobility around the chromophore, i. e., it is due to the predominant incorporation of the azochromophoric units in the hard phase the mobility of chain segments within the hard domain is widely suppressed in this system, cousing a nearly complete immobilization of the chromophore in the hard phase. This behaviour is comparable to systems where the photochrome was incorporated in highly crystalline polyamide or polyimide (2,20,21). 

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