INDEX amorphous polymer thin films

Amorphous polymers characteristically possess excellent optical properties. Unlike all the other commercially available fluoropolymers, which are semicrystalline, Teflon AF is quite clear and has optical transmission greater than 90% throughout most of the UV, visible, and near-IR spectrum. A spectrum of a 2.77-mm-thick slab of AF-1600 is shown in Figure 2.5. Note the absence of any absorption peak. Thin films of Teflon AF have UV transmission greater Ilian 95% at 200 mm and are unaffected by radiation from UV lasers. The refractive indexes of Teflon AF copolymers are shown in Figure 2.6 and decrease with increasing FDD content. These are the lowest refractive indexes of any polymer family. It should be noted that the abscissa could also be labeled as glass transition temperature, Tg, since Tg is a function of the FDD content of the AF copolymer. Abbe numbers are low 92 and 113 for AF-1600 and AF-2400. 

The disposition of phenyl groups in the solid amorphous polymer is of relevance to the photophysical properties. Following refractive index measurements Prest has proposed (1 5) that the phenyl units are oriented in the plane of the film and indeed a thin layer of highly strained polymer can be produced by solvent casting. Since the bulk of excitation radiation is absorbed in the surface of a sample the major component of the emission can occur from a birefringent surface layer, so that the observed polarisation/depolarisation can be dominated by this influence rather than by e.e.m., and therefore by sample preparation. 

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