Cooling optical properties

One of the requirements of this process is that the melt maintain good contact with the chill roU, ie, air must not pass between the film and the roU. Otherwise, air insulates the plastic and causes it to cool at a rate different from the rest of the plastic and this spoils the appearance of an otherwise satisfactory product. The melt should not emit volatiles, which condense on the chill roU, reduce heat transfer, and mar the film s appearance. The cast film process allows the use of a higher melt temperature than is characteristic of the blown film process. The higher temperature imparts better optical properties. 

The macromolecular nature provides an interesting feature of LC polymeric cholesterics, namely the possibility of obtaining monochromic films. Thus for polymeric liquid crystals the helix pitch is practically not altered with temperature below Tg, when a cholesteric phase is frozen in a glassy matrix (Fig. 23a). This implies that fast cooling of polymeric films from a mesomorphic state (shown with arrows) fixes their optical properties, which makes it possible to use them at ordinary temperatures as selective monochromic reflectors. On the other hand, such polymeric films display the extraordinary polarizing properties of cholesterics, i.e. the different absorption... 

The laser recorded information is preserved for a long time if the sample is cooled below Tg. From this viewpoint LC polymers differ usefully from low-molecular liquid crystals. The information storage time in the devices based on low-molecular liquid crystals do not usually exceed a few days. The described effects demonstrate the capabilities for the control of structural and optical properties of LC polymeric materials. 

The process for making a cast film involves drawing a molten web of resin from a die onto a roll for controlled cooling. The cast film process is used to make a film with gloss and sparkle. The melt temperature in the cast film process is higher than in the blown film process. The higher the melt temperature the better are the optical properties of the film. 

Tompkin, W. R., Malcuit, M.S., Boyd, R.W. (1990). Enhancement of the nonlinear optical properties of fluorescein doped boric-acid glass through cooling. Appl. Opt. 29, 3921-3926. 

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