New photoalignable polyimides and their ability to control liquid-crystal alignment - Polyimides and Other High Temperature Polymers - page 123

Figure 6. Optical retardations measured from photosensitive polymer films exposed to a linearly
polarized UV light (260-380 nm wavelength) with varying exposure dose: (a), 6F-HAB-CI; (b), 6F-
HAB-ETCOU.

at 132 C lower than that of the 6F-HAB-CI polymer. Both poly-

mers have the same polymer backbone except for the different side groups. The
coumarin moiety is relatively bulkier than the cinnamoyl group. In general, a
bulkier group in a polymer gives higher T g . Such low T g in 6F-HAB-ETCOU
polymer might result from a relatively high flexibility of the spacer in the ETCOU
side group. The flexible spacer causes a relatively high mobility to the coumarin
side group leading, in turn, to an increase in the polymer chain mobility. On the
other hand, UV-exposure is generally known to generate thermal heat in the

° C which is 50 °