New photoalignable polyimides and their ability to control liquid-crystal alignment - Polyimides and Other High Temperature Polymers - page 119

λ max = 306 nm. And, for the maximum absorption peak, its intensity drop due to
the UV exposure is more significant in the cinnamate group containing polymer
than in the coumarin group containing polymer. Therefore, 6F-HAB-CI polymer
may have a relatively higher photoreactivity than 6F-HAB-ETCOU polymer. For
both polymers the variations in the maximum absorption peaks due to the UV ex-
posure remained constant for an exposure dose of >7.5 J/cm 2 . These results sug-
gest that the photoreaction possible in the film is almost completed for ca. 7.5
J/cm 2 exposure dose, regardless of the photosensitive side group.
The polymer films exposed to UV light were further examined by IR spectros-
copy. The most characteristic IR vibrations of 6F-HAB-CI are shown in Figure 4a
and listed in Table 2. Here, the vibrational peaks were assigned based on the re-
sults reported previously from some cinnamoyl derivatives [24-31]. As shown in
Figure 4a, the vibration peaks (1633 and 981 cm -1 ) of vinylene linkage in the cin-
namoyl group are weakened with increasing UV-exposure dose. In general, [2+2]
photodimerization consumes the vinylene linkages, whereas the cis-form [27, 28]
of the vinylene linkage is expected to have a lower extinction coefficient than the
trans-form in the IR absorption. Thus, such intensity drops might result from
[2+2] photodimerization as well as trans-cis photoisomerization occurring possi-
bly by UV-exposure. However, some supportive clues for the photodimerization
were detected as follows. First, the conjugated ester carbonyl (C=O) appears at
1737 cm -1 , but becomes weak and broad as UV-exposure dose increases. Such
peak weakening is attributed to a decrease in the population of the conjugated es-
ter carbonyl by the photodimerization, while the peak broadening results from
generation of non-conjugated ester carbonyl. Second, new IR peaks appear at
1469 and 1461 cm -1 , which correspond to the scissoring motion of cyclobutane
ring which is a product of the photodimerization [32]. So it can be concluded that
the [2+2] photodimerization takes place preferentially in the film by UV-
exposure. In contrast, the IR spectroscopic study did not provide definite clues
about the possibility of photoisomerization in the film.

Table 2.
Characteristic IR bands of photosensitive polyimide films

Wavenumber (cm -1 )

Assignments

6F-HAB-CI 6F-HAB-ETCOU

1788 1785 imide C=O symmetric stretching

1732 1725 imide C=O asymmetric stretching

1737 - ester C=O stretching

- 1735 pyrone C=O stretching

1633 1613 vinylene C=C stretching

1376 1378 imide C-N-C stretching

1255 1255 CF 3 stretching

981 984 vinylene C-H out-of-plane bending