Photodegradation in Polyimide Materials

As polyimide (PI) materials possess high-temperature stability and were usually used in the LCD industry, it was highly desirable to modify them for 

There are several drawbacks for LC photoaligning materials prepared by photochemical reaction and by photodegradation in particular  

The small value of the order parameter (Equation (2.3)) and the corresponding low value of the induced optical anisotropy and dichroism [22, 67]. 

Contamination of the initial substance by the by-products of the photodegradation is possible in certain cases. The by-products can produce ions and initiate the image sticking effect and flicker [72], as well as reduce the thermal stability of the LC alignment [73]. 

Low sensitivity in PI decomposition. The reported sensitivity for photodegrad-able polyimide materials was 3-12.7 J/cm for aromatic Pis, when broadband UV light was used [74], 

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Four main mechanisms of the photoalignment are known (i) reversible photochemical trans-cis isomerization in azo-dye-containing polymers, monolayers, and pure dye films (ii) pure photophysical reorientation of the azo-dye chro-mophore molecules (iii) topochemical crosslinking in cinnamoyl side-chain polymers (iv) photodegradation in polyimide materials. 

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