Photoalignment using polyimide

Polyimide exposed to LPUV aligns LCs as described above. But why does the LPUV exposed polyimide align the LCs Here we will consider the alignment mechanisms for polyimide exposed to LPUV. 

A photodecomposition-type material does not include optically sensitive structures, and the determination of the photochemical reaction for such materials is rather more difficult than for others. Therefore, determination of the photoalignment mechanism for a photodecomposition-type material is difficult to compare with that for the other two mechanisms. 

15 IR spectra of CBDA/ODA Initial transmittance and reduction after 40 Jem of UV exposure. 

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In this method, radicals generated by the UV exposure connect to other parts of molecules and generate a new structure. If this type of material shows a preferred photoalignment, then this new structure can be used to align the LC. This type of photoalignment was found in the benzophenone structured polyimide shown in Fig. 3.9 [32]. 

Figure 5.17 HLPP configuration, used for STN-LCD (bottom substrate) [20, 21]. PI, P2, polarizers LPPl, LPP2, photoaligning layers LCP, phase retarder SI, S2, glass substrates PI, rubbed polyimide layer ITO, transparent electrodes a,ai,fl2. preferred directions of alignment We, optical axis of the phase retarder. Reproduced from [20, 21], Institute of Pure and Applied Physics (1995)...

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