Crosslinking in Cinnamoyl Side-Chain Polymers

Linear photo-polymerization (LPP) leads to a preferred depletion of the cinnamic side-chain molecules along due to the (2 -i- 2) cycloaddition reaction. 

This causes an anisotropic distribution of cyclobutane molecules with their long axis preferably aligned perpendicular to P.  

LC molecules are also aligned perpendicular to Pj (the axis of the preferred alignment of cyclobutane molecules) due to the van der Waals (or dispersion) interaction. 

Many publications concerning the mechanism of the photoalignment in various cinnamate side-chain polymers, which provide a homogeneous LC alignment, were reviewed in [8]. According to some of them, the LC alignment is parallel to the polarization direction of the LPUVL. The effect may take place due to both the photochemical cis-trans isomerization and dimerization processes. 

The superior properties of these materials in comparison with cinnamate side-chain polymers were first reported in [49], The photoalignment orientation of the crosslinking of the polymer LC film with cinnamoyl and biphenyl fragments parallel to the electric field of the incident LPUVL was observed also by Kawatsuki etal. [50-52]. 

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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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