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Effect of a biphenyl side chain of polyimide on the pretilt angle of liquid crystal molecules: molecular simulation and experimental studies

The side chain structure of polyimides (PIs) exerts an important effect on the pretilt angle of liquid crystal molecules. Three PIs with different side chain structures were prepared based on 3,3′,4,4′-biphenyl dianhydride (BPDA) and 1,3-phenylene diamine (m-PDA), 3,5-diaminobenzoic-4′-biphenyl ester (DABBE) and 4′-(tert-butyldimethylsiloxy) biphenyl-4-yl 3,5-diaminobenzoate (DPA). The surface properties of the PI films and the pretilt angles of the liquid crystal molecules on them were examined with experimental and molecular simulation methods, showing a good agreement between two methods. Molecular simulation showed that the BPDA-DPA system had the lowest surface energy for its side chain enriched on the PI surface, which was driven by the low polar silyl end group of side chain. The phenyl ring in the PI backbone tended to be arranged parallel with the surface, while the phenyl ring in the side chain inclined to align vertical to surface. By incorporation of the biphenyl group and the silyl end group, the pretilt angle of the PI increased from 3° to 89°. This was attributed to the enrichment degree of the side chain on the surface and the configuration of the biphenyl group in the side chain. The main interactions between the liquid crystal molecule, 4-n-pentyl-4′ -cyanobiphenyl (5CB) and the PIs was a π–π interaction between the biphenyl group in 5CB and the PIs.