Synthesis of novel polyimides with (tert-butyldimethylsiloxy)biphenyloxy groups and the effect of side-chains content on their properties

A novel aromatic diamine 4′-(tert-butyldimethylsiloxy)biphenyl-4-yl 3,5-diaminobenzoate (DPA) was synthesized through three steps including monoesterification, silylation and hydrogenation. DPA with different ratios was copolymerized with 4,4′-diaminodiphenyl ether (ODA) and 3,3′,4,4′-biphenyl dianhydride (BPDA) to give poly(amic acid)s which were converted into polyimides (PIs) by thermal imidization. The effect of side-chain content on the thermal stability, optical transparency and surface energies was investigated by DSC, TGA, UV–vis spectra and contact angle measurement. With the content of side-chains increasing, the PIs revealed lower thermal stability, higher optical transparency and lower surface energies. The pretilt angles of 4-pentyl-4′-cyanobiphenyl (5CB) on the PI films were measured by a crystal rotation method. For the same rubbing strength, the pretilt angles increased with the increasing of the content of side-chains. When the content of side-chains rose from 5% to 20%, the pretilt angles increased from 2.8° to 15.0° under 28.5 mm of rubbing strength. When the content of side-chains increased to 70%, 80%, 90%, and 100%, the pretilt angles was 75.0°, 82.9°, 87.0°, and 87.7°, respectively, under 28.5 mm rubbing strength.