Preparation of vertical alignment layers by blending polyimide precursors with and without side chains

A series of polyimides (PIs) were prepared from the precursors, blends of a poly(amic acid) and a side-chain poly(amic acid), based on a diamine 4'-(tert-butyldimethylsiloxy)biphenyl-4-yl 3,5-diaminobenzoate (DPA). The blended polyimide (bl-PI) films were relatively tough. The surface properties of the bl-PI films were characterised by surface energy measurement, X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance infrared spectroscopy (ATR-IR). The results showed that the non-polar side chains were highly enriched on the surface and 90° pretilt angles of liquid crystal molecules were generated regardless of side chain content. For comparison, the copolyimides (co-PI) with different DPA contents were synthesised. It was found that the enrichment of the side chain on the surface increased with increasing DPA content and the pretilt angles of liquid crystal molecules rose from 3° to above 20° when DPA content increased from 10% to 30%. For the polyimides based on a diamine without a non-polar end group, 4'-biphenyl-4-yl 3,5-diaminobenzoate (DABBE), both bl-PIs and co-PIs could only generate the same low pretilt angles. In conclusion, by blending poly(amic acids) based on DPA, tough vertical alignment layers could be prepared.