Yi Gu

A novel benzoxazine/bismaleimide blend resulting in bi-continuous phase separated morphology

A novel modified polybenzoxazine with bi-continuous phase separated morphology was successfully prepared from the blend of bisphenol A-aniline benzoxazine (BA-a), N,N′-(2,2,4-trimethylhexane-1,6-diyl)bismaleimide (TBMI) and catalyst imidazole. The thermal and toughness properties of the phase-separated blend cast were improved compared with polybenzoxazine.

Preparation of hemispherical polyimide particles by inverse emulsion technique from poly(amic acid) ammonium salts

Nonspherical polymer particles have attracted increasing attention recently. In this paper, micron-scale hemispherical polyimide (PI) particles were fabricated using water-soluble poly(amic acid) ammonium salts (PAAS) by a novel inverse emulsion technique. In the process, liquid paraffin was used as a continuous phase, the mixed solution of PAAS and water as a dispersed phase and sorbitan monooleate (Span80) as a surfactant. The research suggested that water as a stabilizing agent played an important role in forming stable emulsion. As the amount of water increased, stability of the emulsion increased gradually and morphology of PI particles transformed from sphere to ellipsoid, and finally to hemisphere. The concentration of PAAS solution and Span80 both affected the shape of particles, which changed from spherical to hemispherical by increasing the PAAS/Span80 concentration. The mechanism of forming hemispherical PI particles was discussed based on interfacial tension and interfacial free energy changes. Via adjusting the composition of the system to change the corresponding interfacial tension, we could get the particles with different morphologies. Furthermore, the change in structure characterized by FT-IR spectroscopy demonstrated that PAAS had been converted to PI after adding the dehydrating agent to the emulsion. And TGA results showed that the obtained PI particles had excellent thermal stability.

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.

Preparation of novel polyimides containing aryl ester side chains end-capped with alkoxy groups and studies on their surface properties

A novel diamine containing aryl ester side chains end-capped with alkoxy groups was synthesised and characterised by Fourier transform infrared spectrometry and 1H-nuclear magnetic resonance. Based on the diamine, a series of copolyimide alignment films were prepared by a one-step process. The effects of side chain content of polyimides on surface energy, surface element components and surface morphology were studied by contact angle measurement, attenuated total reflection Fourier transform infrared spectrometry, x-ray photoelectron spectroscopy and atomic force microscopy. These investigations show that the surface energy decreases linearly with the increase of side chain content in a certain range and then levels off. Side chains with alkyl groups can transfer to the surface spontaneously, which makes the content of side chain on the surface notably higher than that in the matrix. However, when side chain content reaches 60 mol%, further transference stops. The enrichment of side chains on the surface of alignment films could be considered as the reason for the increase of the pretilt angle.