Yu Shoji

Thermal diffusivity of hexagonal boron nitride composites based on cross-linked liquid crystalline polyimides.

Hexagonal boron nitride (h-BN) composites with the oriented cross-linked liquid crystalline (LC) polyimide have been developed as high thermally conductive materials. Well-dispersed h-BN composite films were obtained, as observed by scanning electron microscopy. The morphology of the composite films was further investigated in detail by the wide-angle X-ray diffraction. The obtained composite films based on the cross-linked LC polyimide showed that the polymer chains vertically aligned in the direction parallel to the films, while those based on the amorphous polyimide showed an isotropic nature. Moreover, the alignment of the cross-linked LC polyimides was maintained, even after increasing the volume fraction of h-BN. This alignment plays an important role in the effective phonon conduction between h-BN and the matrices. Indeed, the thermal diffusivity in the thickness direction of the composite films based on the LC polyimide measured by a temperature wave analysis method was increased to 0.679 mm(2) s(-1) at a 30 vol % h-BN loading, which was higher than that based on the amorphous polyimide.

Characteristic smectic structures of main-chain liquid-crystalline polyimides driven by a microphase separation between aromatic imide mesogen and a siloxane spacer

We investigated the phase transition behavior and liquid-crystalline structure for main-chain liquid-crystalline polyimides with aromatic imide cores and alkyl and dimethyl siloxane spacers, BPDA/3SiO4-3F. They formed crystal, smectic C (SC), smectic A (SA), and isotropic phases in order of increasing temperature. The characteristic structures of the SA and SC phases were extracted with wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy measurements. In the SC phase, the mesogenic groups were tilted by approximately 40° to the smectic layer and the unusual quasi two-dimensional structural order exists irrespective of the liquid-like packing of aromatic cores within a layer. On heating, the SC phase transformed into the SA phase with a nature of first-order transition. The resulting SA structure was also unusual. The orientational order parameter was 0.43, which was lower than 0.71 in the SC phase. This indicates that a “de Vries”-type of smectic A is formed in which the mesogenic groups are tilted away from the optic axis, but in azimuthally random directions. The average tilt angle was estimated to be 42°. These characteristics were produced because of the strong segregation effect of the dimethyl siloxane spacer groups with a relatively larger molecular area compared to alkyl chain and aromatic imide cores.

Synthesis of aramids by bulk polycondensation of aromatic dicarboxylic acids with 4,4′-oxydianiline

Aramids were successfully prepared by bulk polycondensation of various aromatic dicarboxylic acids [1] and 4,4′-oxydianiline [2] by controlling the feeding ratio R (= [2]/[1]) of 1.10. The oligomerization was first conducted under relatively mild conditions (260 °C for 1 h), followed by polycondensation at 370 °C for 1 h under reduced pressure.

Thermotropic liquid crystalline polyimides toward high heat conducting materials for 3D chip stack

Novel thermotropic liquid crystalline semi-aliphatic polyimide containing polysiloxane unit has been developed. The polyimide film exhibits high thermal stability and liquid crystallinity with lower transition temperature. Poly(amic acid) solution used as a precursor of the polyimide has very good surface wetting properties for a silicon wafer, resulting in homogeneous polyimide thin film formation on the wafer after thermal imidization below 200 °C. The silicon wafer coated with the polyimide thin film can be bonded to another silicon wafer, or to another polyimide film on heating at 270 °C with relatively low pressure. Moreover the bonding is repairable on heating again. It is proposed that the polyimide can be utilized as a polymer adhesive for three dimensional chip stack.