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The mechanical properties of SiCf/SiC composites have been investigated, based on detailed analyses of their microstructure. SiCf/SiC composites were prepared from fiber preforms by a slurry infiltration technique, in which a mixture with SiC, Al2O3, and Y2O3 particles was impregnated into the fabric structure. SiCf/SiC composites were consolidated by liquid phase sintering process, associated with the creation of secondary phases by the addition of Al2O3 and Y2O3 particles. The reinforcing material was a plain weave Tyranno SA SiC fabric with a carbon interfacial layer. The sintered density and the pore volume fraction of SiCf/SiC composites were about 3.0 Mg/m3 and about 10%, respectively. These SiCf/SiC composites had an average flexural strength of about 250 MPa at room temperature. They exhibited pseudo-ductile fracture behavior, due to the carbon interfacial layer. The introduction of the carbon interfacial layer greatly improved the fracture energy of SiCf/SiC composites.

A new series of thermotropic polyurethanes containing biphenyl units was synthesized by polyaddition reaction of diisocyanates such as 2,6-tolylene diisocyanate, 2,5-tolylene diisocyanate, 2,4-tolylene diisocyanate, and 1,4-phenylene diisocyanate, with 4,4□-bis(9-hydroxynonoxy)biphenyl (BP9). Structures of the monomer and the corresponding polymers were identified using FT-IR and 1 H NMR spectroscopic methods. BP9 exhibited a smectic type mesophase, however, nematic phase was found for all synthesized liquid crystalline polyurethanes except for 1,4-phenylene diisocyanate/BP9 based polyurethane. Their phase transition temperatures and thermal stability were investigated by differential scanning calorimetry (DSC), optical polarizing microscopy, and X-ray scattering. The infrared study indicated that the hydrogen bonding among urethane linkages attributed to the mesomorphism. Thermal gravimetric analysis (TGA) of synthesized polyurethanes showed that no weight loss of the polymers observed up to 280°C.

High-molecular-weight para-type homopolyurethanes prepared by polyaddition of 2,5-tolylene diisocyanate with 4,4‘-bis(ω-hydroxyalkoxy)biphenyls showed monotropic liquid crystallinity, when measurements were performed under shearing. For example, a polyurethane PU6T having [η] 0.82 which was obtained from 4,4‘-bis(6-hydroxyhexyloxy)biphenyl showed TI-LC of 201 °C and TLC-K of 181 °C. In addition, thermotropic liquid-crystalline para-type copolyurethanes PUmnT (m and n denoting carbon number in alkylene moiety: 2, 6, 8, and 11) were synthesized by polyaddition of the diisocyanate with two 4,4‘-bis(ω-hydroxyalkoxy)biphenyls having different alkylene lengths. For example, PU68T(75/25) had TI-LC of 188 °C and TLC-K of 173 °C. Blends of two para-type homopolyurethanes also exhibited monotropic liquid crystallinity. The liquid crystallinity was examined by differential scanning calorimetry measurement, polarized optical microscope observation, and X-ray diffractometry.

Synthesis and Thermotropic Properties of Polyurethanes Prepared from 2,5-Tolylene Diisocyanate and 2,6-Bis(ω-hydroxyalkoxy)naphthalenes

Thermotropic liquid-crystalline polyurethanes were synthesized by the polyaddition reaction of para-substituted 2,5-tolylene diisocyanate (2,5-TDI) with 1,4-bis(ω-hydroxyalkoxy)benzenes (BHBm : HO(CH 2 ) m OC 6 H 4 O(CH 2 ) m OH ; m is the carbon number of the hydroxyalkoxy group). Intrinsic viscosities of the polymers were in the range of 0.22-0.41 dL/g. Polyurethanes 2,5-TDI/BHBm's (m = 3, 5, 6, 8) prepared from BHBm and 2,5-TDI, exhibited monotropic liquid crystallinity, although these polyurethanes contained no mesogenic core unit. For example, polyurethane 2,5-TDI/BHB5 with [η] = 0.33 prepared from 1,4-bis(5-hydroxypentoxy)benzene (BHB5) and 2,5-TDI exhibited an anisotropic phase from 119 to 51 °C. In contrast, for the series of 1,4-PDI/BHBm (m = 2, 3, 5, 6, 8, 11) having no substituent in a phenylene unit, no explicit mesomorphic behavior was observed by DSC measurement and polarized microscopic observation.

Thermotropic polyurethanes [O(CH 2 ) m OPHPhO(CH 2 ) m OCONHPh(R)NHCO] n (m= 2-6,8,11; R= H, Me) with intrinsic viscosities in the range of 0.41-0.99 dL/g were synthesized. DSC thermograms for these polymers exhibited two endothermic peaks corresponding to phase transitions of melting and isotropization. IR study indicated that the hydrogen bonding between urethane linkages affected the mesomorphism.