Thermotropic liquid crystalline (LC) semialiphatic polyimides have been developed from diamines containing siloxane spacer units with pyromellitic dianhydride (PMDA) or 3,30,4,40-tetracarboxybiphenyl dianhydride (BPDA). Monomer diamines with siloxane units were prepared by three steps, which include Williamson’s synthesis, hydrosilylation, and reduction. The polymerization was conducted by the conventional method, solution polymerization, followed by thermal imidization. A highly viscose poly(amic acid) solution was obtained and its inherent viscosities were 0.65-1.73 dL/g. These novel polyimides exhibited high thermal stability and a lower crystal-LC transition temperature compared to reported polyimides containing alkylene or oxyethylene spacer units. X-ray studies were performed for investigating the oriented smectic mesophases of each polyimide. In the polyimides derived from PMDA, a smectic A (SmA) phase was formed on gradual cooling from the isotropic liquid phase. On the other hand, the polyimides based on BPDA formed a smectic C (SmC) phase as well as a SmA phase.
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