Thermotropic liquid crystals of main-chain polyesters having a mesogenic 4,4′-biphenyldicarboxylate unit XI.: Smectic liquid crystalline glass

From examination of transition behavior in poly(pentamethylene 4,4'-bibenzoate) (BB-5) forming crystal, S-CA and isotropic phases, it was found that the liquid crystallization proceeds promptly and completely from the isotropic phase while the crystallization takes place partly even from the ordered smectic phase. The solid state of liquid crystal (LC) polymer is thus composed of crystal and smectic LC glass. Similarly as in the isotropic liquid glass of conventional polymers, the glass transition of the smectic LC glass is detected as an oc-relaxation which appears at around T-alpha=50 degrees C by a dynamic mechanical method. The oc-relaxation shows considerable mechanical anisotropy such that the tan delta peak measured in a direction parallel to the layer is larger than that in a perpendicular direction. This anisotropy can be explained according to the structural characteristics of the smectic phase. Dynamic mechanical properties were also examined For a series of BB-n polyesters with n of 5-12 and compared to those for poly(alkylene 2,6-naphthalene dicarboxylate)s (N-n) with the identical values of n. The smectic LC glass formed from BB-n polyesters with n of 5-9 have the lower T-alpha than the isotropic liquid glass of N-n polyesters with the corresponding n. Similar trend is observed in BB-6-co-N-6 copolymer system in which the S-A phase can be formed only from the copolymers with the N-6 compositions less than 60%. We thus concluded that the glass transition temperature of the smectic LC glass is lower than that of the isotropic liquid glass. The temperature of gamma-relaxation due to the local motion of alkylene spacer is relatively higher in the smectic LC glass than that in the isotropic liquid glass. This may reflect the more stretched conformation of the flexible spacer in the smectic structures.