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The solid state morphology of the main-chain liquid crystal BB-6 polyester was studied by the small angle X-ray scattering (SAXS) method. The BB-6 polyester forms isotropic, smectic A, and crystal phases in the order of decreasing temperature; thereby its crystallization is taking place from the smectic A phase. The SAXS for the crystalline specimens prepared by cooling the isotropic melt at a rate of 10 °C min-1 shows the well-defined reflection maxima which are attributable to the stacked lamellar structure. The lamellar spacings are distributed around 250 Å so that an appreciable number of chain foldings are included in a chain. The lamellar size is increased by annealing the crystal like in other crystallizable polymers. In contrast, it is not essentially altered by annealing the smectic A phase. The results show that the chain foldings exist at a thermodynamic equilibrium in the smectic A phase. The correlation length between the chain foldings is assumed approximately to be 250 Å, which corresponds to 15 times the length of the repeat unit.

ABSTRACT:From examination of transition behavior in poly(pentamethylene 4,4'-bibenzoate) (BB-5) forming crystal, ScA 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 ix-relaxation which appears at around T, = 50°C by a dynamic mechanical method. The ix-relaxation shows considerable mechanical anisotropy such that the tan i5 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, than the isotropic liquid glass ofN-n polyesters with the corresponding n. Similar trend is observed in BB-6-co-N-6 copolymer system in which the SA 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 y-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.KEY WORDS Main-Chain Polymer / Polyester / Liquid Crystal / Smectic Liquid Crystalline Glass / Glass Transition / Mechanical Property / The thermotropic liquid crystalline behavior of polymers with rigid mesogenic units interconnected through flexible spacers is well known from reports over the past two decades. The polymers are termed main-chain liquid crystalline (LC) polymers, which have generated much interest because of challenge of understanding such systems and their many industrial and commercial applications.In this series of studies, 1 -9 we have treated the following main-chain LC polyesters, poly(alkylene 4,4'-bibenzoate )s:The polymers are designated as BB-n (n; carbon number of the methylene spacer). These BB-n polyesters invariably form smectic mesophases when n varies from 3 to 9. In BB-n with an even n, a SA phase is formed with both axes of the polymer chain and biphenyl mesogen lying perpendicular to the layers. In contrast, the smectic structure of BB-n with an odd n was identified as a ScA phase, in which the tilt direction of the mesogenic group is the same in every second layer but opposite between neighboring layers. 2 · 3 · 6 • 9 Despite detailed studies on liquid crystals, the structure and prop...

Shear alignment behavior was examined for smectic CA (SCA) phase of main-chain BB-5(3-Me) polyester using a cone-and-plate fixture. In the smectic temperature region from 100 to 140 °C, two distinct orientations have been identified by wide-angle X-ray diffraction. At temperatures lower than 130 °C, the smectic layers arrange with unit normal perpendicular to the shear direction and parallel to the velocity gradient direction (so-called parallel orientation). Shearing at high temperatures near the isotropization temperature leads to the orientation of smectic layers with unit normal perpendicular to both the flow and velocity gradient directions (perpendicular orientation). At the intermediate temperatures, both orientations coexist. The SR-SAXS profile of the parallel oriented sample measured with irradiation along the vorticity direction includes a small peak with a spacing of around 80 Å in the velocity gradient direction that is parallel to the chain axis. The long spacing is roughly 5 times larger than the length of repeat unit, i.e., the smectic layer thickness (16.4 Å), showing the existence of chain-folded lamellae. The parallel orientation is attributed to mutual slide of the chain-folded lamellae. At higher smectic temperatures, on the other hand, the flow of the molecules within a smectic layer takes place preferentially to the mutual slide of lamellae, which is responsible for perpendicular orientation.

We have prepared a polydomain smectic liquid crystalline (LC) elastomer by introducing 2 mol % 1,3,5-benzenetricarboxylic acid as a cross-linker into a main-chain BB-5 polyester. The elastomer designated as BB-5/E2 forms isotropic and smectic CA (SmCA) LC phases in order of decreasing temperature similarly to the un-cross-linked BB-5 polyester. BB-5/E2 in the isotropic liquid phase shows a typical rubberlike elasticity, whereas that in the SmCA phase shows a characteristic elongation behavior accompanying a polydomain−monodomain transition: the initial strain up to 100% orients the smectic layer with its normal parallel to the elongation direction, and the further elongation does not change the layer orientation and results in permanent deformation. The corresponding stress−strain curve shows a quasi-plateau, which is a so-called “soft stress plateau”, over a wide strain region from 100% to 450%. These indicate that the polymer chains folded in the oriented smectic LC elastomer at a strain of 100% are stretched on further elongation and locked by the smectic layer order. At a strain of more than 300%, the SmCA phase begins to transform into a crystal because of the entropy reduction due to direct stress to the fully extended polymer chains.

Smectic state morphology of the main-chain liquid crystal PB-14 polyester was studied by small-angle X-ray scattering (SAXS), differential scanning calorimetry, and transmission electron microscope methods. The PB-14 polyester forms isotropic liquid (I), smectic H (SH), and crystalline phases in order of decreasing temperature. The transition temperature between the I and SH phases was 226 °C on heating and 210 °C on cooling when it was measured at a rate of 10 °C min-1. SAXS for the SH phase formed from the isotropic melt showed well-defined reflection maxima which are attributed to the chain-folded lamellar structure. The lamellar size increased from 300 Å to 500 Å with an increase of the liquid crystallization temperature of 190 °C to 220 °C. The relationship between the lamellar thickness and isotropization temperature is well described by the Thomson−Gibbs equation. According to this relationship, the surface free energy of the lamellae and the equilibrium isotropization temperature of SH were elucidated as 72 erg cm-2 and 263 °C, respectively. When the sample was annealed at a SH temperature of 215 °C, the isotropization temperature of the SH phase increased from 228 °C to 257 °C, and simultaneously the enthalpy change increased from 4.9 kcal mol-1 to 6.0 kcal mol-1. These trends can be explained by the lamellar thickening. The overall results thus indicate that the SH liquid crystallization at a certain temperature takes place imperfectly in a finite period due to the chain folding and that the succeeding annealing causes the alteration of the chain conformation from a folded form to an extended one as observed in the crystallization of conventional polymers.

The low-angle bent-shaped molecules with 1,7-naphthalene central core and alkylthio tails can form a novel hexagonal columnar phase and a dark B4 phase. The columnar phase has a large two-dimensional hexagonal lattice with edges of 65–75 Å and exhibits polar switching with spontaneous polarization along the column axis. Calculated from the density (∼1 g·cm-3) and unit volume, the number of molecules that are necessary to fill a 4.6-Å stratum of each column were found to be ∼11. Such a large number of molecules can be accommodated only in the tube-like assembly, which may be the first example as formed by the usual bent-shaped molecule with a single alkylthio tail.

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