LIQUID-CRYSTALLINE POLYETHERS BASED ON CONFORMATIONAL ISOMERISM .32. EFFECT OF MOLECULAR-WEIGHT ON THE PHASE-BEHAVIOR OF LINEAR AND MACROCYCLIC OLIGOETHERS AND OF LINEAR POLYETHERS BASED ON 1-(4-HYDROXY-4'-BIPHENYLYL)-2-(4-HYDROXYPHENYL)BUTANE AND 1,10-DIBROMODECANE

The synthesis and phase behavior of the individual monomer, dimer, trimer, and tetramer, and of the polyethers of 1-(4-hydroxy-4'-biphenylyl)-2-(4-hydroxyphenyl)butane (TPB) with 1,10-dibromodecane (TPB-10) with M(n) = 5200 to 57 200, M(w)/M(n) = 1.16 to 1.47 and with well-defined chain ends are described. The influence of various chain ends on the mesomorphic behavior of monomer and dimer was also discussed. There is a continuous increase of the nematic-isotropic (T(ni)) and glass transition (T(g)) temperatures over the entire range of molecular weights. The increase of T(ni) and T(in) on M(n) is very steep up to M(n) = 15 000 to 20 000. At higher molecular weights the increase of T(ni) is much less steep, while T(in) remains about constant or even decreases slightly. This trend is determined by a continuous increase of the degree of supercooling of the isotropization transition temperature with the increase in molecular weight. Therefore, at low molecular weights the isotropization transition is thermodynamically controlled while at high molecular weights, by analogy with other first-order phase transitions in polymers, it becomes kinetically controlled. T(g) determined from both heating and cooling scans follows an identical trend with T(ni), resulting in a constant T(ni)/T(g) ratio over the entire range of molecular weights. The phase behavior of these linear oligomers and polymers was compared to that of the corresponding macrocyclics reported previously (Macromolecules 1992, 25, 3851). The macrocyclic trimer, tetramer, and pentamer show isotropization temperatures which are up to 46-degrees-C higher than those of their corresponding linear oligomers and high molecular weight polymers. T(g) values of macrocyclics are about 17-degrees-C higher than those of the corresponding linear oligomers. The enthalpy changes associated with the isotropization temperatures of macrocyclics are from 20 up to 2 times lower than those of the linear oligomers and polymers. Also, the crystallization ability of macrocyclics is lower than that of their homologues linear oligomers. This paper provides the most complete set of results on the influence of molecular weight on the phase behavior of linear main chain liquid crystalline polymers and presents the first study which compares the phase behavior of macrocyclics with that of the their corresponding linear architectures.