Periodically Spaced Side-Chain Liquid Crystalline Polymers

The melt-transesterification polymerization of diethyl malonate derivatives bearing a pendant mesogen, with alpha,omega-alkanediols of varying lengths generated a series of side-chain liquid crystalline polymers, wherein the intervening backbone alkylene spacer segment was systematically varied; the effect of the backbone spacer segment on the liquid crystalline property was examined using differential scanning calorimetry (DSC), X-ray scattering, and polarizing light microscopic investigations. Two different mesogen units, based on 4,4'-dialkoxydiazobenzene or 4,4'-dialkoxybiphenyl, were examined; it was seen that most polyesters derived from the diazobenzene mesogen exhibited a stable nematic mesophase, whereas most of those based on biphenyl transformed directly to an isotropic melt. X-ray scattering studies revealed that the polymers carrying biphenyl units formed a well-ordered lamellar structure in the solid state, which was interpreted as being generated via the zigzag folding of the polymer backbone, thus permitting the pendant mesogens and the folded backbone to occupy alternate layers. Based on the slope of the linear variation of the interlamellar spacing as a function of backbone segment length, it was inferred that the backbone is not in an extended all-trans conformation but is disordered; from the value of the intercept, which roughly corresponded to the length of an extended pendant mesogenic segment, it was inferred that the pendant mesogens were interdigitated. The formation of a highly ordered structure in the solid state appears to drive up the melting transition and preclude the formation of the liquid crystalline (LC) phase in the biphenyl series; however, when a flexible hydrophilic tetra(oxyethylene) spacer was incorporated, the polyester revealed a stable mesophase owing to the significant lowering of the melting transition. Comparison of pairs of isomeric polyesters, one having the mesogen within the pendant segment and the other within the backbone, revealed the strong tendency for the main-chain isomer to exhibit smectic mesophases, whereas the side-chain isomer exhibited a nematic mesophase.