Periodically Spaced Side-Chain Liquid Crystalline Polymers

Abstract

The melt-transesterification polymerization of diethyl malonate derivatives bearing a pendant mesogen, with α,ω-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.

Main-chain liquid crystalline polymers bearing periodically grafted folding elements

“…[44][45][46][47][48] Building on these studies, we recently began exploring the possibility of enhancing the folding propensity of PGACs by the inclusion of mesogenic aromatic units, such as biphenyl or azobenzene, either within the pendant segment or within the polymer backbone; the presence of the rigid aromatic mesogens, we expected, would not only serve to enhance the tendency to fold but would also permit us to examine the possibility of regulating the nature of the mesophase formed. In our previous report, we examined the influence of the nature and length of the backbone segment in periodically spaced side-chain LCPs; 49 the studies revealed that when alkylene segments were present in the backbone, only nematic mesophases were formed, even though smectic-type layering was evident in the solid state. However, when polar flexible PEG segments were present in the backbone, lowering of the melting temperature and segment immiscibility drove the formation of smectic mesophases.…”

Main-chain side-chain combined liquid crystalline polymers bearing azobenzene and biphenyl-based mesogens – segregation of the mesogens within two distinct layers

“…Synthesis procedure of diethyl malonate based azobenzene mesogenic monomer bearing C6 spacer and C6 tail segment was described in our previous study. 1 Diethyl malonate based monomer of azobenzene bearing C10 spacer and C10 tail segment was synthesized following same procedure.…”

LC‐Polymers and Smectic Phases with Special Substructures/Nanophase Segregation

“…For them a repeated bending of the main chain spacer between the mesogens (not straight spacer) is observed (Figure 4). [ 76–82 ]…”

“…This regularly bended structure allows now the incorporation of functional, even sterically demanding units as side groups, without disturbing the dense packing of the mesogens and thus without disturbing the LC ordering. [ 76–82 ] Concerning the smectic structure the smectic layer thickness includes the length of the side chains (Figure 4). In smectic elastomers prepared from these systems, the shape variation at the phase transition to the isotropic phase is well detectable, but much lower than in the case of LC‐main chain elastomers with a straight spacer.…”