Effect of backbone flexibility on the transitional properties of side-chain liquid-crystalline polymers

Abstract

A new homologous series of side-chain liquid-crystalline polymers has been synthesized, the poly[4- [[a-(4-cyanobiphenylyl-4'-oxy)-ai-alkyl]oxy]styrene]s, and the thermal behavior of the series investigated. The homologues with butyl or longer spacers exhibit a smectic phase, whereas the propyl homologue does not exhibit liquid crystallinity. The properties of these polymers are compared with those containing other polymeric backbones to study the effect of flexibility on thermal behavior. We propose a new model to account for the liquid crystal properties of side-chain polymers.

Liquid Crystal Dimers

“…For comparative purposes, the clearing entropies for the monomeric nOCB series are also shown in Fig. 3 [17]. The propyl to nonyl members of this series exhibit a nematic-isotropic transition and the associated entropy change is considerably smaller than that observed for either the odd or even membered dimers.…”

“…The dependence of the entropy change associated with the nematic-isotropic transition (d) on the number of methylene units in the¯exible spacer, n, for the BCBOn series [18]. Also shown are the entropy changes associated with the nematic-isotropic (h) and the smectic A-isotropic transitions (j) for the nOCB series [17] phase is insuf®cient to convert bent conformers into linear conformers. Hence, the orientational order is not enhanced and a smaller nematicisotropic entropy would be expected.…”

“…Discotic dimers have a strong tendency to exhibit columnar phases but an example of a nematogenic discotic dimer has been reported by Praefcke et al which was based on two discotic multialkylnyl units [111], 17 Conoscopic studies suggest that this compound in fact exhibits a biaxial nematic phase although these experiments indicated only a small optical biaxiality. Indeed, more recently, NMR spectroscopy using selectively deuterated materials has shown that for other materials the assignment of a biaxial nematic phase on the basis of conoscopy alone is not necessarily reliable [118].…”

“…In particular, and as we shall see later, the transitional behaviour of dimers exhibits a dramatic dependence on the length and parity of the¯exible spacer in a manner strongly reminiscent to that observed for the polymeric systems. More recently it has been suggested that dimers may also be used to model the behaviour of side group liquid crystal polymers [17].…”

Synthesis and Characterisation of Side‐Chain Liquid Crystalline Poly[1‐({[(4‐cyano‐4′‐biphenyl)oxy]alkyl}oxy)‐2,3‐epoxypropane]

“…After this discovery, there have been many systematic studies on the effects of the spacer length on the thermal and physical properties of these SCLCPs. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] It has been shown that increasing the spacer length while keeping the backbone and mesogenic groups constant has two effects: (i) the clearing temperature of the polymer increases and (ii) the entropy change associated with the clearing transition decreases. Most of the research work refers to polyacrylate, [33,34] polymethylacrylate, [35,36] polyurethane, [37] and polystyrene [38][39][40][41] backbones.…”