Nematic Order in Polyesters with Flexible Moieties in the Main Chain

Abstract

ABSTRACT:Properties of polyesters based on 4,4' -dihydroxy-2,2 '-dimethylazoxybenzene and a homologous series of alkanedioic acids with methylene sequence length n = 3-4 are discussed in the light of recent results based on DSC, PMR, magnetic birefringence and X-ray data. Influence of substitution in the mesogen and spacer is also described. Particular attention is given to the oddeven effect in the homologous series and the existence of two distinct levels of molecular order for the n=even and n=odd members of the series. On the basis of X-ray diffraction data performed on oriented samples, a cybotactic micellar model for the n=even polymers is proposed. The cooperative nature of the formation of cybotactic micelles is shown by the study of order parameter and its dependence on the degree of polymerization. Progressive development of layered order in n=even polymers with increase in molecular mass is also shown by SAX patterns of unoriented samples of poly(4,4' -dioxy-2,2' -dimethylazoxybenzenedodecanedioyl). While the Meier and Saupe theory of the nematic state is in good agreement with the S(J) dependence for noncybotactic polymer systems for most of the nematic interval, it diverges in the vicinity of the clearing point. In contrast to low molecular mass nematics, polymeric nematics show a remarkable stability at the clearing temperature and a large pretransitional interval.KEY WORDS Nematic Order I Flexible Thermotropic Polyesters I Cybotactic Nematic Polyesters I Odd-Even Effect at the NII Transition I N +I Bi-Phase I In this paper, properties of a family of thermotropic nematic main chain polymers with regularly alternating rigid mesogenic and flexible moieties are described. An impressive volume of work has been obtained for such systems over the past few years and was reviewed.'-3 However, inadequate knowledge of sequence distribution, homogeneity and molecular weight distribution of polymer samples often lead to mistaken conclusions about liquid crystalline behavior and structure correlations. A need for a well characterized system is therefore of prime importance. We believe that the polymers described in this paper, derived from condensation of 4,4 '-dihydroxy-2,2 '-dimethylazoxybenzene (mesogen 9) and alkanedioic acids provide such a system:The moderate transition temperatures of the dimethylazoxybenzene moiety (minimizing decomposition and/or ester interchange reactions) combine with a strong mesogenic tendency of the azoxy group to provide a family of linear polyesters that is particularly well suited for study and understanding of flexible main chain polymeric liquid crystals.The aim of this article is to discuss the evolution of properties of these compounds in the light of our recent results. Attention will be given to the structural parameters affecting the formation, stability and the degree of order of the nematic mesophase. The conformation of nematic polyesters in the isotropic, nematic and oriented nematic state will be discussed and evidence for two distinct levels of order in polymeric ...

Inherently Flexible Thermotropic Main Chain Polymeric Liquid Crystals

“…The reader is referred to (14,15,25) for a more comprehensive discussion of structure-property correlations in R F PLCs. Structures based on the 2,2'-dimethyl-azoxybenzene (mesogen ME9) display the following trends.…”

Diacetylenic liquid‐crystalline polymers and copolymers. I. Derivatives of diphenyldiacetylene

“…Incorporation of several mesogenic groups into a flexible chain can enhance liquid crystallinity because of development of favorable orientational correlations between mesogen units. 6 Nevertheless, there is very little literature evidence for the development of liquid crystallinity in main-chain diacetylenic polyesters. A few mainchain diacetylenic polyesters have been prepared and investigated.…”

Thermotropic Liquid Crystalline Polyesters Using Aromatic Rigid Diols, Unsaturated Fumaric Acid and Flexible Sebacic Acid

“…Almost all the polyesters of this series display glass transitions and melting transitions as well as two exotherms during cooling indicates that the samples maintain crystallinity even in cooling cycle. Mesomorphism was observed almost in all polyester samples namely 2AZ, 3AZ, 4AZ and 5AZ (except 1AZ) [25][26][27][28]. This series reveals that homopolyesters prepared by using fumaroyl chloride is non-liquid crystalline while others are liquid crystalline.…”