NMR study of some thermotropic nematic polyesters with mesogenic elements and flexible spacers in the main chain

Martins, A. F.; Ferreira, João; Volino, F.; Blumstein, A.; Blumstein, R. B.

doi:10.1021/ma00236a023

Cited by 87 publications

(17 citation statements)

References 15 publications

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“…The fluctuating structure of the liquid fringed micelle would be favored when molecular segments have conformational "versatility" to commute dynamically between nematic and isotropic environments. We have therefore Field Figure 9. Changes in the broad-line proton NMR spectra as the nematic fluid acquires macroscopic orientation in the external magnetic field (left).…”

Section: Resultsmentioning

confidence: 99%

Molecular organization in nematic polymers. 1. Biphasic structures vs the nematic phase

Stupp¹,

Wu²,

Moore³

et al. 1991

Macromolecules

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The objective of this work has been to investigate how molecules are organized in nematic polymers within the temperature range between melting of the solid phase and complete isotropization. The main experimental technique in this study has been broad-line proton NMR and the polymers used were a sequentially aperiodic aromatic-aliphatic copolymer and its isomeric periodic compound. The study included also calculations of conformational energy and molecular shape in seven-mer chemical sequences present in the polymers investigated. NMR measurements as a function of temperature on the aperiodic copolymer provided evidence for the segregation of chains by chemical composition in the nematic and isotropic regions of biphasic patterns observed at high temperatures. The more flexible chains of the system congregate in isotropic regions whereas nematic regions retain the more rigid ones. Experiments at lower temperatures where biphasic patterns are no longer observed by optical microscopy suggest that nematic polymers form orientationally ordered phases that tolerate isotropic motion in their molecular segments. We propose that these motions can occur through "nematic-isotropic fluctuations" with spatial and temporal periodicities which depend on temperature and molecular structure. The physical origin of these fluctuations is not known but molecular weight distribution and the aperiodicity of chemical sequences could be important factors.

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Section: Resultsmentioning

confidence: 99%

Molecular organization in nematic polymers. 1. Biphasic structures vs the nematic phase

Stupp¹,

Wu²,

Moore³

et al. 1991

Macromolecules

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“…The separation do of the main doublet lines in the spectrum is proportional to the molecular degree of order of the (macroscopically) aligned phase. Details on the other features of the spectrum may be seen in [7].…”

Section: Methodsmentioning

confidence: 99%

NMR study of the long time, defect-controlled, magnetic reorientation of a nematic polymer liquid crystal

et al. 1993

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The magnetic field reorientation of an initially aligned sample of a nematic polymer liquid crystal was followed by proton NMR. Evolution to a metastable (banded) state was considered using a Rheo-NMR technique developed previously. Late stage reorientation was studied by taking into account the dynamics of defects following the formation of splay-bend walls. NMR spectra simulation allowed us to obtain the wall density as a function of time. This result, together with a defectcontrolled wall dissolution model proposed by Rey [ 11, was used to complement the Rheo-NMR technique of measuring the viscoelastic parameters of nematic polymer liquid crystals.

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“…SAX patterns were recorded using a Rigaku-Denki scanning diffractometer as described in ref 7. The nematic order parameter was measured by PMR as described in ref 14. Techniques for magnetic birefringence measurements and quenching of aligned samples were described in ref 7, 11, 15, and 16.…”

Section: Methodsmentioning

confidence: 99%

Nematic Order in Polyesters with Flexible Moieties in the Main Chain

Blumstein

1985

Polym J

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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 ...

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NMR study of some thermotropic nematic polyesters with mesogenic elements and flexible spacers in the main chain

Cited by 87 publications

References 15 publications

Molecular organization in nematic polymers. 1. Biphasic structures vs the nematic phase

Molecular organization in nematic polymers. 1. Biphasic structures vs the nematic phase

NMR study of the long time, defect-controlled, magnetic reorientation of a nematic polymer liquid crystal

Nematic Order in Polyesters with Flexible Moieties in the Main Chain

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