Elastic Coefficients and Twist Viscosity in Side-Chain Mesomorphic Polymers

Fabre, Pascale; Casagrande, C.; Veyssié, M.; Finkelmann, Heino

doi:10.1103/physrevlett.53.993

Cited by 56 publications

(24 citation statements)

References 6 publications

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“…This physical link between the mesogenic groups and the polymer chain should in theory make the "polymer" characteristics (viscosity, elasticity) anisotropic, and modify the order of magnitude of the purely "liquid-crystal" characteristics (torsional viscosity yl, curvature elastic constants). The Fredericks transition experiments [2] and the transient electrical birefringence experiments [3,4] performed in the nematic and isotropic phases show that the static properties of these mesophases are close to those of conventional liquid crystals (no substantial modification of either the birefringence or the curvature elastic constants), while their dynamic properties are essentially governed by the chains themselves (increase of viscosities and relaxation times). A certain number of rheology experiments have also been carried out [5], and these confirm that the presence of the liquid-crystal order has no significant role to play in the dynamics of the nematic and isotropic phases, which consequently have viscoelastic properties which are qualitatively similar to those of ordinary flexible polymers.…”

Section: P Kellermentioning

confidence: 84%

Abnormal viscoelastic behavior of side-chain liquid-crystal polymers

et al. 1994

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%e show that, contrary to what is commonly believed, the isotropic phase of side-chain liquid-crystal polymers has viscoelastic properties which are totally different from those of ordinary flexible melt polymers. The results can be explained by the existence of a transient network created by the dynamic association of mesogenic groups belonging to different chains. The extremely high sensitivity of the compound to the state of the surfaces with which it is in contact offers us an unexpected method of studying surface states.PACS numbers: 83.10.Nn, 61.30.Eb, 83.70.Jr Side-chain liquid-crystal polymers (SCLCP) are composed of a polymer chain onto the sides of which mesogenic molecules have been grafted using a flexible chain called a spacer [1]. This physical link between the mesogenic groups and the polymer chain should in theory make the "polymer" characteristics (viscosity, elasticity) anisotropic, and modify the order of magnitude of the purely "liquid-crystal" characteristics (torsional viscosity yl, curvature elastic constants). The Fredericks transition experiments [2] and the transient electrical birefringence experiments [3,4] performed in the nematic and isotropic phases show that the static properties of these mesophases are close to those of conventional liquid crystals (no substantial modification of either the birefringence or the curvature elastic constants), while their dynamic properties are essentially governed by the chains themselves (increase of viscosities and relaxation times). A certain number of rheology experiments have also been carried out [5], and these confirm that the presence of the liquid-crystal order has no significant role to play in the dynamics of the nematic and isotropic phases, which consequently have viscoelastic properties which are qualitatively similar to those of ordinary flexible polymers.However, these experiments have not revealed the rubbery plateau which is characteristic of entanglement efl'ects, and which is replaced by an extended distribution of relaxation times the origin of which is unknown. The polymer's polydispersity or/and a relaxation of the mesogenic groups have been invoked to explain this distribution [5].In this Letter, we present new experiments realized with a rheometer, which enabled us to study orientated samples with very low shear strains. The results obtained reveal a viscoelastic behavior which is quite different from that of conventional polymers, and this suggests the existence in the melt of elastic clusters of varying sizes up to 100 pm. We will also show that the rheological behavior observed is extremely sensitive to the state of the surfaces in contact with the sample. The experimental setup was inspired by that previously developed by Cagnon and Durand in their study of the plastic behavior of certain lamellar liquid crystals [6].The sample, of thickness e between 13 and 120 pm, is placed between two glass or silica slides, adjusted for parallelism to within~5x10 rad. The sample can be observed de tisu through an aperture. A sine ...

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Section: P Kellermentioning

confidence: 84%

Abnormal viscoelastic behavior of side-chain liquid-crystal polymers

et al. 1994

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“…In the presence of an electric field £ dc , the term -y € 0 eafiEa C Fp c must be added to (2), where e 0 is the vacuum permittivity and efy the dc dielectric tensor, e d p is linked to the Q a p order parameter by the relation (e a p) dc = (e) dc $ap+ J (Ae 0 ) dc Qaf>,…”

Section: A(t C -T C *) B and The Latent Heat Ismentioning

confidence: 99%

“…As a result these polymers have characteristics which are similar to those of either conventional liquid crystals (elastic constants) or flexible polymers (viscosity). 2 This being the case, we can ask whether the pretransitional effects occurring above the nematic-isotropic transition are analogous to those of conventional liquid crystals, or whether they are modified by the coupling of the mesogenic elements and the backbone. In order to answer this question, we used techniques measuring the birefringence induced by an electric field (the Kerr effect), the birefringence induced by an acoustic field, and the viscosity.…”

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confidence: 98%

Short-Range-Order Effects in the Isotropic Phase of a Side-Chain Polymeric Liquid Crystal

et al. 1988

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Short-range-order effects in the isotropic phase of a side-chain polymeric liquid crystal are studied by means of electric and acoustic birefringences and the viscosity. The static properties are comparable to those of conventional liquid crystals, but the dynamic properties show quite different behavior, characterized by a relaxation time which diverges as (T-T?) ~1 55 , rather than (T -T?) ~\ T? being a virtual second-order transition temperature. Analysis of the results according to the de Gennes theory enabled the friction coefficients of this theory to be established. 61.41.+e Polymers with mesogenic side chains constitute an important example of systems which form liquid-crystal phases. In view of the fundamental problems they raise and their potential uses, 1 they are at present the object of intensive research. Studies carried out in the nematic phase have shown a coupling between the mesogenic elements and the polymeric backbone. This coupling can also be seen to be more or less manifest according to the physical properties studied. As a result these polymers have characteristics which are similar to those of either conventional liquid crystals (elastic constants) or flexible polymers (viscosity). 2 This being the case, we can ask whether the pretransitional effects occurring above the nematic-isotropic transition are analogous to those of conventional liquid crystals, or whether they are modified by the coupling of the mesogenic elements and the backbone. In order to answer this question, we used techniques measuring the birefringence induced by an electric field (the Kerr effect), the birefringence induced by an acoustic field, and the viscosity. The polymer studied is poly[(acryloyloxy-6-hexyloxy)-4-cyano-4'-biphenyl], abbreviated to PA60CB, whose formula is shown in Fig. 1. This polymer has a molecular weight M w of 62000, and a polydispersity of 3.38. 3 The various polymer samples were degassed prior to the experiments, and the electric and acoustic measurements taken under an inert atmosphere.The birefringence induced by a pulsed electric field was used to simultaneously determine the intensity and the characteristic time of the local orientational order, which appears in the vicinity of the nematic-isotropic transition. The birefringence was measured at 6328 A, with a 5-mW He-Ne laser as a light source, and a 40-mm-long Kerr cell with an interelectrode length of 2.3 mm, the cell temperature being kept constant to within ± 0.005 °C. The electrical pulses were rectangular in shape, with a maximum amplitude of 500 V, and for each temperature, their duration was adjusted in such a way as to ensure stationary birefringence. The rise and decay times of the electric field were less than 0.2 jus. This is much less than the relaxation time of the local orientational order, which could therefore be determined. The variation of An with E 2 was found to be linear for all the electric fields used.The birefringence induced by an acoustic field is associated with the velocity gradient accompanying the ultras...

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“…It appeared that, whereas the elastic constants k , , and k,, were of the same magnitude as in the conventional low molecular weight liquid crystals, y , was found to be several orders of magnitude larger. These very high values of y , are thought to be associated with the main-chain reorientation [8].…”

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confidence: 96%