Inherently Flexible Thermotropic Main Chain Polymeric Liquid Crystals

Blumstein, R. B.; Blumstein, A.

doi:10.1080/00268948808082207

Cited by 33 publications

(33 citation statements)

References 53 publications

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“…For thermotropic LCPs, the biphase structure that occurs during the anisotropic-isotropic transition may remain stable at thermal equilibrium over a certain temperature interval. This behavior has been observed for several nematic LCPs with chemically regular structures and was attributed to the polydispersity in the molecular mass [1][2][3][4][5][6][7][8][9][10]. For some of these polymers, it has been demonstrated that the chains are preferentially distributed in the isotropic and anisotropic phases of the biphasic region according to their length.…”

Section: Introductionmentioning

confidence: 94%

Effect of the Thermal History on the Thermal and Rheological Behavior of a Thermotropic Polyester

Ressia

Quinzani

Vallés

et al. 2004

Molecular Crystals and Liquid Crystals

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The biphasic behavior and phase transitions of a thermotropic mainchain liquid crystalline polyester, the poly(oxytrimetilen-etilen glycol p,p 0 -bibenzoate), was studied by X-ray diffractometry, differential scanning calorimetry (DSC), and rotational rheometry. The liquid crystalline mesophase structure of the polymer evolves from Smectic C to Smectic A at around 100 C and changes to the isotropic state at about 200 C. The annealing of polymer samples at different temperatures within the smectic-isotropic transition gives rise to double endotherm and exotherm peaks on the heating and cooling DSC traces. These peaks are attributed to the preferential segregation of the lower molecular weight molecules from the liquid crystalline to the isotropic phase in the biphasic zone. The rheological studies are consistent with this result. The dynamic moduli measured at constant frequency in temperature ramps using polymer samples with different thermal histories reveal that the type of annealing processes applied in the biphasic zone generates different evolution of the rheological data.

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Section: Introductionmentioning

confidence: 94%

Effect of the Thermal History on the Thermal and Rheological Behavior of a Thermotropic Polyester

Ressia

Quinzani

Vallés

et al. 2004

Molecular Crystals and Liquid Crystals

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“…H is the magnetic field direction (sample film distance 60 mm), (a) Wideangle diffuse ring, (b) diffuse lines, and (c) diffuse spots due to unimportant smectic-C fluctuations (see Ref. [6]) superimposed on the first diffuse line.…”

Section: (4)mentioning

confidence: 99%

“…This polyester was first synthesized [5] and widely studied by Blumstein and Blumstein [6]. This group has obtained the first SANS results [7] and shown that these chains are very extended in the nematic phase.…”

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confidence: 96%

Observation of hairpin defects in a nematic main-chain polyester

Brûlet

Davidson

et al. 1993

Phys. Rev. Lett.

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The conformation of a main-chain liquid crystalline polyester in its oriented nematic phase has been determined by small-angle neutron scattering. The data are fitted by a model of rigid cylinder with orientational fluctuations. For a low degree of polymerization (~9) the chain is almost completely elongated in the direction of the nematic field. For a polymer 3 times longer, the existence of two hairpins is shown at high temperature; this number decreases with decreasing temperature.PACS numbers: 61.30.Eb, 61.12.Ex Liquid crystalline polymers (LCPs) have generated much interest in the recent years both because of the challenge of understanding such systems and because of their important technological applications in high modulus fibers, nonlinear optics, synthetic membranes, and electronic devices. A main-chain LCP is a linear link of repetitive units that may, but often does not, possess intrinsic mesogenic character. The liquid crystalline character of the main-chain LCP is a property of the chain backbone. For a semiflexible main-chain LCP in its nematic phase, the tendency to follow long-range orientational order of the nematic phase enters into competition with the random conformation necessary to maximize the polymer entropy. Depending upon the chain molecular weight M, two possible chain conformations have been predicted by de Gennes [1]: For low M values the chain adopts a linear conformation essentially parallel to the nematic director whereas for high M values it can present hairpin defects where the chain executes a contour reversal (180°) with respect to the director. The number of hairpins is an exponential function of the temperature. Statics and dynamics of hairpins in wormlike chains have been developed [2] by Warner and co-workers and a precise temperature dependence of the chain size has been predicted: The chain size increases with decreasing temperature [3]. From a practical point of view the existence of hairpins can play an important role in the rheology and in the nonlinear dielectric and optical properties of mainchain LCPs. We report, for the first time, on the experimental observation of the hairpin conformation by smallangle neutron scattering (SANS) associated with wideangle x-ray scattering (WAXS). The crucial problem is to determine the dimensions of one chain in the direction parallel and perpendicular to the nematic director. It has been resolved by SANS using for the chain a cylinder model. WAXS has been used to measure the monomer length and the monomer orientational order of the polymer in the nematic phase. The existence of the hairpins is deduced from the comparison of the height of the cylinder to the total length of the chain.SANS allows the determination of the conformation of one chain from a melt of 50% deuterated (D) chains dispersed in protonated (H) ones [4]. Here the difficulties lie in finding soluble nematic polymers (in order to obtain the H and D chain mixture from a solution) with a nematic phase in a low-temperature range (r<200°C). A rather good candidate is t...

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“…The isotropic to nematic (I-»N) phase transition in the liquid crystalline polymers can be observed clearly via DSC and shows relatively small entropy and enthalpy changes during the transition (12). From Figures 2 and 3, the phase diagrams indicate that the I->N transition exists in odd-TPP(n<13)s and even-TPP(n<8)s. Figure 4 shows the relationships between the transition enthalpy change and the number of methylene unites for both TPP(n=odd)s and TPP(n=even)s. This kind of plot was first reported by Blumstein et al for another main-chain liquid crystalline polymer (13,14). It is clear that linear relationships of the transition properties with respect to the number of methylene units are found and the enthalpy changes for the TPP(n=odd)s are relatively small compared with those in TPP(n=even)s. This is due to the odd-even effect which influences the mesogenic group packing and methylene unit conformation.…”

Section: Resultsmentioning

confidence: 66%

Identification of Highly Ordered Smectic Phases in a Series of Main-Chain Liquid-Crystalline Polyethers

Yoon

Savitski

et al. 1996

ACS Symposium Series

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A series of polyethers have been synthesized from 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)propane and ct, o~-dibromoalkanes having different numbers of methylene units [TPPs]. Both odd-and even-numbered TPPs [TPP(n=odd)s and TPP(n=even)s) exhibit multiple transitions during cooling and heating and they show little supercooling dependence, indicating close-to-equilibrium nature of these transitions. Combining the structural characterization obtained via wide angle X-ray diffraction powder and fiber patterns at different temperatures and the morphological observations from mieroseopy techniques, not only the nematie liquid crystalline phase but also highly ordered smeetic F, smectic crystal 0 and H phases have been identified. The phase diagrams for both TPP(n=odd)s and TPP(n=even)s have been constructed [1][2][3]. Thermodynamic properties (enthalpy and entropy changes) during these transitions are studied based on differential scanning calorimetry experiments. The contributions of the mesogenie groups and methylene units to eaeh ordering process can be separated and they indicate the charaeteristies of these processes thereby providing estimations of the transition types.

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Inherently Flexible Thermotropic Main Chain Polymeric Liquid Crystals

Cited by 33 publications

References 53 publications

Effect of the Thermal History on the Thermal and Rheological Behavior of a Thermotropic Polyester

Effect of the Thermal History on the Thermal and Rheological Behavior of a Thermotropic Polyester

Observation of hairpin defects in a nematic main-chain polyester

Identification of Highly Ordered Smectic Phases in a Series of Main-Chain Liquid-Crystalline Polyethers

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