Rheological behavior of thermotropic flexible main‐chain polyesters based on 4,4′‐dioxy‐2,2′‐dimethyl azoxybenzene mesogens. I. Oscillatory shear behavior

Blumstein, A.; Thomas, O.; Kumar, Sandeep

doi:10.1002/polb.1986.180240104

Cited by 30 publications

(11 citation statements)

References 13 publications

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“…Subsequently, the synthesis of thermotropic copolyesters of HBA and 6-hydroxy-2naphthoic acid (HNA) was reported.2 A number of research groups studied the rheological behavior of HBA/ PET copolyesters,3-10 HBA/HNA copolyesters,711-13 and other types of TLCPs. [14][15][16][17][18][19][20][21][22][23] Some unusual temperature dependency of TLCP has been reported in the literature,18'21'22 namely, the viscosity in the isotropic state is higher than that in the nematic state although the temperature is higher in the isotropic state. What makes the matter very complicated is that the rheological behavior of TLCPs is greatly influenced by thermal history.3'7 It should be mentioned that almost all the previous rheological studies mentioned above, with the exception of the study of Wissbrun and Griffin,15 dealt with polymers that have relatively high crystal-nematic transition temperature (Tkn) (also referred to melting temperature, Tm) and high nematic-isotropic transition (also referred to as clearing or isotropization) temperature (Tni) (8ay, above 350 °C, which is close to thermal degradation temperature).…”

Section: Introductionmentioning

confidence: 99%

Effect of thermal history on the rheological behavior of a thermotropic liquid-crystalline polymer

Kim¹,

Han²

1993

Macromolecules

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The effect of thermal history on the rheological behavior of a thermotropic liquid-crystalline polymer was investigated. For the study, an aromatic polyester, poly[(phenylsulfonyl)-p-phenylene-l,10decamethylenebis(4-oxybenzoate)] (PSHQ10), was synthesized. PSHQ10 was found to have (a) a weightaverage molecular weight of 35 000 relative to polystyrene standards and the polydispersity index of 2.0, (b) a glass transition temperature of 80 °C, (c) a melting point of 110 °C, and (d) a nematic-isotropic transition temperature (Tni) of 175 6C. Dynamic storage and loss moduli, G' and G", in the oscillatory shear mode were measured during isothermal annealing for specimens of PSHQ10 having various thermal histories. We found that thermal history had a profound influence on the rheological behavior of PSHQ10; specifically, during isothermal annealing in the nematic region, the values of G', G", and complex viscosity |i)*| of the as-cast specimen increased with time much faster at 130 °C than at 135 and 140 °C, but at temperatures above 140 °C the values of G', G", and |i?*| initially decreased and then increased, finally leveling off after a very long annealing period (for instance, 500 h after the annealing began at 145 °C). Using information obtained from differential scanning calorimetry, we explained the seemingly peculiar behavior in terms of a slow process of "crystal-like phase" formation below ca. 140 °C. However, when the as-cast specimen was first heated to a temperature above Tni, subjected to steady shear flow at y = 0.085 s_1 for 5 min, and then cooled slowly down to the nematic region, the values of G', G", and |t?*| were found to change little during the subsequent annealing for a long period. Also conducted were light scattering experiments on as-cast specimens of PSHQ10 having different thermal histories. We found that during isothermal annealing the maximum intensity of scattered light increased faster as the annealing temperature increased, and the rate of "domain" coarsening, induced by isothermal annealing in the nematic range, increased as the annealing temperature increased. This finding was corroborated by the micrographs taken with a cross-polarized optical microscope.

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Section: Introductionmentioning

confidence: 99%

Effect of thermal history on the rheological behavior of a thermotropic liquid-crystalline polymer

Kim¹,

Han²

1993

Macromolecules

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“…However, this behavior, which is rather typical for conventional flexible polymers, was found experimentally only for a few LCPs. For LC polyethers similar in structure to our polymers, the superposition principle does not work23–25 in either the isotropic or nematic phases. At this moment, the question of which structural units in main‐chain LC polymers are responsible for the applicability of the time–temperature superposition principle cannot be answered.…”

Section: Resultsmentioning

confidence: 74%

“…Although the appearance of entanglements in main‐chain LC polymers and the existence of time–temperature superposition are matters of controversy, some authors interpret their results under these assumptions 25, 26. Furthermore, the form itself of the moduli curves is reminiscent of the form of the curves of flexible polymers.…”

Section: Resultsmentioning

confidence: 99%

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New polymer syntheses. 102. Aspirin-modified LC polyesters based on PET and 4-hydroxybenzoic acid

Kricheldorf

Gerken

Yul'chibaev

et al. 2000

J. Polym. Sci. A Polym. Chem.

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Four series of liquid‐crystalline copolyesters were prepared by the transesterification of poly(ethylene terephthalate) (PET) with 4‐acetoxybenzoic acid (4‐ABA) or mixtures of 4‐ABA and acetylsalicylic acid (ASA). Two series consisted of 30 mol % PET, and the other two series consisted of 40 mol % PET. The molar ratio of 4‐HBA and ASA was varied in all four series from 0 to 25 mol %. One 30% PET series and one 40% PET series were prepared with the addition of acetic acid, which caused a more perfect randomization of the sequence but yielded slightly lower molecular weights. The incorporation of ASA reduced the crystallinity, which vanished completely at a salicylic acid (SA) content greater than 10 mol %. SA also reduced the stability of the nematic phase, but all the copolyesters were thermotropic up to a 20 mol % SA content. Furthermore, the molecular weights decreased with the increasing incorporation of ASA. Despite this negative trend, the melt viscosity and the storage and loss moduli passed a maximum between 5 and 10% SA. Obviously, the incorporation of SA favored the formation of entanglements. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2013–2022, 2000

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“…The magnitude of the complex viscosity displays nearly stepwise decreases as a function of increasing temperature at both the crystalline melting and smectic A-to-nematic transitions and a small increase at the onset of isotropy (Figure 11a); this behavior is qualitatively similar to results for other main-chain liquid-crystalline polymers in the literature. [16][17][18] The rheological response is insensitive, however, to the crystal-crystal transition that occurs in the solid state at 137 °C, despite the large DSC endotherm that is observed at this temperature. Examination of the dynamic moduli over the range 100-160 °C indicates a wholly solidlike behavior (G'» G", G' ^/(o>); see Figure 15), with the two moduli nearly independent of temperature.…”

Section: Resultsmentioning

confidence: 99%

Structural, dielectric, and rheological characterization of a thermotropic polyester displaying smectic A, nematic, and isotropic phases

Kalika¹,

Shen²,

Yu³

et al. 1990

Macromolecules

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We report structural, dielectric, and rheological properties of a thermotropic liquidcrystalline polyester, which exhibits a crystal-crystal transition at 137 °C and crystal-to-smectic A, smectic A-to-nematic, and nematic-to-isotropic transitions at 164, 180, and 220 °C, respectively; initial phase identifications were made by using optical microscopy techniques and confirmed by X-ray diffraction measurements. The dielectric results are highly sensitive to phase state, particularly at the lower frequencies investigated, where interfacial polarization effects dominate due to the broad biphasic regions present in our samples. Higher frequency measurements indicate a sizable increase in dipolar (rotational) mobility at the crystal-crystal transition, no change in the dielectric constant («') at the crystal-to-smectic A transition, and a decrease in e' at the onset of the nematic phase. In contrast to the dielectric results, dynamic rheological measurements are insensitive to the crystal-crystal transition, but large, nearly stepwise decreases in complex viscosity are observed across both the crystal-to-smectic A and smectic A-to-nematic transitions, with a slight increase encountered at the onset of isotropy. Constant-temperature frequency sweeps recorded for the smectic A phase indicate a persistent shear thinning behavior, with the storage modulus (G') greater than the loss modulus (G") over the investigated frequency range. Steady shear viscosity measurements obtained during cooling are consistent with the dynamic results and indicate that the layering of the smectic A phase is apparently not disrupted by steady shearing at low rates. A lack of reversibility is observed for the rheological measurements carried out from 230 °C relative to those initiated at 190 °C; this may be the result of transesterification reactions occurring at the higher temperature.

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Rheological behavior of thermotropic flexible main‐chain polyesters based on 4,4′‐dioxy‐2,2′‐dimethyl azoxybenzene mesogens. I. Oscillatory shear behavior

Cited by 30 publications

References 13 publications

Effect of thermal history on the rheological behavior of a thermotropic liquid-crystalline polymer

Effect of thermal history on the rheological behavior of a thermotropic liquid-crystalline polymer

New polymer syntheses. 102. Aspirin-modified LC polyesters based on PET and 4-hydroxybenzoic acid

Structural, dielectric, and rheological characterization of a thermotropic polyester displaying smectic A, nematic, and isotropic phases

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