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ABSTRACT: Recent studies of the dynamic mechanical behavior of ultra high modulus polyethylene? are discussed in the context of our present understanding of the structure of these materials. In particular, the Takayanagi model is shown to achieve a new status in the light of direct measurements of crystal continuity from wide angle X-ray diffraction data. It is further shown that the Takayanagi model in one formulation is compatible with the Cox model for a short fiber reinforced composite. The fiber composite model offers a simple physical understanding of the fall in modulus due to the α-relaxation in terms of shear lag. This reduces the effectiveness of the continuous crystal fraction postulated in the Takayanagi model. The γ-relaxation is considered to be associated primarily with an amorphous relaxation, consistent with the conclusions of previous workers for materials of lower draw ratio.
Polymer Engineering and Science 08/2004; 20(14):941 - 948. · 1.30 Impact Factor
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ABSTRACT: Measurements of the pyroelectric response in both free and constrained conditions are presented for low-draw ratio, high-draw ratio and voided form I polyvinylidene fluoride sheets. The results are used together with related data for the piezoelectric response and for the thermal expansion behavior to estimate the primary and secondary components of the total macroscopic pyroelectricity. For the high-draw ratio and voided sheets the primary response was approximately 90% of the total pyroelectricity compared with 40% for the low-draw ratio nonvoided sheet. It is proposed that the role of reversible temperature-dependent crystallinity is more pronounced in the high-draw samples. Previous NMR studies provide support for this conclusion.
Journal of Polymer Science Part B Polymer Physics 03/2003; 26(1):127 - 140. · 1.53 Impact Factor
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ABSTRACT: Dielectric measurments made from 1 Hz to 10 kHz and from -80 to 150°C on random copolyesters of 4-hydroxybenzoic acid (HBA) and 2-hydroxy 6-naphthoic acid (HNA) reveal three relaxation processes at low frequencies rather than the two seen at audio frequencies. The higher temperature α process is similar to a glass transition, with a high activation energy ∼ 170 kcal/mol, but the two lower temperature processes are identified as local mode processes associated with the HNA and HBA moieties, respectively. It is shown that the observed value of the permittivity above the glass transition is lower than that which would be expected for ester groups rotating freely about the local chain axis, and it is suggested that this may be due to several factors including immobilization of dipoles in the crystalline regions, correlated motion of dipoles, and librational rather than rotational motion.
Journal of Polymer Science Part B Polymer Physics 03/2003; 26(8):1751 - 1760. · 1.53 Impact Factor
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ABSTRACT: The dynamic mechanical properties of a number of ultra-highly drawn polyethylenes have been studied over a wide range of temperature. It is shown that the materials possess low temperature Young's moduli as high as 1.6 Mbar, a figure which approaches the theoretical and experimental values for the c-axis crystalline modulus of this polymer.The α and γ relaxation processes are still clearly discernible even at highest drawn ratios (ca. 35) and a quantitative analysis of the results, using structural data obtained from broad line nuclear magnetic resonance (NMR) measurements, suggests that the data are consistent with a modified series model.
Journal of Polymer Science Polymer Physics Edition 03/2003; 13(12):2331 - 2343.
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ABSTRACT: Dynamic mechanical measurements on ultrahigh modulus polyoxymethylene have been undertaken over the temperature range −150 to 20°C. Measurements of the longitudinal crystal modulus have also been made by studying changes in the (009) reflection with load, over a similar range of temperatures. The dynamic Young's modulus at 5 Hz reaches a value at low temperatures of 64.5 GPa for the most highly oriented sample. The crystal modulus at low temperatures is 105 GPa, which is almost twice the previously reported room-temperature value.
Journal of Polymer Science Polymer Physics Edition 03/2003; 17(3):351 - 355.
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ABSTRACT: The anisotropy of the α and β relaxations in oriented poly(ethylene terephthalate) has been studied by dynamic mechanical and dielectric relaxation measurements. The α relaxation shows considerable mechanical anisotropy but gives rise to an isotropic dielectric process. The β relaxation, on the other hand, shows pronounced dielectric anisotropy but very little mechanical anisotropy. The implication of these results with regard to possible interpretations of the relaxations are discussed.
Journal of Polymer Science Part A-2 Polymer Physics 03/2003; 10(6):1153 - 1165.
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ABSTRACT: Dynamic mechanical measurements in tension and torsion on oriented tapes and monofilaments of a range of liquid crystalline polyesters have been combined with dielectric measurements to obtain a molecular understanding of the relaxation processes.This paper extends our previous work [1,2,3] on polyesters of hydroxybenzoic acid (HBA) and hydroxynaphthoic acid (HNA) to related polymers containing dihydroxynaphthalene (DHN) and terephthalic acid (TPA) or biphenyl (BP) and TPA in place of the HNA. It confirms the association of the β and processes with naphthyl and phenyl moieties but shows that the processes cannot be due simply to the independent motions of single units.
Polymers for Advanced Technologies 02/2003; 1(1):41 - 47. · 2.01 Impact Factor
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ABSTRACT: A quantitative study was undertaken of the anisotropy of low-strain mechanical behavior for specially oriented polyethylene with controlled crystalline and lamellar orientation. The samples were prepared by the die drawing of injection-molded rods of polyethylene and annealing. This produced a parallel lamellar structure for which a simple, three-dimensional composite laminate model could be used to calculate the expected anisotropy. Experimental data, including X-ray strain measurements of the lateral crystalline elastic constants, showed good quantitative agreement with the model prediction. The X-ray strain measurements confirmed that the amorphous regions exert large constraints on the crystalline phase in the lateral directions, where an order of magnitude difference was found between the measured apparent lateral crystalline compliances in the lamellar-stack sample and the expected values for a perfect crystal. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 755–764, 2000
Journal of Polymer Science Part B Polymer Physics 02/2000; 38(5):755 - 764. · 1.53 Impact Factor
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Applications of Ferroelectrics. 1986 Sixth IEEE International Symposium on; 02/1986
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ABSTRACT: The dynamic mechanical behaviour of highly oriented tapes and monofilaments of thermotropic liquid crystalline polymers based on hydroxynaphthoic acid, terephthalic acid and either aminophenol (designated HNATA) or hydroquinone (designated HNATH) have been studied in tension and torsion. It was found that the mechanical properties of these two polymers were very similar. In the case of HNATA both the tensile modulus and the chain modulus determined from X-ray diffraction measurements were studied as a function of applied stress. The aggregate model, when extended to include non-linearity, modelled the macroscopic tensile and shear behaviour with some success. The X-ray measurements could not be reconciled with the macroscopic measurements, which is tentatively attributed to the fact that these materials possess a complex hierarchical morphology.
Polymer.
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ABSTRACT: The dynamic mechanical moduli have been determined in tension and torsion for oriented thermotropic liquid crystalline polymers formed by copolymerization of 4-hydroxybenzoic acid and 2-hydroxy 6-naphthoic acid. The behaviour has been interpreted in terms of an aggregate model of units of structure. On this basis the fall in tensile modulus with increasing temperature can be quantitatively related to two factors, a decrease in the intrinsic chain modulus and a reduction in the shear modulus.
Polymer.
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ABSTRACT: This paper presents a procedure for simulating the anisotropic small-strain mechanical properties of oriented amorphous poly(ethylene terephthalate) (PET) starting from an atomistic level. A technique for producing oriented amorphous simulation cells of glassy PET has been developed and closely examined against related structural and property measurement data. The simulated elastic constants of these cells, derived by energy minimisation and molecular dynamics strain fluctuation methods, show encouraging agreement with experimental data.
Computational and Theoretical Polymer Science.
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ABSTRACT: Infrared spectroscopy has been used to study the hydrogen bonding conditions in a nematic liquid crystalline poly(ester amide). The results showed that the material formed interchain hydrogen bonding as a result of the inclusion of amide groups in the polymer chains. The hydrogen bonding was found to be stable in the temperature range studied (up to 150°C). However, the studies suggested that the hydrogen bond strength was weaker than in polyamide and did not appear to affect the mechanical properties of the material significantly.
Polymer. 37(26):5817-5824.
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ABSTRACT: Oriented liquid crystal copolyesters are mechanically non-linear at surprisingly low levels of strain when stretched in the orientation direction. As the static strain increases the axial Young's modulus also increases in an asymptotic fashion, the effect being most marked above the glass transition. Several models, each with its own distinct philosophy, have been advanced to account for this behaviour. In order to evaluate critically the various models we have studied the non-linear behaviour of a highly oriented random copolyester film over a wide temperature range. We have also measured the shear modulus of the film and similar fibres (where a wider temperature range can be covered) since this is a key parameter in the various theories. On the basis of comparisons between measured and predicted shear moduli we conclude that the models proposed by Allen and Roche, and by Zhang, Davies and Ward describe the non-linearity best, especially at high temperatures where the units of structure are free to rotate, while the model of Northolt and van der Hout is unsatisfactory. None of the models is applicable well below the glass transition. We also note that all the models are exponential in character, whereas better fits to the data are obtained with other empirical asymptotic expressions.
Polymer.
