Viscoelasticity of oriented poly(ethylene terephthalate)

Time–temperature superposition can be successfully applied to both the stress relaxation and dynamic mechanical properties of oriented PET fibers. Two curves result; one is the time dependence of the modulus at constant temperature, while the other is the shift, log aT, of this curve along the time scale as a function of temperature. This temperature dependence is less than that for both unoriented PET and typical amorphous polymers above T_g. It is about the same as that for oriented nylon 66 and unoriented glassy poly(methyl methacrylate). The isothermal modulus has the same time dependence as that of the unoriented PET; however, it is a factor of 3.3 larger. The modulus curve is almost identical in both shape and magnitude with that of oriented nylon 66. However, a temperature of 82°C. is required to place the viscoelastic dispersion region of PET at the same time scale as nylon 66 at 25°C. This temperature increase is the major difference in viscoelasticity between these two oriented polymers.     

Chain folding in oriented poly(ethylene terephthalate)

Drawn poly(ethylene terephthalate), PET, yarns have been heated for 1 min in silicone oil. The resulting samples were studied by x-ray diffraction and mechanical properties were measured. The results suggest that drawn PET consists of highly extended molecules essentially parallel to one another, with few folds present. On heating, chain folding occurs. This model is very similar to that proposed by Dismore and Statton for drawn nylon 66 yarns.     

Morphology of uniaxially oriented poly(ethylene terephthalate)

The morphological character of uniaxially oriented poly(ethylene terephthalate) (PET) films was investigated as a function of draw ratio. Dynamic mechanical, infrared, and crystallite-size measurements were made on the samples. In addition, selective degradation experiments and molecular weight determinations were employed. The dynamic mechanical measurements indicated a sharp decrease in irregular folds for draw ratios of 3.0 and higher, which also coincided with the essentially complete disappearence of regular folds (from the 988 cm^?1 band in the infrared spectra) in unannealed samples. Infrared studies of drawn samples annealed under different conditions gave evidence in support of a structure in which the chains are stretched out. Apparent crystallite-size measurements showed a sudden increase in length of the crystals in the direction of the draw beyond a draw ratio of 3.0. Molecular weight measurements showed a large increase in average chain length in the residue after selective degradation of amorphous material and folds; undrawn and slightly drawn samples gave a much lower M _n. Based on these observations, it is postulated that for higher draw ratios and present drawing conditions, the crystals are of the straight chain type, somewhat similar to the fringed-micelle crystal concept.     

Towards first-principles modelling of the mechanical properties of oriented poly(ethylene terephthalate)

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.     

Thermal shrinkage of oriented semicrystalline poly(ethylene terephthalate)

The shrinkage of commercial oriented poly(ethylene terephthalate) filaments was studied within the framework of the kinetic theory of rubberlike elasticity. Previous workers had found that the shrinkage and optical behavior of amorphous polymers could be satisfactorily explained in terms of this theory. Such an analysis is now applied to semicrystalline samples of moderate and high draw ratios (from 2× to 6×). It was found in this work that the thermal shrinkage force behavior as well as the optical anisotropy as a function of stretch can be explained in terms of the theory of rubberlike elasticity, if the following reasonable assumption is made: the average number of statistical segments per network chain in the noncrosslinked sample increases as a function of the draw ratio. A possible mechanism for such behavior is the relaxation of some of the chain entaglements due to the strain imposed externally on the fiber.     

Directional anisotropy of dielectric β-relaxation in oriented poly(ethylene terephthalate)

The dielectric β-relaxation in oriented poly(ethylene terephthalate) was investigated over wide ranges of frequency and temperature, with the electric field applied at inclinations of 0°, 45°, and 90° to the draw direction. Pronounced directional anisotropy is observed over the entire range of temperature and frequency. With the external field parallel to the draw direction the dielectric loss is considerably smaller than the value obtained with the field normal to the draw direction. The value obtained with the electric field at 45° to the draw direction is intermediate between the other two. On the other hand, the activation energy is largest for 0° inclination and smallest for 90° inclination. It is suggested that motions of the dipoles involve localized rocking of the molecular chain backbone, particularly when the external electric field is parallel to the chain direction.     

Kinetics of structural relaxation of poly(ethylene terephthalate)

The results of an experimental study of the kinetics of structural relaxation of amorphous poly(ethylene terephthalate) are reported. Samples were prepared by ultraquenching the melt on rotating stainless-steel discs. Two types of measurements by differential scanning calorimetry were made: (1) the dependence of the “fictive” (or “structural”) temperature T_f(q^?) introduced by Tool, on the cooling rate q^? and (2) the dependence of the glass transition temperature T_g on the heating rate q⁺. In this way the value x = 0.47 was obtained for the dimensionless parameter proposed by Narayanaswamy.     

The transport of oxygen through oriented poly(ethylene terephthalate)

The transport of oxygen through oriented PET at 25 and 60°C has been studied using the dynamic diffusion method. It did not prove possible to determine the diffusion and solubility coefficients with reasonable accuracy, and only the permeability coefficients are discussed in terms of the structure of the samples. The results suggest that the lowering of the oxygen permeability on drawing is related to the production of additional material with the trans conformation of the glycol residue, either in crystallites or in amorphous regions, rather than solely to the overall development of orientation or crystallinity.     

Oxygen and water-vapor diffusion through biaxially oriented poly(ethylene terephthalate)

The gas barrier properties of a wide range of biaxially oriented polyethylene terephthalate films have been investigated. The permeability and diffusion coefficients for oxygen and water vapor transmissions were determined for films prepared by both simultaneous and sequential biaxial stretching. The effect of annealing was also studied and a comparison made with previous results for uniaxially oriented films. It was found that the permeability correlated well with the density of the sample, but that the dependence on the gauche/trans conformer ratio shown for uniaxial material was not so clear. A good empirical correlation was also obtained between permeability and a proposed measure of molecular orientation obtained from refractive index measurements. A more detailed interpretation, based on infrared measurements of orientation, showed that there is a systematic reduction in permeability as the planes of the terephthalate residues become parallel to the film surface.     

Effects of thermal treatment of biaxially oriented poly(ethylene terephthalate)

Two distinguishable effects of thermal exposure of biaxially oriented poly(ethylene terephthalate) (PET) have been observed in the temperature range from room temperature to 140°C. Upon heating above the glass transition temperature T_g of the film an irreversible shrinkage of a few percent occurred with a concomitant decrease in the rate of creep. Some loss of orientation in the noncrystalline phase with an attendant slight increase in density is believed to be responsible. Since the film was anisotropic in its plane, different amounts and rates of shrinkage were observed along with differing thermal expansion coefficients in various directions relative to the primary optic axis. Upon cooling the 50% crystalline PET from above T_g to lower temperatures, reversible “physical aging” was observed. Creep rates were found to decrease with the residence time below T_g. As with purely amorphous polymers, the effects of the aging are removed by heating the specimen above T_g where the density of the amorphous phase achieves equilibrium values.     

The three-phase structure and mechanical properties of poly(ethylene terephthalate)

The relation between the mechanical properties and the microstructure of PET has been investigated, combining results from WAXS, SAXS, FTIR, DSC, and uniaxial compression tests. The rigid amorphous fraction in the PET was explicitly taken into consideration in interpreting structure–property relations. WAXS results prove that glass crystallized PET with a high volume fraction of rigid amorphous material and small crystal size, on uniaxial compression shows a considerable loss in crystalline fraction. FTIR results in combination with these WAXS results suggest that during this loss in crystallinity, short-range conformational order is retained, while long-range structural order is lost. At the same time, material with small crystals and a high amount of rigid amorphous material was found to show unexpectedly low yield stress. It is concluded that in the interpretation of these phenomena it is necessary to take the three-phase structure of PET, including the rigid amorphous fraction into account. This is expected to hold for other semicrystalline polymers, where a rigid amorphous fraction is prominent, such as PHB, PBT, PEN, PEEK, etc. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2092–2106, 2004     

Anisotropy of dielectric relaxation in crystal form II of poly(vinylidene fluoride)

The anisotropy of the crystalline relaxation (α relaxation) in oriented poly(vinylidene fluoride) in crystal form II has been studied. The dielectric increment Δε is analyzed on the basis of the two-site model. A linear relation between Δε/χξ and cos²θ is obtained, where χ is the degree of crystallinity, ξ is the ratio of the internal field to the applied field, and θ is the angle between the applied electric field and the molecular axis. The dipole moment changes direction only along the molecular axis in the relaxation in crystal form II; the molecular motion cannot be explained by chain rotation around the molecular axis. Possible models for the α relaxation are proposed: change in conformation with internal rotation can occur in the crystalline chains, and defects in the crystalline regions play an important role in the α relaxation.     

Study of the dielectric β-relaxation in poly(ethylene terephthalate) and ethylene isophthalate terephthalate copolyesters

The dielectric β-relaxation in poly(ethylene terephthalate) and ethylene isophthalate terephthalate copolyesters with various ethylene isophthalate contents was investigated over a wide range of frequencies. Isothermal spectra and normalized plots show that the dielectric β-relaxation is characterized by the molecular structure between the aromatic rings. The equilibrium polarizability of the motional segment, which was estimated from the dielectric increment obtained by the best fit of the Havriliak-Negami equation to the Cole-Cole plot, indicates that torsional vibrations of the main chain are correlated over a length corresponding to one monomer.     

Crystallization in oriented poly(ethylene terephthalate) fibers. I. Fundamental aspects

The nature of crystallization- and mobility-induced changes during annealing of melt-spun poly(ethylene terephthalate) precursor fibers of a range of orientations has been examined. The kinetics of crystallization and the accompanying orientational changes have been studied under conditions of constant, low tensile stress, with the accompanying dimensional changes and under a constraint against shrinkage in length, with the stress developed being monitored. The effects of precursor orientation and externally imposed constraints on the course of the fundamental crystallization and orientational relaxation processes are revealed. Oriented crystallization has been shown to have a significant effect on the stress developed and on the dimensions of oriented precursor fibers, with a strong tendency to spontaneously extend as a consequence of the reorientation of crystallizing segments predominantly along the preferred fiber direction. The sequence in which crystallization and major orientational relaxation, if any, occur is found to have a profound effect on the structure and thus the deformability of oriented fibers after annealing above the glass transition temperature.     

Mesophase structures in poly(ethylene terephthalate), poly(ethylene naphthalate) and poly(ethylene naphthalate bibenzoate)

Films of poly(ethylene naphthalate) (PEN) and poly(ethylene naphthalate bibenzoate) (PENBB) have been drawn under a variety of conditions of temperature and strain rate to determine the conditions under which a nematic-like mesophase structure can be produced. In PEN the combination of low temperature and high-strain rate encourages mesophase formation, while in PENBB the mesophase was formed under all conditions where it proved possible to draw the material at all. A molecular modelling study of the mesophase in PEN and in poly(ethylene terephthalate) (PET) offers possible structures for the mesophase and showed that the mesophase structure could be stable once formed © 1997 John Wiley & Sons, Ltd.     

Photolysis of poly(ethylene terephthalate)

The photolysis of poly(ethylene terephthalate) films was studied in vacuo with light of wavelengths 2537 and 3130 A. A very stable filter system which cuts out the 3025 A. line was developed to isolate 3130 A. from a mercury spectrum. Despite the fact that the penetration of 2537 A. light was limited to a depth of a ca. 10³ A. whereas 3130 A. light was more uniformly absorbed it was possible to demonstrate that the quantum yields for CO and CO₂ formation were in agreement for the two wavelengths. Quantum yields for fractures and crosslinks were estimated by sol-gel analysis. An absorption maximum which develops near 13 μ after exposure of poly(ethylene terephthalate) to light or γ-rays was attributed to the formation of groups formed by elimination of CO and CO₂. ESR spectra for trapped radicals were tentatively assigned to the components p-C₆H₃· and ·O? CH₂? CH₂? . It is suggested that the former radicals combine to form crosslinks. Quantum yields (× 10⁴) with 3130 A. light are: CO, 6; CO₂, 2; crosslinks, 5.5; trapped radicals, 1.5; With 2537 A. light, quantum yields are: CO, 6–9; CO₂, 2–3; the network formed was not characterized as to crosslinks and fractures; trapped radicals were observed to exist but not determined.     

Segmented block copolymers of poly(ethylene glycol) and poly(ethylene terephthalate)

A new series of segmented copolymers were synthesized from poly(ethylene terephthalate) (PET) oligomers and poly(ethylene glycol) (PEG) by a two‐step solution polymerization reaction. PET oligomers were obtained by glycolysis depolymerization. Structural features were defined by infrared and nuclear magnetic resonance (NMR) spectroscopy. The copolymer composition was calculated via ¹H NMR spectroscopy. The content of soft PEG segments was higher than that of hard PET segments. A single glass‐transition temperature was detected for all the synthesized segmented copolymers. This observation was found to be independent of the initial PET‐to‐PEG molar ratio. The molar masses of the copolymers were determined by gel permeation chromatography (GPC). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4448–4457, 2004