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.     

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.     

Molecular weight segregation on surfaces of polyethylene blended films as estimated from nanoscratch tests using scanning probe microscopy

Nanoscratch tests using scanning probe microscopy (SPM) were performed on films prepared from two polyethylene (PE) materials polymerized by using a metallocene catalyst system with different molecular weights (MWs). Blended samples were prepared by dissolving both PE materials at various ratios in hot p-xylene. The pure and blended samples were compression molded into films at 180 degrees C for different holding times in the molten state. The results of SPM nanoscratch tests with an applied load of 30 nN indicated that the lower-MW surface could be easily plowed with wear debris but the higher-MW surface was less deformed. However, the deformation pattern of the blended film surface was similar to that of the lower-MW surface. These results suggest that MW segregation occurs during holding in the molten state as lower-MW components rise to the film surface.     

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.     

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.     

Surface characterization of laser-treated poly(ethylene terephthalate) by optical profilometry and scanning tunneling microscopy

Characteristic changes in the surface topography due to UV laser treatment are of high significance for the determination of material destruction thresholds and surface structure development. E.g. irradiation of poly(ethylene terephthalate) (PET) film with pulsed UV laser light of 248 nm modifies the smoth surface of the polymer into a well oriented structured surface. The development of these structures were studied with scanning tunneling microscopy (STM) and high resolution optical profilometry. Three dimensional data of the surface were taken from the samples after each laser pulse. A change of topographic data was found in relation to fluence and number of pulses applied. © 1993 John Wilcy & Sons, Inc.     

Polarized reflection spectrum and molecular orientation in uniaxially drawn poly(ethylene terephthalate)

The polarized electronic spectrum of oriented poly(ethylene terephthalate) (PET) sheets was obtained from the specular reflection spectrum using the Kramers-Kronig relationship. The surface orientation function of drawn and drawn/annealed PET sheets was determined from the dichroic ratio of the second π* ← π transition observed at 41,000 cm^-1. The bulk orientation functions in the crystalline and amorphous regions were evaluated from wide-angle X-ray diffraction and sonic modulus measurements. On annealing of drawn PET sheets, the crystalline orientation and crystallinity were much improved, but the amorphous orientation function showed a minor decrease. The overall molecular orientation in the surface of the drawn PET sheet was shown to be approximately equivalent to the molecular orientation in the bulk.     

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.     

XPS analysis of the poly(ethylene terephthalate) film grafted with acrylamide

Graft polymerization of acrylamide (AAm) was performed onto the surface of a poly(ethylene terephthalate) (PET) film with the simultaneous UV irradiation method but using no photosensitizer and without degassing. To examine whether polyacrylamide (PAAm) was introduced into the bulk place of PET film by the surface graft polymerization, an x-ray photoelectron spectroscopic (XPS) study was performed on the PAAm-grafted PET films. The distribution of grafted PAAm chains on and in the PET films was estimated from the PAAm/PET ratio calculated from the XPS spectra of PET films with different amounts of grafted PAAm. The results clearly demonstrate that graft polymerization has actually occurred not merely on the outermost surface but also within the thin surface region of the PET film. In addition, the XPS analysis revealed that the PET component was always present in the grafted surface region by a mole fraction of 0.1 to 0.05 even when the amount of PAAm grafted was larger than 10 μg/cm².     

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.     

Flow characteristics of poly(ethylene terephthalate) deduced from pressure drop measurements

A unique capillary die was designed which made possible the measurement of extrusion pressure at various locations along the capillary length. Entrance pressure drops, exit pressures, and other rheological characteristics were determined for the flow of poly(ethylene terephthalate) through this extrusion apparatus. The effect of die entrance angle, extrusion temperature, throughput rate and polymer molecular weight were considered. Two samples differing in molecular weight exhibited power-law behavior at shear rates below 1000s^?1. The entrance pressure drops and exit pressures were observed to increase with increasing molecular weight; furthermore, at a specific temperature, both-increased with increasing shear rate. The values for entrance pressure drop obtained using Bagley analysis were consistently higher than those obtained from direct measurements.     

Anisotropy of mechanical and dielectric relaxation in oriented poly(ethylene terephthalate)

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.     

Investigation of transient species in pulse-irradiated poly(ethylene terephthalate) film

A pulse radiolysis study of poly(ethylene terephthalate), PET, film has been carried out with the main aims of investigating charge trapping. In PET, pulse radiolysis gives electron-positive hole pairs. Both charges can be stabilized by the reaction with polymeric matrix. In the first step, the PET radical anions and cations are formed (transient absorption maxima at ∼ 370 and 530 nm). During the second step, the electron is transferred from a PET radical anion to an ester group, followed by formation of an ester radical anion (transient absorption maximum at 430 nm). The recombination of these ionic species leads to an excited state formation observed during and after the 1-μs pulse. Spectral distribution of luminescence observed for pulse-irradiated PET (emission bands at ∼ 340, ∼ 370, and 400–410 nm) was similar to the one obtained for photoexcited PET. The detailed mechanism of ionic reactions in PET is proposed and discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2853–2862, 1999     

Effect of iodine on small-angle X-ray diffraction from drawn poly(ethylene terephthalate) film

The effect of iodine sorption on the small-angle x-ray diffraction intensity is discussed in terms of the distribution of iodine sorbed in the amorphous regions of polymers. The change in the intensity of the peak corresponding to long periods is measured as a function of the iodine uptake in drawn, and annealed poly(ethylene terephthalate) films. A linear relation with slope K is observed between the square root of relative intensity and iodine uptake. The slope K varies with annealing conditions. Theoretical values calculated for models of the distribution of iodine in amorphous regions, are compared with the observed K. Results can be interpreted on the basis of a “concave” distribution of iodine, i.e., with sorption sites more numerous near a crystal surface than in the interior of an amorphous region.     

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.     

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.