Range Of Draw Ratios Research Articles

Roller drawing of polyoxymethylene

AbstractThe roller drawing of polyoxymethylene (POM) sheets was carried out in the temperature range of 140–157°C. The mechanical properties, the molecular orientation, and the microstructure of the roller‐drawn POM sheets were investigated by means of tensile test, dynamic viscoelasticity, wide‐angle X‐ray diffraction, small‐angle X‐ray scattering, visible dichroic spectrum, electron microscopy, and so on. The Young's modulus and the tensile strength increased with increasing draw ratio up to draw ratio, λ of 14–15. The improvement of the mechanical properties is concerned with structural changes, such as the increase in orientation function in the crystalline and amorphous regions and the formation of taut tie molecules and crystalline bridges in the intercrystallite and interfibrillar regions. In the higher draw ratio range (λ > 15), the increase in Young's modulus and tensile strength was restricted by the formation of interfibrillar microvoids.

Modified affine law for the elastic and optical properties of low density polyethylene

AbstractA unified study of the anomalous mechanical properties and birefringence of low density polyethylene (LDPE) is attempted within the frame work of the rotating element model. The present paper investigates the validity of the affine transformation rule used in the Ward model and suggests a modified affine law which is based on the consideration that the units are only reoriented and, unlike the affine law, are not deformed due to the uniaxial stretching. The relationship between the angles made by the C‐axis of the unit with the drawing direction in the undrawn state (θ) and the drawn state (θ), is deduced as sin θ = f(n) sin θ′. From the birefringence data of different polymers a two parameter relation of the form f (n) = exp [–(n – 1)α/β] is suggested. Using this new orientation function, the agreement of the predicted values of E0 and E90 for LDPE for the entire draw ratio range between temperatures of 60°C to −125°C, is quite satisfactory. An interesting feature is that the abrupt change in the values of the parameters α and β at two different temperatures may be associated with a phase transition of LDPE at an intermediate temperature.

The application of gas barrier and thermal expansion techniques to study structural changes on annealing

AbstractChanges occur in the structure of drawn polyethylene when it is annealed at temperatures close to that of the drawing process. Measurements have been made of the oxygen barrier properties and of the thermal expansion coefficient in the draw direction on samples over a wide draw ratio range both before and after annealing at various temperatures. The results are augmented by density, creep modulus, and shrinkage observations. All samples show a drop in the barrier properties and an increase in the thermal expansion after annealing. However, whereas high‐draw‐ratio samples show relatively small effects, the changes observed at low draw ratios are very large, with some barrier properties even lower than those of the isotropic feedstock. The effects are larger at the higher annealing temperatures. The diffusion and expansion measurements are greatly influenced by the amorphous regions in the polymer; these results are interpreted as a relaxation of internal stresses giving rise to greater accessibility of the intercrystalline regions. Further experimental work is required to exploit these techniques.

Orientation behavior of high‐modulus polyoxymethylene produced by microwave heating drawing

AbstractThe orientation behavior of high‐modulus polyoxymethylene tapes produced by tensile drawing with microwave heating has been investigated over the draw ratio range 10–29. Young's modulus E increases monotonically with draw ratio λ and reaches 55 GPa. The volume fraction of taut tie molecules (TTMs) in the amorphous phase has been estimated by using a Takayanagi model for oriented tapes. The increase in E at draw ratios of less than 10 is mainly due to the increase in crystalline orientation (crystalline orientation function, 0.00 → 0.99). The increase in E at draw ratios of more than 10 is due to the increase both in crystallinity (volume‐fraction crystallinity, 0.84 → 0.95) and in TTM (TTM fraction, 0.14 → 0.40). The maximum Young's modulus obtainable by this method of drawing is estimated to be ca. 72 GPa from the relation between 1/E and 1/λ2.

The mechanical and optical properties of oriented fibres of semicrystalline polymers

The mechanical properties of semicrystalline polymers are very much dependent on the molecular orientation. The orientation function to deal with the elastic moduli data of Hadley et al. (1969) are treated under new aspects. To determine the nature of orientation function from empirical data a relation of the form Sinθ=f (n) Sinθ′ is assumed to describe the change in the orientation angle of a unit from θ′ to θ for the draw ration. Birefringence experimental data are used to find the orientation functionf (n) which is then used to calculate elastic properties. The agreement of the experimental values ofE0 andE90 over the entire draw ratio range is found to be satisfactory for Nylon, low density polyethylene and polyethylene terephthalate. This furnishes strong empirical evidence for the validity of the orientational model for the above substances.

Stress relaxation of polymer melts subjected to large uniaxial tension

The stress relaxation behaviour of two molten amorphous polymers (PMMA and PS), has been investigated. The range of draw ratios extends to about 3.5 for PMMA and 7 for PS. The results have been compared with a modification of the original reptation model. The experimental results are fitted rather well by the theoretical predictions in all the range of tested draw ratios for both the materials used.

Pulsed NMR observation of ultradrawn polypropylene

AbstractProton spin‐spin relaxation times have been measured as a function of temperature for ultradrawn polypropylene with draw ratios λ up to 24. The three relaxation times T2a (the longest), T2i (intermediate), and T2c (the shortest), observed for all the samples, have been ascribed to the relaxations of the amorphous, constrained amorphous, and crystalline components, respectively. T2i and T2a, which reflect the changes in structure and mobility in the noncrystalline regions, decrease with increasing λ; T2i becomes saturated at λ > 9, whereas T2a shows a substantial decrease up to λ = 24. The continued decrease in T2a indicates that the constraint on the amorphous segments keeps increasing up to the highest λ. The associated mass fractions Fa, Fi, and Fc also change with λ. At λ < 9, the increasc in Fi with increasing λ is accompanied by a decrease in Fa, with Fc remaining unchanged. At higher λ, however, Fa is almost constant, and stepwise rises in Fc at about λ = 12 and 24 are accompanied by corresponding drops in Fi. It seems that, in this high draw ratio range, some of the taut molecules are fully extended and are in sufficiently good lateral register to transform into crystalline bridges. This conjecture is supported by the similarity in the λ dependence of Fc and the mass‐fraction crystallinity obtained from the heat of fusion.

The crystal modulus of ultra high modulus linear polyethylenes

Attempts have been made to determine the longitudinal crystal modulus for linear polyethylene by studying the change in the 0 0 2 reflection with load for a series of uniaxially oriented films. The specimens examined covered a wide range of draw ratios, and included a number of ultra high modulus films. In low draw ratio specimens, the assumption of homogeneous stress gave a value for the crystal modulus in good agreement with previous work. The results for the ultra high modulus specimens, on the other hand, were more consistent with continuity of strain, although there were still discrepancies which require further study.

Creep in oriented thermoplastics

Abstract Sheets of oriented low-density polyethylene possessing transverse symmetry were prepared by simple tensile drawing at room temperature. The degree of anisotropy was varied by varying the draw ratio. Classic elasticity theory shows that five constants are necessary to characterize the deformation behavior of an elastic material possessing such symmetry. The time-dependent equivalents of these constants have been determined from the simultaneous measurement of longitudinal and lateral strain during tensile creep of specimens cut at 0, 45, and 90 deg to the draw direction. Special creep apparatus, able to handle small, flexible specimens, was developed for this study. The shear compliance obtained from the tensile creep studies was in good agreement with the value obtained from torsional creep measurements on a 0-deg specimen, for the entire range of draw ratios. The contraction measurements have been used to provide additional evidence for deformation mechanisms suggested by the longitudinal strain...

Mechanical and optical anisotropy in low-density polyethylene

Abstract Polymers develop optical and mechanical anisotropy on drawing. The measurement of the extensional modulus of drawn polymer films in directions O, 45, and 90† to the initial draw direction (called E0, E45, and E90, respectively) gives an indication of the extent of mechanical anisotropy. On the basis of studies on fibers and films it has previously been shown that nylon, polyethylene terephthalate, polypropylene, and high-density polyethylene give a “conventional” pattern of anisotropy; i.e., E0 increases steadily with increasing draw ratio, and at high orientations E0 is much higher than E90 and E45. Low-density polyethylene is known to give an anomalous pattern in that E0 shows a large minimum at low draw ratios, whereas at high orientations E0 is fairly close to E90 and both are much larger than E45. The present study was undertaken to throw some light on the unique behavior shown by low-density polyethylene. A series of oriented low-density polyethylene films was made by drawing isotropic, melt-pressed sheets to a wide range of draw ratios. For each draw ratio, E0, E45, and E90 were measured over the temperature range -125 to 60†C. The pattern of anisotropy changes markedly with temperature, and the following features emerge. First, the minimum in E0 which can be observed at low draw ratios is largest at 20†C and is absent within experimental error at —125†C. Second, considering only the most highly oriented sample, at —125†C E45 and E90 are comparable and about one-third as great as E0., whereas at room temperature E45 is an order of magnitude less than E0 or E90. The pattern of anisotropy observed at room temperature has been explained by a theoretical model in which the polymer is regarded as an aggregate of anisotropic units whose properties are those of the most highly oriented film. The unusual pattern observed on this model is attributed to the very low shear modulus of highly oriented low-density polyethylene. It is shown here that the aggregate model gives reasonable agreement with the experimental data over the whole range of temperatures. The absence of a minimum in E0 at the low temperature is consistent with the relatively larger shear modulus of the highly orierited polyethylene at that temperature. Reasons for the discrepancies between the experimental data and the predictions of the aggregate model are discussed. In addition, some tentative molecular implications of the behavior are suggested, based on the correlation between the low shear modulus and the presence of branch points in the structure.

Mechanical and optical anisotropy in polypropylene fibres

Synthetic textile fibres in the drawn or extended state show considerable optical and mechanical anisotropy. A series of polypropylene fibres was made by drawing an isotropic spun fibre over as wide a range of draw ratios as possible and the extensional modulus, torsional modulus and optical birefringence were measured at each draw ratio. An attempt to explain the development of both the mechanical and optical anisotropy on a theoretical basis, in which the polymer is considered as an aggregate of anisotropic units of structure, has met with limited success. The results suggest that in the early stages of drawing the development of anisotropy can be considered as simply related to the change of orientation of such units. At higher draw ratios, however, it appears that there are changes in morphology and molecular mobility taking place simultaneously which also affect the mechanical properties. A second series was produced by drawing fibres which were already anisotropic; their mechanical properties are compared with those of the first series and show significant differences. In particular there is evidence for the breakdown of the morphological structure even at the lowest draw ratios.

Studies on the drawing of polyamide fibers. IV. Effect of drawing on the degree of longitudinal swelling

AbstractThe longitudinal dimensional change of dried nylon 6 monofilaments of various draw ratios swollen in the atmosphere of 24°C. and 100% R.H. was studied. The degree of longitudinal swelling in equilibrium changes with the conditions of drawing and the draw ratio of the sample; for the samples drawn immediately after spinning, it increases, passes through a maximum near a draw ratio of 2, and gradually decreases as the draw ratio increases; for the samples drawn many days after spinning the maximum point is at a draw ratio near 2.8, while for the samples drawn after heat‐treatment the degree of longitudinal swelling does not change appreciably with the draw ratio of the sample. The rate of longitudinal swelling remains nearly constant or increases slightly in the range of draw ratios of 1–3. It then decreases with the draw ratio at draw ratios higher than 3.

Mechanical and optical anisotropy in polyethylene terephthalate fibres

Polyethylene terephthalate fibres in the drawn or extended state show considerable optical and mechanical anisotropy. In the undrawn state the fibres are essentially isotropic and the degree of subsequent anisotropy in the drawn state may be controlled by varying the amount of extension imposed. A series of polyethylene terephthalate fibres were made with a range of draw ratios, all from one sample of isotropic fibre, and their mechanical and optical properties were measured; the mechanical measurements were mainly those of extensional and torsional moduli whereas the optical properties were those of refractive indices and hence birefringence. An attempt to explain the development of both mechanical and optical anisotropy on a theoretical basis in terms of the orientation of the polymer molecules has met with limited success.

The Anisotropy of the Shear Moduli of Drawn Polyethylene

Low density polyethylene was cold-drawn by simple tensile loading to produce transversely isotropic material. The elastic behaviour of such materials can, according to classical elasticity theory, be defined by five independent elastic constants. Three of these can be derived from Young's modulus measurements in various directions and the remaining two from shear moduli. The Young's modulus determinations have been previously reported, and the present paper concerns the determination of the shear moduli by means of torsional experiments (carried out at room temperature) on rectangular prisms of polyethylene. The draw ratios ranged from undrawn (isotropic) to draw ratio 4.6. The shear modulus governing torsion about the symmetry direction fell from 4.6 × 108 dyn cm-2 for the isotropic material to about one-third of this value for the highest draw ratio. The shear modulus for the other two principal directions increased over this range of draw ratios to about 3 times the isotropic value. All the experimentally determined shear moduli fell within the range calculated from the Young's moduli and the assumption that all deformations occurred at constant volume. Thus, although polyethylene is a viscoelastic material, for the small, elastic (completely recoverable with time) deformations here considered the well-established treatments of classical elasticity theory can be usefully employed. The five elastic compliances were calculated and, for the highly drawn polyethylene, showed a wider numerical range of values than most crystals and polymers previously reported. No molecular interpretation of these results can at present be suggested.