R. Jakeways

Lattice modulus and crystallite thickness measurements in ultra-high modulus linear polyethylene

Abstract Measurements have been made of the apparent Youngs modulus of the crystalline part of a number of ultra-high modulus polyethylene drawn tapes by observing the change in Bragg angle of the 002 X-ray reflection when the tapes are placed under stress. The variation with temperature has been measured and it is argued that the common limiting value of modulus reached by all samples at low temperature represents the true crystalline modulus and that the room temperature value, which is some 40% lower, is strongly suggestive of a morphology in which a considerable fraction of the material is non-crystalline and is located, from the mechanical point of view, essentially in parallel with the crystalline fraction. Accurate measurements of the linewidths of the 200, 020 and 002 X-ray reflections have been made in order to deduce the mean thickness of the crystalline elements in the three principal directions. The ‘ a ’ and ‘ b ’ thicknesses vary very little with draw ratio but the ‘ c ’ thickness increases at high draw ratios to a value more than twice as great as the (constant) long period determined from small-angle scattering.

Structural order in alkali-degraded PAN as seen in studies of low-temperature thermal conductivity

Abstract The thermal conductivity, κ, of polyacrylonitrile (PAN), subjected to varying degrees of degradation, has been measured in the temperature range 2–100 K as a means of studying the structural order of the samples. Although, at higher temperatures, the variation in κ throughout the range of specimens is only ∼ 30%, at 2 K there is an order of magnitude increase in conductivity between the undegraded and most highly degraded samples. Such a change is indicative of a pronounced decrease in crystallinity, and this interpretation is supported by estimates of crystallinity as seen through X-ray analysis.

Hysteresis of the stress-induced crystalline phase transition in poly(butylene terephthalate)

Abstract The stress-induced crystalline phase transition in poly(butylene terephthalate), (4GT), has been investigated by X-ray diffraction and laser Raman spectroscopy. A well defined stress hysteresis effect has been demonstrated and a continuum theory has been developed which satisfactorily describes the temperature dependence of the phenomenon.

The crystal modulus and structure of oriented poly(ethylene terephthalate)

Abstract Crystal modulus measurements have been made on a number of samples of drawn poly(ethylene terephthalate) tape. The apparent crystal modulus was found to vary systematically with the degree of tilt of the crystalline c axis from the fibre axis. Using this fact, a value of 110 ± 10 GPa was deduced for the intrinsic crystal modulus in the chain direction. It is considered that this value is more realistic than previously reported values since specimen morphology has been taken into account carefully in a systematic fashion.

The existence of a mesophase in poly(ethylene naphthalate)

Poly(ethylene naphthalate) (PEN) is a polymer with potentially useful industrial properties. During a study of the morphology and properties of drawn PEN fibres it became apparent that an ordered structure existed in some specimens that was not the well-known crystalline form. This structure is considered to be a mesophase in which substantial lengths of individual chains are fully extended but do not pack laterally in crystalline register. The existence of this intermediate phase is strongly dependent upon processing conditions and could have implications for the properties of commercially produced fibres, since it appears to be stable and not easily converted to the crystalline form at elevated temperatures.

The existence of a stable β form in oriented poly(butylene terephthalate)

Abstract The existence of stable β form crystallites in poly(butylene terephthalate) has been investigated. Wide angle X-ray measurements suggest that such crystallites are only stable provided that they are of comparatively small dimensions and that they are then poorly connected to the bulk polymer. A simple theoretical model is presented to explain the stability of the crystals in terms of the contributions to the free energy from both the crystallites and the surrounding amorphous material.

The morphology of highly drawn polyoxymethylene and evidence for a temperature dependent crystal modulus

Abstract Measurements of the crystalline modulus of highly oriented polyoxymethylene have been undertaken for a range of different samples over the temperature range from −150°C to 50°C. The crystalline modulus measurements have been combined with measurements of mechanical modulus, and X-ray structural measurements. A consistent interpretation of all the results has been given in terms of a three phase model in which an oriented amorphous phase acts in parallel with a parallel lamellar structure of crystalline and amorphous phases in series. Clear evidence for the temperature dependence of the crystalline modulus has been obtained from highly annealed samples which show the simpler two phase parallel lamellar structure.

A study of the effects of annealing on the mechanical stiffness and structure of ultra high modulus linear polyethylene

Abstract Measurements have been made of the changes in Youngs modulus and structure which occur when ultra high modulus linear polyethylene is subjected to different annealing treatments. The structural measurements include the determination of the linewidths of the (200), (020) and (002) X-ray reflections, and the long period obtained from small angle X-ray scattering. It has been found that there is an excellent correlation between the Youngs modulus and the longitudinal crystal thickness. This correlation holds for all annealing treatments including those which involve annealing followed by rapid quenching which leads to an initial fall in modulus followed by a significant rise, as reported by previous workers.

Tensile drawing, morphology, and mechanical properties of poly(butylene terephthalate)

The concept of the drawing of molecular network has been employed to derive a total network draw ratio from the combination of the two deformations occurring in the production of poly(butylene terephthalate), PBT, fibers by the consecutive processes of melt spinning and cold drawing. The mechanical properties of PBT can then be more readily explained in terms of increases in this total network draw ratio. However, the preorientation and crystallization that occurs in the melt-spinning process can occur at different strain rates and temperatures, depending on the wind up speed employed, on the extensional viscosity of the polymer, and on the variation of the extensional viscosity with temperature. Therefore, for polymers such as poly(butylene terephthalate), which can exist in two crystalline forms, the morphology of the final drawn fiber might be expected to depend on the first melt-spinning stage of the process as well as on the total network draw ratio. In this work, density, birefringence, mechanical measurements, and WAXD measurements, which have been made on the melt-spun fibers and on the drawn fibers, are described. Small differences in some of the drawn yarn mechanical properties at the same overall network draw ratio are related to the crystallinity and in particular to differences in the proportion of the α and β phases present in the drawn yarn. These in turn are related to differences in the temperature and stress during melt spinning and drawing.