J. Clements

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.

An n.m.r. study of absorbed water in polybenzimidazole

Abstract Polybenzimidazole (PBI) is known to absorb 15 wt% water at equilibrium. This sorbed water greatly affects the mechanical properties of this polymer. Both FT i.r. and broadline n.m.r. have been used to study the interaction between water and PBI. The results clearly show that the water in PBI is mobile and therefore does not hydrogen bond to the polymer chain.

A broad line n.m.r. study of oriented poly(vinylidene fluoride)

Abstract Broadline nuclear magnetic resonance measurements have been carried out for the proton resonance in oriented poly(vinylidene fluoride). The spectra in general show two component lines with distinctly different line widths. The broad component shows a high degree of molecular orientation and can be satisfactorily assigned to the crystalline regions of the polymer. The narrow component corresponds to an oriented non crystalline phase which is sufficiently constrained to allow motion about the chain axis only. An appreciable decrease in the value of the rigid mass fraction was observed in both of the samples over the temperature range examined. Our calculations indicate that this could play an important role in the temperature dependence of the pyroelectric response of this material.

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.

Superheating effects on the melting of ultra-high modulus linear polyethylene

Abstract New results are reported on the superheating behaviour of ultra high modulus polyethylenes which have been restrained from shrinkage during the melting programme. Very high dagrees of superheating were observed, which were somewhat dependent on the polymer molecular weight. It is considered that the ultra high modulus polyethylenes do not contain extended chain crystalline material, and that the superheating effects are associated with the presence of a molecular network which can be prevented from relaxing by mechanical restraint.

A broad-line nuclear magnetic resonance study of a vinylidene fluoride/trifluoroethylene copolymer

Abstract Broad-line nuclear magnetic resonance measurements have been performed for the proton resonance in an oriented 70:30 copolymer of vinylidene fluoride and trifluoroethylene. The spectra show two components with distinctly different linewidths. The broad component is identified with the crystalline regions and the narrow component with the amorphous. An appreciable reversible change in crystallinity with temperature was observed, which could significantly contribute to the pyroelectric response of this material. In addition, measurements through the Curie transition show the increasing libration of the chains prior to the transition, which could also contribute to the pyroelectric response.

Crystallite thickness measurements on ultrahigh modulus linear polyethylene: effect of draw temperature

Abstract The effect of draw temperature on the longitudinal crystal thickness L 002 and long spacing L has been determined for a series of drawn linear polyethylene samples. It was found that both L 002 and L increase almost exactly in proportion with increasing draw temperature. The structural significance of the results is discussed, especially with regard to the interpretation of mechanical stiffness.

A broad-line nuclear magnetic resonance study of stress-induced crystallinity changes in poly(vinylidene fluoride)

Abstract Many mechanisms have been proposed to explain the piezoelectric response of poly(vinylidene fluoride) (PVDF), but none of these are adequate to explain why d 32 is so much less than d 31 . Although it has been suggested that this difference could relate to the anisotropy of Poissons ratios, direct measurement of the latter show that this explanation is not correct. The other viable explanation is that there are changes in crystallinity under stress. This possibility is tested in the present investigation using broad-line nuclear magnetic resonance measurements to estimate the change in crystallinity when drawn oriented samples are subjected to a tensile stress applied parallel or perpendicular to the draw direction. The results show that the crystallinity increases for a stress applied parallel to the draw direction and decreases for a stress applied perpendicular to the draw direction. The crystallinity changes give a positive contribution to d 31 and a negative contributon to d 32 , which can explain the relatively low figure for d 32 compared with d 31 .

The dynamic mechanical behaviour of oriented ethylene-hexene-1 copolymer

Abstract A study has been made of the structure and dynamic mechanical properties of a highly oriented ethylene-hexene-1 copolymer. Samples were prepared by tensile drawing, and by drawing followed by annealing at either constant length or unconstrained. Following previous investigations on oriented polyethylene homopolymers, the −50°C plateau moduli were determined, together with estimations of the degree of crystal continuity obtained from X-ray diffraction measurements. Although general correlations were observed between the plateau moduli and the average crystal lengths, our previous intercrystalline bridge model cannot successfully model both homopolymer and copolymer data and it is concluded that for these copolymers there is a contribution to the mechanical moduli from the presence of oriented taut tie molecules as well as crystalline bridges.