X-Ray Determination of the Disorder in Poly(oxymethylene) Crystals

Abstract

S:Ruland's concept of the isotropic disorder function is used to estimate the disorder parameter and degree of crystallinity in r-ray irradiated and solution-grown poly(oxymethylene)s. The degree of crystallinity increases with increasing irradiation dosage, but the disorder parameter remains constant. The disorder is attributed to thermal motions, the root mean squared amplitude being about 0.2 A at room temperature. The increase in crystallinity is mainly caused by degraded molecular chains in the disordered region. The disorder parameter of the solution-grown crystal is larger than that of r-ray irradiated samples and decreases with annealing. Increase in the disorder parameter in the solution-grown crystal may be caused by the lattice imperfections in the crystal. In both samples the degree of crystallinity decreases above 70°C, and at 120°C it is 5% less than the value at room temperature. In this temperature range, partial melting at the surface layer of the crystal may take place.KEY WORDS X-Ray I Crystallinity I Lattice Disorder I Thermal Motion I Irradiation I Solution-Grown Crystal 1 Annealing I Partial Melting I Poly(oxymethylene) I Ruland 1 ' 2 has developed an X-ray method of crystallinity determination which takes into account the diffuse scattering due to thermal motions and lattice imperfections in a crystal. Using his method, the degree of crystallinity and the disorder parameter in polyethylene/· 3 • 4 polypropylene, 1 and other crystalline polymers 2 • 5 • 6 have been measured. It has been found that the disorder parameter is predominantly determined by the thermal motions in linear polymers in which the interchain attraction is due to van der Waals force only. 1 The influences of crystallization conditions and the effect of r-ray irradiation on the disorder in a crystal and on the degree of crystallinity are investigated. The disorder parameter and degree of crystallinity vary with the r-ray irradiation, crystallization conditions and the annealing. The temperature dependence on the disorder parameter and on the degree of crystallinity is also shown. The mechanism of those variations is discussed in terms of thermal motions and lattice imperfections.Recently, Davis, et al.,7 reported that a unit ·cell dimension of polyethylene is not unique but varies systematically with crystallization ·conditions, annealing and deformation. It is interesting to investigate whether the disorder parameter also varies with such treatment.In the present study, r-ray irradiated and solution-grown poly(oxymethylene)s (POM) were used. It would be useful to measure the disorder parameter of the solution-grown crystal, since the disorder parameter has not previously been discussed in respect of samples crystallized from solution. x., =K"-"-0 ----

Study of Glass Transition and Partial Melting in Melt-crystallized Poly(oxymethylene) by X-Ray Small-angle Scattering

“…In a previous paper, 24 we reported that the degree of crystallinity increases by r-ray irradiation. As shown in Table I, the density also increases by r-ray irradiation.…”