X-ray diffraction studies on reverse-annealed polyethylenes

Linear low and high density polyethylene sheets were compression molded and crystallized at a 5-10 degreesC/min cooling rate. Parts of the sheets were annealed at different temperatures up to 2 degreesC below the melting temp erature. The small angle X-ray scattering (SAXS) and the wide angle X-ray s cattering intensities of the annealed samples were studied. SAXS intensitie s showed particle scattering with a bimodal size distribution. The estimate d radii of gyration were 15-17 nm and 5-7 nm, respectively. The crystallini ty and the radius of gyration increased slightly with increasing annealing temperature for some samples, others did not show any change. No peaks char acteristic of intercorrelated lamellar crystallinity in the SAXS intensitie s developed during the annealing. The original broad peak of high density p olyethylene disappeared from the SAXS recordings on annealing. The length o f the perfect chain versus melting temperature was calculated by the Thomso n-Gibbs formula and Flory's concept of melting temperature depression where methyl groups and tertiary carbon atoms at the branches were regarded as s econd components (solvent). Linear relationships were found for both cases. Experimental data for a linear low density polyethylene obtained from the literature were in between the two functions. A lamellar model of crystalli zation corresponding to the data is proposed. (C) 2001 John Wiley & Sons, I nc.