Neutron diffraction polyethylene

The extensional dynamic storage modulus E and the loss factor tan 8 for a series of linear polyethylene tapes of different draw ratios are shown in Fig. 30(a) and (b). There are two features worthy of particular note. First, the modulus at low temperatures is about 160 GPa, which is about one half of the theoretical modulus and the maximum value obtained from neutron diffraction and other measurements. Secondly, the a and y relaxations are both dearly visible even in the highest draw ratio material, although the magnitude of tan 5 for the y relaxation reduces with increasing draw ratio. 

The technique can also be applied to the examination of semi-crystalline polymer systems, for example. Fig. 2.29 shows the WAXS and broad Q neutron diffraction from the same sample of d-PVC. In the X-ray scattering data (Fig. 2.29a) the material appears very disordered, whilst the neutron scattering from the same perdeuterated sample of PVC shows a typical melt structure factor similar to polyethylene although the first peak is somewhat sharper reflecting a higher level of order which perhaps is a consequence of the more polar structure (Mitchell 2011). 

Neutron structure analyses were successfully carried out on two crystalline polymers, atactic poly(vinyl alcohol) and polyethylene-d4. Neutron structure analysis supports Bunn s model for atactic poly(vinyl alcohol). Three peaks found on the difference synthesis were assigned to the hydrogen atoms to be associated with the intra molecular hydrogen bond in an isotactic sequence of atactic polymer and the intermolecular hydrogen bonds. Neutron diffraction study clarified the detailed crystal structure of polyethylene, in which the azimuthal angle

molecular plane with respect to the b-axis is 45° within the accuracy of the standard deviation 1°. Furthermore, translational and librational motions of the molecule were estimated. The nature of the static disorder in polyethylene was also clarified. 

The program just described, for Rietveld analyses using generalized coordinates, has been used in the structural analysis of isotactic polypropylene recently undertaken both with x-ray and with neutron powder diffraction data. We believe this analysis (Immirzi, in preparation) to be the first Rietveld analysis of a polymer done from x-ray data. Rietveld analyses of polymers from neutron data have been done but, at least in the polyethylene case reported by Willis and co-workers (15), there was no use of generalized coordinates. 

Neutron scattering from polyethylene is primarily due to hydrogen nuclei, whereas x-ray scattering from polyethylene is primarily due to carbon atoms. Accordingly, the Debye-Wall factor for x-ray diffraction depends on vibrational displacements of carbon atoms. The Debye-... 

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