Relationship between the molecular structure and properties of polythylene

Marsden, Eric Paul (1975) Relationship between the molecular structure and properties of polythylene. University of Southampton, Doctoral Thesis.

Abstract

The molecular structure of melt crystallized, linear polyethylene has been studied by analysing those features of the-tibrational spectrum which have structural significance. In particular, these are, in the :aman spectrum, the longitudinal acoustic mode, the lattice modes and the region near A%7= 1400cm:1, whilst in the infra-red spectrum the correlation split,CH2 rocking and twisting modes and those bands which arise from vibrations in the disordered regions of polyethylene are of particular interest. The vibrational spectra have been complemented, where appropriate, by data from wide angle X-ray diffraction and differential scanning; calorimetry. Thee assignment of the vibrational spectrum of polyethyier_e is considered. Using a polyethylene sample of single crystal texture, the polarization of bands in the Raman spectrum whose symmetry properties had not been previously unambiguously assigned, were determined. The longitudinal acoustic motion is shown not to couple with the disordered lamellar surface zones, but to be confined, as in the alkanes, to the e all trans sector of the lamellar unit.A study of correlation splitting in the infra-red spectrum of polyethylene allowed the observation of well known structural transitions which had previously only been observed by dynamo-mechanical methods. Tentative suggestions, as to the structural changes which cause such transitions are made. Additionally, bands arising from vibrations which generate a dipole rio,r~ent in the chain axis direction, have been used both as a measure of structural order in, and the thermal expansion of, the lamellar core. Weak bands in the infra-red spectrum, which occur at frequencies more typical of the Raman spectrum of polyethylene are shown to arise from vibrations of all trans sequences within the disordered zones, not from a breakdown of the selection rules of the factor group,, due to the finite dimensions of the lamellar core. The melting and crystallization of polyethylene is studied. The dataa on melting, support the defect model of polymer melting, proposed initially by Reneker, rather than the pre-melting model due to Fischer, Peterlin and coworkers. Melt crystallization of polyethylene has' been investigated by preparing long chain dicarboxy acids by the nitric acid oxidation of polyethylenes of various thermal histories. Using these structures an attempt was made to find the minimum length at which chainfolding occured. Surprisingly, none of these long chain structures folded when melt quenched, but formed non-folded crystallites whose lamellar thickness was identical to that of a 'normal' polyethylene subjected to the same quenching routine. Contrary to the accepted viewpoints, the non-folded lamellar structures exhibited a spherulitic morphology and were readily annealed to regain viii their extended chain form. The deformation of polyethylene by. cold drawing and cold rolling has been investigated. Bands which appear in the infra-red spectrum only after the sample has necked down, are attributed to vibrations on 'tie molecules.'. It is proposed that such molecules have a monoclinic crystalline structure. Structural changes which occur during cold rolling are shown to be qualitatively the same as for cold drawing. It is possible to study lamellar breakdown much more easily on cold rolled samples than on cold drawn samples, as structural changes can be more readily controlled. Annealing has been used to give information on the restraints on. structural re-organization at elevated temperature imposed by the deformation process.The data support, in general terms, the molecular mechanism, for the deformation of polymers proposed by Peterlin_.fI IIt is concluded, that to give a complete insight into structure it is necessary to complement vibrational data with that from other techniques which measure structural order, e.g. X-ray diffraction and differential scanning calorimetry.

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