Lattice-Frequency Studies of Crystalline and Fold Structure in Polyethylene

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The assignment of the 71 cm−1 band in the infrared spectrum of polyethylene to the B1u translational lattice vibration has been confirmed by dichroism studies on an a-axis oriented sample. This permits confident use of the Tasumi-Krimm calculations for the dependence of this frequency of unit-cell parameters. The results of these calculations have been applied to the analysis of the observed lattice frequency differences between odd and even n-paraffins, and it is shown that the frequency shifts are interpretable in terms of changes in unit cell parameters resulting from different methyl end-group packing. The variation of the lattice frequency with degree of branching observed in a series of low-density polyethylenes can be understood on the basis of incorporation of branches in the lattice. The dependence of the lattice frequency in high-density polyethylene on the physical state of the specimen can be correlated with different constraints imposed by the fold regions on the chain packing. These results indicate that the folds must be considered to be ``tight'' rather than ``loose.''