Chain trajectory in solution grown polyethylene crystals: correlation between infra-red spectroscopy and small-angle neutron scattering

Infra-red (i.r.) measurements of the CD₂ bending vibration have been made for polyethylene single crystals of mixed isotopic composition under a wider range of conditions than hitherto. A comparison with data for mixed paraffin crystals shows that the polymer molecules exist in a compact conformation, consistent with the ‘superfolded’ structure which has emerged from small angle neutron scattering (SANS) measurements. However, the correlation splitting of the CD₂ bending vibration, a prominent feature of previous studies, is not generally observed. At ‘guest’ concentrations of 1%, SANS data indicate minimal isotopic fractionation, and it is only at such low concentrations that the band-shape of the CD₂ bending vibration primarily reflects intra-chain effects due to chain conformation. Previous SANS data at wider angles ($\text{10}{}^{\mathrm{?1}}\text{> s > 10}{}^{\mathrm{?2}}\text{A}\text{?}{}^{\mathrm{?1}}$) lead to an estimate of about 0.5 for the fraction of stem sites along a folded ribbon occupied by a single molecule. A study of the i.r. peak broadening leads to a similar value for the local concentration.