Investigation of the molecular structure in drawn polyethylene based upon nmr fourth moment measurements

Measurements of the anisotropy in the nuclear magnetic resonance fourth moment for proton resonance of a number of drawn polyethylene films are reported. A general expression is derived for the fourth moment of a partially drawn polymer in terms of a set of coefficients which describe the molecular chain orientation, on the basis of a model in which the drawn film is regarded as an aggregate of units aligned by the drawing process. The orientation coefficients have the form IMG1 where Δ is the angle which a typical unit makes with the draw axis. The IMG2 and IMG3 coefficients determined from fourth moment considerations are in good agreement with values derived in an earlier paper from second moment data, and these coefficients therefore provide a firm basis for the description of the distributions of molecular chains in the drawn polymer. Higher order orientation coefficients are estimated on the basis of a smoothing procedure which suggests itself from x-ray diffraction measurements on the drawn polymer.

The distributions predicted by the combined nmr and smoothed orientation coefficients are consistent with distributions obtained from x-ray diffraction data. The results confirm that the anisotropy of the nmr signal can be attributed to preferred orientation of the crystalline regions of the polymer. The predicted distributions are very non-affine in character.

Finally there is reasonable agreement between the measured mechanical anisotropy and that predicted on the basis of the aggregate model using both nmr and x-ray diffraction data.