Small-angle neutron-scattering studies of partially labelled crystalline polyethylene

Abstract

Partially deuterated (DPE) and conventional (PE) polyethylene are used to study chain conformations in melt-crystallized blends. The deuterium fraction γ ranged from 0.19 to 0.59, resulting in melting temperature differences ΔT m=1–2°C between PE and DPE. Small-angle neutron scattering (SANS) from molten or quench-crystallized blends yields single-chain radii of gyration R g which apparently depend on γ and are as much as 15% larger than values reported from experiments with fully labelled (γ=1) polyethylene. Quenched PE/DPE blends give Gaussian-like scattering patterns out to q=O.l A ̊ −1 , the limit of these studies. This implies considerably more intramolecular correlations at distances of 10–50 A than derived from a similar experiment employing fully deuterated chains. The present result is thought to be more representative of semicrystalline polyethylene since DPE and PE are nearly perfectly matched. Crystallization by slow cooling creates non-statistical concentration fluctuations which result in excess forward SANS, so a melting point difference of 1°C is still sufficient to trigger `clustering' or ‘segregation’ of PE and DPE. Intensity decays at q > 0.05 A ̊ −1 to the same value as in quenched samples, regardless of crystallinity changing from 0.52 to 0.74. These results are shown to be consistent with dominant ‘near-adjacent re-entry’ during crystallization of polyethylene from the melt.