Free and constrained amorphous phases in polyethylene: Interpretation of 1H NMR and SAXS data over a broad range of crystallinity

Polyethylene (PE) is available in various grades ranging from high-density to very-low density (co-)polymers. The morphology of PE strongly influences its properties and is extensively studied, including the content of phases. Most studies are usually concerned only with the crystallinity and non-critically adopt the classical three-component model of PE, which considers a crystalline (rigid) and an amorphous (mobile) phase separated by an interfacial (semi-rigid) component. We provide evidence that this assignment of the semi-rigid and mobile components can be incorrect at lower PE densities. The phase composition of industrial PE samples with crystallinities from 35 to 82 wt. % was studied using low-field 1H NMR and the characteristic dimensions of the lamellar stacks were measured using small-angle X-ray scattering (SAXS). Based on our results, we propose a new interpretation of PE morphology at crystallinities below 60 wt. %, which considers the rigid crystalline phase, the constrained (semi-rigid) amorphous phase inside the lamellar stacks and the free (mobile) amorphous phase outside the stacks. Interfacial chain segments are also present, but not detected as a separate component. Moreover, experimental data are compared with model predictions based on reconstructed 3D replicas of PE spherulites and an excellent agreement in the phase content is obtained. Our findings are important for the estimation of application properties (e.g., effective diffusivity), which strongly depend on the spatial distribution of the amorphous and crystalline phases.