Investigation of the structure of cold -drawn high -density polyethylene using solid-state NMR

In this dissertation, the cold-drawing response of a commercial high-density polyethylene (HDPE) resin has been studied using solid-state nuclear magnetic resonance (NMR) spectroscopy and variety of other complementary techniques. Melt-crystallized, isotropic samples of the HDPE have been drawn to various extensions at ambient temperature (21°C) and at a relatively slow strain rate (0.0013 s−1). Using solid-state NMR, the first unambiguous evidence for a major morphological component intermediate to the crystalline and amorphous domains in the cold-drawn HDPE microstructure has been found. Employing an ‘inverse 13 C T1 filter’ and other filtering techniques, signals from the various components have been selected and compared. The intermediate component comprises chains of all-trans conformation but with significant disorder in packing. The chains show fast, intermediate-amplitude motions about their axes and are generally aligned with the draw direction, but with a greater distribution of orientation angles relative to crystalline phase. A quantitative 13C NMR procedure has been utilized in the analysis of morphological component composition during cold drawing. In the undeformed material, the NMR-derived composition shows excellent agreement with other common techniques. The mass fraction of the intermediate component has been measured by NMR to be as high as 35% in the cold-drawn HDPE, greater than the contributions from the amorphous domains and monoclinic crystals. The intermediate component content dramatically increases by 240% just after necking, along with a doubling in the monoclinic crystals. At the same time, decreases of about 25% in the total crystalline and amorphous phases occur. A general re-ordering in the microstructure takes place during neck propagation and strain hardening. The total crystallinity rises by about 8%, with a corresponding decrease in the monoclinic crystals (50%) and amorphous material (30%). Based on 1H spin diffusion data, a microstructural model of cold-drawn HDPE is offered. The spin diffusion data identify the intermediate component with tie-molecule bundles that connect small ‘mosaic block’ crystallites (ca. 10–15 nm side dimension) along the draw direction. The bundles consist of about 30 chains and are estimated to be about 2.5 nm in diameter and 3 nm in length