Probing the Natural Draw Ratio of High-Density Polyethylene

Abstract

The Natural Draw Ratio (NDR) is a physical property of semi-crystalline polymers that has important ties to other properties such as strain-hardening and stress crack growth. This thesis examines the Natural Draw Ratio of a variety of commercial high-density polyethylenes. A quantitative understanding is sought for the relationship between the Natural Draw Ratio and molecular weight, polydispersity, and microstructural properties. The variation of Natural Draw Ratio within individual samples due to creep behavior is also assessed. Each high-density polyethylene was compression-molded into sheets using two different thermal histories, and five samples were stamped out of each sheet for uniaxial tension testing. Microstructural parameters such as long spacing and crystal thickness are measured using a combination of Differential Scanning Calorimetry and Small-Angle X-ray Scattering. Significant variation of the NDR due to creep behavior is observed depending on where the NDR is measured relative to the site of neck initiation. The Natural Draw Ratio is generally observed to decrease with increasing weight-average molecular weight (Mw) across both thermal histories. Finally, molecular weight data are combined with microstructural data to approximate tie molecule frequency, represented by r0/d+tc. This parameter, when plotted against NDR, captures the effects of Mw and thermal history, allowing for a greater predictability of NDR if Mw and thermal history are known.