Structure-property reltionships for solid-state deformed high density polyethylene

One of the fundamental methods of orienting the molecular chains in a semi-crystalline polymer involves the solid-state deformation of the polymer after crystallization has occurred. The purpose of this experimental research was to investigate two important solid-state deformation processes and determine their effects on the structure and properties of high density polyethylene (HDPE). Solid-state extrusion and rolling were the processes studied.

The microstructures of the samples were studied by several methods: wide angle and small angle x-ray diffraction, scanning electron microscopy and thermal analysis. The mechanical properties of the samples were determined by use of tensile tests, tear tests and puncture tests. The results obtained from the mechanical property tests were correlated with the structures present in the samples.

It was found that solid-state extrusion of a tubular HDPE sheet produced a uniaxially oriented material with excellent tensile properties and a unique resistance to splitting. The sheet, while highly oriented, did not exhibit a fibrous texture. A laminate structure was present in the sample, and when torn, the sheet delaminated along the plane of the sheet rather than fibrillating.

Rolling of HDPE sheet also produced a material that was highly uniaxially oriented and possessed a marked resistance to splitting. This material also contained a laminate structure. Detailed x-ray analysis of hot and cold rolled HDPE indicated that the laminate structure was due to a preferred orientation of the chain fold planes in the lamellar structure of the HDPE.

The laminate structure in rolled HDPE was found to develop as an intermediate stage in the transition of the spherulitic structure into a fibrous one. During the formation of the laminate structure, (100)[001] slip occurs in the crystallites. Slip occurs between the chain fold planes, and is responsible for the large change in unit cell orientation seen in cold rolled HDPE. The slip process can also account for the appearance of a monoclinic phase of HDPE during cold rolling.