Microhardness and mechanical anisotropy of elongational flow injection molded polyethylene

High-strength linear polyethylene was prepared by injection molding using an axial elongational flow component during the filling process. Microhardness (MH), tensile and bending stress ([sgrave]), optical birefringence (Δn), differential scanning calorimetry (DSC), and shrinkage measurements were used to characterize the changes in microstructure, molecular orientation, and mechanical properties, occurring throughout the range of processing temperatures used (Tm=140–200°C) It is shown that molecular orientation, mechanical strength, and indentation anisotropy (ΔMH) are notably lower at the surface and in the core of the moldings while they are much higher between both zones. A conspicuous parallel variation of ρ, ΔMH, and Δn with Tm, showing a well-defined maximum at Tm∼150°C, is detected within the core of the moldings. Orientation and local elastic recovery measured by MH at the moldings surface do not, however, depend upon Tm. Results are discussed in terms of the structure of shish-kebab fibrils, oriented along the injection direction. The relative amount of shish-fibers as revealed from the contribution of the two appearing melting peaks, is shown to determine the average value of AMH. The shish-kebab structure is most conspicuously shown at low processing temperatures.