Microindentation hardness of oriented chain‐extended polyethylene

The Knopp and Vickers microindentation hardnesses of oriented chain‐extended polyethylene (cold‐drawn, then hydrostatically annealed at 5·35 kbar) have been measured to establish correlations with some of the morphological features previously investigated (lamellar thickness, orientation). The irreversible deformation increases with time under load and tends towards a characteristic limiting value for each annealing temperature. This limiting value of microhardness is an increasing function of annealing temperature, TA, and consequently of chain extension, 1. A conspicuous feature of the investigated oriented polyethylene samples is the clear anisotropy exhibited by the indentation pattern. The Vickers microhardness is a maximum when the indenter diagonal lies parallel to the fiber direction and minimum when normal to it. The opposite occurs for the Knoop case. This contrary behavior is due to the shape difference between the two indenters coupled with the high anisotropy of the material. This behavior supports morphological experiments and is a further illustration of the value of this technique in polymer textural studies.