Influence of pressure and crystallization rate on the surface microhardness of high-density polyethylene

Abstract

The indentation microhardnessMH of high-density polyethylene crystallized at different pressuresp and crystallization ratesν _c has been investigated. The results confirm thatMH is an increasing function of lamellar thickness and therefore of densityρ. The rate of increase depends on the crystallization conditionsp andν _c. Crystallization at high pressure leads toMH values considerably higher than those of samples prepared under atmospheric pressure. The results are discussed in terms of compressed amorphous regions contributing to an elastic release after removal of the indenter. For samples crystallized under different pressures the hardness value turns out to be nearly independent of the elastic modulusE. For samples crystallized at different ratesν _c there is a steady decrease of microhardness with increasingν _c (decrease of lamellar thickness) but no simple relationship withE. With higher ratesν _c the appearance of strained tie molecules provokes a rapid increase ofE whileMH decreases further because the crystals become smaller.