Vickers microhardness related to mechanical properties of polypropylene

Abstract

Vickers micro-indentation of polymers is a method that yields information about their microstructure and some aspects of their mechanical behaviour. In the case of polyethylenes of different types, a monotonic increase of the microhardness with increasing degree of crystallinity has been observed and it has been proved that the microhardness is related to the elastic modulus and the yield stress [1, 2]. Microhardness has been also shown to be an adequate technique for establishing precisely the glass-transition temperature of isotactic polypropylene [3] or for studying the delayed elastic recovery of polyethylenes [4, 5]. Moreover, microhardness is a suitable tool for investigating oriented polymers, because it has been demonstrated that the microhardness anisotropy can be related either to changes in the preferred orientation in injection-moulded poly(4-methyl pentene-1) [6] or to birefringence and draw ratio in stretched poly(ethylene terephthalate) [7]. The dependence of the microhardness on structure and the relationships between the microhardness and the mechanical properties have not been analysed in polypropylene. For this reason the main purpose of this work was to present the results obtained for this polymer and to discuss these results in terms of structural parameters. A series of nine commercial isotactic polypropylenes whose nominal densities and melt indices ranged from 0.897 to 0.904gcm -3 and from 0.82 to 29.7, respectively, were studied. The materials were placed between the plates of a Collin press, heated to 5°C above their melting temperatures and pressed at 1.5 MPa for 10min. The mouldings were quenched in ice-water and fihns about 200/~m thick were obtained. The densilLy of the samples was measured by using a gradient column filled with ethanol-water which was calibrated with glass floats, and the degree of crystallinity was calculated by means of the twophase model using the values vj = 1.142cm3g -I and vc = 1.070cm3g L for the specific volumes of the amorphous and crystalline phases, respectively [8]. Dumb-bell shaped specimens with a gauge length of 2cm and a width of 0.4cm were cut out of the moulded sheets. The drawing of the samples was carried out on an Instron tensile testing machine at 20 ° C and at a strain rate of 0.25 min- ~. The moduli of elasticity were calculated from the initial slope of the stress-strain curves and the yield points were considered to occur where the stress went through a maxi