Correlation of hardness and microstructure in unoriented lamellar polyethylene

Abstract

The local deformation of the lamellar microstructure of isothermally melted crystallized u noriented polyethylene has been investigated using microindentation hardness (MH). The polymer can be visualized as a composite material consisting of hard and weak elements. The former, the lamellae, are considered to consist of mosaic blocks with liquidlike lattice distortions (paracrystallites). The latter are the interlamellar "amorphous" regions and the mosaic block lateral grain boundaries. The deformation mechanisms beneath the indenter are discussed in the light of current models of plastic deformation. MH is shown to depend on the packing density of the macromolecules in both phases and, as a result, it can be clearly correlated with the macroscopic density of the material. The unit cell expansion and lattice distortions increase in parallel as a consequence of increasing incorporation of chain defects within the lattice. This provokes a conspicuous decrease in the microhardness of the crystals. The increase in lattice distortions is consistent with the concurrent decrease of lamellar thickness and, hence, of the coherently diffracting lattice volume. These results unambiguously emphasize the physical significance of the mosaic block character of the lamellae in determining the micromechanical properties of the material. Finally it is shown that the strain boundary which defines the zone of crystal destruction under the indenter also depends on the average volume of the paracrystallites and on the volume fraction of crystalline material.

Coincidence Doppler broadening spectroscopy in polyvinyl chloride after doping with Al2O3

“…48 It is a measure of the local resistance of the material against the indenter penetration and is connected with the local irreversible deformation. Figure 6 shows the Vickers hardness of the samples as a function of Al 2 O 3 concentration on PVC.…”

Equilibrium and non-equilibrium melting of branched polyethylene relating to defect incorporation within crystals

“…The energy per defect (14-16 Kcal/mol) derived from the slope of the plot for X < 0.025 is in accordance with the range of values derived from the analysis of equilibrium data in Figure 2. However, owing to the rapid quenching of these samples to low temperature, a larger inclusion of defects within the crystals than in the equilibrium crystallization experiments might be here involved [5]. Since only a minor fraction of the total number of branches are penetrating the crystals, the majority of these chain irregularities ought to be located at the interlamellar regions, on the crystal surface and within the lateral grain boundaries [2,30].…”

“…The exposure of PE to fuming nitric acid after long exposures times has confirmed the level of defects remaining in the crystalline cores, using infrared spectroscopy [4]. It has, further, been shown that crystallization conditions may selectively affect the penetration of defects within the crystal lattice [5].…”

Mechanical Properties of Ultra High Molecular Weight Polyethylene Obtained with Different Cocatalyst Systems

“…MHV and MHT, determined for a load of 40 g, the values of the parameter n and the microhardness of the crystal phase (MHV c ), calculated by the parallel model, 38 are listed in Table III. The parameter n, obtained from the slope of Mayer s lines has a value around 2, which means that MHVcan be considered as Comparing the graphs for the three samples it could be seen that MHV increases with increasing the degree of crystallinity and decreasing the molecular weight.…”