Structural Changes in UHMWPE Reactor Powders during Sintering under Different Conditions

To clarify the influence of processing conditions on the structure of the ultra-high-molecular-weight polyethylene (UHMWPE) nascent reactor powder, its change in the course of sintering at various temperatures and pressures has been studied with the help of DSC technique. The basic morphological units of the UHMWPE reactor powder studied, as evidenced by scanning electron microscopy, are crystalline lamellae. An analysis of the shape of the polymer melting peak on the DSC curves made it possible to obtain the data on the thickness distribution of the lamellae since the heat flux dH/dT (where is the melting enthalpy and T is the sample temperature) is proportional to the mass fraction of lamellae melting at a certain temperature. Such an analysis of the UHMWPE structure has not been carried out yet. It has been found that the percentage of the thick (>15 nm) and the thin (<10 nm) lamellae in the compacted samples and in those sintered at a temperature of 130oC (lower than the equilibrium melting temperature of an infinitely long polyethylene crystal T0 = 142oC), remains nearly the same. However, in the samples sintered at temperatures above the T0 of PE with subsequent cooling under different conditions, significant changes in the content of the lamellae of different thicknesses have been observed. At the same time, their melting points remained unchanged, provided that heating from 130°C to 145°C has been carried out when the samples were kept under pressure.