Crystallisation behaviour of high density polyethylene blends with bimodal molar mass distribution 1. Basic characteristics and isothermal crystallisation

Abstract

Isothermal crystallisation of high density polyethylene (HDPE) blends and their parent polymers was investigated. The blends having broad bimodal molar mass distributions and various compositions were prepared by blending a high molar mass ( M w=330 kg/mol, M w/ M n=4.8) and a low molar mass HDPE ( M w=34 kg/mol, M w/ M n=10) in different ratios in xylene solution. The blends and their parent components were characterised by size-exclusion chromatography, dynamic rheological and density measurements. Crystallisation kinetics were studied using a polarised light microscope equipped with an in-house built hot stage and by differential scanning calorimetry. The Avrami theory was applied for crystallisation kinetics analysis. Such crystallisation kinetics parameters as nucleation rate, nucleation density, the Avrami index and cystallisation rate contant were determined for the blends and their parent polymers. According to the results obtained an increasing polydispersity of the sample had a slight increasing effect on the Avrami index, indicating gain in prevalence of the thermal nucleation over the athermal one. In all samples nucleation density increased continuously during crystallisation verifying that the presence of a certain thermal nucleation was typical for all the materials studied. Both the crystallisation rate constant and the nucleation rate decreased with increasing molar mass of the sample. The nucleation density increased proportionally to the increase in average molar mass and the values were larger at lower crystallisation temperatures. The formed supermolecular structure was found to be sensitive to the blend composition and crystallisation temperature. Irregular banded or non-banded spherulites were observed in the materials. Banding of spherulites was typical for the samples having higher average molar mass. The superstructures observed in this work were smaller and vaguer than the superstructures reported in the earlier studies of polyethylene materials having similar average molar mass but narrow molar mass distribution.