Polyethylene/graphene nanocomposites: effect of molecular weight on mechanical, thermal, rheological and morphological properties

Abstract

Three different polyethylene samples (molecular weights PE1 < PE2 < PE3) were synthesized using coordination polymerization, in order to study the effect of the variation in molecular weight on the properties of the nanocomposites, using thermally reduced graphene as filler. The polymer samples had similar crystal structure but had 61, 56, and 52 % degree of crystallinity respectively. For PE1, the degree of crystallinity enhanced on adding 2 and 4 % graphene due to nucleation induced by the graphene nanoplatelets; however, the crystallinity was observed to decrease in PE2 and PE3. Peak melting points also exhibited similar trend, although the overall change was limited to 3–4 °C. The filler was observed to be in the form of stacks with few platelets distributed uniformly in the matrices, and the degree of dispersion was better in PE1 composites. Due to efficient heat transfer to the filler platelets owing to their better dispersion in PE1, the highest extent of improvement in the initiation of degradation temperatures was observed. PE1 composite with 4 % filler fraction was observed to be have initiation delayed by ∼40 °C as compared to pure polymer. Due to the higher degree of crystallinity, PE1 had higher tensile modulus of 1334 MPa, which was improved to maximum extent of 35 % in the composite with 4 % filler content. Thus, the mechanical properties were enhanced due to combination of crystallinity and filler effect. The peak stress as well as elongation of the composites was improved in most cases as compared to pure polymers. The high temperature modulus of the composites was also improved on addition of graphene, with PE1 composites exhibiting the highest magnitude of modulus. The PE1 composites also exhibited lowest melt viscosity among the composites probably due to the lower degree of entanglements, which indicated ease of processing similar to the pure polymer.