Preparation and properties of micro- and nanocomposites based on high density polyethylene

The primary objectives of this thesis were to investigate and compare the fracture and creep behaviour of micro- and nanocomposites based on polyethylene (PE) produced by melt mixing. To achieve these objectives, different fillers were considered: micrometric copper particles, nanometric carbon blacks (CBs) and multiwalled carbon nanotubes (MWCNTs). In the first part of this dissertation, the fracture behaviour of PE-CB composites was investigated via the Essential Work of Fracture (EWF) approach by producing composites with different CBs in order to investigate the effect of the filler particle size. Moreover, several processing (i.e., extrusion) parameters were varied to obtain different degrees of filler dispersion. Experimental results reveal that fracture toughness increases significantly when CB particle size is smaller and as the extent of dispersion of the filler in the polymer matrix is better. Fracture toughness depends on the thermo-mechanical degradation of the polymer matrix that occurs during extrusion. In the second part of this project, creep behaviour of PE-based composites was investigated at several temperatures with assistance from the principle of time-temperature superposition. In particular, the effect of filler dimensions was analyzed by comparing viscoelastic results for composites that contain micrometric copper particles (with an average diameter of 15 and 45 μm) and nanometric carbon blacks (with an average diameter of 15 and 30 nm). In general, these fillers substantially increase the creep resistance of PE, and this phenomenon was more prominent at smaller particle size. This effect was detectable in the linear viscoelastic region (i.e. at low stresses or temperatures), and it became more evident in the non-linear viscoelastic region (i.e. at high stresses or temperatures). In particular, creep compliance and creep rate decrease at smaller particle size. It is postulated that filler particles function as physical crosslink junctions which hinder polymer chain motion and reduce creep deformation. When particle size is reduced at constant filler volume fraction, the physical crosslink density increases such that chain mobility decreases significantly under stress. Finally, the creep behaviour of PE-MWCNT composites were investigated via direct dispersion of MWCNT in the polymer matrix and by using a commercial masterbatch of MWCNT. In all cases, the increase in creep resistance is statistically significant in the linear viscoelastic region (i.e. at low stresses or temperatures) when sufficient dispersion of the nanotubes is achieved. Interestingly enough, creep resistance increases in the non-linear viscoelastic region (i.e. at high stresses or temperatures) regardless of the degree of nanotube dispersion in the matrix. This phenomenon is attributed to nanotube orientation induced by high levels of stress.