Structure and dynamics of polyethylene/clay films

The relationships between structure and rheology of polyethylene/clay hybrid composite blown films were investigated through rheological tests both in shear and elongational flow. Two polymer matrices (low density polyethylene, LDPE and linear low density polyethylene, LLDPE) with different relaxation kinetics were used. Independently from the matrix, morphological analyses (TEM, XRD, and SEM) indicate that the hybrid structures are similarly constituted of delaminated platelets or tactoids having a relevant degree of orientation along the draw direction. This strongly affects the rheological behavior of materials. However, despite the similarities emerged from morphological analyses, both shear (steady shear and oscillatory) and elongation measurements show a negligible effect upon the rheology of LDPE-based nanohybrids. Conversely, relevant increases of shear viscosity, dynamic moduli and melt strength of LLDPE-based nanohybrids have been detected. The effects of homopolymer relaxation kinetics have been investigated by means of stress relaxation tests. The results obtained seem to be consistent with the existence of a roughly bimodal population of dynamical species: a matrix component behaving like the homopolymer, and a fraction interacting with the filler, whose rheological behavior is controlled by the particles and their interactions with the polymer. Mechanical properties of hybrid films were also investigated. Differently from what happens in the melt state, the solid-state properties mainly depend on the filler amount. The relative increases of tensile modulus and melt strength are of the same order of magnitude for both the matrices used, indirectly confirming the similarities in hybrids structures