The effect of molecular architecture on the deformation behaviour of drawn bimodal polyethylene

The most common failure mode for polyolefin pipes is slow crack growth. A crack is preceded by a craze, a voided wedge of material bridged by highly deformed fibrils. Upon failure of the fibrils, the crack propagates. Both the tendency of the material to form voids and the strength of the fibril at the craze - crack interface are governed by the effective entanglement network. The effective entanglement network comprises all the intermolecular junctions in the material that can effectively transfer load at the time scale of the experiment. In this work, the effective entanglement network of bimodal polyethylene is probed through tensile and creep measurements. Bimodal polyethylene is the industrial standard material for polyethylene pressure pipes, and consists of a high molecular mass, branched fraction and a low molecular mass, linear fraction. The former is responsible for the resistance to slow crack growth, the latter for enabling processing. In the first part of the work, the influence of molecular mass and branch content of the high molecular mass fraction on the effective molecular network is studied. It is found that only a combination of high molecular mass and high branch content increases the resistance of the network. In the second part of the work, the high molecular mass fraction of the bimodal polyethylenes is isolated. Again, a combination of high molecular mass with high branch content results in a higher effective entanglement network, and overall the resistance to deformation is higher in these materials than in the bimodal materials. It is concluded that the resilience of the network depends on the available network density and the friction caused by side chain branches. Independently of the morphological origin of this friction in the solid material, it can be expected to vary with the monomeric friction in the melt

A simple method for the melt extrusion of cellulose nano-crystal reinforced hydrophobic polymer

International audienceThe rheological properties of a dispersion of cellulose nanocrystals (CNCs) in an aqueous solution of polyoxyethylene (PEO) have been investigated. A peculiar behavior is reported. Upon adding CNC, the viscosity of the suspension first decreases and then increases. Adsorption of PEO chains on the surface of the nanoparticles has been suspected. Freeze-drying of this PEO-adsorbed CNC dispersion was performed, and the ensuing lyophilizate was extruded with low density polyethylene. Compared to neat CNC-based nanocomposites, both improved dispersibility and thermal stability were observed. This simple and physical method constitutes an approach of choice for the melt processing of CNC-based nanocomposites with a hydrophobic polymeric matrix applicable at the industrial scale

A Morphology-Based Model to Describe the Low-Temperature Impact Behaviour of Rubber-Toughened Polypropylene

The roles of the rubber particle size, the rubber particle size distribution and the constitutive behaviour of the isotactic polypropylene matrix have been studied by combining the Lazerri–Bucknall energy criterion for cavitation with the Van der Sanden–Meier–Tervoort ligament model adapted for impact conditions. It is concluded that an optimised morphology offers great potential to achieve enhanced mechanical properties with far less rubber and hence achieve a superior stiffness/toughness/processing balance

Failure behaviour of self-reinforced polypropylene at and below room temperature

Self-reinforced polypropylene is a very tough material. It is even thought that its impact resistance increases with decreasing temperature. This was investigated by examining the constituent tapes and matrix. Tensile tests on both drawn polypropylene tapes and self-reinforced polypropylene were similar: the stiffness increased and the failure strain slightly decreased at low temperatures. The matrix, however, embrittled below room temperature due to the glass transition. In contrast with literature data on Izod impact resistance, the penetration impact resistance did not increase at low temperatures. At lower temperatures, the damaged area after non-penetration impact was significantly reduced. This was caused by a change in the damage mode from tape–matrix debonding to matrix cracking, as the matrix went through its glass transition. These conclusions provide the first understanding of the failure behaviour of self-reinforced polypropylene below room temperature, and can be exploited to further optimise the excellent impact resistance of self-reinforced polymers.publisher: Elsevier articletitle: Failure behaviour of self-reinforced polypropylene at and below room temperature journaltitle: Composites Part A: Applied Science and Manufacturing articlelink: http://dx.doi.org/10.1016/j.compositesa.2014.06.003 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved.status: publishe

A simple method for the melt extrusion of cellulose nano-crystal reinforced hydrophobic polymer

International audienceThe rheological properties of a dispersion of cellulose nanocrystals (CNCs) in an aqueous solution of polyoxyethylene (PEO) have been investigated. A peculiar behavior is reported. Upon adding CNC, the viscosity of the suspension first decreases and then increases. Adsorption of PEO chains on the surface of the nanoparticles has been suspected. Freeze-drying of this PEO-adsorbed CNC dispersion was performed, and the ensuing lyophilizate was extruded with low density polyethylene. Compared to neat CNC-based nanocomposites, both improved dispersibility and thermal stability were observed. This simple and physical method constitutes an approach of choice for the melt processing of CNC-based nanocomposites with a hydrophobic polymeric matrix applicable at the industrial scale