Fracture toughness of interface of polyethylene modified in bulk

The fracture toughness of the interface, G(a), of the self-healed joints of poly(ethylene) (PE) was measured using the wedge method. Samples of PE mod ified by mixing with three additives (branched low-molecular weight PE, a g raphite filler, and polypropylene oil) were investigated. The development o f the strength of partially healed joints formed by several hours of contac t at a welding temperature of 105 degrees C can be represented in all cases by the linear dependence of the G(a) parameter on the square root of time, in accordance with the diffusion mechanism of the interface formation. The presence of the additive in samples was found to enhance the fracture toug hness of a joint for a given welding time. In the graphite composites, an i nduction period of welding was observed. In contrast, an instant nonzero st rength occurred in joints of PE with PP oil samples. The results confirmed that the concept of the chain entanglement control of fracture toughness de veloped originally for the glassy polymers is well transferable to the semi crystalline PE. However, additional mechanisms due to the crystallization o f PE upon cooling are also effective in the development of the joint streng th. These mechanisms denoted as cocrystallization, transcrystallization, lo cal crystallization, mechanical interlocking, etc., are substantially affec ted by the concentration of additives. (C) 1999 John Wiley & Sons, Inc.