Toughness amorphous matrix polymers

Low density polyethylene (LDPE) was the earliest polyolefin to be manufactured by the free radical polymerisation of ethylene. It is not pure poly(methylene). The latter is a very linear polymer and can be synthesised in the laboratory by decomposition of diazomethane. As a result of its method of manufacture, LDPE contains alkyl groups of varying lengths pendant to the polymer chain and some carbon-carbon double bonds. These irregularities reduce its crystallinity compared with poly(methylene) so that it normally consists of 50-60% of crystalline domains dispersed in amorphous polymer. The amorphous matrix is a very viscous liquid which acts like a rubber and gives the polymer its very high resistance to impact. LDPE is still widely used because of its exceptional toughness. 

Polymer composites have a fracture toughness, ultimate tensile strength and ultimate elongation that can be varied by changing the glass transition temperature of the composite. The thermodynamic properties of the amorphous matrix change at Tg. 

Toughened Polymers with Semicrystalline Matrix. It is well known that toughness of semicrystalline polymers such as PA (polyamide) and PP can be increased similar to the amorphous poljuners by the addition of relatively small amounts of rubber particles such as EPR or EPDM. As in HIPS and ABS, the modifier particles act as stress concentrators, initiating a plastic deformation of matrix strands between the particles as the main energy absorption step. In impact-modified PA and PP at room temperature, plastic deformation takes place through shear deformation (mechanism of multiple shear deformation). 

The adduced results have shown that loosely packed matrix of devit-rificated amorphous phase and disordered in deformation process crystalline phase part are structural components, defining impact energy dissipation and hence, impact toughness of semicrystalline polymers. The fractal analysis allows correct quantitative description of processes, occurring at HDPE impact loading. It is important, that the intercommunication exists between polymer initial structure characteristics and its changes in deformation process [2, 3]. 

Many amorphous thermoplastics are brittle, limiting their range of applications. Toughening with rubber is well known to enhance fracture resistance and toughness. Many major chemical industries are based on toughened plastics, such as ABS, HIPS and ionomers [30-34]. Important issues in the design of fracture resistant polymers are compatibility, deformation, toughening mechanisms and characterization. Particle size distribution and adhesion to the matrix must be determined by microscopy to develop structure-property relationships. 

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