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Processing and Morphological Fractality PS/HDPE Blends with SEBS Block Copolymer as Compatibilizer

Processing and compatibilization effects on the phase morphology and the tensile behaviour of blends of polystyrene (PS) and high-density polyethylene (HDPE) were investigated. The emulsifying effect by the addition of a commercially available styrene/ethylene-butylene/styrene (SEBS) compatibilizer was found not to be negligible. The influence of blends composition on the morphological structure, rheological and mechanical properties was studied. The rheological behaviour of the blends melt during processing was followed by measuring the torque (TQ), output (Q) and melt back-pressure (pback) in a twin extruder, while the rheological properties of extruded and moulded samples were measured with Dynamic Mechanical Analyser (DMA). The process parameters were decreased with the HDPE content. The results show that SEBS compatibilizer can yield compatibilization by substantially reducing torque and increasing the back-pressure. However, the Hurst roughness of melt processing parameters are increased. Compared to the 80/20 PS/HDPE blend without SEBS addition by twin screw melt extrusion, the torque Hurst roughness constant of the same blend with 5% and 7 % addition of SEBS is increased by 4 and 2 %, respectively. In the case of a 40/60 PS/HDPE blend with 5% and 7 % SEBS addition, the torque Hurst constant changed by 9 and 3 %, respectively. Creep relaxations and creep modulus were measured in the creep fatigue regime at temperatures 25, 30, 35, 40 and 45 °C. Tensile strength at break, elongation at break and notched impact strength (at room temperature) were determined. The creep increased with increasing amount of HDPE in the blends of PS/HDPE/SEBS, while the creep modulus and storage modulus decreased. As the content of HDPE in the blends PS/HDPE/SEBS increased the tensile strength at break decreased, while the elongation at break and impact strength increased. SEM microphotographs of PS/HDPE blends revealed better compatibility of PS and HDPE phases in the presence of SEBS. On the base of the results obtained and their correlation with the fractal indices obtained from the SEM microphotographs of investigated systems, the correlation between the blend compositions, morphological structure, mechanical and rheological properties was established and discussed. Box counting analysis and wavelet methods were used to determine the fractal dimension, D, from the SEM microphotographs. Fractal geometry is a non-Euclidean geometry which has been developed to analyze irregular or fractional shapes. In this paper, fracture in blends is analyzed as a fractal process. This means that fracture is viewed as a self-similar process. We have examined the fracture surfaces of uncompatibilized and compatibilized blends to test for fractal behavior. We found a correlation between increasing the fractional part of the fractal dimension and increasing toughness. In other words, as the toughness increasing the fracture surface increases in roughness. However, more than just a measure of roughness, the applicability of fractal geometry to fracture implies a mechanism for generation of the fracture surface. The results presented here imply that brittle fracture is a fractal process ; this means that we should be able to determine fracture processes, by observing the microscopic scale, and by finding the generator shape and the scheme for generation inherent in the fractal process.

PS/HDPE/SEBS blends; morphological structure; rheological properties; tensile behaviour; Hurst roughness; fractal dimension

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