|Title:||The effect of molecular orientation on the mechanical behavior of cold-rolled semicrystalline polymers|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Polymers -- Mechanical properties
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xv, 126,  leaves : ill. ; 30 cm|
|Abstract:||The cold rolling of semicrystalline polymers is a useful method to modify the mechanical and other physical properties of polymers. The effect of rolling on polyethylene has been studied since the 1960's. Cold forming of plastics has several advantages over warm forming. These include increased strength, toughness and shorter cycle times. Usually high impact resistance and good formability of semicrystalline polymer materials could be obtained due to the molecular chains becoming favorably aligned after the rolling process. In recent years, many metal-plastic laminates and sandwich sheets have been developed for reducing vehicle weight and improving the sound-deadening properties. Although there is a bright future for this sandwich material used as automotive panels, the mechanism of rolling on semicrystalline polymers and the microstructure transformation are still not clear. Therefore, the study on the microstructures and mechanical properties of cold-rolled semicrystalline polymers is of considerable academic and practical interest. An understanding of the relationship between microstructures and mechanical properties of polymeric materials can provide the key to control the processing parameters and to obtain the desired mechanical properties. In this thesis the effects of rolling on polypropylene sheets are studied from the molecular morphology changes in microstructure. A modified two-phase model is proposed to quantify the relationship between the microstructure and the mechanical properties. Firstly, the microstructure of polypropylene (PP) sheets with different rolling reduction ratios are examined by polarized light microscopy, scanning electron microscopy, differential scanning calorimetry, density measurement and Raman spectroscopy. Quantitative experiments to determine the orientation in rolled PP sheets were carried out by wide-angle X-ray diffraction. This shows that the degree of orientation increases with rolling. Based on the experimental observation and the background of two-phase model for semicrystalline polymers, a modified two-phase model is proposed to account for the mechanical properties of semicrystalline polymers. The proposed model considers chains in semicrystalline polymer being composed of crystalline and amorphous regions to form two intersecting networks. The spherulites in the crystalline regions are deformed to ellipsoids which abide by the Langevin function and the soft amorphous phase is described by a Nagai-type network behavior. The variation in mechanical and physical properties of the rolled PP samples were obtained by tensile tests. The orientation distributions and orientation factors are calculated from the data obtained from polar angle scan experiments. A formulation to qualify the orientation distribution was deduced. The theoretical stress-strain curve of semicrystalline polymers was derived from the proposed two-phase model and compared with experimental results. The relationship between modulus and orientation factor is obtained. Good correlations are found. The proposed modified two-phase model is based on an original structure and a realistic movement of the polymer chains of semicrystalline polymer. It extends the quantitative measurement of the polar angle scan on cold-rolled PP sheets to obtain the orientation factor. The stress-strain curves which are calculated from the proposed modified two-phase model, and the experimental data are in good accordance. The constitutive equation provides a way, by controlling the processing conditions, to achieve the desired mechanical properties.|
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