Study On The Relationship Between Mechanical Properties And Spherulite Structures Of Isotactic Polypropylene

The relationship between the structures and mechanical properties of polymers has been extensively investigated, for the mechanical property is determined by the final structures of polymers. The deformation behavior is not only one of important mechanical properties of polymers, but also related with temperature and tensile rate. Update, the best way to descript the deformation behavior of polymers is phenomenology, and there is not only one mechanism of deformation to explain the deformation behavior.In this paper, in-situ synchrotron radiation small-angle X-ray scattering (SAXS) was carried out to investigate the structure of isotactic polypropylene (i-PP) during isothermal crystallization process at various crystallization temperatures. The maximum intensity of scattering in SAXS scattering curves indicated the development of ordered structure in the crystallization process, and the scattering intensity of i-PP increases with time. The calculated parameters of structure including long period and the thickness of amorphous layer decreased gradually along with time, while the lamellar remained unchanged. The same trend is more obvious at higher crystallization temperatures, and the long period and lamellae thickness increased with crystallization temperature.The temperature related morphology was observed and the strain-stress curves of i-PP were recorded with a tensile hot stage equipped with polarizing microscope during tensile process. The samples were prepared at different temperatures for isothermal crystallization before the investigations. The obtained results indicate that the fracture mode is brittle fracture, and fracture occurred between the spherulites; while crystallized at lower temperatures, the fracture mode is ductile fracture. The brittle fracture was observed for the samples isothermally crystallized at moderate temperature with lower tension temperatures, and the fracture occurred between the spherulites. When the tension temperature is higher, yield point can be found, complete rupture in the strain softening, fracture also occurred between the spherulites. Crystallization temperature determined spherulite size is essentially the same as the original in the drawing process.Tensile hot stage polarizing microscope was used to study the influence of temperature on the mechanical properties of i-PP with different stretching rate and sample thickness. The results showed that the effects of raising tensile rate and reducing the temperature are equivalent. Young's modulus, yield stress and fracture stress of i-PP decreased with the increase of temperate or decrease of tensile rate, while elongation at break increased. The yield stress of i-PP and tensile rate are of logarithmic relationship, i.e.(?) ~ ln_V, which can be explained by crystallographic theory and Eyring theory. The Young's modulus is almost not affected by the thicknesses of the samples (1mm and 0.1mm). It is considered that the difference in mechanical properties caused by different thickness of samples compared with the tensile properties of the samples with same thickness, error is in the acceptable range.