Structural Evolution And Mechanical Behavior Of Ultrasonic Micro-Injection-Molded Polypropylene

With the rapid development of micro-manufacturing,ultrasonic micro-injection molding as a new processing technique has attracted a lot of attention from academia and industry field,but there are relatively few studies on the structural formation and evolution as well as mechanical properties of the ultrasonic micro-injection molded products.Therefore,this thesis begins by combining a customized micro-injection apparatus with a high-flux synchrotron X-ray radiation source to record the structural formation and evolution of isotropic polypropylene during the injection processing in real time at a high temporal resolution.Then by using a commercial ultrasonic microinjection molding machine,we prepared the ultrasonic micro-injection molded samples and studied the effect of the processing parameters on the structural morphology and distribution of the samples.In addition,the structural evolution during heating and stretching was investigated by in-situ X-ray experiments to elucidate the thermal stability and deformation mechanism of the ultrasonic micro-injection molded polypropylene.In-situ synchrotron small-angle X-ray scattering experiments reveal the nonequilibrium flow-induced crystallization behavior and the structure evolution during the micro-injection molding process,and the results indicated that the shish-kebab structure and parent-daughter lamellae structure were formed in sequence during the micro-injection processing.The changes of the structural parameters as a function of time showed that the structural formation and evolution were significantly dependent on the mold temperature and the molecular weight due to the chain mobility.In addition,the mapping experiments of the wide-angle X-ray diffraction demonstrated that there were a large number of mesophase domains in the core layer of the ultrasonic microinjection molded polypropylene,and the form and distribution of crystal changed as the processing parameters(such as the mold temperature,the filling velocity and the thickness of cavity)varied.This is because the crystallization behavior of the oriented melt during the micro-injection molding process is significantly different from that of the static melt due to the effect of molecular chain orientation.In terms of the properties of the ultrasonic micro-injection molded products,firstly,the in-situ heating tests revealed that the thermal stability of the structure was enhanced with the increase of the mold temperature,and the degree of the molecular chain orientation was affected by the multi-layered structure and phase transition.For the samples molded at the lower mold temperature,the molecular chains orientation remained basically unchanged or even slightly increased during heating process.Then the plastic deformation and stress whitening of the ultrasonic micro-injection molded polypropylene were investigated by using the in-situ X-ray scattering experiments.It can be inferred from the trend of the structure with strain that the significant reduction in molecular chain orientation during stretching along the flow direction was due to the slippage and breakage of the crystal lamellae,and the stress whitening phenomenon is caused by the cavitation of sample during deformation.Furthermore,the lamellae slippage and cavitation is more likely to occur at a lower stretching temperature.The higher mold temperature and the thinner thickness of the sample,the greater the reduction in molecular chain orientation and the more obvious the stress whitening of sample.This thesis focuses on the structure and properties of ultrasonic micro-injection molded polypropylene.Not only were we able to modulate the structure of products by varying the processing parameters,but we can also have a better understanding on the non-equilibrium flow-induced crystallization theory during processing and the tensile deformation of micro-products with multi-layered structure,which is expected to contribute to the practical processing and application of the materials.