| Combination of low density and high strength is the eternal objective for advanced materials, especially the structural materials. Due to their much lower densities than traditional metallic and inorganic materials, polymeric materials are becoming more and more important for social and economic life, defense security, and scientific progresses. However,, these materials exhibit inferior strength and toughness compared to metals and some natural composites (e.g., nacre, bones, etc.).Since the first proposal of toughening theory for polymeric materials in1950s, people have built various effective methods for toughening polymers (among which the rubber toughened ABS material is the most successful one). However, most of these methods take the cost of strength and stiffness. On the contrary, many natural materials could entitle themselves with simultaneously improved toughness, strength and stiffness by the formation of hierarchical ordered microstructures (such as the oriented layered structures of multi-level in nacre). Therefore the exploration of an effective method to simultaneously boost the toughness, strength and stiffness for polymeric materials is not only meaningful in the theoretical aspects, but also very important for potential applications in industry.Inspired by the hierarchical ordered structures in seashell nacre, we came up with a novel method, pressure-induced flow processing (PIF-processing) to obtain high performances for commodity semicrystalline polymeric materials. It is well-known that semicrystalline polymers exhibit hierarchical structures, because they consist of crystalline regions and amorphous regions, the former being composed of lamellae of folded chains with thickness of several nanometers, and then form spherulites. With PIF-processing, the isotropic crystalline spherulites can be deformed into oriented ellipsoids, in which the stacks of lamellae undergo re-construction and re-arrangement, so as to form oriented hierarchical structures in semicrystalline polymers. These typical structures would introduce improvements on the toughness, strength and stiffness for the materials at the same time. In this thesis, we systematically investigated the evolvement of hierarchical microstructures during PIF-processing for typical polymeric materials, and suggested the structural evolution model and related toughening and strengthening mechanisms for semicrystalline polymers with PIF-processing, and confirmed the validity of this method by comparative studies on many semicrystalline polymeric materials. The main results are as follows.1. We discovered the principles in the evolvement of the hierarchical microstructures for semicrystalline polymers during PIF-processing, and revealed the molecular mechanisms for plastic deformation.In spite of the long-established plastic deformation theories since1970s, many issues during the plastic deformation from the spherulites to oriented molecular chains (e.g., how are the spherulites deformed and destroyed, how are the lamellae moving and destroyed, how are molecular chains moving in the crystalline regions and amorphous regions) remain controversial. With the newly developed methods for characterizing the hierarchical structures of semicrystalline polymers (such as synchrotron radiation small angle X-ray scattering, wide angle X-ray diffraction, and the combination of low-temperature microtoming, chemical etching and full splicing of AFM images), we carefully studied the structural evolutions in crystalline (including spherulite, lamellae and their stacks, and fold chains) and amorphous regions during PIF-processing, and confirmed that most of lamellae are broken into nano-sized crystalline fragments, which are then inter-connected by tie molecules to form crystal-rich areas of aligned layers. These crystalline fragments remain the long periods of original lamellae, however, the fold chains of them exhibit a preferred orientation along the flow direction. These new findings enriched the content of molecular mechanism for plastic deformation.2. We proposed a comprehensive structural model to illustrate the toughening and strengthening mechanisms of PIF-processing for semicrystalline polymeric materials.We made a systematic analysis on the fracture type as well as the morphologies and microstructures for the fractured surfaces, by means of optical microscope (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations. Based on the motor-brick model for seashell nacre, we proposed the molecular mechanisms of PIF-processing on toughening and strengthening the semicrystalline polymeric materials. Specifically, the motion of lamellae and deformation of spherulites result in well aligned hierarchical stratified structures with oriented multi-leveled weak boundaries between those layers. These weak boundaries would initiate abundant cracks and promote their propagations along flow direction, and thus leading to a greatly increased sum distance of energy-dissipating paths in the materials. On the other hand, the formation of "confined amorphous regions" consisting of tie-molecules increases the effective stress within lamellae stacks. The above two factors cooperatively contribute to the much improvement of both impact and tensile strength for the PIF-processed samples.3. We set up a novel method for simultaneously enhancing the toughness, strength and stiffness for semicrystalline polymeric materials.The new processing method of PIF-processing offers a novel route to entitle semicrystalline polymeric materials with high performances. For seven commodity polymeric materials including some semicrystalline polymers and their blending or hybrid materials (i-PP, PLLA, PA66, PA6, PA6/PP, PA6/MMT, PP/MMT), in details we discussed the effects of various processing parameters (such as temperature, pressure, compression ratio, and pressure holding time) on the microstructures and resultant properties, and found that the toughness could be improved by2-22folds, while the strength could be improved by2-3.5folds and stiffness by2-3folds. We confirmed that hierarchical oriented layered microstructures could be formed in semicrystalline polymeric materials, hence greatly boosting the impact strength without hurting the tensile strength and stiffness. |
PDF Full Text Request