Micro-structural Evolution Of Isotactic Polypropylene During Die Drawing

Die-drawing,as one of the most promising solid-state processing techniques,has been widely applied to lots of semi-crystalline polymer materials.However,the development of die-drawing has been depending on the tremendous and tedious data accumulation over the decades.The absence of in-depth understanding of the deformation mechanism during the die-drawing process has hindered its further thriving.For semi-crystalline polymer materials,their macroscopic performances are fundamentally determined by the micro-structures.Hence,it is crucial to understand the deformation mechanism during the die-drawing for adjusting the micro-structures of materials as well as their macroscopic performance.In this dissertation,isotactic polypropylene(iPP)was employed.With the help of the X-ray scattering techniques,we monitored the micro-structure evolution of iPP along with the deformation parameters,such as the deformation temperatures and speeds during die-drawing.Consequently,it enabled us to understand the deformation mechanisms during the die-drawing process,based on the plastic flow and cavitation behaviors of iPP.Meanwhile,the die-drawing setup was employed to prepare die-drawn samples with certain micro-structures to clarify the relationship between their mechanical properties and the micro-structures.We look forward to shedding light on the underlying deformation mechanism during the die-drawing process and bringing new insights on manufacturing products with high performances.The main points of this thesis are concluded as follows:1.The die is the essence of the die-drawing technique,which is also the main reason that samples would go through a relative optimal deformation path during the die-drawing process.However,the previous literatures focused only on the shape changing of the materials during die-drawing by means of simulation experiments and lacked the exploration of the materials in the aspects of micro-structures,such as the orientation of molecular chains and the evolution of lamellae stacks.Hence,several die-drawn samples prepared at different deformation conditions were employed and cut into different layers based on their relative distance to the die wall.The wide-angle X-ray diffraction(WAXD)experiments were performed to determine the orientation of molecular chains at different positions.Based on the orientation distributions of the molecular chains,a force model inside the die was proposed.The influence of the die could be considered as a force perpendicular to the elongation direction and a force along the opposite direction of the drawing force.These forces were largest at the die entrance and slowly wore off as the polymer necking down.Eventually these forces disappeared when the iPP sample left the die wall.2.The parent-daughter lamellae structure was a unique feature of the ? form iPP.The dihedral angle between a set of parent and daughter lamellae was either 80° or 100°.Hence,the parent-daughter lamellae could be considered as a quasi bi-oriented structure This structure could not be distinguished as the iPP at the isotropic state due to the random distributions of molecular chains.In theory,the oriented parent-daughter lamellae structure could be induced when the orientation of the molecular chains was elevated by free stretching.Yet in practice,this structure was always destroyed since all the molecular chains tended to realign along the stretching direction.We obtained samples with large fractions of oriented parent-daughter lamellae via die drawing.Adding ?-nucleating agents or elevating the deformation temperatures were also found to be in favor of the preservation of lamellae with normal perpendicular to the die drawing direction.With the help of the small angle X-ray scattering(SAXS)technique,we proved that the preservation of this quasi bi-oriented lamellae structure originated from the effect of the die.The force induced by die,both the perpendicular one and the parallel one,would weaken the drawing force.Consequently,the lamellae with normal perpendicular to the elongation direction could hardly be "molten",Adding ?-nucleating agents or elevating the deformation temperatures could effectively thicken the original lamellae,as a result the thickened lamellae was more likely to be preserved.Thus,more quasi bi-oriented structures could be found in the die-drawn samples.3.There has been a long debate upon the initiation sites of cavitation at small strain regime during uniaxial stretching of semi-crystalline polymers.For clarifying this debate,we compared the small-strain cavitation presented in the ?-iPP through free stretching and die-drawing processes.The cavitation was found to be much more severe in the free tensile-stretched ?-nucleated iPP than in the die-drawn one under the same conditions.It turned out that the die-drawing process could fundamentally decrease the number of growable cavity nuclei,by inspecting the deformed samples via ultra-small angle X-ray scattering(USAXS)techniques.Moreover,we found that the cavitation at small strain region initially born in the lamellae with normal perpendicular to the elongation direction by analyzing the cavitation behaviors of the further free stretched die-drawn samples.The suppressed cavitation found in the die-drawn samples was ascribed to the enhanced interactions between crystalline blocks and increased critical stress for the nucleation of cavities induced by the die.4.The small-strain cavitation in semi-crystalline polymers was further proven to be initiated in the crystalline layer by stretching die-drawn samples with certain micro-structures.The large-strain cavitation,on the other hand,was authentically related to the highly oriented amorphous network formed at late deformation stage.The pre-orientation directions of the molecular chains could influence the above-mentioned network.Thus,the critical stress for the large-strain cavitation as well as the mechanical performances were different for the die-drawn samples with different pre-orientation directions as being further free stretched.