Crystallization Behavior And Crystallization Kinetics Of β-Polypropylene

In this thesis,the crystallization behavior and crystallization kinetics of someβ-nucleated polypropylene systems as well as the methodology for increasing the relative proportion of theβform were investigated.Compared with the commonαpolypropylene,theβpolypropylene usually presents a better performance in both the heat distortion temperature and the impact strength.Several means have been proposed to increase the amount ofβform and the addition of so-calledβnucleating agents is found to be the most effective and practical method.In this thesis,we conduct our experiments and analysis with three systems,includedβnucleated propylene homopolymer,βnucleated propylene/ethylene copolymer and theβnucleated polypropylene/polycarbonate blends,with the aim of further understanding the crystallization behavior ofβpolypropylene and obtaining the methods to increase the proportion ofβform in polypropylene.The thesis contains six chapters.Chapter one is the introduction of the whole research work.In chapter two,two kinds ofβ-nucleating agents,named a rare earth complex(WBG) and N,N'-dicyclohexylterephthalamide(TMB5),were introduced into propylene homopolymer and their effect on crystallization and melting behavior of polypropylene were comparatively investigated.Wide angle X-ray diffraction measurements revealed that both the two additives were highly effective in inducingβform.At their respective saturation concentrations of 0.08 wt%for WBG and 0.06 wt%for TMB5,the relative amount ofβform for both nucleated polypropylene was up to more than 90%.However,the Lauritzen-Hoffman theory analysis revealed that WBG was more effective not only in increasing the nucleus number,but also in accelerating the growth rate of crystallization.After completing isothermal crystallization process,the subsequent melting behavior examination suggested that addition of WBG expanded the upper limit temperature of formingβform,and therefore was more effective in delaying theβ-αtransformation than TMB5.In chapter three,the crystalline structure,morphology and nonisothermal crystallization behavior of propylene homopolymer with and without a rare earthβnucleating agent(WBG) were investigated with wide angle X-ray diffraction,polar optical microscopy and differential scanning calorimeter.It was found that WBG could induce the formation ofβform and a higher proportion ofβform could be obtained by the combined effect of optimum WBG concentration and a lower cooling rate.The relative content ofβform could reach more than 90%in 0.08 wt%WBG nucleated sample at cooling rates lower than 5℃/min.Polar optical microscopy observation showed that the present WBG led to substantial changes in both the morphological development and crystallization process of polypropylene.Under all the studied cooling rates,the temperature at which the maximum rate of crystallization occurred was increased by about 10℃with the presence of the nucleating agent.The nonisothermal crystallization kinetics analysis also revealed that the introduction of WBG significantly shortened the overall crystallization time.In chapter four,the polymorphism and crystallization behavior of a propylene/ethylene copolymer with or withoutβnucleating agent were investigated via wide angle X-ray diffraction and differential scanning calorimetry measurements. For theβnucleated copolymers,the formation ofγform is preferred under lower cooling rates or higher isothermal crystallization temperatures within the investigated conditions,whereas the relative amount ofβform increases with the increase of the cooling rates or the decrease of the isothermal temperatures.It is interesting to note that the content ofβform shows the absolutely opposite tendency asγform under both isothermal and nonisothermal conditions.On the other hand,it is observed that the amount ofβform as a function of cooling rate presents an opposite trend for theβnucleated propylene/ethylene copolymer and theβnucleated propylene homopolymer. These behaviors could be ascribed to the competition between the nucleation ofβnucleating agent inducing the formation of theβform and theγnucleation ability of the co-monomer defects.The composition of polymorphs in propylene/ethylene copolymer may be tailored by addingβnucleating agent and crystallizing at appropriate crystallization conditions.In chapter five,we studied the crystallization behavior of theβnucleated polypropylene/polycarbonate blends.The compatibilizer(MAPP) has been introduced into the blends.In non-compatibilizedβnucleated polypropylene/polycarbonate blends,the polypropylene matrix consisting mainly ofαform even with low polycarbonate content.On the other hand,a predominantlyβ-polypropylene matrix developed in the presence of certain amount of MAPP compatibilizer.The results obtained by calorimetric and polar optical microscopy measurements proved clearly that the compatibilizer assisted the partitioning of theβnucleating agent in both components of the blends and promote of the formation of a matrix rich inβ-polypropylene.Moreover,the size of polycarbonate domains is significantly reduced and the interaction between different phases is greatly improved in polypropylene/polycarbonate blends with addition of the compatibilizer.Chapter six is the conclusion to the whole research work.