Cold Crystallization Behavior of Polybutene-1/Polypropylene Blend

Author:Zhong Zhen Xing

Supervisor:su chao zuo


Degree Year:2019





Isotactic polybutene-1(PB-1)is a typical semi-crystalline polyolefin material with complex polymorphic behavior.Comparing with other polyolefin materials,PB-1 exhibits remarkable physical and mechanical properties,superior creep and impact resistance,excellent environmental stress cracking resistance,good chemical corrosion resistance and relatively high heat distortion temperature,which in turn make it promising for commercial applications in tubes and packaging films.PB-1 exhibits several polymorphs depending on the solidification conditions.Form II crystal is kinetically favored and invariably obtained by cooling PB-1 material from the melt.However,form Ⅱ is metastable and spontaneously transforms into the thermodynamically stable form Ⅰ at room temperature.This transformation requires several days or even several weeks to complete and the process is accompanied by undesirable effects such as shrinkage of the molded objects,which has greatly increase the production period and considerably hindered the commercial application of PB-1.Polybutene-1/polypropylene(PB-1/iPP)blend has draw continuing attention these years and it has been found that iPP component not only can accelerate the transformation process from form ⅠI to form Ⅰ,but also induce PB-1 to generate form Ⅳ crystals directly in the blend.PB-1/iPP blend provide a simple,effective and low cost strategy to produce form Ⅰ’ and avoid the phase transformation in industrial process and it also becomes an attractive academic subject because of the complex crystallization behavior and morphology of PB-1/iPP blend.In this thesis,PB-1/iPP blend was prepared and the cold crystallization behavior of the blend was studied systematically by Fourier transform infrared spectroscopy(FTIR),differential scanning calorimetry(DSC),and wide-angle X-ray diffraction(WAXD),and an effective method was found to prepared form Ⅰ’ as the dominant crystals with a small amount of iPP in the blend.The main research results and conclusions are summarized as follows:1.Cold crystallization behavior of PB-1/iPP blendPB-1/iPP(90/10 w/w)blend sample was prepared by solution precipitation method.The blend film was quenched into liquid nitogen directly from the melt.The quenched sample was first heated to low temperature T1 and annealed for a time t,followed by annealing at a high temperature Th for 30 min,and the cold crystallization process was investigated by FTIR,DSC,and WAXD.Then,the effect of low temperature T1,annealing time t,and high temperature Th on the cold crystallization behavior of the PB-1/iPP blend was studied systematically,and the structure evolution and crystal formation during two-step cold crystallization was measured by in-situ FTIR spectroscopy.It is found that form Ⅰ’ crystals can be obtained directly in the blend with the existence of a small amount of iPP after a two-step cold crystallization process.Upon annealing at low temperature,form Ⅱformation is suppressed and form Ⅰ’ nuclei prevail,which then develop into crystalline PB-1 dominated by form Ⅰ’ in the subsequent annealing at room temperature or above.The formation of form Ⅰ’ as the major polymorph in PB-1/iPP blend containing only a small amount of iPP is attributed mainly to the confinement on PB-1 imposed by the dispersed iPP chains,which hinders form Ⅱ nucleation.2.The effect of molecular weight on the cold crystallization of PB1/iPP blendPB-1/iPP blend samples of 90/10(w/w)composition were prepared from PB-1s and iPPs with different molecular weight,and the cold crystallization was studied by FTIR,WAXD,and DSC.It was found that by decreasing the molecular weight of either PB-1 or iPP,especially the latter,the content of form Ⅰ’ crystals can be increased signifcantly.In PB-1 blends with iPP of the lowest molecular weight of 12 kg/mol,90-95%of the crystalline PB-1 obtained was form Ⅰ’.The observed molecular weight effects on the PB-1 polymorphism in the cold crystallization are attributed to miscibility between the PB-1 and iPP components in the molten state.The degree of mixing between PB-1 and iPP can be enhanced by decreasing the molecular weight and a more confined structure can be obtained after quenching,which lead to a more inhibition effect on the formation of form Ⅱ.3.The crystallization mechanism of form Ⅳ crystals in PB-1/iPP blend.PB-1/iPP and PB-1/aPP blend samples have been prepared and the two-step cold crystallization process was investigated systematically.It was found that form Ⅰ’crystals can be formed directly and become the dominant species in PB-1/iPP blends after cold crystallization and there’s only form Ⅰ’ crystals obtained with the content of iPP components increased to 30 wt%.For PB-1/aPP blends,form ⅠI crystals are still favored and become the dominant species after cold crystallization process.Comparing the results of PB-1/iPP and PB-1/aPP blends after cold crystallization,we can concluded that the iPP crystals can induced the nucleation of form Ⅰ’ in the blend.When iPP crystallized first at high temperature followed by the cold crystallization of PB-1,form Ⅱ crystals are obtained and no form Ⅰ’ crystals generated,indicating form ⅠI nucleation can also be promoted by iPP crystalline when confined structure is absent in the blend.Comparing these results,it shows that the confined structure plays a decisive role on the formation of form Ⅰ’,and the existence of iPP crystals can also promote the nucleation of form Ⅰ’.