Study On Crystallization And Solid-solid Phase Transition Of Isotactic Polybutene-1 With In-situ X-ray Diffraction Technique

Isotactic polybutene-1（iPB-1）has attracted extensive attention due to its high mechanical properties,high temperature creep resistance,environmental stress cracking resistance,good chemical corrosion resistance and wear resistance,etc.iPB-1 is a typical polymorphic semi-crystalline polymer,whose melt crystallization in the actual material processing process usually does not directly form the thermodynamically stable form Ⅰ,but preferentially forms the metastable form Ⅱ with faster kinetics.Afterwards,form Ⅱ spontaneously transforms into form Ⅰ at room temperature,and the relative melting point,density and mechanical properties will be significantly improved after the transformation.However,that phase transformation will be accompanied by deformation of the sample,and it takes a long time to accomplish,which severely limits the commercial application of iPB-1.It focused on exploring the way to accelerate the phase transition or directly generate thermodynamically stable crystal form in this paper.On the one hand,the effect of melt state and crystallization pressure on the crystallization and phase transition behavior of iPB-1 were studied by differential scanning calorimetry（DSC）and in-situ wide-angle X-ray diffraction technique（WAXD）,and the formation mechanism of the stable form under high pressure was discussed and analyzed.On the other hand,the influence of nano-fillers on the crystallization and phase transformation kinetics of iPB-1 were also studied by DSC and WAXD,and the influence mechanism of nano-fillers on phase transformation was also analyzed.The specific research results are as follows:（1）The influence of melt structure and crystallization pressure on the crystallization and phase transition behavior of iPB-1.By controlling the melting temperature(T_melt)and crystallization pressure(P_cry)of the sample,the influence of structural melt on the crystallization behavior of iPB-1 under different crystallization conditions was studied,where WAXD and DSC were used to measure the crystal structure.Under atmospheric pressure condition,the structure melt significantly affects the temperature and kinetics of iPB-1 recrystallization,while its crystal structure is not affected the recrystallization still forms form Ⅱ.Under high pressure condition,the structural melt selectively recrystallizes into form Ⅱ or stable form Ⅰ’,depending on T_melt and P_cry.The content of form Ⅰ’decreases with increasing T_melt or decreasing P_cry.Meanwhile,the critical pressure for the complete formation of form Ⅰ’rises with increasing T_melt.The formation of form Ⅰ’is attributed to the memory effect of the melt which preserved some ordered sequence of crystal and the high pressure(P_cry)which suppressed the nucleation and growth of the kinetically favored form Ⅱ,which promoted the formation of form Ⅰ’.In addition,the melt crystallized form Ⅱ transforms to form Ⅰ under high pressure conditions.The relative contents of the three crystalline forms for different T_melt and P_cry were also obtained in this work.Based on the research results,a crystallization phase diagram of iPB-1 in T_melt-P_cry space is drawn.（2）The influence of nano-fillers on the crystallization and phase transformation behavior of iPB-1.By controlling the content of nano-fillers,the effect of nano-fillers on the crystallization and phase transformation kinetics of iPB-1 were studied.WAXD and DSC were used to study the influence of nano-fillers on the crystallization of form Ⅱ crystals obtained from the melt crystallization in iPB-1 and the subsequent phase transition kinetics.The addition of boron nitride（BN）promotes heterogeneous nucleation of iPB-1 matrix,which increases the nucleation site of form Ⅱ,thus accelerates its crystal nucleation,and then increases its crystallization rate and crystallization temperature.The existence of BN restricts the extension of form Ⅰ crystals in the normal direction of lamellar,which leads to a weakened ability of the fixed form Ⅰ crystals to regulate the movement of amorphous region segments connected with the crystal region.Therefore,the synergistic motion between form Ⅱ crystals and amorphous region segments at the late stage of the phase transition is enhanced,which provides a more favorable environment for the transformation of form Ⅱ at the late stage of the phase transition.The higher the content of BN,the synergistic motion between form Ⅱ crystals and amorphous region segments at the late stage of the phase transition is stronger,thus the time of complete phase transition is greatly shortened.The presence of BN did not affect the lamellar thickness of the initial form Ⅱ,while the lamellar thickness of form Ⅰ obtained during the transformation process gradually increases,which is also attributed to the spatial restriction effect of BN on the extension of form Ⅰ crystals in the normal direction of lamellar during the transformation process.In addition,compared with one-dimensional nano-fillers,two-dimensional nano-fillers have a stronger spatial restriction effect on the extension of form Ⅰ crystals in the normal direction of lamellar,which may be related to the inherent characteristics of the nano-fillers themselves.