The Study On Kinetics And Mechanism Of Polybutene-1 Phase Transition From Form ? To Form ? With In-situ X-ray Diffraction Technique

Isotactic polybutene-1(iPB-1)is a typical polymorphic polyolefin material.iPB-1 can form four crystal modifications according to different crystallization conditions.Among these crystal modifications,form I and form ? are the most important crystal forms.iPB-1 generally forms form ? when it is crystallized from the melt,and form ? will spontaneously transform into form I.The phase transition endows iPB-1 with a higher melting point and density,and improves its mechanical properties.It takes several weeks to for iPB-1 to complete the phase transition,resulting in a substantial increase in production cycle and production costs.Therefore,it is of great significance to accelerate this transition.Studies have shown that applying strain is an effective method to accelerate this phase transition.The transformation of iPB-1 from form ? to form I during solid stretching has been reported,but no attention has been paid to the phase transition during annealing after stretching to different strains.In addition,the literatures only studied the structural evolution during extension of iPB-1 melt,and did not pay attention to the phase transition during annealing after extension.This dissertation focuses on the phase transformation of iPB-1 during the annealing process after solid stretching or melt stretching to different strains,and analyzes the structural factors and phase transformation mechanisms that affect the phase transformation.The main research results are as follows:(1)The effect of solid pre-stretched form ? on its further phase transformation was studied combined with in-situ wide-angle X-ray diffraction(WAXD).Three tensile modes were considered.In mode A,the sample was stretched to their breaking point,and the phase transition was wholly completed.In mode B,the sample was stretched to the set pre-strain,and WAXD was used to study the phase transition during the annealing process after the stretching.Both ends of the sample were fixed during the annealing process.Mode C was studied under the same conditions as mode B,but the sample was allowed free shrinkage during the phase transition.The in-situ WAXD results show that as the pre-strain increases,the phase transition kinetics of the remaining form ? accelerates,and the final degree of phase transition also increases.The Avrami analysis shows that the growth dimension of form I decreases with the increase of pre-strain,which is attributed to the increase in the confinement effect of tensile-induced form I as the pre-strain increases.Although the phase transition of the sample pre-strained in the elastic region still has an induction period,the phase transition is facilitated,compared with the unstretched sample.The pre-stretching may affect the conformation or the lattice arrangement of the segments in form ?,thereby accelerating the subsequent phase transition.Through the comparative study of stretching mode B and mode C,it is found that the stress after stopping the stretching in mode B does not significantly affect the phase transition,which is because the stress is lower than the critical stress for triggering the nucleation of crystal form I.(2)The phase transition kinetics of the oriented form ? induced by melt extension was studied using a homemade extensional rheometer and in-situ WAXD.The effects of strain and strain rate on the structure and phase transition kinetics of form ? were systematically studied.The results show that strain has a more significant effect on the structure and phase transition kinetics of form ? than strain rate.In-situ WAXD analysis results show that the orientation degree of form ? gradually increases and the phase transformation kinetics increases with increasing pre-strain.Small-angle X-ray scattering(SAXS)analysis shows that the pre-strain before the strain hardening point produces form ? with a highly oriented lamella structure,and the sample stretched beyond the strain hardening point has a shish-kebab structure.Combining with the mechanical response curves in the extension process,it can be inferred that the chain are disentangled after yielding.The chain disentanglement may further accelerate the phase transition through improving the mobility of chains in the amorphous region.Therefore,the phase transition kinetics of the sample after yielding is increased compared to the sample before it is stretched to yield.The SAXS analysis of the samples extended at different extension rates shows that the shish-kebab structure produced after strain hardening.We found that the shish plays a very important role in the phase transition,and which is attributed to the high internal stress transfer efficiency and more tauter tie chains as a result of the transition of shish precedes kebab.