Kinetics And Structure Change Mechanisms Of The Form ? To ? Transition In Polybutene-1

Polybutene-1(PB-1)is a typical polymorphic polymer,and exhibits excellent mechanical properties compared with other polyolefins,such as superior resistance to high temperature creep and environmental stress cracking,good chemical corrosion resistance and abrasion resistance.It is mainly used as pipes and films,with a service life up to 50-100 years.However,during processing in the industry,PB-1 materials crystallize into the metastable form II upon cooling,and would spontaneously transform into the stable form I during the storage at room temperature,with a significant enhancement of mechanical properties.It needs several weeks to complete such crystal-to-crystal transition,thus greatly increases the production period and cost.PB-1 is thereby known as"plastic gold"due to its'excellent performance and high price.In thi.s thesis,four PB-1 samples with different molecular weight were used to investigate the kinetics and mechanism of structure change of the form II to form I transition in PB-1,by means of differential scanning calorimetry(DSC),small angle/wide angle X-ray scattering(SAXS/WAXD)techniques.The main results are summarized as follows:1.Kinetics of the nucleation and growth of form II to I phase transition in PB-1.Kinetics of form II to I phase transition in PB-1 and its annealing temperature dependencies were investigated by means of DSC and in-situ WAXD techniques.The PB-1 samples were isothermally crystallized into metastable form II crystalline modification followed by annealing at a lower temperature(Tl)and at a higher temperature(Th)subsequently or at a single temperature(Ts)for a same period to promote polymorphic transition from form II to form I.This solid-to-solid phase transition was shown to be a two-step process including nucleation and growth,and more form I was obtained after being annealed at Tl and Th than being annealed at Ts for the same period.Annealing at Tl benefits nucleation of form I in form II crystal matrix due to internal stress induced by unbalanced shrinkage of amorphous and crystalline phases,while annealing at Th is beneficial to growth owing to rapid segmental diffusion at that temperature.It shows that nucleation and growth in phase transition have independent temperature dependencies,show Gaussian distribution with different center at-10 ? and 40 ?,respectively.Our results decomposed the polymorphic transition into nucleation and growth for the first time and provided a simple and effective way for rapid transition of form ? to I in PB-1.2.Intercrystalline links dependency of the phase transition kinetics.The effect of intercrystalline links containing tie molecules and entangled loops on polymorphic transition from form ? to form I in PB-1 of different molecular weights has been investigated.The PB-1 samples were isothermally crystallized at a range of temperatures respectively to develop metastable form II crystalline modification of different lamellar thickness,long spacing and number of intercrystalline links in amorphous phase.Stepwise annealing was applied to promote a faster polymorphic transition from form ? to ?,and the final fractions of transformed form I after annealing were obtained.At a given annealing condition,the transition rate in high molecular weight sample is found to increase with the increase of the prior form II crystallization temperature,while it shows a negative correlation between transition rate and crystallization temperature in low molecular weight sample.The results can be understood as follows.Long spacing of high molecular weight PB-1 is smaller than the radius of gyration of the chains in the melt,leading to formation of folded-chain crystals and high possibility of generating intercrystalline links.Higher internal stress induced by unbalanced shrinkage of amorphous and crystalline phases would be built up during cooling from higher crystallization temperature to the first temperature stage of annealing.For low molecular weight samples,the probability of forming intercrystalline links decreases with the increasing lamellar thickness at an elevated crystallization temperature.3.Nature of the phase transition without intercrystalline links and supercooling.The form II to I phase transition behavior in PB-1 in the absence of intercrystalline links and supercooling was investigated.As for the solid-solid transformation,the intercrystalline links in amorphous phase and the temperature difference between crystallization and phase transition were thought to be essential in previous studies,for generating and transmitting the internal stress to promote nucleation and growth of the phase transition.In this work,we report experimental evidences showing that such transition can be accomplished at the temperature of crystallization directly without supercooling,in a low molar mass sample where there are almost no intercrystalline links.This result reveals two significant aspects of the form ? to ? transition in polybutene-1 against conventional wisdom.1.The nucleation barrier of the transition is very low and can be overcome by thermal fluctuation;2.Entanglements and intercrystalline links in high molar mass systems play a role of stabilizing the metastable form ? via hindering the translational movements of chains in crystalline phase and slowing the relaxation of amorphous chains during phase transition.Direct formation of form I' from melt begins in the early stage of the phase transition,and proceeds during the phase transition.The melting temperatures of these form I' crystals increase with the aging temperature.Moreover,the structure and melting point of transformed form I highly depend on the annealing temperature.4.Kinetics of the phase transition at the late stage.The retardation of polybutene-1 form II to form I transition in the late stage has been investigated by means of time-resolved WAXD.Form ? samples with different lamellar thickness and constituent in amorphous phase were generated via varying crystallization temperature and molecular weight and aged at room temperature for the form II to form I transition.The II to I polymorphic transition in polybutene-1 undergoes two stages,where slow nucleation and rapid growth proceed in the first stage,and extremely slow secondary nucleation and growth take place in late stage.The degree of transition reaches a plateau value in late stage of transition,which depends highly on crystallization temperature and molecular weight,but is less affected by the kinetics in the first stage.Low molecular weight and high crystallization temperature always contributes to a greater degree of transition.According to the crystal unit cell parameters of form ? and form ?,there should be an extension in normal direction of lamellae and a shrinkage in lateral direction during phase transition,which thus would induce an additional pressure in normal direction and the tendency to expansion in lateral direction on the residual from II crystallites.The chain segments of lamella move in coordination with the molecular chains in the amorphous region,which are constrained by the transformed fixed form?.As a result,the nucleation and further growth of the ? to I transition are retarded.Low molecular weight and high crystallization temperature leads to a larger content of chain ends and higher chain mobility in the amorphous phase,hence the samples are more likely to release the unfavorable factors restraining generation of form I nuclei to some extent.5.Structure change during the phase transition at lamellar level.The structure change of polybutene-1 during the form II to 1 transition at lamellar level was investigated by small-angle X-ray scattering and interface distribution function fitting program.Scattering curves from samples possessing form II crystals showed Porod behavior and had a good fit,while the fitting process did not work on the scattering results of form I samples.It showed additional scattering contribution at the high scattering vector region besides the periodic lamellar structure for form I scatterings.Lamellar thickness of form I(dc,1)was much less than the theoretical estimation(1.12dc,?)according to the unit cell parameters of the two forms.There must be some hierarchal structure beyond the lamellar layer structure in form I and a reduce of the number of repeating units in lamellar chains.The observed results could be understood as follows:it is difficult for a lateral extended lamella to shrink integrally.Therefore,lamellae would split into blocks creating new interstitial spaces with lamellae.Such lateral microstructure of lamellae interferes with the general scattering of lamellar layer structures,resulting in bad fitting of scattering data from form I samples.As a result,one observed an increase of linear crystallinity but decrease of mass crystallinity after transition.