Influence Of Lamellar Thickness On The Crystalline Transformation In Isotactic Polybutylene-1 And Its Nanocomposites

In general,more than half of polymers can crystallize and many polymers can dis-play complex polymorphic forms.Isotactic polybutylene-1(PB-1),as one of typical polymorphic polymers,is of particular interest in commercial and scientific fields as a result of temperature resistance,puncture resistance,low creep and so on.Generally speaking,the kinetically metastable form ?(with 11/3 helical structure)obtained from the melt crystallization usually undergoes the solid-to-solid phase transformation into stable form ?(with 3/1 helical structure)with excellent properties for several weeks during the storage at room temperature,resulting in the production period prolonging and cost increasing.Therefore,probing the internal factors to affect the transformation from crystalline form ? to form ? for PB-1,and further obtaining the possible solu-tion to accelerate the transformation are of great importance in both scientific value and practical implications.Carbon nanotube(CNT)acting as heterogeneous nucleat-ing agent not only accelerates the crystallization and but also improves mechanical and dielectric properties for nanocomposites.Based on it,this thesis explores in detail the formation of form ? and its transformation in PB-1 and PB-1/CNT nanocomposites.Lamellar thickness is one of the most important parameters describing the hierarchical order structure for polymer crystals,and strongly affects the crystalline transformation from form ? to form ? for PB-1.Therefore,the thesis designs different treatment meth-ods to obtain different lamellar thickness of form ? and investigates the influence of lamellar thickness on the transformation from form ? to form ? in neat PB-1 samples and PB-l/CNT nanocomposites by means of differential scanning calorimetry(DSC),in-situ small-angle X-ray scattering(SAXS),wide-angle X-ray diffraction(WAXD),polarized optical microscopy(POM),rheometer.The main results are summarized as follows:1.The Crystalline Transformation of PB-1 in the Condition of the Coexisted Thin and Thick LamellaeTo explore the influence rule of lamelllar thickness on the crystalline transforma-tion,we have adjusted the isothermal crystallization temperature and time to obtain the coexistence of the thin and thick lamellae of form ?.The higher isothermal temperature is,the thicker lamellae are.We obtain the thick lamellae of form ? with different crys-tallinity by changing the isothermal crystallization time at high temperature(100?).Subsequently,the thick lamellae of sample with different crystallinity immediately an-neals at 25? to obtain the thin lamellae of form ? and then the transformation from coexisted thin and thick lamellae of form ? into form ? occurs when keeping isothermal at 25?.In fact,the form ? to form ? crystalline transformation in PB-1 after isothermal crystallization goes through two stages,in which the first stage is slow nucleation and rapid growth proceeding and the late stage is extremely slow secondary nucleation and growth.It is seen that the transformation of thick lamellae is always superior to that of thin lamellae.The crystal transformation time is shortened to?48 h for the samples with only thick lamellae of form ?,which also weakens the retardance of form ? to form? transition at late stage.By directly observing the crystalline morphology via POM,we find that crystalline form ? formed at higher isothermal temperatures possesses loose and irregular spherulites,which facilitates the transition in contrast with the dense and regular spherulites formed at lower temperatures.Moreover,the large temperature dif-ference during cooling from higher isothermal temperature to room temperature also accelerates the transformation due to increasing internal stress.This work builds up connections among the crystalline morphology,the lamellar thickness,and the transfor-mation behavior,which may help understand the crystalline transformation from form? to form ? in PB-1.2.The influence of lamellar thickness on the transformation in PB-1/CNT nanocom-positesWe have been investigated the influence of lamellar thickness on the transformation from form ? to form ? in PB-1/CNT nanocomposites by DSC,POM and SAXS tech-niques.The crystallization kinetics of PB-1/CNT nanocomposites at different isother-mal temperatures(Tc)indicates that CNT significantly accelerates the nucleation and growth of form ?,acting as heterogeneous nucleating agent.However,the influence of CNT on the form ?-? transformation strongly relies on the lamellar thickness formed at different Tc,verified by the change of melting points(Tm)and the SAXS results of form?.The addition of CNT accelerates the transformation and elevates the Tm of completed form ? when the Tc is lower than?88?,while slows the transformation and slightly decreases the Tm when the Tc is higher than?88?.This is probably due to the fact that the incorporation of CNT facilitates the increase of lamellar thickness of form ?formed at lower Tc but decreases the lamellar thickness of form ? formed at higher Tc.This study illustrates that the lamellar thickness is one of the key points to determine the transformation from form ? to form ? in PB-1 and its nanocomposites.3.The effect of percolation network formed by nanoparticles on the transformation for PB-1/CNT nanocompositesWe have been investigated the effect of percolation network on the transformation in PB-1/CNT nanocomposites by rheometer,DSC,and field-emitting scanning electron microscope(FESEM).Oscillatory frequency sweep were carried out to determine the rheological percolation threshold which is?1.0%.By contrast,we equally confirm the rheological percolation threshold of the CNT/PB-1 nanocomposites at nearby?1.0%by means of van Gurp Palmen plots and Cole-Cole plots obtained from small amplitude oscillatory shear(SAOS).The crystallization kinetics for CNT/PB-1 nanocomposites under isothermal(Tc=100?)and non-isothemal crystallization indicates that the addi-tion of CNT still accelerates crystallization of form ? due to heterogeneous nuclei and the best effect with 1.0%,but the effect of percolation network becomes weak within limited range(0.05%?2.0%).The formation of percolation network has different ef-fects on the transformation for thicker lamellae of form ? formed at Tc=100? and thinner lamellae of form ? formed during the non-isothermal crystallization.Incorpo-ration of CNT with different contents slows the transformation of thicker lamellae as a whole and the addition of CNT with a small amount(0.05%)accelarates the transfor-mation of thinner lamellae,which is similar with the previous chapter' results.When the content of CNT further increases to more than 1.5%,the transformation is evidently restrained by the formation of percolation network.The percolation network restrains chain mobility due to steric-hindrance effect and leads to a less effective transmission of the internal stress,further slowing the crystalline transformation.From this,perco-lation network formed by nanoparticles is not conducive to the transformation of form? in PB-1.