Solvent Vapor Annealing Induced Polymorphic Transformation of Polybutene-1

Solvent annealing is a facile method for changing the aggregated microstructure and physical properties of polymer materials. In this paper, we addressed the effects of solvent vapor annealing, including chloroform and water vapor, on the polymorphic transformation in both hot-pressed film and electrospun nonwoven of isotactic polybutene-1 （PB-1） by means of in situ Fourier transform infrared spectroscopy （FTIR）. The pretty rapid transition rate caused by the increased motion of molecular chains under chloroform vapor is associated with a lowest crystallinity. Also, a decreased crystallinity with the crystal transition occurred in electrospun nonwovens resulting from the relaxation of the stretched molecular chains into amorphous state rather than realignment into crystal form I predominating the crystal transition process.

Fabrication of Trans-1,4-polyisoprene Nanofibers by Electrospinning and Its Crystallization Behavior and Mechanism

Trans-1,4-polyisoprene（TPI） nanofibers have been fabricated successfully through electrospinning technology.Through the control of electrospinning parameters, highly crystallized TPI fresh fibers composed mainly of β phase were produced. Morphology and diameter of TPI nanofibers can be controlled by adjusting the electrospinning conditions. The in situ observations of FTIR spectra revealed that the crystallinity of the TPI fibers decreased with aging. While for TPI nanofibers aging at 45 °C for 24 h, a decrease in crystallinity as well as β to a transformation was observed with aging and these changings happened in the first 50 h during aging. The mechanism for β-TPI formation during electrospinning process and the reduced crystallinity with aging were proposed.

Differential Polymorphic Transformation Behavior of Polybutene-1 with Multiple Isotactic Sequences

For the solid-solid transformation from form Ⅱ to form Ⅰ of isotactic polybutene-1(iPB),the temperature dependence of form Ⅰ nucleation and growth was deemed to control the transformation process.However,the relationship between formⅠ formation and form Ⅱ disappearance in the transformation process is not clear.In this work,the spontaneous crystal transformation from form Ⅱ to Ⅰ of iPB with 81 mol%mmmm sequence concentration is studied firstly by tracking the two processes,the decay of form Ⅱ and the yielding of form Ⅰ in a wide range of temperature spanning from 0℃ to 50℃ and in a long transformation time ranging from 5 min to 65 days with in situ FTIR and WAXD.Unlike the literature reports,the decay rate of form Ⅱ is firstly found to be lower than the yielding rate of form Ⅰ at all studied temperatures,especially at low transition temperature.This is attributed to the amorphous chains which locate near crystal lamella participating into the nucleation of form Ⅱ.The regular chain folding and growth of i PB form Ⅰ from amorphous chains containing short isotactic sequences also lead to an increase in crystallinity of form Ⅰ compared with that of initial form Ⅱ crystallized at 60℃.An increase in the annealing temperature results in decrease in crystallinity and increase in lamellae thickness of i PB formⅠ.

Polymerization Kinetics of Propylene with the MgCl_(2)-Supported Ziegler-Natta Catalysts——Active Centers with Different Tacticity and Fragmentation of the Catalyst

The catalytic activity and stereospecificity of olefin polymerization by using heterogeneous TiCl_(4)/MgCl_(2) Ziegler-Natta(Z-N) catalysts are determined by the structure and nature of active centers, which are mysterious and fairly controversial. In this work, the propylene polymerization kinetics under different polymerization temperatures by using Z-N catalysts were investigated through monitoring the concentration of active centers [C*] with different tacticity. SEM was applied to characterize the catalyst morphologies and growing polypropylene(PP) particles. The lamellar thickness and crystallizability of PP obtained under different polymerization conditions were analyzed by DSC and SAXS. The PP fractions and active centers with different tacticity were obtained with solvent extraction fractionation method. The catalytic activity, active centers with different tacticity and propagation rate constant kp, fragmentation of the catalyst, crystalline structure of PP are correlated with temperature and time for propylene polymerizations. The polymerization temperature and time show complex influences on the propylene polymerization. The higher polymerization temperature(60 ℃) resulted higher activity, kp and lower [C*], and the isotactic active centers Ci* as the majority ones producing the highest isotactic polypropylene(iPP) components showed much higher kp when compared with the active centers with lower stereoselectivity. Appropriate polymerization time provided full fragmentation of the catalyst and minimum diffusion limitation. This work aims to elucidate the formation and evolution of active centers with different tacticity under different polymerization temperature and time and its relations with the fragmentation of the PP/catalyst particles, and provide the solutions to the improvement of catalyst activity and isotacticity of PP.

Linear Viscoelastic Behaviors of Polybutene-1 Melts with Various Structure Parameters

The effects of weight-average molecular （Mw）, molecular weight distribution （MWD）, and isotacticity on the linear viscoelastic behavior of polybutene-1 melts are studied. It is observed that the linear viscoelastic region becomes slightly narrower with increasing frequency. In frequency sweeps, the transition of the polymer melts flow from Newtonian flow to power-law flow can be observed. The melts with higher Mw and：or broader MWD, as well as higher isotacficity exhibit higher complex viscosity, zero shear viscosity, viscoelasticity moduli, relaxation modulus, broader transition zone, while lower critical shear rate, non-Newtonian index, and the frequency at which elasticity begins to play an important role. The relationship of zero shear viscosity on Mw has been established, which agrees with the classical power law. Furthermore, it is found that the cross-over frequency decreases with increasing Mw and the cross-over modulus increases with narrowing MWD.