PRELIMINARY INVESTIGATION ON THE MISCIBILITY OF ISOTACTIC POLYPROPYLENE （iPP） AND SYNDIOTACTIC POLYPROPYLENE （sPP） BLENDS

Experimental miscibility studies were performed on different compositions of iPP/sPP blends, where sPP has a low syndiotacticity （[rrrr] = 81%）. Combining optical microscopy, rheology, and solid state NMR spectroscopy, the miscibility of the blends was investigated at different scales in the traditionally thought to be ＂immiscible＂ iPP/sPP blends. For the composition of iPP/sPP （90/10） blend, it shows to be miscible in the melt, and furthermore, the existence of intermolecular chain interactions between sPP and iPP components was detected in the solid state.     

EFFECT OF COMPATIBILITY ON PHASE MORPHOLOGY AND ORIENTATION OF ISOTACTIC POLYPROPYLENE （IPP） BLENDS OBTAINED BY DYNAMIC PACKING INJECTION MOLDING

The effect of compatibility on phase morphology and orientation of isotactic polypropylene （iPP） blends under shear stress was investigated via dynamic packing injection molding （DPIM）. The compatibility of iPP blended with other polymers, namely, atactic polypropylene （aPP）, octane-ethylene copolymer （POE）, ethylene-propylene-diene rubber （EPDM） and poly（ethylene-co-vinyl acetate） （EVA）, have first been studied using dynamic mechanical analysis （DMA）. These blends were subjected to DPIM, which relies on the application of shear stress fields to the melt/solid interfaces during the packing stage by means of hydraulically actuated pistons. The phase morphology, orientation and mechanical properties of the injection-molded samples were characterized by SEM, 2D WAXS and Instron. For incompatible iPP/EVA blends, a much elongated and deformed EVA particles and a higher degree of iPP chain orientation were observed under the effect of shear. However, for compatible iPP/aPP blends, a less deformed and elongated aPP particles and less oriented iPP chains were deduced. It can be concluded that the compatibility between the components decreases the deformation and orientation in the polymer blends. This is most likely due to the hindering effect, resulting from the molecular entanglement and interaction in the compatible system.     

Molecular origin of demixing, prior to crystallization, of atactic polypropylene/isotactic polypropylene blends upon cooling from the melt

An amorphous 50/50 atactic polypropylene (aPP)/isotactic polypropylene (iPP) mixture at 125 degrees C was simulated using a second nearest neighbor diamond lattice and a three states rotational isometric state model. The result suggests that at the liquidlike density that corresponds to the atmospheric pressure, aPP prefers to interact with other aPP chains rather than with iPP chains. The result is consistent with the inference of Keith and Padden [J. Appl. Phys. 35, 1286 (1964)] that aPP and iPP will tend to separate from one another in their melt at 125 degrees C, before the onset of crystallization of iPP. The tendency for immiscibility of the amorphous aPP/iPP blend is likely attributed to the presence of short syndiotactic sequences in the aPP chains adopting all-trans conformations. The attractive intermolecular interaction of pairs of such subchains at 125 degrees C promotes the separation of aPP from iPP. This interaction is weakened at higher temperature, where aPP and iPP become miscible. The result also shows that miscibility of the blend increases with increasing pressure. However, the origin of the pressure effect is not clear.     

Miscibility behavior and single chain properties in polymer blends: a bond fluctuation model study

Computer simulation studies on the miscibility behavior and single chain properties in binary polymer blends are reviewed. We consider blends of various architectures in order to identify important architectural parameters on a coarse grained level and study their qualitative consequences for the miscibility behavior. The phase diagram, the relation between the exchange chemical potential and the composition, and the intermolecular pair correlation functions for symmetric blends of linear chains, blends of cyclic polymers, blends with an asymmetry in cohesive energies, blends with different chain lengths, blends with distinct monomer shapes, and blends with a stiffness disparity between the components are discussed. For strictly symmetric blends the Flory‐Huggins theory becomes quantitatively correct in the long chain length limit, when the χ parameter is identified via the intermolecular pair correlation function. For small chain lengths composition fluctuations are important. They manifest themselves in 3D Ising behavior at the critical point and an upward parabolic curvature of the χ parameter from small‐angle neutron scattering close to the critical point. The ratio between the mean field estimate and the true critical temperature decreases like √χ/(ρb³) for long chain lengths. The chain conformations in the minority phase of a symmetric blend shrink as to reduce the number of energeticaly unfavorable interactions. Scaling arguments, detailed self‐consistent field calculations and Monte Carlo simulations of chains with up to 512 effective segments agree that the conformational changes decrease around the critical point like 1/√N. Other mechanisms for a composition dependence of the single chain conformations in asymmetric blends are discussed. If the constituents of the blends have non‐additive monomer shapes, one has a large positive chain‐length‐independent entropic contribution to the χ parameter. In this case the blend phase separates upon heating at a lower critical solution temperature. Upon increasing the chain length the critical temperature approaches a finite value from above. For blends with a stiffness disparity an entropic contribution of the χ parameter of the order 10^–3 is measured with high accuracy. Also the enthalpic contribution increases, because a back folding of the stiffer component is suppressed and the stiffer chains possess more intermolecular contacts. Two aspects of the single chain dynamics in blends are discussed: (a) The dynamics of short non‐entangled chains in a binary blend are studied via dynamic Monte Carlo simulations. There is hardly any coupling between the chain dynamics and the thermodynamic state of the mixture. Above the critical temperatures both the translational diffusion and the relaxation of the chain conformations are independent of the temperature. (b) Irreversible reactions of a small fraction of reactive polymers at a strongly segregated interface in a symmetric binary polymer blend are investigated. End‐functionalized homopolymers of different species react at the interface instantaneously and irreversibly to form diblock copolymers. The initial reaction rate for small reactant concentrations is time dependent and larger than expected from theory. At later times there is a depletion of the reactive chains at the interface and the reaction is determined by the flux of the chains to the interface. Pertinent off‐lattice simulations and analytical theories are briefly discussed.     

Analysis of polyolefin blends by CRYSTAF

Crystallization analysis fractionation (CRYSTAF) has been introduced for the analysis of the composition of polyolefin blends and the chemical composition distribution of polyolefins. Blends of syndiotactic and isotactic polypropylene (sPP and iPP) and of sPP/High density polyethylene (HDPE) have been fractionated by CRYSTAF and the results been compared to those from DSC. While the blends of sPP and HDPE cannot be separated by DSC a quantitative determination of both components is possible by CRYSTAF over the whole range with the detection limit being 1% on both ends. Furthermore it is demonstrated that the separation of ternary blends of sPP, iPP and HDPE is possible by CRYSTAF.     

Monte Carlo simulation study of the effect of chain tacticity on demixing of polyethylene/polypropylene blends

The molecular origin of the demixing behavior for 50: 50 (wt/wt) polyethylene/polypropylene (PE/PP) with different tacticity of PP at the melts (473 K) was investigated by Monte Carlo simulation of coarse-grained polymer model. Isotactic (iPP), atactic (aPP) and syndiotactic (sPP) polypropylenes were used for blending with PE. Coarse-graining polymer chains were represented by 50 beads, corresponding to C₁₀₀H₂₀₂ and C₁₅₀H₃₀₂ for PE and PP, respectively. The simulation was performed on a high coordination lattice incorporating short-range intramolecular interactions from the Rotational Isomeric State (RIS) model and long-range intermolecular interactions Lennard-Jones (LJ) potential function of ethane and propane units. Chain dimensions, the characteristic ratio (C _n ) and self-diffusion coefficient (D) of PE in the blends are sensitive to the stereochemistry of PP chains. Compared with neat PE melts, PE dimension was relatively unchanged in PE/iPP and PE/aPP blends but slightly decreased in PE/sPP blends. PP dimension was increased in PE/iPP and PE/aPP mixture but decreased in PE/sPP blend in comparison with neat PP melts. In addition, diffusion of PE and PP chains in PE/PP mixture was decreased and increased, respectively, compared to the pure melts. Interchain pair correlation functions were used to detect the immiscibility of the blends. The tendency of demixing of PE/aPP and PE/iPP blends were weaker than that of PE/sPP blend.     

Crystallization and phase morphology of injection‐molded isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends

The crystallization and phase morphology of the injection‐molded isotactic polypropylene (iPP)/syndiotactic polypylenen (sPP) blends were studied, focusing on the difference between the skin layer and core layer. The distribution of crystallinity of PPs in the blends calculated based upon the DSC results shows an adverse situation when compared with that in the neat polymer samples. For 50/50 wt % iPP/sPP blend, the SEM results indicated that a dispersed structure in the skin layer and a cocontinuous structure in the core layer were observed. A migration phenomenon that the sPP component with lower crystallization temperature and viscosity move to the core layer, whereas the iPP component with higher crystallization temperature and viscosity move to the skin layer, occurred in the iPP/sPP blend during injection molding process. The phenomenon of low viscosity content migrate to the low shear zone may be due to the crystallization‐induced demixing based upon the significant difference of crystallization temperature in the sPP and iPP. This migration caused the composition inhomogeneity in the blend and influenced the accuracy of crystallinity calculated based upon the initial composition. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2948–2955, 2007     

Polymer blends based on the β-modification of polypropylene

iPP/sPP, iPP/rPP, iPP/PVDF and iPP/PA-6 blends, and their β-nucleated forms were prepared in the present study. The components of iPP/sPP and iPP/rPP blends are compatible in the molten state. The phase structure of the melt of iPP/PVDF and iPP/PA-6 blends is heterogeneous. The melting and crystallisation characteristics as well as the structure and polymorphic composition of these blends were studied by polarised light microscopy (PLM) and differential scanning calorimetry (DSC). When semicrystalline polymers are added to iPP, the most important factor of the formation a blend with β-crystalline phase is the α-nucleation effect of the second polymer. In the case of polymers with an α-nucleating effect, the temperature range of their crystallisation should be lower than that of β-iPP. β-nucleated iPP/PVDF and iPP/PA-6 blends are extreme examples showing that completely β-iPP matrix can not form even in the presence of a highly effective β-nucleant, because of the strong α-nucleating ability and higher crystallisation temperature range of PVDF and PA-6. We found that the β-crystallisation tendency of random propylene copolymers can be enhanced by adding an iPP homopolymer.     

Conformation and diffusion behavior of ring polymers in solution: a comparison between molecular dynamics, multiparticle collision dynamics, and lattice Boltzmann simulations

We have studied the effect of chain topology on the structural properties and diffusion of polymers in a dilute solution in a good solvent. Specifically, we have used three different simulation techniques to compare the chain size and diffusion coefficient of linear and ring polymers in solution. The polymer chain is modeled using a bead-spring representation. The solvent is modeled using three different techniques: molecular dynamics (MD) simulations with a particulate solvent in which hydrodynamic interactions are accounted through the intermolecular interactions, multiparticle collision dynamics (MPCD) with a point particle solvent which has stochastic interactions with the polymer, and the lattice Boltzmann method in which the polymer chains are coupled to the lattice fluid through friction. Our results show that the three methods give quantitatively similar results for the effect of chain topology on the conformation and diffusion behavior of the polymer chain in a good solvent. The ratio of diffusivities of ring and linear polymers is observed to be close to that predicted by perturbation calculations based on the Kirkwood hydrodynamic theory.     

Photo‐Oxidation of sPP/Organoclay Nanocomposites

Photo‐oxidation of syndiotactic polypropylene–sPP/organoclay nanocomposites was performed. Nanocomposites were prepared in situ by melt compounding of sPP, compatibilizer (iPP grafted with maleic anhydride–iPP‐g‐MAN) and organoclay filler ME C18 (modified with octadecyl ammonium chains in intergaleries of layered silicate, of which silicate layers (about 1 nm thin) were exfoliated). The influence of ME C18 nanoparticles alone (in content region 1 to 15 wt%) and together with compatibilizer iPP‐g‐MAN on the photostability of the sPP nanocomposite was studied. It was found that the silicate ME C18 nanoparticles alone catalyze the photooxidation and shorten the induction period of photo‐oxidation to one fourth (at the content of 5 wt% of ME C18) in comparison with unfilled sPP) and the presence of compatibilizer supports the photo‐oxidation of sPP nanocomposite. The ME C18 nanoparticles decrease the efficiency of UV stabilizers. The rate of photo‐oxidation of sPP/clay nanocomposite after the induction period is significantly higher than unfilled sPP. The mechanism of photo‐oxidation is discussed.     

Polypropylene/poly(butyl methacrylate) blends prepared by diffusion and subsequent polymerization of butyl methacrylate in isotactic polypropylene pellets

Polypropylene/poly(butyl methacrylate)(PP/PBMA) blends were prepared by diffusion and subsequent polymerization of butyl methacrylate(BMA) in commercial isotactic polypropylene(iPP) pellets.The diffusion kinetics,diametrical distribution of PBMA in a pellet and phase morphology of a typical PP/PBMA blend were investigated.     

X-ray investigations of the formation of core-fibrils crystallized out of the oriented melt

Crystallization models for the formation of core-fibrils crystallized from the oriented melt are discussed by comparing results from X-ray experiments with theoretical predictions. Two sets of polymer blends, the systems iPP/PB-1 and iPP/aPP have been measured. From wide angle X-ray scattering, the thicknesses of the core-fibrils, lattice distortions, and unit-cell parameters have been determined. Interface distribution functions have been used to evaluate the axial morphology of the fibers from meridional small angle X-ray scattering curves. It appears that the morphological predictions made by the crystallization models of Pennings and of Hoffman cannot be confirmed by the experimental findings. The diffusion model proposed by Petermann partially describes the morphological properties of the core-fibrils.     

Solid‐state modification of polypropylene (PP): grafting of styrene on atactic PP

Grafting of unsaturated vinyl monomers onto polypropylene (PP) is a convenient route to develop new polymeric materials with synergistic properties. Particular attention must be paid to the formation of graft copolymer relative to the formation of homopolymer, since the final properties are dependent both on the dispersion of the new polymer into the iPP matrix, which is controlled by the degree of grafting, as well as on the chemical nature of the in‐situ formed polymer chains. In the present work the grafting reaction of styrene on atactic PP (aPP), considered as good model system for the more studied solid‐state modification of isotactic PP (iPP), has been investigated in the presence of two different radical initiators in order to get a first insight into the grafting reactions onto PP. Several grafting reactions were carried out by changing the chemical compositions of the starting polymerization mixture, whose homogeneity was accurately investigated by Raman spectroscopy. Infrared‐spectroscopy (FT‐IR) was used for qualitative and quantitative characterization of the reaction product. A quantitative separation procedure, based on the concept of selective solvent extraction, has been established which enables the determination of the grafting efficiency (Φ) as well as the exact chemical composition of the final product. Finally, all products were characterized by means of Differential Scanning Calorimetry (DSC) in order to study their thermal behaviour.     

In silico studies of 1,3(R):2,4(S)‐dibenzylidene‐D‐sorbitol as a gelator for polypropylene

The organic gelator 1,3(R):2,4(S)‐dibenzylidene‐D‐sorbitol (DBS) self‐organizes to form a 3‐D network at relatively low concentrations in a variety of nonpolar organic solvents and polymer melt. In this work, we have investigated the interactions between DBS molecules in polypropylene (PP) by molecular modeling. We have used quantum mechanics to elucidate the preferred geometry of one molecule and a dimer of DBS, and molecular mechanics and molecular dynamics to simulate pure DBS, pure PP, and mixture of DBS and PP as condensed phases, at various temperatures. It was found that inter‐ and intramolecular H‐bonds between DBS molecules are formed in PP in a much more pronounced manner than those formed in pure DBS. The most significant intermolecular H‐bonds are formed between the terminal hydroxyl groups. The most significant intramolecular H‐bonds are formed between O5 /H‐O6 groups. Due to the H‐bonds, DBS molecules form a rigid structure similar to liquid crystal forming molecules, which might explain their tendency to create nanofibrils. It seems that the aromatic rings do not contribute significantly to the intermolecular interactions. Their main role is probably to stiff the molecular structure. Temperature dependences of inter‐ and intramolecular interactions are different. Whereas intermolecular interactions peak heights decrease when temperature increases for pure DBS, the intramolecular interaction almost does not change. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamics of polymers in a particle-based mesoscopic solvent

We study the dynamics of flexible polymer chains in solution by combining multiparticle-collision dynamics (MPCD), a mesoscale simulation method, and molecular-dynamics simulations. Polymers with and without excluded-volume interactions are considered. With an appropriate choice of the collision time step for the MPCD solvent, hydrodynamic interactions build up properly. For the center-of-mass diffusion coefficient, scaling with respect to polymer length is found to hold already for rather short chains. The center-of-mass velocity autocorrelation function displays a long-time tail which decays algebraically as (Dt)(-3/2) as a function of time t, where D is the diffusion coefficient. The analysis of the intramolecular dynamics in terms of Rouse modes yields excellent agreement between simulation data and results of the Zimm model for the mode-number dependence of the mode-amplitude correlation functions.     

类蛋白质分子扩散系数的模拟计算

蛋白质分子是一类比较特殊的高分子，一方面它的性质与氨基酸序列密切相关，另一方面其二级结构（如α螺旋以及β折叠等）极大地影响着它的性质．另外它的内部结构又非常紧密，与一般的高分子链完全不同．同时随着计算机技术的发展，计算机模拟日益作为一种有效的手段广泛应用于高分子科学的研究．但是由于蛋白质分子比一般的高分子结构更加复杂，如氨基酸之间有复杂的互相作用等，因此对蛋白质性质的研究往往建立在简化模型的基础上，如基于格点的HP紧密高分子模型等．虽然建立在格点模型上的蛋白质分子和真实的蛋白质分子存在着一定的差异，但基本上能体现蛋白质分子的主要特征，因此把这一类分子称为类蛋白质分子．     

Local friction in polyolefin blends

Processes on different length scales affect the dynamics of chain molecules. The friction experienced by a short chain segment depends on both small-scale chain properties and on the local environment of the segment. As a consequence, the (monomeric) friction coefficients of the two components of a binary polymer blend will, in general, differ from each other and from the friction coefficients of the corresponding melts. In this work, we investigate local friction in polyolefin blends with the aid of a small-scale simulation approach. The polymer chains, in united atom representation, are assumed to occupy the sites of a partially filled simple cubic lattice. The simulation focuses on short chain sections with straight backbones and enumerates all possible binary contacts and relative movements of such sections. By evaluating the exact enumeration results in conjunction with equations of state for the blends, we are able to make predictions about the variation of the friction coefficients with local chain architecture and thermodynamic state (temperature, pressure, and composition). We calculate relative values of friction coefficients at temperatures well above the glass transition for blends of PEP, an alternating copolymer of polyethylene and polypropylene, with polyethylene and polyisobutylene and for blends of polyethylene and atactic polypropylene. We also investigate a blend of PEP with head-to-head polypropylene and compare our results with experimental data.     

Synthesis of branched polypropylene with isotactic backbone and atactic side chains by binary iron and zirconium single‐site catalysts

This article reports the use of a binary single‐site catalyst system for synthesizing comb‐branched polypropylene samples having isotactic polypropylene (iPP) backbones and atactic polypropylene (aPP) side chains from propylene feedstock. This catalyst system consisted of the bisiminepyridine iron catalyst {[2‐ArN?C(Me)]₂C₅H₃N}FeCl₂ [Ar = 2,6‐C₆H₃(Me)₂] ( 1 ) and the zirconocene catalyst rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂ ( 2 ). The former in situ generated 1‐propenyl‐ended aPP macromonomer, whereas the latter incorporated the macromonomer into the copolymer. The effects of reaction conditions, such as the catalyst addition procedure and the ratio of 1 / 2 on the branching frequency, were examined. Copolymer samples having a branching density up to 8.6 aPP side chains per 1000 iPP monomer units were obtained. The branched copolymers were characterized by ¹³C NMR and differential scanning calorimetry. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1152–1159, 2003     

Molecular-dynamics simulation of crystallization in helical polymers

The molecular mechanism of crystallization in helical polymers is a fascinating but very difficult subject of research. We here report our recent efforts toward better understanding of the crystallization in helical polymers by use of molecular-dynamics simulation. With straightforward approaches to the problem being quite difficult, we adopt a different strategy of categorizing the helical polymers into two distinct types: one type is a simple bare helix which is essentially made of backbone atomic groups only and has smoother molecular contours, and the other is a more general helix having large side groups that would considerably hamper molecular motion and crystallization. Both types of helical polymers are here constructed by use of the united atom model, but they show quite distinct crystallization behavior; the crystallization of the former-type polymer is rather fast, while that of the latter-type polymer is extremely slow. We find that the bare helix, when rapidly cooled in free three-dimensional space, freezes into partially ordered state with limited intramolecular and intermolecular orders, and that remarkable improvement of order and growth of an ordered chain-folded crystallite occurs by very long-time annealing of the partially ordered state around the apparent freezing temperature. We also study crystallization of the bare helix upon a growth surface; the crystallization in this case proceeds much faster through highly cooperative process of the intermolecular and the intramolecular degrees of freedom. On the other hand, crystallization of the realistic model of isotactic polypropylene (iPP) having pendant methylene groups is found to be extremely sluggish. By restricting the spatial dimension of the system thereby fully disentangling the chain, we observe that the molecule of iPP crystallizes very quickly onto the crystal substrate made of the same iPP chain. Quite remarkable is that the molecule of iPP strictly recognizes the helical sense of the substrate chain and efficiently selects its chirality during crystallization.