Constraint release effects on the dynamics of polymer melts

The dynamical behavior of polymers with molecular weight distribution is analyzed from the standpoint of reptation and tube renewal. In a binary blend where the entanglements between longer chains are prominent, the shorter chain relaxes only by reptation, whereas the longer chain shows more complex behavior, i.e., reptation in the original tube, short-range tube renewal causing the tube enlargement, and thereafter reptation again in the expanded tube. Dynamic moduli data from literature are used for determining the compositional dependence of the relaxation times. Also on the basis of the relaxation mechanism considered here, the critical composition due to the onset of entanglements between different longer chains is proposed as a function of their component molecular weighl ratio.     

A rheological study of long branching in polyethylene by blending

Steady shear viscosities, dynamic viscosities and moduli, and the corresponding activation energies for flow were examined for a branched polyethylene, a linear polyethylene, and three of their blends at 150° and 190°C. The polyethylenes were chosen to have closely matched molecular weights and distributions. An R-17 Weissenberg rheogoniometer and an Instron capillary rheometer were used. At lower stress, the branched polymer had a higher viscosity than the linear one, possibly because of the contribution of long branches to entanglements. At high stress, this contribution is reduced and the inherently smaller coil dimensions likely become responsible for the lower viscosity of the branched polymer. The activation energy for the branched polymer is high and decreases with stress, in contrast to the low and almost-constant value for the linear polymer. The effects here of pressure on compression are considered. The entanglements of long branches may also decrease with increasing temperature. With decreasing stress, the activation energy for branched polymer tends to become constant, corresponding to an absence of pressure effects and an equilibrium entanglement of long branches for a given temperature range. The linear relationship between activation energy and blend composition problably means that any compressional effects, like free volume, are additive and that long-branch entanglements rearrange with added linear molecules. The linearity may be the result, in part, of a broad distribution for the lengths of long branches.     

Structure evolution in polymer blending using microfabricated samples

The transient rheological behavior and morphology evolution of polymethyl methacrylate (PMMA)/polystyrene (PS) binary polymer blends with well-defined initial structure were measured in simple shear flow under isothermal conditions. The size and distribution of the dispersed phase and the composition of the blends were designed and fabricated by Computer Numerical Controlling (CNC) machining, photolithography, and micro-embossing. Compatibilizer can easily be placed at the interface of the two components during sample preparation. The effects of initial dispersed domain size, blend composition, and interfacial tension on rheological behavior and morphology evolution were investigated. It was found that the transient shear stress and first normal stress difference are very sensitive to these parameters. The transient rheological responses up to the breakup point are compared with those predicted by both Doi-Ohta and Vinckier-Moldenaers-Mewis models.     

Studies on binary and ternary blends of polypropylene with ABS and LDPE. I. Melt rheological behavior

Studies are presented on melt rheological properties of binary blend of polypropylene (PP) and acrylonitrile–butadiene–styrene terpolymer (ABS), and ternary blend of PP, ABS, and low-den-sity polyethylene (LDPE). Data obtained in capillary rheometer are presented to describe the effect of blending ratio, shear stress, and shear rate on flow properties, melt viscosity, and melt elasticity. At a blend composition corresponding to 10 wt % ABS content, both binary and ternary blends show maximum in melt viscosity accompanied by minimum in melt elasticity. Pseudoplasticity of the melt decreases with increasing ABS content. In ternary blends, LDPE facilitates the flow at low LDPE contents and obstructs the flow at high LDPE contents. Scanning electron microscopic studies are also presented to illustrate the state of dispersion and its variation with blend composition.     

Influence of elasticity on dispersed‐phase droplet size in immiscible polymer blends in simple shearing flow

The influence of elasticity of the blend constituent components on the size and size distribution of dispersed‐phase droplets is investigated for blends of polystyrene and high density polyethylene in a simple shearing flow. The elasticities of the blend components are characterized by their first normal stress differences. The role played by the ratio of drop to matrix elasticity at fixed viscosity ratio was examined by using high molecular weight polymer melts, high density polyethylene and polystyrene, at temperatures at which the viscosity ratios roughly equaled each of three different values: 0.5, 1, and 2. The experiments were conducted by using a cone‐and‐plate rheometer, and the steady‐state number and volume‐mean averages of droplet diameters were determined by optical microscopy. After steady‐state shearing, the viscoelastic drops were larger than the Newtonian drops at the same shearing stress. From the steady‐state dispersed‐phase droplet diameters, the steady‐state capillary number, Ca, defined as the ratio of the viscous shearing stress over the interfacial tension stress, was calculated as a function of the ratio of the first normal stress differences in the droplet and matrix phases. For the blend systems with viscosity ratio 0.5, 1 and 2, the values of steady‐state capillary number were found to increase with the first normal stress difference ratio and followed a power law with scaling exponents between 1.7 and 1.9.     

PVC/EVA共混物的研究

本文研究了混炼温度和时间对PVC/EVA共混物抗冲性能的影响,发现加入聚乙烯,共混体系的抗冲击强度能进一步提高.用TEM观察了PVC/EVA的形态结构,采用Brabender塑化仪和毛细管流变仪研究了共混物的塑化和熔体流变行为.通过计算机对实验结果进行二元线性回归,建立了共混物的熔体粘度与剪切应力和温度相关联的数学模型.     

Nonhomogeneous flow of micellar solutions: A kinetic—network theory approach

The rheological behavior of micellar solutions is analyzed under nonhomogeneous velocity and stress flow conditions. The framework is based on the extended irreversible thermodynamics and the transient network formulation coupled to the underlying kinetics embodying two relevant processes: formation of wormlike chains from a free micellar solution through a thermally activated process and their flow induced degradation. The second kinetic process consists in the formation of entanglements from the free wormlike chains and their flow‐induced breakage. These processes are modeled in a coupled kinetic scheme constituted by a set of reversible kinetic equations describing the evolution in average of the three microstates (free short rod‐like micelles, free wormlike chains, and entangled wormlike chains) that reflect the complexity of macromolecular interactions. The predictions of the shear stress and first normal stress difference as a function of shear‐rate under banded flow are in good agreement with experimental data. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2277–2292, 2018     

Studies on binary and ternary blends of polypropylene with ABS and LDPE. II. Impact and tensile properties

Studies on impact and tensile properties of binary blend of PP and ABS and ternary blend of PP, ABS, and LDPE are presented. Variation of impact strength and the fracture surface morphology with blend composition is examined and interpreted. Tensile behavior in the yield region is studied and the trends of variation of work of yield and impact strength are compared to ascertain the predominent mechanism of impact toughening in these binary and ternary blends. An analysis of yield–stress data in terms of theoretical models is presented to reveal the differences in these binary and ternary blends, attributable to the role of LDPE component in the ternary blend.     

Enhanced strain hardening in elongational viscosity for HDPE/crosslinked HDPE blend. II. Processability of thermoforming

Rheological properties and processability of thermoforming were studied for high‐density polyethylene (HDPE) and a blend of HDPE with crosslinked HDPE (xHDPE). Blending the xHDPE, which enhances melt strength and strain hardening in elongational viscosity of HDPE, helps the sheet avoid sagging in thermoforming. Moreover, the product of the blend obtained by vacuum forming has uniform wall thickness. Melt strength and strain hardening of the blend were, however, depressed by a processing history in a single‐screw extruder, whereas reprocessing by a two‐roll mill enhanced the melt strength again. It is considered that the processing history by a single‐screw extruder, in which shear‐dominant flow takes place, depresses the trapped entanglements between network chain of xHDPE and linear HDPE molecules, and results in low level of melt strength. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 79–83, 2002     

Experimental and numerical investigations of the shear behavior of binary particle blends

In this study, a Schulze ring shear tester and the discrete element method (DEM) are employed to investigate the effect of polydispersity on the binary shear flows. Both experimental results and DEM simulations show that the preshear stresses are greater for binary blends than for monodispersed particles. The flowability of these mixtures is strongly affected by the solid fraction, with minimal flow function values correlating to maximum packing fraction. However, minimum flow function values are not observed at the same packing fractions where the maximum preshear stress occurs. Using DEM, it is demonstrated that the decrease of angular velocity of larger particles due to the addition of small adhesive particles reduces and the fraction of large-small particle contact both make contributions to shear stress difference. A mechanism is proposed to quantify the effects of these two factors.     

Effect of poly(ethylene oxide) with different molecular weights on the electrospinnability of sodium alginate

Although sodium alginate (SA) could not be electrospun from its aqueous solution, SA-based electrospun nanofibers can be fabricated with the help of polyethylene oxide (PEO). In this study, the influence of PEO on the electrospinnability of SA aqueous solution was investigated and the roles of chain entanglements and conformations of the blend system were emphasized. It was found that a little amount of PEO100 with high molecular weight could improve the electrospinnability of SA aqueous solution. However, a large amount of PEO2 with low molecular weight had no positive effect on the electrospinnability of SA aqueous solution. Dynamic laser light scattering (DLLS) results showed that only when the PEO molecular chains in aqueous solution were in an entangled state, PEO can enhance the electrospinnability of SA aqueous solution. The further study on rheological measurements showed that SA molecular chains could not form significant entanglements for the electrospinning even when the SA solution concentration approached concentrated regime. SA molecular chains are closely “overlapped” due to its rigid and extended conformation and cannot form effective chain entanglement. The main contribution of PEO100 to improve SA electrospinnability is offering entanglement sites and thereby enhancing the applicable entanglement degree of the blend system. Whereas, although the chain interaction between PEO2 and SA may improve slightly the flexibility of SA chains, the significant chain entanglements of the blend solution is not achieved. Three molecular models are proposed to depict visually the effect of PEO with different molecular weights on chain conformations and entanglements of SA.     

Morphology and mechanical properties of impact modified polypropylene blends

Isotactic polypropylene (PP) has been reactively blended with various grades of an ethylene–octene copolymer (EOC) in a twin‐screw extruder. Free radical polymerization of styrene and a multifunctional acrylate during melt extrusion has resulted in an enhancement of mechanical properties over the binary blend. The reactive blend exhibits a notched Izod impact strength over 12 times that of pure polypropylene and greater than double the performance of the binary blend. Electron microscopy shows that by grafting onto the polymers, elastomer particle size and interparticle distance decrease, while particle shape becomes less spherical. The acrylate is crucial to achieve superior performance, as infrared spectra correlate an increase in graft yield to improvements in stress–strain behavior and impact strength. In addition, melt flow index (MFI) and melt strength data indicate a reduction in unwanted side reactions of polypropylene and the presence of long‐chain branching. Dynamic‐mechanical analysis reveals that the reaction promotes miscibility between polypropylene and the EOC and reduces molecular mobility at their glass‐transition temperatures. Mechanical properties, graft yield, and MFI are shown to be highly dependent upon the elastomer's concentration, density, and molecular weight, initiator and monomer concentration, as well as processing temperature. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Melt-rheological properties of PP/EVA blend

This paper describes a study of melt-rheological properties of the binary blend of isotactic polypropylene (PP) and ethylene–vinyl acetate copolymer (EVA) at varying blending ratios (from 0 to 40 wt % EVA content) and using three samples of EVA containing different vinyl acetate contents (VA %), viz. 9, 12, and 19%. Measurements made on a capillary rheometer at three different temperatures (210, 220, and 230°C) in a shear stress range of 10⁴–10⁶ Pa (shear rate 10¹–10⁴ s^?1) are presented and discussed for the effects of blend composition and shear stress on the flow curves, melt viscosity and melt elasticity. Morphology of the blend studied through scanning electron microscopy revealed distinct differences in size and number density of dispersed EVA droplets, which are discussed in terms of the variation of average size and number density of the dispersed EVA droplets as a function of blend composition and shear stress. Melt-rheological properties and morphology of dispersion are correlated and found quite consistent.     

Network alteration theory on Mullins effect in semicrystalline polymers

Three semicrystalline polymers with different molecular structure and crystallinity were investigated to analyze the Mullins effect therein. The polymers exhibited time‐dependent stress resistance and stress softening. Recoverable network alteration in both crystalline and amorphous domains was proposed to explain the cyclic loading deformation and relaxation. The crystallites and the entanglements acted as the joints in the network where stress was transferred. The maximum stress first rapidly decreased and the balance stress was reached after ca 50 cycles. The balance stress was higher than the quasi‐static stress obtained by normal stress relaxation. However, the balance stress could be eliminated by the following stress relaxation and the residual stress was very close to the quasi‐static stress. The different network strength between the strain before and after yielding is also discussed by comparing the balance stress and the quasi‐static stress. The stress resistance of the network before yielding was stronger than that after yielding mainly due to crystallite slip. © 2014 Society of Chemical Industry     

Rheological effects of the incorporation of chlorinated polyethylene compatibilizers in a HDPE/PVC blend

Small‐amplitude oscillatory measurements, creep and recoil experiments, capillary extrusion flow and shrinkage measurements have been performed to elucidate the effect of block and random chlorinated polyethylene (CPE) on the rheological properties of a ternary high density polyethylene (HDPE)/ poly(vinyl chloride) (PVC)/CPE system. It is observed that the storage modulus, the complex viscosity and the steady stale viscosity at low shear rates decrease when a small amount of CPE is incorporated to 50/50 (wt.) HDPE/PVC binary blend. However, at high shear rates, in experiments performed in extrusion flow, the trend is reversed, and the incorporation of CPE to the binary blend increases viscosity. The high melt elasticity of HOPE is severely reduced when this polymer is mixed with PVC, but when CPE is included as a third component, elastic recovery is considerably increased. All these rheological results, which are independent of type (block or random) of CPE used, are explained considering the morphological changes produced by CPE and during extrusion flow.     

Variation of periodic crazing based on polymer blends of an ultra‐high and a low molecular weight poly(methyl methacrylate)

Periodic crazes are caused in a polymer film by the unique mechanical method using bending. Generation of a craze depends on entanglements of the molecular chains of a polymer. Therefore, control of composite morphology of periodic crazes was attempted by varying the entanglements of molecular chains. An effective entanglement network became sparse by polymer blends of an ultra‐high molecular weight polymethylmethacrylate (PMMA) and a low molecular weight PMMA. Consequently, the composite morphology of periodic crazes caused in the blend film varied. In other words, the periodic craze can be used for the evaluation of the effective entanglements. In addition, it was figured out that PMMA of which the number‐average molecular weight (M_n) is less than twice of the effective entanglement molecular weight (M_e^*) works as a plasticizer in the blend film. And also, it was revealed that the mechanical properties of the blend film decreased dramatically at M_n ≒ 6M_e^*. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44332.     

PP/POE/T-ZnOw复合材料流变行为的研究

通过熔融共混的方法在双螺杆挤出机上制备了聚丙烯/乙烯-辛烯共聚物/四针状氧化锌晶须（PP/POE/T-ZnOw）三元复合材料。利用毛细管流变仪分析了纯PP、PP/POE二元共混物和PP/POE/T-ZnOw三元复合材料的流变行为,详细研究了T-ZnOw含量对三元复合材料流变行为的影响。研究结果表明,POE对二元共混物的稠度影响与温度有关,而T-ZnOw含量对三元复合材料的稠度、表观黏度和黏流活化能具有不同的影响。     

Mixture Rules for Critical Shear Values of Extruded Linear PE/Branched PE Blends

Oscillatory flow and elastic turbulence belong to the types of flow instabilities frequently encountered during extrusion of polymer melts. The onset of these defects corresponds to the flow conditions when the critical shear stresses or the critical shear rates are attained. The critical values of shear stresses and shear rates were experimentally determined for linear polyethylene/branched polyethylene blends (IPE/bPE) that were prepared with various weight ratios. Consequently, mixture rules of the logarithmic type are proposed. These rules relate the critical value of shear stress (shear rate) of blend to the critical values of shear stresses (shear rates) of the individual pure components, weight fractions, and interaction parameters. There is a good agreement between the proposed mixture rules and experimentally determined critical values.     

Rheological behavior of highly filled ethylene propylene diene rubber compounds

The rheological behavior of highly filled ethylene propylene diene rubber (EPDM) compounds was studied with respect to the effect of curative system, grafted rubber, shear rate, temperature and die swell using a Monsanto Processability Tester (MPT) to gain an understanding of the molecular parameters that control the surface finish. All systems show pseudoplastic behavior. At a particular shear rate, shear viscosity increases with blend ratio. The dependence of flow behavior on extrusion velocity indicates a surface effect. The extrudate die swell and maximum recoverable deformation are related by a linear relationship, which is independent of sulfur/accelerator ratio, extrusion temperature and shear rates and blend ratio. The principal normal stress difference increases nonlinearly with shear stress. Activation energy decreases with shear rate in most cases. The faster relaxing system produces extrudate of better surface quality.