Effect of die geometry on the structural development of a thermotropic liquid crystalline polymer in a thermoplastic elastomer matrix

This study investigates deformation of a thermotropic liquid crystalline polymer (TLCP) in different die geometries. Blends of aTLCP with a thermiplastic elastomer of EPDM were made in a twin-screw extruder. Morphological observation of the extruded blends demonstrates the complimentary effect of shear in the die exit on dispersed phase deformation and fibril formation. Shear strain can affect fibril formation for a relatively large dispersed phase in the region close to the die wall. However, the main role of shearing is in breaking up the larger particles and initial polydomain structure. A strong elongational deformation on the blended melt after the die exit is required, and fine microfibrils normally obnserved in in situ composites were not easily formed by shear deformation only in the die.     

Dispersive mixing efficiency of an elongational flow mixer on PP/EPDM blends: Morphological analysis and correlation with viscoelastic properties

Polypropylene and ethylene‐propylene‐diene terpolymer (PP/EPDM) blends were melt compounded in a new mixing device, designed in our laboratory under the trademark of RMX®, which predominantly generates elongational flows. Dispersion of the EPDM minor phase in PP was carried out in both RMX® and in an internal mixer (Haake Rheomix 600) at equivalent specific mixing energies and the resultant morphologies obtained by SEM were analyzed and compared. A better dispersive mixing efficiency of the RMX® mixer, i.e., lower D_n and D_v of the dispersed EPDM phase was observed. The impact of elongational flow was more pronounced for blends having a high viscosity ratio p, indicating an enhanced droplet break‐up mechanism, which was attributed to the combination of high shear rates inside the mixing element and important elongational flows in the convergent/divergent zones. The morphology of the blends was correlated with their linear viscoelastic properties by using the Palierne model. Very good agreement was found for the PP/EPDM 80/20 blends but for higher EPDM content, the Palierne model failed to describe the rheological behavior, which was attributed to percolation of the minor phase with increasing the concentration. Higher elasticity at low frequencies was observed for blends processed in the RMX®, which was attributed to a higher generated interfacial area. POLYM. ENG. SCI., 54:1444–1457, 2014. © 2013 Society of Plastics Engineers     

Melt strength,local velocity,and elongational viscosity profiles of low‐density polyethylene filaments affected by the die design and process conditions

An experimental arrangement to simultaneously measure the melt strength, velocity profiles, and elongational viscosity profiles across the cross section of a molten filament that emerged from either a circular or slit die for low‐density polyethylene (LDPE) under nonisothermal and isothermal conditions is proposed. The proposed experimental rig was based on a parallel coextrusion technique of colored LDPE melt layers into an uncolored melt flowing from the barrel into and out of a die to form a continuous filament before they were pulled down by mechanical rollers until the filament failed. The experimental rig was also equipped with a high‐speed data‐logging system and a personal computer for real‐time measurements. The results suggest that the draw‐down forces changed continuously with changing roller speed, and the velocity profiles of the melt were not uniform across the LDPE filament during the stretching of the melt. Greater draw‐down forces and local melt velocities were obtained in the slit die or under the nonisothermal condition. The draw‐down forces and velocity profiles in both dies were affected by the volumetric flow rates from the extruder and the roller speeds used, with the effect being more pronounced for the circular die. The elongational viscosity profiles of the LDPE filament were not uniform across the filament cross section and corresponded well to the obtained velocity profiles. The elongational viscosities of the LDPE filament were relatively higher when the filament was extruded and stretched in the circular die and under the nonisothermal condition. The changes in the elongational viscosity profiles were more sensitive to changes in the volumetric flow rate and roller speed in the circular die. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Viscoelastic stress calculation in multi‐layer coextrusion dies: Die design and extensional viscosity effects on the onset of ‘wave’ interfacial instabilities

Non‐isothermal calculation of flow history‐dependent viscoelastic stresses in an annular coextrusion die is performed using the 8‐mode modified Leonov constitutive equation and the deformation rate field from finite element simulations. The study shows how the recently proposed total normal stress difference (TNSD) sign criterion, which quantifies the relative stretching of coextruded layers across an interface, can be used to investigate the role of the die design and elongational viscosities of coextruded materials from the point of view of interfacial instability. It is shown that both the die geometry and the elongational strain hardening have a crucial effect on the interfacial wave instability.     

表面活性剂对聚合物混合物流变和力学性能的影响

多相聚合物混合物在熔融挤出过程中,由于相界面及界面张力的存在,其流动时存在界面滑移。这样,在相同条件下,混合物的表观剪切粘度可能低于任何单一相的表观剪切粘度。因此可以设想,如果在聚合物混合物界面引入表面活性剂降低界面张力,降低界面滑移因素,那么,聚合物混合物的剪切粘度应当增加,流动性下降。本文证明了以上设想,并为聚合物混合物的增容提供了新的途径。     

Melt rheology and morphology of physically compatibilized natural rubber–polystyrene blends by the addition of natural rubber-g-polystyrene

Blends of natural rubber (NR) and polystyrene (PS) were prepared by melt mixing in a Brabender plasticorder and by solution casting using chloroform as the casting solvent. Earlier studies have indicated that these blends are incompatible and immiscible, and their compatibility can be improved by the addition of a graft copolymer of NR and PS (NR-g-PS). The rheological behavior of these blends has been carried out in the presence and absence of the compatibilizer using a capillary rheometer and a melt flow indexer. The effects of blend ratio, processing techniques (melt mixing versus solution casting), shear stress, and temperature on the rheological behavior have been studied in detail. Both in the presence and absence of the copolymer, the blends showed a decrease in viscosity with an increase of shear stress, indicating pseudoplastic nature. Solution-cast blends showed a higher viscosity as compared to melt-mixed blends. The viscosity versus composition curve of both melt-mixed and solution-cast blends showed negative deviation from the additivity at a higher shear rate region. This is associated with the interlayer slip between the highly incompatible NR and PS phases. The effects of graft copolymer loading and temperature on solution-cast blends were studied, and it was found that as the copolymer loading increases, the shear viscosity increases. This is due to the high interfacial interaction between the two components in the presence of the copolymer. The copolymer, in fact, locates at the interface and makes the interface more broad. However, at higher loading of the copolymer, the viscosity of the blends decreases. This may be associated with the formation of micelles, which have a plasticizing action on the viscosity of the blends. Melt elasticity parameters like principal normal stress difference, recoverable elastic shear strain, and die swell were evaluated. Master curves have been generated using modified viscosity and shear rate functions that contain the melt flow index as a parameter. The extrudate morphology of the blends was studied using a scanning electron microscope. Addition of the copolymer reduces the domain size of the dispersed phase, followed by a leveling off at a higher concentration. The leveling off is an indication of interfacial saturation. The interparticle distance also decreased followed by a leveling off at a higher loading of the copolymer. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2673–2690, 1998     

Viscosity decrease by interfacial slippage between immiscible polymers

The rheological behavior under pressure-driven shear flow was studied using binary blends with a sea-island structure. The addition of a low-viscosity dispersion having a high interfacial tension with the continuous phase greatly reduces the shear viscosity, for example, the addition of atactic polystyrene (PS) with a low viscosity to isotactic polypropylene (PP) and the addition of PP with a low viscosity to PS. The interfacial slippage occurs because of the poor adhesive strength with the enlarged interfacial area and is responsible for the viscosity decrease. When the dispersion has a similar viscosity to the continuous phase, the viscosity decrease is barely detected. This is because the deformation of dispersed droplets is restricted, which creates a small interfacial area. The interfacial tension between the continuous and dispersed phases plays a crucial role on the shear viscosity. In the case of PP, the addition of linear low-density polyethylene with a relatively low interfacial tension to PP has almost no impact on the shear viscosity. This is despite the polyethylene having a low viscosity.     

Melt rheology and elasticity of natural rubber—ethylene–vinyl acetate copolymer blends

The melt rheology of blends of natural rubber (NR) and ethylene–vinyl acetate copolymer (EVA) has been studied with reference to the effects of blend ratio, cross-linking systems, shear stress, and temperature. When EVA formed the dispersed phase, the viscosity of the blends was found to be a nonadditive function of the viscosities of the component polymers at lower shear region, i.e., a positive deviation was observed. This behavior has been explained based on structural buildup of dispersed EVA domains in the continuous NR matrix. The effect of the addition of silica filler on the flow characteristics of the blends has been investigated. The melt elasticity parameters such as die swell, principal normal stress difference, recoverable shear strain, and elastic shear modulus of NR–EVA blends were also evaluated. © 1993 John Wiley & Sons, Inc.     

Nonlinear stress properties of poly(styrene-block-butadiene-block-styrene) melt under elongational and shear deformation

Effects of block copolymerized structure on nonlinear stress properties under elongational and shear deformation were investigated. Samples used in this study were poly(styrene-block-butadiene-block-styrene) (SBS, weight rate of S/B = 40/60) and polystyrene (PS) as a reference. Tensile stress–strain and shear stress relaxation properties were measured at the molten state. SBS showed high elasticity after reaching the yield point under elongational deformation at room temperature. PS melt showed substantial tensile stress increase after the yield point as strain rates increased. However, SBS melt did not exhibit noticeable tensile stress rise at higher elongation, and this property was almost independent of strain rates. Stress relaxation experiments revealed that the damping function of SBS melt was more strain-softening than that of PS melt. The results suggested that the block copolymerized structure decreases melt elasticity under elongational and shear deformation. A transmission electron micrograph indicated that the lack of melt elasticity in SBS melt is caused by orientation of the lamellar structure toward the stretched direction during deformation. © 1995 John Wiley & Sons, Inc.     

Melt Rheology and Morphology of Nitrile Rubber and Ethylene-Vinyl Acetate Copolymer Blends

The melt Theological behavior of nitrile rubber (NBR)/ethylene-vinyl acetate (EVA) copolymer blends was studied with special reference to the effect of the blend ratio, cross-linking systems, and shear rate using a capillary rheometer. At a given shear stress at 90°C, the viscosities of the blends vary slightly with composition. The effect of cross-linking systems [viz., sulfur (S), peroxide (DCP) and mixed (S+DCP) systems] on the viscosity of NBR/EVA blends is negligible. The melt viscosity of the blends decreases with increasing shear rate, showing pseudoplastic behavior. The flow behavior index values also support the pseudoplastic nature of these blends. Various theoretical models were used to predict the melt viscosity of the blends. Parameters such as die swell, principal normal stress difference, recoverable shear strain, and shear modulus were calculated to characterize the melt elasticity of these blends. The melt elasticity of the system was increased by the addition of NBR to EVA. The extrudate deformation at different shear rates was also studied. It was observed that as the shear rate increases, the extrudate surface exhibits a higher degree of deformation. The morphology of the extrudates of the blends at different shear rates has been examined by a scanning electron microscope. The morphology was found to be dependent on the blend ratio and shear rate.     

Melt rheological properties of polypropylene–polyamide6 blends compatibilized with maleic anhydride‐grafted polypropylene

The melt rheological properties of binary uncompatibilized polypropylene–polyamide6 (PP–PA6) blends and ternary blends compatibilized with maleic anhydride‐grafted PP (PP–PP‐g‐MAH–PA6) were studied using a capillary rheometer. The experimental shear viscosities of blends were compared with those calculated from Utracki's relation. The deviation value δ between these two series of data was obtained. In binary PP–PA6 blends, when the compatibility between PP and PA6 was poor, the deformation recovery of dispersed PA6 particles played the dominant role during the capillary flow, the experimental values were smaller than those calculated, and δ was negative. The higher the dispersed phase content, the more deformed the droplets were and the lower the apparent shear viscosity. Also, the absolute value of δ increased with the dispersed phase composition. In ternary PP–PP‐g‐MAH–PA6 systems, when the compatibility between PP and PA6 was enhanced by PP‐g‐MAH, the elongation and break‐up of the dispersed particles played the dominant role, and the experimental values were higher than calculated. It was observed that the higher the dispersion of the PA6 phase, the higher the δ values of the ternary blends and the larger the positive deviation. Unlike uncompatibilized blends, under high shear stress with higher dispersed phase content, the PP‐g‐PA6 copolymer in compatibilized blends was pulled out from the interface and formed independent micelles in the matrix, which resulted in reduced total apparent shear viscosity. The δ value decreased with increasing shear stress. Copyright © 2006 Society of Chemical Industry     

Interfacial slip‐stick transition induced by reactive compatibilization in ethylene‐co‐butyl acrylate elastomer dispersed polyamide 6 blend subjected to high shear flow and extensional flow

The effect of phase interaction induced by reactive compatibilization during high shear and extensional flow in polyamide (PA6) and ethylene‐co‐butyl acrylate (EBA) blends was studied using advanced dual bore capillary rheometer. The viscosity‐composition behavior of the uncompatibilized PA6/EBA blends exhibited negative deviation behavior from log‐additivity rule. The interfacial slip mechanism, operative between the matrix PA6 and dispersed EBA during shear flow was studied by the use of Lin's and Bousmina‐Palierne‐Utracki (BPU) model for viscosity for the blends under the processing conditions. On the other hand, the compatibilized PA6/EBA‐g‐MAH^0.49/EBA blends with varying dispersed phase volume fraction show positive deviation behavior. The reactive compatibilizers EBA‐g‐MAH^0.49 and EBA‐g‐MAH^0.96 increased the phase interaction with adequate reduction in the dynamic interfacial tension, which favored the particle break‐up and stabilized the morphology in the compatibilized blends. The extensional viscosity of the blends has enhanced because of the inclusion of EBA in all the uncompatibilized and compatibilized blends. The melt elasticity and elasticity function were systematically studied from first normal stress coefficient functions (ψ₁). The variation in the recoverable shear strain (γ_R), shear rate dependent relaxation time (λ) and shear compliance (J_c) under various shear rates were thoroughly analyzed for all the blend compositions. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

法向应力差诱导黏弹性多相分层流动界面变形的机理

通过模拟,建立了黏弹性多相分层流动的界面变形、二次流动、第二法向应力差的关联理论,提出了第二法向应力差驱动二次流动诱发界面变形的机理。结果表明,当Giesekus模型参数α由0.1增至0.45时,界面形貌由凹面向下演化为凹面向上;在侧壁区域,熔体界面的第二法向应力差均为负,如下层熔体第二法向应力差的绝对值大于上层熔体,呈向下二次流动,诱发界面向下变形,反之,则呈向上二次流动,诱发界面向上变形;上层熔体的第二法向应力差的绝对值随α的增加而增大,而下层熔体呈先增后减的抛物线分布,导致α小于0.25,二次流动为向下运动,超过0.25,演化为向上运动,必诱发界面变形的正反演化。     

Skin-Core morphology and failure of injection-molded specimens of impact-modified polypropylene blends

The structure-property relationship as well as the failure phenomena of injection molded polypropylene (PP) blends modified with ethylene/propylene/diene terpolymer (EPDM) and thermoplastic polyolefinic rubber (TPO) were investigated. Single and double-gated tensile bars were injection molded by different Injection speeds. Microscopic studies on the failure behavior of knit lines were carried out using microtomed sections taken from the doublegated specimens. It was found that during injection molding, a skin-core morphology is formed in both the continuous PP matrix as well as in the modified PP blends containing rubber particles of various deformation. The characteristics of the latter are in agreement with those described by the Tadmor flow model. The skin consists of a thin pure PP layer, whereas the subsurface layer contains more or less elongated rubbery particles due to the elongational flow at the wall. The deformation of the rubbery particles decreases, but their concentration increases with increasing distance from the skin towards the core. The deformed particles are oriented tengentionally to the flow front profile. Failure during tensile and tensile impact loading is initiated in the shear zone along the skin-core boundary. This zone has a transcrystalline character and favors the formation of crazing. Final fracture of the bars depends, however, on how crazing and shear yielding simultaneously interact. Their interaction is a function of the average particle size of the dispersed phase. Above an average particle size of 0.6 μm, crazing is prevented by shear bands. For injection molding of PP/rubber blends a moderate injection speed is recommended, if the melt viscosities of the components are closely matched. In this way a pronounced dispersion gradient of the rubber particles across the plaque thickness is avoided. However, for the blends modified with rubber of high viscosity ratio and greater melt elasticity, use of higher injection speed is advantageous. Here, the higher shear stress field decreases the average particle size taken into the direction perpen dicular to the lead, since the cross section of the stronger deformed particle decreases.     

Fibrillation of thermotropic liquid crystalline polymer enhanced by nano-clay in nylon-6 matrix

The influence of well-dispersed nano-clay filler on the morphology of thermotropic liquid crystalline polymer (TLCP) in nylon-6 matrix was investigated by melt extrusion process. The good dispersion of clay in the hybrid blends was confirmed by X-ray diffraction, transmission electron microscopy and rheological measurement. Morphological observation showed that the clay platelets had dramatic influences on the dispersion and deformation of TLCP phase. The TLCP droplets got smaller at the clay content ≤3 wt%, and deformed into fibrils at the clay content up to 5 and 7 wt%. The morphology evolution of TLCP in the hybrid blends, especially at 7 wt% of clay loading, was consistent well with the prediction based on the micro-rheology parameters such as the viscosity ratio of the dispersed phase to the matrix (η_d/η_m) and the ratio of capillary number to the critical capillary number (Ca/Ca_crit). This enhanced fibrillation of TLCP droplets was attributed to the role of nano-clay particles as a compatibilizer to improve the interfacial adhesion and to suppress the interfacial slip between TLCP and nylon phases in the melt, so that the shear stress was effectively transferred to the dispersed TLCP phase.     

Effect of interfacial strengthening in blends of reclaimed rubber and polypropylene

Thermoplastic vulcanizates (TPVs) were prepared from polypropylene (PP) and reclaimed ground tire rubber crumbs. Three types of interfacial strengthening agents—degraded PP, hydrosilylated PP, and hydrosilylated PP grafted onto styrene–butadiene rubber—were prepared in melt via a stepwise series of reactions and employed to generate various degrees of interfacial adhesion in the aforementioned blends. The incorporation of the interfacial agents resulted in improvements in the mechanical properties of these TPVs, and the rubber particle size remained constant. The PP chain length and the functional groups present in the interfacial agents affected the magnitude of the improvement in the mechanical properties. The interfacial agents were primarily present on the surface of the rubber particles in the blends, as shown by energy‐dispersive X‐ray spectra. These interfacial agents in the PP/rubber crumb blends led to a unique preyield kink in their stress–strain curves, a plateau, or a sharp turning point in the region of approximately 3% elongation and approximately 4‐MPa stress. These kinks were interpreted similarly to the cold flow of semicrystalline polymers in tension. The addition of the interfacial modifiers decreased the shear viscosity and increased the entrance pressure drop in flow through capillary dies, and this was attributed to changes in the elongational viscosity of the blends. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Role of the interface in the melt‐rheology properties of linear low‐density polyethylene/low‐density polyethylene blends: Effect of the molecular architecture of the dispersed phase

We studied the melt linear viscoelastic and elongational properties of blends consisting of a Ziegler–Natta linear low‐density polyethylene (LLDPE) and different LDPEs. The weight fraction of the LDPE used in the blends was 15%. The linear viscoelastic characterization was performed at different temperatures for all of the blends to determine the thermorheological behavior in the melt state. The blends fulfilled the time–temperature superposition but exhibited a broad linear viscoelastic response, which was further than that expected for miscible blends and even immiscible systems with a sharp interface. A rheological study of the application of the Palierne model revealed that in addition to the droplet shape relaxation, another mechanism was present at lower frequencies. We discuss the results by hypothesizing a strong interaction between the high‐molecular‐weight linear fraction of the LLDPE matrix and a fraction of molecules of the dispersed phase, which formed a thick interface with its own viscoelastic properties. A clear change in this additional mechanism was observed, depending on the dispersed minor‐phase properties, which produced an impact on the processing of the blends, and more precisely, on the values of the melt strength in the melt‐spinning experiments. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Three-dimensional phase morphologies in HDPE/EVA blends obtained via dynamic injection packing molding

The formation of phase morphology of injection molded HDPE/EVA blends, under the effect of shear stress, has been investigated in detail. The shear stress was induced by dynamic packing injection molding, by which a specimen is forced to move repeatedly in the model by two pistons that move reversibly with the same frequency during cooling. Two kinds of EVA with VA content 16 wt% (16EVA) and 33 wt% (33EVA) were used to investigate the effect of interfacial tension. The phase morphology was viewed both parallel and perpendicular to the shear flow direction, so one can get an overall three-dimensional phase morphology. Low shear stress provided by the pistons has a substantial effect on the phase morphology along the flow direction but is insignificant in the direction perpendicular to the flow direction. Generally, a much elongated and layer-like structure is formed along the flow direction, and spherical droplet-like morphology is formed perpendicular to the flow direction, and the degree of deformation of rubber particles also depends upon their size and elasticity as well as on the interfacial properties between matrix and dispersed phase. For static samples of HDPE/16EVA blends (without shearing), only droplet morphology is formed as 16EVA content increases from10 to 40 wt%. However, under the effect of shear stress (dynamic samples), both droplet and cylinder morphologies can be formed depending on the volume ratio. For static samples of HDPE/33EVA blends, not only droplet, but also cylinder and co-continuous morphology (perpendicular to flow direction) can be formed depending on the volume ratio. For dynamic samples of HDPE/33EVA blends, droplet, cylinder and co-continuous network (co-continuous in both parallel and perpendicular to flow direction) can be formed under the effect of shear stress. The formation of phase morphology is discussed based on interfacial interaction, viscosity ratio, shear stress, and phase inversion.     

Shear-induced interactions in blends of HMMPE containing a small amount of thermotropic copolyester HBA/HQ/SA

Shear-induced interactions between high molecular mass polyethylene (HMMPE) melt and a thermotropic liquid crystalline copolyester, HBA/HQ/SA (TLCP) were investigated using large amplitude oscillatory shear and capillary shear. Polarized optical microscopy (POM) observations show that the mono-domain nematic TLCP droplets embedded inside a HMMPE melt may be readily elongated using large amplitude oscillatory shear. The HMMPE melt adjacent to the elongated TLCP filament was observed to crystallize faster than that in the matrix away from the interface. TEM analysis on the 1 wt% TLCP/HMMPE blend quenched after capillary shear shows that there are strong interfacial interactions between the elongated TLCP filament and the HMMPE matrix. Long range PE lamellae orientational order up to 2 μm away from the TLCP filament surface were observed, with all the lamellae surface normal parallel to the TLCP fiber. Additionally, a strong interfacial compatibility between the TLCP filament and the HMMPE matrix with an interfacial thickness of ∼30 nm has also been observed. The enhanced interfacial compatibility is attributed to the -CH₂- group interactions due to chain alignment in both components at their interface. These results provide a fundamental insight to other TLCP containing thermoplastics where compatibilities may be present due to segmental interactions.     

Controlling micro‐ and nanofibrillar morphology of polymer blends in low‐speed melt spinning process. Part III: Fibrillation mechanism of PLA/PVA blends along the spinline

The effects of spinning conditions on the fibrillation process of poly(lactic acid) (PLA) and poly(vinyl alcohol) (PVA) polymer blends in an elongational flow within the fiber formation zone are systematically and thoroughly investigated. By considering the relationship between the changes in filament parameters with the focus on the maximum axial strain rate (ASR) and tensile stress at maximum ASR and the morphological evolution of the dispersed PLA phase along the spinline, the fibrillation process from rod‐like to nanofibrillar structures of the dispersed PLA phase in a binary blend with PVA matrix is elucidated. The final morphology of the dispersed PLA phase in PLA/PVA blends is controlled by the changes in the spinning conditions. The lengths and diameters of the PLA fibrils are caused not only by the deformation of their initial sizes but also by the combination of the deformation, coalescence, and break‐up process. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44259.