Effects of shear rate,viscosity ratio and liquid crystalline polymer content on morphological and mechanical properties of polycarbonate and LCP blends

Studies were conducted on the effects of shear rate, viscosity ratio and liquid crystalline polymer (LCP) content on the morphological and mechanical properties of polycarbonate (PC) and LCP blends. The LCP (LC5000) used was a thermotropic liquid crystalline polymer consisting of 80/20 of parahydroxybenzoic acid and poly(ethylene terephthalate) (PHB/PET). The viscosity ratio (viscosity of LCP: viscosity of matrix) was varied by using two processing temperatures. Due to the different sensitivity of materials to temperature, variation in the processing temperature will lead to varying viscosity of the components in the blends. Based on this principle, the processing temperature could be manipulated to provide a favourable viscosity ratio of below unity for fibre formation. To study the effect of shear rate, the flow rate of the blend and the mould thickness were varied. The shear rate has a significant effect on the fibrillation of the LCP phase. The effect was more prominent when the viscosity ratio was low and the matrix viscosity was high. At 5 wt% LCP, fibrillation did not occur even at low viscosity ratios and high shear rates. It was also observed that the LCP content must be sufficiently high to allow coalescence of the dispersed phase for subsequent fibrillation to occur. © 2002 Society of Chemical Industry     

Viscoelastic properties of polyarylene ether nitriles/thermotropic liquid crystalline polymer blend

Polyarylene ether nitriles (PEN)/thermotropic liquid crystalline polymer (TLCP) blend was prepared via melt mixing. The immiscible phase morphologies, linear and nonlinear, as well as transient viscoelastic properties of the blend were studied using SEM, rheometer, and DMA. The linear dynamic viscoelastic behavior of the blend shows temperature dependence due to further evolution of the immiscible morphology and, as a result, the principle of time‐temperature superposition (TTS) is invalid. In the steady shear flow, the discrete TLCP phase is difficult to be broken up because of the high viscosity ratio of the blend systems, while is easy to be coarsened and followed by elongation, and finally, to form fibrous morphology at high TLCP content and high shear level. During this morphological evolution process, the transient stress response presents step increase and nonzero residual relaxation behavior, leading to increase of the dynamic viscoelastic responses after steady preshear. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Transient and steady-state deformations and breakup of dispersed-phase droplets of immiscible polymer blends in steady shear flow

Transient and steady-state deformations and breakup of viscoelastic polystyrene droplets dispersed in viscoelastic high-density polyethylene matrices were observed in a simple steady shear flow between two transparent parallel disks. By separately varying the elasticities of the individual blend components, the matrix shear viscosity, and the viscosity ratio, their effects on the transient deformation, steady-state droplet size, and the breakup sequence were determined. After the startup of a steady shear flow, the viscoelastic droplet initially exhibits oscillations of its length in the flow direction, but eventually stretches preferentially in the vorticity direction. We find that at fixed capillary number, the oscillation amplitude decreases with increasing droplet elasticity, while the oscillation period depends primarily on, and increases with, the viscosity ratio. At steady-state, the droplet length along the vorticity direction increases with increasing capillary number, viscosity ratio, and droplet elasticity. Remarkably, at a viscosity ratio of unity, the droplets remain in a nearly undeformed state as the capillary number is varied between 2 and 8, apparently because under these conditions a tendency for the droplets to widen in the vorticity direction counteracts their tendency to stretch in the flow direction. When a critical capillary number, Ca_c, is exceeded, the droplet finally stretches in the vorticity direction and forms a string which becomes thinner and finally breaks up, provided that the droplet elasticity is sufficiently high. For a fixed matrix shear stress and droplet elasticity, the steady-state deformation along the vorticity direction and the critical capillary number for breakup both increase with increasing viscosity ratio.     

The effect of mixing on the modes of dispersion and rheological properties of two-phase polymer blends in extrusion

An experimental study was carried out to investigate the effect of mixing on the state of dispersion and rheological properties in the two-phase flow of polymer blends. For the study, blends of polystyrene and polypropylene were used, and two mixing devices were employed: a single-screw extruder combined with a “static mixer,” and a twin-screw compounding machine. Materials of various blending ratios were extruded at a constant temperature (200°C) through a capillary die having an L/D ratio of 20 (D = 0.125 in.). The state of dispersion in the two-phase system was investigated from pictures taken of the microstructure of the extrudate samples. It was found that different mixing devices have a profound influence on the state of dispersion of one polymer in another. Also determined were the rheological properties of the two-phase system investigated, from wall normal stress measurements. Our results show that, when shear stress is used as a parameter, the melt viscosity goes through a minimum, whereas the melt elasticity goes through a maximum. This is regardless of the type of mixing device employed, although the shapes of the curves are affected by the type employed. It is suggested that shear stress, instead of shear rate, be used in correlating the viscoelastic properties of two-phase polymer systems.     

Blend morphology development during melt flow: Correlation of a model concept based on dynamic phase volume with practical observations

An approach is first developed that can be used to identify the global morphology of an immiscible two‐phase polymer–polymer blend under shear flow. The basis for the modeling is the concept of a dynamic phase volume; this is based on relative abilities of the respective phases to flow when under stress, and determined by the actual phase volume fraction and the viscosity ratio between phases. One result of the modeling is a schematic diagram providing guidelines for morphology development during melt processing in a nonuniform stress field. Bisphenol A polycarbonate(PC)/ABS blends were studied as an immiscible system, using variations of component ratio and viscosity ratio at constant composition. Blend morphology was characterized by scanning electron microscopy and solid‐state dynamic mechanical spectroscopy. Model predictions correlate well with experimental observations of the frozen‐in solid‐state morphology following injection molding. Discussion also cover the utility of the model for blend design and limitations of the modeling approach. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 311–318, 1999     

台阶式并行微通道内液液两相流流型及其转变机理

利用高速摄像仪研究了台阶式并行微通道内液液两相流流型及其转变机理。以甘油水为分散相、含3% Span 85的环己烷为连续相,观测到了滴状-滴状流、过渡-滴状流、喷射-过渡流和喷射-喷射流4种流型;以两相流量为坐标轴绘制了流型图,并获得了流型转变线;分析了流型的转变机理。考察了分散相黏度对流型及其转变的影响机制。随着分散相黏度的增大,流型转变线整体向下移动,滴状-滴状流区域变小,喷射-喷射流区域变大。最后,运用介尺度概念分析了并行微通道内液液两相流非均匀结构的动态效应。     

Study of rheological property of nylon‐1212 with Haake Rheometer

Rheological properties of nylon‐1212 have been studied by means of Haake Rheometer. The effect of shear rate and temperature on the apparent vicosity of nylon‐1212 was discussed. A correlation of non‐Newtonian index with the temperature was obtained. The results showed that the apparent viscosity decreases with the increase of the temperature. With increasing shear rate, shear thinning of nylon‐1212 was observed clearly. From the relation of the temperature dependence of the polymer, we obtained the viscous flow activation energy. We conclude that the apparent viscosity is sensitive to temperature at lower shear stress because of higher viscous flow activation energy, and the temperature affect on the apparent viscosity becomes weaker at higher shear stress because of lower viscous flow activation energy. We have investigated the creep and elastic recovery of nylon‐1212. A creep test was carried out to define the linear viscoelastic range as 1.0 and 5.0 Pa for 195 and 190°C nylon‐1212 melts, respectively. A time‐dependent response was found for the creep and recovery phases at a lower applied shear stress. However, at higher shear stress, the creep and recovery phases were time‐independent. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 379–385, 2003     

Steady flow simulation of a polymer‐diluent solution through an abrupt axisymmetric contraction using internally consistent rheological scaling

In this work, a new methodology is developed that describes the viscoelastic scaling of a polymer‐physical foaming agent (PFA) solution in a detailed and internally consistent manner. The approach is new in that while previous researchers have largely focused on scaling down experimentally obtained high pressure polymer‐PFA solution viscosity data onto a master curve for the viscosity of the undiluted polymer melt at a reference temperature and atmospheric pressure, we have generated the shear viscosity data required for our simulations by systematically scaling up the viscosity values obtained from measurements on a pure polymer melt to the desired temperature, pressure, and concentration values characterizing the flow. Simulations have been run for the flow of a polymer‐PFA solution through an extrusion foaming die with an abrupt axisymmetric contraction and good qualitative agreement is obtained with experimental pressure drop measurements obtained previously in our laboratory. The pressure drop rates and temperature rise rates have been estimated at the surface of incipient nucleation. Because of the short residence times in the die for the microcellular foaming process, approximating the flow through the die as a single phase flow in our simulations still gives useful insights into the dynamics of the flow. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Viscoelastic properties of polystyrene–polycarbonate blends in melt

Viscoelastic properties of polymer blend melts of polystyrene–polycarbonate were investigated in a wide range of temperatures, frequencies, and compositions. It was established that the more essential changes in viscoelastic characteristics took place at small concentrations of one of the components and at low frequencies, probably because of a putting down of the slow relaxation processes. The marked decrease in the viscosity of the melts takes place in the region of phase separation due to thermodynamic incompatibility of the components and is in a good correlation with the appearance of excess free volume in the system.     

Prediction of equilibrium polymer blend morphology in dispersed two-phase flow and comparison with experiment

A theory is developed to predict equilibrium polymer blend morphology in dispersed two-phase flow in terms of the viscosity ratio and blend ratio of the constituent components. We formulated a system of equations for the ‘One-Cell Model,’ describing the situation where a spherical droplet of one liquid is dispersed in another liquid, by first considering a pair of Newtonian liquids and then a pair of truncated power-law fluids. The question posed was: which of the two liquids, A or B, will form a droplet dispersed in another liquid? Finite element method was employed to calculate the rate of the energy dissipated per unit volume when a unit cell, having either Morphology I or Morphology II, was subjected to steady-state simple shear flow, where Morphology I has a droplet of liquid A dispersed in the matrix phase of liquid B and Morphology II has a droplet of liquid B dispersed in the matrix phase of liquid A. We used a criterion that the morphology that requires a lower rate of the energy dissipated per unit volume in dispersed two-phase flow is an equilibrium polymer blend morphology. In so doing, we determined the blend composition, at which a matrix inversion takes place, in terms of the viscosity ratio of the constituent components. Theoretically predicted blend morphologies compared favorably with experimental results.     

聚氨酯—聚乙烯醇共混物的动态粘弹性

本文采用溶液共混方法,制取了一组PU-PVA共混物,并应用DDV-Ⅱ仪和自制的高聚物浓溶液动态粘弹仪,分别研究了共混物固体及共混溶液的动态粘弹性。实验结果表明,无论是固体粘弹性或浓溶液粘弹性,随着共混比的变化呈现有规律的变化,两者都在PU/PVA=65/35这一共混比中发生相逆转。在相逆转区,共混材料的力学性质发生了很大的变化。所以研究共混体系相逆转现象,对于控制共混物组成和性质具有指导意义。本文同时也提出了一种应用浓溶液动态粘弹仪研究共混物相结构及相逆转的新方法。     

Effect of hydrodynamics on dynamics of phase separation in polystyrene/poly(vinyl methyl ether) blend

Polystyrene/poly(vinyl methyl ether) (PS/PVME) phase diagram was assessed by rheological tools and by on-line microscopy observations both under quiescent and shear flow conditions. Shear flow was found to induce both mixing and demixing of the mixture depending on the amplitude of the imposed shear rate. Viscoelastic properties of PS/PVME blends were also measured under steady shear flow near the phase separation temperature. At lower shear rate, flow enhances concentration fluctuation and induces phase segregation. At high shear rate, flow suppresses fluctuations and the polymer mixture keeps its miscible state. Several rheological signatures of phase transition were found. In steady shear flow, a secondary plateau in viscosity was observed when the temperature was close to T_s whereas, at the start-up shear flow, transient shear stress showed a second overshoot after a few minutes of shearing.     

Evolution of properties in ABS/PA6 blends compatibilized by fixed weight ratio SAGMA copolymer

Blends of acrylonitrile-butadiene-styrene (ABS) and Nylon 6 (PA6) incorporating styrene-acrylonitrile-glycidyl methacrylate (SAGMA) copolymer as compatibilizer have been studied across five different compositions by varying the PA6 ratio from 15 wt% to 55 wt%. The evolution of morphology from discrete dispersed PA6 particles to phase inversion to co-continuous phases effected due to the compatibilizer have been studied vis-à-vis preliminary melt flow analysis, viscoelastic behavior, physico-mechanical and thermal properties of the blends. Single point viscosity measurements during melt flow analyses are indicative of a significant increase in viscosity upon initial incorporation of PA6 followed by narrow increases with content. It is observed that while there are gradual positive modifications in physico-mechanical properties with increasing PA6 content, the most significant improvements are observed for room temperature izod impact strength and break elongation effected in the region of phase inversion on to the formation of a co-continuous phase. The low temperature impact strength at −40 °C essentially remains comparable to that of control ABS. DMTA analysis evidences partial dissolution of the blend components by the shifts of the damping peaks (Tg) of PB rich phase, SAN and PA6. Broadening of the damping peak of PB rich phase of ABS is attributed to increasing interfacial region due to PA6-g-SAGMA molecular layer at the interface. Thermal stability of the blends were not significantly affected in comparison to control ABS and PA6.     

Processability and properties of novel glass-polymer melt blends

Dynamic mechanical analysis was used to study the viscoelastic properties of two novel highly filled phosphate glass-polymer melt blends to accelerate efforts to optimize their melt processing. The melt of one blend was thermally stable while that of the other blend was not, as evidenced by modulus growth over time of the latter. The frequency dependencies of storage and loss moduli for both blends at 400°C showed evidence of incomplete relaxation. Recrystallization, formation of some fibrillar structures within the polymer phase during flow, transesterification of the polymer phase, or interactions between the polymer and glass are thought to be responsible for the observed differences between the viscoelastic properties of the two blends.     

The reduction of orientation in fibers spun from two-phase polymer blends via the introduction of shear into elongational flow by the presence of a second phase

A model is proposed for the reduction of orientation in spun fibers of two-phase polymer blends. This is based on the introduction of shear into an elongational flow by the presence of a second phase. The requirement is that the dispersed phase should not deform to the same extent as the continuous phase so that the flow field in the region of each particle is perturbed. Around an isolated droplet of minor component, the shear rate in the continuous phase goes through a maximum when the extension rate in the droplet is around half that macroscopically imposed. The dependence of orientation reduction on concentration of dispersed phase is fitted well by assuming that the flow field around a particle is disturbed over a distance two to three times the particle diameter. In this case the maximum average shear rate around the particle is of the same order of magnitude as the elongation rate. The model proposed is consistent with all the observed features of orientation reduction during spinning of two-phase blends.     

Polymer blend de‐mixing and morphology development of immiscible polymer blends during tube flow

This work is an investigation of morphology and de‐mixing of polymer blends during melt flow through a tube. Morphology is the relative size, shape and location of each distinguishable phase present in a polymer blend. De‐mixing is the shear‐induced migration of different types of polymers away from each other during the flow. The ability to tailor de‐mixing during extrusion can potentially result in a new family of plastics waste recycling processes with mixed waste entering an extruder and separate streams of different polymer types leaving it. Also, control of morphology development can lead to the formation of layered structures without the need for two or more extruders and co‐extrusion. This work is directed at elucidating morphology development and de‐mixing of polymer blends in the most simple process design: melt flow through a tube. Shear‐induced migration was quantitatively shown in various polyethylene‐polypropylene, polypropylene‐nylon 6 and polyethylene‐nylon 6 blends. The migration observed was in accord with the hypothesis that the system tends to minimize its rate of energy dissipation for a fixed flow rate. The ratio of the viscosity of the dispersed phased to that of the continuous phase greatly influenced the morphology of polypropylene‐nylon 6 and polyethylenenylon 6 blends: a droplet‐dispersed phase structure occurred at a high viscosity ratio, whereas a multi‐layer structure resulted at viscosity ratios near unity. Shear‐induced deformation and coalescence contributed to formation of the multi‐layer structure.     

Viscometric behavior of ternary concentrated solutions of hydroxypropyl cellulose and ethyl cellulose in m-cresol

The shear viscosity of blend solutions of hydroxypropyl cellulose (HPC) and ethyl cellulose (EC) in m-cresol (both HPC/m-cresol and EC/m-cresol systems form lyotropic liquid crystals) was determined by cone-plate-type and capillary-type viscometers. The textures for the same systems at rest and undergoing shear were also observed with a polarized microscope. At shear rate of 1 s^?1, viscosity exhibited a maximum and a minimum with respect to temperature, and this suggested that the phase of the matrix dominated the viscometric behavior of the ternary systems; the blend composition dependence of the viscosity was not additive, and this suggested that HPC and EC were immiscible. At relatively high shear stress, the blend composition dependence of the viscosity greatly depended on the total polymer concentration of the solutions and was quite different from that at low shear rate; the texture of the anisotropic solutions was also different from that at low shear rate. Our findings suggested that the dependence of viscosity on shear and concentration for pure HPC solution was different from that for pure EC solution.     

Dynamic mechanical properties and adhesive strengths of epoxy resins modified with liquid rubber. I. Modification with ATBN

The dynamic mechanical properties and the adhesive strengths of Epikote 828 and Epikote 828-ATBN blend systems were investigated. The ATBN blend systems were proved to be completely incompatible with the dynamic mechanical measurement and also fitted well with Takayanagi's model which was designed for completely incompatible two-phase systems. The epoxy resin had a nonreacted part when cured at room temperature. The blending of ATBN reduced the nonreacted part of the epoxy resin, and made contributions to the adhesive strengths. In the case of tensile test of crosslap specimens using aluminium as adherends, the adhesive strengths of ATBN blend systems were almost 1.5-fold of those of epoxy resin without blending of ATBN. As for wood adherends, the maximum of the adhesive strengths was found at 60°C for epoxy resin without blending of ATBN, and at 0°C for ATBN blend systems. The facts meant that there were mutual interactions between the adhesive strengths and the viscoelastic behavior of the adhesive polymers in the two-phase systems as observed in completely miscible polymer blends. There was not pronounced distinction between epoxy resins without blending of ATBN and ATBN blend system, as to the shear adhesive strengths.     

Influence of concentration and temperature on the flow behavior of oil-in-water emulsions stabilized by sucrose palmitate

To study the influence that concentration and temperature exert on the viscous behavior of emulsions stabilized by a sucrose ester (SE) of high hydrophilic-lipophilic balance (HLB), flow curves and droplet size distributions were determined. Flow curves of presheared emulsions always exhibited a shear-thinning behavior at intermediate shear rates, a tendency to a limiting viscosity at high shear rates, and a metastable region at low rates. This behavior can be fitted to a Carreau model. Both SE and oil concentrations increase emulsion viscosity as a result of a more structured system with a lower droplet size and polydispersity. An increase in temperature usually leads to a decrease in emulsion viscosity. However, at high oil concentration, coalescence and phase separation take place at low temperature. On the other hand, at high temperature, droplet bursting due to shear forces, leading to an increase in viscosity, may result. Despite the strong structural breakdown caused by steady shear, master flow curves may be obtained by using superposition methods.     

Dynamic shear rheological behavior of PP/EPR in‐reactor alloys synthesized by multi‐stage sequential polymerization process

The rheological behavior, morphology, and mechanical properties of in‐reactor alloy of polypropylene (PP)/ethylene propylene rubber (EPR) synthesized by multi‐stage sequential polymerization process are studied in this article. The relationship between polymerization parameters, morphology, and rheological properties are evaluated by scanning electron microscopy (SEM) and small amplitude oscillation rheometry in the linear viscoelastic region. The electron microscopy of samples is showed that by increasing switching frequency in polymerization time, the size of EPR particles decrease. By increasing switching frequency, the curves of complex viscosity against angular frequency of samples are shifted to higher values at low range of shear rates with no significant change at higher frequencies in Power‐law region. The modified Cole‐Cole plots revealed the enhanced melt elasticity by increasing switching frequency up to 230°C. The plot of phase angle versus absolute value of complex modulus G* is used for the evaluation of matrix‐droplets interaction at various temperatures. It is observed two different behaviors before and after 230°C which is the evidence of the change in relaxation mechanism of the blend components because of coarsening the rubber particles in the phase separation process. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011