Effects of AN-contents and shear flow on the miscibility of PC/SAN blends

The miscibility of polycarbonate (PC) with styrene-co-acrylonitrile random copolymer (SAN) has been systematically investigated as functions of acrylonitrile content and shear flow. Various AN-contents ranged from 11 to 74 wt% and different simple shear flow values up to 90 s⁻¹ have been used to explore the effect of both material and proceeding parameters on the miscibility of PC and SAN blends. The finest phase dispersion of the SAN particles was observed at AN=25 wt% for PC/SAN=70/30 blends under the same processing condition using scanning electron microscope (SEM). The obtained morphologies indicated that PC and SAN could form a partial miscibility blend and the maximum miscibility occurred at AN=25 wt%. This observation was supported by considering the shifts in the glass processes of the two rich phases of the blend using the dynamical mechanical analysis (DMA) measurements. The optimum interaction of the two components at AN=25 wt% calculated from ellipsometric technique was found to be the only responsible parameter for the high miscibility of the blend. The viscoelastic properties of the pure polymer components were found to play a minor role in the obtained morphologies. The effect of simple shear flow on the morphology of PC/SAN-25=70/30 blend has been also investigated using a special shear apparatus of parallel plate geometry. It has been found that the dispersed phase of SAN was elongated and broken-up in the direction of flow with weaker contrast at high shear rates. The shear rate was found to enhance the miscibility of SAN (dispersed phase) in the PC matrix to a great extent as seen in the weak contrast of the two phases observed by transmission electron microscope (TEM).     

Effect of shear history on the morphology and coarsening behaviour of polycarbonate/poly(styrene-co-acrylonitrile) blend

The morphology and coarsening behaviour of polycarbonate (PC) blend with styrene-co-acrylonitrile (SAN) random copolymer of 25 wt% AN have been investigated as a function of shear history using transmission electron microscope (TEM) and time resolved light scattering techniques. Simple shear apparatus of two parallel plates geometry was used to generate different shear rate values depending on the different distances from the centre of the sample disk. The morphology of PC/SAN-25=70/30 blend showed that the dispersed phase of SAN was elongated and broken-up in the direction of flow with weaker contrast at high shear rate values. The shear rate was found to suppress the concentration fluctuations and enhance the miscibility of SAN (dispersed phase) in the PC matrix to a great extent. The average of the dispersed particle diameter was evaluated as a function of different shear memories at 240 °C for different time intervals based on the Debye-Bueche theory. The obtained data were found to be shear memory dependent i.e. the average particle diameter decreases with increasing shear memory. This result indicated that the coarsening process is greatly suppressed by shear memory and the shear could produce a permanent morphological change (irreversible change) over the time scale of the measurement. This behaviour was attributed to the very high melt viscosities of the blend components, which in turn led to a very long relaxation time and consequently a high difficulty to erase the effect of shear at the experimental temperature. Furthermore, the coarsening process for all the measured samples followed the general power law, , regardless of the shear memory of the blend. This behaviour implied that the shear could only retard the rate of domain growth without any effect on the coarsening mechanism.     

Precursor of shish-kebab in isotactic polystyrene under shear flow

We performed polarized optical microscope (POM), depolarized light scattering (DPLS) and small- and wide-angle X-ray scattering measurements on the structure formation process or the crystallization process of isotactic polystyrene (iPS) under shear flow below and above the nominal melting temperature T_m. It was found that an anisotropic oriented structure termed here as a string-like object was formed in μm scale even above the nominal melting temperature and stable for more than 24 h, but melted at around 270 °C far above the nominal melting temperature. The string-like object acts as a nucleation agent for the folded chain lamella crystal (or the kebab), and was assigned to a precursor of the shish-kebab. We also examined the shear rate dependence of the structure formation to find a critical shear rate for the formation of the string-like object, suggesting the relaxation of the chains plays an important role in the formation of the structure. Based on the results we have discussed the inner structure of the string-like object.     

Development and relaxation of orientation in sheared concentrated lyotropic solutions of hydroxypropylcellulose in m-cresol

Shear-induced orientation and the relaxation of orientation after the cessation of shear in 45 and 50 wt% solutions of cholesteric hydroxypropylcellulose (HPC) in m-cresol have been studied in situ by infrared spectroscopy and polarised microscopy. The shearing experiments were conducted at 30-80 °C at shear rates of 1-300 s⁻¹, which covered the director tumbling, wagging and a small part of the steady-state shear rate regimes. The steady-state order parameter was proportional to the shear rate and the proportionality constant increased with increasing HPC concentration and decreasing temperature. The concentrated solutions studied showed steady-state alignment even in the tumbling regime. Three different shear-rate regions with different behaviours of the solutions after the cessation of shear were found in these concentrated HPC solutions. At low shear rates (1-5 s⁻¹, referring to the 50% HPC solution) the polymer remained isotropic during shear but became gradually more oriented a few minutes after the cessation of shear, an observation that was substantiated by polarized microscopy. The order parameter reached a final plateau value and stayed constant at this level for long periods of time (∼24 h). At intermediate shear rates (from 5 to 50 s⁻¹ for the 50% HPC solution), a detectable order parameter was recorded at steady shear and, after the cessation of shear, the structure returned to an almost isotropic state within a few minutes, after which the orientation gradually started to increase to approach a plateau value after about 150 min. At even higher shear rates (∼100 s⁻¹ for to the 50% HPC solution), the initial steady shear order parameter relaxed to an almost isotropic state and remained constant at this level for time periods extending up to 24 h.     

Rheological behavior of SAN/PC blends under extremely high shear rate

Extremely high shear rate processing was applied to the compound system of acrylonitrilestyrene copolymer/polycarbonate (SAN/PC). The viscosity was measured against the shear rate up to 10⁷ s^?1. The first non-Newtonian region, the second Newtonian region and second non-Newtonian region were observed in the compound system. The occurrence of these regions are very similar to those in the parent polymers, SAN and PC. For the calculation of the viscosity-shear rate curve, a concentric multilayer model was proposed. It gives good agreement between the calculated value and the measured one. A new mechanism for the occurrence of the non-Newtonian, second Newtonian, and second non-Newtonian regions was proposed. Nuclear spin–spin relaxation time measured on SAN, T₂, seems to be consistent with the consideration that the occurrence of non-Newtonian region, second Newtonian region, and second non-Newtonian region are caused, respectively, by the disentanglement, near saturation of disentanglement associated with snapping of macromolecules, and reentanglement through recoiling of snapped macromolecules, and further snapping of the macromolecules, which is inconsistent with the proposed explanation.     

Electrical conductivity under shear flow of molten polyethylene filled with carbon nanotubes: Experimental and modeling

This work aims to describe the conductivity evolution of polymer composites (polyethylene filled with carbon nanotubes) during a shearing deformation. Rheo-electric measurements were carried out to observe the shear-induced fillers network modification. Extended steady shear forces the conductivity to evolve asymptotically to a steady level attesting to an equilibrium between structuring and break up mechanisms in the melted polymer. Numerous experiments were conducted to cover a wide range of shear rate from 0.05 to 10 s⁻¹ and for carbon nanotubes concentrations between 1.3 and 2.9 vol%. A model is proposed to predict the conductivity evolution under shear deformation using a simple kinetic equation inserted in a percolation law. Structuring parameter was found to be solely dependent on the temperature whereas shear induced modification terms were found to be mostly driven by the shear rate and the fillers content.     

Enhanced compatibility of SAN and PC in their blends exposed to extremely high shear field

Originally incompatible blends of SAN 30% and PC 70% were extruded with extremely high shear rate up to 10⁷ s⁻¹ as a typical example of the blends. The materials were examined with a scanning electron microscope (SEM), a pulsed NMR, etc. The molecular weight of the blends was also measured with gel permeation chromatography. The blends are of binary systems microscopically in the first run of extruding, in which the minor constituent is present as small spherical particles in the major constituent. The apparent volume fraction of the spherical minor constituent estimated from the microscopic photographs decreases with the shear rate. The fraction is decreased also with the repeated runs. SEM observation reveals that dimple fracture of microsize takes place on SAN sphere dispersed in PC matrix. And at the bottom of the dimple, a small particle, which would be composed of PC, is present. From these, SAN in the blend is thought to be partly ductile even at the temperature of liquid nitrogen. At the fifth run, the blend appears uniform or structureless. Dynamic loss tangent gives two peaks corresponding to that of SAN and that possibly attributed to PC. The latter shifts to lower temperatures with the number of extruding run. These show that some of SAN is mixed with PC in a compatible form. The pulsed NMR analysis supports the conclusion. Furthermore, the analysis suggests that some of SAN is mixed in PC. This result shows the compatibility of SAN with PC is enhanced in extremely high shear rate processing.     

Modeling of polymer melting,drop deformation,and breakup under shear flow

Polyethylene (PE) or polycarbonate (PC) drop deformation and the breakup mechanism in a PE melt under shear flow were investigated using numerical simulations. The volume of fluid (VOF) method in FIDAP was used to track the dynamic interface. Two models were built for the investigation of a PE/PE system and a PE/PC system. Experimental data of polymer properties, such as specific heat capacity, viscosity, and heat conductivity, were incorporated in the simulations. For the PE/PE system, a temperature‐dependent viscosity model was used for the matrix PE and the dispersed PE. For the PE/PC system, generalized viscosity models were used for PE and PC with time‐dependent moving boundaries. An erosion mechanism similar to that observed in previous experiments was found for deformation and breakup of both PE and PC in the PE melt under simple shear flow. Local flow information, such as temperature, shear rate, viscosity, and shear stress, was obtained from the simulation results. The shear stress at the interface was much higher than the shear stress either in the dispersed phase or in the matrix phase, which could explain the erosion breakup mechanism. Polym. Eng. Sci. 44:1258–1266, 2004. © 2004 Society of Plastics Engineers.     

Erosion and breakup of polymer drops under simple shear in high viscosity ratio systems

The deformation and breakup of a single polycarbonate (PC) drop in a polyethylene (PE) matrix were studied at high temperatures under simple shear flow using a specially designed transparent Couette device. Two main breakup modes were observed: (a) erosion from the surface of the drop in the form of thin ribbons and streams of droplets and (b) drop elogation and drop breakup along the axis perpendicular to the velocity direction. This is the first time drop breakup mechanism (a), “erosion,” has been visualized in polymer systems. The breakup occurs even when the viscosity ratio (η_r) is greater than 3.5. although it has been reported that breakup is impossible at these high viscosity ratios in Newtonian systems. The breakup of a polymer drop in a polymer matrix cannot be described by Capillary number and viscosity ratio only; it is also controlled by shear rate, temperature, elasticity and other polymer blending parameters. A pseudo first order decay model was used to describe the erosion phenomenon and it fits the experimental data well.     

Effect of compatibilization on the deformation and breakup of drops in step-wise increasing shear flow

Drop deformation and breakup were investigated in the presence of a block copolymer in step-wise simple shear flow using a home-made Couette cell connected to an Anton Paar MCR500 rheometer. Polyisobutylene (PIB) was used as the matrix, while five different molecular weights of polydimethylsiloxane (PDMS) were selected to provide drops with a relatively wide range of viscosity ratio. A block copolymer made of PDMS-PIB was used for interfacial modification of the drop-matrix system. The copolymer concentration was 2 wt% based on the drop phase. The experiments consisted in analyzing the drop shape and measuring the variation of the length to diameter ratio, L/D, both in steady state and in transient regimes till breakup. This allowed revising of the classical Grace curve that reports the variation of the critical capillary number for breakup as a function of viscosity ratio and providing also a new one for blends compatibilized with an interfacial active agent with a given molecular weight.     

Morphological and rheological responses to the transient and steady shear flow for a phase-separated polybutadiene/polyisoprene blend

The morphological and rheological responses to the transient and steady shear flow for a phase-separated polybutadiene (PB)/low vinyl content polyisoprene (LPI) blend have been investigated. Under steady shear flow where the applied shear rate is not too large, the steady sheared structures become increasingly anisotropic and interconnected with an “en route” to the formation of string phases as shear rate increases. After that, the further increase of shear rate leads to a blurred domain interface. These shear-induced complex structures in turn affect the rheological response greatly and both the shear thinning and shear thickening were observed in the steady shear behavior of the phase-separated PB/LPI blend. Under transient shear flow, the time (or strain) dependence of viscosity and morphology after a shear rate jump were extensively studied in order to obtain the insight into the steady state formation and found to be mainly determined by the final shear rates. Depending on whether the transient string phases which were formed by the transient shear flow can be stabilized and with clear domain interface, three kinds of transient shear viscosity changes have been observed. Some of the observations are quite different from the model immiscible blend and believed to be closely related to the significant shear-induced mixing effect happened in the PB/LPI blend.     

Non-linear shear deformation of hydrophobically modified polyelectrolyte systems

The shear imposed oscillation technique was employed to probe the shear-induced structural changes of 3 wt% hydrophobically modified polyelectrolyte solutions that possess shear-thinning and shear-thickening behavior. The shear-thickening behavior is related to the transformation of predominantly intra-molecular to inter-molecular associations. On the other hand, the shear-thinning behavior under moderate shear deformation is caused by the re-organization of the transient network structure. Under high shear deformation, the shear-thinning behavior is solely caused by the shear-induced effect that increases the chain-end exit rate, which reduces the mechanically active chains and lifetime of the hydrophobe in the micellar junctions.     

An experimental study of the kinetics of polymer crystallization during shear flow

Some principles of rheology are applied to the study of the shear-induced crystallization of molten polymers. A new technique is described for measuring crystallization kinetics during isothermal flow at constant shear rate in a parallel plate rheometer. The crystallization rate is characterized by the time elapsed from the start of shearing until the rise in melt viscosity due to crystallization. The measured-viscosity and induction time for crystallization are shown to be independent of sample geometry. Kinetic data are presented for crystallization of three linear polyethylenes at shear rates of 0.03 – 30 sec^?1. It is shown that shear flow has a strong accelerating effect on crystallization when the deformation rate exceeds a critical value. Comparison of results for the different polyethylenes reveals that higher molecular weight materials crystallize faster at a given shear rate and temperature. Finally, shear-induced crystallization of propylene polymers is shown to be unaffected by the presence of either a carbon black additive or a heterogeneous nucleating agent. It is concluded that the hydrodynamic origin of the shear-induced crystallization is elastic chain extension due to entanglement couplings between molecules. Furthermore, it is suggested that transient orientation effects during the startup of shear flow may have a dominant influence on the observed phenomena.     

Evolution of asphaltene floc size distribution in organic solvents under shear

A mathematical model was developed on the basis of population balance to analyze experimental data on asphaltene floc size distribution in a coagulating suspension. Experiments were carried out in a Couette device under a laminar flow condition. Floc size distributions were measured on-line using optical microscopy and image analysis. The aggregation behavior of asphaltenes was investigated by monitoring the size distribution of flocs for various intensities of agitation (i.e., shear rate, G), solvent composition (i.e., ratio of toluene to n-heptane in the solution, T:H) and particle contents (i.e., volume fraction of particles, ?). The results showed that (i) the floc size distribution can be predicted using a population balance approach, (ii) a steady-state mean floc size is reached for a given shear rate, and (iii) this steady-state floc size increases as ? is increased or T:H is reduced. The relative rates of shear-induced aggregation and fragmentation determine the steady-state size distribution. Similar floc size distributions were obtained at steady state for various shear rates, indicating that the width of the size distribution is independent of shear. However, the experimental observations indicate that the steady-state floc size distribution depends on asphaltene concentration and solvent composition.     

Determination of the elastic component for polymeric melt under steady shear flow

Steady state shear flow in rectilinear coordinates can be generated by a direct shear rheometer. The rheological behavior of polymeric melts under shear flow can be characterized by the inception of steady shear flow and stress relaxation after cessation of steady shear flow. Experiments were performed for bisphenol A polycarbonate with various mol wts. The elastic components, such as the primary normal stress differences and the elastic recoverable shear strain of the polymeric melts, can be determined from the shear moduli. © 1993 John Wiley & Sons, Inc.     

Influence of shear deformation on carbon nanotube networks in polycarbonate melts: Interplay between build-up and destruction of agglomerates

Shear-induced destruction and formation of conductive and mechanical filler networks formed by multi-wall carbon nanotubes in polycarbonate melts were investigated by simultaneous time-resolved measurements of electrical conductivity and rheological properties under steady shear and in the quiescent melt. The steady shear experiments were performed at shear rates between 0.02 and 1 rad/s and for nanotube concentrations ranging from 0.5 to 1.5 wt%. The influence of thermo-mechanical history on the state of nanotube dispersion and agglomeration was studied in detail.For melts with well-dispersed nanotubes a shear-induced insulator-conductor transition was observed, which is explained by the agglomeration of nanotubes under steady shear and the formation of an electrical conductive network of interconnected agglomerates. Simultaneously, a drastic decrease of the shear modulus (G^∗ = G′ + iG″) during steady shear was observed, which can be related to a reduction of mechanical reinforcement due to agglomeration of dispersed nanotubes. These findings indicate a substantial difference in the nature of “electrical” and “mechanical” network and contradict earlier assumptions that steady (or transient) shear is always destructive for the conductive filler network in highly viscous polymer composites.It was also shown that after a certain time of steady shear the filler network asymptotically reaches its steady state characterized by the constant electrical conductivity and shear modulus of the composite melt. Such asymptotic behaviour of composite properties was experimentally shown to be related to the interplay of the destructive and build-up effects of steady shear. For modelling of the electrical conductivity in presence of steady shear a kinetic equation was proposed for filler agglomeration with shear-dependent destruction and build-up terms. This equation was coupled to the generalized effective medium (GEM) approximation for insulator-conductor transition.     

A simple model for non-linear stress relaxation

A simple model for the prediction of non-linear stress relaxation following the cessation of steady shear flow is proposed. The model allows the calculation of the shear and first normal stress difference components of the stress. The mathematical flexibility of the model is reduced to a minimum with the result that no adjustable parameters are employed and only linear dynamic deformation data are required to calculate the non-linear behaviour. Verification of the model was carried out with data available for two viscoelastic fluids and good agreement between the predictions and the experimental results was obtained for the range of shear rates examined (0.167 ? γ ? 16.7 sec^?1).     

Visco-elastic properties related to the phase morphology of 60/40 PC/SAN blend

Stress relaxation and dynamic mechanical measurements have been performed on a 60/40 blend of polycarbonate of bisphenol A (PC) and poly (styrene-co-acrylonitrile) (SAN). This paper clearly demonstrates that the phase morphology of an immiscible co-continuous polymer blend is an important parameter in determining visco-elastic behavior. The dynamic mechanical properties are discussed in terms of the visco-elastic form of a Kerner equation as a function of the reciprocal Chalkey parameter, which has been used to quantify the co-continuous phase morphology. The effective volume fraction of the SAN phase has been found to decrease as the phase structure coarsens during annealing above T_g of both SAN and PC. This is probably the result of phase break-up and subsequent inclusion of SAN domains in the PC matrix during the coarsening process, which modifies the structure produced during melt compounding and injection molding.     

Modifications of the weissenberg rheogoniometer for measurement of transient rheological properties of molten polyethylene under shear. Comparison with tensile data

Improvements to the Weissenberg rheogoniometer are necessary in order to measure the transient rheological properties of polymer melts correctly. The improvements reported concern the mechanical design, a new heating system, a new normal force measuring system, and additional equipment for the relaxation test. Reliable short-time results require sufficiently stiff torque and normal force springs, and a small radius and relatively large angles of the cone-and-plate gap. The behavior of the LDPE melt under test is “linear viscoelastic,” if shear rate or total shear are small: The relaxation modulus, the stress growth at the onset of constant shear rate, the stress relaxation after cessation of steady shear flow, and, in addition, dynamic shear data (from an oscillation viscometer) all show consistent results when correlated by means of formulae from the theory of linear viscoelasticity. Shearing in the nonlinear range with constant shear rate leads to pronounced maxima of the shear stress p₁₂ and of the first normal stress difference p₁₁ ? p₂₂ which occur at constant total shear, almost independent of shear rate. Comparison of shear and tensile data (from extensional rheometer) confirms the Trouton relation in the linear-viscoelastic case. In the nonlinear case, there is a “work softening” in shear and a “work hardening” in extension.     

Effect of molecular and processing parameters on the flow-induced crystallization of poly-1-butene. Part 1: Kinetics and morphology

In this work the effect of molecular parameters (molecular weight (M_W), molecular weight distribution (MWD)) and processing conditions (crystallization temperature, flow conditions) on the isothermal crystallization behavior of three isotactic poly-1-butene (iPB) samples is investigated by means of rheo-optical techniques. The emphasis in this paper will be on the kinetics and the resulting morphology.Turbidity measurements show a strong effect of M_W and the degree of undercooling on the flow-induced crystallization (FIC), whereas the effect of MWD is not quite clear. Scaling relations, proposed in literature, that are based on polymer chain relaxation were found to predict correctly the dependence of FIC, at least when samples of similar MWD are considered. A mastercurve is presented combining effects of M_W, shear strain, shear rate and temperature.Optical microscopy observations provide information on the quiescent growth rate and the morphology of the crystallites. The effect of the different parameters on the observed transition from an isotropic morphology at low shear rates to a rod-like crystalline structure at high shear rates, could again be explained in terms of polymer chain relaxation.