A study on the effects of chaotic mixer design and operating conditions on morphology development in immiscible polymer systems

Self‐similar mixing structures, a novel feature of chaotic mixing, were generated in this study as precursor to an array of mixing microstructures, such as nested layers, elongated fibrils, droplets and their combinations in the blending of two immiscible polymers, polypropylene (PP) and polyamide‐6 (PA6). Simulations based on Newtonian flow model were used to compute Poincaré maps and stretching distribution as the tools for investigation of the effect of shear gap and chaotic mixing parameter, such as angular displacement per period (θ) of rotors, on the degree of mixing and morphology development in a batch chaotic mixing device. It was found that a value of θ = 1440° provided the conditions for fastest conversion of the PP‐phase into droplets for the same total strain. A 25% reduction in shear gap from 0.0127 m to 0.0095 m gave rise to much more uniform mixing of the components and led to faster conversion of the PP‐phase into droplets for the same value of θ and the same total strain. A very large fraction (>90%) of the droplets generated fell below the equilibrium size and were found to be much smaller than those produced by twin‐screw extrusion method. Polym. Eng. Sci. 44:407–422, 2004. © 2004 Society of Plastics Engineers.     

Recovery rheology via rheo-SANS: Application to step strains under out-of-equilibrium conditions

Stress relaxation from a step strain test provides important information about constituent dynamics, but if a material has experienced a complex shear history, the underlying physics is not straightforward to access. We use recovery rheology and rheo-small-angle neutron scattering to probe the nonlinear dynamics of an entangled wormlike micelle solution by applying step strains after complex shear histories enforced by large-amplitude oscillatory shear (LAOS) flow. We show that a universal relaxation modulus can be obtained from step strain tests with complex shear histories, as long as the modulus is defined in terms of the recoverable strain. The shear and normal stresses, as well as the alignment of micellar Kuhn segments, are shown to be positively correlated with the recoverable strain. We identify re-entanglement of polymeric chains after cessation of LAOS and show that this process occurs over the same timescales as linear-regime stress relaxation. This work, therefore, lays the foundation of how to accurately probe out-of-equilibrium rheology in a consistent manner.     

Development of fibrillar morphology in immiscible PP/PS blends under shear flow

The formation dynamics of fibrillar morphology in dilute immiscible polypropylene (PP)/polystyrene blends under simple shear flow is investigated using optical‐shear technique. Two strategies in generating fibrillar droplets under shear flow, namely temperature quench and shear jump, are studied. It is found that the shear‐induced deformation of PP droplets is closely related to the total shear strain and changes of rheological properties of components during the temperature quench or shear‐jump process. The shape evolution of fibrillar droplets under shear flow displays large deviation to the prediction of affine deformation theory based on Newtonian fluids and that of three deformation models, which consider the viscoelastic properties of components. The possible effect of droplet coalescence, breakup, and interfacial slip on the deviation between the experimental data and the prediction values for droplet deformation are discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

E‐Glass/DGEBA/m‐PDA single fiber composites: The effect of strain rate on interfacial shear strength measurements

Precise measurements of fiber break regions have been made during the single fiber fragmentation test (SFFT) procedure on E‐glass/diglycidyl ether of bisphenol‐A (DGEBA)/meta‐phenylenediamine (m‐PDA) test specimens. From these measurements, the location and size of each fiber fragment was determined, and the resulting information was used to construct fragmentation maps of the tested fiber. By comparing these maps, the fragmentation process supports random fragmentation along the length of the fiber. Since the interfacial shear strength (IFSS) or the interfacial shear stress transfer coefficient (I‐STC) is obtained from the fragment length data at the end of the test (saturation), frequency histograms of the fragment length data were constructed to determine the repeatability of the fragmentation process. Since the SFFT is performed by sequential step‐strains of the test specimen, test protocols were developed by controlling the step size of each strain increment and the time between each step‐strain (dwell time). For the testing protocols used in this research, the E‐glass/DGEBA/m‐PDA frequency histograms of the fragment lengths were found to be generally repeatable. However, when the effective strain rate of the test was altered by changing the dwell time between strain increments, the fragment distribution at saturation of the E‐glass/DGEBA/m‐PDA SFFT specimens changed. The direction of the change was found to be inconsistent with the effect one might expect when only the nonlinear viscoelastic behavior of the matrix is considered. However, the magnitude of the change observed in the E‐glass/DGEBA/m‐PDA SFFT specimens is not universal. Fragmentation data obtained on E‐glass/polyisocyanurate SFFT specimens revealed a much smaller change in fragment length distributions with the same change in testing protocols. Consistent with the results obtained on the E‐glass/DGEBA/m‐PDA, fiber fragmentation occurs when the polyisocyanurate matrix exhibits nonlinear viscoelastic behavior. The implication of these results for interfacial shear strength measurements is discussed.     

Performance of Dowel-Welded T-Joints for Wood Furniture

Comparison of two types of furniture joints namely step butt T-joints and mortise and tennon T-joints held together either by one or two welded or glued dowels showed that the shear strength results of welded dowel and glued dowel joints were comparable. For mortise and tennon T-joints there is, in general, no difference if the dowel is inserted at a 45° or 90° angle. Also there is no significant difference between welded and glued dowel joints stiffness values in both step butt and mortise and tennon T-joints of the same geometry. Also, there is no significant difference between dowels inserted at 45° or 90° for mortise and tennon T-joints. Glued dowel and welded dowel step butt T-joints behave quite differently from linear joints. Thus, in step butt T-joints a higher shear strength is obtained if a single dowel is inserted at 45° for both glued and welded dowels. Both shear strength and stiffness increase as the number of dowels increases, namely from one to two. The application of the welded joint technique to joints where the number of dowels is limited by the limited space in which they can be inserted, such as in furniture, can give shear strength results comparable to those obtained by gluing the same dowels. This is particularly the case for mortise and tennon T-joints.     

Stress prediction for polymer blends with various shapes of droplet phase

The excess shear stress after application of large step strains in polymer blend is calculated from observed shapes of deformed droplets in immiscible matrix, based on the Doi-Ohta expression for the interface contribution to the stress. The calculation is made for droplet shapes of flat ellipsoid, rod with end caps, dumbbell and ellipsoid of revolution. The predicted excess relaxation modulus agrees very well with experimental data normalized per one droplet with the volume-averaged radius for a poly(isobutylene)/poly(dimethyl siloxane) blend with narrow distribution of droplet size. Especially, slow stress relaxation in the intermediate stage and faster relaxation thereafter predicted from the rod like and dumbbell shapes are consistent with the experimental data. For a blend of hydroxypropylcellulose solution/poly(dimethyl siloxane) with broad distribution of droplet size, the predicted excess relaxation modulus agrees reasonably well with experimental data by taking account of the size distribution.     

Influence of mixing characteristics for water encapsulation by self-assembling hydrophobic silica nanoparticles

Water encapsulation using silica nanoparticles was assessed using two different types of single step mixing processes. The influential mixing characteristics have been determined. Direct mixing at high rotational speed requires high shear and vigorous stirring properties. Progressive water atomisation using gentle mixing process requires high atomisation pressure and rapid surface refreshing of the mixed material. Mechanisms of powder formation were also proposed. Encapsulation of micrometric water droplets in shell-like structure is respectively obtained by either progressive size reduction of macroscopic particulates or direct coating of pre-formed microscopic droplets. These mechanisms resulting from the interactions between a solid particle and a liquid highly depend on parameters such as particle's hydrophobicity, surface tension or kinetic energy.     

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.     

Research on strain distribution and damage behavior of thermal barrier coatings based on digital image correlation

Due to the complicated structure and serving environments, thermal barrier coatings (TBCs) usually encounter failure in the form of surface coating cracking and interface spalling without warning. At present, although many experimental techniques and equipment were developed to predict their service life, the transfer process of stress between different layers and the strain characteristics of ceramic surface are not clearly explained. In this paper, the nondestructive digital image correlation method was used to observe the character of surface strains of supersonic plasma-sprayed TBCs systems in tensile failure processes. Also, mathematical model was established basing on the principle of minimum function to calculate interface stress and coating strain expressions. The results show the characteristics of strain change in the whole tensile stage can be divided into four stages. At first, strain concentration occurs in the range of 9%-27% of the effective distance between one end of the tensile specimen, second, a certain number of strain fringes are formed and distributed at a certain distance, and then the first crack appears in the initial strain concentration area; as the load continues, more and more cracks on the coating surface reach saturation and finally fail. In the microlinear elasticity stage, the shear strain in the coating and the interface shear stress are in a linear relationship. As the thickness of the single-layer coating increasing, the strain value of the surface strain of the coating decreases, the surface strain value of the single-layer coating is about six times larger than that of the double-layer coating.     

The effect of shear on the crystallization of cocoa butter

This paper examines the effect of shear on the crystallization of cocoa butter using a combination of three different experimental techniques and a single crystallization temperature of 20°C. Rheological measurements were carried out to study the effect of a shear step on the crystallization kinetics of the fat. Without a shear step, little rheological change was observed at 20°C; however, with the application of a shear step the onset of significant rheological change occurred and was strongly influenced by the magnitude of the shear step. Detailed crystallographic measurements could be made with in situ X-ray experiments during flow-induced crystallization. The imposition of continuous shear changed both crystal polymorphic structure and crystallization kinetics in a systematic way. Finally, optical measurements were used to follow changes in crystal morphology as a consequence of continuous shear. These results revealed the form and kinetics of crystal growth. In general the results complemented each other, and an overall picture of the way shear influenced cocoa butter growth could be formed. The observations could be the basis for a future mathematical model of growth kinetics and provide insight into the way shear influences crystallization kinetics, morphology, and polymorphic structure.     

Using multiple‐mode models for fitting and predicting the rheological properties of polymeric melts. II. Single and double step‐strain flows

Several classes of multiple‐mode rheological constitutive equations are examined for predicting the viscoelastic flow properties of a typical polymer melt in single and double step‐strain flows. The phenomenological parameters appearing in these models have been obtained by the fitting of experimental data taken in small‐amplitude oscillatory shear and steady shear flows. The performance of the models for predicting the experimental data in the step‐strain experiments is examined in detail. Specifically, we examine whether or not mode coupling is necessary to describe the experimental behavior under step‐strain flows. Furthermore, it is demonstrated that the reversing double step‐strain experiment is a very powerful tool for testing viscoelastic constitutive equations. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Transient shear and extensional properties of biodegradable polycaprolactone

Transient shear and extensional properties of two grades of partially crystalline biodegradable aliphatic polyesters, poly ε caprolactone (PCL), were investigated at three different temperatures. Uniaxial extensional viscosity at a constant strain rate was obtained using the Meissner apparatus. The magnitudes of the stress-over-shoot during stress growth experiments were smaller than those typically observed for other polymers. Higher melt temperature and higher strain led to faster relaxation, while the lower molecular weight PCL 767 relaxed faster than the higher molecular weight PCL 787. The relaxation moduli are independent of strain for strain values below 0.1. Transient extensional measurements were conducted at strain rates of 0.01 to 1.0 s⁻¹. At small stresses the extensional viscosity has the threefold value of shear viscosity as predicted by Trouton. There appeared to be no steady state regime for either grade of PCL studied and as a result η_e(ε) could not be determined. The departure from the linear limit is fastest for the highest extensional rate. Extension thickening behavior is observed at Hencky strains ranging from 1.0 to 2.0. PCL 767 displayed greater extension thickening than PCL 787 at the same temperatures. The Wagner integral constitutive equation was found to give an acceptable fit to the stress growth data in shear and extension, with the fit being better for PCL 767 than for PCL 787.     

The onset of nonlinear viscoelasticity in multiaxial creep of glassy polymers: A constitutive model and its application to PMMA

A physically based, isostructural, constitutive model is described for simulating the onset of nonlinear viscoelasticity in multiaxial creep of glassy polymers, as needed in stress analyses of load-bearing components. In the linear viscoelastic limit, shear response reduces to that of a generalized Maxwell model, while hydrostatic response is Hookean. Nonlinearity enters through Eyring-type rate process kinetics. The equations of the model are solved numerically using a pseudo-linear approximation through each time step, leading to an incremental equation for stress that would be convenient for use in finite element analyses. The model and its assumptions were tested using tension, shear and combined tension/shear creep experiments on well-aged poly(methyl methacrylate) at 70°C. Reproducibility tests confirmed the assumption of constant glass structure for strains up to ～ 1.5 × 10^?2. Shear and pressure activation volumes were obtained by fitting the dependence of the shear compliance on stress invariants. The data showed unequivocally that shear activation volumes vary with log(relaxation time), and excellent agreement was obtained for a linear variation, consistent with the “compensation rule” of polymer thermo-viscoelasticity. The activation volumes are large (many monome units), indicating the cooperative nature of the elementary flow process. Interestingly, they are of the same order as those applying to yield and plastic flow. Although the model finds success in simulating creep, it fails to describe so accurately the strain recovery on unloading. Possible explanations are suggested.     

E‐glass/DGEBA/m‐PDA single fiber composites: Interface debonding during fiber fracture

In this paper, we examine the regions of debonding between the fibers and the matrix surrounding fiber breaks formed during single fiber fragmentation tests. The fiber breaks are accompanied by areas of debonding between the matrix and the surface of the fiber. With increasing applied strain, the lengths of these debonded regions generally increase. At the end of the test, the matrix tensile strain adjacent to the debond regions is an order of magnitude higher than the applied strain (40% vs. 4%). Although the debond edges typically remain attached at the same locations on the fiber fragments, debond propagation along fiber fragments under increasing strain has been observed in some cases. The phenomenon is termed secondary debond growth, and two mechanisms that trigger secondary debond region growth have been proposed. As expected, tests with bare fibers and with fibers coated to alter interface adhesion indicate that the average size of debonded regions at the end of the test increases as the calculated interfacial shear strength decreases. However, a decrease in the “apparent” interfacial shear strength resulting from an increase in testing rate results in a decrease in the size of the average debond region. This result suggests an increase in the amount of energy stored in the matrix from the fiber fracture process. POLYM. COMPOS. 28:561–574, 2007. © 2007 Society of Plastics Engineers     

Anisotropic aspects of the environmental stress cracking of acrylonitrile-butadiene-styrene and styrene acrylonitrile copolymers in methanol

Critical strains causing environmental stress cracking of injection-molded poly(acrylonitrile-butadiene-styrene) (ABS) and poly(styrene-acrylonitrile) (SAN) plaques were determined upon exposure to methanol. Measurements were obtained for samples strained either parallel or perpendicular to the melt flow direction and for samples located at various distances from the mold gate. Critical strains were significantly higher in the direction parallel to the melt flow compared to the transverse direction. The degree of anisotropy increased with increasing rubber content. For ABS containing 46 percent rubber, the critical strain at one point was determined to be 2.99 percent in the direction of melt flow, but only 0.47 percent in the orthogonal direction. For this material, critical strains determined parallel to the melt flow decreased with distance from the gate; whereas, critical strains for SAN and ABS containing 30 percent rubber remained essentially constant. Orientation of the plaques was assessed using shrinkage determinations and a thermal conductivity technique. Though a straightforward correlation of orientation with critical strain is observed for ABS, a similar relationship is not observed for SAN. These results suggest that although stress cracking occurs in the glassy matrix of ABS, it is the dispersed rubbery phase which controls the magnitude of strain required to initiate cracking.     

Molecular orientation of polydimethylsiloxane induced by elongational and shearing strains

A study examining the molecular orientation of poly(dimethylsiloxane) for different combinations of elongational and shear strains is presented. Three different cases were studied: (1) pure elongational strain; (2) increasing shear and decreasing elongational strains; (3) increasing shear and increasing elongational strains. The experiments were performed in a converging flow cell (at room temperature), where elongational and shearing strain rates achieved values of 370 s^?1 and 640 s^?1 respectively. Values of the Hermans orientation function were obtained from measurements of birefringence and polarization angles while strain rates were estimated from laser Doppler anemometry velocity measurements. Prospects for predicting molecular orientation from the stress-optical laws and rheological flow models are outlined and commented on.     

Deformation and fracture of urethane–methacrylate resins

The tensile properties of three urethane–methacrylate resins that varied in the soft segment content of the urethane were characterized. The strain birefringence at a circular hole was observed during loading–unloading cycles to progressivley higher displacements. The shear strain distribution at the hole was calculated from the isochromatic fringe contours and compared with results from linear elastic analysis. When the onset of nonlinearity, and the subsequent appearance of residual strain at the root of the hole, were correlated with features of the macroscopic stress-displacement curves, three regions of prefracture deformation were defined. A region of linear elastic behavior was observed at the lowest strains. The maximum shear strain at the linear limit was the same in all the resins, and appeared to correlate with the yield condition at the hole. When the shear strain at the hole exceeded about 2.8%, the fringe patterns started to deviate from the elastic prediction. However, strain was fully recoverable in this region as indicated by the absence of residual birefringence at the hole after unloading. This region of nonlinear, recoverable deformation extended to progressively higher strains as the amount of urethane soft segment increased. This feature was attributed to the network structure of the urethane–methacrylate resins. A region characterized by nonrecoverable deformation at the hole followed at higher strains; the urethane soft segment content had a major effect on the amount of permanent deformation sustained before fracture. The fracture surfaces exhibited features typical of brittle fracture without crazing. © 1995 John Wiley & Sons, Inc.     

Influence of non-uniform distribution of shear stress on aerobic biofilms

This work deals with the detachment of biofilm subjected to a shear stress. Biofilms are developed on plates, under very low shear stress for one month and then subjected to an erosion test for 2 h in a Couette-Taylor reactor (CTR). During the erosion test, the plate was fixed on the external cylinder of the CTR. The presence of the plate modifies the velocity field in the CTR. A first zone close to the facing step region is characterized by the detachment of the stream lines. A second zone, downstream, is characterized by a pure shear flow: the distribution of the shear stress is uniform; the residual biofilm mass was measured and the detachment can be classically related to the magnitude of shear stress. In the first zone, the recirculating flow induces a strong non-uniform distribution of shear stress. The residual biofilm mass was also measured and found to be much lower than in the uniform shear stress zone, whereas the magnitude of shear stress is of the same order or even smaller. The assumption of elastic rheology for the biofilm enables the strong detachment observed in the region subjected to non-uniform shear stress to be explained.     

Shear deformation and fracture behaviour of polycarbonate–acrylonitrile/butadiene/styrene blend at high strain rates

Abstract

The purpose of the present research was to study systematically the shear deformation and fracture behaviour of a polycarbonate–acrylonitrile/ butadiene/styrene (PC–ABS) blend subjected to high shear strain at high strain rate, using a torsional, split Hopkinson bar. Thin walled tube specimens were deformed at room temperature under strain rates ranging from 10² to 5 × 10³ s^-1. The effects of strain rate on shear flow response, strain rate sensitivity, thermal activation volume, and shear modulus were evaluated. Damage initiation, propagation, and fracture mechanisms were studied by scanning electron microscopy. Correlations between dynamic flow response and observed fracture features are characterised and discussed in terms of loading conditions. The data indicate that the dynamic shear response of the PC–ABS blend is greatly affected by applied strain rate. An increase in shear stress and shear modulus with strain rate was observed. Fracture strains decrease with increased loading rate. Tearing and shear fracture are the major fracture mechanisms and depend quite strongly on the strain rate.     

Electric Fields Obviate Constrained Sintering

The phenomenon of constrained sintering, where large rigid inclusions of alumina have been shown to significantly reduce the rate of sintering of titania [Bordia and Raj (1988) J. Am. Ceram. Soc. 71, 302–310], is shown to subside nearly completely during flash sintering carried out under modest electrical fields. The result is explained by a different mechanism for volumetric and shear deformation under electric fields. It is proposed that vacancy and interstitials generated within the grains migrate to grain boundaries and pores to produce both volumetric and shear strain at equal rates, since, in this way, the diffusion distance for both modes of deformation becomes the same. In conventional sintering, where transport occurs from one interface to another, the diffusion distance for shear is twice as far as for densification, which retards sintering should it become controlled by shear deformation, as seen in constrained sintering.