Polarization-optical investigation of polymers in fluid and high-elastic states under oscillatory deformation

The relationship was investigated between birefringence and oscillatory shear deformation of linear high molecular mass polymers exemplified by narrow- and broad-distribution polybutadienes and polyisoprenes. Polymer deformation at different frequencies and amplitudes was carried out in an annular gap. The stress field uniformity was not below 95%. It was shown that in oscillatory deformation of polymers in the fluid and high-elastic states, birefringence contains a time-independent steady component and an oscillatory component with a frequency equal to that of the assigned oscillation. A linear interrelation was found to exist between the amplitude of the oscillatory component of birefringence and that of the shear stresses, with a proportionality factor equal to the stress-optical coefficient of the polymers. The phase of the oscillatory component of birefringence coincides with that of the shear stresses. Measurements of the steady component of the birefringence make it possible to find the steady component of the first normal stress difference resulting from the assignment of shear oscillations to the polymer. On the basis of the experimental data obtained for polybutadienes and polyisoprenes, and the literature data for polystyrene solutions, a master curve was constructed that generalizes the dependence of the steady component of the first normal stress difference in the linear and nonlinear deformation regimes on the product of the square of the deformation amplitude and the storage modulus measured at low amplitudes. This dependence is valid in the linear and nonlinear deformation regimes. It is invariant with frequency, amplitude deformation, molecular mass, and molecular mass distribution of the polymers. It is shown by visual observation of deformation that the abrupt drop in resistance of polymer to shear in large-amplitude deformation is due to polymer rupture near the surface of the inner cylinder and is accompanied by a slip-stick process. This is the phenomenon of spurt early observed in capillary viscometers at high shear stresses and recently investigated in coaxial cylinder devices at large amplitude deformation.     

A new load cell for measuring normal and shear stresses

A special stress measuring device for determining the normal and shear stresses acting on silo walls was developed and tested. The load cell can be used for measuring simultaneously the normal stress as well as the intensity and direction of the shear stress.     

Melt compounding of various polymers with organoclay by shear flow

We studied melt compounding of polymer/organoclay composites by shear flow in a rotating cylindrical mixer to investigate an effect of shear stress on the dispersion state of clay. The commercial organoclay, which is intercalated by dimethyl benzyl stearyl ammonium ion between clay platelets, and four kinds of commercial polymer (polystyrene (PS), poly(lactic acid) (PLA), polyamide(PA6), and poly(butylene succinate) (PBS)) were used in this study. According to the TEM photographs, there is exfoliated clay in PA6/organoclay composite, but the exfoliated clay cannot be seen in other polymer/organoclay composites. We calculated the value of clay dispersion from the low magnification TEM photograph, called the dispersion coefficient to consider the micro level dispersion of the clay in a polymer matrix. Generally, the dispersion coefficient increases with shear stress. However, the dispersion coefficients in case using PA6 and PBS as the matrix, whose melts have low viscosity, are larger than those in case using PS and PLA, whose melts have high viscosity. According to XRD results, d₀₀₁ peak originated from organoclay mostly shifted to the low value of angle for PA6/organoclay composite and began shifting to the low value of angle for PBS/organoclay composite. However, there is a peak for PS and PLA based organoclay composites. From the cases of PA6 and PBS in the DMA results, the storage modulus increases with an addition of organoclay. These results imply that low viscosity polymer is typical easy to get the composites with intercalated clay by melt compounding. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Model prediction of non-symmetric normal stresses under oscillatory squeeze flow

The non-symmetric responses of normal stresses in oscillatory squeeze flow have been investigated with model calculations. The simplest and most widely used constitutive equations were employed to predict the non-symmetric normal stresses, which is a distinctive feature of oscillatory squeeze flow. The model prediction was compared with experimental data of polymer solution in terms of stress shape, Lissajous plot, stress decomposition, and Fourier transformation. The upper-convected Maxwell, Giesekus, and exponential Phan-Thien Tanner models predicted the nonsymmetric characteristics of normal stresses under oscillatory squeeze flow. The predictions showed fairly good agreement with experimental data. However, the upper-convected Maxwell model showed unrealistic result in the Lissajous plot of [stress vs. strain] and [stress vs. strain rate]. From stress decomposition, it could be confirmed that the non-symmetric nature arises from the elastic contribution of the normal stress, which was verified in both experiment and model calculation. This study is expected to provide useful insights for further understanding of the nonlinear and non-symmetric characteristics of oscillatory squeeze flow.     

Tensile and shear stresses induced in adhesive shear test specimens

The most widely used adhesive test specimen is the lap shear test. Variations of this test have been standardized by the American Society for Testing Materials (ASTM) and other societies or agencies. The results of these tests are generally given in terms of the failure load divided by the area of overlap, but a finite element analysis indicates that the maximum shear stress and the induced normal stress can be much larger than the average. An argument can be made that the induced secondary Mode I stresses, near the bond termini, are more closely related to failure than are the average shear stress or even the maximum shear stress. Experimental and finite element results are presented in this paper to show that this is indeed the case. In addition, ways to reduce these induced stresses in lap shear tests are discussed and evaluated.     

Quantitative experimental study of shear stresses and mixing in progressive flow regimes within annular-flow bioreactors

Annular-flow bioreactors are normally operated under laminar Couette flow conditions in order to minimise shear-induced damage to cells. In this study, we computed the fluid shear stresses in model annular vessels over a range of laminar flow regimes, from Couette flow to Taylor-vortex flow, and at two geometric scales, using a shear rate model for freely suspended particles, together with experimental Laser Doppler Anemometry data for a 2-D velocity field. The shear stresses were greatest in the boundary layers adjacent to each wall in each case, with values typically 6 times higher than the mean stresses in the annular space; their respective magnitudes were significantly lower in the larger of the two vessels studied, however. Cell viability studies were also performed in which mammalian cells were cultured under dynamic conditions in a functional bioreactor having the same dimensions as the smaller vessel. The results of these studies demonstrated that a significantly greater number of cells remained in suspension in Taylor-vortex flows than in Couette flow, but at the expense of cell viability at higher Taylor numbers. Taken together, these findings suggest that the benefits of enhanced convective mass transport afforded by Taylor-Couette flows could be realised without risk of appreciable shear induced damage of cells and tissues in larger vessels operating under dynamically similar flow conditions.     

Structural change of polydomain in the liquid crystalline polymers by weak shear flow

Steady-state shear stress (τ₁₂) and first normal stress difference (N₁) of liquid crystalline polymers at low shear rates were examined by using a mesoscopic constitutive equation set including the idea of initial domain size. For the applicability to the weak shear flow at low shear rates, a Hinch-Leal closure approximation was adopted in the calculation of the constitutive equation set. The steady-state rheological behaviors predicted by adopting the Hinch-Leal approximation were compared with those by the Doi simple decoupling approximation. It could be predicted from the plot of N₁ versus τ₁₂ that smaller domains distributed isotropically at a quiescent state might maintain the isotropic domain distribution even at the imposition of moderate shear rate, and then could be changed to the ordered (or partially elongated) domain phase by a further increase of shear rate. Such change of the polydomain structure with the increase in shear rate could be proved more precisely by the transient rheological behaviors of N₁ and τ₁₂ after the start-up of shear flow.     

Polymer branching and first normal stress differences in small-amplitude oscillatory shear flow

General rigid bead-rod theory explains polymer viscoelasticity from macromolecular orientation. By means of general rigid bead-rod theory, we relate the normal stress differences of polymeric liquids to the branch position on a backbone branched macromolecule. In this work, we explore the first normal stress differences coefficients of different axisymmetric polymer configurations. When non-dimensionalized with the zero-shear first normal stress difference coefficient, the normal stress differences depend solely on the dimensionless frequency. In this work, in this way, we compare and contrast the normal stress differences of macromolecular chains that are branched. We explore the effects of branch position, length, functionality, spacing, and multiplicity, along a straight chain, in addition to rings and star-shaped macromolecules in small-amplitude oscillatory shear flow.     

Residual stresses in injection-molded amorphous polymers

Nonisothermal flow of a polymer melt into a cold cavity and its rapid cooling give rise to the buildup of flow and thermal stresses in the molded article. In the present investigation the resultant residual stresses (RS) induced by these two sources were studied in two stages. First, the flow-induced stresses were relaxed by proper heat treatment followed by quenching, resulting in only thermal stresses. The experimentally determined RS profiles in polysulfone and amorphous polyamide showed a parabolic shape and were correlated with the initial and final quenching temperatures, the glass transition temperature, and Biot Number. In the second stage, the combined effect of thermal- and flow-induced stresses was studied using injection-molded specimens prepared under a wide speptrum of molding conditions including melt and mold temperatures and injection rate and pressure. Results here indicated that the basic thermal-induced parabolic RS profiles are altered by the flow-induced stresses resulting in complicated profiles including local maxima and unbalanced RS. Finally, the tensile mechanical properties obatained for plaques molded under the various injection-molding conditions were studied and correlated in part with the previously determined RS profiles. Results have shown that a property gradient exists as a function of distance from both the gate and surface of the molded plaque.     

Orientation effects and cooling stresses in amorphous polymers

This paper deals with two kinds of internal stresses in amorphous polymers. First, the internal stresses connected with molecular orientation are discussed. A framework is presented that describes the interrelations of the various orientation effects (anisotropy, birefringence, etc.) and their dependence on the thermomechanical history of the material. The second part of the paper deals with the residual thermal stresses generated by rapid inhomogeneous cooling through the glass transition range. These so-called cooling stresses are well-known in glass technology. The theory of the quenching of flat glass plates is described, generalized to objects of arbitrary geometry, and shown to be applicable to polymers. The practical importance of cooling stresses is discussed.     

Cyclic interaction between normal and shear stresses of an epoxy polymer—Experiments and model predictions

This investigation focuses on the axial‐torsional loading interaction of an epoxy polymer, Epon 826/Epi‐Cure Curing Agent 9551. Thin‐walled tubular specimens were subjected to combined constant tensile (or shear) stress and cyclic shear (or tension) loading schemes. Pure tensile creep and shear creep tests were also performed to compare the creep deformation to that with superimposed cyclic shear or cyclic tension. Test data clearly showed that cyclic shear (or cyclic tension) have a readily discernible effect on the tensile (or shear) creep deformation. Similarly, a superimposed constant tensile (or shear) load affects the hysteresis responses in cyclic shear (or cyclic tension). A nonlinear constitutive model developed by the authors was used to simulate the observed normal‐shear stress interaction. Due to the inclusion of an effective stress parameter in its nonlinear function, this model was able to account for the normal‐shear coupling effect. However, the incorporation of a general loading/unloading rule led to inaccurate simulation of the observed oscillatory creep response. A modification of the general rule was proposed and better predictions on both the cyclic and the creep responses could be obtained. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

A new rheometer measuring infrared dichroism in molten polymers subjected to transient and steady shear flow

A rotational parallel plate rheometer that enables simultaneous measurement of the transient or steady-state rheological properties and infrared dichroism was designed and constructed to study orientation in molten polymers. Measurements can be carried out at shear rates between 0.05 and 300 s⁻¹ and at temperatures between 20 and 300°C. Both shear stress and axial normal force together with dichroism are continuously measured during shear flow. Infrared dichroism data for polypropylene showed excellent agreement with data obtained with a FTIR-instrument. The Hermans orientation function for molten poly(dimethyl siloxane) at steady state showed a strong shear-rate dependence in the region 0.1−20 s⁻¹. Rheological data for molten poly(dimethyl siloxane) agrees with data obtained from a conventional rheometer.     

Shear yield and crazing stresses in selected glassy polymers

The plane strain shear yield stress and the triaxial crazing stress were determined for several commercial glassy polymers as a function of temperature. The polymers considered were: polycarbonate (Lexan^®), polysulfone (Udel^®), polyetherimide (Ultem^®), polyarylate (Arylon^®), and an amorphous nylon (Zytel^® 330). When normalized to T_g the data for the various polymers were similar but not identical. An exception may be the triaxial crazing strains. In the temperature region between [T–T_g] = ?300° and ?50°C the crazing strains were all small (<1.5%), showed little temperature dependence, and appeared identical within the precision of our measurements. For temperatures below T_g and above any major secondary relaxation, Poisson's ratio was found to be constant for all of the polymers examined, 0.42 (±5%). © 1993 John Wiley & Sons, Inc.     

Fracture toughness of polymers in shear mode

This paper presents a new test method that measures fracture toughness of polymeric materials when subjected to in-plane shear loading (mode II), and compares the toughness with that in tension mode (mode I). The new test method uses an Iosipescu device to apply the shear load, and determines the toughness based on the concept of essential work of fracture (EWF). Three physical-based criteria were used to verify the occurrence of mode II fracture. The new test method was then used to evaluate toughness of poly(acrylonitrile–butadiene–styrene) (ABS). The results suggest that for the ABS, the ratio of toughness in mode II to mode I is about 2.5 which leads to the dominance of mode I fracture in most loading conditions. The results also showed that for ABS in mode I fracture, the specific work of fracture (defined as the absorbed energy for fracture divided by the cross sectional area of the ligament between the notch tips) depends on ligament length; while in mode II fracture, it depends on ligament thickness. The study concludes that the new test method has a good potential for evaluation of mode II fracture toughness of polymers, though further study using polymers of different characteristics will be needed to confirm universality of the test method in the measurement of mode II fracture toughness.     

Effect of magnetic field on discotic nematic liquid crystalline polymers under simple shear flow

The effect of magnetic field on the discotic nematic liquid crystalline polymers (LCPs) is analyzed with the extended Doi theory, in which the molecular shape parameter (β) is defined at ?1.0. The evolution equation for the probability function of the discotic nematic LCP molecules is solved without any closure approximations. The transition among flow‐orientation modes, such as tumbling, wagging, and aligning defined similar to the rodlike LCPs, is strongly affected by the magnetic fields. The new aligning flow‐orientation mode observed for the rodlike LCPs under magnetic fields also can be investigated in the lower shear rate region. On the other hand, the effect of magnetic fields parallel to the x‐ and y‐axis on the time‐averaged first and second normal stress differences ( , ) are also studied. It can be seen that the shear rate regions of the sign changes of , are completely contrary to those conclusions achieved for the rodlike LCPs. In addition, the absolute values of increase with the magnetic field strength in the lower shear rate range owing to the new aligning flow‐orientation mode. Finally, the flow‐phase diagram versus β is also discussed. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Electrodiffusion diagnostics of wall shear stresses in impinging jets

The electrodiffusion method of flow diagnostics was applied to the measurement of the wall shear stresses and turbulent characteristics in the vicinity of the stagnation point in impinging jets. The wall shear stress vector was determined by a double electrochemical probe consisting of two electrodes separated by a thin insulation gap. The impingement of an axisymmetric jet on a flat plate at normal incidence and the reattachment of the plane jet to an adjacent solid surface due to the Coanda effect were studied. The influence of injection and suction on the two-dimensional jet reattachment is described. This paper was presented at the International Workshop on Electrodiffusion Diagnostics of Flows held in Dourdan, France, May 1993.     

An experimental investigation of normal and shear stress interaction of an epoxy resin and model predictions

The axial‐torsional interaction of an epoxy resin was investigated by subjecting thin‐walled tubular specimens to combined normal and shear stress components. It is shown that a superimposed normal stress (tensile or compressive) or hydrostatic pressure will influence shear creep behavior. Similarly, a superposed shear stress affects the normal stress response of the resin. The axial‐torsional stress interaction is also observed in transient stress responses under different strain paths, and in the creep deformation with non‐proportional stress histories. Urear viscoelastic constitutive models are unable to predict the aforementioned behaviors. Two typical nonlinear viscoelastic constitutive models are examined with respect to their capabilities to predict the observed response. It Is shown that the predictions of these two models agree only qualitatively but not quantitatively with the experimental results.     

Residual stresses in polymers and their effect on mechanical behavior

Plates of bisphenol-A polycarbonate and poly(methyl methacrylate) have been quenched in ice water from temperatures slightly above their glass transition temperatures. Residual stresses are thus created, Measurement of these residual stresses has been accomplished by the “layer removal” method and the stress distributions through the thickness are presented. Compressive stresses, approximately 3000 psi, exist at the surface while tensile stresses-of at least 1000 psi exist in the interior. It is shown that these residual stresses can influence the notched Izod impact strengths for polycarbonates. The mechanism is thought to be suppression of craze initiation in advance of the notch due to the presence of residual compressive stresses for specimens notched prior to quenching. In the case of poly(methyl methacrylate), it is shown that compressive residual stresses at the surface can cause plastic yielding to occur in bending experiments resulting in permanent deformation and greater energy absorption.