Small-angle scattering of polarized light. VI. Sphere doublets at rest and during shear

The polarized or depolarized light scattering by well-defined monodispersed sphere doublets is investigated. Two configurations of doublets are studied. In the first (at rest) the doublets are randomly oriented in a plane, in the second the doublets are oriented in a preferred direction. This is achieved by submitting a suspension of doublets to a shear flow. The scattering patterns are compared to two theoretical predictions based on simplified geometries. In the first approach, the doublet is approximated by two interpenetrating spheres scattering independently, whereas in the second, an ellipsoid geometry is used. A good qualitative comparison is obtained. However, the HV and VH patterns of a randomly dispersed suspension are not similar. The observation of the flow of a doublet suspension in shear shows that the doublets are spiraling around the vorticity axis. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2005–2013, 1998     

Strain stiffening and negative normal stress in alginate hydrogels

Alginate hydrogels are polysaccharide biopolymer networks widely useful in biomedical and food applications. Here, we report nonlinear mechanical responses of ionically crosslinked alginate hydrogels captured using large amplitude oscillatory shear experiments. Gelation was performed in situ in a rheometer and the rheological investigations on these samples captured the strain‐stiffening behavior for these gels as a function of oscillatory strain. In addition, negative normal stress was observed, which has not been reported earlier for any polysaccharide networks. The magnitude of negative normal stress increases with the applied strain amplitude and can exceed that of the shear stress at large‐strain. Fitting a constitutive relationship to the stress‐strain curves reveals that the mode of deformation involves stretching of the alginate chains and bending of both the chains and the junction zones. The contribution of bending increases near saturation of G blocks as Ca²⁺ concentration was increased. The results presented here provide an improved understanding of the deformation behavior of alginate hydrogels and such understanding can be extended to other crosslinked polysaccharide networks. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1767–1775     

Adhesion of silicon oxide layers on poly(ethylene terephthalate). II: Effect of coating thickness on adhesive and cohesive strengths

Fragmentation tests in the uniaxial mode were performed on poly (ethylene terephthalate) (PET) films coated with a silicon oxide layer of thickness ranging from 30 to 156 nm. The coating's fragmentation process was investigated to reveal the crack onset strain and the crack density at fragmentation saturation. Adhesive strength was modeled from the Kelly-Tyson approach, including a Weibull distribution of the coating strength. The prediction was found to be independent of coating thickness, and equal to the substrate shear stress at saturation. The cohesive strength of the coating was characterized from the crack onset strain. The measured decrease in crack onset strain with coating thickness increase was modeled by means of Weibull and fracture mechanics theories, the latter providing the best predictions. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1463–1472, 1997     

Nonlinear shear of entangled polymers from nonequilibrium molecular dynamics

This study aims to use molecular dynamics (MD) simulations of Kremer–Grest (KG) chains to inform future developments of models of entangled polymer dynamics. We perform nonequilibrium MD simulations, under shear flow, for well‐entangled KG chains. We study chains of 512 and 1000 KG beads, corresponding to 8 and 15 entanglements, respectively. We compute the linear rheological properties from equilibrium simulations of the stress autocorrelation and obtain from these data the tube model parameters. Under nonlinear shear flow, we compute the shear viscosity, the first and second normal stress differences, and chain contour length. For chains of 512 monomers, we obtain agreement with the results of Cao and Likhtman (ACS Macro. Lett. 2015, 4, 1376). We also compare our nonlinear results with the Graham, Likhtman and Milner‐McLeish (GLaMM) model. We identify some systematic disagreement that becomes larger for the longer chains. We made a comparison of the transient shear stress maximum from our simulations, two nonlinear models and experiments on a wide range of melts and solutions, including polystyrene (PS), polybutadiene, and styrene–butadiene rubber. This comparison establishes that the PS melt data show markedly different behavior to all other melts and solutions and KG simulations reproduce the PS data more closely than either the GLaMM or Xie and Schweizer models. We discuss the performance of these models against the data and simulations. Finally, by imposing a rapid reversing flow, we produce a method to extract the recoverable strain from MD simulations, valid for sufficiently entangled monodisperse polymers. We explore how the resulting data can probe the melt state just before the reversing flow. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1692–1704     

A new attempt to reconcile the statistical and phenomenological theories of rubber elasticity

The statistical and phenomenological theories of rubber elasticity are reviewed briefly. Combining recent concepts proposed by Yeoh and Gent, a new theory is proposed. The proposed constitutive model for rubber vulcanizates invokes two mechanisms; one influences behavior at small strains while the other dominates behavior at large strains. Network flaws, such as entanglements, are suspected to be responsible for the first mechanism. Finite extensibility of network chains is identified as the cause of the second. Thus, macroscopic behavior is directly linked to molecular concepts. The proposed theory allows prediction of the stress–strain behavior of a family of four rubber vulcanizates in different modes of deformation (simple extension, compression, and simple shear) from regression analysis of tensile data alone from just one member. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1919–1931, 1997     

Stress-dependent dynamic compliance spectra approach to the nonlinear viscoelastic response of polymers

The present work reports a discrete, stress-dependent dynamic compliance spectra method which may be used to predict the mechanical response of nonlinear viscoelastic polymers during strain-defined processes. The method is based on the observation that the real and complex parts of the discrete dynamic compliance frequency components obtained from creep measurements are smooth, easily fit functions of stress. Comparisons between experimental measurements and model calculations show that the model exhibits excellent quantitative agreement with the basis creep measurements at all experimental stress levels. The model exhibits good quantitative agreement with stress relaxation measurements at moderate levels of applied strain. However, the model underestimates the experimental stress relaxation at an applied strain of 3.26%. The stress relaxation error appears to be a real material effect resulting from the different strain character of creep and stress relaxation tests. The model provides a good quantitative agreement with experimental constant strain rate measurements up to approximately 4% strain, after which the model underestimates the experimental flow stress. This effect is explained by the time dependence of the stress-activated configurational changes necessary for large strains in glassy polymers. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2301–2309, 1998     

Structure build‐up at rest in polymer nanocomposites: Flow reversal experiments

Evolution of the microstructure as well as the shear stress and the normal stress difference of polymer/layered silicate nanocomposites prepared by melt mixing of poly[butylene succinate‐co‐adipate] and organically modified montmorillonite are investigated in transient forward and reverse start‐up shear flows at different clay loading and different shear rates. Special attention is paid to the structure build‐up at rest and to the amplitude of the overshoots observed during the reverse start‐up test in the shear stress and the normal stress difference. The model that we have developed previously is used to suggest an explanation for the observed phenomena. The model is able to capture observed behavior of the shear stress in both forward and reverse start‐up flows. It fails, however, to predict experimentally observed overshoot in the normal stress difference. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1728–1741, 2009     

Rheology and microstructures formation of immiscible model polymer blends under steady state and transient flows

Viscoelastic properties of model immiscible blend were studied here under steady state condition at different initial conditions and transient flow conditions. The flow‐induced microstructure has been studied on these model blends. For this system, the elastic properties of the blend are mainly governed by the interface. Measurement of the dynamic modulus and of the first normal stress difference, both reflecting this enhanced elasticity, have been used to prove the blend morphology. The dynamic moduli after cessation of shear flow, the mean diameter of the disperse phase as generated by the shear flow, have been calculated using the model of Palierne. A procedure based on a direct fitting of the dynamic moduli with the model is compared with the one that uses a weight relaxation spectrum. On the other hand, the steady state normal stress data have been related to the morphology of the blend by means of Doi and Ohta model. The specific interfacial area is found to be inversely proportional to the ratio of interfacial tension over shear stress for the blend. The flow behavior during transient shear flow was also discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3519–3533, 2005     

Melt rheology and morphology of thermoplastic elastomers from polyethylene/nitrile‐rubber blends: The effect of blend ratio,reactive compatibilization,and dynamic vulcanization

A study of the melt‐rheological behavior of thermoplastic elastomers from high‐density polyethylene and acrylonitrile butadiene rubber (NBR) blends was carried out in a capillary rheometer. The effect of the blend ratio and shear rate on the melt viscosity reveals that the viscosity decreases with the shear rate but increases with NBR content. Compatibilization by maleic anhydride modified polyethylene has no significant effect on the blend viscosity, but a finer dispersion of the rubber is obtained, as is evident from scanning electron micrographs. The melt‐elasticity parameters, such as the die swell, principal normal stress difference, recoverable shear strain, and elastic shear modulus of the blends, were also evaluated. The effect of annealing on the morphology of the extrudate reveals that annealing in the extruder barrel results in the coalescence of rubber particles in the case of the incompatible blends, whereas the tendency toward agglomeration is somewhat suppressed in the compatibilized blends. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1104–1122, 2000     

Anisotropic plasticity and chain orientation in polymer glasses

The anisotropic mechanical response of oriented polymer glasses is studied through simulations with a coarse-grained model. Systems are first oriented by uniaxial compression or tension along an axis. Then the mechanical response to subsequent deformation along the same axis or along a perpendicular axis is measured. As in experiments, the flow stress and strain hardening modulus are both larger when deformation increases the degree of molecular orientation produced by prestrain, and smaller when deformation reduces the degree of orientation. All stress curves for parallel prestrains collapse when plotted against either the total integrated strain or the degree of molecular orientation. Stress curves for perpendicular prestrains can also be collapsed. The stress depends on the degree of strain or molecular orientation along the final deformation axis and is independent of the degree of orientation in the perpendicular plane. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1473–1482, 2010     

Athermal simulation of plastic deformation in amorphous solids at constant pressure

An algorithm is introduced for the molecular simulation of constant-pressure plastic deformation in amorphous solids at zero temperature. This allows to directly study the volume changes associated with plastic deformation (dilatancy) in glassy solids. In particular, the dilatancy of polymer glasses is an important aspect of their mechanical behavior. The new method is closely related to Berendsen's barostat, which is widely used for molecular dynamics simulations at constant pressure. The new algorithm is applied to plane strain compression of a binary Lennard-Jones glass. Conditions of constant volume lead to an increase of pressure with strain, and to a concommitant increase in shear stress. At constant (zero) pressure, by contrast, the shear stress remains constant up to the largest strains investigated (ε = 1), while the system density decreases linearly with strain. The linearity of this decrease suggests that each elementary shear relaxation event brings about an increase in volume which is proportional to the amount of shear. In contrast to the stress–strain behavior, the strain-induced structural relaxation, as measured by the self-part of the intermediate structure factor, was found to be the same in both cases. This suggests that the energy barriers that must be overcome for their nucleation continually grow in the case of constant-volume deformation, but remain the same if the deformation is carried out at constant pressure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2057–2065, 2004     

Fiber motion and rheology of suspensions with uniform fiber orientation

One of the main goals in the studies of fiber suspensions is the prediction of fiber orientation in a short fiber composite part, using the processing variables, mold geometry, and material characteristics. The rheological properties of the fiber suspensions are strongly associated with the fiber orientation distribution. The understanding of the relations between the fiber structure in the suspension and its rheological properties is a key step in the design and implementation of processing operations. The fiber motion in shear flow is analyzed in this article. The study is focused on the relation between fiber orientation and rheological properties for a suspension with uniform (delta function) fiber orientation distribution in a Newtonian fluid. The study shows that the rheological properties of the suspension, measured during the start up of steady shear flow, can be used to determine the fiber orientation in the sample. The first normal stress coefficient is the property to measure in order to determine whether or not the suspension has a random fiber orientation. Any of the shear flow transient rheological properties can be used to determine the fiber initial orientation. It was found that the normal stress coefficients can show negative or positive values depending on the fiber orientation. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1788–1799, 2000     

Temperature and strain-rate dependence of yield stress of polyethylene

The yield stress behavior of a range of polyethylene materials which differ with respect to their short chain branch content has been studied. Measurements carried out over a wide range of temperatures have shown that there is a sudden transition in the behavior of the yield stress at a temperature which is dependent on both the grade of material and the applied strain rate. These results are in agreement with previous results found from analysis of the yield strain behavior. Above the transition temperature the materials all behave in a nonlinear viscoelastic manner, and the yield process is considered as being propagation controlled. Below the transition temperature the materials all behave in an elastic-plastic manner, and the yield process is considered as being nucleation controlled. Below the transition temperature the temperature dependence of the yield stress is determined by the thickness of the crystalline lamellae. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2177–2189, 1998     

Organoclay/thermotropic liquid crystalline polymer nanocomposites. III. Effects of fully exfoliated organoclay on morphology,thermal, and rheological properties

A fully exfoliated organoclay in thermotropic liquid crystalline polymer (TLCP) based nanocomposite was prepared by a method combining ultrasonication, centrifugation, solution casting, and heat‐shearing separation. Morphological study showed that the organoclays of 15–25 nm in size dispersed uniformly in TLCP with fully exfoliated structures. The organoclays formed molecular level interactions with TLCP molecules. The interactions did not affect the liquid crystallinity and mesophase structure of TLCP, but they affected the thermal stability and thermal properties of TLCP, increasing the thermal stability and shifting the transition temperatures to the higher ends. Mechanical rheology investigations in the linear viscoelastic region showed that with the exfoliated organoclay in TLCP, more obvious pseudosolidlike behavior appeared in the terminal region. The rigidity of TLCP was enhanced by the presence of the exfoliated organoclay with percolated structures in the TLCP matrix. In steady shear tests, the nanocomposite had the similar shear viscosity and N1 (the first normal stress difference) to those of TLCP in the steady state condition. Percolated structures were easily destroyed by sufficient shear strain and the exfoliated organoclays were oriented along the shear direction, even assisting the neighboring TLCP molecules to align in the flow direction, resulting in a decrease of viscosity and an increase of the N1 slope. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 712–720, 2010     

Experimental methods in polymer melt rheology

Experimental developments in different areas of polymer melt rheology and recent results are reviewed: Shear oscillations were performed with mixtures of polyisobutylenes (PIB) of relatively small molecular mass distributions and with blends of polystyrene (PS) and polymethylmethacrylate (PMMA). For the latter, the interfacial tension between the melts of PS and PMMA can be derived from the results. - In constant shear rate flow, the measurement of the first normal stress difference N₁ in a commercial cone-and-plate rotational rheometer is simple if the test temperature is kept extremely constant. By a further modification a partial integral of the pressure distribution is measured from which the second normal stress difference N₂ can be determined. - The molecular orientation in shear flow results in a rheological anisotropy that can be studied in a parallel-plate shear rheometer in which the polymer melt sample can arbitrarily be sheared in two perpendicular directions. - For melt elongation by rotary clamps constant, arbitrary ratios of the strain rate components can be applied. Planar elongations of the PIB samples indicate an influence of the width of the molecular mass distribution. More general elongations with a change of the strain rate ratio during the test are of especial interest as is documented by the comparison of the resulting stresses with predictions from different network theories. - In a rheometer for simple elongation at 170°C, recovery after melt extension of the PS/PMMA blends reveals a value of the interfacial tension equal to the one obtained from shear oscillations.     

剪切作用下纳米粘土对LDPE熔体粘弹响应的影响

以LDPE/EVA/纳米粘土复合体系为研究模型,考察了剪切作用下,分散良好的纳米粘土对聚合物基体熔体稳态及瞬态粘弹响应的影响.发现剪切作用下,纳米粘土增加了聚合物熔体粘弹特性对剪切速率、剪切应变及剪切作用史的依赖性,改变了相应的依赖关系.稳态剪切时,纳米粘土的加入使体系第一法向应力差(N1)在低剪切速率区变为负值,而在高剪切速率区N1与粘土的含量无关;同时就瞬态剪切应力及N1的应变依赖关系而言,复合体系明显不同于聚合物基体;预剪切对聚合物基体瞬态粘弹响应几乎没有影响,而当纳米粘土的加入量大于3wt%后,与未经预剪切的样品相比较,经预剪切的复合体系的瞬态剪切应力值、应力过冲程度以及稳态剪切应力值均明显下降,且预剪切前后复合体系达到稳态时其瞬态剪切应力差值随纳米粘土含量的增高而线性增加.此外,纳米粘土的添加对聚合物熔体受剪切作用的非线性粘弹响应存在影响.复合体系熔体呈现特异非线性粘弹响应,其缘由被认为是由于纳米粘土在聚合物基体中剥离分散,或聚合物分子链插层于粘土片层间,形成局部有序结构,受剪切作用而排列取向.     

Stress relaxation of polybutadiene at large deformation. Measurements of stress and birefringence in shear and elongation

The rheological behavior of an uncrosslinked polybutadiene on sudden application of finite strain was examined. The shear stress σ, two components of birefringence, and the extinction angle were measured in shear (magnitude of shear γ ≤ 3.5) and tensile stress and the birefringence were measured in uniaxial elongation (elongation ratio λ ≤ 3.8). Measurements were performed at 30°C with a tensile tester equipped with appropriate sample holders. The stress-optical coefficient was 3.01 × 10^?9Pa^?1. The first and second normal-stress differences v₁ and v₂ were separately evaluated with the use of stress-optical law. The Lodge—Meissner relation v₁ = γσ held good. The ratio v₂/v₁ was independent of time and varied from about ?0.3 to ?0.2 with increasing γ in the range of measurements. Each of the stress components was factored into a function of strain and one of time, and the latter was common to all the stress components. Simple formulas were proposed to represent stress components in step deformations.     

Mechanistic model for deformation of polymer nanocomposite melts under large amplitude shear

We report the mechanical response of a model nanocomposite system of poly(styrene) (PS)-silica to large-amplitude oscillatory shear deformations. Nonlinear behavior of PS nanocomposites is discussed with the changes in particle dispersion upon deformation to provide a complete physical picture of their mechanical properties. The elastic stresses for the particle and polymer are resolved by decomposing the total stress into its purely elastic and viscous components for composites at different strain levels within a cycle of deformation. We propose a mechanistic model which captures the deformation of particles and polymer networks at small and large strains, respectively. We show, for the first time, that chain stretching in a polymer nanocomposite obtained in large amplitude oscillatory deformation is in good agreement with the nonlinear chain deformation theory of polymeric networks. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Cross deformation and stress relaxation of biaxially oriented polystyrene films

When a biaxially oriented polystyrene film was stretched along one direction and subsequently stretched along the perpendicular direction, the film showed enhanced ductility with pronounced yield softening and extended strain hardening. In the forward deformation, at least two types of shear bands were observed. The bands at the early stages of yielding did not seem to contribute to the reduction of thickness. They were approximately 200 μm thick and had an intersection angle of approximately 120°. The bands developed in the later stages contributed to the thickness reduction. These bands were smaller and possessed an intersection of approximately 90°. In the cross deformation, new shear bands developed that were likely related to the reverse shearing of the existing bands. Stress relaxation showed a power‐law relationship between the stress rate and relaxation time. The internal stress of the cross deformation was significantly (ca. 3 times) lower than that of the forward deformation at the same strain. The enhancement in ductility may be attributed to the lowering of internal stress during the cross deformation. The internal stress increased with the applied stress and strain. Fracture occurred when the internal stress reached a certain level, about 57–68 MPa for deformation along both directions and approximately 44–47% of the final applied stress. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 687–700, 2003