Components of microstrain in polyethylene

Tensile microstrain was measured in high‐density polyethylene with a precision of 2 × 10^?7 for strains up to 10^?4 in the temperature range of 17–28 °C over a range of strain rates. The total strain was partitioned into three of the following components: (1) elastic, (2) amorphous, and (3) dislocation. Also, their respective dependence on stress was determined. Measurements of the area of the hysteresis loops gave the energy loss per cycle from which the frictional stress on the amorphous flow and the dislocations was determined. Although the amorphous strain originated at zero stress and was dependent on the temperature and strain rate, the dislocation motion was activated above acritical stress and independent of temperature and strain rate within the scope of these experiments. The viscosity of the amorphous flow was determined. The effect of 5 Mrd γ irradiation on the micro‐deformation behavior was not appreciable. About 80 Mrd reduced the amorphous flow by about 30% and increased the stress to activate the motions of the dislocations. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2693–2701, 2002     

Young's modulus of polyethylene in the microstrain region

The stress–strain behavior of various polyethylenes was measured with a strain sensitivity of 2 × 10^?7. Young's modulus was measured as a function of the strain rate. The shapes of the stress–strain curves in the vicinity of room temperature were nonlinear down to the lowest measurable strain. The stress–strain behavior in the microstrain region was well described by the model of the standard linear solid. From the model, the relaxation time was determined along with the relaxed and unrelaxed moduli. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2420–2429, 2001     

Thermal activation parameters for large strain deformation of polyethylene and polycarbonate

Experiments were performed to determine the effects of strain rate, temperature, and pressure on the flow stress of polyethylene and Lexan polycarbonate deformed in shear. The results were analyzed to determine the activation enthalpy and the shear and dilatation activation volumes of the rate-limiting mechanism of the deformation process. Results show that the activation event involves a volume containing several monomer units and that this volume must dilate by as much as 7% during the activation event. The activation enthalpy was approximately 2.5 × 10^?12 erg for polyethylene and 1.1 × 10^?12 erg for polycarbonate. The rate-limiting mechanism for polyethylene seemed to be unchanged by plastic strains of up to 250%.     

Strain rate and temperature dependence of a nanoparticle‐filled poly(dimethylsiloxane) undergoing shear deformation

The mechanical properties in shear of unfilled and nanoparticle‐filled polydimethylsiloxane (PDMS) networks are reported. The effect of silicate‐based nanoparticles on the mechanical response was studied as functions of rate and temperature using the time–temperature superposition principle. An apparent yielding phenomenon was observed in the filled polymer in spite of the more typical elastomeric behavior exhibited by the pure PDMS network. The time–temperature superposition principle was applied to capture the shear strain rate (10^?4–10^?1 s^?1) and temperature (?40 to 60°C) dependence of the stress response of the filled PDMS at different strains and at the yield point. A power‐law relationship was found to adequately describe the resulting master curves for yield stress in shear. Using a triangular shear displacement profile at 10^?2 s^?1, the effect of temperature (?20 to 80°C) on the recovery from a particularly pronounced Mullins effect was investigated as a function of rest time. Given adequate rest time (between 10 and 10² min), recovery was observed for the temperature range studied. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Tensile properties of ultradrawn polyethylene

High-density polyethylene filaments prepared by a solid-state deformation in an Instron capillary rheometer show unusually high crystal orientation, chain extension, axial modulus, and ultimate tensile strength. The Young's modulus and ultimate tensile strength have been determined from stress–strain curves. Gripping of this high modulus polyethylene has been a problem heretofore, but the measurement of ultimate tensile strength has now been made feasible by a special gripping procedure. Tensile moduli show an increase with sample preparation temperature and pressure. Values as high as 6.7 × 10¹¹ dyne/cm² are obtained from samples extruded at 134°C and 2400 atm and tested at a strain rate of 3.3 × 10^?4 sec^?1. The effect of strain rate and frequency on modulus has also been evaluated by a combination of stress–strain data and dynamic tension plus sonic measurements over nine decades of time.     

The effect of strain rate on craze yielding,shear yielding,and brittle fracture of polymers at 77°K

The tensile strength of poly(methyl methacrylate) (PMMA), polycarbonate (PC), polychlorotrifluoroethylene, and polysulfone was measured in liquid nitrogen over the strain rate range of 2 × 10^?4 to 660 min^?1. These polymers deformed by crazing which was induced by the liquid nitrogen. The stress versus log strain rate curve was sigmoidal in that its slope increased and then decreased with strain rate. Above a critical strain rate of about 200 min^?1, which varied somewhat with the polymer, crazing was not observed with the optical microscope; the behavior became brittle, and the tensile strength became constant. The nonlinear behavior of stress versus log strain rate at low strain rates was associated with a decrease in activation volume with increasing strain rate whereas the nonlinear behavior at high strain rates was associated with an increase in density and decrease in length of the crazes with strain rate. The strain rate effect was the basis for calculating the diffusion coefficient of nitrogen into the polymers at 77°K. The shear deformation mode of PC was measured under compression and under tension. The compressive strength versus log strain rate was linear throughout the entire range giving a compression shear activation volume of 360 ų. The shear tensile strength of PC varied only slightly with strain rate when compared to the compressive strength. The brittle fracture stress of PMMA, in the absence of crazing, in compression and in tension, did not vary with strain rate.     

Experimental investigation and modeling of the mechanical behavior of PC/ABS during monotonic and cyclic loading

The purpose of this work is to characterize the mechanical behavior of blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) during monotonic and cyclic loading. Compression experiments were performed using a SHIMADZU universal testing machine (10⁻⁴ to 10⁻² s⁻¹) and a split Hopkinson pressure bar (1600–5000 s⁻¹), with, the test temperatures ranging from 293 to 353 K. The influence of the rate and temperature on the deformation of PC/ABS is discussed in detail. Based on the investigation of numerous constitutive models, a phenomenological model called DSGZ was chosen to describe the compression behavior of PC/ABS. This model could not accurately reproduce the deformation of polymers at high strain rates when utilizing the same material coefficients for the low and high strain–rate deformations. In addition, this model was unable to capture the deformation features during unloading and subsequent reloading when adopting the original stress–strain updating algorithm. Hence, some improvements to the model have been implemented to better predict the deformation. Finally, the model predictions are shown to be consistent with the experimental results.     

Effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene

The effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene (UHMW‐PE) are discussed, based on a combination of in situ X‐ray measurement and stress–strain behavior. The sample films of metallocene‐ and Ziegler‐catalyzed UHMW‐PEs with a similar viscosity average MW of ～10⁷ were prepared by compression molding at 180 °C. Stress profiles recorded at 160 °C above the melting temperature of 135 °C exhibited a plateau stress region for both films. The relative change in the intensities of the amorphous scattering recorded on the equator and on the meridian indicated the orientation of amorphous chains along the draw axis with increasing strain. However, there was a substantial difference in the subsequent crystallization into the hexagonal phase, reflecting the molecular characteristics, that is, MW distribution of each sample film. Rapid crystallization into the hexagonal phase occurred at the beginning point of the plateau stress region in melt‐drawing for metallocene‐catalyzed UHMW‐PE film. In contrast, gradual crystallization into the hexagonal phase occurred at the middle point of the plateau stress region for the Ziegler‐catalyzed film, suggesting an ease of chain slippage during drawing. These results demonstrate that the difference in the MW distribution due to the polymerization catalyst system dominates the phase development mechanism during melt‐drawing. The effect of the processing conditions, that is, the including strain rate and drawing temperature, on the melt‐drawing behavior is also discussed. The obtained results indicate that the traditional temperature–strain rate relationship is effective for transient crystallization in to the hexagonal phase during melt‐drawing, as well as for typically oriented crystallization during ultradrawing in the solid state. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2455–2467, 2006     

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     

Comparison of the transient stress–strain response of rubber to its linear dynamic behavior

Master curves of the small strain and dynamic shear modulus are compared with the transient mechanical response of rubbers stretched at ambient temperature over a seven‐decade range of strain rates (10^?4 to 10³ s^?1). The experiments were carried out on 1,4‐ and 1,2‐polybutadienes and a styrene–butadiene copolymer. These rubbers have respective glass transition temperatures, T_g, equal to ?93.0, 0.5, and 4.1 °C, so that the room temperature measurements probed the rubbery plateau, the glass transition zone, and the onset of the glassy state. For the 1,4‐polybutadiene, in accord with previous results, strain and strain rate effects were decoupled (additive). For the other two materials, encroachment of the segmental dynamics precluded separation of the effects of strain and rate. These results show that for rubbery polymers near T_g the use of linear dynamic data to predict stresses, strain energies, and other mechanical properties at higher strain rates entails large error. For example, the strain rate associated with an upturn in the modulus due to onset of the glass transition was three orders of magnitude higher for large tensile strains than for linear oscillatory shear strains. © 2011 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys, 2011     

Effect of the reference chain dimension on the viscoelastic and stress–strain behavior of poly(n-butyl methacrylate) networks

The linear viscoelastic and stress-strain behavior of poly(n-butyl methacrylate) networks at a content of crosslinking agent (ethyleneglycol dimethacrylate) of c? 0–1 × 10^?4 mole/cm³ was investigated in the main transition and rubberlike region in the temperature interval from 20 to 150°C. The dependence of the unperturbed chain dimensions on temperature was determined from thermoelastic measurements in the rubberlike region; this dependence was unaffected by the content of crosslinking agent. Application of time–temperature superposition to the linear viscoelastic behavior did not give a continuous superimposed curve in the proximity of the rubberlike region; superposition within the whole time region required introducing the change of the unperturbed chain dimensions with temperature. This correction was sufficient for a sample with a higher content of the crosslinking agent. However, for loose networks (c< 0.1 × 10^?4 mole/cm³) it was insufficient, because of another relaxation mechanism in the region of high temperatures. It was found that the intensity and temperature dependence of this relaxation mechanism, which is probably due to a change of the number of entanglements with temperature, are connected with the magnitude and the temperature dependence of the C₂ constant of the Mooney-Rivlin equation.     

Tunable microstructures and mechanical deformation in transparent poly(urethane urea)s

Transparent poly(urethane urea) (TPUU) materials offer an avenue to enable material designs with potential to achieve simultaneous enhancements in both physical and mechanical properties. To optimize the performance required for each application, the molecular features that influence the microstructure, the glass transition temperature (T_g), the deformation mechanisms, and the mechanical deformation behavior must be understood and exploited. In this work, a comprehensive materials characterization of select model PUUs with tunable microstructures is addressed. Increasing the hard segment (HS) content increases the stiffness and flow stress levels, whereas altering the soft segment (SS) molecular weight from 2000 to 1000 g/mol leads to an enhanced phase mixing with a SS T_g shifted ～17 °K toward higher temperatures as well as broadening of the SS relaxation closer to room temperature. As a result, the 1K TPUU materials display greater rate‐dependent stiffening and strain hardening on mechanical deformation over the broad range of strain rates covered in this work (10^?3 to 10⁴ s^?1). In such case of similar urea‐based HS content, the molar content of the urethane linkages, per stoichiometric requirements, is much higher in the 1K TPUUs than the 2K TPUUs. These additional urethane moieties lead to an increase in the extent of intermolecular interactions, via hydrogen bonding between the HS and the SS, providing not only further phase mixing and stronger rate sensitivity but also provide 1K TPUUs with drastically improved barrier properties. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Characterization,morphology, thermal and mechanical properties of conductive polyaniline‐functionalized EPDM elastomers obtained by casting

Conductive elastomeric blends based on ethylene–propylene–5‐ethylidene–2‐norbornene terpolymer (EPDM) and polyaniline doped with 4‐dodecylbenzenesulfonic acid [PAni(DBSA)] were cast from organic solvents. Functionalization of the elastomer was promoted by grafting with maleic anhydride. Vulcanization conditions were optimized with an oscillating disk rheometer. The conductivity, morphology, thermal stability, compatibility, and mechanical behavior of the obtained mixtures were analyzed by in situ direct current conductivity measurements, atomic force microscopy, transmission electron microscopy, wide‐angle X‐ray scattering, thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical thermal analysis, stress–strain and hysteresis tests. The vulcanization process was affected by temperature, the PAni content, and maleic anhydride. A reinforcement effect was promoted by the vulcanizing agent. The formation of links between the high‐molar‐mass phases and oligomers of PAni(DBSA) in the elastomeric matrix enhanced the thermal stability and ultimate properties of the blends. By the appropriate control of the polymer blends' composition, it was possible to produce elastomeric materials with conductivities in the range of 10^?5–10^?4 S · cm^?1 and excellent mechanical properties. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1767–1782, 2004     

A rotational rheometer for rheological studies with prescribed strain or stress history

A rheometer utilizing an eddy-current torque transducer and an air-bearing suspension is described. The rheometer is coupled with a computer-based data acquisition system and permits studies of shear deformation for several strain or stress histories. A sinusoidal stress history is used to determine the shear storage and loss compliances J′(ω) and J″(ω), respectively. Step stress histories are used to determine the shear creep compliance J(t) and the recoverable complaince R(t) or more complicated linear and nonlinear rheological responses related to these. Deformation at constant strain rate is used to determine the stress growth function or the steady-state viscosity. The rheometer may be used over the temperature range – 10–180°C, with torque from 1 to 10⁶ dyn cm, and is adaptabel to use with a variety of sample geometrical shapes (e.g., cone and plate, parallel plate, etc.). Examples of measurements on on viscoelastic fluids and on gels below their yield strain are given.     

On the effect of an increase in the effective viscosity of nanodispersed Aerosil in Vaseline oil in the range of extra low shear rates

Total flow curves of the suspensions of modified (methylated) nanodispersed Aerosil (mean particle size is 40 nm) in Vaseline oil with concentrations of 2–7 wt % are recorded under quasi-equilibrium conditions. The behavior of these structured nanodisperse systems in the range of extra low shear rates (10^?6-10^?3 s^?1) is studied in detail. In this range of shear rates, the effect of an increase in the rise of effective viscosity with increasing shear stress is revealed for concentrated Aerosil suspensions for the first time.     

Rheological properties of cellulose-chitosan-phosphoric acid systems in different phase states

Rheological properties of 7.0–9.1% cellulose-chitosan suspensions, solutions, and gels in aqueous phosphoric acid were examined in various modes of shear flow in the range 0.15–100 s^?1 at 268–323 K. During steady state flow, the appearance of a quasi-Newtonian region was detected at shear rates between 20 and 40 s^?1 owing to orientational ordering of macromolecules in the stream. Under the conditions of transient shear flow at a constant shear rate, rheopexy was observed in both cellulose solutions and cellulose-chitosan solutions. The thixotropic behavior of cellulose-chitosan suspensions, spinning solutions, and gels was characterized during a sharp drop of shear rate from 0.15 to 10 s^?1, that is, under conditions modeling the processes of transport and extrusion of spinning solutions.     

Analyzing the Mechanical Behavior of Polymer and Composite Materials by Means of Unique Method of Deformation Calorimetry

Results are presented from long-term investigations of a wide range of polymer systems, varying from elastomers and thermoplastic elastomers to plastics and fibers. The thermophysical properties of both initial and modifying additive–containing polysiloxanes, block copolymers, and poleolefins that differ in chemical nature, structure, and composition are analyzed. It is shown that deformation calorimetry allows the simultaneous registration of mechanical (from 5 × 10^?3 kg) and thermal effects (at a sensitivity of 2 × 10^?7 J/s), and the determination of changes in enthalpy, internal energy, and intra- and intermolecular contributions to the formation of the tensile stress response. In other words, it provides a unique opportunity to analyze the deformation mechanism of investigated systems and its dependence on the changing parameters.     

Mechanical and optical properties of an anelastic polymer at intermediate strain rates and large strains

The mechanical and optical properties of a polyester–styrene copolymer have been investigated by means of drop tests at strain rates from 38 to 113 sec^?1 for strains less than 50%. Over this range of rates, the optical behavior was found to be linear with strain and independent of strain rate while the elastic–plastic mechanical behavior was only slightly dependent on strain rate. Comparison with the results of similar experiments at lower strain rates achieved by means of an Instron tester reveals that both mechanical and optical properties vary significantly with strain rate. The variation of flow stress with strain rate was found to obey a power law.