Rate sensitivity and deformation mechanism of semi-crystalline polymers during instrumented indentation

Instrumented indentation tests using both constant loading rate (CLR) and continuous stiffness measurement (CSM) operation modes were performed to investigate the deformation mechanism and their sensitivity to the deformation rate in semi-crystalline polymers through the quantitative analysis of load-depth loading and unloading curves. The strain rate was constant during the CSM tests, while the strain rate decreased with the increasing of loading time in CLR tests. The mechanical response mechanism of the semi-crystalline polymers to these tests was very complicated because of the combined effects of strain-hardening in the crystal phase and strain-softening in the amorphous phase. Results show that the loading index m reflects the strain-hardening or strain-softening response during indentation. When m > 2, the mechanical response was due to the strain-hardening, and when m < 2, the response was due to strain-softening. A method based on the measured contact hardness was proposed to obtain the unloading stiffness, and the other mechanical parameters could then be determined according to the unloading stiffness.     

Viscoelastic characterization of polymers using instrumented indentation. I. Quasi‐static testing*

The use of instrumented indentation to characterize the mechanical response of polymeric materials was studied. A model based on contact between a rigid probe and a linear viscoelastic material was used to calculate values for the creep compliance and stress relaxation modulus for two glassy polymeric materials, epoxy and poly(methyl methacrylate), and two poly(dimethyl siloxane) (PDMS) elastomers. Results from bulk rheometry studies were used for comparison with the indentation stress relaxation results. For the two glassy polymers, the use of sharp pyramidal tips produced responses that were considerably more compliant (less stiff) than the rheometry values. Additional study of the deformation remaining in epoxy after indentation creep testing as a function of the creep hold time revealed that a large portion of the creep displacement measured was due to postyield flow. Indentation creep measurements of the epoxy with a rounded conical tip also produced nonlinear responses, but the creep compliance values appeared to approach linear viscoelastic values with decreasing creep force. Responses measured for the unfilled PDMS were mainly linear elastic, with the filled PDMS exhibiting some time‐dependent and slight nonlinear responses in both rheometry and indentation measurements. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1794–1811, 2005     

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     

Research on viscoelastic behavior and mechanism of hydrogel grafted with UHMWPE

Hydrogel has been extensively studied for use as articular cartilage. The static and dynamic viscoelasticity behaviors of hydrogel grafted with ultra-high-molecular-weight polyethylene (UHMWPE) were studied by finite-element method (FEM) and dynamic mechanical analysis in this article. The results show that creep deformation presents an exponential function with the pore fluid velocity of hydrogel material. During the first period of stress relaxation, the internal fluid pore pressure of hydrogel material is less than partial pressure, which leads to the increasing fluid exudation, and the stress relaxation rate changes quickly. With the loss of fluid, the pore pressure and partial pressure achieve balance. Then, finally, stress relaxation reaches relative equilibrium. The storage modulus of hydrogel material increases with the increasing frequency, and there is a logarithmic regression between them. With the decrease in liquid–solid ratio, the storage modulus declines, while the loss modulus first increases and then decreases. When the strain increases, both storage modulus and loss modulus show an upward trend.     

Viscoelastic characterization of polymers using instrumented indentation. II. Dynamic testing*

Dynamic nanoindentation was performed on a cured epoxy, a poly(methyl methacrylate) (PMMA), and two poly(dimethyl siloxane) (PDMS) samples of different crosslink densities. These samples were used to compare dynamic nanoindentation with classical rheological measurements on polymeric samples in the glassy and rubbery plateau regions. Excellent agreement between bulk rheological data and dynamic nanoindentation data was observed for the two glassy materials (epoxy and PMMA) and the less compliant PDMS sample. More divergent results were observed for the more compliant PDMS sample. The theoretical foundation and historical development of the working equations for these two types of instrumentation are presented and discussed. The major difference between nanoindentation and the more classical rheological results is in the treatment of the instrument–sample interface. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1812–1824, 2005     

Flat‐punch indentation of viscoelastic material

The indentation of standard viscoelastic solids, that is, the three‐element viscoelastic material, by an axisymmetric, flat‐ended indenter has been investigated theoretically. Under the boundary conditions of flat‐punch indentation of a viscoelastic half‐space, the solutions of the equations of viscoelastic deformation are derived for the standard viscoelastic material. Their generality resides in their inclusion of compressible as well as incompressible solids. They cover the two transient situations: flat‐punch creep test and load‐relaxation test. In experimental tests of their applicability, nanoindentation and microindentation probes under creep and relaxation conditions yielded a modulus from 0.1 to 1.1 GPa and viscosity from 1 to 37 Gpa · s for a crosslinked glassy polyurethane coatings. For bulk polystyrene, the values vary from 1 to 2 GPa and from 20 to 40 Gpa · s, respectively. The analysis here provides a fundamental basis for probing elastic and viscous properties of coatings with nanoindentation or microindentation tests. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 10–22, 2000     

Nonlinear viscoelastic behavior of styrene‐[ethylene‐(ethylene‐propylene)]‐styrene block copolymer

Studies on the nonlinear viscoelastic behavior of styrene‐[ethylene‐(ethylene‐propylene)]‐styrene block copolymer (SEEPS) were carried out. The nonlinear viscoelastic region was determined through dynamic strain sweep test, and the critical shear strain (γ_c) of transition from linear viscoelastic region to nonlinear viscoealstic region was obtained. The relaxation time and modulus corresponding to the characteristic relaxation modes were also acquired through simulating the linear relaxation modulus curves using Maxwell model, and the damping functions were evaluated. Meanwhile, it is found that the nonlinear relaxation modulus obtained at relatively low shear strains follows the strain–time separation principle, and the damping function of SEEPS can be fit to Laun double exponential model well. Moreover, the successive start‐up of shear behavior, the steady shear behavior, and the relaxation behavior after steady shear were investigated, respectively. The results showed that Wagner model, derived from the K‐BKZ (Kearsley‐Bernstein, Kearsley, Zapas) constitutive equation, could simulate the experiment data well, and in addition, experiment data under the lower shear rates are almost identical with the fitting data, but there exists some deviation for data under considerable high shear rates. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1309–1319, 2006     

On the viscoelastic and plastic behavior of semiaromatic polyamides

Different semiaromatic polyamides (SAPA) have been synthesized by step‐growth polymerization of an aliphatic diamine, M (the 2‐methyl 1,5‐pentanediamine), and isophthalic acid, I, or terephthalic acid, T, or mixtures of these two diacids. The influence of the relative amount of randomly distributed MT units on the viscoelastic properties of the materials was investigated. It was shown that the glass transition T_g, as deduced from DSC thermograms, and the relevant mechanical relaxation T_α raise when the content of MT units increases. In contrast, the broad low‐temperature secondary relaxation, called γ, does not markedly depend on the MT content. Samples systematically studied in the absence of any moisture did not exhibit the intermediate‐temperature secondary relaxation, called β, which is characteristic of the wet polyamides. The study of the plastic behavior was focused on the samples MI and I5, which are strictly amorphous, and contain 0% and 50 mol % of MT units, respectively. Mechanical experiments were carried out in both the compression and traction modes, at temperatures ranging from −80°C to T_g. Analysis of the compression data was based on the inspection of the temperature dependence of elastic modulus, E(T), yield stress, ς_y, plastic flow stress, ς_pf, and strain softening ς_y − ς_pf. Whereas the plots of ς_y as a function of temperature, T, reveal some differences between MI and I5 behavior, a unique master curve was obtained by plotting ς_y/E(T) vs. T − T_g, which means that the plastic behavior of these materials is controlled by their chain packing in the glassy state. The strain softening profile of MI and I5 is similar to that already reported in the case of brittle vinyl polymers. This observation is consistent with the traction data, which give evidence for the occurrence of the tensile yielding of MI and I5 at temperatures rather close to T_g. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1131–1139, 1999     

Simulation of the Long Term Behaviour of Plastics Components

Summary: This paper presents a method to model the mechanical behavior of polymers over a wide time- and load-range by means of finite element analyses. The method includes a material model as well as the determination of material parameters to calibrate the material model. As a special feature of this method the model is calibrated only by using creep data that are commonly available in material data bases. So the procedure improves the simulation of the long time behavior of plastic-components without an additional experimental effort. In combination with time-temperature-superposition principle, the temperature dependency of the long term behavior is represented, too. The simulation results are validated by creep experiments on an example part.     

Effect of chemical activity jumps on the viscoelastic behavior of an epoxy resin: Physical aging response in carbon dioxide pressure jumps

We report the results from tensile creep tests performed on an epoxy resin in the presence of carbon dioxide at different pressures (Pco₂) and at a constant temperature below the glass‐transition temperature. Time‐Pco₂ superposition was applied to the data to account for the plasticization effect because of the interaction between the carbon dioxide molecules and the polymer. In addition, physical aging of the epoxy films was investigated with sequential creep tests after carbon dioxide pressure down‐jumps at constant temperature and after temperature down‐jumps at constant carbon dioxide pressure. The isothermal pressure down‐jump experiments showed physical aging responses similar to the isobaric temperature down‐jump experiments. However, the aging rate for the CO₂ jump was slightly lower than that for the temperature‐jump (T‐jump) experiments, and the retardation time for the Pco₂‐jump experiments was up to 6.3 times longer than for the T‐jump conditions. The results are discussed in terms of classical physical aging and structural recovery frameworks, and speculation about the differences in the energy landscape resulting from the Pco₂‐jump and T‐jump experiments is also made. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2050–2064, 2002     

Analysis of viscoelastic behavior and dynamic mechanical relaxation of copolyester based layered silicate nanocomposites using Havriliak–Negami model

The solid‐state viscoelastic properties are examined for intercalated nanocomposites based on a copolyester and (2‐ethyl‐hexyl)dimethyl hydrogenated‐tallow ammonium montmorillonite. The nanocomposites are prepared via the direct melt intercalation technique using a conventional twin‐screw extruder. Dynamic mechanical thermal analysis of the nanocomposites is conducted using two different test setups. The dynamic mechanical relaxation spectra show an increase in the storage modulus of the nanocomposite over the entire temperature range under study as compared to the pristine polymer (except in the transition region from 70 to 80 °C). These results are analyzed using the empirical Havriliak–Negami (HN) equation. The four temperature independent HN parameters (α, β, E₀, and E_∞) and one temperature dependent parameter (τ, the relaxation time) are determined by solving the HN equation for each temperature over the range of temperatures. The calculated moduli results fit well with the experimental values of the relaxation spectra for the nanocomposites. This study shows that the HN model can be applied to polymer layered silicate nanocomposites, and it can be used to predict their dynamic mechanical properties over a wide range of temperatures and frequencies a priori. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2657–2666, 2004     

嵌段共聚物熔体流变行为研究进展

微相分离的结构特点赋予了嵌段共聚物很多优异的性能,使其广泛应用于汽车部件及工具手柄、电线电缆包皮或绝缘带、医疗制品及食品容器、密封胶、粘合剂、涂料以及聚合物共混改性等领域。聚合物流变特性直接关系到材料加工参数的选择以及产品最终性能,是聚合物结构设计、材料加工参数优化选择及拓展产品应用领域的理论基础。本文对嵌段共聚物的熔体流变行为进行了综述,着重介绍了与嵌段共聚物特殊结构相对应的流变特性,以及流变特性与相行为之间的关联,并提出了嵌段共聚物熔体流变行为研究的前沿与重点。     

Viscoelastic behavior of semicrystalline polymers at elevated temperatures on the basis of a two‐process model for stress relaxation

Viscoelastic behavior at elevated temperatures of high‐density polyethylene and isotactic polypropylene was investigated by using the stress relaxation method. The results are interpreted from the view of an established two‐process model for stress relaxation in semicrystalline polymers. This model is based on the assumption that the stress relaxation can be represented as a superposition of two thermally activated processes acting in parallel. Each process is associated either with the crystal or amorphous phase of a polymer sample. It was found that the temperature dependence of viscosity coefficients and elastic moduli of these two fractions are similar in the two materials. The experimental data was correlated with literature data of α and β processes in polyethylene and polypropylene obtained from dynamic mechanical thermal analysis. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3239–3246, 2000     

Effect of sacrificial bond on molecular dynamics and rheological behavior of hybrid butadiene-styrene-vinylpyridine rubber vulcanizates with reversible sacrificial network

Due to the improvement of strength and toughness at the same time, construction of reversible sacrificial bond network has attracted extensive attention to realize the high performance and functionalization of rubber materials. However, the research on the viscoelastic behavior of vulcanized rubber with reversible sacrificial bond network lags behind seriously. In this work relaxation kinetics, linear rheological behavior and nonlinear rheological behavior of hybrid crosslinked butadiene-styrene-vinylpyridine rubber (VPR) with reversible Zn²⁺-pyridine coordination bond were investigated in detail. It was found that, the tensile results under small strain were more conducive to reflect the contribution of sacrificial bond after sequential tensile-recovery process than those under large strain. Although sacrificial bond could affect both G' and G" at low temperature, the contribution of it to G" was more prominent. When the temperature was higher than the dissociation temperature of Zn²⁺-pyridine coordination bond, the sacrificial bond could only affect G", which was due to the fact that the pyridine group was equivalent to the graft on VPR backbone and acted as dangling chain. In addition, the destruction and reconstruction of coordination bond was the dominant factor in modulus recovery.     

The influence of physical aging on the creep rupture behavior of polystyrene

The effects of postannealing aging time on the brittle fracture behavior of polystyrene were studied. A combination of mechanical properties, including creep and creep rupture under constant load and the behavior under constant extension rate deformation were examined for polystyrene samples of different prior aging times (from 1h to 2 months). The specimens and fracture surfaces were examined by optical microscopy and SEM to observe any change in the fracture behavior. It was found that longer aging times caused not only a change in the time-dependent modulus of the material but also a significant decrease in the creep rupture life and a decrease in strain to failure. It was found that the reasons for this are that although aging delays craze formation, craze breakdown and ultimate failure are accelerated by aging. The importance of these findings are discussed, particularly in relation to failure criteria involving the use of critical strains. © 1993 John Wiley & Sons, Inc.     

绿原酸印迹均匀微球的制备及其色谱行为研究

以绿原酸为模板,甲基丙烯酸为功能单体,二乙烯基苯为交联剂,采用沉淀聚合法制备了绿原酸印迹均匀微球。探讨了模板及功能单体类型和用量对聚合物颗粒均匀性的影响,考察了流动相组成及柱温对分子印迹柱色谱保留和选择性的影响。绿原酸均匀印迹微球的制备条件为：模板-功能单体-交联剂的用量比为0.375:1.5:7.0,摇瓶转速为15r/min时,微球直径约2.5μm,聚合物收率为35.4%。流动相组成及含水量对印迹聚合物的保留行为影响较大,当用乙腈-水混合物为流动相时,含水量增加,各化合物在印迹色谱柱上的容量因子降低：当流动相含水量高于2.5%,分子印迹聚合物失去选择识别能力;但以0.002mol L_-1磷酸缓冲液(pH3.0)-乙腈混合液为流动相时,分子印迹柱对模板的容量因子、选择性及色谱峰形都有较大改进。当二者体积比为19:1,容量因子高达90.69,其选择性相对于没食子酸和原儿茶酸高达23.10和6.562。温度影响分子印迹聚合物的选择保留能力,当温度从25℃提高到45℃时,印迹聚合物对模板的容量因子从3.52降低到2.41,选择因子降低约30%。     

Effect of short glass fiber on structure and viscoelastic behavior of olefinic polymers synthesized with metallocene catalyst

Several composites were prepared on the basis of an ethylene homopolymer and different copolymers of ethylene and 1‐hexene, synthesized with a metallocene catalyst, as matrices and a content of a 5 wt % of short glass fiber. The effect of the fiber incorporation on the structure and mechanical and viscoelastic behaviors was analyzed for the different samples. The glass fibers induced a slightly higher crystallinity, and the crystallite morphology significantly changed (long spacings and crystal orientation). The incorporation of fibers did not reinforce the different matrices under study at this low content; consequently, the mechanical parameters, such as Young's modulus, yielding stress, and microhardness, were lower in the composites as compared with those values found in the neat polyolefins. The location and apparent activation energies of distinct relaxation processes are also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1244–1255, 2003     

Variation of the strain rate during CSM nanoindentation of glassy polymers and its implication on indentation size effect

The indentation strain rate is currently assumed to remain unvaried during continuous stiffness measurement (CSM) nanoindentation where is imposed to remain constant. To probe the validity of this assumption for the nanoindentation of glassy polymers, a series of experiments have been performed at different set values on poly(methyl methacrylate) and polycarbonate using CSM technique. It is firstly shown that the actual value changes drastically at shallow indentation depths and it takes a considerable depth, which is material independent, for this parameter to attain a stabilized value. Furthermore, the strain rate is measured directly as the descent rate of the indenter divided by its instantaneous depth ( ), and indirectly via considering the variations of the load and hardness during the test. Both of these approaches reveal that the strain rate is considerably larger at shallow depths, and the depth beyond which it becomes constant is material and ratio dependent. Finally, by considering the relationship between the hardness and strain rate, it is observed that although the strain rate variation alters the hardness, its contribution is not able to justify the observed indentation size effect; hence, other contributing factors for this phenomenon are discussed for their possible effects. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2179–2187     

Static ultrasonic oscillations induced degradation and its effect on the linear rheological behavior of novel propylene based plastomer melts

The ultrasonic degradation of novel propylene based plastomer (DP) melts with different melt viscosities was conducted in a “static” ultrasonic device where the samples were taken from various distances from the tip of an ultrasonic probe. The effects of ultrasonic time, oscillation temperature, ultrasonic intensity and the distance from the ultrasonic probe tip on the degradation behavior of DP melts as well as the ultrasonic degradation effect on the linear rheological behavior of DP melts were studied. The results show that the increase of initial melt viscosity of DP (higher molecular weight) has greater impact on the ultrasonic degradation of DP melt. The molecular weight and intrinsic viscosity of DP decrease with the increase of ultrasonic oscillation time and they approach to a limiting value. The molecular weight distribution of DP increases after ultrasonic degradation. Decreasing oscillation temperature and distance from probe tip and increasing ultrasonic intensity lead to an increase in the degradation of DP melt. The linear rheological behavior measurements of the samples obtained near the ultrasonic probe tip show that ultrasonic oscillations decrease the complex viscosity, zero shear viscosity, viscoelastic moduli, cross modulus, relaxation time and the slope of log G′ − log G″ for DP melts.