Research on viscoelastic behavior of semi-crystalline polymers using instrumented indentation

In this study, the viscoelastic behavior of a polyamide 12 (PA12) polymer was evaluated using instrumented indentation technology based on a rheological model. The creep compliance and retardation spectra were obtained to analyze the viscoelastic response during the holding stage according to the rheological model under different preloading conditions. The results showed that the viscoelastic responses were independent of the indentation depth or load under step loading conditions. However, the creep compliance increases, and the peak intensity of the retardation spectrum decreases with a decrease in the preloading rate owing to the structural relaxation observed during the preloading stage under ramp loading conditions. Furthermore, softening dispersion can be completed during the loading stage under continuous stiffness measurement (CSM) conditions. As the preloading strain rate changes, the peak of the retardation spectrum gradually decreases until it disappears completely. Moreover, studies on indentation creep using the CSM method are challenging because of the complicated viscoelastic response observed during the preloading stage.     

Morphology and Properties of Poly(oxymethylene) Engineering Plastics

Comparative WAXD/SAXS/SEM/DSC structural studies of a series of semi-crystalline poly(oxymethylene) (POM) engineering plastics, including the commercial products, homopolymer Delrin^® and typical poly(oxymethylene-co-oxyethylene)s, and a few lab-made POM compositions, were performed. The latter differed in their content of functional additives (present in low concentrations) and POM molecular weight characteristics. In parallel, their densities, thermal behavior/laser-interferometric creep rate spectra (DSC/CRS) at 20-180 °C, as well as long-term creep resistance (LTCR) at 20 °C were studied. It has been found that introducing the nucleating agents and oxyethylene units resulted in formation of more fine spherulitic or practically non-spherulitic structure with close- or loose-packed lamellar stacks. The presence of both “thick” (5-10 nm) and “thin” (1.5-3 nm) lamellae in the weight ratio of ∼3:1 was shown in all cases. Close values of real POM crystallinities, not exceeding 50%, were obtained by WAXD and DSC. A predominant role of “straightened out” or slightly bent tie chains in disordered layers of isotropic POMs was presumed, resulting in segmental dynamics differently constrained by crystallites (DSC/CRS data). As a result, certain morphology - density - creep resistance correlations were found.     

Simulation Study of the Conformational Properties of Diblock Polyelectrolytes in Salt Solutions

Coarse-grained molecular dynamics simulations are performed to understand the behavior of diblock polyelectrolytes in solutions of divalent salt by studying the conformations of chains over a wide range of salt concentrations. The polymer molecules are modeled as bead spring chains with different charged fractions and the counterions and salt ions are incorporated explicitly. Upon addition of a divalent salt, the salt cations replace the monovalent counterions, and the condensation of divalent salt cations onto the polyelectrolyte increases, and the chains favor to collapse. The condensation of ions changes with the salt concentration and depends on the charged fraction. Also, the degree of collapse at a given salt concentration changes with the increasing valency of the counterion due to the bridging effect. As a quantitative measure of the distribution of counterions around the polyelectrolyte chain, we study the radial distribution function between monomers on different polyelectrolytes and the counterions inside the counterion worm surrounding a polymer chain at different concentrations of the divalent salt. Our simulation results show a strong dependence of salt concentration on the conformational properties of diblock copolymers and indicate that it can tune the self-assembly behaviors of such charged polyelectrolyte block copolymers.     

Tensile creep of thermoplastics: Time‐strain superposition of non‐iso free‐volume data

The dimensional stability of thermoplastics is characterized by their tensile compliance D(t,σ,T) as a function of time t, stress σ, and temperature T. Creep retardation times are controlled by the free volume available for underlying molecular (segmental) motions. Tensile deformation of polymeric materials, whose Poisson ratio is smaller than 0.5, is accompanied by volume dilatation that can be identified with an increase in available free volume. Consequently, a steady increase in strain with time during tensile creep experiments accounts for shortening of the retardation times. The superposition of as‐received tensile compliance curves is difficult because any point of a curve requires a shift factor along the time axis that differs from those of other points. In this article, tensile creep at a constant stress and temperature is viewed as a non‐iso free‐volume process. A procedure is proposed to transform as‐received data to a pseudo‐iso free‐volume state that eliminates this deficiency and permits construction of a generalized compliance curve for the pseudo‐iso free‐volume state. This curve can be used for calculation of real‐time‐dependent compliance for any selected stress in the range of reversible deformations. As the superposed curve can be generated with several short‐term creep tests (e.g., 100 min) for a series of stresses, the proposed procedure saves experimental time. The effects of physical aging on tensile compliance (observed previously by other researchers) are interpreted in terms of the proposed approach in appendix A . © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 736–748, 2003     

AFM nanoindentation of viscoelastic materials with large end‐radius probes

We used atomic force microscopy (AFM) nanoindentation to measure mechanical properties of polymers. Although AFM is generally acknowledged as a high‐resolution imaging tool, accurate quantification of AFM nanoindentation results is challenging. Two main challenges are determination of the projected area for objects as small as AFM tips and use of appropriate analysis methods for viscoelastic materials. We report significant accuracy improvements for modulus measurements when large end‐radius tips with appropriate cantilever stiffnesses are used for indentation. Using this approach, the instantaneous elastic modulus of four polymers we studied was measured within 30 to 40% of Dynamic Mechanical Analysis (DMA) results. The probes can, despite their size and very high stiffnesses, be used for imaging of very small domains in heterogeneous materials. For viscoelastic materials, we developed an AFM creep test to determine the instantaneous elastic modulus. The AFM method allows application of a nearly perfect stepload that facilitates data analysis based on hereditary integrals. Results for three polymers suggest that the observed creep in the materials has a strong plastic flow component even at small loads. In this respect, the spherical indenter tips behave like “sharp” indenters used in indentation studies with instrumented indenters. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1573–1587, 2009     

A Universal Reduced Rupture Creep Approach for Prediction of Long Term Failure Behavior of Aged Glass Polymers from the Short Term Test of Rupture Creep Compliance by the Unified Master Curved Extrapolation

The prediction of long term failure behaviors and lifetime of aged glass polymers from the short term tests of reduced rupture creep compliance (or strain) is one of difficult problems in polymer science and engineering. A new “universal reduced rupture creep approach” with exact theoretical analysis and computations is proposed in this work. Failure by creep for polymeric material is an important problem to be addressed in the engineering. A universal equation on reduced extensional failure creep compliance for PMMA has been derived. It is successful in relating the reduced extensional failure creep compliance with aging time, temperature, levels of stress, the average growth dimensional number and the parameter in K-W-W function. Based on the universal equation, a method for the prediction of failure behavior, failure strain criterion, failure time of PMMA has been developed which is named as a universal “reduced rupture creep approach”. The results show that the predicted failure strain and failure time of PMMA at di?erent aging times for different levels of stress are all in agreement with those obtained directly from experiments, and the proposed method is reliable and practical. The dependences of reduced extensional failure creep compliance on the conditions of aging time, failure creep stress, the structure of fluidized-domain constituent chains are discussed. The shifting factor, exponent for time-stress superposition at differentlevels of stress and the shifting factor, exponent for time-time aging superposition at different aging time are theoretically defined respectively.     

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     

Influence of the crosslink network structure on stress‐relaxation behavior: Viscoelastic modeling of the compression set experiment

A viscoelastic approach of the compression set test is addressed in this work. This test measures the ability of rubber compounds to retain elastic properties after prolonged action of compressive stresses. Elastic properties were tested by recording the normal stress under a constant deformation of 25% with a laboratory rheometer. Considering the Boltzmann superposition principle, compression set data were modeled from the relaxation of Young's modulus, described by a Maxwell spectrum plus a constant E_∞ defining the elastic properties at the long times. This approach was developed with the copolymer of ethylene and vinyl acetate (EVA) networks crosslinked by radical chemistry and by an exchange reaction between acetate groups and silane compounds as crosslinking agents. Regarding the recovery of the elastic properties, radical chemistry provided better results than the exchange reaction for the identical crosslinking density of the network. Then, the Curro–Pincus molecular approach was developed to understand the influence of the microstructure of the EVA network on the elastic properties. The difference of the elastic properties between the two networks crosslinked by two different chemistry means was accounted for by considering the probability of having a dangling end of n units for a random crosslinking process. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1779–1790, 2003     

氢键型水凝胶自修复行为的Monte Carlo模拟

基于氢键型自修复水凝胶的结构特征构建了其横截面的格气模型, 并利用Monte Carlo模拟方法对其自修复行为进行研究. 首先根据自修复过程中的比热峰值确定了氢键的临界联结分数, 然后重点考察了氢键强度、 合作效应以及平均断面间距对其自修复行为的影响. 在此基础上, 进一步研究了氢键型凝胶的动态修复过程, 讨论了相关因素对自修复时间等动态性质的影响. 结果表明, 氢键型凝胶的自修复在本质上归属于一级热力学相变, 且氢键强度是自修复过程中的最关键因素. 同时, 初始断面间距和格子间的合作效应也可显著影响凝胶的自修复行为.     

Time–temperature-dependent behavior of a substituted poly(paraphenylene): Tensile,creep, and dynamic mechanical properties in the glassy state

The linear viscoelastic behavior of a poly(paraphenylene) with a benzoyl substituent has been examined using tensile, dynamic mechanical, and creep experiments. This amorphous polymer was shown to have a tensile modulus of 1–1.5 Msi, nearly twice that of most common engineering thermoplastics. The relaxation behavior, which is similar to that of common thermoplastics, can be described by the WLF equation. Outstanding creep resistance was observed at low temperatures, with rubbery-like behavior being exhibited as the temperature approached T_g. Physical aging was shown to interact with long-term creep, rendering time–temperature superposition invalid for predicting the long-term properties. The effect of physical aging on the creep behavior was characterized by the shift rate μ. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 70: 2971–2979, 1998     

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     

Fabrication and microstructural characterization of functionally graded porous acrylonitrile butadiene styrene and the effect of cellular morphology on creep behavior

The ability to control material properties in space and time for functionally graded viscoelastic materials makes them an asset where they can be adapted to different design requirements. The continuous microstructure makes them advantageous over conventional composite materials. Functionally graded porous structures have the added advantage over conventional functionally graded materials of offering a significant weight reduction compared to a minor drop in strength. Functionally graded porous structures of acrylonitrile butadiene styrene (ABS) had been fabricated with a solid‐state constrained foaming process. Correlating the microstructure to material properties requires a deterministic analysis of the cellular structure. This is accomplished by analyzing the scanning electron microscopy images with a locally adaptive image threshold technique based on variational energy minimization. This characterization technique of the cellular morphology is analyst independent and works very well for porous structures. Inferences are drawn from the effect of processing on microstructure and then correlated to creep strain and creep compliance. Creep is strongly correlated to porosity and pore sizes but more associated to the size than to porosity. The results show the potential of controlling the cellular morphology and hence tailoring creep strain/compliance of ABS to some desired values. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 795–803     

Formation of Long,Multicenter π‐[TCNE]22− Dimers in Solution: Solvation and Stability Assessed through Molecular Dynamics Simulations

Purely organic radical ions dimerize in solution at low temperature, forming long, multicenter bonds, despite the metastability of the isolated dimers. Here, we present the first computational study of these π‐dimers in solution, with explicit consideration of solvent molecules and finite temperature effects. By means of force‐field and ab initio molecular dynamics and free energy simulations, the structure and stability of π‐[TCNE]₂^2? (TCNE=tetracyanoethylene) dimers in dichloromethane have been evaluated. Although the dimers dissociate at room temperature, they are stable at 175 K and their structure is similar to the one in the solid state, with a cofacial arrangement of the radicals at an interplanar separation of approximately 3.0 Å. The π‐[TCNE]₂^2? dimers form dissociated ion pairs with the NBu₄⁺ counterions, and their first solvation shell comprises approximately 20 CH₂Cl₂ molecules. Among them, the eight molecules distributed along the equatorial plane of the dimer play a key role in stabilizing the dimer through bridging C?H???N contacts. The calculated free energy of dimerization of TCNE ^{. ?} in solution at 175 K is ?5.5 kcal mol^?1. These results provide the first quantitative model describing the pairing of radical ions in solution, and demonstrate the key role of solvation forces on the dimerization process.     

Comment on the dynamic bead size and Kuhn segment length in polymers: Example of polystyrene

Discrepancies between the length of a traditionally defined Kuhn segment length (l_k) and a bead size estimated from experimental data have been reported in a number of articles. In this work we emphasize that the traditional definition of the Kuhn segment is an oversimplification and the characteristic ratio, C_∞, is not the only parameter that defines a bead size. We show that the dynamic bead size in polystyrene (PS) might be as large as ～50 monomers, whereas the traditionally defined Kuhn segment is ～8–10 monomers. It is shown that with 50 monomers (M ～ 5000) as a size of the dynamic bead, one can explain many inconsistencies in the interpretation of compliance and mechanical relaxation data known for low‐molecular weight PS. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3505–3511, 2004     

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     

长棒状T形三组分双亲分子相行为的耗散粒子动力学模拟

采用耗散粒子动力学方法建立了以侧链长度和温度为变量的相图,计算了相关结构参数及每个液晶相对应侧链的有效体积分数,重现了实验上观察到的五边形、六边形柱状相及刚棒在层内位置的有序堆垛层状结构.研究发现,长棒状T形三组分双亲分子相图中的相序列和相结构符合相关报道,并且侧链有效体积分数f L与实验值一致.表明粗粒化模型反映了真实T形分子的主要特性,如分子的拓扑结构、排除体积效应及分相趋势、刚棒液晶基元的长径比和侧链的空间效应.证实了侧链尺寸对主/侧链非线性连接体系的自组装结构及各组成单元的位置分布的重要作用.     

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。     

Effect of branching and molecular weight on the viscoelastic properties of poly(butyl acrylate)

A series of poly(butyl acrylate) samples were prepared by emulsion polymerization with a range of molecular weights and degrees of chain branching. Characterization was performed with NMR (giving the fraction of branching, ranging from approximately 0 to 7%), gel permeation chromatography, viscometry, and determination of the gel fraction. The dynamic mechanical response, that is, the frequency dependence of the storage and loss moduli G′(ω) and G″(ω) was measured from 0.02 to 200 Hz. The occurrence of a significant insoluble fraction in the sample meant that full characterization of the molecular weight distribution was not possible, and so an unambiguous separation of the dependencies of the mechanical response on the degree of long‐chain branching (LCB) and short‐chain branching (SCB) and the molecular weight could not be made; however, trends dependent on the molecular weight alone were insufficient to model the results. At high frequencies, all trends in G′(ω) and G″(ω) could be ascribed to molecular weight dependencies; at low frequencies, the effects of both the molecular weight and total degree of branching could be inferred, with more highly branched samples showing lower storage and loss moduli. Although the relative amounts of SCB and LCB could not be determined, no dynamic features attributable to LCB were observed. The low‐frequency trends could be semiquantitatively fitted with reptation and retraction theory if it was assumed that an increased degree of SCB led to an increased tube size. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3335–3349, 2002