Dynamic viscoelasticity of entangled polymers

The study of dynamic viscoelastic properties of polystyrene samples shows the influence of molecular parameters on characteristic parameters of linear viscoelastic behaviour. An analysis of complex viscosity in a complex plane shows the influence of polydispersity on the broadness of the relaxation spectrum and on the shape of steady-flow curve $\text{η}{}_{\mathrm{<mtext>a</mtext>}}\text{(}\text{γ}\text{?}\text{)}$.     

Interpretive nonlinear viscoelasticity: Dynamic properties of nylon 6, nylon 66, and nylon 12 fibers

Nonlinear response of nylon 6, nylon 66, and nylon 12 fibers to sinusoidal straining under relatively large strain amplitude is analyzed in terms of the changes in properties during the straining, i.e., the change in modulus, change in internal friction and change in structure which involves energy release or absorption in straining. Modulus generally increases with strain but it decreases with increase of strain amplitude, the effect of strain amplitude being largest with nylon 6 and smallest with nylon 66. Mechanical loss increase with the increase of strain amplitudes in nonlinear manner, and the magnitude of change is largest with nylon 66 and smallest with nylon 6. During the extension phase, structural change occurring in nylon 6 is predominantly an increase in order or orientation while that with nylon 66 is crack opening or cavitation. Various aspects of the experiments and analysis of the data are described in detail.     

聚阴离子纤维素(PAC)与羟丙基甲基纤维素(HPMC)混合体系动态粘弹性研究

考察了溶液浓度对PAC以及PAC/HPMC水溶液动态粘弹性的影响。结果表明,溶液储能模量(G’),耗能模量(G")均随溶液浓度的增大而增大,但储能模量随频率增大的幅度大于耗能模量随频率增大的幅度;溶液的损耗角正切(tanδ)值随浓度增大而逐渐减小;G’~f与G"~f两曲线交点随溶液浓度增大向低频方向移动;由于PAC与HPMC分子间存在氢键作用,因此,与纯PAC溶液相比,相同浓度下PAC/HPMC水溶液的G’~f与G"~f交点对应的频率更高;ω=0相对应的初始tanδ值比同浓度下纯PAC溶液更高。     

Dynamic viscoelasticity and phenomenological model of electrorheological elastomers

Electrorheological elastomers (EREs) present a tunable viscoelasticity with the application of an electric field. For their application, it is necessary to investigate the viscoelasticity of the EREs under various loading conditions and establish an accurate constitutive model. In this study, anisotropic silicone‐rubber‐based EREs with 30 vol % TiO₂–urea core–shell particles were prepared under an orientation electric field. We evaluated their viscoelasticities by testing their shear stress–shear strain hysteresis loops under various electric fields, frequencies, and strain amplitudes. On the basis of the experimental data, a nonlinear, revised Bouc–Wen phenomenological model was established, and the parameters in the model were identified. The results indicate that the revised model could accurately describe the viscoelastic properties of the EREs within a low frequency. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45407.     

Dynamic viscoelasticity of PUF under small strain and prediction of dynamic viscoelasticity in a wide frequency range

The dynamic viscoelasticity of polyurethane fiber (PUF) under small strain was studied with dynamic mechanical analysis and differential scanning calorimetry, and its viscoelasticity was predicted in a wide frequency range. The results revealed that the loss factor (tan δ) was about 0.05 and the phase angle (δ) was 2.86°, very close to the δ of a perfect elastomer in the range from −100°C to −80°C. The master curves superimposed by time-temperature superposition (TTS), the frequency range of the storage modulus master curve spanned 17 orders of magnitude from 10⁻⁷ Hz to 10¹⁰ Hz, and the loss modulus master curve spanned 13 orders of magnitude from 10⁻³ Hz to 10¹⁰ Hz. The apparent activation energy of the soft segment relaxation decreased with increasing temperature in the range from −70 to −26°C. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47070.     

Dynamic light scattering and viscoelasticity of semidilute polystyrene/cyclohexane solutions

In the present work, dynamic light scattering (DLS) measurements from polystyrene in cyclohexane semidilute solutions were carried out at 60 °C with scattering angles varied from 30° to 120°. The correlation functions were analyzed with the double KWW function and the CONTIN program. The amplitude of the fast mode A_f of the field correlation function g¹(t) was used to calculate the longitudinal stress modulus M_o using Wang’s theory. A comparison between M_o calculated from DLS data and the shear stress modulus G obtained from mechanical measurement was made.     

Evaluate the nonlinear viscoelasticity of polyolefin melts using the Larson model

Nonlinear rheological properties of polyolefins have long been studied and predicted by using the Larson model with the damping function generally obtained from the stress relaxation measurements. In this study, we investigate the nonlinear rheological properties of high density polyethylene (HDPE) and polypropylene (PP) using the Larson model with damping functions obtained from either the dynamic frequency sweep or the stress relaxation test. Experimental measurements and their corresponding model predictions for the rheological parameters were then compared to evaluate the applicability of the Larson model to the nonlinear rheology, and the following conclusions could be achieved. The steady shear viscosity could be satisfactorily described by the Larson model with the damping functions obtained from the two different methods, except at shear rates higher than 10³ s^?1. The predicted first normal stresses also account for the measured data, except for those using the stress relaxation data showing a little deviation for the PP sample. In addition, the predictions for elongation viscosity are also in good agreement with the experimental results within the short range of elongation rate achieved in this work. POLYM. ENG. SCI., 54:2354–2361, 2014. © 2013 Society of Plastics Engineers     

A numerical approach for obtaining nonlinear viscoelasticity parameters of polymeric materials and composites

A least-squares fitting procedure is developed for application to determine the parameters of the power law for nonlinear creep and recovery behavior of polymeric materials. The nonlinear power law, developed by Schapery from thermodynamic principles, is linearized, and an iterative approach is used to determine the desired parameters. The iteration is stopped after the standard deviation of the calculated points with respect to the fitted points reaches a minimum value. The purpose of this method is to avoid the inherent ambiguities present in the more familiar graphical fitting procedure. In order to study the sensitivity and limitations of an experimental approach for determining the viscoelastic parameters, sets of artificial experimental data points were generated for use as a control. These points were obtained by varying the theoretical functional values with normally-distributed random numbers within a preset error band.     

Model description of the nonlinear viscoelasticity of oriented polyethylene terephthalate and polycaproamide fibres

The possibility of quantitatively describing and predicting the nonlinear viscoelasticity of fibres in the region of low strains based on a mechanical model is demonstrated. __________ Translated from Khimicheskie Volokna, No. 3, pp. 12–15, May–June, 2006.     

Dynamic viscoelasticity and molecular orientation in uniaxially drawn PC/PET blends

The dependence of dynamic viscoelasticity on molecular orientation of the hot-drawn polycarbonate (PC)/poly(ethylene terephthalate) blends was studied experimentally. The oriented states of samples were investigated by wide-angle X-ray diffraction and quantified by Hermans' factor. The viscoelasticity was probed by dynamic mechanical analysis. For the fitting curves of dynamic modulus versus orientation, the PC-rich blends with different compositions had an opposite concavity and convexity compared with the neat PC. The difference revealed the influence of molecular mobility on orientation and viscoelasticity. The assumption on the phase morphological change from sea-island to local sandwich-like structure in blends was proposed and used to interpret the variation of slope of fitting curves. The relationship between orientation and viscoelasticity was observed in PC-rich blends, which can help understand the mechanism of molecular orientation and develop the constitutive relations for simulation. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47514.     

Interaction of nonhomogeneous shear,nonlinear viscoelasticity,and yield of a solid polymer

A constitutive equation for small strain viscoelastic response is considered in which stress relaxation. occurs faster as strain increases. The constitutive equation is of single integral type and has a psuedo- or material time function which is calculated from a strain dependent shift function. First, it is shown that such a constitutive equation can account for yield as observed in polymers for a number of different stress and strain histories. Next, the constitutive equation is used in the analysis of the problem in which a hollow cylinder is fixed at its inner surface and a moment history is applied to its outer surface. This causes the cylindrical surfaces to rotate about the central axis, thereby inducing a radial shear strain distribution. It is shown that there is a time when the material near the inner support begins to yield and a layer of large shear strain gradient begins to grow rapidly. It is also shown that the stress or strain history at a material element will generally not be one of the standard histories used to study yield.     

炭黑填充溶聚丁苯橡胶的动态黏弹性能和流变性能

采用橡胶加工分析仪和恒速型双筒毛细管流变仪研究了炭黑填充不同牌号溶聚丁苯橡胶(SSBR)的动态黏弹性能和流变性能。结果表明,当炭黑用量增加至20份时,SSBR-T 2003混炼胶出现炭黑网络结构,当炭黑用量增加至25份时,SSBR-2305混炼胶出现炭黑网络结构;随着炭黑用量的增加,SSBR-T 2003混炼胶的储能模量(G’)和损耗模量(G″)增大,Payne效应越明显,而SSBR-2305混炼胶的G’和G″也均增大,但未发生突增,Payne效应的变化微弱;不同用量炭黑填充的SSBR混炼胶均呈现假塑性流体的特性,且SSBR-T 2003混炼胶的非牛顿性较显著;SSBR-T 2003混炼胶的流动界面状态基本稳定,而SSBR-2305混炼胶的流动在高剪切速率下发生黏/滑的转变。     

Dynamic viscoelasticity of hybrid kevlar and glass fiber reinforced LLDPE in the molten state

Kevlar and glass fibers were used to reinforce linear low density polyethylene (LLDPE), and composite sheets of 0.8, 1.5 and 2.5 mm thicknesses were obtained by using a compression molding technique. Dynamic viscoelastic properties of non‐hybrid and hybrid composites of various compositions at 200°C are evaluated. Storage modulus (G′) and loss modulus (G″) increase with angular frequency (ω) and reinforcement. Replacement of glass fiber by Kevlar at constant loading of fibers in LLDPE increases the value of G′, G″ and η′. The fractured surface of composite shows the gradient orientation of fibers particularly in 2.5 mm thick sheet. Top and bottom layers show relatively two‐dimensional orientation as compared to the middle layer, which shows random orientation. The orientation of fibers decreases G′ and η′ of Kevlar fiber and hybrid fiber hybrid fiber reinforced LLDPE composites. The effect of change in distance between parallel plate of rheometer (change in strain amplitude) on dynamic rheological properties is studied and reported here.     

Constitutive equations in nonlinear viscoelasticity of glassy polymers based on the concept of traps

Constitutive equations are derived for the nonlinear viscoelastic response of amorphous polymers. The model treats a glassy polymer as an ensemble of independent relaxing regions that randomly hop in their cages being thermally activated. Rearrangement occurs when a flow unit reaches some liquid‐like state. Stress‐strain relations are verified using experimental data in tensile relaxation tests for polyester resins with two types of flexibilizers. Fair agreement is demonstrated between observations and results of numerical simulation.     

Relationships between nonlinear dynamic viscoelasticity and fatigue behaviors of glassy polymer under various fatigue test conditions

Summary Fatigue behavior of amorphous polystyrene (PS) under various cyclic strain conditions was investigated on the basis of nonlinear dynamic viscoelastic measurement at various ambient temperatures. The degree of nonlinear viscoelasticity under tension-compression type cyclic strain was greater than that under tension or compression type ones. The longest fatigue lifetime and the shortest one were observed under compression and tension type fatigue tests, respectively. The fatigue lifetime decreased with an increase in nonlinear viscoelastic parameter (NVP) in the case of the same type of cyclic strain conditions.     

Tackling slip effects in the nonlinear flow properties of gellan fluid gels

Gellan gum is a biopolymer widely used in the food, pharmaceutical, chemical, and agrochemical fields. Its ability to form a strong gel makes it possible to produce fluid gels. These materials present an apparent yield stress, but its value could be influenced by the wall-slip effect when performing the rheological measurements by which it is determined. In this work, the influence of the measuring surface and gap on flow behavior was first determined. The tests revealed the need to use geometries with rough surfaces, although the sample thickness using a parallel plate has no influence. Subsequently, the value of yield stress was obtained by means of creep tests (found to be 4.3 Pa), and, finally, the effect of wall slip on the dynamic viscoelastic behavior was assessed. There was an influence on the extension of the linear viscoelastic region, but not on the viscoelastic functions of the mechanical spectra. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46900.     

Compatibility and nonlinear viscoelasticity of polychloroprene/polyvinyl chloride blends with nitrile butadiene rubber as a compatibilizer

Miscible polychloroprene/polyvinyl chloride (CR/PVC) blends with nitrile butadiene rubber (NBR) as a compatibilizer were prepared. The effect of NBR on the compatibility between CR and PVC was mainly analyzed by studying the thermal behavior and the phase structure of CR/PVC blends. An obvious decrement in the T_g of PVC phase successfully provided an explanation for the compatibilization of NBR. Due to the improved compatibility between CR and PVC, the size of PVC particles in CR/PVC blends decreased a lot according to the scanning electronic microscopic images. The significant improvement of mechanical properties of CR/PVC blends was in good agreement with the better compatibility between CR and PVC phases. The softening effect of NBR on the nonlinear viscoelasticity of CR/PVC blends was also studied by RPA 2000. Temperature sweep test by RPA 2000, a less reported characterization method of T_g, was successfully applied to measure T_g of CR/PVC blends and study the compatibilization of NBR. The reason for better thermal stability and the thermal decomposition mechanism of CR/PVC blends were analyzed according to the results of TGA. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42448.