Nonlinear rheology of model filled elastomers

Submitted to large sinusoidal strains, filled elastomers not only show a decrease in their storage modulus — the Payne effect, but also a nonlinear behavior — their response is not sinusoidal anymore and involves strain‐stiffening. We show in this study that the two effects can be separated thanks to large amplitude oscillatory shear experiments. The stress signal of filled elastomers consisting of a dispersion of silica particles into a polymeric matrix was decomposed into an elastic and a viscous part and we could observe simultaneously the Payne effect and a strain‐stiffening phenomenon. We showed that the strain‐stiffening was correlated with the Payne effect but came from various intricated effects. It most probably also has its origins in the finite extensibility of the polymer chains confined between solid particles, where the strain is larger. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Mullins effect recovery of a nanoparticle‐filled polymer

Particle‐filled elastomers often exhibit a reduction in peak stress after the initial extension under cyclic straining, a phenomenon known as the Mullins effect. In reported literature, long rest periods and elevated temperatures are often necessary conditions to even partially restore stiffness. The stress softening of the polymer nanoparticle composite discussed in this article appeared to be completely reversible in a comparatively short time span (less than 17 h) at room temperature. Although this material acted elastomeric at slow strain rates, significant stiffening, and a pronounced yielding behavior was observed during the first strain cycle at higher strain rates. Subsequent cycling of the material revealed much softer behavior and an absence of the yielding phenomenon, although relatively short rest periods allowed the material to regain its original behavior. The Mullins effect recovery phenomenon was investigated by introducing rest periods of various durations following loading and unloading cycles. Both single and multiple loadings were used in the tests conducted to evaluate recovery. A mechanism of stress recovery was proposed based on the reduction of entanglements and weakening particle‐matrix interactions. New entanglements and particle‐chain interactions were considered major contributors to the recovery of the stiffness. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

A new spectral memory of filled rubbers

We recently discovered that shearing particle‐reinforced rubbers in oscillation at a frequency f_a at a small strain γ_a (e.g., ～1% strain) for time t_a can often produce a spectrum hole or drop in the strain‐dependent dissipation spectra of the materials. The location of the hole (or localized perturbation in the loss modulus or loss tangent) depends on the aging strain amplitude γ_a. The depth of this hole is influenced by both the oscillatory aging frequency f_a and the aging duration t_a, and follows a simple power relationship of the product of f_a and t_a. The exponent for the power relationship is a function of filler concentration. These attributes of the spectral hole in filled rubbers are not sensitive to the frequency used to postanalyze the hole. This new memory effect occurs at very small strains and involves material stiffening during the strain aging, and both of those features are quite different from the Mullins effect in filled elastomers. We interpret this newly discovered memory character of filled rubbers from a much broader concept of structure pinning in a condensed frustrated system and consider that the agglomeration of filler particles in rubber matrix shares common physics with granular materials and glass‐forming materials. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 859–869, 2010     

On the structure of polyurethane hard segments based on MDI and butanediol-1,4: X-ray diffraction analysis of oriented elastomers and of single crystals of a model compound

Single-crystal x-ray diffraction analysis is performed on a model compound (bisurethane of diphenylmethane-4–monoisocyanate and butanediol-1,4) for the thermoplastic polyurethane (TPU) hard segment formed from diphenylmethane-4,4′-diisocyanate and butanediol-1,4. The resulting structure is compared to structure models of corresponding TPU hard segments, especially to the structure proposed by Blackwell and Ross. Our results confirm this structure model showing a planar zigzag of the (CH₂)₄ group and planar hydrogen bonding between the urethane groups of adjacent molecules. X-ray diffraction analyses of polymeric TPU hard segments and of TPU elastomers with noncrystallizing soft segments lead to a revision of the dimensions of the proposed lattice cell, resulting in a more plausible value of 1.3226g/cm³ for the crystal density.     

Exploring preferential association of laponite and cloisite with soft and hard segments in TPU‐clay nanocomposite prepared by solution mixing technique

Thermoplastic polyurethane (TPU) is a versatile polymer exhibiting many engineering applications. In this article, two varieties of clay (Cloisite and Laponite RD) have been used to prepare TPU‐based nanocomposites. They differ in, chemical composition, hydrophobicity, aggregation tendency, and dispersibility in a particular solvent. A detailed investigation of the thermal, morphological, and rheological behavior reflects the affinity of Cloisite towards the soft segment, whereas it is the hard segment for modified Laponite. The maximum improvement in onset degradation temperature has been observed to be 17.5 and 8.3 °C for Cloisite and Laponite, respectively. Five percent Cloisite‐filled sample shows optimum storage modulus in the glassy region where as it is the 10% filled sample at the rubbery region. However, the trend remains indifferent both in rubbery and glassy regions for Laponite, and properties have been found optimum for 3% filled sample. To explore the behavior in the terminal and flow regions, dynamic rheological experiments were performed in low shear rate. Variation in dynamic rheological properties can be explained well on the basis of the combination of partly exfoliated, intercalated, and aggregated structures of the nano clay inside the TPU matrix, depending on their nature and preferential association with different segments. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2341–2354, 2008     

Evaluation of physico-mechanical properties of precipitated polyurethane films in medium of free radical agents

Segmented poly(ester-urethanes) (PU) elastomers based on poly(ethylene diethylene adipate)diols as a soft segment and aromatic diisocyanates in the hard segment were synthesized by a conventional method. The precipitated PU elastomers films have been degraded after a limited exposure to free radical agents. An increase of the ratio of radical agents had an increase in the hard segment content which was associated with increased hard microdomain crystallinity, hardness and improvement in mechanical properties. It is suggested that the superior mechanical performance may be related to a interconnecting hard microdomain texture by a radical cross-linking process. The present study attempts to correlate the physical-mechanical properties of the precipitated PU films with the concentration of the free radical agents. In all cases, the effect of free radical cross-linking was to increase the ultimate tensile strain.     

On the degradation and stability of high abrasion furnace black (HAF)/acrylonitrile butadiene rubber (NBR) and high abrasion furnace black (HAF)/graphite/acrylonitrile butadiene rubber (NBR) under cyclic stress-strain

Acrylonitrile butadiene rubber (NBR) compounds filled with 40 phr of high abrasion furnace black (HAF) and HAF (20 phr)/graphite (20 phr) were experimentally investigated. The stress-strain curves of the composites were studied, which are described by applying Ogden's model. The effect of cyclic fatigue and hysteresis was also examined. The dissipation energy that indicates the vibration damping capacity for all samples was determined. A continuum damage model is used to investigate the fatigue damage behavior for elastomers. Experiments on the cyclic fatigue of a carbon-filled NBR rubber and carbon/graphite filled NBR rubber were conducted to determine the relation between the number of cyclic fatigue and the strain amplitude. The results indicate that the theoretical formula for the number of cyclic fatigue as a function of the strain amplitude, derived from the damage model, can describe experimental data for the prepared samples very well.     

Application of the DSC analysis of thermoplastic polyurethane elastomers to a comparative study of their technological properties

The fusion and crystallization enthalpies, dynamic viscosity of melts and tensile properties of a series of commercially available thermoplastic polyurethane elastomers (TPU) were estimated. The DSC analysis has been proved as useful to prediction of the processability and tensile properties of TPU's with a similiar hard segment content.

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Zusammenfassung Schmelz- und Erstarrungsenthalpie, die dynamische Viskosität der Schmelze und Zugdehnungseigenschaften bei einer Reihe von im Handel erhältlichen thermoplastischen Polyurethanelastomere (TPU) wurden ermittelt. DSC-Anaiyse erwies sich als ein geeignetes Mittel zur Vorhersage der Verarbeitungsfähigkeit und der Zugdehnungseigenschaften von TPU mit ähnlichem Hartsegmentanteil.

     

Use of maleic anhydride compatibilization to improve toughness and other properties of polylactide blended with thermoplastic elastomers

Polylactide (PLA) being a very brittle biopolymer could be toughened by blending with thermoplastic elastomers such as thermoplastic polyurethane elastomer (TPU) and thermoplastic polyester elastomer (TPE); unfortunately, these blends are immiscible forming round domains in the PLA matrix. Therefore, the purpose of this study was to investigate the effects of using maleic anhydride (MA) compatibilization on the toughness and other properties of PLA blended with TPU and TPE. MA grafting on the PLA backbone (PLA‐g‐MA) was prepared separately by reactive extrusion and added during melt blending of PLA/thermoplastic elastomers. IR spectroscopy revealed that MA graft might interact with the functional groups present in the hard segments of TPU and TPE domains via primary chemical reactions, so that higher level of compatibilization could be obtained. SEM studies indicated that PLA‐g‐MA compatibilization also decreased the size of elastomeric domains leading to higher level of surface area for more interfacial interactions. Toughness tests revealed that Charpy impact toughness and fracture toughness (K_IC and G_IC) of inherently brittle PLA increased enormously when the blends were compatibilized with PLA‐g‐MA. For instance, G_IC fracture toughness of PLA increased as much as 166%. It was also observed that PLA‐g‐MA compatibilization resulted in no detrimental effects on the other mechanical and thermal properties of PLA blends. Copyright © 2014 John Wiley & Sons, Ltd.     

Electrospinning‐induced shape memory effect in thermoplastic polyurethane characterization and thermoviscoelastic modeling

Electrospun thermoplastic polyurethane (TPU) nanofibers are known to contract considerably (～40%) on heating up to ～90 °C. This study investigates this thermomechanical behavior and the TPU shape memory capabilities. The shape memory effect was first studied in TPU films as a model system by applying classical thermomechanical cycles (programming and recovery). The films were able to fix the applied deformation during long‐term storage at room temperature, well above the material's calorimetric glass transition temperature and in the absence of a percolated structure of hard domains. Structural analysis (Fourier transform infrared, differential scanning calorimeter, and dynamic mechanical analysis) revealed broad thermal transitions indicating the presence of a mixed phase of hard segments dispersed in the soft segment matrix. Using a linear viscoelastic model together with time–temperature superposition, the shape memory effect was attributed to the thermoviscoelastic properties of TPU. In particular, the mixed phase was found to give rise to a very broad relaxation spectrum dominated by long relaxation times, which explains the suppression of strain recovery at room temperature. Finally, the electrospinning process was examined and was found to be similar to a programming cycle characterized by the strong elongation flow accompanied by massive solvent evaporation, whereas the contraction effect was interpreted as the recovery phase in a shape memory perspective. Thus, the contraction of electrospun TPU mats may be considered to be an electrospinning‐induced shape memory effect. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1590–1602     

Hard‐block degradable thermoplastic urethane‐elastomers for electrospun vascular prostheses

Thermoplastic polyurethane (TPU) elastomers with biodegradable chain extenders were synthesized and tested for mechanical performance and biocompatibility. The design of the TPUs was based on structural modification of a mechanically appropriate aromatic isocyanate‐based TPU. As the aromatic isocyanate was substituted with a less toxic but also less “hard” aliphatic isocyanate, the chain extender plays an important role on the mechanical properties. Here, the terephthalate ester chain extender was found to work better than hydroxyl ethyl lactate in providing polymers with mechanical properties similar to commercial aromatic isocyanate‐based TPUs. The polymers were degraded in aqueous solutions at elevated temperatures and compared to polylactic acid (PLA) to partially simulate biodegradation. The lactate‐based TPUs degraded about twice as fast as PLA while the terephthalate‐based TPU degraded much more slowly. The latter material was processed by electrospinning to give a tubular graft approximately the size of a large rat blood vessel. Initial results from implantation of these TPUs into rats are promising and indicate that biodegradation occurs and is likely beneficial to cell proliferation. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Finite Element Based Micro-mechanical Modeling of Interphase in Filler Reinforced Elastomers

Characteristic properties of elastomers can be tailored by embedding them with filler particles. Along with enhancing the overall properties of the composite, filler particles also induce some inelastic effects. In this paper, a finite element computational model is used to study the effect of microstructure morphology in filled elastomers, on its macroscopic large deformation behavior. A multiphase material model that accounts for the hypothesis of shift in glass transition temperature in the vicinity of the filler particle is developed to simulate the interphase between the fillers and the matrix. It also accounts for the breakdown and re-aggregation of filler networks under cyclic loading. Examples at the microstructural level, demonstrating the dynamics of the interphase using the developed multiphase model have been successfully simulated. The obtained results are in good qualitative agreement with the Mullins effect. Therefore, computational experiments using this methodology enable the prediction of the experimentally observed softening behavior in filled elastomers based on its microstructure evolution.     

On the extreme depth dependence of the hardness of PDMS rubber: A problem of false surface detection

There is significant interest in nanoindentation of materials yet mixed results for material properties have been reported, including modest depth dependence of the surface stiffness in metals and other crystalline materials and polymer glasses, as well as stiffening of several orders of magnitude in some studies of soft materials such as poly(dimethylsiloxane) (PDMS) rubber. At the same time, there are reports that suggest that the observed extreme stiffening in soft materials might be an artifact, and that such materials at most exhibit only mild stiffening. Unfortunately, a quantitative model of potential artifacts has not been provided. In the present work, we examine the problem of one potential artifact in the testing of soft materials, that of the difficulty of detecting the surface or “true zero” in the indentation test. We provide a quantitative estimate of the effect of error in surface detection on the measured force–displacement curves for the Berkovich tip geometry and find that the observed apparent stiffening is in agreement with our analysis. The significance of the results for testing of soft materials by nanoindentation is discussed. It is also shown that for hard/stiff materials the induced errors are smaller, but may still be significant in some circumstances. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 30–38     

Detailed analysis of dynamic mechanical properties of TPU nanocomposite: The role of the interfaces

Organo-modified layered silicates (OMLSs) can largely improve mechanical properties of Thermoplastic polyurethanes (TPUs) as well as affect their microdomain morphology. Nanocomposite TPU containing OMLSs were prepared by melt blending at different concentrations. The addition of OMLS has both induced variation in enthalpy of melting of hard and soft phases, and influenced the glass transition temperature of soft domains, as result of the microdomain phase segregation measured by means of fourier transform infrared spectroscopy (FT-IR). Small angle X-ray scattering (SAXS) analysis has shown that the mean distance between hard domains was mostly unaffected by the filler. However, its distribution broadened with the increasing concentration of the OMLSs, resulting in increased extent of the hard domain interface. The storage modulus of TPU nanocomposites incremented with the silicate content, while the dynamic strain scan tests showed pronounced non linear viscoelastic behavior. The analysis of morphological data obtained by SAXS and FT-IR measurements were correlated to thermal and dynamic mechanical properties of TPU samples suggesting a crucial role of the soft domains interface. The storage modulus and loss tangent of TPU nanocomposites were found to increase with the increasing of the interface area of soft domains with both hard domains and OMLS stacks.     

Numerical search for morphologies providing ultra high elastic stiffness in filled rubbers

We have numerically studied the transverse elastic behavior of a unidirectional composite comprising non-overlapping silica fibers dispersed in a rubber matrix. Some composite morphologies that provided an ultra high transverse shear modulus at rather moderate silica loadings were identified. For these morphologies, predicted elastic stiffening levels were in agreement with those measured at low strains in carbon black and silica filled rubbers, leading one to surmise that such elastic stiffening may also play an important role in the low strain mechanical responses of actual carbon black or silica filled rubbers.     

Improvement in Toughness of Polylactide by Melt Blending with Bio-based Poly（ester）urethane

Polylactide （PLA） was successfully toughened by blending with bio-based poly（ester）urethane （TPU） elastomers which contained bio-based polyester soft segments synthesized from biomass diols and diacids. The miscibility, mechanical properties, phase morphology and toughening mechanism of the blend were investigated. Both DSC and DMTA results manifested that the addition of TPU elastomer not only accelerated the crystallization rate, but also increased the final degree of crystallinity, which proved that TPU has limited miscibility with PLA and has functioned as a plasticizer. All the blend samples showed distinct phase separation phenomenon with sea-island structure under SEM observation and the rubber particle size in the PLA matrix increased with the increased contents of TPU. The mechanical property variation of PLA/TPU blends could be quantitatively explained by Wu＇s model. With the variation of TPU, a brittle-ductile transition has been observed for the TPU/PLA blends. When these blends were under tensile stress conditions, the TPU particles could be debonded from the PLA matrix and the blends showed a high ability to induce large area plastic deformation before break, which was important for the dissipation of the breaking energy. Such mechanism was demonstrated by tensile tests and scanning electron microcopy （SEM） observations.     

Thermodynamics of the deformation of segmented polyurethanes with various hard block contents II. Stress softening and mechanical hysteresis

The thermodynamics of stress-softening and hysteresis in PBUs with different compositions were studied using deformational calorimetry, and the intramolecular energy contribution of the soft blocks was established. It is shown that differing from filled elastomers, the softening and hysteresis losses of PBU, like those of a typical TEP, are accompanied by energy absorption reflecting intermolecular change in the hard domains. Increased hard block content increases energy changes, whereas increasing the temperature produces the reverse effect. The dependence of the deformation mechanism of PBU on the hard block content, temperature and mechanical history is discussed.     

Temperature and aging dependence of strain‐induced crystallization and cavitation in highly crosslinked and filled natural rubber

We have investigated the structural changes occurring in highly crosslinked and carbon‐black filled natural rubber under uniaxial extension by small‐ and wide‐angle X‐ray scattering using synchrotron radiation. The experiments focused on strain‐induced crystallization (SIC) and nanocavitation and were carried out on a model series of materials as a function of temperature and aging conditions. We find that for all materials both SIC and cavitation decrease markedly with temperature and aging. However, the presence of carbon black filler shifts the ceiling temperature where SIC is observed to at least 120°C, presumably by a nucleating effect, maintaining the high strength of the elastomers. Interestingly, although in pure elastomers, the cavitation strength decreases with temperature, we find that in these filled elastomers the critical stress for the onset of cavitation increases significantly with temperature strongly suggesting that cavitation is due to the local confinement between fillers and supporting the idea of a glassy layer near the filler. Aging for 10 days at 110°C in oxygen‐free conditions decreases both SIC and cavitation and reduces the strength of the elastomer at high temperature. This is attributed to the formation of sulfur side chains hindering the crystallization. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 780–793     

热塑性聚氨酯及其复合材料的界面形态与流变特性

以聚酯型热塑性聚氨酯(thermoplastic polyurethane,TPU)、玻璃纤维(glass fiber,GF)和玻璃微珠(glass bead,GB)为主要原料制备了TPU/GF、TPU/GB共混物,考察了复合体系的热性能、微观结构、动态流变特性.研究发现,TPU是温敏型聚合物,其温敏性与材料的硬段含量有关,在加工过程中,除考虑剪切速率的影响外,需重点考虑温度对其加工性能的影响;GF,GB填充TPU体系具有良好的分散形态和界面结合牢度,GF和GB的加入能够增加体系的黏度,降低TPU的温敏性,加宽TPU的加工温度窗口,从而改善其成型加工性,并能一定程度地提高其耐热性.研究还发现,复合体系黏度的增加程度不仅和填料的含量有关,而且与填料的形状有关,可用等效直径表征.另外,从比表面积的角度比较了玻璃纤维和玻璃微珠对体系热稳定性的影响.     

Nonlinear behavior of polyethylene/layered double hydroxide nanocomposites under shear flow

The structural evolution of filler clusters in polyethylene/layered double hydroxide-based nanocomposites is investigated under application of a simple shear flow and is described in the framework of a modified Wagner model. Overall, the structural behavior of these polymer-clay nanocomposites is found to be similar to the behavior of filled elastomers for which breakdown of filler clusters at increasing strain and their reaggregation at decreasing strain were observed under oscillatory shear (Payne effect). Similar to the filled elastomers and other jammed systems, the polymer-clay nanocomposites demonstrate an asymmetric behavior upon approaching the steady state depending on whether the system was initially at higher or lower shear strain. In particular, the reaggregation time of filler structure in the quiescent state is found to be about one order of magnitude larger than the characteristic breakage time in the nonlinear shear regime. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A. 2008, Vol. 50, No. 5, pp. 868–881. This article was submitted by the authors in English.