Morphological Behaviour of Thermoplastic Polyurethanes During Repeated Deformation (original) (raw)
Related papers
Real time SAXS/stress–strain studies of thermoplastic polyurethanes at large strains
Polymer, 2002
Simultaneous small angle X-ray scattering (SAXS) and force measurements have been recorded during tensile deformation of two contrasting polyurethane elastomers. The elastomers comprise the same hard and soft chemical segments; in Sample A, the length of the hard blocks is randomised while in Sample B the hard blocks are monodisperse. During deformation of Sample A, the SAXS halo from the mesophase structure deforms to an ellipse with intensification on the meridian. In Sample B, the halo transforms into a four point pattern. The ellipse patterns of A are interpreted in terms of a model based on particles located on a statistical lattice which is subjected to an affine deformation scheme. According to this model, the SAXS patterns of A are consistent with the hard phase regions behaving as embedded particles which separate from each other in an affine manner and which are not impeded by interconnections during the mechanical yield process. In B, the interconnection of the hard phase prevents affine deformation of the structure and involves the formation of a four point 'lattice' structure which then subsequently deforms in an affine manner. The differences in behaviour are linked with the segment sequencing which result in the phase regions of Sample A having a lower volume fraction and are consistent with variation in applied stress. q
Strain induced strengthening of soft thermoplastic polyurethanes under cyclic deformation
Journal of Polymer Science
We investigate the cyclic mechanical behavior in uniaxial tension of three different commercial thermoplastic polyurethane elastomers (TPU) often considered as a sustainable replacement for common filled elastomers. All TPU have similar hard segment contents and linear moduli but sensibly different large strain properties as shown by X-Ray analysis. Despite these differences, we found a stiffening effect after conditioning in step cyclic loading which greatly differs from the common softening (also referred as Mullins effect) observed in chemically crosslinked filled rubbers. We propose that this self-reinforcement is related to the fragmentation of hard domains, naturally present in TPU, in smaller but more numerous subunits that may act as new physical crosslinking points. The proposed stiffening mechanism is not dissimilar to the strain-induced crystallization observed in stretched natural rubber, but it presents a persistent nature. In particular, it may cause a local reinforcement where an inhomogeneous strain field is present, as is the case of a crack propagating in cyclic fatigue, providing a potential explanation for the well-known toughness and wear resistance of TPU.
Transient Microstructure of Low Hard Segment Thermoplastic Polyurethaneunder Uniaxial Deformation
2008
Microstructure evolution of a low hard segment (<10 mol %) thermoplastic polyurethane (LHS-TPU) has been followed by in-situ wide-angle X-ray (WAX) and small-angle X-ray scattering (SAX) with a focus on elucidating peculiar microstructural changes during uniaxial deformation (λ ) 1-3.5). For the LHS-TPU, the hard segments, due to their low content and chemical structure, do not crystallize but form glassy regions that act as physical cross-links. Two types of soft segment crystallites are resolved upon elongation via DSC, SAX, and WAX experiments. Phase I consists of a small amount of initial crystallites (<2%) that function similar to conventional PU hard segment domains, deforming at small uniaxial strains (λ ) 1-2) to a chevrontype morphology, which exhibit equatorial 4-point patterns in SAX. Phase II evolves at higher deformations (λ > 2) due to strain-induced crystallization. Phase II exhibits a conventional meridional 2-point pattern along the deformation direction with lamellar crystallites aligning in the plane normal to the deformation. WAX, SAX, and DSC confirm that both phases coexist over a small strain window (λ ) 1.9-2.5), demonstrating the independent nature of the two crystalline phases. These findings indicate that the LHS-TPU in this study is similar to poly(butylene adipate) (PBA) in its morphological and structural behavior. This is further substantiated by NMR, which reveals that the LHS-TPU consists of 90% soft segments, which are identified as PBA via crystal structure analysis of a highly aligned fiber. The soft segments in the LHS-TPU dominate the morphology and the X-ray patterns upon deformation.
Multiple-length-scale deformation analysis in a thermoplastic polyurethane
Nature communications, 2015
Thermoplastic polyurethane elastomers enjoy an exceptionally wide range of applications due to their remarkable versatility. These block co-polymers are used here as an example of a structurally inhomogeneous composite containing nano-scale gradients, whose internal strain differs depending on the length scale of consideration. Here we present a combined experimental and modelling approach to the hierarchical characterization of block co-polymer deformation. Synchrotron-based small-and wide-angle X-ray scattering and radiography are used for strain evaluation across the scales. Transmission electron microscopy image-based finite element modelling and fast Fourier transform analysis are used to develop a multi-phase numerical model that achieves agreement with the combined experimental data using a minimal number of adjustable structural parameters. The results highlight the importance of fuzzy interfaces, that is, regions of nanometre-scale structure and property gradients, in determining the mechanical properties of hierarchical composites across the scales. De tec tor tra ns lat ion De tec tor tra ns lat ion x ax is 4, 35 8. 47 m m 12 8. 72 m m ARTICLE NATURE COMMUNICATIONS |
Relationship between nanoscale deformation processes and elastic behavior of polyurethane elastomers
Polymer, 2005
The cyclic deformation of two polyurethane elastomers that differed in soft segment content and molecular weight was investigated. The microphase-separated morphology of the polyurethane with higher soft segment content consisted of hard segment domains dispersed in a soft segment matrix. In the polyurethane with lower soft segment content, the hard segment domains appeared to be partially cocontinuous. Following an initial 'conditioning' cycle, both polyurethanes exhibited reversible elastomeric behavior. Structural changes that occurred during conditioning were investigated using atomic force microscopy and Fourier transform infrared dichroism. The results provided the basis of a structural model for the deformation behavior. Yielding and reorganization of hard domains resulted in a highly oriented microfibrous morphology. Subsequent unloading and reloading were associated with reversible relaxation and reformation of the microfibrous entities. The elastic behavior of the conditioned polyurethanes was satisfactorily described by classical rubber theory with inextensibility. The structural model proposed here extended previous efforts to describe the deformation processes of polyurethanes during cyclic loading.
Polymer, 2010
Cyclic tensile responses of fourteen polyurethane elastomers were studied, with respect to their chemical composition and physical structure. Hard segment, soft segment and chain extender were varied, while keeping the hard segment fraction at ca 40% and soft segment molar mass at 2000 g/mol. Hard segments were generated from 4,4 0-methylene bis(phenyl di-isocyanate) (MDI), or 4,4 0-dibenzyl di-isocyanate (DBDI). Physical structure was characterized by X-ray scattering (SAXS and WAXS), revealing significant variations in degree of phase separation and degree of crystallinity, especially in the DBDI-based polymers. Large differences were found in the mechanical responses during first loading to a given strain. Tensile modulus and work input increased significantly with degree of hard phase crystallinity, but were independent of degree of phase separation. First cycle hysteresis was found to increase with reduced phase separation and with replacement of MDI by DBDI. In second and subsequent load cycles, however, in which the Mullins effect was observed, a remarkable degree of uniformity of response was discovered. A unique linear relation was obtained between second cycle hysteresis and second cycle work input, for all strain levels, and for all materials except for two (with highest phase separation) which showed slightly lower second cycle hysteresis. The results can be explained in terms of pull-out of segments from the hard phase on the first cycle, to form a new series-coupled soft phase, whose constitutive response then appears almost independent of chemical and physical structure.
Unravelling the mysteries of cyclic deformation in thermoplastic elastomers
A study has been made of cyclic deformation in a series of model polyurethane thermoplastic elastomers, in which chemical compositions of hard and soft segments were varied systematically. Mechanical tests were supplemented by structural studies. The results reveal significant evidence for series coupling of hard and soft phases, and for deformation-induced conversion from hard phase to soft phase (presumed to occur by chain pull-out), as proposed previously to explain the Mullins effect. The nm-scale structural evidence provides quantitative information on the molecular environment in the two phases, and explains the pull-out and other features of the response. The results suggest a physically-based constitutive model needs to combine series and parallel coupling of the phases, together with phase conversion. Such a model, even when populated with the simplest possible elastic and elastic-plastic representations of soft and hard phases, naturally demonstrates the Mullins effect and other features of the observed cyclic response.
Stress–strain behavior of a polyurea and a polyurethane from low to high strain rates
Polymer, 2007
The large deformation stressestrain behavior of thermoplasticeelastomeric polyurethanes and elastomeric-thermoset polyureas is strongly dependent on strain rate. Their mechanical behavior at very high strain rates is of particular interest due to their role as a protective coating on structures to enhance structural survivability during high rate loading events. Here we report on the uniaxial compression stressestrain behavior of a representative polyurea and a representative polyurethane over a wide range in strain rates, from 0.001 s À1 to 10,000 s À1 , successively marching through each order of magnitude in strain rate using equipment relevant for testing at each particular rate. These results are further analyzed in association with recently reported compressive data on the same materials by Yi et al. [Polymer 2006;47(1):319e29] and intermediate rate tensile data on the same polyurea by Roland et al. [Polymer 2007;48(2):574e8]. The polyurea tested is seen to undergo transition from a rubbery-regime behavior at low rates to a leathery-regime behavior at the highest rates, consistent with the earlier compression study as well as the recent tension study; the polyurethane tested is observed to undergo transition from a rubbery-regime behavior at the low rates to a glassy behavior at the highest rates. The uniaxial compression data for the polyurea are found to be fully consistent with the recently reported uniaxial tension data over the range of rates studied, demonstrating the consistency and complementary aspects of testing at high rates in both compression and tension.
Macromolecules, 2004
The effect of composition on the true mechanooptical properties of thermoplastic poly-(urethane urea)s was investigated by selectively varying the type and content of soft and hard segments. Real-time stress-strain-birefringence data together with off-line wide-angle X-ray scattering measurements revealed that soft segment and chain extender play dominant roles on the chemical structures of the poly(urethane urea)s. All poly(tetramethylene oxide) glycol-based samples showed the same crystal structure. The samples containing ethylenediamine as the chain extender showed enhanced crystallizability as compared to those with 1,6-diaminohexane no matter which soft segment was used. In general, samples with lower fraction of hard segment exhibited higher crystallizability than their high hard segment counterparts. Long-term holding of poly(ethylene oxide) samples in stretched state was found to increase crystallinity. The strain-induced crystallization in low hard segment content poly(tetramethylene oxide)based samples was only observed at very high deformation levels. On the other hand, crystallization in the samples containing high hard segment was found to evolve gradually over large deformation range. The strain rate has a considerable effect on the crystallization behavior of poly(tetramethylene oxide)based samples. While the low hard segment content poly(tetramethylene oxide) sample experiences decreasing crystallizability as the strain rate increases, its counterpart containing higher fraction of hard segments exhibits opposite behavior. We have investigated linear and nonlinear stress optical behavior and observed that the span of the initial linear stress optical region varied primarily with composition (slope ranging from 0.1 to 2.2 GPa -1 ) and secondarily with the deformation rate. Hysteresis experiments show that there is a considerable loss of energy in cyclic loading of these materials, and hysteresis increases as the chain extender is changed from 1,6-diaminohexane to ethylenediamine.
Constitutive Models for Rubber VIII, 2013
The use of micro-hardness for polymers has been for long restricted to the measurements of a scalar value (hardness or modulus), that can clearly not be used to identify local constitutive models. Nevertheless, a recent study coupling instrumented micro-hardness and an optimization loop using finite element simulations has proven to be efficient for the identification of the constitutive hyper-elastic parameters for unfilled natural rubber. It has also been illustrated that the use of these tools for filled rubbers is up to now limited because of the comparable sizes of the tested volume and of the microstructure characteristic scale. This is a pity, as adding fillers lead to a complex cyclic behaviour for these materials (viscosity, Mullins and Payne effects . . . ) and as the comparison of the cyclic response and parameters at micro and macro scales would clearly bring very interesting data. In this study we therefore decided to investigate some specially synthetized Polyurethanes, that are exhibiting a very complex cyclic behavior at the macro-scale, but with a low micro-structural heterogeneity. In this study, we investigated the cyclic response of these materials under indentation testing for several complex loading histories (loading and unloading interrupted with creep and relaxation stages, cyclic force driven steps with increasing amplitudes, cyclic loops until stabilization), that illustrate responses comparable to the ones seen at the macro-scale. A model is then defined that uses a hyper-elastic contribution based on the Edwards-Vilgis potential and a simple Maxwell visco-elastic contribution.