Multiple-length-scale deformation analysis in a thermoplastic polyurethane (original) (raw)
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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
Materials Today Advances, 2019
The distinct molecular architecture and thermomechanical properties of polyurethane block copolymers make them suitable for applications ranging from textile fibers to temperature sensors. In the present study, differential scanning calorimetry (DSC) analysis and macroscopic stress relaxation measurements are used to identify the key internal processes occurring in the temperature ranges between À10 C and 0 C and between 60 C and 70 C. The underlying physical phenomena are elucidated by the small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) study of synchrotron beams, allowing the exploration of the structure-property relationships as a function of temperature. In situ multiscale deformation analysis under uniaxial cyclic thermomechanical loading reveals a significant anomaly in the strain evolution at the nanoscale (assessed via SAXS) in the range between À10 C and 0 C owing to the 'melting' of the soft matrix. Furthermore, WAXS measurement of crystal strain within the hard regions reveals significant compressive residual strains arising from unloading at~60 C, which are associated with the dynamic shape memory effect in polyurethane at these temperatures.
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.
Enhanced Coarse-Graining of Thermoplastic Polyurethane Elastomer for Multiscale Modeling
Journal of Engineering Materials and Technology, 2016
The objective of this work is to develop a multiscale modeling tool of copolymers with long chains. We propose an enhanced coarse-graining method of thermoplastic polyurethane (TPU) with three beads. The proposed coarse-graining provides an accurate molecular modeling tool to keep the molecular interaction together with computational efficiency. The coarse-grained model with three beads is further improved with pressure-correction of the force-field. The improved coarse-grained model holds similar properties of a bulk model of TPU—varying density with temperature, a close density value of TPU at 1 atm, and the phase separation. Equating potential energy densities of the coarse-grained model to the strain energy functions of the continuum model at volumetric and isochoric deformation modes, bulk and shear moduli of TPU are directly obtained and used to estimate Young's modulus and Poisson's ratio. The molecular simulation with the coarse-grained model of TPU demonstrates its ...
Procedia Engineering, 2011
A study was made of how aspects of the mechanical and thermal responses of thermoplastic polyurethane elastomers vary with composition: the hard segment, soft segment and chain extender were varied systematically with the aim of improving understanding of the relationship between molecular/supramolecular architecture at the nm-scale and macroscopic mechanical properties in such systems. Two hard segments were compared, generated from 4,4'-methylene bis(phenyl isocyanate) (MDI), or 4,4'-dibenzyldiisocyanate (DBDI). Rotation around the-CH 2-CH 2-bridge in DBDI allows alignment of aromatic rings and crystallization within the hard phase, which is not available with MDI. The physical structures were characterized by dynamic mechanical analysis (DMA) and by X-ray scattering (SAXS and WAXS) revealing signi cant variations in degree of phase separation and degree of crystallinity, especially in the DBDI-based polymers. The presence of DBDI hard segments instead of MDI led systematically to increases in: the input strain energy to a given elongation, hysteresis and residual strain under cyclic loading, and stress relaxation. Stress-strain cycles reflect the resistance to plastic deformation occurring in the hard domains. This was enhanced by more pronounced hydrogen bonding achieved in the more mobile DBDI than in MDI. These differences between DBDI and MDI could be attributed to the greater flexibility of DBDI allowing a higher tendency to self-associate by hydrogen bonding. The degrees of hysteresis and stress relaxation were found to be greatly enhanced by hard-phase crystallinity, through its effect of increasing the flow stress. The results provide new insight into the physical origin of inelastic effects in reinforced elastomers.
Strain softening of nano-scale fuzzy interfaces causes Mullins effect in thermoplastic polyurethane
Scientific Reports, 2017
The strain-induced softening of thermoplastic polyurethane elastomers (TPUs), known as the Mullins effect, arises from their multi-phase structure. We used the combination of small-and wide-angle X-ray scattering (SAXS/WAXS) during in situ repeated tensile loading to elucidate the relationship between molecular architecture, nano-strain, and macro-scale mechanical properties. Insights obtained from our analysis highlight the importance of the 'fuzzy interface' between the hard and soft regions that governs the structure evolution at nanometre length scales and leads to macroscopic stiffness reduction. We propose a hierarchical Eshelby inclusion model of phase interaction mediated by the 'fuzzy interface' that accommodates the nano-strain gradient between hard and soft regions and undergoes tension-induced softening, causing the Mullins effect that becomes apparent in TPUs even at moderate tensile strains.
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.
International Journal of Material Forming, 2017
Thermoplastic polyurethane elastomers (TPUs) are the most used engineering thermoplastics combining the properties for both elastomers and glassy materials. TPUs have good physical and mechanical properties, excellent chemical and abrasion resistances. Compared with typical thermoset rubbers, TPUs are easier to be processed and recycled. However, the deformation behaviors of TPUs are very complex due to their nonlinear, hysteresis, rate and temperature dependences, and softening properties. Therefore, development of a constitutive model with microstructure considerations is important for predicting the deformation behavior of TPUs under mechanical loading as well as during forming processes such as rolling and stretching. In this work, TPUs were taken as a two-phase material consisting of both hard and soft phases corresponding to their hard and soft domains. A new composite constitutive model for stress-strain response of TPUs was proposed taking into account the microstructure of TPUs as well as its evolution (hard to soft phase transformation) induced by deformation. Excellent agreement between model predictions and experimental results for the loadingunloading behaviors of two TPUs with different hard and soft segment contents confirmed the efficacy of our proposed composite constitutive model.
2008
A study was made of a family of polyurethane copolymers, in which the chemical components were: a hard segment (giving, on phase separation, hard nano-scale reinforcing particles); a soft segment (giving, on phase separation, an elastomeric matrix), and a diol chain extender. The chemical compositions of all three components were varied systematically and independently, and their mechanical responses were measured in cyclic tensile tests at room temperature, up to stretches in the range 5-6. Particular attention was paid to characterizing the inelastic features – hysteresis, and stress relaxation in interrupted tests – and their variations between the materials. The same materials were also studied by wide-angle X-ray scattering (WAXS), to determine levels of crystallinity. Results showed that hysteresis was increased by increasing hard phase crystallinity. This was the case for polyurethanes based on the novel diisocyanate 4,4’-dibenzyl diiscyanate (DBDI). The extent of stress rela...