Segmented Polyurethane Elastomers Research Articles

Single-Step Synthesis and Characterization of Non-Linear Tough and Strong Segmented Polyurethane Elastomer Consisting of Very Short Hard and Soft Segments and Hierarchical Side-Reacted Networks and Single-Step Synthesis of Hierarchical Hyper-Branched Polyurethane.

Polyurethane elastomers are among the most versatile classes of industrial polymers-typically achieved through a two-step synthesis of segmented block copolymers, comprising very long and soft segments that provide elasticity and significantly long and hard segments that provide strength. The present research focused on the design of a single-step synthesis of a new segmented polyurethane consisting of very short soft and hard segments, crosslinked by preferentially side-reacted hierarchical tertiary oligo-uret network structures, thus exhibiting significant strength, elasticity, and toughness. Despite the theoretically linear structure, both FTIR and solid-state 13C NMR spectroscopy analyses indicated the quasi-equal presence of urethane groups and tertiary oligo-uret structures in the resulting polymer, indicating a preferential consecutive side reaction mechanism. Thermal analysis indicated the significant crystallization of soft segments consisting of only four ethylene oxide units, which was, hereby, demonstrated to occur via an extended chain mechanism. Tensile mechanical properties included significant strength, elasticity, and toughness. Increasing the soft segment length led to a decreased tertiary oligo-uret secondary crosslinking efficacy. The preferential hierarchical side reaction mechanism was, hereby, further confirmed through the synthesis of a completely new type of hyper-branched polymer via diisocyanate and a mono-hydroxy-terminated reagent. The structure-property relations and reaction mechanisms demonstrated in the present research can facilitate the design of new polyurethanes of enhanced performance and processing efficacy for a variety of novel applications.

Understanding water absorption effect on molecular dynamics, microstructures and relaxation behavior of segmented polyurethane elastomers

The influence of water absorption on molecular dynamics, microstructures and relaxation behaviors of polyurethane (PU) elastomers with polyether (poly(tetramethylene glycol), PTMG) soft segments has been studied. Kinetics of water absorption of PU elastomer can be well fitted by Fick's model. However, non-Fickian absorption behavior was observed in the late stage at 60 °C, which was presumed to be related to the plasticization of some hard domains. Broadband dielectric relaxation spectroscopy was used to investigate the molecular dynamics of PU elastomers. The analysis of α-relaxation and Maxwell-Wagner-Sillars interfacial polarization indicated that the water absorption has a great influence on the molecular mobility of soft segment and the interface regions between soft domains and hard domains. Microphase separation structures of PU elastomers consisted of loose, partially regular and compact hard domains. Water molecules could interact with loose hard domains and lead to the plasticization. The results of stress relaxation behavior indicated that water absorption leaded to remarkable increase in the stress relaxation ratio. The deterioration of relaxation properties caused by water absorption was not only related to the soft segments, but also caused by the plasticization of the interface regions and the hard domains.

Linear viscoelastic shear and bulk relaxation moduli in poly(tetramethylene oxide) (PTMO) using united-atom molecular dynamics

A quantitative understanding and prediction of the viscoelastic relaxation behavior of polymer melts is required to build up predictive multiscale models, which can be utilized for the practical design of viscoelastic materials. We report the application of the Green–Kubo method to compute the common shear relaxation modulus and the rarely reported bulk relaxation modulus for a realistic united atom model of poly(tetramethylene oxide) (PTMO), which is one key components of the segmented polyurethane (PU) elastomers. The temperature and molecular weight dependent viscoelastic behavior is investigated in detail by computing the bulk relaxation modulus K(t) along with shear relaxation modulus G(t). Our results provide new rheological data for PTMO melt based on the united atom model, which incorporates the chemical details of the polymer backbone. The predicted stress relaxation are mapped onto master curves using the time–temperature superposition principle with horizontal and vertical shift factors. The computed shift factors agree with the Williams–Landel–Ferry equation and appear to be similar for the shear and bulk relaxation modulus. The underlying dynamics for the shear and bulk relaxation modulus appear to stem from the same mechanistic origin as both processes seem to sample similar kinetic signature of the reference time scale of the underlying thermally activated processes in the liquid. Our results demonstrate furthermore, that MD simulations with the chemical details beyond coarse-grained models can sample the Rouse dynamics and the transitions towards entanglement, which is evident by the emergence of a plateau in shear relaxation modulus and scaling relations in the diffusion behavior of the monomer at larger molecular weights.

Stoichiometric-architectural impact on thermo-mechanical and morphological behavior of segmented Polyurethane elastomers

Stoichiometric-architectural impact on thermo-mechanical and morphological behavior of segmented Polyurethane elastomers

Molecular dynamics, microstructures and mechanical properties of segmented polyurethane elastomers under gamma irradiation

The influence of gamma irradiation on molecular dynamics, microstructures and mechanical properties of polyurethane (PU) elastomers with polyether (poly(tetramethylene glycol), PTMG) and polyester (poly(butylene adipate), PBA) soft segments has been studied. Dynamical mechanical analysis and broadband dielectric relaxation spectroscopy were used to investigate the molecular dynamics of PU elastomers. Interestingly, the α-relaxation exhibited contrasting tendency with irradiation dose for PTMG-PU and PBA-PU, demonstrating that main chain scission took place for PTMG soft segments, but main chain crosslink for PBA soft segments. The results of small-angle scattering technique indicated that the volume fraction of hard microdomains increase rapidly with irradiation dose. The microstructures of PTMG-PU were more susceptible to gamma irradiation compared with PBA-PU. The deterioration of tensile properties was not only related to irradiation damage of soft segments, but also caused by the destruction of the microstructures of hard microdomains.

Segmented Polyurethane Elastomers with Mechanochromic and Self‐Strengthening Functions

AbstractMechanochromic elastomers that exhibit force‐induced cross‐linking reactions in the bulk state are introduced. The synthesis of segmented polyurethanes (SPUs) that contain difluorenylsuccinonitrile (DFSN) moieties in the main chain and methacryloyl groups in the side chains was carried out. DFSN was selected as the mechanophore because it dissociates under mechanical stimuli to form pink cyanofluorene (CF) radicals, which can also initiate the radical polymerization of methacrylate monomers. The obtained elastomers generated CF radicals and changed color by compression or extension; they also became insoluble due to the mechanically induced cross‐linking reactions. Additionally, an SPU containing diphenylmethane units also exhibited highly sensitive mechanofluorescence. To the best of our knowledge, this is the first report to demonstrate damage detection ability and changes in the mechanical properties of bulk elastomers induced by simple compression or extension.

Segmented Polyurethane Elastomers with Mechanochromic and Self-Strengthening Functions.

Mechanochromic elastomers that exhibit force-induced cross-linking reactions in the bulk state are introduced. The synthesis of segmented polyurethanes (SPUs) that contain difluorenylsuccinonitrile (DFSN) moieties in the main chain and methacryloyl groups in the side chains was carried out. DFSN was selected as the mechanophore because it dissociates under mechanical stimuli to form pink cyanofluorene (CF) radicals, which can also initiate the radical polymerization of methacrylate monomers. The obtained elastomers generated CF radicals and changed color by compression or extension; they also became insoluble due to the mechanically induced cross-linking reactions. Additionally, an SPU containing diphenylmethane units also exhibited highly sensitive mechanofluorescence. To the best of our knowledge, this is the first report to demonstrate damage detection ability and changes in the mechanical properties of bulk elastomers induced by simple compression or extension.

Innenrücktitelbild: Segmented Polyurethane Elastomers with Mechanochromic and Self‐Strengthening Functions (Angew. Chem. 15/2021)

Innenrücktitelbild: Segmented Polyurethane Elastomers with Mechanochromic and Self‐Strengthening Functions (Angew. Chem. 15/2021)

Strain-induced structural and dynamic changes in segmented polyurethane elastomers

Polyurethane elastomers have been widely used in the industry and daily life due to their versatile physical and chemical properties. Therefore, a fundamental understanding of the structures and dynamics at a molecular level will provide piercing insights into the precise design and application of new polyurethane materials. In this study, we mainly focused on investigating the strain-induced structural and dynamic changes in a typical polyurethane elastomer composed of poly(ε-caprolactone) (PCL) and 4,4′-diphenylmethylene diisocyanate (MDI) as the soft and hard segments, respectively. Obvious strain-hardening phenomenon was observed during the mechanical tensile test, and a systematic comparison was performed on the fractured and pristine samples. DSC results revealed that the crystallization of PCL chains was still going on after the sample was fractured, and the crystallite structures became stable after physical aging at 25 °C for two days. 1H solid-state nuclear magnetic resonance (NMR) experiment was further employed to determine the fraction of mobile PCL chains that were converted to crystallites during stretching. Besides, the microphase separation was also significantly enhanced in the fractured sample. The mobility of amorphous PCL chains was largely reduced due to the strain-induced crystallization of nearby PCL segments, as revealed by the 1H magic-sandwich echo (MSE) NMR experiments. 1H multiple quantum (MQ) NMR experiments also quantitatively revealed the strain-induced orientation of amorphous PCL chains in the fractured sample, indicating that the PCL crystallites were acting as the physical cross-linkages to prevent the contraction of the elongated PCL chains when the sample is fractured.

Recent developments in the field of barrier and permeability properties of segmented polyurethane elastomers

Abstract Polyurethane (PU) elastomers represent an important class of segmented copolymers. Thanks to many available chemical compositions, a rather broad range of chemical, physical, and biocompatible properties of PU can be obtained. These polymers are often characterized by high tensile and tear strength, elongation, fatigue life, and wear resistance. However, their relatively high permeability towards gases and water as well as their biocompatibility still limits the PU’s practical application, especially for biomedical use, for example, in implants and medical devices. In this review, the barrier and permeability properties of segmented PUs related to their chemical structure and physical and chemical properties have been discussed, including the latest developments and different approaches to improve the PU barrier properties.

Thermoplastic Polyurethane Elastomers with Aliphatic Hard Segments Based on Plant‐Oil‐Derived Long‐Chain Diisocyanates

AbstractNovel plant‐oil‐derived long‐chain (C19 and C23) α,ω‐diisocyanates, optionally in combination with the corresponding long‐chain diols, provide entirely aliphatic hard segments in segmented thermoplastic polyurethane elastomers (TPUs), with carbohydrate‐based poly(trimethylene glycol) soft segments. Compared to materials based on a mid‐chain monomer analog, phase separation is higher due to an increased flexibility of the aliphatic segments. Although melting points are slightly lower than for HDPE, the long‐chain TPU's solid‐state structure is still dominated by hydrogen‐bonding.

The Abrasive Wear Resistance of the Segmented Linear Polyurethane Elastomers Based on a Variety of Polyols as Soft Segments.

The presented results make an original contribution to the development of knowledge on the prediction and/or modeling of the abrasive wear properties of polyurethanes. A series of segmented linear polyurethane elastomers (PUR)—In which the hard segments consist of 4,4′-methylene bis(phenylisocyanate) and 1,4-butanodiol, whilst polyether, polycarbonate, or polyester polyols constitute the soft segments—Were synthesized and characterized. The hardness and wear performance as functions of the variable chemical composition of polyurethane elastomers were evaluated in order to define the relationship between studied factors. The microstructure was characterized in detail, including analysis of the hydrogen bonding by Fourier transformed infrared (FT-IR) spectroscopy and the phase structure by X-ray scattering (WAXS) and differential scanning calorimetry (DSC) methods. The presented studies provide the key features of the polymer composition affecting the abrasive resistance as well as attempts to explain the origin of the differences in the polyurethane elastomers’ performance.

Coupling of poly(lactic acid) with a polyurethane elastomer by reactive processing

A segmented polyurethane elastomer (PU) was synthesized in poly(lactic acid) (PLA) melt by reactive processing. The isocyanate component was anticipated to react with the end-groups of PLA resulting in the formation of block-copolymers. The stoichiometry of the functional groups was optimized in the preliminary experiments. Two different processing methods were compared in the further experiments: conventional mixing of PU with PLA (PLA/PU), and reactive blending (PLA-b-PU). The comparison of the structure and properties of compatibilized reactive blends and conventional physical blends clearly shows the benefits of reactive processing. Coupling resulted in a finer dispersion of the particles in the matrix leading to better mechanical properties in the reactive blend. The successful synthesis of PLA-b-PU block copolymers was confirmed by NMR spectroscopy. The isocyanate component was found to react only with the hydroxyl end-groups of PLA, while the formation of amide and acylurea groups was not detected on the carboxyl end.

Internal structure and crystallinity investigation of segmented thermoplastic polyurethane elastomer degradation in supercritical methanol

This work is the first to investigate the internal structure and crystallinity variance of segmented thermoplastic polyurethane elastomer (TPU) degradation in supercritical methanol. Molecular weight distribution (MWD) and molecular structure were determined using GPC, FT-IR and NMR. DSC and XRD were employed to explore the thermal behaviour and morphologic change of TPU, respectively. Results indicated that the segmented TPU has unique degradation behavior because of the three kinds of morphology of hard segments. The short range hard segments dissolved in soft blocks were amorphous. The long range hard segments mixed with and separated from soft blocks formed crystallites. These crystallites ruptured from the soft blocks in initial time period and the soft blocks dissolved in methanol, as indicated by the disappearance of the undecomposed TPU peak in MWDs. The remaining residues were hard segments crystallites which had narrow sharp MWDs. The following process was in two phases. In solid phase, hard segments crystalline structures were destroyed gradually, the melting points decreased correspondingly; in liquid phase, soft segments broken from short range hard segments and fractured.

SHAPE MEMORY BEHAVIOR OF CARBON NANOTUBE FILLED SEGMENTED POLYURETHANE COMPOSITES UNDER VARIABLE STRESSES

ABSTRACT Composites of the segmented polyurethane (SPU) elastomer with different loadings of multiwall carbon nanotubes (MWCNTs) were prepared. Atomic force microscopy and X-ray diffraction were used to study the surface topography and structural aspects of the samples. Thermogravimetric analysis was performed in order to estimate the thermal stability of the composite samples. Two sets of cyclic thermomechanical experiments, both at sub-ambient conditions, were conducted for the evaluation of shape memory performance of the samples using modulated thermomechanical analysis. The first set of experiments was designed to estimate the shape memory performance under stress free or unconstraint conditions, while the second set was planned for the constraint conditions by application of variable stresses. The experimental parameters were so chosen that differences in shape memory response of matrix and composites were clearly distinguished and demonstrated. It was found that MWCNTs strongly affect the shape memory performance of the SPU and reduce the recovery speed under unconstraint conditions. While under constraint conditions, the extent of recovery and recovery speed of composites was found to depend on the applied load and nanotube concentration in the composites.

Strain-induced molecular aggregation states around a crack tip in a segmented polyurethane film under uniaxial stretching

The development of molecular aggregation structures during stretching deformation and local structure mapping around a crack tip of a segmented polyurethane elastomer film were investigated by in situ simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) and by micro-beam WAXD, respectively. Both the hard segment (HS) domain orientation and the strain-induced crystallization of poly(ε-caprolactone) chains were tracked by the WAXD pattern due to the high crystallinity of the symmetric non-bulky aliphatic HS structure. Strain-induced crystallization occurred locally at the vicinity of the crack tip, and the crystallinity immediately decayed in the exterior, whereas the HS domain orientation was observed throughout the stretched film. The mechanisms underlying crack arrest and the mechanical strength of polyurethane elastomer films are associated with the local strain-induced crystallization at the crack tip, the mechanical stability of the HS domains, and the avoidance of local stress concentration through the HS domain orientation.

Tailoring the Morphology and Melting Points of Segmented Thermoplastic Polyurethanes by Self-Nucleation

It is demonstrated that the melting behavior and the morphology of three segmented thermoplastic polyurethane elastomers (TPUs) can be tailored by applying self-nucleation (SN) procedures. The self-nucleating temperature ranges for each of the TPU have been first determined by differential scanning calorimetry (DSC), while their morphology was studied by polarized light optical microscopy (PLOM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS). When the samples are cooled at slow to moderate rates after SN, the crystallization temperature of the TPUs increases by up to 45 °C, when the samples are ideally self-nucleated. This large reduction in supercooling increases the melting points of the samples by approximately 20 °C. At the same time, SAXS and AFM experiments demonstrate the growth of thicker lamellae under these slow to moderate cooling conditions as compared to untreated samples. When ideally self-nucleated samples are rapidly quenched (e.g., at rates of 100 °C min–1 or larg...

Mechanical mapping and morphology across the length scales unveil structure-property relationships in polycaprolactone based polyurethanes

Segmented polyurethane elastomers for biomedical applications were synthesized and studied at macroscopic (by mechanical testing) and meso/nanoscopic length scales (by atomic force microscopy, AFM). The polyurethanes are composed of 4,4'-methylenebis(phenyl isocyanate), 1,4-butanediol and an e-polycaprolactone diol. The stoichiometric ratio of the isocyanate and hydroxyl groups is constant, but the polymer diol to total diol—varies from 0 to 100 %. We show the representative features of the morphology from phase separation to mixed phases, how this is related to the mechanical properties in the bulk and locally, at exposed free surfaces and at the nanoscale. We propose a morphological model considering the molecular structure, the length of hard segments, and the dimensions of both the soft and the hard phases, respectively. Understanding such structure–property relations is pivotal to establishing designer materials and controlling the performance of the final product to achieve optimal properties in polyurethane based medical devices.

Nonisothermal crystallization kinetics of carbon nanotubes containing segmented polyurethane elastomer

Nanocomposites of the segmented polyurethane (SPU) elastomer with different concentrations of multiwall carbon nanotubes (MWCNTs) have been prepared. Scanning electron microscopy has been used to visualize the surface morphology and distribution of the nanotubes inside the matrix. Differential scanning calorimetry has been utilized to investigate the effects of MWCNTs on the crystallization characteristics of the SPU by collecting data at four cooling rates namely 5, 10, 15, and 20°C/min in the temperature range between 200°C to ambient. The results reveal that MWCNTs act as effective nucleating agent for crystallization of the hard segment of SPU and advance the onset and peak temperatures of crystallization by 38 and 23°C, respectively. The associated enthalpy and extent of crystallization are also increased by 34%. Different crystallization kinetic parameters have been calculated using both modified Avrami and combined Ozawa‐Avrami models to suggest a three dimensional growth of crystallization of SPU and its nanocomposites. The activation energy has been calculated using Kissinger method, which indicates that activation energy decreases with increasing concentration of MWCNTs. The calorimetric results have further been correlated with thermomechanical analysis and glass transition temperature of the nanocomposites corresponding to soft segment is found to increase by 20°C. POLYM. ENG. SCI., 56:1248–1258, 2016. © 2016 Society of Plastics Engineers

Low temperature shape memory characteristics of segmented polyurethane-nanoclay composites

Nanocomposites of the segmented polyurethane elastomer with different grades of Cloisite® nanoclay have been prepared. Different characterization techniques viz., X- ray diffraction, scanning electron microscopy, atomic force microscopy and Fourier transform infrared spectroscopy have been utilized to investigate the nature of physico-chemical interaction and dispersion of the nanoclay with the polymer matrix. The dimensional stability and the shape memory characteristics of the nanocomposites have been estimated using thermomechanical analysis between −80 and 20 °C by performing two sets of experiments. The experimental parameters e.g., initial temperature, isothermal time and final temperature were varied keeping the applied load common i.e., between 0.25 and 0.75 N in steps of 0.25 N for both sets of experiments. While the dimensional stability of the nanocomposites was found to improve significantly due to addition of nanoclay, the shape fixity and the shape recovery values were found to be almost reversible under the applied loading/de-loading conditions.