Polyurethane Elastomers Research Articles

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.

Self-healing polyurethane elastomer with wider damping temperature range by synergistic interaction of suspended chains and dynamic disulfide bonds

In order to overcome the sharp damping peak that limits the application of polyurethane elastomers (PUE) and the cut that affects its service life as functional floor coating, a self-healing PUE with broader damping temperature range and high self-healing efficiency was prepared by incorporation of prepolymer synthesized from polycaprolactone diol and isophorone diisocyanate and chain extenders including 4,4′-diaminodiphenyl disulphide (DAPS) and double-hydroxyl suspended chains synthesized from IPDI, polyethylene glycol monomethyl ether and 2-amino-1,3-propanediol. The effects of different functionalities were carefully investigated at the fixed dosage of 30%. DAPS with dynamic disulfide bonds were revealed of higher efficiency in improving the damping properties and self-healing rate. The synergistic effects of the two functionalities were studied by varying the usage of DAPS from 10% to 50% when the suspension chain amount fixed at 30%. The effective damping (tan δ > 0.3) domain presents a range of 131.57 °C with 100% increase of tan δ compared with the base sample, and the self-healing efficiency reaches 95% when the usage of DAPS and suspension chain were 30% and 50%, respectively. The modified material achieved prominent improvement in damping properties and cut self-healing ability, showing potential application in aerospace and acoustic coatings for vibration and noise reduction.

Data‐Driven Approach to Tailoring Mechanical Properties of a Soft Material

AbstractData‐driven, machine learning (ML)‐assisted approaches have been used to study structure‐property relationships at the atomic scale, which have greatly accelerated the screening process and new material discovery. However, such approaches are not easily applicable to modulating material properties of a soft material in a laboratory with specific ingredients. Moreover, it is desirable to relate material properties directly to the experimental recipes. Herein, a data‐driven approach to tailoring mechanical properties of a soft material is demonstrated using ML‐assisted predictions of mechanical properties based on experimental synthetic recipes. Polyurethane (PU) elastomer is used as a model soft material to demonstrate the approach and experimentally varied mechanical properties of the PU elastomer by modulating the mixing ratio between components of the elastomer. Twenty‐five experimental conditions are selected based on the design of experiment and use those data points to train a linear regression model. The resulting model takes desired mechanical properties as input and returns synthetic recipes of a soft material, which is subsequently validated by experiments. Lastly, the prediction accuracies of different machine learning algorithms is compared. It is believed that the approach is widely applicable to other material systems to establish experimental conditions and material property relationships for soft materials.

Embedding ionic hydrogel in 3D printed human-centric devices for mechanical sensing

Flexible sensor applications have increasingly focused on ionically conductive hydrogels due to their notable deformability and easily tunable properties compared to rigid materials. These hydrogels possess electrical properties, thanks to their high water content and porous structure that facilitate effective ion transfer. Despite their attractive features, hydrogels have limitations in terms of water retention and shape fidelity, and they are more typically inspected as two dimensional films and patches. In this paper, 3D printed thermoplastic polyurethane (TPU) elastomer frames with various geometries were injected with ionic conductive polyacrylamide (PAAm) based hydrogels to create durable, robust soft mechanical sensors for detecting strain, pressure, and bending through changes in their electrical resistance. After the effectiveness of the TPU encasement in maintaining the hydrogel water content was demonstrated, hydrogel embedded frames with varying geometries were designed. Their response to mechanical loading was investigated in relation to their dimensions and geometric shape. Finally, glove-shaped frames were fabricated to fit human fingers and injected with ionic hydrogel for sensing abilities. The wearable sensors accommodated free movement of the fingers in multiple directions and were able to detect simultaneous and independent bending and stretching of the fingers. Through comprehensive observation of the electrical behavior of all soft ionic sensors in response to different kinds of mechanical stimuli, it was concluded that the resistance change following mechanical loading was dependent on the specific geometric features of each individual hybrid sensor. Thus, ionic hydrogel-embedded TPU encasement could be designed with targeted geometry to dictate the type and direction of mechanical sensing with regard to its application. This work presents a facile approach to fabricating multi-component soft geometric sensors with potential to be used for wearable electronics and human–machine interactions.

Carbon nanotubes grafted by polyurethane chains with dopamine-mediation to enhance the mechanical and damping properties of polyurethane elastomer

A novel strategy of combining mussel inspired dopamine chemistry and polycondensation reactions to construct unique functionalized carbon nanotubes (CNTs) with polyurethane (PU) chains was demonstrated. The polydopamine (PDA) was firstly functionalized to the surface of carbon nanotubes, then polydopamine acted as a bridge and covalently reacted with polyurethane chains to form a network structure named CNTs-PDA-PU, which improved the ability of polyurethane chains grafted onto carbon nanotubes. The functionalized carbon nanotubes significantly enhanced the mechanical and damping properties of nanocomposites. It was found that the highest mechanical properties were achieved at 0.1% loading of CNTs-PDA-PU, leading to the tensile strength and elongation at break increased by 97% and 25%, respectively, compared with matrix. More interestingly, the functionalized carbon nanotubes could form multiple physical and chemical interface interactions with matrix, thus alter the microphase separation structure of nanocomposites. This novel carbon nanotubes functionalization and elastomer synthesis method can provide ideas for the application of nanocomposites.

Regulation and control of discoloration efficiency of mechanochromic polyurethane elastomers by microstructure design

Regulation and control of discoloration efficiency of mechanochromic polyurethane elastomers by microstructure design

A Stacked FKM/PU Triboelectric Nanogenerator for Discrete Mechanical Energy Harvesting.

To combine the advantages of elastic and nonelastic triboelectric materials, this work proposes a new type of triboelectric nanogenerator (TENG) based on stacking -the stacked FKM/PU TENG. By stacking the elastomer polyurethane (PU) and the nonelastomer fluororubber (FKM), the FKM/PU TENG combines the inherent triboelectric characteristics of both materials and the unique elasticity of PU to achieve an output performance that is much higher than that of the FKM-TENG or the PU-TENG. The maximum instantaneous open-circuit voltage and short-circuit current of the FKM/PU TENG reach 661 V and 71.2 μA, respectively. Under the limiting conditions of 3 Hz and maximum compression, this device can attain a maximum power density of 49.63 W/m3 and light more than 500 LEDs. Therefore, stacking materials with different properties gives the FKM/PU TENG high output performance and great application potential, which can contribute to future development of discrete mechanical energy harvesting.

Impact of a Novel Phosphoramide Flame Retardant on the Fire Behavior and Transparency of Thermoplastic Polyurethane Elastomers.

In many application fields of thermoplastic polyurethane (TPU), excellent flame retardancy and transparency are required. However, higher flame retardancy is often at the expense of transparency. It is difficult to achieve high flame retardancy while maintaining the transparency of TPU. In this work, a kind of TPU composite with good flame retardancy and light transmittance was obtained by adding a new synthetic flame retardant named DCPCD, which was synthesized by the reaction of diethylenetriamine and diphenyl phosphorochloridate. Experimental results showed that 6.0 wt % DCPCD endowed TPU with a limiting oxygen index value of 27.3%, passing the UL 94 V-0 rating in the vertical burning test. The cone calorimeter test results showed that the peak heat release rate (PHRR) of the TPU composite was dramatically reduced from 1292 kW/m2 (pure TPU) to 514 kW/m2 by adding only 1 wt % DCPCD. With the increase of DCPCD contents, the PHRR and total heat release gradually decreased, and the char residue gradually increased. More importantly, the addition of DCPCD has little effect on the transparency and haze of TPU composites. In addition, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were carried out to investigate the morphology and composition of the char residue for TPU/DCPCD composites and explore the flame retardant mechanism of DCPCD in TPU.

Strain-Hardening, impact protective and Self-Healing supramolecular polyurethane nanocomposites enabled by quadruple H-Bonding, disulfide bonds and nanoparticles

Developing protective materials with intelligent and stimuli-responsive properties is indispensable for safety protection. However, combining contradictory characteristics into a single polymer is challenging, such as high mechanical strength, impact protection, and self-healing properties. Herein, a strain-hardening supramolecular polyurethane nanocomposite (SPN) with high mechanical strength and unique impact protection characteristics is synthesized by incorporating quadruple hydrogen bonds (H-bonds), disulfide bonds, and modified silica nanoparticles into polyurethane elastomer. Benefiting from the dynamic break and re-form characteristic of quadruple H-bonds and the stress dissipation behavior of nanoparticles, SPN exhibits high energy absorption efficiency and impact protection properties. During the drop ball striking test, the impact energy absorption efficiencies reached 90%, and the buffer time of impact was significantly longer than common buffering materials. The SPN exhibited excellent stretchability and self-healing ability, ascribing to disulfide metathesis and H-bonds association. Moreover, the prepared SPN could be easily processed through a hot press and showed negligible creeping. The developed SPN could serve as a novel self-standing impact protective material that offers promising prospects in sports, transportation, and aerospace fields.

A strategy for improving negative-Poisson's-ratio constraint effect and shape accuracy of metamaterials by regulating mechanical properties of elastomer with facile filler incorporation

Negative Poisson's ratio (NPR) metamaterials are widely used for their light weight and good shear resistance. However, the essential concave hollow structures in NPR materials may easily distort irregularly when the constraint effect of their solid materials is weak, especially for elastomer-based materials. Here, we introduced nano-scale carbon black (CB) particles into the NPR material based on thermoplastic polyurethane (TPU) elastomers to control the static and dynamic mechanical deformation behavior of the material, improving their constraint effect on the NPR structure. The results show that the modulus of the NPR structure is improved by 269% by adjusting the unit and rigid particle load. The Poisson's ratio of the NPR structure reaches as great as −1.3 via controlling the thickness/width ratio (TWR) of the unit. Meanwhile, finite element analysis verifies that when TWR is less than 0.5, increasing the modulus and yield strength of the composites can enhance the constraint on the NPR unit. Theoretical calculation shows that CB introduced into TPU can improve the shape accuracy of NPR structure. Therefore, the work could offer a new mechanical enhancing strategy for NPR materials, and its application potential is verified as a form of exoskeletons.

Synthesis and characterization of non-invasively traceable poly(ether urethane)s for biomedical applications

The ease of real-time visibility of biomedical implants and minimally invasive medical devices is indispensable in radiological imaging to avoid complications and assess therapeutic success. Herein, we prepared a series of polyurethane elastomers with inherent radiopacity, enabling them to be imaged under fluoroscopy. Through an appropriate selection of less toxic intermediates such as 1,6-Diisocyanatohexane (HDI), poly (tetramethylene glycol) (PTMG), and a chain extender, iodinated hydroquinone bis(2-hydroxyethyl) ether (IBHE), new radiopaque polyether urethanes (RPUs) containing about 10.8 to 20.6% iodine contents were synthesized. RPUs were characterized for the physicochemical, thermomechanical and radiopacifying properties. It was observed that the concentration of IBHE had a profound impact on the radiopacity of polyurethanes. RPUs exhibited similar or better radiopacity than an aluminum wedge of equivalent thickness. In-vivo imaging revealed that the RPUs were easily distinguishable from the surrounding tissues. Irrespective of iodine content, all the RPUs were cytocompatible, indicating the suitability of these materials for medical and allied applications.

Bismuth oxychloride microcrystals decorated carbon foams based on waste polyurethane elastomer for enhanced removal of methylene blue

Multicomponent materials containing nanoscale structures, the so-called “nanocomposites” are one of the fastest-growing areas of photocatalytic research. In this study, the nanocomposites of bismuth oxychloride (BiOCl) with porous carbon foam (CF) derived from waste polyurethane (PU) elastomers were developed by a simple hydrothermal method, and the photocatalytic performance of the as-prepared materials was investigated. The structure, morphology and optical properties of the composites were characterized in detail. The photocatalytic activity of the samples was evaluated by studying the degradation of methylene blue (MB) under UV-A irradiation. Even at a high MB concentration (0.5 mmol/L), excellent photocatalytic activity was observed, with an overall removal efficiency of 99.0% in 100 min of irradiation. Kinetic studies were also carried out for the degradation of MB by pristine BiOCl and CF-BiOCl. The photodegradation rate constants were evaluated by fitting the kinetic data with a pseudo-first-order model, and the rate constants of CF-BiOCl composites were higher than that of pristine BiOCl. The enhanced activity of the composites was due to the well-connected heterojunction interface, improved hydrophilicity, more reactive sites, and effective separation of photo-generated charges. Thus, our work combines the valuable waste PU-derived CFs with BiOCl, which provides a strategy to achieve circular economy and environmental remediation objectives.

Toughness modification of waterborne epoxy emulsified asphalt by waterborne polyurethane elastomer

Waterborne epoxy emulsified asphalt (w-EA) is prone to cracking at low temperatures after curing, making it vulnerable to damage in plateau zone. In order to solve poor performance at low-temperature of w-EA, waterborne polyurethane (w-PU) is used as a toughening agent to modify waterborne epoxy emulsified asphalt (w-PUEA). Furthermore, the microstructure, creep resistance, dynamic mechanical properties, low-temperature performance, and tensile mechanical properties of both w-EA and w-PUEA are studied in this paper. As the amount of w-PU is increased, the resin network structure in w-PUEA becomes more complex. In addition, w-PU significantly improves the long-term creep behavior of w-EA. The glass transition temperature of w-PUEA gradually shifts to a higher temperature with increasing amounts of w-PU. It is worth noting that the damping properties of w-EA are also significantly improved. The results from low-temperature stress relaxation tests show that the relaxation rate of 10% w-PUEA (0.27 MPa·s−1) is 1.93 times higher than that of w-EA (0.14 MPa·s−1). The flexible long chain of w-PU improves the elongation at break and toughness of 10% w-PUEA by 300% and 171%, respectively, in comparison with w-EA. The pavement performance of the mixture indicates that the addition of 10 wt% w-PU can significantly enhance the low-temperature crack resistance of w-EA.

Study on the penetration resistance of a honeycomb composite structure coated with polyurethane elastomer

In this study, the ballistic performance of the composite structure was investigated and studied by experiment and finite element analysis in order to study the anti-penetration performance of a polyurethane elastomer-coated honeycomb sandwich construction. Six kinds of target plate structures with different configurations were prepared by changing the hard segment content and coating position of polyurethane elastomer. The first-stage light gas gun device is used to conduct impact experiments at various speeds, and the finite element simulation computation is performed using LS-DYNA. Based on the experimental and finite element results, the ballistic limit velocity and specific energy absorption of different target configurations were analyzed, and the failure forms, damage areas, and failure mechanisms were discussed. The findings demonstrate that the polyurethane elastomer coating enhances the honeycomb sandwich structure’s ballistic limit performance by 40.6% to 51% and that the honeycomb core of the target plate with the coating on the front panel has a larger area of collapse. This fully exploits the benefits of the honeycomb structure in the energy absorption process and makes up for the weaknesses of the honeycomb sandwich structure’s subpar energy absorption effect under local impact loads. Therefore, the front panel coating has better ballistic limit performance. The coated polyurethane with medium hardness demonstrated the best ballistic limit performance for the various polyurethane hardness. The ballistic limit speeds of the front panel and back panel coatings increased by 51% and 48.8% respectively, in comparison to the uncoated target plate. After comparing the structure of polyurea coating, it is found that the ballistic limit performance of polyurethane coating with medium hardness is higher than that of polyurea coating structure, and the negative growth of the ballistic limit performance of the whole structure caused by the coating of rigid polyurea on the back panel is effectively avoided.

Room-temperature self-healing and recyclable polyurethane elastomers with high strength and superior robustness based on dynamic double-crosslinked structure

Mechanically robust elastomers with the functionalities of room-temperature self-healing ability and even recyclability provide new opportunities to revolutionize next-generation flexible electronics. However, it remains a great challenge to synthesize such materials due to the conflicting requirements of the self-healing ability and mechanical robustness for the dynamicity of crosslinks. Herein, a room-temperature self-healing and recyclable polyurethane (PU) elastomer with ultrahigh mechanical performance is constructed by synergistically incorporating dynamic disulfide bonds and multiple hydrogen bonds (H-bonds) into a PU chain. The introduction of diazolidinyl urea (DU) motifs forms strong H-bonds and branching crosslinking points, improving the robustness of the crosslinked structure, and the fast metathesis of the aromatic disulfide bonds mainly contributes to the dynamicity of the crosslinked network. The resulting PU elastomers exhibit high tensile strength (14.08 MPa), toughness (64.6 MJ m−3), superior elastic restorability, outstanding self-healing capability (∼81% at room temperature), and multiple recyclability. Furthermore, a wearable sensor based on this elastomer is constructed to monitor different human motions, demonstrating the potential application in wearable electronics.

Liquefied chitin-derived super tough, sustainable, and anti-bacterial polyurethane elastomers

Considerable amounts of waste chitin are produced every year from the fishing industry; however, the efficient utilization of natural chitin remains a challenge. To expand the applications of natural chitin, we herein report the thermal liquefaction of chitin to promote chain extension and synthesize a liquefied chitin-based polyurethane (LCPU) elastomer with a desirable tensile strength (∼43 MPa), superior toughness (∼417.66 MJ m−3), and an extremely high fracture energy (∼567.89 kJ m−2). In this study, liquefied chitin was used for the first time as a chain extender to replace traditional polyols and polyamino compounds in the preparation of tough PU materials. The acquired extraordinary mechanical features mainly originated from the highly efficient energy dissipation of strong hydrogen-bonding clusters and strain-induced crystal structures. The prepared LCPU materials exhibited considerable recyclability and self-healing properties owing to their dynamic hydrogen bonding. In addition, they exhibited excellent anti-bacterial properties and a good biocompatibility, thereby indicating their potential for application in biomedicine, such as in the contexts of minimally invasive surgical procedures and anti-bacterial medical instruments. We believe that this strategy will encourage researchers in academia and industry to develop strong, tough, degradable, and biosafe elastic materials from natural resources.

Nanocellulose-Based Thermoplastic Polyurethane Biocomposites with Shape Memory Effect

In 2020, we published a review on the study of semi-crystalline thermoplastic polyurethane elastomers and composites based on the shape memory effect. The shape recovery ability of such polymers is determined by their sensitivity to temperature, moisture, and magnetic or electric fields, which in turn are dependent on the chemical properties and composition of the matrix and the nanofiller. Nanocellulose is a type of nanomaterial with high strength, high specific surface area and high surface energy. Additionally, it is nontoxic, biocompatible, environmentally friendly, and can be extracted from biomass resources. Thanks to these properties, nanocellulose can be used to enhance the mechanical properties of polymer matrices with shape memory effect and as a switching element of shape memory. This review discusses the methods for producing and properties of nanocellulose-based thermo-, moisture-, and pH-sensitive polyurethane composites. The synergistic effect of nanocellulose and carbon nanofillers and possible applications of nanocellulose-based thermoplastic polyurethane biocomposites with shape memory effect are discussed. A brief description of nanocellulose terminology is also given, along with the structure of shape memory thermoplastic polyurethanes. There is significant interest in such materials for three primary reasons: the possibility of creating a new generation of biomaterials, improving the environmental friendliness of existing materials, and exploiting the natural renewability of cellulose sources.

Stress Relaxation Behaviour Modeling in Rigid Polyurethane (PU) Elastomeric Materials.

Polyurethane (PU) has been used in a variety of industries during the past few years due to its exceptional qualities, including strong mechanical strength, good abrasion resistance, toughness, low-temperature flexibility, etc. More specifically, PU is easily "tailored" to satisfy particular requirements. There is a lot of potential for its use in broader applications due to this structure-property link. Ordinary polyurethane items cannot satisfy people's increased demands for comfort, quality, and novelty as living standards rise. The development of functional polyurethane has recently received tremendous commercial and academic attention as a result. In this study, the rheological behavior of a polyurethane elastomer of the PUR (rigid polyurethane) type was examined. The study's specific goal was to examine stress relaxation for various bands of specified strains. We also suggested the use of a modified Kelvin-Voigt model to describe the stress relaxation process from the perspective of the author. For the purpose of verification, materials with two different Shore hardness ratings-80 and 90 ShA, respectively-were chosen. The outcomes made it possible to positively validate the suggested description in a variety of deformations ranging from 50% to 100%.

A Strategy of Thiolactone Chemistry to Construct Strong and Tough Self-Healing Supramolecular Polyurethane Elastomers via Hierarchical Hydrogen Bonds and Coordination Bonds

Self-healing elastomers with extended service life and enhanced reliability have attracted extensive attention in recent years due to their broad application in fields such as protective coatings, wearable electronics, shape memory materials, and health-care monitoring. However, it is challenging to simultaneously optimize the mechanical properties (strength and toughness) and self-healing capacity of the elastomers. Herein, thiolactone chemistry was delicately used as a bridge to address this conundrum by incorporating 2-ureido-4[1H]-pyrimidinone (UPy) motifs and imidazole ligands into the side chains of linear polyurethane. The synergistic cross-linking of UPy H-bonds and Zn2+-imidazole coordination afforded a robust supramolecular network to significantly improve the strength. Meanwhile, these dynamic cross-linking points acted as sacrificial bonds for energy dissipation under external stress, which played a dominant role in toughening. Therefore, the resultant elastomer (PU-Im-UPy-Zn) exhibited a tensile strength of 9.1 MPa, a breaking elongation of 989%, and toughness up to 62.1 MJ m–3. In addition, the reversible exchange of hierarchical hydrogen bonds (single H-bonds between the carbamate groups and quadruple H-bonds of the UPy dimers) and coordination bonds as well as the high mobility of side chains were conducive to repairing under mild conditions. The superficial scratches completely disappeared at 60 °C, and a satisfactory healing efficiency of 78% was observed after 24 h. This work provides some insights into future design of self-healable supramolecular polyurethane elastomers integrated with conspicuous strength and toughness.

Regulating the structure of thermoplastic polyurethane elastomers with a diol chain extender to strength performance

AbstractThe structural defects of thermoplastic polyurethane elastomer (TPU) caused by the uneven distribution of hard segments limiting their potential application in special industrial fields such as aerospace or defense equipment. Optimizing the TPUs' structure is a useful method to adjustable uneven distribution of hard segments and enhance the performance of TPUs. In this work, a chain extender (BMB) embedded in carbamate‐derive units was successfully synthesized by 4,4′‐diphenylmethane diisocyanate (MDI) and 1,4‐butanediol (BDO). Using BMB and as chain extender, a modified BMB‐TPU was prepared, and its properties were systematically evaluated. Compared with conventional thermoplastic polyurethane elastomer (BDO‐TPU), BMB‐TPU had a regular structure with uniform hard segments, narrower molecular weight distribution and stronger intra/inter‐chain hydrogen bonding interactions, and thus better microphase separation. The BMB‐TPU exhibited an excellent tensile strength of 35 MPa, 46% higher than 24 MPa for the control BDO‐TPU. Moreover, the heat resistance of BMB‐TPU was also reinforced compared to BDO‐TPU, with an increase of 7.2°C for the degradation temperature of 5% loss and 9.6°C for the viscous flow transition temperature. We believe our paradigm can provide a feasible guide for designing high‐performance TPUs.