Influence of Dangling Chains on Molecular Dynamics of Polyurethanes

Controllable Design and Synthesis of Polyurethane Elastomers Containing Polar Dangling Chains with High Mechanical Properties and Wide Damping Temperature Range

Vibration and noise severely affect the operation of mechanical equipment and is also detrimental to human health. Therefore, the development of high performance damping materials is crucial. However, current methods to improve damping properties often come at the expense of mechanical properties, resulting in inferior mechanical performance of materials. In order to address the issue of imbalance between damping properties and mechanical properties in polyurethane damping elastomers. In this study, polyester dangling chains containing polar groups are synthesized and introduced into polyurethane. The obtained polyurethane exhibited an effective damping temperature range of 154 °C (-54 °C to 100 °C) and a tensile strength of 15.82 MPa. Furthermore, dynamic mechanical analysis and broadband dielectric relaxation spectroscopy are combined to investigate the influence of polar dangling chains on the structure and properties of polyurethane. The degree of microphase separation increases after the introduction of polar dangling chains, indicating enhances intermolecular interaction forces, facilitating the formation of hydrogen bond between the main chain and dangling chains, thereby increasing molecular chain friction and energy dissipation. This work overcomes the challenge of balancing the damping and mechanical properties of polyurethane, providing a new strategy for designing high performance polyurethane damping elastomers.

Thermoresponsive On-Demand Adhesion and Detachment of a Polyurethane-Urea Bioadhesive

The development of bioadhesives with strong adhesion and on-demand adhesion-detachment behavior is still critically important and challenging for facilitating painless and damage-free removal in clinical applications. In this work, for the first time, we report the easy fabrication of novel polyurethane-urea (PUU)-based bioadhesives with thermoresponsive on-demand adhesion and detachment behavior. The PUU copolymer was synthesized by a simple copolymerization of low-molecular-weight, hydrophilic, and biocompatible poly(ethylene glycol), glyceryl monolaurate (GML, a special chain extender with a long side hydrophobic alkyl group), and isophorone diisocyanate (IPDI). Here, GML was expected to not only adjust the temperature-dependent adhesion behavior but also act as an internal plasticizer. By simple adjustment of the water content, the adhesion strength of the 15 wt % water-containing PUU film toward porcine skin is as high as 55 kPa with an adhesion energy of 128 J/m2 at 37 °C. The adhesion strength dramatically decreases to only 3 kPa at 10 °C, exhibiting switching efficiency as high as 0.95. Furthermore, the present PUU-based adhesive also shows good on-demand underwater adhesion and detachment with a cell viability close to 100%. We propose that biomaterial research fields, especially novel PUU/polyurethane (PU)-based functional materials and bioadhesives, could benefit from such a novel thermoresponsive copolymer with outstanding mechanical and functional performances and an easy synthesis and scaled-up process as described in this article.

Self-Healing Elastomers with Unprecedented Ultrahigh Strength, Superhigh Fracture Energy, Excellent Puncture Resistance, and Durability Based on Supramolecule Interlocking Networks Formed by Interlaced Hydrogen Bonds

Due to the multiple different properties in self-healing elastomers that are mutually exclusive based on the different and contradictory molecule chain structures, simultaneously achieving the ultrahigh mechanical performance and high durability of self-healing elastomers is a great challenge and the goal that has always been pursued. Herein, we report a novel strategy to fabricate a self-healing elastomer by introducing interlaced hydrogen bonds with superhigh binding energy. Distinguishing from the quadruple hydrogen bonds reported already, the interlaced hydrogen bond with a lower repulsive secondary interaction and higher binding energy is composed of two molecule units with different lengths and steric hindrance. Connected by the interlaced hydrogen bonds, a supramolecule interlocking network is formed to lock the polymer chains at room temperature, endowing the poly(urethane-urea) elastomer with an unprecedented ultrahigh strength (117.5 MPa, even higher than some plastics), the superhigh fracture energy (522.46 kJ m-2), and an excellent puncture resistance (puncture force reached 181.9 N). Moreover, the elastomers also exhibited excellent self-healing properties (healing efficiency up to 95.8%), high transparency (the average transmittance up to 91.0%), and good durability (including thermal decomposition resistance, thermal oxidation aging resistance, water resistance, and solvent resistance), providing a theoretical basis and technical reference in the development and broadening the application prospects of self-healing elastomers.

Analysis of Poly(thiourethane) Covalent Adaptable Network through Broadband Dielectric Spectroscopy

The dielectric spectra of the poly(thiourethane) network, HDI-S3, have been analyzed to know the nature and the cooperativity of each of the six dielectric processes observed. At low temperatures, γ₁, γ₂, and β dielectric relaxations were attributed to noncooperative local motions in the glassy state, in which apparent activation energies are 30, 36, and 60 kJ·mol^-1, respectively. At higher temperatures, three dielectric relaxations are observed (α_Tg, α*, ρ). The α_Tg relaxation is attributed to the glass transition, and it is overlapped with the α* relaxation. The molecular origin of α* relaxation is associated with the bond exchange reaction. Finally, the ρ relaxation is ascribed to the heterogeneity of the sample although its origin is uncertain. The DC conductivity (σ_DC) is found to be an appropriate variable to analyze the bond exchange reaction. Accordingly, the HDI-S3 has a molecular exchange mechanism of dissociative nature.

Stretchable Self-Healing Plastic Polyurethane with Super-High Modulus by Local Phase-Lock Strategy

In this work, a multiblock polyurethane (PU-Im) consisting of polyether and polyurethane segments with imidazole dangling groups is demonstrated, which can further coordinate with Ni²⁺ . By controlling the ligand content and metal-ligand stoichiometry ratio, PU-Im-Ni complex with vastly different mechanical behavior can be obtained. The elastomer PU-2Im-Ni has extraordinary mechanical strength (61MPa) and excellent toughness (420 MJ m^-3 ), but the plastic PU-4Im-Ni exhibits super-high modulus (515 MPa), strength (63 MPa), and good stretchability (≈800%). The metal-ligand interaction between polyurethane segments and Ni²⁺ is proved by Raman spectra, dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM). The polyurethane segments domain formed by microphase separation is dynamically "locked" by Ni²⁺ coordinated with imidazole, revealing a local phase-lock effect. The phase-locking hard domains reinforce the PU-Im-Ni complex and maintain stimuli-responsive self-healing ability, while the free polyether segments provide stretchability. Primarily, the water environment with plasticization effect serves as an effective and eco-friendly self-healing approach for PU-Im-Ni plastic. With the excellent mechanical performance, thermal/aquatic self-healing ability, and unique damping properties, the PU-Im-Ni complexes show potential applications in self-healing engineering plastic and cushion protection fields.

The Alkyne Zipper Reaction: A Useful Tool in Synthetic Chemistry

The alkyne zipper reaction is an internal-to-terminal alkyne isomerization reaction with many interesting applications in synthetic chemistry, as it constitutes an efficient means of achieving acetylene functionalization. A review of its applications in synthesis processes is presented in this paper, with a brief overview of the mechanistic features of the alkyne zipper reaction, as well as a brief overview of its future potential.

Tandem metathesis depolymerization and cyclopolymerization toward flexible-rigid block copolymer with unique damping properties

Metathesis depolymerization (MDP) of natural rubber (NR) was readily conducted to afford depolymerized NR (dNRx) bearing the living chain end, which can initiate metathesis cyclopolymerization (MCP) of 1,6-heptadiyne monomers to...

Synthesis and molecular dynamics study of high-damping polyurethane elastomers based on the synergistic effect of dangling chains and dynamic bonds

A molecular dynamics study of a high-damping polyurethane elastomer is performed by introducing MPEG dangling chains and dynamic bonds.

Mechanically robust and reprocessable imine exchange networks from modular polyester pre-polymers

Covalent adaptable networks (CANs) containing dynamic imine cross-links impart recyclability to thermoset materials, and the distribution of these cross-links greatly affects their observed thermomechanical properties.

Controlling Healing and Toughness in Polyurethanes by Branch-Mediated Tube Dilation

In this work, we propose the use of regular branching of polyurethanes as a way to regulate chain dynamics and govern crystallization in highly dense hydrogen-bonded systems. As a result, robust and healable polyurethanes can be obtained. To this end, we synthesized a range of aliphatic propane diol derivatives with alkyl branches ranging from butyl (C4) to octadecanyl (C18). The series of brush polyurethanes was synthesized by polyaddition of the diols and hexamethylene diisocyanate. Polyurethanes with very short (C < 4) and very long (C = 18) brush lengths did not lead to any significant healing due to crystallization. An intermediate amorphous regime appears for polymers with middle branch lengths (C = 4 to 8) showing a fine control of material toughness. For these systems, the side chain length regulates tube dilation, and significant macroscopic healing of cut samples was observed and studied in detail using melt rheology and tensile testing. Despite the high healing degrees observed immediately after repair, it was found that samples with medium to long length brushes lost their interfacial strength at the healed site after being heated to the healing temperature for some time after the optimal time to reach full healing. Dedicated testing suggests that annealed samples, while keeping initial tackiness, are not able to completely heal the cut interface. We attribute such behavior to annealing-induced interfacial crystallization promoted by the aliphatic branches. Interestingly, no such loss of healing due to annealing was observed for samples synthesized with C4 and C7 diols, which is identified as the optimal healing regime. These results point at the positive effect of branching on healing, provided that a critical chain length is not surpassed, as well as the need to study healing behavior long after the optimal healing times.

Effects of Diisocyanate Structure and Disulfide Chain Extender on Hard Segmental Packing and Self-Healing Property of Polyurea Elastomers

Four linear polyurea elastomers synthesized from two different diisocyanates, two different chain extenders and a common aliphatic amine-terminated polyether were used as models to investigate the effects of both diisocyanate structure and aromatic disulfide chain extender on hard segmental packing and self-healing ability. Both direct investigation on hard segments and indirect investigation on chain mobility and soft segmental dynamics were carried out to compare the levels of hard segmental packing, leading to agreed conclusions that correlated well with the self-healing abilities of the polyureas. Both diisocyanate structure and disulfide bonds had significant effects on hard segmental packing and self-healing property. Diisocyanate structure had more pronounced effect than disulfide bonds. Bulky alicyclic isophorone diisocyanate (IPDI) resulted in looser hard segmental packing than linear aliphatic hexamethylene diisocyanate (HDI), whereas a disulfide chain extender also promoted self-healing ability through loosening of hard segmental packing compared to its C-C counterpart. The polyurea synthesized from IPDI and the disulfide chain extender exhibited the best self-healing ability among the four polyureas because it had the highest chain mobility ascribed to the loosest hard segmental packing. Therefore, a combination of bulky alicyclic diisocyanate and disulfide chain extender is recommended for the design of self-healing polyurea elastomers.

Enhancement of microphase ordering and mechanical properties of supramolecular hydrogen-bonded polyurethane networks

Enhanced thermoreversible and mechanical properties in supramolecular polyurethanes have been realised by the incorporation of flexible DBDI derived hard segments.

The network structure and properties of multifunctional epoxy/anhydride systems

In this study, bi-, tri-, and tetra-functional epoxy systems were chosen to design network structures using hexahydrophthalic anhydride as curing agent and tris-(dimethylaminomethyl) phenol as accelerant with bifunctional 1,4-butanediol diglycidyl ether as the flexible reactive diluents to further adjust the network structure. The cross-link density and activation energy ( Ea) of glass transition were calculated by dynamic mechanical thermal analysis (DMTA) data. The packing of molecular chain and average distance between molecular chains were analyzed by DMTA and X-ray diffraction measurements, respectively. As a result, the loss modulus, glass transition temperature ( Tg), cross-link density, average distance between molecular chains and Ea were found to be positively correlated with the degree of functionality. Meanwhile, the storage modulus of cured epoxy products at glass state was found to vary as follows: diglycidyl ether of bisphenol F (DGEBF) > tetraglycidyl diaminodiphenylmethane > triglycidyl- p-aminophenol (TGPAP). The loading of diluents decreased Tg and Ea and increased the storage modulus at glass state of DGEBF and TGPAP systems.

Mesoscopic simulations of hydrophilic cross-linked polycarbonate polyurethane networks: structure and morphology

Polyurethane (PU) cross-linked networks are frequently used in biomedical and marine applications, e.g., as hydrophilic polymer coatings with antifouling or low-friction properties and have been reported to exhibit characteristic phase separation between soft and hard segments. Understanding this phase-separation behavior is critical to design novel hydrophilic polymer coatings. However, most of the studies on the structure and morphology of cross-linked coatings are experimental, which only assess the phase separation via indirect methods. Herein we present a mesoscopic simulation study of the network characteristics of model hydrophilic polymer networks, consisting of PU with and without methyl-polyethylene glycol (mPEG) dangling chains. The systems are analyzed using a number of tools, such as the radial distribution function, the cross-link point density distribution and the Voronoi volume distribution (of the cross-linking points). The combined results show that the cross-linked networks without dangling chains are rather homogeneous but contain a small amount of clustering of cross-linker molecules. A clear phase separation is observed when introducing the dangling chains. In spite of that, the amount of cross-linker molecules connected to dangling chains only, i.e., not connected to the main network, is relatively small, leading to about 3 wt% extractables. Thus, these cross-linked polymers consist of a phase-separated, yet highly connected network. This study provides valuable guidelines towards new self-healing hydrophilic coatings based on the molecular design of cross-linked networks in direct contact with water or aqueous fluids, e.g., as anti-fouling self-repairing coatings for marine applications.

The synthesis and curing kinetics study of a new fluorinated polyurethane with fluorinated side chains attached to soft blocks

Fluorinated polyurethane with fluorinated side chains attached to soft blocks was successfully synthesized, which exhibited unique properties. And also the curing kinetics was studied using a rotational rheometer.

Stimulation of Wound Healing by Electroactive, Antibacterial, and Antioxidant Polyurethane/Siloxane Dressing Membranes: In Vitro and in Vivo Evaluations

A series of novel polyurethane/siloxane-based wound dressing membranes was prepared through sol-gel reaction of methoxysilane end-functionalized urethane prepolymers composed of castor oil and ricinoleic methyl ester as well as methoxysilane functional aniline tetramer (AT) moieties. The samples were fully characterized and their physicochemical, mechanical, electrical, and biological properties were assayed. The biological activity of these dressings against fibroblast cells and couple of microbes was also studied. It was revealed that samples that displayed electroactivity by introduction of AT moieties showed a broad range of antimicrobial activity toward different microorganisms, promising antioxidant (radical scavenging) efficiency and significant activity for stimulation of fibroblast cell growth and proliferation. Meanwhile, these samples showed appropriate tensile strength and ability for maintaining a moist environment over a wound by controlled equilibrium water absorption and water vapor transmission rate. The selected electroactive dressing was subjected to an in vivo assay using a rat animal model and the wound healing process was monitored and compared with analogous dressing without AT moieties. The recorded results showed that the electroactive dressings induced an increase in the rate of wound contraction, promoted collagen deposition, and encouraged vascularization in the wounded area. On the basis of the results of in vitro and in vivo assays, the positive influence of designed dressings for accelerated healing of a wound model was confirmed.