Cross-linked Polyurethane Networks Research Articles

Thermoanalytical studies on cross-linked polyurethane networks: effect of polyol molecular weight and structure of cyclodextrins

Thermoanalytical studies on cross-linked polyurethane networks: effect of polyol molecular weight and structure of cyclodextrins

Thermal conductive enhanced phase change composites with high latent-heat for constant temperature thermal management

Phase change materials (PCMs) can absorb and release heat without the temperature changing to realize the constant temperature thermal management. The low thermal conductivity (K) of PCM molecules leads to slow heat absorbing and releasing. In this study, a polyethylene glycol (PEG) based cross-linked polyurethane network is synthesized to wrap PCMs microcapsules. The composites have a high latent heat (152–164 J g−1) at 38 °C. Carbon fiber (CF) and Al powder with high K is composted to improve the thermal conductivity of the PCMs. After doping with CF, the K increases to two-fold to 0.306 W m−1 K−1. The experimental result reveals that the heat transfer inside the PCMs is improved obviously after doping with CF, which is further verified by a simulation result, and the composites show excellent shape stability and latent heat retention rate (>92 %) under 30 days' cycle test. This thermal conductivity enhanced PCMs composite is also formable which may have the potential applications for the thermal management of electronic devices, such as chips, fast charging, and battery.

Sterically Hindered Oleogel-Based Underwater Adhesive Enabled by Mesh-Tailoring Strategy.

Underwater adhesives hold significant relevance in daily life and numerous industrial applications. Despite considerable efforts, developing scalable, high-performance underwater adhesives through a simple and effective method remains a formidable challenge. This study presents a novel mesh-tailoring strategy for in situ, rapid, and ultrastrong oleogel-based underwater adhesives (OUA), which comprises a highly crosslinked polyurethane network with a matching mesh size (≈2.22nm) that precisely entraps bio-based epoxidized soybean oil (ESO) molecules (≈2.31nm) by steric hindrance effect. This oleogel exhibits unprecedented robust mechanical properties (≈35MPa) and maintains stability under extreme conditions, including high temperatures (100°C), high pressures (30MPa), and immersion in various solvents (water, ethanol, or ESO). In particular, this oleogel displays high hydrophobicity, rapid curing, and strong interface affinity, resulting in ultrahigh underwater adhesion strength (up to 2.13MPa) and exceptional substrate universality. Moreover, the remarkable environmental adaptability and stability of OUA enable its use in harsh aqueous environments, including acidic/alkaline, saline, and extreme temperature solutions. The comprehensive capabilities of the OUA underscore its potential for building underwater structures, repairing leaky containers, and sealing broken submarine pipelines. This research establishes the foundation for the designing of next-generation underwater adhesives and offers fresh perspectives for exploring oleogel-based materials.

Rigid polyurethane foams refined by the lignin oligomers from catalytic upstream biorefining process

Bio-rigid polyurethane foams (RPFs) were synthesized using the poplar sawdust lignin oligomers that were fractionized by catalytic upstream biorefining (CUB) as the substitute of unsustainable petrol-based polyols. On this basis, as the primary hydroxyl group providers for the RPFs, the lignin oligomers were isolated from the crude CUB oils that were obtained using the pulping liquors with different n-propanol/H2O ratios. Results demonstrated the CUB lignin oligomers were generally possessed low molecular weight, good miscibility, and high hydroxyl groups content, irrespective of the pulping liquor constituents, affording to highly cross-linked polyurethane networks. Moreover, the CUB pulping liquor's composition and the lignin oligomer substitution rate were proved as two decisive factors to the morphology and mechanical properties of the resulted RPFs. Compared to other groups, the RPF substituted with the lignin oligomers obtained from R7/3 (refer to n-propanol/H2O of 7/3 (v/v) in the pulping liquor) and a polyol substitution rate of 30% exhibited better mechanical properties and a highly assembled nanosized lignin complex. Additionally, the compressive strength and the apparent density of RPFs were highly dependent on the hydroxyl group content in CUB lignin oligomers, while the inner-molecular bonds such as the syringyl/guaiacyl ratios and β-O-4 bonds were also affected to the compressive strength of RPFs. The RPFs prepared by CUB lignin oligomer in the current work provides a foundation for future valorization of the side-streams from lignin-first biorefining processes.

Insertion of Supramolecular Segments into Covalently Crosslinked Polyurethane Networks towards the Fabrication of Recyclable Elastomers

Insertion of Supramolecular Segments into Covalently Crosslinked Polyurethane Networks towards the Fabrication of Recyclable Elastomers

Enhanced mar/scratch resistance in automotive clear coatings by modifying crosslinked polyurethane network with branched flexible oligomers

Mar and scratch resistance are a major concern for automotive and other industrial sectors from causing inconvenient aesthetic changes on clear coats. Then, a series of two-component solvent-borne polyurethane coatings were formulated with branched polycaprolactone (PCL) oligomers. The effect of the partial substitution of acrylic polyols by tri- and tetra-functional PCL polyols on adhesion, chemical resistance, thermo-mechanical properties and aesthetic response was analysed. The PCL polyols drove to plasticised networks with lower glass transition temperature (Tg) and promoted rubbery state at room temperature. The associated growth in chain mobility did not appreciably affect the key performance of the coatings, apart from the chemical resistance of formulations with PCL triols. Furthermore, remarkable improvement of wear resistance from promoted recovery response was demonstrated. The clear coats with moderated content of PCL polyols yielded a reduced loss of specular gloss with time after repeated scrubbing operations by an abrasive paper. The proposed approach, thus, is presented as a versatile strategy to tailor the mechanical surface properties of clear coats.

Lipase-Catalyzed Transamidation of Urethane-Bond-Containing Ester.

Significant improvement in mechanical properties and shape recovery in polyurethanes can be obtained by cross-linking, usually performed in a traditional chemical fashion. Here, we report model studies of enzymatic transamidations of urethane-bond-containing esters to study the principles of an enzymatic build-up of covalent cross-linked polyurethane networks via amide bond formation. The Lipase-catalyzed transamidation reaction of a urethane-bond-containing model ester ethyl 2-(hexylcarbamoyloxy)propanoate with various amines is discussed. A side product was formed, that could be successfully identified, and its synthesis reduced to a minimum (<1%). Furthermore, a noncatalyzed transamidation that is performed without CalB as the catalyst could be observed. Both observations are due to the known high reactivity of amines with urethane bonds.

In‐situ Robust Polymeric Networks Prepared via Facile Uncatalyzed Huisgen Cycloaddition of Alkyne‐terminated Polyurethane with Terminal Azides

AbstractWe carried out catalyst – and solvent‐free Huisgen azide‐alkyne cycloadditon (AAC) to develop the polyurethane‐based networks crosslinked through triazole moieties at chain‐ends. An asymmetrical divalent compound carrying the hydroxyl and electron‐deficient alkyne as terminal groups was newly synthesized to prepare the alkyne‐terminated polymer containing urethane moieties within polymer backbone. Additionally, three kinds of terminal‐azide crosslinkers were introduced to react with alkyne‐terminated polyurethanes without any promoters. Kinetics of the uncatalyzed AAC were monitored by using real‐time FT‐IR technique at 50 °C temperature. The rate of AAC using aromatic azide dipole was faster than other dipoles. Compared to the non‐polyurethane‐based networks crosslinked via either urethane or triazole moieties at chain‐ends, triazole chain‐ends crosslinked polyurethane networks conveyed the excellent mechanical properties. However, thermal properties at low temperature were not as good as those of non‐polyurethane networks. Especially, Tg of polyurethane networks formed by an oligomeric azide crosslinker was much higher than others.

Catalyst-Free Thermoset Polyurethane with Permanent Shape Reconfigurability and Highly Tunable Triple-Shape Memory Performance.

Thermoset shape memory polymer (SMP) with dynamic covalent bonds in the network is a new class of SMPs for which the permanent shape can be reconfigured via topological rearrangement (plasticity). Catalyzed transcarbamoylation has recently been established as an effective exchange reaction for plasticity in cross-linked polyurethane networks. However, ensuring the plasticity severely constrains the network design which adversely affects the ability to tune other classical shape memory properties for practical applications. Facing this new challenge, we design an amorphous polyurethane system for which the cross-linking density can be adjusted in a wide range. We discovered that the use of an aromatic diisocyanate in the synthesis of the polyurethanes facilitates achieving plasticity without requiring any catalyst. The overall network design leads to tunable recovery stress and shape memory transition temperatures without sacrificing the plasticity. The versatility of our polyurethane SMP is further reflected in its triple-shape memory performance. We anticipate that our tunable polyurethanes will benefit a variety of potential SMP device applications.

Effects of guar gum content on structure and properties of multi-crosslinked polyurethane composite films

Films from multi-crosslinked polyurethane with different contents of guar gum (1–10wt%) were prepared through a solution-casting method, followed by a thermal treatment or natural light exposure crosslinking procedure. Acrylic terminated prepolymers were prepared by capping the NCO groups of polyurethane prepolymers with pentaerythritol triacrylate. The effect of the guar gum content on the miscibility, morphology and physical properties of the blend films was investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, surface properties and tensile tests. The results reveal that small guar gum content is well embedded in the crosslinked polyurethane network and forms films with good mechanical properties, thermal behavior and hydrophilic properties. These results prove guar gum to be eco-friendly filler for multi-crosslinked polyurethane network. In the case of polyether urethanes, incorporating up to 3wt% guar gum leads to a slight enhancement of the elongation at break, while for polyester urethanes incorporation up to 3wt% guar gum slight increases stress at break values. Incorporating guar gum into the polyurethane networks leads to an increase in the hydrophilic nature of the polymer blends and improved surface structure. The properties of guar gum – which can be abundantly found in nature – make it desirable for polymer blends for biomedical applications.

Synthesis and characterization of poly(carbonate urethane) networks with shape‐memory properties

AbstractIn this study, a series of shape‐memory polyurethanes were prepared from polycarbonate diol (PCDL) with a molecular weight of 2000, trimethylol propane, and isophorone diisocyanate (IPDI). The properties of crosslinked poly(carbonate urethane) (PCU) networks with various compositions were investigated. The chemical structures and thermal properties were determined with Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. FTIR analysis indicated that PCU had the structures of IPDI and PCDL and the amido formyl ester in polyurethanes. The gel content of PCU showed that PCU could be effectively formed as crosslinked polyurethane networks. The glass‐transition temperatures of the PCU networks increased slightly with decreasing soft‐segment content in the networks. The values of Young's modulus in the networks at 25°C increased with decreasing soft‐segment content, whereas the tensile stress and breaking elongation decreased significantly. PCU showed shape‐memory effects with a high strain fixity rate. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Atmospheric Pressure Liquefaction of Dried Distillers Grains (DDG) and Making Polyurethane Foams from Liquefied DDG

In this study, dried distillers grains (DDG) was liquefied in acidic conditions at atmospheric pressure, and polyurethane foams were subsequently prepared from the liquefied DDG. Liquefaction was examined over a range of conditions including liquefaction time of 1-3 h, temperature of 150-170 degrees C, sulfuric acid (as catalyst) concentration of 1.0-3.0 wt%, and liquefaction solvent (ethylene carbonate) to DDG ratio of 3:1-5:1. The bio-polyols in the liquefied DDG were rich in hydroxyl groups, which can react with methylene diphenyl diisocyanate (MDI) to form cross-linked polyurethane networks. The biodegradability of the prepared polyurethane foams was also evaluated. This study strives to broaden the application of DDG as a feedstock for bio-polyurethane preparation.

Mechanism of photostabilization of perfluoropolyether coatings by hindered amine stabilisers

Perfluoropolyethers show promising properties for coil coating application and a suitable layer is obtained by incorporating them in a crosslinked polyurethane network. However, photo-oxidative stability of urethane linkages is intrinsically low and an appropriate stabiliser must be added to the reactants prior to curing. The N-methyl HAS bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate (Tinuvin765) has been selected for this model study. The chemical structure of the network strongly depends on the curing conditions: coil coating conditions (280 °C/1 min in air) lead to a polyurethane network also containing some urea-bridges and unreacted isocyanate. HAS further modifies the network increasing the urea-bridges at the expenses of the unreacted isocyanate. This kind of network is shown to be intrinsically more resistant to the photo-oxidation than pure polyurethane. In curing conditions the N-methyl HAS is partially transformed into –NH derivatives which are still active antioxidants. During photo-oxidation HAS acts as a radical scavenger but slowly evaporates from the film.

Optical intensity modulator based on a novel electrooptic polymer incorporating high /spl mu//spl beta/ chromophore

We have synthesized a novel electrooptic (EO) polymer based on a high /spl mu//spl beta/ chromophore incorporating tricyanobutadiene acceptors. A crosslinked polyurethane network was also adopted to enhance its thermal stability. In order to find the optimum poling condition for the polymer, the influence of the electric poling profile on optical characteristics such as the EO effect, thermal stability, and damage was investigated. Then a high-speed intensity modulator using the EO polymer was designed and fabricated. The measured half-wave voltage V/sub /spl pi// was 4.5 V at the wavelength of 1.31 /spl mu/m. Accordingly, the achieved EO coefficient r/sub 33/ was as high as 25 pm/V, and the thermal stability of the poled polymer was as high as 95/spl deg/C. Finally, the modulator was successfully operated up to 40 GHz.

Fluorescence energy transfer studies in a cross-linked polyurethane network

A series of cross-linked polyurethane samples, labeled with dyes suitable for fluorescence energy transfer experiments, were prepared (donor, phenanthrene; acceptor, anthracene). Fluorescence decay profiles for these samples were measured as a function of acceptor concentration. These decays obey Förster nonradiative energy transfer kinetics, with an energy transfer critical distance (R0) of 26.7 Å. Fluorescence intensities, calculated from the decays by integrating the decay profiles, also fit the Perrin model, with a quenching radius (Rs) of 25.6 Å. The fluorescence decay profiles were further examined with a distribution analysis method, which also revealed uniformly distributed donors and acceptors in the polymer matrices. Keywords: fluorescence quenching, fluorescence decay, phenanthrene, anthracene, polyurethane.

Degradable polyurethane networks based on d,l-lactide, glycolide, ε-caprolactone, and trimethylene carbonate homopolyester and copolyester triols

Several trifunctional, hydroxy-telechelic polyester and poly(ester-carbonate) homopolymers and copolymers were synthesized by the triol-initiated, ring-opening bulk polymerization of d,l-lactide, glycolide, ε-caprolactone, and/or trimethylene carbonate. The molar compositions of the copolymers were determined from 13C n.m.r. spectra; hydroxy equivalent weights were determined by acetylation titration, and the glass transition temperatures ( T g) and melting points were determined by differential scanning calorimetry (d.s.c.). Crosslinked polyurethane networks were prepared by reacting the hydroxy-telechelic prepolymers with tolylene diisocyanate. Network characterization included determination of sol content by solvent extraction, T g by d.s.c., tensile properties by stress-strain measurements, and degradation properties by hydrolytic degradation studies. Equilibrium swelling results indicated that the per cent weight gains correlated well with the prepolymer hydroxy equivalent weights, with the exception of the poly(glycolide- co-trimethylene carbonate) network. The poly( d,l-lactide) and poly( d,l-lactide- co-trimethylene carbonate) (PLTMC) networks had the highest tensile strengths of 49.60 and 41.27 MPa, respectively, and glass transition temperatures of 51.3 and 21.3°C, respectively. All other networks were highly flexible with tensile strengths of 12 MPa or less. Hydrolytic degradation studies were conducted by placing network samples in a solution of phosphate-buffered saline (pH = 7.4) at 37°C. Weight uptake of buffer and weight loss of the networks were monitored gravimetrically over time. Tensile properties, monitored as a function of degradation time, indicated that the poly(ε-caprolactone- co- d,l-lactide) and PLTMC networks displayed a linear loss of strength with respect to weight during the first 30 days of degradation; the other networks degraded either too slowly or too quickly to establish such a linear relationship.

Autoclavable highly cross-linked polyurethane networks in ophthalmology

Highly cross-linked aliphatic polyurethane networks have been prepared by the bulk step reaction of low molecular weight polyols and hexamethylenediisocyanate (HDI). These polyurethane networks are optically transparent, colourless and autoclavable amorphous glassy thermosets, which are suited for use in ophthalmic applications such as intraocular lenses and keratoprostheses. The properties of these glassy polyurethanes, obtained from the reaction of the low molecular weight polyols triisopropanolamine (TIPA) or tetrakis(2-hydroxypropyl)ethylenediamine (Quadrol) and HDI in stoichiometric proportions, have been investigated in more detail. The glassy Quadrol/HDI-based polyurethane exhibits a reduction in ultimate glass transition temperature from 85 to 48 C by uptake of 1% of water, and good ultimate mechanical properties (tensile strength 80–85 MPa, elongation at break ca 15%, modulus ca 1.5 GPa). IR spectra of these hydrophobic polyurethane networks revealed the absence of an isocyanate absorption, indicating that all isocyanates, apparently, had reacted during the crosslinking reaction. The biocompatibility could be increased by grafting tethered polyacrylamide chains onto the surface during network formation. These transparent cross-linked polyurethanes did not transmit UV light up to 400 nm, by incorporation of a small amount of the UV absorbing chromophore Coumarin 102, and could be sterilized simply by autoclaving. They were implanted in rabbit eyes, either in the form of small circular disks or in the form of a keratoprosthesis (artificial cornea). It was shown that the material was well tolerated by the rabbit eyes. Serious opacification of the cornea, a direct result of an adverse reaction to the implant, was never seen. Even 1 yr after implantation of a polyurethane keratoprosthesis the eye was still ‘quiet’.

Hydroxy‐terminated poly(ε‐caprolactone‐co‐δ‐valerolactone) oligomers: Synthesis, characterization, and polyurethane network formation

AbstractDifunctional hydroxy‐terminated poly(ε‐caprolactone‐co‐ε‐valerolactone) (PCV) oligomers were synthesized by the diol‐initiated bulk copolymerization of ε‐caprolactone (C) and δ‐valerolactone (V). The two homopolymers were semicrystalline, with almost identical melting temperatures; copolymerization significantly lowered the melting point (Tm) compared to either homopolymer. Copolymer melting points were found to decrease with decreasing molecular weight and to be dependent on composition, i.e., the incorporation of a comonomer into either homopolymer resulted in a decrease in Tm, with the maximum decrease occurring at a copolymer composition of about 60 mol % ε‐caprolactone. The molar compositions of the copolyesters were determined from 13C‐NMR spectra. The reactivity ratios of the two monomers (M1 = C, M2 = V) were determined to the r1 = 0.25 and r2 = 0.49. Number average molecular weight (M̄n) of the PCV diols was inversely proportional to the initial diol concentration within the studied molecular weight range of 900 to 11,100 g/mol. Crosslinked polyurethane networks were prepared by reacting PCV diols with triphenylmethane triisocyanate. Network characterization included determination of sol content by solvent extraction, glass transition (Tg) and Tm by DSC, and tensile properties by stress‐strain measurements. Completely amorphous networks resulted from PCV diols of M̄n ≤ 2,400; semicrystalline networks resulted from PCV diols of M̄n ≥ 3,600.