Hydrogen-bonded Polymer Research Articles

Local Constraints on Junctions to Strengthen Near-Infrared Phosphorescence of Organic Dyes.

A strategy involving the effect of the local constraint on junctions for doping-induced phosphorescence was proposed to increase the rigidity of hydrogen-bonded polymer to inhibit the nonradiative decay of the organic phosphorescent dyes and was verified by bromophenol blue (BPB) derivatives as the near-infrared (NIR) phosphorescent dye. It is shown that the effect of local constraints on junctions of β-cyclodextrin in the poly(vinyl alcohol) (PVA-LCPN) matrix can effectively improve the quantum yields of NIR phosphorescence of BPB derivatives. On the basis of the verification and optimization of the system through response surface analysis, the quantum yield of TBPB@PVA-LCPN film based on NIR emission could be increased up to 77% compared with that of TBPB@PVA, reaching 5.3%, and the quantum yield in the NIR region could be improved to 3.6%. The results of response surface analysis are consistent with the phenomenon of our proposed strategy, which can inspire the production of organic materials with NIR RTP emission. Together, this could inform efficient and cheap strategies for increasing the quantum yield of the doping RTP materials.

Linear and nonlinear viscoelasticity of self-associative hydrogen-bonded polymers

We conducted a nonequilibrium dissipative particle dynamics simulation on the linear and nonlinear viscoelastic behaviors of polymer melt associated with hydrogen bonds. The effects of the number of hydrogen bonding groups and the strength of hydrogen bonding on viscoelasticity were examined. The simulation results show that the polymers with strongly associating hydrogen bonds exhibit supramolecular networks, and their motion displays some similarities to that of polymer melts following the Reptation model. The strain hardening is due to the strain-induced stretching of polymer chains in the supramolecular networks. The number and lifetime of hydrogen bonding have a combined impact on the dynamic and linear viscoelastic behaviors of self-associative polymers. The storage moduli increase linearly with increasing the numbers of hydrogen bonding groups as the lifetime of hydrogen bonds is comparable to or larger than the relaxation time of polymer chains. In contrast, hydrogen bonding has a less pronounced influence on the dynamics and viscoelasticity as the lifetime of hydrogen bonds is minimal. The results are closely aligned with the experimental findings and provide a deep insight into the viscoelasticity of self-associative polymers via hydrogen bonds.

A Molecular Replication Process Drives Supramolecular Polymerization.

Supramolecular polymers are materials in which the connections between monomers in the polymer main chain are non-covalent bonds. This area has seen rapid expansion in the last two decades and has been exploited in several applications. However, suitable contiguous hydrogen-bond arrays can be difficult to synthesize, placing some limitations on the deployment of supramolecular polymers. We have designed a hydrogen-bonded polymer assembled from a bifunctional monomer composed of two replicating templates separated by a rigid spacer. This design allows the autocatalytic formation of the polymer main chain through the self-templating properties of the replicators and drives the synthesis of the bifunctional monomer from its constituent components in solution. The template-directed 1,3-dipolar cycloaddition reaction between nitrone and maleimide proceeds with high diastereoselectivity, affording the bifunctional monomer. The high binding affinity between the self-complementary replicating templates that allows the bifunctional monomer to polymerize in solution is derived from the positive cooperativity associated with this binding process. The assembly of the polymer in solution has been investigated by diffusion-ordered NMR spectroscopy. Both microcrystalline and thin films of the polymeric material can be prepared readily and have been characterized by powder X-ray diffraction and scanning electron microscopy. These results demonstrate that the approach described here is a valid one for the construction of supramolecular polymers and can be extended to systems where the rigid spacer between the replicating templates is replaced by one carrying additional function.

Tannic Acid-Stabilized Self-Degrading Temperature-Sensitive Poly(2-n-propyl-2-oxazoline)/Gellan Gum Capsules for Lipase Delivery.

In recent years, stable hydrogen-bonded stimuli-responsive polymer capsules have been receiving great interest for the encapsulation and release of sensitive molecules such as lipase enzymes. Compartmental capsules having a liquid gel core stabilized with temperature-responsive hydrogen-bonded multilayers are advantageous over other conventional systems because of their ability to maintain hydrophilic lipase and other hydrophobic compounds in compatible protected molecular vehicle environments and prolong their native properties, e.g., in the body. In this work, we report a methodology to stabilize an aqueous liquid gellan gum (GG) core in a capsule using neutral and nontoxic building blocks, namely, poly(2-n-propyl-2-oxazoline) (PnPrOx) and tannic acid (TA), to fabricate temperature-responsive capsules, comprising both lipase and hydrophobic oil droplets. The capsules were fabricated by adding GG droplets to a PnPrOx suspension at a temperature (T) higher than its cloud point temperature (TCP). Notably, the formed capsules were not stable in water without TA stabilization via hydrogen bonding. Scanning electron microscopy (SEM) investigations of the GG/building block interphase revealed that the collapsed PnPrOx globules that are present above the TCP stabilized the GG interphase as a Pickering emulsion, while undergoing a configurational transformation into its linear form by interacting with TA in the next step of capsule formation resulting in a smooth PnPrOx/TA capsule wall. The encapsulation efficiencies of the capsules for model fluorescent molecules were found to be 52, 54, and 24% for FITC-dextran, rhodamine, and Nile red, respectively. The stability experiments exhibited swelling and shell thinning at certain locations followed by complete rupture of the capsules at 37 °C, while the capsules were stable for several weeks at temperatures below the TCP of PnPrOx. The capsules were found to be stable in stimulated gastric fluid (SGF) for several hours at 37 °C while successfully releasing the encapsulated lipase and Nile red (model hydrophobic compound) in stimulated intestinal fluid (SIF). The released lipase was found to retain almost 100% of its activity. The reported capsules have high potential for use as carriers for encapsulation and release of a variety of payloads ranging from proteins and vitamin supplements to enzymes and probiotics through the oral route of administration.

Thermoreversible Gelation with Two-Component Mixed Cross-Link Junctions of Variable Multiplicity in Ternary Polymer Solutions.

Theoretical scheme is developed to study thermoreversible gelation interfering with liquid–liquid phase separation in mixtures of reactive f-functional molecules R{A} and g-functional ones R{B} dissolved in a common solvent. Formed polymer networks are assumed to include multiple cross-link junctions containing arbitrary numbers and of functional groups A and B of each species. Sol-gel transition lines and spinodal lines are drawn on the ternary phase plane for some important models of multiple cross-link junctions with specified microscopic structure. It is shown that, if the cross-link structure satisfies a certain simple condition, there appears a special molar ratio of the two functional groups at which gelation takes place with a lowest concentration of the solute molecules, as has been often observed in the experiments. This optimal gelation concentration depends on f and g (functionality) of the solute molecules and the numbers and (multiplicity) of the functional groups in a cross-link junction. For cross-links which allow variable multiplicity, special attention is paid on the perfectly immiscible cross-links leading to interpenetrating polymer networks, and also on perfectly miscible cross-links leading to reentrant sol-gel-sol transition. Results are compared with recent observations on ion-binding polymer solutions, polymer solutions forming recognizable biomolecular complexes, polymer/surfactant mixtures, hydrogen-bonding polymers, and hydrophobically-modified amphiphilic water-soluble polymers.

Photo-responsive Behaviors of Hydrogen-Bonded Polymer Complex Fibers Containing Azobenzene Functional Groups

Photo-responsive Behaviors of Hydrogen-Bonded Polymer Complex Fibers Containing Azobenzene Functional Groups

Transparent and mechanically strong hydrogen-bonded polymer complex elastomers with improved self-healability under ambient conditions

A major challenge in designing self-healing polymeric materials is to combine robust mechanical properties and high healing efficiency. Herein, we report a kind of transparent, hydrogen-bonded polymer complex elastomers with both good mechanical strength and rapid self-healing capability under ambient conditions. Such elastomers were simply fabricated by in situ photoinitiated random copolymerization of acrylic acid (AA) and N-acryloyl-6-aminocaproic acid (A6ACA), in the presence of a linear polyethylene oxide (PEO). By evaporating water molecules from the samples via free drying, the dynamical hydrogen bonding between PEO and poly(A6ACA-co-AA) were substantially formed, which endowed the polymer chains to form physically crosslinked three-dimensional polymer network, as well as the topological chain entanglements. The resulting optically transparent elastomers exhibited both good mechanical strengths (at break) as high as 1.73 MPa, and excellent stretchability that could be stretched up to 24 times their initial length, respectively. Comparing with the elastomers based on PEO and PAA homopolymer without the A6ACA comonomer, the introduction of A6ACA as a comonomer afforded complex elastomers with comparable mechanical strength and, more importantly, significantly improved self-healing efficiency even under ambient conditions, probably due to the significantly decreased glass transition temperature that facilitated chain mobility and reformation of hydrogen bonds. It is therefore believed that the copolymerization strategy offers a practical method to tune the compositions and to tailor the properties of hydrogen-bonded polymer complexes for engineering elastomers with balanced mechanical and self-healing performance under ambient environment for various applications.

Discordant Supramolecular Fibres Reversibly Depolymerised by Temperature and Light.

Synthetic stimuli responsive supramolecular polymers attract increasing interest for their ability to mimic the unique properties of natural assemblies. Here we focus on the well‐studied benzene‐1,3,5‐tricarboxamide (BTA) motif, and substitute it with two (S)‐3,7‐dimethyloctyl groups and an azobenzene photoswitch. We demonstrate the UV (λ=365 nm) induced depolymerisation of the helical hydrogen‐bonded polymers in methylcyclohexane (MCH) through circular dichroism and UV‐vis spectroscopy in dilute solution (15 μm), and NMR and iPAINT super‐resolution microscopy in concentrated solution (300 μm). The superstructure can be regenerated after thermal depolymerization, whilst repeated depolymerisation can be reversed without degradation by irradiating at λ=455 nm. Molecular dynamics simulations show that the most energetically favourable configuration for these polymers in MCH is a left‐handed helical network of hydrogen‐bonds between the BTA cores surrounded by two right‐handed helices of azobenzenes. The responsiveness to two orthogonal triggers across a broad concentration range holds promise for use in, for example, photo‐responsive gelation.

Effective Optical Diffuser Based on Interfacial Hydrogen-Bonding Polymer Complexation

Optical diffusion is an essential photon management process for various photoelectronic systems. This work proposes an approach to fabricate optical diffuser by interfacial hydrogen-bonding polymer...

A hydrogen-bonded polymer constructed from mechanically interlocked, suit[1]ane monomers

A T-shaped 2,4,7-substituted benzimidazolium “axle” with two ester functionalities and a 24-membered crown ether “wheel” with appendages containing terminal olefin groups were threaded — axle through wheel — to form a [2]pseudorotaxane. Grubbs’ ring-closing metathesis (RCM) was then used to form a third loop and create a bicyclic cage that fully encapsulates the axle and permanently interlocks the two molecular components creating a suit[1]ane. There are no bulky groups on the axle to prevent unthreading, but the axle is trapped due to the cage-like nature of the newly created polyether host. After hydrolysis of the esters groups to carboxylic acids, this novel mechanically interlocked molecule (MIM) polymerizes in the solid state. The structure of the resulting supramolecular polymer was determined by single-crystal X-ray diffraction and contains linear one-dimensional tapes of suit[1]ane monomers linked by intermolecular hydrogen bonding between the carboxylic acid groups.

Mimicking Noble Metals Using Hydrogen-Bonded Conducting Polymers for Electrocatalysis

The most active and efficient catalysts for the electrochemical hydrogen evolution reaction rely on noble metals, a fact that increases the cost of producing hydrogen and thereby limits the widespread adoption of this fuel. Here we present metal-free polydopamine and polyguanine as selective organic hydrogen electrocatalysts1–3. The conducting functional polymers incorporate selective hydrogen-affine hydrogen bonds that possess a similar hydrogen binding energies and work function as e.g. platinum. We report the synthesis of hydrogen-selective electrocatalytic polyguanine and polydopamine and demonstrate the enhancement of the rate-determining step in the proton reduction. We further present mechanistic spectral IR-operando studies on the catalytic hydrogen bonded motifs as well as the continuous electrolysis to molecular hydrogen using polyguanine and polydopamine electrodes for several 100 hours without notable degradation.(1) Coskun, H.; Aljabour, A.; Schöfberger, W.; Hinterreiter, A.; Stifter, D.; Sariciftci, N. S.; Stadler, P. Cofunction of Protons as Dopant and Reactant Activate the Electrocatalytic Hydrogen Evolution in Emeraldine‐Polyguanine. Adv. Mater. Interfaces 2019, 1901364 DOI: 10.1002/admi.201901364.(2) Coskun, H.; Aljabour, A.; Uiberlacker, L.; Strobel, M.; Hild, S.; Cobet, C.; Farka, D.; Stadler, P.; Sariciftci, N. S. Chemical Vapor Deposition - Based Synthesis of Conductive Polydopamine Thin-Films. Thin Solid Films 2018, 645 (August 2017), 320–325 DOI: 10.1016/j.tsf.2017.10.063.(3) Coskun, H.; Aljabour, A.; Luna, P. De; Sun, H.; Nishiumi, N.; Yoshida, T.; Koller, G.; Ramsey, M. G.; Greunz, T.; Stifter, D.; Hassel, A. W.; Sariciftci, N. S.; Sargent, E. H.; Stadler, P. Hydrogen-Bonded Polymers Mimic Noble Metal Electrocatalysts. Adv. Mater., submitted.

First-Order and Third-Order Nonlinearities from Medium-Amplitude Oscillatory Shearing of Hydrogen-Bonded Polymers and Other Viscoelastic Materials

Medium-amplitude oscillatory shear experiments are employed to study the supramolecular relaxation modes in hydroxyl-terminated polydimethylsiloxane. For these associating melts, this allowed us to identify the nonlinear rheological signature of hydrogen-bond-mediated structures occurring in addition to those of the covalent chains. First-order and third-order nonlinearity parameters are determined for the supramolecular and chainlike modes to compare them with the predictions of an array of rheological models. Fingerprints of the two supramolecular modes can be recognized in the nonlinear response. By collecting third-order nonlinearity parameters 3Q0 for about 35 materials including polymer melts, polymer solutions, and other viscoelastic fluids, it is demonstrated that the maximum value of 3Q0 correlates with the magnitude of the shear modulus at the frequency at which this maximum occurs. The implications of this experimental finding are discussed.

Hydration States and Blood Compatibility of Hydrogen-Bonded Supramolecular Poly(2-methoxyethyl acrylate).

The practical use of the viscous liquid polymer, poly(2-methoxyethyl acrylate) (PMEA), was expanded from thin films with excellent blood compatibility to thick coatings and free-standing films without essentially sacrificing its blood compatibility. This was undertaken by creating multiple hydrogen-bonding polymer networks by introducing a functional methacrylic monomer bearing a 6-methyl-2-ureido-4[1H]-pyrimidone group in the PMEA backbone via free radical copolymerization. The hydrogen-bonded PMEA (H-PMEA) contained about 6 mol % of the functional monomer in the copolymer. These functional monomers as physical cross-links are distributed in the PMEA matrix with a Tg of -35 °C, making H-PMEA a solid rubber-like material with recoverable tensile strain. Additionally, mechanical tests revealed its tensile strength, and thermogravimetric analyses confirmed its higher thermostability. The dry and hydration states of H-PMEA were assessed by differential scanning calorimetry, contact angle, and atomic force microscopy measurements. Comparison with viscous PMEA was made. For the first time, we included PVC alongside PET, the surface we usually use as a negative control, in the platelet adhesion test with human blood, and found out that 1.5 times more platelets adhered onto the PVC surface than onto the PET surface, while H-PMEA proved to have a clear edge. Thus, H-PMEA may serve as a suitable replacement for polymers in products contacting blood as it shows potential for making free-standing films, thick coatings, implants, and articles with various geometries for the medicinal industry.

A Divergent Alkyne Diol Directs [2 + 2] Photoreactivity in the Solid State: Cocrystal, Supramolecular Catalysis, and Sublimation Effects.

2-butyne-1,4-diol (1,4-bd) is used as a divergent ditopic template that directs trans-1,2-bis (n-pyridyl) ethylene (n,n′-bpe, where n = n′ = 3 or 4) to undergo an intermolecular [2 + 2] photodimerization in the solid state. The components of cocrystals [(1,4-bd)·(4,4′-bpe)]n and [(1,4-bd)·(3,3′-bpe)]n form 1D hydrogen-bonded polymers with n,n′-bpe assembled as infinite parallel stacks. The alkenes undergo [2 + 2] photocycloadditions to form rctt-tetrakis (n-pyridyl) cyclobutane (where n = 3 or 4). We demonstrate that the reactive solid involving 4,4′-bpe exhibits supramolecular catalysis.

Automated measuring of mass transport through synthetic nanochannels functionalized with polyelectrolyte porous networks

Here we demonstrate the fabrication of porous networks through self-assembly of polyelectrolytes inside nanoconfined geometries. For this purpose, nanochannel membranes fabricated through ion track-etching technique are decorated with an alternate layer-by-layer (LbL) assembly of cationic blend of poly(allylamine hydrochloride)/poly(4-vinylpyridine) (PAH/PVP) and anionic poly(acrylic acid) (PAA) polyelectrolytes. The porous network is prepared by first cross-linking the electrostatic element (PAH and PAA), followed by the removal of hydrogen-bonded polymer (PVP) from the multilayers. In membranes with a single nanochannel, the modification process is monitored by measuring changes in electrolyte ion flux, while the membrane is separating the two compartments of an electrochemical cell. In case of LbL assemblies on multiporous membranes, changes in analyte permeation are presented using an automated analysis of mass transport. Significant decrease in ion current/analyte diffusion across the membranes is caused by LbL assemblies, while an increase is noticed upon removal of hydrogen-bonded PVP from the multilayers, indicating the formation of porous networks. We believe that our presented method for the automated recording of analyte transport in combination with a new modification procedure can be potentially applicable in ionic/molecular separation, drug delivery processes, and for the monitoring of triggered releases of drugs.

Crystal structures of an imidazo[1,5-a]pyridinium-based ligand and its (C13H12N3)2[CdI4] hybrid salt.

The monocation product of the oxidative condensation-cyclization between two mol-ecules of pyridine-2-carbaldehyde and one mol-ecule of CH3NH2·HCl in methanol, 2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridinium, was isolated in the presence of metal ions as bis-[2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridin-2-ium] tetra-iodo-cadmate, (C13H12N3)2[CdI4], (I), and the mixed chloride/nitrate salt, bis-[2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridin-2-ium] 1.5-chlor-ide 0.5-nitrate trihydrate, 2C13H12N3 +·1.5Cl-·0.5NO3 -·3H2O, (II). Hybrid salt (I) crystallizes in the space group P21/n with two [L]2[CdI4] mol-ecules in the asymmetric unit related by pseudosymmetry. In the crystal of (I), layers of organic cations and of tetra-halometallate anions are stacked parallel to the ab plane. Anti-parallel L + cations disposed in a herring-bone pattern form π-bonded chains through aromatic stacking. In the inorganic layer, adjacent tetra-hedral CdI4 units have no connectivity but demonstrate close packing of iodide anions. In the crystal lattice of (II), the cations are arranged in stacks propagating along the a axis; the one-dimensional hydrogen-bonded polymer built of chloride ions and water mol-ecules runs parallel to a column of stacked cations.

Functionalization-Directed Stabilization of Hydrogen-Bonded Polymer Complex Fibers: Elasticity and Conductivity

Elastic, repairable and conductive fibers are desirable in the newly emerging field of soft electronic and wearable devices. Here, we design a multifunctional fiber by incorporation of different components to optimize its performance. The combination of the poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO) through hydrogen bonding endows the fiber with high elasticity and repairability. Polydopamine (PDA) significantly increases the stability of the fiber, thus the fiber will not dissolve in alkaline solutions and still keep the repairable ability. The fiber shows a reversible swelling-shrinking property as pH values go up and down. Further, the conductive component, carbon nanotube, is adsorbed at the swelling state and then is fastened with fiber shrinking.

Effect of a Competitive Solvent on Binding Enthalpy and Chain Intermixing in Hydrogen-Bonded Layer-by-Layer Films

This work is focused on the effect of a small-molecule/hydrogen-bonding competitor on the heat of complexation of hydrogen-bonding polymers in aqueous solutions, as well as on deposition of these molecules within layer-by-layer (LbL) films. Specifically, binding between poly(methacrylic acid) (PMAA) and poly(vinylpyrrolidone) (PVP) under acidic conditions in the presence of dimethyl sulfoxide (DMSO) has been explored. Isothermal titration calorimetry showed that with increasing concentration of DMSO in aqueous solution, the positive enthalpy of formation of PMAA/PVP complexes decreased and switched sign at ∼25 vol % DMSO. This change is interpreted as a transition from endothermal association between PMAA and PVP in water, driven by release of solvation water molecules, to exothermic association driven by the enthalpy of interpolymer hydrogen bonding. This switch is interpreted as destruction of polymer hydration shells by DMSO molecules. The content of DMSO also strongly impacted the growth of LbL films. With an increase in DMSO concentration from 0 to 60 vol %, polymer mass deposited within the bilayer increased, while larger DMSO concentrations suppressed film deposition. The effect of DMSO was also revealed in neutron reflectometry measurements, which showed enhanced interpenetration of deuterated PMAA chains within a hydrogenated LbL matrix at increasing concentration of DMSO. The dependence of film intermixing on DMSO content was strongly history dependent. While assembly of LbL films in aqueous solutions followed by exposure to a mixed solvent post-assembly did not significantly influence the internal film structure, the effect of DMSO on film intermixing was strongest when DMSO was present in the film deposition solutions.

Bioadhesive and biodissolvable hydrogels consisting of water‐swellable poly(acrylic acid)/poly(vinylpyrrolidone) complexes

Films that can form bioadhesive hydrogels on wet biotissues absorbing blood or body fluids are useful for medical devices such as hemostats, adhesion barriers, wound dressings, and drug release devices. We focused on a hydrogen-bonding polymer complex consisting of poly(acrylic acid) (PAA) and poly(vinylpyrrolidone) (PVP). PAA is known as a tissue-adhesive polymer. However, simple mixing of aqueous PAA and PVP solutions resulted in the formation of an insoluble nonadhesive precipitate. We developed a novel solid/solution interface complexation method to afford a PAA/PVP complex that forms a strongly bioadhesive hydrogel with low cytotoxicity. The complex hydrogel can be slowly dissociated and dissolved in the body. The formation of the complexes as well as their swelling and degradation behavior depended strongly on the molecular weights and cross-linking densities of the component polymers. When the complex film was applied to a clipped incised jugular vein of a rat, it immediately formed a hydrogel and closed the incision. After removal of the clip, blood flowed through the vessel without any leakage. Application of the complex film to the surface of an incised mouse liver resulted in firm adhesion and the hemorrhage was effectively stopped. Such bioadhesive and biodissolvable materials consisting of low-toxicity synthetic polymers have high potential for implantable medical devices.

Hydrogen-bonded thin films of cellulose ethers and poly(acrylic acid)

Etherification of cellulose can get a series of water soluble derivatives, which can form hydrogen-bonded polymer complex with poly(acrylic acid) (PAA). Three cellulose ethers hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC) and methyl cellulose (MC) are LbL assembled with PAA to fabricate thin films. The hydrogen bonding of the films is characterized with FT-IR. The film growth behaviors are monitored by a quartz crystal microbalance (QCM) and a spectroscopic interferometer, which shows a different growth rate for different system and a strong dependence on pH values. The stability of the films can be increased by heat treatment. Once the cross-linked structure is formed, and the dissolution of film can be prevented even in the high pH value. In addition, the films show good water adsorption ability from environment. Owing to pH responsive behavior and good water adsorption, CE/PAA films can be explored for various applications, especially for biomedical application.