RNA-inspired phosphate diester dynamic covalent networks

Neighboring group assisted rearrangement substantially increases relaxation rates in dynamic covalent networks, allowing easier (re)processing of these materials. In this work, we introduce a dynamic covalent network with anionic phosphate diesters as the sole dynamic group, incorporating β-hydroxy groups as a neighboring group, mimicking the self-cleaving backbone structure of RNA. The diester-based networks have slightly slower dynamics, but significantly better hydrolytic (and thermal) stability than analogous phosphate triester-based networks. Catalysis by the β-hydroxy group is vital for fast network rearrangement to occur, while the nature of the counterion has a negligible effect on the relaxation rate. Variable temperature 31P solid-state NMR demonstrated a dissociative bond rearrangement mechanism to be operative.

Benzene Tetraamide: A Covalent Supramolecular Dual Motif in Dynamic Covalent Polymer Networks

In dynamic polyamide networks, 1,2,4,5-benzene tetraamide (B4A) units act simultaneously as a dynamic covalent cross-linker and as supramolecular stacking motif. This results in materials with a rubbery plateau modulus that is about 20 times higher than that of a corresponding reference network in which the supramolecular interaction is suppressed. In branched polyamides with the same B4A dynamic motif, hydrogen bonding and stacking lead to strong and reversible supramolecular networks, whereas a branched polyamide with the nonstacking reference linker is a viscous liquid under the same conditions. Wide-angle X-ray scattering and variable-temperature infrared experiments confirm that covalent cross-linking and stacking cooperatively contribute to the dynamics of the network. Stress relaxation in the reference network is dominated by a single mode related to the dynamic covalent chemistry, whereas relaxation in the B4A network has additional modes assigned to the stacking dynamics.

Hepatic Spheroid Formation on Carbohydrate-Functionalized Supramolecular Hydrogels

Two synthetic supramolecular hydrogels, formed from bis-urea amphiphiles containing lactobionic acid (LBA) and maltobionic acid (MBA) bioactive ligands, are applied as cell culture matrices in vitro. Their fibrillary and dynamic nature mimics essential features of the extracellular matrix (ECM). The carbohydrate amphiphiles self-assemble into long supramolecular fibers in water, and hydrogels are formed by physical entanglement of fibers through bundling. Gels of both amphiphiles exhibit good self-healing behavior, but remarkably different stiffnesses. They display excellent bioactive properties in hepatic cell cultures. Both carbohydrate ligands used are proposed to bind to asialoglycoprotein receptors (ASGPRs) in hepatic cells, thus inducing spheroid formation when seeding hepatic HepG2 cells on both supramolecular hydrogels. Ligand nature, ligand density, and hydrogel stiffness influence cell migration and spheroid size and number. The results illustrate the potential of self-assembled, carbohydrate-functionalized hydrogels as matrices for liver tissue engineering.

Synthesis of polyisocyanurate prepolymer and the resulting flexible elastomers with tunable mechanical properties

Polyurethane (PU) is used in a wide range of applications due to its diverse chemical and physical properties. To meet the increasing demands on thermal and mechanical properties of PU...

Towards high-performance polyurethanes: a mechanism of amine catalyzed aromatic imide formation from the reaction of isocyanates with anhydrides

The mechanism of aromatic imide formation from reaction between isocyanate and anhydride using secondary amine as a pre-catalyst is driven by deprotonated urea, which is obtained from the reaction of isocyanate and amine with the help of bases.

Photo-Crosslinked Coumarin-Containing Bis-Urea Amphiphile Hydrogels

The design of photo-responsive supramolecular hydrogels based on coumarin dimerization and de-dimerization is described. The photo-responsive coumarin unit is chemically incorporated into an oligo(ethylene glycol) (OEG) bis-urea amphiphile that is capable of co-assembling with non-functionalized OEG amphiphile, to form supramolecular fibers. UV light with two different wavelengths (365 nm and 254 nm) is employed to induce a photo-reversible dimerization and de-dimerization process of coumarin units, respectively. The co-assembled solutions could be photo-crosslinked to induce a sol-to-gel transition through dimerization of coumarin with 365 nm UV light, and de-dimerization occurs with 254 nm UV light, to provide a weaker gel. In this system, the mechanical strength of supramolecular hydrogels can be tuned using the irradiation time, providing precise control of gelation in a supramolecular hydrogelator.

Polymer Mechanochromism from Force-Tuned Excited-State Intramolecular Proton Transfer

Real-time monitoring of strain/stress in polymers is a big challenge to date. Herein, we for the first time report an ESIPT (excited-state intramolecular proton transfer)-based mechanochromic mechanophore (MM). The synthesis of target MM PhMz-4OH [(2-hydroxyphenyl)benzimidazole with four aliphatic hydroxyls] is quite facile. PhMz-4OH possesses characteristic dual emissions, and its ESIPT activity is greatly affected by steric hindrance. Then, PhMz-4OH was covalently linked into polyurethane chains (PhMz-4OH@PU). Upon stretching, the PhMz-4OH@PU films showed fluorescence color change and spectral variation with the increase in enol emission and blueshift of keto emission due to the force-induced torsion of the dihedral angle between the proton donor and the proton acceptor. The PhMz-4OH@PU films with high mechanophore concentrations (>0.36 mol %) might undergo a two-stage force-responsive process, including torsion of the dihedral angle via force-induced disaggregation and direct chain-transduced force-induced torsion of the dihedral angle. The intensity ratio of enol emission to keto emission (I_E/I_K) shows a quantitative correlation with elongation, and real-time strain sensing is achieved. PhMz-4OH is a successful type II MM (without covalent bond scission) and displays high sensitivity and excellent reversibility to stress. Two control structures PhMz-NH₂ and PhMz-2OH were also embedded into PU but no spectral or color changes were detected, further confirming that mechanochromism of PhMz-4OH@PU films arises from the chain-transduced force. Density function theory (DFT) calculation was performed to study the force-tuned ESIPT process theoretically and rationalize the experimental results. This study might lay the foundation for real-time stress/strain sensing in practical applications.

Dynamic Polyamide Networks via Amide-Imide Exchange

The diamide-imide equilibrium was successfully exploited for the synthesis of dynamic covalent polymer networks in which a dissociative bond exchange mechanism leads to high processibility at temperatures above ≈110 °C. Dynamic covalent networks bridge the gap between thermosets and thermoplastic polymers. At the operating temperature, when the network is fixed, dynamic covalent networks are elastic solids, while at high temperatures, chemical exchange reactions turn the network into a processible viscoelastic material. Upon heating a dissociative network, the viscosity may also decrease due to a shift of the chemical equilibrium; in such materials, the balance between processibility and excessive flow is important. In this study, a network is prepared that upon heating to above ≈110 °C dissociates to a significant extent due to a shift in the amide-imide equilibrium of a bisimide, pyromellitic diimide, in combination with poly(tetrahydrofuran) diamines. At room temperature, the resulting materials are elastic rubbers with a tensile modulus of 2-10 MPa, and they become predominantly viscous above a temperature of approximately 110 °C, which is dependent on the stoichiometry of the components. The diamide-imide equilibrium was studied in model reactions with NMR, and variable temperature infrared (IR) spectroscopy was used to investigate the temperature dependence of the equilibrium in the network. The temperature-dependent mechanical properties of the networks were found to be fully reversible, and the material could be reprocessed several times without loss of properties such as modulus or strain at break. The high processibility of these networks at elevated temperatures creates opportunities in additive manufacturing applications such as selective laser sintering.

Efficient Exchange in a Bioinspired Dynamic Covalent Polymer Network via a Cyclic Phosphate Triester Intermediate

Bond exchange via neighboring group-assisted reactions in dynamic covalent networks results in efficient mechanical relaxation. In Nature, the high reactivity of RNA toward nucleophilic substitution is largely attributed to the formation of a cyclic phosphate ester intermediate via neighboring group participation. We took inspiration from RNA to develop a dynamic covalent network based on β-hydroxyl-mediated transesterifications of hydroxyethyl phosphate triesters. A simple one-step synthetic strategy provided a network containing phosphate triesters with a pendant hydroxyethyl group. 31P solid-state NMR demonstrated that a cyclic phosphate triester is an intermediate in transesterification, leading to dissociative network rearrangement. Significant viscous flow at 60-100 °C makes the material suitable for fast processing via extrusion and compression molding.

Elucidating dynamic behavior of synthetic supramolecular polymers in water by hydrogen/deuterium exchange mass spectrometry

A comprehensive understanding of the structure, self-assembly mechanism, and dynamics of one-dimensional supramolecular polymers in water is essential for their application as biomaterials. Although a plethora of techniques are available to study the first two properties, there is a paucity in possibilities to study dynamic exchange of monomers between supramolecular polymers in solution. We recently introduced hydrogen/deuterium exchange mass spectrometry (HDX-MS) to characterize the dynamic nature of synthetic supramolecular polymers with only a minimal perturbation of the chemical structure. To further expand the application of this powerful technique some essential experimental aspects have been reaffirmed and the technique has been applied to a diverse library of assemblies. HDX-MS is widely applicable if there are exchangeable hydrogen atoms protected from direct contact with the solvent and if the monomer concentration is sufficiently high to ensure the presence of supramolecular polymers during dilution. In addition, we demonstrate that the kinetic behavior as probed by HDX-MS is influenced by the internal order within the supramolecular polymers and by the self-assembly mechanism.

Correction: Mechanochemical tools for polymer materials

Correction for 'Mechanochemical tools for polymer materials' by Yinjun Chen et al., Chem. Soc. Rev., 2021, 50, 4100-4140, DOI: 10.1039/D0CS00940G.

Role of Acetate Anions in the Catalytic Formation of Isocyanurates from Aromatic Isocyanates

The formation of isocyanurates via cyclotrimerization of aromatic isocyanates is widely used to enhance the physical properties of a variety of polyurethanes. The most commonly used catalysts in industries are carboxylates for which the exact catalytically active species have remained controversial. We investigated how acetate and other carboxylates react with aromatic isocyanates in a stepwise manner and identified that the carboxylates are only precatalysts in the reaction. The reaction of carboxylates with an excess of aromatic isocyanates leads to irreversible formation of corresponding deprotonated amide species that are strongly nucleophilic and basic. As a result, they are active catalysts during the nucleophilic anionic trimerization, but can also deprotonate urethane and urea species present, which in turn catalyze the isocyanurate formation. The current study also shows how quantum chemical calculations can be used to direct spectroscopic identification of reactive intermediates formed during the active catalytic cycle with predictive accuracy.

Nanoporous Films with Photoswitchable Absorption Kinetics Based on Polymerizable Columnar Discotic Liquid Crystals

A photoresponsive nanoporous polymer film has been produced from the templated self-assembly of a columnar liquid crystal containing azo units. A liquid crystalline complex of polymerizable azobenzoic acid and a tris-benzimidazolyl benzene template molecule was cross-linked via thiol-ene radical copolymerization with dodecanedithiol. Subsequent removal of the template yielded nanoporous polymer films with pores of approximately 1 nm in diameter. Both trans-cis and cis-trans photoisomerizations of azobenzoic acid took place in the porous films. At room temperature, the cis isomer was sufficiently long-lived to establish a difference in dye absorption kinetics of the two isomers. The cationic dye rhodamine 6G was bound to both isomers, but the rate of binding to films enriched in the cis isomer was 8 times faster.

Reversible crosslinking and fast stress relaxation in dynamic polymer networks via transalkylation using 1,4-diazabicyclo[2.2.2] octane

A dynamic covalent network using transalkylation of benzyl-DABCO crosslinkers features fast relaxation with a very strong temperature dependence. The network is de-crosslinked by an excess of DABCO.

Mechanochemical tools for polymer materials

This review aims to provide a field guide for the implementation of mechanochemistry in synthetic polymers by summarizing the molecules, materials, and methods that have been developed in this field.

Phosphate Triester Dynamic Covalent Networks

Dynamic covalent networks are a class of polymeric materials that combine the merits of classical thermosets and thermoplastics, in terms of mechanical properties and reprocessability, in one material. Various dynamic covalent chemistries have thus been implemented in polymeric materials with recent interests shifting toward chemistries that would allow rearrangements in network topology without the aid of external catalysts. Here we introduce transesterification in phosphate triesters as a new dynamic covalent chemistry in polymeric networks. A simple one-step synthetic strategy has been utilized to synthesize polytetrahydrofuran networks with phosphate triester cross-links. The materials showed finite viscous flow at elevated temperatures via transesterification at the cross-links without externally added catalyst. This approach provides an easy method for cross-linking OH-end-functionalized polyethers and has the potential for general use with other OH-functionalized polymers.

Intramolecularly Catalyzed Dynamic Polyester Networks Using Neighboring Carboxylic and Sulfonic Acid Groups

Dynamic covalent bonds in a polymer network lead to plasticity, reshapability, and potential recyclability at elevated temperatures in combination with solvent-resistance and better dimensional stability at lower temperatures. Here we report a simple one-step procedure for the catalyst-free preparation and intramolecularly catalyzed stress-relaxation of dynamic polyester networks. The procedure is based on the coupling of branched OH-end functional polyesters (functionality ≥ 3) by pyromellitic dianhydride (PMDA) or 2,5-bis(methoxy-carbonyl) benzenesulfonic acid resulting in ester linkages with, respectively, a COOH or a SO₃H group in a position ortho to the ester bond. This approach leads to an efficient external catalyst-free dynamic polyester network, in which the topology rearrangements occur via a dissociative mechanism involving anhydrides. The SO₃H-containing network is particularly interesting, as it shows the fastest stress relaxation and does not suffer from unwanted additional transesterification reactions, as was observed in the COOH-containing network.

Fractography of poly(N-isopropylacrylamide) hydrogel networks crosslinked with mechanofluorophores using confocal laser scanning microscopy

Employing mechanofluorophores in polymer fractography to obtain new information on force-induced events when analyzed by confocal laser scanning microscopy.

Mimicking Active Biopolymer Networks with a Synthetic Hydrogel

Stiffening due to internal stress generation is of paramount importance in living systems and is the foundation for many biomechanical processes. For example, cells stiffen their surrounding matrix by pulling on collagen and fibrin fibers. At the subcellular level, molecular motors prompt fluidization and actively stiffen the cytoskeleton by sliding polar actin filaments in opposite directions. Here, we demonstrate that chemical cross-linking of a fibrous matrix of synthetic semiflexible polymers with thermoresponsive poly( N-isopropylacrylamide) (PNIPAM) produces internal stress by induction of a coil-to-globule transition upon crossing the lower critical solution temperature of PNIPAM, resulting in a macroscopic stiffening response that spans more than 3 orders of magnitude in modulus. The forces generated through collapsing PNIPAM are sufficient to drive a fluid material into a stiff gel within a few seconds. Moreover, rigidified networks dramatically stiffen in response to applied shear stress featuring power law rheology with exponents that match those of reconstituted collagen and actomyosin networks prestressed by molecular motors. This concept holds potential for the rational design of synthetic materials that are fluid at room temperature and rapidly rigidify at body temperature to form hydrogels mechanically and structurally akin to cells and tissues.

Suppressing depolarization by tail substitution in an organic supramolecular ferroelectric

Despite being very well established in the field of electro-optics, ferroelectric liquid crystals so far lacked interest from a ferroelectric device perspective due to a typically high operating temperature, a modest remnant polarization and/or poor polarization retention. Here, we experimentally demonstrate how simple structural modification of a prototypical ferroelectric liquid-crystal benzene-1,3,5-trisamide (BTA) - introduction of branched-tail substituents - results in materials with a wide operating temperature range and a data retention time of more than 10 years in thin-film solution-processed capacitor devices at room temperature. The observed differences between linear- and branched-tail compounds are analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. We conclude that morphological factors like improved packing quality and reduced disorder, rather than electrostatic interactions or intra/inter-columnar steric hindrance, underlay the superior properties of the branched-tailed BTAs. Synergistic effects upon blending of compounds with branched and linear side-chains can be used to further improve the materials' characteristics.

Tuning PBT vitrimer properties by controlling the dynamics of the adaptable network

Vitrimers, which form a bridge between thermosets and thermoplastics, are a class of materials with promising opportunities for modern material innovations.

Tracking Local Mechanical Impact in Heterogeneous Polymers with Direct Optical Imaging

Structural heterogeneity defines the properties of many functional polymers and it is often crucial for their performance and ability to withstand mechanical impact. Such heterogeneity, however, poses a tremendous challenge for characterization of these materials and limits our ability to design them rationally. Herein we present a practical methodology capable of resolving the complex mechanical behavior and tracking mechanical impact in discrete phases of segmented polyurethane-a typical example of a structurally complex polymer. Using direct optical imaging of photoluminescence produced by a small-molecule organometallic mechano-responsive sensor we observe in real time how polymer phases dissipate energy, restructure, and breakdown upon mechanical impact. Owing to its simplicity and robustness, this method has potential in describing the evolution of complex soft-matter systems for which global characterization techniques fall short of providing molecular-level insight.

Strain-Stiffening in Dynamic Supramolecular Fiber Networks

The cytoskeleton is a highly adaptive network of filamentous proteins capable of stiffening under stress even as it dynamically assembles and disassembles with time constants of minutes. Synthetic materials that combine reversibility and strain-stiffening properties remain elusive. Here, strain-stiffening hydrogels that have dynamic fibrous polymers as their main structural components are reported. The fibers form via self-assembly of bolaamphiphiles (BA) in water and have a well-defined cross-section of 9 to 10 molecules. Fiber length recovery after sonication, H/D exchange experiments, and rheology confirm the dynamic nature of the fibers. Cross-linking of the fibers yields strain-stiffening, self-healing hydrogels that closely mimic the mechanics of biological networks, with mechanical properties that can be modulated by chemical modification of the components. Comparison of the supramolecular networks with covalently fixated networks shows that the noncovalent nature of the fibers limits the maximum stress that fibers can bear and, hence, limits the range of stiffening.

Physical reality of the Preisach model for organic ferroelectrics

The Preisach model has been a cornerstone in the fields of ferromagnetism and ferroelectricity since its inception. It describes a real, non-ideal, ferroic material as the sum of a distribution of ideal 'hysterons'. However, the physical reality of the model in ferroelectrics has been hard to establish. Here, we experimentally determine the Preisach (hysteron) distribution for two ferroelectric systems and show how its broadening directly relates to the materials' morphology. We connect the Preisach distribution to measured microscopic switching kinetics that underlay the macroscopic dispersive switching kinetics as commonly observed for practical ferroelectrics. The presented results reveal that the in principle mathematical construct of the Preisach model has a strong physical basis and is a powerful tool to explain polarization switching at all time scales in different types of ferroelectrics. These insights lead to guidelines for further advancement of the ferroelectric materials both for conventional and multi-bit data storage applications.

Mechanics of elastomeric molecular composites

A classic paradigm of soft and extensible polymer materials is the difficulty of combining reversible elasticity with high fracture toughness, in particular for moduli above 1 MPa. Our recent discovery of multiple network acrylic elastomers opened a pathway to obtain precisely such a combination. We show here that they can be seen as true molecular composites with a well-cross-linked network acting as a percolating filler embedded in an extensible matrix, so that the stress-strain curves of a family of molecular composite materials made with different volume fractions of the same cross-linked network can be renormalized into a master curve. For low volume fractions (<3%) of cross-linked network, we demonstrate with mechanoluminescence experiments that the elastomer undergoes a strong localized softening due to scission of covalent bonds followed by a stable necking process, a phenomenon never observed before in elastomers. The quantification of the emitted luminescence shows that the damage in the material occurs in two steps, with a first step where random bond breakage occurs in the material accompanied by a moderate level of dissipated energy and a second step where a moderate level of more localized bond scission leads to a much larger level of dissipated energy. This combined use of mechanical macroscopic testing and molecular bond scission data provides unprecedented insight on how tough soft materials can damage and fail.

Carbon Nanotube Reinforced Supramolecular Hydrogels for Bioapplications

Nanocomposite hydrogels based on carbon nanotubes (CNTs) are known to possess remarkable stiffness, electrical, and thermal conductivity. However, they often make use of CNTs as fillers in covalently cross-linked hydrogel networks or involve direct cross-linking between CNTs and polymer chains, limiting processability properties. Herein, nanocomposite hydrogels are developed, in which CNTs are fillers in a physically cross-linked hydrogel. Supramolecular nanocomposites are prepared at various CNT concentrations, ranging from 0.5 to 6 wt%. Incorporation of 3 wt% of CNTs leads to an increase of the material's toughness by over 80%, and it enhances electrical conductivity by 358%, compared to CNT-free hydrogel. Meanwhile, the nanocomposite hydrogels maintain thixotropy and processability, typical of the parent hydrogel. The study also demonstrates that these materials display remarkable cytocompatibility and support cell growth and proliferation, while preserving their functional activities. These supramolecular nanocomposite hydrogels are therefore promising candidates for biomedical applications, in which both toughness and electrical conductivity are important parameters.

In Situ Network Formation in PBT Vitrimers via Processing-Induced Deprotection Chemistry

Although the network dynamics and mechanical properties of poly(butylene terephthalate) vitrimers can to some extent be controlled via chemical and physical approaches, it remains a challenge to be able to process PBT vitrimers with the same processing conditions via, for example, injection molding as neat PBT. Here, it is shown that the use of protected pentaerythritol as a latent cross-linker and the use of a Zn(II) transesterification catalyst allows for the in situ dynamic network formation in PBT during processing, with a delayed onset of gelation. This process can be controlled by adjusting the processing temperature, (protected) cross-linker content, and the type of protection group. This solvent-free deprotection strategy opens the way to high production rates of PBT vitrimer products via injection molding with the combination of low viscosity during processing and vitrimer characteristics in the final product.

Homeotropic Self-Alignment of Discotic Liquid Crystals for Nanoporous Polymer Films

Nanostructured polymer films with continuous, membrane-spanning pores from polymerizable hexagonal columnar discotic liquid crystals (LCs) were fabricated. A robust alignment method was developed to obtain homeotropic alignment of columns between glass surfaces by adding a small amount of a tri(ethylene glycol) modified analogue of the mesogen as a dopant that preferentially wets glass. The homeotropic LC alignment was fixated via a photoinitiated free radical copolymerization of a high-temperature tolerant trisallyl mesogen with a divinyl ester. Removal of the hydrogen-bonded template from the aligned columns afforded a nanoporous network with pores of nearly 1 nm in diameter perpendicular to the surface, and without noticeable collapse of the nanopores. The effect of pore orientation was demonstrated by an adsorption experiment in which homeotropic film showed a threefold increase in the initial uptake rate of methylene blue compared to planarly aligned films.

Effects of Surfactant and Urea on Dynamics and Viscoelastic Properties of Hydrophobically Assembled Supramolecular Hydrogel

Physically associated hydrogels based on strong hydrophobic interactions often have attractive mechanical properties that combine processability with elasticity. However, there is a need to study such interactions and understand their relation to the macroscopic hydrogel properties. Therefore, we use the surfactant sodium dodecyl sulfate (SDS) and urea as reagents that disrupt hydrophobic interactions. The model hydrogel is based on a segmented copolymer between poly(ethylene glycol) (PEG) and hydrophobic dimer fatty acid (DFA). We show that both agents influence viscoelastic properties, dynamics, and relaxation processes of the model hydrogel. In particular, the relaxation time is significantly reduced by urea, as compared to SDS, whereas the surfactant causes a decrease of the modulus of the hydrogel more efficiently. The reversibility of the effects of SDS and urea can be exploited, for instance, by using an injectable sol that solidifies when the SDS or urea diffuses out of the sample. Surfactant-induced processability may be advantageous in future applications of hydrophobically assembled physical hydrogels.

True ferroelectric switching in thin films of trialkylbenzene-1,3,5-tricarboxamide (BTA)

We have investigated the ferroelectric polarization switching properties of trialkylbenzene-1,3,5-tricarboxamide (BTA), which is a model system for a large class of novel organic ferroelectric materials. In the solid state BTAs form a liquid crystalline columnar hexagonal phase that provides long range order that was previously shown to give rise to hysteretic dipolar switching. In this work the nature of the polar switching process is investigated by a combination of dielectric relaxation spectroscopy, depth-resolved pyroelectric response measurements, and classical frequency- and time-dependent electrical switching. We show that BTAs, when brought in a homeotropically aligned hexagonal liquid crystalline phase, are truly ferroelectric. Analysis of the transient switching behavior suggests that the ferroelectric switching is limited by a highly dispersive nucleation process, giving rise to a wide distribution of switching times.

Stress-induced colouration and crosslinking of polymeric materials by mechanochemical formation of triphenylimidazolyl radicals

Under mechanical stress, the hexaarylbiimidazole (HABI) motif can cleave to triphenylimidazolyl radicals when incorporated into a polymer matrix. The mechanically produced coloured radicals can initiate secondary radical reactions yielding polymer networks. Thus, the HABI mechanophore combines optical reporting of bond scission and reinforcement of polymers in a single molecular moiety.

Pore size dependent cation adsorption in a nanoporous polymer film derived from a plastic columnar phase

Nanoporous polymer films were fabricated from a plastic columnar phase that showed pore size and shape (whether open or collapsed) dependent cation adsorption properties.

Strain Stiffening Hydrogels through Self-Assembly and Covalent Fixation of Semi-Flexible Fibers

Biomimetic, strain-stiffening materials are reported, made through self-assembly and covalent fixation of small building blocks to form fibrous hydrogels that are able to stiffen by an order of magnitude in response to applied stress. The gels consist of semi-flexible rodlike micelles of bisurea bolaamphiphiles with oligo(ethylene oxide) (EO) outer blocks and a polydiacetylene (PDA) backbone. The micelles are fibers, composed of 9-10 ribbons. A gelation method based on Cu-catalyzed azide-alkyne cycloaddition (CuAAC), was developed and shown to lead to strain-stiffening hydrogels with unusual, yet universal, linear and nonlinear stress-strain response. Upon gelation, the X-ray scattering profile is unchanged, suggesting that crosslinks are formed at random positions along the fiber contour without fiber bundling. The work expands current knowledge about the design principles and chemistries needed to achieve fully synthetic, biomimetic soft matter with on-demand, targeted mechanical properties.

Tough Supramolecular Hydrogel Based on Strong Hydrophobic Interactions in a Multiblock Segmented Copolymer

We report the preparation and structural and mechanical characterization of a tough supramolecular hydrogel, based exclusively on hydrophobic association. The system consists of a multiblock, segmented copolymer of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dimer fatty acid (DFA) building blocks. A series of copolymers containing 2K, 4K, and 8K PEG were prepared. Upon swelling in water, a network is formed by self-assembly of hydrophobic DFA units in micellar domains, which act as stable physical cross-link points. The resulting hydrogels are noneroding and contain 75-92 wt % of water at swelling equilibrium. Small-angle neutron scattering (SANS) measurements showed that the aggregation number of micelles ranges from 2 × 102 to 6 × 102 DFA units, increasing with PEG molecular weight. Mechanical characterization indicated that the hydrogel containing PEG 2000 is mechanically very stable and tough, possessing a tensile toughness of 4.12 MJ/m3. The high toughness, processability, and ease of preparation make these hydrogels very attractive for applications where mechanical stability and load bearing features of soft materials are required.

Tailoring Pore Size and Chemical Interior of near 1 nm Sized Pores in a Nanoporous Polymer Based on a Discotic Liquid Crystal

A triazine based disc shaped molecule with two hydrolyzable units, imine and ester groups, was polymerized via acyclic diene metathesis in the columnar hexagonal (Colhex) LC phase. Fabrication of a cationic nanoporous polymer (pore diameter ∼1.3 nm) lined with ammonium groups at the pore surface was achieved by hydrolysis of the imine linkage. Size selective aldehyde uptake by the cationic porous polymer was demonstrated. The anilinium groups in the pores were converted to azide as well as phenyl groups by further chemical treatment, leading to porous polymers with neutral functional groups in the pores. The pores were enlarged by further hydrolysis of the ester groups to create ∼2.6 nm pores lined with -COONa surface groups. The same pores could be obtained in a single step without first hydrolyzing the imine linkage. XRD studies demonstrated that the Colhex order of the monomer was preserved after polymerization as well as in both the nanoporous polymers. The porous anionic polymer lined with -COOH groups was further converted to the -COOLi, -COONa, -COOK, -COOCs, and -COONH4 salts. The porous polymer lined with -COONa groups selectively adsorbs a cationic dye, methylene blue, over an anionic dye.

Mechanoluminescent Imaging of Osmotic Stress-Induced Damage in a Glassy Polymer Network

A chemiluminescent mechanophore, bis(adamantyl-1,2-dioxetane), is used to investigate the covalent bond scission resulting from the sorption of chloroform by glassy poly(methyl methacrylate) (PMMA) networks. Bis(adamantyl)-1,2-dioxetane units incorporated as cross-linkers underwent mechanoluminescent scission, demonstrating that solvent ingress caused covalent bond scission. At higher cross-linking densities, the light emission took the form of hundreds of discrete bursts, widely varying in intensity, with each burst composed of 107-109 photons. Camera imaging indicated a relatively slow propagation of bursts through the material and permitted analysis of the spatial correlation between the discrete bond-breaking events. The implications of these observations for the mechanism of sorption and fracture are discussed.