Electro‐conductive double‐network hydrogels

Novel electro‐conductive and mechanically‐tough double network polymer hydrogels (E‐DN gels) were synthesized by polymerization of 3, 4‐ethylenedioxythiophene in the presence of a double network hydrogel (DN gel) matrix. The E‐DN gels showed not only excellent mechanical performance, having a fracture stress of 1.4–2.1 MPa, but also electrical conductivity as high as 10^?3 S cm^?1, both under dry and water‐swollen states. The fracture stress and fracture energy of the E‐DN gel was increased by 1.7 and 3.4 times, respectively, as compared with the DN gel. From scanning electron microscope and AFM observations, it was found that electro‐conductive poly(3,4‐ethylenedioxythiophene) (PEDOT) was incorporated into DN gel matrix, apparently due to the formation of a poly‐ion complex with sulfonic acid group of the DN gel network. Thus, PEDOT incorporated into the DN gel matrix greatly improves not only electronic conductivity, but also mechanical properties, reinforcing the double network gel matrix. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Phenylboronate‐diol crosslinked polymer/SWCNT hybrid gels with reversible sol–gel transition

This work demonstrates the successful incorporation of functionalized single‐walled carbon nanotubes (f‐SWCNTs) into the phenylboronate‐diol crosslinked polymer gel to create a hybrid system with reversible sol–gel transition. The phenylboronic acid‐containing and diol‐containing polymers were first separately prepared by the reversible addition–fragmentation chain transfer polymerization. Covalent functionalization of single‐walled carbon nanotubes (SWCNTs) with an azide‐derivatized, diol‐containing polymer was then accomplished by a nitrene addition reaction. Subsequently, the hybrid gels were prepared by crosslinking the mixture of f‐SWCNTs and diol‐containing polymer with the phenylboronic acid‐containing polymer. The hybrid gel has been characterized by scanning electron microscopy (SEM) and rheological analysis. The SEM measurement demonstrated a homogeneous dispersion of f‐SWCNTs within the gel matrices. Rheological experiments also demonstrated that the hybrid gel exhibited storage moduli significantly higher than those of the native gel obtained from the phenylboronic acid‐containing and diol‐containing polymers. The hybrid gel can be switched into their starting polymer (f‐SWCNTs) solutions by adjusting the pH of the system. Moreover, the hybrid gel revealed a self‐healing property that occurred autonomously without any outside intervention. By employing this dynamic character, it is possible to regenerate the used gel, and thus, it has the potential to perform in a range of dynamic or bioresponsive applications Copyright © 2013 John Wiley & Sons, Ltd.     

Poly(acrylic acid)‐chitosan @ tannic acid double‐network self‐healing hydrogel based on ionic coordination

In this work, poly(acrylic) acid‐chitosan @ tannic acid‐aluminum ion (PAA‐CS@TA‐Al³⁺) double‐network hydrogel was prepared via prefabrication, blending method, and Al³⁺ immersion method. The interaction between chitosan and tannic acid (CS@TA) was analyzed using Fourier transfer infrared spectra and UV‐Vis spectra. The UV‐Vis spectrum was also used to confirm the formation of ionic coordination in the gel. Then, the possible coordination modes were studied and analyzed. The microscopic pore structure and macroscopic strain behavior of the gel were analyzed using SEM and tensile testing, respectively, which verified that the tensile strength (≈32 KPa) and elongation at break (≈1700%) of the gel primarily resulted from its crosslinking structure. In addition, the gel also demonstrated a good self‐healing performance with recovery ≈92.2% at 60 minutes. Hence, the proposed novel self‐healing gel can provide inspiration for the preparation of future self‐healing gels.     

pH‐Triggered Self‐Assembly of Cellulose Oligomers with Gelatin into a Double‐Network Hydrogel

Multicomponent systems for self‐assembled molecular gels provide huge opportunities to generate collective or new functions that are not inherent in individual single‐component gels. However, gelation tends to require careful and complicated procedures, because, among a myriad of kinetically trapped structures related to the degree of mixing of multiple components over a wide range of scales, from molecular level to macroscopic scale, a limited number of structures that exhibit the desired function need to be constructed. This study presents a simple method for the construction of double‐network (DN) hydrogels with improved stiffness composed of crystalline cellulose oligomers and gelatin. The pH‐triggered self‐assembly of cellulose oligomers leads to the formation of robust networks composed of crystalline nanofibers in the presence of dissolved gelatin, followed by cooling to allow for the formation of soft gelatin networks. The resultant DN hydrogels exhibit improved stiffness; the improvement in gel stiffness with double networking is comparable to that of previously reported DN hydrogels produced via a time‐consuming enzymatic reaction.     

Solvent Controlled Multi‐Morphological Self‐Assembly of Amphiphilic Graft Copolymers

Summary: Amphiphilic graft polyphosphazenes (EtTrp/PNIPAm‐PPP) with different mole ratios of hydrophobic groups to hydrophilic segments were synthesized by ring‐opening polymerization and subsequent substitution reactions. The self‐assembly behavior of these graft copolymers was studied in detail by TEM, SEM, CLSM, and AFM. Depending on the copolymer composition and common organic solvent employed in dialysis process, supramolecular aggregates ranging from network, nanospheres, high‐genus particles to macrophage‐like aggregates were produced with graft copolymers.

     

DNA‐Assisted Self‐Assembly of Pyrene Foldamers

Folding in the tides : Upon hybridization, pyrene molecules assemble through interstrand stacking interactions to form double‐stranded, helical structures. Structural organization of the pyrene molecules is an intrinsic property of the oligoaryl part and takes place independently of the sequence of the attached DNA. Pyrene helicity is most pronounced in a bi‐segmental chimera, in which a DNA stem is present only at one end of the pyrene section.

     

An Injectable Self‐Healing Hydrogel Based on Chain‐Extended PEO‐PPO‐PEO Multiblock Copolymer

Injectable hydrogels have been commonly used as drug‐delivery vehicles and tried in tissue engineering. Injectable self‐healing hydrogels have great advantage over traditional injectable hydrogels because they can be injected as a liquid and then rapidly form bulk gels in situ at the target site under physiological conditions. This study develops an injectable thermosensitive self‐healing hydrogel based on chain‐extended F127 (PEO₉₀‐PPO₆₅‐PEO₉₀) multi‐block copolymer (m‐F127). The rapid sol–gel transition ability under body temperature allows it to be used as injectable hydrogel and the self‐healing property allows it to withstand repeated deformation and quickly recover its mechanical properties and structure through the dynamic covalent bonds. It is hoped that the novel strategy and the fascinating properties of the hydrogel as presented here will provide new opportunities with regard to the design and practical application of injectable self‐healing hydrogels.

     

Self‐Assembly of Amphiphilic Anthracene‐Functionalized β‐Cyclodextrin (CD‐AN) through Multi‐Micelle Aggregation

Multi‐micelle aggregation (MMA) mechanism is widely acknowledged to explicate large spherical micelles self‐assembly, but the process of MMA during self‐assembly is hard to observe. Herein, a novel kind of strong, regular microspheres fabricated from self‐assembly of amphiphilic anthracene‐functionalized β‐cyclodextrin (CD‐AN) via Cu(I)‐catalyzed azide‐alkyne click reactions is reported. The obtained CD‐AN amphiphiles can self‐assemble in water from primary core–shell micelles to secondary aggregates with the diameter changing from several tens nm to around 600–700 nm via MMA process according to the images of scanning electron microscopy, transmission electron microscopy, and atomic force microscopy as well as the dynamic light scattering measurements, followed by further crosslinking through photo‐dimerization of anthracene. What merits special attention is that such photo‐crosslinked self‐assemblies are able to disaggregate reversibly into primary nanoparticles when changing the solution conditions, which is benefited from the designed regular structure of CD‐AN and the rigid ranging of anthracene during assembly, thus confirming the process of MMA.

     

Synthesis and Characterisation of Self‐Assembled and Self‐Adjuvanting Asymmetric Multi‐Epitope Lipopeptides of Ovalbumin

Designing a lipopeptide (LP) vaccine with a specific asymmetric arrangement of epitopes may result in an improved display of antigens, increasing host‐cell recognition and immunogenicity. This study aimed to synthesise and characterise the physicochemical properties of a library of asymmetric LP‐based vaccine candidates that contained multiple CD4⁺ and CD8⁺ T‐cell epitopes from the model protein antigen, ovalbumin. These fully synthetic vaccine candidates were prepared by microwave‐assisted solid phase peptide synthesis. The C12 or C16 lipoamino acids were coupled to the N or C terminus of the OVA CD4 peptide epitope. The OVA CD4 LPs and OVA CD8 peptide constructs were then conjugated using azide–alkyne Huisgen cycloaddition to give multivalent synthetic vaccines. Physiochemical characterisation of these vaccines showed a tendency to self‐assemble in aqueous media. Changes in lipid length and position induced self‐assembly with significant changes to their morphology and secondary structure as shown by transmission electron microscopy and circular dichroism.     

Synthesis and Self‐Assembly of NCN‐Pincer Pd‐Complex‐Bound Norvalines

The NCN‐pincer Pd‐complex‐bound norvalines Boc‐D /L ‐[PdCl(dpb)]Nva‐OMe ( 1 ) were synthesized in multigram quantities. The molecular structure and absolute configuration of 1 were unequivocally determined by single‐crystal X‐ray structure analysis. The robustness of 1 under acidic/basic conditions provides a wide range of N‐/C‐terminus convertibility based on the related synthetic transformations. Installation of a variety of functional groups into the N‐/C‐terminus of 1 was readily carried out through N‐Boc‐ or C‐methyl ester deprotection and subsequent condensations with carboxylic acids, R¹COOH, or amines, R²NH₂, to give the corresponding N‐/C‐functionalized norvalines R¹‐D /L ‐[PdCl(dpb)]Nva‐R² 2 – 9 . The dipeptide bearing two Pd units 10 was successfully synthesized through the condensation of C‐free 1 with N‐free 1 . The robustness of these Pd‐bound norvalines was adequately demonstrated by the preservation of the optical purity and Pd unit during the synthetic transformations. The lipophilic Pd‐bound norvalines L ‐ 2 , Boc‐L ‐[PdCl(dpb)]Nva‐NH‐n‐C₁₁H₂₃, and L ‐ 4 , n‐C₄H₉CO‐L ‐[PdCl(dpb)]Nva‐NH‐n‐C₁₁H₂₃, self‐assembled in aromatic solvents to afford supramolecular gels. The assembled structures in a thermodynamically stable single crystal of L ‐ 2 and kinetically stable supramolecular aggregates of L ‐ 2 were precisely elucidated by cryo‐TEM, WAX, SAXS, UV/Vis, IR analyses, and single‐crystal X‐ray crystallography. An antiparallel β‐sheet‐type aggregate consisting of an infinite one‐dimensional hydrogen‐bonding network of amide groups and π‐stacking of PdCl(dpb) moieties was observed in the supramolecular gel fiber of L ‐ 2 , even though discrete dimers are assembled through hydrogen bonding in the thermodynamically stable single crystal of L ‐ 2 . The disparate DSC profiles of the single crystal and xerogel of L ‐ 2 indicate different thermodynamics of the molecular assembly process.     

Self‐Assembly of a Modified Amyloid Peptide Fragment: pH‐Responsiveness and Nematic Phase Formation

The self‐assembly of peptide YYKLVFFC based on a fragment of the amyloid beta (Aβ) peptide, Aβ16–20, KLVFF has been studied in aqueous solution. The peptide is designed with multiple functional residues to examine the interplay between aromatic interactions and charge on the self‐assembly, as well as specific transformations such as the pH‐induced phenol–phenolate transition of the tyrosine residue. Circular dichroism (CD) and Fourier‐transform infrared (FTIR) spectroscopies are used to investigate the conditions for β‐sheet self‐assembly and the role of aromatic interactions in the CD spectrum as a function of pH and concentration. The formation of well‐defined fibrils at pH 4.7 is confirmed by cryo‐TEM (transmission electron microscope) and negative stain TEM. The morphology changes at higher pH, and aggregates of short twisted fibrils are observed at pH 11. Polarized optical microscopy shows birefringence at a low concentration (1 wt.‐%) of YYKLVFFC in aqueous solution, and small‐angle X‐ray scattering was used to probe nematic phase formation in more detail. A pH‐induced transition from nematic to isotropic phases is observed on increasing pH that appears to be correlated to a reduction in aggregate anisotropy upon increasing pH.

     

Dynamic Formation of Hybrid Peptidic Capsules by Chiral Self‐Sorting and Self‐Assembly

Owing to their versatility and biocompatibility, peptide‐based self‐assembled structures constitute valuable targets for complex functional designs. It is now shown that artificial capsules based on β‐barrel binding motifs can be obtained by means of dynamic covalent chemistry (DCC) and self‐assembly. Short peptides (up to tetrapeptides) are reversibly attached to resorcinarene scaffolds. Peptidic capsules are thus selectively formed in either a heterochiral or a homochiral way by simultaneous and spontaneous processes, involving chiral sorting, tautomerization, diastereoselective induction of inherent chirality, and chiral self‐assembly. Self‐assembly is shown to direct the regioselectivity of reversible chemical reactions. It is also responsible for shifting the tautomeric equilibrium for one of the homochiral capsules. Two different tautomers (keto‐enamine hemisphere and enol‐imine hemisphere) are observed in this capsule, allowing the structure to adapt for self‐assembly.     

Anionic homo‐ and copolymerization of double‐tailed macromonomers: A route to novel macromolecular architectures

The synthesis of model linear and star double‐comb polybutadienes, as well as molecular double‐brush polybutadienes, with two tails emanating from each branch point, is presented. The synthetic approach involves (1) the selective reaction of living polybutadienes with the two chlorines of 4‐(dichloromethylsilyl)styrene to make double‐tailed macromonomers and (2) the homopolymerization or copolymerization of the double‐tailed macromonomer, in situ without isolation, to produce the double brushes and double combs, respectively. The star double comb was synthesized by the reaction of living double‐comb polybutadiene with trichloromethylsilane. Characterization carried out by size exclusion chromatography, with differential refractometer and light scattering detectors, indicated that the synthesized polybutadienes had a high degree of molecular and structural homogeneity. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4070–4078, 2005     

Self‐healing in Nanocomposite Hydrogels

Polymer hydrogels with characteristics distinct from those of solid materials are one of the most promising candidates for smart materials. Here, we report that a nanocomposite hydrogel (NC gel) consisting of a unique polymer/clay network structure, can exhibit complete self‐healing through autonomic reconstruction of crosslinks across a damaged interface. Mechanical damage in NC gels can be repaired without the use of a healing agent, and even sections of NC gels separated by cutting, from whichever the same or different kinds of NC gel, perfectly (re‐)combine by just contacting the cut surfaces together at mildly elevated temperatures. In NC gels, the autonomic fusion of cut surfaces as well as the self‐healing could be achieved not only immediately after being cut but also after a long waiting time.

     

Polymerization and self‐assembly of thermally responsive in‐chain functionalized double‐hydrophilic macromonomers

Double‐hydrophilic in‐chain functionalized macromonomers consisting of poly(N‐isopropylacrylamide) (PNIPAM) and poly(ethylene oxide) were prepared by a multistep procedure including esterification of PNIPAM monoester of maleic acid with α‐methoxy‐ω‐hydroxypolyoxyethylene or its amidation with α‐methoxy‐ω‐aminopolyoxyethylene. The polymerization of the macromonomers was carried out in aqueous solutions. The temperature was the key parameter controlling the polymerization process that was performed in the organized domains formed by the macromonomers below and above the phase transition temperature (T_tr). Polymacromonomers with higher degrees of polymerization were prepared at temperatures just below the T_tr. Static light scattering measurements on dilute aqueous solutions of thermally‐responsive macromonomers and their polymerization products demonstrated that they formed aggregates below the T_tr. Supramolecular structures with low density cores, formed by the polymacromonomers at room temperature, were imaged by SEM. Morphological tuning was achieved by varying both the composition of the copolymer and the concentration of the aqueous solution. The rheological behavior of the polymacromonomers in 25 wt % aqueous solution was compared to that of the respective macromonomers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4720–4732, 2007     

N‐Boc‐Protected 1,2‐Diphenylethylenediamine‐Based Dendritic Organogels with Multiple‐Stimulus‐Responsive Properties

A new class of poly(benzyl ether) dendrimers, decorated in their cores with N‐Boc‐protected 1,2‐diphenylethylenediamine groups, were synthesized and fully characterized. It was found that the gelation capability of these dendrimers was highly dependent on dendrimer generation, and the second‐generation dendrimer (R,R)‐G₂DPENBoc proved to be a highly efficient organogelator. A number of experiments (SEM, TEM, FTIR spectroscopy, ¹H NMR spectroscopy, rheological measurements, UV/Vis absorption spectroscopy, CD, and XRD) revealed that these dendritic molecules self‐assembled into elastically interpenetrating one‐dimensional nanostructures in organogels. The hydrogen bonding, π–π, and solvophobic interactions were found to be the main driving forces for formation of the gels. Most interestingly, these dendritic organogels exhibited smart multiple‐stimulus‐responsive behavior upon exposure to environmental stimuli such as temperature, anions, and mechanical stress.     

Ordered Hybrids from Template‐Free Organosilane Self‐Assembly

Despite considerable achievements over the last two decades, nonporous organic–inorganic hybrid materials are mostly amorphous, especially in the absence of solvothermal processes. The organosilane self‐assembly approach is one of the few opportunities for creating a regular assembly of organic and inorganic moieties. Additionally, well‐established organosilicon chemistry enables the introduction of numerous organic functionalities. The synthesis of periodically ordered hybrids relies on mono‐, bis‐, or multisilylated organosilane building blocks self‐assembling into hybrid mesostructures or superstructures, subsequently cross‐linked by siloxane Si‐O‐Si condensation. The general synthesis procedure is template‐free and one‐step. However, three concurrent processes underlie the generation of self‐organized hybrid networks: thermodynamics of amphiphilic aggregation, dynamic self‐assembly, and kinetically controlled sol–gel chemistry. Hence, the set of experimental conditions and the precursor structure are of paramount importance in achieving long‐range order. Since the first developments in the mid‐1990s, the subject has seen considerable progress leading to many innovative advanced nanomaterials providing promising applications in membranes, pollutant remediation, catalysis, conductive coatings, and optoelectronics. This work reviews, comprehensively, the primary evolution of this expanding field of research.