A Hybrid Supramolecular Polymeric Hydrogel with Rapid Self‐Healing Property

A hybrid supramolecular polymeric hydrogel is conveniently constructed via host–guest interaction of a host cyclodextrin polymer (poly‐CD) with a guest α‐bromonaphthalene polymer (poly‐BrNp) and mixing with 6‐thio‐β‐cyclodextrin (β‐SH‐CD) modified gold nanoparticles (GPCDs) in aqueous solution. According to the dynamic oscillatory data, the hydrogel exhibits markedly enhanced stiffness compared with the GPCD‐free one (both G′ and G“ values are almost twice as high as those of the original GPCD‐free hydrogel) due to the introduction of the inorganic gold nanoparticles. This hybrid supramolecular polymeric hydrogel has a rapid and excellent self‐healing property (only about 1 min, and the G′ and G” of the self‐healed hydrogel almost turned back to their original levels after 1 hour) in air (without adding any solvent or additive).     

Formation of a Polypseudorotaxane via Self‐Assembly of γ‐Cyclodextrin with Poly(N‐isopropylacrylamide)

A polypseudorotaxane (PPR) comprising γ‐cyclodextrin (γ‐CD) as host molecules and poly(N‐isopropylacrylamide) (PNIPAM) as a guest polymer is prepared via self‐assembly in aqueous solution. Due to the bulky pendant isopropylamide group, PNIPAM exhibits size‐selectivity toward self‐assembly with α‐, β‐, and γ‐CDs. It can fit into the cavity of γ‐CD to give rise to a PPR, but cannot pass through α‐CD and β‐CD under the same conditions. The ratio of the number of γ‐CD molecules to entrapped NIPAM repeat units is kept at 1:2.2 or 1:2.4, determined by ¹H NMR spectroscopy and TGA analysis, respectively, indicating that there are more than 2 but less than 3 NIPAM repeat units included by one γ‐CD molecule. This finding opens new avenues to PPR‐based supramolecular polymers to be used as solid, stimuli‐responsive materials.     

Self‐Healing,Expansion–Contraction,and Shape‐Memory Properties of a Preorganized Supramolecular Hydrogel through Host–Guest Interactions

Supramolecular materials cross‐linked between polymer chains by noncovalent bonds have the potential to provide dynamic functions that are not produced by covalently cross‐linked polymeric materials. We focused on the formation of supramolecular polymeric materials through host–guest interactions: a powerful method for the creation of nonconventional materials. We employed two different kinds of host–guest inclusion complexes of β‐cyclodextrin (βCD) with adamantane (Ad) and ferrocene (Fc) to bind polymers together to form a supramolecular hydrogel (βCD‐Ad‐Fc gel). The βCD‐Ad‐Fc gel showed self‐healing ability when damaged and responded to redox stimuli by expansion or contraction. Moreover, the βCD‐Ad‐Fc gel showed a redox‐responsive shape‐morphing effect. We thus succeeded in deriving three functions from the introduction of two kinds of functional units into a supramolecular material.     

Self‐Healing,Expansion–Contraction,and Shape‐Memory Properties of a Preorganized Supramolecular Hydrogel through Host–Guest Interactions

Supramolecular materials cross‐linked between polymer chains by noncovalent bonds have the potential to provide dynamic functions that are not produced by covalently cross‐linked polymeric materials. We focused on the formation of supramolecular polymeric materials through host–guest interactions: a powerful method for the creation of nonconventional materials. We employed two different kinds of host–guest inclusion complexes of β‐cyclodextrin (βCD) with adamantane (Ad) and ferrocene (Fc) to bind polymers together to form a supramolecular hydrogel (βCD‐Ad‐Fc gel). The βCD‐Ad‐Fc gel showed self‐healing ability when damaged and responded to redox stimuli by expansion or contraction. Moreover, the βCD‐Ad‐Fc gel showed a redox‐responsive shape‐morphing effect. We thus succeeded in deriving three functions from the introduction of two kinds of functional units into a supramolecular material.     

Construction of Chemical‐Responsive Supramolecular Hydrogels from Guest‐Modified Cyclodextrins

A methodology for preparing supramolecular hydrogels from guest‐modified cyclodextrins (CDs) based on the host–guest and hydrogen‐bonding interactions of CDs is presented. Four types of modified CDs were synthesized to understand better the gelation mechanism. The 2D ROESY NMR spectrum of β‐CD‐AmTNB (Am=amino, TNB=trinitrobenzene) reveals that the TNB group was included in the β‐CD cavity. Pulsed field gradient NMR (PFG NMR) spectroscopy and AFM show that β‐CD‐AmTNB formed a supramolecular polymer in aqueous solution through head‐to‐tail stacking. Although β‐CD‐AmTNB did not produce a hydrogel due to insufficient growth of supramolecular polymers, β‐CD‐CiAmTNB (Ci=cinnamoyl) formed supramolecular fibrils through host–guest interactions. Hydrogen bonds between the cross‐linked fibrils resulted in the hydrogel, which displayed excellent chemical‐responsive properties. Gel‐to‐sol transitions occurred by adding 1‐adamantane carboxylic acid (AdCA) or urea. ¹H NMR and induced circular dichroism (ICD) spectra reveal that AdCA released the guest parts from the CD cavity and that urea acts as a denaturing agent to break the hydrogen bonds between CDs. The hydrogel was also destroyed by adding β‐CD, which acts as the competitive host to reduce the fibrils. Furthermore, the gel changed to a sol by adding methyl orange (MO) as a guest compound, but the gel reappeared upon addition of α‐CD, which is a stronger host for MO.     

Host–Guest Binding‐Site‐Tunable Self‐Assembly of Stimuli‐Responsive Supramolecular Polymers

Despite the remarkable progress made in controllable self‐assembly of stimuli‐responsive supramolecular polymers (SSPs), a basic issue that has not been consideration to date is the essential binding site. The noncovalent binding sites, which connect the building blocks and endow supramolecular polymers with their ability to respond to stimuli, are expected to strongly affect the self‐assembly of SSPs. Herein, the design and synthesis of a dual‐stimuli thermo‐ and photoresponsive Y‐shaped supramolecular polymer (SSP2) with two adjacent β‐cyclodextrin/azobenzene (β‐CD/Azo) binding sites, and another SSP (SSP1) with similar building blocks, but only one β‐CD/Azo binding site as a control, are described. Upon gradually increasing the polymer solution temperature or irradiating with UV light, SSP2 self‐assemblies with a higher binding‐site distribution density; exhibits a flower‐like morphology, smaller size, and more stable dynamic aggregation process; and greater controllability for drug‐release behavior than those observed with SSP1 self‐assemblies. The host–guest binding‐site‐tunable self‐assembly was attributed to the positive cooperativity generated among adjacent binding sites on the surfaces of SSP2 self‐assemblies. This work is beneficial for precisely controlling the structural parameters and controlled release function of SSP self‐assemblies.     

Triterpenoid‐Based Self‐Healing Supramolecular Polymer Hydrogels Formed by Host–Guest Interactions

Pentacyclic triterpenoids, a class of naturally bioactive products having multiple functional groups, unique chiral centers, rigid skeletons, and good biocompatibility, are ideal building blocks for fabricating versatile supramolecular structures. In this research, the natural pentacyclic triterpenoid glycyrrhetinic acid (GA) was used as a guest molecule for β‐cyclodextrin (β‐CD) to form a GA/β‐CD (1:1) inclusion complex. By means of GA and β‐CD pendant groups in N,N′‐dimethylacrylamide copolymers, a supramolecular polymer hydrogel can be physically cross‐linked by host–guest interactions between GA and β‐CD moieties. Moreover, self‐healing of this hydrogel was observed and confirmed by step‐strain rheological measurements, whereby the maximum storage modulus occurred at a [GA]/[β‐CD] molar ratio of 1:1. Additionally, these polymers displayed outstanding biocompatibility. The introduction of a natural pentacyclic triterpenoid into a hydrogel system not only provides a biocompatible guest–host complementary GA/β‐CD pair, but also makes this hydrogel an attractive candidate for tissue engineering.     

Light‐Switchable Self‐Healing Hydrogel Based on Host–Guest Macro‐Crosslinking

A self‐healing hydrogel is prepared by crosslinking acrylamide with a host–guest macro‐crosslinker assembled from poly(β‐cyclodextrin) nanogel and azobenzeneacrylamide. The photoisomerizable azobenzene moiety can change its binding affinity with β‐cyclodextrin, therefore the crosslinking density and rheology property of the hydrogel can be tuned with light stimulus. The hydrogel can repair its wound autonomously through the dynamic host–guest interaction. In addition, the wounded hydrogel will lose its ability of self‐healing when exposed to ultraviolet light, and the self‐healing behavior can be recovered upon the irradiation of visible light. The utilizing of host–guest macro‐crosslinking approach manifests the as‐prepared hydrogel reversible and light‐switchable self‐healing property, which would broaden the potential applications of self‐healing polymers.

     

Highly Flexible,Tough, and Self‐Healing Supramolecular Polymeric Materials Using Host–Guest Interaction

Flexible, tough, and self‐healable polymeric materials are promising to be a solution to the energy problem by substituting for conventional heavy materials. A fusion of supramolecular chemistry and polymer chemistry is a powerful method to create such intelligent materials. Here, a supramolecular polymeric material using multipoint molecular recognition between cyclodextrin (CD) and hydrophobic guest molecules at polymer side chain is reported. A transparent, flexible, and tough hydrogel (host–guest gel) is formed by a simple preparation procedure. The host–guest gel shows self‐healing property in both wet state and dry state due to reversible nature of host–guest interaction. The practical utility of the host–guest gel as a scratch curable coating is demonstrated.

     

Supra‐dendron Gelator Based on Azobenzene–Cyclodextrin Host–Guest Interactions: Photoswitched Optical and Chiroptical Reversibility

A novel amphiphilic dendron ( AZOC₈GAc ) with three l ‐glutamic acid units and an azobenzene moiety covalently linked by an alkyl spacer has been designed. The compound formed hydrogels with water at very low concentration and self‐assembled into chiral‐twist structures. The gel showed a reversible macroscopic volume phase transition in response to pH variations and photo‐irradiation. During the photo‐triggered changes, although the gel showed complete reversibility in its optical absorptions, only an incomplete chiroptical property change was achieved. On the other hand, the dendron could form a 1:1 inclusion complex through a host–guest interaction with α‐cyclodextrin (α‐CD), designated as supra‐dendron gelator AZOC₈GAc/α‐CD . The supra‐dendron showed similar gelation behavior to that of AZOC₈GAc , but with enhanced photoisomerization‐transition efficiency and chiroptical switching capacity, which was completely reversible in terms of both optical and chiroptical performances. The self‐assembly of the supra‐dendron is a hierarchical or multi‐supramolecular self‐assembling process. This work has clearly illustrated that the hierarchical and multi‐supramolecular self‐assembling system endows the supramolecular nanostructures or materials with superior reversible optical and chiroptical switching.     

Responsiveness and Morphology Study of Dual Stimuli‐Controlled Supramolecular Polymer

The synthesis and characterization of a linear supramolecular polymer formed by dual host–guest recognitions is presented. The polymer linked by the orthogonal interactions of azobenzene with β‐cyclodextrin and methyl viologen with sulfonatocalix[4]arene is constructed, and the morphology change along with the formation and vanishment of host–guest interaction is investigated. The reversible disassembly–reassembly of the polymer induced by light and the redox process are monitored by UV–vis and cyclic voltammetry, respectively. The interesting morphology differences between the monomer guest (G), supramolecular polymer (P), and light dissembled product pseudorotaxane (D1) are observed and analyzed. G conducts self‐assembly into a short rod with average width of 83 nm due to the molecular amphipathy and π–π interaction between naphthalene nucleuses, while P exhibits 20 nm wide line morphology. Irradiating P with 365 nm light, the corresponding aggregation D1 shows as 35 nm wide short rod.     

Construction of different supramolecular polymer systems by combining the host–guest and hydrogen‐bonding interactions

Poly(ethylene glycol) (PEG) can form either the inclusion complex with α‐cyclodextrins (α‐CDs) through host–guest interactions or the interpolymer complex with poly(acrylic acid) (PAA) through hydrogen‐bonding interaction. Mixing α‐CD, PEG, and PAA ternary components in an aqueous solution, the competition between host–guest and hydrogen‐bonding interactions occurs. Increasing feed ratio of α‐CD:EG:AA from 0:1:1 to 0.2:1:1 (molar ratio), various interesting supramolecular polymer systems, such as hydrogen‐bonding complex, dynamic polyrotaxane, crystalline inclusion complex, and thermoresponsive hydrogel, are successively obtained. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1114–1120, 2008     

Multistimuli‐Responsive Supramolecular Assembly of Cucurbituril/Cyclodextrin Pairs with an Azobenzene‐Containing Bispyridinium Guest

A linear supramolecular architecture was successfully constructed by the inclusion complexation of α‐cyclodextrin with azobenzene and the host‐stabilized charge‐transfer interaction of naphthalene and a bispyridinium guest with cucurbit[8]uril in water, which was comprehensively characterized by ¹H NMR spectroscopy, UV/Vis absorption, fluorescence, circular dichroism spectroscopy, dynamic laser scattering, and microscopic observations. Significantly, because it benefits from the photoinduced isomerization of the azophenyl group and the chemical reduction of bispyridinium moiety with noncovalent connections, the assembly/disassembly process of this supramolecular nanostructure can be efficiently modulated by external stimuli, including temperature, UV and visible‐light irradiation, and chemical redox.     

Photo‐Controlled Reversible Microtubule Assembly Mediated by Paclitaxel‐Modified Cyclodextrin

The design and construction of multi‐stimuli‐responsive supramolecular nanoassemblies that can mimic and regulate the fundamental biological processes have become a focus of interest in supramolecular chemistry. In this work, a perfect combination has been achieved between naturally occurring microtubules and artificially macrocyclic receptors. The self‐assembling morphology of microtubules can be photo‐tuned by the host–guest interaction of paclitaxel‐modified β‐cyclodextrin (PTX‐CD) and photochromic arylazopyrazole (PTX‐AAP). Moreover, the supramolecularly aggregated microtubules in a cellular environment can induce a pronounced cell morphological change and cell death. This supramolecular approach based on the secondary PTX‐AAP?PTX‐CD complexation provides us a facile method to reversibly control the intertubular aggregation behaviors of microtubules, which may bring new perspectives in the treatment of diseases related to improper protein aggregation.     

Heat‐Induced Supramolecular Organogels Composed of α‐Cyclodextrin and “Jellyfish‐Like” β‐Cyclodextrin–Poly(ε‐caprolactone)

In general, the complexation and gelation behavior between biocompatible poly(ε‐caprolactone) (PCL) derivatives and α‐cyclodextrin (α‐CD) is extensively studied in water, but not in organic solvents. In this article, the complexation and gelation behavior between α‐CD and multi‐arm polymer β‐cyclodextrin‐PCL (β‐CD‐PCL) with a unique “jellyfish‐like” structure are thoroughly investigated in organic solvent N,N‐dimethylformamide and a new heat‐induced organogel is obtained. However, PCL linear polymers cannot form organogels under the same condition. The complexation is characterized by rheological measurements, DSC, XRD, and SEM. The SEM images reveal that the complexes between β‐CD‐PCL and α‐CD present a novel topological helix porous structure which is distinctly different from the lamellar structure formed by PCL linear polymers and α‐CD, suggesting the unique “jellyfish‐like” structure of β‐CD‐PCL is crucial for the formation of the organogels. This research may provide insight into constructing new supramolecular organogels and potential for designing new functional biomaterials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1598–1606     

Ultrasound‐Driven Secondary Self‐Assembly of Amphiphilic β‐Cyclodextrin Dimers

The controlled secondary self‐assembly of amphiphilic molecules in solution is theoretically and practically significant in amphiphilic molecular applications. An amphiphilic β‐cyclodextrin (β‐CD) dimer, namely LA‐(CD)₂, has been synthesized, wherein one lithocholic acid (LA) unit is hydrophobic and two β‐CD units are hydrophilic. In an aqueous solution at room temperature, LA‐(CD)₂ self‐assembles into spherical micelles without ultrasonication. The primary micelles dissociates and then secondarily form self‐assemblies with branched structures under ultrasonication. The branched aggregates revert to primary micelles at high temperature. The ultrasound‐driven secondary self‐assembly is confirmed by transmission electron microscopy, dynamic light scattering, ¹H NMR spectroscopy, and Cu²⁺‐responsive experiments. Furthermore, 2D NOESY NMR and UV/Vis spectroscopy results indicate that the formation of the primary micelles is driven by hydrophilic–hydrophobic interactions, whereas host–guest interactions promote the formation of the secondary assemblies. Additionally, ultrasonication is shown to be able to effectively destroy the primary hydrophilic–hydrophobic balances while enhancing the host–guest interaction between the LA and β‐CD moieties at room temperature.     

Self‐Healing Supramolecular Materials Constructed by Copolymerization via Molecular Recognition of Cavitand‐Based Coordination Capsules

The repeating guest units of poly‐(R)‐ 2 were selectively encapsulated by the self‐assembled capsule poly‐ 1 possessing eight polymer side chains to form the supramolecular graft polymer (poly‐ 1 )_n?poly‐(R)‐ 2 . The encapsulation of the guest units was confirmed by ¹H NMR spectroscopy and the DOSY technique. The hydrodynamic radius of the graft polymer structure was greatly increased upon the complexation of poly‐ 1 . The supramolecular graft polymer (poly‐ 1 )_n?poly‐(R)‐ 2 was stably formed in the 1:1 host–guest ratio, which increased the glass transition temperature by more than 10 °C compared to that of poly‐ 1 . AFM visualized that (poly‐ 1 )_n?poly‐(R)‐ 2 formed the networked structure on mica. The (poly‐ 1 )_n?poly‐(R)‐ 2 gelled in 1,1,2,2‐tetrachloroethane, which led to fabrication of distinct viscoelastic materials that demonstrated self‐healing behavior in a tensile test.     

Tailoring Polymeric Hydrogels through Cyclodextrin Host–Guest Complexation

A close correllation between molecular‐level interactions and macroscopic characteristics of polymer networks exists. The characteristics of the polymeric hydrogels assembled from β‐cyclodextrin (β‐CD) and adamantyl (AD) substituted poly(acrylate)s can be tailored through selective host–guest complexation between β‐CD and AD substituents and their tethers. Dominantly, steric effects and competitive intra‐ and intermolecular host–guest complexation are found to control poly(acrylate) isomeric inter‐strand linkage in polymer network formation. This understanding of the factors involved in polymeric hydrogel formation points the way towards the construction of increasingly sophisticated biocompatible materials.

     

Electrically driven self‐healing polymers based on reversible guest–host complexation of β‐cyclodextrin and ferrocene

A multifunctional ferrocene‐modified poly(glycidyl methacrylate) (PGMA‐Fc) and a difunctional β‐cyclodextrin derivative (bis‐CD) has been prepared for the construction of an electrically driven removable and self‐healing polymeric materials based on the complexation reaction between ferrocene and β‐CD groups. The chemical structures of PGMA‐Fc and bis‐CD have been characterized with Fourier transform infrared, ¹H nuclear magnetic resonance, and X‐ray photoelectron spectroscopy. The effects of electrical voltages and medium conductivity on the decrosslinking efficiency of the crosslinked PGMA‐Fc/CD polymer have been examined. The PGMA‐Fc/CD network has shown removable feature and properties for application as a reworkable crosslinked material. Moreover, the crosslinked PGMA‐Fc/CD sample has shown electrically driven self‐healing behavior. The self‐healing performance could be enhanced with wetting the sample to increase the electrical conductivity. As a result, the material could serve as a self‐healing agent for commercial painting products. Preparation and application of a novel and efficient self‐healing polymer have been demonstrated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3395–3403     

Programmable Polymer‐Based Supramolecular Temperature Sensor with a Memory Function

A new class of polymeric thermometers with a memory function is reported that is based on the supramolecular host–guest interactions of poly(N‐isopropylacrylamide) (PNIPAM) with side‐chain naphthalene guest moieties and the tetracationic macrocycle cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) as the host. This supramolecular thermometer exhibits a memory function for the thermal history of the solution, which arises from the large hysteresis of the thermoresponsive LCST phase transition (LCST=lower critical solution temperature). This hysteresis is based on the formation of a metastable soluble state that consists of the PNIPAM–CBPQT⁴⁺ host–guest complex. When heated above the transition temperature, the polymer collapses, and the host–guest interactions are disrupted, making the polymer more hydrophobic and less soluble in water. Aside from providing fundamental insights into the kinetic control of supramolecular assemblies, the developed thermometer with a memory function might find use in applications spanning the physical and biological sciences.