Supramolecular elastomers: Switchable mechanical properties and tuning photohealing with changes in supramolecular interactions

To study light‐triggered self‐healing in supramolecular materials, we synthesized supramolecular thermoplastic elastomers with mechanical properties that were reversibly modulated with temperature. By changing the supramolecular architecture, we created polymers with different temperature responses. Detailed characterization of the hydrogen‐bonding material revealed dramatically different temperature and mechanical stress response due to two different stable states with changes in the hydrogen bonding interactions. A semi‐crystalline state showed no response to oscillatory shear deformations while the melt state behaved as a typical energy dissipative material with a clear crossover between storage and loss moduli. Comparison studies on heat generation after light excitation revealed no differences in photo‐thermal conversion when an Fe(II)‐phenanthroline chromophore was either physically blended into the H‐bonding polymer or covalently attached to the supramolecular network. These materials showed healing of scratches with light‐irradiation, as long as the overlap of material absorbance and laser excitation was sufficient. Differences in the efficiency and rate of photohealing were observed, depending on the type of supramolecular interaction, and these were attributed to the differences in the thermal response of the materials' moduli. Such results provide insight into how materials can be designed with chromophores and supramolecular bonding interactions to tune the light‐healing efficiency of the materials. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1003–1011     

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     

Supramolecular Silicone Elastomers with Healable and Hydrophobic Properties Crosslinked by “Salt‐Forming Vulcanization”

In this article, supramolecular silicone elastomers with self‐healing function were first prepared by simple and controllable “salt‐forming vulcanization” of polyaminopropylmethylsiloxane with acids. Their structures and micrographs were verified by Fourier transform infrared spectra, Small‐angle X‐ray scattering experiments and atomic force microscope. The experimental results showed that the ion‐association complexes were formed during vulcanization, and the obtained elastomers displayed self‐healing and good mechanical properties even if the cross‐linking agent was excessed. The thermogravimetric analysis showed that the elastomers crosslinked by inorganic acid were stable under high temperature. Unexpectedly, bionic structures were observed in the elastomers, which further changed the hydrophobicity of the surfaces of the elastomers physically. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 903–911     

Facile synthesis of self‐healing gel via magnetocaloric bottom‐ignited frontal polymerization

A series of the self‐healing gels facilely fabricated by VI (N‐vinyl imidazole) and MAH‐β‐CD (β‐cyclodextrin grafted vinyl carboxylic acid groups) via bottom‐ignited frontal polymerization (BIFP) initiated by magnetocaloric effect. Once ignited the bottom phase, the heat upward propagates to generate the “front” in the upper phase. Then, no further energy is added to maintain the reaction and the whole polymerization process experiences within minutes. In this system, the dependence of frontal velocity and temperature, along with morphology, swelling capacity, mechanical property, and self‐healing efficiency, on the preparation parameters is investigated. Interestingly, the gels show good swelling capacity in the organic solvent, comparatively almost no absorption in water. Moreover, the as‐prepared gels exhibit excellent auto‐healing properties without any external stimuli at ambient temperature. The healed sample possesses 97% recovery of its tensile strength after 8 h healing time, which relies largely on the host–guest interaction between VI and MAH‐β‐CD. The results demonstrate that FP can be utilized as an efficient and energy‐saving method to synthesize self‐healing supramolecular gels. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2585–2593     

Self‐Healing Supramolecular Hydrogels for Tissue Engineering Applications

Self‐healing supramolecular hydrogels have emerged as a novel class of biomaterials that combine hydrogels with supramolecular chemistry to develop highly functional biomaterials with advantages including native tissue mimicry, biocompatibility, and injectability. These properties are endowed by the reversibly cross‐linked polymer network of the hydrogel. These hydrogels have great potential for realizing yet to be clinically translated tissue engineering therapies. This review presents methods of self‐healing supramolecular hydrogel formation and their uses in tissue engineering as well as future perspectives.     

Self‐Healing Polyester Urethane Supramolecular Elastomers Reinforced with Cellulose Nanocrystals for Biomedical Applications

Stretchable self‐healing urethane‐based biomaterials have always been crucial for biomedical applications; however, the strength is the main constraint of utilization of these healable materials. Here, a series of novel, healable, elastomeric, supramolecular polyester urethane nanocomposites of poly(1,8‐octanediol citrate) and hexamethylene diisocyanate reinforced with cellulose nanocrystals (CNCs) are introduced. Nanocomposites with various amounts of CNCs from 10 to 50 wt% are prepared using solvent casting technique followed by the evaluation of their microstructural features, mechanical properties, healability, and biocompatibility. The synthesized nanocomposites indicate significantly higher tensile modulus (approximately 36–500‐fold) in comparison to the supramolecular polymer alone. Upon exposure to heat, the materials can reheal, but nevertheless when the amount of CNC is greater than 10 wt%, the self‐healing ability of nanocomposites is deteriorated. These materials are capable of rebonding ruptured parts and fully restoring their mechanical properties. In vitro cytotoxicity test of the nanocomposites using human dermal fibroblasts confirms their good cytocompatibility. The optimized structure, self‐healing attributes, and noncytotoxicity make these nanocomposites highly promising for tissue engineering and other biomedical applications.     

Controlling supramolecular polymerization through multicomponent self‐assembly

The self‐assembly into supramolecular polymers is a process driven by reversible non‐covalent interactions between monomers, and gives access to materials applications incorporating mechanical, biological, optical or electronic functionalities. Compared to the achievements in precision polymer synthesis via living and controlled covalent polymerization processes, supramolecular chemists have only just learned how to developed strategies that allow similar control over polymer length, (co)monomer sequence and morphology (random, alternating or blocked ordering). This highlight article discusses the unique opportunities that arise when coassembling multicomponent supramolecular polymers, and focusses on four strategies in order to control the polymer architecture, size, stability and its stimuli‐responsive properties: (1) end‐capping of supramolecular polymers, (2) biomimetic templated polymerization, (3) controlled selectivity and reactivity in supramolecular copolymerization, and (4) living supramolecular polymerization. In contrast to the traditional focus on equilibrium systems, our emphasis is also on the manipulation of self‐assembly kinetics of synthetic supramolecular systems. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 34–78     

Hydrogen‐bonded supramolecular polymers from derivatives of α‐amino‐ε‐caprolactam: A bio‐based material

Hydrogen‐bonded supramolecular polymers were prepared from the derivatives of α‐amino‐ε‐caprolactam (ACL), obtained from a renewable resource. Several self‐complimentary bis‐ or tetra‐caprolactam monomers were synthesized by varying the number of carbons of the spacer between the hydrogen‐bonding end groups. Physical properties of these hydrogen‐bonded polymers were clearly demonstrated by differential scanning colorimetry, solid‐state NMR, and X‐ray powder diffraction analyses. The supramolecular behavior was also supported by fiber formation from the melt for several of these compounds, and stable glassy materials were prepared from the physical mixtures of two different biscaprolactams. The self‐association ability of ACL was also used by incorporating ACL at the chain ends of low‐molecular weight Jeffamine (M_n = 900 g/mol) using urea and amide linkages. The transformation of this liquid oligomer at room temperature into a self‐standing, transparent film clearly showed the improvement in mechanical properties obtained by the introduction of terminal hydrogen‐bonding groups. Finally, the use of monomers with a functionality of four gave rise to network formation either alone or combination with bifunctional monomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Developing a Self‐Healing Supramolecular Nucleoside Hydrogel Based on Guanosine and Isoguanosine

Recently, supramolecular hydrogels have attracted increasing interest owing to their tunable stability and inherent biocompatibility. However, only few studies have been reported in the literature on self‐healing supramolecular nucleoside hydrogels, compared to self‐healing polymer hydrogels. In this work, we successfully developed a self‐healing supramolecular nucleoside hydrogel obtained by simply mixing equimolar amounts of guanosine (G) and isoguanosine (isoG) in the presence of K⁺. The gelation properties have been studied systematically by comparing different alkali metal ions as well as mixtures with different ratios of G and isoG. To this end, rheological and phase diagram experiments demonstrated that the co‐gel not only possessed good self‐healing properties and short recovery time (only 20 seconds) but also could be formed at very low concentrations of K⁺. Furthermore, nuclear magnetic resonance (NMR), powder X‐ray diffraction (PXRD), and circular dichroism (CD) spectroscopy suggested that possible G₂isoG₂‐quartet structures occurred in this self‐healing supramolecular nucleoside hydrogel. This co‐gel, to some extent, addressed the problem of isoguanosine gels for the applications in vivo, which showed the potential to be a new type of drug delivery system for biomedical applications in the future.     

Dendronized supramolecular polymers self‐assembled from dendritic ionic liquids

The synthesis, structural, and retrostructural analysis of a library of self‐assembling dendrons containing triethyl and tripropyl ammonium, pyridinium and 3‐methylimidazolium chloride, tetrafluoroborate, and hexafluorophosphate at their apex are reported. These dendritic ionic liquids self‐assemble into supramolecular columns or spheres which self‐organize into 2D hexagonal or rectangular and 3D cubic or tetragonal liquid crystalline and crystalline lattices. Structural analysis by X‐ray diffraction experiments demonstrated the self‐assembly of supramolecular dendrimers containing columnar and spherical nanoscale ionic liquid reactors segregated in their core. Both in the supramolecular columns and spheres the noncovalent interactions mediated by the ionic liquid provide a supramolecular polymer and therefore, these assemblies represent a new class of dendronized supramolecular polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4165–4193, 2009     

Biobased poly(2,5‐furandimethylene succinate‐co‐butylene succinate) crosslinked by reversible Diels–Alder reaction

We synthesized biobased poly(2,5‐furandimethylene succinate‐co‐butylene succinate) [P(FS‐co‐BS)] copolymers by polycondensation of 2,5‐bis(hydroxymethyl)furan, 1,4‐butanediol, and succinic acid. These copolymers could be crosslinked to form network polymers by means of a reversible Diels–Alder reaction with bis‐maleimide. The thermal properties, mechanical properties, and healing abilities of the P(FS‐co‐BS)s and the network polymers were investigated. The mechanical properties of the network polymers depended on the comonomer composition of the P(FS‐co‐BS)s and the maleimide/furan ratio in the network polymers. Some of the copolymers exhibited healing ability at room temperature, and their healing efficiency was enhanced by solvent or heat. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 216–222     

Moisture‐mediated intrinsic self‐healing of modified polyurethane urea polymers

Functional materials having the ability to self‐heal cracks or scratches after damage are of great interest for a huge scope of applications. Herein, we report a self‐healing polyurethane urea‐based material with implemented 1‐(2‐aminoethyl) imidazolidone (UDETA) as a chain terminating molecule and for hydrogen bond network formation. Both, UDETA content and moisture affected the self‐healing process. The reversible change in the materials properties was proven by detailed analyses of hardness and thermomechanical behavior in dependence of the water uptake of the samples. FT‐IR analysis revealed that water is able to act as a plasticizer interrupting hydrogen bonding interactions within the polymer network and thus, influencing glass transition temperature and hardness of the samples. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 537–548.     

A Homotelechelic bis‐terpyridine macroligand: One‐step synthesis and its metallo‐supramolecular self‐assembly

A homotelechelic macroligand bearing two 2,2′:6′,2″‐terpyridin‐4′‐yl units, as chain ends, is used as building block for the preparation of a linear metallo‐supramolecular chain‐extended polymer. The macroligand has been prepared by nitroxide‐mediated polymerization (NMP) of styrene using a bis‐terpyridine‐functionalized NMP initiator. The controlled character of the NMP process has been confirmed by detailed characterization of the polymer by size‐exclusion chromatography, nuclear magnetic resonance spectroscopy as well as mass spectrometry. Subsequently, the self‐assembly with Fe^II ions into the chain‐extended metallopolymer and the disassembly thereof, in the presence of a strong competitive ligand, has been studied by UV–vis absorption spectroscopy and diffusion‐ordered NMR spectroscopy. The reversibility of the formation of the metallo‐supramolecular material, when addressed by external stimuli, could be proven. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Self‐Healing Materials from V‐ and H‐Shaped Supramolecular Architectures

Integrating self‐healing capability into supramolecular architectures is an interesting strategy, and can considerably enhance the performance and broaden the scope of applications for this important class of polymers. Herein we report the rational design of novel V‐shaped barbiturate (Ba) functionalized soft–hard–soft triblock copolymers with a reversible supramolecular healing motif (Ba) in the central part of the hard block, which undergoes autonomic repair at 30 °C. The designed synthesis also offers a suitable macromolecular building block to further self‐assemble with heterocomplementary α,ω‐Hamilton wedge (HW) functionalized polyisoprene (PI; HW‐PI‐HW), resulting in an H‐shaped supramolecular architecture with efficient self‐healing capabilities that can recover up to around 95 % of the original mechanical performance at 30 °C within 24 h.     

Synthesis of self‐healing polyurethane urea‐based supramolecular materials

Supramolecular polyurethane ureas are expected to have superior mechanical properties primarily due to the reversible, noncovalent interactions such as hydrogen bonding interactions. We synthesized polyurethane prepolymers from small molecular weight of poly(tetramethylene ether)glycol and isophorone diisocyanates, which were end capped with propylamine to synthesize polyurethane ureas with high contents of urea and urethane groups for hydrogen‐bonding formations to facilitate self‐healing. The effects of polyurethane urea molecular weight (3000 ≤ M_n ≤ 9000), crosslinking, and cutting direction were studied in terms of thermal, mechanical, and morphological properties with an emphasis on the self‐healing efficiency. It was found that the thermal self‐healability was more pronounced as the molecular weight of polyurethane urea decreased, showing a maximum of more than 96% with 3000 M_n when the sample was cut along the stretch direction. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 468–474     

A novel route to in‐situ incorporation of silver nanoparticles into supramolecular hydrogel networks

A novel strategy was developed for the in situ incorporation of silver nanoparticles into the supramolecular hydrogel networks, in which colloidally stable silver hydrosols were firstly prepared in the presence of an amphiphilic block copolymer of poly(oxyethylene)‐poly(oxypropylene)‐poly(oxyethylene) and then mixed with aqueous solution of α‐cyclodextrin. The analyses from rheology, X‐ray diffraction, and scanning electron microscopy confirmed the formation of the supramolecular‐structured hydrogels hybridized with silver nanoparticles. In particular, the colloidal stability of the resultant silver hydrosol and its gelation kinetics in the presence of α‐cyclodextrin as well as the viscoelastic properties of the resultant hybrid hydrogel were investigated under various concentrations of the used block copolymer. It was found that the used block copolymer could act not only as the effective reducing and stabilizing agents for the preparation of the silver hydrosol but also as the effective guest molecule for the supramolecular self‐assembly with α‐cyclodextrin. In addition, the effects of silver nanoparticles on the gelation process and the hydrogel strength were also studied. Such a hybrid hydrogel material could show a good catalytic activity for the reduction of methylene blue dye by sodium borohydride. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 740–749, 2009     

Synthesis of self‐healing supramolecular rubbers from fatty acid derivatives,diethylene triamine,and urea

We describe the synthesis of supramolecular self‐healing elastomers from vegetable oil fatty acid derivatives, diethylene triamine, and urea. Our strategy to obtain materials that are self‐healing but do not flow relies on the use of a wide molecular distribution of randomly branched oligomers equipped with self‐complementary and complementary hydrogen bonding groups. We prepared such oligomers with a two steps procedure. In the first step, diethylene triamine was condensed with dimer acids. In the second step, the oligomers obtained were allowed to react with urea. The molecules were characterized by NMR and IR spectroscopies and Monte‐Carlo simulations were used to analyze the molecular size distribution. The sensitivity to small variations of the experimental conditions has been examined and the robustness of the synthetic procedure optimized. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7925–7936, 2008     

Theoretical consideration and modeling of self‐healing polymers

Bioinspired self‐healing polymers have attracted more and more interests. Imparting self‐healing ability to existing polymers or developing new polymeric materials capable of self‐healing is considered to be a solution for improving their long term stability and durability. This article reviews achievements in the field of theoretical researches on re‐establishment of bonding between broken surfaces of self‐healing polymers from microscopic and macroscopic point of view. Chains interaction, mechanical models related to healing procedures and effect of healing, design of novel self‐healing composite systems, and so forth are summarized and analyzed in detail. Both thermoplastics and thermosets are included to offer a comprehensive knowledge framework of the smart function. The scientific challenges are also highlighted, which are related to the production of more advanced self‐healing polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Multicomponent supramolecular thermoplastic elastomer with peptide‐modified nanofibers

Bioactive nanofibers present a promising synthetic niche for in vivo applications due to their morphological and functional resemblance to the extracellular matrix. Potentially interesting nanofibers are constructed from the hard‐segment regimes in well‐defined thermoplastic elastomers (TPEs). The supramolecular interactions between these hard segments cause physical crosslinking by the formation of nanofibers and provide excellent mechanical properties. Here, we make use of a new class of biocompatible supramolecular TPEs, in which both the formation of the main chain and the hard block is based on multiple hydrogen‐bonding interactions. A self‐assembly process is explored to arrive at well‐defined peptide‐modified nanofibers embedded in a biocompatible soft matrix. Crucial for the success in the synthetic design is the use of an exact match between the molecular recognition units of the peptide and the supramolecular unit that takes care of forming the supramolecular nanofibers of the TPE. Evidence for the strong anchoring of the modified peptides in the hard‐segment nanofibers of the supramolecular TPE is provided by simple extraction experiments. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis,characterization, and micellization studies of coil‐rod‐coil and ABA ruthenium(II) terpyridine assemblies with π‐conjugated electron acceptor systems

A series of Ru^II heterodinuclear complexes of ABA ‐type with electron‐deficient bis‐terpyridines as building blocks was synthesized by (R‐tpy)Ru^IIICl₃ complexation. These compounds were characterized by NMR spectroscopy, MALDI‐TOF, ESI‐TOF mass spectrometry, and elemental analysis. The results were compared with a coil‐rod‐coil Ru^II metallo‐supramolecular copolymer, which was synthesized by bis‐complex formation between a hydrophilic ω‐terpyridine poly(ethylene glycol) Ru^II mono‐complex and a hydrophobic bis‐terpyridine‐functionalized rigid core. This amphiphilic Ru^II triblock copolymer showed self‐assembly to clusters and micelles in aqueous solution, which was studied by transmission electron microscopy and dynamic light scattering. Applying velocity sedimentation experiments the number of amphiphilic Ru^II ABA triblock copolymer molecules within the micelles could be estimated. Finally, the photophysical properties of the Ru^II supramolecular assemblies were investigated by UV–vis spectroscopy. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011