Xiaojuan Liao

Photoresponsive Pseudopolyrotaxane Hydrogels Based on Competition of Host–Guest Interactions

Reversibility is a basic and crucial feature of supramolecular systems. In designing and fabricating new supramolecular materials, the realization of reversibility is particularly important as it could enable these substances to be superior to conventional materials. An excellent example is an elastomer reported recently, made of small functional molecules which form both long chains and cross-linkers through intermolecular multiple hydrogen bonding. When broken or cut, the elastomer can be simply repaired by just bringing together the fractured surfaces and allowing the recovery of the hydrogen bonding. There are plenty of reports on supramolecular assemblies of polymers showing reversible responses to environmental changes; however, these are mostly based on the inherent stimuli-sensitive properties of the building blocks, such as the temperature-induced coil–globule transition of poly(N-isopropyl acrylamide) (PNIPAM) and pH-induced protonation of poly(vinyl pyridine), rather than the reversibility of the supramolecular interactions. Therefore, taking full advantage of the reversibility of the noncovalent interactions to construct supramolecular materials is still a big challenge. This has drawn increasing interest in recent years, and has led to a series of promising results. For example, some hydrophobically modified water-soluble polymers realized sol–gel transition as a result of the reversible interactions of cyclodextrin and alkyl chains. The assembly and disassembly of polymeric vesicles can be controlled by photosensitive interactions between cyclodextrin and azobenzene compounds. The self-assembly of polyethylene glycol (PEG) and a-cyclodextrins (a-CDs) to form linear pseudopolyrotaxane (PPR), with PEG as the axis and a-CDs as threaded rings, was reported by Harada et al. Since the ability of PPR to form physical hydrogels was first reported in 1994, the system has been extensively studied as the hydrogels show very promising uses as biomedicine materials. There is now good understanding of the formation mechanisms and structures of such PPR hydrogels, but little has been explored concerning their dissociation and reassembly, though increasing temperature and shearing could make the hydrogel turn to a sol. Herein, we demonstrate a facile photocontrollable supramolecular route to realize the disassembly and reassembly of the PPR hydrogels. The addition of a photoresponsive compound containing an azobenzene moiety to the PPR hydrogel was found to be effective in converting the hydrogels to transparent solutions. By subsequent alternation of UVand visible irradiation, reversible sol-to-gel and gel-to-sol transitions were observed. Thus, the widely investigated PEG/aCD PPR hydrogel is proved to be “active” in supramolecular chemistry, and the reversible nature of supramolecular materials is fully realized. In our study, the PPR hydrogel (Figure 1a) was prepared by using PEG10K (molecular weight 10000) and a-CD in water as described in reference [7c]. The concentrations of

Hydrogels locked by molecular recognition aiming at responsiveness and functionality

The principle of molecular recognition originating from the concept of lock-and-key has been one of the foundation stones for modern chemistry and biology. Molecular recognition in either biomolecules or synthetic molecules leads to non-covalent linkages, which are featured by responsiveness, reversibility and competition, differing from the covalent bonds. Therefore, recently, this concept has been introduced to and employed in the field of functional materials with great success. In this review, these materials will be examined from the molecular recognition point of view, without considering the origins of the binding pairs involved. First, the structural characters of hydrogels locked by molecular recognition are discussed in detail with emphasis on the chemical structure and architectures of the interaction pairs and the corresponding polymers. As the new hydrogel materials inherit the reversible advantages from non-covalent interactions as well as the specificities of the host–guest or ligand–acceptor pairs, their corresponding responses to various stimuli are discussed in the second part of this review. Compared to the smart materials made of responsive polymers, the hydrogels locked by molecular recognition are featured by the precise control of the responsiveness to various environmental stimuli via sophisticated design of the interaction sites by changing their chemical structures, density location and linking chemistry to the polymer backbones etc. Finally, representative applications of these hydrogels are briefly described.

Pseudopolyrotaxanes on Inorganic Nanoplatelets and their Supramolecular Hydrogels

In this paper, we demonstrate the first hybrid suprastructure of pseudopolyrotaxanes (PPRs) on clay nanoplatelets. Simple end-modification of poly(ethylene glycol) with pyridinium (PEG-N(+)) enabled the chains to form brushlike conformation on clay surfaces. Thus, the PEG chains were able to thread into the cavities of α-cyclodextrins (α-CDs), leading to hybrid PPR hydrogels. This was very different from the unmodified PEG chains, which were absorbed onto the clay surface and thus made the PPR formation impossible. The hydrogels made of this PPR-on-clay structure displayed a dynamic modulus 1 order of magnitude higher than those of the native PPR hydrogels. Furthermore, based on the competitive host-guest interactions, such hybrid hydrogels showed fully photoreversible sol-gel transition after a competitive guest containing azobenzene moiety was introduced.

Thermoresponsive AuNPs Stabilized by Pillararene-Containing Polymers

Pillararene-containing thermoresponsive polymers are synthesized via reversible addition-fragmentation chain transfer polymerization using pillararene derivatives as the effective chain transfer agents for the first time. These polymers can self-assemble into micelles and form vesicles after guest molecules are added. Furthermore, such functional polymers can be further applied to prepare hybrid gold nanoparticles, which integrate the thermoresponsivity of polymers and molecular recognition of pillararenes.

Novel amphiphilic and photo-responsive ABC 3-miktoarm star terpolymers: synthesis, self-assembly and photo-responsive behavior

Novel amphiphilic and photo-responsive ABC 3-miktoarm star terpolymers consisting of hydrophilic poly(ethylene glycol) monomethyl ether (MPEG), hydrophobic polystyrene (PS) and azobenzene-containing poly[6-(4-methoxy-azobenzene-4′-oxy) hexylmethacrylate] (PMMAZO) were synthesized by a combination of atom transfer radical polymerization (ATRP) and click chemistry. MPEG was first end-capped by an epoxide ring, which was opened by sodium azide for the preparation of the modified MPEG bearing reactive azide group and hydroxyl group (MPEG–(OH)(N3)). Click chemistry was then performed to conjugate α-alkynyl-ω-diethylamino-PS and MPEG–(OH)(N3) for the preparation of a block copolymer with reactive hydroxyl group at the junction point (MPEG(–OH)–b–PS), which was further esterified with 2-bromoisobutyryl bromide to prepare the macroinitiator with reactive bromo group at the central point (MPEG(–Br)–b–PS). Finally, an azobenzene-containing PMMAZO arm was introduced into the diblock copolymer by ATRP of 6-(4-methoxy-azobenzene-4′-oxy) hexylmethacrylate monomer (MMAZO) initiated with macroinitiator MPEG(–Br)–b–PS for the preparation of ABC 3-miktoarm star terpolymer (MPEG)(PS)(PMMAZO). The self-assembled morphologies of the star terpolymers in selective solvents changed from bowl-shaped structures to multibowl-shaped structures with the lengthening of the hydrophobic chains (PS or PMMAZO). Photo-responsive investigation showed that the different aggregation states had a great effect on the photo-induced isomerization behaviors. These results may provide guidelines for the design of effective photoresponsive anisotropic materials.

Synthesis and conductivity of hyperbranched poly(triazolium)s with various end-capping groups

A series of novel hyperbranched poly(triazolium)s with different terminal groups were synthesized by alkylation and anion exchange reactions of the corresponding hyperbranched poly(triazole)s, which were obtained from an AB2-type monomer via Cu(I)-catalyzed azide–alkyne cycloaddition polymerization. The hyperbranched poly(triazolium)s showed high thermal stability with decomposition temperatures of 328–361 °C, and good flexibility, with glass transition temperatures (Tg) ranging from −6.2 to −14.9 °C. Among them, an oligo(ethylene glycol)-terminated hyperbranched poly(triazolium) presented the lowest Tg of −14.9 °C, the highest ionic conductivity (7.70 × 10−6 S cm−1 at 30 °C and 1.02 × 10−3 S cm−1 at 110 °C) and a wide electrochemical stability window of 6.0 V. Therefore, these hyperbranched poly(triazolium)s could act as new electrolyte materials.

Precisely designed perylene bisimide-substituted polyethylene with a high glass transition temperature and an ordered architecture

Acyclic diene metathesis polymerization of a structurally symmetrical perylene bisimide (PBI)-containing α,ω-diene has been performed, yielding an unsaturated polymer with increased molecular weight (Mn = 21.2–87.6 kDa) and decreased polydispersity index (PDI = 2.31–1.76) as the reaction time was prolonged. The subsequent hydrogenation of the as-synthesized polymer was readily accomplished, affording the desired polyethylene (PE) with a saturated backbone and precisely repeating substituted bulky PBI branches. This PBI-substituted PE derivative displayed high glass transition temperatures (Tg = 51.8–75.8 °C), a relatively wide range of light absorption (λ = 230–590 nm), and a highly ordered architecture, which should facilitate electron mobility and be suitable for utilization in optoelectronic devices. It can therefore serve as a superior model for simple construction of functional PE polymers with precisely repeating bulky branches and a soluble PBI polymer with an ordered architecture.

Hyperbranched poly(triazole) with thermal and metal ion dual stimuli-responsiveness

Oligo(ethylene glycol)-terminated hyperbranched poly(triazole) (hb-PTA-OEG) with thermal and metal ion dual stimuli-responsiveness was synthesized by Cu(I)-catalyzed azide–alkyne cycloaddition polymerization, which possessed a high molecular weight (Mn = 203 kDa) and good thermal properties (Td = 367 °C, Tg = −14.1 °C). Importantly, hb-PTA-OEG exhibited a relatively low cloud point (CP) temperature ranging from 34.4 to 31.1 °C as the concentration was increased from 0.02 to 2.0 wt%. Simultaneously, hb-PTA-OEG has the ability to coordinate with various metal ions, and the type of metal ions influenced the CP of hb-PTA-OEG solution to some extent as a result of their different association strengths with the polymer, particularly, Ag+ ion showed a conspicuous contribution to increasing the CP among the several selected ions. As a result, hb-PTA-OEG can act as an absorber of metal ions, and the selective absorption of Ag+ ion could be reached by tuning temperature.

A high-performance dielectric block copolymer with a self-assembled superhelical nanotube morphology

A block copolymer consisting of functional polynorbornene (PNBE) and polyacetylene (PA) segments was synthesized by tandem metathesis polymerization, and self-assembled into a superhelical nanotube morphology from achiral building blocks. Bearing insulated PNBE and conductive PA segments with unique nanostructures, the block copolymers displayed high dielectric permittivities of 25–29, low dielectric losses of 0.03–0.04, and excellent stored/released energy densities of 2.0–1.9/1.7–1.2 J cm−3 under the breakdown fields of 270–200 MV m−1, due to strong dipolar and nanointerfacial polarizations as well as stereoregular chain microstructure contributions. This strategy provided a new way for the development of advanced nanodielectric materials.

High-performance dielectric ionic ladderphane-derived triblock copolymer with a unique self-assembled nanostructure

Functional poly(bisnorbornene)-based ladderphanes, P1, P2, and P3, as well as the derived triblock copolymers containing two blocks of poly(N-3,5-difluorophenyl-norbornene pyrrolidine) (PFNP), PFNP-b-P2-b-PFNP and PFNP-b-P3-b-PFNP, were synthesized by ring-opening metathesis polymerization. The dielectric constants of the polymers were 5, 7, 21, 12, and 18 accompanied with the dielectric losses of 0.03, 0.03, 0.11, 0.02, and 0.04 at a frequency range of 100 Hz to 1 MHz, respectively. In particular, the ionic copolymer PFNP-b-P3-b-PFNP could self-assemble into a unique tree ring-like nanostructure, in which the ionic P3 blocks were isolated between the insulating PFNP blocks, and the long distance migration of ions was difficult to achieve, resulting in lower dielectric dissipation and higher charge–discharge efficiency than those of the ionic homopolymer P3. This research presented a practical way to effectively improve the dielectric properties by combining the ionic, dipolar, and nano-interfacial polarizations, as well as the stereoregular microstructure of the polymer chain.