Creating Coordination‐Based Cavities in a Multiresponsive Supramolecular Gel

Creating cavities in varying levels, from molecular containers to macroscopic materials of porosity, have long been motivated for biomimetic or practical applications. Herein, we report an assembly approach to multiresponsive supramolecular gels by integrating photochromic metal–organic cages as predefined building units into the supramolecular gel skeleton, providing a new approach to create cavities in gels. Formation of discrete O‐Pd₂L₄ cages is driven by coordination between Pd²⁺ and a photochromic dithienylethene bispyridine ligand (O‐PyFDTE). In the presence of suitable solvents (DMSO or MeCN/DMSO), the O‐Pd₂L₄ cage molecules aggregate to form nanoparticles, which are further interconnected through supramolecular interactions to form a three‐dimensional (3D) gel matrix to trap a large amount of solvent molecules. Light‐induced phase and structural transformations readily occur owing to the reversible photochromic open‐ring/closed‐ring isomeric conversion of the cage units upon UV/visible light radiation. Furthermore, such Pd₂L₄ cage‐based gels show multiple reversible gel–solution transitions when thermal‐, photo‐, or mechanical stimuli are applied. Such supramolecular gels consisting of porous molecules may be developed as a new type of porous materials with different features from porous solids.     

Stimuli‐responsive reversible physical networks. I. Synthesis and physical network properties of amphiphilic block and random copolymers with long alkyl chains by living cationic polymerization

The living cationic polymerization of octadecyl vinyl ether (ODVE) was achieved with an 1‐(isobutoxy)ethyl acetate [CH₃CH(OiBu)OCOCH₃]/EtAlCl₂ initiating system in hexane in the presence of an added weak Lewis base at 30 °C. In contrast to conventional polymers, poly(octadecyl vinyl ether) underwent upper‐critical‐solution‐temperature‐type phase separation in various solvents, such as hexane, toluene, CH₂Cl₂, and tetrahydrofuran, because of the crystallization of octadecyl chains. Amphiphilic block and random copolymers with crystallizable substituents of ODVE and 2‐methoxyethyl vinyl ether (MOVE) were synthesized via living cationic polymerization under similar conditions. Aqueous solutions of the copolymers yielded physical gels upon cooling because of strong interactions between ODVE units, regardless of the copolymer structure. The product gels, however, exhibited different viscoelastic properties: A 20 wt % solution of a block copolymer (400/20 MOVE/ODVE) became a soft physical gel that behaved like a typical gel, whereas the corresponding random copolymer gave a transparent but stiff gel with a certain relaxation time. Differential scanning calorimetry analysis confirmed that the crystalline–amorphous transition of the octadecyl chains was a key step for inducing such physical gelation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1155–1165, 2005     

Supramolecular Polymerization of C3‐Symmetric Organogelators: Cooperativity,Solvent, and Gelation Relationship

A systematic study of the influence of solvent and the size of C₃‐symmetric discotics on their supramolecular polymerization mechanism is presented. The cooperativity of the self‐assembly of the reported compounds is directly related to their gelation ability. The two series of C₃‐symmetric discotics investigated herein are based on benzene‐1,3,5‐tricarboxamides (BTAs) and oligo(phenylene ethynylene)‐based tricarboxamides (OPE? TAs) that are peripherally decorated with achiral ( 1 a and 2 a ) or chiral N‐(2‐aminoethyl)‐3,4,5‐trialkoxybenzamide units ( 1 b and 2 b ). The supramolecular polymerization of compounds 1 a , b and 2 a , b has been exhaustively investigated in a number of solvents and by using various techniques: variable‐temperature circular dichroism (VT‐CD) spectroscopy, concentration‐dependent ¹H NMR spectroscopy, and isothermal titration calorimetry (ITC). The supramolecular polymerization mechanism of compounds 2 is highly cooperative in solvents such as methylcyclohexane and toluene and is isodesmic in CHCl₃. Unexpectedly, chiral compound 1 b is practically CD‐silent, in contrast with previously reported BTAs. ITC measurements in CHCl₃ demonstrated that the supramolecular polymerization of BTA 1 a is isodesmic. These results confirm the strong influence of the π‐surface of the central aromatic core of the studied discotic and the branched nature of the peripheral side chains on the supramolecular polymerization. The gelation ability of these organogelators is negated in CHCl₃, in which the supramolecular polymerization mechanism is isodesmic.     

Supramolecular polymer gels formed from carboxy‐terminated telechelic polybutadiene and polyamidine through amidinium‐carboxylate salt bridge

In this article, we report the preparation and properties of the bulk supramolecular polymer gels prepared from a polybutadiene based on the amidinium‐carboxylate salt bridge, highlighting the difference from a well‐established network system based on carboxylic acid and amine. We have prepared the amidinium‐carboxylate salt bridge‐based supramolecular polymer gels from a carboxy‐terminated telechelic polybutadiene and a linear polyamidine having N,N′‐di‐substituted acetamidine group in the main chain. FTIR analysis along with Small angle X‐ray scattering measurements indicated that the salt bridge was attributed to the gelation through three‐dimensional network formation. Virtually no fluidity was observed for the supramolecular gel containing equimolar amounts of the carboxyl group and the amidine group, which showed a high G′ value of about 1 MPa at room temperature and a T_gel of 37 °C. For comparison, the supramolecular polymer gels crosslinked by ammonium‐carboxylate salt were prepared using a linear polyethyleneimine instead of the polyamidine. The gel with equimolar amounts of the carboxyl group and the secondary amino group showed liquid‐like fluidity with a G′ value of about 0.01 MPa at room temperature, which was attributed to the fact that a certain amount of the carboxyl group remained as its free form without salt formation, as evidenced by FTIR analysis. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1815–1824     

A Natural Glycyrrhizic Acid‐Tailored Light‐Responsive Gelator

The construction of stimuli‐responsive materials by using naturally occurring molecules as building blocks has received increasing attention owing to their bioavailability, biocompatibility, and biodegradability. Herein, a symmetrical azobenzene‐functionalized natural glycyrrhizic acid (trans‐ GAG ) was synthesized and could form stable supramolecular gels in DMSO/H₂O and MeOH/H₂O. Owing to trans–cis isomerization, this gel exhibited typical light‐responsive behavior that led to a reversible gel–sol transition accompanied by a variation in morphology and rheology. Additionally, this trans‐ GAG gel displayed a distinct injectable self‐healing property and outstanding biocompatibility. This work provides a simple yet rational strategy to fabricate stimuli‐responsive materials from naturally occurring, eco‐friendly molecules.     

Effects of pigments on the volume‐phase‐transition properties of N‐isopropylacrylamide gels as light‐modulation materials

The effects of pigments contained in N‐isopropylacrylamide (NIPAM) gels on their volume‐change properties were investigated. All the NIPAM gel particles, containing various kinds and concentrations of pigments, showed a volume phase transition at 34 °C. No pigment affected the volume‐phase‐transition temperature of the NIPAM gels. As the concentration of the pigment in the NIPAM gels was increased, the amount of the volume change of the NIPAM gels was reduced. The water absorptivity of the NIPAM gels in the swollen state decreased as the pigment concentration increased, whereas the water absorptivity in the shrunken state was almost constant. Reducing the initial monomer concentration of the polymerization of the NIPAM gel increased the water absorptivity in the swollen state. With an increase in the water absorptivity, the volume changes of the NIPAM gels containing pigments were increased. Prototype light modulators in which the NIPAM gel particles containing pigment were dispersed between glass plates were fabricated. The light modulator using the gel particles with improved diameter change (d/d₀ = 2.3, where d and d₀ are the equilibrium diameter and the diameter of the fully shrunken state at 50 °C, respectively) exhibited a larger transmittance change from 8 to 79% than that using the gel particles before the improvement (d/d₀ = 1.7; from 38 to 79%) according to temperature changes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4644–4655, 2006     

Europium-Containing Organic Gels and Organic and Carbon Aerogels. Preparation and Initial Applications in Catalysis

We prepared organic gels and organic and carbon aerogels doped with europium through sol–gel processes. Eu-gels were prepared by sol–gel polymerization of the potassium salt of 2,4-dihydroxybenzoic acid with formaldehyde followed by ion-exchange with Eu(OTf)₃. Eu–organic aerogels were obtained after CO₂ supercritical drying of the gels and Eu–carbon aerogels were obtained by pyrolysing the organic aerogels. The Eu-gels containing 12% europium proved to be efficient as recoverable catalyst in Michael additions.     

Europium-Containing Organic Gels and Organic and Carbon Aerogels. Preparation and Initial Applications in Catalysis

Summary. We prepared organic gels and organic and carbon aerogels doped with europium through sol–gel processes. Eu-gels were prepared by sol–gel polymerization of the potassium salt of 2,4-dihydroxybenzoic acid with formaldehyde followed by ion-exchange with Eu(OTf)₃. Eu–organic aerogels were obtained after CO₂ supercritical drying of the gels and Eu–carbon aerogels were obtained by pyrolysing the organic aerogels. The Eu-gels containing 12% europium proved to be efficient as recoverable catalyst in Michael additions. Deceased on February 20, 2006     

Porous Colloidal Hydrogels Formed by Coordination-Driven Self-Assembly of Charged Metal-Organic Polyhedra

Introduction of porosity into supramolecular gels endows soft materials with functionalities for molecular encapsulation, release, separation and conversion. Metal-organic polyhedra (MOPs), discrete coordination cages containing an internal cavity, have recently been employed as building blocks to construct polymeric gel networks with potential porosity. However, most of the materials can only be synthesized in organic solvents, and the examples of porous, MOP-based hydrogels are scarce. Here, we demonstrate the fabrication of porous hydrogels based on [Rh₂(OH-bdc)₂]₁₂, a rhodium-based MOP containing hydroxyl groups on its periphery (OH-bdc=5-hydroxy-1,3-benzenedicarboxylate). By simply deprotonating [Rh₂(OH-bdc)₂]₁₂ with the base NaOH, the supramolecular polymerization between MOPs and organic linkers can be induced in the aqueous solution, leading to the kinetically controllable formation of hydrogels with hierarchical colloidal networks. When heating the deprotonated MOP, Na_x[Rh₂₄(O-bdc)_x(OH-bdc)_24-x], to induce gelation, the MOP was found to partially decompose, affecting the mechanical property of the resulting gels. By applying a post-synthetic deprotonation strategy, we show that the deprotonation degree of the MOP can be altered after the gel formation without serious decomposition of the MOPs. Gas sorption measurements confirmed the permanent porosity of the corresponding aerogels obtained from these MOP-based hydrogels, showing potentials for applications in gas sorption and catalysis.     

Studies on the chemical syntheses and on the characteristics of polyaniline derivatives

Syntheses of parent polyaniline and methyl, methoxy, and ethoxy ortho-substituted polyanilines were performed using the conventional chemical methodology and monitored using the new open-circuit-potential (V_oc) profile technique. The intermediate pernigraniline oxidation state was identified and isolated at the V_oc maximum (A) during the conventional chemical synthesis of poly(o-methoxyaniline) in the emeraldine oxidation state. The introduction of the substituent on the aniline ring leads to longer polymerization times and lower V_oc values. Syntheses in the presence of two different monomers in solution were also investigated and showed preferential polymerization of the monomer with the lowest V_oc potential. All polymers produced were characterized by elemental analysis, gel permeation chromatography, UV-VIS spectroscopy, and cyclic voltammetry. The influence of the substituent on the V_oc profile and on the polymer characteristics are rationalized in terms of steric and electronic effects. © 1995 John Wiley & Sons, Inc.     

Stimuli‐responsive reversible physical networks. II. Design and properties of homogeneous physical networks consisting of periodic copolymers synthesized by living cationic polymerization

The design and precision synthesis of physical networks consisting of copolymers with crystallizable pendant groups are described in this work. Amphiphilic periodic, statistical, and gradient copolymers consisting of octadecyl vinyl ether (ODVE) units were synthesized via living cationic polymerization. The synthesis involved the copolymerization of ODVE and 2‐methoxyethyl vinyl ether (hydrophilic) with an 1‐(isobutoxy)ethyl acetate [CH₃CH(OiBu)OCOCH₃]/Et_1.5AlCl_1.5 initiating system in the presence of a weak Lewis base to yield copolymers with very narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight ? 1.2). All aqueous solutions of the copolymers behaved as a viscous liquid above 50 °C. When cooled below 25 °C, the solutions turned into transparent, transient physical gels (exhibiting terminal flow), regardless of the sequence distribution. Viscoelastic studies showed that a periodic copolymer gave a hard gel that was more brittle than the gels obtained from the corresponding statistical and gradient copolymers. This difference and the differences in the relaxation time and relaxation mode distribution of the copolymer gels were consistent with the sequence distributions of ODVE in the respective copolymers. These results indicate that the mechanical properties of a physical network can be controlled by the primary polymer structures. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2712‐2722, 2005     

Interplay between H‐Bonding and Preorganization in the Evolution of Self‐Assembled Systems

Cooperative π–π interactions and H‐bonding are frequently exploited in supramolecular polymerization; however, close scrutiny of their mutual interplay has been largely unexplored. Herein, we compare the self‐assembly behavior of a series of C₂‐ and C₃‐symmetrical oligophenyleneethynylenes differing in their amide topology (N‐ or C‐centered). This subtle structural modification brings about drastic changes in their photophysical and supramolecular properties, highlighting the reciprocal impact of H‐bonding vs. preorganization on the evolution and final outcome of supramolecular systems.     

Multiple‐Stimulus‐Responsive Supramolecular Gels of Two Components and Dual Chiroptical Switches

A new class of L ‐glutamic gelators, LG₁₂(CH₂)_nCOOH, containing different lengths of methylene spacer were synthesized. It was found that the gelation ability of these compounds themselves was very weak. However, when another compound, p‐xylylenediamine (XEA), was introduced, the gelation ability was improved greatly. In particular, LG₁₂(CH₂)₁₀COOH showed super‐gelation ability in the presence of XEA, which could immobilize almost all of the solvents except methanol. Moreover, the formed supramolecular gels even could be molded. Interestingly, some supramolecular gels of LG₁₂(CH₂)_nCOOH and XEA could respond to multiple stimuli, such as heating, shaking, sonication, and acid/base. The studies of CD spectra suggested that the supramolecular chirality induced by self‐assembled chiral gelator molecules in gels could be tuned by the length of methylene spacer. In addition, the supramolecular chirality could be regulated as on/off by heating–cooling or external NH₃/HCl. This would facilitate the development of dual chiroptical switches by temperature and acid/base.     

Crystal growth in syndiotactic poly(vinyl alcohol) hydrogels

Aqueous solutions of syndiotacticity-rich poly(vinyl alcohol) (s-PVA) form gels easily. The optimum condition of growth of the calcium tartrate crystal formed by diffusing calcium chloride into hydrogels containing tartaric acid was studied with use ofs- PVA of a syndiotacticity of 56 % and a degree of polymerization of 1460. The crystal grew in the gel of the concentrations of 2 % s-PVA and of 0.5 N tartaric acid at pH=4. The relation between the formation of Liesegang rings and shear modulus of a gel was studied by diffusing silver nitrate into gels containing potassium chromate. The distance between rings decreased with increasing shear modulus of a gel in the range from 670 to 7500 dyne/cm². The Liesegang rings were not formed for the shear modulus gel for 280 and 16200 dyne/cm₂.     

Urea derivatives as low-molecular-weight gelators

This paper reports an overview of low-molecular-weight gelators (LMWGs) that have a ureide moiety as a hydrogen-bonding site. Various mono-, bis-, tris-, and tetrakis-urea compounds can form supramolecular gels with organic solvents. The author developed a C ₃-symmetrical tris-urea molecule that can form a ubiquitous framework of LMWGs. The supramolecular organogel of the tris-urea molecule exhibited a chemical-stimuli-responsive reversible gel–sol phase transition. Supramolecular hydrogels are constructed from self-assemblies of amphiphilic urea derivatives. Sugar-connected amphiphilic tris-urea was found to form a gel with water, and the hydrogels showed chemical-stimuli-responsive gel–sol phase transitions. The potential of supramolecular hydrogels as matrices of electrophoresis has been demonstrated through the supramolecular gel electrophoresis (SUGE) of protein samples using our developed amphiphilic tris-urea LMWG.     

Mechanical Behavior of Polymer Gels for RDCs and RCSAs Collection: NMR Imaging Study of Buckling Phenomena

Anisotropic NMR parameters, such as residual dipolar couplings (RDCs), residual chemical shift anisotropies (RCSAs) and residual quadrupolar couplings (RQCs or Δν_Q), appear in solution‐state NMR when the molecules under study are subjected to a degree of order. The tunable alignment by reversible compression/relaxation of gels (PMMA and p‐HEMA) is an easy, user‐friendly, and very affordable method to measure them. When using this method, a fraction of isotropic NMR signals is observed in the NMR spectra, even at a maximum degree of compression. To explain the origin of these isotropic signals we decided to investigate their physical location inside the NMR tube using deuterium 1D imaging and MRI micro‐imaging experiments. It was observed that after a certain degree of compression the gels start to buckle and they generate pockets of isotropic solvent, which are never eliminated. The amount of buckling depends on the amount of cross‐linker and the length of the gel.     

Synthesis of transition‐metal‐ion‐selective poly(N‐isopropylacrylamide) hydrogels by the incorporation of an Aza crown ether

A functionalized cyclam was synthesized by the attachment of a polymerizable acryloyl group to one of the four nitrogens on the cyclam molecule. The polymerization of the functionalized cyclam was performed with N‐isopropylacrylamide and N,N′‐methylene bisacrylamide, and the gels obtained were studied in the presence of different transition‐metal‐ion solutions. There was a drastic difference in the phase‐transition temperature (T_c) of the poly(N‐isopropylacrylamide) (PNIPAAm)/cyclam gel in comparison with the pure PNIPAAm gel. For the described system, a T_c shift of 15 °C was obtained. The presence of functionalized cyclam increased the hydrophilicity and T_c of the aforementioned polymer gels in deionized water (at pH 6) because of the presence of protonated amino moieties. The PNIPAAm/cyclam gels showed a dependence of the swelling behavior on pH. T_c of the pure PNIPAAm gel was weakly influenced by the presence of any transition‐metal ions, such as Cu²⁺, Ni²⁺, Zn²⁺, and Mn²⁺. The addition of Cu²⁺ or Ni²⁺ to the PNIPAAm/cyclam gel reduced T_c of the polymer gel, and a shift of approximately 12 °C was observed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1594–1602, 2003     

Influences of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile solutions around the sol-gel threshold

Viscoelastic experiments were performed to study the influence of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile (PAN) solutions around the sol-gel threshold. The dynamic critical parameters around the sol-gel threshold were determined using dynamic rheometer. The sol-gel transition takes place at a critical gel temperature at which the scaling law of G′(ω) ∼ G″(ω) ∝ ωⁿ holds, allowing an accurate determination of the critical gel temperature by means of the frequency independence of the loss tangent. Although the gel points of PAN solutions increase with increasing H₂O content, the results show that the scaling exponent n at the gel point is found to be universal for all ternary PAN solutions, which is independent of temperature and H₂O content, indicating the similarity of the fractal structure in the critical PAN gels. The gelation of ternary PAN solutions induced by adding a nonsolvent and by decreasing the temperature is demonstrated to be a thermoreversible process, which implies that the PAN gels are physical gels. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2637–2643, 2008     

Supramolecular polymer gels formed from polyamidine and random copolymer of n-butyl acrylate and acrylic acid

We have prepared the amidinium-carboxylate salt bridge-based supramolecular polymer gels from random copolymer of n-butyl acrylate and acrylic acid and a linear polyamidine having N,N′-di-substituted acetamidine group in the main chain. The supramolecular polymer gel with equimolar amounts of carboxy and amidine groups shows a high G′ value of 1.6 MPa at 25°C. In contrast, the gel prepared from the carboxy polymer and linear polyethyleneimine instead of the polyamidine shows liquid-like fluidity with a G′ value of 0.01 MPa at 25°C. The robustness of the amidine-based supramolecular polymer gels is attributed to the high stability of the amidinium-carboxylate salt bridge. Replacing the random copolymer with carboxy-terminated telechelic poly(n-butyl acrylate) results in a significant decrease in G′ as well as |η*|, which may arise from the difference in the network structure due to the arrangement of carboxy groups.     

Experimental investigations of relaxation times of gel electrolytes during polymerization by MR methods

In this article, the properties of gel electrolytes based on the NaClO₄ inorganic salt are described. For electrolytes with various concentrations of inorganic salts, relaxation times T ₁ and T ₂ were measured by means of MR techniques at regular intervals during the polymerization. To measure relaxation time T ₁, the inversion recovery (IR) technique was used. The measurement of relaxation time T ₂ was performed using the spin–echo (SE) technique. The reduction of relaxation times during the polymerization, namely, T ₁ from the value of 5.5 to 1.7 ms and T ₂ from the value of 5.5 to 1 ms, indicates a change in the internal structure of the gels and a change in their chemical composition. The waveforms measured will contribute to further studies of gel electrolytes aimed at increasing their electrical conductivity and establishing the influence of water molecules on the conductivity.