Rheological Study on Rapid Recovery of Hydrogel Based on Oligomeric Electrolyte

Rheology of oligomer melts in the nematic and isotropic states

Oligomers prepared by chain extension of liquid crystalline monomers are thermotropic. The alignment of liquid crystalline oligomers to shear flow via direct ink write printing is an increasingly popular approach to prepare aligned and 3-D printed liquid crystalline elastomers (LCEs). Here, we are concerned with the contribution of order and thermal history on the rheological properties of liquid crystalline. When the oligomers begin in a polydomain nematic state, the transition to an aligned nematic state occurs gradually over a wide range of shear rates. Conversely, when the oligomers begin in an isotropic state they behave as a Newtonian fluid until a critical shear rate is reached, at which point they align in a critical manner. It is shown that by either decreasing liquid crystalline content or increasing temperature, the viscosity of the oligomer melt decreases while this critical shear rate increases. In addition, the normal stress of oligomers is positive over all shear rates but decreases significantly in magnitude with increasing temperature. By combining the analysis of both temperature and liquid crystalline content, it is demonstrated that the temperature relative to the nematic-isotropic transition temperature is key to the oligomers' unique flow behaviors.

Universal Behavior of Fractal Water Structures Observed in Various Gelation Mechanisms of Polymer Gels, Supramolecular Gels, and Cement Gels

So far, it has been difficult to directly compare diverse characteristic gelation mechanisms over different length and time scales. This paper presents a universal water structure analysis of several gels with different structures and gelation mechanisms including polymer gels, supramolecular gels composed of surfactant micelles, and cement gels. The spatial distribution of water molecules was analyzed at molecular level from a diagram of the relaxation times and their distribution parameters (τ-β diagrams) with our database of the 10 GHz process for a variety of aqueous systems. Polymer gels with volume phase transition showed a small decrease in the fractal dimension of the hydrogen bond network (HBN) with gelation. In supramolecular gels with rod micelle precursor with amphipathic molecules, both the elongation of the micelles and their cross-linking caused a reduction in the fractal dimension. Such a reduction was also found in cement gels. These results suggest that the HBN inevitably breaks at each length scale with relative increase in steric hindrance due to cross-linking, resulting in the fragmentation of collective structures of water molecules. The universal analysis using τ-β diagrams presented here has broad applicability as a method to characterize diverse gel structures and evaluate gelation processes.

Multiresponsive and biocompatible self-healing hydrogel: its facile synthesis in water, characterization and properties

Multiresponsive and biocompatible self-healing ε-PL/A-Pul/BPEI hydrogels were prepared in aqueous solution by Schiff base reaction with aldehyded pullulan (A-Pul), ε-poly-l-lysine (ε-PL) and branched polyethyleneimine (BPEI) as materials. The imine bonds were rapidly cross-linked into a hydrogel network within 80 s. Scanning electron microscopy images showed that the hydrogels exhibited a cross-linked structure with the average pore size from 58 to 82 μm. Rheology tests indicated that the hydrogels maintained good mechanical properties. Water contact angles and swelling studies suggested that the hydrogels could swell in water, with a max swell ratio of 1559%, and pH and temperature had an influence on the equilibrium swelling ratio. The hydrogels could be injected either before or after gelation, and they displayed a self-healing process in ddH₂O at room temperature based on the dynamic uncoupling and recoupling of the imine bonds. The MTT assays implied that the hydrogels were non-cytotoxic on mice bone marrow mesenchymal stem cells. Therefore, the hydrogels showed potential application in biomedical fields, and consequently further work was performed using the self-healing hydrogels as drug carriers in in vitro/vivo antitumor studies.

Self-Healing Hydrogels

Over the past few years, there has been a great deal of interest in the development of hydrogel materials with tunable structural, mechanical, and rheological properties, which exhibit rapid and autonomous self-healing and self-recovery for utilization in a broad range of applications, from soft robotics to tissue engineering. However, self-healing hydrogels generally either possess mechanically robust or rapid self-healing properties but not both. Hence, the development of a mechanically robust hydrogel material with autonomous self-healing on the time scale of seconds is yet to be fully realized. Here, the current advances in the development of autonomous self-healing hydrogels are reviewed. Specifically, methods to test self-healing efficiencies and recoveries, mechanisms of autonomous self-healing, and mechanically robust hydrogels are presented. The trends indicate that hydrogels that self-heal better also achieve self-healing faster, as compared to gels that only partially self-heal. Recommendations to guide future development of self-healing hydrogels are offered and the potential relevance of self-healing hydrogels to the exciting research areas of 3D/4D printing, soft robotics, and assisted health technologies is highlighted.

A closure relation to molecular theory of solvation for macromolecules

We propose a closure to the integral equations of molecular theory of solvation, particularly suitable for polar and charged macromolecules in electrolyte solution. This includes such systems as oligomeric polyelectrolytes at a finite concentration in aqueous and various non-aqueous solutions, as well as drug-like compounds in solution. The new closure by Kobryn, Gusarov, and Kovalenko (KGK closure) imposes the mean spherical approximation (MSA) almost everywhere in the solvation shell but levels out the density distribution function to zero (with the continuity at joint boundaries) inside the repulsive core and in the spatial regions of strong density depletion emerging due to molecular associative interactions. Similarly to MSA, the KGK closure reduces the problem to a linear equation for the direct correlation function which is predefined analytically on most of the solvation shells and has to be determined numerically on a relatively small (three-dimensional) domain of strong depletion, typically within the repulsive core. The KGK closure leads to the solvation free energy in the form of the Gaussian fluctuation (GF) functional. We first test the performance of the KGK closure coupled to the reference interaction site model (RISM) integral equations on the examples of Lennard-Jones liquids, polar and nonpolar molecular solvents, including water, and aqueous solutions of simple ions. The solvation structure, solvation chemical potential, and compressibility obtained from RISM with the KGK closure favorably compare to the results of the hypernetted chain (HNC) and Kovalenko-Hirata (KH) closures, including their combination with the GF solvation free energy. We then use the KGK closure coupled to RISM to obtain the solvation structure and thermodynamics of oligomeric polyelectrolytes and drug-like compounds at a finite concentration in electrolyte solution, for which no convergence is obtained with other closures. For comparison, we calculate their solvation structure from molecular dynamics (MD) simulations. We further couple the 3D-RISM integral equation with the 3D-version of the KGK closure, and solve it for molecular mixtures as well as oligomeric polyelectrolytes and drug-like molecules in electrolyte solutions.

Poly(vinyl alcohol)-Poly(ethylene glycol) Double-Network Hydrogel: A General Approach to Shape Memory and Self-Healing Functionalities

A double-network polymer hydrogel composed of chemically cross-linked poly(ethylene glycol) (PEG) and physically cross-linked poly(vinyl alcohol) (PVA) was prepared. When the hydrogel (70 wt % of water) is subjected to freezing/thawing treatment under strain, the enhanced physical network as a result of crystallization of PVA chains can stabilize the hydrogel deformation after removal of the external force at room temperature. Subsequent disruption of the physical network of PVA by heating allows for the recovery of the initial shape of the hydrogel. Moreover, the double-network hydrogel exhibits self-healing capability stemming from the physical network of PVA by virtue of the extensive interchain hydrogen bonding between the hydroxyl side groups. This study thus demonstrates a general approach to imparting both the shape memory and self-healing properties to chemically cross-linked hydrogels that otherwise do not have such functionalities. Moreover, by making use of the fixed hydrogel elongation, the effect of anisotropy arising from chain orientation on the self-healing was also observed.

2-Ureido-4-pyrimidone-based hydrogels with multiple responses

Functionalisation of poly[2-(dimethylamino)ethyl methacrylate] (a responsive methacrylate) with light-activatable 2-ureido-4-pyrimidone units allows a supramolecular hydrogel to be obtained that combines temperature, light and pH response with self-healing properties. Whereas the self-healing properties of this system were described previously, this report focuses on its response to different external stimuli, which is studied by quartz crystal microbalance analysis of thin films of the material. Reversible collapse with increasing temperature, reversible swelling with decreasing pH and irreversible shrinkage with UV exposure are demonstrated. These three stimuli are combined to have externally gated or tuned responses. Thermo-induced swelling and shrinkage can be reversibly inhibited by changing the pH and irreversibly regulated by exposure to light of different doses. These materials represent the first general strategy to obtain responsive self-healing hydrogels in which the response and the self-healing properties are decoupled from each other and can be tuned independently.

Poly(vinyl alcohol) Hydrogel Can Autonomously Self-Heal

It is discovered that poly(vinyl alcohol) (PVA) hydrogel prepared using the freezing/thawing method can self-repair at room temperature without the need for any stimulus or healing agent. The autonomous self-healing process can be fast for mechanically strong PVA hydrogel yielding a high fracture stress. Investigation on the effect of the hydrogel preparation conditions points out that hydrogen bonding between PVA chains across the interface of the cut surfaces is at the origin of the phenomenon. The key for an effective self-healing is to have an appropriate balance between high concentration of free hydroxyl groups on PVA chains on the cut surfaces prior to contact and sufficient PVA chain mobility in the hydrogel.

Highly Efficient and Specific Gelation of Ionic Liquids by Polymeric Electrolytes to Form Ionogels with Substantially High Gel-Sol Transition Temperatures and Rheological Properties Like Self-Standing Ability and Rapid Recovery

We synthesized gel-forming polyelectrolytes having N,N'-(trans-cyclohexane-1,4-diyl)dibenzamide linkages, with chloride, bis(trifluoromethanesulfonyl)amide, bis(fluorosulfonyl)amide, or tetrafluoroborate anions that could gelatinize a variety of ionic liquids at very low concentrations. The temperatures at which these ionogels transitioned into isotropic fluids were greater than 100 °C even at concentrations as low as 5 g/L. In addition, the ionogels exhibited high mechanical strength without a significant loss in their ionic conductivities, along with the rapid recovery.

Effect of salt content on the rheological properties of hydrogel based on oligomeric electrolyte

Dynamic light scattering and oscillatory rheology experiments were performed to study the effects of various salts on the hydrogel consisting of an oligomeric electrolyte gelator, poly(pyridinium-1,4-diyliminocarbonyl-1,4-phenylenemethylene chloride) (1-Cl). Sol-gel transition temperature increased with increasing salt concentration that suggested the salt-in behavior. The concentration dependence of the dynamic shear moduli showed power-law scaling behavior and was compared with the predictions made by the fractal gel model. The brittleness was increased by increasing salt concentration, indicating that 1-Cl hydrogel became better packed into stronger networks in ionic solutions. After certain salt concentrations, 1-Cl hydrogel started precipitation that might be due to the excessive network formation resulting in collapse of the network structure. The recovery of the mechanical properties of 1-Cl hydrogel was completely reduced in the presence of salts.

Robust organogels from nitrogen-containing derivatives of (R)-12-hydroxystearic acid as gelators: comparisons with gels from stearic acid derivatives

Thirteen members of a new class of low molecular-mass organogelators (LMOGs), amides, and amines based on (R)-12-hydroxystearic acid (HSA; i.e., (R)-12-hydroxyoctadecanoic acid) and the properties of their gels have been investigated by a variety of structural and thermal techniques. The abilities of these LMOGs, molecules with primary and secondary amide and amine groups and the ammonium carbamate salt of 1-aminooctadecan-12-ol, to gelate a wide range of organic liquids have been ascertained. Their gelating efficiencies are compared with those of HSA and the corresponding nitrogen-containing molecules derived from stearic acid (i.e., HSA that lacks a 12-hydroxyl group). Several of the HSA-derived molecules are exceedingly efficient LMOGs, with much less than 1 wt % being necessary to gelate several organic liquids at room temperature. Generally, the self-assembled fibrillar networks of the gels consist of spherulitic objects whose dimensions depend on the protocol employed to cool the precursor sol phases. X-ray studies indicate that the LMOG molecules are packed in lamellae within the fibers that constitute the spherulites. In addition, some of the organogels exhibit unusual thixotropic properties: they recover a large part of their viscoelasticity within seconds of being destroyed by excessive strain shearing. This recovery is at least an order of magnitude faster than for any other organogel with a crystalline fibrillar network reported to date. Correlations of these LMOG structures (as well as with those that lack a hydroxyl group along the n-alkyl chain, a headgroup at its end, or both) with the properties of their gels, coupled with the unusual theological properties of these systems, point to new directions for designing LMOGs and organogels.