We report here the gelation behavior of two novel L-cysteine-based amphiphiles bearing a poly(ethylene glycol) tail. The amphiphiles were found to form transparent organogels in both apolar and aprotic polar solvents at reasonably low concentrations. In chloroform, dichloromethane, and benzene solvents, the organogels are formed at room temperature without the requirement of heating-cooling cycle due to strong hydrogen-bonding interaction between gelator molecules. The swelling kinetics, however, becomes faster on heating. Unlike most organogels of low-molecular-mass gelators, these organogels do not exhibit a gel-to-sol transition on heating but instead become rigid when heated. Surprisingly, in polar solvents, the gelation required a heating-cooling cycle, and the sol-to-gel transition was found to be reversible. The gelation abilities of the amphiphiles were correlated with the hydrogen-bonding parameters of the solvents. Intermolecular H-bonding interaction was found to be the major driving force for the organogelation. The morphology of the organogels was investigated by the use of optical as well as electron microscopy and was found to be dependent on the nature of solvent. The mechanical strengths of the organogels were studied by rheological measurements.
The development of multifunctional hydrogels with high strength and stretchability is one of the most important topics in soft-matter research owing to their potential applications in various fields. In this work, a dual physically cross-linked network was designed for the fabrication of ultrastretchable tough hydrogels. The hydrogels were prepared through in situ polymerization of acrylic acid and acrylamide in the presence of positively charged quaternary poly(ethylene imine) (Q-PEI) and micelle-forming Pluronic F127 diacrylate, thus introducing electrostatic interactions between the positively charged Q-PEI and negatively charged poly(acrylic acid-co-acrylamide). For further mechanical reinforcement, Ca 2+ and Cu 2+ ions were introduced into the hydrogel network to construct coordination bonds, significantly enhancing tensile strength as well as stretchability. The hydrogel prepared with Ca 2+ ion coordination bonds was found to be stretchable to 108 times its original length and exhibited a maximum toughness of 177 MJ•m −3 , representing one of the most robust systems with both extraordinary toughness and superstretchability prepared to date. The hydrogels also exhibited excellent recovery of dimensions and reproducibility in terms of mechanical properties, providing a promising ultrastretchable soft-matter system with outstanding mechanical strength.
Amphiphile containing l-cysteine covalently linked with poly(ethylene glycol) (PEG) chain (PEG360-Cys) was observed to produce transparent gel at room temperature in polar aprotic solvents not only by heating-cooling (HC) but also when subjected to ultrasound (US). It was observed that a suspension of PEG360-Cys when treated with US readily formed gel at much lower critical gelation concentration. US irradiation has been established to control the gel properties at the molecular level. The morphological change of the organogels produced by the HC and US methods was confirmed from scanning as well as transmission electron microscopy. The organogels produced by the two external stimuli (HC and US) were characterized in detail by FTIR spectroscopy, differential scanning calorimetry, and rheology to shed light on the molecular packing during gelation. It is important to note that the US-induced organogels showed almost 13-fold increase in gel strength compared to the organogels obtained by the HC method. Further, US-induced gels were found to be thermally more stable than the heat-set gels. All these studies clearly demonstrate that hydrogen-bonding interaction is the main driving force for both the gelation processes, but the mode of hydrogen bonding at the molecular level is different.
Hydrogels are used for a variety of technical and medical applications capitalizing on their three-dimensional (3D) cross-linked polymeric structures and ability to act as a reservoir for encapsulated species (potentially encapsulating or releasing them in response to environmental stimuli). In this study, carbohydrate-based organogels were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of a β-D-glucose pentaacetate containing methacrylate monomer (Ac-glu-HEMA) in the presence of a di-vinyl cross-linker; these organogels could be converted to hydrogels by treatment with sodium methoxide (NaOMe). These materials were studied using solid state 13C cross-polarization/magic-angle spinning (CP/MAS) NMR, Fourier transform infrared (FTIR) spectroscopy, and field emission scanning electron microscopy (FE-SEM). The swelling of the gels in both organic solvents and water were studied, as was their ability to absorb model bioactive molecules (the cationic dyes methylene blue (MB) and rhodamine B (RhB)) and absorb/release silver nitrate, demonstrating such gels have potential for environmental and biomedical applications.
Sustained release of doxorubicin from a β-aminoacid-containing cytocompatible tripeptidic hydrogel which shows thixotropic behaviour after PVA-induction.
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