Low Critical Gelation Concentrations Research Articles

Responsive organogels formed by supramolecular self assembly of PEG-block-allyl-functionalized racemic polypeptides into β-sheet-driven polymeric ribbons.

A chemically reactive hybrid diblock polypeptide gelator poly(ethylene glycol)-block-poly(dl-allylglycine) (PEG-b-PDLAG) is an exceptional material, due to the characteristics of thermo-reversible organogel formation driven by the combination of a hydrophilic polymer chain linked to a racemic oligomeric homopeptide segment in a range of organic solvents. One-dimensional stacking of the block copolymers is demonstrated by ATR-FTIR spectroscopy, wide-angle X-ray scattering to be driven by the supramolecular assembly of β-sheets in peptide blocks to afford well-defined fiber-like structures, resulting in gelation. These supramolecular interactions are sufficiently strong to achieve ultra low critical gelation concentrations (ca. 0.1 wt%) in N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and methanol. The critical gel transition temperature was directly proportional to the polymer concentration, so that at low concentrations, thermoreversibility of gelation was observed. Dynamic mechanical analysis studies were employed to determine the organogel mechanical properties, having storage moduli of ca. 15.1 kPa at room temperature.

N-stearoyl amino acid derivatives: Potent biomimetic hydro/organogelators as templates for preparation of gold nanoparticles

New potent N-(2-aminoethyl)-α-[(1-oxoheptadecyl)amino]acetamide derivative gelators were synthesized and their self-assembly in various common solvents was examined. These compounds exhibit high gelation ability in many organic solvents, such as toluene or acetonitrile, with a lower critical gelation concentration of 0.15 wt.% in some cases. The organogels were analyzed by 1H nuclear magnetic resonance (NMR) and Fourier transform-infrared (FT-IR) spectroscopies, and their phase transition temperatures (Tgel) were determined using differential scanning calorimetry (DSC). The homogeneity of the gel networks was observed using field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) and a lamellar structure was confirmed by X-ray diffraction (XRD) analysis. A correlation between the more stable conformations of N-acyl glutamine derivative determined by molecular modeling and the gelation behavior afforded evidence for the influence of the dimethylene side chain on the gelation ability. The organogels were employed as templates for the in situ preparation of stable gold nanoparticles (GNPs) using different reduction methods of HAuCl4 dispersed in the gel matrix, and the resulting GNPs solutions were studied using UV–Visible absorption and TEM.

A Crown Ether Appended Super Gelator with Multiple Stimulus Responsiveness

A crown ether appended super gelator is designed and synthesized. It can gel a variety of organic solvents and shows excellent gelation properties with both low critical gelation concentration and short gelation time. Due to the introduction of the crown ether moiety and a secondary ammonium unit, the supramolecular gels show reversible gel-sol transitions. The supramolecular gels can also be molded into shape-persistent and free-standing objects.

Perylene bisimide organogels formed by melamine·cyanurate/barbiturate hydrogen-bonded tapes

Ditopic melamines possessing one (1) or two (2) perylene bisimide (PBI) chromophores were synthesized and their self-aggregation and coaggregation with dodecyl cyanurate (dCA) or barbital (Bar) were investigated. Optically transparent organogels were formed through self-aggregation of 1 and coaggregation of 1 or 2 with dCA or Bar in nonpolar solvents. X-ray diffraction of the xerogels exhibited typical diffraction patterns assignable to lamellar structures, suggesting the formation of tapelike hydrogen-bonded motifs. Remarkably low critical gelation concentrations (cgc) of 8.0 × 10−4 M were revealed for all organogels, thus they can be classified as supergelators. Comparison of the thermal stabilities of the gels revealed that the gels containing 2 have higher melting temperatures (Tm) than those containing 1. Scanning electron microscopy and atomic force microscopy showed that the gels containing 1 are composed of sheetlike microstructures, whereas those containing 2 are composed of fibrous nanostructures, consistent with the difference in their thermal stabilities. The different self-assembled structures of our PBI aggregates can be related to whether extended stacks of PBI dyes along the hydrogen-bonded strands are possible or not. Ditopic hydrogen bonding melamines possessing perylene bisimide chromophores were synthesized and their self-aggregation and coaggregation with complementary guest molecules were investigated. In this system, 2-D lamellar structures of perylene bisimides were constructed through hierarchical organization of hydrogen-bonded tapes, which can gelate aliphatic and aromatic solvents. The resulting gels are highly transparent and exhibit remarkably low critical gelation concentrations of 8.0 × 10−4 M. The thermal stabilities and the mesoscopic morphologies of these organogels could be reasonably explained by the number of perylene bisimide chromophores introduced in the melamine components.

Additive-free hydrogelation of graphene oxide by ultrasonication

Graphene oxide hydrogels have been prepared by ultrasonication of precursor aqueous dispersions. The ultrasonication fractures the nanosheets, reducing their dimensions and exposing new sheet edges that do not possess the stabilizing carboxyl functional groups found along the edge of the as-prepared material. Ultrasonication does not affect the overall chemical functionality of the graphene oxide nanosheets, as spectra (carbon-13 nuclear magnetic resonance spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy) of samples before and after ultrasonication are nearly identical. Gelation is induced after only 30min of ultrasonication to achieve a relatively weak gel with a shear modulus of 0.3kPa; however, extension of ultrasonic treatment to 120min yields a more robust hydrogel with a shear modulus of 1.6kPa. Such enhancement in the gel’s physical properties can be attributed to the lack of stabilizing carboxyl groups on newly generated nanosheet fragments from the interior regions of the original nanosheets. As prepared, these hydrogels exhibit exceptionally low critical gelation concentrations ranging from ∼0.050 to ∼0.125mgmL−1 that can be tuned according to the extent of ultrasonic treatment.

Synthesis and Characterization of a Multi‐Sensitive Crosslinked Injectable Hydrogel Based on Pluronic

This paper reports the construction of a novel multi-sensitive chemically crosslinked injectable hydrogel with strong mechanical strength by modifying Pluronic F127 responsive against temperature, pH and redox potential. Crosslinked polymer between benzaldehyde grafted Pluronic (P-A) and amine end capped Pluronic having disulfide linkage (P-B) have been synthesized and characterized with 1H NMR spectroscopy and GPC. The hydrogel under physiological conditions significantly altered sol-gel transition behaviors with much lower critical gelation concentrations and temperatures, compared to Pluronic hydrogels. The rheological characterization demonstrated that the moduli of the hydrogels were able to be tuned depending on molecular weight as well as pH, redox and temperature conditions.

On the Gelation of Graphene Oxide

Graphene oxide (GO) has been recognized as a unique two-dimensional building block for various graphene-based supramolecular architectures. In this article, we systematically studied the three-dimensional self-assembly of GO sheets in aqueous media to form hydrogels. The gelation of GO can be promoted by different supramolecular interactions, including hydrogen bonding, π-stacking, electrostatic interaction, and coordination. Furthermore, the lateral dimensions of GO sheets also have strong influences on GO gelation. The resulting GO hydrogels exhibited low critical gelation concentrations and good reversibility upon chemical stimulations. These findings indicate that GO has rich supramolecular properties, and its hydrogels may have a variety of technological applications.

Tailoring Micelle Formation and Gelation in (PEG−P(MA-POSS)) Amphiphilic Hybrid Block Copolymers

We demonstrate that hydrophobic POSS (polyhedral oligomeric-silsesquioxane) nanoparticles, added to block copolymer solutions of poly(ethylene glycol) (PEG) and poly(methacrylisobutyl polyhedral oligomericsilsesquioxane) P(MA-POSS) as the hydrophilic and hydrophobic blocks respectively, could be employed to tailor their micelle formation, gelation, and rheological performance. For example, the hydrodynamic size of the micelles, formed by PEG5k-b-P(MA-POSS)3.6, increased from 13.6 ± 1.0 to 56.9 ± 3.7, an increase of more than four times, with 0.1 wt % (with respect to block copolymer) POSS nanoparticles in solution. However, the micelles retain their spherical morphology with core−shell structure as evidenced by calculating the values of dimensionless ratios, Rg/Rh for micelles as a function of added POSS nanoparticles content. For P(MA-POSS)-b-PEG10k-b-P(MA-POSS) triblock copolymers, which is associative in nature, addition of POSS nanoparticles resulted in formation of more robust and stronger hydrogels with a significantly higher storage modulus, G′, yield strengths, σy, and lower critical gelation concentration, cg, as compared with those for the pristine triblock copolymer. The presence of eight vinyl groups, attached to the POSS nanoparticles under investigations, is also exploited for further enhancement of rheological properties of the hydrogels with UV treatment. Finally, gel formation is induced in aqueous solutions of PEG5k-b-P(MA-POSS)3.6 diblock copolymer by introducing P(MA-POSS)-b-PEG10k-b-P(MA-POSS) triblock copolymer chains, and the rheological performance of the produced hydrogels, with certain compositions, is even superior to that of pure triblock copolymer gel.

Enzyme-mediated cross-linking of Pluronic copolymer micelles for injectable and in situ forming hydrogels

A new class of injectable and erodible hydrogels exhibiting highly robust gel strength at body temperature was fabricated by enzyme-mediated cross-linking between Pluronic copolymer micelles. Tyramine-conjugated Pluronic F-127 tri-block copolymers at two terminal ends of polyethylene oxide (PEO) side chains were synthesized and utilized to form self-assembled micelles in aqueous solution. Tyrosinase was employed to convert tyramine-conjugated micelles to highly reactive catechol conjugated micelles that could further cross-link individual Pluronic copolymer micelles to form a highly stable gel structure. The enzyme cross-linked Pluronic hydrogels showed far lower critical gelation concentration, concomitantly showing enhanced gel strength compared to unmodified Pluronic copolymer hydrogels, suitable for sustained delivery of bioactive agents. Rheological studies demonstrated that the enzyme cross-linked hydrogels exhibited a fast and reversible sol–gel transition in response to temperature while maintaining sufficient mechanical strength at the gel state. In situ formed hydrogels were eroded gradually, releasing FITC-labeled dextran in an erosion-controlled manner. Moreover, they showed tissue-adhesive properties due to the presence of unreacted catechol groups in the gel structure. Enzyme cross-linked Pluronic hydrogels could be potentially used for delivery applications of drugs and cells.

New thermogelling copolymers composed of heptakis(2,6-di-O-methyl)-β-cyclodextrin, poly(propylene glycol), and poly(ethylene glycol)

A series of new thermosensitive polyurethane copolymers composed of heptakis(2,6-di-O-methyl)-β-cyclodextrin (DM-β-CD), poly(propylene glycol) (PPG), and poly(ethylene glycol) (PEG) blocks using 1,6-hexamethylene diisocyanate (HMDI) as a coupling agent were synthesized. Their chemical structures and molecular characteristics were studied using GPC analysis and 1H NMR and 13C NMR spectral analysis. We found that the copolymers had high molecular weight and the DM-β-CD, PPG, and PEG blocks were bonded by a hexamethylene dicarbamyl (HMDC) junction. DSC analysis showed that the molar motion of the PPG and PEG segments was restricted in the copolymers. Moreover, the copolymers are amphiphilic and they have very low critical micelle concentration (CMC) values ranging from 2.63–8.51 g L−1. Upon increasing temperature, the aqueous solutions of the copolymers were found to undergo a reversible clear sol–gel–turbid sol transition. The results from phase diagrams showed that the copolymers had very low critical gelation concentration (CGC) values ranging within 3–6 wt%. The copolymers synthesized in the study are easily soluble in water at room temperature, thus hydrophobic pharmaceutical agents could be entrapped into the hydrophobic cavity of CDs or loaded in the core of micelles. Then, these pharmaceutical agents are encapsulated into the gel when the copolymer forms a gel depot at around body temperature. Hence, these copolymers have potential to be used as injectable drug delivery systems.

Thermo-sensitive and biodegradable hydrogels based on stereocomplexed Pluronic multi-block copolymers for controlled protein delivery

Injectable sol–gel transition hydrogels based on thermo-sensitive polymers are of great interest as potential biomaterials for sustained delivery of therapeutic molecules. A novel temperature-sensitive and in-situ forming hydrogel system based on Pluronic F127 was developed and evaluated. A series of multi-block Pluronic copolymers linked by d-lactide and l-lactide oligomers with different spacer lengths were synthesized. A pair of multi-block copolymers having the corresponding enantiomeric d- or l-lactide oligomer spacer was blended to form stereocomplexed hydrogels. The resultant physically crosslinked Pluronic hydrogels exhibited significantly altered sol–gel phase transition behaviors with much lower critical gelation concentrations and temperatures, compared to the uncomplexed multi-block or Pluronic homopolymer hydrogels. The stereocomplexed hydrogels also had far increased mechanical strength with high resistance to rapid dissolution in aqueous medium. When human growth hormone (hGH) was incorporated in the strereocomplexed multi-block Pluronic copolymers, hGH was released out in a sustained and zero-order fashion for 13 days by a diffusion/erosion coupled mechanism.

New Biodegradable Thermogelling Copolymers Having Very Low Gelation Concentrations

New biodegradable multiblock amphiphilic and thermosensitive poly(ether ester urethane)s consisting of poly[(R)-3-hydroxybutyrate] (PHB), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) blocks were synthesized, and their aqueous solutions were found to undergo a reversible sol-gel transition upon temperature change at very low copolymer concentrations. The multiblock poly(ether ester urethane)s were synthesized from diols of PHB, PEG, and PPG using 1,6-hexamethylene diisocyanate as a coupling reagent. The chemical structures and molecular characteristics of the copolymers were studied by GPC, 1H NMR, 13C NMR, and FTIR. The thermal stability of the poly(PEG/PPG/PHB urethane)s was studied by thermogravimetry analysis (TGA), and the PHB contents were calculated based on the thermal degradation profile. The results were in good agreement with those obtained from the 1H NMR measurements. The poly(PEG/PPG/ PHB urethane)s presented better thermal stability than the PHB precursors. The water soluble poly(ether ester urethane)s had very low critical micellization concentration (CMC). Aqueous solutions of the new poly(ether ester urethane)s underwent a sol-gel-sol transition as the temperature increased from 4 to 80 degrees C, and showed a very low critical gelation concentration (CGC) ranging from 2 to 5 wt %. As a result of its multiblock architecture, a novel associated micelle packing model can be proposed for the sol-gel transition for the copolymer gels of this system. The new material is thought to be a promising candidate for injectable drug systems that can be formulated at low temperatures and forms a gel depot in situ upon subcutaneous injection.

Thermoreversible gelation of polyethylene/decalin mixtures by self-nucleated crystallization

The critical gelation concentration of polyethylene gels formed in decalin by self-nucleation was studied as a function of the gelation temperature, the polymer dissolution temperature, and the time at the dissolution temperature. A comparison was also made with gels formed after cooling from a high temperature. The morphology of the various gels was examined to elucidate the gelation behavior. Self-nucleation of crystalline polymers in solution produces efficient gels that have a low critical gelation concentration and are relatively stronger at higher polymer concentrations than gels produced otherwise. The efficiency arises from the structural unit being a single lamella (but not necessarily a single polymer crystal) and by neighboring lamellae tending to rotate in solution to approach coplanarity, which allows an extensive attachment area between the lamellae. In addition, the lamellae in the gels appeared to all have the same size, and the critical gelation concentration was found to be relatively independent of the lamellar size.