Hydrogen-bonding-mediated vesicular assembly of functionalized naphthalene-diimide-based bolaamphiphile and guest-induced gelation in water

This paper describes the spontaneous vesicular assembly of a naphthalene-diimide (NDI)-based non-ionic bolaamphiphile in aqueous medium by using the synergistic effects of π-stacking and hydrogen bonding. Site isolation of the hydrogen-bonding functionality (hydrazide), a strategy that has been adopted so elegantly in nature, has been executed in this system to protect these moieties from the bulk water so that the distinct role of hydrogen bonding in the self-assembly of hydrazide-functionalized NDI building blocks could be realized, even in aqueous solution. Furthermore, the electron-deficient NDI-based bolaamphiphile could engage in donor-acceptor (D-A) charge-transfer (CT) interactions with a water-insoluble electron-rich pyrene donor by virtue of intercalation of the latter chromophore in between two NDI building blocks. Remarkably, even when pyrene was located between two NDI blocks, intermolecular hydrogen-bonding networks between the NDI-linked hydrazide groups could be retained. However, time-dependent AFM studies revealed that the radius of curvature of the alternately stacked D-A assembly increased significantly, thereby leading to intervesicular fusion, which eventually resulted in rupturing of the membrane to form 1D fibers. Such 2D-to-1D morphological transition produced CT-mediated hydrogels at relatively higher concentrations. Instead of pyrene, when a water-soluble carboxylate-functionalized pyrene derivative was used as the intercalator, non-covalent tunable in-situ surface-functionalization could be achieved, as evidenced by the zeta-potential measurements.     

Dynamic multicomponent hemiaminal assembly

A simple approach to generating in situ metal-templated tris-(2-picolyl)amine-like multicomponent assemblies with potential applications in molecular recognition and sensing is reported. The assembly is based on the reversible covalent association between di-(2-picolyl)amine and aldehydes. Zinc ion is best for inducing assembly among the metal salts investigated, whereas 2-picolinaldehyde is the best among the heterocyclic aldehydes studied. Although an equilibrium constant of 6.6×10(3) M(-1) was measured for the assembly formed by 2-picolinaldehdye, di-(2-picolyl)amine, and zinc triflate, the equilibrium constants for other systems are in the 10(2) M(-1) range. X-ray structural analysis revealed that zinc adopts trigonal-bipyramidal geometry within the assembled ligand. The diversity and equilibrium of the assemblies are readily altered by simply changing concentrations, varying components, or adding counteranions.     

Anisotropic self‐assembly and gelation in aqueous methylcellulose—theory and modeling

Recent experimental studies demonstrated that the aqueous methylcellulose (MC) polymer chains in water can form nanoscale fibrils (diameter ～14 nm, persistence length ～60 nm), and those fibrils can organize into networks at higher temperatures and/or concentrations, forming the commonly observed gel. Here we propose that the fibrils are one‐dimensional self‐assemblies of stacked, fused polymer rings that are formed at elevated temperatures due to the changing nature of the MC‐water hydrogen bonding. This mechanism is analogous to the coil‐helix transition in polypeptides, although it is not clear whether the MC fibrils possess chirality. We perform coarse‐grained molecular simulations of MC chain structure at temperatures both above and below the hypothesized coil‐to‐ring transition, with CG forcefield tuned by atomistic molecular dynamics simulations, and observe the expected conformational change. We then develop a statistical mechanical theory to predict the fibril self‐assembly, gelation and rheology as function of temperature and concentration. The findings are in reasonable agreement with experimental data and could be generalized to other carbohydrate polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1624–1636     

Molecular hydrogel-immobilized enzymes exhibit superactivity and high stability in organic solvents

This communication describes the use of a molecular hydrogel to immobilize enzymes for catalyzing reactions in an organic solvent to attain superactivity and exceptional stability.     

Viscosity divergence and gelation

A new model of gelation is presented in which the viscosity is observed to diverge without the introduction of shear elasticity when a critical concentration of suspended particles is reached. This viscosity divergence results from the hydrodynamic interaction between the suspended particles and can be explained using the Random Resistance Network model. A comparison of the data and with other models is also presented.     

Dynamic covalent assembly of stimuli responsive vesicle gels

We describe the reversible formation of stimuli-responsive vesicle gels from polymerised dynamic covalent surfactants, by simple mixing of soluble surfactant precursors in water under ambient conditions.     

Dynamic assembly of anionic surfactant into highly-ordered vesicles

A highly-efficient dynamic assembly method for the transformation of the initial spongy lamellar structure of concentrated linear alkylbenzene sulfonate, LAS, incorporated with sodium silicate, into spherulitic vesicles is presented. A combination of drag and pressure flows, via twin screw extrusion, was used to mitigate the ubiquitous viscoplasticity and the wall slip behavior of the anionic surfactant paste and gave rise to the dynamic assembly of stable vesicular nanostructures within a narrow size range, that was not possible with either pure drag or pure pressure flows. Concomitantly with the structure transformation of the paste during assembly under the combination of pressure and drag flows, significant changes in its viscoelasticity, i.e., order of magnitude increases in storage and loss moduli and magnitude of complex viscosity, were observed. The demonstrated dynamic assembly of stable vesicular nanostructures, with vesicle diameters within the relatively narrow range of 300-600 nm, from a commodity surfactant is relevant to myriad templating and encapsulation applications, as well as shedding light on the mechanisms of the deformation-induced planar lamellar to vesicle transformation of concentrated amphiphiles.     

Strong fluorescence emission induced by supramolecular assembly and gelation: luminescent organogel from nonemissive oxadiazole-based benzene-1,3,5-tricarboxamide gelator

Supramolecular aggregation of a novel nonfluorescent gelator yields highly luminescent organogels in aprotic organic solvents through intermolecular hydrogen bonding, which is a key motif for both self-assembly and photophysical process control.     

Dynamic mechanical behavior of ultradrawn polyethylene films produced by gelation/crystallization from solution

This paper is concerned with the temperature dependence of mechanical properties of ultradrawn polyethylene in terms of storage modulus E' and loss factor tan by the measurement of the complex dynamic tensile modulus over ranges of temperature from 20 to 140 C. Interestingly, E' of a specimen with drawn ratio of 300 is about 120 GPa at 140 C, when the measurement is carried out at a frequency of 100 Hz. This is a very high value. In addition, the drawn specimens were irradiated to try to produce ultra-drawn polyethylene films with more excellent mechanical and thermal properties. However, the melting peak shifts to a lower temperature with increasing radiation dose. This result is probably attributed to the considerably radiation-induced scission of extended chains constructing crystals.     

Dynamic thickening investigation of the gelation process of PAM/PEI system at high temperature and high pressure

The gelation process of organically polymer gel was investigated by dynamic thickening measurements. Rheological measurements were used to evaluate the viscosity of the gel. During the gelation process, high temperature resulted in higher rate of crosslinking. Rigid and stable gel was formed in neutral and alkaline media, and the higher of the pH value, the faster of the gelation process. However, gel could not be formed in acid condition. Moreover, the rate of crosslinking increased with the increase of concentration of polymer and crosslinker. The addition of NH₄Cl elongated the gelation time significantly, but played a negative role in the gel strength, while a rigid gel was formed in the presence of Sodium acetate or trisodium citrate dehydrate. This paper summarizes the results and discusses how various parameters affect the gelation process of the gel.     

Casein structure,self-assembly and gelation

The literature on recent studies of casein structure and function is reviewed. Where appropriate, we try to reconcile conflicting views on the issue of secondary structure in these proteins, steering a middle course where possible. A suggestion is put forward that a coarser view, treating the caseins as block copolymers may be sufficient to rationalise much of the behaviour of these proteins in self-association, adsorption and micellar assembly.     

Sequential two-step multienzyme synthesis of tumor-associated sialyl T-antigens and derivatives

A series of α2-3-sialylated β1-3-linked galactosides, including sialyl T-antigens, 3'-sialyl galacto-N-biose, 3'-sialyl lacto-N-biose, and their derivatives containing natural and non-natural sialic acid forms have been synthesized from simple monosaccharides using an efficient sequential two-step multienzyme approach.     

A lattice model of vitrification and gelation

In this work we introduce a simple lattice model with T-shaped molecules in two dimensions that exhibits a rich range of morphological behaviors. Depending on the volume fraction and quench path, this system can adopt uniform liquid, solution, and phase-separated states, as well as inhomogeneous glass or gel-like states, as revealed by dynamic mean-field simulations. An important characteristic of this system is the existence of a large number of degenerate low-energy states with small barriers that leads to a broad, kinetically explored landscape. The mean-field stability and phase diagram of this model is constructed and provides a useful guide for understanding the complex behaviors of the system. One striking feature is that there is a cascade of instabilities that converge to mark the onset of what we identify as the glass transition. Both dynamic mean-field and Monte Carlo simulations reveal glass-like relaxation dynamics. Our results lead to a picture of gelation as a continuation of the glass transition into the two-phase region, or equivalently, as an incomplete phase separation arrested by the onset of the glass transition.     

Novel thymidine-based organogelators and their gelation behaviours

We report on a synthesis of novel thymidine based organogelators and a study of their gelation types in relation to structure and solvent, using various data acquired through FT-IR, SEM images and differential scanning calorimetry.     

Aggregation and gelation of micellar casein particles

Micellar casein particles (submicelles) are formed by removing calcium phosphate from native casein. The submicelles aggregate and eventually form a gel with a rate that increases strongly with increasing temperature and casein concentration. At low casein concentrations the gel is very weak and collapses under its own weight so that a precipitate is formed. The structure of the aggregates is studied using light scattering and cryo-electron microscopy. It is found that the aggregates have a self-similar structure with fractal dimension 2. The viscoelastic properties of the gel are studied by frequency scans of the loss and storage moduli during the gelation process. The bonds between the submicelles probably involve calcium phosphate complexes.     

Effect of composition and gelation conditions on structural changes accompanying the gelation of PAN,PVA and gelatin solutions

Summary For three types of polymer-solvent systems: polyacrylonitrile in dimethylformamide, polyvinyl alcohol in water and gelatin in water the isothermal gelation process was followed with viscometry, light scattering and X-ray diffraction techniques. Effects of thermal history (quenching, melting) and solvent composition were investigated. It was found that the variation of structure as revealed by these methods (molecular aggregation, crystallization etc.) is not specific for the gelation itself but associated with some side-processes which result from retarded phase separation. The molecular mechanism of “pure gelation” proceeding at unchanged supramolecular structure is discussed.

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Zusammenfassung Für drei Systeme Polymer/L?sungsmittel, n?mlich für Polyacrylnitril in Dimethylformamid, für Polyvinylalkohol in Wasser und für Gelatine in Wasser, wurde die isotherme Gelierung viskosimetrisch, mit Lichtstreuung und mit R?ntgenbeugung beobachtet. Die Wirkungen der thermischen Vorgeschichte (Abschrecken, Schmelzen) und der L?sungsmittelzusammensetzung wurden untersucht. Es ergab sich, da? die ?nderung der Struktur, wie sie mit diesen Methoden aufgedeckt wurde (Molekularaggregation, Kristallisation usw.) nicht spezifisch für die Gelatinierung ist, sondern mit einigen Nebenprozessen, die aus verz?gerter Phasentrennung resultieren, verknüpft ist. Der molekulare Mechanismus einer reinen Gelatinierung, der bei unver?nderter Supermolekularstruktur stattfindet, wird diskutiert.

     

Screen-printed multienzyme arrays for use in amperometric batch and flow systems

Screen-printing technology for electrode fabrication enables construction of amperometric devices suitable for combination of several enzyme electrodes. To develop a biosensor array for characterisation of wastewaters, tyrosinase and horseradish peroxidase (HRP) or cholinesterase-modified electrodes were combined on the same array. The behaviour of the tyrosinase-modified electrode in the presence of hydrogen peroxide (required co-substrate for the HRP-modified electrode) and acetylthiocholine chloride (required co-substrate for cholinesterase) was studied. Performance of bi-enzyme biosensor arrays in the batch mode and in the flow-injection system are discussed.     

Topology evolution and gelation mechanism of agarose gel

Kinetics as well as the evolution of the agarose gel topology is discussed, and the agarose gelation mechanism is identified. Aqueous high melting (HM) agarose solution (0.5% w/v) is used as the model system. It is found that the gelation process can be clearly divided into three stages: induction stage, gelation stage, and pseudoequilibrium stage. The induction stage of the gelation mechanism is identified using an advanced rheological expansion system (ARES, Rheometric Scientific). When a quench rate as large as 30 deg C/min is applied, gelation seems to occur through a nucleation and growth mechanism with a well-defined induction time (time required for the formation of the critical nuclei which enable further growth). The relationship between the induction time and the driving force which is determined by the final setting temperature follows the 3D nucleation model. A schematic representation of the three stages of the gelation mechanism is given based on turbidity and rheological measurements. Aggregation of agarose chains is promoted in the polymer-rich phase and this effect is evident from the increasing mass/length ratio of the fiber bundles upon gelation. Continuously increasing pore size during gelation may be attributed to the coagulation of the local polymer-rich phase in order to achieve the global minimum of the free energy of the gelling system. The gel pore size determined using turbidity measurements has been verified by electrophoretic mobility measurements.