Glassy Gel Research Articles

Glassy gels toughened by solvent.

Glassy polymers are generally stiff and strong yet have limited extensibility1. By swelling with solvent, glassy polymers can become gels that are soft and weak yet have enhanced extensibility1-3. The marked changes in properties arise from the solvent increasing free volume between chains while weakening polymer-polymer interactions. Here we show that solvating polar polymers with ionic liquids (that is, ionogels4,5) at appropriate concentrations can produce a unique class of materials called glassy gels with desirable properties ofboth glasses and gels. The ionic liquid increases free volume and therefore extensibility despite the absence of conventional solvent (for example, water). Yet, the ionic liquid forms strong and abundant non-covalent crosslinks between polymer chains to render a stiff, tough, glassy, and homogeneous network (that is, no phase separation)6, at room temperature. Despite being more than 54 wt% liquid, the glassy gels exhibit enormous fracture strength (42 MPa), toughness (110 MJ m-3), yield strength (73 MPa) and Young's modulus (1 GPa). These values are similar to those of thermoplastics such as polyethylene, yet unlike thermoplastics, the glassy gels can be deformed up to 670% strain with full and rapid recovery on heating. These transparent materials form by a one-step polymerization and have impressive adhesive, self-healing and shape-memory properties.

Kinetic insights into glassy hydrogels with hydrogen bond complexes as the cross-links

Revealing the structure formation kinetics is a long-term challenging issue in the development of tough hydrogels, although they are significant for understanding structure-property relationship and toughening mechanism. Here, we report a series of tough and stiff poly(methacrylamide-co-methacrylic acid) hydrogels, with a focus on the structure-property relationship and structure formation kinetics. These hydrogels in a glassy state possess moderate water content and excellent mechanical properties with Young's modulus up to 200 MPa. The microstructure of the gels changes from uniform to bicontinuous and then to colloidal network as the fraction of methacrylamide, fam, increases, accounting for the ductile-brittle transition of the mechanical performances. Sequential gelation and vitrification take place in the systems with relatively low fam to form transparent gels with a homogeneous matrix, whereas colloidal jamming and coarsening occur in the systems with high fam to form opaque gels with a colloidal network structure. The structure and properties of the glassy gels are determined by the hydrogen bond complexation and the microphase separation that are strengthened by the increase in fam. Based on these findings, the mechanical properties of hydrogels with high fam can be improved by suppressing the microphase separation during the gel synthesis. Understanding the structure-property relationship and regulation strategy of both microstructure and macroscopic performance of these glassy hydrogels should be inspirative for designing other tough materials with diverse applications as structural elements in biomedical and engineering fields.

Photodirected Morphing Structures of Nanocomposite Shape Memory Hydrogel with High Stiffness and Toughness.

Shape memory hydrogels have drawn increasing attention in recent years. Practical applications require these hydrogels to have good mechanical properties as well as contactless stimulations to trigger the shape deformations. Here we report a stiff and tough shape memory hydrogel that can transform to various configurations sequentially by phototriggered site-specific deformations. Response of the shape memory hydrogel to near-infrared (NIR) light irradiation was achieved by incorporating gold nanorods (AuNRs) into the glassy gel matrix of poly(methacrylic acid-co-methacrylamide) without compromising the excellent mechanical properties. Owing to the photothermal effect of the AuNRs, the localized temperature rise led to a dramatic decrease in Young's modulus (from 200 to 2 MPa) of the prestretched hydrogel and bending deformation with a programmable direction and amplitude. More complex three-dimensional configurations can be obtained by multidirectional prestretching and shape memorizing the individual parts of the nanocomposite hydrogel. Furthermore, the AuNRs embedded in the gel were aligned along the prestretching direction, leading to anisotropic plasmon resonance. These photomediated programmable deformations of tough shape memory hydrogels should find applications in the biomedical and engineering fields.

Investigations on the blue luminescence enhancement of organically modified SiO2-TiO2-PDMS glass matrix

A novel class of Organically Modified Silicates (ORMOSILs) has been synthesized by non-hydrolytic sol–gel method at low temperature, incorporating the organic polymer, polydimethylsiloxane (PDMS) in tetraethylorthosilicate (TEOS) and titanium (IV) isopropoxide (TIP) based glassy gels. The structural modification of pure SiO2-TiO2 matrix by PDMS and its effects on optical and photoluminescence spectroscopy were investigated in detail. XRD pattern and HR-TEM images confirm the structural modification of the SiO2-TiO2-PDMS glass matrix. The formation of TiO2 nanocrystals of about 5–6 nm in size, embedded in the amorphous SiO2 matrix, has been confirmed from HR-TEM and SAED patterns. The TG–DTA analysis revealed the improved thermal stability of the ORMOSIL and the DSC analysis shows that its glass transition temperature decreases to 55.23 °C. A distinctive enhanced blue emission at 470 nm has been observed in photoluminescence studies of the prepared samples. FT-IR studies confirmed the formation of Ti–O–Ti, Si–O–Ti and Si–CH3 bonds by the PDMS modification of pure SiO2-TiO2 glass. These bonds and TiO2 nanocrystals embedded in the amorphous glass matrix together play an important role in the photoluminescence properties of the ORMOSIL. SiO2-TiO2 matrix has a luminescence quantum efficiency of 22.2% and the PDMS incorporation has increased the quantum efficiency to 55.8%. Thus SiO2-TiO2-PDMS ORMOSIL can be used as an efficient blue emitting host matrix for WLED fabrication.

Linear shear rheology of aging β-casein films adsorbing at the air/water interface

In this work, the viscoelasticity of fragile β-casein films has been followed using different macro- and microrheological techniques. The modulus of the complex surface viscosity |η∗| varies with time, allowing for the monitoring of the protein adsorption and annealing. β-casein adsorption creates a soft glassy gel at the interface that experiences an aging process. Macrorheological experiments with multiple probe sizes in addition to microrheological experiments demonstrated the consistency of the surface rheological properties over a broad range of viscosities. Surface pressure measurements were performed to complement the characterization of the processes.

Polystyrene nanoparticles with tunable interfaces and softness

Polystyrene nanoparticles, cross-linked with divinylbenzene (PS-(DVB)x) and having radii, Rp ≤ 10 nm, have been synthesized using batch and semi-batch radical microemulsion polymerizations. The nanoparticles were characterized thoroughly using 1H NMR, size exclusion chromatography, differential scanning calorimetry, and various small-angle scattering techniques (light, X-rays, and neutrons). Control over network and interface morphologies of the PS-(DVB)x nanoparticles is readily achieved by varying the concentration of divinylbenzene and the polymerization technique (batch vs. semi-batch). Small-angle neutron scattering (SANS) allowed us to identify three distinct nanoparticle morphologies: (i) fuzzy soft gels with flexible chain-segments tethered/looped at the interface of a homogeneous core, (ii) smooth soft gels without a fuzzy interfacial layer, and (iii) dendritic glassy gels exhibiting an inhomogeneous core with an ill-defined interface. Atomic force microscopy imaging supports the morphologies and the softness of the nanoparticles as indicated by SANS.

Thermo-reversible gelation of atactic poly(methyl methacrylate) in poly(ethylene glycol) oligomers

The temperature-concentration behavior of physical gel by atactic poly(methyl methacrylate) (aPMMA) in poly(ethylene glycol) oligomer (PEG400) was investigated. A liquid-liquid demixing interferes with a glass transition during cooling. The combination of demixing and T g leads to the formation of amorphous gels at low temperature. We suggest that the gelation of aPMMA/PEG400 is a glassy gel, in which short-range attractive depletion interaction in the polymer/oligomer system was the driving force at molecular level.

Anomalously Large Bend Elastic Constant and Faster Electro-Optic Response in Soft Glassy Gels Formed by a Dipeptide

In this article, results of static/dynamic Freedericksz transformation and rheological studies on anisotropic thermoreversible gels formed by gelation of a nematic liquid crystal (NLC) with a monodisperse dipeptide are presented. Freedericksz transformation studies reveal a surprising result: the gel state possesses a very large Frank bend elastic constant value, order of magnitude higher than the sol state, whereas, the splay elastic constant shows relatively a small increase. The studies also demonstrate that the anisotropic gel to anisotropic sol transition can be tracked by simply monitoring the Freedericksz transformation. Other attractive features of the gel state include the vanishing of the undesirable backflow effect, and nearly an order of magnitude decrease of switching speed. Further, rheological studies bring out the soft glassy characteristics of these gels.

Shear-banding phenomena and dynamical behavior in a Laponite suspension

Shear localization in an aqueous clay suspension of Laponite is investigated through dynamic light scattering, which provides access both to the dynamics of the system (homodyne mode) and to the local velocity profile (heterodyne mode). When shear bands form, a relaxation of the dynamics typical of a gel phase is observed in both bands soon after the flow stops. Periodic oscillations of the flow behavior, typical of a stick-slip phenomenon, are also observed when shear localization occurs. Both results are discussed in the light of various theoretical models for soft glassy gels.

Anomalous dynamical light scattering in soft glassy gels.

We compute the dynamical structure factor S( q,tau) of an elastic medium where force dipoles appear at random in space and in time, due to "micro-collapses" of the structure. Various regimes are found, depending on the wave vector q and the collapse time theta. In an early time regime, the logarithm of the structure factor behaves as (q tau)(3/2), as predicted in (L. Cipelletti et al., Phys. Rev Lett. 84, 2275 (2000)) using heuristic arguments. However, in an intermediate-time regime we rather obtain a (q tau)(5/4) behaviour. Finally, the asymptotic long-time regime is found to behave as q(3/2)tau. We also give a plausible scenario for aging, in terms of a strain-dependent energy barrier for micro-collapses. The relaxation time is found to grow with the age t(w), quasi-exponentially at first, and then as t(w)(4/5) with logarithmic corrections.

Synthesis of ZnO nanopowder via an aqueous acetate–citrate gelation method

The synthesis of nanoparticulate ZnO via an aqueous carboxylate gelation route is presented. Starting from a solution of zinc acetate with citric acid as a complexing agent, a solid glassy gel is obtained after drying that is converted into a fine powder by calcination. It is found that a very homogeneous precursor is indispensable when preparing very fine particles with a narrow size distribution. Cryo-transmission electron microscopy (Cryo-TEM) investigation is used as a feedback tool to prevent early precipitation during gelation. Study of the thermal decomposition of the gel shows that ZnO is formed before the final decomposition step takes place. After removing the organic backbone, very small oxide particles are found. The influence of the thermal treatment parameters on the particle size is investigated and a particle growth process is found. By a proper adjustment of the final calcination temperature in dry air, the mean particle size can be controlled between ∼11 and 175 nm. It was also seen that even in inert atmosphere, ZnO is formed and that particle morphology is greatly influenced by the calcination atmosphere.

Equilibrium and kinetic studies of copper(II) ion uptake by chitosan-tripolyphosphate chelating resin

Spherical chitosan-tripolyphosphate (TPP) chelating resins are successfully synthesized by an in-liquid ionotropic crosslinking method, and are then employed to examine their uptaking ability for copper(II) ion. The crosslinking characteristics of the beads were improved by the modification of the in-liquid curing mechanism in different pH of TPP solution. Sorption capacities and kinetics of the gel beads are greatly influenced by the polymeric physical or physicochemical characteristics of the chitosan-TPP networks, such as ionic crosslinking and swelling properties. Ionic crosslinking not only reduced the crystallinity of the beads but also decreased the amino binding site on chitosan for metal ions uptakes, thus resulting in a decrease of Cu 2+ adsorption. The copper(II) ions adsorbed by the chitosan beads at lower pH (pH 2 and 3) are limited by the poor complex reaction between metal ions and macromolecules at swelling state. The rate of uptake of metal ions to the chitosan beads at the original pH of the Cu 2+ solution (pH ∼ 5) is conditioned by the intraparticular diffusion of metal ions into the glassy gel phase, which has been examined by EDS analysis. The excellent saturation adsorption capacity of chitosan-TPP beads for Cu 2+ ion at the best working condition (pH ∼ 5) was about 200 mg Cu 2+ ion per gram at an initial concentration of 1000 ppm of Cu 2+ ions.

B-Site Doped Lanthanum Strontium Manganites by the DAAS Technique

AbstractMagnetic perovskites of the general form La0.7Sr0.3Mn1−yFeyO3 (y = 0, 0.05, 0.10, and 0.15) have successfully been synthesized using deposition by aqueous acetate solution (DAAS). Crystalline, iron-doped, lanthanum strontium manganite (Fe-doped LSMO) powders are obtained by preparing an aqueous solution of metal acetate precursors in the proper stoichiometry, drying the solution to generate a glassy gel, consolidating the gel, and then firing the gel for short periods of time (<2hrs). This novel technique has the potential for depositing large area thin films with high throughput and low cost. Powder samples of La0.7Sr0.3Mn1−yFeyO3 prepared by DAAS and annealed for 100 minutes at 1200°C are of high purity, are single phase, and exhibit excellent electrical and magnetic characteristics. Powders annealed at 1200°C or greater exhibit sharp metalinsulator transitions. Increasing the iron dopant concentration in these powders from 0% to 15% decreases the metal-insulator transition temperature of these samples from ∼360K to about 140K. The resistivity of these powders also increases with increasing substitution of the lattice B-site with iron, as does the unit cell volume of the lattice. Preparation of an iron doped LSMO powder that exhibits a maximum magnetoresistance at 305K was successfully carried out via a careful selection of iron content and anneal temperature. This compound, La0.7Sr0.3Mn0.93Fe0.07O3, exhibits a magnetoresistance of 40% at 305K and an applied field of 5 Tesla.

Structural analysis of poly(dimethylsiloxane) modified silica xerogels

A new type of inorganic-organic hybrid material incorporating poly(dimethylsiloxane) (PDMS) in tetraethoxysilane (TEOS) based glassy gels has been produced by a sol–gel process. SAXS and SEM studies indicate that these PDMS modified silica xerogels possess multiple size-scale morphologies, ranging from Ångstroms to micrometers. The multiple-level hybrid materials show improved structural integrity relative to pure sol–gel glasses (xerogels). The effect of reaction time, water content, as well as content and molecular weight of PDMS on the structure of such materials were studied. These processing variables affect the relative rates of hydrolysis and condensation reactions, and consequently change the microstructure of the final product.

Enhanced loading and activity retention of bioactive proteins in hydrogel delivery systems

A simple, general and effective technique is developed for increasing the loading of bioactive macromolecules into hydrogels using the principles of aqueous two-phase extraction. Model proteins, ovalbumin and α-amylase, were loaded into hydrated gels by soaking the gels in a buffered solution of protein containing 12 wt% PEG-10 000 and 0.22 M salt (KCl, KBr or KI). The PEG and salts were expected to enhance protein sorption according to aqueous two-phase extraction heuristics. In the absence of the solution additives, the gels absorbed little protein. But protein loading up to 270 mg ovalbumin/g polymer and 67 mg α-amylase/g polymer was obtained when the PEG and salt were added; loading was not significantly dependent upon salt type. Ovalbumin release from hydrated gels was diffusion-controlled. The diffusion coefficient was 1·10 −7 cm 2/s, consistent with protein absorption into the gel rather than adsorption onto the surface. Release kinetics of both proteins from dried, glassy gels matched conventional behavior for release of absorbed drugs from glassy polymers. Finally, α-amylase activity was retained even after drying the loaded gel at 65°C, conditions which denatured the enzyme when not absorbed in the gel.

Magnetoresisnve La-Sr-Mn-O Powders and Films by Daas Technique

ABSTRACTMagnetic perovskites of the general form La1-xSrxMnO3 have successfully been synthesized using deposition by aqueous acetate solution (DAAS). This novel technique, which has the potential for depositing large area min films with high throughput and low cost, involves the preparation of an aqueous metal acetate precursor solution, drying the solution to generate a glassy gel, consolidating the gel, and then firing it for short periods of time (<2hrs) to produce crystalline lanthanum strontium manganate (LSMO). The DAAS method has been used to prepare both powders and thin films of LSMO. Powder samples of La0.83Sr0.17MnO3 annealed for 100 minutes at 1200°C were high purity, single phase, and exhibited excellent electrical and magnetic characteristics. Thin films of La0.7Sr0.17MnO3 deposited onto both sapphire and strontium titanate substrates and annealed at 900°C were also found to be crystalline, and substrate choice was found to influence thin film crystal structure. These films exhibited sharp metal-insulator transitions, and in the case of LSMO films on strontium titanate, magnetoresistance was observed at the unusually high temperature of ∼360K.

Synthesis and characterization of pH‐ and/or temperature‐sensitive hydrogels

AbstractA series of pH‐sensitive hydrogels that exhibit volume phase transition phenomena have been synthesized in aqueous solution and characterized with respect to their dynamic swelling behaviors. Positively charged hydrogels were prepared by copolymerizing varying ratios of N‐isopropylacrylamide and NN′‐dimethylaminopropylmethacrylamide. The hydrogels based on a temperature‐sensitive hydrogel demonstrate a large change of equilibrium swelling in response to small variations of pH and/or temperature. These hydrogels exhibit different lower critical solution temperature (LCST) ranges depending on the environmental pH values. Below their LCST, they exhibit small and broad pH sensitivities normally observed in most hydrophilic polyelectrolyte gels, but above their LCST, they exhibit sharp pH dependent phase transition behaviors. The pH‐dependent phase transition is strongly affected by temperature, while the temperature‐dependent transition is, in turn, largely influenced by the pH. As the temperature is raised, the transitional degree of gel swelling change becomes sharper and larger, and the phase transition pH value shifts to a lower pH. It was also found that swelling is faster than deswelling for these cationic hydrogels, which suggests the existence of a water diffusion barrier during the deswelling. The swelling kinetics of initially dry and glassy gels were strongly dependent on both the pH value and temperature.

Kinetics and mechanisms of water sorption in hydrophobic, ionizable copolymer gels

AbstractThe aqueous kinetic swelling properties of a class of cross‐linked hydrophobic polyamine copolymer gels based on n‐alkyl esters of methacrylic acid (nAMA) and N,N‐dimethylaminoethyl methacrylate (DMA) have been studied as a function of solution ionic composition (pH, ionic strength, and buffer content), gel composition, and temperature. Water uptake and swelling in these gels are driven by ionization of the DMA amine groups, which overcomes the hydrophobic tendency of these gels to exclude water in the unionized state. Sorption kinetics in initially glassy gel disks are generally biphasic, characterized by an initial phase of relatively slow water uptake followed by an accelerated phase during which significant volume expansion of the gel occurs. This sorption/swelling behavior strongly suggests a moving penetrant front mechanism. The initial rate of water sorption increases markedly as (1) solution pH decreases, (2) gel nAMA comonomer content decreases, (3) gel nAMA side‐chain length decreases, and (4) temperature increases. Furthermore, the initial phase of sorption in initially glassy gels is generally non‐Fickian and approaches zero‐order behavior as (1) pH increases, (2) nAMA content increases, and (3) temperature decreases. In direct contrast, sorption in initially dry, rubbery gels is monophasic, but non‐Fickian, and approaches zero‐order behavior as temperature increases. This behavior is contrary to the Fickian sorption behavior normally observed in polymers above their glass transition temperatures. Finally, sorption kinetics critically depend upon the nature of the ions in solution: Kinetics are significantly faster in the presence of weak electrolytes than that of strong electrolytes. We discuss the importance of the rate of ion transport in determining the overall sorption kinetics and how sorption kinetics can be non‐Fickian in rubbery gels. Also, we propose a mechanism for the observed enhanced kinetics in the presence of weak electrolytes.

Temperature-concentration behavior of solutions of polydisperse, atactic poly(methyl methacrylate) and its influence on the formation of amorphous, microporous membranes

The temperature-concentration behavior of Solutions of atactic poly(methy1 methacrylate) in 1-butanol and cyclohexanol was investigated. A liquid-liquid demixing interferes with a glass transition. In the high-concentration region, outside the demising region, Tg decreanes with decreasing polymer concentration. For concentrations in the demixing region, a slight increase of TI with decreasing polymer concentration is observed. This is ascribed to the influence of the polymer polydispersity on the concentration of the coexisting phases. The extent of this increase depends strongly on the polydispersity of the polymer and on the nature of the solvent. The combination of these two thermal transitions leads to the formation of amorphous, glassy gels at room temperature. After elimination of the solvent, different morphologies can be obtained. They range from glassy particle dispersions to porous, glassy materials. The morphology can be controlled by overall polymer concentration and cooling rate.

Evaluation of simple precipitation test for systemic lupus erythematosus.

An unusual precipitate observed during the determination of serum cholesterol in a patient who later developed disseminated lupus erythematosus led to the elaboration of a test for this disease. The test reagent is a 12 % solution of p-toluenesulfonic acid in glacial acetic acid. The test consists in adding 0.1 ml. of serum or plasma to 2 ml. of the reagent. No precipitate appeared when this test was applied in 65 out of 66 healthy people; in one, a light precipitate appeared and persisted for 20 minutes, but redissolved on shaking. Applied in 30 patients in various stages of systemic lupus erythematosus, the test gave precipitates, the amount and character of which paralleled the severity of the disease. In 12 of the 13 acutely ill patients there was a heavy precipitate with this test. When this appeared as a glassy gel that resisted rubbing and shaking it was graded 4+. Altogether 760 tests were done on 311 subjects. Very few false-positive results were found. The test seems to be of value in the differential diagnosis of systemic lupus erythematosus and to afford a measure of the severity of the disease.