Ion exchange and ion transport properties of sulfonated organically modified silica hydrogels

Sulfonated ormosil hydrogels (~80% water) were prepared using tetramethyl orthosilicate as a silica precursor and 2(4-chlorosulfonylphenyl)ethyltrichlorosilane to provide sulfonate functionality for ion-exchange and ion conductivity. Ruthenium(III) hexamine was used as a redox probe in electrochemical studies performed on porous carbon fibre paper electrodes impregnated with the gel. The gel-modified electrodes extracted Ru(NH₃)₆³⁺ from solutions in 0.1 M CF₃CO₂Na_(aq) with a partition coefficient of ~36, and with ~100% of the sulfonate sites being accessible for ion exchange. The Ru(NH₃)₆^3+/2+ couple exhibited reversible and facile electrochemistry in the gel, with a Ru(NH₃)₆²⁺ diffusion coefficient of 4.9×10^–8 cm² s^–1 determined by chronoamperometry. This is an order of magnitude higher than the mobility of this complex in Nafion. The hydrogel-modified electrodes were stable for days, and could be repeatedly loaded with Ru(NH₃)₆³⁺.Special Issue to celebrate the 70^th birthday of Professor Zbigniew Galus     

Preparation and properties of biocompatible polymer-grafted silica nanoparticle

Grafting of biocompatible polymer onto the surface of silica nanoparticles was achieved by radical graft polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC), initiated by azo groups previously introduced onto the surface or by a system consisting of Mo(CO)₆ and trichloroacetyl groups on the silica surface. Both of these systems have the ability to initiate graft polymerization of MPC, resulting in the formation of poly(MPC)-grafted silica, but the percentage of poly(MPC) grafting for the latter initiating system was much higher than that of the former. The amount of moisture that could be adsorbed onto the silica surface was found to increase with increasing poly(MPC) grafting. This indicates that grafting of poly(MPC) onto the silica surface markedly increases the hydrophilic nature of the surface. The contact angle of water in composites prepared from poly(vinyl alcohol) and poly(MPC)-grafted silica was found to decrease with increasing poly(MPC)-grafted silica content. When poly(MPC)-grafted silica was added to water containing a small amount of chloroform, it was found to act as stabilizer for droplets of chloroform. In addition, according to tests by the Lee-White method, poly(MPC)-grafted silica shows non-thrombogenic characteristics.     

Micro- to nanoscale structure of biocompatible PLA-PEO-PLA hydrogels

We observe large-scale structures in hydrogels of poly(l-lactide)-poly(ethylene oxide)-poly(l-lactide) (PLLA-PEO-PLLA) ranging in size from a few hundred nanometers to several micrometers. These structures are apparent through both ultra-small angle scattering (USAS) techniques and confocal microscopy. The hydrogels showed power law scattering in the USAS regime, which is indicative of scattering from fractal structures. The fractal dimension of the scattering from hydrogels revealed that the gels have large size aggregates with a mass fractal structure over the nanometer-to-micrometer length scales. The aggregates also seem to become more "dense" with an increase in the molecular weight of crystalline PLLA domains. Visualization through confocal microscopy confirms that the gels have a microstructure of interspersed micrometer-sized polymer inhomogeneities with water channels running between them. The presence of micrometer-sized water channels in the hydrogels has very important implications for biomedical applications.     

New biocompatible thermo-reversible hydrogels from PNiPAM-decorated amyloid fibrils

New biocompatible temperature-responsive hydrogels have been obtained by using unprecedented low concentration of amyloid fibril-PNiPAM hybrids. The viscoelasticity of the hydrogels can be finely controlled by tuning the PNiPAM layers without changing the structure or concentration of the amyloid fibrils.     

Microstructure and properties of organosoluble polyimide/silica hybrid films

Organosoluble polyimide/silica hybrid materials were prepared via the sol-gel process and their pervaporation properties were studied. The organosoluble polyimide (PI) was based on 4,4′-oxydiphthlic dianhydride (ODPA) and 4,4′-diamino-3,3′-dimethyldiphenylmethane (DMMDA). The surface chemical structure of polyimide/silica films was analyzed by Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) and the results show that the completely hydrolysis of alkoxy groups of precursors and formation of the three-dimensional Si-O-Si network in the hybrid films. The morphology and the silica domain thus obtained were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The silica particle size in the hybrid is in the range of 40-100 nm for the hybrid films when the amount of silica is less than 20 wt%. The strength and the modulus of the hybrid films are improved and the mechanical properties were found to be strongly dependent on the density of the crosslink. The glass transition temperature (T_g) of the hybrid films was determined by dynamic mechanical analysis (DMA) and the value increased 15-20 °C as the silica content increased. Furthermore, the pervaporation performances of the prepared hybrid films were also investigated for the ethanol/water mixtures at different temperature.     

Synthesis and properties of crosslinked polyvinylformamide and polyvinylamine hydrogels in conjunction with silica particles

Polyvinylamine hydrogels with silica particles encapsulated (PVAm/silica) were produced by a two‐step synthesis. In the first step, polyvinylformamide/silica (PVFA/silica) hybrids were synthesized from vinylformamide (VFA) and 1,3‐divinylimidazolidin‐2‐one (1,3‐bisvinylethyleneurea, BVU), as the crosslinker, by radical copolymerization in silica/water suspensions using different compositions of VFA/BVU. The target product PVAm/silica was obtained by acidic hydrolysis of the PVFA/silica hydrogels in a second step. The chemical structures of both hydrogels, PVFA/silica and PVAm/silica, respectively, were revealed by solid‐state ¹³C(¹H) cross‐polarity/magic‐angle spinning NMR spectroscopy. Both hydrogels swelled significantly in water. The swelling capacity of the two systems was characterized by the correlation length ξ (or hydrodynamic blob size) of the network meshes with small‐angle neutron scattering experiments. ξ is significantly larger for PVAm/silica than for PVFA/silica, which corresponds to the observed higher swelling capacity of this polyelectrolyte material. Furthermore, the swelling behavior of the hybrid hydrogels was quantitatively described in terms of free swell capacity, centrifuge‐retention capacity, adsorption against pressure, and free swell rate as compared with values of the corresponding copolymer hydrogels. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3144–3152, 2002     

Microstructure and optical properties of PMMA/gel silica glass composites

FT-Raman and near infrared (NIR) spectroscopies have been used to investigate PMMA/gel silica composites prepared by the post-doping method. The changes of the vibrational features of silanols on the silica surface and ester carbonyls from PMMA indicate that the two phases in the organic-inorganic hybrids interact with each other through hydrogen-bonding. Unlike unmodified gel silica, the as-prepared composites are hydrophobic due to the elimination of the accessible adsorbing sites provided by the surface silanols. Residual water inherited from the preparation process is not evenly distributed in the composites but locally enriched on the silica surface by hydrogen-bonding to limited surface silanols. The conversion efficiency of MMA-to-PMMA depends not only on the polymerisation conditions applied, but also on the initial structure of the sol-gel substrate used. The composites show high optical transmittance in the near UV and visible region due to a reduction of scattering from an originally porous structure. However, their application in the near IR is restricted due to the combined vibrational energy absorptions by different silanol and water species, and in particular by C-H functional groups from organic modifiers.     

E-beam crosslinked,biocompatible functional hydrogels incorporating polyaniline nanoparticles

PANI aqueous nanocolloids in their acid-doped, inherently conductive form were synthesised by means of suitable water soluble polymers used as stabilisers. In particular, poly(vinyl alcohol) (PVA) or chitosan (CT) was used to stabilise PANI nanoparticles, thus preventing PANI precipitation during synthesis and upon storage. Subsequently, e-beam irradiation of the PANI dispersions has been performed with a 12 MeV Linac accelerator. PVA-PANI nanocolloid has been transformed into a PVA-PANI hydrogel nanocomposite by radiation induced crosslinking of PVA. CT-PANI nanoparticles dispersion, in turn, was added to PVA to obtain wall-to-wall gels, as chitosan mainly undergoes chain scission under the chosen irradiation conditions. While the obtainment of uniform PANI particle size distribution was preliminarily ascertained with laser light scattering and TEM microscopy, the typical porous structure of PVA-based freeze dried hydrogels was observed with SEM microscopy for the hydrogel nanocomposites. UV?visible absorption spectroscopy demonstrates that the characteristic, pH-dependent and reversible optical absorption properties of PANI are conferred to the otherwise optically transparent PVA hydrogels. Selected formulations have been also subjected to MTT assays to prove the absence of cytotoxicity.     

Mechanically robust hydrophobized double network hydrogels and their fundamental salt transport properties

Water swollen polymer networks are attractive for applications ranging from tissue regeneration to water purification. For water purification, charged polymers provide excellent ion separation properties. However, many ion exchange membranes (IEMs) are brittle, necessitating the use of thick support materials that ultimately decrease throughput. To this end, novel double network hydrogels (DNHs) with variable water content are prepared and characterized in terms of mechanical and ion transport properties to evaluate their potential utility as tough membrane materials. The first network contains fixed anionic charges, while the other is comprised of a copolymer with varied ratios of hydrophobic ethyl acrylate (EA) and hydrophilic dimethyl acrylamide (DMA) repeat units. Characterization of freestanding DNH films reveals a reduction in water content from 88 to 53 wt% and a simultaneous increase in ultimate stress and strain by ~3.5× and ~4.5×, respectively, for 95%/5% EA/DMA, relative to 100% DMA. Fundamental salt transport properties relevant to water purification, including permeability, solubility, and diffusivity, are measured and systematically compared with conventional membrane materials to inform the development of DNHs for membrane applications. The ability to simultaneously reduce water content and increase mechanical integrity highlights the potential of DNHs as a synthetic platform for future membrane applications.     

Microstructure and properties of polyamideimide/silica hybrids compatibilized with 3-aminopropyltriethoxysilane

In this work, we prepared and characterized polyamideimide (PAI)/silica hybrids compatibilized with 3-aminopropyltriethoxysilane (APTES). PAI/silica nanohybrid thin films were prepared using an in situ sol-gel process, followed by thermal imidization. We have investigated the microstructures and properties of the PAI/silica hybrids using FT-IR spectroscopy, X-ray diffraction, small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). We also measured their tensile properties, thermal properties, refractive indices, and dielectric constants. In general, the properties of the PAI/silica hybrids were optimized when the silica content was 6 wt.%.     

Effect of sol–gel synthesis conditions on the physical properties of silica hydrogels

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Synthesis of biocompatible nanocomposite hydrogels as a local drug delivery system

Nanocomposite biocompatible hydrogels (NCHG) were synthesised as model systems for in situ cured potentially local drug delivery devices for curing periodontal infections. The composite consists of the following components: nanoparticles (NPs), matrix gel, and chlorhexidine (CHX) as antibacterial drug. The NPs were obtained by free radical initiated copolymerization of the monomers, 2-hydroxyethyl methacrylate (HEMA) and polyethyleneglycol dimethacrylate (PEGDMA), in aqueous solution. The same monomers were used to prepare crosslinked matrices by photopolymerization. NCHGs were obtained by mixing NPs, monomers, and drug in an aqueous solution then crosslinked by photopolymerization. Mechanical properties, swelling behavior, and the kinetics of drug release have been investigated. It was found that compression strength values increased with increasing ratio of the crosslinker PEGDMA. Incorporation of NPs into the matrix resulted similar compression strength as the matrix hydrogel. The hydrated NCHGs swelled more slowly but admitted more water. The drug was incorporated in NPs by swelling in CHX aqueous solution or added to the solution of monomer mixture followed by photopolymerization. Studies of release kinetics revealed that on average 60% of the loaded drug was released. The most rapid release was observed over a 24 h period for matrix gels with low crosslinking density. For NCHGs, the release period exceeded 48 h. An unexpected result was observed for NCHGs without drug in the NPs. In this case, increasing release was observed for the first 24 h. Thereafter, however, the apparent quantity of detectable drug decreased dramatically.     

Comparison of the structure and the transport properties of low-set and high-set curdlan hydrogels

Curdlan, a bacterial polysaccharide, can form different types of thermogels, having the very same chemical composition, but whose structures depend on the incubation temperature. Structural characterization of 10% (w/v) low-set and high-set curdlan gels was carried out by Fourier transformed infrared (FT-IR) imaging and environmental scanning electron microscopy (eSEM) in the hydrated state. Considerable differences were observed between the two gels, the high-set one being overall more homogeneous. The self-diffusion coefficients of a series of analytes of different sizes (water, phosphate, glucose-6-phosphate, polyphosphate, polyethylene glycol, and dextran labelled with rhodamine B) were measured in aqueous solution (D(s)(sln)) and in both types of curdlan gels (D(s)(gel)) using (1)H and (31)P pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. The mutual-diffusion coefficients (D(m)(gel)) of dextran in the curdlan gels were determined from release experiments based on fluorescence spectroscopy. The dependence of the relative diffusion coefficient (D(s)(gel)D(s)(sln)) on the size of the analyte, expressed by its hydrodynamic radius (R(h)), could be expressed by D(s)(gel)D(s)(sln) ∝ exp(-R(h)(0.46)), valid for both types of gels. The self-diffusion measurements for the largest investigated analytes were not compatible with a single diffusion coefficient and, therefore, were analysed using an approach based on a normal distribution of self-diffusion coefficients. In the hydrogels, broadening of the self-diffusion coefficient distribution increased as a function of the analyte size. This phenomenon was associated with the limited distance travelled by the analytes during the measurements, and it is inferred that the distribution of diffusion coefficients is representative of the distribution of local environments of the individual analyte. It was found that the structural differences observed between both types of curdlan gels are not correlated with the gel transport properties, highlighting the complexity of the relationship between structural details and transport properties in gels.     

Rheological and transport properties of sulfonated polyacrylamide hydrogels for water shutoff in porous media

In this research, an optimal hydrogel, based on sulfonated polyacrylamide, was synthesized by statistical design of experiments using central composite method. This new hydrogel composed of sulfonated polyacrylamide (AN125VLM) and chromium triacetate as copolymer and crosslinker, respectively. The bottle and rheological tests were conducted to investigate the gelation time, thermal stability, gel strength and also ultimate elastic modulus, complex modulus, and yield stress. It was found that copolymer concentration had the main effect in both rheological and transport properties of hydrogels. The sample prepared at optimum condition, i.e. copolymer concentration of 26,340 ppm and crosslinker/copolymer ratio of 0.12, had an ultimate elastic modulus of 29.9 kPa, yield stress of 800 Pa, and complex modulus of 32 kPa. A coreflooding test through fracture was carried out to examine the optimum gel performance in a porous media. A value of 483 for the residual resistance factor ratio of water to oil confirmed the high ability of the hydrogel in reducing the relative permeability of water to oil in fractured media. Copyright © 2014 John Wiley & Sons, Ltd.     

Multi-fractal imaging and structural investigation of silica hydrogels and aerogels

In this work, we report on the preparation of TEOS gels by means of the sol–gel method employing different TEOS/ethanol/water ratios at room temperature. Small-angle X-ray scattering (SAXS) measurements provide the first-generation structural parameters, which indicate that particle size and fractal dimension are synthesis and depth dependent. These results are supported by the second- and third-generation aggregates observed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The fiber-like first-generation clusters aggregate as spherical second-generation particles which, in turn, in the third generation rearrange into complex structures.     

Mechanical properties of silica hydrogels prepared and aged at physiological conditions: testing in the compression mode

The mechanical properties for silica hydrogel prepared at physiological conditions are reported in this paper. The mechanical testing was performed in the compression mode determining the mechanical characteristics as a function of aging time in TRIS buffer up to 14 days. In addition to a typically used gradient method for Young’s modulus determination from the stress–strain curves, a new phenomenological model was proposed to describe the experimental data. The mechanical properties were stabilized after 2 days of aging, which was concluded from an increase in Young’s modulus between 90 and 400 kPa, an increase in stress at break between 50 and 100 kPa and by a decrease in relative deformation at break from 0.26 to 0.16. The height of samples was constant in the first three days of aging followed by a decrease by ~20%. Dissolving of silica hydrogel characterized through determination of silica content in TRIS buffer employing the molybdenum method was not found to be responsible for this phenomenon. The phenomenological model is proposed to be used for a reliable evaluation of mechanical properties of silica as well as other hydrogels exhibiting low Young’s modulus.     

Microstructure and photoluminescence of Ge-doped mesoporous silica

Nanostructured Ge-doped mesoporous silica powder and thin film were prepared with a cetyltrimethylammonium bromide self-assembled template to investigate the doping effects on the structure and optical properties of mesoporous silica. The X-ray diffraction, transmission electron microscopy and photoluminescence (PL) results suggest that the Ge-doped mesoporous silica with Ge/Si molar ratio of 0.01 was characterized by the strongest PL intensity without phase separation. Worm-like Ge-doped porous silica with specific area up to 987 m²/g could be obtained in this study, in which some Si atoms were replaced by Ge atoms according to the X-ray photoelectron spectroscopy analyses. The PL intensity of mesoporous silica could be increased by germanium-induced oxygen-related defects, but for the samples with Ge/Si molar ratios larger than 0.01, the PL intensity decreased due to the phase separation of germanium oxide.     

Diffusional transport from structurally variant hydrogels

Diffusional release of solutes from polymer matrices undergoing structural changes has been analysed by incorporating the dependence of solute diffusivity on time. The functional dependence of diffusivity with time has been experimentally verified and its utility and limitations are discussed. Criteria for predicting release characteristics have been arrived at based on two model parameters,K and β. HereK represents the reciprocal of the time required for the structural change and β is the ratio of the solute diffusivity prior to the onset and after the completion of the structural change. The criteria, which are independent of the mechanistic details of the structural change, have been validated by analysing solute release from polymeric matrices undergoing diverse structural changes. The approach should be useful in predicting the relase characteristics of solutes on the basis of the physicochemical characteristics of the polymer-solute systems. It should also help in tailoring the polymers to obtain the desired release kinetics.     

High-strength hydrogels: Microstructure design,characterization and applications

Hydrogels are a class of polymeric network materials embedded in a water-rich environment. They are widely applied in drug delivery, actuator, and sensor. However, conventional hydrogels encountered limits from their poor mechanical property. Recent researches in hydrogels have been focusing on mechanical enhancement, ranging from design of microstructures to adjustment of compositions in hydrogels. Here, the design and fabrication strategies of high-strength hydrogels, as well as major progress in their typical strength-support applications are systemically reviewed. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1325–1335