Externally tunable dynamic confinement effect in organosilica sol-gels

The feasibility of using the pores of organosilica sol-gels for dynamic confinement of a luminescent molecule is demonstrated in this paper. The porous organosilica sol-gels act as reversible thermoresponsive materials which exhibit reduced pore volume at higher temperature and enlarged pores at lower temperature, which causes dynamic alterations in the nanoenvironment of encapsulated entities. A particularly remarkable feature of this system is that the temperature-dependent confinement effect provided by the gels is reversible and can be efficiently modulated by simply changing the temperature.     

Thermally-regulated molecular selectivity of organosilica sol-gels

The feasibility of using temperature as a control mechanism for altering the selectivity of organosilica sol-gels for a specific molecule is demonstrated in this communication. The porous organosilica sol-gels act as reversible thermoresponsive materials which become hydrophobic at higher temperature and hydrophilic at lower temperature. When exposed to a mixture of molecules, the gels selectively intake the more hydrophobic species at higher temperature. A particularly remarkable feature of these gels is their ability to preferentially sequester the hydrophobic molecule at high temperature and the hydrophilic species at low temperature. Finally, these gels selectively intake hydrophobic molecules at high temperature and then preferentially release them when the temperature is lowered.     

Sol–Gel Encapsulation of Molecules to Generate Functional Optical Materials: A Molecular Programming Approach

The extraordinary opportunities offered by integrating solution chemistry of molecular entities with the solid-state nature of the gel provide the basis for designing a number of novel molecular materials. Herein, we present a strategy based on encapsulation of suitable response active species to impart useful optical properties to sol–gel glasses. The basic concept of this molecular programming approach is based on deliberate incorporation of response-active species in the silica gel framework to elicit specific optical responses. Design of molecular materials for device applications depends on selection of molecules which exhibit well-defined electronic or optical response, and assembly of these molecular components into a geometric structure that retains the rigidity, addressability, and stability necessary for practical applications. The approach is based on using molecules as active species and sol–gel glass as structural matrix in which the molecules are selectively integrated. A designer approach that employs specific molecules for generating optical signals is described. As such the properties of these silica-based glasses can be tuned by varying the composition of encapsulated species. These modified glasses exhibit substantially altered optical properties as compared to pristine silica sol–gels. The optical response of these materials provide initial examples toward designing novel materials whose optical and/or photonic responses can be modulated by structural integration of specific dopant entities.     

Positron annihilation lifetime studies of the volume phase transition of polyacrylamides

Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.     

A different diffusion mechanism for drug molecules in amorphous polymers

Polymer materials are widely used in controlled drug release, and the diffusion property of drug molecules in these materials is of great importance. In this work, the diffusion behavior of a model drug (aspirin) in different ratios of poly(lactic acid-co-ethylene glycol) (PLA-PEG) was investigated by molecular dynamics simulations. Two major factors, which influence the diffusion of aspirin in polymer matrix: the wriggling of the polymer chain and the free volume of the polymer matrix, are discussed. The wriggling of the polymer chain mainly controls the diffusion of aspirin molecules. Free volume becomes the secondary effect. For two different polymers having a similar degree of wriggling, the free volume controls the diffusion of the aspirin molecules. Comparing with the diffusion behavior of small gas molecules in polymer matrix, a different mechanism was proposed for the drug molecules. The drug molecules can only diffuse along with the wriggling of the polymer matrix.     

Bis(trimethoxysilylpropyl)amine and tetraethoxysilane derived gels as effective controlled release carriers for water-soluble drugs of small molecules

The controlled release of sodium salicylate (SS) from the gels derived from bis(trimethoxysilylpropyl)amine (TSPA) or the mixture of TSPA with tetraethoxysilane (TEOS) was investigated. The experimental results suggest that the release of SS can be easily controlled by adjusting the ratio between TSPA and TEOS. Increasing the ratio between TSPA and TEOS lowers the surface area and pore volume of the gels, while enhances the interactions between the drug and the gel matrix. Therefore, reduces the release rate of the drug effectively. The overall release process is found to be diffusion controlled, and the release behavior can be well explained by considering the effects of the textual properties of the gels and the interactions between the drug and the gel matrix. TSPA is found to be a very convenient and effective precursor for the preparation of gels for controlled release of both hydrophilic and hydrophobic drugs, especially water-soluble drugs of small molecules.     

Ammonia-Sensitive Rubbery Ormosils Materials

Generally, conventional sol-gels are rather hydrophilic and well suited for sensing some ions such as H⁺. A more hydrophobic material for ammonia sensing ormosils materials can be obtained by preparing organically modified siloxanes ormosils. When dimethyldimethoxysilane (DiMe-DMOS) is mixed with an alkoxide such as tetramethoxysilane (TEOS) in a suitable soluent and catalyst, gels can be prepared which exhibit rubbery elasticity.     

Hard and Soft Capsules: From Branched Polymers to Controlled Release via Gels

Summary: Supramolecular gels, made from the combination of telechelic poly (ethyleneglycols) (PEG's) and trivalent poly(isobutylenes) with terminal matching hydrogen bonds were prepared and their rheological properties were investigated. Addition of poly(N-isopropylacrylamide) as thermosensitive polymer was included into the gel-capsules in amounts of 10 w%, producing gels with strongly thermally- responsive properties. Additionally, dansyl-dyes with two different endgroups (either an hydrophobic endgroup or a multiple hydrogen bond) were encapsulated into the gels as mimicks for pharmaceutically active substances. Monitoring their diffusion by UV-dependent measurements allowed to tune the diffusion and thus the release properties of the gel. The generated supramolecular gels will allow the selective encapsulation and triggered release of various pharmaceuticals and biologically active targets.     

Dynamic light scattering studies of ionic and nonionic polymer gels with continuous and discontinuous volume transitions

We have performed dynamic light scattering experiments on poly(acrylamide)‐poly(acrylic acid) copolymer gels with controlled crosslink density and copolymer composition, by varying the temperature, amount and valency of added salt, pH, and solvent quality. Our systematic study provides several insights. The correlation length for the monomer density fluctuations, as inferred from the measured diffusion coefficient, is too small to be identified as the mesh size of the gel. The correlation length in an ionic gel, which is found to be smaller than that for an equivalent gel without ionization. Comparison of swelling ratio with the diffusion coefficient shows that these quantities are not simply geometrically related to each other. When a discontinuous volume phase transition is induced by gradually varying the solvent quality, the diffusion coefficient exhibits a pretransitional reduction by two orders of magnitude even before the gel collapse. These findings provoke a need for new theoretical approaches for describing the elastic modes of polyelectrolyte gels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Facile route to synthesize polyurethane hollow microspheres with size-tunable single holes

Synthesis of polymer hollow spheres with single holes has been a challenging task in materials science. In this work, polyurethane (PU) hollow microspheres with size tunable single holes were fabricated through a facile self-assembly-diffusion process. Firstly, the as-synthesized PU polymers were dissolved in chloroform. By adding 10 times volume of water, the PU polymers were self-assembled into micelles in which the chloroform was encapsulated. The PU micelles were then transferred to methanol solution to allow the diffusion of chloroform. The formation of the single holes in shells of the resultant PU hollow spheres was induced by the out-diffusion of the encapsulated chloroform in methanol. By adjusting the diffusion time from 5 to 20 hours, the hole size could be tuned from 75 to 210 nm. The resultant PU hollow spheres with single holes demonstrated high loading efficiency of various guest materials such as nanospheres, nanoparticles, and chemical species, suggesting their promising applications in controlled release, chemical protection, catalysis, and so forth.     

In situ synthesis and photoresponsive rupture of organosilica nanocapsules

This communication describes in situ synthesis of thiocyanato-functionalized organosilica nanocapsules by a sol-gel reaction using 3-thiocyanatopropyltriethoxysilane as a precursor. Moreover, the nanocapsules are photoresponsive and burst under light irradiation, resulting in release of the encapsulated drug or dye.     

Size effect on competition of two diffusion mechanisms for drug molecules in amorphous polymers

Diffusion of drug molecules in polymer materials is of great importance in controlled drug release, and the investigation of the mechanism of drug release from the polymer matrix would help us to understand the release behavior of the controlled release system. In this work, molecular dynamics simulations were employed to investigate the diffusion mechanisms of penetrant molecules with different sizes in poly(lactic acid-co-ethylene glycol) (PLA-PEG). The size effect on the diffusion mechanism of penetrant molecules in polymer matrixes was discussed in detail. A competition mechanism in a two-step diffusion process-(1) motion within the cavities (free volumes), and (2) jumps between cavities or movement of the cavity itself originated from the wriggling of the polymer chains-was observed, and the contributions of these two factors to the diffusion coefficient were successfully separated. With the medium volume of penetrant molecules (e.g., benzene), a competition between these two steps was observed. Step (2) controlled the diffusion when penetrant molecules became bigger.     

Positron annihilation lifetime study of continuous volume phase transition of poly(N-isopropylacrylamide) gel induced in methanol–water mixed solvent

The macroscopic volume shrinkage and swelling of poly(N-isopropylacryl-amide) (PNIPA) gel induced by the compositional change in the methanol–water mixed solvent is correlated to the change in the nanoscopic free volume size and numerical concentration formed in the PNIPA gels. The free volume size and numerical concentration are estimated from the longest component appearing in the positron annihilation lifetime curves. The apparent free volume fraction calculated by the free volume size and numerical concentration, and dispersion of the free volume deduced by the size distribution are utilized to analyze the origin and location of the free volumes. The free volume parameters obtained by analysis of the positron annihilation data show various nanoscopic phases occuring within the PNIPA gels during the volume change, implying the variation of the strength of the interactions among the solvent molecules and the polymer chains of the PNIPA. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1141–1151, 1998     

Adsorption characteristics of organosilica based mesoporous materials

Hybrid organosilica mesoporous materials with pores of ordered three-dimensional hexagonal structure were prepared by the hydrolysis and co-condensation of 1,2-bis(triethoxysilyl)ethane with various concentrations of a surfactant as structure directing agents. The materials had high pore volume of 1-1.5 mL/g and high surface area from 1057 to 1445 m(2)/g. Adsorption measurement and adsorption calorimetry revealed that the prepared materials exhibited high hydrophobicity and high affinity toward nonpolar organic vapor such as n-hexane. The dynamic adsorption properties of the materials for n-hexane in the presence of water vapor showed that these hybrid organosilica materials preferentially adsorbed n-hexane vapor and were stable in the presence of water compared to the siliceous MCM48.     

In Situ and Post Reaction Cobalt-Incorporation into Periodic Mesoporous Ethanesilica Materials

Cobalt incorporated periodic mesoporous organosilica materials were synthesized in situ by condensation of 1,2-bistrimethoxysilylethane (BTME) in the presence of cetyltrimethylammonium bromide (CTAB) and cobalt nitrate and by incipient wetness addition of Co to BTME mesoporous silica. BET and pore measurements as well as DTA and TGA analysis of the materials revealed that the mesoporous structure is retained under the synthetic conditions used, after solvent extraction and importantly to temperatures of ca. 400°C under N₂. Raman spectroscopy also revealed the presence of the ethane moiety in the Co materials and that the surfactant was removed by solvent extraction. BET and pore measurement studies revealed that the surface area and pore volume of the materials decreased with increasing cobalt loading. The studies indicate that the new Co materials have potential as Fischer-Tropsch catalysts.     

Release of small molecules from aggregates consisting of an amphiphilic oligonucleotide functionalized by a photochromic azobenzene unit

Stimuli-responsive nanocarriers offer favorable properties for the target-specific delivery of drugs. Herein, we employed photoirradiation as an external stimulus for the construction of a molecular system that encapsulated small molecules, which were released upon photoirradiation. These nanocarriers consisted of DNA amphiphiles (ODAz 1), in which an oligodeoxynucleotide and an alkyl chain were employed as the hydrophilic and hydrophobic parts, respectively, and these two parts were linked by a photochromic azobenzene unit. In aqueous solutions, ODAz 1 formed nanosized aggregates that encapsulated hydrophobic molecules in their hydrophobic core. Photoirradiation induced isomerization of the azobenzene unit led to changes in aggregate size and the immediate release of the molecules. The aggregate smoothly penetrated the cell membrane, and the photochemical release and delivery of small molecules into living cells were achieved. Thus, ODAz 1 aggregates represent promising photosensitive nanocarriers that may be applicable to drug delivery and targeting.     

Investigation of swelling and diffusion in polymers by NMR imaging

The kinetics of diffusion in polymers ranges from simple Fickian diffusion to higher order diffusion, such as Case II diffusion^1-2). The conventional method for determining the characteristics of solvents into polymer matrices is by measuring the mass uptake of the polymer as the solvent penetrates the matrix. However, since such measurements perform observations at a macroscopic level, little information has been obtained relating to the nature of the solvent in the polymer matrix and the mechanisms of the processes that control the diffusion. Nuclear magnetic resonance (NMR) imaging (‘MRI’) has been used to observe the penetration of solvents into solid systems in realtime. The method provides a one- or more-dimensional image of the density and the mobility of the solvent in a material or of the network changes of the material itself due to the softening influence of the solvent. The first (imaging of the solvent) can be used for a quantitative measurement of the diffusion whereas the observation of the network gives information about the changing of the network (mobility, de-crystallization…) during the swelling process. For example the diffusion of organic solvents in some polymeric materials (natural rubber, water gels (PNIPAAm), and nematic diblock-copolymers) are investigated.     

Inclusion and fractionated release of nucleic acids using microcapsules made from plant cells.

The encapsulation and fractionated release of nucleic acids on vesicular packing (VP) materials have been investigated. The earlier described dependence of the permeation of nucleic acid molecules through the vesicle membranes on the salt concentration is a necessary precondition for both encapsulation and fractionation. Encapsulation is achieved by applying a suitable sample onto a VP column that has been equilibrated with a high-salt buffer. In that buffer the sample molecules are permeable. Immediately after sample application, elution is started with a low-salt buffer, from which the sample molecules are excluded. At the front between the two buffers the permeability changes, and some of the sample molecules distributed inside the vesicles cannot pass through the membranes. These encapsulated molecules can be released by increasing the salt concentration in the eluent. If the encapsulated nucleic acid sample is polydisperse, a stepwise or linear increase in the salt concentration leads to a fractionated release. The fractions obtained differ in their molecular size composition.     

Water sorption and transport in a series of polysulfones

Water sorption and transport properties for a series of polysulfones are presented and interpreted in terms of the changes in the structure of the repeat unit compared to that of bisphenol A polysulfone. The differences between the sorption and diffusion of water and of permanent gases in these materials are also discussed. Water has the ability to interact with the polymer and with itself through hydrogen bonding in a way that permanent gases cannot. The equilibrium solubility of water in the polymer, unlike permanent gases, does not have a simple dependence on free volume but correlates more strongly with the frequency of hydrogen bonding sites on the polymer. Analysis of the sorption isotherms using the method of Zimm and Lundberg suggests that water molecules cluster in these polysulfones to various extents. For each polysulfone except polyethersulfone, the water diffusion coefficient decreases with increasing activity, which also suggests water clustering. For most of these materials, the water diffusion coefficient is larger than that of bisphenol A polysulfone and is directly related to the polymer free volume. Water permeability in these materials broadly correlates with the polymer free volume, but a favorable water-polymer interaction can be an overriding factor. © 1996 John Wiley & Sons, Inc.     

Preparation and characterization of dicarboxylic acid modified starch-clay composites as carriers for pesticide delivery

Modified starch based composites were prepared with bentonite/organobentonite for their potential use as carriers for pesticide delivery. Soluble starch was esterified with dicarboxylic acid chlorides. Mean degree of substitution of modified starches varied from 0.101 to 0.353. Composites were prepared by solution intercalation method using these esterified starches and bentonite or cetyltrimethylammonium bentonite as filler materials. TEM analysis of composites showed well exfoliated/intercalated structures. Atrazine as a model pesticide was encapsulated in them and encapsulation was found to be 83.52–92.83%. These composites were also found to be acted as controlled release matrices for atrazine and release pattern was diffusion controlled. As compared to the commercially available wettable powder formulation, the composites were able to release encapsulated atrazine in water for a longer period. The research showed that the prepared composites could be further investigated as carriers for controlled release formulations of pesticides to minimize dose and number of applications.