Agnieszka Kierys

Synthesis and characterization of nanostructural polymer–silica composite: Positron annihilation lifetime spectroscopy study

The swelling of poly(TRIM) spherical particles in TEOS is assessed as a potential way for obtaining polymer-silica nanocomposite materials. Silica deposition was achieved by simply stirring of swollen polymer particles in acidic hydrochloric-water solution. This procedure leads to spherical composite particles with dispersed silica gel within the polymer matrix. The resulting material exhibits the same morphology as the initial polymer. Nanocomposite particles are silica rich (about 17 wt.%). Characterization of the nanocomposites was performed using scanning electron microscopy, FT-IR spectroscopy, (29)Si CP MAS NMR spectroscopy and thermogravimetry. Moreover, the use of positron annihilation lifetime spectroscopy PALS to characterize the structural properties of the nanocomposites is presented. This technique gave more realistic pieces of information about the pore structure of the investigated samples in contrast to nitrogen adsorption studies.

Polymer/silica composite of core-shell type by polymer swelling in TEOS.

Monodisperse polymer/silica composite material with a polymer as the core and hydrophilic silica gel as the shell was prepared by a two-stage procedure. In the first stage, the swelling of Amberlite XAD7HP particles in tetraethoxysilane (TEOS) was performed. Subsequently a portion of the XAD7HP particles impregnated with TEOS were transferred to acidic aqueous solution to facilitate a sol-gel process of the silica precursor. This procedure is assessed as a potential route to a composite material with a core-shell morphology. Scanning electron microscopy and (29)Si MAS NMR indicated the formation of silica microfibers on polymer beads. The silica microfibers were anchored in the polymer matrix. In consequence, the silica shell exhibited relatively high mechanical stability. The swelling of the polymer and the formation of the silica phase substantially changed the porosity of the initial polymer material. The final composite surprisingly exhibited very homogeneous porosity. The textural characteristics of the investigated materials were defined by nitrogen adsorption-desorption at 77K.

Positron annihilation and N2 adsorption for nanopore determination in silica-polymer composites

Silica has two different, fragmented and smooth, visual appearances when deposited onto porous Amberlite polymer substrate from TEOS at acidic and basic pH, respectively. The low temperature N2 adsorption isotherms on both materials seem to be a combination of Type 1 and Type 4 isotherms with H2 hysteresis, indicating the presence of both micro (D < 2 nm diameter) and meso (2 nm < D < 50 nm) pores. Their mesopore distributions, computed by both BJH and DFT methods, show maxima near D∼5 nm with a narrower pore size distribution range than the organic support alone. For comparison, pores were also tested by positron annihilation lifetime spectroscopy (PALS) which can scan the full micro to mezo size range in one measurement without cooling and using any adsorbate molecules. These PALS results indicated the presence of D∼0.5, 0.8 and 1.5 nm diameter micropores and an average D∼5 nm diameter mesopores in both the base and the acid set materials. When the organic substrate is burned out from the two differently made composites, the new adsorption isotherms, BET surface areas, and differently measured pore sizes became distinctly different both from each other and from those of the parent materials.

Insight into the structure of polymer-silica nano-composites prepared by vapor-phase

Using a new synthesis technique, in which mesoporous Amberlite XAD7HP resin beads swollen with TEOS were exposed to vapors of either (H2O+HCl) or (H2O+NH3), we obtained smooth, porous, mechanically stable silica gel spheres after burning out the sacrificial organic template. Combined N2 sorption, SEM, TEM, (29)Si NMR, and Raman measurements were used to characterize the physical properties and molecular structures of the intermediate and final gels. Our atomically resolved TEM pictures provide the first visual demonstration of the presence of 3 to 6 member siloxane rings predicted by our Raman studies and other indirect methods. It is demonstrated that the physical appearance, morphology and porosity of the acid and base set gels are different from each other and also from those silica gels that were earlier polymerized from TEOS or Na-silicate saturated Amberlite XAD7HP with aqueous NH4OH or HCl solutions in liquid phase. We show that the different physical properties of the vapor-phase set gels are associated with different gelling rates at acidic and basic conditions, which generates molecular differences both in the intermediate and the final products.

Synthesis of aspirin-loaded polymer-silica composites and their release characteristics.

This study describes a novel approach to the synthesis of polymer-drug-silica nanocomposites via encapsulation/isolation of drug molecules, introduced into the polymer matrix by the silica gel. For the first time, tetraethoxysilane (TEOS) gelation in the vapor phase of the acidic catalyst is presented as an efficient method to enter the silica gel nanoparticles into the polymer-aspirin conjugate. The conducted studies reveal that the internal structure of the polymer carrier is significantly reorganized after the embedding of aspirin molecules and the silica gel. The total porosity of the polymer-drug-silica nanocomposites and the molecular structure of the silica gel embedded in the system strongly depend on the conditions of the silica source transformation. Additionally, the release of the drug was fine-tuned by adapting the conditions of hydrolysis and condensation of the silica gel precursor. Finally, to prove the usefulness of the proposed synthesis, the controlled release of aspirin from the polymer-drug-silica nanocomposites is demonstrated.

Encapsulation of diclofenac sodium within polymer beads by silica species via vapour-phase synthesis.

The present study concerns the preparation of ternary composites via the in situ encapsulation of solid dispersion of diclofenac sodium within the acrylic polymer beads. The encapsulating species were produced through the hydrolysis and condensation of the silica precursors (tetraethoxysilane or ethyltriethoxysilane) introduced into the solid dispersion. The transformation of precursors occurred in the vapor phase of ammonia. A great advantage of the presented vapor-phase method is preventing the desorption of the highly soluble drug during gelation of silica precursors, which stands in contrast to the conventional sol-gel processes occurring in the solution. The conducted studies, involving the low temperature N2 sorption together with spectroscopic techniques, provide insight into the structural differences of drug loaded particles. They reveal that the formation of silica gel accompanies the conversion of the drug into its amorphous form. Finally, the desorption profiles of diclofenac sodium demonstrate that the deposition of silica gel successfully diminishes the degree of the initial drug desorption while significantly modifying its release rate.

Synthesis of the mesostructured polymer-silica composite and silicon dioxide through polymer swelling in silica precursor

Tetrabutoxysilane (TBOS) transition introduced into the preformed porous polymer was found as an effective method in the preparation of highly porous silica adsorbents. The swelling of the acrylic polymer XAD7HP in TBOS causes the total infiltration of polymer beads by the silica precursor. Transformation of TBOS in aqueous solution at presence of the surfactant (hexadecyltrimethylammonium bromide, CTAB) produces two composite phases: silica-polymer beads and fine silica-CTAB particles. After their calcination, two pure silica phases exhibiting extremely high porosity were obtained. The paper presents the structural properties of the composites and silica materials. All materials were characterized by scanning and transmission electron microscopy (SEM and TEM), the low temperature nitrogen adsorption, X-ray diffraction (XRD) and 29Si NMR spectroscopy.

Effect of silica precursor transformation on diclofenac sodium release

The present paper describes the preparation of a new type of ternary composite where pure silica gel or polysilsesquioxane was deposited on a polymer carrier loaded with a high dose of diclofenac sodium. The silica species were prepared by in situ gelation of the precursors, tetraethoxysilane (TEOS) or ethyltriethoxysilane (ETEOS), in the presence of an acidic catalyst in the vapour phase. The conducted studies (low temperature nitrogen sorption, XRD, SEM, EDX) reveal that the introduction of drug molecules, as well as silica species, significantly changes the internal structure of the host material. The total porosity of the ternary composites strongly depends on the type of applied silica precursor. Additionally, it is shown that the exposure of the TEOS-saturated or ETEOS-saturated solid dispersion of drug within the polymer to acid vapors is sufficient to cause the irreversible transformation of diclofenac sodium into sodium chloride and a derivative of phenylacetic acid. Furthermore, TEOS prevents the transformation of the drug into its acidic form more effectively than the ETEOS precursor. Finally, the in vitro release of the drug is demonstrated, which clearly indicates that after the embedding both of the silica species, the rate of drug release is modified and the degree of initial drug delivery is successfully diminished. The obtained data are analyzed using different kinetics models to give insight into the possibility of prolonged release of a drug and the probable mechanism of drug release from the investigated samples.

N-heptane adsorption and desorption in mesoporous materials

Positron Annihilation Lifetime Spectroscopy (PALS) was used for an in situ monitoring of adsorption and desorption processes. The disordered and ordered porous silica as well as the porous polymer were used as adsorbents, while an adsorbate in all the cases was n-heptane. The lifetimes and particularly the intensities of the ortho-positronium(o-Ps) components depend strongly on the adsorbate pressure. The analysis of these dependencies allows us to identify several processes, which are taking place during sorption. At low pressure, an island-like growth of the first layers of the adsorbate on the silica, in a contrary to a swelling of the polymer, is observed. A size of the pores, which remain empty, is estimated at the subsequent stages of the adsorbate condensation and evaporation. The adsorbate thrusting into micropores is deduced at p/p0 > 0.6 whilst the mesopores are still not completely filled.

Polymer-amino-functionalized silica composites for the sustained-release multiparticulate system

This study presents an interesting and promising strategy for producing an oral multiparticulate formulation of the sustained-release of diclofenac sodium (DS) consisting of subunits closed inside hard gelatin capsules (each capsule contains ~50mg of diclofenac sodium). The subunits in the form of beads were produced through the encapsulation of diclofenac sodium dispersed within a nondisintegrating polymer carrier by a silica gel functionalized with the 3-aminopropyl groups. The hybrid silica gel, which plays the role of enteric coating, was fabricated by the gelation of the liquid silica precursors mixture (i.e. tetraethoxysilane (TEOS) and (3-aminopropyl)triethoxysilane (APTES)) in the vapor phase of ammonia. The conducted studies reveal that the introduction of the hybrid silica gel into the solid DS dispersion facilitates prolonged release in the neutral environment of the intestine. Since the ability of the multiparticulate formulation to control the release of the drug depends on the properties of its subunits, studies involving the low temperature N2 sorption, DSC analysis together with spectroscopic techniques (XRD, SEM, 29Si MAS NMR) were conducted.