Jacek Goworek

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.

Porosity evolution of VP-DVB/MCM-41 nanocomposite

The porous structure of nanostructured vinylpyrrolidone-divinylbenzene (VP-DVB)/MCM-41 composite was characterized using N(2) adsorption at 77K and positronium annihilation lifetime spectroscopy (PALS), atomic force microscopy (AFM) and Raman spectroscopy. Positron annihilation lifetime spectra were measured during outgassing procedure. The voids of differentiated dimensions were detected in the composite material. The number of free volumes and their dimensions depend on the degree of evacuation of volatile components from the samples.

Vacuum removal of the template in MCM-41 silica studied by the positron annihilation method.

Positron annihilation lifetime spectra were measured for ordered silica before template (octadecyltrimethyl ammonium) removal. An intermediate lifetime component, well pronounced in the spectra, indicates the presence of structural defects in the template being the size of a surfactant molecule. At the temperature near 420 K a radical change of template structure occurs, consisting in appearance of disorder and the degradation of template; evaporation of the fragments begins. It seems that moderate heating (up to about 500 K only) in vacuum for several hours is an effective and simple method of template removal, leaving no pure carbon residue in the pore structure. The results are compared with the data from the liquid nitrogen adsorption/desorption method and elemental analysis, confirming the positron annihilation results.

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.

Adsorption of aliphatic alcohols from benzene solutions on silica gel

Abstract The adsorption equilibria of binary solutions of benzene-aliphatic alcohols on silica gel have been studied at 298°K. Surface layer capacities have been calculated on the basis of the theory of adsorption from solutions on heterogeneous solid surfaces. Taking into account the molecular areas occupied by the components in the adsorbed phase the possibility of the formation of adsorption multilayer has been tested.

Temperature changes of the template structure in MCM-41 type materials; positron annihilation studies

Abstract The lifetime spectra of positrons annihilating in MCM-41 ordered silica were measured as a function of temperature. It was found that in alkyltrimethylammonium templates degradation in vacuum begins at 380–400 K, that consists in breaking the long micelle cylinders, rise of gaps between the fragments and finally pore evacuation. The latter process is not completed and particularly in C12TAB the spectrum component belonging to fragmented template preserves its high intensity up to 520 K (the highest temperature in our experiment). The pore surface area over 1000 m 2 /g is achieved already at temperatures about 490 K. The lifetime of the longest-lived component indicates that practically all ortho -positronium atoms leave the pores before annihilation.

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.

Adsorption of polar components from ternary mixtures on porous silica gels

Abstract The adsorption of acetone from binary and ternary mixtures withn-pentanol, benzene, andn-heptane on completely hydroxylated and silanized silica gel was investigated. The structure of the surface phase and the competition of liquid components for adsorbent surface are discussed on the basis of individual adsorption isotherms. It has been found that the formation of acetone-n-pentanol complexes plays the main role in adsorption on the investigated adsorbents. The structure of the surface layer strongly depends on the character of the hydrocarbon solvent as well as the degree of polarity of the adsorbent surface.

Measuring porosity of polymeric adsorbents by temperature programmed desorption of liquids

The total pore volumes and pore size distributions were calculated for organic polymers on the basis of the temperature programmed desorption (TPD) data. The results were corrected with respect to the swelling process of polymer skeleton.