Josef Pleštil

Formation and structure of the epoxy-silica hybrids

The organic-inorganic hybrid interpenetrating network (IPN) composed of an epoxide-amine network and silica was prepared and studied. Formation of the inorganic phase from tetraethoxysilane (TEOS) by sol-gel process was characterized by 29Si n.m.r. spectroscopy, gas chromatography, small-angle X-ray scattering and electron microscopy. Kinetics of the silica structure build-up in the organic matrix, its final structure and morphology depend on the method of IPN hybrid preparation. The large compact silica aggregates, 100–300 nm in diameter, are formed during the one-stage polymerization. The two-stage process with the acid prehydrolysis of TEOS leads to an acceleration of gelation and formation of more open and smaller silica structures: 50–100 nm in diameter. The most homogeneous hybrid morphology with the smallest silica domains of size 10–20 nm, appears in the sequential IPN. The development of the silica structure is restricted by a rigid reaction medium of the preformed epoxide network.

Structure evolution in epoxy–silica hybrids: sol–gel process

Abstract The evolution of heterogeneous structure during polymerization in the epoxy–silica hybrid was followed by small-angle X-ray scattering using a position-sensitive detector. The organic–inorganic hybrid was composed of an epoxide–amine system and the silica formed by the sol–gel process from tetraethoxysilane (TEOS). Silica structure evolution is determined by catalytic conditions and the way of preparation: one- or two-stage process. The one-stage polymerization was base-catalyzed by an amine used as a cross-linker of the epoxide. The reaction results in formation of large overlapping polysiloxane clusters from the very beginning of the reaction. During polymerization more branched domains gradually appear within the structure. The polymer shows a compact structure with fractal dimension increasing during the polymerization to Dm=2.5. The two-stage procedure consisting in acid prehydrolysis of TEOS and basic catalysis in the second step leads to an acceleration of gelation. Primary particles are formed in the first step followed by aggregation into clusters in the second step. The inner structure of the clusters described by a fractal dimension does not change during the polymerization. The diffusion-limited cluster–cluster reaction may be responsible for a more open structure with a fractal dimension Dm=1.7.

Macromolecular HPMA-based nanoparticles with cholesterol for solid-tumor targeting: detailed study of the inner structure of a highly efficient drug delivery system.

We report a rigorous investigation into the detailed structure of nanoparticles already shown to be successful drug delivery nanocarriers. The basic structure of the drug conjugates consists of an N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer bearing the anticancer drug doxorubicin (Dox) bound via a pH-sensitive hydrazone bond and a defined amount of cholesterol moieties that vary in hydrophobicity. The results show that size, anisotropy, and aggregation number N(aggr) of the nanoparticles grows with increasing cholesterol content. From ab initio calculations, we conclude that the most probable structure of HPMA copolymer-cholesterol nanoparticles is a pearl necklace structure, where ellipsoidal pearls mainly composed of cholesterol are covered by a HPMA shell; pearls are connected by bridges composed of hydrophilic HPMA copolymer chains. Using a combination of techniques, we unambiguously show that the Dox moieties are not impregnated inside a cholesterol core but are instead uniformly distributed across the whole nanoparticle, including the hydrophilic HPMA shell surface.

The multifunctional role of ionic liquids in the formation of epoxy-silica nanocomposites

This work addresses the use of ionic liquids (ILs) as additives for formation of epoxy-silica nanocomposites, via the simultaneous sol–gel process and epoxy network build-up. The application of different methylimidazolium based ILs allows controlling the silica structure and modifying interphase interaction, thus producing hybrids with diverse morphologies and improved mechanical properties. Both the anionic and cationic components of the ILs affected the hybrid formation and the final properties. The application of 1-decyl-3-methylimidazolium tetrafluoroborate ionic liquid together with HCl as an acid catalyst promotes both hydrolysis and condensation in the sol–gel process as well as the self-assembly ordering of the IL. This system produces a very fine hybrid morphology with well dispersed silica nanodomains and a significantly increased rubbery modulus due to physical crosslinking by the ordered domains of decyl-substituents.

Layered zinc hydroxide salts: delamination, preferred orientation of hydroxide lamellae, and formation of ZnO nanodiscs.

Delamination of layered zinc hydroxide salts (LZH) into hydroxide layers provides nanobuilding blocs of a two-dimensional anisotropy. The methodology, extent of delamination, the size and stability of hydroxide lamellae are described in detail. The ability of lamellae to restack to form oriented hydroxide films depends on the solvent, original LZH salt, and conditions used for delamination. The most interesting results were obtained using LZH intercalated with dodecyl sulfate anions and LZH nitrate delaminated in butanol at 60 °C and in formamide at room temperature, respectively. The former method produces hydroxide lamellae of a lateral size of ca. 10-20 nm. The inner structure of the hydroxide layers is conserved and separated lamellae restack to the original layered structure of LZH dodecyl sulfate. The latter method yields lamellae with a size decreasing from 73.3 nm to 10 nm after a 2-week aging, while their thickness is nearly constant (2.6-3.8 nm). However, the use of formamide is complicated by the formation of Zn(II) formate. The major part of LZH intercalated with dodecyl sulfate anions is transformed during the delamination procedure to anisotropic ZnO nanoparticles, either needle-like particles prolonged in the [0 0 1] direction or disc-like particles flattened along the (0 0 1) plane.

Heat-induced transition of polystyrene- block-poly(ethylene-co-propylene) micelles in decane and in dioxane

Abstract Dissolution of polystyrene-block-poly(ethylene-co-propylene) (PS—PEP) in decane, a selective solvent for the PEP block, and dioxane, a selective solvent for the PS block, at room temperature leads to formation of metastable micellar structures, which convert into stable micelles with molar mass reduced several times after heating above ∼50°C. By comparing small-angle X-ray scattering (SAXS) and light scattering (LS) data, the following models of micellar structures are suggested: At room temperature, dissolution of PS— PEP in decane yields aggregates of micelles. These are detected by LS, while SAXS reflects the molar mass of individual micellar cores formed by PS blocks. At elevated temperature, these compound particles disaggregate and then reorganize into equilibrium micelles of reduced molar mass. Solutions of PS—PEP in dioxane at room temperature have a different character. The molar masses of the observed particles determined by SAXS are in a good agreement with LS results, so we can exclude aggregation of micelles in this solvent. Nevertheless, also here, after heat treatment, both SAXS and LS yield a molar mass that is about 5 times lower than that determined for unheated solutions. Metastable structures surviving from the solid-state morphology after dissolution in a selective solvent at room temperature may convert to classical equilibrium micelles only after heating.

Nickel hydroxide ultrathin nanosheets as building blocks for electrochemically active layers

Layered nickel hydroxides (LNHs), intercalated with lactate and nitrate anions, were synthesised using controlled precipitation and anion exchange methods. The present study reports a novel approach for the delamination of LNHs in water into nickel hydroxide nanosheets. The thickness of a single nanosheet varied between 0.7 and 1.0 nm with a lateral dimension between 50 and 80 nm. The nanosheets formed colloidal solutions or gels, retained the original hydroxide structure of LNHs, and were stable for weeks. The nanosheets were re-assembled into large-scale, well-oriented films with adjustable thickness using drop casting and spin coating techniques. The prepared nanostructured films were electrochemically active with stable and reproducible charge–discharge properties in an alkaline electrolyte. These results suggest that the nickel hydroxide nanosheets, prepared by the present methods, have potential as building blocks in the design of nanocomposite materials for energy applications.

SANS study of multilayer nanoparticles based on block copolymer micelles

A novel type of three-layer nanoparticles was studied using small-angle neutron scattering (SANS). The particles were prepared by adding methyl methacrylate monomer to a polystyrene-block-poly(methacrylic acid) micellar solution in aqueous buffer and subsequent polymerization by γ-irradiation. The contrast-matching SANS experiments revealed that upon polymerization, the PMMA chains form a layer on the surface of the PS cores of the original micelles. This is in agreement with the finding that at room temperature, MMA monomer molecules do not penetrate into the micelle core in detectable amounts. In the presented example, the mean core radius of original micelles and the layer thickness were determined to be 99 and 17 A, respectively. Both characteristics are expected to be controllable by the choice of block copolymer micelles and the parameters of the polymerization process.

Structure of inhomogeneous polymer networks prepared from telechelic polybutadiene

AbstractThe structure and dynamics of segmented polyurethane networks prepared from poly(butadiene) diols (PBD) of different molecularweights M n ¼ 1200; 2160, 2650, 4690 and 10,300 g mol 21 , 4,4 0 -diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT)ðM n ¼ 710 g mol 21 Þ have been investigated by dynamic mechanical analysis, dielectric relaxation spectroscopy, differential scanningcalorimetry (DSC), small- and wide-angle X-ray scattering (SAXS and WAXS). Microphase separation on nanometre scale has beenconfirmed by SAXS in all networks. Except for the highest molecular weight of PBD, the networks consist of microdomains of similar sizeand composition dispersed in the soft phase of polybutadiene segments. Dielectric spectroscopy revealed the existence of three types of non-crystalline phases, each characterised by a well-developed dynamic glass transition. Absence of crystallinity has been proven by WAXS andDSC. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyurethane network; Microphase separation; Dielectric spectroscopy

Structure and elasticity of polyurethane networks based on poly(butadiene) diol, 4,4′-diphenylmethane diisocyanate and poly(oxypropylene) triol

Abstract Relations between structure and equilibrium mechanical properties of polyurethane networks based on poly(butadiene) diol, 4,4′-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) prepared at various initial ratios of reactive groups were studied. An inhomogeneous structure of the networks was revealed by small-angle X-ray scattering. The microdomains consist of POPT molecules crosslinked with MDI and they have a diameter of about 4–5 nm. The characteristic distance of microdomains is about 9 nm. The poor mutual miscibility of the reaction components and the existence of hydroxyl group association in the reaction mixture are the probable reasons for microdomain formation. The strain dependence of the equilibrium elastic force obeyed the Mooney-Rivlin equation. The experimental moduli of dry and equilibrium swollen samples were higher than those calculated for the affine limit of the Flory junction fluctuation model on the assumption of a homogeneous network formation.