Libor Matějka

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.

Cage-like structure formation during sol-gel polymerization of glycidyloxypropyltrimethoxysilane

Abstract The sol–gel polymerization of 3-glycidyloxypropyltrimethoxysilane (GTMS) was followed by size exclusion chromatography and 29Si NMR. Extensive non-random cyclization under formation of polyhedral cycles – cubic cages – predominates at the beginning of the reaction. Structure growth of polysilsesquioxanes proceeds by combining the incompletely condensed cage frameworks. The extent of the cage formation increases with dilution and the amount of water and depends appreciably on a catalyst. The cage fraction was isolated from a reaction mixture using preparative size exclusion chromatography and identified by 29Si NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). High content of polyhedral cages prevents gelation of the trifunctional GTMS monomer. Reaction of pendant epoxy groups is much slower; however, at a late reaction stage the epoxy hydrolysis can be significant. Under some conditions, like base catalysis, polysilsesquioxane clusters are crosslinked by intermolecular condensation of SiOH with hydrolyzed epoxy groups and the system gels. The cage with epoxide functionalities may serve as a rigid precursor of crosslinking.

Rheology of epoxy networks near the gel point

SummaryCuring and gelation of diglycidyl ether of Bisphenol A with poly(oxypropylene)-diamine (Jeffamine D 400) was followed using dynamic mechanical measurements. The gel point was determined rheologically and the characteristic value of the loss factor tan δ at the gel point, (tanδ)gel, was found to depend on the structure of the polymer. A negative normal force is generated in the shear dynamic mechanical measurement at the gel point.

Photomechanical effects in crosslinked photochromic polymers

Abstract The effect of radiation on photochromic crosslinked polymers containing azo group side chains is investigated. After irradiation at constant sample length, the swollen gels of light-sensitive polymers exhibit a reversible increase in the elastic retractive force. The light induced conformational changes are also accompanied by changes in the swelling equilibrium and temperature of the sample due to absorption of radiation and this added complication to the interpretation of the data. Interactions of the polymer with the solvent and the heat effect were eliminated by determining the temperature dependences of the elastic retractive force for the irradiated and unirradiated rubbery dry networks. The photomechanical effect increased with an increase in the content of photochromic groups and for the polymer with 5.4 mol % of azo groups, the photoinduced contraction of the sample amounted to 1%.

Network formation in the free-radical copolymerization of a bismaleimide and styrene

Abstract Network formation during copolymerization of styrene with a model bismaleimide, p -maleimidobenzoic anhydride, or with mixture of p -maleimidobenzoic anhydride and methyl p -maleimidobenzoate was studied. The concentrations of pendent double bonds in copolymers, calculated from the consumption of monomers and copolymer composition, follow the general trend typical for vinyl divinyl copolymerizations. A substantial fraction of pendent maleimide bonds remains in the system at the end of polymerization. The gel point conversions are much higher than for ring-free copolymerizations due to cyclization and steric hindrance of pendent double bonds. The gel point conversions increase with increasing content of the monomaleimide in the copolymerization mixture reaching 100% at 4 mol% of bismaleimide (12 wt% solution). The gel point conversions also increase with increasing dilution reaching 100% for the bismaleimide polymerization in 3 wt% solution. The steric hindrance effect on pendent double bond reactivity seems to be somewhat higher than for conventional vinyl-divinyl copolymerizations.

The thermal effect in the photomechanical conversion of a photochromic polymer

SummaryThe effect of radiation on the mechanical behaviour of the crosslinked copolymer of maleic anhydride and styrene containing photochromic azo groups was investigated. The analysis of results of the observed photomechanical conversion proved a considerable influence of the thermal effect in addition to the effect of photoisomerization of the photochrome. The interpretation of some photomechanical experiments described earlier may be obscured by a rise in temperature caused by the absorption of radiation.

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.

Description of the tensile stress–strain behaviour of filler-reinforced rubber-like networks using a Langevin-theory-based approach. Part II

Abstract A combination of the Langevin-theory-based James–Guth equation with the phenomenological C 2 term of the Mooney–Rivlin equation (modified by introducing an additional empirical parameter) is shown to represent the tensile stress–strain dependencies obtained on retraction of a number of carbon-black- and silica-reinforced butadiene–styrene networks. The stress–strain behavior at increasing strain of both pre-strained and virgin specimens is more complex but it can be satisfactorily described using the concept of a strain-dependent finite extensibility parameter (introduced previously for unfilled networks). The accuracy of data description is better than ca. 4%. Similarly to unfilled networks, the increase in the finite extensibility parameter with increasing strain is ascribed to strain-induced changes in network topology (increase in network mesh size). On retraction, such changes probably take place to a much lesser degree if at all.

Formation of polyurethane networks studied by the gel point method

SummaryDetermination of critical stoichiometric ratio necessary for gelation in polyurethane systems composed of poly(oxypropylene)triol and diol, trimethylolpropane and 4,4′-diphenylmethane diisocyanate revealed the effects of a different functionality, reactivity of functional groups, tendency to cyclization and the course of the crosslinking process.