Karel Dušek

Network structure formation during crosslinking of organic coating systems

Abstract Crosslinking is a very important process in coating film formation and the structure of the formed network determines the application properties of the coating film. In this review, general features of crosslinking and evaluation of network structure are described. Network formation theories and their applicability to special chemical systems are analyzed. The kinetics of network formation is usually controlled by chemical reactivity of functional groups. A transition to the regime controlled by segmental mobility is typical for film formation especially for ambient temperature drying. Formation of inhomogeneities in multicomponent systems determined by composition and group reactivities and/or by thermodynamic segregation may also play an important role. In the last section, special features of formation of crosslinked structured from designed precursors — polyfunctional molecules with designed backbone architectures, such as telechelic polymers, stars, combs, microgels, dendrimers, or hyperbranched polymers are reviewed. Introduction of precursors of special architecture allows us not only to modify the processing and material properties of the film, but also brings about new problems in explaining and modeling the formation and properties of coating films.

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.

Features of Network Formation in the Chain Crosslinking (Co) Polymerization

SummaryAnalysis of experimental results on the chain (co) polymerization of bisunsaturated monomers suggests that cyclization plays a decisive role already at the onset of the reaction. At medium and elevated concentrations of the bisunsaturated compound, compact microgel-like particles are formed, which give rise to a gel mainly by participating in the polymerization reaction only with pendant double bonds in their surface layer, and not with double bonds in the interior. This mechanism is reflected in an apparent decrease in reactivity of the pendant double bonds.

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.

Are cured epoxy resins inhomogeneous

Abstract The alternating mechanism of network formation in the curing of epoxy resins from bisphenol A diglycidyl ether (BADGE) and amine curing agents does not offer any special opportunity for the formation of inhomogeneities caused by partial segregation or inhomogeneous crosslinking. Etched fracture surfaces of resins cured with 4,4′-diaminodiphenylmethane, hexamethylenediamine and hexahydrophthalic anhydride at various initial ratios of BADGE, studied by electron microscopy, reveal globular structures 20–40 nm in size. However, similar structures are observed with etched surfaces of amorphous polystyrene and poly(methyl methacrylate). The small-angle X-ray scattering curves for cured epoxy resins do not differ in principle from those of common amorphous polymers; swelling in a solvent of a lower electron density does not lead to an increase in scattering within the particle size range 10–10 2 nm. It is pointed out that the physical structure of simple cured epoxy resins does not essentially differ from that of common amorphous polymers. With more complicated systems, a more pronounced inhomogeneity might be caused by thermodynamic incompatibility or by non-alternating mechanisms of the curing reaction.

The structure and elasticity of polyurethane networks: 1. Model networks of poly(oxypropylene) triols and diisocyanate

Abstract The equilibrium mechanical and optical behaviour of networks prepared from poly(oxypropylene) triols (PPT) and 4,4′-diphenylmethane diisocyanate (MDI) at various initial molar ratios of reactive groups, r H = [ OH ] [ NCO ] , in the range 0.6 A = 1 3 (phantom network). (4) The difference between the eperimental and theoretical moduli for A = 1 3 may be adequately described by using Langleys concept of trapped entanglement contribution (for networks of the longer triol quite satisfactorily, for those of shorter PPT with some systematic deviation) with the same proportionality constant. (5) For correlations between the experiment and theory a generalized plot of the reduced modulus of the weight fraction of the gel proved to be useful.

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%.

The structure of low conversion polymers of ethylene dimethacrylate

Abstract Results of study of the pregel chain polymerization of ethylene dimethacrylate in the presence of various amounts of diluent suggest the formation of compact microgel-like particles even at the very beginning of polymerization, as a consequence of predominant cyclization. This view is based on following experimental findings. The fraction of units with pendant vinyls (70 mol %) is independent of conversion and depends only slightly on dilution; critical conversion at gelation is displaced by a factor of about 10 2 ; molecular weights at the beginning correspond approximately to molecular weights of poly(methyl methacrylate) obtained under the same conditions; intrinsic viscosities are very low and much lower than those of strongly branched poly(methyl methacrylates).

Correspondence Between the Theory of Branching Processes and the Kinetic Theory for Random Crosslinking in the Post-Gel Stage

SummaryThe kinetic theory of crosslinking by step reactions (Kuchanov 1978) may be extended beyond the gel point, and the structure and amount of sol may be described. For random reactions, the kinetic theory yields results identical with the theory of branching processes based on cascade substitution (Gordon 1962, 1975, Dobson 1965). The extension of the kinetic theory may be employed in order to compare deviations between the two theories expected for the dependent reactivities of functional groups.

Processes and states during polymer film formation by simultaneous crosslinking and solvent evaporation

Coating film formation with simultaneous crosslinking and solvent evaporation, accompanied by passage of the polymer film through glass transition region, is a complex process by which temporary or permanent anisotropic and gradient network structures can be formed. Evaporation and crosslinking are processes that are interdependent. The changes in structure (growth of branched molecules and network evolution) are a function of reaction kinetics, which gets diffusion controlled when the system passes through the glass transition region. Structural changes are determined by branching, gelation, and network build-up and depend on the architecture of network precursors. Thermodynamic interactions of polymer with solvents affect the solvent activity which determines the vapor pressure of the solvent over the film and thus the evaporation rate. The glass transition temperature increases as a result of both the decreasing solvent content and conversion of functional groups into bonds. By interplay of these two factors more or less solvent can be locked in by vitrification. The roles and intensity of these basic processes and interrelations are discussed. Some older results are reviewed and new experimental evidence is added. The interrelations are illustrated by time dependences of solvent evaporation and conversion of functional groups for solvent-based high-solids polyurethane systems composed of a hydroxyfunctional star oligomer and triisocyanate and by the role of the ratio of evaporation to crosslinking rates. Evidence was obtained of gradient formation in which appearance of a glassy surface layer is an important event in the history of film formation that determines solvent retention and other film characteristics.