Miroslava Dušková-Smrčková

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.

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.

Polyurethane networks with controlled architecture of dangling chains

Network formation from A x + B y precursors (A + B → A - B) with functionality and molecular weight distributions is described by the statistical theory of branching processes. Network formation is described in terms of sol and gel fractions, dangling chains, elastically active network chains (EANC), elastically active crosslinks, free chain ends and branch points partitioned between sol and gel. Various definitions of an EANC are considered. The general relations are applied to a special case of A1 + A2 + A3/B2 system. It is show how the size and weight fractions of dangling chains can be varied indepently by varying the functionality or molecular weight distributions. This is demonstrated experimentally analyzing corresponding polyetherurethane networks prepared from mixtures of polyoxypropylene polyols. The width of the main transition region correlates with the fraction of material in dangling chains. The dependence of the equilibrium shear modulus on the concentration of EANCs indicates much weaker intermolecular interactions for networks with many short dangling chains compared with systems having few longer dangling chains.

Network structure dependence of volume and glass transition temperature

A series of polyurethanes was used to determine the molar contributions of chain ends (CE) and branch points (BP) to free volume and glass transition temperature Tg. The polyurethanes were copolymers of diphenylmethane diisocyanate and poly(propylene oxide) (PPO) with hydroxyl functionalities of one, two, and three. The equivalent weights of all the PPOs were equal, such that the chemical composition of the chain segments was essentially identical. Therefore, the only distinctions among polymers were differences in CE and BP concentration. Theory of branching processes computer simulations were used to determine the concentration of CE due to imperfect network formation. Other CE contributions were from the monofunctional PPO. Polymer volumes and Tgs were correlated to CE and BP concentrations, and the contributions of these species were determined from least squares fits. The molar volume and Tg contributions were then used to determine free volume thermal expansion coefficients. These values were compar...

Macroporous 2-hydroxyethyl methacrylate hydrogels of dual porosity for cell cultivation: morphology, swelling, permeability, and mechanical behavior

AbstractMacroporous hydrogels of dual porosity based on cross-linked poly(2-hydroxyethyl methacrylate) (pHEMA) were prepared, and their swelling, mechanical responses, and hydraulic permeability, as well as cell adhesion and proliferation, were examined on gel-based scaffolds. Large pores on the order of tens and hundreds of microns were generated by adding a solid porogen (fractionated particles of sodium chloride) to the polymerization mixture, which was subsequently washed out of the gel. Small pores on the order of a few microns were formed by the reaction-induced phase separation mechanism caused by addition of a diluent (1-dodecanol), which is a poor solvent for pHEMA. Morphological studies using light and electron microscopy techniques revealed that the large pores were embedded in the pHEMA matrix containing small fused spherical pores. Mechanical behavior of the dual-porosity hydrogels characterized by oscillatory shear measurements revealed that despite very high pore volume, the hydrogels were sufficiently stiff and self-supportive. The cell cultivation pilot experiment showed that the morphology (dual porosity) demonstrated marked effects on the promotion of cell adhesion, growth, and proliferation, not only on the gel surface but in the large pores within the gel bulk as well. Graphical AbstractDual porosity hydrogel was formed using washable templates (large pores) and utilizing the phase separation within the hydrogel walls (fine porosity) in one step. The hydrogel structure was visualized by scanning electron microscopy (gel without cells) - the image confirmed dual porosity.

Branching theories and thermodynamics used to help designing precursor architectures and binder systems

SummariesCross-linking hydroxy-functional stars and copolymer precursors with a triisocyanate and associated structural changes were studied theoretically and experimentally. The model of network formation was based on the statistical generation of structures from units which were in different reaction states. The differences in gel point conversion and cross-link density were interpreted in terms of precursor functionality and functionality distribution, and differences in the reactivity of functional groups. Hydroxy-functional copolymer systems containing precursors of the same number-average functionality gel at lower conversions than stars which are monodispersed mainly due to functionality distribution. The structure of copolymers characteristic of several internal cross-links contributes more to the concentration of elastically-active network chains than stars with a single cross-link. Cyclization accompanies intermolecular cross-linking; its intensity is higher for copolymer systems compared with stars.RésuméOn a étudié des étoiles réticulantes à fonction hydroxy et des précurseurs polymériques qui avaient subi des changements de structure dus à un triisocyanate et des changements de structure associés. Ces études étaient théoriques et expérimentales. Le modèle de la formation du réseau était basé sur la génération statistique de structures à partir des unités qui étaient dans des états de réaction variés. Les différences de la densité de la réticulation et de la conversion au point de gél étaient interprétées du point de vue fonctionnalité précurseur, distribution de la fonctionnalité et différences de la réactivité des groupes fonctionnels. Les systèmes copolymériques à fonction hydroxy et contenant des précurseurs de la même fonctionnalité moyenne en nombre gélifient à des conversions plus basses que les étoiles qui sont monodispersées, principalement à cause de la distribution de la fonctionnalité. La structure des copolymères, c’est-à-dire la pluralité de leurs reticulations internes, contribue plus à la concentration des chaines de réseau qui sont plus actives élastiquement que les étoiles avec leur seule réticulation. La cyclisation accompagne la réticulation inter-molaire : son intensité est plus élevée pour les systèmes copolymériques que pour les étoiles.ZusammenfassungDie Kreuzverbindung von hydroxyfunktionellen Stem- und Kopolymerausgangsstoffen mit einem Triisozyanat und die damit verbundenen strukturellen Veränderungen wurden theoretisch und experimentell studiert. Das verwendete Modell der Netzwerkbildung basierte auf statistisch generierten Strukturen von Einheiten in verschiedenen Reaktionszuständen. Die Unterschiede in der Gelpunktumwandlung und Kreuzverbindungsdichte wurden durch die Ausgangsstoffunktionalität und Funktionalitätsverteilung, sowie durch Unterschiede in der Reaktivität der funktionellen Gruppen interpretiert. Hydroxy-funktionelle Kopolymersysteme, die Ausgangsstoffe mit der gleichen durchschnittlichen numerischen Funktionalität gellieren bei niedrigeren Umwandlungen als Sterne, die durch ihre funktionelle Distribution größtenteils monodispergiert sind. Die für mehrere interne Kreuzverknüpfungen typische Kopolymerstruktur trug mehr zur Konzentration von elastisch aktiven Netzwerkketten bei als Sterne mit einer einzelnen Kreuzverknüpfung. Die Zyklisierung geht mit intermolekularen Kreuzverknüpfungen einher; die Intensität ist grösser für Kopolymersysteme als für Steme

Scratch-and mar-resistant refinish two-pack clear coats -linear versus branched acrylics

SummariesThe new generation clear coats introduced in the automotive industry have improved scratch and mar resistance. It is believed that those properties are related to the introduction of new binder technologies leading to higher film cross-link densities. This was relatively easy to accomplish in the automotive industry because baking temperatures are well above the glass-transition temperature (Tg) of the final clear coat film. In car repair coatings, the cross-linking takes place under conditions where the intermediate and/or final film Tg values are above the curing temperature. Such formulations result in vitrification during the drying process, a transition during which chemical cure and solvent release are slowed down substantially. The chemical cure can be improved through more reactive and/or higher functional binders, but this shifts the gel point to shorter times in the drying cycle which may lead to more solvent entrapment, resulting in low film hardnesses and paint defects. This paper describes the overall properties of 2K-refinish clear coats based on linear versus branched, random and graft acrylic polyols. The authors will try to correlate overall properties with theoretical predictions and practical results.RésuméLa nouvelle génération de revêtements transparents qui a apparu dans l’industrie automobile a une résistance améliorée aux éraflures et aux dégradations. On pense que ces qualités sont associées à l’introduction de nouvelles technologies dans le domaine des liants, ce qui entraine une densité de réticulation plus élevé dans le film. Tout cela était relativement facile à accomplir dans l’industrie automobile parce que les températures de cuisson sont beaucoup plus élevées que la température de la transition vitreuse (Tg) du film de revêtement transparent final. En ce qui concerne les revêtements de réparation pour les autos, la réticulation se produit sous des conditions où les valeurs Tg de film intermédiaires et/où finales sont plus élevées que celles de la température de séchage. De telles formulations provoquent la vitrification pendant le processus du séchage, transition pendant laquelle le séchage chimique et la libération du solvant sont beaucoup ralentis. Le séchage chimique peut être amélioré gráce à des liants plus réactifs et/où plus hautement fonctionnels, mais cela fait que le point de gel arrive plus tôt dans le cycle de séchage, situation qui pourrait entraîner un rétention du solvant, donc une basse dureté de film et des défauts de revêtement. Cet article décrit les propriétés générales de 2K revêtements de refinition transparents : ceux qui sont basés sur les polyols linéaires par rapport à ceux qui sont basés sur les polyols acryliques branchés, aléatoires et greffés. Les auteurs essayeront de corréler les propriétés générales aux prévisions théoriques et aux résultats pratiques.ZusammenfassungDie neue Generation von Klarlacken für die Auto industrie weist sich durch verbesserte kratz- und schadensresistenz aus. Es wird angenommen, daß diese Vorteile der Einführung von neuen Bindemitteln zu verdanken sind, die die Dichte der Kreuzverbindungen erhöht haben. Dieses war bei der Lackierung von neuen Autos relativ leicht zu erreichen, weil die Lackhärtungstemperaturen deutlich über der Glastransitionstemperatur (Tg) des letzten Klarlackes liegen. Bei Reparaturlacken findet die Kreuzvernetztung unter anderen Bedingungen statt: hier ist der Tq-Wert des Zwischen- oder obersten Klarlackfilmes höher, als die Härttemperatur. Der Lack wird während des Trocknung-sprozesses verglast (vitrifiziert) - ein Vorgang, der die Freigabe der Lösungsmitteln verlangsamt und die Härtungszeit deutlich verlängert. Die chemische Härtung kann durch die Beisetzung von Bindemitteln mit größerer Reaktivität oder verstärkter Funktionalitat verbessert werden, aber dieses verringert die Zeit zum Gellierungspunkt, was zur unvollständigen Freigabe der Lösungsmittel führen kann, was wiederrum in verringerter Filmhärte und Lackfehlem resultiert. Diese Arbeit beschäftigt sich mit den allgemeinen Eigenschaften von 2K Refinish-Klarlacken auf Basis von linearen, verzweigten, vermischten und graft acrylischen Polyolen. Die Autoren versuchen, die beobachteten Eigenschaften mit theoretischen Vorhersagen und praktischen Ergebnissen zu korellieren.

Hydrogel tissue expanders for stomatology. Part I. Methacrylate-based polymers

In order to create a soft tissue surplus, implantable volume expanders are often utilized in dental surgery. Implanted tissue expanders should gradually increase their volume, exerting a constant pressure on the surrounding tissue for weeks. Current tissue expanders are based predominantly on externally inflatable balloons or on osmotically active tissue expanders that use soft hydrogels wrapped in perforated plastic coatings, which limit fluid entry and swelling. We have designed and examined tissue expanders based on the controlled rate expansive hydrogels synthesized from copolymers of selected methacrylates and N-vinylpyrrolidone, cross-linked with a combination of non-degradable (glycol dimethacrylates) and hydrolytically degradable (N,O-dimethacryloylhydroxylamine) cross-linkers. These copolymers have close-to-linear volume expansion rates (up to 6-9 times their original volume) and exert an increasing swelling pressure in vitro. The anesthetic benzocaine has been incorporated into the hydrogels, and kinetic release experiments have shown that most of the drug (90%) was released within 48 h. Our proposed hydrogel expanders are homogeneous and have suitable mechanical properties, thus simplifying the surgical manipulations required. Further studies will be needed to completely evaluate their biocompatibility and tissue response to the implants.

Swelling of Coating Films

The factors and theoretical approaches to equilibrium swelling of cross-linked polymer systems were analyzed in relation to the application of the equilibrium swelling method for characterization of structure of cross-linked coating films. Special attention was paid to the possibility of characterizing the cross-link density and its changes during film formation. For swelling of highly cross-linked networks (coating films) in good solvents, the finite extensibility of network chains was respected. The effect of adhesion of the film to the substrate was also considered. If equilibrium swelling method (swell test) is used for characterizing changes in cross-link density of drying (reacting) films, the results are often not meaningful for several reasons such as (a) dependence of interaction parameter on conversion; (b) if the sample is in its glassy state and contains unreacted functional groups, increase of conversion occurs during the test when swelling induces transition from the glassy to the rubbery state.

Role of Distributions in Binders and Curatives and Their Effect on Network Evolution and Structure

Precursors of cross-linked polymer systems are blends of many compounds, some of them forming series (distributions) of compounds of increasing molecular weights, type and number of functional groups, or some other property. The distributions may arise from impurities in the raw materials, or a result of side reactions. In most cases, the distributions are generated intentionally; functional copolymers, hyperbranched polymers, off-stoichiometric highly branched polymers, chain extended systems, or precursors prepared in several stages can serve as examples. The distributions affect processing and materials properties. We show ways to generate these distributions from bond formation kinetics and reaction mechanisms. Statistical branching theories based on assemblage of branched molecules and gel structure from building units in different reaction states are used to model the evolution of the cross-linked system.