A novel class of smart materials - European …A novel class of smart materials The blending of...
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Quelle/Publication: Ausgabe/Issue: Seite/Page: European Coatings Journal 06/2003 28 A novel class of smart materials The blending of non-covalent interactions with traditional polymer chemistry can lead to a novel class of smart (responsive) materials allowing a new access to high-tech applications, such as switchable camouflage coatings or anticorrosive surfaces. An overview on supramolecular polymer chemistry will be presented in combination with a discussion of several potential systems, based mainly on hydrogen bonding and metal-ligand interactions. For a particular system (terpyridine-metal systems), the "switchable" properties are discussed in detail. Ulrich S. Schubert, Georg Hochwimmer, Stefan Schmatloch, Harald Hofmeier At present, smart or responsive materials observe an extremely high world-wide interest both in academia and industry [1]. Smart or responsive materials are structured systems that show a response to an external stimulus. They react to outside (environmental) conditions, such as temperature, light, stress, pH, strain, electric or magnetic fields in a selective way. In particular responsive films, adhesives or coatings represent an enormously important area with highly important high-tech applications, such as antifouling or foul-release coatings (ships, houses), switchable surfaces for cell adhesion, switchable camouflage (transmission vs. reflection) for airplanes, switchable adhesives, sensors, electrochromic films, anticorrosive surfaces, MEMs, signalling coatings, corrosion sensing coatings, smart coatings for in-situ monitoring of engine components or coatings with included nanomachines. Besides the intensive efforts at universities, the (US) military sees smart coatings as the fundamental tool for enabling vehicles, if corroded or scratched, to detect and heal themselves, changing color on the battlefield, creating instant camouflage and reporting problems as well as finally repairing them. In order to construct such systems, the combination of tailor-made macromolecules (covalent chemistry) with supramolecular interactions (non-covalent chemistry) or "addressable" molecular parameters (phase transition, E-Z isomerization, charge, dipole moment) are required. In this contribution we highlight first steps towards the engineering of self-healing and self-repairing materials on the basis of non-covalent systems (e.g. based on metal-ligand interactions, hydrogen bonding, ionic interactions) and subsequently the construction of coatings and paints with ambient intelligence (e.g. adjustable camouflage, self-cleaning features). The main focus will be on terpyridine-metal containing systems. All reagents and solvents were obtained by commercial suppliers and were purified by standard techniques prior to use. The compounds were synthesized and characterized as described elsewhere [2]. New non-covalent bonds by self-organization The combination of supramolecular chemistry with macromolecular chemistry is of major interest in modern materials science and nanotechnology. Utilizing supramolecular interactions, new non-covalent bonds can be formed by self-organization processes. In contrast to the typical bond strength of covalent bonds of around 350 kJ/mol (breaking a C-C bond into the two radicals), the secondary interactions are much weaker, ranging from less then 5 kJ/mol for van der Waals forces, through 5 to 65 kJ/mol for a hydrogen bond to 250 kJ/mol for an ion-ion interaction. In addition to that thermodynamic view, the reversibility of the supramolecular linkage is of major importance. Utilizing heat, pH, electrochemistry or shear forces, the connection can be opened up - and reformed later again. This could be of special interest, e.g. in the direction of recycling purposes or "switchable" responsive materials. A recent example from the combination of hydrogen-bonding and polymer chemistry is represented by the work of E. W. Meijer's group in Eindhoven [3]: Stable polymers are constructed from a low-molecular weight poly(ethylene-co-butylene) with telechelic 2-ureido-4[1H]-pyrimidinone end groups. The two supramolecular end groups can bind each other by the formation of a quadruple hydrogen bond resulting in a high molecular weight polymer with high molecular weight. Due to the reversible binding behavior of hydrogen bonds, a strong temperature dependence of the melt viscosity due to depolymerization could be observed [4]. In view of the binding strength of supramolecular interactions, metal-to-ligand interactions seem to be a preferred candidate [5]. Already in 1993 the formation of well-defined structures by self-assembly processes became a field of research in polymer science. Telechelic polymers consisting of bipyridine and bis(bipyridine) end groups were prepared utilizing PU chemistry [6]. As shown in Figure 1, both types of polymers can be elongated on addition of Cu(I) or Ag(I) ions by formation of the corresponding metal complexes. The bis(bipyridine) functionalized polymers form helical structures similar to their monomeric analogs. The resulting coordination polymers were characterized in detail with respect to their mechanical properties and morphological behavior. The authors found demonstrated a distinct microphase-separation in bulk, with nano to mesoscopic superstructures consisting of copper-bipyridine complex aggregates in a polyether matrix. Furthermore the polymer ion complex revealed a temperature dependence of the storage modulus G' and tan δ typical for a two-phase thermoplastic elastomer with an extended plateau region between 260 and 450 K. At 450 K the metal complexes opened up resulting in a sharp decrease of G' and tan δ (Figure 2). A different approach to thermo-switchable materials was described recently: In order to obtain materials with tunable and manipulable physical or chemical properties, a certain specific fragility of the coordinative species is required. In this case, terpyridine ligands in combination with iron(II) ions were selected. For such systems a sensitivity to thermal or pH alteration can be observed. First investigations on the temperature sensitivity of an iron(II)-poly(lactide) complex film showed the disappearance of the purple color at ~160 °C. The color returns after cooling (Figure 3). This process can be repeated several times, until degradation takes place. Chujo and his group described a further example of thermo-switchable materials. The authors demonstrated the formation of thermal and oxidative reversible hydrogels based on bipyridine-functionalized telechelics. Temperature stimulated interconversion of intermolecular bipyridine-cobalt(III) complexes into kinetically favored intramolecular complexes lead to formation of a soluble polymer from an insoluble gel [7]. Another possibility to open and close metal complexes was represented by redox processes. The switching between kinetically stable cobalt(III)-bipyridine and kinetically labile cobalt(II)-bipyridine macromolecular complexes represents an example of redox reversible hydrogel system [8]. A novel class of thin films with tunable viscosity Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Transcript of A novel class of smart materials - European …A novel class of smart materials The blending of...
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A novel class of smart materialsThe blending of non-covalent interactions with traditionalpolymer chemistry can lead to a novel class of smart(responsive) materials allowing a new access to high-techapplications, such as switchable camouflage coatings oranticorrosive surfaces. An overview on supramolecularpolymer chemistry will be presented in combination with adiscussion of several potential systems, based mainly onhydrogen bonding and metal-ligand interactions. For aparticular system (terpyridine-metal systems), the"switchable" properties are discussed in detail.Ulrich S. Schubert, Georg Hochwimmer, Stefan Schmatloch,Harald HofmeierAt present, smart or responsive materials observe anextremely high world-wide interest both in academia andindustry [1]. Smart or responsive materials are structuredsystems that show a response to an external stimulus. Theyreact to outside (environmental) conditions, such astemperature, light, stress, pH, strain, electric or magneticfields in a selective way. In particular responsive films,adhesives or coatings represent an enormously importantarea with highly important high-tech applications, such asantifouling or foul-release coatings (ships, houses),switchable surfaces for cell adhesion, switchablecamouflage (transmission vs. reflection) for airplanes,switchable adhesives, sensors, electrochromic films,anticorrosive surfaces, MEMs, signalling coatings, corrosionsensing coatings, smart coatings for in-situ monitoring ofengine components or coatings with includednanomachines. Besides the intensive efforts at universities,the (US) military sees smart coatings as the fundamentaltool for enabling vehicles, if corroded or scratched, to detectand heal themselves, changing color on the battlefield,creating instant camouflage and reporting problems as wellas finally repairing them.In order to construct such systems, the combination oftailor-made macromolecules (covalent chemistry) withsupramolecular interactions (non-covalent chemistry) or"addressable" molecular parameters (phase transition, E-Zisomerization, charge, dipole moment) are required.In this contribution we highlight first steps towards theengineering of self-healing and self-repairing materials onthe basis of non-covalent systems (e.g. based onmetal-ligand interactions, hydrogen bonding, ionicinteractions) and subsequently the construction of coatingsand paints with ambient intelligence (e.g. adjustablecamouflage, self-cleaning features). The main focus will beon terpyridine-metal containing systems.All reagents and solvents were obtained by commercialsuppliers and were purified by standard techniques prior touse. The compounds were synthesized and characterizedas described elsewhere [2].
New non-covalent bonds by self-organizationThe combination of supramolecular chemistry withmacromolecular chemistry is of major interest in modernmaterials science and nanotechnology. Utilizingsupramolecular interactions, new non-covalent bonds canbe formed by self-organization processes. In contrast to thetypical bond strength of covalent bonds of around 350kJ/mol (breaking a C-C bond into the two radicals), thesecondary interactions are much weaker, ranging from lessthen 5 kJ/mol for van der Waals forces, through 5 to 65kJ/mol for a hydrogen bond to 250 kJ/mol for an ion-ioninteraction.In addition to that thermodynamic view, the reversibility ofthe supramolecular linkage is of major importance. Utilizing
heat, pH, electrochemistry or shear forces, the connectioncan be opened up - and reformed later again. This could beof special interest, e.g. in the direction of recycling purposesor "switchable" responsive materials.A recent example from the combination ofhydrogen-bonding and polymer chemistry is represented bythe work of E. W. Meijer's group in Eindhoven [3]: Stablepolymers are constructed from a low-molecular weightpoly(ethylene-co-butylene) with telechelic2-ureido-4[1H]-pyrimidinone end groups. The twosupramolecular end groups can bind each other by theformation of a quadruple hydrogen bond resulting in a highmolecular weight polymer with high molecular weight. Dueto the reversible binding behavior of hydrogen bonds, astrong temperature dependence of the melt viscosity due todepolymerization could be observed [4].In view of the binding strength of supramolecularinteractions, metal-to-ligand interactions seem to be apreferred candidate [5]. Already in 1993 the formation ofwell-defined structures by self-assembly processes becamea field of research in polymer science. Telechelic polymersconsisting of bipyridine and bis(bipyridine) end groups wereprepared utilizing PU chemistry [6]. As shown in Figure 1,both types of polymers can be elongated on addition of Cu(I)or Ag(I) ions by formation of the corresponding metalcomplexes. The bis(bipyridine) functionalized polymers formhelical structures similar to their monomeric analogs. Theresulting coordination polymers were characterized in detailwith respect to their mechanical properties andmorphological behavior. The authors found demonstrated adistinct microphase-separation in bulk, with nano tomesoscopic superstructures consisting of copper-bipyridinecomplex aggregates in a polyether matrix.Furthermore the polymer ion complex revealed atemperature dependence of the storage modulus G' and tan δtypical for a two-phase thermoplastic elastomer with anextended plateau region between 260 and 450 K. At 450 Kthe metal complexes opened up resulting in a sharpdecrease of G' and tan δ (Figure 2).A different approach to thermo-switchable materials wasdescribed recently: In order to obtain materials with tunableand manipulable physical or chemical properties, a certainspecific fragility of the coordinative species is required. Inthis case, terpyridine ligands in combination with iron(II) ionswere selected. For such systems a sensitivity to thermal orpH alteration can be observed. First investigations on thetemperature sensitivity of an iron(II)-poly(lactide) complexfilm showed the disappearance of the purple color at ~160°C. The color returns after cooling (Figure 3). This processcan be repeated several times, until degradation takesplace.Chujo and his group described a further example ofthermo-switchable materials. The authors demonstrated theformation of thermal and oxidative reversible hydrogelsbased on bipyridine-functionalized telechelics. Temperaturestimulated interconversion of intermolecularbipyridine-cobalt(III) complexes into kinetically favoredintramolecular complexes lead to formation of a solublepolymer from an insoluble gel [7]. Another possibility to openand close metal complexes was represented by redoxprocesses. The switching between kinetically stablecobalt(III)-bipyridine and kinetically labile cobalt(II)-bipyridinemacromolecular complexes represents an example of redoxreversible hydrogel system [8].
A novel class of thin films with tunable viscosity
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Moreover, the combination of non-covalent interactions withtraditional thermal and UV cross-linking methods can lead toa novel class of thin films with tunable viscosity behaviorand recycling possibilities or a new access to multi-layersystems. In this case, the specific non-covalent linkage (e.g.coordinative bonds or ionic interactions etc.) can be formedalready at low temperatures in water, in solution or in 100%pure systems (incl. powders), providing partially cross-linkedmaterials with adjustable viscosity behavior (directly tunableby the number and position of the non-covalently linkingunits and the kind of cross-linkers). The linkage can still bemanipulated and completely recycled, providingpre-coatings with excellent processibility properties (Figure4).For this purpose, thermal- or UV-curable moieties should beintroduced into one polymer together with supramolecularunits. As a first example we prepared recently a PMMAterpolymer consisting of epoxide or oxirane units as well asterpyridine ligands (Figure 5) [9]. Addition of suitable metalions (such as iron(II) or zinc(II)) immediately resulted in aformation of the corresponding metal complexes andtherefore in a non-covalent cross-linking. The coupling witha UV-cross-linking process can "fix" then in a second stepthe non-covalent cross-linking by the formation of covalentbonds (Figure 6).
The reversibility can also be adjustedBesides the fine-tuning of materials properties (e.g.influencing viscosity and processability), an entire switchingbetween monomeric and polymeric units is possible via thechange of temperature, pH-value or redox chemistry. Suchproperties might be in particular important for recyclingpurposes. Not only the degree of polymerization ofcoordination polymers can be adjusted by the proper choiceof complexing ligand and metal-ion, but also the reversibility.Based on metal-ligand interactions we have recentlysynthesized coordination polymers based onbis(2,2':6',2''-terpyrid-4'-yl) di(ethylene glycol) and iron(II)ions [10]. The coordination polymer was fully characterizedby 1H-NMR and UV/VIS- spectroscopy, the polymericcharacter could be proven by viscosity measurements. Themetal-ligand bond is stable towards heating up totemperatures of about 210 °C. The fading of the colorindicates a breakage of the metal-ligand bond at thosetemperatures. After cooling, a reformation of the complexescould be observed. Moreover, upon addition of a 20-foldexcess of the chelating ligand HEDTA(hydroxyethylethylenediaminetriacetic acid), the coordinationpolymer could be fully cleaved down to the monomeric units;the monomers could be isolated and recomplexed (seeFigure 7). The breakage and reformation of the polymercould be followed by 1H-NMR spectroscopy. Firstexperiments have been successfully undertaken toelectrochemically switch between coordination polymers andmonomeric units by changing of the oxidation state of iron.Finally, we could demonstrate the construction ofmetallo-supramolecular block copolymers on the basis ofterpyridine functionalized telechelics and ruthenium(II) ions[11]. In a first step a terpyridine functionalized polymericbuilding block A was endcapped via the well-definedformation of ruthenium(III)chlorid monocomplexes. In asecond step, the ruthenium(III) complex was reduced andsimultaneously reacted with anotherterpyridine-functionalized telechelic B, in order to form ablock copolymer AB. Utilizing poly(styrene) orpoly(ethylene-co-butylene) and poly(ethylene oxide) asbuilding blocks for the block copolymer, supramolecularmicelles could be constructed [12]. The formation ofsupramolecular micelles could be proven via dynamic light
scattering (DLS), AFM and TEM. Even so the bindingstrength of the ruthenium(II) terpyridine bond is rather high,a cleavage could be achieved in certain cases via theaddition of a large excess (104 mol per mol terpyridineligand) of the chelating ligand HEDTA(hydroxyethylethylenediaminetriacetic acid). The terpyridinefunctionalization of the isolated poly(styrene) micelle corecould be demonstrated by a color change upon the additionof iron(II) ions (see Figure 8). Moreover, the "switching"should be possible utilizing redox processes.
AcknowledgementsThe research was supported by the DeutscheForschungsgemeinschaft (SFB 266, SFB 486, SFB 563),the Fonds der Chemischen Industrie, the BASF AG and theDutch Polymer Institute.
References[1] (a) R. Penterman, S. I. Klink, H. de Koning, G. Nisato, D.J. Broer, Nature, (2002), 417 p. 55; (b) J. Lahann, S.Mitragotri, T.-N. Tran, H. Kaido, J. Sundaram, I. S. Choi, S.Hoffer, G. A. Somorjai, R. Langer, Science, (2003), 299,371; (c) J. Hiller, J. D. Mendelsohn, M. F. Rubner, NatureMaterials (2002), 1, p. 59[2] See, e.g.: (a) U. S. Schubert, C. Eschbaumer, G.Hochwimmer, Tetrahedron Lett. (1998), 39, p. 86; (b) G.Hochwimmer, O. Nuyken, U. S. Schubert, Macromol. RapidCommun. (1998), 19, p. 309; (c) U. S. Schubert, O. Hien, C.Eschbaumer, Macromol. Rapid Commun. (2000), 21, p.1156; (d) U. S. Schubert, G. Hochwimmer, Macromol. RapidCommun. (2001), 22, p. 274; (e) M. Heller, U. S. Schubert,Macromol. Rapid Commun. (2001), 22, p. 1358; (f) U. S.Schubert, M. Heller, Chem. Eur. J. (2001), 5, p. 5252; (g) M.Heller, U.S. Schubert, Macromol. Symp. (2002), 177, p. 87;(h) U. S. Schubert, C. Eschbaumer, Macromol. Symp.(2001), 163, p. 177; (i) U. S. Schubert, C. Eschbaumer, O.Hien, P. R. Andres, Tetrahedron Lett. (2001), 42, p. 4705; (j)U. S. Schubert, S. Schmatloch, A. A. Precup, Design.Monom. Polym. (2002), 5, p. 211; (k) U.S. Schubert, C.Eschbaumer, Angew. Chemie. (2002), 114, p. 3016; Angew.Chem. Int. Ed. (2002), 41, p. 2892.[3] B. J. B. Folmer, R. P. Sijbesma, R. M. Versteegen, J. A.J. van der Rijt, E. W. Meijer, Adv. Mater. (2000), 12, p. 774[4] L. Brunsveld, B. J. B. Folmer, E. W. Meijer, R. P.Sijbesma, Chem. Rev. (2001), 101, p. 4097[5] U. S. Schubert, C. Eschbaumer, Angew. Chem. (2002),41, p. 3016; Angew. Chem. Int. Ed. (2002), 41, p. 2892[6] (a) C. D. Eisenbach, U. S. Schubert, Macromolecules(1993), 26, p. 7372; (b) C. D. Eisenbach, W. Degelmann, A.Göldel, J. Heinlein, M. Terskan-Reinhold, U. S. Schubert,Macromol. Symp. (1995), 98, p. 565; (c) C. D. Eisenbach, A.Göldel, M. Terskan-Reinhold, U. S. Schubert, Macromol.Chem. Phys. (1995), 196, p. 1077; (d) C. D. Eisenbach, A.Göldel, M. Terskan-Reinold, U. S. Schubert, Kautsch.Gummi Kunstst. (1998), 51, p. 424; (e) C. D. Eisenbach, A.Göldel, M. Terskan-Reinold, U. S. Schubert, in PolymericMaterials Encyclopedia, Vol. 10 (Ed.: J. C. Salamone), CRCPress, Boca Raton, (1996), p. 8162[7] Y. Chujo, K. Sada, T. Saegusa, Macromolecules, (1993),26, p. 6315[8] Y. Chujo, K. Sada, T. Saegusa, Macromolecules, (1993),26, p. 6320[9] (a) Abdelkrim El-ghayoury, Harald Hofmeier, Barteld deRuiter, Ulrich S. Schubert, Macromolecules (2003), 36 inpress[10] (a) S. Schmatloch, U. S. Schubert, Polym. Preprints(2001), 42, p. 395; Schmatloch, C. Brändli, H.-H.Nguyen-Ngoc, U. S. Schubert Polymeric Materials: Science& Engineering, (2002), 87, p. 237; (c) S. Schmatloch, U. S.
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Schubert, Polym. Preprints, (2002), 43, p. 1155; (d) S.Schmatloch, M. Fernández-Gonzalez, U. S. Schubert,Macromol. Rapid Commun. (2002), 23, p. 957; (e) S.Schmatloch, U. S. Schubert, Macromol. Symp. (2002), inpress[11] B. G. G. Lohmeijer, U. S. Schubert, Angew. Chem.(2002), 114, p. 3980; Angew. Chem. Int. Ed. (2002), 41, p.3825[12] (a) J.-F. Gohy, B. G. G. Lohmeijer, U. S. Schubert,Macromolecules, (2002), 35, p. 4560; (b) J.-F. Gohy, B. G.G. Lohmeijer, S. K. Varshney, U. S. Schubert,Macromolecules, (2002), 35, p. 7427; (c) J.-F. Gohy, B. G.G. Lohmeijer, U. S. Schubert, Macromol. Rapid Commun.(2002), 23, p. 555; (d) B. G. G. Lohmeijer, U. S. Schubert, J.Polym. Sci. Part A: Polym. Chem. (2003), p. 1414
Result at a glanceIn order to engineer new materials with "smart" properties(e.g. responsive coatings), the combination ofsupramolecular chemistry with polymer chemistry is ofspecial interest. Besides the formation of the supramolecularlinkage by self-organization processes, the potential"switchability" of the linkage opens new avenues for theconstruction of novel materials. Two ways can bedistinguished: (a) "Pure" supramolecular systems, whereonly supramolecular interactions play a role in order toconnect and cross-link covalent polymer chains; (b)combined systems, where supramolecular interactions areconnected with traditional thermal and UV-cross-linkingmethods. Both strategies are at present investigated indetail regarding their impact onto the creation of smart andresponsive coatings and thin films.
The authors:-> Ulrich S. Schubert studied chemistry at the Universities ofFrankfurt and Bayreuth (both Germany). Since June 2000he is Full Professor at the Eindhoven University ofTechnology. The major focus of his research interest relatesto supramolecular materials, non-covalent polymers,combinatorial material research, surface interactions,organic heterocyclic chemistry, nanoscience and tailor-mademacromolecules.-> Georg Hochwimmer graduated in 1998 in Chemistry,Machinery Engineering and Computer Science at theTechnische Universität München and in Economy at theFernuniversität Hagen. In 2000 he received his PhD at theTechnische Universität München in the field of polymerresearch. He founded the Georg HochwimmerUnternehmensberatung in 1994, the general ResearchGmbH in 2000 and among other positions he is CEO of thesupraMat technologies AG.-> Stefan Schmatloch graduated in Applied Chemistry at theUniversity of Strathclyde (UK) in 1995 (B.Sc) and inChemistry at the University of Regensburg (Germany) in1998. In 2001 he received his PhD in the field oforganometallic chemistry and homogeneous catalysis.Currently he is working at the Eindhoven University ofTechnology (Prof. Dr. U. S. Schubert) and the DPI (DutchPolymer Institute) in the field of metallo-supramolecularpolymers and combinatorial material research.-> Harald Hofmeier graduated in 1999 in Chemistry at theUniversity of Heidelberg (Germany) and is currently workingon his PhD thesis at the Eindhoven University of Technologyin the field of metallo-supramolecular polymers (Prof. Dr. U.S. Schubert)
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Figure 1: Addition of Cu(I) ions resulted in the formation of supramolecular(ABA)n-systems.
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Figure 2: Dynamic mechanical properties of the Cu(I)-complexed triblock copolymerwith one bipyridine (curve 1) and two bipyridine segments (curve 2), respectively. In
addition two cross-linked multiblock copolymers (curve 3; mole ratio bpy units/Cu(I) =2:1.25; curve 4; mole ratio bpy units/Cu(I) 2:1) are shown. G' = storage modulus; tan
delta = loss factor.
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Figure 3: Top: Schematic presentation of the chemical "switching" between thecomplexed dimer and the uncomplexed system; bottom: Temperature dependency ofmetallo-supramolecular polymer films: Left) room temperature, middle left) ~130 °C,
middle right) ~160 °C, right) room temperature again.
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Figure 4: a) Schematic representation of a traditional cross-linked polymer coating; (b)concept of two step curing utilizing temperatur-, redox- or pH-responsive coatings; (c)
non-covalent interactions opens possibilities towards self-repair and self-healingcoatings.
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Figure 5: Synthesis of a functional terpolymer via a free radical copolymerization ofterpyridine-functionalized, epoxy-functionalized and unfunctionalized methacrylate
monomers.
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Figure 6: Schematic representation of a two-step curing: Towards smart coatings.
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Figure 7: Schematic representation of the formation and cleavage of themetallo-supramolecular coordination polymer
bis(2,2':6',2''-terpyrid-4'-yl)-FeCl2-di(ethylene glycol).
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Figure 8: Top: Metallo-supramolecular polymer based on poly(ethylene oxide) andpoly(ethylene-co-butylene) telechelics; bottom: Opening and recomplexation of themetallo-supramolecular micelles; the functionalized micelles might serve as filling
material for defects in non-covalent coating systems.
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