X思 D0L10.1002adma.200701608 A Simple and Efficient Route to Transparent Nanocomposites** By Simon H.Stelzig,Markus Klapper,*and Klaus Millen ICATION Hybrid organic-inorganic materials have attracted substan tial attention as new materials since strong synergetic effects of the propertieso the organic and the inorganic lar enviro ment by u ing t mi mposites or scratc-resistant materials via the incor copolymers.The key issue was to find mixtures of polar and poration of inorganic nanoparticles into a polymer matrix is nonpolar solvents which are able to stabilize the polar parti- t extraord al inter and the hydropl of nanoparticles in suchfilms is still a challenging problem. as this would immediately lead to the formation The formation of aggregates of the additives results in a loss of aggregates,which are difficult to redisperse.The amphi be designed f the ch a way that they their polar part,in order to hydrophobize matrix have been proposed in recent years, additional functionality towards interaction with a polymer proach does not imply the use of a confined space to trap the in micellar stru Each of The proces ince the am partieer egates which leads totion of the under the chosen solvent conditions As a representative ex uperor syn mple,such a mposition will b e presented for silica(SiOz) to the the fo edpitbyoihorgahicwpanicdesshchye icles w that possess excellent hydrophobize them by ith the of the particles in the composite and suppress the formation The amphiphilic copolymers were easily prepared by fre e radi fthcgoductior this to occur. (DMAEMA)in various com Most inorganic nanoparticles are readily available inaque ons However he quan the ed out in a mulucomponen vent sy medium destabilizes the dispersion and aggregation is ob- ing which indicated a maximum size for the amphiphilic co ymer of ca.2 nm. t for the amphiphili mixture.which r ion with the surface of the SiO particles.Hydrophobization Adv.Mater2008,20,929-932 2008 WILEY-VCH Verag GmbH Co.KGaA,Weinhein interScience 929 DOI: 10.1002/adma.200701608 A Simple and Efficient Route to Transparent Nanocomposites** By Simon H. Stelzig, Markus Klapper,* and Klaus Müllen* Hybrid organic–inorganic materials have attracted substan￾tial attention as new materials since strong synergetic effects of the properties of the organic and the inorganic components are expected.[1–4] In particular, the production of transparent nanocomposites or scratch-resistant materials via the incor￾poration of inorganic nanoparticles into a polymer matrix is of extraordinary industrial interest for coating applications.[5] However, achieving the necessary homogeneous distribution of nanoparticles in such films is still a challenging problem. The formation of aggregates of the additives results in a loss of transparency and poor mechanical properties, which are detrimental to industrial applications.[6,7] Numerous approach￾es for the compatibilization of nanoparticles with a polymeric matrix have been proposed in recent years,[8] for example, using surfactants, grafting-from or -to methods [9,10] and emul￾sion/miniemulsion techniques.[11–16] Amphiphilic block co￾polymers have also been used to functionalize nanoparticles in micellar structures.[17,18] Each of these methods suffers from at least one of the following drawbacks. Either they are syn￾thetically demanding or they show a high tendency for the for￾mation of larger aggregates, which leads to segregation of the particles after incorporation into a matrix.[19,20] Superior syn￾thetic strategies that render nanoparticles applicable for trans￾parent composite materials are thus required. Essential to the broad applicability of inorganic nanoparticles is the ability to hydrophobize them by a simple but effective procedure. Furthermore, chemical or physical interactions with the poly￾mer matrix must be present to increase the migration stability of the particles in the composite and suppress the formation of aggregates. The introduction of appropriate functionality on the surface of the hydrophobized particles should allow this to occur. Most inorganic nanoparticles are readily available in aque￾ous dispersions. However, the quantitative transfer of these particles from an aqueous phase directly into a nonpolar envi￾ronment is complicated because the change in polarity of the medium destabilizes the dispersion and aggregation is ob￾served. The approach described herein enables the transfer of inorganic particles from an aqueous dispersion into a nonpo￾lar environment by using a latent miscible multicomponent solvent system in combination with surface-active amphiphilic copolymers. The key issue was to find mixtures of polar and nonpolar solvents, which are able to stabilize the polar parti￾cles, the amphiphilic copolymers, and the hydrophobized products. Any precipitation during the hydrophobization had to be avoided as this would immediately lead to the formation of aggregates, which are difficult to redisperse. The amphi￾philic copolymers had to be designed in such a way that they were able to adsorb to the surface of the particles through their polar part, in order to hydrophobize them and to offer additional functionality towards interaction with a polymer matrix. In this regard, an exact balance of all of the nonpolar and polar components had to be found. However, this ap￾proach does not imply the use of a confined space to trap the inorganic particles, as in emulsions or micelles. The process described herein represents a homogeneous solution process, since the amphiphilic copolymers do not form an emulsion under the chosen solvent conditions. As a representative ex￾ample, such a composition will be presented for silica (SiO2)- based systems. To demonstrate the efficiency of this concept, the hydrophobized particles were applied in the formation of polyurethane/SiO2 nanocomposites that possess excellent transparency. SiO2 nanoparticles, with an average diameter of approxi￾mately 10 nm, were used as a 30 wt.% aqueous dispersion. The amphiphilic copolymers were easily prepared by free radi￾cal polymerization of 2-(ethylhexyl)methacrylate (EHMA), poly(ethylene oxide) methacrylate (PEOMA), and 2-(di￾methylaminoethyl)methacrylate (DMAEMA) in various com￾positions (Scheme 1). The surface hydrophobization of the SiO2 particles was carried out in a multicomponent solvent sys￾tem that consisted of an alkane, water, and ethanol. The alco￾hol plays a decisive role as it serves to compatibilize the medi￾um for the normally immiscible alkane and water. No emulsion was generated with the applied amphiphilic copolymers and the chosen solvents. This was verified by dynamic light scatter￾ing, which indicated a maximum size for the amphiphilic co￾polymer of ca. 2 nm. A basic requirement for the amphiphilic copolymers is their solubility in the nonpolar solvent as well as the solvent mixture, which requires a minimum concentration (approx. 70 mol.-%) of the nonpolar EHMA part of the copolymer. Furthermore, a minimum concentration of the PEOMA (ap￾prox. 5 mol.-%) is necessary to achieve the essential interac￾tion with the surface of the SiO2 particles. Hydrophobization COMMUNICATION Adv. Mater. 2008, 20, 929–932 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 929 – [*] Prof. K. Müllen, Dr. M. Klapper, S. H. Stelzig Max-Planck-Institute for Polymer Research Ackermannweg 10, 55128 Mainz (Germany) E-mail: muellen@mpip-mainz.mpg.de; klapper@mpip-mainz.mpg.de [**] Financial support from Merck KGaA, Darmstadt, is gratefully ac￾knowledged. We also thank Gunnar Glasser and Katrin Kirchhoff for the SEM and TEM measurements. Supporting Information is avail￾able online from Wiley InterScience or from the author