北京化工大学：《材料导论》课程阅读材料（复合材料）A_Simple_and_Efficient_Route_to_Transparent_Nanocomposites.pdf_大学文库

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 substantial 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 incorporation 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 approaches 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 emulsion/miniemulsion techniques.[11–16] Amphiphilic block copolymers 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 synthetically demanding or they show a high tendency for the formation of larger aggregates, which leads to segregation of the particles after incorporation into a matrix.[19,20] Superior synthetic strategies that render nanoparticles applicable for transparent 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 polymer 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 aqueous dispersions. However, the quantitative transfer of these particles from an aqueous phase directly into a nonpolar environment is complicated because the change in polarity of the medium destabilizes the dispersion and aggregation is observed. The approach described herein enables the transfer of inorganic particles from an aqueous dispersion into a nonpolar 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 particles, 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 amphiphilic 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 approach 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 example, 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 approximately 10 nm, were used as a 30 wt.% aqueous dispersion. The amphiphilic copolymers were easily prepared by free radical polymerization of 2-(ethylhexyl)methacrylate (EHMA), poly(ethylene oxide) methacrylate (PEOMA), and 2-(dimethylaminoethyl)methacrylate (DMAEMA) in various compositions (Scheme 1). The surface hydrophobization of the SiO2 particles was carried out in a multicomponent solvent system that consisted of an alkane, water, and ethanol. The alcohol plays a decisive role as it serves to compatibilize the medium 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 scattering, which indicated a maximum size for the amphiphilic copolymer 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 (approx. 5 mol.-%) is necessary to achieve the essential interaction 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 acknowledged. We also thank Gunnar Glasser and Katrin Kirchhoff for the SEM and TEM measurements. Supporting Information is available online from Wiley InterScience or from the author

of the SiO2 nanoparticles in such monophasic systems provides the possibility of isolating the resulting hydrophobized particles in an alkane phase by a subsequent phase-separation process (Scheme 1). This phase-separation process was induced by varying the concentrations of the solvents used.[21–23] As shown in Scheme 1, the amphiphilic copolymer was dissolved in an alkane and combined with the aqueous dispersion of SiO2 particles, which had previously been diluted with ethanol. This gave rise to a clear, monophasic solution. Phase separation was then induced by increasing the water concentration. After complete separation of the two phases, the SiO2 particles were present exclusively in the nonpolar solvent. The resulting hydrophobized SiO2 could be isolated simply by removal of the hydrocarbon solvent and were fully redispersible in common organic solvents. Additionally, they did not show any aggregation during the hydrophobization step, as demonstrated by dynamic light scattering and scanning electron microscopy measurements (see Supporting Information). To prove that the modified SiO2 particles were capable of forming transparent nanocomposites, they were incorporated into a polyurethane matrix. Polyurethane (PU) was chosen due to its great importance as a coating material. The polyurethane matrix was formed from two standard components: the first consisted of hexamethylene diisocyanate derivatives (DD 3390 BA/SN from Bayer Material Science (BMS)) and the second of a polyesterpolyol (DP 680 BA from BMS). By using this two-component PU and the SiO2 particles modified with amphiphilic copolymer 1, transparent nanocomposite materials were prepared, as indicated by UV-vis spectroscopy (see Supporting Information). The homogeneity of the SiO2 particles within the matrix was established by examining transmission electron microscopy (TEM) images of the PU nanocomposite film (Fig. 1C). Not only were the particles homogenously distributed in the matrix, but the particle size was also kept below 100 nm, which is necessary for maintaining the composite’s transparency.[24,25] To prevent the SiO2 particles from migrating and aggregating within the matrix, some form of interaction (covalent or noncovalent) between the incorporated particles and the matrix was necessary. For this reason, the third monomer DMAEMA, which is capable of hydrogen bonding to the polyurethane matrix (Fig. 2), was incorporated into the amphiphilic copolymers 1 and 2. Indeed in case of copolymer 1, with the highest content of DMAEMA, a homogeneous distribution of the nanoparticles COMMUNICATION 930 www.advmat.de © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 929–932 Scheme 1. Functionalization of inorganic nanoparticles using a multicomponent solvent system by a two step procedure. Quantitative transfer of the hydrophilic nanoparticles into the nonpolar phase is achieved after the phase separation. The aqueous dispersion of SiO2 nanoparticles was first diluted with ethanol before mixing with the copolymer solution. Ethanol suppressed the miscibility gap between the nonpolar phase and the aqueous dispersion of SiO2, whereas the further addition of water counteracted this effect

was observed. In the case of copolymer 2, which contained a lower amount of DMAEMA, a significant amount of aggregation occurred and structures in the range of 200 to 500 nm in diameter appeared (Fig. 1B). For copolymer 3, containing only EHMA and PEOMA, the aggregates that formed were in the range of several micrometers in diameter, and opaque films resulted (Fig. 1A, and Supporting Information). These results clearly supported our concept of a trifunctional surface-active compound. The absence of the third monomer (copolymer 3) is accompanied by significant aggregation within the PU film. The introduction of the third monomer (copolymer 2) reduces the extent of the aggregation and when its concentration exceeds a certain threshold, aggregation is completely suppressed. The observed aggregation (copolymer 3) is expected to be a result of a consecutive precipitation process. The structure of the aggregates (Fig. 1A) suggests that during the formation of the polymer matrix, the hydrophobized particles stayed dissolved in the remaining solvent (applied in the polymerization of the polyurethane matrix). This resulted in the trapping of the particles in large aggregates (Fig. 1A). The monomer DMAEMA (copolymer 1-2) provided interactions of the particle with the polymer matrix, which prevented the formation of large aggregates. Summarizing these results, the role of EHMA was to impart hydrophobicity to the SiO2 particles, PEOMA provided the interaction of the polymer with the SiO2 surface, and DMAEMA anchored the particles to the matrix. To date, this procedure has been scaled-up without complications from the original 100 mg scale to 20 g of hydrophobized nanoparticles per batch. It should be noted that inorganic particles other than SiO2, for example alumina (Al2O3) or ceria (CeO2), can be processed similarly by such copolymers in the same solvent mixtures. However, tuning of the surface-active copolymer, with regard to the specific surface properties of the inorganic particles, is required. The specific tuning of the amphiphilic copolymers concerns only the hydrophilic moiety, responsible for the interaction with the surface of the inorganic particles. To functionalize CeO2 and Al2O3 nanoparticles, PEOMA was replaced by 4-vinylpyridine and copolymerized with EHMA. The copolymer was reacted with 1,3-propanesulton, introducing a sulfonic acid group. The amount of EHMA in this copolymer was 85 mol% (number-average molecular weight, Mn=9000 g mol-1, polydispersity index, PDI=1.8). The conditions of the functionalization step were identical to those used in the case of copolymers 1-3 (see Supporting Information). In each case, the resulting hydrophobized particles were again fully redispersible in organic solvents without the formation of any aggregates. In conclusion, a highly efficient method to hydrophobize inorganic nanoparticles, demonstrated in detail here for nanoparticles of SiO2, has been presented. The formerly hydrophilic nanoparticles were directly transferred into a nonpolar environment in one step without the formation of any aggregates. The key feature was the choice of an appropriate mixture of solvent. The mixture contains polar and nonpolar solvents and is precisely optimized to avoid a phase separation and to allow treatment of the hydrophilic particles with amphiphilic copolymers in a homogenous phase, thus preventing COMMUNICATION Adv. Mater. 2008, 20, 929–932 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 931 Figure 1. TEM image of the PU/SiO2 nanocomposite film showing a significant aggregation (A) of the SiO2, which were hydrophobized with copolymer 3. In contrast to (A) the aggregation tendency is diminished if copolymer 2 is used to hydrophobize the SiO2 nanoparticles (B), and a homogeneous particle distribution of SiO2 is obtained (C), if copolymer 1 is used to modify the SiO2 particles, within the matrix. Figure 2. Illustration of the role of the monomers in the copolymers 1 and 2. The group A in the picture represents the anchor group NMe2 of the third monomer DMAEMA

aggregation. Using this approach, readily available aqueous dispersions of inorganic particles can be hydrophobized in a very efficient way without the need of any further synthetic manipulation. Additionally, the introduction of multiple functionalities, especially those necessary for the interaction with the matrix polymer, has been demonstrated to be compatible with the hydrophobization process. Also of benefit is that this procedure is environmentally friendly due to its energy-saving synthesis and the possibility of completely recycling all of the solvents used. Modified nanoparticles have been successfully incorporated into a PU matrix without any aggregation of the inorganic particles, leading to fully transparent nanocomposite films. By this method, a versatile technique for the hydrophobization of nanoparticles in a large scale that has the potential to drastically broaden access to homogeneous organic/ inorganic hybrid materials is realized. Experimental Materials and Analytical Methods: All reagents and solvents were purchased from commercial sources and were used as received, unless otherwise stated. 2-(Ethylhexyl)methacrylate (EHMA) and 2-(dimethylaminoethyl)methacrylate were dried over calcium hydride and distilled prior to use. NMR measurements were performed on a Bruker 250 or 300 MHz spectrometer. Gel permeation chromatography (GPC) vs. poly(methyl methacrylate) (PMMA) standard was carried out at 30 °C using MZ-Gel SDplus 10E6, 10E4, and 500 columns, an ERC RI-101 differential refractometer detector, and tetrahydrofuran (THF) as eluent. SEM images were taken using a Zeiss Gemini 912 microscope. TEM images were obtained using a Technai F20 microscope and samples were prepared by ultramicrotoming. UV-VIS spectra were recorded on a Perkin-Elmer Lambda 15 spectrometer. Dynamic light scattering measurements were performed on a Malvern Zetasizer 3000 instrument. Synthesis: Copolymers 1-3 were prepared by a free radical polymerisation of EHMA, PEOMA, and DMAEMA in toluene, using mercaptoethanol to adjust the molecular weight. The mixture was degassed by several freeze-pump-thaw cycles followed by flushing with argon. The reaction was carried out in a Schlenk flask sealed with a rubber septum at 70 °C using azobisisobutyronitrile (AIBN) as the initiator. The crude copolymer was precipitated twice out of methanol and was dried at room temperature under reduced pressure. The surface modified inorganic particles were prepared by dissolving copolymers 1-3 in n-hexane (6 mL). Separately, the aqueous silica dispersion was diluted with ethanol (2 mL). The concentration of the copolymers in n-hexane was 17 g L–1 and the concentration of the SiO2 dispersion in ethanol was 40 g L–1. The two mixtures were combined to give a monophasic system. Phase separation was induced by the addition of water (0.2 mL). After complete phase separation, the nonpolar phase exclusively contained the hydrophobized silica nanoparticles. The polyurethane/SiO2 nanocomposites were prepared by mixing the hydrophobized silica nanoparticles, redispersed in n-butyl acetate to afford a 20 wt % dispersion, with the two polyurethane components DD 3390 BA/SN (hexamethylene diisocyanate derivatives) and DP 680 BA (polyesterpolyol). The polyurethane/SiO2 nanocomposite films were prepared by drop-casting this mixture on a glass substrate and heating to 80 °C for 60 min. The SiO2 content in the nanocomposite material was approximately 5 wt%. Received: June 29, 2007 Revised: October 15, 2007 Published online: February 5, 2008 – [1] P. Gomez-Romero, Adv. Mater. 2001, 13, 163–174. [2] C. Sanchez, B. Julián, P. Belleville, M. Popall, J. Mater. Chem. 2005, 15, 3559–3592. [3] F. Mammeri, E. Le Bourhis, L. Rozes, C. Sanchez, J. Mater. Chem. 2005, 15, 3787–3811. [4] Z. Mlynarcíkova, D. Kaempfer, R. Thomann, R. Mühlhaupt, E. 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