Surface Modification of Oxide Nanoparticles for

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Surface Modification of Oxide Nanoparticles for Polyacrylate Reinforcement Frank Bauer*1, Horst Ernst2, Dietmar Hirsch1 Matthias Pelzing3, Volker Sauerland3, and Reiner Mehnert1 1

2

Leibniz-Institut für Oberflächenmodifizierung, Leipzig, Germany Universität Leipzig, Inst. für Experimentelle Physik I, Leipzig, Germany 3 Bruker Daltonik GmbH, Leipzig, Germany

Abstract Reinforced clear coats were prepared using nanosized silica and alumina particles in UV/EB curable acrylate formulations. For a firm embedding of the oxide nanofillers via covalent bonds to the network acrylates their surfaces were modified by polymerization-active trialkoxysilanes, e.g., methacryloxypropyltrimethoxysilane and vinyltrimethoxysilane. The cured nanocomposite clear coats showed improved scratch and abrasion resistance. However, oxide modifications accomplished by silanes having polymerization-inactive methyl, n-propyl, and isobutyl functionalities yield coatings with similar scratch and abrasion resistance. To explain these findings, infrared and multinuclear MAS NMR n experiments, MALDI-TOF and ESI-MS mass spectroscopy, and atomic force microscopy results were used to reveal the structure of surface-anchored organosilanes and their interaction with the acrylate matrix. Ladder-like polysiloxane chains chemically grafted onto the filler particles have been proposed. These ladder-like structures build a short range interpenetrating network with the polyacrylates. This results in a durable link between the organic and inorganic phase. 1 Introduction The development of organic/inorganic coatings based on surface-modified oxide nanoparticles provides an attractive route to upgrade the properties of polymer coatings. Transparent reinforced top coats are especially needed for decorative furniture papers and wooden floorings where the surface of the coating should remain in good conditions over its lifetime [1-3]. Many of the organic/inorganic hybrid materials are thermally curable or need additional solvents, which contribute to the VOC's, whereas the presented nanocomposite coating consists of 100 % reactive acrylic resins and can be rapidly cured by UV or EB radiation. The scratch and abrasion resistance of such coatings is clearly dependent on the content, the hardness, and the dispersion of the inorganic filler. A complete miscibility of fumed oxide nanoparticles with the surrounding organic fluid medium even at a higher filler content is accomplished by organosilane coupling agents which offer the advantage of chemical bonding of functional silane groups onto the surface of hydrophilic inorganic oxides. Surface modification by polymerization-active functionalities is commonly employed to yield a firm interfacial embedding of the modified solid substances within polymer chains. For example, grafting of trialkoxysilanes having acrylic, epoxy, and isocyanate groups is recommended for the reinforcement of acrylic, epoxide, and urethan resins, respectively. In addition, the polymerizable nanoparticles may act as crosslinking centers and thereby improve the network density. UV curable acrylate formulations containing up to 35 wt.-% fumed nanosized silica have been adjusted for parquet coating applications and revealed excellent scratch and abrasion resistance [4]. In the this study, liquid-phase silylation of silica nanopowder was performed using the acrylate resin as solvent. This in situ procedure avoids reagglomeration of nanoparticles after modification which may occur during drying of modified nanofillers prepared in common solvents [5]. To enhance the reaction rate of silane grafting, water in stoichiometric amounts and a polymerization-active organic acid as catalyst were added prior to the nanofiller batch. Specifically, in this "aqueous" environment the alkoxy groups of functionalized organosilanes undergo hydrolysis forming reactive silanol groups. As a result, intercondensation reactions between neighboring silane molecules form oligomeric siloxanes before (and after) depositing onto the substrate. Therefore, the average coverage of about 5 silane mole2 cules/nm , found by thermogravimetric measurements after silica modification by different trialkoxy2 silanes, is higher than the surface silanol density of 2-3 OH groups/nm on fumed silica [5]. Moreover, 29 Si CP MAS NMR spectroscopy indicates a fraction of only about 33 % of the total surface OH groups that react with the polysiloxanes formed. As a consequence, this silylation technique leads to localized

polysiloxane networks on the nanoparticle surface instead of a monomolecular coverage via tridentate silane anchoring. To reveal the structure of the condensed trialkoxysilanes, matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectroscopy and electrospray ionization (ESI) coupling to an ion trap mass spectrometer (with its MSn capabilities) have been applied. In addition, the surface topology of a silicon wafer, with a thin SiO2 layer, was visualized before and after silane grafting by atomic force microscopy (AFM). 2 Experimental 2.1 Materials The grafting procedure of trialkoxysilanes, e.g., methacryloxypropyltrimethoxysilane (MEMO), vinyltrimethoxysilane (VTMO), methyltrimethoxysilane (MTMO), n-propyltrimethoxysilane (PTMO), and isobutyltrimethoxysilane (IBTMO) - available from Degussa (Germany), onto the surface of nanosized silica (Aerosil 200 and TT 600, Degussa) and alumina (Aluminiumoxid C, Degussa) was described in detail earlier [6]. In addition to the modification of nanoparticles, an identical silylation procedure was performed with silicon wafers for AFM and contact angle measurements. Typical acrylate formulations, e.g. tetrahydroxyethylpentaerythritol tetraacrylate (Sartomer 494, Cray Valley) contain about 15-35 wt.-% modified nanopowder. Thin films (up to 100 µm) were prepared on various substrates, e.g. wood, polymer and glass plates. The wet coatings were cured by using a 180 keV electron accelerator with a curing dose of 80 kGy. UV-induced polymerization was accom-1 plished with the aid of a conventional medium pressure Hg lamp (120 W⋅cm ) and photoinitiators, e.g. 2 wt.-% Darocur 4265 and 1 wt.-% Irgacure 500 (Ciba). 2.2 Equipment 13

Solid-state C MAS NMR spectra were recorded on a Bruker MSL 500 spectrometer. Magic-angle 13 spinning was performed at a spinning rate of 5 kHz. Solution C NMR spectra were performed in CDCl3 on a Bruker Avance DPX 400 spectrometer. For MALDI TOF MS, sample preparations were carried out by mixing a dispersion of the silylated nanoparticles in tetrahydrofurane with a solution of Dithranol as MALDI matrix and LiCl. MS was performed on a Bruker Biflex III mass spectrometer in the reflectron mode using pulsed ion extraction. Using electrospray ionization in positive ion mode, n MS spectra (n 4) of precondensed silane oligomers were obtained on an ion-trap mass spectrometer Esquire 3000 (Bruker Daltonik). The change of structural features of SiO2 surfaces after silylation has been visualized by a VEECO di NanoScope IIIa atomic force microscope (AFM). The system was operated in tapping mode with standard silicon tips (frequency around 300kHz). The scanning velocity was chosen between 0.6 and -1 1.0 µm s . The root-mean-square surface roughness (Rrms) was averaged from 2×2 µm areas of the wafer. 3 Results and discussion 3.1 Effect of silane functionality on abrasion resistance Grafting of polymerization-active functionalities and their copolymerization with a desired monomer are assumed to be a prerequisite for a durable link between the silylated filler and the polymer matrix. Otherwise, the embedded filler particles could be released during abrasive surface treatment reducing thereby the scratch and abrasion resistance of the coating. Crosslinks between the surface acrylic groups of silica nanoparticles modified by MEMO and a tetraacrylate polymer matrix have been shown 13 by C NMR spectroscopy [6]. Signals of olefinic carbons (120 - 140 ppm) present both in the modified silica and the resin are completely disappeared after electron beam curing. Similar curing studies using VTMO modified silica reveal quite different reactivities of vinyl and acrylic groups, however. Only 35 % of the grafted vinyl groups are incorporated in polymer chains whereas acrylic carbons of the tetraacrylate SR 494 have been converted to about 90 % as can be obtained by NMR spectroscopic 13 studies (Fig. 1). The C MAS NMR spectrum of the EB cured nanocomposite displays a strong signal at 174.9 ppm and a weak signal at 166.4 ppm assigned to carboxyl groups in the neighborhood of aliphatic and olefinic carbons, respectively. These intensities were used to estimate the degree of acrylic conversion. The relatively low reactivity of surface vinyl groups is indicated by the ratio of the 13 broad C signals remaining after EB curing, i.e., at 136.9 ppm (assigned to SiHC=CH2) and 130.5 ppm (overlapping of OC-C=CH2, OC-C=CH2, and SiHC=CH2).

Figure 1 13 C NMR spectrum of a SR 494 tetraacrylate formulation containing 35 wt.-% VTMO modified silica 13 nanoparticles (a) and C CP MAS NMR spectrum of the final nanocomposite cured by electron beam (b). (Asterisks denote spinning sidebands).

signal intensity [a.u.]

174.9

b)

136.9 166.4

167.0

a) 200

131.5

130.5 *

128.7

136.5

150 100 50 chemical shift [ppm]

0

Clearly, MAS NMR spectroscopy points to an unsatisfactory copolymerization and crosslinking of the VTMO modified nanoparticles. As a result, a reduced abrasion resistance of the VTMO/silica/SR494 system would be expected (Table 1). Nevertheless, good mechanical surface properties of the reinforced acrylate coatings were obtained even in those cases where polymerization-inactive trialkoxysilanes were used as coupling agents. Hence, the prerequisite of a firm interfacial embedding of the nanoparticles must also be fulfilled by other features then chemical bonding, e.g., by positive locking of modified particles and polymer. Therefore, advanced analytical techniques have been applied to elucidate the structure of the interface between filler surface, silane, and acrylate matrix. Table 1:

Taber Abraser test of SR 494 nanocomposite films (15 wt.-% silica nanopowder modified by equimolar amounts of different trialkoxysilanes). coupling agents

abrasion (mg)

methacryloxypropyltrimethoxysilane vinyltrimethoxysilane

20 19

methyltrimethoxysilane propyltrimethoxysilane

21 19

isobutyltrimethoxysilane

22

3.2 Characterization of modified nanoparticles Anchoring of trialkoxysilanes onto the surfaces of silica particles is obtained by condensation reactions between OH groups present on the oxide surface and silanol groups formed by hydrolysis of trialkoxy29 silanes. Si MAS NMR spectroscopy allows to characterize both the inorganic and the organic silicon i i atoms, i.e. Q and T structures, respectively. Signals of the grafted organosilanes are typically observed in the -45...-50 ppm, -55...-60 ppm, and -65...-70 ppm regions and are assigned to 1 2 3 mono(T )-, bi(T )-, and tri(T )-fold Si-O-linked silicons, respectively. From Fig. 2, it is apparent that the 2 3 prevailing structures of the grafted trialkoxysilanes under study are formed by T and T silicon atoms. Regardless of the effect of the reaction condition on the rate of silane hydrolysis and condensation, 3 the different proportions of the T structures show that the degree of condensation is effected by the silane functional group. 3.2 MALDI-TOF Mass Spectroscopy Due to ablation and desorption of the grafted species by laser irradiation, MALDI TOF mass spectrometry permits the elucidation of the structure of the oligomers covering the particle surface. Oligomers of MEMO, VTMO, and PTMO with characteristic repeating mass units of the silanes under study were observed [7]. For IBTMO having a repeating mass unit of 118 Da, there is a stronger effect of the

Figure 2 29 Si CP MAS NMR spectra of MEMO/alumina (a) and IBTMO/silica (b) nanoparticles.


-58.8 -68.6

b)

-66.3

-58.4

a) -40

-60 -50 -70 chemical shift [ppm]

-80

aliphatic group on silane hydrolysis/condensation resulting in lower condensed species (Fig. 3). Up to five mass signals differing in 18 mass units have been observed for each oligomer. These findings are similar to those of filler-free MEMO homocondensates which exhibit lower degrees of silane condensation compared to oligomers formed in the presence of silica and alumina [7]. Based on the NMR and MALDI-TOF MS data, a ladder-like structure of two linked siloxane chains has been proposed (see Fig. 6). Such moderate condensation degrees as observed for IBTMO can be 3 attributed a Si-O-Si skeleton of connected 8-membered rings (T structure) with side chains of low 1 2 condensed groups (T and T structures).

14

15

16

17


13

Monomeric IBTMO units 18 19 20 21

1500

1700

1900

2100

2300

m/z

Figure 3 MALDI-TOF mass spectrum of IBTMO modified silica nanoparticles. All molecules were observed as + Li attached ions. 3.3 MS/MS investigations To obtain another argument for a polysiloxane skeleton, silane homocondensates soluble in acetonitrile were characterized by an ion-trap mass spectrometer. Electrospray ionization at 4 kV in combination with mild fragmentation in the ion trap leads to detailed information about the end groups of the

siloxane chains (Fig. 4). MS/MS experiments of a MEMO oligomer (molecular weight 1075 Da) reveal the subsequent splitting off of six subunits groups of 86 Da each. This fragmentation pathway can be explained by successive decomposition reactions forming methacrylic acid, i.e., the oligomer under study exactly consists of 6 MEMO molecules. Moreover, the fragmentation depth points to a symmetric arrangement of the silane functionalities. 86

86

86

86

86

86 988.7

902.6

MS/MS : 1075 816.6


1077.5

730.8

646.9

902.4

MS/MS/MS : 1075/989 647.0

816.6 730.5

991.5

817.2

MS/MS/MS/MS : 1075/989/902

731.0 647.0

904.6 1106.8

500

600

700

800

900

1000

1100 m/z

n

Figure 4: MS (n 4) mass spectra of a MEMO hexamer (molar weight 1075 Da).

3.5 Surface topology Silicon wafers typically covered by a SiO2 layer exhibit a very smooth surface and are excellently suitable for AFM investigations to visualize the changes of the surface morphology due to silylation. Whereas the native wafer revealed a surface roughness (Rrms) of 0.29 nm, the wafer silylated by MEMO is much rougher (Rrms = 0.69 nm) and contains noticeable nanosized clusters (Fig. 5). These clusters are randomly distributed and separated with a minimum distance of about 5 nm. The AFM images show clusters of 1 - 3 nm in height and 5 - 20 nm in diameter. For VTMO grafted on hydroxylated silicon wafers, Nguyen et al. [7] observed bigger clusters of 10-20 nm height and 22 nm diameter. These clusters are believed to be local polysilane networks grown on the surface, resulting from a minimal intermolecular condensation of neighboring molecules under anhydrous conditions. Unfortunately, there are no 29Si NMR data to confirm the low silane condensation degree claimed.

a)

b)

20 nm

20 nm 2.0 1.5

0

1.0

0.5 1.0

1.5

0.5

surface distance (µm)

2.0 1.5

0 0.5

1.0 1.0 1.5

0.5

surface distance (µm)

Figure 5: Comparison of 3D images of (a) native silicon wafer and (b) silylated silicon wafer.

To confirm the formation of a ladder-like structure as a result of trialkoxysilane condensation, molecular modeling was used to calculate the size of MEMO oligomers. As shown in Fig. 6, the height of a polysiloxane subunit formed by a 8-membered ring is about 0.31 nm. Therefore, the observed chain length of about 1 nm can be assigned to MEMO oligomers formed by about 8 silane molecules. Such octamers have been found at high proportions in the MALDI-TOF measurements of various modified nanoparticles [7].

2.1 nm

Si O C H 0.31 nm

OH Si

Si

HO OH

Si

Si

Figure 6 Proposed ladder-like arrangement of silicon atoms in MEMO oligomers grafted on silica surface.

4

Conclusions

Surface modification of inorganic nanoparticles by trialkoxysilanes is the prerequisite for embedding a high filler content into radiation curable acrylate formulations. Acid catalyzed prehydrolysis of trialkoxysilanes results in grafting of precondensed polysiloxanes onto the filler surface. The findings of solidstate MAS NMR, MS/MS, and MALDI-TOF mass spectrometry suggest a ladder-like arrangement of two linked polysiloxane chains. More than a simple organophilation of the inorganic filler, the grafted siloxane oligomers form an interpenetrating network with the polyacrylate chains in the near-surface range. Therefore, a firm and durable hybrid system of organic and inorganic phases is obtained even for such organosilanes which cannot form crosslinks with the acrylate matrix. References [1] [2] [3] [4] [5] [6] [7] [8]

Vu, C., LaFerté, O., Eranian, A., European Coatings Journal, 1-2, 64 (2002). Roscher, C., Conference Proceedings 7th Nürnberg Congress 2003, 417. Frahn, S., Valter V., Leder, G., Conference Proceedings 6th Nürnberg Congress 2001, 145. Borup, B., Edelmann, R., Mehnert, R., European Coatings Journal, 6, 707 (2003). Hinterwaldner, R., Gläsel, H.-J., Hartmann, E., Mehnert, R., EP 1123354 A1 (1999) assigned to Institut für Oberflächenmodifizierung. Gläsel, H.-J., Bauer, F., Ernst, H., Findeisen, M., Hartmann, E., Langguth, H., Mehnert, R., Schubert, R., Macromol. Chem. Phys., 201, 2765 (2000). Bauer, F., Sauerland, V., Gläsel, H.-J., Ernst, H., Naumov, S., Mehnert, R., Macromol. Chem. Phys., 204, 375 (2003). Nguyen, V., Yoshida, W., Cohen, Y., J. Appl. Polymer Sci., 87, 300 (2003).