Sung-Il Lee, Duk-Bae Kim, Jung-Hyun Sin, Youn-Sik Lee, and Changwoon Nah Division of Environmental and Chemical Engineering, Nanomaterials Research Center, Chonbuk National University, Chonju 561-756, Korea *Division of Materials Engineering, Chonbuk National University, Chonju 561-756, Korea Received February 16, 2007; Accepted April 24, 2007 Abstract: A hydrophilic fumed nanosilica surface was modified with γ-glycidoxypropyltrimethoxysilane, and subsequently with diethanolamine. The chemically modified silicas were used for the in-situ preparation of polyurethane/silica composites. The doubly modified silica gave rise to a greater enhancement of the tensile strength and elongation at break of the polymer composite films, as compared to the singly modified silica. The more improved tensile properties were attributed to the more increased number of covalent links between the modified silica surface and polyurethane chains, and the better dispersion of the silica particles in the polyurethane matrix. Keywords: modified silica, polyurethane/silica composite, glycidoxypropyltrimethoxysilane, diethanolamine Introduction 1) Polyurethane (PU) is a versatile polymeric material, which can be tailored to meet the various demands of modern technologies, such as coatings, adhesives, reaction moldings, plastics, fibers, foams, rubbers, thermoplastic elastomers, and composites [1-3]. Nanocomposites are a new class of materials with improved physical properties, such as thermal, mechanical and barrier properties, as compared with conventional composites (microcomposites), due to the much stronger interfacial interactions between the dispersed nanometer-sized domains and the matrices [4-6]. Since elastomeric PU/clay nanocomposites were first reported by Pinnavaia and coworkers, they have been extensively studied [7]. These nanocomposites have been prepared via in-situ polymerization, using either modified clay with long chain onium ions or with an onion form of chain extenders [8-10]. However, exfoliated PU/ clay nanocomposites were obtained only in a limited number of cases. Furthermore, the preparation of organoclays, especially clay-chain extender hybrids, usually requires a complicated procedure [11,12]. To whom all correspondence should be addressed. (e-mail: yosklear@chonbuk.ac.kr) Recently, Wu and coworkers reported the synthesis of polyester-based PU/silica nanocomposites [13], whose hardness, glass transition temperature, adhesion, and viscosity properties were enhanced. Lee and coworkers reported the preparation of polyether-based PU/silica nanocomposite films [14], whose tensile strength and elongation at break were much greater than those of pure PU. Pereira and coworkers studied the effects of the surface silanol group (Si-OH) concentration of nanosilicas (silica I: highly concentrated, silica II: less concentrated) on the mechanical properties of composites prepared using the nanosilica fillers [15]. The polymer hardness and Young s modulus increased with increasing the silica I content, but did not vary significantly with increasing the silica II content. The reinforcing effect of silica I was attributed to the additional physical crosslinks generated within the polymer network, due to the interactions between the silanol groups and polyurethane carbonyl groups. The tensile strength of the composite prepared using silica I reached a maximum value at 1 phr and then decreased as the filler concentration increased further. The increase in the tensile strength was attributed to the filler particles which act as barriers preventing fracture growth, while the decrease in the tensile strength at high concentrations was attributed to the increased number of voids in the polymer layer next to the filler surface.
Polyurethane/silica Composites, Prepared Via in-situ Polymerization in the Presence of Chemically Modified Silicas 787 Table 1. Recipes for the Modification of the Hydrophilic Fumed Nanosilica (Aerosil A200), Using Various Amounts of GPTS and DEA Code EtOH/H 2O (ml) A200 (g) GPTS (g) Silane content (%) GS-silica (g) DEA (g) Weight loss (%) GS60 200 20 12.12 60.6 6.7 GS90 200 20 18.18 90.9 7.2 GS60D 10 0.64 10.5 GS90D 10 0.59 9.5 composites, and some basic characterizations of the resulting composites. Experimental Materials Poly(tetramethyl ether glycol) (PTMEG, MW 2000), 4,4'-diphenylmethane diisocyanate (MDI), diethanolamine (DEA), and 1,4-butanediol (BD) were purchased from Aldrich, and used as received. Aerosil A200 (particle size 12 nm, surface area 200 m 2 /g) was purchased from Degussa-Huls. γ-glycidoxypropyltrimethoxysilane (GPTS, coating area 330 m 2 /g) was purchased from Shinetsu Co. Scheme 1. Chemical modification of Aerosil A200, using GPTS and DEA. In this research, a hydrophilic fumed nanosilica surface was chemically modified in two steps, consisting of modification first with γ-glycidoxypropyltrimethoxysi- lane (GTPS) and then with diethanolamine (DEA). The hydrophilicities of the modified silicas are expected to be lower than that of the initial nanosilica. Furthermore, in addition to silanol groups (Si-OH), the doubly modified silica surface can have primary and secondary hydroxyl groups (CH 2 -OH, CH-OH), which are derived from the chemical reactions of the silanol groups with GPTS and DEA. The primary hydroxyl groups are expected to be much more reactive in coupling reactions with isocyanates than the silanol groups, due to the less steric hindrance. This paper describes the chemical modification of a hydrophilic fumed nanosilica, preparation of PU/silica Chemical Modification of Nanosilica (Aerosil A200). The experimental procedure employed for the chemical modifications of the hydrophilic fumed nanosilica surface (Aerosil A200) is summarized in Scheme 1. The nanosilica was dispersed in ethanol/water (7/3), followed by the addition of acetic acid to adjust the ph of the mixture to be about 4 5. The reaction temper- ature was gradually increased to 65 o C during the addition of GPTS, and the coupling reaction was allowed to continue for 9 h. The amount of GPTS (W GPTS,full ) required for the full coverage of the nanosilica surface was obtained from the following equation: W GPTS,full (g) = (W s A s )/A GPTS where W s,a s,anda GPTS represent the amount of nanosilica to be modified, the surface area of the nanosilica (200 m 2 /g), and the specific wetting surface of GPTS (330 m 2 /g), respectively [14,15]. The surface-modified silica particles with GPTS were filtered, washed with ethanol, and dried in an oven at 70 o C for 24 h. The resulting silica particles were designated as GS30, GS60, GS90, and GS120, where GS means the GPTS-treated silicas and each number refers to the percentage of GPTS actually used for the surface modification, relative to W GPTS,full. The recipes for the chemical modifications of Aerosil A200 are presented in Table 1.
788 Sung-Il Lee, Duk-Bae Kim, Jung-Hyun Sin, Youn-Sik Lee, and Changwoon Nah Table 2. Recipes for the Preparation of Pure PU and PU/silica Composites Code PTMEG (mol) GS90 (wt%) GS90D a (wt%) MDI (mol) BD (mol) Pure PU 1 - - 2.0 1 PU/GS90-1 1 1-2.0 1 PU/GS90-2 1 2-2.0 1 PU/GS90-3 1 3-2.0 1 PU/GS90D-1 1-1 2.05 1 PU/GS90D-2 1-2 2.05 1 PU/GS90D-3 1-3 2.05 1 a The wt% values are based on PTMEG. Among the resulting singly modified silicas, only GS60 and GS90 were used for further chemical modification with DEA, and dispersed in toluene, followed by the addition of DEA (2 equiv. based on the amount of glycidoxy groups on the silica surfaces). The reactions were allowed to continue at 60 o C for 3 4 h. The reaction mixtures were washed with ethanol and dried in an oven at 70 o C for 24 h. The doubly modified silicas were designated as GS60D and GS90D, respectively. Preparation of PU/silica Composites Prior to use, PTMEG, BD and the silicas were dried in a vacuum oven at 60 o C for 48 h. The recipes for the preparation of the PU/silica composites are presented in Table 2. For comparison, the GTPS-treated silicas were also employed for the in-situ preparation of PU/silica composites. PTMEG and toluene were transferred to a fourneck flask, followed by the addition of silica (Aerosil A200, GS60, GS90, GS60D, and GS90D). The mixture was stirred at 90 o C and 500 900 rpm for 2 h, and then cooled to room temperature. MDI was added and the mixture temperature was gradually increased to 80 o C. After 3 4 h, BD was added, followed by the addition of dibutyltin dilaurate as a catalyst (0.05 0.10 wt%). The reaction mixture was stirred for 1 h, cooled to about 60 o C, and precipitated from ethanol. Measurements The presence of certain important functional groups in the polymers was confirmed by FT-IR spectra, obtained from a JASCO FT-IR spectrometer. The thermal transitions were observed by differential scanning calorimetry (DSC, TA 2910), at a heating rate of 10 o C/min in a nitrogen atmosphere. An energy filtering transmission electron microscope (FE-TEM, Leo 912 Omega) with an acceleration voltage of 120 kv was employed to observe the morphology of the composites. The tensile strength and elongation at break of the composite films were measured, using a universal testing machine (Loyd Co.), Figure 1. FT-IR spectra of Aerosil A200, GS90, and GS90D. at a strain rate of 500 mm/min. The test samples for the measurements were prepared using a hot press (thickness 50 µm), according to ASTM D412-92. Results and Discussion In order to confirm the chemical modifications of the fumed nanosilica surface, the FT-IR spectra of Aerosil A200 and the modified silicas were compared (Figure 1). In the FT-IR spectrum of Aerosil A200, the absorption peak for silanol groups clearly appeared at about 3450 cm -1. On the other hand, in the FT-IR spectrum of the silica modified with GPTS, the absorption peaks of the C-H stretching vibrations appeared just below 3000 cm -1. This observation indicates that the fumed nanosilica surface was chemically modified to some degree via the covalent linkage between silanol groups on the silica surface and GPTS. In the FT-IR spectrum of the doubly modified silica, the absorption peaks of -OH groups and C-H stretching vibrations were slightly more pronounced, as expected. Figure 2 presents the thermal degradation behaviors of the chemically modified silicas. As the amount of GPTS used for the reaction of Aerosil A200 was increased from 30 to 60 %, the weight loss of the modified silicas at 700 o C increased from 4.7 to 6.7 wt%, and then increased more slowly as the amount of GPTS was further increased. For example, when the amount of GPTS was increased from 90 to 120 %, the weight loss of the resulting modified silicas increased only from 7.2 to 8.0 wt%. This small increase in the weight loss is due to the small increase in the amount of GPTS linked to the silica surface, because the GPTS moieties already linked to the silica surface can interfere with the chemical reactions of the silanol groups situated nearby. Thus, only GS60 and GS90 were used for further chemical modification with
Polyurethane/silica Composites, Prepared Via in-situ Polymerization in the Presence of Chemically Modified Silicas 789 A Figure 2. TGA curves of modified silicas along with that of Aerosil A200. B Figure 3. Photographs of PU composites, prepared using three different fillers (1 wt%): (a) CaCO 3 (50 µm), (b) Aerosil A200, and (c) GS90D. DEA. The surface areas of the modified silicas that are covered by glycidylpropylsilyl group can be easily calculated from the TGA data. In the case of GS90, for example, pure silica and glycidylpropylsilyl group contents are 92.8 wt% and 7.2 wt%, respectively. The surface area of GS90 covered by glycidylpropylsilyl group is calculated to be 12.8. This calculation result indicates that the modified silicas still contain silanol groups at high levels on their surfaces. As described above, the weight losses of GS60 and GS90 at 700 o C were 6.7 and 7.2 wt%, but those of GS60D and GS90D at 700 o C were 9.5 and 10.5 wt%, respectively. This means that the weight losses due to the reaction of DEA with the glycidoxy groups on the surfaces of GS60D and GS90 are 2.8 and 3.3 wt%, respectively. The theoretical maximum amounts of DEA which can react with the glycidoxy groups on the silica surfaces of GS60 and GS90 to produce GS60D and GS90D were calculated to be 2.4 and 2.9 wt%, respectively. This good agreement between the calculated and experimental values indicates that most of the glycidoxy groups immobilized on the surfaces of GS60 and GS90 reacted with DEA. We attempted to prepare PU/silica composites using the various silicas (Aerosil A200, GS60, GS90, GS60D, and GS90D). However, the PU/silica composites prepared using Aerosil A200 were not transparent, and did not form films which could be used for measurements, in- Figure 4. DSC thermograms: PU/GS90D (A), PU/GS90, and pure PU (B). dicating that they are not nanocomposites, but microcomposites (Figure 3). This was probably due to the aggregation of the hydrophilic nanosilica particles in the PU matrix, because of the poor compatibility between Aerosil A200 and the PU chains. Thus, the PU/silica composites prepared using Aerosil A200 were not characterized any further. Figure 4 presents the DSC thermograms of the PU/silica composites. No significant change was found in the glass transition temperatures of the pure PU and PU/silica composites in the range of -66 to -62 o C. However, the melting enthalpy of the soft segments (PTMEG) at around 24 o C was considerably changed by the addition of the modified silicas. The strong melting peak observed for the pure PU was almost absent or significantly reduced in the composites with 1 wt% of the modified silicas, and increased somewhat as the silica loading was further increased. This strongly indicates that the crystallization of PTMEG is suppressed in the PU/silica composites, especially at lower concentrations of the modified silicas. The TEM morphology also showed the excellent dispersion of the modified silica particles in the PU ma-
790 Sung-Il Lee, Duk-Bae Kim, Jung-Hyun Sin, Youn-Sik Lee, and Changwoon Nah Table 3. Tensile Strengths and Elongations at Break of PU/silica Composite Films (a) (b) Figure 5. SEM images: (a) GS90 and (b) GS90D. Code Tensile strength (kgf/cm 2 ) Elongation at break (%) Pure PU 35 700 PU/GS90-1 137 850 PU/GS90-2 115 683 PU/GS90-3 21 500 PU/GS90D-1 174 950 PU/GS90D-2 74 850 PU/GS90D-3 44 200 (a) (b) Figure 6. TEM images: (a) PU/GS90-1 and (b) GS90D-1. trix (Figure 6). Thus, the decrease in the intensity of the melting peak can be explained by the fact that the formation of hydrogen bonds between -OH groups on the silica surface and soft segments (PTMEG) is predominant [16,17]. A greater amount of hydrogen bonding is to be expected for highly dispersed composites, such as the 1 wt% filled PU composites, because of the much higher probability of contact being made between the silica surface and PTMEG segments. It is generally agreed that nanoparticle-filled polymer composites show the best performance with a maximum particle loading of only a few percent, due to the formation of nanocomposites without any aggregation of the nanoparticles. When the silica content is too high, however, the silica particles become aggregated and the interfacial area between silica surface and soft segments is reduced. In the resulting composites, the relative amount of free PTMEG segments from the interfacial interactions increases, leading to the formation of larger crystalline domains, which consequently exhibit larger melting peaks [14,18]. Interestingly, the melting enthalpies of PU/GS90 were smaller than those of the corresponding PU/GS90D, but the reason for this result is not known at the present time. The FE-SEM images of GS90 and GS90D are presented in Figure 5. The particles in GS90D appeared to be more aggregated than those in GS90. The greater aggregation tendency of GS90D results from the additional hydrogen-bonding interactions among the -OH groups on the silica surface, which are derived from the reactions of the silanol groups with GPTS and DEA. The dispersion morphologies of PU/GS90-1 and PU/GDS90-1 were studied using FE-TEM, and the results are presented in Figure 6. The silica particles in PU/GS90D appeared to be less aggregated than those in PU/GS90. This result clearly suggests that GS90D is more dispersive in the PU matrix than GD90, probably due to the additional covalent bonds formed between the isocyanates and the -OH groups on the silica surface derived from the chemical reactions of the silanol groups with GPTS and subsequently with DEA. The tensile strength and elongation at break of the composites are listed in Table 3. In general, the tensile strength of nanocomposites containing 1 or 2 wt% modified silica is greater than that of pure PU. The enhanced tensile strength has been attributed to the additional physical crosslinks within the polymer network, due to interactions between -OH groups on the silica surface and PU carbonyl groups [15]. On the other hand, the tensile strength of PU/silica composites usually decreases as the silica content increases, due to the formation of more voids in the PU layer next to the silica surface. Similar results were obtained with our current composites, since the tensile strength of the composites reached a maximum value at a silica content of 1 wt% and then decreased as the silica content was further increased. The composites prepared using the doubly modified silica exhibited a higher tensile strength (GS90D-1, 5.0 times) than that of the composite prepared using the singly modified silica (GS90-1, 3.9 times). The more enhanced tensile strength of GS90D-1, as compared to GS90-1, probably resulted from the additional covalent bonds formed between the isocyanates and the -OH groups on the silica surface derived from the reactions of the silanol groups with GPTA and subsequently with DEA. Similarly, the elongations at break of the composites reached their maximum values at a silica content of 1 wt% and then decreased as the silica content was increased further. This is consistent with the results reported in the literature [14,15,19,20]. In our current system, the enhanced elongations at break of PU/GS90-1 and PU/GS90D-1 may be due to the dispersed silicas, which can act chain extenders through chemical bonds
Polyurethane/silica Composites, Prepared Via in-situ Polymerization in the Presence of Chemically Modified Silicas 791 A B Figure 7. FT-IR spectra of PU/GS90-1 (A) and PU/GS90D-1 (B) before and after washing with chloroform. between the -OH groups on the silica surface and the isocyanates. The enhanced phase separation between the soft and hard segments of the PU chains, due to preferential interactions of the silica surface with the soft segments as compared to the hard segments, may also contribute, at least in part, to the enhancement of the elongation at break [17]. PU/GS90-1 and PU/GS90D-1 were immersed in chloroform for 24 h in order to separate the free PU chains from the PU chains covalently linked to the silica surfaces. There were some floating particles without any precipitate in the mixture. The floating material was assumed to be the PU chains covalently linked to the silica surface, since the silica particles were insoluble in chloroform. The floating material was isolated, dispersed in chloroform, subjected to ultra-sonication and then again isolated from the dispersed mixture. The FT-IR spectrum of the initial PU/GS90-1 clearly shows an absorption peak at 1730 cm -1 (free urethane carbonyl groups), but that of the floating material isolated from PU/GS90-1 showed an absorption peak at 1708 cm -1 (Figure 7), which corresponds to the hydrogen bonded urethane carbonyl groups. On the other hand, the FT-IR spectra of the initial PU/GS90D-1 and the isolated floating material are very similar to each other. This result indicates that the relative amount of PU chains covalently linked to the surface of GS90D-1 was more than that of the PU chains covalently linked to the surface of GS90-1. We would expect the PU chains to be covalently linked to the surface of Aerosil A200 via the coupling reaction between silanol groups on the silica surface and the isocyanates. However, the coupling reaction cannot be efficient, due to the steric hindrance of the silica surface on which silanol groups are directly attached without any spacer group, as evidenced by the reaction of Aerosil A200 with GPTS. On the other hand, the primary -OH groups, derived from the reactions of the silanol groups with GPTS and subsequently with DEA, may be much more reactive in the coupling reaction with the isocyanates, since the -OH groups are far away from the silica surface (Scheme 1). Thus, the more reactive -OH groups on the doubly modified silica surface can probably act more efficiently as chain extenders. In the resulting composites, more silica surfaces may be covalently linked to the PU chains and, consequently, well dispersed in the PU matrix. Thus, the more enhanced tensile properties of the composites prepared using the doubly modified silica are probably mainly due to the more increased number of covalent links between the silica surface and the PU chains, and the better dispersion of the silica particles in the PU matrix. Conclusion The hydrophilic surface of a fumed nanosilica was successfully modified firstly with GPTS and subsequently with DEA. The PU/silica composites, prepared using silica modified with only GPTS, exhibited significantly enhanced tensile strength, as compared to the pure PU. When the doubly modified silica was employed, the tensile strength of the resulting composites was even more enhanced. The doubly modified silica particles were more uniformly dispersed in the PU matrix, and formed a greater amount of covalent bonds with the PU chains, as compared to the singly modified silica. Therefore, the more enhanced tensile properties of the composites prepared using the doubly modified silica resulted from the greater number of covalent links between the silica surface and PU chains, and the better dispersion of the silica particles in the PU matrix. Acknowledgment This research was supported by the program for cultivating graduate students in regional strategic industry of Korea (2003).
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