silica nanohybrids by means of tetraethoxysilane sol–gel condensation onto waterborne polyurethane particles

Progress in Organic Coatings 77 (2014) 1436–1442

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Polymer/silica nanohybrids by means of tetraethoxysilane sol–gel condensation onto waterborne polyurethane particles H. Sardon, L. Irusta, R.H. Aguirresarobe, M.J. Fernández-Berridi ∗ POLYMAT, Department of Polymer Science and Technology, University of the Basque Country (UPV/EHU), P.O. Box 1072, 20080 Donostia/San Sebastián, Spain

a r t i c l e

i n f o

Article history: Received 4 July 2013 Received in revised form 10 February 2014 Accepted 29 April 2014 Keywords: Waterborne polyurethane/silica hybrids Tetraethoxysilane Sol–gel process Functionalization Homogeneous distribution

a b s t r a c t Stable waterborne polyurethane/silica hybrid dispersions were obtained by sol–gel reaction of tetraethoxysilane added to previously synthesized waterborne polyurethane nanodispersions. Two series of polyurethane/silica nanostructures with different silica contents were synthesized using pure polyurethane particles and polyurethane particles previously functionalized with (3aminopropyl)triethoxysilane (APTES) as colloidal templates. The optimum experimental conditions for tetraethoxysilane sol–gel reaction (T = 75 ◦ C and semi batch polymerization conditions) leading to the formation of silica/polyurethane aqueous nanodispersions were established. The presence of silica was confirmed using TGA, FTIR, 29 Si NMR and TEM. TEM images showed an excellent final dispersion of the silica nanoparticles in the polymer matrix when silane functionalized polyurethane nanoparticles were used. © 2014 Published by Elsevier B.V.

1. Introduction Polyurethane materials show a unique combination of high performance properties, including excellent abrasion resistance, flexibility or hardness, which makes them suitable for many useful applications such as surface and textile coatings, adhesives, elastomers, foams and dispersions [1–4]. In the past, polyurethanes were mainly synthesized in organic solvents [5]. However, due to current regulations, organic solvents must now be replaced by water based systems [6–8]. Unfortunately, it is difficult to synthesize polyurethanes in aqueous media due to the incompatibility between water and isocyanates [9]. As a consequence polyurethane dispersions are almost exclusively obtained using a two-step procedure where the first polymerization step is carried out in an organic solvent followed by its dispersion in aqueous media [10,11]. One of the main drawbacks of these water-based systems is that in most of the cases carboxylic groups must be incorporated in order to stabilize polyurethane dispersions. The presence of these polar groups increases the susceptibility of the polymer toward hydrolysis, considerably reducing mechanical and thermal properties [11,12]. One of the most efficient synthetic approaches to overcome these negative effects has been the incorporation of inorganic moieties such as silica

∗ Corresponding author. Tel.: +34 943018194; fax: +34 943015270. E-mail address: [email protected] (M.J. Fernández-Berridi). http://dx.doi.org/10.1016/j.porgcoat.2014.04.032 0300-9440/© 2014 Published by Elsevier B.V.

nanoparticles [13–15] due to their synergetic effect on polymer properties [16,17]. However, the interfacial interaction between the polymer matrix and dispersed silica particles is a crucial factor in order to improve the properties [18]. It is generally accepted that the preferred hybrid structure is one in which the organic and inorganic materials are linked by means of covalent or ionic bonds [19,20]. The use of the sol–gel process to prepare highly intermingled inorganic–organic hybrid polymer networks using coupling agents, such as aminopropyl triethoxysilane (APTES), is of current scientific interest [21–23]. However, Sardon et al. have shown that using this strategy it was possible to incorporate only 2 wt% of silica without affecting the homogeneity of the films [20]. Several attempts have been made in order to incorporate greater amounts of silica into the polyurethane matrix without affecting the homogeneity of the films [24–28]. One possibility is the surface modification of the inorganic particles by chemical reaction with organic materials. This method has been found to be an effective strategy to improve homogeneity [29,30]. Similarly, Jeon et al. [31] incorporated functionalized silanes into polyurethane particles to act as a coupling agent and subsequently reacted them with preformed silica particles by the sol–gel process. Although their properties were slightly enhanced, homogeneous silica distribution in the polyurethane matrix was not obtained. Yeh et al. [32] polymerized tetraethoxysilane (TEOS) by the sol–gel process in the presence of amine terminated polyurethane, which can also act as an internal base catalyst for the TEOS sol–gel process. Even though

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the condensation of TEOS was successfully obtained, the films were not totally homogeneous. In order to enhance the silica distribution in the polyurethane matrix, the best solution is to polymerize silica “in-situ” in the presence of polyurethane particles [33]. Tissot et al. [34] showed that TEOS could be polymerized on the surface of polystyrene particles using 3-(trimethoxysilyl)-propyl methacrylate (MPS) as a functional comonomer. They showed that the silanol groups coming from the functional monomer allowed the creation of silica coating onto the surface of the polystyrene particles. Following a similar strategy, stable functionalized polyurethane dispersions containing silanol groups were generated in order to enhance TEOS grafting [20]. This work presents the first successful polymerization of TEOS onto waterborne polyurethane nanodispersions. The prepared polyurethane/silica colloids were studied by Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). Dried hybrid films were characterized using 29 Si solid-state NMR, FTIR spectroscopy and TEM. Firstly, the best conditions for TEOS polymerization in the presence of polyurethane particles were established. Secondly, polyurethane silica systems containing different silica amounts were synthesized from functionalized and non-functionalized polyurethane particles. Finally, the coupling agent effect on the homogeneity of the films obtained by casting was evaluated. 2. Experimental part 2.1. Materials Isophorone diisocyanate (IPDI), 2-bis(hydroxymethyl) propionic acid (DMPA), 1,4-butanediol (BD), poly(1,4-butylene adipate) end capped diol (PBAD) (Mn ca. 1000 g mol−1 ), triethylamine (TEA), dibutyltin diacetate (DBTDA), (3-aminopropyl)triethoxysilane (APTES), tetraethoxysilane (TEOS), acetone (HPLC grade), ethanol (HPLC grade) and methanol (HPLC grade) were purchased from Aldrich Chemical Corporation. All materials were used as received. 2.2. Preparation of hybrid dispersions Waterborne polyurethane dispersions without and with APTES (10 wt%) were obtained (WPU and f-WPU respectively) by means of the so-called “acetone-process” as previously described [35]. Briefly, 45.0 g (0.045 mol) of polyol (PBAD), 3.0 g (0.022 mol) of internal emulsifier (DMPA) and 3.0 g (0.030 mol) of TEA, were fed into the flask reactor together with 0.2 g of DBTDA (0.00055 mol) and 70 g of acetone. When the reaction temperature reached 60 ◦ C, 25 g of IPDI (0.11 mol) were added drop wise at 1 mL min−1 . The reaction was carried out for 3 h and afterwards, the chain extender BD 3.9 g (0.045 mol) was introduced and reacted for 3 h in order to obtain WPU (mol ratio NCO:PBAD:BD:DMPA = 1:0.40:0.40:0.2). For the synthesis of functionalized polyurethane dispersions (fWPU), 2.3 g of BD (0.026 mol) were added together with 8.4 g of APTES (0.038 mol) and reacted for 3 h to obtain f-WPU (mol ratio NCO:PBAD:BD:DMPA:APTES = 1:0.40:0.23:0.2:0.34). In both cases, the temperature of the reactor was dropped to 25 ◦ C and the mechanical stirring was raised to 400 rpm to help the dispersion process [36]. Water (180 g) was added drop wise to the reactor at 3 mL min−1 , and the stirring rate was kept constant for an additional 30 min. Finally, acetone was removed under vigorous stirring using distillation equipment under vacuum. The resulting dispersion had 30–35 wt% of solids. Non-functionalized and functionalized polyurethane/silica nanostructures were obtained by adding TEOS to the dispersions. First, the solid content of the polyurethane dispersion was adjusted

Fig. 1. Route for the synthesis of silica/polyurethane hybrid aqueous nanodispersions.

to 25 wt% and the pH was raised with triethylamine to 10. An aliquot of 40 g (containing 10 g of polymer) was charged into the reactor and mechanical stirring was adjusted to 200 rpm. In Fig. 1 the strategy to form covalently linked hybrid dispersion is shown. Three different TEOS amounts (0.73 g (0.0035 mol), 1.5 g (0.0070 mol), 2.9 g (0.014 mol)) at three different sol–gel process temperatures (25, 50 and 75 ◦ C) were used in order to evaluate their effect on TEOS polymerization process. Moreover the effect of the TEOS addition method (batch or semi-batch (drop wise at 0.70 mL h−1 )) on the silica distribution was evaluated by TEM. 2.3. Instrumentation Gas chromatography (GC) experiments were carried out to study the reaction kinetics of TEOS in the presence of polyurethane particles. The apparatus used was a Shimadzu GC-14A. The chromatographic column was an Agilent 30 meters DB-waxetr of 0.53 mm internal diameter. The temperature of the detector and injector was set at 200 ◦ C. The following temperature program was used for the GC oven: isothermal at 45 ◦ C for 5 min, temperature ramp at 10 ◦ C min−1 to 200 ◦ C, where it was maintained for a further 10 min. Nitrogen was used as the carrier gas at a pressure of 150 kPa. The analysis was performed in split mode with a ratio of 40:5. Methanol was used as the internal standard and a calibration curve was performed using known concentrations of ethanol and methanol in order to obtain the accurate ethanol concentration. The samples were prepared diluting 0.015–0.05 g of sample, taken directly from the reactor, in 10 g of water using between 0.004 and 0.010 g of methanol in a closed vial. The pH of all dispersions was adjusted to 10 with TEA prior to the sol–gel process. Under the present experimental conditions the condensation reaction is favored with respect to the hydrolysis and therefore all hydrolyzed alkoxy groups will be condensed. The

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Table 1 Description of the different polymerization conditions. Sample

WPU (g)

TEOS (g)

SiO2 (wt%) theoretical

T (◦ C)

Pol. process

WPU-1 WPU-2 WPU-3 WPU-4 WPU-5 WPU-6

10.0 10.0 10.0 10.0 10.0 10.0

1.5 1.5 1.5 0.7 2.9 1.5

4.2 4.2 4.2 2.0 7.7 4.2

25 50 75 50 50 75

Batch Batch Batch Batch Batch Semi-batch

conversion of the reaction can be calculated using the following expression [37]. Conversion (wt%) =

gethanol(GC) gethanol(max .)

× 100

where gethanol(GC) is the weight of ethanol (g) of the sample measured by GC and gethanol(max .) represents the maximum ethanol weight that can be formed during the sol–gel process. Fourier transform infrared (FTIR) spectra were obtained at room temperature using a Nicolet 6700 FTIR spectrometer at a resolution of 2 cm−1 , and a total of 64 interferograms were signal averaged. The spectra were obtained from solution casting onto KRS-5 windows. 29 Si solid Nuclear magnetic resonance (Si-NMR) spectra were performed to study the final structure after drying the films at room temperature for at least one week. The experiments were recorded in a Fourier Transform Bruker 300 MHz (model Avance 300 DSX) spectrometer operating at 59.58 Hz. The rotor (7-mm) spin rate was 4 kHz, with delay time of 2 s, accumulating 2000 transients. Dynamic light scattering (DLS) measurements were carried out to determine the diameter of the particles, Dp, using a Malvern Zetasizer nano series. The samples were diluted with deionized water before performing the measurements to avoid multiple light scattering. The final value was an average of five measurements. Transmission electron microscopy (TEM) was employed using a Philips Tecnai 20, accelerating voltage 200 kV. For the observation of particles, diluted dispersions (0.005–0.01 wt% depending on the size) were prepared. Moreover, ultrathin sections of the formed films were observed by TEM to study the morphology of the materials. Specimens for TEM were prepared with an ultramicrotome (LEICA ultracut EM UC6) using a diamond knife. Silica content in the hybrid materials was determined by Thermo Gravimetric Analysis (TGA Q500, TA instruments). Samples were heated under nitrogen atmosphere at a rate of 10 ◦ C min−1 from 50 ◦ C to 600 ◦ C. After cooling to 300 ◦ C, nitrogen was replaced by air and another temperature scan was carried out at a rate of 10 ◦ C min−1 to 800 ◦ C. The values of the residual weights were used as the silica content in the hybrid materials. 3. Results 3.1. Optimization of TEOS sol–gel process in the presence of polyurethane aqueous nanodispersions The effect of temperature and TEOS concentration on the sol–gel process was studied. Table 1 shows all the conditions employed for this study. GC was employed to quantify the amount of ethanol released during the sol–gel process. The ethanol amount can be related to the condensation rate, allowing the comparison between different systems. In order to simplify the study, the best polymerization conditions for TEOS were established in the presence of nonfunctionalized polyurethane particles (WPU). In this case, all the ethanol released came from the condensation reaction of TEOS. In order to investigate the effect of temperature on the polymerization

80 WPU-1 (T = 25ºC) Conversion %

1438

60

WPU-2 (T = 50ºC) WPU-3 (T = 75ºC)

40

20

0 0

20

40

60 80 time (h)

100

120

Fig. 2. Conversion – calculated by gas chromatography – as a function of time for tetraethoxysilane systems polymerized at three different temperatures 25, 50 and 75 ◦ C.

rate, three different temperatures 25, 50 and 75 ◦ C were employed using polyurethane seeds containing 25 wt% of polymer and 29 nm of initial particle size. Fig. 2 shows the conversion (%) measured by GC as a function of time for the systems polymerized at different temperatures. As shown in Fig. 2, the higher the polymerization temperature the higher the condensation rate and the shorter the time to obtain the maximum conversion. When TEOS was polymerized at room temperature in the presence of non functionalized polyurethane particles, the maximum conversion was obtained at 95 h achieving an unstable dispersion. However, at higher temperatures stable dispersions were obtained and the maximum conversion time was considerably reduced (8 and 4 h at 50 ◦ C and 75 ◦ C respectively). Moreover, solid 29 Si NMR studies were carried out in order to analyze the structure of the silicon groups in the final product (Fig. 3). Using the Qn notation for describing the chemical environment around Si atom, Q3 (around −100 ppm) and Q4 (around −110 ppm) species were observed. These signals were attributed to highly condensed structures. In our opinion, the reaction did not reach full conversion because some silanol groups could have been trapped in the rigid inorganic network, thus hindering their full condensation. In order to investigate the effect of the initial TEOS concentration on the polymerization rate, dispersions containing 2.0, 4.2 and 7.2 wt% of silica were synthesized. Fig. 4 shows the conversion (%) measured by GC as a function of time for three different TEOS ratios. As can be seen in Fig. 4, as TEOS concentration was increased, the time for reaching the maximum conversion was longer (6, 8 and 10 h for WPU-2, WPU-4 and WPU-5 respectively), and the condensation rate slowed down. It has to be mentioned that in the cases of WPU-4 and WPU-5 the viscosity of the reaction rose, that increase being more pronounced in the case of high TEOS concentration. This behavior was probably due to the trend of alkoxy silane compounds to gel under basic aqueous conditions, limiting the polymerization kinetics. Moreover, it should be pointed out that all the systems were able to reach full conversion. 29 Si NMR spectra of WPU-4 and WPU-5 samples were similar to that of WPU-2 shown in Fig. 3. All the experiments shown so far were performed adding TEOS under batch conditions. Asua et al. showed that particle morphology could be defined by the interplay between thermodynamic and kinetic principles [38,39]. Thermodynamics determines the

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100

Conversion (%)

80

60

WPU-4 (2 wt % SiO )

40

2

WPU-2 (4 wt % SiO ) 2

WPU-5 (8 wt % SiO ) 2

20

0

Fig. 3. CPMAS 29 Si NMR scale expanded spectra of samples synthesized at three different temperatures 25, 50 and 75 ◦ C containing 4 wt% of SiO2 .

particle morphology at equilibrium, according to the minimum surface free energy. Kinetic factors control whether the particle reaches equilibrium morphology or remains at a metastable (kinetically stable) morphology. In order to encourage the deposition of TEOS on the polyurethane surface, TEOS was added drop-wise (0.7 mL h−1 (semi-batch)) performing the polymerization at high temperature. The morphology of the polyurethane/silica nanostructures was evaluated by TEM. Fig. 5 shows the photographs of the polyurethane/silica synthesized under batch and semi-batch conditions. As can be observed in both images agglomerates of small SiO2 particles, in the range of 20 nm were formed. The silica domains were slightly better distributed onto the polyurethane particles when semi-batch polymerization conditions were employed. In our opinion this occurs because in the latter case, silica was forced to interact with the polyurethane surface whereas in the case of batch conditions, TEOS could interact easier with itself. According to these results, the optimum experimental conditions needed to obtain high condensation degrees and good distribution of the inorganic domains onto the polyurethane

0

2

4

6 8 time (h)

10

12

Fig. 4. Conversion (calculated by GC) as a function of time for systems polymerized at 50 ◦ C with three different tetraethoxysilane concentrations.

particles are semi-batch conditions and high condensation temperatures (75 ◦ C), in order to encourage the fast polymerization of TEOS.

3.2. Synthesis of silica/polyurethane hybrid nanodispersions using non-functional polyurethane seeds Under the conditions established in the previous section, different amounts of TEOS were polymerized in the presence of non-functionalized polyurethane particles (WPU). Particle size and distribution of the different systems containing varying amounts of silica (WPU, WPU–2% SiO2 , WPU–4% SiO2 WPU–8% SiO2 ) were determined by means of DLS. As shown in Table 2, the coagulum amount increased with TEOS, together with particle size and the polydispersity index. This increase in the particle size could not be explained assuming that TEOS was totally covering the polyurethane particles. Following the expression proposed by Bourgeat-Lami et al. [34] the increase in the diameter of the coated particles for all the cases must be close to 1 nm. In our opinion silica coming from TEOS promotes the

Fig. 5. TEM images of dispersions (4 wt% of SiO2 ) synthesized using batch and semi-batch polymerization conditions.

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Table 2 Particle diameter (Dp), polydispersity index, coagulum weight and theoretical and experimental silica content for the non functionalized and functionalized systems. Sample

Theoretical (SiO2 wt%)

Experimental (SiO2 wt%)

WPU WPU–2% SiO2 WPU–4% SiO2 WPU–8% SiO2 f-WPU f-WPU–2% SiO2 f-WPU–4% SiO2 f-WPU–8% SiO2

0 2.0 4.2 7.8 2.8 4.8 6.0 10

0 1.6 3.7 7.6 2.4 4.3 6.1 10

± ± ± ± ± ± ± ±

Dp (nm)

0.3 0.2 0.3 0.3 0.2 0.3 0.1 0.1

29 42 58 61 42 50 63 98

± ± ± ± ± ± ± ±

1 3 5 10 1 2 2 4

P.I.

Coagulum (wt%)a

0.15 0.19 0.33 0.43 0.12 0.17 0.22 0.25

0 2±1 6±1 8±1 0 <1 2±2 5±1

f-WPU 8 wt % SiO

2

a Coagulum wt% was calculated by filtering the dispersion through a 0.45 ␮m membrane and weighting out the filtrate content (coagulum wt% = filtered polymer content (g)/total polymer content (g)).

formation of agglomerates increasing the mean particle size, the polydispersity index and coagulum size as observed in Table 2. The SiO2 content of the final products was experimentally calculated from the residual weight after TGA analysis and the values were compared with the theoretical ones assuming total conversion. Representative TGA thermograms can be found in a previous publication [18]. As expected, SiO2 content increased as a function of TEOS concentration confirming that most of the added TEOS was able to polymerize under the employed experimental conditions. Fig. 6 shows FTIR spectra of WPU (without silica) and WPU–8% SiO2 dried at room temperature. When the sol–gel reaction took place, TEOS underwent hydrolysis and condensation reactions. These processes gave rise to a slight modification of the bands in the region where the Si–O–Si stretching vibration absorbs (≈1065 cm−1 ) and the appearance of a small band at 470 cm−1 (Si–O–Si bending). These modifications confirmed that TEOS polymerization reaction in the presence of WPU particles occurred and that the silica content increased with TEOS concentration. 3.3. Synthesis of silica/polyurethane hybrid nanodispersions using silane functional polyurethane seeds Using the same polymerization conditions TEOS was polymerized (6.1, 13.1 and 21.1 wt%) in the presence of silane functionalized polyurethane particles (f-WPUs), which present higher affinity toward TEOS than the pure polyurethane particles.

f-WPU

2000

1800

1600

1400

1200

1000

800

600

400

-1

wavenumber (cm ) Fig. 7. Scale expanded FTIR spectra of samples f-WPU and f-WPU–8 wt% SiO2 after drying.

The particle size and distribution of the final dispersions were determined by means of DLS for the samples f-WPU-0, f-WPU–2% SiO2 , f-WPU–4% SiO2 and f-WPU–8% SiO2 , as shown in Table 2. The general behavior arising from functionalized particles was similar to that of non functionalized ones. Nevertheless, in this case stable dispersions were obtained at large TEOS concentrations (8% SiO2 ) without excessively increasing the coagulum amount. The silica content was also calculated by TGA (Table 2) and the experimental values were similar to the theoretical ones. In Fig. 7 FTIR spectra of two different films (f-WPU-0 and fWPU–8% SiO2 ) dried at room temperature are shown. As described previously for the non-functionalized systems, the bands of silica are shown in the spectrum of f-WPU–8% SiO2 . Moreover, the shoulder at 1650 cm−1 could be assigned to the carbonyl stretching vibration of urea groups, generated as a consequence of the insertion of the coupling agent. Fig. 8 shows the 29 Si NMR spectra of the f-WPU and f-WPU8% SiO2 (before and after TEOS addition). Using the Tn notation for describing the chemical environment around the Si atom the species T2 (around −55 ppm) and T3 (around −66 ppm) were

WPU- 8 wt % SiO

T

2

T

3

Q

2

3

Q

4

f-WPU 8 wt% SiO

2

WPU f-WPU

2000

1800

1600

1400

1200

1000

800

600

400

-1

wavenumber (cm ) Fig. 6. Scale expanded infrared spectra of samples WPU (without silica) and WPU–8 wt% SiO2 dried at room temperature.

0

-50

-100

ppm

-150

-200

Fig. 8. CPMAS 29 Si NMR scale expanded spectra of f-WPU and f-WPU–8 wt% SiO2 .

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Fig. 9. TEM images of films containing 0, 4 and 8 wt% SiO2 for the functionalized and non-functionalized systems.

observed before TEOS addition. After TEOS addition Q3 (around −100 ppm) and Q4 (around −110 ppm) structures were observed together with the previously mentioned T structures. Furthermore, the intensity of the T2 structures decreased after TEOS addition probably due to the reaction between the two inorganic precursors. TGA, FTIR and 29 Si NMR results confirmed the insertion of the silica into the waterborne polyurethane particles. 3.4. Morphology of silica/polyurethane hybrid films obtained using functionalized and non-functionalized polyurethane nanodispersions Finally, the morphology of the hybrid films using functionalized and non-functionalized particles was analyzed by TEM (Fig. 9). As observed, before TEOS addition both films were completely homogeneous. This result was evident for the film without functionalized particles, as it did not contain an inorganic phase. However the film obtained from functionalized particles (containing silicon atoms) also showed a homogeneous morphology that could be explained considering that the silica domains were too small to be detected by TEM. After TEOS addition, nanometric silica particles were observed in TEM images regardless of the functionalization. It is important to remark that in the samples obtained from non-functionalized polyurethane particles, silica formed aggregates and consequently its distribution was not homogeneous. However, the silica distribution in the polyurethane matrix was considerably enhanced when the polyurethane particles had previously been functionalized with the aminosilane type precursor. The size of the silica domains was in the range of 10–20 nm in all the cases regardless of the functionalization and the TEOS initial concentration.

4. Conclusions In conclusion, a series of stable waterborne polyurethane/silica dispersions, containing 2, 4 and 8 wt% SiO2 were prepared. Tetraethoxysilane was successfully polymerized in the presence of functionalized and non-functionalized polyurethane particles, which acted as colloidal templates. The best polymerization conditions for obtaining stable polyurethane silica dispersions were established (75 ◦ C and semi-batch polymerization conditions). TEOS polymerization was confirmed by FTIR, TGA and solid 29 Si NMR. Finally, TEM images of the films showed that silica was homogeneously distributed in the presence of functionalized polyurethane particles. This method shows for the first time the generation of well distributed polyurethane silica hybrids in aqueous media. Acknowledgments The authors acknowledge the University of the Basque Country, UPV/EHU (UFI 11/56), the Ministerio de Ciencia e Innovación (MAT2010-16171) and the Basque Government (Ayudas a grupos de investigación del sistema universitario vasco ITT68-13) for the funding received to develop this work. H.S. gratefully acknowledges financial support through a postdoctoral grant (DKR) from the Basque Government. References [1] [2] [3] [4]

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