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Silicone rubbers filled with TiO2: Characterization and photocatalytic activity
Materials Chemistry and Physics 113 (2009) 395–400
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Silicone rubbers filled with TiO2 : Characterization and photocatalytic activity V.P. Silva, M.P. Paschoalino, M.C. Gonc¸alves, M.I. Felisberti, W.F. Jardim, I.V.P. Yoshida ∗ Instituto de Química, Universidade Estadual de Campinas, 13083-970 Campinas, São Paulo, Brazil
a r t i c l e
i n f o
Article history: Received 28 February 2008 Received in revised form 16 June 2008 Accepted 27 July 2008 Keywords: TiO2 Silicone Rubbers Photoactivity
a b s t r a c t Compounding of silicone rubbers with TiO2 was carried out in a two-roll mill from a mixture of TiO2 and poly(dimethylsiloxane) (PDMS) with several other additives. The silicone matrix was crosslinked by a platinum-catalyzed hydrosilylation reaction. Characterization of the rubbers was performed by thermogravimetric and dynamic mechanical analyses, swelling measurements in toluene and in a salicylic acid/water solution, UV–vis spectroscopy and field emission scanning electron microscopy. The photocatalytic activity of the rubbers was evaluated with respect to the photodegradation of salicylic acid in aqueous solution under UV radiation, by measuring total organic carbon. The thermal stability of the PDMS matrix was enhanced by the addition of 10 phr (parts per hundred parts of rubber, by weight) of TiO2 , but a further increase in the quantity of TiO2 favored thermal degradation of the matrix. The stiffening effect promoted by the incorporation of TiO2 into the matrix was more pronounced at higher TiO2 amount. The increase in the quantity of TiO2 also promoted a decrease in the crosslinking density of the matrix, favoring the swelling of the rubbers. The rubber with 78 phr of TiO2 degraded 56% of the salicylic acid from an aqueous solution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide has been extensively investigated as a heterogeneous photocatalyst for water purification. However, since this photocatalyst is often applied as a suspension, problems associated with catalyst leaching, flotation and the need for eventual catalyst separation by filtration during post-treatment, make its large-scale application difficult and expensive [1]. In order to solve these problems, research on TiO2 immobilization in different supports has attracted wide attention [1–4], particularly using polymeric supports, which are easily molded in a variety of shapes. Jardim and coworkers [2] described the use of poly(dimethylsiloxane), PDMS, and orthophthalic polyesters, OP, as supports for TiO2 (Degussa P-25). OP/TiO2 films presented the most suitable characteristics to be used as a fixed bead in gas-phase reactors. Rizzo et al. [3] prepared an efficient photocatalytic reactor made of stacked poly(methylmethacrylate) (PMMA) rings coated with a thin-film of TiO2 for the removal of methylene blue from water. Most of the organic pollutants in the environment cannot be determined and/or degraded without the application of concentration techniques. PDMS-based membranes have been successfully used in the concentration of organic traces in water [5–8], and in the pervaporative separation of organic contaminants from aque-
ous solutions [9,10], with good selectivity for organics removal. On the other hand, PDMS-based membranes are usually constituted by a crosslinked PDMS matrix loaded with filler, which can promote an appropriated mechanical reinforcement to these membranes. Although the most popular filler in PDMS membranes is SiO2 , these membranes can also be filled with TiO2 photocatalyst. Thus, PDMS matrices are advantageous for the incorporation of TiO2 , in relation to other polymeric supports used in photocatalysis, due to the high flexibility and mobility of the PDMS chains and their consequent high permeability to gases [11,12] and some organic volatiles [13–16]. These characteristics are important to the diffusion of organic pollutants into the polymeric material. In addition, PDMS is also transparent to UV–vis radiation, which allows access of this radiation to the TiO2 photoactive sites incorporated into the PDMS matrix [1]. In this study, rubbers obtained by the incorporation of TiO2 into PDMS gum, crosslinked by a hydrosilylation reaction, were evaluated in relation to their photocatalytic activities, with respect to the photodegradation of salicylic acid in aqueous solution under UV radiation. These rubbers were also characterized in relation to their morphological, thermal, dynamic mechanical, swelling and UV–vis absorption properties. 2. Experimental 2.1. Materials
∗ Corresponding author. Tel.: +55 19 35213130; fax: +55 19 35213023. E-mail address: [email protected] (I.V.P. Yoshida). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.104
Titanium dioxide photocatalyst, P-25 TiO2 , with an anatase/rutile ratio of about 80/20 and mean primary particle size of about 21 nm, was purchased from Degussa
396
V.P. Silva et al. / Materials Chemistry and Physics 113 (2009) 395–400
(São Paulo, SP, Brazil). Poly(dimethylsiloxane), PDMS, with an average molar mass of about 106 g mol−1 and a (SiCH3 CH CH2 O)/(Si(CH3 )2 O) molar ratio of <1/1000, used as matrix for the rubbers, 1,3,5,7-tetramethyl,1,3,5,7-tetravinylcyclotetrasiloxane, D4 V, utilized as crosslinking inhibitor, poly(methylsiloxane), PMS, with an average molar mass of 5096 g mol−1 , used as crosslinker, and the catalyst complex platinum divinyltetramethyldisiloxane were all supplied by Dow Corning (Hortolândia, SP, Brazil). Toluene, used in the swelling measurements, and salicylic acid, SA, used in the swelling and photocatalysis experiments, were both acquired from Synth (Diadema, SP, Brazil). 2.2. Compounding procedure Unfilled PDMS and TiO2 /PDMS rubbers were prepared by using 0, 10, 30, 40 and 78 phr (parts per hundred parts of rubber, by weight) of TiO2 , 2 phr of D4 V, 1 phr of platinum catalyst and 6 phr of PMS. Compounding was carried out in a Copé open two-roll mill for 20 min at 25 ◦ C. The rotors were operated at a speed ratio of 1:1.4. The compounds were compression molded in a Parabor PL350 at 155 ◦ C and 22 MPa for 10 min, followed by a post-cure treatment in a vacuum oven at 120 ◦ C for 8 h. 2.3. Characterization The morphology of the rubbers was investigated by field emission scanning electron microscopy (FESEM), performed in a JEOL JSM-6340 F microscope operated at 3 kV. FESEM samples were cryogenically fractured and the fracture surface was sputter-coated with very thin carbon and gold layers. Thermal stability of the rubbers was analyzed by thermogravimetric analysis (TGA) in a TA 2950 thermobalance from 25 to 1000 ◦ C, at a heating rate of 20 ◦ C min−1 under an argon flow. Dynamic mechanical analysis (DMA) was carried out in a Rheometric Scientific DTMA V, in a temperature range between −135 and 40 ◦ C, at a frequency of 1 Hz, and a heating rate of 2 ◦ C min−1 . Swelling measurements were performed in toluene and in aqueous solutions of SA, following ASTM D471 [16]. Rubber strips with dimensions of (30 mm × 10 mm × 2 mm) were weighed and immersed in 25 cm3 of the solvent, and, at regular intervals, the surface of the swollen strip was gently dried with a filter paper, and the weight of the sample was registered. The above procedure was repeated until a constant swollen weight was obtained. Afterwards, samples were dried in a vacuum oven at 50 ◦ C for 24 h and weighed again. For each rubber, three samples were tested and the average results are reported. The diffuse reflectance UV–vis spectra were recorded with a Cary 5G UV–vis-NIR spectrophotometer, using Teflon as reference. The photocatalytic activity of the rubbers was evaluated with respect to the photodegradation of SA in aqueous solution (3.6 10−4 mol L−1 ) under UV radiation (Sankyo 30 W black light lamp, = 365 nm) for 2 h by TOC (total organic carbon) measurements in a Shimadzu TOC-5000. In duplicate for each rubber, samples perfectly fitted into Petri dishes were covered by the SA solution or by water (in order to evaluate possible carbon leaching from the matrix). Petri dishes containing only the SA solution (for photolysis evaluation) were also tested. All dishes were kept below the UV lamp for 2 h, according to the schematic experimental setup shown in Fig. 1. Dishes with the rubbers containing the SA solution were also kept in the dark, in order to evaluate the SA adsorption effect.
3. Results and discussion The compounding process used to prepare silicone rubbers filled with TiO2 is easy and appropriate to disperse variable amounts of
Fig. 1. Schematic of the experimental setup used in the photocatalysis experiment showing (a) UV lamp and Petri dishes containing; (b) rubbers covered by the SA solution; (c) rubbers covered by water and (d) dishes with only the SA solution.
this filler into a high viscosity PDMS matrix (PDMS gum). The morphology of the rubbers with 10, 30 or 40 phr of TiO2 (TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40, respectively) was similar, characterized by agglomerates of TiO2 particles, expressed by white points with sizes ranging between 60 and 200 nm, uniformly dispersed in the PDMS matrix, as can be seen in the micrograph of TiO2 /PDMS40 in Fig. 2(a). For the rubber with 78 phr of TiO2 , TiO2 /PDMS78, larger agglomerates of TiO2 particles, in the range of 60–1000 nm, were observed, as shown in Fig. 2(b). All the rubbers presented good adhesion between the PDMS matrix and the TiO2 agglomerates, which is an important aspect for a successful application of these rubbers in photocatalysis, because poor adhesion may cause detachment of TiO2 particles from the PDMS matrix, leading to the TiO2 catalyst leaching into the aqueous phase and resulting in a loss of activity during reuse of these rubbers [2]. TGA and differential thermogravimetric (DTG) curves for the TiO2 /PDMS rubbers can be seen in Fig. 3(a) and (b), respectively. It is well known that the thermal degradation of end-blocked PDMS in inert atmosphere occurs by depolymerization over the range of 400–650 ◦ C to produce cyclic oligomers [17]. The effect of crosslinking during the thermal degradation of the unfilled PDMS can be ignored owing to its very low crosslinking density. In earlier studies [18], it has been found that the occurrence of intra and intermolecular rearrangements, through cyclic transition state, can explain the elimination of cyclic dimethylsiloxane and the shortening of the residual chain length until it is too short to cyclize. This process can be associated with the first step of the degradation process of the unfilled PDMS, with maximum weight loss rate temperature (Tmax ) at 475 ◦ C, as can be seen in Fig. 3(b). Following this step,
Fig. 2. FESEM micrograph of (a) TiO2 /PDMS40 and (b)TiO2 /PDMS78.
V.P. Silva et al. / Materials Chemistry and Physics 113 (2009) 395–400
397
Fig. 3. (a) TGA and (b) DTG curves for unfilled PDMS, TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40 and TiO2 /PDMS78.
the second peak with Tmax at 554 ◦ C can be related to the evaporation of the shortened residual linear fragments of the PDMS chains, which competes with the cyclization. This second degradation step can also be associated with the degradation of the PDMS network domains with higher crosslinking density, which restrict the mobility of the PDMS chains and make difficult their intra or intermolecular rearrangements [19]. The presence of TiO2 in the PDMS matrix resulted in multiple degradation steps. This behavior is related to a change in the thermal degradation mechanism of the PDMS in the presence of TiO2 . In addition, the barrier effect of the TiO2 particles on the diffusion of volatiles may have promoted the shift of the main weight loss processes to higher temperatures, as compared to the unfilled PDMS, and the overlap of different processes. For the rubbers filled with TiO2 , the amount of residue at ∼900 ◦ C was higher than the TiO2 amount added to the PDMS matrix. For example, the sample containing 10 phr of TiO2 (8 wt.%) produced 49% of residue and that one containing 40 phr of TiO2 (27 wt.%) produced 63% of residue, which can be associated with the formation of SiCx Oy residue [20]. These results reinforce the hypothesis of change of the degradation mechanism of the rubber matrix, probably catalyzed by a rich Ti–OH environment Swelling measurements of the rubbers were performed in toluene and in aqueous salicylic acid solutions. Toluene was used in order to obtain appropriate physical parameters such as the average molar mass between crosslinking points (MC ) and the crosslinking density of the polymeric network (nFR ). These parameters can be useful for the interpretation of the photocatalysis data. Swelling of the PDMS rubbers in the salicylic acid solution was studied with the main objective of understanding the possible influence of swelling on the photocatalytic degradation of SA. The equilibrium swelling ratio was determined by the balance between the osmotic pressure of the solvent, which forces the polymeric chains to take an elongated conformation, and the elastic force of the polymeric network. Thus, the greater the crosslinking density of the network, the smaller the average molar mass between crosslinking points and the lower the swelling [21]. The swelling ratio (SR ) values of the PDMS rubbers in toluene and in SA solution were calculated using Eq. (1). Because TiO2 particles do not swell, the swelling ratio of the polymeric matrix alone was
calculated discounting the amount of TiO2 in each sample [21], SR =
[mS − (˛filler md )] [˛pol md ]
(1)
where ms is the weight of the swollen sample at equilibrium, md is the weight of the dried sample after swelling, and ˛filler and ˛pol are the TiO2 and polymer weight fractions, respectively. The curves of SR as a function of time for toluene and SA solution are shown in Fig. 4(a) and (b), respectively. For the swelling in toluene, an increase in SR with an increase in the amount of TiO2 was observed. As expected, swelling of all these rubbers in SA solution was very small, due to the low affinity of the siloxane matrix (hydrophobic) for the aqueous solution, but a higher SR for the rubber with 78 phr of TiO2 was observed, as compared to the other rubbers, probably associated with higher water adsorption in the larger TiO2 agglomerates of TiO2 /PDMS78 [22]. The average molar mass between crosslinking points (MC ) was calculated using the Flory–Rehner equation (Eq. (2)) for a tetrafunctional polymeric network [21],
MC =
1/3
−dpol Vsol pol − pol /2
2 ] [ln(1 − pol ) + pol + pol
(2)
where dpol is the density of the PDMS matrix (0.93 g cm−3 ), Vsol is the molar volume of toluene (105.91 cm3 mol−1 ), is the Flory–Huggins polymer-solvent interaction parameter (0.35) and ˚pol is the volume fraction of the polymer in the swollen sample at equilibrium [21]. ˚pol was calculated using the following equation [21]: dpol 1 = 1 + SR(eq) pol dsol
(3)
With the MC values obtained for each sample, the crosslinking density of the polymeric network (nFR ) was calculated using the following equation [21]: nFR =
dpol MC
(4)
The SR(eq) , MC , and nFR values calculated from swelling measurements of the samples in toluene are presented in Table 1.
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Fig. 4. Variation of the swelling ratio (SR ) with time for swelling of unfilled PDMS, TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40 and TiO2 /PDMS78 in (a) toluene and (b) salicylic acid solution.
According to these results, an increase in the amount of TiO2 promoted a decrease in the crosslinking density of the matrix, which contributed to an increase in the MC and SR(eq) values. This can be explained by the occurrence of condensation reactions between the Ti–OH groups at the TiO2 surface and the Si–H groups on the crosslinker chains, as described in the reaction illustrated below, which reduces the availability of the Si–H groups for matrix crosslinking reactions. The data in Table 1 also suggests that the introduction of 10 phr of TiO2 in the PDMS matrix was not enough to significantly disturb the crosslinking density of this matrix. Ti–OH + H–Si
→
Ti–O–Si
region plateau and can be attributed to the hydrodynamic effect arising from the inclusion of rigid TiO2 particles in the matrix [23], despite the decrease in the crosslinking density of the matrix, which contributes to the reduction in E values at the elastic region plateau. UV–vis absorbance spectra of powdered TiO2 and of the rubbers (Fig. 6) revealed a small absorption for the unfilled rubber in the range of around 250–400 nm, associated with the PDMS matrix,
+ H2
In addition, a higher quantity of TiO2 promoted physical restrictions to the hydrosilylation reaction between PDMS gum and the PMS crosslinker. Si–CH CH2 + H–Si
→
Si–CH2 CH2 –Si
The storage modulus (E ) curve for the unfilled PDMS (Fig. 5) showed a little pronounced drop at approximately −120 ◦ C related to the glass transition and a more pronounced drop at about −40 ◦ C attributed to the melting process. These transitions were not affected by the presence of TiO2 , however, a slight increase in the modulus values from ∼−130 ◦ C to −50 ◦ C with the increase in the TiO2 amount was observed, suggesting a reinforcing effect of the filler. This stiffening effect was more pronounced at the elastic Table 1 Equilibrium swelling ratio (SR(eq) ), crosslinking density (nFR ) and average molar mass (MC ) between crosslinking points, for swelling of the rubbers in toluene Sample
SR(eq)
Unfilled-PDMS TiO2 /PDMS10 TiO2 /PDMS30 TiO2 /PDMS40 TiO2 /PDMS78
3.74 3.67 4.12 4.66 8.46
nFR (10−4 mol cm−3 ) ± ± ± ± ±
0.00 0.01 0.02 0.01 0.01
1.8 1.8 1.4 1.2 0.4
MC (g mol−1 ) 5247 5247 6574 7433 21,926
Fig. 5. Variation of storage modulus (E ) with temperature for unfilled PDMS, TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40 and TiO2 /PDMS78.
V.P. Silva et al. / Materials Chemistry and Physics 113 (2009) 395–400
Fig. 6. UV–vis absorbance spectra for powdered TiO2 , unfilled PDMS, TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40 and TiO2 /PDMS78.
which promoted a higher absorbance of the rubbers with TiO2 in this range, as compared to the powdered TiO2 . On the other hand, the increase in the quantity of TiO2 promoted no significant differences in the absorption of the rubbers with this filler. Fig. 7 shows the values of TOC decrease as a function of the quantity of TiO2 in the rubbers after the photocatalysis experiments. These values were calculated by subtracting the TOC decrease in the dark (of about 10%), which is associated with the adsorption of the SA molecules onto the surface of the TiO2 particles and also to the entrapment of these molecules in the PDMS matrix. There was no degradation of SA associated with photolysis, in the absence of TiO2 . The values from Fig. 7 show that the rubber with 10 phr of TiO2 presented a decrease of 20% in TOC values, while for rubbers with 30 or 40 phr of TiO2 a TOC decrease of 15% was observed. These results indicate a similar efficiency of TiO2 /PDMS10, TiO2 /PDMS30 and TiO2 /PDMS40 rubbers for the photodegradation of SA, which is in accordance with the similar swelling behavior in SA solution presented by these rubbers (Fig. 4(b)). Also in agreement with the swelling data, the TiO2 /PDMS78 rubber, having the highest MC and SR values, presented the best photocatalytic performance, promoting the photodegradation of
56% of the salicylic acid in aqueous solution. These results also show that when TiO2 is incorporated in elastomeric matrices, swelling measurements can be a useful tool for predicting the best candidate material for photocatalysis, since the parameters MC and nFR were shown to influence the sorption and diffusion of the
399
Fig. 7. Decrease in TOC as a function of the quantity of TiO2 in the rubbers.
contaminated aqueous solution through the matrix and the consequent access of the pollutant molecules to the photoactive sites of TiO2 . It is well known that the primary step of the photocatalytic process consists of electron (e− ) and hole (h+ ) generation taking place when photons with energies higher than the band gap are adsorbed. These charge carriers can undergo recombination, can be trapped by suitable hole–electron scavengers or surface defects, or react with organic compounds present. There is agreement that the main oxidizing agent involved in photodegradation reactions of organic compounds is the hydroxyl radical, • OH. These species can be generated by reaction of the holes with OH surface groups and of the electrons with adsorbed O2 followed by an attack of water [24]. When salicylic acid is the organic pollutant, the chain photooxidation reactions can also be initiated by direct reaction of the holes with the TiO2 -chemisorbed salicylate complexes. The adsorption and photooxidation of salicylic acid on dispersed TiO2 , with the formation of different titanium(IV) salicylate surface complexes has been reported [25,26]. These results suggested that the overall photooxidation rate of salicylic acid depends on the contributions due to all hydroxyl groups at the TiO2 surface and also to the contributions of all titanium(IV) salicylate surface complexes. The following complex is expected to be the most reactive one [25].
As a consequence, the good swelling properties in relation to organics and the uniform distribution of the TiO2 filler in the PDMS matrix make the TiO2 /silicone rubbers promising materials to be prepared on a large scale as coatings, self-supported films or in different sizes and shapes with great photocatalytic activities in decontamination processes.
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4. Conclusions Easily molded and shaped TiO2 /PDMS rubbers with good thermal stability and appropriate moduli were prepared. The increase in the amount of TiO2 in the PDMS matrix promoted a decrease in the crosslinking density of the matrix and a higher swelling of the rubbers. TiO2 /PDMS78 rubber promoted the photodegradation of 56% of the salicylic acid in aqueous solution and showed good photocatalytic performance for the decontamination of water. Acknowledgments This study was supported by FAPESP (processes 03/09926-1; 04/04810-8), CNPQ (processes 142432/2006-7; 305916/2006-8), and CAPES (V.P. Silva fellowship). The authors also thank Dow Corning for providing polymers and Prof. Carol Collins for revising the English version of this manuscript. References [1] [2] [3] [4] [5] [6]
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Silicone rubbers filled with TiO2 : Characterization and photocatalytic activity V.P. Silva, M.P. Paschoalino, M.C. Gonc¸alves, M.I. Felisberti, W.F. Jardim, I.V.P. Yoshida ∗ Instituto de Química, Universidade Estadual de Campinas, 13083-970 Campinas, São Paulo, Brazil
a r t i c l e
i n f o
Article history: Received 28 February 2008 Received in revised form 16 June 2008 Accepted 27 July 2008 Keywords: TiO2 Silicone Rubbers Photoactivity
a b s t r a c t Compounding of silicone rubbers with TiO2 was carried out in a two-roll mill from a mixture of TiO2 and poly(dimethylsiloxane) (PDMS) with several other additives. The silicone matrix was crosslinked by a platinum-catalyzed hydrosilylation reaction. Characterization of the rubbers was performed by thermogravimetric and dynamic mechanical analyses, swelling measurements in toluene and in a salicylic acid/water solution, UV–vis spectroscopy and field emission scanning electron microscopy. The photocatalytic activity of the rubbers was evaluated with respect to the photodegradation of salicylic acid in aqueous solution under UV radiation, by measuring total organic carbon. The thermal stability of the PDMS matrix was enhanced by the addition of 10 phr (parts per hundred parts of rubber, by weight) of TiO2 , but a further increase in the quantity of TiO2 favored thermal degradation of the matrix. The stiffening effect promoted by the incorporation of TiO2 into the matrix was more pronounced at higher TiO2 amount. The increase in the quantity of TiO2 also promoted a decrease in the crosslinking density of the matrix, favoring the swelling of the rubbers. The rubber with 78 phr of TiO2 degraded 56% of the salicylic acid from an aqueous solution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide has been extensively investigated as a heterogeneous photocatalyst for water purification. However, since this photocatalyst is often applied as a suspension, problems associated with catalyst leaching, flotation and the need for eventual catalyst separation by filtration during post-treatment, make its large-scale application difficult and expensive [1]. In order to solve these problems, research on TiO2 immobilization in different supports has attracted wide attention [1–4], particularly using polymeric supports, which are easily molded in a variety of shapes. Jardim and coworkers [2] described the use of poly(dimethylsiloxane), PDMS, and orthophthalic polyesters, OP, as supports for TiO2 (Degussa P-25). OP/TiO2 films presented the most suitable characteristics to be used as a fixed bead in gas-phase reactors. Rizzo et al. [3] prepared an efficient photocatalytic reactor made of stacked poly(methylmethacrylate) (PMMA) rings coated with a thin-film of TiO2 for the removal of methylene blue from water. Most of the organic pollutants in the environment cannot be determined and/or degraded without the application of concentration techniques. PDMS-based membranes have been successfully used in the concentration of organic traces in water [5–8], and in the pervaporative separation of organic contaminants from aque-
ous solutions [9,10], with good selectivity for organics removal. On the other hand, PDMS-based membranes are usually constituted by a crosslinked PDMS matrix loaded with filler, which can promote an appropriated mechanical reinforcement to these membranes. Although the most popular filler in PDMS membranes is SiO2 , these membranes can also be filled with TiO2 photocatalyst. Thus, PDMS matrices are advantageous for the incorporation of TiO2 , in relation to other polymeric supports used in photocatalysis, due to the high flexibility and mobility of the PDMS chains and their consequent high permeability to gases [11,12] and some organic volatiles [13–16]. These characteristics are important to the diffusion of organic pollutants into the polymeric material. In addition, PDMS is also transparent to UV–vis radiation, which allows access of this radiation to the TiO2 photoactive sites incorporated into the PDMS matrix [1]. In this study, rubbers obtained by the incorporation of TiO2 into PDMS gum, crosslinked by a hydrosilylation reaction, were evaluated in relation to their photocatalytic activities, with respect to the photodegradation of salicylic acid in aqueous solution under UV radiation. These rubbers were also characterized in relation to their morphological, thermal, dynamic mechanical, swelling and UV–vis absorption properties. 2. Experimental 2.1. Materials
∗ Corresponding author. Tel.: +55 19 35213130; fax: +55 19 35213023. E-mail address: [email protected] (I.V.P. Yoshida). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.104
Titanium dioxide photocatalyst, P-25 TiO2 , with an anatase/rutile ratio of about 80/20 and mean primary particle size of about 21 nm, was purchased from Degussa
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V.P. Silva et al. / Materials Chemistry and Physics 113 (2009) 395–400
(São Paulo, SP, Brazil). Poly(dimethylsiloxane), PDMS, with an average molar mass of about 106 g mol−1 and a (SiCH3 CH CH2 O)/(Si(CH3 )2 O) molar ratio of <1/1000, used as matrix for the rubbers, 1,3,5,7-tetramethyl,1,3,5,7-tetravinylcyclotetrasiloxane, D4 V, utilized as crosslinking inhibitor, poly(methylsiloxane), PMS, with an average molar mass of 5096 g mol−1 , used as crosslinker, and the catalyst complex platinum divinyltetramethyldisiloxane were all supplied by Dow Corning (Hortolândia, SP, Brazil). Toluene, used in the swelling measurements, and salicylic acid, SA, used in the swelling and photocatalysis experiments, were both acquired from Synth (Diadema, SP, Brazil). 2.2. Compounding procedure Unfilled PDMS and TiO2 /PDMS rubbers were prepared by using 0, 10, 30, 40 and 78 phr (parts per hundred parts of rubber, by weight) of TiO2 , 2 phr of D4 V, 1 phr of platinum catalyst and 6 phr of PMS. Compounding was carried out in a Copé open two-roll mill for 20 min at 25 ◦ C. The rotors were operated at a speed ratio of 1:1.4. The compounds were compression molded in a Parabor PL350 at 155 ◦ C and 22 MPa for 10 min, followed by a post-cure treatment in a vacuum oven at 120 ◦ C for 8 h. 2.3. Characterization The morphology of the rubbers was investigated by field emission scanning electron microscopy (FESEM), performed in a JEOL JSM-6340 F microscope operated at 3 kV. FESEM samples were cryogenically fractured and the fracture surface was sputter-coated with very thin carbon and gold layers. Thermal stability of the rubbers was analyzed by thermogravimetric analysis (TGA) in a TA 2950 thermobalance from 25 to 1000 ◦ C, at a heating rate of 20 ◦ C min−1 under an argon flow. Dynamic mechanical analysis (DMA) was carried out in a Rheometric Scientific DTMA V, in a temperature range between −135 and 40 ◦ C, at a frequency of 1 Hz, and a heating rate of 2 ◦ C min−1 . Swelling measurements were performed in toluene and in aqueous solutions of SA, following ASTM D471 [16]. Rubber strips with dimensions of (30 mm × 10 mm × 2 mm) were weighed and immersed in 25 cm3 of the solvent, and, at regular intervals, the surface of the swollen strip was gently dried with a filter paper, and the weight of the sample was registered. The above procedure was repeated until a constant swollen weight was obtained. Afterwards, samples were dried in a vacuum oven at 50 ◦ C for 24 h and weighed again. For each rubber, three samples were tested and the average results are reported. The diffuse reflectance UV–vis spectra were recorded with a Cary 5G UV–vis-NIR spectrophotometer, using Teflon as reference. The photocatalytic activity of the rubbers was evaluated with respect to the photodegradation of SA in aqueous solution (3.6 10−4 mol L−1 ) under UV radiation (Sankyo 30 W black light lamp, = 365 nm) for 2 h by TOC (total organic carbon) measurements in a Shimadzu TOC-5000. In duplicate for each rubber, samples perfectly fitted into Petri dishes were covered by the SA solution or by water (in order to evaluate possible carbon leaching from the matrix). Petri dishes containing only the SA solution (for photolysis evaluation) were also tested. All dishes were kept below the UV lamp for 2 h, according to the schematic experimental setup shown in Fig. 1. Dishes with the rubbers containing the SA solution were also kept in the dark, in order to evaluate the SA adsorption effect.
3. Results and discussion The compounding process used to prepare silicone rubbers filled with TiO2 is easy and appropriate to disperse variable amounts of
Fig. 1. Schematic of the experimental setup used in the photocatalysis experiment showing (a) UV lamp and Petri dishes containing; (b) rubbers covered by the SA solution; (c) rubbers covered by water and (d) dishes with only the SA solution.
this filler into a high viscosity PDMS matrix (PDMS gum). The morphology of the rubbers with 10, 30 or 40 phr of TiO2 (TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40, respectively) was similar, characterized by agglomerates of TiO2 particles, expressed by white points with sizes ranging between 60 and 200 nm, uniformly dispersed in the PDMS matrix, as can be seen in the micrograph of TiO2 /PDMS40 in Fig. 2(a). For the rubber with 78 phr of TiO2 , TiO2 /PDMS78, larger agglomerates of TiO2 particles, in the range of 60–1000 nm, were observed, as shown in Fig. 2(b). All the rubbers presented good adhesion between the PDMS matrix and the TiO2 agglomerates, which is an important aspect for a successful application of these rubbers in photocatalysis, because poor adhesion may cause detachment of TiO2 particles from the PDMS matrix, leading to the TiO2 catalyst leaching into the aqueous phase and resulting in a loss of activity during reuse of these rubbers [2]. TGA and differential thermogravimetric (DTG) curves for the TiO2 /PDMS rubbers can be seen in Fig. 3(a) and (b), respectively. It is well known that the thermal degradation of end-blocked PDMS in inert atmosphere occurs by depolymerization over the range of 400–650 ◦ C to produce cyclic oligomers [17]. The effect of crosslinking during the thermal degradation of the unfilled PDMS can be ignored owing to its very low crosslinking density. In earlier studies [18], it has been found that the occurrence of intra and intermolecular rearrangements, through cyclic transition state, can explain the elimination of cyclic dimethylsiloxane and the shortening of the residual chain length until it is too short to cyclize. This process can be associated with the first step of the degradation process of the unfilled PDMS, with maximum weight loss rate temperature (Tmax ) at 475 ◦ C, as can be seen in Fig. 3(b). Following this step,
Fig. 2. FESEM micrograph of (a) TiO2 /PDMS40 and (b)TiO2 /PDMS78.
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397
Fig. 3. (a) TGA and (b) DTG curves for unfilled PDMS, TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40 and TiO2 /PDMS78.
the second peak with Tmax at 554 ◦ C can be related to the evaporation of the shortened residual linear fragments of the PDMS chains, which competes with the cyclization. This second degradation step can also be associated with the degradation of the PDMS network domains with higher crosslinking density, which restrict the mobility of the PDMS chains and make difficult their intra or intermolecular rearrangements [19]. The presence of TiO2 in the PDMS matrix resulted in multiple degradation steps. This behavior is related to a change in the thermal degradation mechanism of the PDMS in the presence of TiO2 . In addition, the barrier effect of the TiO2 particles on the diffusion of volatiles may have promoted the shift of the main weight loss processes to higher temperatures, as compared to the unfilled PDMS, and the overlap of different processes. For the rubbers filled with TiO2 , the amount of residue at ∼900 ◦ C was higher than the TiO2 amount added to the PDMS matrix. For example, the sample containing 10 phr of TiO2 (8 wt.%) produced 49% of residue and that one containing 40 phr of TiO2 (27 wt.%) produced 63% of residue, which can be associated with the formation of SiCx Oy residue [20]. These results reinforce the hypothesis of change of the degradation mechanism of the rubber matrix, probably catalyzed by a rich Ti–OH environment Swelling measurements of the rubbers were performed in toluene and in aqueous salicylic acid solutions. Toluene was used in order to obtain appropriate physical parameters such as the average molar mass between crosslinking points (MC ) and the crosslinking density of the polymeric network (nFR ). These parameters can be useful for the interpretation of the photocatalysis data. Swelling of the PDMS rubbers in the salicylic acid solution was studied with the main objective of understanding the possible influence of swelling on the photocatalytic degradation of SA. The equilibrium swelling ratio was determined by the balance between the osmotic pressure of the solvent, which forces the polymeric chains to take an elongated conformation, and the elastic force of the polymeric network. Thus, the greater the crosslinking density of the network, the smaller the average molar mass between crosslinking points and the lower the swelling [21]. The swelling ratio (SR ) values of the PDMS rubbers in toluene and in SA solution were calculated using Eq. (1). Because TiO2 particles do not swell, the swelling ratio of the polymeric matrix alone was
calculated discounting the amount of TiO2 in each sample [21], SR =
[mS − (˛filler md )] [˛pol md ]
(1)
where ms is the weight of the swollen sample at equilibrium, md is the weight of the dried sample after swelling, and ˛filler and ˛pol are the TiO2 and polymer weight fractions, respectively. The curves of SR as a function of time for toluene and SA solution are shown in Fig. 4(a) and (b), respectively. For the swelling in toluene, an increase in SR with an increase in the amount of TiO2 was observed. As expected, swelling of all these rubbers in SA solution was very small, due to the low affinity of the siloxane matrix (hydrophobic) for the aqueous solution, but a higher SR for the rubber with 78 phr of TiO2 was observed, as compared to the other rubbers, probably associated with higher water adsorption in the larger TiO2 agglomerates of TiO2 /PDMS78 [22]. The average molar mass between crosslinking points (MC ) was calculated using the Flory–Rehner equation (Eq. (2)) for a tetrafunctional polymeric network [21],
MC =
1/3
−dpol Vsol pol − pol /2
2 ] [ln(1 − pol ) + pol + pol
(2)
where dpol is the density of the PDMS matrix (0.93 g cm−3 ), Vsol is the molar volume of toluene (105.91 cm3 mol−1 ), is the Flory–Huggins polymer-solvent interaction parameter (0.35) and ˚pol is the volume fraction of the polymer in the swollen sample at equilibrium [21]. ˚pol was calculated using the following equation [21]: dpol 1 = 1 + SR(eq) pol dsol
(3)
With the MC values obtained for each sample, the crosslinking density of the polymeric network (nFR ) was calculated using the following equation [21]: nFR =
dpol MC
(4)
The SR(eq) , MC , and nFR values calculated from swelling measurements of the samples in toluene are presented in Table 1.
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Fig. 4. Variation of the swelling ratio (SR ) with time for swelling of unfilled PDMS, TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40 and TiO2 /PDMS78 in (a) toluene and (b) salicylic acid solution.
According to these results, an increase in the amount of TiO2 promoted a decrease in the crosslinking density of the matrix, which contributed to an increase in the MC and SR(eq) values. This can be explained by the occurrence of condensation reactions between the Ti–OH groups at the TiO2 surface and the Si–H groups on the crosslinker chains, as described in the reaction illustrated below, which reduces the availability of the Si–H groups for matrix crosslinking reactions. The data in Table 1 also suggests that the introduction of 10 phr of TiO2 in the PDMS matrix was not enough to significantly disturb the crosslinking density of this matrix. Ti–OH + H–Si
→
Ti–O–Si
region plateau and can be attributed to the hydrodynamic effect arising from the inclusion of rigid TiO2 particles in the matrix [23], despite the decrease in the crosslinking density of the matrix, which contributes to the reduction in E values at the elastic region plateau. UV–vis absorbance spectra of powdered TiO2 and of the rubbers (Fig. 6) revealed a small absorption for the unfilled rubber in the range of around 250–400 nm, associated with the PDMS matrix,
+ H2
In addition, a higher quantity of TiO2 promoted physical restrictions to the hydrosilylation reaction between PDMS gum and the PMS crosslinker. Si–CH CH2 + H–Si
→
Si–CH2 CH2 –Si
The storage modulus (E ) curve for the unfilled PDMS (Fig. 5) showed a little pronounced drop at approximately −120 ◦ C related to the glass transition and a more pronounced drop at about −40 ◦ C attributed to the melting process. These transitions were not affected by the presence of TiO2 , however, a slight increase in the modulus values from ∼−130 ◦ C to −50 ◦ C with the increase in the TiO2 amount was observed, suggesting a reinforcing effect of the filler. This stiffening effect was more pronounced at the elastic Table 1 Equilibrium swelling ratio (SR(eq) ), crosslinking density (nFR ) and average molar mass (MC ) between crosslinking points, for swelling of the rubbers in toluene Sample
SR(eq)
Unfilled-PDMS TiO2 /PDMS10 TiO2 /PDMS30 TiO2 /PDMS40 TiO2 /PDMS78
3.74 3.67 4.12 4.66 8.46
nFR (10−4 mol cm−3 ) ± ± ± ± ±
0.00 0.01 0.02 0.01 0.01
1.8 1.8 1.4 1.2 0.4
MC (g mol−1 ) 5247 5247 6574 7433 21,926
Fig. 5. Variation of storage modulus (E ) with temperature for unfilled PDMS, TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40 and TiO2 /PDMS78.
V.P. Silva et al. / Materials Chemistry and Physics 113 (2009) 395–400
Fig. 6. UV–vis absorbance spectra for powdered TiO2 , unfilled PDMS, TiO2 /PDMS10, TiO2 /PDMS30, TiO2 /PDMS40 and TiO2 /PDMS78.
which promoted a higher absorbance of the rubbers with TiO2 in this range, as compared to the powdered TiO2 . On the other hand, the increase in the quantity of TiO2 promoted no significant differences in the absorption of the rubbers with this filler. Fig. 7 shows the values of TOC decrease as a function of the quantity of TiO2 in the rubbers after the photocatalysis experiments. These values were calculated by subtracting the TOC decrease in the dark (of about 10%), which is associated with the adsorption of the SA molecules onto the surface of the TiO2 particles and also to the entrapment of these molecules in the PDMS matrix. There was no degradation of SA associated with photolysis, in the absence of TiO2 . The values from Fig. 7 show that the rubber with 10 phr of TiO2 presented a decrease of 20% in TOC values, while for rubbers with 30 or 40 phr of TiO2 a TOC decrease of 15% was observed. These results indicate a similar efficiency of TiO2 /PDMS10, TiO2 /PDMS30 and TiO2 /PDMS40 rubbers for the photodegradation of SA, which is in accordance with the similar swelling behavior in SA solution presented by these rubbers (Fig. 4(b)). Also in agreement with the swelling data, the TiO2 /PDMS78 rubber, having the highest MC and SR values, presented the best photocatalytic performance, promoting the photodegradation of
56% of the salicylic acid in aqueous solution. These results also show that when TiO2 is incorporated in elastomeric matrices, swelling measurements can be a useful tool for predicting the best candidate material for photocatalysis, since the parameters MC and nFR were shown to influence the sorption and diffusion of the
399
Fig. 7. Decrease in TOC as a function of the quantity of TiO2 in the rubbers.
contaminated aqueous solution through the matrix and the consequent access of the pollutant molecules to the photoactive sites of TiO2 . It is well known that the primary step of the photocatalytic process consists of electron (e− ) and hole (h+ ) generation taking place when photons with energies higher than the band gap are adsorbed. These charge carriers can undergo recombination, can be trapped by suitable hole–electron scavengers or surface defects, or react with organic compounds present. There is agreement that the main oxidizing agent involved in photodegradation reactions of organic compounds is the hydroxyl radical, • OH. These species can be generated by reaction of the holes with OH surface groups and of the electrons with adsorbed O2 followed by an attack of water [24]. When salicylic acid is the organic pollutant, the chain photooxidation reactions can also be initiated by direct reaction of the holes with the TiO2 -chemisorbed salicylate complexes. The adsorption and photooxidation of salicylic acid on dispersed TiO2 , with the formation of different titanium(IV) salicylate surface complexes has been reported [25,26]. These results suggested that the overall photooxidation rate of salicylic acid depends on the contributions due to all hydroxyl groups at the TiO2 surface and also to the contributions of all titanium(IV) salicylate surface complexes. The following complex is expected to be the most reactive one [25].
As a consequence, the good swelling properties in relation to organics and the uniform distribution of the TiO2 filler in the PDMS matrix make the TiO2 /silicone rubbers promising materials to be prepared on a large scale as coatings, self-supported films or in different sizes and shapes with great photocatalytic activities in decontamination processes.
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4. Conclusions Easily molded and shaped TiO2 /PDMS rubbers with good thermal stability and appropriate moduli were prepared. The increase in the amount of TiO2 in the PDMS matrix promoted a decrease in the crosslinking density of the matrix and a higher swelling of the rubbers. TiO2 /PDMS78 rubber promoted the photodegradation of 56% of the salicylic acid in aqueous solution and showed good photocatalytic performance for the decontamination of water. Acknowledgments This study was supported by FAPESP (processes 03/09926-1; 04/04810-8), CNPQ (processes 142432/2006-7; 305916/2006-8), and CAPES (V.P. Silva fellowship). The authors also thank Dow Corning for providing polymers and Prof. Carol Collins for revising the English version of this manuscript. References [1] [2] [3] [4] [5] [6]
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