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Humidity and pH sensor based on sulfonated poly-{styrene–acrylic acid} polymer.
Materials Science and Engineering C 29 (2009) 599–601
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m s e c
Humidity and pH sensor based on sulfonated poly-{styrene–acrylic acid} polymer. Synthesis and characterization Luciano da Silva a, Fernando Effiting da Silva a, César Vitório Franco b, Rafael Bianchini Nuernberg c, Thauan Gomes c, Rodrigo Miranda c, Marcos Marques da Silva Paula c,⁎ a b c
Laboratório de Pesquisa em Energia – LAPEN – Universidade do Vale do Itajaí, 88122-000, São José, Brazil Laboratório de Síntese Inorgânica e Materiais Nanoparticulados – NanoSiN – Universidade Federal de Santa Catarina, 88000 000, Florianópolis, Brazil Laboratório de Síntese de Complexos Multifuncionais – PPGCS – Universidade do Extremo Sul Catarinense, 88806-000, Criciúma, Brazil
a r t i c l e
i n f o
Article history: Received 29 May 2008 Received in revised form 27 September 2008 Accepted 7 October 2008 Available online 18 October 2008 Keywords: pH sensor Ion conducting polymer Humidity sensor Polystyrene sulfonated Acrylic acid
a b s t r a c t Solid polymer electrolytes have been developed to interact with several different substances. In particular, humidity sensors and ionic conductivity have recently attracted increased attention due to their applications in food quality control and storage as well as environment humidity for air conditioning systems. In this work we report the preparation and characterization of sulfonated poly-{styrene–acrylic acid} to assess studies as humidity and pH sensors. Copolymers poly-{styrene–acrylic acid} were synthesized employing different styrene/acrylic acid monomer ratios. The copolymers were sulfonated using H2SO4 in CH2Cl2 as described elsewhere. Conductivity measurements were performed at different values of relative humidity (RH%). In a 10–100 RH% range the conductivity showed linear dependence with values ranging from 5.7 × 10− 2 S to 1.0 × 10− 1 S. The pH vs. λmax dependence can be observed at the maximum absorption band with changes from 435 nm (pH 2.6) to 460 nm (pH 12.0). However, in the pH 5.5 to 9.0 range, the λmax is constant and close to 444 nm. These results suggest the possible use of this material as a sensor in food quality control. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polymer electrolytes that perform the function of humidity sensors have been largely used in many areas such as health care, the processing industry, environmental monitoring, storage, and others. Polymeric sensitive materials have been employed in humidity sensor devices for a long time [1,2]. This class of sensors which are mainly based on polyelectrolytes and doped conjugated polymers, has become one of the most important humidity sensors, with the advantages of easy preparation, low cost, fast response and good compatibility [3–7]. Conducting polymers are promising materials for the humidity sensor, [8] however they have shown good performance in only a certain range of relative humidity. For the measurement of high relative humidity, the stability of sensors is usually a problem, as well as the chemical modifications of the polymer sensitive materials [9]. It is known that polyelectrolytes have high solubility in water due to the presence of ionic groups, therefore it is expected that such materials would be “soluble” under high humidity. Otherwise, such humidity sensitive properties soon deteriorate [10,11]. This problem has been solved by introducing hydrophobic monomers [12–14] that ⁎ Corresponding author. PPGCS – Laboratório de Síntese de Complexos Multifuncionais, UNESC, Av. Universitária, 1105, 88806-000, Criciúma, SC, Brazil. Tel.: +55 48 3431 2577; fax: +55 48 3431 2750. E-mail address: [email protected] (M.M. da Silva Paula). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.10.018
build a cross-link reducing water solubility and allowing for better durability and stability of sensitive properties [15,16]. Furthermore there are conducting polymers that are able to absorb water, thus changing their conductivity, giving those materials the features of a humidity sensor. These features have been widely used in many areas, such as health care, the processing industry, environmental monitoring, and storage. In spite of their prominent advantages like low cost, easy preparation and ready modulation of the sensitive properties, some drawbacks still remain, including long response time, large hysteresis, limited sensing range, poor durability in high humidity, and drift humidity response [17]. Therefore, the synthesis of poly(styrene–co-acrylic acid) is proposed in different molar ratios and different conditions of sulfonation, thus seeking the control of water absorption by increasing the sulfonation degree. The increase of acrylic acid content also amplifies the pH sensor characteristic. To improve the mechanical properties divinyl benzene could be incorporated in the polymeric chain network. The formed cross-link network aids in reducing the solubility without entirely affecting the water uptake. 2. Experimental 2.1. Materials The monomers styrene and acrylic acid were used without previous purification and were kindly donated by RESICOLOR. Benzoyl
600
L. da Silva et al. / Materials Science and Engineering C 29 (2009) 599–601
peroxide was purchased from ALDRICH. Solvents were purchased from different providers. Concentrate sulfuric acid (96%) was employed as a sulfonating agent. The UV–Visible measurements were performed in an Ocean Optics USB 4000 and vibrational spectra in a FTIR Prestige-21 spectrophotometer. Conductivity and pH measurements were performed with a Multimeter fluke-10 and pH meter PHS3B, respectively. 2.2. Synthesis of polymers Poly-{styrene–acrylic acid} copolymers were synthesized employing monomers styrene/acrylic acid at various molar ratios (95:5, 92:8, 90:10, 88:12, 85:15). Benzoyl peroxide was added in the monomer to the reactional medium and was maintained under vigorous stirring at 110 °C, under inert atmosphere of argon for 2 h. Then, the product was dissolved in acetone and precipitated in methanol. The solid product was collected, filtered and dried under vacuum at 70 °C. Fig. 2. Conductivity vs. RH% plot to 92:8 poly-{styrene sulfonated–co-acrylic acid} at pH 3.0. The conductivity values correspond to the average of three measurements (n = 3).
2.3. Sulfonation of poly-{styrene–acrylic acid} The copolymer was dissolved in CH2Cl2 and poured in a reactor. Then 15 ml H2SO4 was added drop wise and the reaction was maintained for 90 min, at 40 °C in inert atmosphere. After this, water was added into the reactional medium to quench the reaction. The product was collected by precipitation as a yellow solid stuff. The sulfonated copolymer was washed until the pH of the collected washed water was neutral, then dried and purified by dissolution in acetone and re-precipitated in acetonitrile. Fig. 1 shows the schematic synthesis reactions. 2.4. Characterization Among the obtained copolymers only one was chosen as the optimal material to assess the sensor properties. In this step the solubility of copolymers in dichloromethane (solvent used in sulfonation) was performed. The characterization of the sulfonated copolymer was made by FTIR and 1H NMR spectroscopy. Conductivity measurements were performed using a two-probe technique, with a coupling multimeter at different relative humidities (RH%). Samples were dried under vacuum in a dissector. Before each measurement the samples were hydrated, weighed and their conductivity measured. Each measurement was performed three times (n = 3). To test the pH sensor performance of the copolymer the wavelength (λmax) of the material was performed in function of pH. The copolymer was initially acid, and dissolved in acetone (50%) and water (50%) and subjected to pH changes by adding NaOH (0.1 M). Monitoring was done with a pH meter PHS-3B instrument. The sulfonation degree was estimated by volumetric titration with sodium
Fig. 3. Conductivity vs. RH% plot to 92:8 poly-{styrene sulfonated–co-acrylic acid} at pH 12.0. The conductivity values correspond to the average of three measurements (n = 3).
hydroxide solution (0.05 M). The sulfonation degree (mol%) was defined by equation described elsewhere [18]. 3. Results and discussion Poly-{styrene–acrylic acid} copolymers were synthesized employing monomers styrene/acrylic acid at various molar ratios. The copolymers
Fig. 1. Schematic synthesis reactions.
L. da Silva et al. / Materials Science and Engineering C 29 (2009) 599–601
Fig. 4. The λmax vs. pH plot to 92:8 sulfonated poly-{styrene–acrylic acid}.
with 85:15 styrene/acrylic acid, 88:12 styrene/acrylic acid and 90:10 styrene/acrylic acid ratios were discarded because they showed poor solubility in the sulfonation reactional medium. Copolymer 92:8 styrene/acrylic acid ratio presented the best performance in mechanical properties, solubility and was then made appropriate for sulfonation. The vibrational FTIR transmission spectra of sulfonated poly-{styrene– acrylic acid} polymer showed a strong −SO3H stretch at 1060 cm− 1, indicating that sulfonyl groups are present in the polymeric chain. In addition, a strong absorption band at 1785 cm− 1 corresponding to the carbonyl stretch and a very weak absorption band at 1450 cm− 1 associated with the O–H stretch in −CO2H were observed. In comparison, the pH dependence on the FTIR transmission spectra was made. In pH 3, the spectra showed absorption bands associated with the carboxylic acid and sulfonyl groups, a very strong H3O+ absorption at ~1700 cm− 1, and a broad band centered at 2210 cm− 1 likely associated with water. Besides these peaks, there were no remarkable features in the spectra at the basic pH range. The sulfonated poly-{styrene–acrylic acid} copolymer was effective as a humidity sensor since according to the amount of the water uptake the conductivity increases in a linear fashion. The optimum sulfonation degree for the tested copolymer was estimated as 20.8% (molar percentage yield). Fig. 2 shows the ionic conductivity dependence as a function of RH% and which was normalized to the maximum water uptake at pH 3.0. In the 10–100 RH% range the conductivity showed linear dependence changing from 5.7 × 10− 1 S to 1.0 × 10-2S. The equation corresponding to the best straight calculations by linear fitting shows slope = 0.55 and linear coefficient = 51.06. The correlation coefficient is 0.99. The conductivity dependence of humidity profile climbs up in an abrupt way up to 10 RH% and then increases mildly. Fig. 3 shows the ionic conductivity dependence as a function of RH% at pH 12.0. In the 40–100 RH% range the conductivity showed linear dependence, with the equation corresponding to the best straight calculations by linear fitting showing slope = 0.13 and linear coefficient = 7.16 × 10− 4. The correlation coefficient is 0.99. Three samples of poly-{styrene–acrylic acid} underwent successive hydration and dehydration cycles, where initial and final mass
601
and respective conductivity were evaluated at each cycle. After 10 cycles, neither variation of initial mass was verified (0% RH) nor of final mass (100% RH) and electrical properties, indicating reversibility of the process. Stability studies, using a greater number of cycles at different temperatures are ongoing. Response time of the polymer to environmental humidity variations were shown to be dependent especially on mass and geometry of samples. To test the sulfonated poly-{styrene–acrylic acid} as pH sensor the pH vs. λmax dependence profile was performed and can be seen in Fig. 4. Bathochromic shift is observed in the maximum absorption band from 435 nm (pH 2.6) to 460 nm (pH 12.0). However, in the pH range from 5.5 to 9.0, the λmax is constant and close to 444 nm. These results strongly suggest the employment of this material as a pH sensor in environmental monitoring in a reversible way. The color change can be perceived without instrumental help when the polymer is submitted to a basic medium. The color slowly changes from green to pink in a reversible way. The copolymer 92:8 styrene/ acrylic acid ratio was tested in different pH before sulfonation and did not show any absorption band in the visible region. 4. Conclusion The copolymer sulfonated poly-{styrene–acrylic acid} was prepared in different monomeric ratios with good yield. Better results in the sensory properties were observed with a 92:8 styrene:acrylic acid ratio, suggesting the possibility of using this material as a pH and humidity sensor. Acknowledgments This study was supported by grants from Fundação de Apoio à Pesquisa Científica e Tecnológica do Estado de Santa Catarina (FAPESC) and by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The authors would also like to thank CNPq and UNESC for fellowship support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
C.Y. Lee, G.B. Lee, Sens. Lett. 3 (2005) 1. Z.M. Rittersma, Sens. Actuators A: Phys. 96 (2002) 196. P.G. Su, C.L. Uen, Talanta 66 (2005) 1247. C.W. Lee, S.W. Joo, M.S. Gong, Sens. Actuators B: Chem. 105 (2005) 150. I. Fratoddi, P. Altamura, A. Bearzotti, A. Furlani, M.V. Russo, Thin Solid Films 458 (2004) 292. G. Casalbore-Miceli, M.J. Yang, Y. Li, A. Zanelli, A. Martelli, S. Chen, Y. She, N. Camaioni, Sens. Actuators B: Chem. 114 (2006) 584. R. Nohria, R.K. Khillan, Y. Su, R. Dikshit, Y. Lvov, K. Varahramyan, Sens. Actuators B: Chem. 114 (2006) 218. Y. Li, Y. Chen, C. Zhang, T. Xue, M Yang, Sens. Actuators B: Chem. 125 (2007) 131. Y. Li, L. Hong, Y. Chen, H. Wang, X. Lu, M. Yang, Sens. Actuators B: Chem 123 (2007) 554. P.G. Su, C.L. Uen, Talanta 66 (2005) 1247. Z.W. Yao, M.J. Yang, Sens. Actuators B: Chem. 117 (2006) 93. C.W. Lee, O. Kim, M.S. Gong, J. Appl. Polym. Sci. 89 (2003) 1062. Y. Li, M.J. Yang, Y. She, Sens. Actuators B: Chem. 107 (2005) 252. M.S. Gong, S.W. Joo, B.K. Choi, J. Mater. Chem. 12 (2002) 902. M.S. Gong, S.W. Joo, B.K. Choi, Sens. Actuators B: Chem. 86 (2002) 81. S.H. Park, J.S. Park, C.W. Lee, M.S. Gong, Sens. Actuators B: Chem. 86 (2002) 68. A.L.A. Silva, I. Takase, R.P. Pereira, A.M. Rocco, Eur. Polym. J. 44 (2008) 1462. Y.A. Elabd, E. Napadensky, Polymer 45 (2004) 3037.
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m s e c
Humidity and pH sensor based on sulfonated poly-{styrene–acrylic acid} polymer. Synthesis and characterization Luciano da Silva a, Fernando Effiting da Silva a, César Vitório Franco b, Rafael Bianchini Nuernberg c, Thauan Gomes c, Rodrigo Miranda c, Marcos Marques da Silva Paula c,⁎ a b c
Laboratório de Pesquisa em Energia – LAPEN – Universidade do Vale do Itajaí, 88122-000, São José, Brazil Laboratório de Síntese Inorgânica e Materiais Nanoparticulados – NanoSiN – Universidade Federal de Santa Catarina, 88000 000, Florianópolis, Brazil Laboratório de Síntese de Complexos Multifuncionais – PPGCS – Universidade do Extremo Sul Catarinense, 88806-000, Criciúma, Brazil
a r t i c l e
i n f o
Article history: Received 29 May 2008 Received in revised form 27 September 2008 Accepted 7 October 2008 Available online 18 October 2008 Keywords: pH sensor Ion conducting polymer Humidity sensor Polystyrene sulfonated Acrylic acid
a b s t r a c t Solid polymer electrolytes have been developed to interact with several different substances. In particular, humidity sensors and ionic conductivity have recently attracted increased attention due to their applications in food quality control and storage as well as environment humidity for air conditioning systems. In this work we report the preparation and characterization of sulfonated poly-{styrene–acrylic acid} to assess studies as humidity and pH sensors. Copolymers poly-{styrene–acrylic acid} were synthesized employing different styrene/acrylic acid monomer ratios. The copolymers were sulfonated using H2SO4 in CH2Cl2 as described elsewhere. Conductivity measurements were performed at different values of relative humidity (RH%). In a 10–100 RH% range the conductivity showed linear dependence with values ranging from 5.7 × 10− 2 S to 1.0 × 10− 1 S. The pH vs. λmax dependence can be observed at the maximum absorption band with changes from 435 nm (pH 2.6) to 460 nm (pH 12.0). However, in the pH 5.5 to 9.0 range, the λmax is constant and close to 444 nm. These results suggest the possible use of this material as a sensor in food quality control. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polymer electrolytes that perform the function of humidity sensors have been largely used in many areas such as health care, the processing industry, environmental monitoring, storage, and others. Polymeric sensitive materials have been employed in humidity sensor devices for a long time [1,2]. This class of sensors which are mainly based on polyelectrolytes and doped conjugated polymers, has become one of the most important humidity sensors, with the advantages of easy preparation, low cost, fast response and good compatibility [3–7]. Conducting polymers are promising materials for the humidity sensor, [8] however they have shown good performance in only a certain range of relative humidity. For the measurement of high relative humidity, the stability of sensors is usually a problem, as well as the chemical modifications of the polymer sensitive materials [9]. It is known that polyelectrolytes have high solubility in water due to the presence of ionic groups, therefore it is expected that such materials would be “soluble” under high humidity. Otherwise, such humidity sensitive properties soon deteriorate [10,11]. This problem has been solved by introducing hydrophobic monomers [12–14] that ⁎ Corresponding author. PPGCS – Laboratório de Síntese de Complexos Multifuncionais, UNESC, Av. Universitária, 1105, 88806-000, Criciúma, SC, Brazil. Tel.: +55 48 3431 2577; fax: +55 48 3431 2750. E-mail address: [email protected] (M.M. da Silva Paula). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.10.018
build a cross-link reducing water solubility and allowing for better durability and stability of sensitive properties [15,16]. Furthermore there are conducting polymers that are able to absorb water, thus changing their conductivity, giving those materials the features of a humidity sensor. These features have been widely used in many areas, such as health care, the processing industry, environmental monitoring, and storage. In spite of their prominent advantages like low cost, easy preparation and ready modulation of the sensitive properties, some drawbacks still remain, including long response time, large hysteresis, limited sensing range, poor durability in high humidity, and drift humidity response [17]. Therefore, the synthesis of poly(styrene–co-acrylic acid) is proposed in different molar ratios and different conditions of sulfonation, thus seeking the control of water absorption by increasing the sulfonation degree. The increase of acrylic acid content also amplifies the pH sensor characteristic. To improve the mechanical properties divinyl benzene could be incorporated in the polymeric chain network. The formed cross-link network aids in reducing the solubility without entirely affecting the water uptake. 2. Experimental 2.1. Materials The monomers styrene and acrylic acid were used without previous purification and were kindly donated by RESICOLOR. Benzoyl
600
L. da Silva et al. / Materials Science and Engineering C 29 (2009) 599–601
peroxide was purchased from ALDRICH. Solvents were purchased from different providers. Concentrate sulfuric acid (96%) was employed as a sulfonating agent. The UV–Visible measurements were performed in an Ocean Optics USB 4000 and vibrational spectra in a FTIR Prestige-21 spectrophotometer. Conductivity and pH measurements were performed with a Multimeter fluke-10 and pH meter PHS3B, respectively. 2.2. Synthesis of polymers Poly-{styrene–acrylic acid} copolymers were synthesized employing monomers styrene/acrylic acid at various molar ratios (95:5, 92:8, 90:10, 88:12, 85:15). Benzoyl peroxide was added in the monomer to the reactional medium and was maintained under vigorous stirring at 110 °C, under inert atmosphere of argon for 2 h. Then, the product was dissolved in acetone and precipitated in methanol. The solid product was collected, filtered and dried under vacuum at 70 °C. Fig. 2. Conductivity vs. RH% plot to 92:8 poly-{styrene sulfonated–co-acrylic acid} at pH 3.0. The conductivity values correspond to the average of three measurements (n = 3).
2.3. Sulfonation of poly-{styrene–acrylic acid} The copolymer was dissolved in CH2Cl2 and poured in a reactor. Then 15 ml H2SO4 was added drop wise and the reaction was maintained for 90 min, at 40 °C in inert atmosphere. After this, water was added into the reactional medium to quench the reaction. The product was collected by precipitation as a yellow solid stuff. The sulfonated copolymer was washed until the pH of the collected washed water was neutral, then dried and purified by dissolution in acetone and re-precipitated in acetonitrile. Fig. 1 shows the schematic synthesis reactions. 2.4. Characterization Among the obtained copolymers only one was chosen as the optimal material to assess the sensor properties. In this step the solubility of copolymers in dichloromethane (solvent used in sulfonation) was performed. The characterization of the sulfonated copolymer was made by FTIR and 1H NMR spectroscopy. Conductivity measurements were performed using a two-probe technique, with a coupling multimeter at different relative humidities (RH%). Samples were dried under vacuum in a dissector. Before each measurement the samples were hydrated, weighed and their conductivity measured. Each measurement was performed three times (n = 3). To test the pH sensor performance of the copolymer the wavelength (λmax) of the material was performed in function of pH. The copolymer was initially acid, and dissolved in acetone (50%) and water (50%) and subjected to pH changes by adding NaOH (0.1 M). Monitoring was done with a pH meter PHS-3B instrument. The sulfonation degree was estimated by volumetric titration with sodium
Fig. 3. Conductivity vs. RH% plot to 92:8 poly-{styrene sulfonated–co-acrylic acid} at pH 12.0. The conductivity values correspond to the average of three measurements (n = 3).
hydroxide solution (0.05 M). The sulfonation degree (mol%) was defined by equation described elsewhere [18]. 3. Results and discussion Poly-{styrene–acrylic acid} copolymers were synthesized employing monomers styrene/acrylic acid at various molar ratios. The copolymers
Fig. 1. Schematic synthesis reactions.
L. da Silva et al. / Materials Science and Engineering C 29 (2009) 599–601
Fig. 4. The λmax vs. pH plot to 92:8 sulfonated poly-{styrene–acrylic acid}.
with 85:15 styrene/acrylic acid, 88:12 styrene/acrylic acid and 90:10 styrene/acrylic acid ratios were discarded because they showed poor solubility in the sulfonation reactional medium. Copolymer 92:8 styrene/acrylic acid ratio presented the best performance in mechanical properties, solubility and was then made appropriate for sulfonation. The vibrational FTIR transmission spectra of sulfonated poly-{styrene– acrylic acid} polymer showed a strong −SO3H stretch at 1060 cm− 1, indicating that sulfonyl groups are present in the polymeric chain. In addition, a strong absorption band at 1785 cm− 1 corresponding to the carbonyl stretch and a very weak absorption band at 1450 cm− 1 associated with the O–H stretch in −CO2H were observed. In comparison, the pH dependence on the FTIR transmission spectra was made. In pH 3, the spectra showed absorption bands associated with the carboxylic acid and sulfonyl groups, a very strong H3O+ absorption at ~1700 cm− 1, and a broad band centered at 2210 cm− 1 likely associated with water. Besides these peaks, there were no remarkable features in the spectra at the basic pH range. The sulfonated poly-{styrene–acrylic acid} copolymer was effective as a humidity sensor since according to the amount of the water uptake the conductivity increases in a linear fashion. The optimum sulfonation degree for the tested copolymer was estimated as 20.8% (molar percentage yield). Fig. 2 shows the ionic conductivity dependence as a function of RH% and which was normalized to the maximum water uptake at pH 3.0. In the 10–100 RH% range the conductivity showed linear dependence changing from 5.7 × 10− 1 S to 1.0 × 10-2S. The equation corresponding to the best straight calculations by linear fitting shows slope = 0.55 and linear coefficient = 51.06. The correlation coefficient is 0.99. The conductivity dependence of humidity profile climbs up in an abrupt way up to 10 RH% and then increases mildly. Fig. 3 shows the ionic conductivity dependence as a function of RH% at pH 12.0. In the 40–100 RH% range the conductivity showed linear dependence, with the equation corresponding to the best straight calculations by linear fitting showing slope = 0.13 and linear coefficient = 7.16 × 10− 4. The correlation coefficient is 0.99. Three samples of poly-{styrene–acrylic acid} underwent successive hydration and dehydration cycles, where initial and final mass
601
and respective conductivity were evaluated at each cycle. After 10 cycles, neither variation of initial mass was verified (0% RH) nor of final mass (100% RH) and electrical properties, indicating reversibility of the process. Stability studies, using a greater number of cycles at different temperatures are ongoing. Response time of the polymer to environmental humidity variations were shown to be dependent especially on mass and geometry of samples. To test the sulfonated poly-{styrene–acrylic acid} as pH sensor the pH vs. λmax dependence profile was performed and can be seen in Fig. 4. Bathochromic shift is observed in the maximum absorption band from 435 nm (pH 2.6) to 460 nm (pH 12.0). However, in the pH range from 5.5 to 9.0, the λmax is constant and close to 444 nm. These results strongly suggest the employment of this material as a pH sensor in environmental monitoring in a reversible way. The color change can be perceived without instrumental help when the polymer is submitted to a basic medium. The color slowly changes from green to pink in a reversible way. The copolymer 92:8 styrene/ acrylic acid ratio was tested in different pH before sulfonation and did not show any absorption band in the visible region. 4. Conclusion The copolymer sulfonated poly-{styrene–acrylic acid} was prepared in different monomeric ratios with good yield. Better results in the sensory properties were observed with a 92:8 styrene:acrylic acid ratio, suggesting the possibility of using this material as a pH and humidity sensor. Acknowledgments This study was supported by grants from Fundação de Apoio à Pesquisa Científica e Tecnológica do Estado de Santa Catarina (FAPESC) and by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The authors would also like to thank CNPq and UNESC for fellowship support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
C.Y. Lee, G.B. Lee, Sens. Lett. 3 (2005) 1. Z.M. Rittersma, Sens. Actuators A: Phys. 96 (2002) 196. P.G. Su, C.L. Uen, Talanta 66 (2005) 1247. C.W. Lee, S.W. Joo, M.S. Gong, Sens. Actuators B: Chem. 105 (2005) 150. I. Fratoddi, P. Altamura, A. Bearzotti, A. Furlani, M.V. Russo, Thin Solid Films 458 (2004) 292. G. Casalbore-Miceli, M.J. Yang, Y. Li, A. Zanelli, A. Martelli, S. Chen, Y. She, N. Camaioni, Sens. Actuators B: Chem. 114 (2006) 584. R. Nohria, R.K. Khillan, Y. Su, R. Dikshit, Y. Lvov, K. Varahramyan, Sens. Actuators B: Chem. 114 (2006) 218. Y. Li, Y. Chen, C. Zhang, T. Xue, M Yang, Sens. Actuators B: Chem. 125 (2007) 131. Y. Li, L. Hong, Y. Chen, H. Wang, X. Lu, M. Yang, Sens. Actuators B: Chem 123 (2007) 554. P.G. Su, C.L. Uen, Talanta 66 (2005) 1247. Z.W. Yao, M.J. Yang, Sens. Actuators B: Chem. 117 (2006) 93. C.W. Lee, O. Kim, M.S. Gong, J. Appl. Polym. Sci. 89 (2003) 1062. Y. Li, M.J. Yang, Y. She, Sens. Actuators B: Chem. 107 (2005) 252. M.S. Gong, S.W. Joo, B.K. Choi, J. Mater. Chem. 12 (2002) 902. M.S. Gong, S.W. Joo, B.K. Choi, Sens. Actuators B: Chem. 86 (2002) 81. S.H. Park, J.S. Park, C.W. Lee, M.S. Gong, Sens. Actuators B: Chem. 86 (2002) 68. A.L.A. Silva, I. Takase, R.P. Pereira, A.M. Rocco, Eur. Polym. J. 44 (2008) 1462. Y.A. Elabd, E. Napadensky, Polymer 45 (2004) 3037.