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Shape memory-enhanced water sensing of conductive polymer composites
Materials Letters 161 (2015) 189–192
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Shape memory-enhanced water sensing of conductive polymer composites Hongsheng Luo n, Yuanyuan Ma, Wenjin Li, Guobin Yi n, Xiaoling Cheng, Wenjin Ji, Xihong Zu, Shengjie Yuan, Jiahui Li Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 July 2015 Received in revised form 20 August 2015 Accepted 22 August 2015 Available online 24 August 2015
Electrically conductive polymer composites have been widely utilized as the sensory materials being capable of response to various types of stimuli. A shape memory-enhanced water sensing was unprecedentedly reported for polymer composites comprising of carbon nanotubes (CNTs) and shape memory polyurethane (SMPU). The morphological, thermal, mechanical and micro-structural properties of the CNT–SMPU composites were investigated. Two swelling processes were specifically designed to measure the water sensing of the composites. It was found that the shape memory thermo-mechanical programming enlarged magnitudes of the resistivity variation, which may be attributed to the combination of the swelling effect and the re-orientation/local movements of the CNTs along with stress/strain energy release. The findings may greatly benefit the applications of the smart polymers in the fields of sensory materials and flexible electronics. & 2015 Elsevier B.V. All rights reserved.
Keywords: Conductive Water sensing Shape memory Carbon nanotubes Composites
1. Introduction Polymeric composites containing various types of nanofillers have been widely explored towards implanting with the reinforced mechanics as well as novel functions [1,2]. The carbon nanotubes (CNTs) are one set of the most prominent candidates to construct functional polymer composites. The CNT-containing polymer composites commonly possess superior electrical conductivity, which have been applied as the sensory materials, such as gas [3], pressure [4] and strain [5] sensing. The combination of the shape memory polymers (SMPs) and nanofillers lead to shape memory polymer composites. The previous studies of the conductive shape memory polymer composites emphasized on the mechanical reinforcement and electro-responsiveness. However, there has been little literature concerning the stimuli sensing of the polymer composites by far. The electrical conductivity of the composites varies along with the shape changes in a typical shape memory thermo-mechanical programming. By virtue of the shape-dependent conductivity, we once exploited a novel temperature sensing silver nanowire-shape memory composites [1]. Herein, a shape memory-enhanced water sensing was unprecedentedly reported for CNT-containing polymer composites. Compared to the direct immersion into water, the composites n
Corresponding authors. Fax: þ 86 20 39322231. E-mail addresses: [email protected] (H. Luo), [email protected] (G. Yi).
http://dx.doi.org/10.1016/j.matlet.2015.08.105 0167-577X/& 2015 Elsevier B.V. All rights reserved.
experiencing shape memory programming exhibited enlarged magnitudes of the variation in resistivity. The findings may greatly benefit the development of the conductive shape memory polymer composites in the fields of the sensory materials and flexible electronics.
2. Experimental The CNT–SMPU composites were fabricated via transfer process according to our previous study [2], which was briefly described as below: multi-wall CNTs suspension in methanol was dip-coated onto a glass substrate before adding the SMPU solution in dimethylacetamide; the composite films were peeled off after solidification in vacuum. The composite films, cut into the size of 15 mm*3 mm*60 μm (length*width*thickness), were labeled as PUC-01, PUC-02 and PUC-03, representing for the CNTs contents of 5, 12 and 20 wt% respectively. The investigations with Scanning Electron Microscope (SEM, Hitachi, S-4800), tensile tests (SUST, China), micro-Raman spectrometer (JY HR800, excited by He Ne at 786 nm and a beam size of 1 μm) and Integrated Thermal Analyzer (STA 409 PC, NETZSCH) were performed. The composites in dry state were extended by different percentages at 90 °C at the rate of 10 mm/min. The variation of the electrical resistivity (Rs) during the immersion was recorded with an electrochemical workstation (CHI660D, ChenHua, PR China) under a constant
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H. Luo et al. / Materials Letters 161 (2015) 189–192
voltage of 1 V.
3. Results and discussion 3.1. The morphological, mechanical and structural investigations The CNT–SMPU composites were in bi-layer structure, consisting of the conductive layer constructed by the CNTs and the SMPU matrix (are shown in Fig. 1a and b). It was clearly found that the fibril-like CNTs network whose one part was embedded into the matrix meanwhile the other part was left on the surface, making the composites single-side electrically conductive [2]. Acid treatment endowed the CNTs with an abundant of polar groups. The thickness of the CNTs layer could be well controlled by adjusting the dip-coating process, which was determined in the range of 5–30 μm. The composites maintained the stretchability originating from the SMPU matrix. The elongation in break was 27%, 33% and 34% in turn for the composites of PUC01, PUC02 and PUC03 (is shown in Fig. 1c). Meanwhile, the strength of the composites was greatly improved, which was consistent with the results of the previously reported CNT-containing polymer systems [6,7]. The maximum stress increased from 44 MPa to 70 MPa with the increase of the CNTs content. Fig. 1d shows the Raman spectra of the composites. Three dominant peaks locating at 1329, 1591 and 2109 cm 1 were found. The former two peaks were indicative of the D and G bands of the CNTs while the latter peak was assigned to the –NH–CO groups of the PU matrix [8]. 3.2. Water sensing process design and thermal gravity analysis Unlike the traditional strategy, the SMPC in this study responded to the water stimuli via changes in the resistivity. In order to disclose the influence of the shape memory programming on water sensing, two processes were designed, which are
schematically illustrated in Fig. 2a. Compared to the direct immersion into the water (Process A), the composites experienced a two-step thermo-mechanical programming, namely extension from the original shape to the temporary shapes at a high temperature and cooling down to fix. Afterwards, the extended composites were immersed into water (Process B). The variation of the Rs through the two processes should be quantitatively measured and discussed in the next section. Fig. 2b and c presents the TGA and differentiation thermal gravity (DTG) curves of the composites swollen with water, respectively. The composites were subjected to dramatic weight loss at nearly 100 °C, resulting from the evaporation of the absorbed water. The PUC-03 had the largest weight loss of nearly 20% compared to the other composites, suggesting that the composites with more CNTs contents were able to absorb much amount of water. Furthermore, the peaks locating at 355–365 °C were assigned as the thermal decomposition of the composites. The decomposition temperatures slightly shifted upwards as the CNTs contents increased, reflecting the interaction of the CNTs with the SMPU matrix via hydrogen bonding [7]. 3.3. Resistivity variation under water stimulation The dry-state composites had different Rs of 0.048, 0.036 and 0.022 Ω m for PUC-01, PUC-02 and PUC-03 respectively. The Rs rapidly increased within the initial 500 s, which followed a nearly linear increase as the composites experienced process A (are shown in Fig. 3a and b). The resistivity variation may result from the disconnection of the CNTs network due to the swelling effect during solvent absorption [6,9]. The variation magnitude of the Rs was determined to be 31.4%, 25.0% and 27.2% in turn. By contrast, a shape memoryenhanced water sensing was found in process B. PUC-03 and PUC01 were chosen and arbitrarily extended by different percentages. The corresponding Rs–time curves are presented in Fig. 2c and d. The PUC-03 had the Rs of 0.026 and 0.036 Ω m as extended by 5%
Fig. 1. SEM images of the surface (a) and cross section (b) of the composite PUC03; Stress–strain curves (c) and Raman spectra (d) of the composites.
H. Luo et al. / Materials Letters 161 (2015) 189–192
191
Fig. 2. a. Schematic illustration of water sensing processes; TGA (b) and DTG (c) curves of the swollen composites.
and 15%, which encountered an increase in the magnitudes of 27.4% and 38.9% after experiencing process B. Moreover, the PUC01 had the variation magnitudes of 40.3% and 63.2% as extended by 5% and 50% respectively. The proposed mechanism was
described as the following: shape memory thermo-mechanical programming enabled the composites in the temporary shapes to store stress and strain energy [10]. As immersed into water, the absorbed water led to the CNTs enlarged their distance with each
Fig. 3. Plots of the Rs against time through process A (a) and the corresponding differentiation curves (b); The Rs–time curves in process B (c, PUC-03, d, PUC-01).
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H. Luo et al. / Materials Letters 161 (2015) 189–192
other due to the swelling effect. On the other hand, the absorbed water played the role of plasticizer, facilitating the local movements of the PU molecules in the stretched states. As a result, the CNTs may locally change the orientation and locations along with the stress/strain energy release. The combination of the swelling effect and the re-orientation/location resulted into the deformation of the conductive network, magnifying the water sensing of the composites. The sensitivity of the resistance type sensors could be defined by the following equation: Sensitivity ¼d[(Rs–Ri)/Ri]/dt, where the Rs and Ri represent the resistivity at t moment and the initial resistivity respectively. The tunability on the magnitudes of the Rs variation may benefit the improvement of the sensitivity and the performance of the water sensors.
4. Conclusions CNT–SMPU composites in bi-layer structure were fabricated via transfer process. The CNTs constructed a percolating conductive network, mechanically reinforcing the composites. Moreover, the CNTs content promoted water absorption percentages and improved the thermal stability of the composites. The water sensing of the composites was comparatively studied, exhibiting a shape memory enhanced effect. The underlying mechanism may be the re-orientation and local movements of the CNTs driven by release of stress/strain energy during the immersion.
Acknowledgments The authors thank the National Natural Science Foundation of China (Nos. 51203191, 51273048, and 51203025), the Special
Funded Project of Pearl River in Guangzhou City of Nova of Science and Technology (2014J2200090), and Science and Technology Planning Project of Guangdong Province, China (2013B021700001) for providing financial support.
References [1] H. Luo, Z. Li, G. Yi, Y. Wang, X. Zu, H. Wang, et al., Temperature sensing of conductive shape memory polymer composites, Mater. Lett. 140 (2015) 71–74. [2] H. Luo, Z. Li, G. Yi, X. Zu, H. Wang, H. Huang, et al., Multi-stimuli responsive carbon nanotube–shape memory polymeric composites, Mater. Lett. 137 (2014) 385–388. [3] A. Boyd, I. Dube, G. Fedorov, M. Paranjape, P. Barbara, Gas sensing mechanism of carbon nanotubes: from single tubes to high-density networks, Carbon 69 (2014) 417–423. [4] M. Wang, I.V. Anoshkin, A.G. Nasibulin, J.T. Korhonen, J. Seitsonen, J. Pere, et al., Modifying native nanocellulose aerogels with carbon nanotubes for mechanoresponsive conductivity and pressure sensing, Adv. Mater. 25 (2013) 2428–2432. [5] V. Eswaraiah, S.S.J. Aravind, K. Balasubramaniam, S. Ramaprabhu, Graphenefunctionalized carbon nanotubes for conducting polymer nanocomposites and their improved strain sensing properties, Macromol. Chem. Phys. 214 (2013) 2439–2444. [6] D. Ponnamma, K.K. Sadasivuni, M. Strankowski, Q.P. Guo, S. Thomas, Synergistic effect of multi walled carbon nanotubes and reduced graphene oxides in natural rubber for sensing application, Soft Matter 9 (2013) 10343–10353. [7] D. Wang, H.Y. Li, M.F. Li, H.Q. Jiang, M. Xia, Z. Zhou, Stretchable conductive polyurethane elastomer in situ polymerized with multi-walled carbon nanotubes, J. Mater. Chem. C 1 (2013) 2744–2749. [8] C. Mercader, C. Jaillet, N. Lachman, C. Zakri, M. Maugey, H.D. Wagner, et al., Sensitivity of carbon nanotubes to the storage of stress in polymers, Macromol. Rapid Commun. 32 (2011) 1993–1997. [9] H.S. Qi, J.W. Liu, Y.H. Deng, S.L. Gao, E. Mader, Cellulose fibres with carbon nanotube networks for water sensing, J. Mater. Chem. A 2 (2014) 5541–5547. [10] K. Yu, T. Xie, J. Leng, Y. Ding, H.J. Qi, Mechanisms of multi-shape memory effects and associated energy release in shape memory polymers, Soft Matter 8 (2012) 5687–5695.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Shape memory-enhanced water sensing of conductive polymer composites Hongsheng Luo n, Yuanyuan Ma, Wenjin Li, Guobin Yi n, Xiaoling Cheng, Wenjin Ji, Xihong Zu, Shengjie Yuan, Jiahui Li Faculty of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 July 2015 Received in revised form 20 August 2015 Accepted 22 August 2015 Available online 24 August 2015
Electrically conductive polymer composites have been widely utilized as the sensory materials being capable of response to various types of stimuli. A shape memory-enhanced water sensing was unprecedentedly reported for polymer composites comprising of carbon nanotubes (CNTs) and shape memory polyurethane (SMPU). The morphological, thermal, mechanical and micro-structural properties of the CNT–SMPU composites were investigated. Two swelling processes were specifically designed to measure the water sensing of the composites. It was found that the shape memory thermo-mechanical programming enlarged magnitudes of the resistivity variation, which may be attributed to the combination of the swelling effect and the re-orientation/local movements of the CNTs along with stress/strain energy release. The findings may greatly benefit the applications of the smart polymers in the fields of sensory materials and flexible electronics. & 2015 Elsevier B.V. All rights reserved.
Keywords: Conductive Water sensing Shape memory Carbon nanotubes Composites
1. Introduction Polymeric composites containing various types of nanofillers have been widely explored towards implanting with the reinforced mechanics as well as novel functions [1,2]. The carbon nanotubes (CNTs) are one set of the most prominent candidates to construct functional polymer composites. The CNT-containing polymer composites commonly possess superior electrical conductivity, which have been applied as the sensory materials, such as gas [3], pressure [4] and strain [5] sensing. The combination of the shape memory polymers (SMPs) and nanofillers lead to shape memory polymer composites. The previous studies of the conductive shape memory polymer composites emphasized on the mechanical reinforcement and electro-responsiveness. However, there has been little literature concerning the stimuli sensing of the polymer composites by far. The electrical conductivity of the composites varies along with the shape changes in a typical shape memory thermo-mechanical programming. By virtue of the shape-dependent conductivity, we once exploited a novel temperature sensing silver nanowire-shape memory composites [1]. Herein, a shape memory-enhanced water sensing was unprecedentedly reported for CNT-containing polymer composites. Compared to the direct immersion into water, the composites n
Corresponding authors. Fax: þ 86 20 39322231. E-mail addresses: [email protected] (H. Luo), [email protected] (G. Yi).
http://dx.doi.org/10.1016/j.matlet.2015.08.105 0167-577X/& 2015 Elsevier B.V. All rights reserved.
experiencing shape memory programming exhibited enlarged magnitudes of the variation in resistivity. The findings may greatly benefit the development of the conductive shape memory polymer composites in the fields of the sensory materials and flexible electronics.
2. Experimental The CNT–SMPU composites were fabricated via transfer process according to our previous study [2], which was briefly described as below: multi-wall CNTs suspension in methanol was dip-coated onto a glass substrate before adding the SMPU solution in dimethylacetamide; the composite films were peeled off after solidification in vacuum. The composite films, cut into the size of 15 mm*3 mm*60 μm (length*width*thickness), were labeled as PUC-01, PUC-02 and PUC-03, representing for the CNTs contents of 5, 12 and 20 wt% respectively. The investigations with Scanning Electron Microscope (SEM, Hitachi, S-4800), tensile tests (SUST, China), micro-Raman spectrometer (JY HR800, excited by He Ne at 786 nm and a beam size of 1 μm) and Integrated Thermal Analyzer (STA 409 PC, NETZSCH) were performed. The composites in dry state were extended by different percentages at 90 °C at the rate of 10 mm/min. The variation of the electrical resistivity (Rs) during the immersion was recorded with an electrochemical workstation (CHI660D, ChenHua, PR China) under a constant
190
H. Luo et al. / Materials Letters 161 (2015) 189–192
voltage of 1 V.
3. Results and discussion 3.1. The morphological, mechanical and structural investigations The CNT–SMPU composites were in bi-layer structure, consisting of the conductive layer constructed by the CNTs and the SMPU matrix (are shown in Fig. 1a and b). It was clearly found that the fibril-like CNTs network whose one part was embedded into the matrix meanwhile the other part was left on the surface, making the composites single-side electrically conductive [2]. Acid treatment endowed the CNTs with an abundant of polar groups. The thickness of the CNTs layer could be well controlled by adjusting the dip-coating process, which was determined in the range of 5–30 μm. The composites maintained the stretchability originating from the SMPU matrix. The elongation in break was 27%, 33% and 34% in turn for the composites of PUC01, PUC02 and PUC03 (is shown in Fig. 1c). Meanwhile, the strength of the composites was greatly improved, which was consistent with the results of the previously reported CNT-containing polymer systems [6,7]. The maximum stress increased from 44 MPa to 70 MPa with the increase of the CNTs content. Fig. 1d shows the Raman spectra of the composites. Three dominant peaks locating at 1329, 1591 and 2109 cm 1 were found. The former two peaks were indicative of the D and G bands of the CNTs while the latter peak was assigned to the –NH–CO groups of the PU matrix [8]. 3.2. Water sensing process design and thermal gravity analysis Unlike the traditional strategy, the SMPC in this study responded to the water stimuli via changes in the resistivity. In order to disclose the influence of the shape memory programming on water sensing, two processes were designed, which are
schematically illustrated in Fig. 2a. Compared to the direct immersion into the water (Process A), the composites experienced a two-step thermo-mechanical programming, namely extension from the original shape to the temporary shapes at a high temperature and cooling down to fix. Afterwards, the extended composites were immersed into water (Process B). The variation of the Rs through the two processes should be quantitatively measured and discussed in the next section. Fig. 2b and c presents the TGA and differentiation thermal gravity (DTG) curves of the composites swollen with water, respectively. The composites were subjected to dramatic weight loss at nearly 100 °C, resulting from the evaporation of the absorbed water. The PUC-03 had the largest weight loss of nearly 20% compared to the other composites, suggesting that the composites with more CNTs contents were able to absorb much amount of water. Furthermore, the peaks locating at 355–365 °C were assigned as the thermal decomposition of the composites. The decomposition temperatures slightly shifted upwards as the CNTs contents increased, reflecting the interaction of the CNTs with the SMPU matrix via hydrogen bonding [7]. 3.3. Resistivity variation under water stimulation The dry-state composites had different Rs of 0.048, 0.036 and 0.022 Ω m for PUC-01, PUC-02 and PUC-03 respectively. The Rs rapidly increased within the initial 500 s, which followed a nearly linear increase as the composites experienced process A (are shown in Fig. 3a and b). The resistivity variation may result from the disconnection of the CNTs network due to the swelling effect during solvent absorption [6,9]. The variation magnitude of the Rs was determined to be 31.4%, 25.0% and 27.2% in turn. By contrast, a shape memoryenhanced water sensing was found in process B. PUC-03 and PUC01 were chosen and arbitrarily extended by different percentages. The corresponding Rs–time curves are presented in Fig. 2c and d. The PUC-03 had the Rs of 0.026 and 0.036 Ω m as extended by 5%
Fig. 1. SEM images of the surface (a) and cross section (b) of the composite PUC03; Stress–strain curves (c) and Raman spectra (d) of the composites.
H. Luo et al. / Materials Letters 161 (2015) 189–192
191
Fig. 2. a. Schematic illustration of water sensing processes; TGA (b) and DTG (c) curves of the swollen composites.
and 15%, which encountered an increase in the magnitudes of 27.4% and 38.9% after experiencing process B. Moreover, the PUC01 had the variation magnitudes of 40.3% and 63.2% as extended by 5% and 50% respectively. The proposed mechanism was
described as the following: shape memory thermo-mechanical programming enabled the composites in the temporary shapes to store stress and strain energy [10]. As immersed into water, the absorbed water led to the CNTs enlarged their distance with each
Fig. 3. Plots of the Rs against time through process A (a) and the corresponding differentiation curves (b); The Rs–time curves in process B (c, PUC-03, d, PUC-01).
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H. Luo et al. / Materials Letters 161 (2015) 189–192
other due to the swelling effect. On the other hand, the absorbed water played the role of plasticizer, facilitating the local movements of the PU molecules in the stretched states. As a result, the CNTs may locally change the orientation and locations along with the stress/strain energy release. The combination of the swelling effect and the re-orientation/location resulted into the deformation of the conductive network, magnifying the water sensing of the composites. The sensitivity of the resistance type sensors could be defined by the following equation: Sensitivity ¼d[(Rs–Ri)/Ri]/dt, where the Rs and Ri represent the resistivity at t moment and the initial resistivity respectively. The tunability on the magnitudes of the Rs variation may benefit the improvement of the sensitivity and the performance of the water sensors.
4. Conclusions CNT–SMPU composites in bi-layer structure were fabricated via transfer process. The CNTs constructed a percolating conductive network, mechanically reinforcing the composites. Moreover, the CNTs content promoted water absorption percentages and improved the thermal stability of the composites. The water sensing of the composites was comparatively studied, exhibiting a shape memory enhanced effect. The underlying mechanism may be the re-orientation and local movements of the CNTs driven by release of stress/strain energy during the immersion.
Acknowledgments The authors thank the National Natural Science Foundation of China (Nos. 51203191, 51273048, and 51203025), the Special
Funded Project of Pearl River in Guangzhou City of Nova of Science and Technology (2014J2200090), and Science and Technology Planning Project of Guangdong Province, China (2013B021700001) for providing financial support.
References [1] H. Luo, Z. Li, G. Yi, Y. Wang, X. Zu, H. Wang, et al., Temperature sensing of conductive shape memory polymer composites, Mater. Lett. 140 (2015) 71–74. [2] H. Luo, Z. Li, G. Yi, X. Zu, H. Wang, H. Huang, et al., Multi-stimuli responsive carbon nanotube–shape memory polymeric composites, Mater. Lett. 137 (2014) 385–388. [3] A. Boyd, I. Dube, G. Fedorov, M. Paranjape, P. Barbara, Gas sensing mechanism of carbon nanotubes: from single tubes to high-density networks, Carbon 69 (2014) 417–423. [4] M. Wang, I.V. Anoshkin, A.G. Nasibulin, J.T. Korhonen, J. Seitsonen, J. Pere, et al., Modifying native nanocellulose aerogels with carbon nanotubes for mechanoresponsive conductivity and pressure sensing, Adv. Mater. 25 (2013) 2428–2432. [5] V. Eswaraiah, S.S.J. Aravind, K. Balasubramaniam, S. Ramaprabhu, Graphenefunctionalized carbon nanotubes for conducting polymer nanocomposites and their improved strain sensing properties, Macromol. Chem. Phys. 214 (2013) 2439–2444. [6] D. Ponnamma, K.K. Sadasivuni, M. Strankowski, Q.P. Guo, S. Thomas, Synergistic effect of multi walled carbon nanotubes and reduced graphene oxides in natural rubber for sensing application, Soft Matter 9 (2013) 10343–10353. [7] D. Wang, H.Y. Li, M.F. Li, H.Q. Jiang, M. Xia, Z. Zhou, Stretchable conductive polyurethane elastomer in situ polymerized with multi-walled carbon nanotubes, J. Mater. Chem. C 1 (2013) 2744–2749. [8] C. Mercader, C. Jaillet, N. Lachman, C. Zakri, M. Maugey, H.D. Wagner, et al., Sensitivity of carbon nanotubes to the storage of stress in polymers, Macromol. Rapid Commun. 32 (2011) 1993–1997. [9] H.S. Qi, J.W. Liu, Y.H. Deng, S.L. Gao, E. Mader, Cellulose fibres with carbon nanotube networks for water sensing, J. Mater. Chem. A 2 (2014) 5541–5547. [10] K. Yu, T. Xie, J. Leng, Y. Ding, H.J. Qi, Mechanisms of multi-shape memory effects and associated energy release in shape memory polymers, Soft Matter 8 (2012) 5687–5695.