Relationship between electromechanical response and percolation threshold in carbon nanotube/ poly(vinylidene fluoride) composites A. Ferreira a , M.T. Martı´nez b , A. Anso ´ n-Casaos b , L.E. Go ´ mez-Pineda c , F. Vaz a , S. Lanceros-Mendez a,d, * a Center/Department of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal b Instituto de Carboquı´mica ICB-CSIC, Miguel Luesma Casta ´ n 4, 50018 Zaragoza, Spain c Centro de Graduados e Investigacio ´n, Instituto Tecnolo ´gico de Tijuana, 22500 Tijuana, Mexico d INL – International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal ARTICLE INFO Article history: Received 6 January 2013 Accepted 16 May 2013 Available online 24 May 2013 ABSTRACT This paper reports on the piezoresistive response of carbon nanotube/poly(vinylidene fluo- ride), CNT/PVDF, composites prepared with different CNT types with and without function- alization, via in situ-generated diazonium compounds. The results show that for a CNT concentration close to the percolation threshold, tunneling is the main mechanism respon- sible for the electrical response, leading also to a significant increase of the piezoresistance of the composites. Interestingly, this fact is independent of the CNT type or functionaliza- tion, as well as of the percolation threshold concentration. In this way, a close relationship between the percolation threshold and the piezoresistive response was demonstrated. The electromechanical response, as characterized by the gauge factor, reach values up to 3.9, being among the largest obtained for thermoplastic composites and demonstrating the suitability of these materials for sensor applications. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Carbon nanotubes (CNTs) are considered suitable nanostruc- tures for the development of functional composites due to their mechanical reinforcement effects and electrical conduc- tivity [1–3]. The use of this kind of nanostructures allows to develop smart materials and functional composites by the production of conductive polymer composites, using small amount of CNT dispersed in an insulating polymer. In partic- ular, composite materials can be developed with the ability to significantly change the electrical response when subjected to strains, which is suitable for the development of polymer- based strain sensors [4,5]. The effective use of CNT in com- posite applications depends significantly on the ability to dis- perse them throughout the matrix and on the compatibility with the polymer matrix, which can be tailored through suit- able modification of the CNT surface [6]. In order to increase their compatibility with organic polymers and to improve pro- cess ability, CNT are often functionalized with organic chem- ical groups [7]. This functionalization improves dispersion but can, on the other hand, hinder some of the properties neces- sary for the development of applications, such as the electri- cal conductivity [8]. The materials most commonly used as conductive fillers include carbon black [9,10], metal powder [11] and carbon nanofibers [12], among others. In the field of resistance type strain sensors based on carbon composites, two main types of strain sensors have been developed: sin- gle-walled carbon nanotube (SWCNT) buckypaper sensors [13,14]; and sensors based on various CNT/polymer compos- ites, such as SWCNT or multi-walled carbon nanotubes 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.05.038 * Corresponding author. E-mail address: lanceros@fisica.uminho.pt (S. Lanceros-Mendez). CARBON 61 (2013) 568 – 576 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon
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Relationship between electromechanical responseand percolation threshold in carbon nanotube/poly(vinylidene fluoride) composites
0008-6223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2013.05.038
* Corresponding author.E-mail address: [email protected] (S. Lanceros-Mendez).
A. Ferreira a, M.T. Martınez b, A. Anson-Casaos b, L.E. Gomez-Pineda c, F. Vaz a,S. Lanceros-Mendez a,d,*
a Center/Department of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugalb Instituto de Carboquımica ICB-CSIC, Miguel Luesma Castan 4, 50018 Zaragoza, Spainc Centro de Graduados e Investigacion, Instituto Tecnologico de Tijuana, 22500 Tijuana, Mexicod INL – International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal
A R T I C L E I N F O A B S T R A C T
Article history:
Received 6 January 2013
Accepted 16 May 2013
Available online 24 May 2013
This paper reports on the piezoresistive response of carbon nanotube/poly(vinylidene fluo-
ride), CNT/PVDF, composites prepared with different CNT types with and without function-
alization, via in situ-generated diazonium compounds. The results show that for a CNT
concentration close to the percolation threshold, tunneling is the main mechanism respon-
sible for the electrical response, leading also to a significant increase of the piezoresistance
of the composites. Interestingly, this fact is independent of the CNT type or functionaliza-
tion, as well as of the percolation threshold concentration. In this way, a close relationship
between the percolation threshold and the piezoresistive response was demonstrated. The
electromechanical response, as characterized by the gauge factor, reach values up to 3.9,
being among the largest obtained for thermoplastic composites and demonstrating the
suitability of these materials for sensor applications.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Carbon nanotubes (CNTs) are considered suitable nanostruc-
tures for the development of functional composites due to
their mechanical reinforcement effects and electrical conduc-
tivity [1–3]. The use of this kind of nanostructures allows to
develop smart materials and functional composites by the
production of conductive polymer composites, using small
amount of CNT dispersed in an insulating polymer. In partic-
ular, composite materials can be developed with the ability to
significantly change the electrical response when subjected to
strains, which is suitable for the development of polymer-
based strain sensors [4,5]. The effective use of CNT in com-
posite applications depends significantly on the ability to dis-
perse them throughout the matrix and on the compatibility
with the polymer matrix, which can be tailored through suit-
able modification of the CNT surface [6]. In order to increase
their compatibility with organic polymers and to improve pro-
cess ability, CNT are often functionalized with organic chem-
ical groups [7]. This functionalization improves dispersion but
can, on the other hand, hinder some of the properties neces-
sary for the development of applications, such as the electri-
cal conductivity [8]. The materials most commonly used as
conductive fillers include carbon black [9,10], metal powder
[11] and carbon nanofibers [12], among others. In the field of
resistance type strain sensors based on carbon composites,
two main types of strain sensors have been developed: sin-
NAN/112574/2009, and NANO/NMed-SD/0156/2007, as well
as by the Spanish ‘‘Ministerio de Economıa y Competividad’’
through the projects reference EUI2008-00153, TEC 2010-
15736 and PRI-PIBAR-2011-1. AF thanks the FCT for grant
SFRH/BD/69796/2010. The authors also thank the COST Ac-
tions MP1003 (European Scientific Network for Artificial Mus-
cles, ESNAM) and MP0902 (Composites of Inorganic
Nanotubes and Polymers, COINAPO).
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