Research Article www.acsami.org
An Ionic Liquid as Interface Linker for Tuning Piezoresistive Sensitivity and Toughness in Poly(vinylidene fluoride)/Carbon Nanotube Composites Kai Ke,†,‡ Petra Pötschke,*,† Shanglin Gao,† and Brigitte Voit†,‡ †
Leibniz Institute of Polymer Research Dresden (IPF), Hohe Strasse 6, 01069 Dresden Germany Organic Chemistry of Polymers, Technische Universität Dresden, 01062 Dresden, Germany
‡
ABSTRACT: Conductive polymer nanocomposites (CPNCs) have emerged as potential alternatives for metallic foil sensors and semiconductor strain gauges. The simultaneous achievement of high piezoresistive sensitivity and large strain ranges for CPNCs currently presents a great challenge and solving this challenge may extend the applications of CPNCs with self-diagnosis capabilities to many structural health-monitoring (SHM) systems. This paper reports a facile strategy for fabricating highly piezoresistive and tough poly(vinylidene fluoride) (PVDF) based CPNCs by tuning the interactions between the polymer matrix and multiwalled carbon nanotubes (CNT) using an ionic liquid (IL) as an interface linker/modifier. As a result, the presence of IL achieves homogeneous dispersion of CNTs in PVDF but causes a reduced number of CNT−CNT ohmic contacts with higher electrical contact resistance. According to the lower initial resistivity, piezoresistive sensitivity is greatly improved, and the gauge factor (GF) varies from 7 to 60 upon the addition of IL. It is also shown that IL tunes PVDF−CNT interfacial bonding and, as an effective interface linker/modifier, achieves significantly improved sensing strain ranges (increased from ca. 6 to 21%) and toughness (elongation at break increases from 6 to 130%) of CPNCs. These results substantially advance the understanding of the missing relationship between polymer−filler interface interactions and piezoresistive properties and have important implications for future studies of tuning polymer−filler interface bonding properties and piezoresistive sensitivity. KEYWORDS: poly(vinylidene fluoride) (PVDF), carbon nanotubes (CNTs), piezoresistivity, ionic liquid, interface interactions
1. INTRODUCTION Carbonaceous fillers in insulating materials can form electrically conductive networks, which are susceptible to external mechanical stimuli and thus can be applied to structural health monitoring (SHM).1−4 In the past decade, much attention has been given to the application of the piezoresistive behavior of conductive polymer nanocomposites (CPNCs), particularly to the use as strain sensors for monitoring structural damage and microcracks.4−6 Advantages such as facile fabrication, tunable piezoresistive sensitivity, and the capability of sustaining highlevel stress/strain make CPNCs frontrunners in strain sensor applications. With regard to sensor requirements, one challenging research topic is to achieve simultaneously high sensitivity and high ductility/toughness in CPNC-based piezoresistive strain sensors. In general, two main approaches for improving the sensitivity (also known as gauge factor (GF), i.e., the relative resistance change (ΔR/R 0 ) against strain) have been introduced in the literature: (1) using a matrix material with high ductility, which can be easily deformed;7−10 (2) constructing conductive networks/pathways with relatively high electrical resistance and/or loose structure.9,11−21 For the former approach, a facile way is to mix elastomers, taking © 2017 American Chemical Society
advantage of their superior mechanical deformation, with conductive fillers such as carbon nanotubes (CNTs), carbon black (CB), graphite, or its nanoscale derivatives.7,9,10 It was extensively reported that CPNCs based on thermoplastic elastomer composites (e.g., thermoplastic polyurethane, polydimethylsiloxane (PDMS), styrene butadiene styrene (SBS), styrene−ethylene−butadiene−styrene (SEBS) copolymers, etc.) are able to sustain high levels of strain and exhibit high piezoresistive sensitivity, but they cannot be used for highstrength or high-pressure applications. In contrast to elastomer/rubber-based CPNCs, a great deal of attention has been paid to thermoplastics, which can be used for highstrength applications, for example, stress >15 MPa, and are capable of being functional parts of mechanical engineering materials with integrated piezoresistive capabilities. For the latter approach, to achieve high ΔR/R0-strain sensitivity in piezoresistive CPNCs, networks/pathways with high resistance can be applied with the use of hybrid conductive fillers,9,11,12,20 the selection of fillers with different aspect ratios,13−15 or the Received: October 21, 2016 Accepted: January 12, 2017 Published: January 12, 2017 5437
DOI: 10.1021/acsami.6b13454 ACS Appl. Mater. Interfaces 2017, 9, 5437−5446
Research Article
ACS Applied Materials & Interfaces control of filler contents slightly above the electrical percolation threshold (ΦC).15−19 However, it is recognized that the incorporation of conductive fillers into thermoplastic polymers generally results in deteriorated ductility and toughness in the resultant nanocomposites.22 Accordingly, thermoplastic-based CPNCs generally have a relatively low yield strain or a small elastic region in stress−strain curves compared with pure thermoplastics, leading to relatively narrow sensing strain ranges for CPNCs. For enabling reliable measurements in larger strain/stress ranges, it is challenging to obtain increases of piezoresistive sensitivity and sensing strain ranges simultaneously. More recently, one encouraging piezoresistive CPNC material has emerged by utilizing poly(vinylidene fluoride) (PVDF) and conductive fillers, which exhibits great potential for SHM applications in relatively harsh environments such as public infrastructures, wind generators, automobiles, and airplanes, etc. This is the result of not only the advantage of electric resistance change upon strain due to conductive networks of fillers but also PVDF’s good combination of high resistances against chemicals, heat, and ultraviolet radiation as well as excellent mechanical properties. For example, Eswaraiah et al.15 reported that PVDF nanocomposites filled with functionalized graphene exhibit higher piezoresistive sensitivity than those filled with multiwalled carbon nanotubes (MWCNTs) at the same content (2 wt %). Additionally, they found that the sensitivity is higher at a filler weight loading level of 2 wt % compared to 3 wt % or even higher. Accordingly, the highest GF (maximum GF ≈ 12) under monotic tensile load up to 10 kN was obtained when the content of graphene was only slightly higher (2.2 wt %) than ΦC (2.0 wt %).16 Similarly, Ferreira et al.17,18 found a dependence of piezoresistive sensitivity on filler content in regard to ΦC of solution-mixed PVDF/CNT nanocomposites in bending tests. Notably, they found that, at loadings slightly above ΦC, GF is independent of the CNT type, concentration, and surface functionalization of CNTs. It should be mentioned that all of the aforementioned studies focused on the piezoresistive behavior of PVDF nanocomposites under microstrain (20% strain) using a melt-mixing method. This facile strategy 5444
DOI: 10.1021/acsami.6b13454 ACS Appl. Mater. Interfaces 2017, 9, 5437−5446
Research Article
ACS Applied Materials & Interfaces
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is also suitable for other polymer nanocomposites for tuning polymer−filler interfacial interactions toward achievements of high piezoresistive sensitivity and mechanical toughness.
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AUTHOR INFORMATION
Corresponding Author
*(P.P.) E-mail:
[email protected]. ORCID
Petra Pötschke: 0000-0001-6392-7880 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Felix Buchta for help in the nanocomposite preparation, Dietmar Krause for cutting of the dog-bone shaped samples used for tensile testing and Manuela Heber for SEM and TEM observation. In addition, we appreciate the support of Dr. Konrad Schneider, Holger Scheibner and Yilong Li for tensile testing, Dr. Lothar Jakisch for FTIR measurements, Dr. Dieter Fischer and Julia Muche for Raman spectroscopy measurements and Dr. Qiang Wei for the discussion of FTIR results (all from IPF). K.K. also thanks the China Scholarship Council (Grant 201206240047) for funding the Ph.D. study at Leibniz Institute of Polymer Research Dresden.
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DOI: 10.1021/acsami.6b13454 ACS Appl. Mater. Interfaces 2017, 9, 5437−5446
