A Nuomici-Inspired Universal Strategy for Boosting

Subscriber access provided by Nottingham Trent University

Applications of Polymer, Composite, and Coating Materials

A Nuomici-Inspired Universal Strategy for Boosting Piezoresistive Sensitivity and Elasticity of Polymer Nanocomposite based Strain Sensors Kai Ke, Yu Wang, Yilong LI, Jing-hui Yang, Petra Pötschke, and Brigitte Voit ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b13510 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

A Nuomici-Inspired Universal Strategy for Boosting Piezoresistive Sensitivity and Elasticity of Polymer Nanocomposite based Strain Sensors Kai Ke1,2+, Yu Wang3, Yilong Li1,2, Jinghui Yang4, Petra Pötschke1, Brigitte Voit1,2 1Leibniz

Institute of Polymer Research Dresden (IPF), Hohe Str. 6, 01069 Dresden,

Germany 2Organic

Chemistry of Polymers, Technische Universität Dresden, 01062 Dresden,

Germany 3School

of Mechanical and Materials Engineering, Washington State University,

Pullman, WA 99163, USA. 4School

of Materials Science & Engineering, Key Laboratory of Advanced

Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu, 610031, China

Abstract Electrically conductive polymer composites (CPCs) are potential alternatives to conventional strain gauges due to their tunable sensitivity and strain ranges. Currently, to achieve very high piezoresistive sensitivity in thermoplastic-based CPCs with Gauge factors GF above 20 at low tensile strains (ε ≤ 5%) is a big challenge, but critical for structural health monitoring application in infrastructures. Here, inspired by the unique structures of a famous Chinese food, nuomici, we coat carbon nanotubes (CNTs) onto sticky acrylic rubber (AR) granules (ARG) to form nuomici-like CNT@ARG composite granules, which are employed as unique 

Corresponding author: [email protected] (Dr. Petra Pötschke) 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

conductive filler to fabricate highly piezoresistive and flexible CPCs based on poly (vinylidene fluoride) (PVDF). This strategy of localizing CNTs densely on the surface of touching rubbery particles resulted in a much more sensitive elastic conductive network built by the CNT@AR composite and showed a big gain effect. The resultant PVDF/CNT@AR nanocomposites (AR content ranging from 0 to 10 wt.%) show extremely high piezoresistive sensitivity at low strain, depending on the AR content. In particular, the GF value of PVDF with 1.5 wt.% CNT@10 wt.% AR is 41 at 5% strain, which is more than one magnitude higher than that (ca. 3) of traditional PVDF/CNT nanocomposite sensors. Moreover, the elongation at break increases by about 60% with the addition of 1.5 wt. CNT@10 wt. % AR. This study introduces a universal effective strategy for tailoring the mechanical properties and strain-sensitivity of conductive network in CPCs, which is critical for the fabrication of high-performance strain sensors.

Keywords: Piezoresistive, Strain sensor, Poly (vinylidene fluoride) (PVDF), Acrylic rubber particles, Conductive polymer composites.

Introduction Electrically conductive polymer composites (CPCs) are widely used to fabricate strain sensors for structural health monitoring (SHM) in infrastructures.1-5 Since poly(vinylidene fluoride) (PVDF) has excellent environmental resistance and good mechanical properties, its CPCs have potential for composite-based piezoresistive strain sensors toward SHM applications 6-7. To date, carbon nanofillers like graphene based structures and multi-walled carbon 2


Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanotubes (MWCNT) have been widely used to fabricate PVDF-based CPCs for piezoresistive strain sensors used at low- (≤1%)6-10 and high-level (1~10%)11-12 strain sensing. However, the piezoresistive sensitivity, i.e. the dependence of the relative electrical resistance change ΔR/R0 on strain, of the aforementioned PVDF composites is limited by either the filler network structure or the poor PVDF-filler interfacial interactions in the corresponding composites. Although ΔR/R0-strain sensitivity, indicated by the gauge factor GF, could be increased to some degree by using hybrid fillers of MWCNTs and carbon black12, it is still in the range of 2~10. Here the gauge factor is defined as GF=(ΔR/(R0╳ε)), where ΔR is the resistance change, R0 is the initial resistance before stretching and ε is the actual strain.13 Interestingly, the use of commercial PF6- based ionic liquid as interface linker for PVDF and MWCNTs is able to achieve much larger GFs, e.g. 60 at 21% tensile strain14. However, in the aforementioned studies, larger GFs were obtained only if the loading strain was near or above the yield strain of PVDF-based CPCs. In this case, contact resistance between MWCNTs increases significantly due to the breakage of parts of conductive networks in the CPCs. However, it is challenging to achieve high piezoresistive sensitivity for PVDF-based strain sensors at relatively low strains, e.g. ≤5% strain, which is generally the strain range of commercial optic Fiber Bragg Grating sensors used for SHM in infrastructures15-16. Hence, exploration of new strategies to achieve large GFs at strain ε≤5% for PVDF-based piezoresistive strain sensors is of great significance, enabling alarming an earlier structural damage to prevent catastrophes. In general, there are three main ways to increase the resistance-strain sensitivity of 3



Page 4 of 29

CPCs: 1) constructing loose networks of conductive fillers by controlling their compactness and morphology; 2) constructing flexible conductive networks by using rubbers or elastomers as matrix polymer; and 3) improving the piezoresistive effect by enhancing the polymer-filler interfacial interactions. For the first way, up to now, many studies have investigated effects of filler content or initial resistance/resistivity of the CPCs6-11, 17-18, dimension19-20 and hybrids12, 20-23 of carbon nanofillers, as well as network structure23-24 on piezoresistive behavior of CPCs. For the way of using elastomers/rubbers with high elasticity, their CPCs have relatively high piezoresistive sensitivity and are favorable for sensing relatively large strains25-31, e.g. strain ε≥100%, in terms of body motion detection application. However, they also show extremely limited piezoresistive sensitivity for strain ε≤5%, e.g. GF is around 1~228, 30.

Additionally, usually relatively high amounts of carbon fillers are needed in the

fabrication of these CPCs by melt mixing. Therefore, they are generally inadequate for SHM applications in infrastructures. Currently, some attention has been paid to the third way and factors such as using interfacial compatibilizer32-33, functionalized MWCNTs26, 34-35 or reduced graphene oxide36-37, and polymer matrices with different polarities38 have been concerned. Although the piezoresistive sensitivity could be improved to some degree, there are still limitations to reach GFs larger than 20 at strain ε≤5%. This is mainly due to the tiny deformation of conductive networks in thermoplastics/elastomers under such low strain, while the interfacial interaction is often too weak to effectively transfer the stress load from the matrix to the conductive networks. To solve this issue, an effective way is to amplify the piezoresistive 4


Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


response at such low tensile strains (ε≤5%) via designing flexible conductive networks and enhancing the polymer-filler interactions in thermoplastic-based CPCs. As such, conductive networks will deform more easily under tensile strain in CPCs, giving rise to an amplified resistance-strain response, as reported in flexible strain sensors with heterogeneous strain distribution39-40. Herein, a novel strategy inspired by “nuomici” (a Chinese pastry made of sticky rice balls and coconut shreds), i.e. using MWCNT-coated acrylic rubber granules (CNT@AR) as conductive fillers, was used to fabricate highly sensitive piezoresistive strain sensors based on PVDF, a polymer type which due to its general property profile is very suitable for SHM applications.12 Acrylic rubber (AR) was reported to be partially miscible with PVDF41-44, which potentially enhances the PVDF-filler interfacial interactions and facilitates the deformation of the flexible CNT network. The piezoresistive behavior of PVDF nanocomposites under single and cyclic loading conditions both indicates that the piezoresistive sensitivity of PVDF nanocomposites depends on the AR content. To our knowledge, this is the first conceptual study about increasing ΔR/R0-strain sensitivity via amplifying the piezoresistive response through polymer-filler interface design (gain effect). It paves a new way for making thermoplastic-based piezoresistive strain sensors with high sensitivity and wide strain range.

Experimental Materials and Nanocomposite preparation 5



Page 6 of 29

PVDF (Kynar 720, Arkema) used in this work has a weight average molecular weight of 455,800 g / mol (density 1.78 g / cm3). Short thin multi-walled carbon nanotubes, containing amino groups (NC3152, Nanocyl, S.A., Belgium, ca. 0.5% -NH2 group), have a length of ~1 μm and an average outside diameter of 9.5 nm. The selection of this type of MWCNTs is based on their good dispersion in PVDF and strong PVDF-CNT interfacial interactions, as reported in our previous study45. Acrylic rubber (AR, type HyTemp® Polyacrylate Elastomer ACM, grade PV-04, density 1.10 g / cm3) containing acrylic esters was purchased from Zeon Chemicals. This type of AR, also proposed by the manufacturer to be used as binder and adhesive, was selected because it is well compatible with PVDF and has a "sticky" character, which means that the CNTs adhere well to its surface. In addition strong interactions with the used CNTs can be expected. The PVDF nanocomposites containing MWCNT-coated acrylic rubber granules (CNT@AR) were prepared via melt mixing using a twin-screw micro-compounder (DSM Xplore, capacity 15 cm3, The Netherlands) under optimized conditions (210 ºC, 200 rpm and 10 min) as previously reported 45. Acrylic rubber block material was weighed and cut into small sphere-like or cubic-like particles (maximum diameter or length ca. 4 mm) manually by using a tailor scissor and then premixed with CNTs in a mortar by manually grinding them for 15 min prior to melt mixing. At the end of the grinding process, all CNTs adhered well to the surface of the AR granules, which themselves did not adhere to each other, inspired by the Chinese food "nuomici". All nanocomposites have a fixed CNT content of 1.5 wt. % and various AR contents. The 6


Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanocomposites were labeled according to their compositions, e.g., that containing 1.5 wt. % CNTs and 2 wt. % AR was designated as CNT@2AR. Based on the densities given by the provider, the addition of 2, 6, and 10 wt. % AR corresponds to ca. 3, 9, and 15 vol. %. To investigate the piezoresistive behavior of the PVDF nanocomposites, the strands extruded in air without additional cooling were cut into small pieces and compression molded (220 °C, 50 kN, 2 min) into rectangular plates (80╳54╳0.5 mm3, length╳ width╳thickness) using a hot press machine (Model-PW40EH, Paul-Otto Weber GmbH, Germany). Afterward, the plates were cut into dog-bone-shaped specimens according to the DIN EN ISO527-2 standard. Characterization Uniaxial tensile tests were performed at room temperature using a Zwick 2.5 (Zwick Roell, Germany) tensile machine and the resistance change (R) of the specimens during tensile testing was recorded simultaneously using a Keithley 2001 multimeter.12, 14 A crosshead speed of 1 mm/min was used for the tensile testing. To reduce the contact resistance between specimens and electrodes, two silver paste electrode marks (ca. 3-4 mm wide) were painted on the surface of dog-bone specimens. Cyclic testing (12 cycles) was performed similarly to a previously reported study14, i.e. the stress was loaded up to 15 MPa at a rate of 1 mm/min and held for 90 s before unloading to 0.35 MPa at the same rate and held for 90 s. Scanning electron microscopy (SEM) was performed on the compression molded composites using an Ultra Plus Field Emission Gun SEM (FEG-SEM, Carl-Zeiss AG, 7



Page 8 of 29

Oberkochen, Germany). The samples were immersed in liquid nitrogen for 30 min before being fractured and the fracture surface was sputter coated with gold prior to SEM observation. Transmission electronic microscopy (TEM) (TEM LIBRA 200 MC, Carl Zeiss SMT, Oberkochen, Germany) on thin sections (60-80 nm) of the compression molded plates was applied to observe the morphology of the nanocomposites and the localization of the CNTs. The crystalline phases of the PVDF nanocomposites were investigated using a the Tensor 27 (Bruker, GmbH) Fourier transform infrared (FTIR) spectrum (4000 cm-1 to 650 cm-1) in an attenuated total reflection model, as previously reported.45 Raman spectrum analysis of the compression molded composites was done using the Confocal Raman Microscope alpha 300 R (WITec GmbH, Ulm, Germany) equipped with a laser (excitation wavelength 532 nm, a laser power 500 μW). The intensity of D-band to G-band (ID/IG) was obtained by calculating the ratio of intensity height of the Raman spectra, as reported in our previous work.45 Dynamic mechanical analysis (DMA) was performed on rectangular specimens (40× 10×4 mm3, length×width×thickness) using the ARES G2 rheology instrument (TA Inc., US). The test was performed with a fixed strain and frequency of 0.05% and 1 Hz, respectively, in the temperature range of -100 to 150 ºC using nitrogen.

Results and discussion Concept and strategy design As mentioned in the introduction, CPCs with flexible conductive filler networks have 8


Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


potentially high ΔR/R0-strain sensitivity. As such, we anticipated the formation of CNT networks with a preference at the PVDF-AR interface in the nanocomposites since the CNTs have strong interactions to both polymers and PVDF is compatible with AR. Inspired by cooking a kind of Chinese pastry “nuomici”, which is presented in Figure 1(a), we used a novel strategy to realize our expected structure. As seen in Figure 1(a), the cooked sticky rice balls can stick coconut shreds to become “nuomici”. Inspired by this, it is aimed to attach CNTs (similar to coconut shreds) onto ARGs (like sticky rice balls, we draw a ball structure to make an analogy to "nuomici".) due to their sticky surface to form CNT@ARG, as presented in the schematic in Figure 1(b). The attachment of CNTs on the ARG surface is shown in Figure S1. Besides CNTs, certainly other conductive carbon nanofillers can also be applied to form conductive ARGs by this strategy. Acrylic rubber was selected to make the “sticky balls” due to of its good compatibility with PVDF41, which is expected to enable suitable interfacial bonding between CNT containing AR domains with PVDF after the melt mixing. Once the CNT@ARGs are dispersed in the PVDF matrix in such a way to form conductive paths, a more sensitive conductive network structure consisting of CNT@AR within the PVDF matrix will be obtained, which allows some control of the network. As the rubber has a much lower melt viscosity but higher elasticity than PVDF at the processing temperature it can be easily deformed to elongated structures with the CNTs on its surface or in it, so that its content can be selected to be lower than the 16 vol.% which would be needed for percolation of spherical rubber particles. During melt-mixing a dispersion of the ARG 9



Page 10 of 29

into much smaller domains is achieved by the shear stresses and preservation of CNT localization at the surface or within the AR component is expected. During strain sensing experiments of such samples, two processes occur. On the one hand side, the percolated arrangement of the rubber particles will be influenced; on the other hand, the CNT network on the AR surfaces will be modified by the stretching. Both effects result in the separation of the contacts between adjacent CNTs, either between neighbored AR particles or at their surface. In the case of uncontrolled random conductive networks such as those found in matrix-CNT composites, the total increase in resistance is the sum of the increase in each tunnel resistance Rt between adjacent CNTs (Rti, i=1, 2, …, n), as illustrated in Figure 1(c). In contrast, when stretching CNT@AR-controlled conductive networks, since the CNTs are located at the surface of the AR particles, the change in resistance occurs both between the CNTs on each AR particle (resulting in Rai) and in the contact areas between AR particles (resulting in Rti), as shown in Figure 1(d). Thus, the resistance change with respect to a CNT@AR particle is the result of Rai and Rti, which can be considered as one unit, and the total resistance change will be a sum of these units (see Figure 1(d)). This leads to an amplification of the piezoresistive response in comparison to the system with only CNTs, i.e. a large gain effect is expected.

10


Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1 (a) A cooking of the nuomici. (b) Preparation of “nuomici” inspired CNT@ARG. Schematic illustration of the piezoresistive effect in PVDF nanocomposites with (c) only CNTs and (d) CNT@AR.

Morphology analysis of the nanocomposites Figure 2 shows the morphology of PVDF/CNT nanocomposites with CNTs and CNT@AR as analyzed by SEM and TEM. As seen in Figure 2(a), in the PVDF-CNT mixtures the nanotube distribution is uniform, even if some small agglomerates are seen, as supported by the magnified image on the right. The CNTs are seen as small white dots or short parts pultruding out due to the cryofracture. This is in accordance with our previous research45. Besides, the TEM image in Figure 2(c) confirms the uniform distribution and suitable dispersion of the CNTs in the PVDF matrix. For CNT@AR, the SEM image in Figure 2 (b) shows that the CNTs are selectively located within some parts, which can be assigned to the AR component of the blend, and have a suitable dispersion in those parts. The insert in Figure 2(b) as well as the images given in Figure S4 reveal that AR and PVDF cannot be clearly distinguished 11



Page 12 of 29

and that the interface between the two polymers in these cryofractures cannot be clearly identified, both illustrating an excellent phase adhesion between the two polymers. However due to the rubbery behavior of the AR, the nanotubes are seen in more length on the surface of the broken AR particles. The TEM image in Figure 2(d) shows that the AR was dispersed during the melt-mixing procedure and forms elongated particles with length of ca. 0.5 m to 3 m. In addition, it confirms the location of CNTs in either the acrylic rubber domains or the rubber-PVDF interface, which is supported by the further amplified TEM image in Figure S2. No CNTs can be seen outside the AR domains. The desired selective localization of the CNTs is expected to result from the nuomici-inspired strategy and a strong interaction between the functionalized CNTs and the acrylic rubber polymer46-47, whereby the also good interaction with PVDF is assumed to drive the CNTs preferentially towards the blend interface. The size of the observed CNT@AR domains is much lower than that of the initially used granules, illustrating that a dispersion process took place during melt mixing. In addition, the CNT@AR domains in the compression molded samples show a strongly elongated shape with orientation parallel to the plate surfaces which may facilitate the formation of a conductive network through the whole composite in in-plane direction (in which strain sensing is performed) even at relatively low AR contents. Such interesting morphology structure of CNT@AR in the PVDF matrix is expected to influence the piezoresistive behavior significantly, as hypothesized in Figure 1(d).

12


Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2 SEM (a,b) and TEM (c,d) images of PVDF nanocomposites containing CNTs (a,c) and CNT@AR (b,d) The CNT content is 1.5 wt.%, the AR content 10 wt.%.

Piezoresistive behavior of PVDF nanocomposites To measure a piezoresistive response, the nanocomposites have to possess a suitably low initial resistivity. While the electrical percolation threshold for the type of CNTs used in the same type of PVDF was reported at 0.6 wt.%45, all composites with 1.5 13



Page 14 of 29

wt.% CNTs and 2, 6, or 10 wt.% ARP are sufficiently conductive (Figure S3). This illustrates that beside the blend-like structure an electrical network is formed through the samples. Figure 3(a) shows the growth of ΔR/R0 with strain loading for PVDF nanocomposites containing 1.5 wt.% CNT and CNT@AR with different AR contents. As seen in Figure 3(a), for all the nanocomposites, ΔR/R0 increases with strain loading, while CNT@AR systems have much larger ΔR/R0s at the same strain when compared to the composite with only CNTs. This indicates that the use of CNT@AR as an alternative to CNTs can effectively increase the ΔR/R0-strain sensitivity of PVDF nanocomposites. Besides, the CNT@AR systems with higher AR contents have larger ΔR/R0 values, indicating the dependence of piezoresistive sensitivity on the AR content in the CNT@AR. To underline this, for strains of 1% and 5%, the gauge factors (GF), which are generally used to assess the elongation resistance sensitivity of CPCs, are given in Figure 3(b). The GF increases with the AR content at both strains, but a more significant rise of GF is observed at strain ε=5%. For instance, the nanocomposite CNT@10AR has a GF of 41 at ε=5%, which is ca. 14 times of that for the control nanocomposite with only CNTs (GF≈3 at the same strain). To our knowledge, this value is currently the highest for piezoresistive PVDF nanocomposites at such low strain. Although a high piezoresistive sensitivity can be achieved even when using filler contents that are only slightly above the electrical percolation threshold, the nanocomposite with 1.5CNT@10AR has a higher ΔR/R0 value at the same strain than the nanocomposite with 0.75 wt.% CNTs (Figure S5). To 14


Page 15 of 29

verify the efficiency of this strategy, a comparison with other results reported in literature about GFs of piezoresistive CPCs is provided in Figure 3(c). The composite CNT@10AR has a much larger GF than all other materials reported in literature including nanocomposites using either elastomers/rubbers48-54, or thermoplastics like polypropylene55 and polycarbonate56, or cellulose57-58, combined with conductive carbon nanofillers. Therefore, using CNT@ARG as an alternative to CNTs is an effective way to increase the piezoresistive sensitivity of CPCs. Figure 3(d) additionally shows that such nanocomposites exhibit excellent repeatability of piezoresistive behavior under the same cyclic testing conditions. In addition, the nanocomposite CNT@2AR has higher ΔR/R0-strain sensitivity at the same strain amplitude than the control sample, which indicates a higher piezoresistive sensitivity at low strain for the former and is in agreement with the results shown in Figure 3(a). (b) 45

CNT CNT@2AR CNT@6AR CNT@10AR

sensitivity

2.0

1% strain 5% strain

40

1.5

1.0

0.5

35

Gauge factor

(a) 2.5

R/R0

30 25 20 15 10 5 0

0.0 1

2

3

4

5

Strain (%)

6

7

0

8

2

4

6

8

10

AR content in CNT@AR (wt.%) (d) 1.2 1.0

Strain (%)

0

CNT

CNT@2AR

0.8 0.6 0.4 0.2 0.0 -0.2 0

500

1000

0

500

1000

1500

2000

2500

1500

2000

2500

0.02

R/R0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.01

0.00

Time (s)

15



Page 16 of 29

Figure 3 (a) ΔR/R0 plotted vs. strain and (b) GF vs. AR content in CNT@AR of piezoresistive PVDF nanocomposites with 1.5 wt.% CNTs. (c) Comparison of GFs at 5% strain in this work with values of other piezoresistive CPCs reported in literature, Rx refers to the reference in the reference list. (d) Cyclic testing of the nanocomposite with CNTs and CNT@2AR until a maximum stress of 15 MPa.

To understand the piezoresistive behavior for the entire strain range, the development of stress and ΔR/R0 upon strain at break is provided for the two representative nanocomposites with CNTs and CNT@10AR in Figure 4(a) and (b). The PVDF/CNT nanocomposite tends to break quickly beyond the yield point (ca. ε=7%, the stress reaches the highest in the engineering stress-strain curve) and its elongation at break is ca. 10% (Figure 4(a)). In contrast, the stress-strain curve of the nanocomposite CNT@10AR shows a plateau after yielding (see Figure 4(b) and Figure S6), which is completely different from the control nanocomposite shown in Figure 4(a). The nanocomposites with CNT@AR have higher elongation at break but lower Young’s modulus and tensile strength by comparison with the control sample with only CNTs (Figure S6). Besides, the strain at break is reached at 16%, which is due to the toughening effect of the AR to the PVDF nanocomposites. Therefore, AR enhances the flexibility and elasticity of the PVDF nanocomposites, which is favorable for tuning their piezoresistive properties. For the ΔR/R0-strain curve, ΔR/R0 increases with strain loading, but the ΔR/R0 increment becomes steep when the stress exceeds the yield stress, which is due to the rapid increase of tunneling resistance Rt and the 16


Page 17 of 29

subsequent break of parts of conductive paths.12 Interestingly, the ΔR/R0-strain curve for the nanocomposite CNT@10ARshows a significantly stronger increase with strain loading, and becomes linear at strains > 2% (ΔR/R0=-0.85 + 0.56ε), indicating a constant gauge factor for the strain range of 2%~16%. This is expected to be due to the deformation of CNT@AR controlled conductive network in the nanocomposite triggered by the elastic deformation of AR.

(b) 25

0.8

30 0.4

20

0.2

10 0

0.0 0

2

4

6

Strain (%)

8

10

R/R0

0.6

Stress (MPa)

40

10

20

8

15

6

10

4 R/R =-0.85+0.56 0

5

2

0 0

2

4

6

8

10

12

Strain (%)

R/R0

(a) 50 Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

16

0 18

Figure 4 Stress and ΔR/R0 plotted with strain for the PVDF nanocomposites with (a) CNTs and (b) CNT@10AR. (c) Schematic representation of the morphology and its development under strain of PVDF nanocomposites with CNTs and CNT@AR.

A schematic illustration is provided in Figure 4(c) to explain the effects of CNTs and 17



Page 18 of 29

CNT@AR on the piezoresistive behavior of the PVDF nanocomposites. For PVDF composites with CNTs (left column), the deformation of the PVDF matrix at low strains triggers the increase of tunneling resistance (Rt) due to the slight separation of the CNT-CNT contacts, leading to the increase of ΔR/R0 (see Figure 4(c) in the middle schematic). A further increase in strain leads to a rapid increase in Rt due to the increased CNT-CNT contact separation and even parts of contacts open (see Figure 4(c) on the left bottom), corresponding to the steep rise of ΔR/R0. The total resistance is the sum of these resistance changes, as shown in Figure 4(c). In the case of nanocomposites with CNT@AR, however, the resistance change at low strains results from the increase in the tunnel resistances between CNT@AR particles and between CNTs on the AR surface, as can be seen in the right middle schematic in Figure 4(c). Since the AR has good compatibility with the PVDF matrix, the AR is stretched at higher strains, thus the contacts between CNTs on the AR surface will be increasingly lost, giving rise to the quick increase in resistance. The total resistance is the sum of resistance changes between CNT@AR particles and CNT-CNT on the AR surface (considered as one unit), as presented by the equation under the right schema in Figure 4(c). Therefore, the PVDF/CNT@AR system has a much greater piezoresistive effect than the PVDF/CNT system, which represents a large gain effect.

PVDF-filler interfacial interactions In order to investigate the PVDF-filler interactions, dynamic mechanical analysis (DMA) as well as Raman and FTIR spectroscopy of PVDF and its nanocomposites 18


Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were performed as shown in Figure 5. The dependency of the loss tangent (tan ) from DMA studies (Figure 5(a)) shows one glass transition temperature (Tg) for the nanocomposites with CNT and CNT@AR. This can be seen on the peak at about -40 °C which is assigned to the main relaxation of the amorphous phase (called β-relaxation)59. As pure PVDF has a Tg of -42.2 °C and pure AR (according to the data sheet) has multiple Tg values between -33.1 and -25.6 °C, the small increase in the Tg by 8 K (marked by the dashed arrow in Figure 5(a)) when adding up to 10 wt.% AR seems to be an overlap of both Tgs indicating slightly changed molecular chain relaxation. However, due to the similarity in both Tg ranges, no information about immiscibility or partial miscibility can be drawn from DMA. Interestingly, the second relaxation peak of PVDF at about 85 °C which is assigned to the relaxation related to -phase crystallites59-60 is significantly lowered in intensity when CNTs are added and recovers with increasing AR content. Raman spectra can provide support for the influence of AR on PVDF-CNT interactions. Figure 5(b) shows that the D-band (located at ca. 1349 cm-1 for the PVDF/CNT nanocomposite, which is generally related to multiple phonon scattering of defects or amorphous carbon)61-63, slightly shifts to a lower wave number (1345 cm-1) in the nanocomposite CNT@10AR. In addition to that, 2D-band (located at 2692 cm-1 in the control nanocomposite with only CNT), which is also related to multiple phonon scattering of defects or amorphous carbon61-63, shifts to lower wave numbers with increasing AR contents, as shown in Figure 5(b). The graphitic G-band (located at ca. 1590 cm-1), which is related to the stretching of conjugated double 19



bonds and corresponds to sp2 hybridization during the formation of the aryl-nanotube bond61-63, has no shift. However, the ratio of the intensity of D-band to G-band ID/IG shows an obvious increase with the AR content. All of these changes are indicative of the interactions between the CNTs and PVDF or/and AR. Furthermore, the FTIR spectra of the PVDF nanocomposites in Figure 5(c) reveal that the addition of AR has no influence on the formation of -phase crystals (at 840 and 1275 cm-1) of PVDF by comparison with the nanocomposites containing only CNTs, as marked by the dashed cyan lines. By contrast, neat PVDF has no -phase crystal and exhibits the typical -phase (763, 795 and 975 cm-1), which reveals the nucleation effect of the CNTs on the PVDF crystallization due to the filler-PVDF interactions45, 64-68. As reported by Li et al.69 and Abolhasani et al.43, there was no crystal phase transition in their investigated PVDF-AR blends, which is in accordance with the relaxation of crystallites shown in Figure 5a. Given the data shown in Figure 5b and c, there are likely interactions between CNT and PVDF/AR or PVDF and CNT@AR, which induces the formation of -phase PVDF crystals.

0.10 0.08

PVDF CNT CNT@2AR CNT@6AR CNT@10AR

shift

0.12

8K

(a)

Tan 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

0.06 0.04 0.02 -100 -80 -60 -40 -20 0

20 40 60 80 100 120 140 160

Temperature (C)

20


Page 21 of 29

1349 cm-1 1349 cm-1

ID/IG

G-band

1345 cm-1

2D-band -1

2694 cm

-1

2689 cm

D-band

1200

1500

(c)

PVDF CNT CNT@2AR CNT@6AR CNT@10AR

CNT 1.47 CNT@2AR 1.50 CNT@10AR 1.55

Absorbance

(b)

Intensity (i.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




 

 

2684 cm-1

2700



3000

-1

Wave number (cm )

1300

1200

1100

1000

900

Wave number (cm-1)

800

Figure 5 (a) Loss tangent (DMA) vs. temperature, (b) Raman and (c) FTIR spectra of PVDF and its nanocomposites.

Conclusion A novel nuomici-inspired strategy has been reported for the preparation of PVDF based nanocomposites containing CNT@AR to notably improve the strain sensitivity and elasticity of strain sensors. The nuomici-like CNT@AR is composed of CNTs coated on rubber particle surfaces which, when melt mixed with PVDF, form a controlled elastic conductive network that provides a big gain effect in strain sensitivity and superior deformation ability of PVDF-based strain sensors. In particular, a piezoresistive sensitivity with a very high Gauge factor of 41 has been achieved at a low tensile strain of 5% in the CNT@10 wt.%AR composite containing only 1.5 wt.% CNT. This is ca. 14 times the control composite with randomly distributed CNTs. The improvement of piezoresistive sensitivity is strongly dependent on the CNT@AR content, i.e. larger GFs were obtained in nanocomposites with higher AR contents (varied between 2 and 10 wt.%). In addition to the notable improvement in strain sensitivity, the elongation at break was improved by ca. 60% 21



Page 22 of 29

with the addition of 1.5 wt.% CNT@10wt.% AR as compared to the control composite. This nuomici-inspired strategy provides a facile way to control both the 3D structures and mechanical properties of the conductive networks. It is very instructive for the design of advanced piezoresistive strain sensors with high sensitivity at low strain range (e.g. ε ≤ 5%) and good flexibility for structural health monitoring applications. The strategy can be extended to other polymer matrices, rubber types, and conductive fillers and is very versatile.

Associated content Supporting information Digital photo of acrylic rubber (AR) particles manually cut by a tailor scissor and optical microscopy image of the CNTs coated on the AR particle. Scanning electron microscoyp of cryofractures of CNT@2AR. Transmission electron microscopy image of the localization of CNTs at the AR phase in the PVDF composite CNT@10AR. Electrical properties of PVDF composites with various CNT@AR having different AR contents prior to testing. Resistance change plotted with strain loading for the PVDF/CNT composite with a CNT content (0.75 wt.%) slightly above the percolation threshold (0.6 wt.%) and PVDF/CNT@10AR composite. Stress-strain curve of PVDF/CNT and PVDF/CNT@10AR composites.

Acknowledgements

22


Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We thank Mr. Felix Buchta for the melt mixing of PVDF nanocomposites, Dr. Konrad Schneider and Mr. Dietmar Krause for cutting dog-bone shaped specimens for tensile testing and Mrs. Manuela Heber for SEM and TEM observation. In addition, we appreciate the support provided by Dr. Sven Wiessner and Mr. Jurk Rene in selecting and providing the ACM material. Kai Ke thanks the China Scholarship Council for the financial support (grant 201206240047) for his Ph.D. study at the Leibniz Institute of Polymer Research Dresden. The funding from the International Science and Technology Cooperation Project of Sichuan Province (2017HH0066) is also acknowledged.

References 1.

Kang, I.; Schulz, M. J.; Kim, J. H.; Shanov, V.; Shi, D., A Carbon Nanotube Strain Sensor for

Structural Health Monitoring. Smart materials and structures 2006, 15 (3), 737. 2.

Böger, L.; Wichmann, M. H.; Meyer, L. O.; Schulte, K., Load and Health Monitoring in Glass Fibre

Reinforced Composites with an Electrically Conductive Nanocomposite Epoxy Matrix. Composites Science and Technology 2008, 68 (7), 1886-1894. 3.

Thostenson, E. T.; Chou, T.-W., Real-Time in Situ Sensing of Damage Evolution in Advanced Fiber

Composites Using Carbon Nanotube Networks. Nanotechnology 2008, 19 (21), 215713. 4.

Li, C.; Thostenson, E. T.; Chou, T.-W., Sensors and Actuators Based on Carbon Nanotubes and

Their Composites: A Review. Composites Science and Technology 2008, 68 (6), 1227-1249. 5.

Avilés, F.; Oliva‐Avilés, A. I.; Cen‐Puc, M., Piezoresistivity, Strain, and Damage Self‐Sensing

of Polymer Composites Filled with Carbon Nanostructures. Advanced Engineering Materials 2018, 20(7), 1701159. 6.

Eswaraiah, V.; Balasubramaniam, K.; Ramaprabhu, S., Functionalized Graphene Reinforced

Thermoplastic Nanocomposites as Strain Sensors in Structural Health Monitoring. Journal of Materials Chemistry 2011, 21 (34), 12626-12628. 7.

Eswaraiah, V.; Balasubramaniam, K.; Ramaprabhu, S., One-Pot Synthesis of Conducting

Graphene–Polymer Composites and Their Strain Sensing Application. Nanoscale 2012, 4(4), 1258-1262. 8.

Ferrreira, A.; Rocha, J.; Ansón-Casaos, A.; Martínez, M.; Vaz, F.; Lanceros-Mendez, S.,

Electromechanical Performance of Poly (Vinylidene Fluoride)/Carbon Nanotube Composites for Strain Sensor Applications. Sensors and actuators A: physical 2012, 178, 10-16. 9.

Ferreira, A.; Martínez, M.; Ansón-Casaos, A.; Gómez-Pineda, L.; Vaz, F.; Lanceros-Mendez, S., 23



Page 24 of 29

Relationship between Electromechanical Response and Percolation Threshold in Carbon Nanotube/Poly (Vinylidene Fluoride) Composites. Carbon 2013, 61, 568-576. 10. Ferreira, A.; Lanceros-Mendez, S., Piezoresistive Response of Spray-Printed Carbon Nanotube/Poly (Vinylidene Fluoride) Composites. Composites Part B: Engineering 2016, 96, 242-247. 11. Georgousis, G.; Pandis, C.; Kalamiotis, A.; Georgiopoulos, P.; Kyritsis, A.; Kontou, E.; Pissis, P.; Micusik, M.; Czanikova, K.; Kulicek, J., Strain Sensing in Polymer/Carbon Nanotube Composites by Electrical Resistance Measurement. Composites Part B: Engineering 2015, 68, 162-169. 12. Ke, K.; Pötschke, P.; Wiegand, N.; Krause, B.; Voit, B., Tuning the Network Structure in Poly (Vinylidene Fluoride)/Carbon Nanotube Nanocomposites Using Carbon Black: Toward Improvements of Conductivity and Piezoresistive Sensitivity. ACS applied materials & interfaces 2016, 8(22), 14190-14199. 13. Hu, N.; Fukunaga, H.; Atobe, S.; Liu, Y.; Li, J., Piezoresistive Strain Sensors Made from Carbon Nanotubes Based Polymer Nanocomposites. Sensors 2011, 11 (11), 10691-10723. 14. Ke, K.; Pötschke, P.; Gao, S.; Voit, B., An Ionic Liquid as Interface Linker for Tuning Piezoresistive Sensitivity and Toughness in Poly (Vinylidene Fluoride)/Carbon Nanotube Composites. ACS applied materials & interfaces 2017, 9 (6), 5437-5446. 15. Guo, H.; Xiao, G.; Mrad, N.; Yao, J., Fiber Optic Sensors for Structural Health Monitoring of Air Platforms. Sensors 2011, 11 (4), 3687-3705. 16. Li, H.-N.; Li, D.-S.; Song, G.-B., Recent Applications of Fiber Optic Sensors to Health Monitoring in Civil Engineering. Engineering structures 2004, 26 (11), 1647-1657. 17. Avilés, F.; May-Pat, A.; Canché-Escamilla, G.; Rodríguez-Uicab, O.; Ku-Herrera, J. J.; Duarte-Aranda, S.; Uribe-Calderon, J.; Gonzalez-Chi, P. I.; Arronche, L.; La Saponara, V., Influence of Carbon Nanotube on the Piezoresistive Behavior of Multiwall Carbon Nanotube/Polymer Composites. Journal of Intelligent Material Systems and Structures 2016, 27 (1), 92-103. 18. Bautista-Quijano, J. R.; Pötschke, P.; Brünig, H.; Heinrich, G., Strain Sensing, Electrical and Mechanical Properties of Polycarbonate/Multiwall Carbon Nanotube Monofilament Fibers Fabricated by Melt Spinning. Polymer 2016, 82, 181-189. 19. Zhao, J.; Dai, K.; Liu, C.; Zheng, G.; Wang, B.; Liu, C.; Chen, J.; Shen, C., A Comparison between Strain Sensing Behaviors of Carbon Black/Polypropylene and Carbon Nanotubes/Polypropylene Electrically Conductive Composites. Composites Part A: Applied Science and Manufacturing 2013, 48, 129-136. 20. Zheng, Y.; Li, Y.; Li, Z.; Wang, Y.; Dai, K.; Zheng, G.; Liu, C.; Shen, C., The Effect of Filler Dimensionality on the Electromechanical Performance of Polydimethylsiloxane Based Conductive Nanocomposites for Flexible Strain Sensors. Composites Science and Technology 2017, 139, 64-73. 21. Liu, S.; Lin, Y.; Wei, Y.; Chen, S.; Zhu, J.; Liu, L., A High Performance Self-Healing Strain Sensor with Synergetic Networks of Poly (Ɛ-Caprolactone) Microspheres, Graphene and Silver Nanowires. Composites Science and Technology 2017, 146, 110-118. 22. Liu, H.; Gao, J.; Huang, W.; Dai, K.; Zheng, G.; Liu, C.; Shen, C.; Yan, X.; Guo, J.; Guo, Z., Electrically Conductive Strain Sensing Polyurethane Nanocomposites with Synergistic Carbon Nanotubes and Graphene Bifillers. Nanoscale 2016, 8 (26), 12977-12989. 23. Pu, J.-H.; Zhao, X.; Zha, X.-J.; Bai, L.; Ke, K.; Bao, R.-Y.; Liu, Z.-Y.; Yang, M.-B.; Yang, W., Multilayer Structured AgNW/WPU-Mxene Fiber Strain Sensors with Ultrahigh Sensitivity and Wide Operating Range for Wearable Monitoring and Healthcare. Journal of Materials Chemistry A 2019, 7, 15913-15923. 24


Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24. Pu, J.-H.; Zha, X.-J.; Zhao, M.; Li, S.; Bao, R.-Y.; Liu, Z.-Y.; Xie, B.-H.; Yang, M.-B.; Guo, Z.; Yang, W., 2d End-to-End Carbon Nanotube Conductive Networks in Polymer Nanocomposites: A Conceptual Design to Dramatically Enhance the Sensitivities of Strain Sensors. Nanoscale 2018, 10(5), 2191-2198. 25. Kang, I.; Khaleque, M. A.; Yoo, Y.; Yoon, P. J.; Kim, S.-Y.; Lim, K. T., Preparation and Properties of Ethylene Propylene Diene Rubber/Multi Walled Carbon Nanotube Composites for Strain Sensitive Materials. Composites Part A: Applied Science and Manufacturing 2011, 42 (6), 623-630. 26. Costa, P.; Silva, J.; Ansón-Casaos, A.; Martinez, M.; Abad, M.; Viana, J.; Lanceros-Mendez, S., Effect of Carbon Nanotube Type and Functionalization on the Electrical, Thermal, Mechanical and Electromechanical Properties of Carbon Nanotube/Styrene–Butadiene–Styrene Composites for Large Strain Sensor Applications. Composites Part B: Engineering 2014, 61, 136-146. 27. Yan, C.; Wang, J.; Kang, W.; Cui, M.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P. S., Highly Stretchable Piezoresistive Graphene–Nanocellulose Nanopaper for Strain Sensors. Advanced materials 2014, 26 (13), 2022-2027. 28. Boland, C. S.; Khan, U.; Backes, C.; O’Neill, A.; McCauley, J.; Duane, S.; Shanker, R.; Liu, Y.; Jurewicz, I.; Dalton, A. B., Sensitive, High-Strain, High-Rate Bodily Motion Sensors Based on Graphene–Rubber Composites. ACS nano 2014, 8 (9), 8819-8830. 29. Ma, L.-F.; Bao, R.-Y.; Dou, R.; Zheng, S.-D.; Liu, Z.-Y.; Zhang, R.-Y.; Yang, M.-B.; Yang, W., Conductive Thermoplastic Vulcanizates (Tpvs) Based on Polypropylene (Pp)/Ethylene-Propylene-Diene Rubber (Epdm) Blend: From Strain Sensor to Highly Stretchable Conductor. Composites Science and Technology 2016, 128, 176-184. 30. Yu S.L., Wang X. P., Xiang H.X., Zhu L.P., Tebyetekerwa M., Zhu M.F., Superior Piezoresistive Strain Sensing Behaviors of Carbon Nanotubes in One-Dimensional Polymer Fiber Structure. Carbon 2018, 140,1- 9. 31. Li, Y.; Zhou, B.; Zheng, G.; Liu, X.; Li, T.; Yan, C.; Cheng, C.; Dai, K.; Liu, C.; Shen, C., Continuously Prepared Highly Conductive and Stretchable Swnt/Mwnt Synergistically Composited Electrospun Thermoplastic Polyurethane Yarns for Wearable Sensing. Journal of Materials Chemistry C 2018, 6 (9), 2258-2269. 32. Soltani, R.; Katbab, A., The Role of Interfacial Compatibilizer in Controlling the Electrical Conductivity and Piezoresistive Behavior of the Nanocomposites Based on Rtv Silicone Rubber/Graphite Nanosheets. Sensors and Actuators A: Physical 2010, 163 (1), 213-219. 33. Wang, L.; Wang, Z.; Wang, Y.; Wang, X.; Wang, H.; Lu, G.; Zhao, D.; Li, Z., Styrene‐Butadiene‐ Styrene Copolymer Compatibilized Interfacial Modified Multiwalled Carbon Nanotubes with Mechanical and Piezoresistive Properties. Journal of Applied Polymer Science 2016, 133 (5), 42945. 34. Hwang, J.; Jang, J.; Hong, K.; Kim, K. N.; Han, J. H.; Shin, K.; Park, C. E., Poly (3-Hexylthiophene) Wrapped Carbon Nanotube/Poly (Dimethylsiloxane) Composites for Use in Finger-Sensing Piezoresistive Pressure Sensors. Carbon 2011, 49 (1), 106-110. 35. Lin, L.; Liu, S.; Zhang, Q.; Li, X.; Ji, M.; Deng, H.; Fu, Q., Towards Tunable Sensitivity of Electrical Property to Strain for Conductive Polymer Composites Based on Thermoplastic Elastomer. ACS applied materials & interfaces 2013, 5 (12), 5815-5824. 36. Hou, Y.; Wang, D.; Zhang, X.-M.; Zhao, H.; Zha, J.-W.; Dang, Z.-M., Positive Piezoresistive Behavior

of

Electrically

Conductive

Alkyl-Functionalized

Graphene/Polydimethylsilicone

Nanocomposites. Journal of Materials Chemistry C 2013, 1 (3), 515-521. 37. Xiao, Y.-j.; Wang, W.-y.; Lin, T.; Chen, X.-j.; Zhang, Y.-t.; Yang, J.-h.; Wang, Y.; Zhou, Z.-w., Largely Enhanced Thermal Conductivity and High Dielectric Constant of Poly (Vinylidene Fluoride)/Boron 25



Page 26 of 29

Nitride Composites Achieved by Adding a Few Carbon Nanotubes. The Journal of Physical Chemistry C 2016, 120 (12), 6344-6355. 38. Sezen, M.; Register, J. T.; Yao, Y.; Glisic, B.; Loo, Y.-L. In Exploiting the Different Polarity in Piezoresistive Characteristics of Conducting Polymers for Strain Gauge Applications, APS Meeting Abstracts, 2015. 39. Jang, S.; Kim, J.; Kim, D. W.; Kim, J.; Chun, S.; Lee, H. J.; Yi, G.-R.; Pang, C., Carbon-Based, Ultraelastic, Hierarchically Coated Fiber Strain Sensors with Crack-Controllable Beads. ACS applied materials & interfaces 2019, 11(16), 15079-15087. 40. Jiang, Y.; Liu, Z.; Wang, C.; Chen, X., Heterogeneous Strain Distribution of Elastomer Substrates to Enhance the Sensitivity of Stretchable Strain Sensors. Accounts of chemical research 2018, 52(1), 82-90. 41. Abolhasani, M. M.; Naebe, M.; Jalali-Arani, A.; Guo, Q., Influence of Miscibility Phenomenon on Crystalline Polymorph Transition in Poly (Vinylidene Fluoride)/Acrylic Rubber/Clay Nanocomposite Hybrid. PloS one 2014, 9 (2), e88715. 42. Abolhasani, M. M.; Guo, Q.; Jalali‐Arani, A.; Nazockdast, H., Poly (Vinylidene Fluoride)–Acrylic Rubber Partially Miscible Blends: Phase Behavior and Its Effects on the Mechanical Properties. Journal of applied polymer science 2013, 130 (2), 1247-1258. 43. Abolhasani, M.; Jalali-Arani, A.; Nazockdast, H.; Guo, Q., Poly (Vinylidene Fluoride)-Acrylic Rubber Partially Miscible Blends: Crystallization within Conjugated Phases Induce Dual Lamellar Crystalline Structure. Polymer 2013, 54 (17), 4686-4701. 44. Abolhasani, M. M.; Fashandi, H.; Naebe, M., Crystalline Polymorph Transition in Poly (Vinylidene Fluoride)(Pvdf)/Acrylic Rubber (Acm)/Clay Partially Miscible Hybrid. Polymer bulletin 2016, 73(1), 65-73. 45. Ke, K.; Pötschke, P.; Jehnichen, D.; Fischer, D.; Voit, B., Achieving Β-Phase Poly (Vinylidene Fluoride) from Melt Cooling: Effect of Surface Functionalized Carbon Nanotubes. Polymer 2014, 55 (2), 611-619. 46. Bayazit, M. K.; Clarke, L. S.; Coleman, K. S.; Clarke, N., Pyridine-Functionalized Single-Walled Carbon Nanotubes as Gelators for Poly (Acrylic Acid) Hydrogels. Journal of the American Chemical Society 2010, 132 (44), 15814-15819. 47. Kumar, S.; Rath, T.; Mahaling, R.; Mukherjee, M.; Khatua, B.; Das, C., Multi-Walled Carbon Nanotubes/Polymer Composites in Absence and Presence of Acrylic Elastomer (Acm). Journal of nanoscience and nanotechnology 2009, 9 (5), 2981-2990. 48. Natarajan, T. S.; Eshwaran, S. B.; Stöckelhuber, K. W.; Wießner, S.; Pötschke, P.; Heinrich, G.; Das, A., Strong Strain Sensing Performance of Natural Rubber Nanocomposites. ACS applied materials & interfaces 2017, 9 (5), 4860-4872. 49. Slobodian, P.; Riha, P.; Sáha, P., A Highly-Deformable Composite Composed of an Entangled Network of Electrically-Conductive Carbon-Nanotubes Embedded in Elastic Polyurethane. Carbon 2012, 50 (10), 3446-3453. 50. Shin, M. K.; Oh, J.; Lima, M.; Kozlov, M. E.; Kim, S. J.; Baughman, R. H., Elastomeric Conductive Composites Based on Carbon Nanotube Forests. Advanced materials 2010, 22 (24), 2663-2667. 51. Lozano-Pérez, C.; Cauich-Rodríguez, J.; Avilés, F., Influence of Rigid Segment and Carbon Nanotube Concentration on the Cyclic Piezoresistive and Hysteretic Behavior of Multiwall Carbon Nanotube/Segmented Polyurethane Composites. Composites Science and Technology 2016, 128, 25-32. 26


Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


52. Fan, Q.; Qin, Z.; Gao, S.; Wu, Y.; Pionteck, J.; Mäder, E.; Zhu, M., The Use of a Carbon Nanotube Layer on a Polyurethane Multifilament Substrate for Monitoring Strains as Large as 400%. Carbon 2012, 50 (11), 4085-4092. 53. Bautista‐Quijano, J.; Aviles, F.; Cauich‐Rodriguez, J., Sensing of Large Strain Using Multiwall Carbon Nanotube/Segmented Polyurethane Composites. Journal of Applied Polymer Science 2013, 130 (1), 375-382. 54. Selvan, N. T.; Eshwaran, S.; Das, A.; Stöckelhuber, K.; Wießner, S.; Pötschke, P.; Nando, G.; Chervanyov, A.; Heinrich, G., Piezoresistive Natural Rubber-Multiwall Carbon Nanotube Nanocomposite for Sensor Applications. Sensors and Actuators A: Physical 2016, 239, 102-113. 55. Zetina-Hernández, O.; Duarte-Aranda, S.; May-Pat, A.; Canché-Escamilla, G.; Uribe-Calderon, J.; Gonzalez-Chi, P.; Avilés, F., Coupled Electro-Mechanical Properties of Multiwall Carbon Nanotube/Polypropylene Composites for Strain Sensing Applications. Journal of materials science 2013, 48 (21), 7587-7593. 56. Robert, C.; Feller, J. F.; Castro, M., Sensing Skin for Strain Monitoring Made of Pc–Cnt Conductive Polymer Nanocomposite Sprayed Layer by Layer. ACS applied materials & interfaces 2012, 4(7), 3508-3516. 57. Qi, H.; Liu, J.; Gao, S.; Mäder, E., Multifunctional Films Composed of Carbon Nanotubes and Cellulose Regenerated from Alkaline–Urea Solution. Journal of Materials Chemistry A 2013, 1(6), 2161-2168. 58. Qi, H.; Schulz, B. r.; Vad, T.; Liu, J.; Mäder, E.; Seide, G.; Gries, T., Novel Carbon Nanotube/Cellulose Composite Fibers as Multifunctional Materials. ACS applied materials & interfaces 2015, 7 (40), 22404-22412. 59. Sencadas, V.; Lanceros-Méndez, S.; i Serra, R. S.; Balado, A. A.; Ribelles, J. G., Relaxation Dynamics of Poly (Vinylidene Fluoride) Studied by Dynamical Mechanical Measurements and Dielectric Spectroscopy. The European Physical Journal E 2012, 35 (5), 41. 60. Mano, J.; Sencadas, V.; Costa, A. M.; Lanceros-Méndez, S., Dynamic Mechanical Analysis and Creep Behaviour of Β-Pvdf Films. Materials Science and Engineering: A 2004, 370 (1-2), 336-340. 61. Zhao, Q.; Wagner, H. D., Raman Spectroscopy of Carbon–Nanotube–Based Composites. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 2004, 362 (1824), 2407-2424. 62. Lefrant, S.; Baibarac, M.; Baltog, I., Raman and Ftir Spectroscopy as Valuable Tools for the Characterization of Polymer and Carbon Nanotube Based Composites. Journal of Materials Chemistry 2009, 19 (32), 5690-5704. 63. Bokobza, L.; Zhang, J., Raman Spectroscopic Characterization of Multiwall Carbon Nanotubes and of Composites. Express Polymer Letters 2012, 6 (7), 601-608. 64. Yu, S.; Zheng, W.; Yu, W.; Zhang, Y.; Jiang, Q.; Zhao, Z., Formation Mechanism of Β-Phase in Pvdf/Cnt Composite Prepared by the Sonication Method. Macromolecules 2009, 42 (22), 8870-8874. 65. Patro, T. U.; Mhalgi, M. V.; Khakhar, D.; Misra, A., Studies on Poly (Vinylidene Fluoride)–Clay Nanocomposites: Effect of Different Clay Modifiers. Polymer 2008, 49 (16), 3486-3499. 66. Li, Y.; Xu, J.-Z.; Zhu, L.; Zhong, G.-J.; Li, Z.-M., Role of Ion–Dipole Interactions in Nucleation of Gamma Poly (Vinylidene Fluoride) in the Presence of Graphene Oxide During Melt Crystallization. The Journal of Physical Chemistry B 2012, 116 (51), 14951-14960. 67. Xing, C.; Zhao, L.; You, J.; Dong, W.; Cao, X.; Li, Y., Impact of Ionic Liquid-Modified Multiwalled Carbon Nanotubes on the Crystallization Behavior of Poly (Vinylidene Fluoride). The Journal of 27



Page 28 of 29

Physical Chemistry B 2012, 116 (28), 8312-8320. 68. Zhang, W.-b.; Zhang, Z.-x.; Yang, J.-h.; Huang, T.; Zhang, N.; Zheng, X.-t.; Wang, Y.; Zhou, Z.-w., Largely Enhanced Thermal Conductivity of Poly (Vinylidene Fluoride)/Carbon Nanotube Composites Achieved by Adding Graphene Oxide. Carbon 2015, 90, 242-254. 69. Li, Y.; Iwakura, Y.; Zhao, L.; Shimizu, H., Nanostructured Poly (Vinylidene Fluoride) Materials by Melt Blending with Several Percent of Acrylic Rubber. Macromolecules 2008, 41 (9), 3120-3124.

28


Page 29 of 29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A Nuomici-Inspired Universal Strategy for Boosting

Recommend Documents