Tuning the Magnitude and the Polarity of the Piezoresistive Response

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Tuning the Magnitude and the Polarity of the Piezoresistive Response of Polyaniline through Structural Control Melda Sezen-Edmonds, Petr P. Khlyabich, and Yueh-Lin Loo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00101 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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ACS Applied Materials & Interfaces

Tuning the Magnitude and the Polarity of the Piezoresistive Response of Polyaniline through Structural Control Melda Sezen-Edmonds1, Petr P. Khlyabich1, and Yueh-Lin Loo*,1,2 1

Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08544

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Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544

KEYWORDS: flexible electronics, conducting polymers, piezoresistivity, polyaniline, gauge factor, strain gauge, resistive sensors

ABSTRACT

We demonstrate the tunability of both the polarity and the magnitude of the piezoresistive response of polyaniline that is template-synthesized on poly(2-acrylamido-2-methyl-1propanesulfonic acid), PANI-PAAMPSA, by altering the template molecular weight. Piezoresistivity is quantified by gauge factor, a unitless parameter that relates changes in electrical resistance to applied strain. The gauge factor of PANI-PAAMPSA decreases linearly and becomes negative with decreasing PAAMPSA molecular weight. The polarity of PANIPAAMPSA’s gauge factor is determined by macroscopic connectivity across thin films. PANI-

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PAAMPSA thin films comprise electrostatically stabilized particles whose size is determined at the onset of synthesis. An increase in the interparticle spacing with applied strain results in a positive gauge factor. The presence of PANI crystallites increases connectivity between particles; these samples instead exhibit a negative gauge factor whereby the resistance decreases with increasing strain. The tunability of the piezoresistive response of these conducting polymers allows their utilization in a broad range of flexible electronics applications, including thermoand chemo-resistive sensors and strain gauges.

Introduction Conducting polymers are widely used in next-generation electronic devices because they possess the chemical, mechanical, and processing attributes of conventional polymers, yet have electrical conductivities approaching those of metals.1 Further, the mechanical compliance of conducting polymers with flexible substrates makes them excellent candidates for use in conformal electronic devices, either as electrodes2–4 or as sensing units.5,6 Like other conducting materials, however, conducting polymers are inherently piezoresistive; changes in their electrical resistances due to mechanical deformations often limit their use as chemo- or thermo-resistive devices.7 This piezoresistive effect can either arise from changes in the resistance of the material due to changes in the sample geometry with strain or it can be an intrinsic materials property stemming from changes in resistivity of the material due to the structural changes with strain.8 Whereas the changes in the sample geometry under tensile strain always cause the resistance of conducting materials with positive Poisson’s ratio to increase, changes in the material’s structure can cause an increase (e.g., due to the formation of cracks) or decrease (e.g., due to straininduced chain alignment) in the resistance with strain.9 Understanding the intrinsic factors that


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govern the piezoresistivity of conducting polymers is therefore important so materials design can be tailored according to specific application needs. The relationships between the structure of conducting polymers and their piezoresistive properties have mostly been investigated under large strains that inadvertently result in plastic deformation. Such plastic deformation can either result in strain-induced morphological changes that can in turn enhance the polymer’s electrical conductivity, or in cracks and fractures that are detrimental to the polymer’s electrical conductivity.9-11 The solid-state structure of poly(3,4ethylenedioxythiophene):polystyrene sulfonate, PEDOT:PSS, (Clevios pH1000) thin films, for example, is shown to drastically impact how PEDOT:PSS thin films respond to mechanical strain. Applying a 60% tensile strain to pristine PEDOT:PSS thin films causes PEDOT-rich domains to coalesce and form connected pathways for charge transport that results in an increase in the conductivity. Since the films are permanently deformed, the improvement in electrical conductivity is retained even when the strain is removed. On the other hand, the addition of 5 wt% dimethyl sulfoxide, DMSO, to PEDOT:PSS dispersion before deposition eliminates any conductivity improvement with subsequent strain. DMSO is thought to uniformly disperse PEDOT chains, so stretching does not further alter domain connectivity.9 In order to utilize conducting polymers in resistive sensing applications, where reversible changes in the resistance are correlated with changes in the environment, their piezoresistive response under elastic deformation must be understood. Strain-induced changes in the resistance of conducting polymers, for example, can be utilized to detect mechanical deformation of buildings or bridges or for monitoring human motion.12,13 In strain-sensing applications, the mechanical deformation is quantified through a unitless parameter called the gauge factor, GF, which is defined as:


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GF

∆ = ε

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Equation 1

where Ro is the resistance at zero strain, ∆R is the change in the resistance, and ε is the strain.14 Maximizing GF optimizes for sensitivity in the strain direction. Conversely, if the conducting polymers are used for flexible thermo-resistive or chemo-resistive sensing applications, small changes in resistance in response to mechanical deformations – even reversible – will cause error in sensor output.15,16 Minimizing the GF of conducting polymers is thus important to minimize mechanical deformation-related resistance drifts during such sensor use. Polyaniline, PANI, is a conducting polymer that has shown promise in flexible or conformable electronics applications.5,17-19 Template polymerization of aniline along the polymeric acid poly(2-acrylamido-2-methyl-1-propane-sulfonic acid), PAAMPSA, results in the formation of water-dispersible and electrically conducting polymer complex PANI-PAAMPSA with its chemical structure shown in Scheme 1.20 With its biocompatibility and stability at ambient conditions, PANI-PAAMPSA is especially suitable for wearable electronics and human health monitoring applications.19,21 As-synthesized PANI-PAAMPSA forms electrostatically stabilized colloidal particles, and this particulate nature is retained when PANI-PAAMPSA is cast as thin films from its water dispersions. Previous studies in our group showed that decreasing the molecular weight of PAAMPSA results in the formation of smaller PANIPAAMPSA particles, and increases PANI’s crystallinity concurrently. These structural changes have resulted in an increase in the conductivity of PANI-PAAMPSA from 0.4 S/cm to 1.1 S/cm when a template of 724 kg mol-1 PAAMPSA is replaced with 45 kg mol-1 PAAMPSA.20 In this work, we show the tunability of PANI-PAAMPSA’s piezoresistive response by changing the molecular weight of PAAMPSA at the onset of synthesis. As we lower the molecular weight of PAAMPSA, the GF of PANI-PAAMPSA decreases progressively, and even becomes negative


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for PANI-PAAMPSA that is synthesized with the two lowest molecular-weight PAAMPSA (45 kg mol-1 and 106 kg mol-1). Although negative and positive GFs (GF of -4.9 and 22) have been reported for a commercial grade of PANI (PANIPOL T), the source of this difference in the polarity of GF was not fully understood.22,23 With PANI-PAAMPSA as our model system, we show how the structure of PANI affects the polarity and the magnitude of their piezoresistive properties. We conclude that the change in the GF polarity with decreasing PAAMPSA molecular weight is a result of increased crystallinity of PANI that is likely to connect the individual PANI-PAAMPSA particles, which can then undergo strain-induced alignment under tensile deformation. Control of the particle size and the extent of crystallization by altering the molecular weight of PAAMPSA has enabled the tuning of both the magnitude and the polarity of PANI-PAAMPSA’s GF. Such tunability allows us to address the need of high GF for strain sensing and near-zero GF for thermo- or chemo-resistive sensing applications with a single class of material. Results & Discussion In order to test the piezoresistive response of PANI-PAAMPSA-X, where X represents the molecular weight of PAAMPSA in kg mol-1, we prepared serpentine patterns of PANIPAAMPSA-X on flexible polyimide substrates (Figure S1). A serpentine pattern is used in order to maximize sensitivity in the direction of the applied strain and minimize transverse strain during our uniaxial tensile measurements. Figure 1 shows the change in resistance of PANIPAAMPSA-X patterns with respect to their zero-strain resistance, Ro, (black line) under uniaxial cyclic strain (blue line). The films were stretched in their strain-sensitive direction. The recovery of Ro after the strain is released indicates that these tests were performed in the elastic deformation region of the conducting polymer patterns. Figures 1a and b show that the


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resistances of PANI-PAAMPSA-724 and PANI-PAAMPSA-255 patterns are in-phase with applied strain. An opposite trend is observed for PANI-PAAMPSA-106 and PANI-PAAMPSA45 patterns (Figures 1c and d); resistances of these conducting polymer patterns are out-of-phase with applied strain. Interestingly, we observe hysteresis in the piezoresistive response of PANIPAAMPSA-255 between the loading and unloading cycles. Such hysteresis is commonly observed in the piezoresistive response of carbon nanotube/polymer composites due to their thermoplastic nature.24 Hysteresis was also reported in the piezoresistive response of nanoarchitectured TixCuy films due to structural defects in the crystals.8 We do not believe these to be the origin of the hysteresis we observe in PANI-PAAMPSA-255. While the exact source of the hysteresis observed in the piezoresistive response of PANI-PAAMPSA-255 patterns is still under investigation, we have previously shown that such hysteresis can be eliminated through postdeposition dichloroacetic acid, DCA, treatment.19 Similarly, we are able to eliminate this nonlinearity in the piezoresistive response of PANI-PAAMPSA-255 through structural modification with DCA treatment (Figure S2). We quantified the piezoresistive response of PANI-PAAMPSA-X patterns by calculating their GFs from the slope of the normalized resistance change with strain (Figure S3) using Equation 1. Figure 2a shows the change in GF of PANI-PAAMPSA-X (black data points) depending on the molecular weight of the PAAMPSA template used in the synthesis. The GF of PANI-PAAMPSA-X decreases and becomes negative with decreasing PAAMPSA molecular weight, changing from 3.7 + 0.28 to -0.60 + 0.15 as the PAAMPSA molecular weight is decreased from 724 kg mol-1 to 45 kg mol-1. The tunability of the GF enables us to select the appropriate grade of PANI-PAAMPSA-X per application needs. For example, PANIPAAMPSA-724 should be used among the different PANI grades for strain sensing. At a GF of


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3.7, PANI-PAAMPSA-724 strain gauges are almost twice as sensitive to strain as commercially available metal-foil strain gauges (GF = 2).25 In examples of chemo-resistive or thermo-resistive sensors in which any electrical response to mechanical strain should be suppressed to maximize signal-to-noise output, however, PANI-PAAMPSA-106 should instead be used given that it has the smallest GF among the different PANI grades tested (GF = -0.34). The linear relationship between GF and the molecular weight of PAAMPSA also provides an opportunity to specify a priori new PANI-PAAMPSA with tailored piezoresistive responses, and synthesize according to these specifications. Our empirical correlation suggests that PANI synthesized with PAAMPSA having a molecular weight of 140 kg mol-1 should yield conducting patterns having a GF of zero. At GF = 0, we can completely suppress errors associated with resistance change due to mechanical deformation in flexible chemo- or thermo-resistive sensors. Unique to PANI-PAAMPSA-X is our easy access to both negative and positive GFs of different magnitudes. Instead of synthesizing a new grade of PANI-PAAMPSA, existing PANIPAAMPSA-X, having different GF polarities with similar magnitudes (e.g., PANI-PAAMPSA106 and PANI-PAAMPSA-255), can thus be combined as an alternative approach to accessing near-zero net GFs.19 This approach for creating net-zero GF sensors is much more straightforward compared to prior reports of placing the active component of the sensor at its neutral bending plane or lithographically patterning the active component in mesh-like structures to reduce strain.16,26,27 These near-zero GF conducting patterns can also be combined with highGF PANI-PAAMPSA strain sensors in order to account for the non-strain related drifts in the electrical resistance of PANI-PAAMPSA. Additionally, PANI-PAAMPSA-X grades having similar magnitudes of GF but opposite polarities can also be incorporated in Wheatstone bridge


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circuits to build strain gauges that can quantitatively decouple thermal, shrinkage, and mechanical strains.28 We previously observed an anti-correlation between the electrical conductivity of PANIPAAMPSA-X films and the molecular weight of PAAMPSA template.29 The improvements in the electrical conductivity of PANI-PAAMPSA-X films with decreasing PAAMPSA molecular weight are correlated with the improved PANI crystallinity and the particle connectivity of the resulting films.20,29 The dependence of PANI-PAAMPSA-X films’ GF on the molecular weight of PAAMPSA is reminiscent of how the electrical conductivity of PANI-PAAMPSA-X is correlated with PAAMPSA molecular weight. We thus surmised that this trend in the piezoresistive response of PANI-PAAMPSA-X films to also have structural origins. A positive GF is known to be a result of increasing separation between conducting domains under tensile deformation.30,31 In fact, we previously observed a positive GF in PANIPAAMPSA-724 films, and attributed this positive GF to increased separation between PANIPAAMPSA-724 particles upon stretching.19 As synthesized, PANI-PAAMPSA-X forms electrostatically stabilized colloidal particles, and this particulate nature is retained in thin films of PANI-PAAMPSA-X that are cast from its water dispersions.20,29 Figure 2c (blue data points) shows that the hydrodynamic diameters of the as-synthesized PANI-PAAMPSA-X particles decrease with decreasing PAAMPSA molecular weight. Figure 3a quantifies the correlation between GF and the hydrodynamic diameter of particles of as-synthesized PANI-PAAMPSA-X. The GF is positive for the PANI-PAAMPSA-X grades comprising the largest particles. As the hydrodynamic diameter of as-synthesized PANI-PAAMPSA-X decreases, the GF becomes progressively negative. The observation that the GF decreases with particle size for PANIPAAMPSA-724 and PANI-PAAMPSA-255 is consistent with previous reports that the GF of


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films composing gold nanoparticles decrease from 250 to 50 with decreasing nanoparticle diameter.31 This trend is attributed to smaller particles being more interconnected compared to larger particles, and therefore films comprising smaller particles are less sensitive to straininduced interparticle separation.31,32 The discontinuity in the data correlating the GF to the hydrodynamic diameter of PANI-PAAMPSA-X and the switch in the GF polarity suggest another mechanism to also contribute to PANI-PAAMPSA-X films’ piezoresistive response. We also tested PANI that is doped with polystyrene sulfonate, PSS. Dynamic light scattering experiments revealed that PANI:PSS forms particles with Dh = 205 + 14 nm (Figure S5). When cast from its aqueous dispersions, PANI:PSS forms films that are topographically similar to PANI-PAAMPSA-45 films.20,33 If particle size is the only determining factor in the piezoresistive response of PANI films, we would expect PANI:PSS, like PANI-PAAMPSA-45, to exhibit a negative GF per Figure 3a data. Figure 4 shows the relative change in the resistance of a PANI:PSS serpentine pattern under cyclic strain in its strain-sensitive direction. PANI:PSS patterns instead have a resistance that is in-phase with applied strain, yielding a positive GF of 3.0 + 0.1 (Figure S6). The observation that PANI:PSS exhibits a positive GF despite its small particle size is further evidence that factors other than the particle size are at play in determining the piezoresistive response of PANI films. Interestingly, powder X-ray diffraction, XRD, studies show that PANI:PSS, unlike PANI-PAAMPSA-45 but similar to PANI-PAAMPSA-724, is mostly amorphous (Figure S7). We therefore hypothesize that PANI crystallinity could also play a role in determining the piezoresistive response of PANI films. In order to examine the differences in the crystallinity of different PANI-PAAMPSA-X grades, we performed grazing-incidence X-ray diffraction, GIXD, on PANI-PAAMPSA-X thin films. Figure 2b shows the azimuthally integrated XRD traces of as-cast PANI-PAAMPSA-X


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films. All traces show a broad halo centered around q = 1.5 Å-1 that stems from amorphous PAAMPSA, and a reflection at q = 1.84 Å-1 that stems from PANI sub-chain alignment.29 Additional reflections associated with PANI appear in PANI-PAAMPSA-X that were synthesized with lower molecular weight PAAMPSA. We quantified the fractional crystallinity of PANI-PAAMPSA-X films through the ratio of the area under the reflections associated with crystalline PANI to the total area under the XRD trace. Figure 2c (black data points) shows that PANI becomes more crystalline as the template PAAMPSA molecular weight decreases. Figure 3b shows the dependence of GF on the fractional crystallinity of PANIPAAMPSA-X thin films. A negative GF is observed in the more crystalline PANI-PAAMPSA-X grades (PANI-PAAMPSA-45 and PANI-PAAMPSA-106). Not unlike the data shown in Figure 3a, we observe a discontinuity in the data that correlates the GF of PANI-PAAMPSA-X films to the fractional crystallinity; the GF switches to positive as PANI crystallinity decreases. We believe the negative GF in these higher crystallinity samples is a result of further alignment of crystalline PANI upon stretching that in turn promotes conduction in the direction of applied strain. Such strain-induced chain alignment has previously been observed in PANI that is doped with volatile acids under large strains (>400% strain), and plastic deformation of the polymer was shown to result in a decrease in the resistance of PANI.34 Here, we observe a similar decrease in resistance with stretching, even in the elastic deformation regime of the polymer. Taken together, our data suggests that PANI-PAAMPSA’s piezoresistive response is dominated by PANI’s crystallinity when the samples are sufficiently crystalline. In less crystalline samples, PANI-PAAMPSA’s piezoresistive response is dominated by the interparticle separation, which is in turn related to the particle size. However, having crystalline PANI and the ability for PANI crystals to align under strain alone cannot be sufficient to obtain negative GFs.


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Supporting this circumspection is the fact that the GF of gold nanoparticles, which are crystalline, can be entirely predicted based on its particle diameter alone.31 This observation indicates that the macroscopic piezoresistive response of gold nanoparticles is independent of the crystallinity within individual particles. We are thus left to surmise that the PANI crystals in the more crystalline PANI-PAAMPSA-X films must bridge neighboring particles in order for preferential alignment of PANI under strain to influence its piezoresistive response. This hypothesis is consistent with our previous report that the conducting PANI is preferentially localized on the exterior of the PANI-PAAMPSA particles.20 It should thus not be surprising that crystalline PANI enhances the particle connectivity in PANI-PAAMPSA thin films. Consistent with the notion that negative GF is accessed when PANI is sufficiently crystalline and when the conducting domains are connected, solvent annealing of PANIPAAMPSA-X in dichloroacetic acid, DCA, results in patterns having uniformly negative GF (blue data points in Figures 2a) that is independent of the molecular weight of PAAMPSA used in the synthesis. DCA treatment eliminates the particulate nature and induces further crystallization in PANI-PAAMPSA films.19,33 This structural change is confirmed by the data in Figures 2b (black line) and Figure 3b (red data point). The simultaneous elimination of the particulate nature of PANI-PAAMPSA and increase in PANI crystallinity with DCA treatment enable preferential alignment of PANI during strain to dominate their piezoresistive response. Post-deposition DCA annealing has thus provided access to negative GFs, even in PANIPAAMPSA-724 and PANI-PAAMPSA-255 that previously exhibited positive GFs in their ascast states. Conclusions


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We demonstrated systematic tunability of both the polarity and the magnitude of the piezoresistive response of PANI-PAAMPSA-X by controlling its thin-film morphology. By altering a single parameter, the molecular weight of PAAMPSA, at the onset of synthesis, we change the particle size of the resulting polymer complex and the crystallinity of PANI. The interplay between these parameters coupled with the PANI crystals’ ability to bridge the individual PANI-PAAMPSA-X particles determine the piezoresistive response of PANIPAAMPSA-X films. The tunability of the piezoresistive response of these water-dispersible, electrically-conducting polymers, especially accessing negative GFs, allow us to utilize them in a broad range of flexible and conformable electronics applications. Optimizing for the GF enables utility of PANI-PAAMPSA as sensitive strain sensors, and minimizing – even eliminating – GF enables the use of PANI-PAAMPSA as flexible resistive-temperature or chemical sensor that work accurately under mechanical deformations. Experimental Section Polymer synthesis: In order to synthesize PANI-PAAMPSA-X (where X represents the poly(ethylene oxide) equivalent number-average molecular weight of PAAMPSA used in the synthesis), we used aniline (Sigma-Aldrich, 99.5%), ammonium peroxydisulfate (98.9%, Fisher Scientific), and PAAMPSA (Scientific Polymer Products, 10.36 wt% in water, reported MW 800 kg mol-1) as purchased. We determined the poly(ethylene oxide), PEO, equivalent molecular weight of the as-purchased PAAMPSA to be 724 kg/mol. Lower molecular weight PAAMPSA (PAAMPSA-45, PAAMPSA-106 and PAAMPSA-255) were synthesized by conventional freeradical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid, AAMPSA, (Aldrich. 99%) according to previously published procedures.29 PANI-PAAMPSA-X was synthesized by template polymerization of aniline along PAAMPSA-X at 1:1 monomer-to-acid molar ratio in


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deionized water as described by Yoo, et al.29 PANI:PSS was synthesized by template polymerization of aniline along PSS according to published procedures.33 5 wt% aqueous dispersions of PANI-PAAMPSA-X and PANI:PSS were prepared, and stirred for 2 weeks prior to deposition. Tensile tests: Serpentine patterns of PANI-PAAMPSA-X or PANI:PSS on polyimide substrate were prepared as described in our previous work.19 Dichloroacetic acid, DCA, (Sigma-Aldrich, 99%) treatment was performed by vigorously shaking the samples at 100 oC DCA for 3 minutes. Samples were annealed at 170 oC for 30 minutes and kept under vacuum for at least 3 hours to remove any residual DCA.33 Cyclic tensile tests were performed by stretching the samples uniaxially in the strain-sensitive direction of the serpentine pattern using an Instron electromechanical universal testing machine (Model 5969) following previously published procedures.19 We performed cyclic tensile tests in the elastic deformation region of the conducting polymers. The resistances of the samples were recorded during the tensile tests using a four-probe geometry with Agilent 4145B Semiconductor Parameter Analyzer. At least 3 different films were tested for each sample, and each film was exposed to at least 10 strain cycles. Gauge factors were determined from linear fits to the normalized change in the resistance of the serpentine patterns with applied strain. Structural characterization: GIXD experiments were conducted at the G1 station (9.82 ± 0.05 keV) of the Cornell High Energy Synchrotron Source. The beam was chosen to be 0.05 mm tall and 1 mm wide. PANI-PAAMPSA-X aqueous dispersions were spin-coated on pre-cleaned glass substrate at 1000 rpm for 60 seconds, and DCA treatment was performed as described above. The width of each sample was 5 mm. The X-ray beam was aligned above the conducting polymer film’s critical angle but below that of the substrate, at a 0.17 o incident angle with the


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substrate. X-ray scattering was collected with a 2D CCD detector, positioned 113.9 mm from the sample. All GIXD images have been background subtracted. The fractional crystallinity of PANI-PAAMPSA-X films was calculated from their 2D-GIXD patterns by dividing the crystalline area by the total area under the azimuthally integrated traces. In order to calculate the total crystalline area, the area under the broad amorphous halo was subtracted from the total area. Powder XRD data was collected using Rigaku Miniflex X-ray diffractometer (with Cu-kα radiation at 30 kV and 15 mA) at a scan rate of 0.6 o min-1 by loading 100 mg of PANIPAAMPSA-45, PANI-PAAMPSA-724 and PANI:PSS. Dynamic light scattering, DLS, experiments were performed using ZetaSizer Malvern DLS instrument (Malvern Instruments, Malvern, UK) with a 633 nm laser and backscatter detection angle of 173 o on 0.001 wt % of PANI:PSS in 0.1 M NaCl aqueous solutions at 25 oC as described by Yoo et al.20 0.1 M NaCl was added in order to screen interparticle interactions. Hydrodynamic diameters of assynthesized PANI-PAAMPSA-X are obtained from previous publications.20


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FIGURES

Scheme 1. Chemical structure of electrically conducting PANI-PAAMPSA.


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Figure 1. Relative change in the resistance (black) of as-cast a) PANI-PAAMPSA-724, b) PANI-PAAMPSA-255, c) PANI-PAAMPSA-106, and d) PANI-PAAMPSA-45 serpentine patterns under cyclic strain (blue) along their strain-sensitive direction.


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Figure 2. a) GF of as-cast (black) and DCA-treated (blue) PANI-PAAMPSA-X serpentine patterns as a function of PAAMPSA molecular weight. Dashed lines represent linear fits to the data. b) Azimuthally integrated XRD traces of as-cast PANI-PAAMPSA-X and DCA-treated


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PANI-PAAMPSA-724 extracted from their 2D-GIXD patterns. Labels indicate the PAAMPSA template molecular weight used in the synthesis. c) Fractional crystallinity of as-cast PANIPAAMPSA-X thin films (black data points) and hydrodynamic diameter of the as-synthesized PANI-PAAMPSA particles (blue data points) as a function of template molecular weight. Fractional crystallinity is obtained from the azimuthally integrated XRD trace given in b), by calculating the ratios of the crystalline area to total area. Hydrodynamic diameter values are adapted from reference 20. Dashed lines connecting the data points are included as a guide to the eye.


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Figure 3. Change in the GF of PANI-PAAMPSA-X serpentine patterns with a) hydrodynamic diameter of the as-synthesized PANI-PAAMPSA particles, and b) fractional crystallinity of PANI-PAAMPSA-X films (red data point represents DCA-treated PANI-PAAMPSA-724). Linear fits to the data (dashed lines) are included showing the different slopes in the negative and the positive gauge factor regions.


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Figure 4. Relative change in the resistance (black) of as-cast PANI:PSS serpentine pattern under cyclic strain (blue) along its strain-sensitive direction.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional figures as described in main text (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Portion of this work was conducted at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR-1332208. The authors acknowledge Prof. Craig B. Arnold (Princeton University) for access to the Instron, Brian K. Wilson and Prof. Robert K. Prud’homme (Princeton University) for access to Dynamic Light Scattering instrument, and Dr. Joung-Eun Yoo for help in synthesising the low-molecular weight PANI-PAAMPSA-X grades. This work was partially supported by the Anonymous Fund, sponsored by the School of Engineering and Applied Science at Princeton University and by Princeton Center for Complex Materials that was funded by NSF-MRSEC under NSF award DMR-1420541.


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REFERENCES (1)

Epstein, A. J. Electrically Conducting Polymers: Science and Technology. MRS Bulletin.

1997, 22, 16–23. (2)

Tait, J. G.; Worfolk, B. J.; Maloney, S. A.; Hauger, T. C.; Elias, A. L.; Buriak, J. M.;

Harris, K. D. Spray Coated High-Conductivity PEDOT:PSS Transparent Electrodes for Stretchable and Mechanically-Robust Organic Solar Cells. Sol. Energy Mater. Sol. Cells. 2013, 110, 98–106. (3)

Khodagholy, D.; Doublet, T.; Gurfinkel, M.; Quilichini, P.; Ismailova, E.; Leleux, P.;

Herve, T.; Sanaur, S.; Bernard, C.; Malliaras, G. G. Highly Conformable Conducting Polymer Electrodes for in Vivo Recordings. Adv. Mater. 2011, 23, 268–272. (4)

Takamatsu, S.; Lonjaret, T.; Ismailova, E.; Masuda, A.; Itoh, T.; Malliaras, G. G.

Wearable Keyboard Using Conducting Polymer Electrodes on Textiles. Adv. Mater. 2015, 3, 3– 6. (5)

Jang, J.; Ha, J.; Cho, J. Fabrication of Water-Dispersible Polyaniline-Poly(4-

Styrenesulfonate) Nanoparticles for Inkjet-Printed Chemical-Sensor Applications. Adv. Mater. 2007, 19, 1772–1775. (6)

Freund, M. S.; Lewis, N. S. A Chemically Diverse Conducting Polymer-Based

“Electronic Nose”. Proc. Natl. Acad. Sci. U. S. A. 1995, 92, 2652–2656. (7)

Vijay, V.; Rao, A. D.; Narayan, K. S. In Situ Studies of Strain Dependent Transport

Properties of Conducting Polymers on Elastomeric Substrates. J. Appl. Phys. 2011, 109, 084525. (8)

Ferreira, A.; Borges, J.; Lopes, C.; Martin, N.; Lanceros-Mendez, S.; Vaz, F.

Piezoresistive Response of Nano-Architectured TixCuy Thin Films for Sensor Applications. Sensors Actuators A Phys. 2016, 247, 105–114. (9)

Lee, Y.-Y.; Lee, J.-H.; Cho, J.-Y.; Kim, N.-R.; Nam, D.-H.; Choi, I.-S.; Nam, K. T.; Joo,

Y.-C. Stretching-Induced Growth of PEDOT-Rich Cores: A New Mechanism for StrainDependent Resistivity Change in PEDOT:PSS Films. Adv. Funct. Mater. 2013, 23, 4020–4027.


22

Page 23 of 28

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


(10)

Pouget, J. P.; Jozefowicz, M. E.; Epstein, A. J.; Tang, X.; MacDiarmid, A. G. X-Ray

Structure of Polyaniline. Macromolecules. 1991, 24, 779–789. (11)

Lee, Y.-Y.; Choi, G. M.; Lim, S.-M.; Cho, J.-Y.; Choi, I.-S.; Nam, K. T.; Joo, Y.-C.

Growth Mechanism of Strain-Dependent Morphological Change in PEDOT:PSS Films. Sci. Rep. 2016, 6, 25332. (12)

Liu, N.; Fang, G.; Wan, J.; Zhou, H.; Long, H.; Zhao, X. Electrospun PEDOT:PSS–PVA

Nanofiber Based Ultrahigh-Strain Sensors with Controllable Electrical Conductivity. J. Mater. Chem. 2011, 21, 18962-18966. (13)

Mateiu, R.; Lillemose, M.; Hansen, T. S.; Boisen, A.; Geschke, O. Reliability of Poly

3,4-Ethylenedioxythiophene Strain Gauge. Microelectron. Eng. 2007, 84, 1270–1273. (14)

Tung, S.-T.; Yao, Y.; Glisic, B. Sensing Sheet: The Sensitivity of Thin-Film Full-Bridge

Strain Sensors for Crack Detection and Characterization. Meas. Sci. Technol. 2014, 25, 075602. (15)

Zhang, F.; Zang, Y.; Huang, D.; Di, C.; Zhu, D. Flexible and Self-Powered Temperature–

Pressure

Dual-Parameter

Sensors

Using

Microstructure-Frame-Supported

Organic

Thermoelectric Materials. Nat. Commun. 2015, 6, 8356. (16)

Hattori, Y.; Falgout, L.; Lee, W.; Jung, S.-Y.; Poon, E.; Lee, J. W.; Na, I.; Geisler, A.;

Sadhwani, D.; Zhang, Y.; Su, Y.; Wang, X.; Liu, Z.; Xia, J.; Cheng, H.; Webb, R. C.; Bonifas, A. P.; Won, P.; Jeong, J.-W.; Jang, K.-I.; Song, Y. M.; Nardone, B.; Nodzenski, M.; Fan, J. A.; Huang, Y.; West, D. P.; Paller, A. S.; Alam, M.; Yeo, W.-H.; Rogers, J. A. Multifunctional SkinLike Electronics for Quantitative, Clinical Monitoring of Cutaneous Wound Healing. Adv. Healthc. Mater. 2014, 3, 1597–1607. (17)

Hong, S. Y.; Lee, Y. H.; Park, H.; Jin, S. W.; Jeong, Y. R.; Yun, J.; You, I.; Zi, G.; Ha, J.

S. Stretchable Active Matrix Temperature Sensor Array of Polyaniline Nanofibers for Electronic Skin. Adv. Mater. 2016, 28, 930–935. (18)

Du, P.; Liu, H. C.; Yi, C.; Wang, K.; Gong, X. Polyaniline-Modified Oriented Graphene

Hydrogel Film as the Free-Standing Electrode for Flexible Solid-State Supercapacitors. ACS Appl. Mater. Interfaces. 2015, 7, 23932–23940.


23


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

(19)

Page 24 of 28

Sezen, M.; Register, J. T.; Yao, Y.; Glisic, B.; Loo, Y.-L. Eliminating Piezoresistivity in

Flexible Conducting Polymers for Accurate Temperature Sensing under Dynamic Mechanical Deformations. Small. 2016, 21, 2832–2838. (20)

Yoo, J. E.; Krekelberg, W. P.; Sun, Y.; Tarver, J. D.; Truskett, T. M.; Loo, Y.-L. Polymer

Conductivity through Particle Connectivity. Chem. Mater. 2009, 21, 1948–1954. (21)

Mano, N.; Yoo, J. E.; Tarver, J.; Loo, Y.-L.; Heller, A. An Electron-Conducting

Crosslinked Polyaniline-Based Redox Hydrogel, Formed in One Step at pH 7.2, Wires Glucose Oxidase. J. Am. Chem. Soc. 2007, 129, 7006-7007 (22)

Pereira, J. N.; Vieira, P.; Ferreira, A.; Paleo, A. J.; Rocha, J. G.; Lanceros-Méndez, S.

Piezoresistive Effect in Spin-Coated Polyaniline Thin Films. J. Polym. Res. 2012, 19, 9815. (23)

Lillemose, M.; Spieser, M.; Christiansen, N. O.; Christensen, A.; Boisen, A. Intrinsically

Conductive Polymer Thin Film Piezoresistors. Microelectron. Eng. 2008, 85, 969–971. (24)

Costa, P.; Ribeiro, S.; Lanceros-Mendez, S. Mechanical vs. Electrical Hysteresis of

Carbon Nanotube/Styrene-Butadiene-Styrene Composites and Their Influence in the Electromechanical Response. Compos. Sci. Technol. 2015, 109, 1–5. (25)

Herrmann, J.; Müller, K. H.; Reda, T.; Baxter, G. R.; Raguse, B.; De Groot, G. J. J. B.;

Chai, R.; Roberts, M.; Wieczorek, L. Nanoparticle Films as Sensitive Strain Gauges. Appl. Phys. Lett. 2007, 91, 183105. (26)

Sekitani, T.; Iba, S.; Kato, Y.; Noguchi, Y.; Someya, T.; Sakurai, T. Ultraflexible

Organic Field-Effect Transistors Embedded at a Neutral Strain Position. Appl. Phys. Lett. 2005, 87, 173502. (27)

Widlund, T.; Yang, S.; Hsu, Y.-Y.; Lu, N. Stretchability and Compliance of Freestanding

Serpentine-Shaped Ribbons. Int. J. Solids Struct. 2014, 51, 4026–4037. (28)

Glisic, B.; Verma, N. In Innovative Developments of Advanced Multifunctional

Nanocomposites in Civil and Structural Engineering; Loh, K. J.; Nagarajaiah, S.; Woodhead Publishing Ltd. (Elsevier): 2016; Chapter 12, pp 281-302.


24

Page 25 of 28

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


(29)

Yoo, J. E.; Cross, J. L.; Bucholz, T. L.; Lee, K. S.; Espe, M. P.; Loo, Y.-L. Improving the

Electrical Conductivity of Polymer Acid-Doped Polyaniline by Controlling the Template Molecular Weight. J. Mater. Chem. 2007, 17, 1268. (30)

Lin, Y.; Liu, S.; Chen, S.; Wei, Y.; Dong, X.; Liu, L. A Highly Stretchable and Sensitive

Strain Sensor Based on Graphene–elastomer Composites with a Novel Double-Interconnected Network. J. Mater. Chem. C. 2016, 4, 4304–4311. (31)

Segev-Bar, M.; Ukrainsky, B.; Konvalina, G.; Haick, H. In Situ and Real-Time

Inspection of Nanoparticle Average Size in Flexible Printed Sensors. J. Phys. Chem. C. 2015, 119, 27521–27528. (32)

Han, B. G.; Han, B. Z.; Yu, X. Effects of the Content Level and Particle Size of Nickel

Powder on the Piezoresistivity of Cement-Based Composites/Sensors. Smart Mater. Struct. 2010, 19, 65012. (33)

Yoo, J. E.; Lee, K. S.; Garcia, A.; Tarver, J.; Gomez, E. D.; Baldwin, K.; Sun, Y.; Meng,

H.; Nguyen, T.-Q.; Loo, Y.-L. Directly Patternable, Highly Conducting Polymers for Broad Applications in Organic Electronics. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 5712–5717. (34)

Fischer, J. E.; Tang, X.; Scherr, E. M.; Cajipe, V. B.; MacDiarmid, A. G. Polyaniline

Fibers and Films: Stretch-Induced Orientation and Crystallization, Morphology, and the Nature of the Amorphous Phase. Synth. Met. 1991, 41, 661–664.


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TABLE OF CONTENTS GRAPHIC


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TOC graphic 35x15mm (300 x 300 DPI)


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Tuning the Magnitude and the Polarity of the Piezoresistive Response

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