Elastomer Composites with Tailored Interface Network toward Tunable

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Elastomer Composites with Tailored Interface Network toward Tunable Piezoresistivity: Effect of Elastomer Particle Size Zhen Sang, Kai Ke, and Ica Manas-Zloczower ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00241 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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ACS Applied Polymer Materials

Elastomer Composites with Tailored Interface Network toward Tunable Piezoresistivity: Effect of Elastomer Particle Size Zhen Sang, Kai Ke*, Ica Manas-Zloczower* Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, OH 44106-7202, United States Keywords: Thermoplastic polyurethane (TPU), piezoresistivity, strain sensor, segregated structure, carbon nanostructures (CNS). ABSTRACT: Wearable strain sensors have significant potential applications for the development of Internet of Things. As such, sensors based on conductive elastomer composites (CECs) for various sensing applications require different piezoresistive properties, i.e. strain sensitivity and sensing ranges. Herein, we report a facile strategy to fabricate thermoplastic polyurethane (TPU)/carbon nanostructure (CNS) composites designed for different applications based on different conductive interface network morphology via forming the filler network at segregated TPU particles with various sizes. This strategy renders the composites tunable electrical conductivity (4 orders of magnitude change at low filler content), mechanical and piezoresistive properties upon changing the TPU particle size. The larger the TPU particle size, the denser and stronger the conductive network, leading to higher electrical conductivity, better mechanical reinforcement and more stable piezoresistive behavior. By contrast, using smaller TPU particle size gives rise to relatively lower conductivity, but higher elongation at break and much higher strain sensitivity. Composites with 0.7 wt. % CNS using TPU particle size up to 212 μm have a gauge factor of 7668 at 300% tensile strain and elongation at break of 990%, whereas, when using TPU particles with 1000~1400 μm, the gauge factor is 175 for 300% tensile strain, and the elongation at break is 780%. These CEC composites have potential applications for a variety of flexible sensors.

1. Introduction Conductive elastomer composites (CECs), consisting of electrically conductive nanofillers and elastomers, have shown significant potential for wearable electronics due to their good combination of conductivity and stretchability. They have been widely used for stretchable displays1-2, conductors3-4, actuators5-6, strain sensors7-12, wearable sensors in human’s healthcare, such as human body motion detection13-17, health monitoring18 and therapeutics12, 19-20. CEC-based strain sensors can be effectively applied in the aforementioned applications by correlating the resistance change with mechanical strain. For practical applications, good electrical conductivity, i.e. robust conductive networks, are essential to sustain stable and reproducible piezoresistive response to external mechanical stimuli, especially for large strains (e.g. strain ε>100%).21 However, large amounts of conductive fillers are required to ensure this in conventional processing methods like melt22-25 and solution mixing26-31, causing deterioration in the elongation at break for the composites. For the design of CEC-based stretchable piezoresistive sensors, it is important to simultaneously achieve good electrical conductivity and high elongation at break. Motivated by the brick-wall structure, we have recently reported an environment-friendly strategy to facilely fabricate stretchable strain sensors by constructing segregated carbon nanostructures (CNS, also known as branched carbon nanotubes) networks at the thermoplastic elastomer interface.32 Similar strategies of building segregated networks reported for polydimethylsiloxane33 and natural rubber34 use tetrahydrofuran solvent or freeze drying for fabricating pressure and liquid sensors. The polymer-filler interface, which is of essential significance for piezoresistive strain

sensor fabrication27, 35-37, has been rarely analyzed for the CEC systems with segregated filler network. At certain filler content, polymer powders with larger size give rise to the formation of much denser filler networks at the polymer interface, resulting in higher electrical38-40 and thermal41-42 conductivities of the composites. In principle, both the distribution of conductive fillers at the polymer-polymer interface and the interface areas controlled by the polymer particle size influence the elongation at break and electrical conductivity of conductive polymer composites with segregated structure. Therefore, piezoresistive properties, closely related to the electrical and mechanical properties of CECs, are also potentially affected by the morphology and structure of the segregated filler network. CEC-based stretchable strain sensors, with high strain sensitivity and wide strain sensing range are of great significance.43-45 Although forming a segregated filler network at the elastomer interface by using low filler concentration is an efficient way to fabricate stretchable strain sensors32, 34, it is challenging to simultaneously obtain highly piezoresistive sensitivity and wide sensing strain ranges in such systems. For CECs with segregated network structure, the elastomer-filler interface is key for tuning piezoresistive sensitivity. Thus, tailoring the interface by using different elastomer particle size is playing a crucial role in tuning strain sensitivity and sensing ranges for stretchable strain sensors. In this work, stretchable TPU/CNS composites with tunable piezoresistive properties were successfully prepared by a facile and environment-friendly approach, namely constructing segregated structure through direct hot press. Using TPU powders with different size, enables obtaining composites with

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different electrical, mechanical and piezoresistive properties. Smaller TPU particle sizes render composites exhibiting both higher elongations at break and greater piezoresistive sensitivity, for the fabrication of highly stretchable strain sensors. On the contrary, composites with larger TPU particle sizes, exhibiting more stable and reproducible piezoresistive behavior, are more desirable for potential application of flexible conductors. This is the first report on the effect of polymer-filler interface in CECs with segregated networks on their piezoresistive properties. 2. Experimental 2.1 Materials and composite preparation A commercially available p-phenylene diisocyanate (PPDI)based thermoplastic polyurethane, reported previously, 58 was used here. Branched carbon nanotubes, also known as carbon nanostructures (flake, 70 μm long, 10 μm thick and ca. 9 nm nanotube diameters), were supplied by Applied Nanostructured Solutions LLC (Lockheed Martin Corporation, MD, US). TPU powders with a series of diameters, i.e. 0~212, 212~300, 300~425, 425~500, 500~1000(1k) and 1000~1400(1.4k) μm, respectively, were obtained by sifting ground TPU particles using a sieve shaker (W.S. Tyler RX86). TPU powders were manually mixed with CNS in the mortar with a limited amount (3 ml) of ethanol and dried in an oven for compression molding, as reported in our previous work. 32 The TPU nanocomposites were labeled based on their composition, e.g. 0.7 C/T 0-212 denotes the nanocomposite containing 0.7 wt. % CNS and TPU powder size of 0-212 μm. 2.2 Characterization CNS network morphology of the composites was observed using Olympus BX51TF optical microscopy used in previous work.32 Prior to observation, thin sections (3 μm thick) were cut from the nanocomposites using a LEICA microtome (EM FC6) at -80 ºC under liquid N2 atmosphere. Fracture surface morphology of TPU nanocomposites was analyzed using a field emission scanning electron microscope (FE-SEM) Helios NanoLabTM 650 (Hillsboro, OR, US). The samples were fractured after being immersed in liquid nitrogen for 30 min. Palladium was coated on the fractured surface before SEM observation. Non-coated samples with low resistance, i.e. the composites with 4 wt. % CNS, were used for SEM analysis under 1 kV voltage in a charge contrast imaging (CCI) mode, to visualize the carbon nanofiller network in the polymer matrix, as reported in literature46-49. The 4 C/T 0-212 and 4 C/T 1k-1.4k composites were employed to investigate the tensile broken cross-sections after stretching. The stretched samples were broken and submerged into liquid N2 to freeze the structure for SEM analysis. The electrical resistance of the samples was measured by a resistance set (PRS-801, Prostat corporation), as reported in the literature50-51. Dumbbell-shaped samples (40 mm × 4.94 mm × 0.75 mm, length × width ×thickness) cut from compression-molded films were tested. To evaluate the volume resistivity, the samples were coated with silver and placed between two clamps spaced 10 mm apart and at least seven samples were tested to report the average results. Tensile properties at room temperature were tested using a MTS instrument (Model 2525-806, MTS System Corporation, MN, US) at a cross head speed of 50 mm/min. At least seven specimens were tested and the average results were reported.

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Piezoresistive properties were evaluated by measuring the instantaneous resistance change using a digital multimeter (Keithley 2701, OH, US) during the tension testing (MTS instrument, Model 2525-806) at room temperature. Three dumbbell-shaped samples were selected and tested under a loading speed of 5 mm/min until the electrical resistance was beyond the maximum measuring range (110 MΩ) of the digital multimeter. Cyclic piezoresistive behavior was investigated using a tensile instrument (Zwick Roell Z0.5, Germany). All samples were tested under a maximum stress of 9 MPa (for strain ~ 70%) under a constant loading speed of 5 mm/min for 15 cycles. There was a 2s holding time between loading and unloading. The 0.7 C/T 0-212 composites were employed for the human motion detection in finger bending and respiration monitoring with size of 20×3×0.3 mm3 and 150×5×0.3 mm3, respectively. They were attached on the index finger and chest periphery with the aid of sports tape and resistance change was measured by the same Keithley multimeter. To eliminate the contact resistance, conductive silver paste was coated on the contacts of the sample and copper clamps. To visually observe the piezoresistive effect of the composites, the brightness change of an LED bulb powered by a DC voltage supplier (BK Precision 1667, CA, US) was tested upon stretching a strip sensor (150×3×0.3 mm3) to different strains using a home-made stretching equipment. 3. Results and discussion 3.1 Segregated CNS network at the TPU interfaces Figure 1 gives the schematics of the CNS network structure at the TPU interface for powders with different size. For the same TPU powder size, with higher CNS loading, the filler interface layers are expected to be thicker and denser, conducive to more and stronger conductive pathways. For larger TPU particle size at the same CNS loading, the filler layer at the TPU-TPU interface is thicker and denser. The optical microscope images provided in Figure 2 showing as dark regions the CNS segregated network in various systems confirm the hypothesis in Figure 1 and illustrate an increase in interface layer thickness, especially at high CNS contents for larger TPU particle size. The interface layer morphology will significantly influence mechanical and electrical properties as well as the piezoresistive behavior of TPU/CNS composites.

Figure 1 Schematic illustration of the CNS network at the TPUTPU interface in composites with small and large TPU powders.

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Figure 2 Optical microscopy images of TPU composites with 0.1, 0.7 and 4 wt. % CNS and TPU powder sizes of 0-212 and 1k-1.4k μm.

To further illustrate the morphology of the segregated conductive network an FE-SEM analysis in the CCI mode for the 4 C/T 0-212 composite is provided in Figure 3. With a further amplification of a selected region (dashed red frame), the formation of CNS-rich regions between TPU powders (region B marked by the yellow dot circle) and obscure boundaries are observed. Besides, as confirmed in the amplification of the blue pentagon dash frame, the blurry interface between CNS and TPU matrix illustrates the penetration of TPU chains into the CNS network. Notable are the aligned carbon nanotubes due to the dense nanotube array structure in CNS as well as the overlapping and stacking of carbon nanotubes at such high filler content (4 wt. %). Similar morphologies can also be observed in the composites with TPU powder size of 300-425 and 1k-1.4k μm as illustrated in Figure S1. These images reveal the densely-interconnected structure of carbon nanotubes in the CNS and the CNS agglomerates in the TPU composites at high filler content.

Figure 3 CNS conductive network formed in the 4 C/T 0-212 μm composite visualized by FE-SEM in a CCI mode. Amplifications of the selected regions by red dashed square and blue pentagon, respectively. Marks A and B represent the TPU-rich region and CNS-rich region, respectively.

the composite with TPU particle size no more than 425 μm)32, the volume conductivity of TPU composites increases dramatically with increasing TPU powder size. The composite with TPU powder size of 1k~1.4k μm has a volume conductivity 4 orders of magnitude higher than that with TPU powder size of 0~212 μm, indicating that at the same filler content, larger TPU particle size form more and denser CNS networks at the TPU interface resulting in higher conductivity. At 0.7 wt. % CNS content, the increase in volume conductivity (about 1 order of magnitude) can be observed once the TPU powder size increases above 425~500 μm. However, at high filler loading (4 wt. % CNS), the volume conductivity seemingly reaches a plateau indicating saturation of the conductive network at the TPU-TPU interface.

Figure 4 Electrical conductivity of TPU/CNS composites with various TPU powder sizes. The connecting lines make for easier reading.

3.3 Mechanical properties To understand the effect of TPU-CNS interface morphology on the mechanical properties of TPU composites, tensile data for the TPU composites are provided in Figure 5. Young’s modulus of the composites was normalized by that of the neat TPU. With increasing the TPU particle size, the normalized Young’s modulus increases especially at higher filler content. The elongation at break decreases with increasing the polymer particle size and with higher filler content. SEM analysis of cross-section surface morphology for the tensile-fractured TPU composites in Figure S2 provides further explanation for the results in Figure 5. The composites with smaller TPU powder size (0~212 μm) display rough fracture surfaces (Figure S2 (a)). Clearly observed are the penetration of TPU into CNS and the filler orientation induced by uniaxial stretching. By contrast, a brittle fracture surface is shown in Figure S2(b) for the composite with TPU powder size of 1~1.4k μm, related to the weak interplay between CNS and TPU as a result of less surface area of larger size and filler agglomeration at high filler loading.

3.2 Electrical properties CNS segregated network morphology in the TPU composites will affect the system electrical and mechanical properties. As shown in Figure 4, at 0.1 wt. % CNS, which is closer to the percolation threshold of CNS (ca. 0.06 wt. % for



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networks, as proved by their electrical conductivity especially at low filler loading. As a result, those composites with more robust networks have higher resistance to strain, giving rise to a wider strain sensing range and a delay of resistance increase upon stretching.

Figure 5 (a) Normalized Young’s modulus and (b) the elongation at break of TPU and its composites. The connecting lines make for easier reading.

3.4 Piezoresistive behavior Differences seen in electrical and mechanical properties for various TPU particle sizes contribute directly to dissimilar piezoresistive behavior of TPU composites. As seen in Figure 6(a), for CNS filler content of 0.7 wt. %, the composite with TPU powder size of 0-212 μm demonstrates a relatively stable resistance change (ΔR/R0) at strains lower than 100% but a significant increase beyond that up to a strain of 310 % where the resistance is beyond the range of the multimeter (100 MΩ). Composites with TPU powder sizes of 300-425 μm and 1k1.4k μm show a delay in the sharp rise of ΔR/R0 to strains of 120% and 300%, respectively, which continue up to 360 % and 420 %, respectively. Similar phenomena can be observed at 4 wt. % CNS loading for the composites with TPU powder size of 0-212 μm and 300-425 μm (Figure 6(b)). The 4 C/T 0212 composite exhibits an earlier sharp resistance change and wider sensing range up to 390 % by comparison with the 4 C/T 300-425 composite. The 4 C/T 1k-1.4k breaks at low strain (140 %). Such phenomena are a direct consequence of the TPU-CNS interface morphology as hypothesized in Figure 1. As reported previously32, the penetration of melted polymer chains into the CNS network leads to efficient stress transfer and the formation of robust conductive pathways of the composites. For certain filler loading, the composites with smaller size TPU particles have less robust network due to the large total interface area, giving rise to higher piezoresistive sensitivity but lower sensing ranges. In contrast, composites with lager TPU powder size have less total interfacial area, which allows the formation of a relatively thicker filler layer at the TPU-TPU interface, leading to rather dense conductive

Figure 6 (a, b) Stress and ∆R/R0 plotted vs. strain for TPU/CNS composites. (c) ∆R/R0 plotted as a function of cycle number under strain ε= 70% for 15 test cycles.

To further understand the piezoresistive sensitivity of these composites, the gauge factor (GF), calculated using equation 1, can be also used to evaluate sensitivity for piezoresistive strain sensors.52-53 GF = (∆R/R0)/ε (1) where ∆R and R0 are an instantaneous change in resistance and initial resistance, respectively, and ε represents the instant strain. Conventional strain sensors have a GF about 2-5.54-55 Various GF at different strains (50%, 100%, 160%, 200% and 300%) were used to compare piezoresistive sensitivity as shown in Table. 1. With relatively close TPU powder sizes, 0.7 C/T 0-212 and 0.7 C/T 300-425 composites show GFs of 24.8 and 39.5 at strain= 50% and GFs of 50.1 and 54.4 at strain= 100%, indicating relatively similar GFs at small strain. However, they are higher than values reported in the literature for other TPU composites (around 20~25) for strains less than 100%.26, 29, 56-57 Nevertheless, the 0.7 C/T 0-212 composite


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ACS Applied Polymer Materials shows larger GF values by comparison with the 0.7 C/T 300425 composite at strain of 160% and higher as a result of the thinner conductive interface of CNS induced by the larger surface area of smaller particle size. With thicker and denser CNS interface, the GFs of 0.7 C/T 1k-1.4k remain relatively the same at strains below 160% and increase to 88.7 and 174.5 at strains of 200% and 300%, respectively. More interestingly, the GF of the 0.7 C/T 0-212 composite can reach ca. 7868 at strain of 300 %, while those for the 0.7 C/T 300-425 and 0.7 C/T 1k-1.4k composite are only 1800.4 and 174.5, respectively. Further increasing the strain leads to larger GFs, e.g. for the 0.7 C/T 300-425 composite the GF is 8636 at 356% strain and for the 0.7 C/T 1k-1.4k composite the GF is 4250 at 400% strain. However, the network in the 0.7 C/T 0-212 composite can only sustain 310% strain (the resistance is beyond the limit of the Keithley multimeter), while the other two composites can sense higher level of strain due to their more robust networks. Besides, they also show better piezoresistive sensitivity by comparison with other CEC-based stretchable strain sensors reported in the literatures 32, 34 in terms of strain sensing range and strain sensitivity, indicating potential application for stretchable strain sensors. As a result, this strategy of building composites with segregated structure is efficient to simultaneously tune piezoresistive sensitivity and strain sensing ranges. As real application of these strain sensors often requires reversible and reproducible strain sensing behavior, cyclic piezoresistive testing for the 0.7 C/T 0-212 and 0.7 C/T 1k1.4k composites are shown in Figure 6(c). The composite 0.7 C/T 1k-1.4k shows relatively better sensing stability and reproducibility due to the denser and stronger CNS network at TPU-TPU interface by comparison with the composite 0.7 C/T 0-212. However, the 0.7 C/T 0-212 composite exhibits a higher ∆R/R0 change (up to 3.5), but a mild decrease trend in the first 6 cycles becoming stable after 7 cycles. Noteworthy, for both composites during dynamic loadings the double-peak patterns, they are observed also in other piezoresistive CECs48, 58 which most likely related to the creep and slow stress relaxation of CECs under fast loading and unloading at room temperature.58-61 The possible mechanisms for the different piezoresistive behaviors may result from the stability of conductive pathways upon cyclic stretching. The smaller TPU powder size, more severe damage of the conductive pathways in the CNS interfaces will occur upon stretching by comparison with the larger TPU particle composites at the same filler content. Therefore, during cyclic testing, the 0.7 C/T 0-212 composite will show a higher ∆R/R0 change with a progressive decrease with additional cycles. Table 1 Gauge factors for TPU composites with 0.7 wt. % CNS at various strains. 0.7 C/T 0.7 C/T 0.7 C/T 0-212 300-425 1k-1.4k GF@ strain=50% 24.8 39.5 1.5 GF@ strain=100% 50.1 54.4 4.0 GF@ strain=160% 140.2 110.3 10.7 GF@ strain=200% 350.0 207.7 88.7 GF@ strain=300% 7668.2 1800.4 174.5

3.5 Strain sensor applications Examples of strain sensor applications for the 0.7 C/T 0-212 are shown in Figure 7. Strain sensors mounted on the joint of

the index finger and the chest periphery can clearly monitor finger bending (Figure7 (a)) and the respiration rate while exhaling and inhaling (Figure 7(b)) indicating the potential use for personal healthcare. Moreover, the brightness change of an LED bulb upon stretching the sensor to different strains using a home-made stretching equipment can visually show the piezoresistive effect of the sensors (Figure 7 (c)). The brightness of the LED bulb remains the same when subjected to 50% strain and slightly dims upon further stretching to 100%. However, at 160% strain, the LED bulb is much dimmer indicating the potential application for stretchable resistors at strains lower than 50% and as piezoresistive sensors at larger strains. These examples demonstrate promising applications of these stretchable composites for wearable sensors toward the development of human-machine interface devices for the Internet of Things.7, 57, 62

Figure 7 Relative resistance change versus cycle numbers of 0.7 C/T 0-212 composites for (a) finger bending (θ = 60 °) and (b) monitoring respiratory rate; (c) illustration of the stretching equipment with corresponding brightness changes of an LED bulb upon different strains. 4. Conclusion CECs with tunable piezoresistive sensitivity and strainsensing ranges were fabricated in an environment-friendly procedure by forming segregated conductive networks of CNS at TPU-TPU interfaces using different TPU particle sizes. The polymer-filler interface features, such as the CNS interface layer thickness and the total interface area, can be tailored by changing the TPU particle size, enabling tuning the composites electrical and mechanical properties as well as the piezoresistive sensitivity. For the same filler loading, larger



TPU particle sizes enable the formation of denser and stronger CNS networks at the TPU-TPU interface, resulting in higher electrical conductivity and Young’s modulus and more stable piezoresistivity. By contrast, higher piezoresistive sensitivity and elongation at break can be observed in the composites with smaller TPU particle size, indicating potential application for stretchable strain sensors. For instance, the 0.7 C/T 0-212 composite has a gauge factor of 7668.24 at 300% strain with actual elongation at break of 990%. These composites can be used as strain sensors for human motion detection (i.e. finger motion and respiration movement during inhaling/exhaling) as well as stretchable resistors (change in LED bulb brightness upon stretching). This study strengthens understanding of the relationship between filler network morphology and piezoresistive properties for the design of CEC-based stretchable strain sensors.

ASSOCIATED CONTENT Supporting Information Available: SEM images of 4 C/T 300-425 and 1k-1.4k composites, SEM images of broken cross-section of 4 C/T 0-212 and 1k-1.4k composites.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (K.K.); [email protected] (I.M.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to acknowledge the financial support from Parker Hannifin Corporation. We also thank Applied Nanostructured Solutions LLC for providing the CNS. The authors thank Mrs. Nanthawan Avishai for SEM analysis in Swagelok Center for Surface Analysis of Materials at Case Western Reserve University.

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