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Applied Nanoscience https://doi.org/10.1007/s13204-019-00958-x

ORIGINAL ARTICLE

Fabrication and characterization of porous CNF/PDMS nanocomposites for sensing applications

Wenyuan Luo1 · Mohammad Charara1 · Mrinal C. Saha1  · Yingtao Liu1

Received: 25 October 2018 / Accepted: 9 January 2019 © King Abdulaziz City for Science and Technology 2019

AbstractThis paper presented the development, fabrication, and characterization of porous nanocomposites using carbon nanofiber (CNF) and polydimethylsiloxane (PDMS) elastomer for sensing applications under compressive loads. Sugar particles coated with CNFs were first used to manufacture templates and then used for the nanocomposite fabrications. All the manufactured nanocomposites were characterized to understand the CNF distribution, porous morphology, and electrical conductivity. The piezoresistive sensing functions were investigated by characterizing the nanocomposites under compressive cyclic loads at various applied maximum strains and strain rates. Long-term sensor performance was evaluated via durability tests for 12 h. The developed nanocomposite sensors have great potential for applications in a broad range of engineering fields.

Keywords Carbon nanofiber · PDMS · Porous nanocomposite · Piezoresistive sensing · Durability · Gauge factor

Introduction

Highly flexible and electrically conductive nanocompos-ites have recently gained considerable attention in both the scientific community and industry, resulting in a variety of promising applications, such as structural health monitor-ing (Eswaraiah et al. 2011; Luo et al. 2017), wearable elec-tronics (Amjadi et al. 2016, 2015), flexible pressure sensors (Kim et al. 2017; Wang et al. 2009), and robotic manipula-tion (Massaro et al. 2011; Tanaka et al. 2003). In particular, polydimethylsiloxane (PDMS) has been broadly employed as the base polymer of nanocomposites due to its high flex-ibility, biocompatibility, chemical stability, and tailorable mechanical properties (Hocheng et al. 2010; Johnston et al. 2014; Yilgör and Yilgör 2014). Compared to solid PDMS polymers, porous PDMS foam has better flexibility, and can be easily integrated with various types of nanoparticles for nanocomposite applications.

A wide range of approaches have been employed to fabricate porous PDMS foam using sacrificial templating

(Han et al. 2013; Wu et al. 2017), freeze-drying backfilling methods (Zhai et al. 2015), water droplet methods (Huang et al. 2018), and 3D printing (Duan et al. 2016). Zhai et al. fabricated porous PDMS foam by backfilling super hydro-phobic poly (vinyl alcohol) (PVA) aerogel with the PDMS pre-polymer using a vacuum-assisted liquid filling method for up to 4 days at low temperature (Zhai et al. 2015). Lee et al. synthesized porous PDMS foam by inducing phase separation of polymethylmethacrylate (PMMA) in PDMS and later removing the PMMA using a solvent, obtaining well-controlled pore networks (Lee et al. 2012). Huang et al. reported the fabrication of porous PDMS foam by mixing distilled water into the PDMS pre-polymer (Huang et al. 2018). Among the mentioned techniques, an increasing number of researchers have employed sacrificial templat-ing using salt or sugar (Han et al. 2013; Wu et al. 2017). These porogens are dissolved in solvents, leaving a PDMS skeleton with an interconnected porous structure. The poros-ity of PDMS foam is adjustable based on the size of salt or sugar crystals. The low cost, environmental friendly and simplicity of this method make this a promising approach for the development of porous nanocomposites.

Porous and conductive PDMS nanocomposites enhanced by carbon and piezoelectric nanoparticles have been reported in many applications, including highly sensi-tive sensors (Sepulveda et al. 2011), flexible conductors (Wu et al. 2017), fluidic electronics (Han et al. 2013), and

* Mrinal C. Saha msaha@ou.edu

Yingtao Liu yingtao@ou.edu

1 University of Oklahoma, 865 Asp Ave., Norman, OK 73019, USA

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energy harvesting and storage devices (McCall et al. 2014). Development of electrically conductive porous PDMS nano-composites requires dispersion of nanoparticles within the skeleton of the porous PDMS. Various conductive nano-particles, such as copper nanoparticles (Athanassiou et al. 2006), carbon nanotubes (CNT) (Han et al. 2013), carbon nanofiber (CNF) (Amjadi et al. 2014; Wu et al. 2017), and graphene (Boland et al. 2016) have been employed to create these nanocomposites. In general, carbon-based nanocom-posites are light in weight due to the low density of carbon nanoparticles. Moreover, it is easier to disperse CNFs than CNTs in a polymer matrix or solvent due to CNFs’ relatively large particle size. Recent research has shown that the dis-persion of CNFs into a 3D interconnected porous structure can significantly improve materials’ electrical conductivity and provide piezoresistive sensing capability (Amjadi et al. 2014). This method requires the manufacturing of three-dimensional porous PDMS foams prior to dispersing any nanoparticles. For example, Wu et al. characterized the sens-ing performance of porous CNF/PDMS strips with less than 1 wt% CNF under cyclic tensile test (Wu et al. 2017). These structures exhibited failure strain as high as 120% and sens-ing response with a gauge factor as high as 6.5. Rinaldi et al. fabricated porous graphene/PDMS foam using the dipping infiltration method (Rinaldi et al. 2016). The piezoresistive response at different strain rates was demonstrated to vali-date the robustness of the sensor when strain rate varied over orders of magnitude. To the best of our knowledge, most previous work dispersed nanoparticles after the fabrication of porous PDMS polymers, after which the piezoresistive sensing capability of the carbon-based porous nanocompos-ites was tested under tensile or bending loads.

This paper reported a simple method to fabricate highly deformable and electrically conductive porous CNF/PDMS nanocomposites using the sugar template technique. CNFs were first coated on sugar particles and used for the fabrica-tion of sugar templates. Then, porous CNF/PDMS nano-composites were manufactured by submerging the prepared sugar templates in PDMS pre-polymer. The porous nano-composites were characterized to understand their micro-structure, morphology, and electrical resistance. Finally, the piezoresistive sensing capabilities of the fabricated nano-composites were tested under compressive cyclic loads.

Experimental

Materials

CNFs with average diameter of  100  nm (Pyograf-III PR-24XT-LHT, Applied Science, Inc.) were used as the electrically conductive nanoparticles. Tetrahydrofuran (THF) was used as the organic solvent during fabrication.

PDMS polymer (Sylgard 184 Dow Corning Co. Ltd.) was used as the base polymer. Pure cane sugar crystals (Florida Crystals, Inc.) were used as porogen. Sugar crystals were fil-tered using a strainer to eliminate agglomerates. The average size of the filtered cane sugar crystal was 500 µm.

Nanocomposite fabrication

A pre-determined amount of CNFs was introduced into a 100 ml THF solvent solution and subsequently dispersed using a high-intensity ultrasonic probe sonicator for 6 h to create the CNF–THF suspension. The suspension was stirred at 200 rpm to partially evaporate the solvent, until the vol-ume of CNF/THF suspension reduced to 25 ml. Sugar par-ticles were added into the CNF/THF suspension, and mixed for additional 10 min. The remaining solvent was evapo-rated on a hot plate. Once all THF was completely removed, the sugar/CNF particles were used to manufacture sugar cube templates using an aluminum mold. Prepared CNF/sugar crystal mixtures were compressed into the mold at 120 psi, heated at 120 °C for 30 min in an oven, and cooled back to room temperature before removal from the mold. PDMS pre-polymer was prepared by mixing the pre-polymer base (part A) and a crosslinking curing agent (part B) at a 10:1 weight ratio. The prepared sugar templates were submerged into PDMS pre-polymers for 24 h, allowing polymer to fully fill the pores in the sugar templates. The excess PDMS on sugar templates was trimmed off along the sugar surfaces. The manufactured samples were submerged in water to dis-solve the sugar crystals, and dried at 80 °C for 4 h. Pris-tine porous PDMS foam was also fabricated using the same method as the CNF/PDMS nanocomposites (without coating the sugar crystals with CNF) to identify the weight of PDMS in the CNF/PDMS nanocomposites using the TGA method (described in “Characterization of CNF/PDMS nanocom-posites”). It should be noted that a negligible amount of CNF was removed from the nanocomposites during the sugar dis-solving process. The schematic of nanocomposite fabrica-tion process is shown in Fig. 1.

Characterization of CNF/PDMS nanocomposites

The morphology of the porous CNF/PDMS nanocomposites was examined by a field-emission scanning electron micro-scope (FESEM) at 20 kV. Each examined sample surface was sputter coated with a thin layer of gold–palladium alloy to improve the electrical conductivity.

To calculate the CNF content in the PDMS/CNF nano-composite, the PDMS/CNF samples underwent thermo-gravimetric analysis (TGA) burn-off tests. However, CNF content calculation was not trivial because the burn-off remains from this TGA test were a combination of CNF and silica from the PDMS. To differentiate the amount

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of CNF and silica, a second TGA burn-off test was per-formed using pristine PDMS foam. The TGA burn-off remains from this test were assumed to be all silica. The weight of remaining silica was taken as a percentage of the total weight of pristine PDMS foam. This percentage was used to calculate the amount of silica in the burn-off remains of the PDMS/CNF TGA test. From here, the CNF content could be easily calculated.

The interfacial bonding between the PDMS skeleton and CNF was validated using adhesive tape. Porous CNF/PDMS samples were placed on a piece of double-sided tape and compressive quasi-static load was applied three times. The samples were then peeled from the tape to observe the residual CNF left on the tape.

Electrical resistivity of the developed porous nanocom-posites is critical for piezoresistive sensing applications. The two-probe method was used to measure the samples’ electrical resistivity by sandwiching the cubic samples between two copper plates, with each copper plate con-nected to one of the multi-meter leads (setup imaged and diagrammed in Fig. 2). A multi-meter (Agilent 34401A) connected to a data logger (RS-232) was used to record

the electrical resistance data. The electrical resistance data were recorded for 20 s in each measurement.

Piezoresistive sensing of porous CNF/PDMS nanocomposites

The porous CNF/PDMS nanocomposites were placed between two copper plates on an Instron 5900 single col-umn testing machine, as shown in Fig. 2. A 0.1 N initial force was applied to ensure full contact on top and bottom sample surfaces throughout all tests. Cyclic compressive load was applied, deforming the sample with maximum strains of 1.25%, 2.5%, 5%, 10%, 20%, and 40%, with a crosshead load rate of 1 mm/min. The force, deformation, and electrical resistance were recorded continuously by the Instron testing machine and a multi-meter (Agilent 34401A). Load rate effects were investigated by testing the piezoresistive sensing capabilities at six different strain rates: 0.1%/s, 0.2%/s, 1%/s, 2%/s, 4%/s, and 6%/s, and with a maximum strain of 20%. To verify the robustness of the samples, a compressive test of 240 cycles was performed with a crosshead load rate of 1 mm/min for over 12 h. The repeatability of the developed piezoresistive sensors was validated in this experiment.

Results and discussion

Morphology characterization using FESEM

In this paper, the quality of the fabricated sugar templates and CNF/PDMS nanocomposites was examined first. Fig-ure 3a shows the cross-section of the CNF-coated sugar template that was cut from the middle of the cubic sample. The white dots were identified as sugar particles that were dissected when cutting. It should be noted that sugar crystals were compressed and packed tightly, with small gaps left among sugar particles providing space for the liquid PDMS pre-polymer to penetrate during nanocomposite fabrication (Fig. 3b). Figure 3c shows the CNF distribution on the sugar particle surface. There is no visible CNF in the cross-sec-tion of sugar particles, revealing that no CNFs penetrated the sugar crystals during the creation of the CNF coated sugar cube. Figure 3d and e illustrates the open-cell pores in the manufactured CNF/PDMS nanocomposites. Sugar crystals were completely removed from the CNF/PDMS nanocomposites, leaving an open-cell microstructure with interconnected channels in the nanocomposites. As shown in Fig. 3f–i, the electrically conductive network was gener-ated by well-distributed CNF nanoparticles. Although the majority of CNFs were embedded within PDMS, a small amount of CNFs were still visible from the surface of the

Fig. 1 Schematic of porous nanocomposite fabrication process using CNF-coated sugar template

Fig. 2 Experimental setup for piezoresistive sensing measure-ment under compressive cyclic load

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porous walls. There were no entangled or agglomerated CNFs observed in the PDMS matrix, revealing a uniform CNF dispersion throughout the fabricated nanocomposites.

Interfacial adhesion study

To investigate the interfacial adhesion property between porous PDMS skeleton and CNF, a double-sided tape press-ing experiment was performed. As shown in Fig. 4, there are minimal CNFs left on the tape after repeated compres-sion loading, demonstrating good bonding between CNFs and the PDMS matrix in the nanocomposites. Since PDMS

is a typical elastomer polymer with relatively low fracture strength of 3.51–7.65 MPa, residual CNF on the tape was caused by the fracture of CNF/PDMS nanocomposites instead of CNFs being peeled off the nanocomposites (John-ston et al. 2014).

Characterization of electrical conductivity

Electrical properties are critical for good piezoresistive sens-ing function. The electrical conductivity of porous CNF/PDMS was calculated using

where � is the electrical conductivity, R is the electrical resistance, A is the cross-sectional area of the sample, and L is the height of nanocomposites. Nanocomposites with CNF concentrations from 1 to 9% were characterized in this paper. The measured electrical conductivity of the manufactured nanocomposites is shown in Fig. 5. It should be noted that high CNF concentration resulted in increased electrical con-ductivity due to more electrical pathways formed by CNF networks, as depicted in the FESEM figures. In addition, the electrical conductivity of the nanocomposites increased

(1)� =L

AR,

Fig. 3 Morphology characterization of sugar template and nanocom-posites: a optical image of CNF-coated sugar template, b, c FESEM images of CNF-coated sugar particles, d, e FSEM images showing

the pores in fabrication nanocomposites, f–i FESEM images showing the CNF distribution on PDMS surface in nanocomposites

Fig. 4 Interfacial adhesion tests showing the strong bonding between CNFs and PDMS

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rapidly when the CNF concentrations increase from 1 to 3 wt%, but increase slowly when the CNF concentrations increased from 3 to 6 wt%. It is expected that a saturation of conductivity would be reached when the CNF concen-trations continuously increase in this type of CNF/PDMS nanocomposites.

Piezoresistive sensing capability of fabricated CNF/PDMS nanocomposites

Piezoresistive sensors convert external load, such as pressure or deformation to a change of electrical resistance. High-per-formance sensors have high sensitivity and repeatability to external load. To explore the sensing capability of the devel-oped nanocomposites, the piezoresistive sensing performance was characterized under cyclic compressive loads. Note that although the conductivity results for nanocomposites with 1 wt% CNF were measured, samples with this CNF loading were found to have a noisy electrical response to mechanical stimuli. This can be due to discontinuous electrical networks within the material due to low conductive filler concentration.

As a result, the experiment further explores only the 3 wt%, 6 wt%, and 9 wt% CNF nanocomposites. The sensing function of the porous CNF/PDMS nanocomposites was characterized using nanocomposites with 3 wt%, 6 wt%, and 9 wt% CNF concentrations under compressive cyclic loading conditions. Figure 6 illustrates the relationship of relative electrical resist-ance change and applied compressive strain for five cycles for the sample with 3% CNF concentration. The relative resist-ance change increased with larger compressive strain. When the sample was deformed, the average distance between CNFs varied, resulting in a change in resistance due to tunneling effects in the CNF networks. When the compressive force was unloaded, the CNFs returned to their original location, reversing the tunneling effect of the electrical networks and increasing the electrical resistance back to the original value. Similar experiments were conducted using CNF/PDMS nano-composite samples with 6% and 9% CNF concentrations, and the piezoresistive sensing results are shown in Figs. 7 and 8, respectively.

Piezoresistive sensitivity, also known as gauge factor, is one of the most commonly used parameters to evaluate

Fig. 5 Electrical conductivity of nanocomposites with different CNF concentrations

Fig. 6 Sensing functions of porous nanocomposites of 3 wt% CNFs under different max compressive strains

Fig. 7 Sensing functions of porous nanocomposites of 6 wt% CNFs under different max compressive strains

Fig. 8 Sensing functions of porous nanocomposites of 9 wt% CNFs under different max compressive strains

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sensor performance. In general, the gauge factor of a sensor under applied strain is defined in Eq. (2):

where GF is the gauge factor, Ro is the original electrical resistance of nanocomposites, R is the instant electrical resistance at a certain time, and ε is the applied strain at the time of measurement. The calculation for GF is applicable when the response of the sensor (i.e. the slope of the rela-tive resistance change vs. strain curve) is linear, as evident by having a consistent GF over that particular strain range. However, when the sensor’s electrical response is not linear for all strains, Eq. (2) can be used to calculate the sensitiv-ity of the sensor at a particular strain. Table 1 lists the sen-sitivity for each type of nanocomposite sensors at various cyclic max strains. For nanocomposites with 3 wt% and 6 wt% sensors, the sensitivity continuously decreased when compressive strain increased up to 20%. The sensitivity then increased slightly when the applied maximum strain reached to 40%. However, for the nanocomposite with 9 wt% CNFs, the sensitivity decreased when the maximum strain increased up to 10%, and slightly increased at maximum strains of 20% and 40%. The nanocomposites with 3% CNF concentrations exhibited the highest sensitivity at the smaller strains up to 5% compared to composites with 6% and 9 wt% CNFs.

Aside from compressive strain sensing, the relationship between applied stress and piezoresistive sensing function was investigated in this paper. The compressive stress–strain curves and piezoresistive sensing response for all prepared nanocomposites with different CNF concentrations are shown in Figs. 9, 10, and 11. The stiffness of nanocom-posites, as represented by the slope of stress–strain curve, increased slightly for nanocomposites with higher CNF con-centrations, resulting in increasing maximum stress at the peak load condition. The maximum stress of the nanocom-posites with 3 wt%, 6 wt%, and 9 wt% CNF concentrations was 0.058 MPa, 0.061 MPa, and 0.063 MPa, respectively, at 40% maximum compressive strain. In addition, the nano-composites with 9 wt% CNF concentrations demonstrated

(2)GF =

(R − Ro)∕Ro

�

the best piezoresistive sensing linearity in the entire tested load range. Therefore, the further experimental characteri-zations of the nanocomposites were completed using the nanocomposites with 9 wt% CNF concentrations.

The effects of strain rate on the piezoresistive sensing capability of nanocomposites were tested using nanocom-posites with 9 wt% CNFs at 20% maximum compressive strain at strain rates from 0.001 to 0.06 s−1. Figure 12 shows the piezoresistive sensing results for five load cycles using each applied strain rate. Although the applied strain

Table 1 Sensitivity of CNF/PDMS nanocomposites under cyclic compressive loads

Applied strain (%)

Sensitivity of nanocomposites with 3 wt% CNF

Sensitivity of nanocomposites with 6 wt% CNF

Sensitivity of nanocom-posites with 9 wt% CNF

1.25 3.28 2.64 1.842.5 2.96 2.08 1.525 2.36 1.82 1.5210 1.65 1.52 1.4920 1.27 1.46 1.8740 1.47 1.75 2.06

Fig. 9 Variation of resistance and stress as a function of applied com-pressive strain of nanocomposites with 3 wt% CNF up to 40% maxi-mum strain

Fig. 10 Variation of resistance and stress as a function of applied compressive strain of nanocomposites with 6 wt% CNF up to 40% maximum strain

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rate increased by a factor of 60, the maximum piezoresis-tive response only changed about 3% between the low-est and highest strain rates. As the applied strain rate increased, the maximum piezoresistive sensor response slightly reduced. This variation of sensor response can be due to the lag of realignment of CNF conductive network under the applied cyclic load. However, the nanocomposite sensor displayed excellent repeatability throughout the five cycles at all the tested strain rates.

Durability tests under compressive cyclic load condi-tions were also conducted using the nanocomposites with 9 wt% CNFs, at a maximum compressive strain of 20% and crosshead speed of 1 mm/min for over 12 h. The results for the durability test are shown in Fig. 13. The relative piezoresistive changes were relatively constant, at around 33% (Fig. 13a), and produced a repeatable sensing response for 240 cycles, with a deviation of less than 5% (Fig. 13b).

Piezoresistive sensing mechanisms of porous nanocomposites

Two mechanisms played a role in the performance of the developed piezoresistive sensors using CNF/PDMS nano-composites: the matrix effect and pore effect. The matrix effect is the change in resistance due to the rearranging of CNF conductive network embedded within the PDMS matrix (Fig. 14a). When the nanocomposite was compressed in a low strain range, the positions of CNFs in PDMS are adjusted and the average distance of CNFs is reduced, resulting in reduced electrical resistance due to tunneling effects within the CNF conductive networks. New conductive paths could be formed and CNF overlapping could occur to further reduce the elec-trical resistance during the loading process. In this paper, we observed highest sensitivity from nanocomposites with 3 wt% CNFs at compressive strain below 5% during sensor characterization. These results agreed with other published piezoresistive sensors using solid nanocomposites (Hu et al. 2011). However, the porous effect becomes dominant when large deformation was applied on the porous nanocompos-ites. CNFs on the pore walls touch each other when large deformation collapsed the pores, and the contact resistance of CNFs on the wall of pores becomes the dominant effect of the piezoresistive property (Fig. 14c). Since higher CNF

Fig. 11 Variation of resistance and stress as a function of applied compressive strain of nanocomposites with 9 wt% CNF up to 40% maximum strain

Fig. 12 Effects of applied strain rates on piezoresistive nanocompos-ite sensors

Fig. 13 Relative resistance change throughout durability test for the porous CNF/PDMS fabricated via the direct method (9% CNF load-ing)

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concentrations results in more CNFs contacts after the crash of PDMS pores, the sensitivity of nanocomposites with high CNF concentrations increases under large deformation and high maximum strain. This phenomenon was also observed during the characterizations of nanocomposites with 9 wt% CNF concentrations.

Conclusions

Porous nanocomposites sensors consisting of CNFs and PDMS were fabricated and characterized in this paper. CNFs were coated on the surface of sugar particles and used to make templates for nanocomposite fabrication. By adjust-ing the amount of CNFs used, the weight concentrations of CNFs in nanocomposites were controlled between 1 wt% and 9 wt%. The morphology and CNF distributions in the nanocomposites were verified using an FESEM. The adhe-sive tape tests demonstrated strong bonding between CNFs and PDMS. The electrical conductivity of nanocomposites increased significantly when CNF concentrations increased. The piezoresistive sensing capability of CNF/PDMS nano-composites was characterized under cyclic loading at six dif-ferent maximum strains. Nanocomposites with 9 wt% CNFs showed the highest sensitivity at the 40% maximum strain load condition. The effect of applied strain rate on piezore-sistive sensing capability was studied, and a variation of only 3% was observed. The repeatability of the developed sen-sors was tested via a durability test for 12 h. Experimental results showed variations of 5% in the piezoresistive sensing properties. The sensing mechanisms of the developed CNF/PDMS nanocomposites were discussed.

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