Flexible conductive blend of natural rubber latex with PEDOT:PSS

Flexible electronics is a promising field of research with potential use in bioelectronics,¹ solar cells,^2,3 light-emitting diodes (LEDs), organic electrochemical transistors,⁴ and memory storage devices.^5,6 The application of flexible and soft electronics ranges from wearables^7–9 to biomedical devices,^1,10 embracing fields that require tolerance to bending and stretching. Flexible materials with high stability such as conductive polymers mixed with elastomers^5,11 such as silicone rubber, PDMS,¹¹ and parylene¹ have shown interesting properties such as biocompatibility⁷ and high conductivity.¹² 

PEDOT:PSS [poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate)] is a polymer mixture widely used in bioelectronics due to its exceptional properties such as thermal stability, high transparency, and excellent conductivity.¹³ Solvent addition such as dimethyl sulfoxide (DMSO), DMF, and ethylene glycol (EG) can increase the number of small grains and the interconnectivity of PEDOT domains that enhances the conductivity of the films.^14–16 The excellent electrical properties of PEDOT:PSS enable its large applicability in organic electronic devices as electrodes in transistors¹⁷ or organic light-emitting diodes (OLEDs)^18,19 and as active layers in neuromorphic devices.^20,21

Natural rubber latex (NRL) is obtained from the tree Hevea brasiliensis and composed basically of 1,4 polyisoprene (∼35%) and water (∼55%).^22,23 NRL is an electrical insulating material with exceptional mechanical properties. In the absence of tensile stress, the polymer chains are amorphous; however, when stretched, an ordered arrangement is formed by the alignment of its chains. Higher crystallinity provides greater strength to the material, which turns NRL into a “self-reinforcing” material.²⁴ Although NRL presents remarkable mechanical and biocompatible properties, it is not widely used in electronics.

In this work, we report a blend of natural rubber latex (NRL) and PEDOT:PSS with high flexibility and conductivity. Different NRL:PEDOT:PSS ratios and curing temperatures were investigated. A detailed study was performed in the most promising membrane.

High-conductivity PEDOT/PSS Orgacon^TM S315 (0.5%–1% in water) was bought from Merck Brazil, and ethylene glycol (purity of 99.0%) was bought from Synth. The natural rubber latex (NRL) was bought from BDF Latex (Guarantã/SP, Brazil) with high-ammonia content (10%) and about 60% of dry rubber content,²² extracted in May 2018. The microscope slides used were from Olen (k5-7102). Reagents were used without further purification.

NRL was centrifuged for 1 h at 5000 rpm to separate proteins from the latex extract. The latter was used in a blend with 5% (v:v) ethylene glycol doped PEDOT:PSS in a ratio of latex/PEDOT:PSS (v:v) of 1/2 and 1/4 and stirred for 5 min. Then, the mixture was centrifuged for 1 min at 3000 rpm to reduce air and bubbles in the mixture. Prior to deposition, the mixture was stirred for 1 min again to mix the latex and PEDOT:PSS separated in the centrifugation process. A volume of 200 μl of NRL/PEDOT:PSS mixture with different N/P ratios was deposited by drop-casting on a glass substrate with an area of 1.0 × 2.5 cm² (totally 80 μl/cm²), with thermal curing at 60 °C for 30 min for some samples and curing at ambient temperature for other sets of samples (NTC) for 24 h. Three optimized samples of each composition/curing temperature were produced and investigated for reproducibility evaluation.

A schematic that synthesizes and illustrates the preparation process and electrical characterization of the flexible conductive polymer NRL/PEDOT:PSS is presented in Fig. 1.

The electrical characterization of the blend was carried out in a Metrohm Autolab PGSTAT302 equipped with an FRA32 impedance module. Measurements of chronoamperometry were performed with an applied voltage of 0.2 V on samples under mechanical stretching using two electrodes on the extremes of the membrane. The initial electrical resistance (R₀) of the pristine membranes was taken after stabilization of the electrical current under 0.2 V using Autolab. The electrical conductivity (σ) was obtained by the equation σ = L/RA, where A is the initial cross-sectional area of the membrane and L is the distance between the two electrodes. A is the product of the width (10 mm) and thickness of the membranes, taken by using the caliper and ranging from 100 μm to 250 μm. A summary of these values, for different samples, is given in Table S1 of the supplementary material.

I vs V curves were obtained up to 0.2 V, and impedance spectroscopy was also evaluated for offset voltages of 0 mV and 200 mV with 10 mV rms and frequencies from 1 Hz to 10⁷ Hz. The blend was applied in an electrical circuit in series with a commercial LED, under 6 V, up to 500% strain. Current was measured using Autolab.

Mechanical stress was applied to samples by stretching by steps of 1 mm, and the strength measurements were acquired in the equipment Xplorer GLX PS-2002 from PASCO. The morphology and atomic composition were obtained by using a Scanning Electron Microscope (SEM) (Zeiss EVO) and an Energy Dispersive Spectrometer (EDS) (Oxford Instruments) attached to the SEM. Thermogravimetry/Differential Thermal Analysis (TG/DTA) was performed under room atmosphere in a Netzsch STA449 F3.

The strain produced on the membranes and the Gauge Factor (GF) were calculated through the following equations, respectively:

where ΔL is the elongation of the membrane, L₀ is the initial length of the pristine sample (L₀ = 12 mm), ΔR is the resistance change, and R₀ is the initial electrical resistance of the sample before stretching.

To understand the significance and role of PEDOT:PSS (5% EG) in the electrical conductivity of the flexible membrane, blends of two different concentrations (v/v:1/2 and 1/4) thermally cured (TC) and not thermally cured (NTC) were produced. Triplicates of four different samples, 1/2 NTC, 1/2 TC, 1/4 NTC, and 1/4 TC, were used to calculate the mean values of thickness, elongation at break, and initial conductivity for each blend (Table S1). The electrical conductivity of pristine samples, before stress–strain tests, is shown in Fig. 2(a). As expected, sample 1/2 NTC presents considerably lower electrical conductivity than sample 1/4 NTC, whereas samples thermally cured (TC) show higher electrical conductivity at the same NRL/PEDOT:PSS ratios. Mechanical stress–strain tests [Fig. 2(b)] show that samples with higher PEDOT:PSS concentration and thermal curing present higher strength to tensile deformation [Fig. 2(b)]. Figure 2(a) shows the elongation at break for each membrane. Note that all blends support elongations above 700% before break. As the concentration of PEDOT:PSS is increased, there is a reduction in the elongation at break.

Thermal curing improves not only the conductivity but also the mechanical strength of the membrane, which is further discussed in the next paragraphs.

TG/DTA was performed in one sample of each blend, as shown in Fig. 3(a); the first mass loss registered in TG (at 140 °C) with an exothermic peak in the DTA spectrum is attributed to water evaporation.²⁵ The second mass loss that begins at 300 °C in TG with an exothermic peak in the DTA spectrum is associated with the degradation of 3,4-ethylenedioxythiophene²⁶ and with the degradation of latex.²⁷ The last mass loss at 500 °C registered in TG may be related to the degradation of highly reticulated polymeric chains, as already observed for pure NRL.²⁷ 

Derivative Thermogravimetry (DTG) in Fig. 3(b) shows that the blend 1/4 NRL/PEDOT:PSS lost more water at 120 °C compared to samples 1/2 TC (or NTC), which is expected due to the larger volume of water present in the PEDOT:PSS solution. Independent of the thermal curing, samples 1/4 present similar water elimination. This result suggests that the thermal curing at 60 °C is not responsible for water elimination but only to accelerate the blend polymerization.

DTG has a peak at 350 °C, with the most intense values for membranes 1/2, TC or NTC, indicating a dominant degradation of NRL components above 300 °C. On the other hand, the most intense peak at 500 °C for membranes 1/4 TC (or NTC) indicates that PEDOT:PSS is responsible for inducing cross-linking, forming highly reticulated polymeric chains in the blend.²⁷ 

Therefore, with these results, we assume that the higher conductivity and strength of the thermally cured 1/4 TC blends are related to PEDOT:PSS and thermal curing at 60 °C. These results also indicate a good thermal stability of the blends that may be operated at temperatures as high as 300 °C with no or weak degradation.

In pristine samples, the electrical current increases under a bias of 0.2 V before reaching a plateau, achieved after about 10 min, as shown in the inset of Fig. 4. This effect probably occurs due to the movement of PEDOT in the blend from unstable to more stable positions, enhancing its percolation, which leads to a change in electrical conductivity.²⁸ To better understand this effect, impedance spectroscopy was performed on sample 1/4 TC prior and after current saturation (CS), with 0 mV and 200 mV offset voltage. Prior to CS, the Nyquist plot in Fig. 4, with V_OFFSET = 200 mV, presents a small reduction in the complex impedance followed by a narrowing of the semicircle due to a more significant reduction in the real impedance, compared to V_OFFSET = 0. After CS of 0.2 V bias applied for 10 min, a plateau of current is reached. The sample does not return to its original conduction state as it is demonstrated with similar impedance curves taken after CS.

Such improvement of electrical conductivity indicates an improved percolation of PEDOT:PSS chains and charge release processes from interfacial states in the blend, led by the current passing through the blend. The Bode plot (shown in Fig. S1 of the supplementary material) confirms the behavior observed in the Nyquist plot, a reduction in the impedance after current saturation for both bias.

Applying tensile stress on the samples causes strain that is responsible for an increase in the electrical resistance of the flexible membrane. It is noticed in the inset of Fig. 5(a) that the addition of small stress initially largely increases the electrical resistance, right before the relaxation of the PEDOT:PSS chains that recovers partially the previous resistance. Sample 1/4 TC presents the largest relative resistance change, ΔR/R₀ = ∼240, at 500% of its original length L₀, whereas sample 1/2 NTC shows the lowest ΔR/R₀ = 13.5 at the same distention. Thermal curing of the blend produces more conductive flexible membranes with more interconnected conductive polymeric chains, as discussed earlier [see Figs. 2(a) and 3]. These percolations are substantially affected under tensile stress that breaks these connections and significantly increases the electrical resistance.²⁹ Figure 5(b) shows the derivative of the current (−di/dt) of sample 1/4 TC under stress and strain. The electrical current strongly decreases at stretching up to about ΔL/L₀ = 100% and presents additional significant electrical changes up to ΔL/L₀ = 200%. Such results indicate that the conductive polymeric percolation is strongly reduced under 100% strain.

The conductivity variation of sample 1/4 TC under repetitive stretching–relaxation cycles is shown in Fig. 5(c). Taking as a reference the lowest current after the first stretching for strains of 50% and 100%, the conductivity relaxes to values near to the reference, with a standard deviation of 1.1% and 0.7%, respectively. It reveals good stability of the blend under repetitive elongations. In the distention cycles, once the sample is relaxed from 100% strain to 0%, the rise in current is not sufficient to reach the original values (pristine sample). This result indicates that the repair in the percolation path is not fully achieved due to the plastic deformation of the blend.

Figure 5(d) shows the Nyquist plot for sample 1/4 TC in the range of 0%–200% strain. One can see that the sample significantly increases its series resistance (R_s) due to a disruption of the PEDOT:PSS percolation network. The horizontal and flat characteristics of the curve, i.e., green curve at 200% strain (inset), may be explained by the Williams–Watts dielectric relaxation model, which considers that the relaxation processes in a medium with many polar groups or molecules occur due to the interaction of the dipole-moments with thermally activated mobile defects. In polymers, these defects may be a local conformational abnormality induced by the interaction of the polymer chain with itself or with other chains, which introduces local strains into the system.^30,31 Thus, this flattened semicircle is an indication that the straining of the N/P blend forces the interaction of adjacent chains of PEDOT:PSS, sacrificing the percolation and privileging the formation of polar molecules that contributes even further to this mechanism. A tail appears in measurements up to 100% strain in the low-frequency range and 0 V offset. This extent of the real part of the impedance, absent at 0.2 V offset bias, suggests carrier charge liberation from traps generated by the natural rubber in the blend. Above 100% strain, this effect fades away because of the continuous creation of defects and polar groups. Figure S2 of the supplementary material shows the Bode plot for the same measurements, and a clear increase in impedance is observed for larger strains and an offset of 0 V.

The flexible blend 1/4 TC, with higher conductivity and stress–strain response, was integrated into a circuit with a LED, as shown in Fig. 6. The electrical current variation in the circuit is taken in steps of 25% strain in a non-reversible process since strains larger than 50% do not increase the electrical current to initial values I₀, as shown in Fig. 5(c). As the electrical resistance increases due to geometrical factors such as the reduction in cross-sectional area and higher longitudinal extension, the combination with the reduction in the percolation system diminishes the LED brightness and keeps on up to 475% strain. Although the gauge factor shown in Fig. 6(b) is as low as GF = 150 at 475% strain, these values may still be interesting for sensing distension in the membrane.²⁸ 

Figure 7(a) presents surface microscopies of different samples, and Fig. 7(b) shows the compositional analysis of samples 1/2 TC and 1/4 TC, obtained by EDS. C, O, and S are the main atoms found in the blend, and the S content is higher in the sample with higher PEDOT:PSS concentration, as expected. This higher concentration of sulfur combined with thermal curing may be the factor responsible to enhance the stiffness and conductivity observed in Fig. 2. Sulfur is known to induce cross-linking that improves the strength of the rubber (sulfur vulcanization).³² A plot of the oxygen and sulfur concentration for each sample is presented in Fig. S4 of the supplementary material.

Figure 7(c) presents images of samples 1/4 TC pristine and 1/4 TC relaxed after stretching, as well as an artistic image of the percolation reduction of PEDOT:PSS in the blend after large deformations. In this assumption, pristine samples present chains of conductive polymer more homogeneously distributed in the material, with high percolation and cross-link. After stretching and relaxing, the conductive polymer PEDOT:PSS agglomerates and reduces its percolation chains in the membrane.³³ As previously discussed, both the electrical conduction and mechanical strength are enhanced on samples with higher percolation chains of PEDOT:PSS. Higher gauge factors for samples with higher PEDOT:PSS percolation also indicate that the stretching breaks more percolation paths in these membranes. Moreover, stretching–relaxation cycles [Fig. 5(c)] show that the blend presents stable electrical response, which implies that the percolation is reduced only during the first stretch.

Table I summarizes the results presented here and compares with other flexible conductive materials reported in the literature with some common parameters. As shown, the NRL/PEDOT:PSS blend presents elevated stretching (ΔL/L₀ ≥ 700%), elevated conduction (63 ± 26 S/m at L₀), and a maximum gauge factor of 150 at L/L₀ = 475%. These values balance high stretchability with considerable high conductivity when compared to most of the other blends of silicone rubbers with PEDOT:PSS, graphene, or carbon nanotubes as conductive fillers. Besides these interesting and balanced characteristics, the biocompatibility of the components of the NRL/PEDOT:PSS blend indicates promising applications in biocompatible electronics. Although the blends present extreme high strength to stretching, the elastic regime where the membrane totally returns to its initial length is still low, below ΔL/L₀ = 10%. A way to improve the elastic regime could be vulcanization, but the addition of chemicals normally used in the process could sacrifice its conductivity and/or its biocompatibility for use in many bioelectronic applications.

In this paper, we describe a highly conductive and stretchable polymer based on the blend of PEDOT:PSS and natural rubber latex (NRL). The blend presents great interconnection between the electrical and mechanical properties, attributed to a good percolation of the conductive polymeric chains with thermal curing, which resists strain as high as 700%. The composition of NRL/PEDOT:PSS investigated in this work with the best conductive and highest gauge factor is obtained with a 1/4 (v:v) NRL/PEDOT:PSS ratio cured at 60 °C for 1 h. These samples showed excellent conductivity and mechanical stability, which indicates that this blend is perfectly suitable for many bioelectronic devices that work with strains up to 100% without significant electrical resistance variation, such as strain gauges, electronic skin, or flexible conductors.

Additional electrical, mechanical, and compositional data are presented in table and figures in the supplementary material.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

The authors would like to thank the funding support from FAPESP (Grant Nos. 2016/01743-5, 2013/07296-2, 2017/20809-0, and 2019/18481-1) and CAPES, PROPG Edital 07/2020; Professor G. Bannach and C. Gaglieri for TG measurements in the Laboratory of Thermal Analysis; Multiuser Lab for electron microscopy; and Professor M. V. C. Rudge and J. F. Floriano.