Electrical Conductivity Enhancement of Thermoplastic

Electrical Conductivity Enhancement of Thermoplastic PolyurethaneGraphene Nanoplatelet Composites by Stretch-Release Cycles Pietro Cataldi1*, Luca Ceseracciu1, Sergio Marras2, Athanassia Athanassiou1 and Ilker S. Bayer1* 1

Smart Materials, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

2

Nanochemistry, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

Materials Graphene nanoplatelets (GnPs) were kindly donated by Directa Plus (grade Ultra G+, for details on thickness and lateral size see this work(1)). Thermoplastic polyurethane (TPU) was purchased from BASF (Elastollan® 1185A). The dispersion of TPU and GnPs was prepared in chloroform (Sigma Aldrich) employing 1 g of polymer every 6 ml of solvent and adding nanoflakes wt.% relative to the weight of TPU depending on the final load desired. The obtained liquid solution was dense and viscous enough to avoid Marangoni effect(2, 3). The dispersion was then tip sonicated (750 W, 40% Amplitude, 20 kHz, 3 times for 30 s) using a Sonics & Materials, Inc. (Model Num. VCX750), solvent casted in Teflon petri dishes (7.5 cm diameter) and dried overnight. To ensure complete solvent evaporation, the films were placed in the oven at 120°C for 4 hours. Immediately after, dried films were hot pressed (15 bar, Specac-Atlas Power Presses T8) for 4 minutes at a temperature of 180 °C to eliminate various structural defects such as bubbles. Teflon anti-adhesive films (Advent Research Material, Art. Num. FP823338) were employed during the hot-pressing process to avoid sticking of the composites on the platen of the press. Measurements The SEM images were obtained with a JEOL microscope (model JSM-6490LA) operating at an acceleration voltage of 15 kV. The specimens for cross-sectional SEM images were cut at liquid nitrogen temperature (−195.79 °C) by tearing them with two tweezers, minimizing the deformation of the films. The sheet resistance percolation measurements were performed with a four-probe resistance system (Keithley 2611A sourcemeter). Minimum five samples were measured for each composite type. Silver paste (SPI Conductive Silver Paint) 1

electrodes of 5 mm were realized on the specimens, keeping apart 5 mm spacing. The effect of static and cyclic deformation on the conductance of composites was characterized on two uniaxial testing machines (Instron 3365) coupled with the sourcemeter. At least five samples were tested for each GnPs wt.% concentration. The composites were mounted on the testing machine and electrodes were connected at the specimens extreme. A tension of 1 V was applied and the current i0 at zero deformation was recorded. The strain was increased stepwise in 2 mm steps with the rate of 5 mm/min between steps. At each step, deformation was held constant for about 2 minutes to allow partial relaxation of the samples and current was recorded. Other measurements (see figure S7) were performed releasing the sample after each stepwise incremental deformation. The current was recorded at each release (i0i) and deformation step (ii). The LED chips were made employing Cree lights components (model CLP6, white, 4.4 V, 32 000 mlm). They were assembled on copper plates resembling IIT logo. All our cyclic measurements were conducted in a closed lab room with controlled relative humidity (40%) and temperature 22oC. Samples were removed from the testing room and stored under variable ambient conditions. Upon ageing for a few days the samples did not display any variations in baseline conductivity or other variations during in conductivity during stretch-release cycles. The effect of a high number of cycles was also evaluated on a micro uniaxial testing stage (Deben, UK, custom design). Samples were cut in (20 ∙ 4) mm2 strips and mounted on the stage. The clamps starting span was 10 mm. Electrodes were connected to both ends. Cyclic deformation was applied with the rate of 5 mm/min, both in loading and unloading, to the maximum extension of 0.5, 1, 2, or 4 mm, corresponding to 5, 10%, 20% and 40% strain, respectively. 1000 cycles were performed for each test. Current was recorded real time during the tests and for a few minutes at the end of the cyclic deformation. At least five samples were tested for each GnPs wt.% concentration. As expected, the samples were chemically unaltered by the cycles (see Figure S8 for FTIR details). TEM images were collected by means of a JEOL JEM 1011 electron microscope, operating at an acceleration voltage of 100 kV, with a 11 Mp fiber optical charge-coupled device (CCD) camera (Gatan Orius SC-1000).

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X-ray diffraction (XRD) patterns were recorded on a PANalytical Empyrean X-ray diffractometer equipped with a 1.8kW CuKα ceramic X-ray tube, PIXcel3D 2x2 area detector and operating at 45 kV and 40 mA. The diffraction patterns were collected in air at room temperature using a zero diffraction quartz substrate, ParallelBeam (PB) geometry and symmetric reflection mode. XRD data analysis was carried out using HighScore 4.5 software from PANalytical. FTIR spectra were recorded through a Bruker Vertex 70v (from 600 to 4000 cm-1). Raman spectra were obtained with a Horiba HR800UV, LabRAM 600 spectrometer (diffraction grating of 600 line per mm, excitation wavelength of 632.8 nm HeNe laser, maximum power 20 mW). Thermogravimetric analysis of the films was performed using a TA instruments machine (model Q500) in N2 flow. Thermogravimetric Analysis We performed Thermogravimetric Analysis (TGA) of two different GnPs composites with different filler content (10 and 20 wt.%) and of the pure TPU film. The results are graphed in figure S1. The weight percent loss is monitored as a function of the temperature and graphed in Fig. S1a. No relevant weight loss is registered until approximately 250°C. Therefore it was possible to hot press the films at a temperature of 180°C not incurring in any thermal degradation. Increasing the GnPs concentration from 10 wt% to 20 wt%, leads to a proportional increase of the final residual mass after 450°C. At 600°C, GnPs has almost unaltered mass, while the TPU component of the films degrades completely(4). Both composites display a left-shift of the initial point of the thermal degradation, probably due to the nanoflakes content: in fact, the oxidation of the amorphous carbon present in the sample could generate such kind of shift(5). The first derivative of the weight loss as a function of the temperature is reported in Fig. S1b. The pure TPU undergo two degradation steps relative to the hard segment (between approx. 280 and 380 °C) and the soft segment (between approx. 380 and 440°C) (6, 7). Introducing 10 and 20 wt.% of nanoflakes content, leads to a decrease in the starting point of the degradation. This behavior was already found in literature when high loads of GnPs were employed(8).

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Figure S1: Thermogravimetric analysis of pure TPU and of 10 and 20 wt.% GnPs loaded composites. In a) is shown the weight percent loss as a function of temperature. In b) is reported the first derivative of the weight loss with respect to temperature. Two degradation steps are present.

Morphology In Figure S2 is presented the SEM morphology of the pure TPU matrix (Figure S2a) and of the TPU-GnPs freestanding composite at 20 wt.% filler concentration (Figure S2b). Both the surfaces are flat due to the hot pressing process.

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Figure S2: Electron Microscopy images of the surface of a) the pure TPU and b) the composite with 20 wt.% GnPs.

Raman Figure S3 shows the Raman spectra of representative GnPs flakes, of the pure TPU matrix and of the 20 wt.% TPU-GnPs composite. The pure filler spectrum present features of multilayer graphene materials with mainly three peaks: the 2D, the G and the D peaks. The first and the last (located at approx. 2700 and 1360 cm-1) are activated with the presence of defects and are respectively the second and the first order relative to the breathing modes of sp2 carbon bonds(9, 10). The 2D peak is constituted of a single component in single layer graphene, while it presents many components in the form of graphene platelets or graphite(11). The G peak (located at approx. 1585 cm-1) is related to the E2g phonon at the Brillouin zone center(11, 12). GnPs Raman peaks are visible in the 20 wt.% composite. GnPs are more Raman active than the polymer matrix. In this previous work(1) we estimated the thickness of the GnPs to be ≥ of 9 layers.

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Figure S3: Raman spectra of pure GnPs, pure TPU and the composite with 20wt.% GnPs.

Percolation comparisons with other carbon nanofillers The threshold of percolation for GnPs does differ from higher aspect ratio carbon nanostructures such as CNTs or CNFs. In our previous works (see P. Cataldi et al., Foldable Conductive cellulose Fiber Networks Modified by Graphene Nanoplatelets-Bio-Based Composites, 2015, 1(12), 1500224; P. Cataldi et al., Cellulosic Graphene Biocomposites for Versatile High-Performance Flexible Electronic Application, 2016, 2(11), 1600245) different thermoplastic polymers blended with the same GnPs employed in this work showed percolation threshold between 5 and 7 weight percent similar to the value obtained for the thermoplastic polyurethane-graphene nanoplatelets composite. In all these cases, 3D percolation mechanism occurs in the form of direct continuum contact of the conductive filler inside the matrix. Comparing these results with other types of carbon 6

nanomaterial fillers such as CNTs leads to the conclusion that particles in the form of tubes percolates at lower filler concentration (see e.g. M. Martin-Gallego et al., Comparison of Filler Percolation and Mechanical Properties in Graphene and Carbon Nanotubes Filled Epoxy Nanocomposites, 2013, 49(6), 1347-1353; S. Chatterjee et al., Comparing Carbon Nanotubes and Graphene Nanoplatelets as Reinforcement in Polyamide 12 Composites, 2011, 22(27), 275714).

Stretch-release measurements at 40 wt.% elongation We measured the electrical resistance of the material as a function of stretch-release cycles from the initial to a fixed length also with an elongation of the 40%. We report only the result for 10 wt.% load of GnPs. The setup was identical to the one employed for the measurements in the main text (see Figure 2c and 2d and measurements for details). In figure S4 is plotted the ratio Ri/R0 as a function of the number of cyclic deformation: Ri represent the resistance recorded during the 1000 cycles of stretch and R0 is its initial value. Ri/R0 jumps to a value above 200 after the first stretch cycle. On the contrary, it decreases augmenting the

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number of deformation cycles. It never reach values lower than one, ending at a value around 4.

Figure S4: Real time variation of the ratio between the resistance during stretch-release cycles (Ri) and the resistance at zero cycles (R0) as a function of number of cycles performed on the 10 wt.% composite. The material is elongated at 40% every cycle of stretch.

Stress Strain Curves As can be seen in Fig. S5, GnPs significantly affect the mechanical properties of the pristine TPU thermoplastic. Although the elastic modulus is increased significantly as a function of increased GnP concentration (to achieve best electrical conductance), significant decline in elongation values are observed. This also does not allow us to test a similar behavior under longer elongation values due to rupturing of the nanocomposites at or before 100% elongation. More specifically, in Figure S5 we present typical stress-strain curves of pure TPU and composites, with and without cyclic deformation conditioning (5% strain, 1000 cycles). The pure TPU curves present the 8

classical non-linear behavior of elastomeric materials, with an elbow at about 500% strain. The addition of GNPs leads to higher stiffness and to an evident change in the deformation response, with a linear elastic deformation followed by a change in slope that can be attributed to the progressive rupture of the GNPs network. Interestingly, after 1000 cycles at 5% strain, the response of pure TPU is unchanged, while the composites present lower elongation, demonstrating that cycling deformation modifies the graphene particles arrangement only.

Figure S5: Stress-strain curves of the sample before and after 1000 cycles at 5% maximum elongation.

Resistance changes at constant elongation 9

In Figure S6 is shown the resistance of our composite when a single stretch event is performed. We elongated at 5% relative to the initial length. The composites investigated have 10 and 20 wt.% GnPs content. R i is the resistance during stretch while R0 is the resistance before stretch. The x axis reports the time of the stretch which corresponds to the time employed to make 1000 cycles in the stretch-release experiments. It is possible to notice that from the initial value of Ri/R0=1 there is an increase to higher values for both composites. This value is slightly decreased with time, probably due to a relaxation of the material. Nonetheless, the final value of the resistance is far from unity.

Figure S6: Ratio between Ri (resistance of the material elongated at 5% of the initial length) and R0 (resistance before stretching). The x axis is the time correspondent to the realization of 1000 cycles.

Resistance changes due to increasing elongation in stretch-release cycles

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Figure S7 reports the stress induced resistances variation of the 10 and 20 wt.% GnPs loaded composites. The resistance at zero deformation (R0) was recorded. Then 2% elongation steps were performed till reaching an extension of 24%. Before each step the material was released to the initial length and the resistance (R0i) was measured. The resistance was also measured after each nonconsecutive stretch event (R i). The upper plot shows the ratio R0i/R0. This quantity monitors how the resistance at zero deformation varies after each elongation step. The first three steps (till 6% elongation) slightly damage the electrical properties (percolation) inside the material. After 6% stretch there is a more important increase in the value of the ratio especially for the 10wt.% GnPs load. At 24% extension, the resistance at zero deformation increase 5 times for the 20wt.% loaded sample and 12 times for the 10 wt.%. The lower plot graphs the ratio Ri/R0. At maximum reported elongation (24%), this quantity increases to approx. 6 for the specimen whit higher filler loads while jumps to approx. 35 for the 10wt.% nanoflakes composite. As in figure 2 of the main text, the samples with more GnPs are more stretch stable.

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Figure S7: Stress induced variation of the composites resistance. The resistance at zero deformation (R 0) was recorded. Then 2% non-consecutive steps of elongation were performed: before each step the material was released to the initial length. The resistance was measured when stretched (Ri) and released(R0i). The upper plot shows the ratio R0i/R0 while the lower plot graphs the ratio Ri/R0.

Decay Constant Results due to percent elongation In Table S1 are reported the exponential decay constants for 10% and 20% GnPs nanocomposites under different stress-release cycles extracted from the curves in Figure 2c-d . The exponential decays was best fitting with a double exponential from which we obtained two decay constants. This decay constant corresponds to a fast rearrangement approximately within the first 15 cycles of stretch (t1) and a slow one (corresponding to the plateau in Figure 2c-d) increasing the number of cycles (t2). Table S1: Decay constants extracted from Figure 2c-d.

Sample (Percent elongation)

Decay Constant (t1)

Decay Constant (t2)

10% GnPs- (5%)

8

147

10% GnPs- (10%)

3

79

10% GnPs- (20%)

1

7

20% GnPs- (5%)

12

223

20% GnPs- (10%)

6

115

20% GnPs- (20%)

7

255

FTIR Analysis To verify that the matrix was not modified by the cyclic stretch release events, we investigated the FTIR spectra of the cycled and uncycled TPU, finding no difference between the spectra (Figure S8). In both cases, the typical peaks at approx. 3330 cm− 1 (N-H group in urethane) and at approx. 2930 and 2850 cm− 1 (respectively the asymmetric and symmetric vibration of the C-H2 group) are present(13). Also the peak at approx. 1733 cm−1(relative to urethane carbonyl) and at approx. 1720 cm−1 (hydrogen bonded carbonyl) are unchanged 12

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. A detailed discussion of the FTIR peaks is out of the scope of this paper. For further details on FTIR see

these reports(15-18).

Figure S8: FTIR spectra of the pure TPU film cycled (Blue) and uncycled (Cyan)

Electrical conductivity as a function of the monotonic stretching. From the results of electrical resistance as a function of the deformation upon uniaxial stretching shown in Fig. 2b, the variation in intrinsic conductivity can be calculated according to the following equation(19) 𝜎 𝑅0 = (1 + 𝜀)2 𝜎0 𝑅

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Which leads to the plot in Fig. S9. The conductivity of both materials decreases steadily as the percolation paths are progressively interrupted.

Conductivity (/0)

100

TPU 20% GnP TPU 10% GnP

10-1

Repeated stretch-release cycle experimental zone

10-2 10-3 10-4 0

20

40 60 80 Elongation (%)

100

120

Figure S9: Conductivity as a function of the elongation for the composite materials.

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