Electro-Mechanical Properties of Metallized Sodium Alginate Foils at the Limit of the Electrical Conduction

Abstract

In recent years, much attention has been given to biopolymers and renewable raw materials obtained from nature to find alternatives to petroleum-based materials. In this context, we developed a free-standing and flexible conductive substrate by sputtering a thin layer of gold onto a foil of sodium alginate, producing conductive self-standing substrates. These substrates have been utilized for the fabrication of eco-designed solution-processed optoelectronic devices. Herein, we report experimental work to study the mechanism behind the dependence of electrical resistance on the mechanical deformation. Data obtained from mechanical measurements, such as strain, stress, deformation, and bending, are correlated with morphological (Atomic Force Microscopy and Transmission Electron Microscopy) and structural (X-ray Diffraction) data relative to both the surface and the subsurface regions of the metallized substrates. Collectively, these data enabled the elucidation of both the composition and spatial distribution of the metal clusters implanted within the polymer matrix. The substrates present an anisotropic Young modulus, making them more stretchable in-plane with respect to out-of-plane. In the elastic regime, the reproducibility of the electrical resistance variations with respect to the stress applied makes these substrates robust candidates for the realization of strain sensors.

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Experimental setup for electro-mechanical tests: samples consisting of 50 mm × 25 mm stripes were fixed to a paper frame through double tape and sandpaper. Two copper streaks were embedded as electrodes. The samples were thus clamped in an INSTRON 4465: the electrical resistance was measured during tensile tests.

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Topographic images 15 × 15 μm² of the SAD side exposed to air (a) and in contact with the Petri dish (b). Higher magnification images 5 × 5 μm² of the Petri dish surface (c) and the SAD side in contact with it (d). The Petri dish is characterized by linear threads (in the middle of the yellow dashed lines) and disordered regions (transparent blue trapezoid areas) that are negatively transferred to the SAD surface. (e) ACF calculated from the AFM images of the Petri dish surface (black line) and the SAD side in contact with it (red line). Rectangles and triangles point at periodic features, while the black circle indicates the first zero-crossing point of the two ACFs.

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Topographic AFM images 7 × 7 μm² of pristine (a) and metalized (b) SAD surfaces. Corresponding phase images at higher magnifications, 1 × 1 μm², obtained in the repulsive regime on pristine (c) and metallized (d) SAD surfaces with an average tip–sample force of ≈200 pN and 1.3 nN, respectively. Inset: digital zoom ≈140 × 140 nm² to better emphasize the dimensions of the dark spots governing the diameter distribution (circled).

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(a) GIXRD patterns for the metallized SAD surface, collected at different incident angle ω values. Below, Au (I _Au) and SA (I _SA) peak intensities (b) and their ratio (c) are plotted vs ω.

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Gray-scale pictures of the solvent-assisted transfer-printing technique applied to a metallized SAD surface. A water drop is dispensed on a circular coverslip (a), and a piece of the metallized SA surface is placed on it (b). The piece of the metallized surface is then gently removed, leaving residual material on the coverslip (c). AFM topographic image of the region transferred onto the coverslip in contact mode (d). Below, the height profiles of three steps are indicated with black segments in the AFM image.

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(a) TEM bright field image of the thicker portion obtained with the solvent-assisted transfer-printing technique. (b) High-resolution image of a small region: The diffraction patterns are visible and measured with FFT (inset). (c) Bright field images of a vertical section at a lower (inset) and a higher magnification.

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(a) Stress–strain curves before failure. (b) Nanoindentation curves, the average of more than 50 measurements. Data obtained for pristine (red) and metallized (blue) SADs.

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(a) Electrical resistance R normalized with respect to the unstressed value R ₀ (ε = 0) plotted vs strain ε. (b) Cyclic tests: ε and R plotted as a function of time t. (c) Stress–strain curves: 10 cyclic tests are plotted here; the black ellipses indicate the relaxation of the material. (d) Linear trends of R vs ε: Two consecutive loading and unloading curves are plotted for illustrative purposes.

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(a) Simulations of a stripe with the metallized side pointing up: for each curve, the H _max and L values are the measured ones. (b) Experimental resistance R data plotted vs ROC values (metallized side pointing up). The ROC values are extracted from the simulations (see Supporting Information). The red and blue arrows indicate forward and backward bending paths, respectively. Since an unstressed sample has ROC → ∞, the dashed green line indicates the R value ‘Before Compression’ (BC), while the dashed fuchsia line indicates the R value ‘After Compression’ (AC). (c) Experimental resistance R data plotted vs ROC values (metallized side pointing down). Since ROC → ∞ for an unstressed sample, (b, c) plots must be read from right to left. The relative experimental errors of the R data are negligible, 10^–4–10^–5%.

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(a) Digital zoom of a bright field TEM image highlighting the two regions with a different density of Au clusters. (b) Sketch of cluster density: Purple colors are used to highlight regions with different densities at different depths.