Solvent-free electrically conductive Ag/ethylene vinyl acetate (EVA) composites for paper-based printable electronics

Solvent-free electrically conductive composites have been applied to flexible electronics to obtain high electrical conductivity. However, some of the proposed composites have low electrical conductivities and are unable to meet the requirements of commercial printable electronics. In this study, solvent-free electrically conductive Ag/EVA (ethylene vinyl acetate) composites for paper-based printable electronics were prepared by a thermal melting method. The properties of these electrically conductive Ag/EVA composites, including particle sizes, morphologies and phase purities of the flake silver flake powders, were investigated using a particle size analyzer, scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results showed that nanometer-thick flake silver flake powders with smooth and flat surfaces were made by the nanofilm transition technique. These obtained powders were able to form smooth face-to-face contacts, which facilitated the formation of an excellent conductive network in the conductive system. Dynamic mechanical analysis (DMA) was conducted to investigate the mechanical properties of EVA and Ag/EVA composites. A Fourier transformation infra-red (FTIR) spectrometer, laser micro-Raman spectrometer and thermogravimetric analyzer were used to analyze the organic functional groups, glass transition temperatures and thermal weight losses of the EVA resin and solvent-free electrically conductive composites. The solvent-free electrically conductive Ag/EVA composite, which contained 55 wt% of the as-prepared flake silver flake powders, was found to have an extremely low volume resistivity of 1.23 × 10−4 Ω cm as well as excellent bending performance and adhesion. These features indicate the great potential of these composites for application in printed electronics.


Experimental Section Weight percent selection of silver powders of solvent-free electrically conductive Ag/EVA composites
The reason why the loading of silver flakes was fixed between 40-70 wt% is as follow: Once the silver loading of Ag/EVA composites was 35 wt%, the volume resistivity of as-prepared, FAgL 6500 and FAgL6501 reached 0.82 Ω·cm, 16.8 Ω·cm and 36.9·Ω·cm,respectively.The volume resistivity increases by three orders of magnitude when compare with the Ag/EVA composites with the loading of 40 wt%.
When the loading of silver surpass 70 wt%, electrical conductivity continues to deteriorate; furthermore, the production costs of them have been greatly raised; thirdly, the stirring of Ag/EVA composites becomes much more difficult; finally, the mechanical property of cured film of Ag/EVA composites deteriorate.Considering the electrical conductivity, mechanical property and production cost, the Ag/EVA composites filling with 40-70 wt% silver powders are chosen as the research subject.
The electrical percolation of the three Ag/EVA composites are different, The electrical percolation of Ag/EVA composites filling with as-prepared flaky silver powders is around 25 wt%, of FAgl6500 and FAgL 6501 is about 30 wt%.Note: The density of EVA is 1.0 g/cm 3 .

SEM micrographs of cured conductive films
Figure S3 SEM of solvent-free electrically conductive Ag/EVA composites filled with as-prepared flake silver powder (a) 55 wt% Ag (b) 65 wt% Ag.

XRD analysis
X-ray diffraction was carried out to study the crystalline structures of the home-made and commercial silver powders.Figure S4 shows all the samples have four characteristic peaks for crystalline metallic silver at about 38.1°, 44.5°, 64.5°, 77.4° corresponding to the Bragg's reflection indices of ( 111), ( 200), (220), and (311) planes in a fcc structure, proving that all the silver powders haven't been oxidized.

ATR-FTIR of paper substrate
It was observed that the peak at 880 cm -1 was assigned to C-H bending vibrations of glycosidic linkage. 1 The peak at 1055 can be corresponded to the C-O stretch of glucose ring.The peak at 1110 and 1160 cm -1 which could be associated to the -C-O-C-asymmetric stretch and vibration of glucose ring stretch in cellulose. 2The Shoulder peak at 1343 and 1413 cm -1 were assigned to the asymmetric CH 2 bending and wagging. 3The band appearing at 1632 cm -1 can be assigned to the conjugated C=O.The small peak which appeared at 1730 cm -1 can be attributed to the -C-Ostretching of the cellulose. 4The band at 2902 cm -1 appeared due to C-H stretching in cellulose.The band at 3345 cm -1 can be ascribed to the stretching of H-bonded of -OH groups. 5

Adhesion test
From Figure S6(c) we can see that no noticeable conductive films were removed by the tape, a 5B level of adhesion strength was obtained.This can be ascribed to the excellent bonding of the conductive Ag/resin composites towards the paper substrate.

Figure S1 .
Figure S1.Optical images of solvent-free electrically conductive Ag/EVA

Figure
Figure S4 X-ray diffraction (XRD) patterns of flake Ag powders.

Figure S5
Figure S5ATR-FTIR spectroscopy of paper substrate.

Figure S6
Figure S6 Optical images of (a) cured Ag/EVA conductive film after crosshatched; (b)

Table S1
The conversion relationship between weight fraction and volume fraction of solvent-free electrically conductive Ag/ethylene vinyl acetate (EVA) composites filling with different silver powders.