Development and experimental characterization study of cesium doped zinc oxide polymer conductive films for sensing applications

In this research study we report the successful preparation of cesium doped zinc metal oxide nanoparticles by solution combustion technique further incorporated into polyurethane films synthesized from bio-degradable castor oil, for study of piezo-conductive property. The composite films prepared with filler weight percentages of 0.5, 1.0, 2.0 and 4.0 are studied for structural, mechanical, thermal, electro-mechanical and weatherability properties. Studies revealed successful formation of urethane links and good dispersion of nanoparticles in the prepared films. Films under tensile and compression loading showed promising electro active results with maximum volume conductivity values of 1.226E−7Scm−1 at 30N compression load. The developed films show good compatibility to be employed in corrosive and acidic environments with safe working temperature upto 160°C.


Introduction
Modern electro active material applications demand the need of communicating conductivity along with stretchability and excellent flexibility to suit the application areas of structural health monitoring, human body part motion sensing, sports accessories and recreational gadgets.There is a huge demand for development of materials which can demonstrate and continue to exhibit good electrical conductivity properties while simultaneously being in stretched, twisted and bending positions.Materials with self-healing properties are high on demand for wearable applications [1][2].Advancements in the development of conductive polymers with nano metal oxides have gained importance due to their unique properties and applications in the field of microelectronics and soft robotics.[3] reported achieving higher conductivity values at low doping concentrations of Cs-ZnO in prepared solar cells with organic polymers.[4] achieved thrice in orders of conductivity in magnitude with Cs doping in ZnO with 80% transparency for photovoltaic applications.[5] prepared polyurethane foams with MWCNTs and Lignin to achieve very good change in resistance performance for a wide range from zero to ninety percent with 2000 cycles of repeated compression test and detection pressure values upto 275 kPa.[6] reported to have achieved low sheet resistance to conductivity with 1mol% Cs doped ZnO on glass substates with sol gel spinning technique.[7] achieved an increase in organic solar cell efficiency by 5.6% with ZnO-PVA thin film coatings.[8] studied the utilization of Cs-ZnO thin films on SnO2 substrates for NH3 detection.[9] investigated photoluminescence studies and reported that doping Cs into ZnO leads to quenching and shifts of optical band gap near band edge for thin film.[10] synthesized ZnO nano particles into PVA: PEDOT: PSS matrix, sandwiched between Al and demonstrated a class of flexible resistive switching memory device with retained I-V performance under bending between 60 to 120angle.[11] reported to have achieved high transmittance with diminished absorbance in infrared and visible region by films prepared with Ag-ZnO.[12] synthesized ZnO nanoparticles into polymers PMMA, PVDF, PVA and PS and achieved low transmittance in UV region and also reported that concentration of ZnO particles is inversely proportional to transmittance.Most of the research studies are on the utilization of Cs doped ZnO nanoparticle thin films for photovoltaic applications and very minimal reports on utilization for piezo-electric applications.In the current work an attempt is made to utilize cesium doped zinc oxide (Cs-ZnO) nanoparticles as conductive fillers into polyurethane matrix synthesized from castor oil and studied to be used as flexible sensors.

Preparation of conductive polymer films
Solution combustion technique is employed to prepare cesium doped zinc metal oxide nanoparticles(Cs-ZnO-NPs). The primary oxidizer was zinc nitrate with cesium nitrate being the dopant, glycine and sucrose fuelled the combustion process.200ml double distilled water was used to dissolve the nitrates along with fuel maintained at 60C temperature with continuous stirring.Once the solution mixture condensed to less than 20ml, temperature was raised with stirring stopped to achieve auto-ignition of the gel mixture with precipitates.The precipitates were transferred to furnace maintained at 800C to achieve complete combustion of left out solution particulate mixture.The prepolymer polyol is prepared from naturally available and purified castor oil synthesized adding toluene isocyanate, ethyl solvent and dibutyl catalyst.The polyol solution was prepared in nitrogen inert atmosphere maintained at 60C water bath temperature.To prepare the conductive films calculated weight percentage ofmetal oxide nanoparticles 0.5 1.0, 2.0 and 4.0 were dispersed into the prepolymer solution and sonicated.The nano polymer solution was transferred to glass mould for normal atmospheric room temperature drying of 12hrs and then for hot air drying of 8hrs at 70C [13,14].Films are further prepared according to standard dimensions for various characterizations and testing.Specimens for electro-mechanical testing are attached with copper electrodes.

Characterizations
Scanning electron microscopy along with energy dispersive x-ray spectroscopy was conducted to find the morphological structure details and chemical compositions in the conductive films.Fourier transform infrared spectroscopy and x-ray diffraction spectrums were conducted to identify the composition and crystallinity.Electro-mechanical properties of the films in longitudinal stretching and surface volume compression are studied using universal testing machine and keithley dc source attachment.High temperature degradation is checked by thermo gravimetric analysis.Chemical compatibility and hydrophilicity tests are conducted by chemical resistivity tests and contact angle measurements.ASTM standards are followed for mechanical testing [14].

FTIR Characterization
Fourier transform infrared spectra for pure and metal oxide filled polyurethane composite films for all the filler weight percentage is presented in Figure 1.The absence of peaks in the band 2260cm -1 -2310cm -1 range confirms the absence of free isocyanate in the polymer structure and successful completion of complete urethane chain formation [15][16][17][18][19]. Presence of disturbance bands at 1072cm -1 shows C-N stretching vibrations, 1550cm -1 -1580cm -1 represents C-N stretching and N-H bending, 1600cm -1 features C-C stretching, 1720cm -1 -1730cm -1 C-O stretching vibrations from urethane groups, 2820cm -1 -3040cm -1 CH2 symmetric and anti-symmetric stretching vibrations and 3380cm -1 free O-H and N-H stretching from urethane group stretching [7,[19][20][21][22][23].By the addition of synthesized nanoparticles,the peaks become narrow at C-O stretching indicate the interaction of metal oxide with urethane links.At higher band range 4000cm -1 to 3200cm -1 with the increase in filler weight percent stretching vibrations have increased [7].
Figure1: FTIR spectra of pure PU and cesium zinc oxide nano filler dispersed composites films.

X-Ray Diffraction Studies
It is revealed from the figure2, the intensity of diffraction peaks increases with increasing weight percentage of nanoparticles, indicatingthe enhancement of the sample film crystallinity.Pure PU films under investigation show peaks at 2 value at 20.3 of the reflection plane with interchain 'd' spacing of 4.369A confirming the presence of short range regular ordered urethane structure with hard and soft domains and also presence of disordered amorphous phase in polyurethane matrix [15,25].Prepared films show further diffraction peaks at 2 values of 31 and 36 indicating presence of ZnO, peaks at 40, 47, 50, 56, 62 and 66 represent presence of cesium zinc oxide in the composite films [6,7,9,[25][26][27].
Figure2.X-Ray Diffraction Spectrum of Pristine PU and cesium zinc oxide filled polymer composite films.

SEM Characterization and EDX
Scanning Electron Microscopy image of pure PU films confirms formation of neat films with minimal agglomeration figure 3(a).Figure3(b) and 3(c), exhibit not just the presence of primary zinc oxide particles but also larger aggregated entities.A closer inspection reveals that the primary zinc oxide particles have an average agglomerate diameter of 180nm.These observations are in good agreement with the findings by many researchers [7,28,29] who have attributed the reason for presence of larger agglomerates due to the presence of high surface energy inherent with these particles.High surface energy leads to the formation of agglomerates, especially in conditions where the growth rate of the polymer blend is relatively slow.The EDX spectra proves the presence of polyurethane elements, notably Oxygen (O), Nitrogen (N), Carbon (C), filler elements Cesium (Cs), and Zinc (Zn).On further study, distinct energy peaks are observed at various keV levels.The energy peak at 0.52 keV can be ascribed to the K-shell of oxygen, emissions at 1.0 keV, 8.7 keV, and 9.6 keV correspond to the Kshell of Zinc and is stated by references [10][11][12].Peaks at 3.8 keV, 4.15 keV, 4.3 keV, 5.3 keV, and 5.55 keV can be directly linked to cesium's atomic structure, as evidenced by findings in references [30,31].

Electro-Mechanical Studies of Nanocomposite Films
Pure films subjected to tensile elongation test following ASTM-D882 standard, show tensile strength of 0.152MPa and films broke down after a maximum elongation of 59%.It was observed that with filler dispersion into films longitudinal stretchability of the films increased exponentially for instance composite films with 2.0wt% of filler concentration showed stretchability upto 180%.To study the change in current conductivity in the films while being stretched the dc source meter readings were recorded and analysed.Source meter was maintained at 50volts potential during all the testing.Conductivity readings were recorded for stretching increments of 0.0mm 1.25mm, 2.5mm, 5.0mm and 10.0mm.As the filler weight concentration is increased films show proportional decrease in resistance values at 0.0mm stretching.With the increase in elongation distance there is increase in resistance due to decrease in current conductivity in the films due to increase of inter-particulate distance within the film.Figure 4(a) and 4(b) reveal linear relationship between tensile stretching and conductivity by the films filled with 1.0wt% fillers and above.Conductive films below 1.0wt% of filler concentration the exhibit asymptotic conductivity behaviour for any value of elongation.From the graph figure 4(a) it can be concluded that with the variation of filler percentage there is no appreciable change in resistance values limiting the film utilization for practical purposes.Prepared films exhibit minimal change in conductivity values above 5mm stretching for all weight percentages.Volume conductivity of the films was determined by loading the specimens under compressive nature.It can be observed from the figure4(c) all the conductive films show minimal change in resistance values upto 5N loading above which linear changes are noted with increasing loading.Figure 4(d) shows that films upto

Thermogravimetry Study
Thermo gravimetric analysis spectrum of pure and nano filler dispersed films is shown in figure5.Pure films undergomajor weight loss between temperature range of 302C and 382C with 10% of weight loss recorded at 302C and 50% at 382C.This is due todisintegration of ester linkage between fatty acids and glycerol backbone of castor oil [12,28].80% of the test sample is lost at temperature of 460C.Cesium zinc oxide filled conductive films show 10 percent weight loss at 292C and 50% weight loss at 410C followed by 80% loss at 470C.Films show less than 1% of degradation upto 160C of temperature.Post test sediment matter of 1.3% was present in the testing crucible at 900C. Figure 5. Thermo gravimetric analysis spectra of pure and nano filler reinforced polyurethane composite films

Chemical compatibility and hydrophilic testing
Prepared film samples were exposed to chemical environment by immersing it in different chemical reagents namely sodium hydroxide, sulphuric acid, hydrogen peroxide, acetic acid, sodium chloride, distilled water and potassium permanganate with 5% concentration for 10 days at atmospheric conditions.Before and after sample weight was recorded to find degradation of the films.Test samples survived test conditions with less than 5%weight loss in all the solutions but completely degraded in potassium permanganate solution.Contact angle testing of the samples reveal hydrophilic nature of all the films with surface energy of 90.0mJm -2 and 84.8mJm -2 and surface contact angle of 74.7 and 84.8 for pure polymer and conductive films respectively.

Conclusion
Cesium doped zinc oxide dispersed polyurethane nanocomposites thin films were prepared using prepolymer method for incremental filler weight percentage.Matrix polyol is successfullyprepared from biodegradable castor oil with toluene isocyanate.Infraredand XRD studies revealed the successful formation of urethane linkages and presence of metal oxides.SEM and EDX analysis reveal the presence of poly crystalline structure and nanofiller dispersion with compactness and agglomeration in the films.Electro-mechanical studies reveal the limited applicability of conductive films for longitudinal stretching applications, as the films do not exhibit major change in resistivity values with respect to increase in filler concentration.Films tested for compressive loading show promising results with proportional change in current conductivity values with the increase in filler concentration and loading values above 5N.Prepared films exhibit good high temperature resistance upto 292C with 10% degradation.Composite films are hydrophilic in nature and have good chemical compatibility with acidic and alkaline environment.

Figure3.
Figure3.SEM images of (a) pure PU film (b) PU films with 2.0wt% filler dispersion (c) PU films with 4.0wt% filler dispersion (d) image showing cesium and zinc oxide particulate compactness and agglomeration in PU film (e) EDX image of nanocomposite film.

Figure4:
Figure4:Graphical representation of (a)filler concentration versus current conductivity plot under tensile loading (b) film elongation versus conductivity plot (c) compressive load versus conductivity plot (d) filler concentration versus conductivity of the films under compressive loading.