University of Illinois Urbana-Champaign

Local electromechanical behavior of elastomeric conductive composites

Moronkeji, Oluwadara Emmanuel

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MORONKEJI-THESIS-2023.pdf

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https://hdl.handle.net/2142/120321

Description

Title
Local electromechanical behavior of elastomeric conductive composites
Author(s)
Moronkeji, Oluwadara Emmanuel
Issue Date
2023-05-04
Director of Research (if dissertation) or Advisor (if thesis)
Chasiotis, Ioannis
Department of Study
Aerospace Engineering
Discipline
Aerospace Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
M.S.
Degree Level
Thesis
Keyword(s)
Microscale testing, elastomers, finite strains, digital image correlation, flexible electronics
Abstract
Although several studies have investigated the bulk mechanical and electrical properties of elastomeric conductive composites, there is still limited understanding of the relative contribution of the local mechanisms for electron transport at the micro and nanometer length scales to the bulk electrical conductivity below and above the electrical percolation threshold, and their evolution with applied stress. This research investigated the microscale and bulk mechanical and electrical properties of polydimethylsiloxane (PDMS)-matrix composites with 12 wt% CB (carbon black) (below the electrical percolation threshold) and 20 wt% CB (above the electrical percolation threshold) that were subjected to uniaxial tension. All specimens utilized in this study were subjected to 12 loading cycles. Due to Mullin’s effect, the initial 1-2 loading-unloading cycles were expended to reach a repeatable mechanical and electrical response in all subsequent loading-unloading cycles. The composite modulus (obtained from the initial loading) increased with filler content, with values 1.91 MPa, 3.19 MPa and 3.49 MPa for neat PDMS, 12 wt% CB/PDMS composite, and 20 wt% CB/PDMS composite, respectively. In the 12th loading, the composite modulus was 2.72 MPa for the 12 wt% CB/PDMS composite and 2.97 MPa for the 20 wt% CB/PDMS composite. Furthermore, microscale full-field strains derived from 50×25 µm2 AFM images during in situ loading were compared to macroscopic values computed from optical images through Digital Image Correlation (DIC). The two measurements derived from specimen dimensions differing by a factor of 100 were in very good agreement in the entire range of applied strain (up to 60% engineering strain), therefore the mechanical behavior of 50×25 µm2 specimen domains was considered representative of the bulk composite. The rapid imaging capabilities of non-contact AFM were also explored by quantifying the uncertainty in the Lagrange strains εxx, εyy and εxy as computed via DIC from AFM images acquired from a 3×3 µm2 specimen area and for AFM cantilever scanning frequencies ranging between 1 Hz and 20 Hz. The mean uncertainty in εxx was <0.4%, which is excellent for finite strain measurements from highly deformable elastomers. It was also shown that measurements obtained for AFM cantilever scanning frequencies ranging between 3-20 Hz contained the least overall uncertainty in terms of mean εxx, εyy and εxy thus providing excellent repeatability of microscale full-field strain measurements. Prior to preconditioning, the longitudinal DC resistance of PDMS composites with 20 wt% CB was found to be 14 times smaller than for 12 wt% CB (67.5 Ω vs. 0.94 kΩ per mm of specimen length), while the through-thickness DC resistance of 20 wt% CB/PDMS composites was 11 times smaller than for 12 wt% CB (78 Ω vs. 0.86 kΩ per mm of specimen thickness). A non-linear increase in DC resistance along the specimen length with applied strain was recorded and attributed to the loss of CB particle contact upon stretching, reaching 87 Ω and 1.34 kΩ per mm of specimen length for 20 wt% and 12 wt% CB, respectively, at 50% applied logarithmic strain. Conversely, the 12 wt% CB and 20 wt% CB specimens showed a linear reduction in through-thickness resistance per unit specimen thickness due to the Poisson’s effect, which in the unstretched state was 0.86 kΩ/mm for 12 wt% CB and 78 Ω/mm for 20 wt% CB, decreasing to 0.64 kΩ/mm and 56 Ω/mm, respectively, at 50% longitudinal logarithmic strain. The microscale electrical conductivity was also quantified via Conductive AFM (C-AFM) through the sum of the total current in C-AFM images and the fraction of conductive image pixels, Ac, registering a current value above the noise floor of 300 pA. A detailed study was carried out to reduce the effect of contact resistance between the AFM tip and the sample surface, yielding 70 nN contact force and 0-250 mV bias as the optimum parameters for C-AFM imaging. Local C-AFM measurements as a function of applied strain showed a bi-linear increase in the total current as a function of strain for the 12 wt% CB composite, attributed to the combination of Ohmic and tunneling conductance. In comparison, the 20 wt% CB/PDMS composite demonstrated a relatively linear increase in the total current owed to predominantly Ohmic conductance. A study to identify a Representative Surface Element (RSE) for strain fields and local current measurements via AFM showed that a specimen domain of 25×25 μm2 could serve as an RSE for both types of measurements. Finally, important insights were obtained from nanoscale I–V characteristics that provided clear evidence for the transition from primarily tunneling conductance in the unstretched state to Ohmic conductance upon the application of 35% logarithmic strain to 12 wt% CB/PDMS composites. In contrast, nanoscale I–V characteristics obtained at the scale of individual CB particles from 20 wt% CB/PDMS composites showed predominantly Ohmic conductance in the unstretched material state, with the vast majority of conducting surface sites becoming Ohmic upon the application of 35% logarithmic strain.
Graduation Semester
2023-05
Type of Resource
Thesis
Handle URL
https://hdl.handle.net/2142/120321
Copyright and License Information
Copyright 2023 Oluwadara Moronkeji

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