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Numerical Analysis and Experimental Study of the Piezoresistivity of the Flexible Polymer Nanocomposites

Zarei Darani, Sajjad | 2025

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 58540 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Naghdabadi, Reza; Sohrabpour, Saeed
  7. Abstract:
  8. This research investigates the piezoresistivity of MWCNT/thermoplastic polyurethane (MWCNT-TPU) nanocomposite through both experimental and numerical modeling approaches. Piezoresistive materials are those whose electrical conductivity changes with deformation. Key challenges in the experimental phase included the potential for non-uniform nanoparticle dispersion and agglomeration, as well as the method of measuring electrical resistance under strain, given the inherently high electrical resistance of polymeric materials. In the modeling phase, challenges comprised the high computational cost due to the microstructure's geometry and properties, and the necessity of accounting for the distribution of nanoparticles and their contact effects. In the experimental section, the MWCNT-TPU nanocomposite was fabricated. Following the preparation of standard specimens and testing, the mechanical, electrical, and piezoresistive properties of the nanocomposite were examined at various MWCNT content. The addition of a small amount of CNT to the polymer not only enhanced the mechanical properties by 50% (increasing the Young's modulus from 17.5 to 29 MPa and the yield stress from 4 to 6.7 MPa) but also drastically increased its electrical conductivity. Scanning electron microscope (SEM) images of the nanocomposite with a 2 wt% filler revealed that the degradation in mechanical and electrical properties was due to poor dispersion and agglomeration of nanoparticles within the polymer matrix. In the modeling section, the electrical conductivity and piezoresistivity of the nanocomposite were studied using a three-dimensional numerical Monte Carlo method. A representative volume element (RVE) of the nanocomposite was generated using the Monte Carlo method with a random distribution pattern of CNTs in the matrix, incorporating the properties of both the nanotubes and the polymer. By establishing conductive pathways within the RVE, which included both intrinsic nanotube resistances and tunneling resistances, the equivalent electrical conductivity and piezoresistivity of the nanocomposite were determined. Using the properties of the CNTs and matrix from the fabricated samples as model input parameters, the modeling results showed good agreement with the experimental data. The influence of key CNT parameters such as length, diameter, electrical conductivity, and weight percentage as well as matrix parameters like the potential barrier height on the electro-mechanical behavior of the nanocomposite, were examined. It was determined that the CNT's aspect ratio has the most significant impact on the electrical properties of the nanocomposite; increasing the length and diameter of the nanotubes leads to an increase and decrease in electrical conductivity, respectively. It was also found that increasing the weight percentage of CNTs increases electrical conductivity similar to the experimental results, with its effect diminishing at higher percentages. To account for agglomeration effects, an RVE with agglomerates (Agg-RVE) was modeled. By considering agglomerates of different sizes and percentages within the RVE, the effects of the number and percentage of agglomerates on electrical conductivity and piezoresistivity were investigated. Based on the analysis of SEM images from a sample with agglomerates, the initial size and percentage of agglomerates were predicted. By comparing the modeling results with experimental data, a model for the distribution of agglomerates in a nanocomposite sample with 2 wt% CNTs was developed. The results indicated that an increase in the agglomeration percentage and a decrease in agglomerate diameter both reduce the electrical conductivity and increase the strain sensitivity of the nanocomposite. The electrical conductivity of the nanocomposite has an inverse relationship with its strain sensitivity (piezoresistivity), meaning that factors which reduce electrical conductivity lead to an increase in the material's strain sensitivity
  9. Keywords:
  10. Monte Carlo Method ; Agglomeration ; Multi-Walled Carbon Nanotube ; Flexible Nanocomposite ; Piezoresistivity ; Polymer Nanocomposits

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