Effect of Particle Size, Shape, and Composition on Physical Properties of Derived Polymer Nanocomposites (Experimental and Numerical Studies)

Abstract

Electrically conductive polymer composites (ECPCs) are transformative materials, combining the flexibility, lightweight, and processability of polymers with the electrical conductivity of conductive fillers. This thesis investigates polyurethane-based composites, emphasizing the effects of filler type, size, shape, and preparation methods on their electrical and mechanical properties. Four composite systems were studied: micro-copper polyurethane (micro-Cu-PU), micro-silver polyurethane (micro-Ag-PU), green-synthesized silver nanoparticles polyurethane (AgNPs-PU), and copper nanowires polyurethane (CuNWs-PU). These composites were prepared using advanced solution mixing and spin-coating techniques, enabling precise control of filler dispersion and composite structure.
Micro-Cu-PU composites were developed by incorporating copper particles (<25 μm) at concentrations ranging from 0.5 to 20 vol.%. Conductivity measurements revealed a pressure-induced percolation transition (PIPT), where applied pressure reduced the polymer film thickness and facilitated conductive network formation. Maximum conductivity reached 7.2 × 10⁻¹ S·m⁻¹ at pressures of 20 kPa for filler concentrations above 2.6 vol.%. Numerical simulations using a finite element method (FEM) and a semi-analytical model accurately predicted percolation thresholds between 7.1 vol.% and 1.4 vol.%, reducing experimental efforts while guiding composite design.
For micro-Ag-PU composites, silver particles (<3.5 μm) were incorporated at concentrations up to 14 vol.%, with conductivity measured under compression (0.5–20 kPa). Spin-coated films demonstrated lower percolation thresholds and cast films higher conductivity, achieving a maximum conductivity of 2.45 S·m⁻¹. Ultrasonic mixing improved filler dispersion, reducing agglomeration and enhancing composite performance. Numerical predictions correlated well with experimental results. Green-synthesized AgNPs-PU composites utilized biotechnological approaches to synthesize AgNPs from Urtica dioica extracts. This method ensured environmentally friendly and cost-effective production, reducing the reliance on hazardous chemicals. Incorporating these nanoparticles into PU matrices yielded a maximum conductivity of 1.4 S·m⁻¹ at 43.3 wt.% under 20 kPa pressure. Washing the nanoparticles prior to integration improved dispersion and electrical performance by reducing residual stabilizers, as confirmed through FEM simulations.
CuNWs-PU composites were fabricated using copper nanowires synthesized via a solution-based method at 198°C. These nanowires, with lengths of ~15 μm and diameters of 70 nm, were integrated into PU matrices at concentrations up to 18.5 wt.%. Electrical characterization revealed a significant conductivity increase—up to 0.61 S·m⁻¹—with applied pressure. The percolation threshold shifted with pressure, dropping from 15 wt.% at 1 kPa to 3 wt.% at 20 kPa. The reversible electrical behaviour demonstrated potential for pressure-sensitive applications like flexible sensors and piezo-resistive devices.
Additionally, the mechanical properties of the composites were thoroughly evaluated. Tensile strength, elongation at break, and dynamic mechanical analysis (DMA) demonstrated a clear correlation between filler content and mechanical performance. Micro-Cu-PU composites exhibited enhanced tensile modulus at higher filler concentrations, while AgNPs-PU and CuNWs-PU composites demonstrated superior flexibility, attributed to uniform filler dispersion. The study also explored the relationship between electrical and mechanical properties, highlighting challenges such as filler-induced brittleness and potential strategies for achieving balanced performance.
This research highlights the critical influence of filler morphology, synthesis methods, and processing techniques on the electrical and mechanical performance of PU-based ECPCs. By employing experimental, and simulation approaches, the study advances the understanding of percolation phenomena and the design of high-performance composites. These materials hold immense promise for applications in wearable electronics, energy storage, electromagnetic interference shielding, and environmental sensing.

Date of Award	30 Apr 2025
Original language	English
Supervisor	Yolanda Sanchez Vicente (Supervisor), Khalid Lafdi (Supervisor) & Sergio Gonzalez Sanchez (Supervisor)