Relationship between Polystyrene Molecular Weight and the Concentration of ZnO Nanoparticles on Inhibition of Dewetting Behavior

Abstract

Background and Objectives : Polymer thin films are materials with a thickness of less than 100 nanometers and play a crucial role in electronic devices, including solar cells, capacitors, and sensors. Currently, the technology industry is trending toward the development of smaller electronic devices, enhancing portability and reducing energy consumption. This trend has led to insulation polymer film becoming thinner as well. As the polymer film becomes thinner, its adhesion to the substrate decreases. Exposed to environmental factors like contact, humidity, or heat can cause these polymer films to separate, develop holes, and lose adhesion to the substrate. This reduces the performance of the polymer thin film and shortens the device’s lifespan. The addition of nanoparticles is one technique used to improve the adhesion of thin films. The positioning of nanometer-scale particles within the polymer thin film depends on the surface energy of the substrate, the surface energy of the nanoparticles, and their concentration, all of which influence the film ability to resist degradation. This research aims to study the dewetting behavior of polystyrene thin films under heat stimulation and to identify the optimal concentration of ZnO nanoparticles to inhibit the dewetting behavior of polystyrene thin films with varying molecular weights.

Methodology : Solutions of polystyrene with molecular weights of (PS13K, PS30K, and PS50K) and a suspension of ZnO nanoparticles (~ 50 nm) were prepared in toluene at a concentration of 0.5 wt.%. The ZnO nanoparticle suspension was added to the polystyrene solution at concentrations of 0, 0.3, 0.5, 0.7, and 1.0 wt.%. Polystyrene thin films were fabricated on silicon substrates using the spin-coated process. The contact angles between the liquid droplets and the surface of the sample films were measured using a contact angle measurement, and the surface energies of the thin films were calculated. The thin films were then heated to stimulate dewetting behavior. The surface morphologies of the thin films, both before and after heating, were examined using an optical microscope. Finally, the percentage of the area affected by dewetting of the thin films was calculated.

Main Results : The polystyrene (PS) thin film had an average surface energy of 45.083 mJ/m², whereas the PS-ZnO thin film had an average surface energy of 45.263 mJ/m². This result supports the assumption that the adding of ZnO nanoparticles does not significantly alter the physical properties of the polystyrene thin film and that the inhibition of dewetting behavior is not due to changes in surface energy. However, surface energy is used to predict the movement of ZnO nanoparticles within the polystyrene thin film. Heat stimulation caused all the polystyrene thin films to exhibit dewetting behavior, leading to the formatting of holes on their surfaces. With continued heating, these holes expanded and eventually merged. Ultimately, the thin films degraded into polystyrene droplets. An optimal concentrations of ZnO nanoparticles improves stability and suppresses thin film degradation. The ZnO nanoparticles concentrations of 0.3, 0.7, and 0.7 wt.% were found to be the most effective in preventing the dewetting of PS13K, PS30K, and PS50K thin films, respectively.

Conclusions : PS and PS-ZnO thin films were fabricated on silicon substrates using spin-coating. As the solvent rapidly evaporated, the thin films solidified, generating internal stress within the films. When continuously exposed to heat, the polymer chains in the thin films release this internal stress by moving and clustering together. This process generated voids, initiating nucleation holes that expanded over time, ultimately resulting in the loss of adhesion between the polystyrene thin films and the substrate. Both PS and PS-ZnO thin films exhibited dewetting behavior under continuous heat exposure. However, the addition of a small amount of ZnO nanoparticles enhanced the adhesion stability of the polystyrene thin films on the silicon substrate and effectively inhibited dewetting behavior. Notably, there was no significant variation in the surface energy of the thin films, indicating that the addition of ZnO nanoparticles did not alter the physical properties of the polystyrene thin film. Surface energy reflects the movement tendencies of nanoparticles within the polystyrene thin film. The ZnO nanoparticles were anticipated to migrate to the interface between the silicon substrate and the polymer, compressing the polymer chains, restricting their movement, and reducing hole expansion. The molecular weight of the polymer significantly influences the temperature required for heat stimulation, chain mobility, and the characteristics of hole formation on the surface. Higher molecular weight thin films exhibit greater heat resistant compared to lower molecular weight films, as their polymer chains move more slowly and become more entangled. In low molecular weight thin films, hole formation is asymmetrical, with wave-like features at the hole edges due to the higher mobility of shorter polymer chains, which can move through the gaps between nanoparticles. In contrast, hole formation in high molecular weight thin films is symmetrical, as the longer polymer chains are more restricted in mobility, leading the film to form new holes instead of expanding existing ones.