There are many ways of fabricating porous polymer membranes. One way is to form self-arranged structures on the surface and in the volume of evaporated polymer films to achieve the improved properties desirable for a wide range of applications. For this approach, the internal structures of the resulting polymer films are affected by different parameters such as humidity. Generally, the formation of self-arranged structures on the surface, otherwise known as small holes, is believed to be due to the condensation of the humidity from the surrounding air with the small water droplets. However, the underlying physical mechanism has remained a rarely explored topic.
To date, several studies have been conducted to explain the theory of breath figures – the appearance of small holes in patterned polymer films. These theories are mainly based on the physical estimations of the evaporation of water droplets. Research has revealed that the formation of the breath figures is greatly affected by the chemical composition of both polymer films and solvents. In particular, the pore sizes are highly affected by the molecular weight of the solvent. However, the heat transfer problem and the effects of temperature on the formation of self-arranged structures on the film surface have not been fully explored.
Typically, the formation of thin polymer films by solvent evaporation occurs in two major phases, namely, evaporative cooling of the film followed by convective heating. However, the complex interactions between these processes make modeling challenging and difficult. To address this problem, Dr Leonid Dombrovsky from the Joint Institute for High Temperatures of the Russian Academy of Science (Moscow) together with a team of researchers at Ariel University: Dr Mark Frenkel, Dr Irina Legchenkova and Professor Edward Bormashenko investigated the effects of thermal properties of substrates on the formation of self-arranged structures on the evaporated polymer films. Their main objective was to study the various thermal regimes of the evaporating polymer films and propose a physically sound model for a quantitative explanation of the formation of specific patterns on the film surface. Their work is published in the International Journal of Heat and Mass Transfer.
In their approach, the authors developed a semi-empirical computational model for heat transfer. The proposed model was based on the analysis of several laboratory experiments with different substrates such as copper foil and steel plates as well as heat transfer modeling. The data was primarily the measured time dependencies of the film temperature. This approach enabled the authors to obtain the key parameters affecting the thermal process.
The experimental data were in good agreement with the numerical calculations. The results confirmed that the self-arranged structures on the evaporated polymer films are significantly affected by both heat conduction along the substrate and its corresponding heat capacity. Only some holes qualified to be classified as breath figures because others were due to the evaporation of the solvent at the film volume. Moreover, the formation of large-scale surface patterns analogous to Bernard-like cells was evidenced and was attributed to the convective instability of the boundary layer flow induced by the rapid solvent evaporation. Under the surface crust of the polymer film, the volume evaporation of the solvent also led to the formation of orifices grouped at the cells’ boundaries.
In summary, Dr Leonid Dombrovsky and colleagues conducted a series of laboratory experiments to investigate the role of heat conduction and heat capacity along the substrate on the self-structured patterns of the polymer film surface. Both computational and experimental analysis allowed the authors to assume that an instability of the boundary layer is responsible for the formation of large-scale surface patterns. Mainly, the formation of the internal porous structures of the polymer films was associated with the suppression of surface patterns using metal-containing substrates. In a statement to Advances in Engineering, the authors explained their study advance developing high-performance polymer films.
Dombrovsky, L., Frenkel, M., Legchenkova, I., & Bormashenko, E. (2020). Effect of thermal properties of a substrate on formation of self-arranged surface structures on evaporated polymer films. International Journal of Heat and Mass Transfer, 158, 1-9.