Nanocellular Foams with a Superior Thermal Insulation Property

It has been estimated that the world will require 50% more primary energy by 2030. The effectiveness of the current thermal insulation materials which are often produced from highly-expanded polystyrene foam has been already optimized. Nanocellular plastics will be the next generation thermal insulation materials that will significantly reduce energy loss. However, there are a few substantial and decisive requirements in the fabrication through foaming with an inert and green physical blowing agent. Among those requirements, having a nanostructured medium and increasing the gas solubility are the most important ones by which the cell nucleation can be enhanced dramatically. For the first time, It was demonstrated that a low-density nanoporous medium with a cell density in the order of 0.96E16 and the average cell size of ~70nm, out of isotactic polypropylene could be successfully prepared. This was accomplished by the introduction of a complex nano-fibrillar network into the matrix to alter the crystallization kinetics and to manipulate the nanostructure of polypropylene. Importantly, this recyclable medium had a remarkably 8-fold lower thermal conductivity compared to its solid counterpart owing to the diminution of the heat conduction through the solid fraction as well as the trapped air inside the nanopores with the Knudsen effect. In the same context, polycarbonate is a special polymer that is not only strong but also it shows very unique characteristics that make it a promising candidate for the nanocellular foam fabrication. Among amorphous polymer, PC is the one that can be crystallized with the assistance of CO2, and the nanocrystals can be harvested as the cell nucleating agent to increase the cell density. Moreover, in this study, it is shown that the solubility of CO2 in PC increases dramatically at a very low temperature (e.g., up to -40 oC), where PC undergoes a transition from a solid to a rubbery state due to swelling and plasticization, so-called the retrograde vitrification behavior. This provides a unique processing window adjacent to the lower glass transition temperature where the gas solubility and the amount of stress variation inside the polymer are considerably high. So far, only few amorphous polymers have shown this behavior. In this study, a theoretical model based on Henry’s law, Arrhenius equation, and Doolittle equation was developed to predict how the glass transition temperature depresses in the PC/CO2 system at a different pressure. The experimentally measured solubility data were used to determine the coefficient of the model and to quantify the retrograde vitrification behavior of the PC/CO2 system. To validate this model, the lower glass transition temperatures of PC/CO2 , at a few different pressures, were measured by using a high-pressure DSC. The results were in agreement with the theoretically predicted data. Furthermore, I demonstrated the rubbery state below the lower glass transitions by inducing foaming and crystallization in the theoretically predicted rubbery region. Also, systematic foaming experiments at low temperature were conducted, and the crystallization kinetics of PC with the assistance of CO2 was studied and characterized.

Keywords: Crystallization
Free Volume
Nanocellular Foam
Polycarbonate
Retrograde Vitrification
Thermal Conductivity
Advisor: Park, Chul B.