(108b) Melting and Crystallization Temperatures and Foaming of Poly(?-caprolactone) in Carbon Dioxide and Nitrogen

In this presentation we will discuss results of a recent study that has been carried out using High-Pressure Torsional Braid Analysis (HP-TBA) to assess the melting and crystallization temperatures of Poly(ε-caprolactone) (PCL) in carbon dioxide and in nitrogen and their importance in selection of foaming conditions with these physical blowing agents.

HP-TBA is a relatively new technique developed in our laboratory [1]. It involves an inertial mass suspended from a polymer-impregnated glass braid in a high-pressure chamber that is subjected to an initial oscillation. The resulting oscillation is describable by a decaying sine or cosine wave. The changes in the frequency and the period of the oscillations that are monitored allow the evaluations of the changes in the rigidity and mechanical loss as a function of temperature or pressure. As the transition temperatures are approached, mechanical damping goes through a maximum which is accompanied with a significant change in relative rigidity.

Experiments are typically carried out in a temperature scan mode while the polymer is exposed to carbon dioxide or nitrogen at selected pressures. The system is heated at a controlled rate up to a target temperature while recording the oscillations from which the melting transition temperature (T_m) is determined. Then the system is cooled, while again monitoring the oscillations, from which the crystallization transition temperature (T_c) is identified. The pressure is maintained constant during both the heating and cooling stages with the aid of a programmable syringe pump.

Even though PCL melting temperature and its depression in carbon dioxide has been previously reported [1, 2], there is no published information on how the melting point is altered in nitrogen. Similarly, data on the crystallization temperature of PCL in either carbon dioxide or nitrogen is not well documented. The present results show that in contrast to carbon dioxide, exposure to high pressure nitrogen does not lead to a depression of T_m or T_c. Instead, these transition temperatures tend to increase, indicative of a greater influence of hydrostatic as opposed to diluent effects in nitrogen. Hydrostatic effects become observable in carbon dioxide only at high pressures, in this case above 100 bar as reflected by an upward turn in the T_m values.

We will present and compare the results observed in CO₂ and N₂ over a pressure range up to 125 bar. We will also present results of foaming of PCL at selected pressures and temperatures in the interval between T_c and T_m the polymer displays in the presence of CO₂ or N₂, illustrating the importance of knowing the T_m and T_c in the physical blowing agent in selecting the foaming conditions.

References

[1] E. Kiran, J. C. Hassler, High-pressure torsional braid analysis (HP-TBA): A new technique for assessment of thermal transitions and changes in moduli of polymers exposed to supercritical or compressed fluids, J. Supercritical Fluids 143 (2019) 223-231; https://doi.org/10.1016/j.supflu2018.07.015

[2] E. Kiran, J. A. Sarver, J. C. Hasler, Solubility and diffusivity of CO₂ and N₂ in polymers and polymer swelling, glass transition, melting, and crystallization at high pressures: A critical review and perspectives on experimental methods, data, and modeling, J. Supercritical Fluids, 185 (2022)105378, https://doi.org/10.1016/j.supflu.2021.105378