The interfacial tension of molten polylactide in supercritical carbon dioxide

• The surface tension and density difference of CO2/PLA mixture was investigated.
• The surface tension is inversely proportional to temperature.
• Molecular configuration of the molecular chains did not affect the surface tension.
• The SS-EOS and the SL-EOS were used to find theoretical density difference of CO2–PLA.
• The SS-EOS was found to be closer to the experimental results.

The interfacial tension and density of molten polylactide/carbon dioxide (PLA/CO2) mixtures were determined by analyzing sessile drops in a high-pressure and high-temperature visualization chamber. The temperature and pressure ranged from (453 to 473) K and (6.89 to 20.68) MPa, respectively. The outcomes showed that the density difference between the PLA/CO2 mixture and the CO2 decreased proportionally with increased CO2 pressure. Overall, the PLA’s interfacial tension decreased with an increase in CO2 pressure. The effect of temperature on interfacial tension depended on the CO2 pressure level; that is, at a lower pressure, the interfacial tension decreased with the temperature while at a higher pressure it increased with the temperature. These opposing trends were attributed to two competing CO2 mechanisms: hydraulic pressure and polymer swelling. The effects of the molecular chain configuration (that is, the varying D-content) were also investigated. It was found that the PLA’s interfacial tension remained almost unchanged with a change in the D-content.