Experimental Study for the Sorption and Diffusion of Supercritical Carbon Dioxide into Polyetherimide

Supercritical carbon dioxide (SCCO₂) is a non-toxic and environmentally friendly fluid and has been used in polymerization reactions, processing, foaming, and plasticizing of polymers. Exploring the behavior and data of SCCO₂ sorption and dissolution in polymers provides essential information for polymer applications. This study investigated the sorption and diffusion of SCCO₂ into polyetherimide (PEI). The sorption and desorption processes of SCCO₂ in PEI samples were measured in the temperature range from 40 to 60 °C, the pressure range from 20 to 40 MPa, and the sorption time from 0.25 to 52 h. This study used the ex situ gravimetric method under different operating conditions and applied the Fickian diffusion model to determine the mass diffusivity of SCCO₂ during sorption and desorption processes into and out of PEI. The equilibrium mass gain fraction of SCCO₂ into PEI was reported from 9.0 wt% (at 60 °C and 20 MPa) to 12.8 wt% (at 40 °C and 40 MPa). The sorption amount increased with the increasing SCCO₂ pressure and decreased with the increasing SCCO₂ temperature. This study showed the crossover phenomenon of equilibrium mass gain fraction isotherms with respect to SCCO₂ density. Changes in the sorption mechanism in PEI were observed when the SCCO₂ density was at approximately 840 kg/m³. This study qualitatively performed FTIR analysis during the SCCO₂ desorption process. A CO₂ antisymmetric stretching mode was observed near a wavenumber of 2340 cm^-1. A comparison of loss modulus measurements of pure and SCCO₂-treated PEI specimens showed the shifting of loss maxima. This result showed that the plasticization of PEI was achieved through the sorption process of SCCO₂.