A Modified Magnetic Suspension Balance for Assessment of Sorption and Swelling of Polymers in CO2

Magnetic suspension balance is a powerful technique to assess the sorption of gases by polymers under pressure. However, the determinations require knowing the swelling of the polymer to incorporate buoyancy corrections. In the absence of direct measurements of swelling, these corrections are made based on predictions of swelling using an equation of state model.

We have modified our Rubotherm magnetic suspension balance, by incorporating a custom designed bottom cell with a pair of large (1 inch-diameter) sapphire windows. With the use of a dedicated camera, swelling of the polymer is monitored as a function of temperature or pressure by following the location of the interface of the polymer loaded in a glass vial.

Glass vials, with dimensions of 1.9 cm diameter and 2.3 cm height, are used for effective viewing across the sapphire windows. The polymer to be investigated is loaded (about a 2 gram sample) into the glass vial and melted in a vacuum oven. A Teflon plunger is employed to keep the polymer surface leveled in the process. The sample vial is then placed in a custom-made, lightweight wire basket with a hook to mount in the instrument. This sample preparation method allows for a clear sample with an easy to observe interface through the sapphire windows.

After mounting the sample basket, the system is loaded with carbon dioxide with the aid of an ISCO syringe pump and pressurized to a target pressure (typically 100 or 200 bar). Temperature is then increased by a set interval (typically 2.5°C or 5°C) every 3 hours, which provides enough time for the system to reach thermal equilibrium. Just before the temperature is increased, an image of the polymer interface in the sample vial is taken using the dedicated camera and saved for analysis of swelling.

Mass readings are taken every 2 minutes throughout the duration of the experiment. Using the swelling data and the fluid density, buoyancy corrections are performed to derive the corrected mass of the sample, from which the sorption levels are then assessed at each set of pressure and temperature conditions.

In this poster, we will present the results of swelling and sorption of a set of rubbery polymers, poly(ethylene-co-vinyl acetate-co-carbon monoxide) (EVACO), with different degrees of ethylene content exposed to carbon dioxide. The specific copolymers were EVACO 0010; EVACO 1010 and EVACO 2010 with ethylene/vinyl acetate/carbon monoxide compositions of 90/0/10; 80/10/10 and 70/20/10.