Molecular Dynamics and Nuclear Magnetic Resonance Studies of Supercritical CO2 Sorption in Poly(Methyl Methacrylate)

To measure the NMR spectra of ¹³C at supercritical parameters of the CO₂ state, a system to create and maintain high pressure in real time was used (see Figure 1), based on a unique scientific installation “Fluid Spectrum” (https://ckp-rf.ru/usu/503933/ accessed on 6 December 2022) of G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences. The installation connects a high-pressure cylinder (Figure 1 pos. 1) containing carbon dioxide gas, with high-pressure NMR cell due to a high-pressure capillary. Filling and emptying high-pressure NMR cell is possible because of the system of taper seal valves (Figure 1 pos. 3, 6). Pressure monitoring is implemented using a pressure gauge (Figure 1 pos. 2) and electronic pressure transmitters (Gems™ Sensors&Controls, Basingstoke, UK) (Figure 1 pos. 5), a pressure adjustment in high-pressure NMR cell is carried out by means of a manual press (Figure 1 pos. 4). The accuracy of maintaining the pressure was ± 0.05 MPa.

High-pressure NMR cell (Figure 2b pos. 4) [36,37,38] is a single crystal sapphire tube of high-pressure production (Daedalus Innovations, Aston, Pennsylvania, USA). The small size of the ampoule, which has an inner diameter of 3 mm, an outer diameter of 5 mm and a total length of 87 mm, allows it to work at pressures up to 30 MPa. The sapphire ampoule is equipped with an upgraded [39] cell fixation system (Figure 2b pos. 5), capillary mounts (Figure 2b pos. 6, 7) and sealing elements (Figure 2a pos. 1, 2, 3) to supply CO₂ continuously. The temperature range of the cell is limited by the temperature range of the NMR spectrometer sensor.

NMR spectra were obtained using a Bruker Avance III 500 (Bruker Co., Karlsruhe, Germany) spectrometer equipped with a 5 mm Bruker TBI probe. The temperature was controlled by a VT-2000 prefix (Bruker Co., Karlsruhe, Baden-Württemberg, DE) with a Bruker BCU cooling unit (Bruker Co., Karlsruhe, Germany), the air consumption was 535 L/hour. Temperature calibration was carried out using a standard K-type thermocouple (Bruker Co., Karlsruhe, Germany). ¹H NMR spectra of methanol were used as a method of temperature verification and calibration [40].

Polymethylmethacrylate (PMMA) powder was used as a sorbent (CAS No: 9011-14-7) of the Aldrich^TM (Sigma–Aldrich, Moscow, Russia). The impregnation was carried out in a carbon dioxide environment (CAS No: 124-38-9) of the primary standard, GOST 8050-85 (CO₂—99.995%, H₂O— < 0.001%) produced by Linde Group Russia (Balashikha, the Linde Group, RF). All substances were used without additional purification.

¹³C NMR spectra of scCO₂ were obtained using a pulse program included in the software package of the TopSpin 3.6.1 for NMR spectrometer. The spectra were measured at a temperature of 50 °C and a pressure of 250 bar in the spectral range of 284 ppm to reduce the statistical error, 1024 scans were performed to obtain each spectrum, followed by averaging. A total number of 93 NMR ¹³C spectra were obtained with a fixed time interval between them equal to 90 min.

To select the parameters of the experiment, we turned to the experimental data on the kinetics and sorption of PMMA in CO₂, given in the literature [41]. As shown in [41] CO₂ is better sorbed by the polymer matrix with increasing pressure. Thus, to increase the sensitivity of the ¹³C NMR experiment, a pressure over 20 MPa should be chosen. At the same time, the dependence of the sorption logarithms on pressure is demonstrated by the presence of two sorption modes [41].

The first sorption mode is observed at relatively low pressure up to 10 MPa and is characterized by low values of sorption intensity and capacity. The opposite mode is observed for pressure above 10 MPa and is characterized by relatively high values of capacitance and intensity, which makes it preferable for ¹³C NMR studies. However, when selecting the temperature values, some ambiguities arose. On the one hand, the sorption intensity in the high-temperature regime increases with increasing temperature, on the other hand, the sorption capacity decreases by an order of magnitude, which, of course, can significantly reduce the sensitivity in the ¹³C NMR experiment. With this in mind, when setting up the ¹³C NMR experiment, a temperature of 50 °C was used. It is due to a reduction in the duration of the NMR kinetics experiment from two weeks to one and the possibility of observing the process in a reasonable time. It is worth noting that the obtained parameters of in situ measurement of sorption and swelling in the volume of polymer powder can be used to optimize the production processes of polymer impregnation in supercritical liquids.