Encapsulation of Perfluoroalkyl Carboxylic Acids (PFCAs) Within Polymer Microspheres for Storage in Supercritical Carbon Dioxide: A Strategy Using Dispersion Polymerization of PFCA-Loaded Monomers

Abstract

The removal of per- and polyfluoroalkyl substances (PFAS) from global aquatic environments is an emerging issue. However, little attention has been paid to addressing accumulated PFAS through their removal. This study demonstrates the encapsulation of perfluoroalkyl carboxylic acids (PFCAs) within polymer microspheres that dissolve in supercritical carbon dioxide (scCO₂). PFCAs were effectively captured by a hindered amine-supported monomer, 2,2,6,6-tetramethyl-4-piperidyl methacrylate (TPMA), in methanol (MeOH) through a simple acid-base reaction. The PFCA-loaded TPMA underwent dispersion polymerization in MeOH in the presence of poly(N-vinylpyrrolidone) (PVP) as a surfactant, producing microspheres with high monomer conversions. The microsphere size depended on the molecular weight and concentration of PVP, as well as the perfluoroalkyl chain length of the PFCAs. X-ray photoelectron spectroscopy (XPS) revealed that the perfluoroalkyl chains migrated from the interior to the surface of the microspheres when exposed to air. These surface perfluoroalkyl chains facilitated dissolution of the microspheres in scCO₂, with cloud points observed under relatively mild conditions. These findings suggest the potential for managing PFCA-encapsulated microspheres in the scCO₂ phase deep underground via CO₂ sequestration.

Keywords: PFCA-loaded monomer; acid-base reaction; cloud point; dispersion polymerization; electrostatic crosslinking; encapsulation; microspheres; perfluoroalkyl carboxylic acids (PFCAs); perfluoroalkyl chains; supercritical carbon dioxide.

Figure 1

Schematic procedure for the synthesis of microspheres via dispersion polymerization of TPMA bearing PFCA.

Figure 2

NMR analyses of the PFNA–TPMA salt: (a) ¹H NMR spectra of the PFNA/TPMA equimolar mixture (top), TPMA (middle), and PFNA (bottom); (b) ¹³C NMR spectra of the same samples; (c) ¹⁹F NMR spectra of the mixture (top) and PFNA (bottom), using 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as an internal standard (−76.40 ppm) [46]. Solvent: CD₃OD.

Figure 3

GPC profiles of microspheres containing PFCA.

Figure 4

FE-SEM images of PFCA-encapsulated microspheres obtained by dispersion polymerization with PVP (M_w, wt%) for designated polymerization times: (a) PFNA, without PVP; (b) PFNA, PVP (40,000, 5 wt%), 18 h; (c) PFNA, PVP (360,000, 5 wt%), 18 h; (d) PFNA, PVP (40,000, 10 wt%), 18 h; (e) PFNA, PVP (40,000, 60.5 wt%), 6 h; (f) PFNA, PVP (360,000, 10 wt%), 18 h; (g) PFNA, PVP (40,000, 5 wt%), 6 h; (h) PFNA, PVP (360,000, 5 wt%), 6 h; (i) PFHA, PVP (40,000, 5 wt%), 24 h; (j) PFPA, PVP (40,000, 5 wt%), 18 h; (k) PFPA/PFAZ, PVP (40,000, 5 wt%), 18 h; (m) PFAZ, PVP (40,000, 5 wt%), 18 h.

Figure 5

Effect of PVP molecular weight on microsphere stabilization: (a) M_w = 40,000; (b) M_w = 360,000.

Figure 6

DSC spectra of microspheres, TPMA polymer, and PFNA.

Figure 7

TG curves of the microspheres and TPMA polymer.

Figure 8

NMR analyses of PFNA–TPMA microspheres placed in aqueous NaOH at a molar ratio of NaOH/PFNA = 2.0 for 3 h (top), 2 h (middle), and 0 h (bottom): (a) ¹⁹F NMR spectra with HFIP as an internal standard; (b) ¹H NMR spectra of the same samples. Solvent: CD₃OD.

Figure 9

Schematic illustration of microspheres: (a) in MeOH; (b) in air.

Figure 10

XPS spectra of PFNA-containing microspheres: (a) surface; (b) interior at a depth of 100 nm.

Figure 11

Plots of CO₂ density and pressure at the cloud point of microspheres versus the number of carbons in the PFCA perfluoroalkyl chain. [Microspheres]₀ = 5.0 g/L.