NMR Microscopy: A Tool for Measuring Monomer Diffusion in Supercritical CO₂

Abstract

Summary: The diffusion of styrene into linear low density polyethylene in a solution of supercritical CO2 was investigated using NMR microimaging. For both pure styrene and styrene dissolved in supercritical CO2, the diffusion was found to follow Fickian kinetics. Supercritical CO2 was found to enhance the rate and extent of diffusion of styrene into the substrate by up to three times under the conditions of this investigation, compared to pure styrene. NMR imaging was used to measure the concentration profiles of the styrene penetrants in real time, and the results were fitted to a Fickian model for diffusion. At a CO2 pressure of 150 bar and temperature of 40 °C, the diffusion coefficient of a 30 wt‐% solution of styrene into LLDPE was calculated to be 1 × 10−11 m2 · s−1. This is significantly faster than the diffusion coefficient measured for pure styrene diffusion at 40 °C (3 × 10−12 m2 · s−1). The diffusion coefficients determined by gravimetric analysis were slightly higher than those determined by the imaging method. This was probably due to residual styrene and/or polystyrene adhering to the surface of the films in the gravimetric technique.NMR image of styrene in linear low density polyethylene following supercritical fluid enhanced impregnation.imageNMR image of styrene in linear low density polyethylene following supercritical fluid enhanced impregnation.

Diffusion NMR of Fluids Confined to Mesopores under High Pressures

“…It has to be noted that design of high-pressure and high-temperature NMR sample cells have already been reported in the literature [7,8]. While in many cases (e.g., for supercritical water) high temperatures are needed in addition to high pressures, for a few industrially important materials, such as carbon dioxide or ethane, the supercritical state is attained at moderate temperatures.…”

Supercritical CO2 Directional‐Assisted Synthesis of Low‐Dimensional Materials for Functional Applications

“…The density inhomogeneity of supercritical fluid can be characterized by the solvation color shift obtained by various spectral means (such as UV-vis, fluorescence, infrared, and nuclear magnetic resonance, etc.). [51][52][53][54][55] Solvation color rendering means that the characteristic spectrum of probe molecules shifts with the change of solvent environment. However, most probe molecules contain chromophores composed of polar or unsaturated functional groups, and their characteristic spectra will directly reflect their interactions with surrounding solvent molecules.…”

Solute Diffusion into Polymer Swollen by Supercritical CO2 by High-Pressure Electron Paramagnetic Resonance Spectroscopy and Chromatography

“…Under normal conditions, the diffusion rate of a solute in a medium such as solvent-swollen polymers or porous materials soaked with a solvent is measured using different methods, such as the diffusion (diaphragm) cell and infinite cylinder/semi-infinite slab methods (with either uptake measurement or sectioning/imaging to obtain the concentration profile) [25], electrochemical methods [26], NMR methods (Pulsed Field Gradient NMR, NMR imaging, etc.) [27,28], EPR methods (concentration broadening measurement, EPR imaging) [29,30], IR and Raman spectroscopy/microscopy [31][32][33][34], FRAP technique [25], forced Rayleigh scattering [35,36], and so on. All these methods have advantages and disadvantages in terms of required time and effort, sample opacity, need for special molecular probes, cost, and availability of the equipment.…”

“…Raman spectroscopy has been used to study the diffusion of ethanol in aerogels during supercritical drying [34], and is also able to characterize the state of the solute resolved in space. High-pressure NMR imaging and forced Rayleigh scattering were also used to measure tracer diffusivities in CO 2 plasticized polymers [28,46].…”