Enhancement of Poly(vinyl ester) Solubility in Supercritical CO2 by Partial Fluorination: The Key Role of Polymer–Polymer Interactions

Molecular Dynamics Simulation on Thickening and Solubility Properties of Novel Thickener in Supercritical Carbon Dioxide

Supercritical CO2 has wide application in enhancing oil recovery, but the low viscosity of liquid CO2 can lead to issues such as poor proppant-carrying ability and high filtration loss. Therefore, the addition of thickening agents to CO2 is vital. Hydrocarbon polymers, as a class of green and sustainable materials, hold tremendous potential for acting as thickeners in supercritical CO2 systems, and PVAc is one of the best-performing hydrocarbon thickeners. To further improve the viscosity enhancement and solubility of PVAc, here we designed a novel polymer structure, PVAO, by introducing CO2-affine functional groups to PVAc. Molecular dynamics simulations were adopted to analyze viscosity and relevant solubility parameters systematically. We found that PVAO exhibits superior performance, with a viscosity enhancement of 1.5 times that of PVAc in supercritical CO2. While in the meantime, PVAO maintains better solubility characteristics than PVAc. Our findings offer insights for the future design of other high-performance polymers.

Branching in molecular structure enhancement of solubility in CO2

Most compounds of some 1,000 amu molecular weight (MW) and higher are poorly soluble in carbon dioxide (CO2). Only at very high pressure, there may be mild solubility. This limits the use of CO2 as a solvent and modifications of CO2 properties through additives. We have developed a coarse-grained molecular model to investigate the dependency of the solubility of hydrocarbon oligomers (MW of ∼1,000 amu) in CO2 and on the molecular structure. The coarse-grained model is optimized by the particle swarm optimization algorithm to reproduce density, surface tension, and enthalpy of vaporization of a highly branched hydrocarbon oligomer (poly-1-decene with six repeating units). We demonstrate that branching in molecular structure of oligomers significantly increases solubility in CO2. The branching in molecular structure results in up to 270-time enhancement of solubility in CO2 than an n-alkane with the same MW. The number of structural edges (methyl group) is a key in improved CO2-philicity. The solubility of poly-1-decene with nine repeating units (MW of 1,264.4 amu) is higher in CO2 than poly-1-dodecene with six repeating units (MW of 1,011.93 amu) because it has more structural edges (10 vs. 7). These results shed light on the enhancement of CO2-philicity by altering molecular structure rather than modifying chemical composition in compounds.

Organocatalyzed Controlled Radical Copolymerization toward Hybrid Functional Fluoropolymers Driven by Light

Photo-controlled polymerizations are attractive to tailor macromolecules of complex compositions with spatiotemporal regulation. In this work, with a convenient synthesis for trifluorovinyl boronic ester (TFVB), we report a light-driven organocatalyzed copolymerization of vinyl monomers and TFVB for the first time, which enabled the controlled synthesis of a variety of hybrid fluorine/boron polymers with low dispersities and good chain-end fidelity. The good behaviors of "ON/OFF" switch, chain-extension polymerizations and post-modifications further highlight the versatility and reliability of this copolymerization. Furthermore, we demonstrate that the combination of fluorine and boron could furnish copolymer electrolytes of high lithium-ion transference number (up to 0.83), bringing new opportunities of engineering high-performance materials for energy storage purposes.

Effect of Branching on Mutual Solubility of Alkane-CO2 Systems by Molecular Simulations

Mutual solubilities of hydrocarbon-CO₂ systems are important in a broad range of applications. Experimental data and theoretical understanding of phase behavior of large hydrocarbon molecules and CO₂ are limited. This is especially true in relation to the molecular structure of hydrocarbons when the carbon number exceeds 12. In this work, the continuous fractional component Gibbs ensemble Monte Carlo simulations are used to investigate mutual solubility of different alkane and CO₂ systems and the molecular structure. We investigate the mutual solubility of n-decane, n-hexadecane, n-eicosane, and the corresponding structural isomers in the CO₂-rich and hydrocarbon-rich phase. The focus will be solubility of the heavy normal alkanes and their structural isomers in CO₂. The simulation results are verified by comparing the experimental data when measurements are available. The simulation of phase behavior of the n-decane-CO₂ system agrees with the experiments. We also present simulation results of n-hexadecane-CO₂ and n-eicosane-CO₂ systems away from the critical region partly due to the finite size effect. We establish that solubility of the hydrocarbons in CO₂ is improved by change of the molecular structure in heavier alkanes. The enhanced solubility is limited in decane isomers, but the isomers of hexadecane and eicosane show 2- to 3-time solubility enhancement. The molecular dynamics simulations suggest that the improvement is from a higher coordination number of CO₂ for methyl (CH₃) rather than for methylene (CH₂) groups. This study sets the stage for molecular engineering and synthesis of hydrocarbons that are soluble in CO₂ not only by considering functionality but also by changing the molecular structure. The solubility enhancement is the first step in viscosification of CO₂ which broadens the use of CO₂.

Dispersibility of Poly(vinyl acetate) Modified Silica Nanoparticles in Carbon Dioxide with Several Cosolvents

The dispersibility and stabilization of silica nanoparticles with surface-capped poly(vinyl acetate) (PVAc) chains are examined in carbon dioxide with four different cosolvents. Three surface coverages of silica-PVAc were formed by using different weight ratios of the silica and PVAc. The dispersibilities of three silica-PVAc nanoparticles in CO₂ with the four cosolvents were tested in a rotatable high-pressure variable-volume view cell. The effects of surface coverage, cosolvent type, pressure, and particle concentration on dispersion were investigated. Results show that, in the experimental pressure range (5.5 to 20 MPa), the pressure has no significant effect on the dispersion of nanoparticles, and the cosolvent is the key factor in dispersing silica-PVAc particles in CO₂. 1-Butanol is an adequate cosolvent to disperse silica-PVAc in CO₂ with any coverage of PVAc on the surface of the particles when the concentration of particles is smaller than 0.31 wt %. Ethanol can only improve the dispersibility of particles with a high surface coverage of PVAc when the concentration of particles is smaller than 0.14 wt %. 1-Hexanol and ethyl acetate cannot disperse the particles in CO₂ with any coverage of PVAc. Molecular dynamics simulations were carried out to study the nanoparticle-CO₂-cosolvent dispersions. Results suggest that 1-butanol has a good solubility in the CO₂ condensed phase and can effectively absorb onto the nanoparticle surface, which help to prevent the formation of nanoparticle aggregation. The precipitation of nanoparticles in the nanoparticle/1-hexanol/CO₂ and nanoparticle/ethyl acetate/CO₂ systems is attributed to the relatively low solubility of CO₂ in 1-hexanol and ethyl acetate. The precipitation of nanoparticles in the nanoparticle/ethanol/CO₂ system is the result of less hindrance of ethanol molecules to the aggregation of nanoparticles.

Investigation on modified polyether as an efficient CO2 thickener

Based on the CO₂-philic silicon group and the CO₂-phobic aryl group, silicon modified polyether exhibits both high solubility in carbon dioxide and a high thickening ability toward CO₂.

Insight into the Intermolecular Interaction and Free Radical Polymerizability of Methacrylates in Supercritical Carbon Dioxide

High pressure in situ Fourier transfer infrared/near infrared technology (HP FTIR/NIR) along with theoretical calculation of density functional theory (DFT) method was employed. The solvation behaviors and the free radical homopolymerization of methyl methacrylate (MMA), methacrylate acid (MAA), trifluoromethyl methacrylate (MTFMA) and trifluoromethyl methacrylate acid (TFMAA) in scCO2 were systematically investigated. Interestingly, the previously proposed mechanism of intermolecular-interaction dynamically-induced solvation effect (IDISE) of monomer in scCO2 is expected to be well verified/corroborated in view that the predicted solubility order of the monomers in scCO2 via DFT calculation is ideally consistent with that observed via HP FTIR/NIR. It is shown that MMA and MAA can be easily polymerized, while the free radical polymerizability of MTFMA is considerably poor and TFMAA cannot be polymerized via the free radical initiators. The α trifluoromethyl group (-CF3) may effectively enhance the intermolecular hydrogen bonding and restrain the diffusion of the monomer in scCO2. More importantly, the strong electron-withdrawing inductive effect of -CF3 to C=C may distinctly decrease the atomic charge of the carbon atom in the methylene (=CH2). These two factors are believed to be predominantly responsible for the significant decline of the free radical polymerizability of MTFMA and the other alkyl 2-trifluoromethacrylates in scCO2.

Thickening Supercritical CO₂ with π-Stacked Co-Polymers: Molecular Insights into the Role of Intermolecular Interaction

Vinyl Benzoate/Heptadecafluorodecyl acrylate (VBe/HFDA) co-polymers were synthesized and characterized as thickening agents for supercritical carbon dioxide (SC-CO₂). The solubility and thickening capability of the co-polymer samples in SC-CO₂ were evaluated by measuring cloud point pressure and relative viscosity. The molecular dynamics (MD) simulation for all atoms was employed to simulate the microscopic molecular behavior and the intermolecular interaction of co-polymer⁻CO₂ systems. We found that the introduction of VBe group decreased the polymer⁻CO₂ interaction and increased the polymer⁻polymer interaction, leading to a reduction in solubility of the co-polymers in SC-CO₂. However, the co-polymer could generate more effective inter-chain interaction and generate more viscosity enhancement compared to the Poly(Heptadecafluorodecyl) (PHFDA) homopolymer due to the driving force provided by π-π stacking of the VBe groups. The optimum molar ratio value for VBe in co-polymers for the viscosity enhancement of SC-CO₂ was found to be 0.33 in this work. The P(HFDA_0.67-co-VBe_0.33) was able to enhance the viscosity of SC-CO₂ by 438 times at 5 wt. %. Less VBe content would result in a lack of intermolecular interaction, although excessive VBe content would generate more intramolecular π-π stacking and less intermolecular π-π stacking. Both conditions reduce the thickening capability of the P(HFDA-co-VBe) co-polymer. This work presented the relationship between structure and performance of the co-polymers in SC-CO₂ by combining experiment and molecular simulations.

Modeling Solvation in Supercritical CO2

In recent decades, a microscopic understanding of solute-solvent intermolecular interactions has been key to advances in technologies based on supercritical carbon dioxide. In many cases, computational work has provided the impetus for new discoveries, shedding new light on important concepts such as the local structure around the solute in the supercritical medium, the influence of the peculiar properties of the latter on the molecular behavior of dissolved substances and, importantly, CO₂ -philicity. In this Review, the theoretical work that has been relevant to these developments is surveyed and, by presenting some crucial open questions, the possible routes to achieving further progress based on the interplay between theory and experiments is discussed.

Incorporating a silicon unit into a polyether backbone—an effective approach to enhance polyether solubility in CO2

A series of poly(silyl ether)s were prepared by condensation polymerization and hydrosilation polymerization through incorporating a silicon unit into a polyether backbone.

Microcosmic understanding on thickening capability of copolymers in supercritical carbon dioxide: the key role of π–π stacking

Thickening capability evaluations and microscopic understanding of St–HFDA copolymers in SC-CO₂.

Structure-Property Relationships in CO2-philic (Co)polymers: Phase Behavior, Self-Assembly, and Stabilization of Water/CO2 Emulsions

This Review provides comprehensive guidelines for the design of CO2-philic copolymers through an exhaustive and precise coverage of factors governing the solubility of different classes of polymers. Starting from computational calculations describing the interactions of CO2 with various functionalities, we describe the phase behavior in sc-CO2 of the main families of polymers reported in literature. The self-assembly of amphiphilic copolymers of controlled architecture in supercritical carbon dioxide and their use as stabilizers for water/carbon dioxide emulsions then are covered. The relationships between the structure of such materials and their behavior in solutions and at interfaces are systematically underlined throughout these sections.

Solvation of Esters and Ketones in Supercritical CO2

Vibrational Raman spectra for the C═O stretching modes of three esters with different functional groups (methyl, a single phenyl, and two phenyl groups) were measured in supercritical carbon dioxide (scCO2). The results were compared with Raman spectra for three ketones involving the same functional groups, measured at the same thermodynamic states in scCO2. The peak frequencies of the Raman spectra of these six solute molecules were analyzed by decomposition into the attractive and repulsive energy components, based on the perturbed hard-sphere theory. For all solute molecules, the attractive energy is greater than the repulsive energy. In particular, a significant difference in the attractive energies of the ester-CO2 and ketone-CO2 systems was observed when the methyl group is attached to the ester or ketone. This difference was significantly reduced in the solute systems with a single phenyl group and was completely absent in those with two phenyl groups. The optimized structures among the solutes and CO2 molecules based on quantum chemical calculations indicate that greater attractive energy is obtained for a system where the oxygen atom of the ester is solvated by CO2 molecules.

Exploration of CO2-Philicity of Poly(vinyl acetate-co-alkyl vinyl ether) through Molecular Modeling and Dissolution Behavior Measurement

Hydrocarbon CO2-philes are of great interest for use in expanding CO2 applications as a green solvent. In this work, multiscale molecular modeling and dissolution behavior measurement were both applied to explore CO2-philicity of the poly(vinyl acetate) (PVAc)-based copolymer. Introduction of a favorable comonomer, i.e., vinyl ethyl ether (VEE), could significantly reduce the polymer-polymer interaction on the premise that the polymer-CO2 interaction was not weakened but enhanced. The ab initio calculated interaction of the model molecules with CO2 demonstrated that the ether group in VEE or VBE was the suitable CO2-philic segment. From the molecular dynamics (MD) simulations of polymer/CO2 systems, the interaction energy and Flory-Huggins parameter (χ12) of poly(VAc-alt-VEE)/CO2 supported that poly(VAc-alt-VEE) possessed better CO2-philicity than PVAc. The dissolution behaviors of the synthesized poly(VAc-co-alkyl vinyl ether) copolymers in CO2 showed the best CO2-phile had the VEE content of about 34 mol %. The MD simulations also indicated that the interaction of random poly(VAc-co-VEE) containing about 30 mol % VEE with CO2 was the strongest and the χ12 was the smallest in these polymer/CO2 systems. Not only could the VEE monomer reduce the polymer-polymer interaction, but it could also enhance the polymer-CO2 interaction with an optimized composition. Introducing a suitable comonomer with a certain composition might be a promising strategy to form the synergistic effect of polymer-polymer interaction and polymer-CO2 interaction for screening the hydrocarbon CO2-philes.

Evaluation of CO2-philicity of poly(vinyl acetate) and poly(vinyl acetate-alt-maleate) copolymers through molecular modeling and dissolution behavior measurement

Multiscale molecular modeling and dissolution behavior measurement were both used to evaluate the factors conclusive on the CO2-philicity of poly(vinyl acetate) (PVAc) homopolymer and poly(vinyl acetate-alt-maleate) copolymers. The ab initio calculated interaction energies of the candidate CO2-philic molecule models with CO2, including vinyl acetate dimer (VAc), dimethyl maleate (DMM), diethyl maleate (DEM), and dibutyl maleate (DBM), showed that VAc was the most CO2-philc segment. However, the cohesive energy density, solubility parameter, Flory-Huggins parameter, and radial distribution functions calculated by using the molecular dynamics simulations for the four polymer and polymer-CO2 systems indicated that poly(VAc-alt-DBM) had the most CO2-philicity. The corresponding polymers were synthesized by using free radical polymerization. The measurement of cloud point pressures of the four polymers in CO2 also demonstrated that poly(VAc-alt-DBM) had the most CO2-philicity. Although copolymerization of maleate, such as DEM or DBM, with PVAc reduced the polymer-CO2 interactions, the weakened polymer-polymer interaction increased the CO2-philicity of the copolymers. The polymer-polymer interaction had a significant influence on the CO2-philicity of the polymer. Reduction of the polymer-polymer interaction might be a promising strategy to prepare the high CO2-philic polymers on the premise that the strong polymer-CO2 interaction could be maintained.

Enhanced Solubility of Polyvinyl Esters in scCO2 by Means of Vinyl Trifluorobutyrate Monomer

The novel monomer, vinyl trifluorobutyrate (VTFBu), when polymerized in a controlled fashion by RAFT/MADIX polymerization with a xanthate transfer agent, yields poly(vinyl ester)s with improved solubility in supercritical carbon dioxide. The thermodynamic parameters controlling the solubility of VTFBu/vinyl acetate statistical copolymers are discussed based on ab initio calculations, glass transition temperatures of the copolymers, and surface tension measurements. The enhanced solubility of this new class of CO₂-philic polymer combined with its good chemical stability render it attractive for the preparation of next-generation macromolecular surfactants for the formation of water-scCO₂ emulsions.

Controlled copolymerization of n-butyl acrylate with semifluorinated acrylates by RAFT polymerization

A new class of well-controlled semifluorinated copolymers were successfully synthesized via RAFT polymerization. We found that ESI-TOF MS was particularly useful for readily analysing our semifluorinated copolymers. The reactivity ratios of each pair of co-monomers are close to one.

Interactions of CO2with various functional molecules

We report the CO₂-interactions with diverse functional molecules. Useful functional molecules such as melamine showing very large adsorption enthalpy for CO₂are reported.