Exploration of CO2-Philicity of Poly(vinyl acetate ... - ACS Publications

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Exploration of CO2‑Philicity of Poly(vinyl acetate-co-alkyl vinyl ether) through Molecular Modeling and Dissolution Behavior Measurement Dongdong Hu, Shaojun Sun, Pei-Qing Yuan, Ling Zhao, and Tao Liu* State Key Laboratory of Chemical Engineering, Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China S Supporting Information *

ABSTRACT: 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 CO2philicity 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 CO2philic 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 CO2philicity 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.

1. INTRODUCTION Supercritical carbon dioxide (scCO2) is of interest as a potential alternative to traditional organic solvents in many industrial processes due to its nontoxic, nonflammable, inexpensive, and environmentally benign properties, as well as its moderate critical conditions (T = 31.1 °C, P = 7.38 MPa).1−3 At the supercritical state, many properties such as density and polarizability can be easily tuned by adjusting its pressure and temperature. However, a major drawback of scCO2 is its unfortunately feeble solvent strength to dissolve polar materials and high molecular weight polymers4 because of its low polarizability, cohesive energy density (CED), and dielectric constant5 relative to those conventional petrochemical solvents. The low solvating power of scCO2 restricts its potential applications as a green solvent. In recent years, significant efforts have been devoted to the design of CO2-philic polymers that could modify the solvent character of CO2 and expand the use of dense CO2 in many processes,3−22 such as the thickeners to control the CO2 mobility in enhanced oil recovery (EOR),23−25 surfactants to stabilize water-in-CO2 emulsions/microemulsions,26−31 and chelating agents of metal processing in scCO2.32,33 Fluorinated5−7,34,35 and silicone-based polymers8 show relatively high © 2015 American Chemical Society

solubility in scCO2. Unfortunately, the polymers containing fluorine and silicone are relatively expensive, making them unsuitable as CO2-philic materials for application on an industrial scale. In addition, the fluorinated polymers are difficult to degrade, which may cause environmental issues and negate the superiority associated with scCO2.3 Therefore, the design of nonfluorous CO2-philes attracted wide attention and interest. Some hydrocarbon polymers only composed of carbon (C), hydrogen (H), and oxygen (O) atoms had been identified as relatively highly CO2-soluble materials,16,17,19,21,22 such as poly(vinyl acetate) (PVAc), 16 poly(vinyl ethyl ether) (PVEE),17 poly(propylene oxide), and poly(vinyl methoxymethyl ether).21 The favorable polymer−CO2 interaction is greatly helpful for enhancing the compatibility between polymer and CO2, which may provide a reference for screening the CO2-philic polymers. The polymer−CO2 interaction could be enhanced as a benefit from the Lewis acid−Lewis base (LA−LB) interaction by introducing some favorable functional groups into the polymer. The weak attractive interaction existed indeed between CO2 Received: August 28, 2015 Published: September 2, 2015 12490

DOI: 10.1021/acs.jpcb.5b08393 J. Phys. Chem. B 2015, 119, 12490−12501

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The Journal of Physical Chemistry B and carbonyl-containing compounds from FT-IR spectroscopy results by Kazarian et al.36 Furthermore, the O atom of the carbonyl group could increase the solubility of polymers in CO2 due to the weak LA−LB interactions, confirmed by Beckman et al.14 Molecular modeling is another feasible approach to characterize the interactions between CO2 and polymers with specific functional groups. Nelson et al.37 used ab initio calculations to investigate the interaction of CO2 with the carbonyl group in small molecules. Raveendran et al.13 confirmed the existence of H-bonds between the H atoms bonded to C atoms in the CO2-philic molecule and the O atoms of CO2. The H-bonds were weaker than typical H-bonds, but still enhanced the binding with CO2. Wang et al.21 systematically studied the interactions between the CO2-philic candidate small molecules and CO2 to explore nonfluorous CO2-soluble polymers. The favorable moieties and functional groups were found by calculating the binding energy using the ab initio methods. Modeling by Kilic et al.17 suggested that the binding energy between ether O atom and CO2 was the same order of magnitude as that between carbonyl O atom and CO2 so that the ether group might also play an active role in polymer−CO2 interaction. Until now, PVAc is still recognized as one of the most CO2-philic hydrocarbon materials,16,17 which is attributed to the ester group−CO2 interactions that were an LA−LB interaction associated with a weak H-bond interaction.14,15 However, the solubility of PVAc in scCO2 is still limited by its molecular weight and is significantly lower than that of the fluorinated polymers. In a polymer/CO2 system, the solvent−solvent, solute− solvent, and solute−solute interactions should all be considered to enhance the solubility of polymer in scCO2. Therefore, a weak solute−solute interaction is necessary in addition to a strong solute−solvent interaction. O’Neill et al.4 found a moderate polymer−polymer interaction was conducive to the dissolution of polymers in scCO2, indicating that the polymers with low CED were favorable to weaken the solute−solute interaction and might have high CO2-philicity. It was consistent with the fact that CO2-philic polymers containing fluorine and silicone had a lower CED and a relatively higher solubility in scCO2 than PVAc. A series of attempts were made in weakening the polymer−polymer interaction by introducing other repeat units into the homopolymers to increase their compatibility with CO2. Howdle et al. introduced dibutyl maleate,22 vinyl butyrate,38 and vinyl pivalate (VPi)39,40 monomer into the PVAc. The cloud point pressures (Pc’s) of the copolymers in scCO2 are significantly reduced in comparison with those of the PVAc homopolymer. Destarac et al. introduced vinyl trifluoroacetate (VTFAc)5 and 1(trifluoromethyl) vinyl acetate (CF3VAc)6 to synthesize partial fluorinated PVAc-based copolymers poly(VAc-stat-VTFAc) and poly(VAc-stat-CF3VAc) through reversible addition−fragmentation chain transfer (RAFT) polymerization. The compatibility between the copolymers and CO2 was enhanced in comparison with PVAc, suggesting that the reduction of polymer−polymer interactions dominated to increase the solubility in scCO2. In particular, the lowest Pc was observed in the copolymers with an optimal VAc/CF3VAc molar composition of 67:33. They surmised that the polymer−CO2 interactions were preserved at low VTFAc or CF3VAc contents, and the polymer−polymer interactions were the main driving force until the surface tension reached a minimum value. Then, the mixing entropy might play the secondary driving force when continuing to increase VTFAc or CF3VAc content in the copolymer.5,6

Actually, the surface tensions could be correlated with the CED of polymers.4 In addition, the CED can be evaluated by MD simulation, providing a means to predict the strength of polymer−polymer interactions related to the compatibility of the polymer/CO2 system. Although molecular modeling had been applied to calculate the binding energy between CO2 and small molecular units of polymers, research was rarely reported to evaluate the polymer chain−CO2 interactions in the polymer/CO2 system. In our previous work,10 the polymer chain−CO2 interactions, CED, and Flory−Huggins parameter of poly(VAc-alt-maleate) series copolymer/CO2 systems were calculated through MD simulations and used to evaluate the CO2-philicity of polymers. In comparison with the experimental results, it was confirmed that the polymer−polymer interaction had a decisive influence on the CO2-philicity of poly(VAc-altmaleate) copolymers. Unfortunately, the polymer−CO2 interaction was weakened since the maleate comonomer was not CO2-philic as VAc although the polymer−polymer interaction was more favorable in the copolymer than that in the PVAc homopolymer. In addition, the effect of copolymer composition on the CO2-philicity was not considered by using molecular simulations until now. This work aims at exploring favorable comonomers containing only C, H, and O atoms introduced into the PVAc backbone as well as their compositions for the reduction of polymer−polymer interaction on the premise that the strong polymer−CO2 interaction is maintained by using multiscale molecular modeling and dissolution behavior measurement, which will help to screen suitable CO2-philic polymers. Vinyl ethyl ether (VEE) and vinyl butyl ether (VBE) were selected as the additional comonomers. Three model structures (VAc, VEE, and VBE) and five dimeric model structures were constructed to calculate the binding energies with CO2 by using the ab initio methods. Five polymer chains, i.e., PVAc, PVEE, PVBE, poly(VAc-alt-VEE) (PVAEE), and poly(VAc-alt-VBE) (PVABE), were also built to evaluate their interactions with CO2 and the CED of systems by using MD calculations. PVAc and a library of poly(VAc-co-VEE) and poly(VAc-co-VBE) copolymers with varying monomer compositions were synthesized through RAFT polymerization, and their Pc values in scCO2 were measured. To gain a deeper understanding of the relationship between the polymer composition and its CO2philicity, some random poly(VAc-co-VEE)/CO2 systems were calculated to verify the optimum VEE content. Then, the respective contributions of polymer−CO2 interaction and polymer self-interaction for CO2-philicity were evaluated.

2. SIMULATION PROTOCOLS 2.1. Ab Initio Methods. As a high-level quantum mechanical approach, ab initio method is an alternative to calculate the interaction energies between CO2 and model molecules of interest which can be implemented in Gaussian 09 software.41 In this way, the affinities of CO2-philic candidates with CO2 could be evaluated, and the CO2-philic structure can be found to design CO2-philic hydrocarbon polymer. Three model molecules, i.e., isopropyl acetate, ethyl isopropyl ether, and 1-isopropoxy-butane, were chosen to represent VAc, VEE, and VBE, as shown in Figure 1a−c, respectively. In a typical calculation, an initial guess of configuration was set by using a CO2 molecule close to the O atom of the model molecule. The MP2 method with 6-31+g(d) basis set was used to optimize the configuration of the bimolecular system. The final stable configuration related to the CO2 molecule at various initial 12491


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2.2. Molecular Dynamics Simulations. Although ab initio calculations can give accurate interaction energy between a molecular segment and CO2, two obvious shortages exist in this approach: integral polymer models cannot be established, and temperature properties are incalculable. MD calculations make up the disadvantages of ab initio to some extent. MD simulations were used to calculate the intermolecular interactions in polymer/CO2 systems, which were implemented in Material Studio 6.1 developed by Accelrys. In the calculations, the COMPASS force field was chosen to express the molecular potential energy function,44 which was capable of both hydrocarbon polymer44,45 and CO2.46 The parameters of a force field in this work could be found in Sun and Rigby et al.’s work,44−49 and were also gathered in our previous work.10 When the polymer chain simulation is involved, many thermodynamic parameters have a strong dependence on the number of repeating units. The previous work50,51 has shown that the thermodynamic properties, such as solubility parameter and cohesive energy density, become insensitive to the molecular weight when the number of repeating unit reaches 30−40. Considering the thermodynamic properties approaching the actual value and the acceptable computational complexity, the PVAc chain was built with 50 repeat units of VAc (Mn = 4300 g/mol). The other model polymers including PVEE, PVBE, PVAEE, and PVABE with similar molecular weight were also constructed. Simulation cases containing different monomer compositions of poly(vinyl acetate-co-alkyl vinyl ether) and CO2 are listed in Table 1. Four kinds of systems were calculated by using MD simulation, namely, pure polymer systems with one or six polymer chains, and polymer/ CO2 systems with 1000 CO2 molecules and one or six polymer chains. Thereafter, some random copolymers systems with different VEE contents were built and calculated by using MD simulation to verify the optimum VEE content for CO2philicity. The simulation cases with different VEE contents are listed in Table 2. The abbreviations of PVAEE15, PVAEE30,

Figure 1. Three model molecules for the calculation of interaction energies with CO2. Panels a−c represent the models of VAc, VEE, and VBE, respectively.

positions. Thereafter, a larger basis set, aug-cc-pVDZ at MP2 level, was applied to compute the interaction energy between CO2 and the model molecule in the stable configuration. Simultaneously, basis set superposition error (BSSE) should be considered to obtain more accurate interaction energy that could be corrected by the counterpoise correction (CP).42,43 The detailed simulation procedures had been described in Wang et al.21 and our previous work.10 In addition, five dimeric form molecules were also constructed for studying the interaction between multiple functional groups and CO2, including VAc dimer (VAc2), VEE dimer (VEE2), VBE dimer (VBE2), VAc-VEE (VAEE), and VAc-VBE (VABE), as illustrated in Figure 2a−e, respectively. The systems containing a dimeric molecule and a CO2 molecule were optimized, and the interaction energies were calculated by using the same method above.

Figure 2. Five dimeric forms of model molecules for the calculation of interaction energies with CO2. Panels a−e represent the models of VAc2, VEE2, VAEE, VBE2, and VABE, respectively.

Table 1. Systems of Poly(VAc-alt-alkyl vinyl ether) with Different Monomer Compositions and CO2 Considered in MD Simulations system

composition

Mn of chain

no. of chain

no. of VAc unit

no. of another unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

CO2 PVAc PVEE PVBE PVAEE PVABE PVAc/CO2 PVEE/CO2 PVBE/CO2 PVAEE/CO2 PVABE/CO2 PVAc PVEE PVBE PVAEE PVABE PVAc/CO2 PVEE/CO2 PVBE/CO2 PVAEE/CO2 PVABE/CO2

4302 4320 4302 4254 4280 4302 4286 4302 4254 4312 4302 4320 4302 4254 4280 4302 4286 4302 4254 4312

1 1 1 1 1 1 1 1 1 1 6 6 6 6 6 6 6 6 6 6

50

0 60 43 27 23 0 60 43 27 23 0 60 43 27 23 0 60 43 27 23

no. of CO2 1000

27 23 50

27 23 50

27 23 50

27 23 12492

1000 1000 1000 1000 1000

1000 1000 1000 1000 1000 DOI: 10.1021/acs.jpcb.5b08393 J. Phys. Chem. B 2015, 119, 12490−12501

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reaction, the mixture was cooled to room temperature. The product was precipitated from the reaction mixture by adding cold n-hexane, further purified by dissolving in ethyl acetate and precipitating in n-hexane again, and then dried to constant weight under vacuum at 30 °C. The RAFT end group was retained in the polymer products. 3.3. Molecular Weight and Structure Characterization. The molecular weight and molecular weight distribution of the polymerization products were characterized by gel permeation chromatography (GPC, PL-GPC50, Agilent) using THF as a mobile phase at 1 mL/min with the monodispersed polystyrene (20 000 g/mol) as calibration standards. The samples were also analyzed by 1H NMR spectra which were recorded with a 500 Hz nuclear magnetic resonance instrument (NMR, 500 Bruker, Avance) in deuterochloroform (CDCl3), and the number of scans was 16. The composition and the content of VAc in the samples were determined by analyzing the intensity and position of peaks in the 1H NMR spectra. All spectra were referenced to the solvent residual peak (CDCl3 at 7.26 ppm). 3.4. Cloud Point Pressure Measurements in CO2. The solubility of polymers in CO2 was determined by measuring the cloud point pressure with a high-pressure view cell system, which consisted of a variable-volume view cell (from 25.0 to 50.0 mL) equipped with two sapphire windows in the rightangle position of each other in the same horizontal plane. The details and the control method of temperature and pressure were shown in the Supporting Information. The apparatus was pressurized to 40 MPa with CO2 using a booster pump after introducing the polymer sample at a certain temperature. Then, the polymer−CO2 mixture would form a single and transparent phase at the high pressure. After the system was stable, the cell volume was enlarged slowly to decrease the system pressure. Suddenly, the solution became cloudy viewed from the glass sapphire window, and phase separation appeared to form a twophase system at a certain CO2 pressure, which was the cloud point pressure, denoted Pc. The phase transition was measured and recorded by a CCD camera transmitted to a computer. The Pc of sample in CO2 was measured at least three times to ensure the experimental reliability, and the experimental error was less than ±1%.

Table 2. Systems of Random Poly(VAc-co-VEE) with Different Monomer Compositions Considered in MD Simulations run

sample

Mn of chain

no. of VAc unit

no. of VEE unit

VEE content

1 2 3 4

PVAEE15 PVAEE30 PVAEE45 PVAEE60

4362 4422 4310 4342

44 38 30 22

8 16 24 34

15% 30% 45% 60%

PVAEE45, and PVAEE60 (the subscript refers to the mole content of VEE), represent poly(VAc44-co-VEE8), poly(VAc38co-VEE16), poly(VAc30-co-VEE24), and poly(VAc22-co-VEE34), respectively. In a typical simulation procedure, a cubic box with periodic boundary condition was constructed in Amorphous Cell module52 after a model polymer chain was optimized in a Smart Minimizer. Then, the configuration with the lowest energy was selected for annealing 5 circles from 300 to 500 K. A 100 ps NVT MD simulation (35 °C) and a subsequent 300 ps NPT simulation (35 °C, 20 MPa) were carried out on the basis of the configuration after annealing. The final 10 configurations were analyzed to obtain the thermodynamic parameters. More details of MD simulation procedures had been described in our previous work.10

3. EXPERIMENTAL SECTION 3.1. Materials. VAc (Aldrich, 99.0%) was washed by saturated sodium bisulfite solution (NaHSO3), sodium carbonate solution (Na2CO3), and deionized water; dried with anhydrous sodium sulfate (Na2SO4) for 12 h; and obtained ultimately through decompression distillation. VEE (Aladdin Chemical Reagent, 98%), VBE (Aladdin Chemical Reagent, 98%), and argon (Ar, Air Product Co., 99.99%) were used as received. Isopropylxanthic disulfide (DIP, Adamas Reagent Ltd., 98%) and α,α′-aazobis(isobutyronitrile) (AIBN, J&K Chemical Ltd., 99%) were recrystallized twice from methanol and dried under vacuum for 24 h. Ethyl acetate (EAc, Sinopharm Chemical Reagent Co., >99.5%) was dried by using molecular sieve. The CO2 with a purity of 99.99 wt % was purchased from Shanghai Chenggong Gases Co., China. All the other chemicals were used as received. 3.2. RAFT Copolymerization of Poly(VAc-co-alkyl vinyl ether). The copolymers, poly(VAc-co-VEE) and poly(VAc-coVBE), were synthesized by using RAFT polymerization in solvent ethyl acetate, as illustrated in Figure 3. In a typical

4. RESULTS AND DISCUSSION 4.1. Models and Simulation. 4.1.1. CO2−Polymer Segment Interaction. VEE and VBE were selected as the potential monomers to calculate the interaction energy with CO2 for the comparison with VAc since the binding energy between the ether O atom and CO2 might be the same order of magnitude with that between the carbonyl O atom and CO2.17 The CO2−polymer segment interaction was preassessed by using three model molecules to represent the backbones of VAc, VEE, and VBE, respectively. Optimized configurations of three systems (VAc-CO2, VEE-CO2, and VBE-CO2 complexes) were simulated at the MP2 level with the 6-31+g(d) basis set. The most stable binding mode between CO2 and the model molecule in the three complex systems is illustrated in Figure 4a−c, respectively. The other stable configurations in these systems can be found in Figure S1 of the Supporting Information. It was worth noting that the CO2 position was close to the carbonyl O rather than the ether O in the favorable binding mode of the VAc-CO2 complex, indicating that the binding mode of CO2-carbonyl O was more stable than that of CO2−ether O. The results of interaction energies in the aug-ccpVDZ basis set at the MP2 level, listed in Table 3, clearly

Figure 3. Synthesis procedure of random poly(VAc-co-VEE).

experiment, a mixture of VAc (4.30 g, 50 mmol), VEE (3.60 g, 50 mmol), AIBN (0.082 g, 0.5 mmol), and DIP (0.081 g, 0.3 mmol) was dissolved in 20 mL of ethyl acetate in a 100 mL begoon-shape flask with a valve. The reaction flask was degassed by three freeze−pump−thaw cycles, and repressurized with Ar to atmospheric pressure ultimately. The mixture was sealed and heated to 80 °C in an oil bath with stirring. After 12493


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Figure 4. Most stable modes in three monomer−CO2 complex systems.

Table 3. Interaction Energy between the Model Monomer Molecule and CO2 interaction energy Einter (kJ/mol) complex

A

B

C

D

E

av

IPA (VAc) EIE (VEE) BIE (VBE)

−11.53 −12.64 −12.88

−11.86 −13.53 −15.26

−12.38 −15.09

−12.03

−12.63

−12.08 −13.75 −14.07

Figure 5. Most stable modes in five dimer−CO2 complex systems.

showed that the binding energies of CO2 with these model molecules were in the same order of magnitude. The isolated ether group in VEE or VBE had larger interaction energies with CO2 than the ester group in VAc. According to the interaction points (the dashed line as shown in Figure 4), the binding modes of the VEE-CO2 complex and VBE-CO2 complex had five interaction points, which was more than that in the binding mode of the VAc-CO2 complex. The interatomic distance of the interaction points was roughly less than 3 Å.21 Due to the cumulative effect of interaction points, the interaction was stronger in the VEE-CO2 and VBE-CO2 complexes. In a comparison of the interaction energies of CO2 with VEE and VBE, the stronger interaction energies of VBE with CO2 than that of VEE were attributed to the shorter distance between the O atom of the ether group and the C atom of CO2 and the greater NBO charges of O atom in VBE (2.812 Å, −0.710 e) than those in VEE (2.846 Å, −0.705 e). The result indicated that VEE and VBE had a favorable ether bond for the interactions with CO2 and might be suitable monomer candidates to enhance the solubility of polymer in CO2. Furthermore, optimized configurations of five dimer-like systems (VAc2-CO2, VEE2-CO2, VBE2-CO2, VAEE-CO2, and VABE-CO2 complexes) were also obtained at the same basis set level. The most favorable configurations in the five dimer−CO2 complex systems are shown in Figure 5a−e, respectively. The other stable configurations in these systems can be found in Figure S2 of the Supporting Information. Simultaneously, the interaction energies in the aug-cc-pVDZ basis set at the MP2 level are gathered in Table 4. The results showed that the interaction energy of VAc2-CO2 was slightly larger than those of the other complexes, indicating that VAc2-CO2 was a more

Table 4. Interaction Energy between the Model Dimer Molecule and CO2 interaction energy Einter (kJ/mol) complex

A

VAc2 VEE2 VBE2 VAEE VABE

−21.34 −14.96 −18.74 −15.85 −16.99

B −15.35 −22.71 −17.47 −18.61

C −17.66 −18.31

D −18.79

av −21.34 −16.43 −20.73 −17.21 −17.80

stable complex in favor of the CO2−polymer interaction. To some extent, CO2 might act as a Lewis base (O atom) while acting as a Lewis acid (C atom). The binding between C atom in CO2 and O atom in model molecules could form the Lewis acid−Lewis base interaction. The binding of O atom in CO2 and H atom in model molecules could also appear as a weak Hbond in the complexes.21 With a combination of these two interactions, the structure of a quasi-six- or five-membered ring could form to make the system configuration relative stable. The C atom of CO2 was closed to only one ether O atom in VEE2 and VBE2, resulting in the number of quasi-sixmembered rings being relatively small. In comparison to the VAc2-CO2 complex, CO2 was located at a suitable position (the C atom of CO2 was placed between a carbonyl O atom and an ether O atom) to allow for multiple roles of Lewis acid and base,15 and creation of more six-membered rings to enhance the binding energy with the target molecule. The distance between the key O atom of the model molecule and the C atom of CO2 was also consistent with the conclusion that the VAc2-CO2 system provided stronger binding energies than the 12494


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the two simulations being modeled in different scales. In the ab initio method, the binding energy was only considered by using an average value without taking into account the cumulative effect. The binding of VAEE with CO2 had three stable configurations, and that of VABE with CO2 only had two stable modes. In the VAEE-CO2 complex, the C atom of CO2 in three configurations was placed between a carbonyl O and an ether O (Figure 5d), only close to an ether O (Figure 6a), and only close to a carbonyl O (Figure 6b), respectively. In the VABECO2 complex, the C atom of CO2 in two configurations was placed only a carbonyl O or an ether O, as shown in Figure 5e and Figure 6c. However, only one stable binding mode was obtained in the VAc2-CO2 complex (Figure 5a). In MD simulations, more binding modes caused more probability that the CO2 molecules could place in different positions near the interaction points, resulting in stronger interaction between CO2 and polymer chain. After selection of the appropriate functional groups on the basis of ab initio calculations, MD simulations could make a more reliable prediction of the interaction energy between polymer chains with CO2 while considering the interaction point numbers, the complex chain structure, and the self-interactions between CO2 molecules. From the calculation of CO2−polymer chain interaction, PVAEE might be more CO2-philic than the PVAc homopolymer for enhancing the compatibility with CO2. Note that the result presented here did not consider the contribution of polymer−polymer interactions so that the solubility of these polymers in CO2 could not be directly compared and predicted from the above-mentioned simulations. 4.1.3. Polymer−Polymer Interaction. The interaction between polymer chains has a significant impact on the thermodynamic properties of polymers such as cohesive energy density, solubility parameter, and mixing entropy. Therefore, the polymer−polymer interaction should be considered to predict its solubility in CO2. On the basis of the results of previous publications,3,4,10 the weaker polymer−polymer interaction would lead to a lower cohesive energy density in the polymer, which might be a key driving force to enhance the CO2-philicity and increase the solubility of polymers in CO2. The cohesive energy density (ecoh) could be estimated from the potential energy difference between the isolated polymer chain and parent polymer chains. The expression of solubility parameter (δ) was given by the following equation:

other systems, as shown in Figure 5. The results also agreed well with the conclusion that PVAc was a CO2-philic homopolymer containing only C, H, and O.16,17 Moreover, the binding energies of the other four molecules with CO2 were a little smaller than that of VAc2, indicating that these target structures might also show good CO2-philicity. 4.1.2. CO2−Polymer Chain Interaction. The ab initio calculation presented above was only applied to accurately evaluate the interaction between small molecules. However, it was not used to calculate the polymer−CO2 interaction due to the limited computing resource. For a more comprehensive evaluation of the polymer−CO2 interaction, MD simulation was carried out to account for the interaction energies of a polymer chain with CO2. Through the calculation of interaction energies, the intermolecular interaction between polymer and CO2 could be quantitatively evaluated. Three types of systems, systems 1−11 listed in Table 1, were designed to calculate the interaction energies in NPT ensemble. The interaction energy in each CO2−polymer chain system was calculated by using the simulation results in Forcite energy, expressed as follows: E inter = ECO2 /chain − (ECO2 + Echain)

(1)

Here Einter referred to the interaction energy of CO2 with a single polymer chain, and ECO2 and Echain were the system energies of CO2 and the polymer chain, respectively. ECO2/chain represented the system energy of the CO2−polymer chain complex. The calculated interaction energies between CO2 and five kinds of polymer chain were shown in Table 5. More Table 5. Interaction Energy between CO2 and a Single Polymer Chain at 35 °Ca

a

system

Echain/CO2

Echain

ECO2

Einter

PVAc/CO2 PVEE/CO2 PVBE/CO2 PVAEE/CO2 PVABE/CO2

−8873.84 −6986.97 −5953.53 −8231.06 −7306.86

−5089.58 −3386.17 −2223.23 −4335.41 −3601.42

−2616.24 −2616.24 −2616.24 −2616.24 −2616.24

−1168.02 −981.46 −1114.06 −1279.41 −1089.20

Unit: kJ/mol.

detailed simulation results of the system energy are listed in Tables S1 and S2 of Supporting Information. The interaction energy between the PVAEE chain and CO2 (1279.41 kJ/mol) was the largest in the five CO2−polymer chain systems, which was significantly higher than that of the other three ethercontaining polymer chains (981.46, 1114.06, and 1089.20 kJ/ mol for PVEE, PVBE, and PVABE, respectively), even larger than that of the PVAc chain (1168.02 kJ/mol). The variation tendency of the Einter between CO2 and different polymer chains was different from the ab initio calculation results due to

δ=

ecoh

(2)

The values of ecoh and δ could evaluate quantitatively the intermolecular forces between polymer chains, which exerted an important influence on the solubility of polymer in CO2. To investigate the polymer−polymer interaction, five model polymers (systems 12−16) were constructed to calculate the

Figure 6. Binding modes of the other stable configurations for VAEE-CO2 and VABE-CO2 systems: (a) mode A of VAEE, (b) mode B of VAEE, (c) mode A of VABE. 12495


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The Journal of Physical Chemistry B ecoh and δ by MD simulation in Material Studio. Each system contained six polymer chains, and the compositions of polymer chains are shown in Table 1. The last 10 trajectory files in the NPT dynamic simulation were used to calculate the system properties including density, ecoh, and δ at 35 °C and 20 MPa. The statistic results were listed in Table 6 for the five polymer

Here ΔEmix referred to the mixing energy. The Vm represented the referenced mole volume used, and its value could be considered as the average molar volume of the polymer repeat unit. R and T were the molar gas constant and the Kelvin temperature. The φA and φB represented the volume fractions of components A and B in the binary system, respectively. Specifically, in this work, φA and φB were the volume fractions of CO2 and polymer in the simulation cell. The mixing energy ΔEmix could be calculated by the equation as follows:

Table 6. Simulated Cohesive Energy Density and Solubility Parameter at 35 °C and 20 MPa system PVAc PVEE PVBE PVAEE PVABE CO2

density (g/cm3)

ecoh (J/m3)

δ ((J/cm3)1/2)

1.149 0.977 0.933 1.056 1.035 0.854

× × × × × ×

17.94 15.83 14.96 16.13 16.25 13.10

3.22 2.51 2.24 2.60 2.64 1.72

8

10 108 108 108 108 108

ΔEmix = φA ecoh,A + φBecoh,B − ecoh,mix

where ecoh,A and ecoh,B were the cohesive energy density of CO2 and polymer, and ecoh,mix was the cohesive energy density of the binary system when the polymer dissolved in CO2. According to the Flory−Huggins theory, the critical value of miscibility, denoted as χc, was defined as follows: χc =

2 1⎛ 1 ⎞ ⎜1 + ⎟ 2⎝ n⎠

(5)

Here n referred to the polymerization degree of polymer. If χ12 < χc, the polymer−CO2 binary system was regarded as a miscible system. Also, if χ12 ≈ χc, the components would exhibit partial miscibility. Otherwise, if χ12 > χc, the system would be an immiscible system. This provided a quantitative proposal to predict the compatibility extent of small molecule/polymer system through the comparison of χ12 and χc estimated from the simulation data. The values of χ12, χc, and Vm were calculated by using the approach as described above to predict the compatibility of polymer with CO2, as shown in Table 7. The results showed

systems. The PVAc had the largest δ and ecoh, indicating that the polymer−polymer interaction between PVAc chains was stronger than the other four systems. The strong intermolecular interaction would weaken the compatibility of PVAc with CO2 and hinder the dissolution of PVAc chain into CO2. In contrast, the other four kinds of polymer chain had lower δ and ecoh, indicating that the intermolecular forces were more moderate in these systems to cause lower interfacial tension and promote the stability of polymer/CO2 solution. From the viewpoint of polymer self-interaction, the other polymers might have a higher solubility in CO2 than PVAc did. The calculation of ecoh for poly(VAc50-stat-CF3Ac50) (2.28 × 108 J/m3) and poly(VAc10-stat-VPi90) (2.09 × 108 J/m3) with 4300 g/mol also confirmed that lower δ caused lower interfacial tension (34 and 30 mN/m,6 respectively) and the reduction of the polymer− polymer interaction was a promising strategy to prepare the highly CO2-philic polymers on the premise of the strong polymer−CO2 interaction (1157.4 and 1065 kJ/mol, respectively). According to the principle of “like dissolves like”, the difference of δ values (|Δδ|)53 between polymers and CO2 also supported the viewpoint that PVAc might be not a favorable candidate as a CO2-philic polymer. However, this judging criterion neglected some influential factors such as molecule polarity, H-bonding, and the difference in molecule scale between polymer and CO2. 4.1.4. Flory−Huggins Interaction Parameter. The Flory− Huggins interaction parameter, denoted as χ12, is an extensively approved theoretical criterion to evaluate and predict the compatibility of small molecules and polymer chains in the absence of experimental data. Especially, it is more effective than the criterion of solubility parameter when H-bonding interaction and electrostatic force cannot be ignored in a binary system. Not only the influence of polymer−polymer interaction on the formation of polymer−CO2 solution but also that of the polymer−solvent interaction and solvent−solvent interaction are all considered in the parameter χ12, which provides a more complete and profound understanding of the polymer−CO2 compatibility. The χ12 would be evaluated from the following equation:54 ⎛ ΔE ⎞ mix ⎟⎟Vm χ12 = ⎜⎜ ⎝ RTφA φB ⎠

(4)

Table 7. Flory−Huggins Parameter and Critical Value of Miscibility Calculated by MD Simulations system

Vm (cm3/mol)

χ12

χc

PVAc/CO2 PVEE/CO2 PVBE/CO2 PVAEE/CO2 PVABE/CO2

92.6 95.0 129.1 93.8 110.8

4.21 4.81 4.98 2.98 3.44

0.65 0.64 0.66 0.65 0.66

that all the χ12 of the five polymer/CO2 systems were higher than the χc, indicating that it was difficult to be miscible and CO2 might be a feeble solvent for these polymers. The conclusion was consistent with the fact that polymers with high molecular weight had a poor compatibility with CO2. By the comparison of the χ12 in the five systems, the χ12 of PVAEE was lower than that of the other systems, including the PVAc/CO2 system, proposing that PVAEE was the best CO2-philc polymer due to its stronger interaction with CO2, which agreed well with the simulated results of polymer−CO2 interactions by MD simulations. 4.2. Dissolution Behavior of Poly(VAc-co-alkyl vinyl ether) in scCO2. 4.2.1. Synthesis and Characterization of Poly(VAc-co-alkyl vinyl ether). The poly(VAc-co-alkyl vinyl ether) copolymers were synthesized in solvent ethyl acetate by RAFT polymerization at 80 °C with AIBN as the initiator and isopropylxanthic disulfide as a RAFT agent. The molecular weight and polydispersity index (PDI) of the purified products were determined from the GPC traces. The molecular weight and PDI results were listed in Table 8, and the GPC traces of the samples were also shown in Figure S5 of the Supporting

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molar ratio of VEE:VAc = 1:3.6 and the molar content of VEE in the copolymer was 21.7%. The same calculation method was applied to the composition analysis of poly(VAc-co-VBE), but the peak position of the methyl group (CH3) in VBE was 0.84 ppm. The calculated VEE or VBE content of the samples was listed in Table 8. More detailed results of 1H NMR spectrum are demonstrated in Figures S6−S8 of the Supporting Information. 4.2.2. Cloud Point Pressure of Poly(VAc-co-alkyl vinyl ether) Copolymers in scCO2. The solubility of poly(VAc-coalkyl vinyl ether) copolymers in scCO2 was characterized by using cloud point pressure measurements. The Pc of samples in CO2 were determined at temperature ranging from 35 to 65 °C. Figure 8a−d shows the results of Pc in 35−65 °C, respectively. The characterizations were performed at a sample concentration of 0.2 wt % in CO2 to allow a comparison with the results of previous publications.5,6,22 Herein, as a reference sample, the Pc of PVAc of 4500 g/mol was 26.7 MPa at 35 °C and 30.3 MPa at 45 °C, whereas the Pc of PVAc of 6700 g/mol was 34.3 MPa at 35 °C and 37.2 MPa at 45 °C.10 Moreover, according to the literature result, 17 the P c of PVEE homopolymer of 3800 g/mol was 32.0 MPa at 22 °C. Comparison of the Pc of poly(VAc-co-VEE) with Mn ranging from 6600 to 6900 g/mol showed that the cloud point pressures decreased dramatically with increasing the content of VEE monomer units. With the gradual incorporation of VEE units from 0, 7, 11, 22, to 34 mol % into polymer chains, the Pc decreased gradually from 34.3, 29.6, 28.5, 26.2, to 25.3 MPa at 35 °C, as shown in Figure 8. Then, the Pc rebounded to 25.8 MPa with VEE content of 38 mol % incorporated into the copolymer. The minimum Pc was evidenced at the VAc-VEE mole ratio of 2:1, which was consistent with the optimum monomer ratio in Destarac et al.’s work.6 The CO2-philicity of the optimum random copolymer (Pc = 26.2 MPa) was comparable with that of poly(vinyl acetate-alt-dibutyl maleate) of 7600 g/mol (Pc = 26.1 MPa).10 The CO2-philicity of poly(VAc-co-VEE) with 24.4% VEE content (Mn = 8300 g/ mol, Pc = 29.9 MPa at 35 °C) was similar to that of the optimal poly(VAc10-stat-VPi90) (Mn = 8900 g/mol, Pc = 37.9 MPa at 35 °C).40 The variation trend of the Pc of poly(VAc-co-VEE) of 4300−4500 g/mol was also consistent with that of poly(VAcco-VEE) of 6600−6900 g/mol, and the Pc of the optimal sample was 24.1 MPa at 35 °C. Although the Pc was larger than the optimal poly(VAc50-stat-VTFAc50) (Pc = 17.7 MPa), poly(VAc70-stat-CF3VAc30) (Pc = 22.1 MPa), and poly(VAc10stat-VPi90) (Pc = 22.0 MPa) with similar Mn, the poly(VAc-coVEE) was a noteworthy nonfluorous CO2-phile candidate. The Pc increased with the temperature since the density of CO2 decreased with the temperature at the same pressure. The Pc increased with the Mn in accordance with the thermodynamic consistency. According to the previous work,10,39 the Pc would increase more than about 2 MPa when the Mn increases 1000 g/mol in the range 4000−8000 g/mol. From the results in Figure 8, the poly(VAc-co-VEE) with 6600−6900 g/mol had lower Pc than that of the poly(VAc-co-VBE) with 5500−5700 g/mol at similar monomer content. And the poly(VAc-co-VEE) with 4400 g/mol had the lower Pc than the poly(VAc-co-VBE) with 3500 g/mol at about 20 mol % VEE or VBE content. Therefore, the poly(VAc-co-VEE) possessed a higher CO2philicity than poly(VAc-co-VBE) at the same monomer ratio. Note that the poly(VAc-co-VEE) exhibited a most favorable solubility behavior in CO2, and an optimum composition for CO2-philicity of such copolymer existed. The variation

Table 8. Synthesis of PVAc-Based Copolymers Using 1 wt % AIBN as Initiator run

sample

Mn (g/mol)

PDI

molar ratio of VEE or VBE (%)

1 2 3 4 5 6 7 8 9 10 11 12

PVAc poly(VAc-co-VEE) poly(VAc-co-VEE) poly(VAc-co-VEE) poly(VAc-co-VEE) poly(VAc-co-VEE) poly(VAc-co-VEE) poly(VAc-co-VEE) poly(VAc-co-VEE) poly(VAc-co-VEE) poly(VAc-co-VBE) poly(VAc-co-VBE)

4580 4540 4360 4470 4430 6820 6750 6610 6890 6680 3480 5740

1.26 1.12 1.22 1.25 1.29 1.29 1.30 1.22 1.44 1.28 1.49 1.67

10.7 19.8 35.1 41.0 6.9 10.8 21.7 33.6 37.5 20.3 14.0

Information. The poly(VAc-co-VEE) samples had the Mn in a range 4300−4500 g/mol and 6600−6900 g/mol and a relatively narrow molecular weight distribution due to the RAFT polymerization. Thus, the comparison of their Pc in CO2 depended on these two classes of molecular weight. In the reaction of VAc and VEE, the reactivity ratio of VAc, r1(VAc), = 3.25, indicating that the VAc units were more inclined to carry out self-polymerization instead of copolymerization with VEE. In contrast, the reactivity ratio of VEE, r2(VEE) = 0.13,55 explaining that VEE units tended to copolymerize with VAc units although it could conduct selfpolymerization. The product of r1 and r2 was less than 1 so that the samples obtained should be random copolymers. The similar result would occur in the reaction of VBE and VAc due to r1(VAc) = 1.18 and r2(VBE) = 0.57.55 The compositions of the purified products were confirmed by using 1H NMR spectrum. Herein, only the 1H NMR spectrum of the PVAEE in run 8 of Table 8 is shown for analyzing the composition in Figure 7. As can be seen, the chemical shift of H atom in 4.86

Figure 7. 1H NMR spectrum for a purified poly(VAc-co-VEE) copolymer.

ppm belonged to the methylidyne group (CH) of VAc in point c while that in 1.12 ppm belonged to the methyl group (CH3) of VEE in point g. Thus, the composition of samples could be calculated with the intensity of these two peaks of the H atom from VAc and VEE. The formula was as follows: contentVEE =

1 3

⎛

⎞

∫1.12 CH3/⎝ 13 ∫1.12 CH3 + ∫4.86 CH⎠ ⎜

⎟

(6)

The H atom peak area ratio of the CH3 group of VEE in 1.12 ppm to the CH group of VAc in 4.86 ppm was 1:1.2, so that the 12497


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Figure 8. Cloud point pressures of poly(VAc-co-VEE) and poly(VAc-co-VBE) copolymers in CO2 at 35, 45, 55, and 65 °C (0.2 wt % polymer in CO2). Cloud point pressures of pure PVAc were from our previous work.10

chain quantitatively evaluated through the analysis of the simulation results. The interaction energies were enhanced in the copolymer when the content of VEE was larger than 15%, indicating that an appropriate addition amount of VEE monomer in the copolymer would improve the interaction between the copolymer and CO2. It was worth mentioning that the interaction energy of PVAEE30 with CO2 had the highest value (1502.47 kJ/mol) and was significantly larger than that of PVAc (1168.02 kJ/mol) and PVEE homopolymers (981.46 kJ/ mol), which agreed well with the experimental results that the minimum Pc was evidenced at the VAc:VEE ratio of 2:1. Combined with the ab initio calculations and MD simulations, the copolymers had more binding modes with CO2 and more CO2 molecules could place in different positions near the polymer chain, resulting in the greater number of interaction points than that with the homopolymers. The cumulative effect of interaction points led to the stronger interaction in CO2− copolymer chain, though the interaction energy in a single interaction point between poly(VAc-co-VEE) and CO2 was not necessarily stronger than that between the PVAc homopolymer and CO2. Thus, it was a promising strategy to enhance the polymer−CO2 interaction for preparing the highly CO2-philic polymer by copolymerization of VAc and another correct monomer with a certain composition. 4.3.2. Flory−Huggins Interaction Parameter. As an important criterion to understand the compatibility of polymer and CO2, the Flory−Huggins parameter χ12 was also calculated in four random polymer/CO2 systems to evaluate and analyze comprehensively the influence of polymer−polymer interaction and polymer−solvent interaction. The cohesive energy densities of pure polymer systems (PVAEE15, PVAEE30, PVAEE 45 , and PVAEE 60 ) and polymer/CO 2 systems (PVAEE 15 /CO 2 , PVAEE 30 /CO 2 , PVAEE 45 /CO 2 , and PVAEE60/CO2) were statistically obtained from the simulation

tendency of CO2-philicity in these copolymers agreed basically with the simulation results of Flory−Huggins parameters. From similar results between the multiscale molecular modeling and the experiments, it is noteworthy that although the CO2philicity of polymers related to the polymer self-interaction, the strong polymer−CO2 interaction could not be ignored in the selection of CO2-philies. 4.3. Molecular Dynamic Calculations of Random Copolymer−CO2 Systems. The MD simulations above did not consider the influence of monomer composition on the polymer Pc in CO2. To gain a deeper understanding of the observed dissolution behaviors and the optimum monomer composition of poly(VAc-co-VEE), the respective contributions of polymer−CO2 interaction and polymer self-interaction were evaluated by the MD calculations. 4.3.1. CO2−Polymer Chain Interaction. MD simulations were applied to calculate the interaction between CO2 and a single chain of four kinds of random copolymers, including PVAEE15, PVAEE30, PVAEE45, and PVAEE60. The procedures were similar to the previous calculations of the binding energies of CO2−polymer chain interaction, and the compositions of designed polymer chains are shown in Table 2. Table 9 shows the molecular interaction energies between CO2 and polymer Table 9. Interaction Energy between CO2 and a Single Random Polymer Chain at 35 °Ca

a

system

Echain/CO2

Echain

ECO2

Einter

PVAEE15/CO2 PVAEE30/CO2 PVAEE45/CO2 PVAEE60/CO2

−8666.34 −8842.69 −8231.98 −7910.25

−4932.77 −4723.98 −4386.23 −4075.09

−2616.24 −2616.24 −2616.24 −2616.24

−1117.33 −1502.47 −1229.51 −1218.92

Unit: kJ/mol. 12498


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ecoh, A+ φB·ecoh, B and ecoh, mix, i.e., ΔEmix, was the smallest, which was favorable for enhancing the compatibility between polymer and CO2. Simultaneously, the value φA·φB was largest in the PVAEE30/CO2 system, and Vm was very similar to that of other systems. The lowest ΔEmix and the largest φA·φB led to the smallest value of χ12 in the PVAEE30/CO2 system, proposing that PVAEE30 was a relatively better CO2-philic polymer due to the stronger interaction with CO2 and favorable polymer− polymer interaction. The simulated results of random polymer/ CO2 systems agreed well with the cloud point pressure results, especially the optimum content of VEE in the copolymer.

results, as illustrated in Figure 9. The ecoh of pure polymer systems (ecoh, B) had a rapid decline when adding a low content

5. CONCLUSION Two potential comonomers, VEE and VBE, were introduced in the PVAc backbone, and the compatibility of the copolymers with CO2 was explored through multiscale molecular modeling and dissolution behavior measurement. The ab initio calculations were used to evaluate the interaction energy of CO2 with three monomer molecules (VAc, VEE, and VBE) and five dimeric model structures (VAc2, VEE2, VBE2, VAEE, and VABE). The ether group in VEE or VBE had larger interaction energy with CO2 than the ester group in VAc, but the interaction in the VAc2-CO2 complex was slightly stronger than the others due to the greater number of interaction points. Five polymer chains (PVAc, PVEE, PVBE, PVAEE, and PVABE) were also constructed, and their interactions with CO2 by using MD calculations were calculated. The interaction energy between PVAEE chain and CO2 was the largest due to the cumulative effect of interaction points and greater probability that the CO2 molecules could interact with the polymer chain. In addition, the CED and χ12 of polymer/CO2 systems were evaluated to predict the polymer−CO2 compatibility. The strong interactions of PVAc chains would weaken the compatibility with CO2. The χ12 of PVAEE was lowest, which was favorable to its dissolution in CO2. The poly(vinyl acetate-co-alkyl vinyl ether) copolymers were synthesized by RAFT polymerization, and their Pc values in CO2 were measured. Random poly(VAc-co-VEE) containing 34 mol % VEE content showed the best CO2-philicity, agreeing well with the simulated results that the PVAEE/CO2 system had the lowest χ12. Random poly(VAc-co-VEE)/CO2 systems containing different VEE contents were calculated to verify the optimum VEE content. Adding 30 mol % VEE, the interaction energy of copolymer with CO2 could be significantly enhanced. More importantly, the χ12 was smallest in the PVAEE30/CO2 system due to the stronger interaction with CO2 and favorable polymer−polymer interaction, which was highly consistent with the experimental results. The greater CO2-philicity of poly(VAc-co-VEE) benefited from a combination of two factors: weaker polymer−polymer interaction and stronger polymer−CO2 interaction, rather than improvement of only one factor. It is suggested that a favorable comonomer could not only reduce the polymer−polymer interaction but also enhance the polymer−CO2 interaction with a suitable composition. Addition of a favorable comonomer with a certain composition to design a new hydrocarbon-based copolymer might obtain the synergistic effect of polymer− polymer interaction and polymer−CO2 interaction to enhance the CO2-philicity, which is an attractive strategy to explore the CO2-philic polymer and expand the application range of CO2.

Figure 9. Cohesive energy density of pure polymer systems and polymer/CO2 systems in MD simulations.

of VEE into the copolymer, and then the drop slowed with increasing VEE content. The change trend of ecoh in polymer with varying compositions was similar to that of surface tension in PVAc-based fluorine polymer chains in Destarac et al. work,6 confirming the intrinsic connection between the surface tension and the polymer−polymer interaction. However, the ecoh of polymer/CO2 systems (ecoh,mix) almost decreased linearly and slowly with the addition of VEE content. The difference between ecoh, B and ecoh, mix would affect ΔEmix at a large extent. The χ12 could be calculated with the eq 3 after obtaining the data of ΔEmix (the mixing energy), φB (the volume fraction of polymer in the binary system), and Vm (the referenced molar volume) from the MD simulations, as listed in Table 10. The Table 10. Flory−Huggins Parameter and Critical Value of Miscibility of Random Copolymer/CO2 Systems Calculated by MD Simulations system

ΔEmix (J/cm3)

Vm (cm3/mol)

φB

χ12

χc

PVAEE15/CO2 PVAEE30/CO2 PVAEE45/CO2 PVAEE60/CO2

22.8 8.2 14.5 17.5

92.94 93.29 93.70 94.05

0.68 0.65 0.73 0.74

3.45 1.24 2.20 2.67

0.65 0.65 0.64 0.64

results showed that PVAEE30/CO2 systems had the lowest χ12 value, indicating that PVAEE30 was the most CO2-philic one in the copolymers with different VEE contents. The conclusion was consistent with the experimental results that the most CO2philic polymer in the poly(VAc-co-VEE) series had the optimal VAc:VEE ratio at about 2:1. With the PVAEE30/CO2 system as an example, the ΔEmix depended on the difference between φA·ecoh, A + φB·ecoh, B and ecoh, mix, which was related to the ecoh of PVAEE30 (ecoh, B), PVAEE30/CO2 (ecoh, mix), and the volume fraction of PVAEE30 (φB), as shown in eq 4. The φB in the simulation box of PVAEE30/CO2 system (0.65) was smaller than the other three polymer/CO2 systems (0.68, 0.73 and 0.74) due to the stronger interaction between polymer chain and CO2. The high volume fraction of polymer also indirectly showed that the polymers overall had a poor compatibility with CO2. The difference between ecoh, B and ecoh, mix in the PVAEE30/CO2 system was significantly less than that of the PVAEE15/CO2 system and similar to that in PVAEE45/CO2 and PVAEE60/CO2 systems. Thus, in the PVAEE30/CO2 system, the difference between φA· 12499


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b08393. Energies of polymer chain−CO2 system, simulated cohesive energy density and solubility parameter, optimized structures in model molecule−CO2 systems, GPC traces, 1H NMR spectra, the apparatus for cloud point pressure measurements, and force field details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 21 64253470. Fax: +86 21 64253528. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21376087), Research Fund for the Doctoral Program of Higher Education of China (20130074110013), the 111 Project (B08021), and the Fundamental Research Funds for the Central Universities.

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