Partition Coefficients of Methyl Methacrylate and Acetone between

Partition coefficients of acetone between polymer and compressed gaseous CO2: (symbol) experimental results; (lines) calculation results of eq 6. The ...
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Ind. Eng. Chem. Res. 2009, 48, 7354–7358

Partition Coefficients of Methyl Methacrylate and Acetone between Polymer and Compressed Carbon Dioxide Phases by an in situ Infrared Spectroscopy Method Jun He* and Bo Wang Institute of Chemistry, Chinese Academy of Sciences, Beijing, China 100190

Partition coefficients of methyl methacrylate and acetone between some polymer and compressed gaseous carbon dioxide phases were determined by an in situ Fourier transform infrared (FTIR) spectroscopy method with a special designed high pressure IR cell at 318.2 K and 2.0-7.0 MPa. At the same temperature and pressure, the partition coefficient for methyl methacrylate was higher than that for acetone between the same polymer and subcritical carbon dioxide. For most solute/polymer/CO2 systems, the partition coefficient of solute between the polymer and compressed gaseous carbon dioxide decreased with the pressure increase. Only for a methyl methacrylate/poly(methyl methacrylate)/CO2 system, the partiton coefficient was not sensitive to the pressure change at pressures lower than 3.9 MPa. A linear solvation free energy relationship was used to estimate the main tendency of the pressure dependency. With the pressure dependent polarizability-dipolarity parameter of carbon dioxide, the calculated results were in accord with some experimental results. 1. Introduction Due to their special characteristic properties, supercritical and subcritical carbon dioxide have widely been used as clean alternatives for conventional organic solvents in modifying and processing polymers, such as the following: purifying, drying, dying, foaming, blending, and making polymer complexes.1,2 In processes involving extraction and impregnation, the partition of solute between the polymer and fluid phases becomes a key parameter for design. Although most of the polymers have very limited solubility in CO2, CO2 has appreciable solubility in many polymers and can swell the polymer matrix.3,4 As the solvent power of CO2 can be modulated with pressure, it is possible to adjust the distribution of solute between the two phases by merely changing the pressure of the fluid phase. There are already several investigations in this field. Johnston et al. measured the partition of some aromatic compounds between supercritical carbon dioxide (scCO2) and cross-linked poly(dimethylsiloxane) (PDMS) by inverse supercritical fluid chromatography.5 Inomata et al. determined the partition of n-hexane between poly(butadiene) (PB) and scCO2 phases by a supercritical fluid chromatography approach.6 Eckert’s group did a series of investigations.7-11 West et al. investigated the partitions of dyes and deuterated water between poly(methyl methacrylate) (PMMA) and scCO2 by using in situ spectroscopy methods (ultraviolet (UV) and Fourier transform infrared (FTIR)).7 Using the same method, Kazarian, Vincent, and coworkers studied the partitions of some conventional cosolvents (methanol, 2-propanol, and acetone) between PDMS and scCO2;8,9 Brantley et al. investigated the influence of cosolvents on the partition behavior of dilute aromatic solutes;10 Ngo et al. studied the partition of azo-dyes between PMMA and scCO2.11 With a tracer response technique, Funazukuri et al. measured the partitions of acetone or several aromatic compounds between poly(ethylene glycol) (PEG) and scCO212 and measured the partition ratio of β-carotene or R-tocopherol between PEG and scCO2.13 Banchero et al. determined the partition of some dyes between polyethylene terephthalate (PET)/scCO2 and the cosolvent effect of methanol by measuring the dye uptake in the polymer and solubility of dyes in * To whom correspondence should be addressed. Tel.: 86-1062562821. Fax: 86-10-62562821. E-mail: [email protected].

scCO2.14,15 The pressure for most of these works were higher than 7.0 MPa, and the pressure dependence of the partition coefficient or cosolvent effect in this pressure range were consistent and well understood. But until now, the investigation for the partition of solute between a polymer and compressed gaseous carbon dioxide was very limited and the regulation was not well-understood. In ref 6, the partition coefficient (Kpartition) of n-hexane between PB and CO2 phases was comparatively unchanged in the fluid density range of 0.14-0.18 g/mL (about 6.0-7.0 MPa for pure CO2). In ref 9, the Kpartition of 2-propanol between PDMS and CO2 had a maximum at about 4.5 MPa in 2.0-6.0 MPa; and the Kpartition of acetone between PDMS and subcritical carbon dioxide decreased smoothly with the pressure increase. In this work, the partition coefficients of methyl methacrylate (MMA) and acetone between some glassy polymer and compressed gaseous CO2 were measured by an in situ FTIR spectroscopy method at 318.2 K and at 2.0-7.0 MPa. The solutes are the following: methyl methacrylate, a monomer which can be polymerized under the action of some initiators but can not be polymerized by heating; and acetone, a conventional cosolvent, which can enhance the solubility of some solute in carbon dioxide but may also compete with solute in polymer phase. The polymers are poly(methyl methacrylate), poly(styrene), and poly(carbonate biphenol A), glassy polymers with different glass transition temperatures. The main factors that influence the partition of solute at constant temperature were discussed. And, a linear solvation free energy relationship was used to estimate the general tendency of the pressure dependency. 2. Experimental Section 2.1. Materials. Methyl methacrylate (MMA) (g0.98), acetone (g0.995), and ethanol (g0.997) were produced by Beijing Chemical Regent Company. MMA was distilled under reduced pressure and stored in refrigerator at 277 K. Carbon dioxide with a purity g0.9995, provided by Beijing Analytical Instrument Factory, was used without further purification. Poly(methyl methacrylate) (PMMA), V020, was produced by Atofina. Poly(styrene) (PS), 666D, was provided by Yansan

10.1021/ie801824y CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009

Figure 1. Structure of the IR cell (a) and details of basket/stopper (b): (1) stainless steel body; (2) Teflon seal; (3) stopper; (4) heater; (5) nut; (6) ZnS window; (7) platinum resistance; (8) basket.

Figure 2. Schematic map of the system for preparing the solution: (1) water bath; (2) 50 mL stainless steel vessel; (3) temperature controller; (4) pressure gauge; (5 and 7) needle valves; (6) two-way connector; (8) carbon dioxide cylinder or IR cell; (9) magnetic stirrer.

Petrochemical Company, SINOPEC Corp. Poly(carbonate biphenol A) (PC), A2700, was produced by Idemitsu Petrochemical Co. Ltd. 2.2. Procedures and Equipment. 2.2.1. Film Preparation. To ensure the partition equilibrium and the accuracy of the measurement, the polymer grains were pressed into thin film. The polymer granules were flushed with ethanol, dried with nitrogen flow, and heated in vacuum at a temperature 10 K higher than the glass transition temperature of corresponding polymer for 24 h. Then the polymers were hot pressed into films. The thickness of the films of PMMA, PS, and PC were all 0.05 ( 0.01 mm. 2.2.2. Instruments. An IR cell was special designed for determining the solute partition coefficients for such solute/polymer/ carbon dioxide systems: the IR absorption band for the characteristic functional group of the solute is overlapped with the IR absorption of the polymer but not overlapped with the IR absorption of carbon dioxide, or the polymer may deform in the experiments. The specially designed high pressure IR cell (Figure 1) was used for all spectra measurement. The inner volume of the IR cell was 6.5 mL, and the path length of the cell was 26 mm. The temperature of the cell was controlled at 318.2 ( 0.5 K with a heater controlled by a controller made by Beijing Tianchen Electronic Company and a platinum resistance probe. To measure the partition coefficient, a Bruker Tensor 27 FTIR spectrometer was used to record the IR spectrum of the fluid phase in this cell. The apparatus used to prepare solute/CO2 solution was displayed in Figure 2. The inner volume of the stainless steel vessel was 50 mL. And, the pressure gauge in the system was composed of a pressure transducer (FOXBORO/ICT, model 93) and an indicator, which was accurate to 0.02 MPa in the pressure range of 0-20 MPa. 2.2.3. Preparation of Compressed Gaseous Solute/CO2 Solution. At first, the solute concentration was calculated and the solution was prepared before the partition experiment. Suitable amounts of solute and CO2 were charged into the 50 mL stainless steel vessel. Then the stainless steel vessel was put in the water bath controlled at 318.2 ( 0.1 K and the content was stirred with an electromagnetic stirrer. After the ther-

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moequilibrium, the fluid phase was adjusted to the desired pressure by turning the screw to change the inner volume of the stainless steel vessel. The solute concentration was kept lower than 0.001 molar fractions, which was lower than the solubility of MMA and acetone in compressed gaseous CO2 in our experimental temperature and pressure range.16-19 Thus the solute/CO2 solution was in a homogeneous compressed gaseous state in all of our experiments. 2.2.4. Determination of the Partition Coefficient. The specially designed IR cell was cleaned with ethanol and dried with nitrogen flow. The basket of the IR cell was weighted ((0.1 mg), about 50 mg polymer film was cut into small pieces and put into the basket, and then the basket was weighted again ((0.1 mg), thus the mass of polymer used was obtained. The basket was connected with the stopper and installed into the IR cell. Then the cell was sealed, evacuated for 5 min, and heated to 318.2 K. When both the 50 mL stainless steel vessel and the IR cell were heated to 318.2 K, they were equilibrated for another 30 min. Then the IR cell and the 50 mL vessel were connected together as in Figure 2, valves 5 and 7 were opened for 1-2 min, and then closed at once when the pressure was constant. The IR absorption spectrum of the fluid phase in the IR cell was recorded at once as the average of 32 scans until the peak absorbance of carbonyl groups did not change with time within 2 h. At 318.2 K and a series of pressures, the molar absorptivity of solute in compressed gaseous carbon dioxide was calibrated with MMA/CO2 and acetone/CO2 solutions of know solute molar concentration by using the same IR cell. From the peak absorbance of the >CdO group, the solute moalr concentrations in the fluid phase at the beginning and at the end of the absorption process were obtained. Therefore, from this concentration change, the volume of the IR cell, and the amount of the polymer used, the equilibrium concentration of solute in the polymer could be calculated. Thus the partition coefficient (Kpartition) could be calculated by eq 1: Kpartition ) Cpoly /CCO2

(1)

In which, Cpoly (mol/kg) and CCO2 (mol/L) were the equilibrium concentration of the solute in the polymer and in compressed gaseous CO2 phases, respectively. The experiment was repeated twice for each condition, the deviation is within (10%. In this work, the pressure drop in the partition process, the decrease of peak absorbance of solute in the blank experiment without polymer film, and the amount of solvent residue in the polymer films were also observed. 3. Results and Discussion 3.1. Molar Absorptivity of MMA/CO2 and Acetone/ CO2 Solution. MMA and acetone have a single sharp peak at 1740 cm-1 which is attributed to the >CdO group in the molecule. The dependence of this absorbance (A) on the molar concentration (C, mol/L) of solute in solute/CO2 solution at 318.2 K and constant pressures is shown in Figure 3. Beer’s law was obeyed, and thus, the molar absorptivity (, L/(mol · cm)) of MMA/CO2 or acetone/CO2 solution at 1740 cm-1 was obtained. The dependence of this molar absorptivity on fluid pressure (P, MPa) is shown in Figure 4. The relationship could be represented by eqs 2 or 3 For MMA, ε/(L/(mol·cm)) ) 55 + 35P/MPa

(2)

For acetone, ε/(L/(mol·cm)) ) 117 + 6.3P/MPa

(3)

The concentration was lower than 2.0 × 10-3 mol/L for MMA and was lower than 2.5 × 10-3 mol/L for acetone, and the peak

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Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009

Figure 3. Dependence of the peak absorbance of the >CdO group (A) on the solute molar concentration in compressed gaseous CO2 (CCO2).

Figure 5. IR absorbance of the >CdO group in the partition process of MMA between PC and CO2 at 318.2 K and 4.5 MPa. From the top down,the absorption time is 5 min, 10 min, and 24 h.

Figure 4. Dependence of the molar absorptivity of solutes in compressed gaseous CO2 on the pressure.

Figure 6. Partition coefficients of MMA between polymer and compressed gaseous CO2: (symbol) experimental results; (line) calculation results of eq 6.

absorbance of the solute was kept no higher than 1.1 in all of our experiments. 3.2. Peak Absorbance Change in the Partition Procedure. The intensity change of IR resonance for carbonyl group in the distribution process of MMA between PC and compressed gaseous CO2 phases at 318.2 K and 4.5 MPa is shown in Figure 5. The peak absorbance of the solute in the fluid phase decreased remarkably. At the same time, the pressure drop in the measurement was no more than 0.1 MPa; no observable change in the peak absorbance of carbonyl group was detected in the blank experiments; and the IR absorbance of >CdO and sCH3 groups in the fluid phase were lower than 0.05 for all kinds of our polymer films in the extraction experiments at 6.7 MPa. 3.3. Partition Coefficient. The partition coefficient of MMA between polymer and compressed gaseous CO2 at 318.2 K and 2.0-7.0 MPa is shown in Figure 6, and the partition coefficient for acetone is shown in Figure 7.

same temperature and pressure. And, the influences of the polymer on the partition coefficient are different for different solutes. The pressure dependence of Kpartition for different solute/ polymer/compressed gaseous CO2 system is distinctive. At constant temperature, the Kpartition of MMA between PMMA and subcritical CO2 is insensitive to pressure change in low pressure range; however above 3.9 MPa, the Kpartition drops dramatically with the pressure increase. For MMA/PS/CO2 or MMA/PC/ CO2, the Kpartition decreases with the pressure increase. For the acetone/polymer/CO2 system, the Kpartition all decreases with the pressure increase. The magnitude of Kpartition for acetone between PMMA and compressed gaseous carbon dioxide is close to that for acetone between the PDMS and compressed gaseous carbon dioxide phases;9 the partition coefficient determined by Eckert et al. also decreased with the pressure increase in a similar pressure range. Several factors may influence the pressure dependence of the partition of solute between polymer and compressed gaseous CO2, such as the following: the dissolution of solute in CO2; the competition of CO2 with solute in the polymer phase; and

The partition coefficient depends greatly on the solute. Between the same polymer and compressed gaseous CO2 phases, Kpartition for acetone is much lower than that for MMA at the

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Table 1. Parameters Used for Quantitative Structire-Activity Relationship (QSAR) Correlation Rx acetone MMA CO2 PMMA PS PC a

Vx

π a

a

0.179 0.173a

0.547a 0.845a

0.71 0.60a b 0.71a 0.66a 1.10a

References 26, 27, and 29-32. b Equations 4 and 5.

Table 2. Calculation Results of Equation 6

Figure 7. Partition coefficients of acetone between polymer and compressed gaseous CO2: (symbol) experimental results; (lines) calculation results of eq 6.

the change of the property of polymer induced by CO2 and solute absorbed in the polymer matrix. The first two influences are closely related with the solvency of compressed gaseous CO2. The solubility of lower molecular weight organic compounds in CO2 was found to be directly proportion to the density of the gaseous phase.20,21 And, the solubility of CO2 in polymer increased monotonously with pressure increase when the fluid pressure was lower than 7.0 MPa.22-25 An empirical quantitative structure-activity relationship (QSAR) was used to analysis the main tendency of the pressure dependence of the determined Kpartition. QSAR has been successfully used to analysis many solvation processes with the solvation parameters26,27 and was used to correlate the partition coefficients of six organic solutes between water and scCO2 with r g 0.99.28 Smith and Leffler et al. investigated the influence of pressure on the solvation parameter of some supercritical fluid.29,30 They found that the dipolarity-polarizability parameter π3 of CO2 changed dramatically with the pressure, while the other parameters were comparably unchanged: π3 ) 1.15Fr - 0.98(Fr < 0.7)

(4)

π3 ) 0.173Fr - 0.37(Fr > 0.7)

(5)

In which, Fr is the reduced density of CO2. In our solute/polymer/carbon dioxide systems, the Π/nelectron pair interaction contribution for MMA and acetone are almost the same; no obvious hydrogen bonding lies between solute-CO2, solute-polymer, and polymer-CO2. Then, our experimental partition coefficients were correlated using eq 6: log Kpartition /(L/kg) ) A + BVx2 + Cπ2(π1-π3)

(6)

Where, subscripts 1, 2, and 3 stand for polymer, solute, and CO2, respectively; π is the dipolarity-polarizability parameter describing the polar interaction contribution; Vx is the McGowan’s character volume of the solute, describing the sum of the dispersion interaction contribution and the endoergic cavity term. The parameters used are listed in Table 1. The calculation results are shown in Table 2 and Figures 6 and 7. The calculated partition coefficient decreases smoothly with pressure increase for the entire system. These findings are in accord with the general tendency of our experimental results and deviate from

solute/polymer/CO2

A

B

C

∆maxa

MMA/PMMA/CO2 acetone/PMMA/CO2 MMA/PS/CO2 acetone/PS/CO2 MMA/PC/CO2 acetone/PC/ CO2

-3.20 -3.20 -3.27 -3.27 -4.68 -4.68

2.65 2.65 2.64 2.64 3.86 3.86

3.11 3.15 3.45 3.15 3.11 3.20

0.26 0.07 0.08 0.05 0.09 0.04

a ∆max is the maximum absolute difference between the calculated and experimental log(K/(L/kg)).

the experimental pressure dependence for the partition coefficient of MMA between PMMA and compressed gaseous CO2. Thus for solute/glassy polymer/compressed gaseous carbon dioxide systems, the partition coefficient of the solute between the polymer and subcritical carbon dioxide phases should decrease smoothly with pressure increase, if there is no other strong influence. 4. Conclusions For MMA/PS/CO2, MMA/PC/CO2, acetone/PMMA/CO2, acetone/PS/CO2, and acetone /PC/CO2 systems, the partition coefficient of solute between the polymer and compressed gaseous carbon dioxide all decreased with the pressure increase. Only for the MMA/PMMA/CO2 system was the partiton coefficient not sensitive to the pressure change at pressures lower than 3.9 MPa. At the same temperature and pressure, the partition coefficient for MMA was higher than that for acetone between the same polymer and compressed gaseous carbon dioxide. The solvent power of CO2, which intensively depends on the CO2 density at constant temperature, contributes a lot to the determined pressure dependence of the partition coefficient. Acknowledgment The authors are grateful to the Nature Science Foundation of China (No. 20373080). Literature Cited (1) Sun, Y. P. Supercritical Fluid Technology in Materials Science and Engineering: Syntheses, Properties and Application; Marcel Dekker: New York, 2002. (2) Kemmere, M. F.; Meyer, T. Supercritical Carbon Dioxide: in Polymer Reaction Engineering; Wiley-VCH: Weinheim, 2005. (3) Prabhakar, R. S.; Angelis, M. G. D.; Sarti, G. C.; Freeman, B. D.; Coughlin, M. C. Gas and Vapor, Sorption, Permeation, and Diffusion in Poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether). Macromolecules 2005, 38, 7043–7055. (4) Eslami, H.; Mu¨ller-Plathe, F. Molecular Dynamics Simulation of Sorption of Gases in Polystyrene. Macromolecules 2007, 40, 6413–6421. (5) Condo, P. D.; Sumpter, S. R.; Lee, M. L.; Johnston, K. P. Partition Coefficients and Polymer-Solute Interaction Parameter by Inverse Supercritical Fluid Chromatography. Ind. Eng. Chem. Res. 1996, 35, 1115–1123.

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ReceiVed for reView November 27, 2008 ReVised manuscript receiVed May 31, 2009 Accepted June 9, 2009 IE801824Y