Ind. Eng. Chem. Res. 2006, 45, 3381-3387
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Cloud Points of Poly(E-caprolactone), Poly(L-lactide), and Polystyrene in Supercritical Fluids Ji-Young Park,† Soo Young Kim,‡ Hun-Soo Byun,§ Ki-Pung Yoo,‡ and Jong Sung Lim*,‡ Department of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290, Department of Chemical and Biomolecular Engineering, Sogang UniVersity, Seoul 121-742, South Korea, and Department of Chemical Engineering, Yosu National UniVersity, Yosu, Chonnam 550-749, South Korea
Cloud points were measured, using a high-pressure variable volume cell apparatus for poly(-caprolactone) (PCL), poly(L-lactide) (PLA), and polystyrene (PS) in chlorodifluoromethane (HCFC-22), trifluoromethane (HFC-23), difluoroethane (HFC-32), 1,1-difluoroethane (HFC-152a), dimethyl ether (DME), and HCFC-22 + CO2. The cloud points were characterized as functions of pressure, temperature, and polymer molecular weight. The cloud points of these polymers were studied at temperatures ranging up to 413.2 K with various polymer concentrations and the solubility of the above-mentioned three polymers in DME was compared. Among all systems, the cloud point data for PCL + HCFC-22, PCL + DME, PLA + HCFC-22, PLA + HFC-23, PLA + DME, PS + DME, PCL + HCFC-22 + CO2, and PLA + HCFC-22 + CO2 exhibited lower critical solution temperature (LCST) behavior, whereas those for PLA + HFC-152a and PLA + HFC-32 showed upper critical solution temperature (UCST) behavior. The cloud point pressure of PLA and PCL in the HCFC-22 + CO2 system increased as the amount of CO2 increased. This suggests that the nonpolar CO2 could not solubilize the polar polymer. Thus, the cloud point of PLA and PCL can be controlled by the amount of adding CO2 as an antisolvent, and this can be applied to nanoparticle formation processes. 1. Introduction Much attention has been given to the use of supercritical fluids as solvents in a variety of polymer processes, including extractions, separations, fractionations, particle formations, and other reactions.1-5 Interest stems primarily from the ability to change the bulk properties of supercritical fluids (e.g., density and solubility) dramatically with small variations in temperature and pressure, such that supercritical fluids have liquidlike dissolving power while exhibiting the transport properties of a gas.6 One application of the continuous research of chemists and engineers into supercritical fluids is the combination of supercritical and nanotechnology to form a nanometer-sized polymer particle in the supercritical state. Nanoparticle formation is categorized according to the use of supercritical fluids as solvent or antisolvent. Precipitation from a supercritical fluid solvent is called the rapid expansion of supercritical solution (RESS) process,7 whereas the use of supercritical fluids to reduce the solubility of a solute in an organic solvent is called the supercritical antisolvent (SAS) process.8 The latter precipitation process uses various equipment designs. Recently, the SAS process has been studied extensively as a way of producing microsized to nanosized particles of drugs, polymers, and/or pigments. Phase behavior and solubility data of a chemical in various solvents in and near the supercritical state are important to the efficient design and production of nanoparticles of polymer via the SAS process. Recent studies in several fields have generated useful data in regard to the phase behavior of chemicals in various solvents; moreover, the phase behavior of polymer + supercritical fluid systems has been measured and is reported in refs 9 and 10. For this study, poly(-caprolactone) (PCL), poly(L-lactide) (PLA), and polystyrene * To whom correspondence should be addressed. Tel.: (822)7058918. Fax: (822)705-7899. E-mail:
[email protected]. † University of North Carolina. ‡ Sogang University. § Yosu National University.
(PS) were selected, which are of interest in the area of sutures, dental, orthopedic, drug delivery devices, tissue engineering, and many other industrial purposes. In this work, the phase behavior data of PCL, PLA, and PS in various solvent systems were presented. The systems were as follows: PCL in chlorodifluoromethane (HCFC-22) and dimethyl ether (DME); PLA in 1,1-difluoroethane (HFC-152a), difluoroethane (HFC-32), trifluoromethane (HFC-23), HCFC-22, and DME; PS in DME; PCL, PLA, and PS in DME; PLA in a HCFC-22 + CO2 mixture; and PCL in a HCFC-22 + CO2 mixture. The cloud points of these systems were measured as functions of the polymer molecular weight at temperatures up to 413.2 K. The cloud point and solubility data obtained by this study are important thermodynamic properties of supercritical fluids, because the SAS process is directly affected by solubility. 2. Experimental Section 2.1. Chemicals. DME (Aldrich Chemical Co.), HCFC-22 (Solvay Fluorides), HFCs (Du Pont), and CO2 (Duck Yang Co.) had a minimum purity of 99.9% and were used as received. PCL (obtained from Aldrich Chemical Co.), PLA (obtained from the Biomaterials Research Center at KIST), and PS (obtained from Aldrich Chemical Co.) were used; these components had molecular weights of 2000, 14 000, 45 000, and 80 000, respectively. The chemical structures of PCL, PLA, and PS are shown in Chart 1. Table 1 lists the physical properties of the chemicals used in this study. 2.2. Experimental Apparatus. A schematic diagram of the apparatus is shown in Figure 1. This apparatus was designed to measure the cloud point temperature (up to 423.15 K) and the pressure (up to 200 MPa). It contains a variable-volume view cell, the internal volume and pressure of which can be changed via a movable piston. The variable-volume view cell was designed by ourselves and manufactured by our machine shop. A pressure generator (High-Pressure Equipment Co., model 37630, with a pressure rating of 206.8 MPa) was used to pressurize
10.1021/ie050710j CCC: $33.50 © 2006 American Chemical Society Published on Web 12/03/2005
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Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006
Table 1. Physical Properties of Solvents Used in This Studya solvent
chemical formula
molecular weight, MW
critical temperature, Tc (K)
critical pressure, Pc (MPa)
dipole moment (D)
carbon dioxide, CO2 dimethyl ether, DME chlorodifluoromethane, HCFC-22 trifluoroethane, HFC-23 difluoroethane, HFC-32 1,1-difluoroethane, HFC-152a
CO2 CH3OCH3 CHClF2 CHF3 CH2F2 CHF2CH3
44.01 46.06 84.46 70.01 52.02 66.05
304.10 400.00 369.30 299.07 356.26 386.41
7.38 5.24 4.97 4.84 5.78 4.52
0.0 1.3 1.4 1.6 2.0 2.2
a
Data taken from refs 13 and 14.
Chart 1. Chemical Structure of Polymers: (a) Poly(E-caprolactone), (b) Poly(L-lactide), and (c) Polystyrene
the contents of the cell. Visual observation of the phenomena occurring inside the cell was made through a sapphire window (thickness of 19.05 mm, diameter of 19.05 mm), using a borescope (Olympus, model R080-044-000-50) and CCD camera (Watec, WAT-202B). These were connected to a video cassette recorder/television (VCR/TV) (Anam Co.) monitor and computer. One advantage of using the variable-volume cell is that the concentration of the system remains constant during the experiment. A stirrer, rotated at variable speeds by an external magnet, was used to mix the solvent and polymer in the high-pressure reactor. A simple thermostatic air bath was used to maintain the system temperature to within (0.1 K. The temperature in the reactor was measured with a K-type thermocouple and digital indicator (Omega Co.) calibrated by KRISS (the Korea Institute of Standards and Science). The pressure in the reactor was measured to within (0.1 MPa, using a pressure transducer (Sensotec, model TJE, which a straingauge-based sensor- bonded foil type apparatus, with an
accuracy of (0.1%) and digital indicator (Sensotec, model L20000WM1) calibrated by a dead weight gauge. 2.3. Experimental Procedure for Measuring the Cloud Points of Polymer in Various Solvents. The reactor cell was evacuated and a certain amount of polymer was loaded into the cell. The amount of the polymer loaded into the cell was determined using a sensitive balance (Ohaus, model E04130) that could measure to (0.1 mg. To remove any trapped air, the cell was purged a minimum of three times with low-pressure solvent (
