Polymer Plasticization Using Supercritical Carbon Dioxide - American

To model the Tg behavior of diluent-polymer systems, a model that couples the lattice- ... vitrification for the systems PPO-CO2, PVP-CO2, and P(VP-VA...
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Polymer Plasticization Using Supercritical Carbon Dioxide: Experiment and Modeling I. Kikic,* F. Vecchione, P. Alessi, A. Cortesi, and F. Eva Department of Chemical, Environmental and Raw Materials Engineering University of Trieste, Piazzale Europa 1, 34127 Trieste, Italy

N. Elvassore Department of Chemical Engineering, University of Padova, Via Marzolo 9, 35131 Padova, Italy

The most important effect of sorption of compressed gases and supercritical fluids into glassy polymers is the reduction of the glass transition temperature (Tg). This plasticization effect causes changes in mechanical and thermophysical properties of the polymers. In this work, a thermodynamic study based on experimental and theoretical results is addressed. New data were carried out for poly(2,6-dimethylphenylene oxide) (PPO), poly(acrylic acid) (PAA), and the copolymer vinylpyrrolidone-vinyl acetate [P(VP-VA)] using an inverse gas chromatographic technique. To model the Tg behavior of diluent-polymer systems, a model that couples the latticefluid equation of state and the Gibbs-DiMarzio criterion was used. A parametric study of the influence of the physical properties (lattice coordination number, molecular weight, and binary interaction parameter) on the Tg behavior is presented. The thermodynamic model fairly describes the experimental data measured in this work and gives a phenomenological representation of the retrograde vitrification for the systems PPO-CO2, PVP-CO2, and P(VP-VA)-CO2. Introduction The solubilities and rates of diffusion of high-pressure gases and supercritical fluids in polymers have been interesting subjects of investigation for many applications. In particular, polymer separation and fractionation, impurities and additives extraction, drug impregnation, membrane conditioning, gas separation membranes, productions of microparticles, macro- and microcellular foams, gels, and fibers, and reactions of polymerization in the presence of supercritical fluids1-4 are processes and applications in which the knowledge of the mechanical and physical property changes of polymers is of enormous importance. The sorption of gas into polymers results in the increasing of the free volume and polymer segment mobility. These molecular level phenomena can be macroscopically observed by the depression of the glass transition temperature of the polymer-penetrant mixture. The extent of the depression in the glass temperature depends on the pressure of the gas and, consequently, on its concentration in the polymeric matrix. The sorbed gas acts as a kind of “lubricant”, making easier for chain molecules to slip over one another and thus causing polymer softening.5 This condition, when a gas under pressure increases its permeability in a glassy polymer, is also referred as plasticization. For example, because of the CO2 swelling and plasticization of the polymeric matrix in the CO2/CH4 separation process, the penetration of the CH4 is accelerated and, as a consequence, the polymer loses its selectivity. To overcome this problem, the CO2 plasticization should be minimized.6 The knowledge of the Tg-P behavior of * To whom correspondence should be addressed. Tel.: +39 040 5583433. Fax: +39 040 569823. E-mail: Ireneok@ dicamp.univ.trieste.it.

a given polymer-gas system is an important parameter for developing processing for various applications. For example, in the production of the microcellular polymeric foam, it provides information on the conditions under which the cell nucleation and growth will take place.7 The same information is also required for characterizing the optimum temperature-pressure condition for the gas separation membrane,6 the extraction of unreacted species, or the purification of the polymeric matrix. On the other hand, applications of supercritical fluids in polymer processing include fractionation, impregnation, and purification of a polymer and formation of porous or powdered polymers. Polymers swollen by supercritical fluids may be impregnated rapidly with additive. Sorption and desorption are rapid, and the operating condition can be adjusted continuously with pressure and temperature.8 Dyeing of polymer fibers is an example of impregnation of polymers.9 One of the commonly used supercritical fluids is CO2 because it is an inexpensive, nontoxic, nonflammable, environmentally benign solvent. Moreover, its small size allows the penetration of the gas into a polymer easier than larger liquid solvents. For the optimization of all of the processes mentioned above, it is extremely important to understand and predict the thermodynamic conditions at which the polymer plasticization occurs. In this work, the influence of high-pressure gas on the Tg depression of polymeric substances from both experimental and theoretical points of view is studied. A number of techniques such as NMR, dielectric relaxation, or high-pressure calorimetry have been used to study the polymer plasticization by compressed gases.10 However, in most of these techniques, the thermodynamic state of the glass-rubber transition is not welldefined. One of the most used methods is differential

10.1021/ie020961h CCC: $25.00 © 2003 American Chemical Society Published on Web 05/14/2003

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Figure 1. Four different types of behavior.

scanning calorimetry (DSC), which provides fast and accurate information. On the other hand, a very simple and inexpensive technique recently proposed by Alessi et al.10 is based on the use of a conventional highpressure gas chromatograph. In this technique, the Tg behavior as a function of the pressure of the system can be measured by using inverse gas chromatography (IGC). New data for poly(2,6dimethylphenylene oxide) (PPO), poly(acrylic acid) (PAA), and the copolymer vinylpyrrolidone-vinyl acetate [P(VPVA)] under CO2 pressure are presented here. The experimental data will be discussed and interpreted on the framework of the thermodynamic model proposed by Condo et al.8 The model combines the lattice-fluid (LF) theory and the Gibbs-DiMarzio criterion, which asserts that the total entropy of the system is zero at the glass transition. It requires only one interaction parameter, and it is able to find four different types of glass transition behavior, as shown in Figure 1, depending on the value of the binary interaction parameter (i.e., on the solubility of the diluent in the polymer).11 Interestingly, the model also

predicts the “unusual” phenomenon of the retrograde vitrification: at a constant pressure, an increase of the temperature causes a liquid to glass transition, opposite to the known behavior. A phenomenological interpretation of this behavior can be related to the competition between the density of the system and the diluent solubility in the polymer phase. In fact, decreasing the temperature (i.e., the density of the system and polymer mobility) causes the polymer to undergo a liquid to glass transition as expected. However, depending on the solvent-polymer interaction, the solubility of the gas into the polymer phase can be favored. For high values of the interaction parameter, the solubility of the diluent increases (i.e., higher solubility at constant pressure), so that decreasing the temperature at constant P, an unexpected behavior called the retrograde vitrification, causes the glass to become a liquid again. In this framework, a parametric study is presented to investigate the influence of the lattice coordination number, the molecular weight, and the binary interaction parameter on the Tg behavior. Successively the

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model is used to predict the Tg behavior for the experimental data measured in this work and literature data for carbon dioxide (CO2)-polystyrene (PS) and trifluoromethane (CHF3)-PS.12

Table 1. Comparison between Tg Values at Atmospheric Pressure: Measured by the IGC Technique and Reported in the Literature or Measured by a DSC Apparatus

Experimental Section Gas chromatography (GC) is a technique based on the partition of a volatile solute between a mobile gas phase and a stationary phase. In IGC, used in this work, the stationary phase is formed by the polymer. In the ICG technique, the retention mechanism of a solute in a polymer depends on the polymer state: the plot of the retention volume logarithm versus the reciprocal of the temperature is called the retention diagram, from which it is possible to observe the phase transitions of the polymer investigated. The specific retention volume of a solute injected in the stationary phase at different temperatures is defined as follows: 0

V0g

Fm Po - PH2O 273 ) (t - ta) j w 760 Tm R

(1)

where Fm is the mobile gas-phase flow, w the stationary phase weight, Po the outside column pressure, Tm the 0 the water vapor pressure flowmeter temperature, PH 2O at Tm, tR - ta the net retention time, ta the inert retention time, and j the James-Martin factor, which considers the pressure drop in the chromatographic column. For a semicrystalline polymer, three characteristic temperatures are evidenced: the glass transition temperature, Tg, the softening temperature, Ts, and the melting temperature, Tm. In the IGC conditions for temperature range below Tg, the penetration of the solute in the polymer bulk phase is precluded and so the retention mechanism is only due to the surface adsorption.13 At Tg, the solute penetration begins and the retention volume increases with the temperature. Because of an initially slow rate of diffusion of the solute, nonequilibrium conditions prevail: as the temperature increases, the diffusion coefficient rises sharply and equilibrium conditions are reached. Below Tm, retention proceeds by bulk sorption but the interactions between polymer and solute are restricted to the amorphous domains of the stationary phase. For temperatures above Tm, the behavior is linear, corresponding to the bulk adsorption in the totally amorphous polymer.13 High-pressure partition chromatography is a modification of the IGC technique: it is used for Tg depression determinations of a polymer, used as a stationary phase, in compressed CO2 that acts as the mobile phase, and in which an appropriate solute is injected. Considering the partition of a solute as the ratio of its fugacity in the mobile and polymer phases, it is necessary to point out that, at fixed pressure, the fugacity of the solute in the CO2 phase changes linearly with the temperature: therefore, this modification does not determine the characteristic “Z” shape of the retention diagram.14,15 Material and Methods. The polymers used are PPO, PAA, and P(VP-VA) supplied by Sigma-Aldrich and PVP K25 (Mw ) 29 000, Tg around 434 K) and PVP K90 (Mw ) 90 000, Tg around 438 K) supplied by BASF. The used organic solvents, 1-butanol, acetone, benzene, and

a

polymer

Tg (IGC) (K)

Tg (lit.) (K)

PPO PVP K90 PVP K25 PAA P(VP-VA)

482.1 438.2 434.0 373.2 369.1

484.1 442.1a 440.1a 379.1

Values determined in the present work by the DSC technique.

ethyl acetate, were provided by Sigma-Aldrich. CO2 was supplied by SIAD (purity of 99.98%). Experimental Setup and Procedure. The apparatus used for the experimental measurements is a supercritical fluid chromatograph (SFC 300, Fisons Instruments) consisting of a syringe pump, an oven containing the column with the polymer, and an UV detector (Spectromonitor 3200, Thermo Separation Products). Carbon dioxide is sent by the pump to a coil, where it reaches the system temperature, and then to the column (length 250 mm, i.d. 3.7 mm, o.d. 4 mm, filled with the polymer supported with Chromosorb 100/ 120). Both the coil and the column are contained in an oven. The solutes are injected in the fluid phase just before the column by means of an automatic actuation injecting valve. At the end of the column, a pressure transducer (DS Europe) monitors the pressure value. The outlet stream passes through the high-pressure cell of the UV detector and finally through a flowmeter. The experimental measurements were made at two pressures (8 and 10 MPa), while the temperature were chosen according to the Tg of the polymer. Before beginning the investigation of the CO2 plasticizing effects, a Tg determination, for all of the polymers, by IGC in the presence of an inert gas (nitrogen at 0.5 MPa), was carried out: these measurements have been made to test the validity of the IGC method by comparing the experimental values with the Tg supplied by Sigma-Aldrich and those determined by a DSC technique in our laboratory. For both the partition chromatographies at low pressure with N2 and high pressure with CO2, the solutes injected have been chosen among polar compounds such as alcohols, esters and aromatics. The choice was made on the basis of the evidenced retention time, response sensitivity and reproducibility of the peak maximum. After the screening, the solutes injected are: 1 butanol for PPO, acetone for PVP K90 and PVP K25, ethyl acetate for PAA and benzene for P (VP-VA). In Table 1 the Tg values provided by Sigma-Aldrich are compared with those obtained experimentally with nitrogen at 0.5 MPa and by DSC technique. Table 1 shows the substantial agreement for the Tg values of the three sets of data. Thermodynamic Model The glass transition temperature of polymers can be estimated by combining the LF and the GibbsDiMarzio theories.8 In this work, the Gibbs-DiMarzio argument, which assumes that the total entropy of the system becomes zero at an ideal glass transition temperature obtained by infinitely slow cooling, is adopted. This ideal transition temperature, at which the system becomes frozen and has zero entropy, has been found to be proportional to the glass transition temperature of the polymer. For practical purposes, one can identify

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this ideal glass transition temperature with the glass transition temperature once the flex energy and scaling parameters of the entropy fit the experimental glass transition temperature of the pure polymer. Under these assumptions, the density of the gaspolymer mixtures, the sorption of the gas in the polymer, and the glass transition temperature can be calculated by simultaneously solving the equation of state, the equilibrium condition for the gas, and the Gibbs-DiMarzio criterion. The Gibbs-DiMarzio criterion for mixtures assumes that the mixture entropy (not the entropy of the pure polymer) is zero at the glass transition. As the equation of state, the well-known SanchezLacombe LF16 has been used:

[

(

F˜ 2 + P ˜ +T ˜ ln(1 - F˜ ) + 1 -

1 F˜ ) 0 r

)]

Table 2. Parameters of the LF Equation of State for the Gases and Polymers substance

T* (K)

P* (MPa)

F* (kg/m3)

ref

CO2 CHF3 PS PPO PMMA PVP K90 PVP K25 PC P(VP-VA)

308.6 282.3 735.0 739.0 696.0 950.0 950.0 755.0 806.0

574.0 496.3 357.0 517.0 503.0 585.0 585.0 534.0 554.6

1505 1803 1105 1161 1269 1428 1428 1275 1370

6 a 6 18 6 b b 20 c

a Parameters fitted on both vapor-liquid equilibrium and PVT data.17 b Parameters obtained by molecular dynamics prediction.19 c Parameters for copolymer are obtained using the segment ratio average of the pure homopolymers.

(2)

T ˜, P ˜ , and F˜ are the reduced temperature, pressure, and density, respectively, defined as

T ˜ )

T P F ; P ˜ ) ; F˜ ) T* P* F*

(3)

T*, P*, and F* are the three characteristic equation of state parameters defined by

T* )

Mw * * ; P* ) ; F* ) k v* rv*

(4)

where Mw is the molecular weight. In the one-fluid approximation, the average interaction energy per segment, *, for a binary mixture constituted of components 1 and 2 is defined as follows:

* ) Φ111* + Φ222* - Φ1Φ2kTX12

Figure 2. Retention diagram for PVP K90-acetone at 8 MPa.

where z is the lattice coordination number, whereas the equilibrium fraction of flexed bonds i is given by the expression

(5)

X12 ) (11* + *22 - 212*)/kT

(6)

12* ) ζ12(11*22*)1/2

(7)

Φ1 and Φ2 are the segment fractions; ii* is the interaction energy of component i that correspond to energy required for the creation of a vacancy in component i; ζ12 is the binary interaction parameter. The equilibrium condition is given by equating the chemical potential of the pure solvent in the fluid phase and one of the same component in the polymer-rich phase. The system entropy is composed of two terms: the first takes into account the conformational contribution, whereas the second term is the internal or flexibility contribution to the entropy:

fi )

(z - 2) exp(-∆i/kT) 1 + (z - 2) exp(-∆i/kT)

(10)

∆i represents the increase of the intramolecular energy due to the flexing of a bond in a type i chain molecule; ∆i ) 0 represents a completely flexible chain without excluded volume. Tg may be calculated by setting, accordingly with the Gibbs-DiMarzio criterion, the entropy of the system (eq 8) equal to zero and by solving in the same time the LF equation of state (eq 2) and the equilibrium condition for the gas for a fixed value of the binary interaction parameter. In Table 2 the characteristic parameters of the LF equation of state for the two plasticizing gases and for the polymers investigated used in this work are reported. Results

conf

S)S

flex

+S

(8)

The conformational entropy is derived from the equation of state as reported by Condo et al.,8 whereas the expression for the flexibility contribution follows:

{( ) [ ( ) [

-Sflex/k ) rN

]

Φ1 ∆1 (r1 - 2) ln(1 - f1) - f1 + r1 kT Φ2 ∆2 (r2 - 2) ln(1 - f2) - f2 r2 kT

]}

(9)

Experimental Results. An example of the retention diagram for PVP K90 and PAA is reported in Figures 2 and 3, respectively. In the first case, the appearance of two minima corresponding to the two values of glass transition temperatures of the retrograde vitrification is evident. On the other hand, for PAA (Figure 3) only one value of the glass transition temperature was determined at 8 MPa. The experimental measurements obtained for the different polymers are summarized in Table 3. PPO, PVP K25, and PVP K90 show two values of the glass

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Figure 3. Retention diagram for PAA-ethyl acetate at 8 MPa. Figure 5. Influence of the molecular weight on the Tg versus P behavior for the system PC-CO2 4 with ζ12 ) 1.181.

Figure 4. Influence of the lattice coordination number, z, on the Tg versus P behavior for the system PMMA-CO2.21,22 Table 3. Experimental Tg Data at Different Pressures polymer PPO PVP K90 PVP K25 PAA P(VP-VA)

P (MPa)

Tg1 (K)

Tg2 (K)

8.0 10 8.0 10 8.0 8.5 8.0 8.0

429.0 399.2 393.3 378.9 373.6 373.9 321.1 309.2

373.9 344.1 368.9 353.9 354.1 315.1

transition temperature for the same value of pressure. This fact can be explained with the appearance of the retrograde vitrification phenomenon. Vice versa, PAA shows only one value of the glass transition temperature. It is also worthwhile to note that for PPO at 10 MPa only one value of Tg was detected. Theoretical Results. First, some considerations about the influence of the polymer characteristics and of the model scaling parameters on the Tg depression will be drawn through a parametric study of the model. In Figure 4, the study of the influence of the lattice coordination number, z, using as model parameters the values that fairly reproduce the experimental data obtained by Wissinger and Paulaitis for the system CO2-poly(methyl methacrylate) (PMMA) is reported.21,22 Five different values of z are considered (z ) 4, 5, 6, 8, and 10), while the interaction parameter ζ12 is set equal to 1.135. z can be considered as an adjustable parameter, but its value is limited by physical significance; thus, z should lie somewhere between 6 and 12.

For practical reasons, in Figure 4, the point corresponding to a zero value of ∂P/∂Tg is defined as the glass transition inversion point (GTIP). For temperatures above 323 K, the Tg behavior as a function of the pressure (upper glass transition line) is not influenced by z, whereas in the range 323-273 K, only a slight variation appears for the lower glass transition line. In all considered cases, the lattice coordination number slightly influences the Tg versus P behavior; consequently, a more physically significant value of z ) 10 was adopted for all of the calculations reported in this work. In the work of Condo et al., z was set equal to 5. It is interesting to note how the model predicts the Tg depression for polymers with different molecular weights. Figure 5 shows the relative glass transition depression predicted by the model. As an example, a CO2-polycarbonate (PC) system with molecular weight ranging from 500 to 5000 Da is considered. For high molecular weight polymers (above 2000), the polymer number of segments, r, of the LF equation of state is very large, leading the fourth term in eq 2 to be zero. Therefore, the reduced density does not change significantly, and for this reason, the predicted glass transition temperature depression is not affected by the polymer molecular weight. On the other hand, for low molecular weight polymers (below 2000 Da), the glass transition behavior is strongly dependent on the molecular weight. Interestingly, it can be concluded that the prediction of the model representation of the Tg depression behavior for high molecular weight polymers cannot be affected by the molecular weight distribution of the polymers. The thermodynamic approach used in this work is able to describe four different types of Tg versus pressure behavior.8 For sake of clarity in Figures 6 and 7, the glass transition temperature depression behaviors for the systems CO2-PMMA and CHF3-PS for different values of the binary interaction parameter are reported. Similar results were also obtained by Condo et al.8 for the system CO2-PMMA using a different lattice coordination number. Condo et al. classified four types of behavior. For the system CO2-PMMA, a type I was found for low values of the interaction parameter (ζ12), i.e., low solubility of the CO2 in the polymer. A further increasing of the binary interaction parameter gives type II-IV behavior, with the retrograde vitrification typical of the plasticizer with a high solubility in the polymer.

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Figure 6. Influence of the labeled interaction parameter, ζ12, on the Tg versus P behavior for the system PMMA-CO2.14,15

Figure 8. Tg behavior as a function of the pressure for the system PS-CHF3. Points are experimental data from Uosaki et al.12 Lines are obtained by the thermodynamic model presented in this work using interaction parameter ζ12 ) 1.146.

Figure 7. Influence of the labeled interaction parameter, ζ12, on the Tg versus P behavior for the system PS-CHF3.12

Similar results, reported in Figure 7, were obtained for the system CHF3-PS:12 the value of the binary interaction parameter has a large influence on the glass transition temperature behavior also for this system. For lower values of ζ12 (1.148 and 1.150), the system shows a type I behavior, with a decrease of the Tg increasing the pressure; a very small change of the ζ12 value from 1.150 to 1.155 leads to a different behavior (type II), with a pressure maximum for temperature values below the critical temperature of the diluent. For a further increase of the binary interaction parameter (ζ12 ) 1.160 and 1.165), the system exhibits retrograde vitrification (type IV behavior), with the GTIP above the critical temperature of the plasticizer. Fitting of Experimental Data. The model was used to correlate experimental data from Kazarian et al.,9 where CHF3 was used as a plasticizer. The best fitting of the experimental data is obtained with the binary interaction parameter ζ12 set equal to 1.148, which is able to predict the type I behavior of the system examined which shows a Tg decrease as the CHF3 pressure increases (Figure 8). In Figure 9, a comparison between the experimental data obtained by Handa et al.2 and experimental data measured in this work for PPO is shown. The new experimental data extend the range of pressure experimentally investigated up to 10 MPa. These new data show the appearance of the retrograde vitrification phenomenon that was not evidenced by the data obtained by Handa et al.2 In Figure 9, the results of glass transition temperature calculations for the system CO2-PPO with three values of the binary interaction

Figure 9. Tg behavior as a function of the pressure for the system CO2-PPO. Points are experimental data: diamonds are from Handa et al.;2 triangles are experimental data measured in this work. Lines are obtained by the thermodynamic model presented in this work using different interaction parameters, ζ12.

parameter ζ12 (1.180, 1.194, and 1.200) are also reported. The model is able to fit simultaneously both sets of data and, consequently, it gives, predicting the retrograde vitrification phenomena, an overall phenomenological representation of the experimental behavior of the glass transition temperature as a function of the pressure for the system data PPO-CO2. Figure 10 shows the experimental data measured in this work and the model calculations for the system CO2-PVP K90 up to 10 MPa. The good agreement between the model calculation and the experimental data presented here is evident. The model fully justifies the presence of two experimental Tg values at 8 MPa resulting from the retrograde vitrification phenomena. Figure 11 shows the experimental data and model calculation for the system CO2-PVP K25. The Tg line as a function of the pressure is calculated using the same interaction parameters as those used in Figure 10 for the same polymer but with different molecular weight. As highlighted also by the parametric study, the model does not take into account the influence of the molecular weight on the Tg behavior for polymers with molecular weights above 5000 Da. On the other hand, the experimental data reported in Figures 10 and 11 show small differences on GTIP for the two polymers. Figure 12 shows the experimental data and model calculation for the system CO2-P(VP-VA). Interest-

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ments are the absorption data. However, using the LF model, correlation of sorption data would be possible only by introducing a temperature dependence on the interaction parameter. In this respect, a better phenomenological description of the absorption and Tg behavior as a function of the pressure can be obtained using a more accurate equation of state. Conclusions

Figure 10. Tg behavior as a function of the pressure for the system CO2-PVP K90. Points are new experimental data. Lines are obtained by the thermodynamic model using two interaction parameters, ζ12.

New experimental data are measured for the systems PPO, PAA, and P(VP-VA) using gas chromatographic techniques. A thermodynamic model that couples the Sanchez and Lacombe equation of state and the GibbsDiMarzio criterion of zero entropy at the glass transition was adopted in order to obtain the glass transition temperature behavior prediction (or equivalently the Tg depression as a function of the pressure of the diluent) for the measured systems. In addition, the parametric study of the model highlights the influence of the lattice coordination number and polymer molecular weight on the Tg depression line. For PAA, the model is able to predict a type I behavior with a Tg decrease as the plasticizer pressure increases both in the presence of CO2. The other measured systems exhibit a type IV behavior. The phenomenon of the retrograde vitrification observed for PPO, PVP, and the copolymer P(VP-VA) justifies the presence of two Tg experimental values founded experimentally with the high-pressure chromatography corresponding to a fixed pressure value. Acknowledgment

Figure 11. Tg behavior as a function of the pressure for the system CO2-PVP K25. Points are experimental data measured in this work. Lines are obtained by the thermodynamic model presented in this work using three interaction parameters, ζ12.

Figure 12. Tg behavior as a function of the pressure for the system CO2-P(VP-VA). Points are experimental data measured in this work. Lines are obtained by the thermodynamic model presented in this work using the interaction parameters, ζ12 ) 1.134.

ingly, in the case of this copolymer, the model fairly represents the experimental data predicting a type III behavior. It is worthwhile to note that the model is very sensitive to the value of the interaction parameter, and for complete prediction of the glass transition temperature behavior, the interaction parameter should be estimated from other experimental information. The most commonly available and easy to obtain measure-

The authors acknowledge MURST for financial support. Literature Cited (1) Condo, P. D.; Johnston, K. P. In situ measurement of the glass transition temperature of polymers with compressed fluid diluents. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 523. (2) Handa, Y. P.; Lampron, S.; O’Neill, M. On the plasticization of poly(2,6-dimethyl phenylene oxide) by CO2. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 2549. (3) Handa, Y. P.; Kruus, P.; O’Neill, M. High-pressure calorimetric study of plasticization of poly(methyl methacrylate) by methane, ethylene, and carbon dioxide. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 2635. (4) Zhang, Z.; Handa, Y. P. In situ study of plasticization of polymers by high-pressure gases. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 977. (5) Kazarian, S. G. Polymer processing with supercritical fluids. Polym. Sci., Ser. C 2000, 42, 78. (6) Bos, A.; Pu¨nt, I. G. M.; Wessling, M.; Strathmann, M. CO2induced plasticization phenomena in glassy polymers. J. Membr. Sci. 1999, 155, 67. (7) Hwang, Y. D.; Cha, S. W. The relationship between gas absorption and the glass transition temperature in a batch microcellular foaming process. Polym. Test. 2002, 21, 269. (8) Condo, P. D.; Sanchez, I. C.; Panayiotou, C.; Johnston, K. P. Glass transition behavior including retrograde vitrification of polymers with compressed fluid diluents. Macromolecules 1992, 29, 6119. (9) Kazarian, S. G.; Brantley, N. H.; West, B. L.; Vincent, M. F.; Eckert, C. A. In situ spectroscopy of polymers subjected to supercritical CO2: plasticization and dye impregnation. Appl. Spectrosc. 1997, 51, 491. (10) Alessi, P.; Cortesi, A.; Kikic, I.; Vecchione, F. Plasticization of polymers with supercritical carbon dioxide: experimental determination of glass transition temperatures. J. Appl. Polym. Sci. 2003, 88, 2189.

Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003 3029 (11) Kikic, I.; Vecchione, F.; Elvassore, N. Thermodinamic analysis of the effect of supercritical fluids on the glass transition temperature. Proceedings of the 2nd International Meeting on High-Pressure Chemical Engineering, Hamburg, Germany, 2001. (12) Braun, J. M.; Guillet, J. E. Study of polymers by inverse gas chromatography. In Advances in polymer science; SpringerVerlag: Berlin, 1976. (13) Uosaki, Y.; Moriyoschi, T. Glass transition behavior of polystyrene in compressed gas. Proceedings of the 7th Meeting on Supercritical Fluids, Particle DesignsMaterials and Natural Products Processing, Perrut, M., Reverchon, E., Eds.; Antibes/ Juan-les-Pins, France, 2000; Vol. 1, p 355. (14) Alessi, P.; Cortesi, A.; Kikic, I.; Vecchione, F. Experimental determination of glass transition temperatures: influence of supercritical carbon dioxide. Proceedings of the 7th Meeting on Supercritical Fluids, Particle DesignsMaterials and Natural Products Processing, Perrut, M., Reverchon, E., Eds.; Antibes/ Juan-les-Pins, France, 2000; Vol. 1, p 323. (15) Alessi, P.; Cortesi, A.; Kikic, I.; Vecchione, F. Plasticization of polymers with supercritical carbon dioxide: experimental determination of glass transition temperatures. Proceedings of the 6th Conference on Supercritical Fluids and Their Applications; Reverchon, E., Ed.; Maiori, Italy, 2001; p 449. (16) Sanchez, I. C.; Lacombe, R. H. An elementary molecular theory of classical fluids. Pure Fluids. J. Phys. Chem. 1976, 80, 2352.

(17) Smith, B. D.; Srivastava, R. Thermodynamic data for pure compounds Part B: Halogenated hydrocarbons and alcohols; Elsevier: Amsterdam, The Netherlands, 1986. (18) Prinos, J.; Panayiotou, C. Glass transition temperature in hydrogen-bonded polymer mixtures. Polymer 1995, 36, 1223. (19) Fermeglia, M.; Pricl, S. Molecular dynamics prediction of PVT behavior and determination of related thermophysical properties of pure polymers. Proceedings of the AIChE Topical Conference on Applying Molecular Modeling and Computational Chemistry, Cox, K. R., Cummings, P. T., Westmoreland, P. R., Eds.; Miami Beach, FL, 1998; p 343. (20) Doghieri, F.; Sarti, G. Nonequilibrium lattice fluids: a predictive model for the solubility in glassy polymers. Macromolecules 1996, 29, 7885. (21) Wissinger, R. G.; Paulaitis, M. E. Swelling and sorption in polymer-CO2 mixtures at eevated pressures. J. Polym. Sci., Part B: Polym. Phys. 1987, 25, 2497. (22) Wissinger, R. G.; Paulaitis, M. E. Glass transitions in polymer/CO2 mixtures at elevated pressures. J. Polym. Sci., Part B: Polym. Phys. 1991, 29, 631.

Received for review December 2, 2002 Revised manuscript received March 25, 2003 Accepted March 28, 2003 IE020961H