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Polymer/Solvent Equilibrium Measurements via Vapor-Phase Infrared Spectroscopy John M. Zielinski,* Brian T. Carvill, Sharon A. Gardner, Michael F. Kimak, Robert Horvath, and Johanna E. Rovira Air Products & Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195
A new analytical technique based on vapor-phase infrared (IR) spectroscopy has been developed to measure vapor-liquid equilibria in binary and multicomponent polymer/solvent systems. A leak-tight glass vial and heater assembly have been designed to enable in situ IR measurements at conditions of experimental interest, and appropriate calibration curves have been measured. Data obtained by this technique are quantitatively compared with toluene/poly(vinyl acetate) and methanol/poly(vinyl acetate) binary data measured by standard gravimetric analysis. Introduction Reliable vapor-liquid equilibrium (VLE) data are essential for the design and optimization of industrial polymer processing unit operations.1 These data are generally required over a wide range of experimental conditions in which temperature, pressure, and concentration are varied. Most experimental investigations use conventional gravimetric2 or volumetric3 techniques to acquire accurate VLE data. These techniques are, however, normally limited to low pressures (P < 1 atm) and almost always restricted to binary systems composed of one polymer and one solvent. Information of practical importance generally involves multicomponent equilibrium data from one polymer and several solvents. Standard gravimetric and volumetric techniques are also commonly constrained in terms of throughput; i.e., they are limited in their ability to generate an abundance of data in a short amount of time. These experimental measurements are conducted by exposing a polymer to a solvent, waiting until an equilibrium condition is achieved, and subsequently perturbing the system to reach a new equilibrium point. Because equilibration can take on the order of 24 h (or longer, depending on the sample thickness, temperature, etc.), experiments conducted in this fashion (in series) do not generate data very rapidly. If one considers the ongoing need for reliable and propitious process engineering data, expedient analytical techniques are continuously being sought. In this study, a new analytical technique based on vapor-phase infrared (IR) spectroscopy has been developed to measure VLE in binary and multicomponent polymer/solvent systems. This methodology allows multiple samples to be prepared at one time and facilitates VLE data acquisition at a much faster rate (minimal technician and instrument time) than by conventional methods. In addition, higher operating pressures are achievable via this method relative to standard gravimetric and volumetric techniques. Experimental Materials The reagent-grade quality solvents employed in this study (methanol and toluene) were purchased from * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (610) 481-7975. Fax: (610) 4816517.
Figure 1. Schematic representation of the vapor-phase IR experiment.
Aldrich and were used without additional purification. Poly(vinyl acetate) (PVAc) was also purchased from Aldrich [18,248-6] and had a weight-average molecular weight of 167 kDa. The 20 mL headspace gas chromatography (GC) vials that were used in this work were purchased from Kimble [60827A-2375]. Experimental Apparatus and Considerations The basic premise of the technique developed here is depicted schematically in Figure 1. Dry polymer of known mass is placed into a conventional headspace vial typically used for GC measurements. A known amount of solvent is syringed into the vial, and the vial is sealed. A Teflon disk replaces the conventional septum because sorption of solvent into a rubber septum can introduce a significant error in this experimental design. The sample cell is placed in a temperature-controlled oven to ensure equilibration. Upon removal, it is then inserted into the top of a heater assembly designed to enable in situ IR measurements at the conditions of experimental interest. An illustration of this heating assembly is presented in Figure 2. An IR spectrum of the vapor phase, measured through the walls of the glass vial, is collected, and the number of moles of solvent in the vapor phase is determined via a calibration curve. Spectra were obtained by co-adding 128 scans at 4.0 cm-1 resolution using a Nicolet Magna 750 FT-IR interferometer equipped with a deuterated triglycine sulfate (DTGS) detector. A background spectrum is obtained on an empty vial from the same lot as that used to prepare the samples. Using the above parameters, a spectrum can be measured in approximately 3 min. It immediately follows that the amount of solvent in the polymer phase can be calculated, and the pressure
10.1021/ie0005957 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/11/2001
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Figure 2. Schematic representation of the heat-block assembly used within the FT-IR sample chamber to enable in situ IR experiments at process temperatures. In addition, 0.25 in. of insulation surrounds the block to mitigate heat loss, and a metal face-plate with Teflon spacers is attached to the block to position it securely in the IR apparatus. The sample vial is inserted at the top of the heater block, and the IR beam passes through the sides.
Figure 3. IR absorbance spectra from six empty headspace GC vials, three each from two different lots.
of the solvent in the vial can be estimated by an equation of state. Although the conceptual description of this IR technique is simple, several critical experimental issues had to be considered and resolved. We discuss these issues in the following sections. Contribution of Glass to IR Spectra. Because the IR beam passes through the walls of a glass headspace GC vial, this experiment requires the systematic elimination of any contribution to infrared spectra by the GC vials themselves during analysis of the vapor-phase composition. This issue introduces two questions: (1) Does the spectrum change with the position of a vial in the sample holder? (2) Does the spectrum change with each new GC vial used? In Figure 3 we provide representative spectra of several empty headspace GC vials. A background spectrum of an open beam path was used for the evaluation of the GC vials. The results in this figure reveal that minimal variation exists in the spectra of headspace GC vials taken from a single lot number. In addition, the spectra were found to be invariant if the vials were rotated (results not shown). Significant variation does exist, however, between vials from different lots. Therefore, it is critical that a background spectrum is obtained on an empty vial from the same lot as that used to prepare the samples. This eliminates spectral effects caused by lot-to-lot variations in the glass. This finding constitutes an important result, because it reduces the overall experimental time by eliminating the need to measure a background spectrum for every sample to be examined. Transmission of the IR beam through the glass of the GC vial is, however, responsible for the predominant experimental limitation: the useful spectral range of IR detection is restricted to ∼4000 to ∼2200 cm-1
because of the absorbance characteristics of glass. This restriction reduces the number of IR bands, e.g., -OH and -CH bands, available for analysis of the vaporphase composition. While this result does not greatly impact VLE measurements for binary systems, it may hinder the analysis of multicomponent VLE for some systems because of band overlap within the available spectral range. Sample Preparation. Because of the sensitivity of the IR technique to water and CO2, samples were prepared in a helium glovebox. The GC vials, caps, and septa were predried in a vacuum oven to remove any residual moisture and then transferred, along with the solvents and syringes, into the glovebox. The solvents were contained in 2 mL GC vials sealed with a rubber septum and cap. Approximately 0.2 g of PVAc was weighed into 20 mL headspace GC vials. Appropriate amounts of liquid solvent were then drawn with 1-10 µL syringes, introduced into the vials containing the polymer, and quickly sealed. Weight measurements were collected to verify the amount of polymer and solvent introduced into the vials. Because the liquid densities of the solvents were known and a very accurate volume of solvent was drawn, the mass of solvent in the vials could be determined with precision. On the basis of this analysis, we verified that minimal solvent was lost during its introduction into the GC vials. The polymer/solvent samples were then placed in an oven maintained at 100 °C for sufficient time to ensure equilibration. Equilibration times are dependent on the sample thickness and the rate of penetrant diffusion through the polymer. On the basis of reported4-6 diffusion coefficients for the binary systems of interest here, we estimated that equilibration would occur within 24 h. This expectation was confirmed in our initial experiments by periodically sampling the system until its vapor-phase IR spectrum became invariant. Calibration Curves. IR spectra of vapor species are strongly influenced by temperature. Consequently, keeping the temperature identical for the background, calibration, and sample spectra is critical for successful analyses. To assess the vapor-phase concentration above the polymer/solvent samples, calibration curves were constructed by injecting known amounts of methanol and toluene into empty headspace GC vials, equilibrating the samples at experimental temperatures of interest (in this case 100 °C), and measuring the intensity of IR band(s) for each solvent. The amount of solvent introduced was always lower than the saturation volume. At 100 °C, this corresponds to approximately 100 µL of methanol and 55 µL of toluene. The variation of absorbance with the vapor-phase concentration is illustrated for methanol in Figure 4. IR bands in this analysis were selected by considering whether the absorbance levels were sufficiently low (typically less that 1.2 absorbance units) to maintain photometric accuracy. In the present work, the 3099.57 cm-1 peak for toluene and the 3628.1 cm-1 peak for methanol were chosen. It is important to note that identical VLE results are obtained when other component-specific peaks are selected. Calibration curves for toluene and methanol at 100 °C are presented in Figures 5 and 6, respectively. Analysis Method. The calibration curves presented in Figures 5 and 6 can be used to calculate the mass of solvent (toluene or methanol) in the vapor phase of the
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(empty) volume of the GC vial and the vapor-phase (freespace) volume and (2) the summation of all of the individual component volumes residing in the polymer phase. These considerations can be expressed as n
Vp )
Figure 4. Variation of IR absorbance intensity with methanol vapor mass density at 100 °C. Spectra shown correspond to 1050 and 70-100 µL of methanol in increments of 10 µL.
∑ i)1
mpi
+
F/i
mp F/p
) VT - Vg
Here, VT and Vg denote the total and vapor (gas)-phase volumes, respectively. The summation in eq 1 runs from 1 to n, where n represents the total number of solvents in the system. For a binary polymer/solvent system, n ) 1. Furthermore, mpi represents the mass of solvent i in the polymer phase, and F/i denotes the pure-component density of solvent i. Similarly, mp and F/p correspond to the mass and density, respectively, of the pure polymer. The mass of solvent i in the polymer phase (mpi) may also be expressed as the difference between the total amount of i introduced into the system and the mass of i residing in the vapor phase. This may be written as
mpi ) mTi - CgiVgMi
Figure 5. Calibration curve for the 3099.57 cm-1 toluene IR peak versus vapor-phase toluene mass density at 100 °C.
n
∑ i)1
Vg )
mpi
n
1-
headspace GC vials provided that the volume of the vapor phase is known. In generating the calibration curves, the vapor-phase volume is simply the entire volume of the vial, which was determined to be 20.4 cm3 by filling vials with liquids of known density and measuring the weight of liquid introduced. During the polymer/solvent VLE measurements, the vapor-phase volume is reduced because of the presence of the polymer. As a first approximation, one can compensate for this reduction of volume by subtracting the volume of the polymer from the volume of the empty vial. In doing so, one presumes that the polymer does not swell as it sorbs solvent. A more realistic approximation of the vapor-phase volume relies on a simple mass balance and assumes volume additivity of the solvent and polymer. The volume of the polymer phase (Vp) can be expressed in two ways: (1) the difference between the total
(2)
where mTi is the total mass of solvent i introduced into the system, Cgi is the vapor-phase concentration of component i (in units of mol/vol), and Mi is the molecular weight of solvent i. For a binary polymer/solvent system, the vapor phase consists of a single component. In this case, the amount of vapor in the free-space (Cg1), although designated as a molar concentration, is understood to be the molar density of the vapor phase. Substitution of eq 2 into eq 1, followed by algebraic manipulation, yields an explicit relation for the freespace volume (Vg), namely
VT -
Figure 6. Calibration curve for the 3628.1 cm-1 methanol IR peak versus vapor-phase methanol mass density at 100 °C.
(1)
+
mp
F/i
∑ i)1
F/p
CgiMi
(3)
F/p
When the vapor-phase signal intensity is measured during an IR experiment, the mass balance can be closed and the mass of solvent in both the vapor phase and the polymer phase can therefore be determined unambiguously. The next aspect of constructing an isotherm plot consists of expressing the activity (or the fugacity, fi) of component i in the vapor phase above the polymer sample. Because the vapor-phase pressures are relatively low, fi can be accurately expressed as the partial pressure (Pi) of component i. In gravimetric and volumetric experiments, the total vapor pressure is measured directly with a pressure transducer in the event that a single solvent is used. For the IR experiment described herein, the system is closed and the vaporphase pressure cannot be measured directly. Because the vapor-phase concentration can be computed from the corresponding calibration curve, we can estimate the vapor-phase pressure above the sample from an equation of state. In this work the ideal gas law, i.e., Pi ) CgiRT, was found to be sufficient. This approach is applicable even when a multicomponent vapor (or gas)phase mixture is present. The solvent activity (Pi/Pio)
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Figure 7. Comparison of toluene/PVAc VLE data measured at 100 °C by the vapor-phase IR technique and with conventional gravimetric sorption instrumentation.
can be subsequently calculated from tabulated values of the pure-component saturation vapor pressure (Pio). Results and Discussion Experimental VLE data for toluene/PVAc measured at 100 °C are presented in Figure 7 and are compared with literature data measured4 by a conventional gravimetric sorption technique. The comparison reveals excellent agreement between these two techniques. Note that two sets of IR data are presented (#1 and #2 on graph). The only difference between these IR measurements is the day when the samples were prepared and analyzed. Such favorable agreement confirms that our IR technique is reproducible and provides an accurate and expedient method of data acquisition. Because the boiling point of toluene is approximately 110 °C, the data illustrated in Figure 7 have been measured at pressures below 1 atm, where conventional gravimetric and volumetric experiments can be readily performed. To examine the robustness of the IR technique and to discern if it could be expanded to pressure regimes typically unreachable by conventional methods, we examined the binary methanol/PVAc system at 100 °C, which is above the normal boiling point of methanol (ca. 65 °C). Because most conventional gravimetric devices are limited to 1 atm of pressure, a maximum activity of about 0.29 is achievable with methanol at 100 °C. The IR data presented in Figure 8 have been acquired up to activities of approximately 0.8, which corresponds to pressures of about 2100 Torr (2.75 atm). At these high pressures, the quality of the cap seal becomes a critical consideration in collecting reliable VLE data due to potential solvent leakage. Included in Figure 8 are (i) data measured in our laboratory by gravimetric sorption and (ii) predicted sorption behavior based on the Flory-Huggins theory7,8 and values of χ derived5,6 from low-activity data. The experimental data measured at 90 and 100 °C have been obtained near the high-pressure limit of our gravimetric unit and highlight the low activity range over which data can be acquired. The sample temperature in the gravimetric analysis has been reduced to 65 °C to achieve solvent activities sufficiently high to serve as comparison points to our data. The IR and gravimetric
Figure 8. Comparison of methanol/PVAc VLE data measured at 100 °C by the vapor-phase IR technique with conventional gravimetric sorption equipment at 65, 90, and 100 °C. Predictions based on the Flory-Huggins theory and literature values5,6 of the χ parameter are included (solid line) to illustrate the strong concentration dependence of χ in this binary system.
data in Figure 8 illustrate the importance of collecting data over a wide activity range and the danger of relying on extrapolations based on low-activity-data correlations. Although not reported here, the IR technique described herein can be employed directly to measure VLE data in polymeric systems containing multiple solvents if the IR peaks for the individual solvents are clearly resolvable without interference from the other components. Even in cases wherein IR peaks do overlap, we believe that sorption isotherms are measurable insofar that at least n - 1 pure-solvent peaks can be distinguished in an n-component system by performing an appropriate analysis of peak ratios. An important consideration also not elaborated upon here is that this vapor-phase IR technique is not limited to polymeric systems; it can be readily applied to measuring sorption isotherms of IR-detectable vapors (or gases) in porous media or other solids. Conclusions A new analytical technique based on vapor-phase IR spectroscopy has been developed to measure VLE in binary and multicomponent polymer/solvent systems. The experimental technique has been validated by measuring standard sorption isotherms for toluene/ PVAc and methanol/PVAc at 100 °C. The technique has been applied up to a solvent pressure of 2.75 atm, which is significantly higher than the pressure attainable during conventional gravimetric and volumetric analyses. The pressure achievable by our IR technique is limited only by the quality of the seal made in capping the sample vials. This IR technique removes the throughput limits imposed by standard techniques because multiple samples, representing the entire sorption isotherm, can be prepared simultaneously and measured in parallel, rather than in series. Because each point along the isotherm can take on the order of a work day (or longer, depending on the sample thickness, temperature, etc.) to measure, conventional sorption experiments do not permit expedient data collection. Considering the ever-
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increasing need for reliable process engineering data in a timely fashion, faster analytical techniques such as the vapor-phase IR technique described here are continuously being sought. Literature Cited (1) For example, see: Albalak, R. J. Polymer Devolatilization; Marcel Dekker: New York, 1996. (2) Duda, J. L.; Kimmerly, G. K.; Sigelko, W. L.; Vrentas, J. S. Sorption Apparatus for Diffusion Studies with Molten Polymers. Ind. Eng. Chem. Fundam. 1973, 12 (1), 133-136. (3) Surana, R. K.; Danner, R. P.; de Haan, A. B.; Beckers, N. New technique to Measure High-Pressure and High-Temperature Polymer-Solvent Vapor-Liquid Equilibrium. Fluid Phase Equilib. 1997, 139, 361-370. (4) Vrentas, J. S.; Duda, J. L.; Ling, H. C.; Hou, A. C. Free-
Volume Theories for Self-Diffusion in Polymer-Solvent Systems. II. Predictive Capabilities, J. Polym. Sci., Polym. Phys. Ed. 1985, 23, 289-304. (5) Vrentas, J. S.; Duda, J. L.; Hou, A. C. Evaluation of Theories for Diffusion in Polymer-Solvent Systems. J. Polym. Sci., Polym. Phys. Ed. 1985, 23, 2469-2475. (6) Kishimoto, A. Diffusion and Viscosity of Polyvinyl AcetateDiluent Systems. J. Polym. Sci., Part A 1964, 2, 1421-1439. (7) Flory, P. J. Thermodynamics of High-Polymer Solutions. J. Chem. Phys. 1942, 10, 51-61. (8) Huggins, M. L. Theory of Solutions of High Polymers. J. Am. Chem. Phys. 1942, 64, 1712-1719.
Received for review June 19, 2000 Revised manuscript received August 28, 2000 Accepted August 30, 2000 IE0005957
