J. Phys. Chem. 1994,98, 4055-4060
4055
Solubility of Poly (ethylene-co-acrylicacid) in Low Molecular Weight Hydrocarbons and Dimethyl Ether. Effect of Copolymer Concentration, Solvent Quality, and Copolymer Molecular Weight Sang-Ho Lee, Minna A. LoStracco, Bruce M. Hasch, and Mark A. McHugh' Department of Chemical Engineering, Johns Hopkins University, Baltimore, Maryland 21 218 Received: September 8, 1993; In Final Form: January 6, 1994'
Cloud-point data to 260 OC and 2600 bar are presented for four different solvents, propane, butane, butene, and dimethyl ether (DME), with polyethylene (PE) and four poly(ethy1ene-co-acrylic acid) copolymers (2.4, 3.9, 6.9, and 9.2 mol 9% acrylic acid). The pressureconcentration isotherms measured for PE-DME and poly(ethy1ene-co-3.9 mol 5% acrylic acid)-butene mixtures have broad maximums that range between 5 and 10 wt %. The cloud-point curves for the acid copolymers in the olefinic and paraffinic hydrocarbons dramatically increase in pressure to as high as 2600 bar with decreasing temperature, especially below 180 OC where intraand interpolymer acid dimerization occurs. All of the cloud-point curves in the olefinic solvents are located at lower temperatures and pressures than the curves in the analog paraffinic solvents. The cloud-point curves for the acid copolymers in D M E are all below 600 bar as a result of the hydrogen bonding between the acrylic acid units and DME. The P E cloud-point curves for the paraffins and olefins all have positive slopes and are located at pressures below 800 bar. In contrast, the PE-DME curve has a negative slope that increases rapidly in pressure at temperatures below 140 OC where polar DME-DME interactions dominate the phase behavior. Cloud-point curves in butene are affected more by the acid content of the copolymer than by molecular weight. At pressures greater than 1000 bar, the curves shift to higher temperatures by approximately 26 OC/mol 5% acid compared with 1.7 OC/lO 000 Mn, up to an M, of 132 000. Also, the curves isobarically shift to higher temperatures with increasing M , rather than M,, indicating that the phase behavior may be more sensitive to the number rather than the weight of acid groups in the backbone. N
Introduction It is well-known that the carboxylic acid copolymers intra- and interpolymer hydrogen bond, and that this characteristic greatly influences the properties of the acid copolymers and the solution properties of acid copolymer-solvent mixtures. The primary tools used to investigate the properties of acid copolymers and their solutions include X-ray diffraction, IR spectroscopy, dynamicmechanical measurements, differential scanning calorimetry, and viscometry. MacKnight and colleagueslq2demonstrate that acrylic acid groups primarily form dimers at room temperature and that the dimer formation decreases substantially at temperatures above 160 OC. They report that increasing the amount of acid in the polymer backbone decreases the crystallinity of the copolymer and that the acid repeat units reside primarily in the amorphous regions of the copolymer below its melting temperature. Otocka and Kwei,3 who also studied the impact of acid content on the pure component properties of acid copolymers, show that the enthalpy of acrylic acid dimerization of the repeat units in the backbone of the copolymer, 1 1.5 kcal/mol of dimer, was almost identical with the value for the dimerization of acetic acid in a nonpolar ~ o l v e n t .Also, ~ the dimerization was reversible, suggesting that it occurs mainly in the amorphous region of the copolymer. Chang and Morawetz5 report that methacrylic acid repeat units in styrene-methacrylic acid copolymers preferentially form intramolecular hydrogen-bonded acid dimers at room temperature. They conclude that the hydrogen bonding between acid segments depends more on the acid content in the copolymer rather than on the overall copolymer concentration in solution. Longworth and Morawetz6 also find that the enthalpy of hydrogen bonding of methacrylic acid groups in poly(styrene-co-methacrylic acid) copolymers is similar to that for pivalic acid, the monomer analog for the methacrylic acid repeat unit. In this paper we investigate the effect of acrylic acid content on the phase behavior of poly(ethy1ene-co-acrylic acid)-
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* Author to whom correspondence should be sent.
Abstract published in Advance ACS Abstracts, March 15, 1994.
0022-3654/94/2098-4055$04.50/0
hydrocarbon solvent mixtures. The acrylic acid content in the backbone of the copolymers is zero (PE), 2.4 mol % (EAAg8,2), 3.9 mol % (EAA96/4), 6.9 mol % (EAA93/7), and 9.2 mol % (EAAgIp). Table 1 lists the physical properties of these copolymers. The melting temperature and crystallinity of the copolymers decrease with increasing acid content, in agreement with the trends reported in the literature.2.3 Fractionated samples of one particular acid copolymer, EAAwpa, are used to determine the impact of copolymer molecular weight and polydispersity on the phase behavior. Table 2 lists the physical properties of the parent E A A ~ ~and J ~ the A fractions obtained by fractionating with supercritical dimethyl ether.' Note that E A A ~ ~ has /~A slightly lower number and weight average molecular weights and a slightly higher molecular weight polydispersity than EAA%/4. In addition, EAA96/4A contains 4.1 mol % rather than 3.9 mol % acrylic acid. Table 2 shows that the fractionated samples are much less polydisperse than the parent material. Table 3 lists the properties of the five solvents used in this study. Theabilityofthesesolvents todissolvetheacid copolymers depends on whether they have sufficient strength to overcome acid dimerization in the backbone of copolymer. It is important to note that thecopolymers contain greater than 90 mol % nonpolar ethylene repeat units. Hence, the normal paraffins such as propane and butane should have sufficient solvent strength to dissolve these copolymers. However, at low temperatures where acid dimerization becomes prevalent, it may not be possible to dissolve the copolymers in an nonpolar solvent. The two olefinic solvents, propylene and 1-butene, have dipole and quadrupole moments that provide enhanced favorable interactions with the polar groups in the acid copolymer. The olefins are also expected to form weak *-complexes with the acid repeat units in the copolymer which should enhance copolymer solubility."JJ1 The phase behavior of the acid copolymers in the paraffinic and the olefinic solvents will be compared to that in DME. Dimethyl ether is not only polar, it also hydrogen bonds with proton donors such as the acrylic acid repeat units in the copolymer. However, DME does not hydrogen bond to itself. The location of thecloud0 1994 American Chemical Society
4056 The Journal of Physical Chemistry, Vol. 98, No. 15, 1994
TABLE 1: Physical Properties of Polyethylene and Poly(ethylene-ceacrylic acid) Used in This Study' acid crystalcontent linity Tmelt polymer (mol%) (8) ("C) M n Mw Mw/Mn 42.0 123.0 21 000 106000 polyethylene 0.0 5.1 37.5 108.0 19 100 100050 5.2 EAA9up 2.4 36.3 100.7 21 000 123 100 5.9 EAA9t.p 3.9 26.3 90.5 13 400 59 700 4.5 EAA93p 6.9 19.5 83.6 11 100 33 800 3.0 EAA9ip 9.2 a The amount of acrylic acid in the backbone of the copolymers is 2.4 mol % (EAA98/2), 3.9 mol % (EAA96p), 6.9 mol % (EAA93p), and 9.2 mol % (EAA9119). The molecular weights are based on polyethylene standards and are corrected for the acid content of the copolymer. TABLE 2 Physical Properties of the Parent and the Fractionated Samples' of EAA%/4, Used in This Study' acid crystalcontent linity Tmclt polymer (mol%) (%I ("C) M. Mw Mw/Mn E A A ~ ~ A4.1 37.0 102.5 16 600 100500 6.1 fraction 1 4.1 35.9 100.5 13 800 24 300 1.8 fraction 2 4.0 41 800 54500 1.3 fraction 3 4.0 30.4 96.9 132 200 247 200 1.9 0 The molecular weights are based on polyethylene standards and are corrected for the acid content of the copolymer.
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Figure 1. Effect of EAAMI, (3.9 mol % acrylic acid) concentration on the cloud-point curves of EAA9614-butene mixture.
point curves obtained in this study should provide insight into the impact of hydrogen bonding relative to polarity, polarizability, and weak complex formation. The influence of copolymer molecular weight and polydispersity relative to copolymer acid content will also be ascertained by performing selected experiments with fractionated acid copolymers.
OC and f0.4 OC above 200 OC. The mixture inside the cell can be viewed on a video monitor using a camera coupled to a borescope (Olympus Corp., Model F100-024-000-55) placed against the outside of the sapphire window. A fiber pipe connected to a high-density illuminator (Dolan-Jenner Industries, Inc., Model 180) and to the borescope is used to transmit light into the cell. The solution in the cell is well mixed by using a magnetic stir bar activated by an external magnet beneath the cell. The cloud-point pressure is defined as the point at which the mixture becomes so opaque that it is no longer possible to see the stir bar in the solution. The results obtained with this definition of the cloud point have been compared in our laboratories with results obtained using a laser light setup where the cloud point is defined as the condition of 90% dropoff in light transmitted through the solution. The cloud points obtained by both methods gave the same results within the reproducibility of the data. The cloud points with propane, butane, and butene are repeated at least twice a t each temperature and are typically reproducible to within f 5 bar a t the highest temperatures. In the P-T region where the cloud-point pressure increases very rapidly for a small changein temperature, thecloud points are reproducible to within f 1 0 bar. The cloud points in DME are also repeated at least twice at each temperature, and they are reproducible to within f5 bar regardless of operating temperature. Typically the lowest temperature of the cloud-point curves presented in this work represents either the highest operating pressure of the experimental apparatus or the location of the crystallization boundary.
Experimental Section
Materials
Cloud-point curves are obtained using a high-pressure, variablevolume cell, which has a 1.59-cm i.d., an 0.d. of 7.0 cm, and a working volume of -28 cm3. A 1.9 cm thick sapphire window is fitted in the front part of the cell to allow visual observation of the phases. Typically 350 f 2 mg of polymer are loaded into the cell. While being maintained at room temperature, the cell is first purged with nitrogen a t pressures of 30-50 bar and then with the solvent of interest a t 3-6 bar to remove any entrapped air which could act as a free radical initiator. Generally 6-7 f 0.020 g of solvent are transferred into the cell gravimetrically using a high-pressure bomb. The polymer-solvent mixture is compressed to the desired pressure by moving a piston located within the cell. The piston is moved using water pressurized by a high-pressure generator (HIP Inc., Model 37-5.75-60). The pressure of the mixture is measured with a Heise gauge (Dresser Ind., Model CM-108952, 0-3450 bar, accurate to within f3.5 bar). Because the measurement is made on the water side of the piston, a small correction (- 1 bar) is added to account for the pressure required to move the piston. The temperature of the cell is measured using a platinum-resistance thermometer (Thermometrics Corp., Class A) and a digital multimeter (Keithley Instruments, Inc., Model 195T, accuracy f 0.03%). The system temperature is typically maintained to within f 0 . 2 OC below 200
The polyethylene and poly(ethy1ene-co-acrylic acid) were kindly donated by DuPont Corp. Propane (CP grade, 99.0% minimum purity) was obtained from Linde Corp. Butane, I-butene, and dimethyl ether (all CP grade, 99.0% minimum purity) were obtained from MG Industries. All of the solvents were used as received.
TABLE 3 Physical and Thermodynamic Properties of the Solvents Used in This Studys9 crit crit crit dipole temp pressure density polarizability solvent ("0 (bar) (g/cm3) ( cm3) moment (D) 0.217 propane 96.1 42.5 62.9 0.08 46.2 0.236 62.6 0.37 propylene 91.9 0.228 152.1 38.0 81.4 0.0 butane 39.7 0.234 146.4 82.4 0.4 butene 0.258 53.0 52.2 1.3 dimethyl 126.8 ether
'
Results Effect of Copolymer Concentration. The effect of copolymer concentration on the location of the cloud-point curve was determined for butene and dimethyl ether (DME), two very different solvents. Figure 1 shows the effect of EAA96l4 concentration on the cloud-point behavior in butene, which is the strongest non-hydrogen-bonding solvent used in this study. The cloud-point curves for EAA96p concentrations between 5 and 10 wt 96 are virtually identical while the cloud-point curves at 10 and 22 wt % EAA9614 are situated a t progressively lower pressures from those a t copolymer concentrations below 10 wt %. Notice how the curves all increase fairly rapidly in pressure below temperatures of 160 OC. This characteristic of the curves will be more fully explored in subsequent paragraphs. The effect of copolymer concentration on the cloud-point behavior of polyethylene (PE) in DME is shown in Figure 2.
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The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 4051
Solubility of Poly(ethy1ene-co-acrylic acid)
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Although the general shapes of the PE-DME curves are superficially similar to the EAA96~4-butene curves, the curves at the highest concentration of PE do not diverge from one another at low temperatures as did the EAA96/4-butene curves. All of the PE-DME cloud-point curves are essentially parallel even in the low-temperature region of the diagram. Using the data in Figures 1 and 2, it is possible to construct pressure-concentration (P-x) isotherms of in butene, and of PE in DME at various temperatures. For example, Figure 3 shows P-x isotherms for the EAA96/4-butene at 190 and 210 O C . At these high temperatures the amount of acid dimerization should be minimalI.3 so that the acid copolymer can be considered as a moderately polar polyethylene that interacts predominantly by dispersion forces. Notice that the P-x curves of the EAA96pbutene system are essentially flat between copolymer concentrations of 5 and 10 wt %. The P-x isotherms for PE-DME at 180 and 190 O C are also shown in Figure 3. Although the maximums in the P-x curves for the PE-DME curves are not as broad as those of the EAA96~4-butenesystem, the maximums are near 5 wt % polymer in solution. However, with the PE-DME system, the increase in pressure with decreasing copolymer concentration from 20 to 10 wt % is now only 40 bar as compared to 100 bar with the EAA96p-butene system. Figure 4 shows three P-x isotherms for the EAA96p-butene system and the PE-DME system at 130,140, and 150 O C . Notice
that the pressure scale covers a much expanded range from that in Figure 3. At these lower temperatures the P-x curves for the butene system exhibit a pressure increase of up to 900 bar as the concentration of copolymer in solution decreases from 20 to 10 wt %. This is in contrast to the pressure increase in the hightemperature butene isotherms which was only 100 bar over the same copolymer concentration range. The P-x curves for the PE-DME system also shown in Figure 4 do not exhibit nearly as extreme a pressure increase as those of the acid copolymerbutene system. Since PE is nonpolar, it is reasonable to attribute the modest increase in pressure of the PE-DME isotherms to increased polar DME-DME interactions a t these colder temperatures. The rather large pressure increase in the P-x curves for the acid-butene system is more than likely a consequence of the increased intra- and interpolymer hydrogen bonding that occurs at low temperatures and low copolymer concentrations. It is well-known that a polymer-solvent P-x curve will exhibit a pressure kink if the polydispersity of the polymer is large.lz-14 However, the pressure increase in the acid copolymer-butene P-x curves in Figure 4 is much greater than that expected due to polydispersity. More work with "monodisperse" copolymer samples is in progress to resolve this issue. In the remaining sections of this paper we compare cloud-point curves obtained at a fixed copolymer concentration of - 5 wt %, the expected maximum in the P-x curves. Effect of Solvent Quality. Figure 5 shows the P-T behavior of polyethylene (PE), EAAggp (2.4 mol % acrylic acid), and (3.9 mol % acrylic acid) in propane and propylene. The intermolecular interactions between the acid copolymers and propane are expected to be predominantly via dispersion forces. The curves in both propane and propylene appear to converge to the same pressure at high temperatures. This high-temperature behavior suggests that there has been a significant decrease in the amount of hydrogen bonding between the acrylic acid units in the backbone of copolymer. As the temperature decreases below 180 "C, intra- and interpolymer acid dimerization increases and the cloud-point curves begin to increase sharply in pressure. It is not surprising to find that the dimerization of acrylic acid groups dominates the phase behavior since the energy
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The Journal of Physical Chemistry, Vol. 98, No. 15, 1994
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Figures. Cloud-pointcurvesfor polyethylene,EAA%p(2.4 mol W acrylic acid), and EAA%/)(3.9 mol W acrylic acid) in propane and propylene. The cloud-point curves for propylene were obtained by Hasch et al.15
of hydrogen bonding between acrylic acid segments (1 1.5 kcal/ mol)' is roughly an order of magnitude larger than the energy expected for dispersion (-0.5 kcal/mol).l6 In propane, thecloudpoint curve for EAA96/4 turns up a t a higher temperature and more rapidly than the EAA98p curve. This behavior is expected since the number of hydrogen bonds should be in proportion to the acrylic acid content of copolymer. It is noteworthy that these copolymers have greater than 96-98 mol % ethylene repeat units and yet they are not very soluble in propane, which readily dissolves polyethylene. Also, it is not possible to dissolve either EAA93p or EAA9119 in propane even at conditions of 240 OC and 2600 bar. The cloud-point curves for PE, EAA9812, and EAA96p in propylene have the same characteristics as the propane curves. However, the curves are shifted to lower pressures and temperatures, reflecting the increased solvent strength of propylene relative to propane. For example, a t 180 O C the cloud-point pressure for EAA96p is -730 bar lower in propylene than in propane. The acid copolymers are more soluble in propylene compared to propane due to the small dipole moment of propylene that interacts favorably with the acrylic acid groups in the copolymer. However, the shifts in the propylene cloud-point curves relative to the propane curves are much too large to be explained solely by favorable dipole interactions. We conjecture that the increased miscibility in propylene results from the formation of a weak complex between the acrylic acid units in the copolymer and the i electrons of thedouble bond in propylene. This conjecture is based on available spectroscopic studies done with alcohol-olefin and alcohol-aromatic mixtures that clearly demonstrate the occurrence of complex formation.loJ1 Unfortunately, to the best of our knowledge, there are no carboxylic acid-olefin studies available in the literature. The spectroscopic studies of T base-acid complexes in alcohol-solvent mixtures suggest that the strength of the complex can be as much as 40% of the hydrogen-bonding energy between alcohol molecules.1° Relative to dispersion forces, the strength of this complex is quite large, suggesting that ibase-acid complexing is a sufficiently strong interaction to moderate acrylic acid dimerization and significantly improve acid copolymer solubility. Therefore, the proton donor-acceptor complex that occurs in acid copolymer-
TEMPERATURE ("C)
Figure 6. Cloud-pointcurves for polyethylene,EAA98p (2.4 mol W acrylic acid), EAA96p (3.9 mol 5% acrylic acid), and EAA93/7 (6.9 mol % acrylic acid) in butane and butene. Cloud-pointcurvesfor EAA98 2 and EAA%14 in butene were obtained by Hasch, Lee, and McHugh.li
propylene mixtures should reduce the strength of the hydrogen bond between acid repeat units and, thus, make the copolymer more accessible to the solvent. Figure 6 shows the P-T behavior of polyethylene, EAA9812 (2.4 mol % acrylic acid), and EAA9614 (3.9 mol % acrylic acid) in butane. Not surprisingly, the cloud-point behavior of these three polymers in butane is very similar to that in propane shown in Figure 5. Again, the cloud-point curves in butane appear to approach the same pressure as the temperature is increased above 220 OC, and they turn up very sharply in pressure at low temperatures. Compared to those for propane, the cloud-point curves for PE and the three acid copolymers in butane are shifted to lower pressures of 200-400 bar, reflecting the increased solvent quality of butane which has a larger polarizability than propane. The cloud-point curves in butene are also given in Figure 6. These cloud-point curves have the same characteristic temperature dependence as the curves with butane. However, the one-phase regions for EAA9812 and EAA9614 in butene are larger than those in butane. For example, at 160 OC the cloud-point pressure for EAA9812 is 140 bar lower in butene than in butane. Even more dramatic is the 1450 bar decrease in the cloud-point pressure of EM9614 at 160 OC in butene relative to butane. The increased miscibility of the acid copolymers in butene relative to butane is predominantly a result of the weak i base-acid complex expected to form between the acrylic acid groups and the A electrons of the double bond in butene. The proton donor-acceptor complex combines with the large polarizability and the small dipole of butene to make EAA9317 soluble in butene, but not in butane. Figure 7 shows the P-T behavior of polyethylene, EAA9812 (2.4 mol % acrylic acid), EAA9614 (3.9 mol 76 acrylic acid), EAAg317(6.9 mol % acrylic acid), and EAA91p (9.2 mol %acrylic acid) in dimethyl ether (DME). This solvent has the smallest polarizability and the largest dipole moment of the solvents used in this study. Also, DME can hydrogen bond with acrylic acid repeat units but not to itself. The phase behavior of the acid copolymers in DME shows a very different pattern from the behavior in the low molecular weight hydrocarbon solvents. With the exception of the PE and
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Solubility of Poly(ethy1ene-co-acrylicacid)
The Journal of Physical Chemistry, Vol. 98, No. IS. 1994 4059 5,000
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