In Situ Preparation and Evaluation of Open Pore Polyurethane Chromatographic Columns F. D. H i l e m a n and R. E. S i e v e r s l Aerospace Research Laboratories, ARLILJ, Wright-Patterson Air Force Base, Ohio 45433
G. G. Hess Wright State University, Dayton, Ohio 45437
W . D. Ross Monsanto Research Corporation, Dayton, Ohio 45407
The preparation and properties of chromatographic columns composed of in situ formed precipitated open pore polyurethane (OPP) have been examined. The columns are filled with a solvent containing the monomer precursors, and after the polymerization occurs the solvent is removed from the rigid porous polymer. Electron photomicrographs reveal that the polymer consists of highly uniform spherules with diameters in the micron range, yet the matrix of agglomerated spherules is highly permeable. The density of the porous precipitated polyurethane was varied over a range of 0.10 to 0.24 g / c m 3 by changing the conditions of polymerization. The optimum density is ca. 0.15 g/cm3. From 5 to 5 0 % by weight of a liquid phase can be incorporated in the polymer matrix, or the columns can be used without liquid phases. Separations of several classes of compounds including alcohols, metal chelates, aromatics, etc., were effected. While most of the work was done with ordinary 4-mm i.d. glass columns, polymer-filled small bore (0.56- to 0.86-mm i.d. X 100 ft) columns with high sample capacities were also prepared.
Very effective chromatographic solid supports have been formulated from polymeric materials. Hollis ( I ) initiated the use of polystyrene as a chromatographic support, leading to the introduction of Porapak. Others have prepared foam-filled columns (2, 31, and Ross and Jefferson ( 4 ) first prepared open pore precipitated polyurethane columns resulting in several subsequent publications and patents (5-9). The precipitate consisted of a homogeneous conglomerate of microscopic spheres bonded to each other and to the walls of the column. Such columns can now be prepared in a great variety of configurations and sizes, in'Author to whom reprint requests should be sent Hollis, Ana/. Chem.. 38, 309 (1966). ( 2 ) J . J . VanVenrooy, U.S. Patent 3,347,020,Oct. 17, 1969. (3) H . Schnecks and 0. Bieber. Chromatographia, 4, 109 (1971). ( 4 ) W. D. Ross and R . T. Jefferson, J. Chromatogr. Sci.. 8, 386 (1970). (5) I . 0. Salyer, J . L. Schwendeman. and R. T. Jefferson, "Polyure( 1 ) 0. L.
thane Precipitation Foam." presented at the 156th National Meeting. American Chemical Society, Atlantic City, N.J., Sept 8-14, 1968. (6) W. D. Ross and R. T. Jefferson, J. Chromatogr. Sci., 8, 386 (1970). ( 7 ) R . T. Jefferson and I . 0. Salyer, U.S. Patent 3,574,160, issued April 6. 1971. ( 8 ) I . 0. Salyer, R . T. Jefferson, and W. D. Ross, U.S. Patent 3.580.843. issued May 28, 1971 ( 9 ) I . 0. Saiyer, R . T. Jefferson, J . V . Pustinger, and J . L. Schwende-
man. "Preparation and Properties of Open-Pore Polyurethane" presented at the 163rd National Meeting, American Chemical Society, Boston, Mass., April 1972.
1126
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 7, JUNE
1973
cluding capillary types. For uncoated polyurethane gassolid behavior is exhibited; upon the addition of a suitable liquid phase the properties are markedly altered and typical gas-liquid behavior results. This study was aimed a t determining the conditions under which columns of varying permeability can be prepared and evaluating the properties of these columns.
EXPERIMENTAL Materials. Polyurethanes are produced by step-growth polymerization of polyisocyanates and polyols. The mode of propagation involves the addition of a hydroxy group to an isocyanate to yield a substituted amide ester of carbonic acid (polyurethane). H RNCO
+
R'OH
O
I II
R-N-C-OR'
It is well known that the active hydrogen of the polyurethane may continue to react to form a relatively insignificant amount of the allophanate structure. In this research, the isocyanate was Kaiser Chemical's NCO-10, a mixture of 4,4'-diphenylmethanediisocyanate with lesser amounts of the tri-, tetra-, and pentaisocyanates, having the formula
NCO-10 contains an average functionality of -2.3 NCO groups and has an equivalent weight of 132-135. Special care in handling this reagent is necessary since it reacts with moisture in the air and solvents. The pure material was stored in a desiccator and all solvents were dried with molecular sieves. In the preparation of columns, the pure isocyanate was weighed out, diluted in solvent, and used immediately. This retarded deterioration which would unbalance the stoichiometry of the polymerization reaction. The poly01 used was Union Carbide's LA-475, principally a pentahydroxy compound resulting from the total oxypropylation of diethylenetriamine. OH
I
CH3CHCH?
I
OH
I
LA-475
LA-475 contains a n average functionality of -3 and has a molecular weight of approximately 590. The tertiary amine backbone present in this poly01 serves as a self-contained catalyst for the polymerization reaction.
Table I. Characteristic Parameters for I-Meter Polyurethane a n d Chromosorb W Columns Permeability. Theor Dlate Columntype cm2 X i o - ' Porosity. % N0.O ResolutionE Polyurethane dcm' 0.106 0.130 0.154 0.178 0.198 0.243 Chromosorb W mesh size 40/60 60180 80/100 roo/r2o
16.0 6.26 2.92 1.14 1.os 0.44
90.9
b
b
88.2
483 772
4.30
800 700
5.50 5.14 4.65
8.86
89.0 87.6
87.0 85.5 84.5 75.8
4.71 3.14 2.71
5.13
602
Figure 1. Electron photomicrograph of open pore polyurethane
88.6
85.5
0 For dodecane. Serious channeling. Average Of resolution between three pa,rs of n-hydrocarbons In-Ctt and n-C?s.n-Cir and n-C?s. n-C>> and o - C , r l .
The solvent for the polymerization reaction is a toluene-carbon tetrachloride mixture of 60:40 volume per cent, respectively. In this mixture both reactants are soluble hut the polymer precipitates readily. The solvent density, 1.17 g/ml, matches the precipitate density. thus preventing any settling effects in the column. After polymerization was complete, the volatile solvents were easily removed from the polyurethane. Column Preparation. Stoichiometric amounts of poly01 and isocyanate in equal volumes of the dried solvent were mixed together and then injected into 4-mm i.d. glass columns using a 50-cm3 Plastipak syringe. In general, precipitation did not begin for 10 to 15 mi", which allowed time for the very long columns t o be filled. Once the column was filled, small pieces of Tygan tubing on the ends were clamped shut and the column was rotated a t 5 rpm for 18 hr a t room temperature to ensure uniformity. After forcing out the solvent with dry nitrogen, the column was placed in a gas chromatography oven with the detector end of the cotumn disconnected and was conditioned a t 1M) "C with gas flow far 24 hr. A modification of this procedure was adopted for flexible tubing such as 3.2-mm capper or stainless steel. The column was filled vertically and allowed to stand for 18 hr without rotation. Then it was coiled to m y desired shape and the solvent was subsequently purged, leaving the dry polyurethane. When the solvent has been removed the polymer is quite rigid and resists mechanical deformation. However, later addition of solvent C B U S ~ Sthe polymer to become flexible again and columns can he bent to new configurations ifdesired. A designation scheme was devised based on the bulk density of polyurethane. The calculation was made in the following manner: Material
Weight
LA-475 NCO-10 Solvent
4.20 4.70
8.90 grams
wt per unit volume = 8.90 g polymer/57.9 ml
volume
4.1 3.8
50.0 57.9 ml 0.154 g/mi
Such a column would he designated as a 0.154 type; this represents the theoretical density assuming quantitative polymerization and negligible changes in volume on mixing. Measurements were made to determine how well the calculated theoretical density agreed with the actual density. Agreement was excellent (within 2% of the expected density for the more dense preparations and within 10% for the less dense). This scheme has the advantage over previously used weight per cent designations in that it is essentially independent of the nature ofthe solvent system. Apparatus. The equipment used included a Hewlett-Packard Model 810 gas chromatograph equipped with a flame ionization and thermal conductivity detector. A Hewlett-Packard Model 402
Figure 2. Electron photomicrograph (higher magnification)
Of
open pore poiyurethane
gas chromatograph with a eJNi electron capture detector wa8 also used for the study of metal chelates. Flow rates between 25 and 80 ml/min were used. Heat stability tests were carried out on a Du Pont Model 950 thermogravimetric analyzer. Permeability and porosity studies were performed with a pressure drop apparatus consisting of a pressure tank with a two stage regulator, a manometer for reading inlet pressure, a barometer for reading outlet pressure. and a soap bubble flow meter. The porosity percentages indicated represent the total unfilled space in the polymer rather than the interparticle porosity.
RESULTS AND DISCUSSION Table I lists the permeability, porosity, theoretical plate n u m b e r , and resolution for a series of 1-m polyurethane columns of varying hulk density. For comparison, an abbreviated listing is given for a series of Chromosorb W columns. Densities of polyurethane below 0.13 g/cm3 result in poorly filled c o l u m n s w i t h serious voids and cbanneling. Densities of polyurethane much over 0.18 g/cm3 give columns w i t h very low permeability and reduced efficienc y in plate n u m b e r and resolution. Thus intermediate concentrations of 0.15 to 0.18 g/cm3 are ideal for m o s t work. To improve the permeability of the columns when long capillaries are prepared, t h e 0.13 g/cm3 concentration is used. ANALYTICAL CHEMISTRY, VOL. 45, NO. 7. JUNE 1973
1127
1.0-
Table 11. Relative Retention Data (uncoated polyurethane) Compound
2-Propanol Ethanol Methanol
cc14
CHC13 CH2C12
Relative retention time
Boiling point, "C
1.o
82.5
2.0 9.6 1 .o 1.05 1.6
78.3 64.5 7.7 61 40
> E
LJ v,
B
v,
0.5-
6 0
C h
8
Y E
CIS
TRANS
Figure 3. Geometrical isomers of Cr(tfa)3 where the ligand is the anion of 1 , l,1-trifluoro-2,4-pentanedione and M equals chromium
The precipitated polyurethane consists of a homogeneous conglomerate as shown in the electron photomicrographs in Figures 1 and 2. The microscopic spheres are bonded together and adhere to the walls of the column to form a rigid yet highly permeable support. The diameter of the spheres is quite uniform; in the specimen shown the spherules have a much narrower particle size range than can readily be obtained by other techniques. In Figure 2, all of the spherules measure very close to 4 in diameter. Spherules with different diameters from about 1 to 10 can be generated by varying the reaction conditions. By heating the solution of monomers or using catalysts one can obtain smaller spherule sizes. I t should be noted that in spite of the small "particle" size, the permeability of the polyurethane columns is comparable to Chromosorb W columns and is substantially better than micron-range silica columns used in liquid chromatography. High permeability may be a result of the excellent uniformity of the spherules and the fact that they are held rigidly in place since they are bonded to neighboring spherules a t points of contact. The bulk material is rigid and mechanically strong, resisting pressure packing and blockage characteristic of other materials. Another advantage over siliceous supports offered by polyurethane is the absence of highly reactive Si-OH sites with concomitant elimination of the need for silanization. The temperature stability of the polyurethane was measured on a thermogravimetric analyzer. Decomposition of the polymer becomes apparent around 200 "C. For normal work, a sustained upper temperature limit of 160 to 170 "C was used. In short term temperature programmed runs, an upper limit of 200 "C can be used. Separations on uncoated polyurethane columns involve gas-solid adsorption. This was demonstrated in a study of the peak shapes and retention times of dodecane with varying sample sizes. With larger samples, one observes the increased tailing and decreasing retention times typical of gas-solid chromatography (10). A problem with gassolid adsorption is that symmetrical peaks are obtained only with sample sizes less than 2 x 10-7 gram. Overloading becomes a problem with sample sizes over 4 X 10-4 gram. (10) A. B. Littlewood. "Gas Chromatography,'' Academic Press, New York, N.Y.. 1963, p 12 and 42.
1128
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 7, J U N E 1973
0
I
I
I
2
4
6
RETENTION TIME,
rnin.
Figure 4. G C separation of geometrical isomers of Cr(tfa)3 on uncoated OPP column; 4-rnm i.d. Flow rate: He, 40 ml/rnin
A valuable consequence of the gas-solid behavior is the fact that the efficiency of the OPP column does not decrease with increasing flow rate over a very wide range once the minimum HETP has been obtained. The van Deemter plot showed no loss in plate number with flow rates as high as 150 ml/min. This behavior corresponds to a small mass transfer term in the van Deemter equation in gas-liquid chromatography. Thus equilibrium in the adsorption process is achieved very rapidly, suggesting potential usefulness in high speed chromatography. In general, nonpolar compounds elute in order of increasing boiling point. Polar compounds containing protons capable of hydrogen bonding are broadened and eluted in the order shown in Table 11. This elution order results from the abundant tertiary nitrogen atoms and carbonyl groups which make the uncoated polyurethane very susceptible to hydrogen bonding. Since 2-propanol is eluted before ethanol or methanol, it might be possible to do trace analyses of higher alcohols in the presence of large amounts of ethanol or methanol using OPP columns. The polar nature of OPP can be used advantageously in the separation of geometrical isomers of various metal chelates. As an example, the cis and trans isomers of Cr(tfa)a (Figure 3) were completely resolved on a 10-in. uncoated polyurethane column with a total analysis time of less than 6 min (Figure 4). This separation can be effected much more readily on OPP columns that has been possible previously using conventional columns (11, 12). If a less polar column is desired, a liquid phase can be added either by combining the liquid with the reagents prior to polymerization, or by coating the column after polymerization. The coated columns no longer show gassolid behavior and have greater sample capacity, with efficiencies of 300-400 plates per foot. Figure 5 shows a series of metal chelates separated on a column coated with 10% DC-550. Due to the modifying effect of the nonpolar (11) R. E. Sievers, B. W. Ponder, M. L. Morris, and R. W. Moshier. Inorg. Chem., 2, 693 (1963). (12) R . W. Moshier and R . E. Sievers. "Gas Chromatography of Metal Chelates," Pergamon Press, Oxford, 1965.
Be (tfa),
BENZENE I-XYLENE
3LUENE
-
I
IL 0 - X Y LE N E
L 4 8 12
0
I
1
I
I
8 12 Minutes Figure 5. GC separation of four metal chelates on OPP coated with 10% DC-550; 1-mm i.d. X 106 cm. Flow rate: He, 25 ml/min. Col. temp., 136°C
0
n-PROPANOL
O
4
7-BUTANOL
b
W
4
Minutes
Figure 7. G C separation of aromatic hydrocarbons on OPP coated with Carbowax 400; 4-mm i.d. X 1 m. Flow rate: He, 40 ml/
min. Col. temp., 110 "C
For the optimum gas-liquid performance, the weight percent of liquid phase must be greater than 5%. At lower percentages very small samples overload these columns. At the other extreme, liquid phase loadings greater than 50% result in decreased column efficiency due to the large mass transfer term in the van Deemter equation. Small bore columns have been prepared from 0.86-mm and 0.56-mm i.d. Teflon. Since the polyurethane does not adhere to the Teflon as it does to glass or metal, the result is a core-filled tube with gas flow principally around the outer perimeter of the core. Using the 0.13 g/cm3 density polyurethane, 0.86-mm i.d. tubing was filled in lengths up to 30.5 m. Tubing of 0.56-mm i.d. could be filled only in lengths up to 15 m because of problems encountered in forcing the solution through the capillary. It was necessary to include a liquid phase in the small bore columns because of the low capacity of the uncoated polyurethane. With liquid phase loadings of 10 to 20%, good separation was obtained for a variety of compounds with a capacity similar to 3.2-mm (l/s-in.) packed columns.
Minutes
Figure 6. GC separation of c 3 - C ~normal alcohols on OPP coated with Carbowax 400; 4 - m m i.d. X 1 m. Flow rate: He, 55 ml/ min. Col. temp., 100 "C
liquid phase, the cis and trans isomers of Cr(tfa)3 are no longer separated. Here again the separation of metal chelates on O P P columns is superior to that with conventional columns (11, 12). On a column coated with Carbowax 400,methanol and ethanol are eluted in the order of their boiling points, rather than the inverted order observed on the uncoated column. In Figure 6, the separation of several alcohols on a polyurethane column coated with Carbowax 400 is depicted. Very symmetrical peaks were obtained for all of the alcohols studied. Figure 7 illustrates the facile separation of a mixture of aromatic hydrocarbons.
CONCLUSIONS The uniform size of the polyurethane spherules makes the OPP an excellent support. Column permeability can be varied by changing the monomer concentrations and reaction conditions. The optimum density of OPP appears to be ea. 0.15 g/cm3. Even though the spherule size is in the micron range, the OPP columns have permeabilities comparable to Chromosorb W. Although most of this study was aimed a t examination of OPP as a gas chromatographic material, it is clear that it will also be useful in liquid chromatography. Low pressure drops and good mechanical integrity are especially attractive features. Almost any length, size, or shape of column can be easily filled with a uniform porous solid. The diameter of the spherules can be varied by changing the conditions under which the polymerization occurs. At higher temperatures or in the presence of catalysts, smaller spherule diameters result. The good adheANALYTICAL C H E M I S T R Y , VOL. 45, NO. 7, JUNE 1973
1129
sion of the polyurethane to glass and metal prevents channeling along the column walls. If the uncoated polyurethane is bsed, advantage can be taken of the efficiency of the column a t high flow rates. For example, flow programming can be carried out without significant loss in plate number and high speed analysis a t high flow rates with resulting short retention times should be possible. The support can be tailored to many uses with the addition of appropriate liquid phases. Incorporation of the liquid phase in the precursor reagents has the advantage of giving uniform phase distribution. If the liquid phase contains groups capable of reacting with the isocyanate such as hydroxyls, then a bonded liquid phase can be obtained. This should offer the advantage of low column bleed, even for low molecular weight liquid phases. The effective separation of mixtures of metal chelates, alcohols, aromatics, and other compounds with OPP col-
1130
A N A L Y T I C A L C H E M I S T R Y , V O L . 45, NO. 7, J U N E 1973
umns has been demonstrated. The raw materials are relatively inexpensive, an important consideration with increasing cost awareness. The absence of reactive Si-OH groups characteristic of siliceous supports is also noteworthy. Presently under examination is the use of OPP columns as adsorbents to preconcentrate compounds present a t very low levels for trace analysis.
Note added in proof. Kaiser Chemical Co. has recently discontinued manufacture of NCO-10; however, similar diisocyanate products are available from other sources ( e . g . , Mondur MR from Mobay Chemical Co.) and polymerfilled columns can be obtained from Analabs, Inc., Korth Haven, Conn. Received for review November 2 2 , 1972. Accepted February 2,1973.
