than those obtained with the samples from the meteorites, but is typical of the performance obtained with the detector in this work.
%% /
CONCLUSIONS
Evaluations of the photoionization detector developed indicated it to be a highly sensitive and discriminating device of considerable long-term stability. The end of the hollow discharge cathode was clean even after approximately 500 hours of operation. Its response to sample under routine operating conditions was almost 1000 times greater than the commercial flame ionization detector. The detector operated satisfactorily over a wide temperature range and any limitations in this respect would be due to the electrical and mechanical properties of the materials of construction. The detector was versatile and possessed linearity of response up to observed currents of about 5 x 10-7 ampere. Contrary to findings of some previous investigators with other photoionization detectors (12),the performance of the detector developed during this program was significantly influenced by detector geometry, electrode design, and operating parameters. There appear many areas in which the almost universal response and high sensitivity of the photoionization detector may make it a practical device in spite of the complexity of its supporting equipment. One obvious application is in chromatographic systems used in space probes to Mars or other bodies with low atmospheric densities. Other potential ap-
I /BENZENE
I
'15
lPRISTANE 1 J
717
L
J
I
iI
J
u
d
TIME, MINUTES
Figure 13. Chromatogram of C 1 ~ C Ihydrocarbons 7 at 150' using photoioni~tiondetector ohm 700 feet x 0.02-inch id. coated with SF-96 operating at 150" C
plications include air pollution studies and analyses in which only small quantities of sample are available. for review August 10, 1967. Accepted December 20, 1967. Work supported in part by Public Health Service Grant Number AP 00308-04 and also by AEC Contract Number AT(40-1)-3541. RECEIVED
Fractionation of Phenol-Formaldehyde Reaction Mixtures with Gel Permeation Chromatography E. J. Quinn, H. W, Osterhoudt, J. S. Heckles, and D. C. Ziegler Research and Deoelopment Center, Armstrong Cork Co., Lancaster, Pa. I7604 Gel permeation chromatography (GPC) was used to analyze the reaction products formed in a phenolformaldehyde novolak resin (CH20/phenol = 0.60) and i n a novo1ak:alkylene oxide condensate. A calibration curve was obtained relating logarithm of molecular weight with elution volume for various phenolics, polyethylene glycols, and polypropylene glycols. A linear relationship was found for t h e 94 to 2000 molecular weight range. GPC chromatograms for the novolak reaction mixture had seven distinct peaks. Fractions were obtained containing the materials that produced t h e two lowest molecular weight positive peaks. These were shown t o be phenol and a mixture of bis 2- and 4-hydroxyphenylmethane. The remaining peaks were produced by higher molecular weight compositions of phenol-formaldehyde units, each presumably a mixture of geometric or position isomers. GPC elution of a novolak: propylene oxide condensate showed seven distinct peaks. The lowest molecular weight positive peaks were isolated and shown t o be 1-phenoxy-2-propanol and 2-phenoxy-1-propanol. Thus, GPC i s a useful tool for analyzing organic and polymeric reaction mixtures and can even separate position isomers of the same molecular weight.
GEL PERMEATION CHROMATOGRAPHY (GPC) fractionation, which was first described by Moore (1) and has been rapidly adopted in polymer laboratories, is essentially one according
to the molecular size of the sample molecules in dilute solution. As such, it covers an extremely broad range of molecular weights, a range that extends from molecular weights of less than 100 to the upper permeability limit of the columns employed. An early application of GPC for the separation of low molecular weight monomers, solvents, plasticizers, or oligomers from a mixture of any of these and a polymer was the work of Bartosiewicz (2). Complex mixtures of low and high molecular weight materials abound in the coatings and chemical process industries and are successfully analyzed only after considerable effort. This communication reports the GPC fractionation of a common and interesting reaction mixture-viz., that resulting from the phenol-formaldehyde condensation to produce a novolak resin. The analysis of phenolic mixtures by GPC has been recommended by Drumm (3) and studied in some detail by Gardikes and Konrad (4). The latter authors determined the molecular weight distribution of a novolak by fractional precipitation (1) J. C. Moore, J . Polymer Sci., A2, 835 (1964). (2) R . L. Bartosiewicz, J . Paint Techno/., 39 (504), 28 (1967). (3) M. F. Drumm, American Chemical Society, Division of Organic Coatings and Plastics Chemistry, Preprints, 26 (l), 85 (1966). (4) J. J. Gardikes and F. M. Konrad, Zbid., p 131. VOL. 40, NO. 3, MARCH 1968
547
Table I. Samples Used for Calibration Study Sample Mol wt Elution vol Phenolic* 94 39.30 Phenol 0-Hydroxybenzyl alcohol 124 37.87 1.Phenoxy-2-propanol 152 38.02 Bis(2-hydroxypheny1)methane 200 36.50 Bis(4hydroxyphenyl)methane 200 35.71 2,2-Bis(4hydroxyphenyl) propane 228 35.33 306 34.10 Phenolphthalol Polyethylene glycolb 200 36.34 Carbowax 200 300 34.60 Carbowax 300 400 33.48 Carbowax 400 600 32.04 Carbowax 600 1000 30.42 Carbowax 1000 2050 27.88 DOWE-2000 Polypropylene glycolb 134 37.26 Dipropylene glycol 400 33.65 DOWP-400 1200 29.39 DOWP-1200 2000 27.84 DOWP-2000 Elution volume in counts (one count = 5 ml) as determined by peak maximum. * Elution volume in counts as determined by first moment of Q
peak.
and GPC and found that the two methods were in agreement once the GPC columns had been calibrated with appropriate novolak standards. In the present communication we examine not so much the molecular weight distribution of the resin but the fractionation and identification of the low molecular weight constituents of the reaction mixture. Other workers have examined the nature and extent of GPC separations of low molecular weight compounds and recognized the potential usefulness of GPC in this range (5-8). EXPERIMENTAL
Materials. Commercially available chemicals were used throughout this investigation. Polyethylene glycols: Carbowax 200, 300, 400, 600, and 1000 were obtained from Union Carbide Corp. and Dow E-2000 from the Dow Chemical Co. Polypropylene glycols: Dow P-400, P-1200, and P-2000 were obtained from the Dow Chemical Co. Sample numbers indicate average molecular weights as provided by the suppliers. Novolak Sample Preparation. Phenol-formaldehyde novolak resin was prepared by refluxing 94.0 grams (1.0 mole) of phenol, 18.0 grams (0.6 mole) of formaldehyde as 37x formalin solution, and 1.0 gram of oxalic acid in 10 ml of water for 45 minutes. Aqueous hydrochloric acid (1.5 ml concentrated hydrochloric acid in 10 ml of water) was added and the mixture was refluxed for an additional 90 minutes. The reaction mixture was washed several times with water. The insoluble novolak was obtained as a viscous white residue containing 7 2 x solids. Novolak-Propylene Oxide Condensate. Novolak resin prepared as above, 155.6 grams (1.0 mole calculated as a 72 % solution), was stripped under reduced pressure. Potassium hydroxide, 2.8 grams (0.05 mole), was added and the
t
1
xi
42
40
38
36
34
32
30
28
26
Ve (COUNTS)
Figure 1. Gel permeation chromatogram of a phenol-formaldehyde novolak resin: propylene oxide condensate
mixture was treated with 116.2 grams (2.0 moles) of propylene oxide at reflux temperature for 3 hours. The reaction mixture was stripped under reduced pressure at 50-60' C for 20 minutes. The final product was a viscous clear amber liquid (hydroxyl number 270 as determined by acetylation). Apparatus and Calibration. The GPC elutions were made with a Waters Associates, Inc. (61 Fountain St., Framingham, Mass.), gel permeation chromatograph (9) which had been modified in the manner described by Osterhoudt and Ray (10). A combination of five 4-foot columns was used for the sample fractionations. The permeability limits of these columns as supplied by Waters Associates were 1-3000A, 1-800A, 1-400A, and 2-100A. This combination of columns had 19,300 theoretical plates as determined by the injection of a benzene sample of concentration 0.01 gram/ml for 15 seconds. The eluent used was tetrahydrofuran. Elutions were performed at ambient temperature (ca. 25 O C ) and with a flow rate of 1 ml per minute. The columns were calibrated with the compounds listed in Table I. Three types of material-phenolic, poly(ethy1ene glycol), and poly(propy1ene glycolkare represented. For the low molecular weight phenolics, which displayed sharp and nearly symmetrical peaks, the elution volume V, of each standard was taken to be the position of the peak maximum in counts (1 count = 5 ml). This was determined with a precision of 0.03 count. The polyglycols, however, did not always yield sharp and symmetrical peaks because these samples were heterogeneous in molecular weight and the fivecolumn combination was able to separate different molecular weight species only partially. Thus, the assignment of an elution volume for a polyglycol standard was not so straightforward. A mathematical analysis (11) has indicated that when the Ve (elution volume in counts) us. log M (molecular weight) calibration plot is linear, the first moment of the GPC trace is to be correlated with the molecular weight of the sample. Here the first moment Vel is defined as
where H(Ve) is the height of the trace of elution volume V,. A plot of the values listed in Table I showed that the Ve us. log M plot is linear or nearly so over the range of elution volumes examined. Therefore, the elution volume of a polyglycol was taken to be Vel.
( 5 ) J. C. Moore and J. G. Hendrikson, J. Polymer Sci., C8, 233 (1965).
(6) J. G. Hendrikson and J. C. Moore, Zbid.,A l , 4,167 (1966). (7) G. D. Edwards and Q. Y . Ng, American Chemical Society, Division of Polymer Chemistry, Preprints, 8 (2), 1326 (1967). (8) J. Cazes and D. R. Gaskill, Separation Sci., 2 (4), 421 (1967).
548
ANALYTICAL CHEMISTRY
(9) L. E. Maley, J. Polymer Sei., 0 3 , 253 (1965). (10) H. W. Osterhoudt and L. N. Ray, Jr., Zbid., A2,5, 569 (1967). (11) H. W. Osterhoudt, Armstrong Cork Co., Lancaster, Pa., unpublished results, 1967.
4
6
i
10
ri
1'4
WAVELENGTH (MICRONS) Figure 3. Infrared spectra of phenol and fraction 1 obtained by GPC from a phenol-formaldehyde novolak resin
Ve (COUNTS)
Figure 2. Gel permeation chromatogram of a phenol-formaldehyde novolak resin GPC Fraction Collection. The GPC instrument was used as a means of obtaining fractions of the materials causing low molecular weight peaks in the chromatograms. These fractions were subsequently subjected to infrared analysis. To obtain them, a technique was devised by one of us (DCZ) for shifting the emission of 5-ml fraction from the siphon so that the center of the fraction corresponded to the center of the peak. An example of the necessity of doing this can be seen in Figure 1. Here peak A normally is split between two 5-ml fractions, numbers 38 and 39. To ensure that the peak occurs mostly in fraction 39, the sampling of a 5-ml cut must be moved ahead by the addition of approximately 0.80 ml of tetrahydrofuran to the siphon early in the elution. Because so little solute is contained in a 5.0-ml fraction, duplicate or triplicate elutions of the starting resin sample were necessary to obtain enough solute (1 to 10 mg) for infrared analysis. A fraction from the GPC was then evaporated on spectrographic grade potassium bromide. As dryness was attained the mixture was ground and potassium bromide pellets were made. Infrared spectrograms were obtained with a Perkin-Elmer, Model 137B, Infracord with sodium chloride optics, range 2.5 and 15.0 microns. RESULTS AND DISCUSSION
Phenol-formaldehyde novolak resins (CH20/phenol
