973
Ind. Eng. Chem. Res. 1992,31,973-979 Ng, H. J.; Huang, S. S. S.; Robinson, D. B. Equilibrium Phase Properties of Selected m-Xylene Binary Systems, m-XyleneMethane and m-Xylendarbon Dioxide. J. Chem. Eng. Data 1982,27,119-122. Ohgaki, K.; Katayama, T. Isothermal Vapor-Liquid Equilibrium Data for Binary Systems Containing Carbon Dioxide at High Pressures: MethanolCarbon Dioxide, n-Hexandarbon Dioxide, and Benzendarbon Dioxide Systems. J. Chem. Eng. Data 1976, 21, 53-55. Olds, R. H.; Reamer, H. H.; Sage, B. H.; Lacey, W. N. Phase Equilibria in Hydrocarbon Systems. The n-Butane-Carbon Dioxide System. Znd. Eng. Chem. 1949,41,475-482. Osborn, A. G.; Douslin, D. R. Vapor Pressures and Derived Enthalpies of Vaporization for Some Condensed-Ring Hydrocarbons. J. Chem. Eng. Data 1975,20, 229-231. Pandit, A,; Singh, R. P. Vapor-Liquid Equilibrium Calculations for Polar Mixtures with Mixing Rules Using the ASOG Group Contribution Method. Fluid Phase Equilib. 1987,33, 1-12. Patel, N. C.; Teja, A. S. A New Cubic Equation of State for Fluid and Fluid Mixtures. Chem. Eng. Sci. 1982,37,463-473. Reamer, H. H.; Sage, B. H.; Lacey, W. N. Phase Equilibria in Hydrocarbon Systems. Volumetric and Phase Behavior of the Propane-Carbon Dioxide System. Ind. Eng. Chem. 1951, 43, 2515-2520. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. Schmitt, W. J.; Reid, R. C. Solubilities of Monofunctional Organic Solids in Chemically Diverse Supercritical Fluids. J. Chem. Eng. Data 1986,31,2041212. Sebastian, H. M.; Simnick, J. J.; Lin, H. M.; Chao, K. C. VaporLiauid Eauilibrium in Binary Mixtures of Carbon Dioxide + nDecane &d Carbon Dioxide n-Hexadecane. J. Chem. Eng. Data 1980e,25, 138-140.
+
Sebastian, H. M.; Nageshwar, G. D.; Lin, H. M.; Chao, K. C. Vapor-Liquid Equilibrium in Binary Mixtures of Carbon Dioxide Diphenylmethane and Carbon Dioxide + l-Methylnaphthalene. J. Chem. Eng. Data 1980b, 25,145-147. Sebastian, H. M.; Simnick, J. J.; Lin, H. M.; Chao, K. C. Gas-Liquid Equilibrium in Mixtures of Carbon Dioxide + Toluene and Carbon Dioxide + m-Xylene. J. Chem. Eng. Data 198Oc, %, 246-248. Sheng, Y. J.; Chen, Y. P.; Wong, D. S. H. A Cubic Equation of State for Predicting Vapor-Liquid Equilibria of Hydrocarbon Mixtures Using a Group Contribution Mixing Rule. Fluid Phase Equilib. 1989,46, 197-210. Somait, F. A.; Kidnay, A. J. Liquid-Vapor Equilibria at 270.00 K for System Containing Nitrogen, Methane, and Carbon Dioxide. J. Chem. Eng. Data 1978,23,301-305. Suen, S. Y.; Chen, Y.P.; Wong, D. S. H. Calculation of Multiphase Equilibria by a Group Contribution Equation of State. Fluid Phase Equilib. 1989,52, 75-82. Tochigi, K.; Kurihara, K.; Kojima, K. Prediction of High Pressure Vapor-Liquid Equilibrium with Mixing Rule Using ASOG Group Contribution Method. J. Chem. Eng. Jpn. 1985, 18, 60-65. Tremper, K. K.; Prausnitz, J. M. Solubility of Inorganic Gases in High-Boiling Hydrocarbon Solvents. J. Chem. Eng. Data 1976, 21, 295-299. Vera, J. H.; Orbey, H. Binary Vapor-Liquid Equilibria of Carbon Dioxide with 2-Methyl-l-pentene, l-Hexene, l-Heptene, and m-Xylene at 303.15, 323.15, and 343.15 K. J. Chem. Eng. Data 1984,29, 269-272. Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 69th ed.; CRC Press: Boca Raton, FL, 1989.
+
Received for review November 18, 1991 Accepted November 27, 1991
Polymer Solution Properties of a Phenol-Formaldehyde Resol Resin by Gel Permeation Chromatography, Intrinsic Viscosity, Static Light Scattering, and Vapor Pressure Osmometric Methods Moon G. Kim,*it World L. Nieh,t Terry Sellers, Jr.,' and Wilbur W. Wilson* Forest Products Laboratory and Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762
Jimmy W. Mays Department of Chemistry, University of Alabama a t Birmingham, Birmingham, Alabama 35294
A plywood adhesivetype phenol-formaldehyde (PF)resol resin was shown to have structures generally described as polymeric methylene(hydroxymethy1)phenolsas determined by 13C NMR, and static light scattering gave molecular weights of up to about 114 OOO daltons. The molecular weight and intrinsic viscosity data resulted in a Mark-Houwink exponent of 0.21 in tetrahydrofuran and 0.15 in ethyl acetate, which indicated a compact molecular structure in solution due to branching of the polymer chain. With the molecular weight and intrinsic viscosity data, the gel permeation chromatographic results obtained on a cross-linked polystyrene gel in tetrahydrofuran gave a universal calibration curve that agreed well with a similar curve obtained for linear polystyrene standards. This work determined the molecular weight and extent of branching of a wood adhesive PF resol resin synthesized by sodium hydroxide catalysis.
Introduction The (PF) industry resins used extensively as binders in the wood products are polymeric methylene(hydroxymethyl)phenols Of molecular weights properties Of On the synthesis parameters' theseresins Featly In wood binder applications, the PF resins are normally 'Forest Products Laboratorv. MississiDDi State University. Department of Chemistry, Mississip$ State University. -
*
synthesized at 60-100 "C with a formaldehyde/phenol (F/P)ratio of 1.9-2.5, a sodium hydroxide content of up to 0.8 mol/mole of phenol, and a resin solids range of M%. Since the resin viscosity must lie within a certain range for mostapplications, the resin solids level in the reactionmixturehas been the major synthesis variable in the molecular weight In North America the pF r e s . used in plwd adhesi;es nody have a &lids level of 40-45 7 ' 0 &Eluding the sodium hydroxide catalyst and are among the highest molecular weight PF resins manufactured:
0888-588519212631-0973$03.00/00 1992 American Chemical Society
974 Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 QH
Figure 1. Schematic structure of PF resol resins.
Molecular weights of PF resol resins have been determined by gel permeation chromatography (GPC) in alkaline aqueous medium (Wooten et al., 1988; Kim et al., 1990),in tetrahydrofuran (THF) with or without an acid (Riedl et al., 1988), or in dimethylformamide with or without an added salt (Schulz et al., 1982),using various detection methods (Aldersley et al., 1969; Duval et al., 1972; Wellons and Gollob, 1980; Rudin et al., 1983). Also, ultracentrifugation in an aqueous medium of reduced pH (Wooten et al., 1988), 'H and 13CNMR (Kim et al., 1990), vapor pressure osmometric (VPO), and intrinsic viscometric (Tobiason et al., 1973) methods have been used. However, results reported to date are incomplete due to the lack of suitable molecular weight standards. In addition, P F resol polymers have a tendency to undergo molecular associations in organic solvents and in low pH aqueous solvents (Wooten et al., 1988; Schulz et al., 1982), to form hydrogen bonds with solvent (Duval et al., 19721, and to advance in the degree of polymerization during the analytical experiment. Acetylation of the hydroxyl groups provides a means to avoid these difficulties (Schulz et al., 1982). The aqueous alkaline GPC method appears to entail a minimal molecular association effect, but the method has been difficult to verify or calibrate for high molecular weight P F resin components because suitable calibration materials have not been available. The branching extent of PF resol resins appears to be largely determined by the reaction temperature and polymer solvation parameters of the synthesis medium, which are primarily determined by the kind and amount of catalyst and the level of reactant solids. The extensive branching that has been generally assumed to occur because of the trifunctionality of phenol has not been well elucidated. A recent report described the examination of two very different wood adhesive P F resol resins after acetylation and fractionation (Kim and Amos, 1991),and the results showed the difficulty of measuring the branching extent by 13CNMR procedures. The authors of this paper also measured the intrinsic viscosity and VPO molecular weights of the two resins. One resin had a Mark-Houwink exponent of 0.21 in chloroform and 0.12 in benzene. A compact molecular structure was indicated arising from branching of the polymer chain. The second resin showed a heterogenous branching extent among fractions, which suggested the possible difficulties of determining molecular weight through measurement of the hydrodynamic properties of PF resins. This paper reports a study of a plywood-type PF resol resin after acetylation and fractionation using VPO, NMR, GPC, intrinsic viscosity, and light scattering methods in an attempt to develop a basic methodology for the determination of the molecular weight and branching extent of general wood adhesive P F resol resins.
Experimental Section Resin Synthesis. A plywood-type PF resol resin was prepared by utilizing a 5-L reactor with a stirrer, cooling condenser, and cooling coil. The reactor was charged with
1044.4 g of 90% (w/w) phenol (in water) and 120 g of 50% (w/w) sodium hydroxide solution. After the temperature was raised to 70 "C, 1500 g of 50% formaldehyde solution was added dropwise over a period of 1 h while the temperature was kept at 80 "C by intermittent cooling. The reaction was continued at 80 "C until the viscosity,checked at 10-min intervals by using Gardener-Holdt viscometers at 25 "C, reached 275 cP, then 80 g of 50% sodium hydroxide solution and 245 g of water were added, and the reaction was continued until the viscosity reached 375 cP. Again, 80 g of 50% sodium hydroxide solution and 460 g of water were added and the reaction was allowed to continue until the resin viscosity reached 600 cP. The resulting resin, which had a solids level of about 40%, was cooled and stored frozen until analysis. GPC in Alkaline Aqueous Medium. The synthesized (unacetylated) resin was analyzed according to a reported GPC procedure which uses a Sephacryl (high-resolution) gel column (70 cm X 1.6 cm) at a low pressure, 0.1 N sodium hydroxide solution as eluant, and UV detection (280 nm) with a Nelson analytical chromatographic data acquisition system (Wooten et al., 1988). Acetylation. Acetylation of the synthesized resin was performed according to a known procedure (Woodbrey et al., 1965) with minor modifications. Two hundred grams of resin having a 40% resin solids level was mixed with 200 g of water, and the solution was acidified to pH 4.0 in an ice bath by dropwise addition of acetic acid. The precipitated resin was collected and dissolved in 200 g of a 1:l dimethyl sulfoxide-pyridine mixture. The resin solution was stirred in an ice bath, and excess acetic anhydride (200 g) was added dropwise over a period of 1h. The acetylation reaction was continued for an additional 2 h in the ice bath and overnight at room temperature. The reaction mixture was then evaporated on a rotary evaporator at 40 "C. The residue was dissolved in 1500 mL of chloroform, washed with saturated NaCl solution and water, dried over anhydrous sodium sulfate, and evaporated to give about 80 g of a crude product. The crude product was dissolved in acetone and precipitated with water three times to remove impurities. To separate out the low molecular weight components which could unduly complicate the fractionation process, the product was dissolved in 500 mL of ethyl acetate and then 1870 mL of anhydrous ether was added slowly during vigorous stirring. The ether-insoluble precipitates were collected (43.3 g) while the soluble components were recovered by evaporating the solution (36.0 8). Fractionation. Forty grams of the ether-insoluble product was dissolved in 500 mL of ethyl acetate, and 170 mL of hexane was added as the solution was slowly stirred. Some precipitates were produced. The solution mixture was heated to 60 "C to dissolve the precipitates and stirred slowly at room temperature overnight for equilibration. The precipitates were then collected as fraction 1. The procedure was repeated in collecting fractions 2-21 with fraction yields and amounts of hexane used as shown in Table I. GPC of Acetylated Resin Fractions. The GPC procedure was performed using a Waters high-pressure liquid chromatography system equipped with two ultrastyragel columns (30 cm X 0.78 cm with 102-10s-I( pore size) connected in seriespd a differential refractive index detector. Tetrahydrofuran (THF) was used as eluant at a flow rate of 1.0 mL/min. The sample injection volume was 20 r L of a 0.2% (w/w) resin solution in THF. Using the Waters GPC software, the preliminary weight-average molecular weights (M,)of the resin fractions were initially obtained using polystyrenes as calibration standards. These values
Ind. Eng. Chem. Res., Vol. 31, No. 3,1992 975 Table I. Fractionation Data of Acetylated P F Resin" fr no. yield, g hexane, mL 1 2.94 170.0 10.0 0.84 2 10.0 0.77 3 4.0 4 0.90 6.0 1.00 5 8.0 0.70 6 7.0 1.52 7 8.0 8 1.06 10.0 9 1.99 15.0 10 1.72 20.0 2.63 11 12 20.0 1.33 20.0 2.03 13 30.0 2.80 14 35.0 1.36 15 40.0 1.36 16 70.0 1.75 17 150.0 1.40 18 150.0 19 0.90 200.0 0.59 20 b 1.86 21
total
time was 18 ps (90'). Between 4000 and 5000 scans were accumulated and peak areas were integrated using expanded spectra. Calculations were made using reported procedures (Kim and Amos, 1991). Static Light Scattering. A DAWN Model F laser light scattering photometer from Wyatt Technology was used at 25 'C with ethyl acetate or THF as solvent. The intensity of scattered light was simultaneously monitored by photodiode detectors at 15 different fixed angles ranging from 26.6 to 144.5'. The incident light source was a 5mW helium-neon laser emitting vertically polarized light at 632.8 nm. Solvents and polymer solutions were filtered through 0.2-pm filters from Gelman. The polymer concentrations ranged from 1.7 X lo-* to 1.6 X g/mL for all fractions except for fraction 21 of which the concento 5.3 X g/mL. The tration ranged from 1.1 X excess scattering ratios, Re, were related to molecular parameters by the following equations (Flory, 1953): Kc/RB = (l/Mw)[l + (u2/3)(r,2)] + 2A2c
K = 4 ~ ~ n ~ ( d n / d c ) ~ / N A ~ ~ (2) u = (47rn/&) sin (8/2) (3)
31.41 (78.5%)
" Forty grams of the ether-insoluble fraction was dissolved in 500 mL of ethyl acetate followed by the addition of hexane in the indicated amounts. Fraction 21 was obtained by evaporation of the entire solution. were then used to assign the elution volumes for each fraction, which were then incorporated into a universal calibration curve using the light scattering molecular weights and intrinsic viscosity data. Through the universal calibration resulta, the number-average molecular weights, M,,were calculated (reported as MJM, in Table 11). lH and '3c NMR. The 'H NMR spectra were obtained for the resin fractions on a Varian CFT-20 (modified with a 'H Fourier transform probe) on 6.3% (w/w) concentrations in deuterochloroform with tetramethylsilane as frequency reference. The 13C NMR spectra were obtained on a Varian CFT-20 on 26.0% (w/w) solutions in the same solvent. The decoupler was gated 'on" during the acquisition (0.892 s) and "off" during the delay (12.0 s) to suppress the nuclear Overhauser effect (NOE). The pulse
(1)
where c is the polymer concentration in g/mL, Re is the excess scattering a t angle 8, K is the optical constant, no is the refractive index of the solvent at the wavelength A,, N is Avogadro's number, and (r,) is the radius of gyration. The refractive index increment (dnldc) values were measured separately using a C. N. Wood 500 Monophotometer at 633 nm. From the obtained Re values as a function of polymer concentration and scattering angle, the Dawn-F 'AURORA" software package provided the Zimm plot (Zimm, 1948) in terms of Kc/Re versus sin2(8/2) + 12%. Intrinsic Viscosity. Intrinsic viscosity was determined at 25.0 f 0.1 "C using a Cannon-Ubbelohde one-bulb viscometer which was designed for minimizing the kinetic effect (Cannon Instrument Co., State College, PA; No. CUDC, #25). Duplicate measurements of efflux time usually agreed to within 0.2 s with efflux times ranging between 45 and 120 s for four 10-mL dilution measure-
Table 11. Molecular Prowrties of Acetylated PF Resin Fractions" fr no. M W 1O-'Az dnldc MJVPO) Mw/Mn(GPC) 1 114000 -6.5 0.175 7.65 2 69300 -6.3 0.178 9.52 3 31500 3.6 0.180 6.63 4 30400 1.6 0.181 4.20 5 26000 4.5 0.888 3.69 6 21700 1.7 0.182 3.34 7 20300 6.3 0.188 3.65 8 21700 3.2 0.184 5.20 9 22300 2.8 0.178 2.92 10 17000 3.8 0.170 2.37 11 16100 4.6 0.185 2.20 12 8800 4.0 0.170 1.62 13 8700 4.9 0.186 6710 1.64 14 6900 2.9 0.161 2660 1.45 6Ooo 4.4 0.173 2250 1.42 15 16 4900 2.1 0.163 3400 1.44 17 3900 5.4 0.183 2350 1.25 18 3100 3.3 0.161 3160 1.20 19 2200 4.0 0.179 1420 1.24 20 1.19 21 1200 -0.8 0.149 770 1.14 dimerb (660) 1.04
mL/g 8.98
k'mF 0.60
5.66 5.68 6.45 5.93 6.10 6.04 5.81 4.36 5.12 4.86 4.78 4.50 4.17
4.18 1.00 0.40 0.77 0.83 0.51 0.37 4.24 0.57 0.42 0.30 0.64 0.98
3.79
0.18
2.57
1.23
[ql!ll-IF,
[?]EA,mL/g
FA
5.82 6.36 5.55 5.68 4.88 6.73 6.18 5.49 5.51 5.55 4.87 4.68 4.67 3.87 4.09 4.14 4.03 3.84 3.97 3.71
1.90 0.07 0.92 1.02 4.45 3.34 3.33 0.50 0.44 0.66 0.79 0.48 0.34 0.55 2.19 0.83 0.94 0.83 0.14 0.46
"M,,molecular weight by light scattering in ethyl acetate; A2,second viral coefficient from light scattering in mL g-2 mol; dnldc, differential refractive index; MJVPO), molecular weight by vapor pressure osmometry in THF; M,/M,(GPC), polydispersity value obtained from GPC data using Universal calibration results based on the liiht scattering M, and intrinsic viscosity; [qITHF,intrinsic viscosity (mL/g) in THF; k', Huggins' constant in THF; [qlm, intrinsic viscosity (mL/g) in ethyl acetate; k', Huggins constant in ethyl acetate. bDimerwas the acetate of 2,5,3',5-tetrakis(hydroxymethyl)-4,4'-dihydroxydiphenylmethane.
976 Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 Table 111. Number of C H L O Hand -CH2- per Phenolic Ring (NMR) 'H NMR I3C NMR frno. C H 9 0 H CH,d e f C H 9 0 H CH,deP 0.79 0.58 1.04 -0.11 1 0.84 1.13 2 0.87 0.66 0.81 0.94 0.90 0.26 0.75 3 0.85 0.65 0.95 1.03 -0.01 0.82 4 1.02 0.34 0.93 0.97 0.13 0.91 7 1.14 0.04 0.99 0.92 0.17 0.81 10 1.00 0.38 1.17 0.91 0.01 0.88 1.02 -0.11 0.18 1.07 13 1.06 16 0.94 0.78 0.50 1.07 0.93 0.07 0.93 -0.24 18 1.30 1.22 1.00 -0.22 21 1.70 0.83 -0.36 0.80 -0.01 1.41 ~~
"def = 3.0 - [CH,OH] for phenol is 3.0.
I200
- 2[CHz]; functional group requirement
I
180
160
140
120
1W
80
60
r3
20
0
ppm
.5
Sephacryl S200 Eluent 0 IN NaOH Flow rate 17 mlimin Detector 280 nm UV
-
( 5
25
20
35
18
IO
10
35
5
3 2
10
45
1
.5
50
5 5 (min)
elution lime
.1 ( X 1 0 ' ) m o l e d e r wught
t
IO
I
I
Q
I
8
I
6
7
I
5
4
I
3
2
I
O
J
P pm
Figure 2. NMR spectra of PF resin fraction in CDCI,/TMS. S, solvent peaks.
ments, which started with a 15.0-mL stock solution containing 0.25-0.50 g of polymer. The solution densities were measured with a pycnometer, and four specific viscosities, tsp,were obtained from the four dilution efflux times. By applying the Huggins' equation (Flory, 1953) tsp
= [SI + k'[tI2c
(4)
Huggins' constant (k? and intrinsic viscosity, [TI, were obtained with r2 values greater than 0.90 in most cases. The data for intrinsic viscosity, [SI, and light scattering molecular weight (Wyatt et al., 1988),M,,were correlated using the Mark-Houwink equation (Flory, 1953). [VI = KMwa
(5)
where K and a are constants. Vapor Pressure Osmometry. The VPO number-average molecular weights were measured by Galbraith Laboratory using THF as solvent at 45 "C with glucose pentaacetate as the calibration standard. Three or four dilution measurements were made for each sample with most 1.2 values for the linear regression plota being greater than 0.96. Results and Discussion NMR Results and Resin Structure. Typical 'H and '3c N M R spectra for fraction 4 are shown in Figure 2. The small phenolic hydroxyl peaks at about 150 ppm in the 13C N M R spectrum indicated that the acetylation reaction was incomplete. The residual hydroxyl groups might have
Figure 3. Aqueous GPC of the PF resin calibrated with polyatsrrene sulfonataa.
affected the solutesolvent interaction parameters of this study. All fractions showed similar structural features of a PF rem1 consisting of methylol groups, methylene groups, and an aromatic ring. Small amounts of unreacted Cz and C4 carbons were present at 12e125 ppm in I3C NMR spectra. In general, the values for the number of methylene and methylol groups per aromatic ring (Table 111) were not accurate enough to give reliable number-average molecular weight values when compared to the values obtained by the light scattering or VPO methods. The structures deduced from the NMR data were not quantitatively consistent, showing discrepancies of functional sites up to 20% from those expected for the theoretical structures for polymeric methylene(hydroxymethy1)phenols. Them? typea of discrepancieswere reported earlier as occurring due to depressed methylene group signals because of the limited solution mobility of the polymer chain (Kimand Amos, 1991). The extent of branching or the presence of cyclic products could not be ascertained from the NMR spectra. Aqueous GPC. The GPC results obtained using 0.1 N sodium hydroxide solution as eluant for the unacetylated, whole PF resol resin (Figure 3) showed that the earliest eluting resin had a molecular weight of about 35 OOO daltons (Da) based on a point-to-point calibration curve obtained using narrow molecular weight polystyrenesulfonate standards. This value is considerably lower in comparison to the light scattering data of the acetylated resin fractions 1and 2 (see below). The UV extinction coefficient of PF resin polymers, known to decrease as molecular weight increaees (Aldersley et al., 19691, might have contributed somewhat to this discrepancy. However, the branching of the P F resin polymer chains appears to be the primary cause of this discrepancy since it would cause a greatly
Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 977 K
,
C/R8
t
(XlO')
.I
3.00 5.00
0.50
1.00
2.00
1.50
sin28/2 6 3 8 c
3.50
-
3.40
-
b 4z.i
3.30
-
3.1 0
-
I
I 3.00
4.00
4.50
5.00
Log M W
0.0 0
(XlOS)
3.50
1 0.00
I 0.50
1.00
1.50
2.00
sin29/2 3 8 B c
Figure 4. (a) Light scattering resulta of fraction 1 ( Z mplot). (b) Light scattering results of fraction 3 (Zimm plot).
reduced hydrodynamic volume. The alkaline aqueous eluant used here appears to be a good solvent for the P F resin. Otherwise, the association effects would have resulted in erratic molecular weight values. A better calibration method on unacetylated PF resols in this aqueous system is desired, but the instability of the polymers in normal fractionation and molecular weight measurement procedures make it very difficult. Molecular Weight from Light Scattering, VPO, and GPC. To investigate, in part, the validity of the molecular weight values obtained from aqueous GPC, the acetylated resin fractions were studied by light scattering, VPO, and organic-phase GPC procedures. Use of the Dawn Model F for molecular weight determinations has been described for both static and flow (GPC) applications (Wyatt et al., 1988). We have measured the molecular weight and second viral coefficient for several polymer standards typically used for GPC calibration purposes and found good agreement between the measured and reported values for M,. Figure 4s shows the Zimm plot obtained for fraction 1in ethyl acetate. The intercept of the plot gave a M, of about 114000 Da for this fraction. The concentration dependence of the data extrapolated to zero angle (shown by the X's) clearly shows a negative slope, giving a value mol. for the second viral coefficient of A2 = -6.5 X mL-g-2. This behavior suggests that ethyl acetate is a thermodynamically poor solvent for fraction 1,which was obtained with the least amount of nonsolvent (hexane). The negative value for A2 indicates that the highest molecular weight acetylated resin molecules may be self-associating as their concentration increases. In like manner,
Figure 5. Mark-Houwink correlation of PF resin fractions in THF (0's and eq 6). Equations 7-9 as defined in the text.
fraction 2 showed a negative value for A2 with a M, value of 69 300 D a Beginning with fraction 3 of which the Zimm plot is shown in Figure 4b, the M, values dropped consistently from about 31 500 Da for fraction 3 to about 1200 Da for fraction 21. Overall, with the observed normal behavior of most Zimm plots and other experimental considerations, we believe the molecular weights obtained are realistic valuea for these relatively low molecular weight resins. The fact that the third and subsequent resin fractions showed appreciably lower molecular weight values appears to indicate that the molecular weight of the advanced fractions increases rather rapidly during the synthesis as expected from the nature of polyfunctional polymerizations. The obtained second viral coefficients, A2, were somewhat erratic although most fractions showed values in a reasonable range. Since Az is related to the molecular interaction and solvation (Flory, 1953), the irregularity might have arisen from the structural heterogeneity among the fractions due to either the residual hydroxyl groups, which was observed in 13CNMR,or the different branching extents. Overall, however, the results appear to indicate the inadequacy of the calibration method of the aqueous GPC mentioned above. The number-average molecular weight values obtained by VPO were generally erratic, especially the high molecular weight fractions (not reported). The polydispersities of the fractions apparently affected the values. Similarly, calculating the molecular weight values of resin fractions from GPC elution curves using the calibration curve based on polystyrene standards resulted in low values as expected from the hydrodynamic effect of the branched structures. Intrinsic Viscosity, Molecular Weight, and Branching. Intrinsic viscosity data in THF and in ethyl acetate (EA) and the light scattering molecular weight data of the resin fractions were correlated using the MarkHouwink equation (Figure 5):
[TI = 0.77M,0.21 (r2 = 0.92; THF; 25 "C) [TI = 1.16MW0.l5(1.2 = 0.71; EA;25 "C)
(6) (7)
< [77] < 9.0 mL/g (THF) 3.7 < [ T ] < 6.7 mL/g (EA) 2200 < M, < 114000 (acetylated) 3.8
Reported relationships based on diffusion methods for molecular weight determinations for unacetylabd PF r e d resins (MD)and for linear polystyrenes (M,) are (Figure 4)
978 Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 [q] =
800 < MD
0.99MDo"8 (r2 = 0.92; THF) 2.9 < [ a ] < 7.9 mL/g
(8)
2
< 111000 (NaOH-catalyzed PF; Laurent and Gallot, 1982)
[q] =
770 < MD
Mw
1.43MDo,O5(r2= 0.39; acetone) 1.98 < [q] < 2.49 mL/g
10'
(9)
6
5
< 10600
4
(NH,-catalyzed PF; Ishida et al., 1977) = 1.02~~0.725 (THF)
3.0
9
8 1
3
(10)
< [ a ] < 30 mL/g
2
1100 < M , < 50000 (polystyrene; Mays et al., 1991) The a values for linear polymers in good solvents are generally higher than 0.5 (Flory, 1953). Equations 6 and 7 indicate that the acetylated resin fractions have a compact solution structure which agrees with earlier studies (Tobiason et al., 1973; Kim and Amos, 1991). This compact solution structure has been ascribed to branching for PF resol resins. The sodium hydroxide-catalyzed resin (eq 8) showed an a value only slightly lower than the value of the acetylated PF resin. These two resins appear to be very similar, with the solvation being slightly increased by the acetylation. However, since the limiting a value for a linear PF resol is not known, the extent of branching of these resins can be reckoned only in a relative term. The ammonia-catalyzed resin (eq 9) showed a very low a value which indicated a more compact polymer structure that probably resulted from a greater extent of branching. This structure might be due to the incorporation of trifunctional ammonia in the resin chain or perhaps other synthetic reasons such as a reduced solvation power of ammonia during the synthesis in comparison to that of sodium hydroxide. Although the observed Huggins constants (k? are somewhat scattered (Table 11), they are generally much larger than the value of 1/3 expected of linear polymer chains in good solvents. Large k'values have previously been noted for branched polymers (Bodanecky and Kovar, 1982). GPC and Universal Calibration. The high-pressure GPC results were initially analyzed to find the elution volumes of fractions corresponding to the weight-average molecular weights based on the polystyrene standards. The elution volumes (V,) were then correlated with light scattering molecular weights (M,) and intrinsic viscosity values ( [ a ] ) using the universal calibration method, resulting in log M,[a] = -1.024Ve + 24.53 (r2 = 0.94; THF) (11) Equation 11is in relatively good agreement with a similar curve for linear polystyrenes (Mays et al., 1991) obtained under the same chromatographic conditions (Figure 6). This result confirms the adequacy of the light scattering molecular weight data obtained in this study. On the basis of universal calibration results, the polydispersity values for resin fractions were calculated (Table 11). The later eluting resin fractions (12-21) have relatively narrow molecular weight distributions, with the M,/M,, ratios ranging below 1.62, but the early eluting fractions (1-11) showed broader molecular weight distributions which indicated the inadequacy of the fractionation method used.
Conclusion Resin fractions of an acetylated PF resol were described in general as polymeric methylene(hydroxymethy1)phenols
103 8 1 6 5
4 3 102
2
18.5
19.0
19.5
20.0
20.5
21.0
21.5
mL
Elution Volume
Figure 6. GPC calibration of acetylated P F resin fractions (universal calibration method).
from 13CNMR results with molecular weights ranging up to about 114000 Da as determined by static light scattering. The Mark-Houwink exponents of 0.21 in THF and G.15 in ethyl acetate obtained for this resin indicate a compact molecular structure in solution which probably occurred as a result of branching. Also, the universal calibration curve obtained in the GPC of these fractions in THF agreed well with a similar curve obtained from linear polystyrene standards. This work afforded a methodology of determining the molecular weight and comparative branching extent of PF resol resins in general and also indicated the inadequacy of the calibration method for the aqueous GPC method currently used. Future study with typical P F resol resins according to this methodology will help clarify the various resin property differences arising from the different synthesis procedures.
Acknowledgment This work was supported by a USDA-Wood Utilization Research Grant. Registry No. Phenol-formaldehyde (copolymer), 9003-35-4; 3,5,3',5'-tetrakis( hydroxymethyl)-4,4'-dihydroxydiphenylmethane, 13653-12-8.
Literature Cited Aldersley, J. W.; Bertram, V. M. R.; Harper, G. R.; Stark,B. P. Chromatographic studies of some thermosetting resins. Br. Polym. J . 1969, I , 101-109. Bodanecky, M.; Kovar, J. Viscosity of Polymer Solutions; Elsevier: Amsterdam, 1982;p 187. Duval, M.; Bloch, B.; Kohn, S. Analysis of phenol-formaldehyde resole by gel permeation chromatography. J. Appl. Polym. Sci. 1972,16, 1585-1602. Flory, P. J. Principles of Polymer Chemistry; Cornel1 University Press: Ithaca, New York, 1953;pp 266-314. Ishida, S.; Chika,M.; Nishikawa, K.; Kaneko, K. Formation of three dimensional polymers: Studies on the formation mechanisms of phenolic resin by GPC and computer. Asahi Garasu Kogyo Gi-
Znd. Eng. Chem. Res. 1992,31,979-981 siuts Siourei Kenkyu Hog0 1977,30, 279-292. Kim, M. G.; Amos, L. W. NMR and intrinsic viscosity study of two different phenol-formaldehyderesol resins. Znd. Eng. Chem. Res. 1991,30, 1151-1157. Kim, M. G.; Amos, L. W.; Barnes, E. E. Study of Reaction Rates and Structures of a Phenol-Formaldehyde Resin. Znd. Eng. Chem. Res. 1990,29, 2032-2037. Laurent, P.; Gallot, Z. Utilisation du couplage chromatographie sur gel permeable-diffusion de la lumiere pour la caracterisation de resines formophenoliques. J. Chromatogr. 1982, 236, 212-216. Mays, J. W.; Lindner, J. S.; Hadjichristidis, N.; Fetters, L. J. To be published, 1991. Riedl, B.; Calve, L.; Blanchette, L. Size-exclusion chromatography of spray-dried phenol-formaldehyde resins on different columns and solvent systems. Holzforschung 1988,42, 315-318. Rudin, A.; Fyfe, C. A.; Vines, S. M. Gel permeation chromatographic analyses of resole phenolic resins. J. Appl. Polym. Sci. 1983,28, 2611-2622. Schulz, G.; Gnauck, R.; Ziebarth, G. Zur gelpermeationschromatographie von Phenolformaldehydharzen. Plaste Kautsch. 1982,29, 396401.
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Tobiason, F. L.; Chandler, C.; Negstad, P. Molecular Weight Characterization of Resole Phenol-FormaldehydeReains. In Aduances in Chemistry Series; American Chemical Society: Washington, DC, 1973; NO. 125, pp 194-206. Wellons, J. D.; Gollob, L. GPC and light scattering of phenolic resins-Problems in determining molecular weights. Wood Sci. 1980,13,68-14. Woodbrey, J. C.; Higginbottom, H. P.; Culbertson, H. M. Proton Magnetic Resonance Study on the Structures of Phenol-Formaldehyde Resins. J. Polym. Sci. 1965, A3, 1079-1106. Wooten, A. L.; Prewitt, M. L.; Sellers, Jr., T.; Teller, D. C. Gel Filtration Chromatography of Resole Phenolic Resins. J. Chromatogr. 1988,445, 371-376. Wyatt, P. J.; Jackson, C.; Wyatt, G. K. Absolute GPC determinations of molecular weights and sizes from light scattering. Am. Lab. 1988,86-91 and 108-113. Zimm, B. H. The scattering of light and radial distribution of high polymer solutions. J . Chem. Phys. 1948, 16, 1093-1099. Receiued for reuiew September 19, 1991 Accepted December 23, 1991
RESEARCH NOTES Estimation of the Acid Strength of Mixed Oxides by a Neural Network An artificial neural network is applied to the estimation of strength of acid sites synergistically generated in binary mixed oxides. The acid strength is represented as a function of physical/chemical properties of both constituent oxides. The form of the function is represented by a back-propagation neural network which was trained by using the published acid strength data for such cases. The estimated results were in a good agreement with the experimentally observed ones.
Introduction In designing multicomponent catalysts, the synergistically generated catalytic functions have to be taken into consideration. This is especially the case for mixed oxide catalysts, since, while single-component oxides do not provide strong acid sites, mixed oxides can sometimes provide strong acid sites by a synergistical effect in the mixing. It is also the case that such experimental data is scarce mainly because of a combinatorial problem. Therefore, when designing multicomponent oxide catalysts, we encounter the task of estimating the acid strength of the mixed oxides under consideration. Several models have been proposed for acid sites synergistically generated on the surface of binary oxides (Thomas, 1949; Shibata et al., 1973; Tanabe et al., 1974; Aso et al., 1976; Kung, 1984). These models, however, are not reliable for enough catalyst design purpose from our experience of developing an expert system for catalyst design called INCAP (Hattori et al., 1988; Kit0 et al., 1989, 1990). In the present investigation, we present a feasibility study of estimating the acid strength of mixed oxides by using an artificial neural network which is quite a different approach from those presented so far.
Application of Neural Network In the models mentioned above, the generation of acid sites and their strength are explained by taking the following into account: valence (Z)coordination , number (C),
ionic radius (r), electronegativity (x),and electrostatic potential (Z/r)for metal ions, and partial charge of oxygen ion (6,) for the oxides. When considering that valence, ionic radius, and electrostatic potential are not mutually independent, one may expect that the acid strength of mixed oxide, H,(max), could be represented as a function of these parameters: Ho(mm)ij = f(zi,Ci, ri, xi, 6oi, zj, Cj, rj, XI,
6oj)
where subscripts i and j denote the constituent oxides. In this work, an artificial neural network (Rumelhart et al., 1987) which is a simplified model of the human brain has been applied to the estimation of the acid strength of mixed oxides. One of the most remarkable features of a neural network is the ability of self-organization by learning, and the neural network as a function approximator can learn the form of function as a network pattern, if one gives a training set of known pairs of input data (physical/chemical properties) and output data (acid strength). Then, unknown output data can be calculated by substituting corresponding input data into the function. A simulator of a multilayer back-propagation neural network was developed by using C language on a UNIXbased workstation. In a back-propagation network, the activation function for each of the network units has to be a nonlinear and differentiable function. In this respect, a logistic function was used as the activation function which is used in almost every application of such a backpropagation network. The simulator was trained by using the acid strength data of the following 13 binary metal
OSSS-5~S5/92/2631-0979$03.00/00 1992 American Chemical Society
