The number average molecular weight of the unfractionated maltenes as measured by VPO was found to be 950 while that of the asphaltene fraction was 3800 (Table I). From our NMR data, one might expect a molecular weight in the order of 1000 to represent the maximum weight of a molecule in the maltene fraction. As the concentration of the maltenes in the GPC fractions decreases (and the concentration of asphaltenes increases), the number average molecular weight (by VPO) will approach that of the asphaltenes, as is observed in Figure 2. The NMR unit sheet weights should approach a maximum value of -1000 (theoretically the largest unit sheet in the maltenes) and then vary as a function of the decreasing maltene concentration to the average value of the unit sheet of the asphaltene--i.e., mol wt -1400). These data lead to the conclusion that NMR spectra in conjunction with gel permeation chromatography can be used to obtain valid unit sheet weights of asphaltic materials.
port, however, the theoretical picture of the macrostructure of asphaltic materials as proposed by Dickie and Yen (3). These authors have proposed that the molecular weight determined by high-voltage mass spectrometry represents the "disc" weight or the unit sheet weight less the aliphatic fragments. Our low-voltage mass spectrometry data would yield parent ion data or complete (including aliphatic fragments) unit sheet weight. The NMR data would yield the unit sheet weight as well, since the method cannot distinguish between monomers and polymeric species. Thus, the NMR weights and the weights as calculated by mass spectrometry should and do agree. Dickie and Yen also point out that VPO measurements on asphaltics will yield a number average molecular weight which describes the associated unit sheets or particles. Although a difference exists between molecular weights measured by VPO techniques and the unit sheet weights measured by NMR, the data for elution cuts 62 through 84 do follow the same trend. Below cut 64, however, a distinct break is observed in the NMR values while the VPO molecular weights continue to increase. Ferris et al. (2) have observed similar breaks at mol wt -1200 in plots of S, 0,etc., GS. molecular weight. They have attributed this break to the presence of asphaltenes in fractions where the molecular weight is greater than 1200.
ACKNOWLEDGMENT
The authors express their appreciation to Dr. T. C. Paradellis for helpful comments during the course of this research.
z z
RECEIVED for review March 28, 1969. Accepted May 16,1969.
Fractionation of Polyvinyl Alcohol on Deactivated Porous Silica Beads by Gel Permeation Chromatography Karl J . Bombaugh, William A. Dark, and James N. Little Waters Associates, Inc., 61 Fountain St., Framingham, Mass. 01 701
POLYVINYL ALCOHOL is a polymeric material widely used for its mechanical properties in adhesives, plastics, and sizing agents. The mechanical properties are closely related to the molecular weight distribution of the polymer. The molecular weight distribution of polymers is best understood through the gel permeation chromatogram, GPC ( I ) , which provides the envelope of the distribution rather than just a weight and number average molecular weight as is provided by other techniques, such as light scattering and viscosity, respectively. The GPC fractionation of polyvinyl alcohol has not been available to polymer chemists because of the lack of a suitable column packing material which provided the mechanical rigidity required by the high pressure GPC chromatographic system and which can fractionate the polyvinyl alcohol without excessive adsorption. This paper reports the successful GPC fractionation of polyvinyl alcohol on deactivated porous silica, recently developed in our laboratory, which used Porasil ( 2 ) as a base material. EXPERIMENTAL Apparatus. All fractionations were made on the Waters Associates Model 200 gel permeation chromatograph equipped with 4-ft X a/8-in.columns packed with deactivated Porasil (patent pending). Deactivated Porasil is a propriatory material available from Waters Associates, Inc., Framingham, Mass. Porasil 1000, 400, 250, and 60, respectively, were used in the four-column system. The num-
(1) J. C . Moore, J . Polym. Sci., A2, 835 (1964). (2) A. J. DeVries, M. LePage, R. Beau, and C. L. Guillemin, ANAL.
CHEM., 39,935 (1967).
bers designate molecular weight in thousands of polystyrene excluded from the gel pore in tetrahydrofuran solvent. Column plate number as determined at total permeation ( V , ) using galactose was 2700 plates for the 16-ft column. Water was used as a moving solvent at 65 "C, with a flow rate of 1 ml/min. Separation of discrete species to test activity was done on a single column of Porasil 250 at 25 "C. Materials. Samples fractionated include bulk, suspension, and solution polymerized materials from several suppliers. Low molecular weight synthetic yarn size, broad distribution adhesive grade and TV tube adhesive grade polymers were fractionated. Pyridine, strongly adsorbed by silica, and valeric acid, partially excluded from porous silica, were used to compare the absorption characteristics of the newly deactivated material with regular Porasil. Calibration Standards. Commercially available polydextrans of known molecular weight obtained from Pharmacia Fine Chemicals, Piscataway, N. J., were used to calibrate the GPC columns in order to relate retention volumes to molecular weight. Table I shows the molecular weight of the standards used, along with the dispersity of each. The calibration curve for the four-column set was prepared in steps as follows: a plot of retention volume (Vr) of the dextran peak maximum versus log M , was prepared by using M , values shown in Table I. This preliminary calibration curve, in conjunction with the Waters Associates procedure normally used to determine M , and M,, was used to calculate M, and M , values for each dextran standard. The calculated M , and M , values were applied to the preliminary calibration curve to determine V, values corresponding to the respective M , and M , calculated values. The correct calibration curve was then prepared by plotting the published M, and M , values (Table I) cs. the calculated V , values as determined by the procedure described. VOL. 41, NO. 10,AUGUST 1969
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1
,
!
1
175 150 RETENTION VOLUME (ml)
200
225
125
100
Figure 1. GPC chromatogram of Datran standard Column:
4-ft X 3/8-indeactivated Porasil, 4 each in series packed with P. 1000, P. 400, P. 250, P. 60 Waterat65 "C Solvent: Sample: 0.25 for 120 sec = 5 mg Flow rate: 1 ml/min
Average Molecular Weights of Dextran Standards M,
MU 510,000 236,000 150,000 100,500 85,800 39,800 21,800 11,200
185,000 109,000
100
Figure 2. GPC fractionation of Dextran 500
Dextran Calibration. The distribution curve of each dextran is shown in Figure 1. Five of the seven fractions offered as narrow distributions were found to possess wide distributions. The Dextran 500 showed a severe low molecular weight tail, while standards 150, 80, and 40 each showed high molecular weight tails or skews. T o establish that the apparent peak asymmetry was caused by the actual distribution and not by adsorption of the column packing material, a 100-mg sample of Dextran 500 was injected and fractions collected as shown in Figure 2. The respective fractions were reinjected and the results are also shown in Figure 2. It is apparent from the position of the peak centers and the symmetry of the collected peaks that the
Dextran std No. 500 250 150 110 80 40 20 10
t
I
1
I50 125 RETENTION VOLUME (mll
Conditions same as Figure 1 except sample for collection was 0.5 for 120 sec = 10 mg
RESULTS AND DISCUSSION
Table I.
I
175
,
I
I
i
200
MdMn 2.75 2.17
...
...
62,000 43,700 25,600 14,500 5,700
1.69 1.95 1.55 1.50 1.94
I
CGILACTOSE
Figure 3. Retention of pyridine Column: 4' X 3/8" Deactivated Porasil 250 Solvent: Water at 25 "C Flow rate: 1 ml/min
assymetry in the present curve is present in the distribution and is not caused by the fractionating system. These dextran standards, widely used as calibration materials, can themselves be characterized and their actual distribution determined by GPC using deactivated Porasil. This was done to obtain the data shown in Table 11. A plot of log molecular weight DS. retention volume may be prepared from the data shown in Table 11, to approximate molecular weight (size in solution) of the polyvinyl alcohols. Test of Deactivation of Porasil. A comparison of deactivated Porasil with regular Porasil is shown in Figure 3 where pyridine shows a V , of 124 ml and a skew of 15. Skew was
Table 11. Data for Corrected Calibration Curve Dextran std No. 500 250 150 110 80
5
V,"
24.0 28.5 29.8 32.0 33.5 40 38.0 20 40.0 42.8 10 Volume in siphon counts where
1338
Calculated MU 223,500 136,300 125,400 94,900 78,200 46,700 23,200 12,800 each count equals approximately 5 ml.
ANALYTICAL CHEMISTRY
V,
MTi 154,400 110,100 99,700 80,100 59,200 31,900 18,400 4,000
M
W
25.0 27.7 28.8 31.3 32.8 36.2 39.5 42.0
Mfl 26.7 30.0 30.6 32.6 34.8 38.1 40.5 45.3
REGULAR PORASIL 250
zn
ab0
11s 160 R ~ f E N T l O NVOLUME (.I)
1
100
115
Figure 6. GPC chromatograms of medium viscosity TV grade polyvinyl alcohol from three suppliers Conditions same as Figure 1
DEACTIVATED PORASIL 2 5 0 I 1
l 1
l 1
l 1
l 1
l 1
t 1
l 1
l 1
l 1
l 1
l 1
l 1
50 25 RETENTION VOLUME (ml)
1
0
Figure 4. Retention of valeric acid Conditions same as Figure 3
+
It5
200
175
IW "EIEYIlO"
I25
KII
I
It
VOLUME IIII
Figure 7. GPC chromatograms of polyvinyl alcohol for different applications Conditions same as Figure 1 except sample 10 mg
200
175
150 l RETENTION VOLUME (ml)
s
s
Figure 5. GPC chromatograms of polyvinyl alcohol from various methods of production Conditions same as Figure 1
determined as S = (b/a)*where b is the peak width after the maximum and a is the peak width before the maximum. The deactivated Porasil showed a V , of 65 ml and a skew of 2.3. Galactose, used to measure the total permeation volume ( V , ) showed a V , of 60 ml. Because pyridine is one of the most strongly adsorbed materials on silica, the modest adsorption and improved peak symmetry indicate that the new Porasil is virtually inactive, and quite suitable for size separation in aqueous media. A symmetrical peak was also obtained with valeric acid using the deactivated Porasil which supports the conclusion that adsorption of most material is virtually eliminated. Figure 4 shows the comparison of the chromatograms of valeric acid obtained with both deactivated Porasil and regular Porasil. Valeric acid elutes from regular Porasil before galactose (1, = 0.87) with a leading peak, whereas it elutes from deactivated Porasil with virtually the same V , ( t r = 1.05) as galactose with a symmetrical peak. It is probable that the
=
0.5% for 120 sec =
OH groups on regular Porasil limit the access of the valeric acid to the entrapped solvent in the gel pore, causing both skew and early elution. The deactivated Porasil permits statistical access based on size, yielding a gaussian curve with the correct V,. The marked decrease in adsorption of such highly active materials as pyridine and valeric acid by the deactivated Porasil indicates that the material is extremely desirable for GPC separations where separation by steric effects alone is required. Polyvinyl Alcohol. Polyvinyl alcohol, permanently retained on regular Porasil, was reproducibly fractionated on the deactivated Porasil with no evidence of adsorption. Figure 5 shows distribution curves of polyvinyl alcohol prepared by suspension polymerization, solution polymerization, and bulk polymerization. The bulk polymerized material is lower in molecular weight and shows a much narrower distribution than the suspension and solution polymerized material. A low molecular weight polyvinyl alcohol used as a seize for snythetic yarns as shown in Figure 6, along with a broad distribution adhesive grade, such as is used in a commercial glue. The distribution curves of three medium viscosity polymers used as television tube adhesives are shown in Figure 7. The materials were obtained from three different suppliers. The difference in mean molecular weight and distribution is evident from the chromatograms. RECEIVED for review April 4,1969. Accepted June 4,1969. VOL. 41, NO. 10,AUGUST 1969
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