Determination of Number-Average Molecular Weights by Ebulliometry

1 Determination of Number-Average

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Molecular Weights by Ebulliometry CLYDE A. GLOVER Research Laboratories, Tennessee Eastman Co., Division of Eastman Kodak Co., Kingsport, Tenn. 37662

This paper describes an ebulliometric system for routine and special determinations of molecular weights. The system uses a simple ebulliometer, an immersion heater, and a Cottrell-type pump. Temperature sensing is by differential thermopile. Precision varies from about 1 to 6%, and values compare well with those from other laboratories and those from other methods. Values as high as 170,000 have been successfully measured. Some problems encountered in using the ebulliometric method are: selection and effect of reference temperature, limitations of the vapor lift pump and a possible substitute for it, measurement of equilibrium concentrations within the operating ebulliometer, and the experimentally determined ebulliometric constant and some factors which influence its value. "Ebulliometry, one of the classical methods for determining molecular weights, has undergone great improvement in recent years. The re quirements of a successful ebulliometric system are thermal stability, temperature and concentration equilibrium, and temperature sensing. These requirements have been met by a number of investigators i n various ways. The systems of each are reported to have met the need for which they were designed; in spite of this, however, the method does not now appear to be used widely either for routine determinations or i n special problems. Here we present the case for what appears to be a neglected technique which is felt to have great potential. W e describe a system which has been in use for several years for both routine and special molecular weight measurements, discuss some of the results ob tained, and finally consider some of the problems and unanswered ques tions which have been encountered. 1 In Polymer Molecular Weight Methods; Ezrin, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

2

P O L Y M E R M O L E C U L A R WEIGHT METHODS


The Ebulliometric

System

The system consists of a simple ebulliometer, a temperature sensing element, and an electrical recording device. The ebulliometer is essen tially the same as that described by Glover and Stanley (I ) and is shown in Figure 1. It consists of a platinum immersion heater, a vapor lift pump of the Cottrell type, and a recycle arrangement patterned after that de scribed earlier by Ray (2). There are other successful designs, such as those of Ray (2), Lehrle (3), and others, which differ i n detail from the one shown, and there are different types, such as the shaking apparatus of Schultz (4) which was improved by E z r i n (5). Temperature sensing is an important part of ebulliometry. In the work being described, ther mopiles are used. They are very stable i n operation and change little, if any, with age. One advantage is the leveling effect of the multiple

temperature sensing points of the thermopile on rapid temperature fluc tuations caused by uneven boiling or pumping. Thermistors have been used, with equal success, for temperature sensing by many workers. Also, devices such as the quartz crystal thermometer might be adapted to this application. Finally, the signal from the thermopile is amplified i n this system by a Leeds and Northrup stabilized microvolt amplifier No. 9835A and is recorded on a strip chart recorder. Thermal stability is ob tained with a vapor jacket. This jacket normally contains the solvent to be used in the determination and is covered with aluminum foil which serves as a light shield as well as a thermal barrier. The jacket is con nected directly to the ebulliometer to prevent pressure differences, and

In Polymer Molecular Weight Methods; Ezrin, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

1.

GLOVER

3

Ebulliometry

the system is vented to ambient pressure through a surge tank. W i t h a thermopile having 80 measuring junctions i n the ebulliometer, tempera ture differences of 2 X 1 0 " C can be detected with satisfactory accuracy. Operation of the system has been simplified to permit routine deter minations by a laboratory technician with no active supervision. The procedure differs from those published by Dimbat and Stross (6) and others chiefly i n that the ebulliometer is neither washed nor dried be tween determinations. However, after fresh solvent is added, the output of the thermopile ("zero line") for the solvent is recorded. Further, a decreasing heat input program is followed to eliminate the effects of foaming and superheating. Normally three or four weighed portions of the sample are added successively, and the elevation i n boiling tempera ture is recorded for each portion. The molecular weight is obtained from these data by the limiting slopes calculation, or by any of the established calculation procedures (6, 7 ) , with the aid of an I B M 1130 computer programmed to give the molecular weight and an "error" indication for each data point.


5o

About 8000 determinations involving a range of solvents and solutes have been made with the system as described. The most recent and perhaps most interesting development i n this respect is the use of hexafluoro-2-propanol ( H F I P ) as an ebulliometric solvent for the study of polyesters and polyamides. H F I P has an ebulliometric constant ( K ) of 3.0, compared with a value of 3.3 for toluene, and behaves satisfactorily as an ebulliometric solvent. Precision of these measurements varies with molecular weight level, solvent, and to some extent, solute. W i t h a series of polyglycols, as shown i n Table I, the standard deviation was about 1% at a molecular weight level of about 4100. F o r polyethylene, as shown in Table II, precision does vary with molecular weight and is obviously influenced by other factors, such as homogeneity and purity of the sample. 6

Table I.

Precision in Molecular Weight Determination: Poly glycol in Toluene Determination

Molecular Weight

1 2 3 4 5

4189 4138 4112 4112 4164 Mean 4143 Standard Deviation 36 or 0.85% Table III shows some surprising results obtained from the N B S ( National Bureau of Standards) polystyrene sample 705. This material has a very narrow molecular weight distribution; since it is a specially prepared


4

P O L Y M E R M O L E C U L A R WEIGHT METHODS Table II.

Precision in Molecular Weight Determination: Polyethylene in Toluene


α

α

Av. M o l . Wt. η Standard Dev.

18,170 7 756 or 4.2%

Av.

Mol. Wt. η Standard Dev.

24,620 5 1886 or 5.6%

Av. M o l . Wt.

34,280

Standard Dev.

942 or 2.7%

Data from Anal Chem. (1961) 31, 449.

Table III.

Precision in Molecular Weight Determination: in Toluene

Polystyrene

N B S Reference Material 705 M o l . W t . Found 171,900 168,400 169,700 A v . 170,000 Standard Dev. 1756 or 1.03% reference material, it probably is more homogeneous than most samples and thus gives more precise data. Assessment of accuracy presents a problem since few authentic refer ence materials exist i n the molecular weight range of interest. However, our results have been compared with those obtained by other laboratories and those obtained by other methods. One comparison is shown i n Table IV. The accuracy of the method is also shown by the data ob tained from the previously mentioned N B S sample. These data are shown i n Table V . Effect of Experimental

Variables

In spite of the apparently successful performance of the ebulliometric system i n routine use, a number of variables exist i n connection with the operation of the system as it has been described. M a n y of these variables, as many as possible, are overcome by standardization i n routine opera tion. However, they must be recognized if the ebulliometric method is to be understood and applied to special problems. First, the use of the condensing solvent vapors as a reference tem perature can lead to considerable uncertainty, particularly with nonpolymeric solutes, because changes i n the reference temperature, w h i c h



1.

GLOVER

5

Ebulliometry

result from the presence of impurities i n the solvent or the solute or from solute volatility, may go undetected. Such changes have actually been measured with special thermopile circuits and are real and significant. If a boiling liquid reference (twin ebulliometer) is used, the problem is less acute. A n example is shown i n Table V I ; the results were obtained when both types of reference temperatures were used to determine the molecular weight of biphenyl (bp, 256°C) i n toluene (bp, 110°C). These results may explain some of the confusion found i n the literature con cerning the required difference between the boiling temperatures of a solute and a solvent necessary for valid use of the ebulliometric method. Table IV.

A c c u r a c y in Molecular Weight Determination: in Toluene*

Polyethylene

Ebulliometry Sample

Lab A

Lab

1 2

11,500 18,400

B

r r

Lab C

Cryoscopy

11,500 19,200

11,500 —

Vapor Pressure Osmometry

10,700 19,100

10,900 18,800

* 'Advances in Analytical Chemistry and Instrumentation," Vol. 5, Table X I I , Chapter 1, p. 63, Wiley, New York 1966. α

Table V .

A c c u r a c y in Molecular Weight Determination: in Toluene

Polystyrene

N B S Reference Material 705 M o l . W t . (Elbulliometry) 170,000 M o l . W t . (Osmometry—NBS) 170,900 Table V I . Reference Type

Condensing Vapor Boiling Liquid

Molecular Weight of Biphenyl in Toluene Mol. Wt. Found

512 171

Theory

154 154

The Cottrell-type pump presents an additional variable. W i t h it the rate and the heat input to the boiling solution cannot be varied inde pendently. For this reason, the possibility of superheating cannot be rigorously eliminated. To overcome superheating, a no-dead-space me chanical pump ( Figure 2 ) was designed, constructed, and installed i n an experimental ebulliometer as shown i n Figure 3. The unique feature of this pump is the loose fit of the piston which permits continuous flow of liquid through the piston chamber during operation. The problems of superheating and equilibration are currently being reexamined with this apparatus i n a twin configuration.


6

P O L Y M E R M O L E C U L A R WEIGHT METHODS

PLATINUM WIRE

4-MM.-O.D. TUBE 6-MM. ROD


8-MM.-O.D. TUBE

6-MM.-O.D. TUBE

Figure 2.

Mechanical pump for ebulliometer

SAMPLING CAPILLARIES

PLATINUM WIRE

SAMPLING CUP

SOLVENT LEVEL PUMP

Figure 3.

Experimental ebulliometer

A further need is to know the actual solute concentration at the temperature sensing point i n the ebulliometer at operating equilibrium. Amounts of solvent and solute added to the ebulliometer can be accu rately measured and controlled; however, their distribution within the apparatus at equilibrium is difficult to establish. As shown i n Figure 3, provisions have been made for removing samples during operation. T h e problem of solvent-solute distribution and the accompanying problem of analysis are being studied, but so far the studies have been less than completely successful. The last variable to be discussed is that of the experimentally de termined value for the ebulliometric constant, K . Equation 1 indicates b


1.

GLOVER

7

Ebulliometry

that theoretically Δ Γ varies with solute molecular weight to make K constant.

b

6

π _ Solute mol wt (grams) X AT (°C) X solvent wt (grams) ~ 1000 X solute wt (grams) b


6

, . ^'

In the work described here, this has not been found to be true. C o n siderable effort has been expended to eliminate this apparent inconsistency or to explain it as instrumentally induced. To date neither has been successful, and the observations remain unexplained. It appears from this work that the experimentally determined value of K is influenced by both the chemical nature and the molecular weight of the solute. b

(C H ) 6

(C H ) Si

Si(C H )

(C H ) Si

.Si(C H )

6

s

6

5

2

6

2

5

6

(C H ) 6

5

5

(C H ) S r

2

6

5

5

^Si(C H ) 6

2

(C H ) Si

2

6

5

6

(C H ) 6

M O L WT.911 I (C H ) Si

Si(C H )

(C H ) Si

Si(C H )

6

6

5

5

2

2

6

5

6

5

5

Si(C H )

2

2

2

5

s

2

2

2

M O L WT. 1094 II 2

2

MOL. WT. 729 III

Figure 4.

Organosilicon compounds

This is demonstrated by a problem i n which an effort was being made to establish the structure of an unidentified organosilicon compound pre pared by D r . Gilman of Iowa State University. Other data and the his tory of the compound indicated that the structure was either I or II as shown i n Figure 4. The boiling point elevation of the unknown compound was obtained i n toluene with the apparatus shown i n Figure 1. A K which had been established with tristearin (molecular weight 891) was used to calculate the molecular weight of the unknown, and an incon clusive value of 973 was obtained. A new K was then established with an authentic compound of structure III, Figure 4. The new K was used to recalculate the molecular weight of the unknown, and a value of 921 was obtained. The structure was later confirmed by x-ray diffraction as I, Figure 4. This paper has attempted to show that the ebulliometric method can be used successfully for the routine determination of molecular b

b

b


8

POLYMER MOLECULAR WEIGHT METHODS

weights and i n special problems. It still presents some worthy challenges and some opportunities of a theoretical nature.

Literature Cited


1. 2. 3. 4.

Glover, C. Α., Stanley, R. R., Anal Chem. (1961) 33, 477. Ray, Ν. H., Trans. Faraday Soc. (1952) 48, 809. Lehrle, R. S., Majury, T. G., J. Polym. Sci. (1958) 29, 219. Schön, K. G., Schultz, G. V., Z. Physik. Chem. (Frankfurt am Main) (1954) 2, 197. 5. Ezrin, M., Eastern Analytical Symposium, Symposium on Molecular Weight Measurements, New York, 1962. 6. Dimbat, M., Stross, F. H., Anal Chem. (1957) 29, 1517. 7. Lehrle, R. S., "Progress in High Polymers," Robb, J. C., Peaker, F. W., Eds., pp. 57-61, 1961, Academic Press, New York. RECEIVED January 17, 1972.


Determination of Number-Average Molecular Weights by Ebulliometry

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