Polymer Molecular Weight Methods

9 Fractionation of Linear Polyethylene with Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on October 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0125.ch009

Gel Permeation Chromatography. V. IUPAC Samples

NOBUYUKI NAKAJIMA

1

Plastics Division, Allied Chemical Corp., Morristown, N. J. 07960

The number and weight average molecular weights were determined for two samples of linear polyethylenes distributed by the Macromolecular Division of IUPAC. The methods used were GPC, osmotic pressure, infrared analysis, melt viscosity and intrinsic viscosity. Data interpretations are discussed for each method. By comparing the results the average molecular weights were obtained; for one sample, M = 10,500 to 11,000 and MW = 150,000 to 165,000: for another sample, MN = 13,600 to 18,500, and MW = 40,000 to 48,000. N

T n the previous papers of this series (1, 2, 3, 4) calibration and reproducibility of gel permeation chromatography ( G P C ) have been ex tensively examined. This paper describes the application of G P C to two selected samples of linear polyethylenes, one having a narrow molecular weight distribution ( N M W D ) and another a broad molecular weight distribution ( B M W D ) . These samples were distributed by the Macromolecular Division of I U P A C (5) for the "molecular characterization of commercial polymers." The average molecular weights by G P C are com pared with the data obtained from infrared spectroscopy, osmotic pres sure, melt viscosity, and intrinsic viscosity. Problems associated with data interpretation are discussed.

Present address: B. F. Goodrich Co., Development Center, P.O. Box 122, Avon Lake, Ohio 44012. 1

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

9.

ΝΑΚΑ J I M A

Fractionation of Linear Polyethylene

99

Experimental Operating Condition of G P C . The operating conditions were the same as before (2) except that the sample concentration was 0.5%. Sol vent was 1,2,4-trichlorobenzene, temperature was 137 °C, injection time was 120 sec, and the flow rate was 1 cc per minute. Four columns having nominal capacities of 7 Χ 10 , 3 Χ ΙΟ , 10 , and 10 were used.

Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on October 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0125.ch009

6

6

5

3

Calibration. Figure 1 shows the calibration curve based on seven narrow distribution polystyrene standards. This is calibration No. β i n the previous paper (4). A t high molecular weights, where there was no calibration standard, a linear extrapolation was used i n the semilogarithmic plot. This extrapolation is arbitrarily chosen. As long as the extra polation is used, there is some degree of uncertainty (3). Some i m provement resulted by using a broad distribution polyethylene as a supplementary standard—i.e., a control sample ( Figure 2 ). The molecular weight distribution of this standard was predetermined by the following

\ \

10

18

Figure 1.

\

\

22

\

2 65 30 34 COUNT NUMBER

38

42

Calibration curve No. 6 with polystyrene standards

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


100

P O L Y M E R

M O L E C U L A R

W E I G H T

M E T H O D S

COUNT NUMBER

Figure 2. Calibration curve No. 6 corrected by using broad distribution polyethylene as a reference sequence. First, a G P C fractionation curve was obtained. Then, the number average, A , and the weight average molecular length, A , were calculated by using a polystyrene calibration curve ( calibration No. 3 of the previous paper) (4). Then, A and A were calibrated to the num ber average, M , and weight average molecular weight, M , respectively by a constant factor of 17.5. The M value was based on the end group analysis by infrared spectroscopy. The M had been calculated from the low shear Newtonian viscosity of the melt. F o r J h e control sample, polyethylene, two sets of values were obtained for A and A - One set was based on calibration No. 3 and another on No. 6. These sets d i d not have the same values. Therefore, correction factors, f and f , were derived as follows: N

w

N

w

N

w

N

w

N

w

N

f

N

= ί^φ

= 0.774

Sw = St

= 0.840


w

(1)

9.

NAKAjiMA


101

where the subscript numbers refer to the particular calibration. This enables us to convert (A ) and (A )Q to M and M , respectively. N

17.5 f

N

17.5

6

=

(A ), N

fw

N

17.5

ÇT )i N

= 17.5

(A )t w

W

=

(A )z

=

w

(2)

M

N

M

(3)

w

The correction factors, f and f , together with the conversion factor of 17.5 are applied to the samples in the present study. Instead of correcting the average molecular length, each molecular length, At, may be corrected against the reference: N


W

w

(Aj)

z

This was done i n the following way. First, two cumulative distribution curves were prepared for the control sample; one is based on the calibra tion No. 3 and the other on No. 6.

(£ )r(IM h-^ ^ dW

(£ ^ dw

dA

=

A

(5)

(f^ )/ - ^ dA

s

ihsA

(6)

Then, (At)3 and ( A i ) were read from the curves at the same cumulative fraction, 6

( / » .

-

(

7

)

The values of were calculated by Equation 4 for all ranges of the cumu lative fraction. The results of the calculation enabled us to compare the characteristics of G P C performances at two different times. In this treat ment any difference i n the G P C performance—e.g., resolution—was ex pressed in terms of the correction factor, f to the chain length, A4. The calibration curve of Figure 1 is a plot of (log A s ) vs. count number, N . The curve was corrected according to the procedure de scribed above; the new calibration curve is a plot of (log Ai)3 = log ft + (log Αί)β against N . A use of this calibration curve is discussed later with the samples of the present study. Noteworthy is a significantly dif ferent performance of two G P C runs at the molecular lengths from 20,000 to 100,000. W i t h the polystyrene standards alone such detailed difference could not be detected. T o illustrate this point the molecular lengths of the polystyrene standards are indicated on the graph. u

6

6

6


102

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

Results


G P C runs of B M W D and N M W D samples were made within two days after calibration No. 6 was done, and the G P C traces are shown i n Figures 3 and 4. The baseline was stable and reproducible before and

COUNT

Figure 3.

21

NUMBER

GPC trace of BMWD sample

I

I

ι

ι

ι

ι

ι

ι

ι

1

ι

23

25

27

29

31

33

35

37

39

41

43

COUNT

Figure 4.

NUMBER

GPC trace of NMWD sample

after elution of the samples. However, the separation from the baseline at the low molecular weight region is subject to interpretation because of interference b y the additives i n the polymers a n d / o r the impurities i n the system. Treatment of the data i n this region is discussed below. The height reading at every half count was fed into a computer program (6) together w i t h the calibration data of Figure 1 or 2. Table I is a summary


9.

Ν Ά Κ Α j

iMA


103


of the molecular weight data obtained by G P C and other methods. The experimental techniques and the data treatment are discussed individually. G P C D a t a Treatment. As discussed above, treatment of the data at the low molecular weight region presented a considerable problem. Inter pretation 1 treated data as if there were no molecules smaller than chain length A4 = 50; it gave an arbitrary cutoff at the low molecular weight. Interpretation 2 faithfully treated the data as it appeared in Figures 3 and 4. This meant that for B M W D sample there was no interference from the negative peak. For N M W D sample the chain length of as short as 10 A was included. F o r both interpretations the calibration was done according to the curve in Figure 1. These two interpretations gave a large difference i n M values of N M W D sample obviously because of a low molecular weight peak influencing the difference. N

The average molecular weights by interpretation 1 were corrected by the method described previously (4)—that is, the correction factors, f and f , were applied. The results are shown as interpretation 3. The results by interpretation 4 were based on the calibration curve of Figure 2. For N M W D sample the low molecular weight peak at the count number larger than 37 was ignored, assuming that the peak resulted from an impurity. For all four interpretations the conversion factor, 17.5 was used to calculate the molecular weights from the chain lengths. N

w

Osmometry. Measurements were made in tetrahydronaphthalene at 130°C by using a membrane osmometer (Mechrolab model 502). The plots of osmotic pressures at different concentrations are shown in Figure 5. Comparing the osmotic pressure results with those of G P C (interpre tation 1) it appears that molecules larger than A* = 50 may have per meated the membrane. If this is the case, the M by osmometry is too high. N

Infrared Spectroscopy. Spectra were obtained on a Perkin-Elmer model 521 grating infrared spectrophotometer. The amount of branching in these materials is very small and appears to be of the methyl type, which is indicated by the small absorption near 1140 cm" . E t h y l branch ing was also sought but not detected at 770 cm" . The results are sum marized i n Table II. 1

1

Calculation of the methyl branch is based on the assumption that one double bond exists per molecule. The methyl absorbance at 1378 cm" , in excess of that required for methyls terminating the backbone, was interpreted as methyl branches. The value may be incorrect if the as sumption is not valid. Also, a part or all of the methyl may correspond to long branches rather than methyl branch. The number average mo lecular weight by interpretation 1 is based on the assumption that there is one double bond per molecule. This assumes the presence of branches. 1


104

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

GPC


BMWD

Interpretation

Technique

M

1 2

M e l t viscosity 150°C 170°C 190°C Intrinsic viscosity

M

N

1 2

w

Mw/M

193,000 150,000 162,000 165,000

13,500 11,000 10,500 10,600 (17,700) 10,500 (8,600)

1 2 3 4

Osmometry Infrared

Molecular Weights of

14.3 13.6 15.4 15.6

— —

— — —

(112,000) (115,000)

— — — —

200,000 (100,000)

—

ο ζ 0

O.I

0.2

0.3

0.4

0.5

C C O N C E N T R A T I O N OF

Figure 5.

P O L Y M E R , IN

g/dl

Osmotic pressure measurements with BMWD and NMWD samples

Table II.

Infrared Results

BMWD M e t h y l branch/1000 carbons Unsaturation, wt % vinyl trans vinylidene

NMWD

0.6

0.6

0.248 trace 0.008

0.080 0.003 0.006


9.

105


Ν Ά Κ Α j i M A

T w o Commercial Polyethylenes

NMWD


M

M

N

17,500 10,700 13,600 13,700 20,400 (30,200) 18,500

Mw/M

w

56,100 55,600 47,200 45,800

— — —

39,800 41,700 44,700 (87,100) 47,900

N

3.20 5.20 3.47 3.35

— — — — — — — —

The M value for N M W D sample appears to be too large. O n the other hand, if we do not assume the presence of the branching at all, the M values are smaller, as given in interpretation 2. Melt Viscosity. L o w shear melt viscosities were measured by Kepe's cone-plate viscometer (7) at 150°, 170°, and 190°C. No stabilizer was added to the sample. The flow curves are shown in Figure 6. The vis cosities of N M W D are i n good agreement with those observed by others (8); the viscosities of B M W D by our measurements are somewhat lower. The Newtonian viscosities, η , were observed with N M W D sample. W i t h B M W D sample, η was estimated by extrapolation shown in the figure. The extrapolated values are uncertain; they may have been underesti mated. The Newtonian viscosities are listed i n Table III. N

N

0

0

The M

w

values were calculated by an equation from Tung ( 9 ) : log V = 3.4 log M 0

w

+ (1.64 X 10 /T) 3

15.5

Intrinsic Viscosity. Intrinsic viscosity was determined i n tetrahydronaphthalene at 130°C. B M W D = 1.80 d l / g r a m ; N M W D — 0.98 d l / g r a m . These values were converted to the corresponding values i n decahydronaphthalene at 135°C using a conversion factor of 1.16 given by Tung (10). Then, an equation by Billmeyer ( I I ) for whole polymer was used to calculate M (interpretation 1). For N M W D sample the equation for the fractions ( I I ) may be more appropriate; the calculated M values are given as interpretation 2. w

w


106

P O L Y M E R

M O L E C U L A R

W E I G H T

M E T H O D S

I0 c 6


B M W D

N M W D

ο υ

I0

3 L

ισ

10"

3

ΙΟ"* SHEAR

Figure 6.

RATE, f

ιο'

10" -I , IN S E C .

Melt viscosities of BMWD and NMWD samples

Table III.

Newtonian Viscosities of Melt Flow

NMWD

BMWD"

V Χ ΙΟ" 0

Temperature, °C 150 170 190

poise (3.5) (2.5)

5

Vo Χ ΙΟ"

4

poise 1.10 0.80 0.58

° Value estimated

Discussion B M W D Sample. The M of this sample appears to be about 10,50011,000. The values obtained by G P C with interpretations 2, 3, and 4 are in good agreement. They also agree with the result from the infrared analysis with one-double-bond-per-molecule assumption. This automati cally assumes the presence of small amounts of branches i n the sample. If we assume the unbranched chain, M is 8600, which is somewhat smaller than other values. The G P C M by interpretation 1 is larger because of the artificial cut-off at the molecular length larger than 50 A . The osmotic pressure measurements gave a larger value, which is prob ably the result of the diffusion of low molecular weight species through the membrane. The M is more difficult to estimate precisely. The melt viscosity results provide only an order of magnitude which is about 100,000. Since Newtonian viscosity was not observed, no refinement can be made on N

N

N

w


9.

107


NAKAjiMA

the data. The intrinsic viscosity gives an estimate of M of 200,000 ac cording to an equation for whole polymer. If we use the equation for fractions, M is estimated as 110,000. Although the value agrees well with that from the melt viscosity, it must be i n error because this polymer has a broad molecular weight distribution. The M values by G P C are between the above two extremes. Per haps the correct value lies between 150,000 and 165,000 because these are obtained by better data treatment interpretations 2, 3, and 4. Taking the best estimates of M and M , the ratio is calculated as M /M = 13.5 to 15.5. _ N M W D Sample. The M values of this sample are i n the range 10,000 to 30,000. However, 30,200 obtained from the infrared analysis appears to be too high. The value is based on the one-double-bond-per-molecule assumption. It automatically assumes the presence of branches. If no branches are present, M is calculated as 18,500, which is more i n line with other data. T h e G P C M values are between 10,700 and 17,500. The lower value is the result of including the low molecular weight peak between 10 and 35 A . Probably this peak is not of polyethylene fraction because the lowest possible M estimated from the infrared is 18,500. M b y osmometry may be too large for the same reason stated before. The correct M for this sample is probably 13,600-18,500. The M values are i n better agreements than M 's. They are i n the range 40,000-48,000. The M calculated from intrinsic viscosity favors equation for fractions rather than that for whole polymers. The best estimates of M /M ratios are 2.0-3.5. w

w


w

N

w

W

N

N

N

N

N

N

N

w

N

w

W

N

Acknowledgments The author is indebted to R. T. Guliana for the osmotic pressure data and intrinsic viscosities, to G . A . Tirpak for the infrared measurements, and to R. D . Hoffman for the melt viscosities.

Literature Cited 1. Nakajima, N., J. PolymerSci.,Pt A-2 (1966) 5, 101. 2. Nakajima, N., J. Polymer Sci. (1968) C-21, 153. 3. Nakajima, N., SeparationSci.(1971) 6 (2), 275. 4. Nakajima, N., J. Appl. PolymerSci.(1971) 15, 3089. 5. Benoit, H., Macromolecular Division of IUPAC, Centre de Recherches sur les Macromolecules, Strasbourg, France. 6. Pickett, H. E., Cantow, M. J. R., Johnson, T. F., J. Appl. Polymer Sci. (1966) 10, 917. 7. Kepe, Α., J. Polymer Sci. (1956) 22, 409. 8. Wales, J. L. S., Pure Appl. Chem. (1969) 20, 331. 9. Tung, L. H., J. PolymerSci.(1960) 46, 409. 10. Tung, L. H., J. Polymer Sci. (1959) 36, 287. 11. De La Cuesta, M. O., Billmeyer, F. W., Jr., J. Polymer Sci. (1963) A-1, 1721. RECEIVED January 17, 1972.


Polymer Molecular Weight Methods

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