Reproducibility of Molecular Weight Measurements ... - ACS Publications

10 Reproducibility of Molecular Weight Measurements by GPC with Infrared Detectors

Downloaded by DICLE UNIV on November 6, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0125.ch010

J. H. ROSS, Jr. and R. L. SHANK Research and Development Department, Chemicals and Plastics, Union Carbide Corp., South Charleston, W. Va. 25303

The variability of tests for physical properties and structural features of polymers usually exceeds ± 5 % at the 95% confidence interval. Molecular weight measurements of polymers are generally accepted to have large variances, in particular, the interlaboratory round robins. With the advent of gel-permeation chromatography (GPC) and with the increased attention given measurements of this kind, significantly lower levels of variability may be expected. This paper shows that the capability of the GPC technique for measurement of molecular-weight parameters, Mw and Mn, for polyethylenes is better than ±2.5% in repetitive measurements over periods as long as a month. The excellent performance of this GPC system is attributed to the high photometric precision and sensitivity of the infrared detection device, to the data acquisition and computational procedures, and to the stabilizer used. "industry has rapidly accepted the G P C technique and exploited it for a variety of uses including quality control, guidance of product blend ing and polymer syntheses, and establishment of physical and structural property relationships. In each of these areas, requirements for pre cision have increased as more information was obtained. Operation of the Waters Associates model 100 instrument at ele^ vated temperatures, necessitated by dissolution of polyethylene, requires several modifications in order to improve baseline stability and sensitivity ( J ) . Usually the first modification is to reduce the unacceptably high temperature gradients in the oven which may be as much as 20 °C from A

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


10.

ROSS A N D SHANK

Reproducibility of Measurements

109

top-to-bottom when the oven is operated at 130 °C. T h e refractometer requires thermostating to better than ±0.01 °C, and the original re fractometer is generally replaced with the Waters Associates R-4 refrac tometer. W i t h these and even other modifications, it has not been possible to perform the analysis with the precision required by many of the appli cations for the data. Several of the Waters Associates units have been relegated to operation at relatively low temperatures only, including the one i n the authors laboratory. Other workers have accepted the poor precision and the difficult task of operation of the chromatograph at high temperatures and have attempted to draw conclusions from these data even though the reproducibility is not as high as desired ( 2 ) . Statistical studies of the precision of measurements of molecular weight parameters b y G P C are not common ( 3 ) . Interlaboratory round robins of G P C analyses conducted by A S T M and other GPC-oriented groups have shown high variance. Gamble et al. indicate a precision of about ± 5 % for the replicate measurement of butyl rubber analyses of Mw and Mn (4). The test apparatus described here evolved as an effort to maximize precision and accuracy of the G P C technique. W e chose the National Bureau of Standards ( N B S ) Standard Reference Material No. 1475 to indicate both the precision and accuracy capabilities of the system. It was assumed that the N B S sample is homogeneous and that the values of molecular weight parameters have high accuracy, although N B S considers it impossible to provide a statement as to the absolute accuracy at this time ( 5 ) . The purpose of this paper is to demonstrate that the precision and accuracy capabilities of the G P C technique when operating at high temperatures for long periods can be quite high. It is not an attempt to make comparative systematic appraisal of the factors affecting precision and accuracy of this system with that of the Waters Associates chro matograph, since i n the author's laboratory even after quite vigorous effort the precision and accuracy of the latter instrument were so com pletely unacceptable at the high temperatures required for polyethylenes as to make such a comparison impossible. W i t h the high precision and accuracy of the system discussed herein, many more applications for the data are revealed. Experimental Apparatus. A G P C system using infrared spectrophotometric detec tion of the column effluent was described earlier ( 6 ) . Because only branched polyethylenes were examined initially, certain modifications were necessary for the separation of linear polyethylenes. Certain i m proved components have been added although essentially the same appa ratus as that already described was employed. F o r example, the oven

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

110

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

containing the columns was designed to have variations in temperature less than ± 0 . 0 1 ° C from end to end. Porous glass-bead column packing was replaced with the cross-linked polystyrene gels (Styragel) as i m proved columns became available. The four columns contained gels having nominal porosities of ΙΟ , ΙΟ , 10 , and 10 A and had plate counts of better than 700 plates/foot as determined with nonane. A Hughes Series II micropump was used to pump degassed perchloroethylene at a reproducible rate of 1 ± 0.003 ml/minute. This pump produces 300 overlapping pulsations per minute and was used without a pulsation damper because pressure variations were less than ± 1 psi. A Waters Associates automatic injection unit was used to add 2 m l polymer solution containing approximately 1 to 2 mg polymer to the stream for each determination. The concentration of sample is selected so that the height of the curve is about 7 inches. The concentration may range between 0.04 and 0.075%. This injection unit when loaded is capable of automatically injecting seven samples within a 24-hour period. Degrada tion was avoided by adding 100 ppm 3,5-ditertiary butyl catechol to the solution. A schematic of the detection recording and data acquisition system is shown in Figure 1. The detector ( Perkin-Elmer model 112) had a double-pass optical system and a calcium fluoride prism set at 3.4μ. (2940 cm" ). For the lowest signal-to-noise ratio, it was necessary to replace the globar with a Sylvania F T C tungsten-iodine, 375 W lamp operated at 100 W . The stability of this lamp was found to exceed that of other infrared sources by a factor of at least ten. Life time of the tungsten-iodine bulb operating at reduced wattage is at least three


4

5

6

7

1

Figure 1.

GPC infrared detection, recording, and data acquisition system



10.

ROSS A N D SHANK

Figure 2.


111

Recorder output on a 10-inch chart for 2 mg of NBS SRM No. 1475

months and some have exceeded a year. Baseline stability is better than ± 1 % i n a 24-hour period. A special zero suppressor was connected to the output of the pre amplifier so that the transmission from 80 to 100% of the incident energy could be recorded. The relationship of concentration to signal output was found to be essentially linear i n this range. A n example of the re corder output on a 10 inch chart is shown i n Figure 2. Data Acquisition. A Control Data Corp. ( C D C ) 1700 process con trol computer was used for on-line data acquisition and storage without manual intervention, accelerating the collection of experimental data by eliminating the burden of handling the large quantity of data involved. Data reduction, which included smoothing, baseline correction, interpre tation, and average molecular-weight calculations, was performed at a convenient time with simple operator initiation, producing tabular and graphical displays of the molecular weight distribution. The computer usage improved the accuracy of the results by employing a statistical analysis of the data instead of depending upon personal interpretation. The recorder drive gear was connected to a potentiometer containing an applied voltage. The potentiometer output was directly proportional to the infrared absorption. The output signal was sent directly to a process computer where an analog-to-digital converter produced a digital fixed-point quantity. This value was stored i n a current value table, i n main storage, containing 15 locations. The converter output signal was monitored continuously. W h e n no sample was i n the system the signal was zero and the scan frequency was 60 second. When a sample was injected, the signal increased to a previously selected baseline value, the scan interval changed to 5 seconds, and the contents of the current value table, when filled, were stored on



112

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

the computers disk files. A t the end of the run the converter output signal was automatically reduced to zero, and the file for that particular set of data was closed. It was possible to inject another sample immedi ately and store the data in another file. The sample injection and data acquisition were entirely automatic, requiring no manual supervision. Data reduction required manual initiation and was done at a convenient time. This may also be automated, but at the present time software problems have not been completely resolved. The experimental data were smoothed by fitting short segments to a first-degree polynomial. The volume markers were located during the smoothing process. A l l calculations were performed on the smoothed data. Number-, weight-, and Z-average molecular weights were calcu lated using standard methods described in most texts on polymers. Besides the calculation of average molecular weights, several other means of characterizing the distribution were produced. These include tables of percentile fractions vs. molecular weights, standard deviation, skewness, and kurtosis. The data for the tables were obtained on punched cards as well as printed output. The punched cards were used as input to a C A L C O M P plotter to obtain a curve as shown in Figure 3. This plot is normalized with respect to area. No corrections were made for axial dispersion. Calibration. The accepted method of calibrating a G P C system was used. Narrow molecular-weight distribution high-density polyethylene polymers were characterized by light scattering, osmometry, and sediJ I

TIME DRTE

2056. 1222.

§ S

LOG NH

Figure 3.

CAL COMP plot of NBS SRM No. 1475


10.

ROSS A N D SHANK

113


mentation techniques. These data were used to obtain the elution volume-molecular weight relationship. A universal calibration curve was established by plotting the product of the limiting viscosity numbers and molecular weight, M [r{\, vs. the elution volume, EV, for a variety of characterized polymers. The major usefulness of the universal calibration curve was to validate individual molecular-weight values and to provide extended molecular-weight cali bration at the ends of the calibration curve where fractions of narrow dispersion of the polymer being analyzed are not available. The calibra tion curve was monitored daily with polystyrene fractions certified by Pressure Chemicals. The relationship between the polyethylene frac tions and polystyrene fractions was determined using the universal calibration curve. The calibration was represented i n the computer program by a fifth-degree polynomial. The conventional method of least-squares was followed to determine the coefficients of the polynomial. The sensitivity of the normal equations made round-off error a significant factor i n the calculations. The effect of round-off error was greatly reduced when the calculations were performed with double-precision arithmetic. The mo lecular weights corresponding to selected count numbers were calculated from the coefficients. The coefficients were input information for the data-reduction program.


w

Discussion Precision and Accuracy. Table I shows the results of tests on the linear polyethylene N B S Standard Reference Material N o . 1475. This reference material has a pellet-to-pellet coefficient of variation of 3 % in the limiting viscosity number according to the N B S Certificate. A t least 50 pellets are recommended for a representative sample on which limiting viscosity number is obtained. Nine analyses were performed over a period of about one month on the pellets, using approximately three pellets per determination. The analysis of these data is shown i n Table I along with a comparison with the N B S data and their estimate of precision. Subsequently, 3 grams (at least 60 pellets) homogenized by dissolution and precipitation as Wagner of N B S . Table II shows the results of homogenized fine powder performed over a period

of the pellets were suggested by H . L . 13 analyses on the of about 10 days.

These data in Tables I and II show that the variation of the M , M , and M /M values at the 9 5 % confidence interval is less than ± 2 . 5 % for this particular polyethylene. Variation of M appears to be about twice this level. Comparison with the N B S molecular-weight parameters indicates excellent absolute accuracy except for M . The tables also show that pellet-to-pellet variations i n molecular weight could not be detected. n

w

n

z

z


xc

114

POLYMER

MOLECULAR WEIGHT METHODS

Table I.

G P C Analysis of N B S

Time

Date

Loop

M J1000

336 1744 2109 34 400 1404 2056 1506 438

1203 1218 1218 1219 1219 1222 1222 104 105

1 3 4 5 6 3 5 5 3

17.06 17.59 17.71 18.70 17.88 18.29 18.30 17.17 17.90 17.84 17.84 ± 0.437 ± 2.4% 18.3 ± 1 . 2


X

Variability of mol wt parameters by G P C (South Charleston) Variability of mol wt parameters (NBS)

Each analysis performed on approximately three pellets per determination or 0.075 gram. a

Table II.

G P C Analysis of N B S Standard N o .

Time

Date

Loop

M J1O0O

206 857 106 439 1618 234 1513 127 1902 1450 105 135 42

1208 1208 1209 1210 1210 1211 1211 1212 1214 1215 1216 1217 1218

1 3 1 4 1 4 1 4 4 4 1 1 6

18.15 17.74 18.79 16.88 16.80 16.77 17.07 17.09 18.66 17.49 17.98 17.40 18.10 17.61 17.61 ± 0.384 ± 2.2% 18.3 ± 1 . 2

X

Variability of mol wt parameters by G P C (South Charleston) Variability of mol wt parameters (NBS)

1475

Three grams of pellets were dissolved in perchloroethylene, slowly precipitated in cold methyl alcohol, and dried, producing a fine powder. Each analysis was performed on 0.075 gram of the homogenized powder. α

The excellent performance of the instrumentation results from a combination

of contributions from several components, i.e., the high

sensitivity and stability of the infrared detection system, the data acqui sition and computational procedures, and the stabilizer used.


10.

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Standard N o . 1475 M /1000

115

Pellets" M /M

MJ1000

w



W

N

54.68

129.5

53.45

123.0

3.04

55.09

131.3

3.11

3.20

57.04

135.0

3.05

54.83

128.8

3.07

54.34

141.6

2.97

54.75

143.1

2.99

53.14

120.7

3.09

53.25

121.4

2.97

54.51

130.49

3.05

54.51 ±

0.992

± 1.8% 54.2 ± 2.0

1 3 0 . 4 9 =b ±

6.72

5.1%

149 ±

3 . 0 5 =fc 0 . 0 6 1 ±

2.0% 2.96

13

Dissolved and Precipitated in Methyl Alcohol M /1000 W

M /M W

N

56.06

132.9

3.09

56.48

133.8

3.18

57.72

158.5

54.60

135.1

53.35

125.8

3.18

53.53

128.4

3.19

3.07 3.23

52.28

119.7

3.06

54.32

131.5

3.18

58.30

139.9

3.12

56.11

136.7

3.21

52.70

114.7

2.93

54.77

129.4

3.15

54.95

128.4

3.04

55.01

131.91

3.12

1 3 1 . 9 1 =fc 6 . 5 7

3 . 1 3 =fc 0 . 0 5 3

5 5 . 0 1 =fc ±

MM/1000

1.16

2.1%

54.2 ±

2.0

±

5.0%

149 ±

13

±

1-7% 2.96

Acknowledgment W e acknowledge the contributions of the computer section at this laboratory, i n particular J . S. Bodenschatz, L i n d a Jarrett, Richard Settle, and T . J . McGovern. W e gratefully acknowledge the contributions of


116

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

F . Rodriquez for the concept of the zero suppressor and for his technical advice. In addition, the assistance of M . E . Casto and W . E . Coiner are recognized. W e thank T. P. Wilson for his interest i n this work and Union Carbide Corp. for permission to publish these results.


Literature Cited 1. Williamson, G. R., Cervenka, Α., Eur. Polym. J. (1972) 8, 1009. 2. Nakajima, J., private communication, and ADVAN. CHEM. SER. (1973) 125, 98. 3. Johnson, J. F., Porter, R. S., Prog. Polym.Sci.(1970) 2, 201. 4. Gamble, L. W., Westerman, L., Knipp, E., Amer. Rubber Chem. Technol. (1965) 38, 823. 5. Ross, G ., Frolen, L., J. Res. Natl. Bur. Std. (1972) 76A, No. 2, 163. 6. Ross, J. H., Casto, M. E., J. Polym. Sci. (1968) Part C, 21, 153. RECEIVED January 17, 1972.


Reproducibility of Molecular Weight Measurements ... - ACS Publications

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