The Polymer Standard Reference Materials Program at the National

3 The Polymer Standard Reference Materials Program at the National Bureau of Standards

Downloaded by UNIV OF LIVERPOOL on December 7, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0125.ch003

HERMAN L. WAGNER Institute for Materials Research, National Bureau of Standards, Washington, D. C. 20234

The National Bureau of Standards now distributes four polymer Standard Reference Materials designed for use in the calibration of instruments employed in polymer characterization. Polystyrene is available in narrow (SRM 705) and broad (SRM 706) distributions and polyethylene in high -density linear (SRM 1475) and low-density branched (SRM 1476) whole polymer. These materials are characterized with respect to many but not necessarily all of the following properties: weight and number-average molecular weight, limiting viscosity number in several solvents, ASTM density, and ASTM melt flow rate. In addition the molecular-weight distribution of the linear polyethylene is given, making it suitable for the calibration of gel-permeation chromatographs at high temperatures. ^T*he National Bureau of Standards has for more than 60 years provided Standard Reference Materials which have assumed a vital role in many areas of production, commerce, and research. These samples, which provide a universal basis for comparison and standardization of many material properties, are characterized with great care before they are certified and released for use. They also have the advantage of being continuously available, unchanged, for long periods of time. In 1963 the first polymer Standard Reference Materials were issued, polystyrenes S R M 705 and S R M 706. The need for improving the reliability of characterization of poly mers has long been evident. Other efforts in this direction include the round-robin testing conducted at various times, beginning in 1950, under the auspices of the Commission of Macromolecules of I U P A C ( J , 2, 3), as well as the work now in progress in A S T M concerned with establishing standardized methods of characterization. 17 In Polymer Molecular Weight Methods; Ezrin, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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A recent survey has shown that our first polymer standard samples are widely distributed and used in research and industry for the calibra tion of a variety of characterization instruments, particularly gel-permea tion chromatographs. They also serve as materials with well defined properties for research in many areas. These properties should become better defined with time as results accrue i n the literature.


Polystyrene

Standard Reference

Materials

The choice of the first polymer Standard Reference Material (4) was governed by some fairly simple requirements—that it be reasonably stable, that it be soluble in solvents ordinarily used i n characterization work, and that it be readily obtained in the molecular weight range from 40,000 to 500,000. This is the range most readily measured by conven tional molecular-weight techniques. Polystyrene easily meets these cri teria and is of special importance because it is probably one of the most studied amorphous polymers. In addition, an attractive feature of poly styrene is the fact that it can be polymerized anionically to give polymers of very narrow molecular-weight distribution. This is particularly impor tant in characterization work because of the problems caused by diffusion of the low molecular-weight species of broad molecular-weight-distribu tion polymers across the semipermeable membranes during osmotic pressure measurements. S R M 705 is the narrow molecular-weight-distribution sample; the other S R M 706, by contrast, is a broad molecular-weight-distribution polymer, the result of a thermal polymerization which more closely resembles commercially available polymers. Both of these were supplied by the D o w Chemical Co. (Certain commercial equipment, instruments, or materials are identified i n this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose. ) Because sample homogeneity is particularly important for a Standard Reference Material, it was carefully assessed for both polystyrenes using solution viscosity, a measure of molecular weight, as an index. There is essentially no variation with location within the lot, or from pellet to pellet, within the limits of error of the viscosity measurements. Viscosity measurements may be made with a standard deviation of a single deter mination of about 0.3%. The details of the measurements made on these samples are dis cussed elsewhere (4, 5 ) . The certified values from the certificates for all the samples are shown i n the Appendix. For S R M 705 the numberaverage molecular weight by osmometry, 170,900, and the weight-average

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

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Polymer Standard Reference Materials

molecular weight by light scattering, 179,300, are, as expected^ very close to each other, giving an M /M of 1.05. The value of M /Mw/M was determined by fractionation into 36 fractions giving an M /M of 1.07. Weight-average molecular weight by sedimentation equilibrium was about 190,000. Since both the light-scattering and the sedimentationequilibrium results are based on completely separate absolute calibra tions, the 6% difference between the two values is small and certainly well within the expected range of uncertainty for such measurements, which is usually considered to b e ± 1 0 % at best. In addition to the molecular-weight values, limiting viscosity numbers i n benzene at 25° and 35 °C as well as i n cyclohexane at 35 °C are certified. w

n

z

n


w

n

For S R M 706, the broad molecular-weight-distribution polystyrene, the certificate provides weight-average molecular weight both by light scattering and sedimentation equilibrium, with the latter method again showing the higher value. Limiting viscosity numbers in benzene at 25 °C and i n cyclohexane at 35 °C are given as well. Number-average molecular weight by osmometry could not be determined because of the diffusion problems referred to above. However, from the ratio of molecu lar weights found by fractionation into 44 fractions, M /M /M = 2.9/2.1/1.0, and the number average is estimated to be 123,000. Weight- and number-average molecular weights were recently re determined for S R M 705. N o significant difference could be found between the recent and earlier measurements. T h e same holds true for the viscosity numbers. The results of this more recent work are given i n a Special Publication available from the National Bureau of Standards (5). z

Polyethylene

Standard Reference

w

n

Polymers

The next polymer Standard Reference Materials, issued i n 1970, were a linear and a branched polyethylene. They are representative of crystal line olefin polymers, which have assumed great commercial and scien tific importance. The linear material was kindly provided by the Dupont Co. and the branched by Union Carbide Corp. Both samples are i n the form of pellets containing antioxidants as specified on the certificates. They have also been examined for sample homogeneity by dilute-solution viscosity. In the case of the linear material, S R M 1475, although the lot is uniform with respect to location, a measurable pellet-to-pellet variation exists. To make certain that a uniform sample is obtained, at least one gram of pellets should be blended to reduce the expectation of error from pellet variability to less than 0.5%. The branched material on the other hand is quite uniform and does not show pellet-to-pellet variation. Both materials have low ash and volatile content.



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There is no evidence of branching in the linear material, S R M 1475. Infrared measurements show that there is about 0.15 methyl group for 100 carbon atoms. This is consistent with the value expected for a number-average molecular weight of 18,000, assuming two methyl end groups per chain and no short-chain branching. The absence of longchain branching is indicated by the linearity of the log limiting viscosity number-log molecular-weight relationship for the fractions (6). The weight-average molecular weight of S R M 1475 determined by light scattering in 1-chloronaphthalene at 135 °C is 52,000 with an esti mated standard deviation of about 4%. Again, number-average molecu lar weight could not be determined by osmometry for this whole polymer. It was possible, however, to determine the number-average molecular weight and, in fact, the entire distribution by gel-permeation chromatog raphy. The columns of the gel-permeation chromatograph were first calibrated with 9 fractions of polyethylene obtained by column elution. The number- and weight-average molecular weights were determined by osmometry and light-scattering measurements, respectively, and ranged in weight-average molecular weight from 19,000 to 688,000. Two zonerefined linear hydrocarbons, C and C , were also employed to help anchor the lower end of the calibration curve, which is shown i n Figure 1. The distribution is given in tabular form on the certificate (see Appen dix). Hence this linear whole polymer may be conveniently used for the calibration of gel-permeation chromatographs without the necessity of resorting to a whole series of narrow-distribution polymers in the range 9 4

30 ELUTION

3 6

35 COUNT

40

45

Figure 1. Calibration curve for the gel-permeation chromatographic tracing obtained with polyethylene fractions, and n-C H and n - C H 9i

190

3 6

7 4



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of a thousand to a million. This is the range of molecular weights most readily measured by conventional methods. The certificate also gives limiting viscosity numbers in 1-chloronaphthalene, 1,2,4-trichlorobenzene, and decahydronaphthalene. Density and melt flow viscosity under specified A S T M conditions are also included. Hence S R M 1475 may be utilized in the calibration of a variety of instru ments used for characterizing polyethylene. A series of papers describing the methods used to characterize this sample have been published (7). The low density-branched polyethylene, S R M 1476, has been charac terized with respect to limiting viscosity number in the same solvents as the linear material, namely 1-chloronaphthalene, 1,2,4-trichlorobenzene, and decahydronaphthalene. Melt index and density were also determined using A S T M procedures indicated in the certificate. Because of the com plications arising from branched polyethylene in light scattering and gelpermeation chromatography, meaningful molecular weights could not be obtained by these methods for the whole polymer. The material has been passed through an elution column and the fractions are being charac terized for molecular weight and branching. In addition to these studies on the branched polyethylene, fractions of linear polyethylene prepared by large-scale gel-permeation chromatog raphy are being characterized for certification in the near future. These should be useful for gel-permeation chromatography calibration. W e also expect them to be particularly valuable in dilute solution, crystallization, and rheological studies. Appendix Standard Reference Material 705, Polystyrene ( N a r r o w Molecular-Weight Distribution)

Specification

Number-average molecular weight (measured by osmotic pressure) Weight-average molecular weight (measured by light scattering) Weight-average molecular weight (measured by sedimentation equilibrium) Limiting viscosity number (ml/gram) (intrinsic viscosity) Benzene, 25°C Benzene, 35°C Cyclohexane, 35°C Ratios of molecular weight (based on fractionation)

No. of Determin ations

Average

Standard Deviation of Average

12

170,900

580

9

179,300

740

22

189,800

2,100

5 13 6

M /M /M z

w

n

74.4 0.18 74.5 0.23 35.4 0.24 = 1.12:1.07:1


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Standard Reference Material 706, Polystyrene (Broad Molecular-Weight Distribution)

Specification

No. of Determinations

Average

12

257,800

930

288,100

9.600

93.7 39.5

0.19 0.10

Weight-average molecular weight (measured by light scattering)


Weight-average molecular weight (measured by sedimentation equilibrium) Limiting viscosity number (ml/gram) (intrinsic viscosity) Benzene, 25°C Cyclohexane, 35°C

17 4 M /M /M

Ratios of molecular weight (based on fractionation)

2

w

n

Standard Deviation of Average

= 2.9:2.1:1

Standard Reference Material 1475, Linear Polyethylene (Whole Polymer)

Average Value

Specification

Molecular weight Weight-average molecular weight Number-average molecular weight Weight-average molecular weight Z-average molecular w e i g h t y Ratio of molecular weights M /M /M α

z

Limiting viscosity number (dl/gram) 1-Chloronaphthalene, 130°C 1,2,4-Trichlorobenzene 130°C Decahydronaphthalene, 130°C 0

M e l t flow rate (gram/10 min) Density (gram/cm ) 3

e

d

w

n

Estimated Standard Deviation of Average

52,000 2,000 18,310 360 53,070 620 138,000 3,700 7.54:2.90:1 See Table I below 0.890 1,010 1.180 2.07

0.0032 0.0086 0.0032 0.0062

0.97844

0.00004

B y light scattering in 1-chloronaphthalene at 135°C. B y gel-permeation chromatography. "Technical" grade, which assayed at approximately equal proportions of cis- and Jrans-decahydronaphthalenes. B y a procedure similar to Procedure A , A S T M Method D1238-65T, Test Condition D , 190°C, load 325 gram. B y A S T M Method D1505-67; sample prepared by Procedure A , A S T M Method D1928-68. α

6

c

α

e


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Table I.

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Cumulative Molecular Weight Distribution by Gel-Permeation Chromatography

log M

Wt.%

log M

Wt. %

log M

Wt. %

2.800 2.865 2.929 2.992 3.056 3.119 3.181 3.243 3.305 3.366 3,427 3.488 3.548 3.607 3.667 3.725 3.784 3.842 3.900 3.957

0.0 0.005 0.020 0.052 0.105 0.185 0.343 0.475 0.706 0.999 1.38 1.88 2.51 3.30 4.28 5.46 6.87 8.56 10.50 12.7

4.014 4.070 4.126 4.182 4.237 4.292 4,346 4.400 4.454 4.507 4.560 4.612 4.664 4.715 4.766 4.817 4.868 4.918 4.967 5.016

15.2 18.1 21.5 25.2 29.3 33.7 38.5 43.4 48.5 53.5 58.3 62.9 67.3 71.4 75.1 78.5 81.6 84.4 86.7 88.9

5.065 5.113 5.161 5.209 5,256 5,303 5.349 5.395 5.440 5.485 5.530 5.574 5.618 5.662 5.705 5.789 5.87

90.7 92.2 93.7 94.8 95.8 96.6 97.3 97.9 98.4 98.7 99.1 99.3 99.5 99.7 99.8 99.9 100.0

Standard Reference Material 1476, Branched Polyethylene (Whole Polymer)

Specification

Limiting viscosity number (dl/gram) 1-Chloronaphthalene, 130°C 1,2,4-Trichlorobenzene, 130°C Decalin,* 130°C Melt index (gram/10 m i n ) Density (gram/cm ) at 23°C 6

3

C

Average Value

Estimate of Precision

No. of Points or Measure ments

0.8132 0.9024 1.042 1.19 0.9312

0.0033 0.0034 0.0022 0.010 0.0006

14 30 5 35 10

d d d

e

e

° "Technical" grade, a mixture of cis- and fraws-decahydronaphthalene. B y Procedure A , A S T M Method D1238-65T, Test Condition E , 190°C, load 2160 gram. B y A S T M Method D1505-67T; Sample prepared by Procedure A , A S T M Method D1928-68. Standard deviation of the intercept of the least-squares line. * Standard deviation of a single determination. 6

c

d

Literature Cited 1. Frank, H. P., Mark, H., J. Polym. Sci. (1953) 10, 129. 2. Atlas, S. M., Mark, H. P., Report, presented to the Commission on Macromolecules, International Union of Pure and Applied Chemistry, Montreal, July 28, 1961.


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POLYMER MOLECULAR WEIGHT METHODS

Strazielle, C., Benoit, H., Rept. PureAppl.Chem. (1971) 26, 453. McIntyre, D. J., J. Res. Nat. Bur. Stand. (1967) 71A, 43. Wagner, H. L., Nat. Bur. Stand. Special Publ. 260-33 (1972). Wagner, H. L., Hoeve, C. A. J., to be published. Hoeve, C. A. J., Wagner, H. L., Verdier, P. H., and the following papers, J. Res. Nat. Bur. Stand. (1972) 76A, 137. Also available as Nat. Bur. Stand. Special Publ. 260-42. RECEIVED January 17, 1972.


3. 4. 5. 6. 7.


The Polymer Standard Reference Materials Program at the National

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