NBS SPECIAL PUBLICATION 260-33

A111D3 Q&S73E

U.S. jVRTMENT

IMMERCE

I National 160

Standard Reference Materials:

COMPARISON OF ORIGINAL

AND SUPPLEMENTAL SRM 705,

NARROW MOLECULAR WEIGHT

DISTRIBUTION POLYSTYRENE
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress March 3,

1 901 . The Bureau's overall goal is to strengthen and advance the Nation's science and

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onal Bureau of Standards Standard Reference Materials:

Comparison of Original and Supplemental SRM 705,

V Narrow Molecular Weight Distribution Polystyrene

Herman L. Wagner

Polymers Division

Institute for Materials Research

National Bureau of Standards Washington, D.C. 20234

"< '1,

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acti ng Director

Issued May 1972
Library of Congress Catalog Card Number: 7 2-600099

National Bureau of Standards Special Publication 260-33

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-33, 30 pages (May 1972) CODEN: XNBSAV

Issued May 1972

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.10:260-33). Price 35 cents.
PREFACE

^ Standard Reference Materials (SRM's) as defined by the

National Bureau of Standards are "well-characterized materials, produced In quantity that calibrate a measurement

system to assure compatablllty of measurement in the nation."

SPJyi ' s are widely used as primary standards In many diverse

! fields in science. Industry, and technology, both within the

I United States and throughout the world. In many industries

I traceablllty of their quality control process to the national

I measurement system is carried out through the mechanism and

j use of SRM's. For many of the nation's scientists and techj nologlsts it is therefore of more than passing interest to

I know the details of the measurements made at NBS in arriving

j at the certified values of the SRM's produced. An NBS series

of papers, of which this publication is a member, called the

NBS Special Publication - 260 Series is reserved for this

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.

This 260 Series is dedicated to the dissemination of

Information on all phases of the preparation, measurement,

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the user to assess the validity and accuracy of the measurement processes employed, to judge the statistical analysis,

and to learn details of techniques and methods utilized for

work entailing the greatest care and accuracy. It is also

hoped that these papers will provide sufficient additional

information not found on the certificate so that new applications in diverse fields not foreseen at the time the SRM

was originally issued will be sought and found.

Inquiries concerning the technical content of this

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concerned with the availability, delivery, price, and so forth will receive prompt attention from:

Office of Standard Reference Materials

National Bureau of Standards

Washington, D.C. 20234

J. Paul Call, Chief

Office of Standard Reference Materials

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vii

CONTENTS

PAGE

1. Introduction 2 2. Weight Average Molecular Weight by Light

Scattering 3 3. Number Average Molecular Weight by Osmometry.... 8 4. Limiting Viscosity Numbers 12

5. Gel Permeation Chromatography Measurements 13

6. Determination of Pellet to Pellet Variation..... 15

7 . Summary 17

8. References 1^ Appendix - -I . . ^ 19

LIST OF TABLES

PAGE

Table No.

1. Direct Comparison of 705-705A by Light

Scattering 5 2. Additional Determinations of M on 705 6 w

3. Comparison of M for 705-705A 7 w

1/2

4. Number Average Molecular Weight (II/c)

Method 11

5. Limiting Viscosity Number In Cyclohexane

at 35 °C 12

6. G. P. C. Data 15

ix
LIST OP FIGURES

PAGE

Figure No.

1. Comparison of gel permeation chromatograms l4 of 705 and 705A

2. Viscosity numbers for samples of 705 ... 17

X
COMPARISON OF ORIGINAL AND SUPPLEMENTAL SRM 705,

NARROW MOLECULAR WEIGHT DISTRIBUTION POLYSTYRENE

Herman L . Wagner

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 2023^

Because the supply of the original batch of narrow molecular weight distribution polystyrene, SRM 705, Is almost exhausted, another supply of polystyrene, reported

to be from the same batch as the original has been characterized to determine whether It Is the same as the original

as far as the properties listed on the certificate are concerned. No difference In molecular weight as measured

by light-scattering, osmometry, or gel-permeat Ion chromatography could be found. The limiting viscosity numbers

were also the same. It was also shown from solution

viscosity measurements that pellet to pellet variation In molecular weight Is negligible.

Key words: Characterized polymers; narrow molecular weight

polystyrene; polystyrene standard; Standard Reference

Material 705.

1
1. INTRODUCTION

Early In I963 the National Bureau of Standards issued

its first polymer Standard Reference Material, SRM 705 [1],

a narrow molecular weight distribution polystyrene. Recently

the original supply of one of these, SRl^l 705, became very low,

but additional material, which had previously been sent to the Bureau by Dow Chemical Company, was available. According

to the letter by Dr. H. W. McCormick of Dow Chemical Company,

Midland, Michigan, dated September 7, 1966, to Dr. Emanuel

Horowitz, this additional supply is the same as the original.

Because this is a certified material, however, it was nec- essary to com.pare the original and the new material to make

certain that the two are indistinguishable as far as the

properties listed on the certificate are concerned. About

4900 grams of this second sample are available and will be

referred to as SRM 705A in this report only.

This polystyrene, produced by Dr. McCormick's group at Dow Chemical Company by large scale anionic techniques, has

a very narrow molecular weight distribution—M /M of about ^ w n 1.12. It had been certified previously by Dr. D. Mclntyre

and his group at the National Bureau of Standards for weight

average molecular weight by light scattering and sedimentation equilibrium, number average molecular weight by membrane

osmometry, and limiting viscosity number in benzene and

cyclohexane. A copy of the certificate is shown in Appendix
In the present study SRM 705 and 705A were compared for

weight average molecular weight, for number average molecular

weight and for limiting viscosity number. In addition they were compared for molecular weight distribution by gel permeation chromatography, a technique which was not available

at the time the SRM 705 was originally Issued. No comparison

was made In this work using the ultracentrlfuge

.

In the discussion which follows no attempt Is made to describe equipment or techniques where this has already been

done satisfactorily elsewhere. However, when changes were

made In the standard procedure, or when there Is no common

standard procedure, details of the method are given.

2. WEIGHT AVERAGE MOLECULAR WEIGHT BY LIGHT SCATTERING

A comparison of the weight average molecular weight of

SRM 705 and SRM 705A was made using the "Soflca" photo-gonlo

dlff usometer , Model 701 (Bausch and Lomb, Rochester, New

York).* A general description of the light scattering

* Certain commercial equipment. Instruments, or materials

are Identified In this paper In order to adequately specify

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.
method may be found in several places, as for example in

the chapter by Chiang [2]. For a description of the specific techniques used in this laboratory, the paper on light scattering in the series [3] describing the certification of SRM 1475, linear polyethylene, is recommended.

Determinations were carried out in cyclohexane, at the

theta temperature, 35 °C, with unpolarized light at 5460^.

It was generally found that less scatter was obtained in

the data by placing an additional 5460^ light filter just

before the detector photocell. It is possible that this

filter removed extraneous radiation due to fluorescence

in the polymer solution.

The instrument was calibrated with benzene, for which

the Rayleigh ratio is taken as 15 . 8xlO~^cm~"'" [4]. In practice, a special glass rod is used as a working standard,

after comparison of its scattering with the pure benzene.

Six different concentrations, ranging from about

-3 -3 1x10 g/ml to 5x10 g/ml of each polymer were measured.

Each solution was made up independently by weight and then

filtered through "Millipore" filters of 0.22 ym, nominal

pore size, to remove dust and other particulate matter

which might interfere with scattering at low angles. The

solutions were kept at 40 °C until measured at 35 °C.

Replicate measurements were made on two different samples

of each solution.

4
The value of the refractive index increment, dn/dc for

the polystryrene-cyclohexane system at 35 °C was taken as 0.1705 ml/g [5].

The value of c/I for each concentration was plotted

2 against sin 0/^ and extrapolated visually to 0 = 0. All

these extrapolated c/I's were then plotted against con- centration and again extrapolated to c = 0. A similar

procedure was followed for these values of c/1, at each

specified angle, as a function of concentration. The

values of c/1 extrapolated to c = 0 for each angle were

2 then plotted against sin Q/^ to find the value of c/I at 0=0. The mean of this value and the value obtained at

c = 0, which almost always were within 1/2% of each other,

was used to obtain 1/M upon multiplication by the appropriate factor. The results follow:

Table 1. Direct Comparison of 705-705A by Light

Scattering

.

M

w Sample 705: 182,300

183,900

Average 183,100

705A: 183,000

184,100

Average 183,550

Certificate value: 179,300

5
It was possible;, because of the availability of some subsequent determinations on 705, made in another connection,

to obtain a better value for the mean and standard deviation.

These additional determinations gave the following values:

Table 2. Additional Determinations of M on 705. w 183,000

177,500

180,900

182,700

177,100

Mean 180,240

As shown in Table 3, the difference between the mean

for the first set, (Table 1) and the above is 2860, well

within the 95% confidence limits of the difference. Hence

we may assume that the two sets of data may be treated as one set

.

If we compare the mean of this combined set, l8l,lll,

with the mean for 705A, 183,550 (Table 1) we find that the

difference, 2539, between the two is also within the 95^

confidence limits value, ^,900. This is also shown in

Table 3. Hence there is no evidence of a difference between the two materials.

We also conclude that there is no difference between

the value on the certificate and the value now obtained for

705, again as seen in Table 3. It should be noted that

since there is no reason to believe that the standard

6
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7
deviations of the present set of measurements and of the

certificate value are equal, the standard deviations were

not pooled. In addition, the number of degrees of freedom

was estimated by the method shown in the footnotes to Table 3. In other cases shown in Table 3 the standard deviations were pooled.

3. NUMBER AVERAGE MOLECULAR WEIGHT BY OSMOMETRY

Number average molecular weight was determined by

osmotic pressure measurements. Techniques for determining

the number average molecular weight by osmotic pressure are described in the literature [7, 8]. The details of the

method used in this laboratory are included in the paper on osmometry describing the certification of SRM 1^75 [9].

These were made with a Hewlett-Packard Membrane Osmometer,

Model 502, which is characterized by rapid response time

(a few minutes), negligible solvent transfer across the

membrane, and small sample volume (0.3 ml). The semipermeable membranes were gel cellophane, type 450 D,

obtained from Arro Laboratories, Inc. These, when re- ceived, are in an isopropanol-water solution. They are then conditioned by soaking successively in ethanol, 1:1

ethanol-acetone , 1:1 acetone-toluene, and finally pure

toluene, the solvent used for the measurements.

When the osmotic pressure measurements were begun, it was not known that the pellet-to-pellet variability was

8
I.negligible, as shown by the viscosity measurements described

below. To eliminate the effects of possible pellet-to-pellet

variability and to bring both materials to the same physical

ij'condition, particularly moisture content, the following proi!

jj cedure was utilized.

;| Five replicate one-gram blends of pellets of 705 and

I

j 705A were separately dissolved in benzene and freeze dried,

j Prom each of these ten samples, five independent concenI trations , ranging from 0.25 g/100 ml to about 1.1 g/100 ml

I were made up in toluene to give a total of 50 solutions

.

I The osmotic pressure, IT, of each of these was measured

] twice at 25 °C

.

The limiting values of Jl/c as the concentration, c, approaches zero were determined for each of these ten

I solutions in three ways

.

I 1. linear least squares Jl/c vs c 1/2

2. linear least squares (H/c) vs c 2 S 3. least squares - polynomial II = d^c+d^c +d2C

Method 3, which is perhaps the most desirable of the

above methods for treating the data, gave, with our computer

program, at least one result that was extremely sensitive

I to one point that was just off a smooth curve. Further inj vestigation is needed before a satisfactory method of fitting

I such data to a polynomial is found. For our immediate needs,

it was sufficient to compare the results for the two polymers

obtained by method 2 above, i.e., plotting (n/c)^^^ vs c
and determining the limiting value of (II/c) ' from the

Intercept at c = 0 by a linear least squares computation.

This limiting value Is Inversely proportional to the square

' root of the molecular weight. The results are shown In Table 4, where (II/c)^ represents the square of this limiting

value . I To determine whether the difference between the averages , of for the two— 162,^00 for 705 and 165 ,200 for 705A— Is significant, the same procedure was employed as In the light |

! scattering case. A combined standard error of 3,500 Is found > for the two sets of data, and since this, when multiplied ' by a t value of 2.31 for 8 degrees of freedom, gives, for ; 95^ confidence limits, a value of 8,000, we conclude that

the difference of the two number average means of 2,800 Is i not significant. ) If the 705A value Is compared with the certificate ! value of 170,900, determined In cyclohexane at 35 °C with j a standard deviation of the average 58O (number of deterj

I mlnatlons was 12), the estimated standard error of the j

I: difference Is 2,960 with 4 degrees of freedom, calculated

as shown In Table 3- The t value Is 2.78 which when

multiplied by the combined standard error above gives a 95^ confidence limit of 8,200, Indicating that the difference of 5,680 molecular weight units Is not significant.

The standard error of 58O In the certificate Is sur- prisingly low but since the details of this determination

10
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LIMITING VISCOSITY NUMBERS

Limiting viscosity number, L.V.N. , was determined in

the usual way by extrapolation, to zero concentration, of

viscosity number {r]/r\^-l) /c , where n/n^ is the relative

viscosity of solution to solvent, and c is the concentration

in g/ml. The measurements were made in a Ubbelohde viscometer.

A Hewlett-Packard, Model 5901B Autovis comet er , was used to operate the viscometer and measure the flow times, while the

viscometer was thermos tatted in a Hewlett-Packard 5901A

Constant Temperature Bath. With this system, flow times

of about 40 or 50 seconds for a given solution are reproducible to about 0.03 seconds, making it possible to use fairly dilute solutions and thereby increasing the reliability of the extrapolation to zero concentration. Six

solutions of each sample in distilled cyclohexane, ranging

in concentration from .004 g/ml to 0.1 g/ml were measured

at 35 °C. As in the case for light scattering, the solutions were kept at 40 °C until used. The results were: Table 5- Limiting Viscosity Number in Cyclohexane

at 35 °C.

705

705A

35.2 ml/g

35.3

L.V.N. Standard Deviation

0.2

0.1

Certificate value

35.4 0.24*

* "Standard deviation of average, for six samples

12
Again there is no evidence for a difference between 705 and

705A, nor between their average and the certificate value.

5. GEL PERMEATION CHROMATOGRAPHY MEASUREMENTS

The two samples were compared by gel permeation chromatography, a column fractionation method that has come into

general use in recent years for the determination of molecular weight distribution of high polymers.

The gel permeation chromat ograph used was the Model

200 (analytical scale) G.P.C. manufactured by Waters Associates, 61 Fountain Street, Pramlngham, Massachusetts. It was run at 100 °C with 1 , 2 , 4-trichlorobenzene as the solvent,

at a concentration of 0.3%-

Since only the difference between 705 and 705A was of

interest, an absolute, carefully determined independent calibration was not required. Instead two anionically produced

polystyrenes, supplied by Waters Associates, with molecular

weight falling on either side of the SRM 705 value (41,000

and 98,000) and the SRM 705 (certificate value M = 179,300)

w were run to give the following calibration:

log M = 11.89 - 0.204

where is the peak elution volume.

Visual comparison of the charts (figure 1) for runs made under the same conditions for 705 and 705A show that

the peaks occur at the same elution volume. Calculation

of the number, weight and z average molecular weights, given

13
GEL PERMEATION CHROMATOGRAMS

36 35 34 33 32 31 30 36 35 34 33 32 31 30

ELUTION VOLUME. Vg

Figure 1. Comparison of gel permeation chromatograms

of 705 and 705A.

14
in Table 6, show that no significant differences in distribution occur. If the curves in figure 1 were normalized,

they would be superimposable

.

Table 6. G. P. C. Data.

206,000

204,000

705A 1 157,000 l80,200 208,000

2 157,800 181,500 211,000

The replication is insufficient to provide a good measure of the reproducibility among solutions. However,

the difference in molecular weights found between 705 and

705A is of the order of about 2%, which we know from previous experience to be statistically insignificant, so we again conclude that there is no evidence of an important

difference between the two materials

.

Because of the method of calibration, the molecular

weights quoted above should not be considered absolute.

6. DETERMINATION OP PELLET TO PELLET VARIATION

Since the polymer pellets weigh only between 7 to 20 mg, there are only two techniques available to us which take

a small enough sample size, namely G. P. C. and solution

viscosity, with which to obtain a measure of variation in

molecular weight from pellet to pellet. Since in our case

15

M

Run n 705 1 155,600

2 155,900

A.w 178,200

177,900
viscosity measurements are more sensitive to small changes

in molecular weight than G. P. C., the former was used.

It was possible to obtain meaningful results with a "semimicro" Cannon-Ubbelohde viscometer which takes a one milliliter sample. Single pellets were dissolved in 5 to 10 ml

of benzene and measured at 35 °C. Because of variation in

pellet weight, it was not convenient to keep the concentration constant, so that the following procedure was used to estimate the variation in viscosity. The viscosity number

(n/ilQ-l)/c for each pellet is computed from flow times of

solvent and solution, corrected for kinetic energy effects,

and is plotted against concentration. These are shown in

figure 2, together with the line representing the linear

least squares fit of the points. The standard deviation of

the points about the line is about 0.3% of the value of the

intercept on the viscosity number axis, or the limiting

viscosity number. This represents the sum of measurement

error and pellet to pellet variation in viscosity or molecular weight. Prom previous measurements it has been established that measurement error itself leads to a standard

deviation of 0.3%, leading to the conclusion that the

variation in viscosity, and hence molecular weight, from

pellet to pellet, is negligible.

16
7 . SUMMARY

No evidence of a difference between 705 and 705A can

be shown to exist. From solution viscosity measurements

It was shown that pellet to pellet variation In molecular

weight Is negligible.

The author acknowledges the assistance of J. E. Brown,

R. G. Chrlstensen, and J. R. Maurey who carefully performed

i the measurements discussed In this report. He Is also

j grateful to Dr. John Mandel for helpful discussion of the

I statistical analyses employed.

17

i

i
8 . REFERENCES

Mclntyre, D., J. Res. Nat. Bur. Stand. (U.S.), 71A , 43(1967)

.

Chiang, R., Newer Methods of Polymer Characterization,

Bacon, Ke , (ed.), (Intersclence Publishers, New York,

1964) . Frolen, L. J., Ross, G. S., Wlms , A. M., and Verdler,

P. H., J. Res. Nat. Bur. Stand. (U.S.), In preparation.

Coumou, D. J., J. of Colloid Scl . 15, 4o8(1960).

O'Mara, J. H. and Mclntyre, D., J. Phys . Chem. 63., 1435(1959)

.

Davles, 0. L., Statistical Methods In Research and Production, (Oliver and Boyd, London, I96I).

Hookway , H. T., Techniques of Polymer Characterization,

Allen, P. W., (ed.), (Butterworths , London, 1959).

Bonnar, R. V., Dlmbat, M., Stress, P. H., Number Average

Molecular Weights, (Intersclence Publishers, New York,

1958).

Brown, J. and Verdler, P. H., J. Res. Nat. Bur. of Stand.

In preparation.

1.8
APPENDIX I.

U. S. Departritent of Commerce

Maurice H. Stans

Secretary

National BvrcalJ of

THIS IS A PRELIMINARY ISSUE OF THE PRINTED CERTIFICATE

WHICH WILL BE AVAILABLE WITHIN 60 DAYS

Rational ^umu of ^tmdntd^

(SLtvtxixtntt

Standard Reference Material 705

Polystyrene

(Narrow Molecular Weight Distribution)

Donald Mclntyre

Number Standard

of Deter- Deviation minations Average of Average 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 Niimber (ml/g) (Intrinsic viscosity)

Benzene, 25 °C Benzene, 35 "C Cyclohexane, 35 °C Ratios of Molecular Weight

(Based on fractionation)

12

22

170,900^

179,300'

189,800'

74.3^

74.5^

35.4^

580

740

2,100

0.18 0.23

0.24 1.12:1.07:1 *Average of individual determinations made on a pooled sample

combining portions of material taken from the entire lot.

*'*The average includes results from the pooled sample and from a separate study made to determine possible heterogeneity

of the lot. It was found that samples taken from different

locations showed slightly more variatjility than samples taken from adjacent locations. The standard deviation

of the reported average includes the effect of lot heterogeneity. ***The average is obtained from a study made on samples

from six locations to determine possible heterogeneity

(over) 19
of the lot. It was found that samples taken from different

locations showed slightly more variability than samples

taken from adjacent locations. The standard deviation

of the reported average includes the effect of lot

heterogeneity

.

Washington, D. C. 20234

November 6, 1969 Office

(Revised February 25, 1972)

J. Paul Call, Chief

of Standard Reference Materials

The polystyrene sample was prepared by the polymerization

of styrene in benzene using butyllithium as an initiator.

Ash content is 0.05% of sample. Volatile content is about

0.5%. Determinations of molecular weight and intrinsic

viscosity are based on sample weights of the polystyrene

pellets uncorrected for volatiles. Each pellet weighs

approximately 10 mg. Several pellets were always used in the above determinations.

The osmotic pressure measurements were made with

#600 gel cellophane membranes. The light scattering and

sedimentation molecular weight determinations were calculated using the following constants for

polystyrene-cyclohexane solutions at 35 °C: 0.1705 ml/g

for the refractive index increment at 546 nm and

0.930 ml/g for the partial specific volume. The maximum

rate of shear in the Ubbelohde viscometers used to determine

the intrinsic viscosities was about 1500 sec"^ for water. The z -average (M^) , weight -average (M )

, and number- average

(Mj^) , molecular weight ratios are based upon a complete

viscometric analysis and selected osmometric analysis of

36 fractions.

U. S. GOVERNMENT PRINTING OFFICE 1972-481-333

20
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA ^BS SP 260-33

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Standard Reference Materials

:

Comparison of Original and Supplemental SRM 70 5, Narrow Molecular Weight Distribution Polystyrene

5. Publication Date May 1972

6. Performing Organization Code

7. AUTHOR(S) Herman L. Wagner

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/ Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

S ain& 3. s No. 9

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) Becuase the supply of the original batch of narrow molecular

weight distribution polystyrene, SRM 705, is almost exhausted,

3.nother supply of polystyrene, reported to be from the same batch as the original has been characterized to determine

whether it is the same as the original as far as the properties

listed on the certificate are concerned. No difference in

molecular weight as measured by light scattering, osmometry,

or gel-permeation chromatography could be found. The limiting viscosity numbers were also the same. It was also shown from

solution viscosity measurements that pellet to pellet variation

in molecular weight is negligible.

17. KEY WORDS (Alphabetical order, separated by semicolons) Characterized polymerS ; narrOW molecular weight polystyrene; polystyrene standard; Standard Reference Material 705.

18. AVAILABILITY STATEMENT (5 UNLIMITED.

1 1 FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

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30

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