1396
INDUSTRIAL A K D ENGINEERING CHEMISTRY
coarse aggregates R hich are extremely sensitive to mechanical shear. This is of some importance for accurate control of aggregate size prior to latex incorporation, as it is necessary to pump the carbon black slurriee from the make-up and storage tanks to the latex mixing tanks. Pumping operations involve high mechanical shearing forces and can change the size of the dispersed aggregates. The size of the carbon black aggregates observed in the final carbon black and rubber masterbatch is determined not only by the degree of dispersion present in the carbon black slurry but also by the technique of mixing the carbon black and latex and the creaming and coagulation processes, as shown by Adams, Messer, and HoN-land ( 1 ) . There is no obvious reason why the contribution of the initial aggregate size introduced in the slurry to the h a 1 size of the aggregates observed in the master batch should influence the final quality of the master batch. Presumably the aggregates of carbon black present in the slurry and carried over into the masterbatch ail1 disperse during the mixing and milling operations employed in preparing final compounded stocks in a similar manner to the dispersion obtained in ordinary dry mixing of carbon black and rubber. However, other factors involved Rith the coprecipitation of carbon black and latex may have a marked influence on the dispersibility of the aggregates present in the final masterbatch ( 1 ) and can minimize the effect of the initial degree of dispersion present in the carbon black slurry. If the ultimate degree of dispersion produced by violent mechanical agitation could be maintained in the final masterbatch a substantial improvement in quality could be expected in a black masterbatch prepared from a slurry of the type obtained in a braring Blendor. CONCLUSIONS
Vol. 43, No. 6
3. The size of the dispersed carbon aggregates in water is determined by the initial degree of dry densification of the black and the extent of mechanical agitation of the system. ACKNOWLEDGMENT
The authors wish to express their appreciation to William D. Schaeffer of the fundamental research group of Godfrey L. Cabot, Inc., for the dispersing agent adsorption data. They also thank F. Richard 11-illiams, a Sortheastern .Univereity cooperative student, for much of the laboratory work done in connection with this paper. LITERATURE CITED
Adams, J. IT7., hlesser, W.E., and Howland, L. W.,IND.ENG. CHEX.,43, 754 (1951).
Brunauer, S.,Emmett, P. H., and Teller, E., J . Am. Chem. Soc., 60,309 (1938).
Dannenberg, E. M., and Collyer, H. J., IND.ESG. CHEX., 41, 1607 (1949).
Dannenberg, E.bI.,and Stokes, C. A., I b i d . , 41, 812 (1949). General Tire and Rubber Go., Baytown, Tex., private communication. McMahon, W.,and Kemp, A. R., IND.ENG. CHmr., 36, 735 11944).
Madigan, 3. C., and Adams, J. W., Chem. Eng. Progress, 44, 815 (1948).
Marathon Corp., Chemical Division, Rothschild, \Vis., private communication. O’Conner, H. F., and Sweitzer, C. W.,Rubber Age, 54, 423 (1944).
Rongone, R. L., Frost, C. B., and Swart, G. H., Ibid., 55, 577 (1944).
Smith, W.R., “Carbon Black” in “Encyclopedia of Chemical Technology,” Vol. 111,p. 41, New York, Interscience Encyclopedia, 1949. Stokes, C. -4., and Dannenberg, E. XI., IND.ENC. CHERI.,41, 381 (1949).
1. The rate of incorporation of carbon black in water under given conditions of agitation can be influenced by the initial degree of dry densification of the carbon black, the presence of traces of benzene-extractable hydrocarbons, the temperature of the water, and the use of wetting agents. 2. The quantity of dispersing agent necessary to produce a carbon black dispersion of a given solids content having a fluid consistency is related to the surface area, the presence of inorganic soluble ash contaminants, and the degree of surface oxidation of the black.
Sweitzer, C . W.,and Goodrich, W. C., Rubber Age, 55, 469 (1944).
Veselovskii, V. S.,Issledovnniyo Fizika-Khim. tekh. S u s p e n s i i , 1933, 83.
Vold, R. D., and Konecny, C. C., J . Phys. &. Colloid Chem., 53, 1262 (1949).
Wiegand, W. B., IND. ENG.CHEM.,29, 953 (1937). RECEIVED April 24, 1950. Presented before the Division of Rubber Chemistry at the 117th Meeting of the h f E R I C A U CHEMICAL SOCIDTY, Detroit, Mich.
Influence of Molecular Weight on
the Properties of Polystyrene E. H . MERZ, L. E. NIELSEN, AND ROLF BUCHDAHL Monsanto Chemical Co., Springfield, Mass. T h i s work was undertaken to answer the question: “How do the mechanical, and especially dynamic physical properties of polystyrene vary with molecular weight and molecular weight distribution?” For strain-free films of fractionated polystyrene a class of properties was found which vary with molecular weight according to the relation: property = A
B +- ,where A M
n
and B are constants and ,%L is the number average molecular weight. Certain properties depend upon Mu. If both M, and M, for a particular polystyrene specimen are above their respective limiting values, then the shape of the distribution curve does not affect the tested physical properties. These results show that the physical properties of commercial polystyrenes are mainly controlled by the amount
of very low molecular weight species present. Furthermore, these results on strain-free films provide the reference point for studies on molded specimens containing strain and orientation.
T
WO previous papers related t o this subject ( I O , 1 1 ) described necessary preparations for the study of the effect of molecular weight and molecular weight distribution upon the physical properties of polystyrene. In undertaking such a study it is necessary t o decide tR-0 important things: first the physical form and shape of the polystyrene t o be tested; and, secondly, the method of obtaining variations in molecqlar weight and molecular weight distribution. Molding procedures were considered inapplicable to prepare specimens for mechanical testing for three reaeons. First, they required too much material; secondly, the exposure to high tem-
June 1951
*
I
IN D U S T R I A L A N D E N G IN E E2R IN G C H E M IS T R Y
peratures during molding could have altered the polystyrene and limited its re-use. The third reason had to do with the question of what has been termed “orientation.” During a molding operation polystyrene undergoes a change in its state of molecular aggregation which depends upon the conditions of molding and can influence to a large extent the measured properties of the molded piece. Casting a thin film of polystyrene from a solvent, on the other hand, had none of these disadvantages. Thin films required a small weight of material; the casting procedure reported in (IO) exposed the films t o moderate temperatures only briefly during the drying cycle, and the films could be made “strain-free” or with no “orientation”-they showed no colors under crossed polaroids. The use of varying amounts of solvent, regulator, or catalyst, different temperatures or methods of polymerization can lead t o polystyrenes of different average molecular weights. However, the distribution of molecular weight changes simultaneously so that the possible effects of each separately are obscured. The materials which were used throughout this investigation were the polystyrene fractions derived from a specially prepared polystyrene as described by Merz and Raetz (11). These fraotions were portions of the original polystyrene and were nearly homogeneous in molecular weight. Amounts of from 80 to 600 grams of each of the 40 fractions were available for mechanical and physicochemical tests. EXPERIMENTAL
*
Representative fractions which covered the range of molecular weights from 1,500,000 t o 128,000 were chosen to be cast into films and tested. The fractions used, with their molecular weights, are listed in Table I, Part I. The film of fraction 36 shattered on simple air drying, and so fraction 34 was cast to provide a film close to the lower end of the molecular weight scale. I n order to be able to study the effect of polymolecularity and the properties of very low molecular weight polystyrenes (which could not be cast directly) several films were cast from mixtures. The details of their compositions are given in Table I, Part 11. The low polymer was made by polymerizing a 10% solution of styrene in toluene containing 1% of benzoyl peroxide (based on the styrene) a t 94’ C.-the temperature a t which the original polystyrene had been polymerized. Upon isolation of the acetonesoluble polymer by precipitation in 10 volumes of methanol, it was obvious that some fractionation had taken alace, for diluting the methanol-toluene liquors with water gave idditional precipi‘tate. In Table I, Part 111, are listed unfractionated polystyrenes which also were tested. An Instron machine was used to measure film tensile properties. This machine, by means of differential speeds of crosshead separation and recorder motion, could expand the elongation scale by a factor of 500 if necessary; however, most tests were run on 6-inch strips at a jaw separation speed of 0.05 inch per minute while the chart ran a t 2.0 inches per minute. All tests were run a t 77” F. and 50% relative humidity. Dynamic mechanical properties are those measured when the applied force varies with time, in general, in a sinusoidal manner. Two quantities which describe the material under these oondiditions are the dynamic Young’s modulus and the damping. These quantities were measured with an electromagnetic reed vibrator; the dynamic modulus was determined from the resonance frequency of the reed and the damping was obtained from the half-width of the resonance peak (IO). All tests were mad! in the frequency range of 20 to 30 cycles per second. The damping data are expressed as a dissipation factor which is equal to the halfwidth of the resonance peak divided by the square root of 3. Tensile creep properties were measured and have been reported elsewhere (6). A “tensile heat distortion test” was devised to simulate the standard American Society for Testing Materials (A.S.T.M.) flexural heat distortion test ( 3 ) . This was carried out by hanging on the usual 6-inch film strip a small weight which would give a stress of 1.43 X lo7 dynes per sq. em. Upon raising the temperature of the air bath slowly, a temperature (range) would be reached where the sample began to extend rapidly. The heat distortion temperature was arbitrarily defined as that temperature a t which the compliance reached the value of 40 X sq. em. per dyne.
1397
TABLE I. MATERIALS TESTED PARTI
ti^^ No. 1 4
7
9 14 19 24 27 29 34 36
Wt. Av. Molecular Wt.,
No. Av. Molecular Wt.,
gatio-of
Mw 1,500,000 1,190,000 1,010,000 880,000
M7la 1,360,000 992 000 664:OOO’ 765,000* 583 000 481 :OOO* 408,000 288 000 231 ‘OOO* 146’000* 111:000*
Mw,/iMn
660,000
554,000 470,000 320,000 266,000 168,000 128,000
1.10 1.20 1.52 1.15 1.13 1.15 1.15 1.11 1.15 1.15 1.15
PARTI1 Mixture NO.
1 2 3 4 5 6
7 8
Fraction Addend 14 Bibenzyl 14 Bibenayl 14 Bibenzyl 27 Low polymer 27 Low polymer 4 Low polymer 27 Aroclor 1248 27 Tricresyl phosphate
Wt.
%
1.55 5.52 9.97 25.00 50.00 50.00 6.84 6.30
ZW
-
(Calcd.) 650,000 623,000 594,000 263,000 176,900 596,000 330,000 328,000
M7l (Calod.) 11,700 3,300 1,820 10,240 5,120 5,120 5,000 5,840
PART I11 Unfractionated Molecular Wt. Polystyrene Mu A 340,000 B 340,000 C 370,000 D 450,000 EO 690,000
*
T ~of ~ Polymerization Mass Mass Emulsion Mass Mass
~
, Remarks
.. . . . ..
High methanol solubles
.. . . , . .
High in large M ’ s No methanol solubles
a z n ’ s were measured on an oamomegr. Those marked with an asterisk were calculated from the equation M w / M n = 1.15,the Mw’s being known. b Molecular weight estimated from the intrinsic viscosity. C This is the polymer t h a t was originally fractionated.
To measure the second order transition point (SOTP), a simple dilatometer was constructed from a standard taper joint and capillary tubing so that second order transition points could be estimated volumetrically. The advantage of such a dilatometer is that the same instrument can be used many times over without having to seal the glass with an oxygen-gas torch and hence exposing the samples to high temperatures. A mixture of graphite with a small amount of silicone grease was satisfactory as a seal; the dilatometer could be evacuated and then mercury run into it without trapping air within the measured volume due to leakage through the joint. The first attempts were made using the sample in the form of strips of film. The very erratic results were due to air bubbles entrapped between the film strips. The erraticity disappeared when the samples were cut from a compression molded disk. A.S.T.M. heat distortion temperatures ( 3 ) were measured on 0.5 X 0.5 X 5 inch bars compression-molded from some of the fractions. Those molded a t a maximum temperature of approximately 180” C. were held there for 20 minutes. The bars whose maximum molding temperature was 160” C. were brought up to that temperature and then cooled as rapidly as possible (total time per bar was about 1 hour). A Dunbar oil bath was used for the second order transition point and A.S.T.M. heat distortion measurements to obtain a uniform rate of heating. Flow measurements a t temperatures above the second order transition point were performed using a rotational viscometer. These results have been reported separately (6). The standard A.S.T.M. procedures for measuring impact strength ( d ) were not usable for they required t o o much material per specimen and a large number of these specimens so that a valid estimate of experimental error could be determined. A modified Gorley stiffness tester, run so as to approximate the relationship between specimen and impacter in the Izod tester was finally used. The bars from the A.S.T.M. heat distortion test were sawed in half, notched, and run on the Izod impact tester. The density of the films was checked using a density gradient tube (8). The tube was made up by adding water to the top of a saturated solution of ammonium nitrate contained in a litergraduated cylinder. Using a piece of cellulose acetate for calibration it was estimated that differences in density of 0.0004 gram per cc. could be detected. Ultraviolet absorption spectra were determined on a Beckman Model D U spectrophotometer using film specimens.
INDUSTRIAL AND ENGINEERING CHEMISTRY
8398
Vol. 43, No. 6
limiting value, this value being substantially (within experiArea under mental error) reached a t li?, Elongation, Curve Molecular = 20,000. The differences % Inch-LA. Wt. X 10-8 between the properties of the 2.26 * 0.30 2.54 rh 0.55 1360 1.99 0.12 2.36 rh 0.19 765 fractions from 146,000 t o 2.20 0.24 2.53 ;h 0 . 2 1 583 2.26 * 0 . 2 6 231 2.20 * 0.18 1,360,000 in molecular weight 231 1.86 0.08 2.08 A 0.12 were not significant as com2.04 * 0.10 146 2.05 ;h 0.22 pared to experimental error. 1.91 * 0.14 1.88 * 0.24 11.7 The data in Table I1 show 1.35 * 0.28 0.70 * 0.07 1.82 1.42 4 0 . 0 8 1.09 * 0 . 0 5 10.24 that low molecular weight ad0.7 ........ 5.12 dends, at the same number 1.30 * 0.16 0.97 4 0.21 5.00 1.27 * 0.05 1 . 0 2 A 0.09 5.84 average molecular weight (%fn), lower the area under the stress-strain curve, the tensile strength, and the tensile elongation t o the same extent. This is in agreement with the results of Sookne, Harris, and Mark ( I S ) who showed that the properties of mixtures of different @ cellulose acetate fractions varied as the %%. However, this also means that the kind of low molecular weight addend is nearly unimportant; the diminution of mechanical strength depends mostly upon the amount of low molecular weight
TABLE 11. TENSILE TESTDATAAT 68' F. Tensile Strength Lb./Sq. Idch 5180 * 350 5750 * 70 5350 A 220 5970 * 120 5780 * 85 5630 * 1G0
Day of Teut 1
Fraction NO.
1 9 14 29 29 34
a
1
2
a
2
Mixture No.@
Tensile Modulus, Lb./Sq. Inch x 10-8 2.98 A 0.23 3.44 ;t. 0 . 2 3 2.98 4 0.07 3.69 * 0.35 3.66 * 0.09 3.32 0.14
5120 * 260 3.19 * 0.07 2.87 * 0 . 3 5 3480 * 210 4200 ;t. 80 3.38 * 0.21 x 1 6) 2000 7 3:ijo * 0 . 1 1 4250 * 250 3.68 * 0.05 4280 * 130 8 3 Refer to Table I, P a r t 11. for composition of mixtures. b Average of only 2 values; too brittle to test. I 3 4
a 2
3 3 3
;t.
TABLE111. SECOND ORDERTRANSITION POINTS
-4 u 2
Figure
4
1.
6
8 10 12 14 16 18 20 22 STRAIN I ~ I N . 10-3 X
Representative Strain Curves
Stress-
1, m i x t u r e 35 2, fraction 9 ; 3, m i x t u r e 8
Fraction
z n
4
992,000
14
583,000 231,000
SOTP",
c./d.
c.
Average 93.5
93.5 96.0 94.0 96.0 90.0 92.5 90.0 92.5 91.5 93.0 89.5
1 2 2 2 1 2 0.5 1
2 2 34 146,000 2 SOTP,second order transition point.
..
95:3 90.0 Q2.5 90.0 92.5 92:3 89.5
TABLE IV. A.S.T.M. HEATDISTORTION TEMPERATURE
FILMFORMABILITY. During casting the behavior of the film of fraction 36 was no different from the behavior of any of the other fractions. However, upon simple air drying it shattered. Since the ability t o form a film which would cohere under the stresses induced by the removal of solvent was considered t o be of practical importance, the behavior upon drying of the mixtures in Table I1 was closely observed. All of the mixtures made good films except M5 whose weight average molecular weight (M,) of 176,900 was the lowest of any of the mixtures. This film cracked on drying but pieces large enough for the dynamic tests could be salvaged easily. It seemed then that film formability was much more nearly dependent upon the number of molecules large enough t o contribute t o a three-dimensional gel and upon the actual length (weight) of the molecules. The lower molecular weight constituents would then merely be fillers in the gel structure and could only contribute t o the strength of the film when the gel was sufficiently deswollen so that van der Waals-type bonds would have a n appreciable strength. This leads t o a dependence of film formability upon however, since a mixture of ii?, =i 176,900 cracked, but a fraction 34 with an of 168,000 did not, can be taken only as a it is obvious that the dependence on gw guide. Another property, notch sensitivity or brittleness, seemed However, the two were int o be almost as important as separable under the conditions of test. Results of tensile tests are given in TENSILEPROP~RTIES. Table I1 and typical stress-strain curves are plotted in Figure 1. A plot of the tensile data (not shown) indicated that the per cent elongation, area under the stress strain curve, and the tensile strength become zero a t low Nn)s and asymptotically approach a
zw.
0
29
RESULTS
zw;
Rate of Heating
No.
(Oil bath) Fraction No. 1 4 7 14 29 29
35r
Molding Temp.,
Rate of Heating
1,500,000 1,190,000 1,010,000 660,000 266,000 266,000
Heat Distortion Temp.. C. 104.2 105.4 101.6 103.3 103.8 102.2
' C./Mid.
c.
Blo
181 184 160 180 186 160
2.3 2.3 2.2 a.2 2.2 2.2
=
I
zm
20 I 20
I
I
I
I
1
I
I
I
I
30
40
50
60
70
&b
I
I
I
I
90
TEMPERATURE 'C.
Figure 2.
i J
I 100
Dynamic Modulus vs. Temperature for Fractions Numbers on ourvea are fraction nuxubera
110
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1951
miscible addend having a given molecular weight and not on details of ita structure. The special low polymer (ii?, = 2560) had t o be mixed with a polystyrene fraction in amounts up t o 50% by weight in order t o get the of the mixture below 15,000. However, at the same final ii?, of the mixture, low polymers of polystyrene are more efficient in lowering the tensile strength, tensile elongation, and area under the stress-strain curve. On a weight basis low polymers are vastly less efficient than low molecular weight plasticizers. At loadings of 50% by weight of low polymer in the film some doubt arose as t o the homogeneity of the fdm since the reflection from its surface was not specular.
zn
*
1399’
IMPACT PROPERTIES. Impact test results showed that the variation of f i m thickness controlled the response of the sample to the test. However, during tensile testing it was noticed that films of fractions 1 through 24 broke into two pieces, whereas films of fraction 34 and of mixtures 4 and 6 shattered. Some impact strength data on molded bars are given in Table VI. 12 II
10 N
0 9 X
8
E 7
..
0
e 6
N3 \
2
5
g
4
E
3
0 3.0
In 5 2
5 a
20
Q
50
40
‘P
60 60 70 TEMPERATURE *C.
80
90
UO
100
X
9 j e5
Figure 4.
Mechanical Dissipation Factor us. Temperature for Fractions
a
0
Fraations 1, 4, 9. 29, and 34 were teated
z
v)
%
-
TABLE VI. 20
30
Figure 3.
40
50 60 70 TEMPERATURE ‘C.
80
90
Fraction No.
Dynamic Modulus vs. Temperature for Mixtures
1
.
29
DYNAMIC PEOPERT~~OS. The dynamic modulus versus temperature curves for fractions and mixtures are given in Figures 2 and 3. The relative positions of the curves along the modulus axis are unimportant since the error in determining the modulus is * 5 % , whereas the precision of single points on any one curve is &2%; the important things are the shape of the curve and the large change in modulus over a few degrees spread in temperature. Mechanical dissipation factors as a function of temperature for fractions and mixtures are plotted in Figures 4 and 5. HEATDISTORTION PROPERTIES. A dynamic heat distortion point, calculated as the temperature at which the mechanical dissipation factor reaches the value of 0.0578, is plotted versus the number average molecular weight in Figure 6. Measured second order transition points are listed in Table 111. Closely related heat distortion points are listed in Tables IV and V. It can be seen that within the range of molecular weight measured, these transition values do not depend upon molecular weight. From Table IV i t would seem that for compressionmolded bars the higher the molding temperature, the higher is the A.S.T.M. heat distortion temperature. This effect has been attributed ( 1 ) to the decrease of strain and orientation permitted by the lower visaosity of the polystyrene in the mold at the higher molding temperatures.
at0 1,500,000 1,190,000 660,000 266,000
4 14
Numbers on curves are mixture numbers
*
IZOD IMPACT STRENGTH
100
Av. Deviation Based on 2 Bars 0.017 0.007 0.036 0.006
Impaot Strength. Ft./Lb./Inch Notch 0.390 0.376 0.384 0.352
DENSITY.The density of the films can be assumed not to vary with molecular weight since pieces of films from fractions 1 and 34 sank to the same level in the density gradient tube. OPTICALPROPERTIEJS. The optical density versus air in the wave length region from 290 t o 600 mp did not vary with molecular weight in a systematic fashion. 12
c>
4d
I TEMPERATURE
Figure 5.
OC.
Mechanical Dissipation Factor us. Temperature for Mixtures
Numbers on curvea are mixture numbere
TABLEV. TENSILE HEAT DISTORTION TEMPERATURE (T,) Fraction No. 4 14 19 Mixture No.
at9
Tm, * C.
1.190 000 66O:OOO 564,000
101.2 97.3 100.0
650,000
89 .b 85.0 80.6
594,000 328,000
UNFRACTIONATED POLYSTYRENES. The results obtained by testing the unfractionated polystyrenes are collected in Table
VII. DISCUSSION
EFFECTOF MOLECULAR WEIGHT. The effect of molecular weight upon the properties of polystyrene follows a pattern in that, for many properties, the property has a value of zero at
1400
Vol. 43, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE VII. RESULTS OS Material A
A. Tensile Strength, Lb./Sq. Inch
FF7HOLE POLYSTYRENES
TENSILE Elongation,
Young's Modulus Lb./Sq. Inch X 1 0 - 5 6070 d 160 1 . 6 9 * 0.09 3.51 * 0.06 5200 * 290 1.61 * 0.05 3.52 0.18 5770 70 1 . 9 0 * 0.07 3.49 * 0.12 5030 * 240 1.54 * 0.12 3.62 4 0.11 6000 * 230 1.90 * 0.08 3.55 * 0.12 €3. TESSILEHEATDISTORTION Heat Distortion Temp., C 96 102 C. DYNAMIC HEAT DISTORTION^ Heat Distortion Temp., C.
B
C
D E Material
A
C
Material
9%
i:C
90 90 96 97.5
E
the number average molecular weight is not one of the significant variables. The value A corresponds to the value a given property should have a t infinite molecular weight; it is also the value used to determine the upper limiting molecular weight as described later. The calculated values of A lie uniformly in the low portion of the range of optimum values as determined from fractions of high molecular weight, but do fall within this spread in the experimentally accessible range of molecular weight. The fit of this Flory equation cannot be precisely defined as the experimental error in determining ultimate strength properties is large; the scatter of the points above and below the calculated curve cannot be used to estimate if there is a significant trend toward either negative or positive deviations because there are not enough individuals. However, some idea of the fit can be obtained from the reproducibility of the prediction for the lower limiting it is probably better than the data warrant.
z,,;
These were run on the vibrator before rebuilding and hence are not comparable to the values given in Figure 6. Q
14
low molecular weights, increases in value above a certain molecular weight, and then approaches an asymptote with increasing molecular weight. The lower limit of molecular weight, below a,hich the property has a value of zero, is difficult to determine accurately since the production of such low molecular weights introduces n e y unknowns. A case in point is that the lower limit of tensile strength for pure polystyrenes appears to be governed by the brittleness or notch sensitivity of the specimen, whereas when the low molecular weights are achieved by addition of plasticizers the cold flow limits the experimentally accessible range. 100
13
I
H = C O M h i E R C I A L RANGE
5
, I
4
w
LIMITING RANGE
30-
0
1
Figure 7 .
2
3
4
5
M , x 10-5
6
7
8
Tensile Strength US. Weight Average Rlolecular Weight
1, cellulose acetate ( 1 3 ) ; 2, polyvinyl chloride ( 4 ) ; 3, polystyrene (authors' data); 4, Butyl rubber ( 1 4 )
--0
2
Figure 6,
4
6
8
IO
M, x 1 0 - 3
I2
14
16
18
Dynamic Heat Distortion ws. &-umber Average Molecular Weight
Numbers by data points are mixture numbers
However, Flory (7') has shown that the form of the equation governing the variation of tensile strength with molecular weight should be tensile strength
=
A
R += Jfn
where A and B are constants. If the constants A and B are determined from the data, the equation can be used t o find the value which gives the tensile strength a value of zero. of As can be seen from the inspection of the data there is a class of properties which appears to obey this form of equation. For such selected properties, the lower limiting molecular \\.eights are collected in Table YIII. The equation given above is used here primarily as a convenient method t o characterize in an empirical manner the dependence of certain properties on molecular weight and its use is not meant to imply that these properties are actually an explicit function of the number average molecular weight. Particularly in the case of second order transition points (or heat distortion points) there exists abundant evidence to the fact that
The asymptotic value of a property with increasing molecular weight is of greater interest in that it defines t o a large extent the usefulness of polystyrene as an engineering material and sets the upper limit of the property attainable with polystyrene. These upper limiting molecular weights are collected in Table IX. In most cases these limiting molecular weights are extrapolated values; in addition, some of the values represent the lomest molecular weight tested. Further work may well show that such limiting values are too large. The dependence of the property nith kind of average molecular weight was reasoned out for each case of which the following is a sample :
-
-
Fractions of polystyrene (AIwE ~11~) whose molecular weights were 168,000 or greater had the samedensile strengths. A mixture with an of 176,000 but an AI,, of 3000 had only three fifths the tensile strength. Further, mixtures with various plasticizers had the same tensile strength a t a &ven A I n of the mixture, Therefore, tensile this value being unaffected by t h e JI, strength depends mainly upon the Mn.
zu
The only properties measured whose limiting molecular were melt viscosity weights appeared t o be a function of the ( 3 ) and film formability. The general pattern of variation in the value of a property with molecular creight for polystyrene is also found in the behavior of the property among dlfferent polymers (4,6, l g , 1 4 ) . In Figure 7 are collected data shoning this effect \!-hich has been
xu,
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1951
WEIGHTS,gn TABLE VIII. LOWERLIMITINGMOLECULAR Property
Addends
A
Lower Limiting Mn
-B
These values do not have the same meaning as the others sinoe a heat distortion of 0’ C. is completely arbitrary.
TABLE IX. MINIMUMVALUEFOR THE MOLECULAR WEIGHT BEYOXD WHICHAN INCREASE DOESNOT CHANGETHE VALVEOF THE PROPERTY IN A STRAIN-FREE OBJECT Molecular Weight Parameter
Propert:, Film formability Tensile strength Tensile elongation Area under @tress-strainciirve Tensile modulus Dynamic modulus at 25’ C. Dissination factor at 25” C. Tensile heat distortion Dynamic heat distortion A.S.T.M. heat distortim Second order transitior. Film density Lowest molecular weigbt tested
Limiting Molecular Weight 130,000 20,000 20,000 20,000
2,000 5,000 20,000 35,000 30,000 266,000“ 150,000a 168,000a
1401
of molecular weight distribution, F ( M ) , upon the properties of polystyrene has been implicitly discussed in the previous section. Each property has a limiting molecular weight, either rc?, or Pw,that is unaffected by the other average molecular weight. For example, the question “Polystyrenes having what distributions of molecular weight, F(M)’s, exhibit the maximum film tensile strength?” can be answered by considering two things. The first is that in order to obtain a coherent film of polystyrene the %f, must be greater than 150,000; and secondly, the maximum tensile strength is manifested above an of 20,000. The answer is then that polystyrenes of all F(M)’s which simultaneously have gW > 150,000 and > 20,000 will exhibit the maximum film tensile strength. The details of the shape of the distribution curve are unimportant. EFFECTOF ORIENTATION AND STRAIN.The orientability of polystyrene and the resultant effect upon physical propeities and temperature dependencies have been discussed (18). It should be re-emphasized that previous conclusions concern only strain-free and nonoriented polystyrene. At the present, from fragmentary experimental evidence, only qualitative conclusions can be drawn about the effect of molecular weight and molecular weight distribution upon the physical properties of strained objects. The problem - _ has been analyzed into two parts: first the effect of M,, M,, and F ( M ) upon orientability, degree of orientation, and rate of strain release; and secondly, the effectof degree of orientation and amount of strain upon physical properties. Preliminary results on each of these parts have already been published ( 1 , 6, 12).
zfi
zfi
ACKNOWLEDGMENT
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discussed in general by Mark (9). The niolecular weight a t which the value of the tensile strength drops t o zero here is the molecular weight a t which the film becomes unmanageable. For polystyrene, cellulose acetate, and polyvinyl chloride this value is set by the brittleness of the film. Following the line of argument of Mark, it can be seen that the higher the intermolecular attraction, the higher the upper limiting value of tensile strength attained. Superimposed upon this effect, however, is that crystallization raises this value even higher and, in addition, perhaps by tying together small molecules a t crystallites in the solid state increases the effective length of the molecules so that appreciable tensile strength is manifested a t smaller solution molecular weights. The commercial ranges of molecular weight as noted in Figure 7 represent the viscosity average molecular weights of commercial polymers. Since the authors have shown that tensile strength is primarily a function of %ffiand not li?, (to which the viscosity average molecular weight is an approximation), it would be of inof the commercial polymers. terest to note on these curves the Unfortunately, a simple calculation of the for a two-component system shows that, for example, for fractions of polystyrene having molecular weights of 1,000,000 and 200,000 each mixed with 1% of styrene monomer the B:s of the mixtures are practically the same, approximately 10,000. The low molecular weight species control the Bnand these do not contribute t o the f>f,Aof commercial polymers as measured in an osmometer. However, even if the rough ratio of = 2 is used to approximate the M , of the commercial polymers, most commercial polymers do not possess the maximum values of tensile strength they could have were they purified of low molecular weight constituents. EFFIWTOF MOLECULAR WEIGHTDISTRIBUTION. The effect
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The authors wish to thank W. J. Hamburger of the Fabric Research Laboratories, Boston, Mass., for making the Instron machine available to them. They wish t o thank also A. S. Kenyon who measured the weight average ~nolecular weights by light scattering. LITERATURE CITED
Adams, C. H., paper presented at the “Seminar on Fundamental Research in the Plastics Industry,” Polytechnic Institute of Brooklyn, Brooklyn, N. Y., April 19, 1949. American Society for Testing Materials, A.S.T.M. Designation D 256-431‘. Ibid., D 648-451‘. Auerbach, V., and Jelling, M., “Report of Research on Coated Fabrics and Thin Films,” Quartermastcr Corps, QMC-36, 271 (1946). Buchdahl, R., Nielsen, L. E., and Merz, E. H., J . Polymer Sci., 6, 403 (1951). Flory, P. J., IND. ENG.CHEM.,38, 417 (1946). Flory, P. J., J . Am. Chem. SOC.,67.2048 (1945). Linderstrom-Lang, K., and Lang, H., C o m p t . rend. trau. lab. Carlsberg, Ser. chim., 21, 315 (1948). Mark, H., Am. J . Phys., 13, 207 (1945). Merz, E. H., Nielsen, L. E., and Buchdahl, R., J . Polymer Sci., 4, 605 (1949). Merz, E. H., and Raetz, R. W., Ibid., 5, 587.(1950). Nielsen, L. E., and Buchdahl, R., J. Chem. Phys., 17,839 (1949); J . A p p l i e d Phus., 21, 488 (1950). Sookne, A. M., Harris, M., and Mark, H., J . Research AVafl.Bur. Standards, 29, 123 (1942). Zapp, R. L., private communication. RECEIVED April 1.5, 1950. Presented before the Division of Paint, Varnish, and Plastics Chemistry a t the 116th Meeting of the AMERICANCHEVICAL SOCIETY, Atlantic City, N. J
