RADIATION CHEMISTRY
O F ISOTACTIC AND
ATACTICPOLYPROPYLENE
with decreasing tem2erature. Thus, the data in Table IV further support the mechanism outlined above, It is interesting to note that the addition of
883
methyl radicals to the ring is nonselective with respect to the three ring positions, as the yields of o-, m-, and p-xylenes are close to the statistical ratio 2 :2 : 1.
Radiation Chemistry of Isotactic and Atactic Polypropylene. 111. Radiolysis in the Presence of Nitrous Oxide1
by Marmoru Kondo and Malcolm Dole Department of Chemistry and Materials Research Center, Northwestern University, Evanston, Illinois 60801 (Receiaed October 18, 1966)
Using Co60 y rays, isotactic and atactic polypropylene were irradiated in the presence and absence of about 60 em pressure of nitrous oxide gas. G(Hz) and G(CH4) were reduced by the N20 in the same ratio, a fact which is in line with the recent postulates of Dyne. On the other hand, cross-linking and chain degradation were enhanced, cross-linking somewhat more than degradation. The NzO probably acts mainly as an electron scavenger.
Introduction Lyons and Dole2 investigated the gas yields during the radiolysis of polyethylene in the presence of nitric and nitrous oxides and they did one experiment on polypropylene. The latter experiment with nitric oxide showed that the reaction mechanisms in the case of polypropylene were probably the same as in the case of polyethylene, but no experiments were done on polypropylene using nitrous oxide. The latter experiments should be of considerable interest because polypropylene undergoes both cross-linking and chain scission on irradiation. Okada3-5 discovered that in the case of polyethylene, nitrous oxide dissolved in the polymer increased the cross-link yield, but that in the case6 of polyisobutylene, the yield of chain scissions was reduced 30% by the nitrous oxide. It became of interest to see whether or not nitrous oxide could both increase the crosslink yield and at the same time decrease chain degradation in polypropylene during radiolysis. The results given below demonstrate on the contrary that both the cross-link yield and the G value of chain scissions were
increased by the presence of the nitrous oxide. This conclusion is of further interest in connection with the general mechanism of the radiolysis of aliphatic hydrocarbons of Dyne,' who proposed that the major products of radiolysis all stem from an identical reaction intermediate, A*, and that solutes depress radiolysis yields by reacting with the precursor of A*, either directly or as a negative ion after gaining an electron, to give a reaction intermediate which is not A*. We shall see that, in line with Dyne, hydrogen and methane yields were depressed in the same ratio by nitrous oxide but that the cross-link yield was increased relatively more than chain degradation.
(1) The previous paper of this series was R. W. Keyser, B. Clegg, and M. Dole, J . Phys. Chem., 67, 300 (1963). (2) B. J. Lyons and M. Dole, ibid., 68,526 (1964). (3) Y.Okada and A. Amemiya, J . Polymer Sci., 50, 522 (1961). (4) Y.Okada, J . A p p l . Polymer Sci., 7,695,703,1153 (1963). (5) Y.Okada, ibid., 8,467 (1964). (6) Y.Okada, ibid., 7, 1791 (1963). (7) P.J. Dyne, Can. J . Chem., 43, 1080 (1965).
Volume 70, Number 3 March 1966
MARMORU KONDO AND MALCOLM DOLE
884
Experimental Section Materials. The polymers used and some of their physical constants are collected in Table I. We have no explanation for the discrepancy in the M , values of samples 1 and 2.
Table I : Properties of Polypropylene Samples
-_____ 1
Tacticity Manufacturer Designation M , x 10-5 M , x lo-!' Un x lo-",* p, g at 25' Film thickness, mm Ye stabilizer
Isotactic Hercules X9467-70-3 5.24 1.08 0.78 0.897,O. 912" 0.025 0.25
Sample no.------2
3
Isotactic Hercules XA18-1-4 2.0 0.40 0.77
Atactic Esso 1457-83 0.772 0.148 0.141
0,025 0.2
Powder 0
As measured in this laboratory. * As calculated from the equation [ q ] = 2.5 x 10-5Jfn, valid for tetralin as the solvent at 23" (G. Ciampa, Chim. Ind. (Milan), 38, 298 (1956)). I
.04
Irradiation Procedures. A Co60 y-ray cell which initially contained 1288 curies was the radiation source. The radiation intensity and temperature of the radiation cell during this investigation were approximately 4.64 X 1019 ev g-I hr-1 or 0.742 RSrad hr-l and 35'. All irradiations were carried out under vacuum or in the initial presence of 60 em pressure of S z O (gel and viscosity experiments) or 74 em (gas evolution experiment). Before making the viscosity measurements, the film samples were annealed to a temperature just below the softening point to remove any residual free radicals. Gel and Intrinsic Viscosity Measurements. The fraction of insoluble polypropylene created by the irradiation was measured by inserting the polymer sample into a stainless steel basket and extracting with boiling toluene to constant residual weight. The extraction time varied from 150 to 400 hr. The antioxidant 2,6-di-t-butyl-p-cresol was added to the extraction solvent. The G values for cross-linking were calculated in the same manner as in the paper of Schnabel and Doles using the equation of Charlesby and Pinnerg +
=
G(S)/ZG(X)
+ 100Na/rMwG(X)
(1)
assuming fMw to be the original weight-average molecular weight, M,,o, and the molecular weight-distribution to be random. As discussed below, these assumptions are certainly not correct, but the G values are valuable for comparison purposes nevertheless. I n eq 1, s The Journal of Ph@sical Chemistry
.
I
.08
I
I
I
.I2
.I6
-20
.24
f g (e.v. I-' x 10'' Figure 1. Charlesby-Pinner plots: curves 1 and 3, atactic and isotactic polypropylene under vacuum (samples 3 and 2); curve 2, atactic in 60 cm of NzO; curves 4 and 5, isotactic (samples 2 and 1) in 60 cm of NzO. is the fraction of soluble component after a dose r , N A is Avogadro's number, and G(S) and G(X) are the G values for chain scissions and interchain cross-links, respectively, in units of events per 100 ev of energy absorbed in the polymer. The degree of reproducibility of the data is illustrated by the G values collected in Table 11. I n Table I1 we also list I , which is G(S)/ 2G(X), the intercept of the curves of Figure 1, 2, the slope of the straight lines of Figure 1, and rP, the dose to the gel point. The latter was calculated from the ratio 2/(2 - I ) in which x and I were calculated from a least-squares analysis of the data. The intrinsic viscosity measurements were carried out a t 135' using tetralin as the solvent and Ubbelohdetype viscometers. Kinetic energy correction factors to multiply the observed relative viscosity for each viscometer were calculated from the expression
1-1--
P fftz P
at02
(8) W. Schnabel and M. Dole, J. P h y s . Chem., 6 7 , 295 (1963). (9) A. Charlesby and 9. H. Pinner, Proc.Rou. SOC. (London), A269, 367 (1959).
RADIATION CHEMISTRY OF ISOTACTIC AND ATACTIC POLYPROPYLENE
Table 11: Gel Measurement Data and Derived Quantities
__~--
Sample no.--------3-----
,---2----
1
-
Polymer type------
Isotactic
----
--Isotactic--Atactic-Radiation conditions-Vacuum N20 Vacuum
I
0.360
z X
2.63
0.556 0.547 18.94 20,44 13.12 14.06 0.15 0.16 0.18 0.16
14'20
ev g-' T~ X 10-20, ev g-l G(X)
0.44
G(S)
0.32
1.60
0.401 0,390 9.32 10.32 5.82 6.41 0.32 0.29 0.26 0.23
0.367
N20
0.318
45.3
30.52
27.79
18.14
0.17
0.26
0.13
0.16
where t is the time of flow of the solution, to is the same for the solvent, and CY and p are constants of the equation for the viscosity, qo, of the solvent 170
=
woto -
poP/to
in which po is the density of the solvent. Actually, the kinetic energy correction was usually negligible. The solutions contained 1% antioxidant. A computer routine was programmed for calculation of the intrinsic viscosity from a least-squares analysis of plots of (vr l ) / c and In qr/c where vr is the relative viscosity and c is the concentration in grams per 100 ml as a function of the concentration. The intrinsic viscosity was taken as the average of the intercept of these plots a t zero concentration. Gas Analyses. Mass spectrometric gas analyses were carried out on the gases liberated from sample 2 during the vacuum irradiation experiment and the experiment with 74 cm of NzO. From the relative peak heights of masses 2 and 16 in the vacuum experiment the relative concentration of hydrogen and methane were determined. From measurements of the pressure, volume, and temperature of the final gaseous products, G(tota1 gas) of the vacuum experiment was calculated in units of molecules of gas per 100 ev of energy absorbed in the polymer. Knowing the relative moles of hydrogen and methane, it was then easy to calculate G(Hz) and G(CH4). From the peak heights of masses 2 and 15 in the NzO experiment, as compared to the vacuum experiment, the relative yields of hydrogen and methane were calculated, and from these values, G(H2) and G(CH,) for the N 2 0 irradiation experiment were obtained. Inasmuch as NzO may decompose under irradiation partly into N NO and as the mass spectrum of NO has a significant
+
885
peak a t mass 15, it is necessary to consider what contribution, if any, NO may have made to the mass 15 peak which we have assumed to be due entirely to the fragment CH3+ from methane (in methane the mass 15 peak is 86% of the parent peak at mass 16). We concluded that the amount of NO in the final gaseous product was negligible because (a) S O reacts about three times faster with irradiated polypropylene than N20; (b) the ratio of the mass 30 peak (the parent peak of KO) to the 44 mass peak was less than it should have been if all mass 44 were iS20;i.e., the pressure of S O should have made it greater; (c) the mass 44 peak (1130 units) could have included neither a significant contribution from C02 because the maSs 12 peak was so small (1.2units) nor one from propane because its most abundant peak a t mass 29 was only 3.4 units; and (d) the ratio, G(CH4)/G(H2), was practically the same in the vacuum and N20-irradiatedexperiments. The G value for K2 was obtained by subtracting from the total pressure, measured at liquid nitrogen temperature but corrected to room temperature, the pressures of H2 and CH4. The small mass 12 peak demonstrated that the CO content of the gas could not have been more than 1 or 2% of the nitrogen fraction. G(-N20) could not be calculated directly from the mass spectral data because the relative magnitudes of the peaks at masses 44, 30, and 14 did not agree with the known ratios for N2O (the peaks a t masses 30 and 14 were too low in comparison to that at mass 44 even after correcting the latter for possible contributions from COz or propane), but was estimated as follows. The pressure of the gaseous products as measured at 25' at the end of the irradiation was the sum, P H ~ PN? PCH4 PN?O vp(HzO), where the last term in the sum is the equilibrium vapor pressure of water a t that temperature. Subtracting the latter from the sum and also subtracting PH? Ps, PCH~ at 25' as calculated from the pressure measured at liquid nitrogen temperature, we obtained PN?O (final). Subtracting the latter from PS2o(initial) gave APN~o and from the latter G(-NzO) was readily calculated. Although the mass spectral data definitely indicated the presence of water vapor, a quantitative calculation of G(HzO) from them was lower than that estimated from other data, so G(H20) was determined by comparing the increase with dose of the total pressure before the saturation vapor pressure of water was reached with that afterwards. The initial rate of pressure increase can be expressed by the ratio A(PH? PN? PCH, -i- P H ? ~P E ? ~ ) /where ~ I - , is the dose in ev g-l, and the final rate of pressure increase by the PCHl Pslo)/4r. The difratio A(PHn Ps,
+ +
+
+
+
+
+
+ +
+
+
+
Volume 'YO, -\'umber
3
March 1966
886
MARMORU KONDO AND MALCOLM DOLE
ference between these ratios multiplied by the factor 100NAV/gRT, where N A is Avogadro's number, V is the volume of the irradiation cell, g is the weight of the polymer, R is the gas constant, and T is the absolute temperature, gave G(H20). Results and Discussion
G Value of Evolved Gases. Table I11 contains G values in both the vacuum and N20 experiments as well as previously determined G values of Schnabel and Doles for comparison, while in Table IV are tabulated ratios of G values of the N2O experiments to those of' the vacuum-irradiated samples. We consider first the G values of the gases liberated or consumed.
Table I11 : G Values (Molecules or Events per 100 ev of Energy Absorbed in Polymer) Type of polymer Isotactic Atactic Type of irradiation Vacuum S a n d Da K a n d D b NsO Vacuum Nz0
G(total gas) G(H2) G(CH4) G( -NzO) G(Nz) G(Hz0) G(X)-lC G(X)-2 G(X)-3 G( S)-1 G(S)-2 G(S)-3
2,85 2.78 0.072
2.80 2.63 0.17
0.153
2.03 0.14 3.57 2.33 0.45 0.438 0.307
0.170
0.315 0.243
0.272
0.244
+
0.172
0.255
0.126
0.162
From Schnabel and Dole.8 This paper. Average values when available. 'G(X)-l means G(X) for sample 1, G(X)-2 for sample 2, etc.
~~
Table IV: Ratio of G Values Determined in the N20 Experiments to the Vacuum Irradiation Values Polymer
Isotactic Sample 1 Sample 2 Atactic
0.77
0.81
1.61 2.01 1.48
1.29 1.43 1.29
It is interesting to note that the presence of the N2O reduces G(H2) and G(CH4) in the same proportion within the limits of experimental error. This result, as mentioned above, is in line with the proposed mechThe Journal of Physical Chemistry
anism of Dyne.' As suggested previously,2 N20 probably acts as an electron scavenger or reacts directly with a positive charge and prevents the normal subsequent reactions leading to hydrogen and methane. The fact that G(H2) and G(CH4) are reduced in the same proportion by N20 strongly suggests that S20 depresses the concentration of the precursor that normally leads to H2 and CH4. It is interesting to note that Sholes and Simic'O came to the same conclusion in their study of the effect of X20a t different concentrations on reducing G(H2) in liquid cyclohexane. They found that the reduction in G(H2) was only slightly less than G(N2) and they suggested that, at low N,O concentrations, a t least, G(KJ was equal to the number of electrons scavenged by the NzO. I n our case, G(N2) is about four times as great as the decrease in G(H2), but since we are dealing with a semicrystalline solid in which the NzO may only be soluble in the amorphous regions and in which the reaction kinetics may differ considerably from the liquid case, data obtained in liquid cyclohexane could not be expected to agree exactly with data obtained with polypropylene as the reacting species. As pointed out previously,2the N20may decompose partly into N and NO as well as react with an electron to form N2 0as suggested by Scholes and Simic. The NO produced by the decomposition of N2O could then react according to the mechanism previously discussed, yielding about 1 mole of Nz for every 3.5 nioles of NO consumed. However, if the decomposition into X O is only 20% of the total decomposition as found by Harteck and Dondes," for the radiolysis of N20 in the gas phase, then this mode of reaction is not too important for the explanation of the data of this paper. We shall return below to the discussion of reaction mechanisms. Intrinsic Viscosity. Figure 2 illustrates the change in intrinsic viscosity of both isotactic (sample 2) and atactic polypropylene as a function of the radiation dose for both the vacuum-irradiated samples and the same materials irradiated in the presence of 60 cm pressure of N20. The vacuum results are substantially the same as those previously observed' by Keyser, Clegg, and Dole, but the intrinsic viscosity of the atactic polypropylene changed very little with dose either for the vacuum-irradiation case or the S20 experiment. If the number of radiologically produced chain scissions equals the number of cross-links, then there would be no change in the number-average molecular weight and probably little change in the intrinsic viscosity. Although the condition of equal G(X) and G(S) does not (10) G. Scholes and M. Simic, Nature, 202, 85 (1964). (11) P. Hzrteck and S. Dondes, NucZeonics, 14, 66 (1956).
RADIATION CHEMISTRY OF ISOTACTIC AND ATACTIC POLYPROPYLENE
887
tures of the type -CH2CH(CH3)CH(CH3)CH2-. Hence, the initial rapid decrease in [q] with dose of the isotactic sample as compared to the very small change of [ q ] in the atactic experiments is probably the result of more head to head structures in sample 2 than in sample 3. Keyser, Clegg, and Dole found that the intrinsic viscosity with dose followed the empirical equation
-
1.2
In
-
‘0
DOSE ev.g? X
Figure 2. Intrinsic viscosity of polypropylene as a function of radiation dose: solid lines, vacuum experiments; dotted lines, in 60 cm of NzO; two lower curves, atactic sample 3; two top curves, isotactic sample 2 .
exist exactly, this is approximately true and may explain the small dependency of the intrinsic viscosity on the dose. Furthermore, the effect of the nitrous oxide in increasing G(X) and G(S) is nearly the same for both, Table 111; hence, again, the change in the net number of molecules on irradiation would be nearly zero. The small change of the intrinsic viscosity of the atactic sample with dose may also be partly explained by the rather low intrinsic viscosity to begin with and partly by the nature of the polypropylene itself. I n our previous publication we suggested that the initial rapid decrease with dose of the intrinsic viscosity may have been due to the effect of radiation on weak bonds between a few head to head structures such as -CH2CH(CH3)CH(CH3)CH2- in the polypropylene. At that time, we knew of no evidence for such “weak points” in the chains. Recently, Bailey, Liotta, and FungI2 observed 91% propane in the gases evolved during the first minute of pyrolyzing isotactic polypropylene a t 340’. They stated, “This is dramatic evidence that the postulated weak spots in polypropylene exist,” because according to their mechanism of degradation, propane would arise only from struc-
[VI,/ [TIO = Aq
+ Bq2/2
(2)
where q is the square root of the dose and A and B are empirical constants. The applicability of eq 2 to the data of this paper is illustrated in Figure 3 where {log [ q ] l / [ q ] o ] / qis plotted as a function of q. In agreement with a similar plot of Inokuti and the vacuum experiment data give a line of zero slope. However, the line representing the NzO experiment data has a positive slope. Inasmuch as the slope dep e n d ~partly ~ ~ upon G(X) - [G(S)/2b], where b is the ratio of the weight- to number-average molecular weights, it can be seen that for B to have a nonzero value, G(X) must have been increased by the XzO more than G(S)/2b. From the data of Table IV this appears to be the case for all of the G(X) and G(S) values determined from the gel data. Thus the viscosity and gel data are consistent with each other. Quantitative calculations can be made in the following way. Referring to the B constant of eq 2, according to a previous analysis of the intrinsic viscosity of a polymer undergoing simultaneous cross-linking and degradati~n,’~ the B constant is given by the expression (4aMw/100N~) { G(X)
-
[G(S)/2b]]
(3)
where a is the exponent of the Mark-Houwink viscosity eq 4. For sample 2, (4aMw/100N~)is approximately and since the slope of the NzO curve of equal to Figure 3 is 0.059 X it is clear that G(X) [G(S)/2b] must equal 0.059 to explain the results of this work. As suggested in our previous publication,2 in the initial stages of the vacuum irradiation where the B constant of eq 2 is zero, G(X) - [G(S)/2b] must equal zero, and since G(X) is estimated to be 0.15 (Table 11) and 2b about 10, G(S) initially must be 1.5. If this value of G(S) is increased by the presence of nitrous oxide by the factor 1.43, given in Table IV, then in order to make G(X) - [G(S)/Bb] equal to 0.059, see the previous paragraph, G(X) must have increased (12) W. J. Bailey, C. Liotta, and D. Fung, Technical Report, AFMLTR-65-100, April 1965. (13) M. Inokut,i and M. Dole, J. Polymer Sci., Al, 3289 (1963). (14) M.Dole, J. Phys. Chem., 65, 700 (1961).
Volume 70, Number 3 March 1966
MARMORU KONDO AND MALCOLM DOLE
888
[TI
2
0
-X
,o u .06-
O'I
I
.04
.02
-
t
I
to 0.27. This value of G(X) is close to the observed G(X) for the NzO experiment, 0.30, and is evidence in favor of the assumption that NzO affects G(S) in the same ratio at the beginning as well as a t the later stages of the radiolysis. The new value of G(X) was calculated from the expression G(X) = 0.059
+ 1.43G(S) 2b 0.059
+ (1.43)(0.15) = 0.27
We have been unable to interpret the A constant of eq 2, either its absolute magnitude or the small change in A from -0.11 x 10-10 to -0.099 x 10-10 [g/evll" between the vacuum and NzO experiments. Inasmuch as A depends upon the number of branch points produced in the polymer by cross-linking (decreases the intrinsic viscosity), on the decrease in the weight- to number-average molecular weights due to degradation, and on the variation of the K constant with dose of the Mark-Houwink eq 4 The Journal of Physical Chemistry
KM,"
(4) the situation is too complex to unravel at the present time. We would have expected A to be more negative in the NzO experiments than in the vacuum experiments, because of the effect of NzOin increasing G(S). The Gel Data. The fact that the gel data when plotted, Figure 1, according to the Charlesby-Pinner eq 1 follow a linear relationship demonstrates that, by the time that the gel point has been reached, chain degradation has probably been sufficient to produce a random molecular weight distribution. Thus, we imagine the polymer first to be degraded to give this distribution and then to be cross-linked to produce gel. Because of the initial rather high G(S) the initial weight-average molecular weight cannot be used to compute G values for cross-linking G(X). However, the ratio of G(X)N,Oto G(X),,, can be calculated from the inverse ratio of the slopes of Figure 1, Z , ~ ~ / Z N ~ O provided , that the value of the weightaverage molecular weight valid for eq 1 is the same for both the vacuum and N60 experiments. This calculation yields, of course, the same ratio as G(X)N,O/ G(X),,, when M , is held constant, but it is rather impossible to know what should be the correct value of M, to be used in the Charlesby-Pinner equation. I n the NzO experiment, G(S) is apparently greater than in the vacuum experiment, but the dose to the gel point was only about one-third as great. What we need to know in order to estimate a reasonable value of M , is not the initial G(S) values nor the final, but an average value valid over the dose range 0 to rg. Overlooking these uncertainties, the data of Table IV show that in all cases G(X) is increased by the NzO gas more than G(S) which also increases. On the other hand, G(H2) and G(CH,) are decreased by the NzO in practically the same ratio. Material Balance. Another difficulty in understanding the effect of NzO on gel formation is concerned with deriving a material balance from the data. Unfortunately, this has not been possible even in the vacuum-irradiation case because of the uncertainty in the G(X) values due to the uncertainty in knowing the proper value of M , to use in the calculations. Ordinarily one would write G(Hz)
=
=
G(X)
+ G(db)
(5)
where G(db) represents the G value for any double bonds or unsaturation produced. I n the N20 experiments G(H2) decreased but G(X) increased. We know nothing about G(db); it may have decreased more than G(H2) and in this way maintained material balance. However, if there are mechanisms by which
RADIATION CHEMISTRY OF ISOTACTIC AND ATACTIC POLYPROPYLENE
cross-links can be produced without the evolution of molecular hydrogen, then eq 5 would no longer be valid. We consider such mechanisms in the next section. Reaction Mechanisms. I n all probability, molecular hydrogen is produced partly by reactions of the type15
+
-CHzCH-
I
II
CH3f -CH2+-
+ -CHz-
(6)
+
+ e- +-CH-
+ -CH+ H2
(7)
(8)
Nitrous oxide could interfere with these hydrogen evolution reactions by reactions of the type2Il0
+ e- +NzO+ -CHz-+- +Nz + OH. + -CHOH. + -CHz-CH+ H2O XZO
N20-
not quite true, since AG(H~)was 0.60 and G(H20) was 0.45, but considering the difficulty of estimating G(HzO), the agreement is quite satisfactory. G(N2) was considerably greater than AG(H2), 2.33 as compared to 0.60. This indicates that NzO was reacting by some independent mechanisms, not involving a precursor of hydrogen. Another unexplained discrepancy is the fact that G(-NzO), 3.57, was greater than either G(N2) or AG(Hz). Nitrous oxide may have been degraded to N NO and these products may then have reacted directly with the polypropylene without liberating molecular nitrogen. The mechanism by which NzO enhanced degradation can only be speculated on. A greater production of free radicals by N 2 0 which enhanced cross-linking may also have enhanced degradation. Some of this increase in G(S) due to NzO may have been an artifact resulting from the 2.2-fold lower dose to the gel point in the N2O experiments as compared to the vacuum. With a much smaller dose to the gel point, the degradation up to the gel point would be less by a factor 2.2/ 1.43 or 1.54 in the NzO case than in the vacuum, thus giving the possibility of greater degradation beyond the gel point in the N 2 0 experiments. Fewer of the “weak points” would have disappeared a t the gel point, leaving more to be degraded beyond it.
+
CH2 -CH3+-
-CH3+-
+ Hz
+-CHzC-
889
(9) (10) (11)
The two free radicals of eq 10 and 11 could combine to form a cross-link without the evolution of hydrogen gas. The N2O should also depress the formation of unsaturation as well as the production of H2 and CH, gas. However, Okada5 found that, in the case of polyethylene, N2O increased the vinylene unsaturation by 87%, even more than the 44% increase in the number of cross-links. Unfortunately, Okada did not determine the effect of NzO on G(H2) in his polyethylene experiments. The production of one molecule of water should balance either one cross-link or one double bond in the material balance equation. Furthermore, G(H20) should be as large as and probably larger than the reduction in G(H2) (the absolute reduction of G(CH4) is negligible by comparison). This is
Acknowledgments. This research was supported by the U. S. Atomic Energy Commission and by the Advanced Research Projects Agency of the Department of Defense through the Northwestern University Materials Research Center. We are indebted to Dr. J. H. Elliott of the Hercules Powder Co. and to Dr. John Rehner, Jr., of the Esso Research and Engineering Co. for the polypropylene samples. (15) For a review, see M. Dole in “Crystalline Oiefin Polymers,” R. A. V. Raff and K. W. Doak, Ed., Interscience Publishers, Inc., New York, N . Y., 1965, Chapter 16.
Volume 70,Number 3
March 1966
