2184
G. ADLER,D. BALLANTINE, R. RANGASTHAR', AND T. DAVIS
Radiolysis of Solid Solutions of Acrylamide and Propionamide'
by G. Adler, D. Ballantine, R. Ranganthan, Brookhaven National Laboratory, U p t o n , N e w Y o r k
and T. Davis Chemistry Department, N e w York University, N e w Y o r k , N e w York
(Received February 21, 1964)
~~
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A wide range of solid solutions of acrylamide and propionamide were irradiated to various doses and the gaseous decomposition products measured. G-values for H2, CO, and CH4 were determined at several temperatures. The results indicate that either energy transfer reactions or hydrogen atom scavenging are important in determining relative yields. The G-value for CO as a function of composition is unusual and may be influenced by lattice structure. Finally, the hydrogen yield has been found to be primarily the result of a bimolecular reaction.
Introduction Pure acrylamide has been shown to polymerize in the solid state both during and after exposure to ionizing radiation to yield polymers of molecular weight in excess of 50,000.2,3a Morawetzab demonstrated that in solid solution with the isomorphous but nonpolymerizable propionamide the reaction occurred a t equivalent rates but molecular weights were reduced. This behavior was attributed to a classical chain transfer reaction, but later work by Adler4 was suggestive of the fact that some other process may be involved. Since Burton5 and co-workers demonstrated that the radiolysis of liquid organic mixtures may involve complex transfer processes, it was felt that detailed radiolysis of acrylamide-propionamide mixtures might help clarify the previous studies. Recent reports by Dainton6 demonstrate that such energy transfer reactions in solid organic solutions can indeed occur and are greatly affected by crystallinity factors.
Experimental Materials. Eastman Kodak pure grade propionamide was recrystallized twice from acetone and was dried for several days in a vacuum desiccator. The observed melting point was 80.0-81.0°. The acrylamide, obtained from American Cyanamide, was recrystallized from acetone and was dried in the same manner as propionamide. The observed melting point was 83.0-83.5 '. T h e Journal of Physical Chemistry
Labeled propionamide-d7 was obtained from Volk Radiochemical Co., Bkokie, Ill., with an isotopic content, specified by the supplier, of 98.14 atom % deuterium. Sample Preparation and Irradiation. The solid solutions were prepared by weighing the two components in the required proportions, melting them together in a 90" bath and then cooling the melt quickly in a liquid nitrogen bath. It was previously shown3b by X-ray diffraction that mixtures prepared in this fashion formed true solid solutions over the complete composition range. All samples were degassed on a vacuum manifold for 48 hr. at lop5 mm. and the irradiations performed in the Coaoy-pool a t Brookhaven Kational Laboratory. Irradiation dosimetry was done with Fricke dosimeters. Analysis. Total gas analyses were made using a modified Saunders-Taylor apparatus and a conveii( I ) (a) This work was performed under contract for the U.S.A.E.C.; (b) This paper is based on a thesis submitted to New York University by R. Ranganthan in partial fulfillment of the requirements for a Ph.D. degree. ( 2 ) B. Baysal, G. Adler, D. Ballantine, and A. Glines, Polymer Letters, 1, 257 (1962). (3) (a) T. Fadner, I. Rubin, and H. Morawetz, J . Polymer Sci., 3 7 , 549 (1959); (b) T. Fadner and H. Morawetz, ibid., 45, 475 (1980). (4) G. Adler, ibid.,in press. ( 5 ) (a) J. P. Manion and M. Burton, J.P h y s . Chpm., 56, 560 (1952) ; (b) M. Burton and W. N. Patrick, ibid., 58, 421 (1954); (e) M . Burton and R. Lipsky, ibid., 61, 1461 (1957). (6) F. Dainton. et al., Discussions Faraday Soc., 36, 153 (1963).
2185
RADIOLYSIS OF SOLID SOLUTIOSS OF ACRYLAMIDE A N D PROPIONAMIDE
tional gas-measuring vacuum manifold. The gases which were noneondensable a t - 196" were collected ininiediatcly after opening the irradiated sample to the gas-measuring system. The sample was then melted to ensure removal of any trapped gases and a collection of the rioncoriderisable gases a t - 196" repeated. Only these - 196" noncondensable gases were analyzed completely and were composed mainly of hydrogen, methane, and carbon monoxide. The sample was then warmed to room temperature and again noncondensable gases were collected. Gas analysis on the separated fractions was done using a Consolidated mass spectrometer and/or by gas chroniatography using a Perkin-Elmer gas chroniatograph with a modified silica packing (PerkinElmer Column S). Experimsntal Results. The yields of hydrogen and carbon monoxide vs. dose for 100% propionamide are given in Fig. 1 where it is seen that the yields of both gases are linear over the entire dose range. The yield of hydrogen from 100% acrylaniide is also linear in Fig. 2,
but the yield is an order of magnitude lower. The carbon monoxide observed in the radiolysis of pure aerylamide was very low, GCO0,001. For intermediate solid solution composition the yield of hydrogen was always linear with dose, and thc slope decreased with increasing acrylarriide content. The yield for CO 2's. dose was not linear for a solid solution containing greater than 25% acrylarnide as seen in Fig. 3 for a 50-50 mole yo composition, One can only determine then differentialG or an overall G-value for CO. The yields of CO a t several doses over a range of composition are shown in Fig. 3. The data on G-value for Hz, CO, arid CH4 for pure propionarnide and acrylaniide as a function of temperature are shown in Fig. 4 and 5. I t is readily seen that G for H2 and CH4 from pure propionainide is independent of temperature from - 176" to about 60" while
14
-
12
-
I
I
I
25 MEGARADS
'
7.5 MEGARADS1 I
-
ENERGY INPUT I N MEGARADS .
5
7.5
12.5
I
/
I
20 22.5 I I
27.5
I
35 I
/
_4
I 8
I 20 ENERGY INPUT fev per gm ria*) 1
I
I2
16
Figure 1 . Product, yields as a function of total energy input from ?-irradiation of pure propionarnide. ENERGY
INPUT IN MEGARADS
12
u"
l0-
I
-
8 -
64-
I
4
ENERGY INPUTleV per gm x
Figure 2. Product yields as a function of total energy input from ?-irradiation of solid solution of composition 50% propionarnide, 507; acrylamide.
volume 68,.\'umber
8 August, 1864
2186
G. ADLER,D.
I
I
I
I
BALLANTINE,
0.08
I
r '
R. RANGASTHAN, A N D T. DAVIS
I
1
I
I
9
1
0.6
-t
1
O
l -is04
-2000
-200"
-150"
-100" -50" TEMP " C +
0"
50"
100"
l
-1000
r
l
-500 00 TEMP 'C +
r
l
l
504
1000
Figure 5. Hydrogen yield a8 a function of irradiation temperature of acrylamide.
0.01
-1
i O
i I - - A
OO
2
4
6
8
m x IO'
Figure 6.
Plot of data in Table I.
Discussion -200"
-150"
-100'
-50' TEMP O
0" C
50"
100"
-+
Figure 4. Top: product yields as a function of irradiation temperature of propionaniide. Bottom: carbon monoxide yield as a function of irradiation temperature of propionamide.
Gco shows an activation energy which is about 2 kcal., mole based on an Arrhenius plot of the available data. The data from irradiation of niixtures of propionamide-& and propionaniide are given in Table I. Using the argument of Dyne' to differentiate a uniinolrcular from a bimolecular reaction leading to hydrogen forination the data in Table I were plotted in Fig. 6. Thc intercept gives the GD,(uni) and in this instance is equal to about 0.02. A conservative estimate of OfiI,(uiii)would appear to be GH,(uni) = 0.08. Coniparison of GH,(uni) = 0.08 with the observed GTI,(total)for propionaniide of C: = 0.44 indicatcs that the hydrogen is formed principally by a bimolecular process.
The Journal of Physical Chemistry
Any explanation of the yields of the various products should take into account conditions peculiar to the solid state. Factors of molecular size of products and lattice dimensions may affect diffusion and recombination reactions in a manner different from the liquid or gaseous state. The small size of the hydrogen molecule will cause it to be least affected and so it will be considered first. Figure 7 shows the variation of GH, with composition a t two temperatures, 22 and -78". It is a t once apparent that for intermediate conipositions of propionamide and acrylamide G H l is not a simple average value given by t.he mixture rule for two components A and B and represented by the solid straight line in Fig. 7 .
+
GH,(mixture) = GR,(A) X electron fraction of A GH,(B) X electron fraction of 13 Curve B gives the results at -78" and is typical of observations made in many two-component liquid systems. The acrylamide component is providing a (7) 1'. J . Dyne and W. >I. Jenkins, Can. J . Chem., 38, 639 (1960).
RADIOLYSIS OF SOLIDSOLUTIOSS OF ACRYLAMIDE AND PROPIOKAMIDE
PERCENT POLYACRYLAMIDE IN PROPIONAMIDE
71
:3
2;
;1
6;
S,O
‘CY
9;
8,O
7;
10,O
-I
0.5
0.1
-
100
90
I
l
0
IO
80
70 60 x) 40 PERCENT PROPIONAMIDE
J
l
20
30
l
’
20
30
(
l
40 50 60 70 PERCENT ACRYLAMIDE
0
10 l
80
l
l
100
90
Figure 7. Hydrogen yield as a function of composition of solid solution of propionamide-acrylamide.
Table I : Analysis of Total Hydrogen Yield from the Solid Solutions of Propionamide-d, and Propionamide, 7-Irradiation (22”) a t 12.5 Mrads Concentration of .~ r o.~ i o n a n i i d e d , . wt. %
Mole %I HD
Dl
99.06 99.21
0.88
0.06
0.77
0.02
98.46 98.60
1.44 1.35
0.10
4
97.17 97.15
2.66 2.70
0.17 0.15
8
94.49 94.37
5.19 5.27
0.32 0.36
1 2
Mole ’70 R D
m X
0.913
0.921
IO’
7 -
LIZ
0.05
QDS X QEZ
%
0.06
10’
Qm/m
0. 041° 0.012
0.63 0.54
0.02
0.378 0.108
1.83
1.86
0.495 0.557
0.10 0.05
0.495 0.279
0.027 0.015
3.66
3.80
0.490 0.523
0.17 0.15
0.833 0.785
0.022 0.021
7.35
7.93
0.44 0.32 1.41 0.018 0.523 1.88 0.024 0.36 a Not included in Fig. 6 since the basis of collection of gaa in this instance waa different from the rest.
protective effect and such behavior can be explained by a mechanism in which hydrogen is scavenged by an unsaturated component, in this case acrylamide. Curve B can also be explained by what is generally called energy transfer. This transfer may involve either a transfer of ionization or excitation energy to
2187
acrylarnide which has the ability to dissipate the energy without generation of hydrogen. Curve A which gives the variation of GH, a t room temperature is much more complex and difficult to interpret. At room temperature it is necessary to include consideration of GH, from polyacrylamide siricc acrylamide can be converted to a polymer under the conditions employed. I t has been shown that a t this temperature polymerization of the acrylamide approaches completion a t doses below 5 l l r a d s a t all cornpositions containing greater than 10% acrylarnide. The GH, (polyacrylamide) can be determined in two ways and the value obtained depends on the method employed. It has been noted earlier that the yield from acrylarnide is linear to a dose of 25 LIrads. Since it has been shown under similar irradiation temperatures that acrylamide is “essentially completely” polymerized a t less than 3 Mrads, one could conclude that GH,(polyacrylarnide) is the same as GH?(acrylamide), namely G H , = 0.047. This would account for the linear relationship of yield of Hz us. dose which was determined experimentally. Alternatively, the GH2polyacrylamide was determined by irradiation of polynier which had been separated from irradiated acrylamide samples by a selective solvent technique. The GH, from polyacrylamide prepared in this fashion was 0.24 or almost five times greater than that obtained from unseparated polymer. It is believed that in the situation where acrylamide crystals are irradiated continuously beyond the point where polymerization approaches completion a trace amount of monomer becomes trapped among growing polymer molecules and remains unreacted. It is possible that these isolated molecules can act as efficient energy transfer agents or scavengers. Effectively, then, the remaining trace of acrylamide acts as a protective agent for the polyacrylamide which is formed. Such an explanation also would account for the higher GH1from polymer which has been washed with a solvent that can dissolve monomer. Curve A is best considered in comparison to a calculated GH, curve based on GHp from pure propionamide and GHt from precipitated polyacrylamide. Such a calculated curve appears as the dashed line in Fig. 7. All deviations from this calculated line are negative and can be interpreted a t all compositions in terms of an energy transfer from either propionamide or polyacrylamide to acrylamide molecules or by hydrogen scavenging by acrylamide. The temperature dependence of G H ~from pure propionamide and acrylamide is shown in Fig. 4 and 5. The temperature dependence is small for acrylamide, an activation energy of -1 kcal./mole and zero for
Volume 68,Number 8 August, 1864
2188
G. ADLER,D. BALLANTINE, R. RANQANTHAN, AND T. DAVIS
propionamide. The absence of any appreciable temperature effect is often cited as evidence for hot hydrogen atoms as the reactive intermediates. However, if hot atoms are involved in the formation of hydrogen it is difficult to invoke an efficient hydrogen scavenging by acrylamide to account for reduced G H a in the mixed solutions. This result considered alone would tend to favor energy transfer as the reaction which is responsible for the protective role of acrylamide. The interpretation of the data for the other radiolysis products which were identified is more complex. These products, CH,, (AH6, and CO, are of greater physical size and their ability to diffuse through the cage formed by the lattice is less than that of hydrogen. Reactions involving them would be expected to be influenced to a greater degree by the physical domain in which they originate. If one considers the formation of methane, two facts are observed. As the data in Fig. 4 indicate, the formation of methane from propionamide is one of low yield and also uninfluenced by temperature from - 78' to temperatures close to the melting point. The other fact is that in compositions containing greater than 4Oy0acrylamide no methane is found while below 3Oy0 acrylamide the GcHlis relatively constant. The reason for the abrupt change in GCHr with composition may lie in difference in lattice dimensions. In propionamide, acrylamide, and their solid solutions the molecules are bound together as sheets by hydrogen bonding through the amide groups. These sheets have the hydrocarbon part of the molecules outermost. The sheets are stacked to form the crystal structure. Other than chemical saturation the principal difference between acrylamide and propionamide crystals is that the distance between sheets is 11% greater in the propionamide.8 In solid solutions, of course, the average distance is intermediate. It is believed that the larger intersheet distance of propionamide permits freer diffusion along the planes formed by the hydrocarbon portion of the molecules. In the event of bond rupture to produce a CHI radical the cage in the case of pure propionamide would thus be significantly looser and any prompt recombination reaction would be reduced. In combinations containing greater than 40% acrylamide the mobility of methyl radicals appears to be so reduced because of a sufficiently tight cage that the formation of methane is prevented almost completely. This difference in the intermolecular distances of acrylamide and propionamide has also been used to explain8 the inhibition by oxygen of polymerization of acrylamide in mixed crystals containing greater T h e Journal of Physical Chemistry
than 60% propionamide. Here it is suggested that oxygen could not inhibit polymerization in pure acrylamide crystals because of its inability to diffuse into the lattice. However, introduction of propionamide sufficiently increased the intermolecular space to permit oxygen diffusion and hence inhibition. Analogous effects of oxygen on e.s.r. signals decays were observed in the acrylamide-propionamide system for similar reasons. If one considers the change of G C Hwith ~ composition, it follows a different behavior from GH, and the behavior is difficult to reconcile with an energy transfer process. If energy is indeed transferred from propionamide to acrylamide, the GcH, should be expected to follow a pattern similar to that of G H ~ . When on< sonsiders the GCOyields us. composition the picture E,e mes very confused. At compositions of greater than 75% propionamide the yield of CO us. dose is linear but with compositions of less than 75y0 propionamide the yield us. dose is nonlinear. With pure acrylamide the CO yield is linear but the GCO is only 0.001. In Fig. 3 the CO yield for various compositions is shown a t several dose levels and a strange result is observed. There are found two distinct maximums in these curves at concentrations of about 75 and 40% propionamide. It is difficult to relate this to any energy transfer process. I n fact, a t this point no good explanation exists although in very vague ternis it is believed to be related to some critical changes in the average distance between hydrocarbon chains in the lattice. Little is known about the Gc2naa t this time except that in pure propionamide the G c ~ Hcan ~ be as great as 0.3. The fact that this is more than an order of magnitude greater than the GCH, indicates that the carbon-carbon bond cy to the amide group is preferentially scissioned. This result is analogous to those early studies of Breger9 on fatty acids and work on the photolysis of amides.'O Allen and Rae" found acetonitrile in reasonably high yield in studies of the radiolysis of acetamide1 No nitriles were found in this work but the analytica. techniques were not sufficiently extensive or complete to rule out their formation. ~~~
(8) G. Adler, Proceedings of the International Symposium of the Radiation Industry, Polymerization and Graft Copolymerization, T I D 7643, 127 (1962). (9) I. A. Breger and V. L. Burton, J . Am. Chem. Soc., 68, 1629 (1946). (10) G.H.Booth and R. G. Norrish, J . Chem. SOC.,188 (1952). (11) A. 0.Allen and K. N. Rao, BNL Annual Report 40, July 1. 1960.
RADIOLYSIS OF CYCLOHEXANE
2189
The radiolysis of solid solutions of acrylamide and propionarriide yields gaseous products which are mainly Hz,CO, CHs, and (&He. The yields of hydrogen can bt. interpreted on the basis of efficient energy transfer from propionarnide to acrylamide or by a protective scavenging of hydrogen atonis by the acrylaniide
Radiolysis of Cyclohexane.
double bonds. A t any rate, a protective effect similar to that shown in many liquid mixtures can exist in truly crystalline solids. The yields of the other gaseous products are not explainable by energy transfer or scavenging alone and differences in lattice dimensions are believed to play a significant role.
V.
Purified Liquid Cyclohexane and Solutions of Additives’
by S. K. Ho and G. R. Freeman Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received February $8, 1964)
The initial product yields (G values) in the 7-radiolysis of highly purified liquid cyclohexane have been found to be: hydrogen 5.6 f 0.1 : cyclohexene 3.2 f 0.2; 1-hexene 0.40 f 0.05; n-hcxarie 0.08 f 0.02; methylcyclopentaiie 0.15 f 0.01 . ethylcyclohexaiie -0.04; dicyclohexyl 1.76 f 0.05; cyclohexylhcxene 0.12 * 0.02; unidentified CI2-0.05. Cyclohexylcyclohexene was a secondary product. Both oxygen and p-benzoquinone reduced the major liquid product yields to the same liniiting values: cyclohexene 1.5 f 0.1; 1hexene 0.27 f 0.03; dicyclohexyl 0.29 0.03. From the reduction in the yields of these products, and on the assumption that free radicals were bcing scavenged by the additives, a lower limit of 1.1 f 0.3 was obtained for the ratio of the rate constants k ~ i n p r o p o r t i o n a t i o n / kcomhination for cyclohexyl radicals in liquid cyclohexane.
Introduction In the radiolysis of liquid cyclohexane, there is as
benzoquinone as a scavenger as compared with that of oxygen,6 re-examination of these systems should be useful. In view of the discrepancy of results in the yet-no general agreement on the initial yields of the literature, the system of pure cyclohexane has also been various products,2a-G and reports on the nature and yields of some of the minor products are in ~ o n f l i c t . ~ ~ ~ - ~ (1) The revearch for this paper was supported in part by The Defence Only Dyne and Stone4 have reported a good balance Research Board of Canada, Grant No. 1601-17. between hydrogen and hydrogen-deficient products. (2) (a) H. A. Dewhurut, J . P h y s . Chem., 63, 813 (1959); (b) G . R. Freeman, J . Chem. i’hys., 33, 71 (1960). As the first part of an investigation of track reactions (3) T. D. Nevitt and L. P. Rernsberg, J . P h y s . Chem., 64, 969 in liquid cyclohexane by the study of the effect of L.E.T. (1960). o n the distribution of products in solutions of various (4) 1’. J. Dyne and J. A. Stone, C a n . J . Chem., 39, 2381 (1961). additives, the present work deals with the product (5) J. W. Falconer and M .Burton, J . Phys. Chem., 67, 1743 (1963). yields of -y-radiolysis with oxygen and p-benzoquinone (6) E. S. Waight and P. Walker, J. Chem. Soc., 2225 (1960). as scavengers. Because of the conflicting results re(7) A . C . Nixon and R. IC. Thorpe, .I. Chem. P h y s . , 28, 1004 (1958). ported for oxygen2aJ and the different behavior of (8) G. Dobson and G. Hughes, Proc. Chem. Soc.,, 109 (1963). Volume 68, .?‘umber 8
August, 1904
