PRESSURE EFFECTS ON GLASS TRANSITION IN POLYMERS
In benzenes there are two types of linear conformations of atoms, represented by HICICl and H1CIHd. The corresponding linear shrinkage effects may be derived from the eight quantities of eq. 3 4 4 1 in the following way.
1497
Conclusion
(43)
Spectroscopic calculations of shrinkage effects will no doubt become very important in the future as the accuracy of electrondiff raction experiments increases. There have already been performed some tentative refinements of electron-diff raction interatomic distances in benzene, using the presently calculated shrinkage effects.
Numerical values of linear shrinkage effects in benzene and benzene-& are reported in Table XI.
(48) W. V. F. Brooks, B. N. Cyvin, 5. J. Cyvin, P. C. Kvande, and E. Meisingseth, Acta Chem. Scond., 17, 345 (1963).
-6
l i n (r ***)
-61in(l**)
=
-6(r***
= --6(t**)
1
+ 6(d**)
+ 6(d**)
(42)
Pressure Effects on Glass Transition in Polymers
by Umberto Bianchi I s t i t d o d i Chimka Industriale, Sez. V del Centro Nazionule d i Chimica delle Mactomolecole, Vniveraitb, Genova, Italy (Received August 3, 1964)
Heating at constant volume of a glass (polyvinyl acetate) has been followed by measuring internal pressure, Pi,over a range of temperature comprising the glass transition temperature T,O a t 1 atm. The large increase in Pi a few degrees above T,O indicates that the glass transition takes place even if the heating process is performed at constant volume. Free volume considerations are shown to be still capable of offering an explanation of pressure effects on glass transition provided the change in specific volume of the glass a t different T , (and P,) is taken into account.
Introduction Starting from the fundamental consideration that pressure is the thermodynamic variable that, like temperature, can be used to change the volume of a body, many workers have studied the influence of pressure on the glass transition temperature, T,, of polymers.'-' In all of these papers it has been explicitly (or tacitly) assumed that we can speak of a significant glass temperature provided the time scale of the experiments is long enough to assure reproducibility of the most important physical properties (specific volume, thermal expansion coefficient, etc.) below and above T,. Experience has shown that this reproducibility can be achieved by changing the temperature of the sample
under investigation very slowly, at a rate of say 1-2'/ day, In these conditions, it also becomes possible6to apply thermodynamics to glass transition, with results which have proved very useful for a better understanding of some aspects of the transition itself. It is well known that the "free volume (1) N. Hirai and H. Eyring, J . Polymer Ai., 37, 51 (1959). (2) J. E. McKinney and H. V. Belcher, J . Res. Natl. Bur. Sa., 67A, 43 (1963). (3) J. M. O'Reilly, J. Polymer Sci., 57, 429 (1962). (4) K. H. Hellwege, W. Knappe, and P. Lehmann, Kolloid-Z., 183, 110 (1962). (5) G. Gee,P. N. Hartley, J. B. Herbert, and H. A. Laceley, Polymer, 1, 366 (1960).
Volume 69, Number 6
M a y 1966
UMBERTO BIANCHI
1498
gives an explanation of glass transition in terms of chain mobility, which decreases as the “free volume” becomes smaller than a certain critical value. This interpretation has been substantiated by a great deal of experimental work, connected with molecular weight effects on T,,7 correlations between polymer chain structure and Tg,*effects of diluents on T,,getc. However, there is a point in which free volume theory expectations and experiments are out of harmony, and this is just the pressure effect on T,. One of the most straightforward consequences of free volume considerations is expressed by the equationW11
where at is the so-called free volume thermal expansion coefficient, Pf is the free volume compressibility, f is the free volume fraction at the temperature T , f, is the free volume fraction a t the temperature Tg, P is the pressure, and T,O is the glass transition temperature under a pressure of 1 atm. As is well known, there are strong indications that afand Pf are equal, respectively, to a1 - a, and 81 BE, where at,81 and a,, 8, are thermal expansion coefficients and compressibilities above and below Tg.6 From eq. 1 it is easily seen that
therefore would never reach f,, which is necessary to allow the transition glass ---t liquid. It is clearly of interest to conduct an experimental study capable of t,elling us whether or not glass transition happens during the heating of a glass at constant volume; we have performed such an experiment on polyvinyl acetate (PVAc). Nature of the Experiment. Experimental apparatus already used by us for internal pressure (Pi)measurem e n t ~ ’ ~ can ~ ‘ *also be used to keep constant the volume of a polymer over a range of temperature of about 60”. I n addition, by a fortunate coincidence, the Pi value itself has been shown15 to be extremely sensitive to a glass transition. From what we know until now, there appears to be a general, impressive decrease of Pi, defined by
V
- P = T -a- P B
when we pass from a liquid to a glass. Table I collects values of Pi, extrapolated at T , from the liquid and glass regions, for various polymers. Table I: Values of Internal Pressure, Pi,Extrapolated from the Liquid (Pi), and from the Glass (Pi), at T,,for Various Polymers (pi)gs
Polymer
This equation tells us what the increase dT, in T , is due to an increase in pressure dP; of course, it is reasonable to expect that eq. 2 is valid not for very high pressure, but only in a range of a few hundred atmospheres, where compressibilities and expansion coefficients are in a first approximation independent of pressure. The aspect of eq. 2 is very similar to an equivalent equation which could be written if glass transition were treated as a second-order transition; this analogy is to be considered only formal and we think, with Gee6 and many others,12 that it, is desirable to abandon the use of the term “second-order transition’’ to describe the change of glass to liquid. Postponing the comparison of eq. 2 with experimental data to the Discussion part of this paper, we see from eq. 1 that the glass transition can take place only when the free volume fraction f is equal to f,, a characteristic value for each polymer. If we were to start with a glass, i.e., with f < fer and heat it a t constant volume with a suitable increase in pressure, one would think that the free volume fraction present in the material would remain constant and The J o u d of Physical Chemktw
(3)
Natural rubber vulcanized Polymethyl methacrylate Polyvinyl acetate Atactic polystyrene
(Pi)],
T,,OC.
cal./cc.
cal./cc.
Ref.
36 97 21 102
62 68 65 66
122 96 103 100
15 15 15 13,15
It is therefore possible, by using the same apparatus, to keep the volume of a polymer constant and, a t the same time, by measuring Piat various temperatures, (6) F. Bueche, “Physical Properties of Polymers,” Interscience Publishers, Inc., New York, N. Y., 1962. (7) T. G Fox and P. J. Flory, J . Appl. Phys., 21, 581 (1950); J . Polymer Sci., 14,315 (1954). (8) A. V. Tobolsky, “Properties and Structure of Polymers,” John Wiley and Sons, Inc., New York, N. Y.,1962. (9) F. N. Kelley and F. Bueche, J. Polymer Sci., 50, 549 (1961). (lo) J. E. McKinney, H. V. Belcher, and R. S. Marvin, Trans. SOC. Rheol., 4,347 (1960). (11) J. D.Ferry and R. A. Stratton. KoUoid-2.. 171, 107 (1960). (12) A. J. Kovacs, F m h r . Hochpolymer. Forsch., 3 , 394 (1964). (13) U. Bianchi and C. Rossi, Chim. I d . (Milan), 45. 33 (1963). (14) C . Rossi, U. Bianchi, and E. Bianchi, J. Polymer Sci., C4, 699 (1963). (15) G.Allen, D.Sims, and G. J. Wilson, Polynter, 2,375 (1961).
PRESSURE EFFECTS ON GLASSTRANSITION IN POLYMERS
1499
t
Figure 2. Pyrex glass cell used for dilatometry (precision bore capillary is not shown) and internal pressure measurements; A and B, platinum wire contacts. Figure 1. Cathetometer readings against temperature for our sample of PVAc (T,O = 21.5'). Different symbols denote different runs, performed both by increasing and decreasing the temperature.
to know if the material is still a glass or has changed to a liquid.
Experimental Material. Polyvinyl acetate was chosen since its T , is conveniently located near room temperature. A commercial sample of PVAc (Mowilith 30) was mm.) to give "melted" at 80" under vacuum a block of perfectly degassed and transparent polymer. The block was cut and machined to give rods 1.5 cm. in diameter and several centimeters in length; these rods were used for dilatometry and Pi measurements. Figure 1 shows the results of a dilatometric study of our sample; values found for T,O, a,, and a1 at 1 atm. were 21.5", 2.81 X lo-* deg.-', and 7.15 X deg.-', respectively. The heating rate was always l"/day; the cell for dilatometry was the same as that used for Pi measurements (see below). Apparatus and Procedure. Experimental apparatus were identical with those previously described. Therefore we shall give only a brief summary here. Figure 2 shows the inverted cell used to keep the
polymer a t constant volume over a range of temperatures and to measure Pi when desired. The rods of polymer were placed inside the Pyrex cell, which was connected to a vacuum line and finally filled with mercury when the pressure reached mm. The cell was then equipped with electric contacts (A and B) and cooled a t the desired temperature below T,, always keeping the capillary a t the top filled with mercury. When the cell content ceased to contract sensitively, the cell was immersed in a pressure vessel, thermostated with a precision of ~ 0 . 0 0 1 " ,and connected with a Budenberg pressure balance capable of generating pressures up to 600 atm. with a precision of 0.01 atm. (see Figure 3). At this point, the thermostat temperature was raised to a temperature above T, to increase the speed of molecular rearrangements, a t the same time applying an increasing pressure, always keeping the mercury in the capillary just in contact with the platjnum wire B (condition of constant volume). (Unfortunately, it is not possible, with the apparatus described, to keep the volume of the sample strictly constant because of the effect of the mercury inside the cell and of the cell glass itself. However, it is possible to calculate that the change in volume in our case is very small Volume 69,Number 6 M a y 1866
1500
UMBERTO BIANCHI
Q
t
L
b
-
*YI
UT
I
U
7v
5
a
f5
J
Figure 3. Schematic apparatus for Pi determination: UT represents the ultrathermostat, P is the steel pressure vessel, filled with transformer oil and connected with a Budenberg pressure balance, t is a 4-v. a x . transformer, and L is the lamp.
and corresponds to a decrease of polymer volume by increasing T and P.) By measuring Pi, as amply described elsewhere,I8 a t various temperatures, it becomes possible to know whether the polymer is in the glassy or liquid "state." We have performed two cycles of measurements, the first starting with the volume of the glass at 1 atm. and T o = 12", and the second a t 1 atm. and To= 7". In each cycle, Pi was measured by decreasing the temperature from above to below T,O; then the temperature was raised again and a new series of measurements was performed. Each value of Pi is the average of a t least two distinct measurements. Because of the importance of the experimental time scale used during this kind of experiments, we show in Figure 4 a diagram of the thermal history of our sample during the first cycle of measurements; the second cycle was performed along exactly the same line. Table I1 collects Pi values a t constant volume, observed during the two cycles, whereas in Figure 5, Pi is plotted against temperature. The precision is estimated to be f1%. As is apparent from Figure 5, we have to conclude that glass transition takes place even if the heating process is conducted keeping the volume of PVAc constant a t an initial value characteristic of a glass. This result is surprising a t first, since one would expect that by keeping the volume constant a t a value The Journal of Physical Chemistry
35
Figure 4. Thermal history of our PVAc sample during Pi measurements (first cycle).
CrrLl P
---&---
i
.
- 1
w
a
3s
(0
25
30
~
35
w
t, *F
Figure 5. P , behavior aa a function of T at constant volume; dotted line is obtained by superimposition of the two plots.
where free volume is not sufficient to allow segment mobility the glass transition should not take place with a rise in temperature. On the contrary, experimental results show that the transition takes place only a few degrees above the atmospheric pressure transition (Tg0 = 21.5') and a t a temperature which slowly increases by decreasing the temperature a t which sample volume is kept constant.
Discussion Making reference to the Introduction of this paper, it is necessary to point out that (a) the disagreement between experiments and the theoretical eq. 2 and (b) the presence of a glass transition in the measurements described are two facts intimately connected; therefore, having found an explanation for (a), this would also explain (b).
PRESSURE EFFECTS ON GLASSTRANSITION IN POLYMERS
Table 11: Values of P , at Constant Volume for the Two Cycles of Experiments; i and P Denote Average Values of t and P at Which P , Has Been Measured 1, 0
.
(,
.
40.8 37.7 34.9 31.7 28.5 25.5 24.1 22.9 21.7 20.2 18.0 15.7 36.3 33.3 30.7 27.9 26.5 25.0 23.5
-
Pi,
oc.
P, atm.
cal./cc.
I,
First cycle 465 406 359 302 245 193 171 152 135 112 82 52 381 329 284 234 210 185 161
103 8 103 7 104 2 104 5 69 8 65 1 64 6 64 1 65 1 65 7 64 9 65 2 104 0 104 4 98 1 70 7 68 7 65 6 65.1
37 34 33 31 29 28 27 26 42 40 39 37 35 34 32 31 30
6 1 0 8 8 7 9 7 3 8 1 6 9 6 9 3 3
F,atm. Second cycle 515 450 428 408 372 355 341 32 1 600 572 54 1 514 482 458 430 398 377
Pi,
cal./qc.
101.5 94.0 91.1 89.2 65.2 63.8 63.3 63.9 100.5 100.7 100.6 100.8 101.3 99.1 92.3 85.0 65.2
As a first point, we shall discuss the comparison between eq. 2 and the experimental results. There are many p a p e r ~ l - ~ ,in ' ~ the literature about this subject, and it appears that the greatest difficulty in making such a comparison is to find sufficiently reliable values for Ap = PI - P,. However, an important source of compressibility data for polymers can be found in the internal pressure ( P I )measurements on polymers above and below T , . l S Recalling eq. 3 , from the direct measurement of ( d P / b T ) v and cy it is possible to get a very reliable value of isothermal compressibility, p, in any desired condition. After a critical survey of the data existing in the literature, we think that the comparison between eq. 2 and experiments can be reasonably done for only three polymers, ie., polystyrene (PS), polyvinyl acetate (PVAc), and polymethyl methacrylate (PNDMA). We deliberately have not taken into consideration cross-linked polymers, as we believe that for those materials some aspects of glass transition could be altered by the existence of a network. Table IIIl7sl8 collects, for the three polymers cited above, all the experimental data necessary to make the comparison between A p / A a and (bT,/bp),,. Values appearing in the first line for each polymer are considered to be the most reliable. As can be seen from Table 111,we must conclude that
1501
eq. 2 leads to a great overestimation of the pressure effect on T,, thus confirming recent analyses by O'Reilly3 and Goldstein. On the other hand, as eq. 2 is a quitmestraightforward consequence of free-volume considerations, its failure could lead to the conclusion that free volume is not the critical factor governing a glass transition16; this failure could also be considered as a support to Gibbs, the01-y'~that the glass transition has a thermodynamic character and is associated with t'he vanishing of configurational ent'ropy of the polymer. We will show in the following that there is a simple reason which can explain differences found between ( b T , / b P ) , g and A p / A a and also the experimental results obtained here. The point is that in discussing these effects, the change in specific volume of the glass at the glass temperature which takes place when we change T , by application of a pressure has been overlooked. Changes, dV,/dT,, in the specific volume of the glass at T , under different pressures, P,, have been reported by McKinney2 arid Hellwege4 without, however, proper recognition of t'he importance of this fact. Therefore, the true P V T diagram of a glass-forming polymer having dV,/dT, < 0 is like that illustrated, in its simplified but essential aspects, in Figure 6. This type of diagram should be valid in a range of pressures up to a few hundred atmospheres arid not to very high pressures where changes in a and p become important. The line AB gives the change A T , in glass transition temperature due to a change AP in pressure; however, AB is not parallel to the ( P , T ) plane, and segment BC (the specific volume of the glass at t'he glass temperature and pressure of point C) is shorter than AD (the specific volume of the glass at 0 or 1 atni.). The projection of the diagram on the ( V , T ) plane is shown in Figure 7 . The line AB, which is the boundary line between glass and liquid regions, has a negative slope due to the decrease of specific volume at T , wheri T , itself increases. It' is t'herefore quite clear that our experiments consist' of a heating process at constant volume (horizontal broken line in Figure 7 ) starting from T o ; as soon as the point representative of the system arrived at the temperature indicated as T , = T,O dl', it
+
(16) M . Goldstein, J . Chem. Phys., 39, 3369 (1963). (17) G . Allen, G. Gee, G . Mangaray, D . Sirns, anti G . J. Wilson, Polymer, 1 , 467 (1960). (18) N . Shishkin, Soviet Phys.-SoLid State, 2 , 3 2 2 (1960). (19) J. H . Gibbs, J . Chem. Phys., 2 5 , 185 (1956); J . H . Gihhs arid E. A. Di Marzio, ibid.. 28, 373 (1958).
Volume 6 9 , 3umher 6
M a y 1966
1502
UMBERTO BIANCHI
Table 111: Relevant Data for PVAc, PS, and PMMA for the Comparison between (dTg/dP),,,,~ and A ~ / A in This ~ Work
x
T,, O C . a t
x
1 atm.
deg. -1
deg. - 1
5.93
2.28
, . .
, . .
6.74 7.15
2.26 2.81
...
6.85 ... ...
Polymer
PVAc
20.7 25.0 17.0 21.5
PS
107
PMMA
97 ...
V
104,
104.
81 x 106% a t m . -1
4.10 4.0
B, x 105. atrn. -1
aa x de,.
104. -1
(aTg/
~p x
105,
atm. -1
den. atm. - 1
...
0.044 ,..
...
2.54
...
0.061
..
0.031 0,030
...
...
0.065 ...
3.65
... ...
4.48 4.34
...
, . .
2.70
6.31
3.77
4.15
... ..
...
...
,..
,..
...
5.47
2.35
5.12
3.10
3.12
... ...
...
... ...
...
...
...
M/Aa,
deg. atm. -1
0.021 0.020 0,023
2.5 2.0
...
Wexptl.
1.6 2.0
...
2.02 .
.
I
0.023 0.020
...
...
, . .
Ref.
15 3 2 This work 13, 17 1 4 15 4 18
I
Figure 7. Projection of the ( V , T )plane of the diagram of Figure 6.
J Figure 6. Schematic PV?' diagram for a glass-forming polymer, for which (l/V,)(dV,/dT,) < 0; a and p are supposed to be independent of T and P , respectively.
crossed the glass-liquid boundary and we had to have the transition. I t also follows from Figure 7 that by decreasing To, one must have the glass transition at a higher temperature; this is exactly what has been found in the second cycle of our experiments. Table IV collects values of T o and T , obtained experimentally.
Table I V : Values of To, T,, and P , Found in This Work To,O C .
T,, O C .
P,, atm.
12 7
27 30.5 21.5 ( = T,O)
218.6 381.2 1.0
The Journal of Physical Chemistry
In connection with T , values reported in Table IV, it is necessary to point out that there are no a priori reasons why glass temperatures observed dilatometrically (as our Tg0)and those derived from P , behavior should be exactly coincident, and this is due first because of the different nature of the experiments used to find T , and second because the transition itself is never sharp but always occurs in a range of a few degrees. Early works15 have shown, however, that the temperature a t which P , starts to become greater than the value characteristic of the glass is in coincidence with T , derived in the conventional way by dilatometry. This argunient has been therefore used in to derive T , values reported in Table IV. Having shown why the heating at constant volume is still capable of generating a glass transition, we turn back to the point that the decrease of glass specific volume with increasing T , must also explain why eq. 2 is not verified by experimental results.
PRESSURE EFFECTS O N GLASSTRANSITION IN POLYMERS
With reference to Figure 7 , if we imagine the heating of a glass a t Tg0by dT, the total increase in volume over that of the new glass at Tg0 d T is now given by the segment EB; by considering that the portion CD = V,a,dT is due to van der Waals expansion and therefore does not contribute any free volume, we can easily write
+
1503
is in much better agreement with values reported in Table 111. The improvement obtained for PVAc by using eq. 5 instead of eq. 2 can also be obtained for PS and PAIRIA by calculating (l/Vg)(dVg/dTg) from Tables I and I1 of the Hellwege paper.4 Table V collects these data.
Table V : Comparison between Experimental Values of (dT,,W'),,p,~ and Eq. 5
which can be rearranged to 1 dV,
~g '
The new term in the denominator of the right term of eq. 5 (being (l/V,)(dV,/dT,) < 0) tends to decrease the value of the fraction in comparison with A p / A a , therefore bringing into better agreement experimental and calculated values of (bTg/W)jg. It is interesting now to calculate the value of (l/V,). (dV,/dT,) from the measurements reported in this work for PVAc. With reference to Figure 7 , we have
-
v,
d dV (T, - Tgo) = (Tgo- To) dT, dT ~
p g
Therefore 1 dV, Tgo - To __ - -ag v, dT, T , - T,'
-.
Calling (AT)v = T, - To and (AT)P we have the equation 1 dV, __ -
-
v,
dT,
- ffg
=
T,O - To,
(AT)P (AT)v - (AT)P
(6)
By making use of values in Table 11,we get 1 dV,
_ _
V , dT, =
=
-0.53 X
deg.-' (first cycle)
-0.45 X l o w 3deg.-l (second cycle)
Of course, the accuracy of these values is rather low owing to the small values of (AT)p and (AT)v; nevertheless, they are in good agreement with the average which can value, (l/Vg)(dVg/dTg) = -0.6 X be calculated from the dilatometric results under different pressures of ref. 2 , Figure 1. Taking (l/V,)(dV,/dT,) = -0.5 X as an average value for our sample and using our Aa together with Ap from ref. 15 (see Table 111),we can calculate with the aid of eq. 4 (bT,/bP),, = 0.017°/atni., which
dTK
x IO',
Polymer
deg
PVAc PS P4IhfA
-0 5 -0 67 -1 0
-1
a T g / a P from eq. 5, deg / a t m
0 0li 0 024 0 016
(DTg/DP)exptI,
deg / a t m
0 020-0 023 0 030 0 020-0 023
Taking into account the relatively low accuracy of (l/Vg)(dVg/dTg), the improvement offered by eq. 5 must be considered very interesting. From what we have seen up to now, it appears quite clear that the inequality (l/V,)(dV,/dT,) # 0 changes completely the type of conclusions which, a t first sight, one could derive from the inaccuracy of eq. 2 or from our experimental results. In particular, the free vofume point of view, through the substitution of eq. 2 with eq. 5, is still capable of offering a self-appealing explanation of the glass transition, including effects of pressure on T,. On the other hand, it is evident that results presented here lead essentially to a re-formulation of the same problem, which could be represented by the question : how can free volume theory explain the decrease of glass specific volume at T, observed at increasing glass temperatures? Before advancing a tentative explanation, it is very interesting to look a t the magnitude of (l/V,) (dV,/dT,) values for PVAc, PS, and PRIMA reported in Table V. As one can see, these are of the same order of magnitude as the temperature coefficient of unperturbed dimensions 3/2(d In $)/dT.20 Therefore, it seems attractive to recognize in the (d ln Q)/dT factor the cause or one of the causes which are responsible of the change of glass specific volume with glass temperature. If we put V
=
V,
+ Vf
(7)
where V is the volume of a polymer system, Vf is (20) C. Bianchi, J . Polymer Sci., A 2 , 3083 (1964)
Volume 69, .Vumber 6
May 1965
1504
C. A. GOY,D. H. SHAW,A N D H. 0. PRITCHARD
its free volume, and V o = V - Vr the "occupied" volume, we could assign to Vo a temperature coefficient which must be in some way related to 3//z (d In q ) / d T . However, it must be stressed that this tentative interpretation is unfortunately connected with factors which are at present almost totally unknown; for instance, one of these factors is chain mobility below T,. In particular, what is of more concern here is not the intermolecular mobility (ie., movements of one molecule in respect to another, which we know to be almost absent at temperatures below T,), but intramolecular mobility, that is, the ability of chain segments to change
their conformations when temperature changes, below T,. This problem, which we consider of the foremost importance for the study of glass properties, is now receiving attention in our institute. Acknowledgment. The author thanks Prof. C. Rossi for his constant encouragement and advice given during this work, and also thanks his colleagues of this institute for helpful discussions. The author also wishes to express his gratitute to A h . A. Turturro and Rlr. L. Planitario, who have skillfully attended to the experimental work almost day and night for more than 3 months.
The Reactions of CN Radicals in the Gas Phase
by C. A. Goy, D. H. Shaw, and H. 0. Pritchard Chemistry Department, University of Manchester, Manchester IS, England
(Received August 17, 1964)
Competitive rate constants have been obtained for the reaction of C S radicals with methane, ethane, and propane by the photolysis of ICN in the presence of mixtures of these hydrocarbons. It has not proved possible to establish absolute rate constants for any of these reactions, but it would seem that CN radicals abstract hydrogen from hydrocarbons about as easily as do chlorine atoms. Other sources of CN radicals and the addition of C-U radicals to olefins have been studied qualitatively. An improved preparation of solvent-free ICX is reported.
Preliminary Qualitative Experiments Photolytic dissociation of ICN vapor at 2537 8. leads to an iodine atom and a CN radical. The useful temperature range is somewhat limited, roughly 30150", since above 150" the equilibrium 2ICS
12
+ CzNz
(1)
is too far over to the right,' and, below 30", the vapor pressure is too BrCK is much more volatile and is photolyzed in the same way, but nothing is gained because its extinction coefficient is lower3 than that of I C s by a factor of loo* If a mixture Of a hydrocarbon R H and ICN vapor is photolyzed in The Journal of Physical Chemistry
a quartz vessel with a low pressure mercury lamp, the ~ in reaction products, apart from IZand C Z N formed 1, are RI, H C x , a little RCN, and Some paracyaI1ogen. These can be explained by a scheme of reactions including ICS+hv+I+CN
(2)
CN+RH+HCnT+R
(3)
R
+ Iz +R I + I
(4) *
(1) G. N. Lewis and D. B. Keyes, J . Am. Chem. Soc., 4 0 , 4 7 2 (1918). (2) D. R. Stull, I n d . Eng. Chem., 39, 517 (1947). (3) A. E. Gillam, Trans. Faraday Soc., 29, 1132 (1933).
