1852
ARNIMHENGLEIN
should permit direct testing of the exactness and validity of equations 4 and 7, and should provide a sound basis for studying the dependence of the constants (I& and ( N p ) e l on the properties of the systern. The apparent similarities between potassium on active carbon, barium on tungsten, and cesium on
Vol. 63
tungsten may apply to adsorption on metallic conducting surfacesin general. If such is the case, many effects that have been ascribed to surface heterogeneity could simply reflect the formation of adsorbed ions on essentially uniform surfaces, Acknowledgment.-The author thanks G. S. John and C. E. Johnson for helpful discussions.
CROSSLINKING OF POLYMERS IN SOLUTION UNDER THE INFLUENCE OF ?-RADIATION1 ARNIMHENGLEIN~ Contribution from the Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pa. Received April
4, 1969
Polyvinylpyrrolidone, r l y v i n y l acetate and polystyrene simultaneously undergo intermolecular crosslinking and degradation of their main c ains when irradiated in solution. Below a critical concentration which depends on the solvent no continuous network is built up since degradation is the predominant reaction in dilute solutions. Only a few solvents or mixtures of solvents in which this critical concentration is smaller than 10 g./lOO cc. have been found for each of these polymers. No relation exists between the radiation sensitivity of the solvent and the rate of crosslinking of dissolved polymers. However, crosslinking seems to be slightly favored in poor solvents. Observations on the gel dose show that this often increases with increasing polymer concentration in concentrated solutions. Radical scavengers inhibit crosslinking of these polymers and often are incorporated into the polymers. A mechanism is proposed in which the formation of macroradicals and low molecular weight radicals from the solvent by direct action of radiation are the primary steps. Crosslinks are formed by combination of macroradicals. The solvent radicals sensitize or retard crosslinking by attacking the polymer to form additional macroradicals or by deactivating macroradicals, respectively. The increase in gel dose in concentrated solutions is attributed to the decrease in the rate constant for the combination qf the free macroradica!s in viscous solutions as is well known from the autoacceleration observed in the bulk polymerization of a number of vinyl monomers.
Introduction I n secent years considerable research on the radiation chemistry of macromolecular substances has been devoted to changes which occur in polymers when they are irradiated in the solid state.3 Only a few investigations concerned with the effects of ionizing radiation on macromolecules in solution have been reported. These, however, have revealed that polymers in solution undergo changes similar to those observed in solid state irradiations, Le., degradation of their main decomposition of side g r o ~ p sand ~~ also intermolecular crosslinking. lo*l1 However, the mechanisms responsible for these changes are more complex since they may result from either indirect or direct action of radiation or both. I n early investigations on the radiation chemistry of polymers in solution no attempts were made to study the dependence of the reactions on the (1) This work was supported, in part, by the U. S. Atomic Energy Commission. (2) Visiting fellow on leave from the University of Cologne, Cologne, Germany. (3) See, for example, F. A. Bovey, "The Effects of.Ionizing Radiation on Natural and Synthetic High Polymers," lntersoience Publishers, New York, N. Y., 1958. (4) P. Alexander and M. Fox, Trans. Faraday Soc., 60, 605 (1954). (5) P . Alexander and M. Fox, J . chim. phys., 60, 415 (1953). (0) L. A. Wall and M . Magat, ibid., 60, 308 (1953). (7) A. Chapiro, J. Durup, M. Fox and M. Magat, International symposium in macromolecular chemistry, Milan-Turin, 1954, Supplemento a "la Ricerca Scientifioa," 1955, p. 207. (8) A. Henglein and M. Bcysen, Makromol. Chem., 20, 83 (1956). (9) A. Henglein, M . Boysen and W. Sohnabel, 2.physik. Chem. Neue Folge, 10, 137 (1957). (10) P. Alexander and A. Charleaby, J . chim. phys., 6 2 , 094 (1955). (11) P. Alexander and A. Charlesby, J . Polymer Sei., 28, 355 (1957).
nature of the solvent. I n recent communicai t has, however, been shown that the rate as well as the nature of the chemical changes in the dissolved polymer depend strongly on the properties of the solvent. Certain generalizations for the various interactions that take place between the dissolved polymer and the solvent during irradiation have been established. Polymers such as polymethyl methacrylate and polyisobutylene which undergo a degradation of their main chains in the solid state are also degraded in s ~,~~ and Charlesby found ~ o l ~ t i o n . ~Alexander that several water-soluble polymers crosslink when they are irradiated in aqueous solutions at concentrations above 0.5 weight yo. Recent studiesis on the radiation chemistry of polystyrene in solution showed that this phenomenon is not limited to aqueous solutions. It appears that every polymer that crosslinks in the solid state may also be crosslinked in solution under suitable conditions. The studies reported here are concerned with the radiation chemistry of polyvinylpyrrolidone and polyvinyl acetate in solution. Viscosity measurements were carried out to study the changes in these polymers. The results obtained are compared with earlier investigations on the crosslinking of polystyrene in solution. Each of these polymers (12) A. Henglein, Ch. Schneider and W. Sohnabel, I.physik. Chem. Neue Folge, 12, 339 (1957). (13) A. Henglein and Ch. Schneider, ibid., 18, 56 (1958). (14) A. Henglein and Ch. Schneider, ibid., in press. (15) A. Henglein, K. Heine, W. Hoffmeister, W . Sohnabel, Ch. Schneider and H. Url, International Conference on the peaceful mea of atomic energy in Geneva, Sept. 1958 (United Nations), contribution No. 962.
Nov., 1959
CROSSLINKING OF POLYMERS IN SOLUTION INFLUENCED BY ?-RADIATION
can be crosslinked by radiation in the solid state. It will be shown that these polymers simultaneously undergo intermolecular crosslinking and degradation in their main chains when irradiated in solution. The rates of crosslinking as well as of degradation depend on the nature of the solvent and the concentration of the polymer. At suitable concentrations crosslinking in solution occurs a t higher rates than in the solid state. This may be of interest in preparative macromolecular chemistry. Experimental The polyvinylpyrrolidone used for irradiation was an unfractionated product ([TI in methanol: 2.0 g.-1.100 cc.) of the Badische Anilin and Soda Fabrik. The polyvinyl acetate was unfractionated Gelva V-800 ([TI in benzene: 4.2 g.-I.lOO cc.) of the Shawinigan Resins Corporation. The polystyrene used in the experiments shown by Fig. 1 was obtained from a product of the Badische Anilin and Soda Fabrik by rough fractionation. The fraction of highest mean molecular weight ( [ T I in benzene: 4.6 g.-l.lOO cc.) was used. The mean molecular weight of each of these polymers was in the range of 1 X 108 to 2 X 106. Irradiations were carried out in oxygen-free solutions at room temperature. The samples were exposed in the interior of a, Brookhaven type cylindrical cobalt-60 source a t an exposure dose rate of 7.1 X lo4r./h. (approximately 7.0 x 1016 e.v. g.-1 min.-l). The small test-tube like irradiation vessels contained 3-6 cc. of the solutions. Air wa8 removed from the solutions by purging with pure argon before irradiation. Viscosity measurements were carried out with a semimicro viscometer of the Ostwald type at 25 I n order to compare the changes induced in the polymers when they were irradiated in different solvents all viscosity determinations were carried out in the case of polyvinylpyrollidone in methanol solution and in the case of polyvinyl acetate and polystyrene in benzene solution. For the more dilute solutions the polymer was separated after irradiation by evaporation of the solvent and subsequently dissolved in the appropriate solvent. For the more concentrated solutions a high dilution was made with methanol or benzene. vsp/c was determined at a polymer concentration of 0.5 g./100 cc.
.
Results and Discussion The Interpretation of the Viscosity Changes in Irradiated Polymers.-The interpretation of the viscosity changes of polymers irradiated in solution has already been discussed briefly. l3 Figure 1 illustrates typical data previously obtained for polystyrene solutions. l5 The degradation of the main chain causes a decrease in the reduced viscosity whereas the formation of intermolecular crosslinks will increase the reduced viscosity of a polymer. When degradation of the main chain and intermolecular crosslinking occur simultaneously there exists a critical value, Vorit, of the ratio above which no continuous network can build up in a particular polymer of given molecular weight. The ratio V depends on the nature of the solvent and on the concentration of the polymer. For V >> Vcrit the intrinsic viscosity of the polymer gradually decreases with increasing dose. This case is illustrated in Fig. 1 for the irradiation of solutions of polystyrene in chloroform, benzene and toluene. The slope of these viscosity-dose curves decreases with increasing dose. I n the case of V Vorit no continuous network throughout the dissolved polymer will be formed, but essential changes may be expected not only in the mean molecular weight but also in the shape and the structure of the macromolecules by the formation of branched macromolecules and microgels and linear fragments of different chain lengths. The interpretation of the viscosity data must be handled with caution in this case. It seems noteworthy to mention that the formation of microgels can be explained by the simultaneous effects of degradation and intermolecular crosslinking. Berkowitch, Charlesby and Desreux16explained microgel formation by intermolecular crosslinking. This, however, requires that two radical sites formed simultaneously on the same macromolecule. Polyvinylpyrrolidone.-In Figs. 2 and 3 the reduced viscosity of polyvinylpyrollidone is plotted as a function of the irradiation time for solutions having a concentration of 2.5 and 50 grams per liter. Figure 2 illustrates the cases of V > Vcrit or V >> Vorit. At this low concentration the viscosity gradually decreases in all solutions studied. Apparently the rate of degradation of the main chain is dependent on the solvent. I n general a degradation of the polymer by indirect action of radiation does not seem probable in organic solvents such as alcohols since the free radicals formed in these solvents are more likely to deactivate by reactions among themselves or with the solvent than with the polyvinylpyrrolidone present in low concentration. The degradation of the main chain therefore must be due to direct action of the radiation on the dissolved polymer. (16) J. Berkowitoh, A. Charlesby and V. Desreux, J . PoEymer Sei., 25, 490 (1957).
ARNIMH[ENGLEIN
1854
4
Ib
io
210
4 1 0
510
610
Time of irrodiotion (h)
710
do
,b
Fig. 2.-Irradiation of polyvinylpyrrolidone in different solvents: concentration of the polymer, 2.5 g./l.; dose rate, 7.1 X lo4r./h.
Time of irrodiotion (h)
-
Fig. 3.-Irradiation of polyvinylpyrrolidone in different solvents: concentration of the polymer, 50 g./l.; dose rate, 7.1 X lo4r./h.
,,t Methanal
x
100 Water
Fig. 4.-Relative rates of crosslinking of polyvinylpyrrolidone and polyvinyl acetate in mixtures of methanol and water: concentration of the polymers, 50 g./l.
The different rates of the degradation may be caused by energy transfer processes between the
Vol. 63
polymer and the solvents as has already been found for other polymers.'2-14 I n aqueous solution however changes in the the polymer may be expected to be caused by indirect action of radiation. The H atoms and OH radicals formed in this solvent may easily abstract hydrogen atoms from the polymer thus forming reactive macromolecular free radicals which undergo a subsequent stabilization by breaking a bond in the main chain (see eq. 3) or which promote the formation of intermolecular crosslinks. I n fact, the decrease in the reduced viscosity in Fig. 2 is most pronounced for the aqueous solution. Furthermore, this solution becomes strongly turbid during irradiation. This indicates changes in the structure of the polymer which are not due solely to a degradation of its main chain. The turbidity is explained by the formation of microgels as a consequence of simultaneous degradation and intermolecular crosslinking. Figure 3 shows that a t higher concentration of the polymer a continuous network is rapidly formed in the aqueous solution. I n methanol solution a complete network is also built up after a very long time of irradiation. V is higher than Vcrit for all the other solutions shown in Fig. 3 where the viscosity slowly decreases. During the irradiation of chloroform solutions the polymer separated from the solution to give a turbid sol of high viscosity. The amount of polymer precipitated was found to be proportional to the dose. This precipitation is due to the quaternization of the polyvinylpyrrolidone under the influence of the hydrogen chloride resulting from the radiolysis of the chloroform. I n Fig. 4 the rate of crosslinking is plotted for different mixtures of water and methanol. I n every mixture the rate of crosslinking is much smaller than would be expected if it was a linear function of the composition of the mixture. Apparently methanol retards the crosslinking of polyvinylpyrrolidone in aqueous solution. Tetranitromethane a t a concentration of lo-* mole/l. was found to inhibit the crosslinking of polyvinylpyrrolidone in solution. It has been shown earlier 17, l8 that tetranitromethane is an effective radical scavenger for organic free radicals formed in the radiolysis of aqueous solutions of organic materials. The solutions, after irradiation, were colored yellow because of the nitroform formed. Polyvinyl Acetate.-Figures 5 and 6 show similar data on the reduced viscosity of polyvinyl acetate as a function of the irradiation time. The concentrations of the polymer were 20 and 50 g./l., respectively. It can be seen from Fig. 5 that polyvinyl acetate in dilute solutions undergoes a degradation of its main chain. This degradation occurs a t a high rate in chloroform solutions. Chloroform has been found to be an effective solvent for the degradation of other polymers such as polymethylmethacrylate and polystyrene also. 12*l 3 Experi(17) A. Henglein and J. Jaspert, 2. phyeik. Chsm. N e w Folgs, 12, 324 (1957).
(18) A. Henglein, J. Langhoff and G. Schmidt, THISJOURNAL, in press.
-
CROSSLINKING OF POLYMERS IN SOLUTION INFLUENCED BY 7-RADIATION
Nov., 1959
1855
TABLEI IRRADIATION OF POLYSTYRENE, POLYVINYLPYRROLIDONE AND POLYVINYL ACETATEIN DIFFERENT SOLVENTS Concentration of the polymer, 50 gJ1. Polymer
Polystyrene
Polyvinylpyrrolidone
Polyvinyl acetate
a
The rate of
Solvent
Dioxane 5.42 Chloroform 5.37 Benzene 4.62 Toluene 4.50 Carbon tetrachloride 4.40 Ethyl acetate" 2.62 Methyl ethyl ketone 2.42 Isobutyl alc. 2.46 Methanol 2.2G Aniline 2.10 Chloroform 1.84 Water0 1.64 Dimethylformamide 1.40 Aniline 7.30 Chloroform 6.20 Dioxane 6.10 Acetone 4.95 Ethyl acetate 4.90 Dimethylformamide 4.74 Benzene 4.60 Toluene 4.20 Propionitrile 4.18 Methanol" 1.90 crosslinking in these solvents was made equal to 100.
ments on the photo-chlorination of these polymers showed that free chlorine atoms cause degradation of their main chains.l8 Cl.
+ mCH2-CH-CH-CH-
I
A
0 -
A0
co
I
A0
&)
~~
AI
AH*
AH,
5 2
100
30 100
-
Fig. 5.-Irradiation of polyvinyl acetate in different solvents: concentration of the polymer, 20 g./l.; dose rate, 7.1 X lo4 r./h.
+ *CHw A0
100
chloroform
Time of irradiation (h)
stabilization
co
30
4
'I
A
Crosslinks Degrades Degrades Degrades Degrades Crosslinks Crosslinks Degrades Crosslinks Degrades Degrades Crosslinks Degrades Degrades Degrades Degrades Degrades Degrades Degrades Crosslinks Degrades Degrades Crosslinks
F
I
&Ha CHs M.CH~-C=CH~
Turbid Clear Clear Clear Clear Turbid Turbid Clear Clear Clear Clear Turbid Clear Clear Clear Clear Clear Clear Clear Clear Clear Clear Turbid
tu$ 32
hydrogen abstraction
AH* AH1 HC1+ M.CH~-C--CH~-CHW
A
Relative rate of orosslinking
Appearanoe of s o h .
V../C
(3)
The high rate of the degradation under the influence of y-radiation is therefore explained by the action of free chlorine atoms resulting from the radiolysis of the solvent on the polymer. Possibly, for the other solvents in Fig. 5 the degradation of the polymer is caused more by direct than by indirect action of radiation. At a concentration of 50 g./l, crosslinking predominates in methanol, water-methanol mixtures and benzene (Fig. 6). All experimental results are compiled in Table I. It is seen that crosslinking of polymers in rather dilute solutions (50 g,/l.) depends on the nature of the solvent in an extremely specific manner. For (19) W. Hahn and F. Grafmuller, Makromol. Chrm., 91, 121 (1956).
each polymer studied only a few suitable solvents or mixtures of solvents have been found. No solvent exists which is generally suitable for crosslinking of all polymers. Similar observations have been reported by WipplerZ0for the crosslinking of polyvinyl chloride swollen to a high degree by different organic solvents. I n Table I there is also listed the reduced viscosity of the polymers in the different solvents. Some of the solutions were turbid even after filtration or centrifugation. Turbidity was preferentially observed for solutions in which vsp/c of the polymer has a low value. This indicates that part of the polymer (branched structures or macromolecules of very high chain lengths) is poorly soluble in these solvents and that a certain aggregation took place. There is a slight but not unambiguous evidence in Table I that crosslinking occurs in these solutions preferentially. (20) C. Wippler, J . Polymer Sci.. 29, 585 (1958).
ARNIM HENGLEIN
1856
I Ya r
Time of Irradiation [h]
-
Fig. 6.-Irradiation of polyvinyl acetate in different solvents: concentration of the polymer, 50 g./l.; dose rate, 7.1 X lo4 r./h.
O I ,.!
'
-
' "I I ' ' " I 10 Weight per cent of polymer
'
" ' I 100
Fig. 7.-Gel dose as a function of polymer concentration: A, polyvinylpyrrolidone in water (dotted curve, data of Alexander and Charlesby" for PVP of lower molecular weight) 0,polyvinylpyrrolidone in methanol; M, polyvinyl acetate in methanol; 0,polystyrene in ethyl acetate.
It seems, therefore, that the solubility or aggregation of a polymer in solution has some influence on the rate of crosslinking. An influence of the solubility has been reported earlier16in the case of crosslinking of polystyrene in ethyl acetate. The turbidity and rate of crosslinking are strongly increased by adding methanol to such a solution up to a point just below precipitation. A similar effect has been observed for the crosslinking of polyvinvyl acetate in mixtures of water and methanol. Water is known to increase the solubility of polyvinyl acetate in methanol when it is present in low concentrations. However, if water is present a t concentrations higher than about 33 volume % precipitation occurs. Figure 4 shows that the rate of crosslinking at first decreases with increasing concentration of water and increases strongly again before precipitation occurs.
Vol. 63
Radical scavengers such as tetranitromethane, iodine and diphenylpicrylhydrazyl at a concentration of mole/l. were found to inhibit crosslinking of polyvinyl acetate in solution. I n an earlier8 investigation it was observed that diphenylpicrylhydrazyl is incorporated into the polymer under the influence of radiation. The inhibiting action of the radical scavengers, therefore, is assumed to result from their reaction with macromolecular free radicals of the polyvinylpyrrolidone which are the reactive intermediates of crosslinking. Crosslinking of polystyrene in solution is already known to be inhibited by radical scavengers. l a Gel Dose and Polymer Concentration.-Figure 7 shows the dependence of the gel dose on the concentration of the dissolved polymer. Below the concentration where the ratio V passes through its critical value (the critical concentration) no continuous network can be built up in the dissolved polymer. This critical concentration was found to be 0.28, 4.0, 2.5 and 2.0 weight % for the systems polyvinylpyrrolidone in water, polyvinylpyrrolidone in methanol, polyvinyl acetate in methanol and polystyrene in ethylacetate, respectively. Above this concentration the gel dose a t first decreases until it reaches a minimum which occurs a t the concentrations of 0.5, 50, 10 and 20 weight %, respectively. Above these concentrations the gel dose increases again. This increase is especially pronounced for the solutions of polyvinylpyrrolidone and polystyrene. These two polymers need very high doses for network formation is their solid states. However, in the case of polyvinyl acetate the gel dose for the solid polymer is much lower and the minimum gel dose in solution is only slightly smaller than that in the solid state. General Mechanism of Crosslinking in Solution. -The primary reactions occurring in the irradiation of a polymer in solution are the excitation and dissociation of both the polymer P and the solvent S by direct action of radiation PM+-P*
CI
Snw-+S*+2%*
+
.w
+ RI.
h ) ~
(4a)
(4b) (5)
P* and S* being electronically excited or ionized molecules. Dissociation in the main chain of P leads to free "main chain" macroradicals (4a) while decomposition of a side group yields a free "side group" macroradical and a free radical R1 of low molecular weight (4b). Dissociation in the main chain and in side groups may occur from the same or from different excited states P*. A protection of the polymer occurs in the case that energy is transferred from the polymer to the solvent. Energy transfer in the opposite direction will result in a sensitization of the decomposition of the polymer. A sensitization also occurs when free radicals Rz,resulting from the radiolysis (5) of the solvent, attack the polymer probably by hydrogen abstraction Rz. + P +RzH + y w (6) This process will enhance the formation of free side group macroradicals. Depending on the
.
Nov,, 1959
CROSSLINKING OF POLYMERS IN SOLUTION INFLUENCED BY ?-RADIATION
1857
that the “critical” concentration of the polymer is much lower for crosslinking in water than in organic solvents. Highly reactive radicals resulting from the radiolysis of organic solvents have little chance to react with the dissolved polymer. Most of these radicals will react with the solvent, present in much higher concentration, to give free radicals of lower reactivity which do not attack the polymer at all. A strong indirect action of radiation may be expected in a few special cases only, for example, in chloroform solutions (eq. 3). I n organic solutions, therefore, there exists a rather high stationary concentration of free radicals R2 resulting Muw from the decomposition of the solvent. As a : + I (8) consequence, reaction 10 will be favored. The rate of crosslinking, therefore, will be much lower Combinations between side group and end group than in aqueous solutions and the critical concentramacroradicals also may contribute to network tion of the polymer will be much higher. I n formation. Many other reactions of the macro- accordance with these considerations, the experiradicals are likely to compete with their combina- ments show that the critical concentration in tion (8). One of the competing reactions may be organic solutions is higher by a factor of a t least the mutual deactivation by disproportionation ten. wwcw The retardation of crosslinking of polyvinyl+ (9) pyrrolidone in aqueous solution by methanol nw&w (Fig. 4) can be understood easily by the mechaHowever, most of the free macroradicals will be nisms discussed. The reactive H-atoms and OHdeactivated by their reactions with free radicals of radicals resulting from the decomposition of water low molecular weight R2 resulting from the de- are partly scavenged by the methanol before they composition (5) of the solvent abstract hydrogen atoms from the polymer to my+ Rs.+deactivation (10) form free macroradicals according to eq. 6. The The stationary concentration of R2, generally, CH2OH radicals thus formed21 in the methanol will be much higher than that of the macroradicals will scarcely abstract hydrogen atoms from the because of the higher concentration of the solvent polymer. On the contrary, they can contribute to the decrease in the rate of crosslinking by dein the solutions. Network formation in a polymer in solution, activating macroradicals according to (10). (c) The dose required to form a complete nettherefore, is favored if the rates of reactions 4b, 8 and 6 are high while it is retarded or inhibited if work in the dissolved polymer (gel dose) is ex(4a,) (7), (9) and (10) are too fast. The rate of pected to decrease with increasing polymer con(5) depends on the radiation sensitivity of the centration in dilute solutions. I n the case of aqueous solutions, the suppression solvent. Reaction 5 retards or sensitizes crosslinking depending on whether the following re- of reaction 10 by reaction 6 will strongly favor actions 7 and 10 or 6 and 8 are fast. crosslink formation (8) with increasing polymer The following considerations and explanations concentration. However, the number of macroof the experimental results are derived from this radicals formed by reaction 6 per unit weight of the dissolved polymer will decrease when the general mechanism. (a) The degradation of the main chain by re- concentration of the polymer is high enough to actions 4a and 7 is of lower order with respect to scavenge all free radicals R2. The gel dose, therethe polymer concentration than crosslink forma- fore, is expected to increase again with increasing tion (8). A “critical” concentration of the poly- polymer concentration. Figure 7 shows that this mer, therefore, must exist below which reactions increase is already beginning at a very low con7 and 10 suppress the rate of crosslinking (8) so centration of the polymer. much that the ratio V (eq. 1) will become higher I n the case of non-aqueous solutions the dethan the critical ratio Vcrit. This critical con- crease in the gel dose is attributed to the increased centration has been found in all cases where cross- formation of free macroradicals by direct action linking of a polymer in solution occurs. of radiation (4a) with increasing polymer concen(b) Water is expected to be the most effective tration. The number of macroradicals formed by solvent for network formation in a dissolved direct action of radiation per unit weight of the polymer. The highly reactive H-atoms and OH- polymer is constant. However, a higher stationary radicals formed here easily abstract hydrogen concentration of free macroradicals is built up atoms from the dissolved polymer according to during irradiation and this favors reaction 8 eq. 6. I n aqueous solutions, therefore, free side over the competing reactions 7 and 10. group macroradicals will be formed by indirect (d) The gel dose, however, often increases again action in a high yield. I n fact, it can be seen from (21) See for example M. Lefort in “Actions Chimiques et Biologigues Figs. 3 and 7 that extremely fast crosslinking occurs des Radiations,” edited by M . Haissinsky, Masson et Cie., Paris, 1955, in aqueous solutions. Furthermore, Fig. 7 shows serial No. 1 , p. 174. nature of the polymer, degradation of the main chain may be increased by reaction (6) if some of the macroradicals formedst abilize spontaneously by breaking a bond in the main chain &w.wmw----) + (7) For example, this phenomenon occurs in polyvinyl acetate as illustrated by eq. 3. The inhibition of crosslinking by radical scavengers and their incorporation into the dissolved polymer show clearly that crosslinking proceeds by a free macroradical mechanism. The formation of crosslinks, therefore, is attributed to the combination of side group macroradicals hlv
.MW
WW
h w w
WWW
-“*“hw
WWW
1858
A. J. MAJUMDAR AND RUSTUM ROY
with increasing concentration of the polymer in concentrated non-aqueous solutions (Fig. 7). A very pronounced increase also occurs in aqueous solutions at high concentrations of the polymer. A plausible explanation is the decrease in the rate constant of combination (8) of the free macroradicals in concentrated solutions. Such a decrease is well known from the bulk polymerization of several vinyl compounds where autoacceleration occurs at high concentrations of the polymer 23 This phenomenon is attributed to (22) R. G. W. Norrish and R. R. Smith, Nature, 160, 336 (1942). (23) E. Trommsdorff, H. Kohle and P. Lagally, Makromol. Chem., 1, 169 (1948).
Vol. 63
the high viscosities of these solutions in which the diffusion of the macroradicals is very low. The diffusion of species of low molecular weight, however, is not so strongly hindered in viscous solutions. As a consequence, reactions of the macroradicals with low molecular weight compounds become favored over macroradical combinations. I n the case of autoacceleration of polymerizations the intermediate macroradicals do preferentially react with molecules of the vinyl monomer. In the irradiation of concentrated polymer solutions the deactivation of the macroradicals by low molecular weight radicals R1 and R2 (eq. 4b and 5 ) are favored.
EXPERIMENTAL STUDY OF THE POLYMORPHISM OF AgI BY A. J. MAJUMDAR AND RUSTUM ROY Contribution No. 68-110,The College of Mineral Induatries, The Pennsylvania State University, University Park, Pennsylvania Received
April 10, 1060
Polymorphism in AgI has been studied in the temperature range between 25 and 200’ and the gressure range between 1 and 1000 atm. Pressure dependence of the temperature of the transition that takes place at 146.5 at atmospheric pressure has been studied up to 1000 atmospheres, and the volume change has been determined by high temperature X-ray diff raction. From these parameters the enthalpy change at the transition temperature has been calculated. No evidence could be obtained for a definite transition temperature for the important cubic hexagonal polytypic transition. It is concluded that there is no range of thermodynamic stability for the 3C polytype, if indeed i t can be prepared at all. i,
Introduction An examination of the l i t e r a t ~ r e l -reveals ~ the picture of the polymorphism of AgI. At least three forms are said to exist: i, a high temperature cubic form (I) which only forms (and is the stable form of AgI) above 146.5’; ii, a hexagonal form (11) with the wurtzite structure which forms reversibly upon cooling the high temperature cubic form below 146.5’; iii, a second cubic form (111) presumably with the sphalerite structure, which it has been claimed may be stable below 137”. The transition between forms I and I1 is well authenticated and understood even though the high temperature structure is a very rare one in which the Ag+ ions appear to be in random motion in the lattice. The transition between forms I1 and I11 is quite a different matter. It is a polytypic change from the hexagonal close-packing of I1 to cubic close-packing in 111. However, a closer look at the experimental data shows that neither is it certain that an endmember sphalerite-structure form I11 exists, nor is there much justification for any equilibrium transition temperature being assigned for the I1 % I11 reaction. Our interest in the applicability of classical thermodynamics to solid phase transitions caused us to examine two phases of this problem. First, in the I F? I1 transition, in which we have an “ordered” arrangement changing to a “random” arrangement accompanied by a very large entropy (1) L. W. Strook, 2. physik. Chem. B d . , 2 6 , 441 (1934). (2) L. W. Strock and V. A. Brophy, Am. Mineralogist,
change, do the transition parameters fit the Clapeyron relationship. If not, would the equivalent expression for second-order phase transition fit it better? Second, does AgI actually exist in a sphalerite I11 form and if so, what are the stability relations with the wurtzite I1 form. If a stable transition exists, o’ne has a structural mechanism to explain a possible second-order transition, and the thermodynamic relations in such a case should be most interesting. Experimental Procedure
I. Preparation of the Starting Materials.-Verwey, et aZ.,6 pointed out a long time ago that the structural nature of AgI depends to a large extent on its method of precipitation, and this has been the way in which “different” A I structures have been usually made. Reagent grade AgbOa and K I (Fisher), in the present investigation, were used for the preparation of AgI. The following different methods of preparing the AgI starting material were used: ( a )preci itated from solution by the addition of AgNOa to excess $1 with constant stirring; (b) precipitated from solution by the addition of KI to excess AgNOa with constant stirring; recipitated from solution, using aliquot amounts of Ag Osand KI; (d) previously precipitated AgI dissolved in KI, the solution filtered, and subsequently poured into a large excess of water. The precipitate now appearing filtered and washed free of I- by repeated washing with water; (e) previously precipitated AgI melted in a sealed silica tube, quenched and pulverized carefully; ( f ) previously precipitated AgI treated with concentrated NH40H and evaporated to dryness a t 110”; (g) formed by solid state reaction between metallic Ag powder and excess IZa t 150” in a sealed silica glass tube; ( h ) formed by solid state reaction in a sealed tube between 1 2 and excess metallic Ag powder a t 40, 94 300.’; (i) previously precipitated AgI vaporized and redeposited on a glass late; ( j ) previously precipitated AgI boiled with HsO to &ness; (k) AgI powder melted in the Trans-
(1985). (3) M. L. Huggina, Transition i n Siluer Halides, in “Phase formation in Solids,” ed. by Smoluchoweki, et at., John Wiley & Sons, New York, N. Y..1951,Ch. 8. (4) J. W. Manaen, TEIS JOURNAL, 60, 806 (1956).
( 5 ) E. J. W. Verwey and H. R. Kruyt. 2. physik. Chem., 8161, Spez. S. 142 ff. (1933).
. I
