1646
R. J. BOBKA, R. E. DININNY, A. R. SIEBERT AND E. L. PACE
prene were conducted at pressures below 50,000 atm. The production of insoluble gel during heating of 338,000 MW polystyrene at 50,000 atm. at 200 and at 300" probably proceeds by a cross-linking mechanism similar to that operative duxing the shearing experiments. I n this case, however, bond rupture occurs due to the increased temperature. It is interesting to note that in spite of the competing radical recombination reaction, cross linking can occur under these conditions when there is no shear force tending to separate the radicals formed during bond rupture. The degradation due to heating under elevated pressure is less extensive than the shear degradation a t the same pressure. The insoluble gel produced thermally was clear whereas the gel produced during shear degradation was invariably colored a light brown. Changes in molecular weight and crystallinity during the shear degradation of various polyethylenes show that the amorphous portions of the polymer have the predominating influence. The decrease in intensity of the 730-735 cm.-' infrared band, shows that polyethylene crystallinity is reduced by high pressure shear. Since five minutes was the usual time of a shearing experiment with polyethylene it is significant to point out that the reduction of crystallinity as determined by in-
Vol. 61
frared analysis is already complete after one minute of shear. Thus the five minute degradation experiment is predominantly an experiment on material having a crystalline content well below that of the starting material. This predominant influence of amorphous material is shown in Fig. 3 where it is evident that only for crystalline contents above about 80% is there a significant effect of crystallinity on the extent of shear degradation of polyethylene. A comparison of those polymers where no gel was formed gives rise to the following series which is arranged in order of decreasing ease of shear degradation: polyethyl acrylate, polymethyl methacrylate, polyethylene, polyvinyl chloride and the copolymer of vinyl chloride and vinylidene chloride. Due to the many differences among the various systems, however, no generalizations seem warranted in this connection. Acknowledgments.-For the donation of polymer samples used in this investigation thanks are due to C. S. Marvel of the University of Illinois, L. H. Tung and G. Jones of the Dow Chemical Company, to F. C. McGrew and C. A. Sperati of the Polychemicals Department of E. I. du Pont de Nemours and Co., and to R. M. Ross of the Rohm and Haas Company. Mr. H. A. Larsen wishes to thank the Shell Fellowship Committee for financial assistance.
HEAT CAPACITY AND HEAT OF ADSORPTION OF ARGON ADSORBED ON GRAPHON BY R. J. BOBKA, R. E. DININNY, A. R. SIEBERT AND E. L. PACE Morley Chemical Laboralory, Western Reserve University, Cleveland, Ohio Received July 19, 1967
Heat capacities of argon adsorbed on Graphon were determined calorimetrically in the range 55-88"K., for surface coverages of 0.301, 0.609 and 0.921 fractions of a monolayer. The data indicate the existence of maxima at temperatures below 55"K., with the height of each maximum increasing as coverage decreases. The magnitude of the heat capacity at these maxima, particularly at the lowest coverage, could not be justified solely on the basis of a localized t o mobile transition of the type predicted by Hill. An isosteric heat of adsorption was determined calorimetrically for this system at 87.5"K. for surface coverages from 0.165 to 1.25 monolayers. The general shape of the curve agrees with other studies of the system and, in particular confirms the resence of a maximum isosteric heat of adsorption at about 0.75 fraction of a monolayer. This maximum is interpreted as geing due to a prominence of argon-argon interactions on a relatively homogeneous surface.
Introduction The high degree of surface homogeneity of a variety of carbon blacks which have been graphitized at high temperatures in the absence of air has been established by Beebe and Young.' Graphon, graphitized Spheron-6, has a relatively homogeneous surface and a specific surface area high enough to be used with advantage in a calorimetric determination of heat capacity and heat of adsorption of the adsorbed phase. A study of the adsorption of some low-boiling gases on Graphon was initiated with the view that the contribution of this surface toward the properties of the adsorbed phase would be more clearly defined and consequently more easily interpreted than would be the case with other, more complex and less homogeneous surfaces. The heat capacity determination of argon adsorbed (1)
R. A. Beebe and D. M.Young, T ~ r JOURNAL, s 88, 93
(1954).
on Graphon, which was part of this study, is the first reported for the system. Experimental The Graphon which waa used as the adsorbent was a medium processing grade channel black (Spheron-6) which had been heat treated in the absence of air a t 2700'. Its crystal properties from X-ray data areass La 53 A. Lc 34 C 6.93 in which La and Lc are the dimensions of the arallel layer groups of graphite platelets within the carbon brack particle. The dimension c, twice the interplanar distance within the crystallite, may be compared with 6.74 A., which is the comparable distance in graphite. The monolayer capacity for the 31.3 g. of Graphon which was used was determined to be (2) W. D. Schaeffer, personal communication. (3) M. H. Polley, W. D. Schaefler and W. R. Smith, THISJOURNAL, 67, 469 (1953).
t
>i
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i
Dee., 1957
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HEATCAPACITY OF ARGON ADSORBED ON GRAPHON
0.0310 mole of argon, corresponding to a B E T area of 86.1 m.2/g. The argon contained less than 0.4% impurities according to the analysis supplied by the manufacturer, The Matheson Go., N.J., and no further analysis or purification was attempted. The construction of the low temperature adiabatic calorimeter which was used has been described by Pace, et uZ.,~ and the detailed experimental procedure and methods of calculation, including the application of desorption and compression corrections have been reported by Pace, et UZ.,~ and Morrison, et aL6 Activation of the Graphon consisted of heating it at 220" for five days and evacuating until a pressure of 10-Bmm. of mercury was attained. For the heat capacity measurements enough argon was admitted into the calorimeter vessel containing the activated Graphon to approximate a 0.3 monolayer coverage and the vessel permitted to remain at the boiling point of argon for one day before measurements were made. The temperature range 50-90°K. was spanned twice in the same day in four to six degree intervals. Enough additional argon was then admitted to approximate a coverage of 0.6 monolayer and measurements were taken in the same manner as for the lower coverage. The treatment of the 0.9 monolayer was again the same except that the two separate runs for heat capacity measurements were made on two SUCcessive days. The isosteric heat measurements were obtained for eight coverages increasing from 0.00513 mole argon adsorbed up to 0.0388 mole argon. This corresponded to monolayer fractions 0 = 0.165 to 0 = 1.25.
Results and Discussion Heat capacities at fractions of a monolayer 8 = 0.301, 0.609 and 0.921 in the range 55-88'K. are plotted versus the temperature in Fig. 1 both as absolute heat capacities G,,, and as molar heat capacities CN.. That equilibrium conditions were not attained at the highest coverage, 8 = 0.921, is indicated by the distinctly separate curve appearing for run no. 1 which was made one day before run no. 2. The molar heat capacities are unexpectedly high for all coverages, but especially so for 8 = 0.301 and 8 = 0.609. Also, the sharp drop for these two curves up to about 72'K. strongly indicates the occurrence of a transition starting a t temperatures below 55'K. and a maximum in the heat capacity versus temperature plot. A lower and more diffuse maximum under similar conditions of low coverage and low temperature has been observed by Pace, et ul.,' for krypton adsorbed on rutile. Hills has predicted that a localized to mobile transition of adsorbed atoms, besides being altogether likely at low temperatures, would contribute about R cal./mole to the heat capacity of adsorbed argon and result in a maximum in the heat capacity curve. However, as Hill implies, this maximum would be apparent only under conditions of low temperature, low coverage and on a homogeneous surface. Although the latter conditions have presumably been met in the present case it is obvious that a localized to mobile transition of the type post)ulated by Hill would fall far short of accounting for a peak value for CN.of 18 cal./mole. Morrison, et u Z . , ~ have observed anomalously (4) E. L. Pace, L. Pierce and K. 8. Dennis, Rep. Sei. Instr., 26, 20 (1955). (5) E. L. Paae. D. J. Sasmor and E. L. Heric, J . Am. Cham. Soe., 74. 4413 (1952). ( 6 ) J . A. Morrison, J. M. LOBand L. E. Drain, Trans. Faraday Soe. 41, 1023 (1951). (7) E. L. Pace, W. T. Berg sud A. R. Siebert, J . Am. Cham. SOC., '18, 1531 (1956). (8) T. L. Hill, J . Chsm. Phya.. 14, 441 (1946).
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60
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66
TEMPERATURE
Fig. 1.-Heat
capacity of argon adsorbed on Graphon.
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0.5
Fig. 2.-Isosteric
90
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heat of adsorption of argon on Graphon a t 87.5"K.
high heat capacities, but no maximum, at low coverage and low temperature for argon adsorbed on rutile. Since the adsorbent surface was relatively heterogeneous, the absence of a maximum did not preclude the possibility of the occurrence of a localized to mobile transition, but the maximum contribution predicted by Hill was not high enough to account for the observed heat capacities. The interpretation of the maxima in the heat capacities is qualitative in view of the lack of data taken a t lower temperatures which would be necessary to characterize the transition. Although the magnitudes of these maxima cannot be justified solely on the basis of a simple localized t o mobile transition, the fact that these maxima occur in a
MICHIOKONDO AND MASAJIKUBO
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Vol. 61
temperature range predicted by Hill suggests that this heat of adsorption curve is made in Fig. 2 with these data may, in part, represent an example of a the curve of Beebe and Young.' The generally localized to mobile transition. A model for the ad- good agreement between the two curves is obvious sorbed gas is needed which allows for (1) a config- and the presence of a maximum a t a surface coverurational contribution in addition to that consid- age of about 0.75 is confirmed. The slightly more ered by Hill and (2) the interaction between ad- diffuse maximum and less steep decline a t the monosorbed atoms. layer are attributed to a greater surface heterogeThe tendency of the heat capacities for all cover- neity as evidenced by a larger surface area in the ages to approximate the values for the liquid form sample of graphon used here than was used by of the adsorbate is quite apparent in this case as it Beebe and Young. The presence of this maximum is considered to be due to attractive argon interachas been for argon on rutile. 9. lo The isosteric heat of adsorption curve which is tions which are prominent as a consequence of the shown in Fig. 2 covers the range 0.00513 to 0.0388 relatively homogeneous surface which the Graphon moles adsorbed, corresponding to surface coverage possesses. Acknowledgments.-The preceding work was 0 = 0.165 to 0 = 1.25. The temperature to which all heats are referred is 87.5'K. A comparison of supported by the Atomic Energy Commission, under Contract AT(30-1)-824. (9) E. I.. Pace and 8. A. Greene, J . A m . Chem. SOC.,76, 3286 The Graphon along with its analysis and struc(1954). tural data were generously supplied by Dr. W. D. (10) L. E. Drain and J. A. Morrison, Tranr. Faraday Sac., 48, 840 * Schaeffer of Godfrey L. Cabot, Inc., Boston, Mass. (1952).
THE MAGNETIC SUSCEPTIBILITIES OF DICYANAMMINENICI
