NOTES
12,55
Table 111 : Thermal Expansion Data"
Temp., "C.
Temp., OC.
20545 545-750 20-900
( 6 f 1) -(12 zk 3) t12.1 (7 f 2)
0 - ( 6 f 3) +14 3 20-720
(2 f 0.7) +7.6
20-900
Exp. coeff. x 108
(15 f 3)
20-750
20-600 --600-900 20-970
21.2
-11.0 (12 f 3)
t22 0
20-600 -600-900 20-720 20-500
-10 4 (4 f 1 0) + 1 64 -9 4
-m-m Calculated from X-ray data.
'See ref. 3.
loosely packed in this direction and thus more irregular in thcir c-axis thermal expansion. The introduction of tantalum ions in the hexagonal bronzcs also affccts their electrical resistivities. Since the presenco of pentavalent tungsten probably causes the low resistivity of the bronzes, its replacement by tantalum should result in phases with much higher resistanccs. This result was observed. The resistivities of pellets of the tantalum-tungsten bronzes are given in Table IV.
Table IV: Resistivity Data
Comporrnd
Ko 3(T%rWo 7 1 0 3 Kn tiWOi'
a
Itbo 3 ( 1 ' ~ 0 3Wo 7 j 0 3 Ilbo zeW03' CSOdTao 3Wo i ) 0 8 CSO noWO,' A t room temperature 'See ref. a in
Resistivity. ohm. cm."
2
5 7 2 2 6
x x x 8x x 7 x
108 10-2
108 10-2 107
Table I.
In summary, since the pentavalent tungsten ion has a Fid electron which probably causes the blue color and low resistivity and contributes to the distortion of the oxygen octahedra in the hexagonal tungsten bronzes, suhst,itution of pentavalent tantalum, which has no ;id elcctron, for the pentavalent tungsten ions producrs crcain color phascs, with high resistivities and less distortion of the octahedra in thcir structures.
Influence of Temperature on the Radiation-Induced Branching of Polyisobutene Molecules
by D. T. Turner Camille Ihryfus Laboratory, Reaeareh Triangle Institutr, Durham, iVorth Carolina (Rrceivrd iVovabrr 20>1863)
The stoichiometry of polymer degradation is conveniently studied by measurement of solution viscosity with reference t o an appropriate molecular weight relationship established for linear fractions. This procedure is clearly not valid in cases where branching occurs and this possibility must be given careful attention when degradation is caused by exposure to high energy radiation since this almost certainly results in the formation of potential cross-linking sites on the polymcr molecules, for example, by the elimination of hydrogen atoms. It is difficult to estimate, and perhaps even to detect, a small degree of branching in a polymer, but the theories of Zimm, Stockmayer, and Fixman1*2 have been applied to studies of the radiation-induced degradation of polyacrylates and polymethacrylates by Shultz, Roth, and Rathmann.3 A careful comparison of solution viscosity and light scat(1) B. €1. Zimm and W. H. Stockmayer. .I. Cham. Phys., 17, 1301
(1949). (2) W. H.Stockrnayer and M. Fixman. Ann. N . Y . Acad. Sci., 5 7 , 334 (1953). (3) A. R. Shultz. P. I. Rgth. and G . B. Rathrnann. J . f'olymrr Sei., 2 2 , 495 (1956)
Volume 68, Number 6
May3 1964
twing data as a func%ion of radiation dose failed to rcvcal branching ill polymcrs which, by structural analogy with polymem which yield cross-Iinkrd networks on irradiation, would bt expected to form some cross links.4 Recently, Kilb has cxtendrd the earlier theoretical treatments to include thc more relevant case of a polydisperse polymer with a random distribution of tetrafunctional cross links and has siiggcsted that the most practicable inrthod of estimating branching is by comparisoii of the ratio of the limiting viscosity numbers of branched and liricar polymers of the Same weight average molecular weight .5 There follows an account of thc application of this method to a study of the influrnce of trmpcrature on the radiolysis of polyisobutene.
Experimental Commercial samples of polyisobutene (M,,in the rarigc fib 12 X lo6) were purified by double precipitation from pctroleum ether (b.p. 40-60°) with methanol. Solvent then was pumped out of the polymer a t 60' during scvrral weeks. Polymers of lower molecular weight swre prcparcd by polymcrization of isobutene with boron trifluoride a t low temperatures6and purified similarly. Thc high moleculai weight polymers were thoroughly drgasscd and sealed in uucuo in glass ampoules for cxposure to a 4 i\fev. beam of electrons from a linear accelerator a t a dose rate of the order 1 Ahad,/ min. Ampoules were cooled during irradiation by imrnrrsion in nater (20') or in liquid nitrogcn (- 196'). I'olyisobutcne was found to have a refractive index incremrnt in 12-hcxanc a t 22' of 0.1567 f O.OOO36 ml./'g. (11r. F. Rogers, Imperial Chemical Industries Ltd., Weln-yn Garden City, England). Turbidity mcasurtnirnts wcre made with a Brice-Phoenix light scattrring photometer arid M,,was calculated from a Zimm diagi am. Polymer was recovered from aliquotsof solution, by total evaporation of n-hexane, and redissolved i n toluenc for determination of limiting viscosity number a t 2.5' with an Ubbelohde viscometer.
Calculations ITigure 1 shows plots of [7]against ;If, for linear polymcre ( 7 = 0) calculated from the relationship [VI = 2.It5 X 10-4111,n.R7which is derived from data of ]:ox and I'lory for linear fractions of p~lyisobutene.~ l ' h c brokcn curve is for the case where all the polymer molrcwlts arc of the same Irngth, i.e., AI, = M , , and the topmost full curve is for polymer niolecules of random length distril)ution, z.c., M,, = 2A1,/1.85. The full ciirvc has b t w ~used to gencrate a family of curves with thc paranictcr y , a cross-linking index which
2 3 4 WEIGHT AVERAGE MOLECULAR WEIGHT, M ~ x,
5
Figure 1. lielationship t)c:tnet:n ,If, and [ ? ] for various polyisohutenes: i I, polymer prepared by polynierimtion of isobutene with boron trifluoride; 0, polyisot)ut,enes irradiat,ed at, 20' (doses 0.3-4.0 Mrads) ; 0 , polyisobiitenes irradiated at - 198" (doses 4--10 Mrath).
rises toward unity a t the gel point. Ratios of limiting viscosity numbers of the same weight average molecular weight for various values of y were taken from Table TI1 of Kilb's paper.
Discussion The fact that the unirradiated polyisobutenes have (If,,, [VI) coordinates which lie below the curves for y = 0 implies that they are either branched or else have a wider length distribution than corresponds to either of the two distributions considered in 1Jig. 1. Branching cannot be excluded definitely but the latter possibility seems the more likely. Kilb has suggested that his method of estimating branching is not very sensitive to length distribution and this may be seen bJi a comparison of the two curves for y = 0 in Fig. 1. Nevertheless, this factor will assume importance when an attempt is made to estimate low degrees of branching. In these circumstances analysis of the results obtained a t 20' is deferred pending the establishment of a curve for y = 0 which takes appropriate account of length distribution. The (M,, 171) results obtained after irradiation a t - 196' lie sufficiently below thosr obtained a t the higher temperature to demonstrate unequivocally that the polymers are extensively branched. However, a decision as to whether the cross-linking reaction itself (4) A. It. Shultz, 1'. I. Roth, and J . h l . Rerge, paper presented at t,he 135t,h National lleeting of the American Chemical Society. Boston. hlass.. 1959. (5) It. R. Kilb. J . Polymer Sci., 38, 403 (1959). (6) It. M. Thomas. W. J. Sparks, 1'. K. Frolich, Sl. Otto, and M. MNer-('unradi. J . Am. Chem. Soc., 62, 256 (1940). (5) T.G Fox and 1'. J. Vlory, J . Phye. Colloid Chem.. 53, I95 (1949).
NOTES
1257
increases on lowering the temperature of irradiation or whether the number average of branches per molecule is merely enhanced by depression of the fracture reaction must again await precise knowledge of the extent of cross linking at the higher temperature. From the above findings it now clearly emerges that the puzzling linear relationship reported between the yield of fractures in polyisobutene arid the temperature of irradiation in the range - 19620' is an artifact, because it was based on the analysis of viscosity data on the assumption that the polymer is linea^-.^,^ Presumably, other data concerning radiation damage in various biological systems, which has been adduced to make a case that this unusual temperature dependence may be a rather general effect in radiation chemistry, also require reappraisal. In this respect it is noted that l'ollard has interpreted the temperature dependcncc of the inactivation of a number of biological systems on irradiation as a plateau effect.'O Acknowledgment. This work was begun in the laboratories of the Xatural Rubber l'roducers' Research Association and samples were irradiated at Wantage Research Laboratories (A.E.R.E.). It was continued with support from the Camille and Henry Dreyfus Foundation. Dr. A. R. Shulte is thanked for sending valuable information at the beginning of this work. (8) 1'. Alexander, 11. M. Black. and A. Charlesby, Proc. Roy. Snc. (London), A232, 31 (1955). (9) D. T. Turner, J . Polymer Sci.. in press. (10) E. Pollard, Adtan. Biol. Med. Phys., 3. 163 (1963).
Decomposition Pressure of TcZnal
by Ewald Veleckis and Irving Johnson Argonne National Laboratmy. Chemiccrl Engineering Division. Arponrie, IUirwis (Received December 9, 196Y)
Studies2of the technetium-zinc system have revealed the existence of two intermediate phases: a zinc-rich phase which contains approximately 5.5 atom yo technetium and a technetium-rich phase which corresponds to the formula TcZn6. The free energy of formation of this latter compound has been determined from the measurement of the vapor pressures of zinc in equilibria 'l'cZne(s)
=
Tc(s)
+ 6Zn(g)
by the Knudsen effusion method.
(1)
Experimental Vapor pressures were measured by the use of a continuous weighing effusion bala~ice.~A tantalum effusion cell, with two opposing orifices (approximately 0.01 cni. in diameter) located in the cell wall, was employed for all of the measurements. To prepare the alloy (15 atom yo Tc), arc-melted technetium and high purity zinc (99.999%) were heated together at 600" for 31 days. Approximately 150-mg. samples of the powdered (200 mesh) alloy were used in each experiment. Zinc was evaporated isothermally until no further changes in weight were observed. The weight loss us. time graph had orie sharp break a t the sample weight which corresponded to TcZn6. The break was followed by a straight line indicative of the heterogeneous region TcZne-Tc. The decomposition pressure of TcZn6 is proportional to the constant rate of weight loss in this region and was computed using the Knudsen equation Pmm
=
17.14(KA)-'(7'/M)'"(A~/Al) (2)
in which P,, is the vapor pressure of zinc in mm.; K , the Clausing short channel correction factor; A , the area of the orifice in I',the absolute temperat,ure in O K . ; M , the atomic weightof zinc; and A w / A t , the rate of weight loss in grams per second. The effective orifice area, K A = 1.62 X of the cell was determined by calibration with pure solid zinc. The vapor pressure of zinc, reported by Barrow, el ~ l . , ~ was used. Because of the low volatility of technetium only zinc contributes significantly to the vapor. Mass spectrometric6and torsion effusion6 studies have shown that zinc vapor consists mainly of monatomic zinc. The small value of the ratio (orifice area) : (sample area) renders the corrections for noriequilibrium conditions within the cell negligible. Assuming that the evaporation coefficient of Zn from Tc-Zn alloys is approximately the same as that of Cd from U-Cd alloys,' the correction to the vapor pressure is estimated to be less than 0.5%. (1) Work performed under the auspices of the (J. S. Atomic Energy Commiwion. (2) M.Cliasanov. I. Johnson, and R. V. Schablaske, J . Less-Common Metals, in prew (3) E. Veleckis. C . L. Rosen. and H . M .I:eder, J . Phys. Chem.. 65, 2127 (1961); the balance in the original apparatus has been replaced by an Ainsworth. Model KVA-AI:-2. recording vacuum balance. (4) It. F. Barrow, el d..Trans. FaJaday Soc.. 51, 1354 (1955). (5) K . H. Mann and A. W. Thickner. J. I'hys. Chrm., 64, 241 (1980). ( 8 ) G. M .Rownblatt and C . E. Birchenall. J . Chem. Phys.. 35, 788 (1961).
(7) E. Veleckis, I f . M . Is'eder, and I . Johneon. .I. Phiis. Chrm.. 66, 362 (19G2).
Volume 68. Nirmbrr 6
May. 1964
