Nov., 1963
VISCOELASTIC PROPERTIES OF A SIMPLE ORGANIC GLASS
drogen peroxide has been indicated to be an electrochemical reaction taking place on an oxide of platinum, an anodic reaction and a cathodic reaction; however, in this case the platinum oxide acts as an electron acceptor and donor. 14.16.17.18.25 The data in these experiments are explainable in terms of this mechanism. The quinone is a strong enough oxidizing agent to reduce both forms of hydrogen. Therefore, the total surface takes part in .the catalytic reduction of the quinone. The hydrogen peroxide data indicate that the two forms of adsorbed hydrogen are significantly different. (26) I. A. Bagotskaya a n d I. E. Yablokova, Dokl. Akad. Nauk SSSR,95, 1219 (1984).
2439
The decomposition of the hydrogen peroxide occurred on an oxidized electrode. The fact that the rate of decomposition is related only to the second form of hydrogen indicates that the difference in energy for bonding of hydrogen to the metal is basically a property of the metal surface and the same difference extends to the bonding of oxygen on the surface. Acknowledgment.-We wish to acknowledge the fellowship given by Phillips Petroleum Company in support of this work. In addition, the research reported in this paper has been sponsored in part by the Geophysics Research Directorate of the Air Force Cambridge Research Laboratories, Office of Aerospace Research, under contract AF 19(604)-8414.
'VISCOELASTIC PROPERTIES OF A SIMPLE ORGANIC GLASS BY A. V. TOBOLSKY AXD R. B. TAYLOR Department of Chemistry, Princeton University, Princeton, N e w Jersev Received M a y 31, 1963 The shear compliance of a three-dimensional glass-forming substance, Galex, composed largely of dehydroabietic acid, has been studied as a function of time and temperature by a ball indentation method, and the results are expressed in terms of a master curve of shear compliance against time a t 15'. This curve has been analyzed to separate the various components of the compliance and it is found that a glassy compliance, a single retardation time, and a steady flow shear viscosity are sufficient to describe the viscoelastic behavior of the compliance over the entire time scale. This is in striking contrast to the broad distribution of retardation timerr exhibited by polymeric materials. The glass transition temperature has been determined by volume temperature measurements and found to be 7'. 'The variation of the shear viscosity and the viscoelastic shift factors as a function of temperature are also discussed.
Introduction Although many investigations have been reported on the viscoelastic behavior of high polymer systems, very few have dealt with low molecular weight glass-forming materials. Benbowl has studied such systems by dynamic methods using 2-hydroxypentamethyl flavan as material and the rheological behavior of glucose2 has also been investigated. The material chosen for the present study was dehydroabietic acid (Galex). I n molecular structure it may be represented as a roughly spherical molecule, so that the intermolecular interactions must be described as three-dimertsional, similar in kind to those that exist in a molecular crystal structure. However, the regularity of a crystal structure is absent in this glassforming material. Plastic flow by dislocations, so characteristic of crystals, is not pertinent here. I n this study we were particularly interested in the viscoelastic properties of Galex in the region of transition between glass and liquid. Linear amorphous polymers such as polystyrene also undergo a glass-to-liquid transition. The interactions between segments of neighboring polymer molecules are of the same type as found in Galex. I n the region of transition between glass and rubber, however, intramolecular segmental motion is of great significance. This type of molecular motion has been treated by Rouse and Bueche3* as being analogous to a one dimensional array of masses coupled by springs, the entire array being (1) J. J. Renbow, Pror. Phga. SOC.(London), B67, 120 (1954). ( 2 ) G. S. Park8, 1,. E. Barton, M. E. Spanht, and J. W. Richardson, Phusics, 5, 143 (1934). (3) f. E. Rouse, .I. Chem. Phys., 21, 1272 (1953). (4) F. Beuche zbzd. 22. 603 (1954).
immersed in a viscous fluid. This type of one dimensional motion gives rise to a very broad distribution of relaxation or retardation times. Tobolskys has calculated that the distribution of relaxation times in a three dimensional array should be very much narrower than that for a linear array and considered Galex as a substance by means of which this prediction might be verified. Experimental Galex is the trade name of EL dehydrogenated rosin supplied by the National Rosin Oil Products Co. It is a stable nonoxidizing material. The stability is obtained by dehydrogenation of abietic acid, the principal constituent of rosin, t o dehydroabietic acid, which contains the benzenoid nucleus.
Abietic acid (rosin)
Dehydroabietic acid (Galex)
The Galex, contained in shallow glass vessels, was heated in an oven at 100" until completely liquid and allowed to cool slowly to room temperature in order to minimize local stresses due to uneven contraction. This method produced short cylinders having parallel ends. The method chosen t o measure the viscoelastic behavior was a ball indentation method. By applying a force t o a sphere of known diameter and measuring the indentation 8s a function of time, the shear compliance J ( t ) could be calrulated. The incten( 5 ) A.
V. Tobolsky, ibid., 87, r575
(1962).
2440
A. V. TOBCJLSKY AND R. B. TAYLOR -4 i
l
i
Vol. 67
'
l
/
'/ A
/
I
I
I
I
I
4
5
6
7
8
-E
-6
A -7 +J u
-3
c3 0 J
-a -I
I 1
I
I
I
I
2
3
LOG t , ( SECONDS)
-9
Fig. 3.---RIaster curve of log compliance us. log time. In all cases the indentation a t the longest time was less than 10% of the diameter of the particular ball being used. For this condition the compliance is given by the equation8t7
-10
16 R"/" 3 F (d(t))"/"
J(t) = -1 I
L O G t , (SECONDS). Fig. 1.-Double
logarithmic plots of J ( t ) os. t a t various temperatures.
where d(t) is the displacement a t time t resulting from a force F applied a t zero time to a sphere of radius R. This method yielded excellent reproducibility in the region
