Oct., 1959
SONICDEGRADATION OF HIGH POLYMERS IN SOLUTION
about the pro-hypro peptide linkage sets the hydrogen-bonded configuration which the remainder of the chain may assume. The so-called "collagen fold" is thus directly related t o the trans pro-hypro form. The data presented here cannot answer another important question-whether the proline and hydroxyproline are concentrated in only one of the three collagen peptide chains, as has been suggested in connection with X-ray diffraction data.I6 (16)
P. M. Cowan. 5. h9cGarin and A. C. J. North, Nature,
116,
1062 (1955).
1725
However, this seems to be unlikely in view of the viscosity data in the 40-100% FA range. On the basis of our data it does appear that there is a decided possibility that long segments of each of the polypeptide chains are essentially free of proline and hydroxyproline. Grassman has found large peptide fragments in gelatin that do not have a full complement of hydroxyproline. This means that other parts of the chain must have more than their average share.17 (17) W. Grassman, K. Hannig, H. Endres and A. Riedel, 2.physiol. Chem., 806, 123 (1956).
SOKIC DEGRADATION OF HIGH POLYMERS I N SOLUTION BY J. R. THOMAS California Research Corporation, Richmond, California Received Februan! IO. 1969
The rates of sonic degradation of polymethyl methacrylate, polyisobutene polylauryl methacrylate and polystyrene are determined by use of 2,2-diphenyl-l-picryl hydrazyl to measure free radicai fragments. Using fractionated samples it is found (1) that the rate of degradation per polymer backbone bond, under constant conditions of cavitation, is directly proportional to the degree of polymerization; ( 2 ) that large side chains accelerate the rate of degradation; and (3) that a 10-2070 change in the carbon-carbon bond dissociation energy of the polymer chain has little effect upon the rate. A simple model is roposed whereby the rupturing stress arises from the radial velocity gradient surrounding a collapsing cavity. The modef yields a kinetic expression in agreement with the experimental results.
It has been shown that the degradation of high polymers in solution by sonic or ultrasonic radiation is primarily due to hydrodynamic forces arising from cavitation of the fluid.' Henglein2 has reported the use of 2,2-diphenyl-1picryl hydrazyl (DPPH) to detect free radical fragment products from the degradation of polymethyl methacrylate in solution by ultrasonics. We have confirmed his observation and have used this technique to study the degradation of polymethyl methacrylate, polystyrene, polyisobutene and polylauryl methacrylate. This paper describes the results of this study and presents a simple model to account for the observed rates of degradation. Experimental Degradation of the polymer solutions was carried out in a Raytheon DF 101 sonic oscillator operating at 10 kc. The machine was equipped with Teflon gaskets to avoid reaction of DPPH with the rubber cpmmonly used. Standard runs were included in each series of experiments so that daily variations in the power output of the oscillator could be corrected for. Samples were degassed and run under dry nitrogen since oxygen is known to interfere with the quantitative use of DPPH as a radical trap. Small aliquots were withdrawn as a function of time and analyzed for DPPH by optical density measurements at 5250 A. Polystyrene, polymethyl methacrylate and polylauryl methacrylate were pre ared by bulk polymerization with dibenzoyl peroxide. $he polybutene was a commercial polymer (Paratone N). The polymers were fractionat,ed in the usual fashion with appropriate non-solvents. The fractionations were conducted in dilute solution (0.5Y0 or less). Initial precipitates were redissolved by warming the solutions, and the fractions taken were reprecipitated by cooling the solution to its original temperature. The fractions taken were all less than 10% of the total material initially present. The molecular weights of the polystyrene, polymethyl methacrylate and polyisobutene were determined from intrinsic viscosity measurements. The molec(1) H. H. G. Jellinek, "Degradation of Vinyl Polymers," Chapter 4, Academic Press, Inc., New York, N. Y., 1955, p. 231. (2) Von Arnim Henglein, Makromol. Chem., 18, 188 (1955).
ular weights of the polylauryl methacrylate fractions were determined from ultracentrifuge sedimentation rates. Sedimentation data on the polybutene polymers gave molecular weights in agreement with those determined by viscosity measurements. DPPH was prepared by the method of Lyons and Watsona and was purified by recrystallization from hexane. Optical density measurements were made with either a Cary Model 14 or Beckman DU spectrophotometer. C.P. benzene, dried with calcium hydride, was used as solvent in all cases.
Results Typical results are shown in Fig. 1 where the DPPH consumption is plotted against time of sonic irradiation. In the absence of polymer, the DPPH consumption was essentially zero during the time of these experiments. While the DPPH-radical adduct has a much lower extinction coefficient than the DPPH, its extinction coefficient is not zero. The small correction necessary for this was made by determining the extinction coefficient of the adduct in a sample of DPPH and polymer irradiated to the point that no further change in optical density took place. Effect of DPPH Concentration.-The rate of DPPH consumption was found to be independent of DPPH concentration over the range 5 to 80 mg./100 cc. In all experiments reported here the DPPH concentration was 20 mg./100 cc. Effect of Polymer Concentration.-The effect of polymer concentration is shown in Fig. 2 where the initial rate of DPPH consumption (determined as described below) is plotted versus weight concentration of polyisobutene. The failure of the rate to be strictly first order in polymer concentration probably results from the influence of viscosity upon the cavitation process. In very viscous solutions, cavitation cannot be induced a t a1ll1and polymer degradation ceases. I n the following experi(3) J. A. Lyons and W. F. Watson, J. Polymer Sci., 18, 141 (1955).
J. R. THOMAS
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as those given in Fig. 1. With low molecular weight fractions, where the rate of degradation is low and the rate of change of the degree of polymerization is small, the initial slopes could be extracted quite accurately. With the high molecular weight samples, the initial slope is good to about f10%. I n all cases rate data refer to initial rates. The initial rates of carbon-carbon bond breakage in the various polymer fractions are plotted in Fig. 3 versus the degree of polymerization of the polymer. The rates are given in terms of bonds broken per minute per polymer backbone bond available, assuming the destruction of two D P P H molecules per broken bond. As discussed above, it is expected TIME, MIN. that the rate of bond rupture would be first order Fig. 1.-DPPH consumption versus time of sonic irradiation with respect to polymer concentration at constant -2% polyisobutene (mol. wt. = 1.3 X 106). solution viscosity. Each point represents the average of a t least two independent determinations. I n the interpretation of the data, it is assumed that, the counting efficiency of D P P H is the same for all polymer radicals studied. Discussion Three points of interest are disclosed by the data given in Fig. 3. First, for all four polymers the rate of degradation per polymer backbone bond is directly proportional to the degree of polymerization. Second, the rate of degradation of polylauryl methacrylate is 3.5 times as fast as that of polymethyl methacrylate of similar degree of polymerization showing an influence of side chain length. Third, V I I 2 3 the rates of degradation of polymethyl methacryPOLYISOBUTENE CONCENTRATION, PERCENT, I N BENZENE. late, polyisobutene and polystyrene are the same Fig. 2.-Effect of polymer concentration (polyisobutene) within experimental error, indicating a t most a upon rate of DPPH consumption. small effect of the dissociation energy of the back14-bone bonds upon the rate. While there has been considerable discussion of I possible mechanisms of polymer degradation in a cavitating fluid,l no specific detailed mechanism has been proposed. Most of the published data which might be used to determine the mechanism of degradation are based upon viscosity change as a result of degradation. The complicated dependence of viscosity upon molecular weight distribution makes it very difficult to interpret the results from such studies. The experiments reported here give a diPOLYSTYRENC rect measure of the important quantity, namely, the A POLYMETHYLMETHACRYLATE 0 POLYLAURYLMETHACRYLATE rate of carbon-carbon bond breakage. Henglein's results, also obtained by use of DPPH, are difficult ' A IO 15 20 to interpret because his measurements were made OEGREE OF POLYMERIZATION x IO-^ after considerable degradation had occurred, during Fig. 3.-Rate of bond breakage versus degree of polymer- which time in many cases the average degree of ization. polymerization changed manyfold, and because the ments, all solutions were prepared to have the same measurements were not made a t constant solution reinitial viscosity so that the cavitational intensity viscosity. Recently, Ovenall, Allen, el d4J would be constant. The polymer concentrations ported a study of polymer degradation using ranged from about 3% by weight for the lowest de- DPPH. They arrive at the conclusion, based upon gree of polymerization samples to 0.2% by weight their own data and their interpretation of Henglein's data that the fundamental rate equation for for the highest degree of polymerization samples. Treatment of Data.-The decrease in rate of polymer degradation is DPPH consumption as a function of time noted in Fig. 1 is primarily due' to the decrease in average degree of polymerization of the polymer, as will be (4) D. W . Ovenall, G. W. Hastings and P. E. M. Allen, J. Polymer discussed later. I n order t o obtain rates under con83, 207 (1958). ditions where the degree of polymerization is Soi., M. Allen, C.M. Burnett. G. W. Hastinga, H.W. Melville known, the initial slopes were taken from data such and(5)D.P.W.E. Ovenall, ibid., 88, 213 (1958). I
IO,
"'I/
0
SONICDEGRADATION OF: HIGHPOLYMERS IN SOLUTION
Oct., 1959
where dBi/dt is the rate of breakage of molecules of degree of polymerization Pi, ni is the number of such molecules and Pe is a degree of polymerization below which molecules no longer degrade. The assumptions made in use of equation 1 are that only P i - P e units are capable of degradation and that all bonds within this degradable section are equally likely to break. Our data, plottedin Fig. 3, are not consistent with this rate expression but fit instead the expression dBi = kPi2ni dt
= k’PInb
(2)
where nb is the number of polymer bonds. For this discussion, the existence of a degree of polymerization P, below which degradation does not take place is unimportant and P e has been assumed small with respect to Pi. Equation 2 is of the form originally proposed by Schmid.6 It is difficult to reconcile the discrepancy between the results of Ovenall, Allen, et al., and those of the present study. For the reasons stated above, the use of Henglein’s data to determine the functional dependence of the rate of degradation upon the degree of polymerization appears hazardous. The interpretation of the results of the present study with regard to the functional dependence of rate upon the degree of polymerization appears to be more straightforward and more free of ad hoc assumptions than that which Ovenall, Allen, et al., were forced to use. Having obtained experimental data which appear to show correctly the effects upon the rate of bond rupture of the degree of polymerization, the length of the side chains, and change in the carbon-carbon bond dissociation energy of the polymer backbone bonds due to changes in chemical composition, it is reasonable to consider the mechansm of degradation in some detail. The principal point of interest concerns the manner by which the stresses arising in a cavitating fluid are able to rupture the chemical bonds in a macromolecule. We propose the following mechanism which accounts reasonbly well for the experimental observations. The collapse of a cavity, or a void, in a liquid is a process wherein very high velocities of collapse can be obtained resulting in large hydrodynamic pressures and velocity gradients in the surrounding Since a polymer molecule in solution occupies a relatively large volume (a region of diameter several hundred to several thousand Angstroms), i t is apparent that, with sufficiently large velocity gradients, the side of the polymer coil near a collapsing cavity will move at a higher velocity than the side away from the collapsing cavity. Assuming that the relaxation time of the polymer segments is short, the velocity gradient existing over the volume of the polymer will distort it from its initial spherical shape along a radius of the collapsing cavity. The unfolding of the polymer coil will continue until a geometry is reached which is incapable of further relaxation At this time, a tensive force operates on the polymer chain due to the relative (6) G. Schmid, Z. phyaik. Cham., A186, 113 (1939). (7) Lord Rayleigh, Phil. Mag.. 88, 94 (1917). (8) B. E. Noltingk and E. A. Neppiras, Proc. Roy. Soc. (London), E63 074 (1950); B64, 1032 (1951).
xhl------A-
1727
A’
EXTENDED POLYMER COLLAPSING CAVITY
Fig. 4.-Model
for degrading polymer.
motion of polymer segments and solvent. For simplicity, we assume that this critical geometry can be approximated by the linearly extended polymers presented in Fig. 4. The extended polymer will flow through the solution a t a velocity intermediate between the solvent velocities a t the two ends. At the point of maximum tension, designated as rmax in Fig. 4, the polymer velocity and solvent velocity will be equal. Making the usual assumptions that the hydrodynamic behavior of the polymer resembles that of a string OF spherical beads,Qthe frictional force dF on a polymer segment of length ds is ds
d F = 3?rd?lVrei2
(3)
where q is the viscosity, Vrel is the relative velocity of solvent with respect to polymer, d is the diameter of the polymer, and a is the length of a monomer unit in the polymer chain. This treatment assumes that each monomer unit has the frictional behavior of an independent macroscopic sphere of diameter d. At any point, T distant from the center of the cavity, the solvent velocity is v = UR2/r2
(4)
where U is the velocity of the collapse of the cavity and R is the cavity radius. Integrating (3) from r* (defined in Fig. 4) to rmax,the total force on this part of the polymer is seen to be
Integrating the force on the other part of the molecule from rmaxto r* 1 and equating the twoforces, one obtains, where 1 is extended polymer length
+
T2mar
=
T*(T*
+ 1)
(6)
For r* > 1 it is readily seen that the position of rmax is near the center of the molecule and that the tensive force is (7)
The rate of polymer degradation will be determined by the product of: (a) the volume of solution surrounding the cavity where FT > F” (the yield strength of the polymer), (b) the number concentration of the polymer molecules, and (c) a quantity defining the rate and intensity of cavitation. That is
2
- =
nivFT>F*K
(8)
where dB/dt is the rate of bond rupture, ni is the number concentration of polymer, VFT>F* is the solution volume where the tensive force is greater (9) P. J. Flory, “Principles of Polymer Chemistry,” Cornel1 University Press, Ithaca, N. Y., 1953, p. 602.
J. R. THOMAS
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Vol. 63
than the yield strength and K is a constant defining the cavitation. From equation 7
The calculated values, assuming the consumption of two DPPH’s per broken bond, are only in fair agreement with the observed values with the obdl= VFT>F+ F7 served loss being twice that of the calculated loss. Hammond, et al.,“ and BevingtonI2 have shown For the degrees of polymerization under considera- that DPPH is only about 65% effective in trapping tion lZb, the number of concentration of polymer the radicals produced by decomposition of azo-bisbonds, is isobutyronitrile. Using this lower trapping efficiency the calculated viscosity loss is only about nb 01 nil (10) 30% higher than that observed. If bhe bond cleavand the rate of bond rupture per polymer bond is age is random, the loss in viscosity caused by a given from (8) number of breaks will be only about 60% that ob1 d B dl served for cleavage restricted to the center of the nbTtffF polymer chain. Uncertainty in the trapping effiThis expression gives the linear dependence of ciency and lack of knowledge about the polydisrate upon degree of polymerization which is found persity of the fractionated samples precludes a choice experimentally as shown in Fig. 3. Further it cor- between random and center of the chain cleavage. rectly predicts the influence of increasing the side However, the data are consistent with the idea of chain length in changing from methyl methacrylate largely center cleavage and are good enough to exto lauryl methacrylate. Assuming that the side clude a mechanism whereby the cleavage takes place chains are fully extended, the diameter of the lauryl selectively near the ends of the polymer chain. While the proposed model gives kinetic behavior methacrylate should be about 4.7 times that of the consistent with experiment, it is difficult to say with methyl methacrylate. This agrees well with the experimentally observed variation in degradation certainty that the shear gradient over the region occupied by a polymer molecule is large enough to ruprate of 3.5. With regard to the influence of the C-C bond dis- ture a C-C bond. Using de Boer’s’3 value of the sociation energy upon the rate, equation 11 predicts force necessary to rupture a C-C bond, i.e., 6 X an inverse relationship. Using dissociation energies lod4 dynes, it can be seen from (7) that sufficient from simple molecules having somewhat similar force is available for a polymeroof degree of polystructures, it is reasonable to expect the D(C-C) for merization of 2000 a t about lo6A. from the cavity if polyisobutene to be about 70-75 kcal./mole; the collapsing cavities can achjeve near sonic vewhereas that of polystyrene would be expected to be locities a t radii the order of lo6A. The calculations about 60 kcal./mole. lo With similar diameters one of Noltingk and Neppirass indicate that these valwould only expect a 10-20% difference in rate. ues may be realized. While the data shown in Fig. 3 are not precise The notion that a random coil polymer is considerenough to show this difference, it is clear that a dif- ably distorted a t the time of its rupture receives ference in D(C-C) the order of 10-15 kcal./mole qualitative support from the observation that a causes very little effect upon the rate. This is con- polylauryl methacrylate fraction and a polyisobufirming evidence that polymer degradation is not a tene fraction suffered the same relative loss in thermally activated reaction. The influence of side (vsp/C)/(q8p/C)o in a solvent where their intrinsic chain length further substantiates this conclusion. viscosities differed by 50% as they did in a solvent With the assumption r* > 1, which appears rea- where their intrinsic viscosities were identical. sonable for a majority of the degradation events, The pearl necklace hydrodynamic model used equation 6 predicts that the molecule will break very here is highly idealized and cannot be rigorous. I n near the center. On this assumption the change in addition to the simplifying assumptions normally reduced specific viscosity, qap/C,as a function of deg- introduced in its application to polymer solutions, radation can be calculated from the number of we have explicitly assumed that each monomer unit bonds broken as determined by DPPH consump- behaves as a sphere, with macroscopic fractional cotion. Calculated values of (qap/C)/(vap/C)O a t 1% efficient, of radius equal to the side chain length. A are compared in Table I with observed values for a better representation might be thought to be the polylauryl methacrylate and a polymethyl metha- identification of a “bead” as a polymer segment of crylate fraction. length equal to the side chain length. This, however, leads to the implausible result that the fricTABLE T tional force is independent of the polymer diameter. CALCULATED A N D OBSERVED CHANGES I N (vsu/C)/(vs,,/C)o Treatment of the polymer chain as a string of conWITH DEGRADATIOS nected non-interacting cylinders leads to essentially (?t.P/C) / (?lSl>/C)0 the same results as found above except that the polPolymer Time, min. Obad. Calod. ymer diameter appears in a log function. TreatPLMA 1 0.981 0.984 ment of the polymer chain as a single macroscopic 3 ,884 .926 cylinder in a velocity gradient appears to involve 6 ,752 ,860 formidable mathematical problems. The kinetic PMM 1 0.959 0.964 treatment used here is also highly idealized since it 3 ,844 ,927 (Y
6 ,719 ___(10) J. S. Roberta and H. A. Skinner, T r a n s . Faraday (1949).
,871 Soc., 46, 339
(11) G . S. Hammond, J. N. Sen and G. E. Boozer, J . A m . Chem. SOC., 1 7 , 3244 (1955). (12) J. C. Bevington, Nature, 478 (1955). (13) J. H. de Boer, T r a n s . Faraday Sa.,38, 10 (1936).
I
Oct., 1959
ADSORPTION OF FLUORINATED COMPOUNDS ON SOLIDPLANAR SURFACES
correlates observed rate data with a model based upon monodisperse cavities of uniform size; whereas, actually, a distribution of initial cavity aizes probably exists. Throughout, we have ignored molecular entanglements between polymer molecules which have been considered important by various investigators. Our justification is that the polymer concentrations ape moderately low and that the proposed mechanism gives satisfying results without considering this additional complication. Allen, Burnett, et dl6 discuss at some length the limiting degree of polymerization below which a polymer will not degrade in a cavitating fluid and
1729
the discrepancies in the determination of this quantity by various investigators. Since this discrepany involves a range in the limiting degree of polymerization of 200-2000, it is obvious from the scatter in Fig. 3 that our data will not aid in answering this question. It seems worthwhile to point out, however, that the mechanism we propose would predict a strong dependency of this limiting value upon the intensity and characteristics of the cavitational process. It is not surprising, on this basis, that the different experimental conditions of the various investigators lead to different values of the limiting degree of polymerization.
A RADIOACTIVE TRACER STUDY OF THE ADSORPTION OF FLUORINATED COMPOUNDS ON SOLID PLANAR. SURFACES. I. PERFLUOROOCTANOIC ACID’ BY J. W. SHEPARD AND JOHN P. RYAN Contribution No. 146from Central Research Department, Minnesota Mining and Manufacturing Company, St. Paul, Minnesota Received March 10, 1060
A method for measuring areas of flat surfaces involving solution adsorption techniques is described and its limitations discussed. A rarbon-14 labeled fluorochemical acid ( C T F I ~ C ~ Owas H ) used. Isotherms for the adsorption of the acid onto plane surfaces of glass, quartz, aluminum and platinum have been determined. The adsorption is not reversilde. Desorption studies showed that the rate and extent of desorption was a function of the polarity of the desorbing solvent. Contact angles using hexadecane were found to be a poor measure of the extent of surface coverage by the adsorbate. Surfaces of platinum and quartz were unreactive and surface area measurements corresponded closely to the geometric area. Soft glass and aluminum showed signs of chemical reaction with the perfluoro acid. An exchange phenomenon was observed between the adsorbed acid molecules and those in solution. The rate and extent of exchange for the surfaces studied increased in the order: glass, aluminum, platinum.
Introduction The use of radioactive tracer techniques in studying surface phenomena has expanded considerably in recent years.2 One technique which has considerable merit since i t lends a quantitative aspect to the usual measurements of surface properties is the adsorption of long chain polar organic compounds labled with C-14 or H-3 onto solid surfaces. These “oleophobic” films, when properly prepared, have the interesting property of not being wetted by the solution or the pure solvent.* When the film-covered surfaces are withdrawn from the solution, the liquid recedes, leaving a dry surface made up, in most cases, of a closely packed monolayer of the organic compound oriented with the polar group on the surface of the solid and the hydrocarbon chain perpendicular to the substrate. Preliminary work in our laboratory with a benzene solution of C-14 labeled stearic acid indicated that this adsorbed acid imparted only a low degree of “oleophobic” character to the surface and the samples emerged from the adsorption cell wet with (1) Presented at the Symposium on “Surface Chemical Propertiea
of Fluorochomioals,” 134th Meeting of the American Chemical Society, Division of Colloid Chemistry, Chicago, September, 1958. (2) F. P. Bowden and A. C. Moore, Trans. Faradav Sor., 47, 900 (1951); J. E. Willard. THIS JOURNAL, 67, 129 (1953); D. E. Reischer, ibid., 67, 134 (1953); E. Rideal and J. Tadayon, Proc. Roy. Soc. (Lond o n ) , 226A, 346 (1954); J. E. Young, Aslr. J . Chem., 8, 173 (1955); H. A. Smith and T. Fort, Jr., THISJOURNAL, 62, 519 (1958); H. D. Cook and H. E. Ries, Jr., Miami ACS Meeting, April, 1957. (3) W. C . Bigelow, D. L. Pickett and W.A. Zisman, J . Colloid Sci., 1, 513 (1946).
solution. Other workers have also noted this beh a ~ i o r . I~n view of this difficulty, a system was chosen which would eliminate this “carry-out” problem. A surface composed of closely packed, oriented -CFs groups has extreme oleophobic character5 and fluorocarbons as a class have the lowest free surface energy of any known compounds.6 A solution of C-14 labled perfluorooctanoic acid
(C7Fd26OH) in n-decane’ was chosen as the adsorp t,ion system. The initial objective of this research was to develop a simple method for measuring the specific surface area of plane solid surfaces. The importance of the actual surface area when studying adhesion, catalysis, lubrication, etc., is well known. Techniques for measuring specific surface areas of flat surfaces have found limited application. Gas adsorption techniques have been e x p l ~ r e dbut ~~~ the method is primarily limited by low sensitivity. (4) W.C . Bigelow and L. 0. Brookway, i b X , 11, 60 (1956); H. A. Smith and K. A. Allen, THISJOURNAL, 68,449 (1954). (5) F. Schulman and W. A. Zisman, J. ColZoid Sci., 7, 465 (1952); E. F. Hare, E. G. Shafrin and W. A. Zisman, THISJOURNAL, 68,236 (1954). (6) H. M. Scholberg, R. A. Guenthner and R. I. Coon, ibid., 17, 928 (1953). ( 7 ) J. W. Shepard and John P. Ryan, ibid., 60, 127 (1956). (8) P. H. Emmett, “Pittsburgh Conference on Surface Reactions,” Corrosion Publishing Co., Pittsburgh 12, Pa.,1948, p. 82. (9) C. Brown and H. H. Uhlig, J . Am. Chem. Soc., 69, 462 (1947); R. L. Burwell, P. A. Smudski and T. P. May, J . Am. Chem. Soc., 69, 1925 (1947); T. Rhodin, ibdd., 72, 4343 (1950); Rauscb, 2. phyrik. Chem., 201, 32 (1962).
W.
