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where maximum light emission occurs. Adding a smaller size halogen ion increases the crystal's ability to thermoluminesce while adding a larger halogen ion decreases it. This same behavior of ion size was observed for NaCl containing one mole per cent. of various alkali ions. summary It is well established that F-centers are due to missing halogen ions and since it has been found that the coloration of the crystal disappears when the major high temperature peak disappears, it is felt that this peak in the glow curves is related to missing halogen ions. In support of this relationship, it is known that the absorption spectrum of F-centers shifts to longer wave lengths as the lattice constants increase, in the same manner as the temperature of the major high temperature peak drops t o lower temperatures as the lattice constants increase. No adequate explanation can be offered now for the other peaks. Saturation experiments show that the height and the shape of glow curves depend upon the
Vol. 61
amount of radiation which the crystals receive. For a study of the influence of the crystal on the thermoluminescence the minimum amount of radiation should be used in activating the crystal so as to obtain a characteristic glow curve uninfluenced by the radiation itself. The thermoluminescence of sodium chloride induced by intense X-rays has been studied by Hill and Schwed,'O and the thermoluminescence of lithium fluoride, potassium chloride and potassium bromide induced by intense y-radiation has been studied by Ghormley and Levy.l' Mixed crystal experiments showed that adding foreign alkali or halogen ions to NaCl varied its ability to thermoluminesce but did not affect the temperatures of the peaks. Therefore, the minor peaks in the alkali halides, if related to impurities, are due to impurities other than those of the alkali or halogen family. The authors are glad to acknowledge support of this research by the Atomic Energy Commission. (11) J. A. Ghormley and H. A. Levy, THISJOURNAL, 66, 548 (1952)
EVIDENCE FOR MECHANICAL SHEAR DEGRADATION O F HIGH POLYMERSlBYA. B. BESTUL National Bureau of Standards, Washington, D. C. Neceiued September 7, 1966
The author has previously reported the decrease in molecular weight of high molecular weight polyisobutenes during the flow of their concentrated solutions through capillaries a t high rates of shear a t moderate temperatures. The present paper reports the results of several experiments which support the conclusion that this molecular weight decrease results from mechanical shear degradation. The existence, rate and extent of degradation is uniquely dependent on the severity of shearing, since the results are unaffected by interruption of shearing a t any stage of the degradation process, and the degradation remains characteristic of rate of shear when solutions are subjected successively to different rates of shear in varied orders. The observed phenomenon is in fact degradation, and not an artifice of solution non-equilibrium, since there is no recurrence during the shearing of a solution of polymer precipitated and dried after previous degradation. The degradation is not a pyrolytic one energized by the adiabatic collapse of cavities of any kind, since it is not promoted by the presence of a highly volatile compound in solution. The degradation occurs for the higher molecular weight fractions from a previously degraded polymer, and unfractionated polymers will suffer the degradation a t lower average molecular weights than will fractions. Therefore the degradation ruptures only the higher molecular weight individual molecules and occurs only above a minimum concentration of these. The degradation appears to occur with the same general mechanism in polymers of several different chemical compositions, since generally similar results are obtained with polystyrene and polymethyl methacrylate as well as polyisobutene.
1. Introduction I n earlier papers4-e the author has reported measurements on the degradation of high molecular weight polyisobutene molecules during the flow of their concentrated solutions through capillaries a t high rates of shear. It is the purpose of the present paper to assemble the results of several previously unpublished experiments which further indicate that the process concerned is in fact mechanical shear degradation. (1) Extracted partly from a thesis submitted t o the Graduate School of the University of Maryland in partial fulfillment of, the requirements for the degree of Doctor of Philosophy. (2) This work was performed as part of a research project last sponsored by the National Science Foundation in connection with the Government Synthetic Rubber Program. (3) Presented before the sessions of the Division of Polymer Chemistry at the 128th National Meeting of the American Chemical Society in Minneapolis, Minnesota, on September 11-16, 1955. (4) A. B. Bestul, J . A p p l . P h y s . , 22, 1069 (1954). (5) P. Goodman and A. B. Bestul, J . Polymer Bci., 18,235 (1955). (6) .1.E. Bestul, J . Chem. Phys.. 24, 1196 (1956).
2. Experimental Procedures and Results The experimental techniques and the calculations used in obtaining the results reported here were similar in all cases to those stated in reference 4. Polymer solutions were sheared by being repeatedly forced through a capillary shearing tube in the McKee Consistometer.788 The shear load, which is proportional to the pressure drop established along the capillary when the solution is forced through it, was observed every other passage of the solution through the capillary. The effect of shearing ?n the molecular weight of the polymer is followed by intrmsic viscosity determinations before and after a shearing run or a t any stage of the run where it is desired to know the molecular weight. If an intrinsic viscosity determination is to be made during the course of a run i t is necessary to sacrifice the run at that point. 2.1 Successive Shearing at Different Shear Rates.I n two companiop runs solutions of 9.4wt. % ' Vistanex B-100 polyisobutene (M,, viscosity average molecular weight = 1,740,000) in n-hexadecane were sheared initially a t rates of shear of 66,000 and 33,000 sec.-l, respectively, and sub(7) s. A. McKee and H. s. White, ASTM Bull. No. 153,90 (1948). (8) 8. A. McKee and H. 6. White, J . Research Natl. Bur. Standards, 46, 18 (1951).
EVIDENCE FOR MECHANICAL SHEARDEGRADATION OF HIGHPOLYMERS
April, 1957
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Fig. 1.-Shear load decreases for 9.4 wt. yo Vistanex B-100 polyisobutene in n-hesadecane a t 40". Filled circles represent shearing first a t D (nominal rate of shear) equal 66,000 sec.-l and subsequently a t 33,000 sec.-l Open circles represent shearing first a t 33,000 see.-' and subsequently a t 66,000 sec.-l
I-HOUR INTERRUPTION 5-MINUTE INTERRUPTION MINUTE INTERRUPTION
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Fig. 2.-Shear load decrease plots for runs, with interruptions (10 wt. yo Vistanex B-100 polyisobutene in nhexadecane at 40" and D = 66,000 sec.-l). sequently at interchanged rates. For these two runs Fig. 1 shows the shear load observed during the successive passes of the solution through the capillary. Shearing a t 66,000 see.-' before or after shearing a t 33,000 see.+ reduced Mv to 1,150,000. Shearing at 33,000 sec.-l before shearing at 66,000 sec.-l reduced 8, to 1,420,000. After shearing a t 66.000 sec.-l subseauent shearing- at 33,000 sec.-l effected nofurther reduckion. As discussed in reference 4 almost all of the molecular weight decrease occurs while the shear load maintains its
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Fig. 4.-Runs
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relatively high value during the earlier passes a t a given rate of shear. The runs represented in Fig. 1 are atypical in that the decrease of shear load following the early relatively high values is more rapid than usually observed. A more typical, slower decrease of shear load is illustrated in Fig. 2. The difference is not related to the concentration difference between 9.4 and loyo. The over-all molecular weight decrease is quantitatively the same whether the shear load decrease is unusually rapid, as in Fig. 1, or slower a8 in Fig. 2. Therefore, the difference in the rate of shear load decrease following the relatively high early values is immaterial as .......~~ . regards the estent of molecular weight decrease.
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2.2 Interruption of Shear Degradation Process.-Five runs on 10 wt. % solutions of Vistanex B-100 polyisobutene at 66,000 were interrupted for various intervals of time a t various stages of the shearing process and then resumed. The shear load as a function of the number of passes for these runs is shown in Fig. 2, which also shows the position and duration of the interruptions. The behavior of the shear load and the extent of molecular weight reduction in these cases is the same as in uninterrupted runs made under the same conditions otherwise. 2.3 Shearing of Solution of Recovered Degraded Polymer.-A solution of 10 wt. % Vistanex B-100 polyisobutene in n-hexadecane was sheared a t 40" and 66,000 sec.-l until the shear load became nearly constant for successive passes. The polymer was then recovered by dilution with toluene, precipitation with methanol and vacuum drying to constant weight. It was then again dissolved at 10 wt. % concentration in n-hexadecane and again sheared under the previously stated conditions. The shear load during successive passes throughout this procedure is shown in Fig. 3. During the 80 passes before recovery of the olymer, 2i?, decreased from 1.74-million to 1.13 million. $here was no further decrease in M,on subsequent shearing after recovery and redissolution. Correspondingly there wa8 no large decrease of shear load with number of passes during shearing after recovery and redissolution. The small shear load decrease with shearing after recovery and redissolution is commonly observed during the initial Lasses for a solution in cases where there is no decrease of M, and is probably a slight thixotropic effect. It is not pertinent to the present consideration. The difference in final shear loads before and after recovery and redissolution may be the result of solution non-equilibrium before recovery. 2.4 Degradation in Presence of Highly Volatile Compound.-A solution of 10 wt. % Vistanex B-100 polyisobutene, 5% hexane and 85% n-hexadecane was sheared at 40' and 66,000 sec.-1. -The shear load for successive passes is shown in Fig. 4. M, decreased from 1.74 million to 1.13 million just as it does in degradation under identical shearing conditions in a solution identical except with the hexane replaced by n-hexadecane. The shear load curve for the solution with hexane lies generally about 10% below the curve for an atypical run I
1,550,000 M, = 1,320,000
o M, 0
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Fig. 5.-Degradat'ion of fractions from degraded olydisperse polymer ( M o = molecular weight before ,%earing).
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without hexane represented by the barred circles in Fig. 4. The shift between the two is probably the direct result of replacing 5% of the hexadecane with hexane. The atypical shape of the shear load curve seems to be reproducible for the solutions with hexane, though it is not so for solutions without hexane. For solutions without hexane the atypical shape of curve is obtained only very seldom and as a rule the typical shape of curve shown by the open circles in Fig. 41s obtained. 2.5 Shearing of Fractions from Degraded Polymer.-A sample- of Vistanex B-100 polyisobutene shear degraded from M , 1.74 million to 1.13 million was fractionated bv dilution with toluene and fractional precipitation with met& anol. The two highest molecular weight fractions were each about 20% of the unfractionated polymer and had M, values of 1,550,000 and 1,320,000, respectively. These fractions, dissolved at 10 wt. % concentration in n-hexadecane, were sheared at 40" and 66,000 sea-1. The resulting shear load curves are shown in Fig. 5. The M , values after this shearing were 1,500,000 and 1,320,000, respectively. The limits of uncertainty of the intrinsic viscosity determinations are about &I%. Although the measured M,of the 1.32 million @ . , fraction is not reduced after shearing, the sharp drop of shear load after the first pass for this fraction indicates that a Rmall amount of shear degradation, within the limits of uncertainty of the intrinsic viscosity determinations, may have occurred during the shearing of the fraction. Any such degradation could not have amounted to more than one bond rupture per 100 polymer molecules. 2.6 Degradation of Polymer Fractions.-A sample of Vistanex B-100 polyisobutene was fractionated. Three of the fractions obtained represented 20-30,40-50 and 60-65% precipitation. They had M v values of 3.05, 1.80 and 0.90 million, respectively. These fractions were dissolved a t 10 wt. % concentration in n-hexadecane. The solutions were sheared at 40" and 66,000 sec.-I, after which the Mv values were 1.57, 1.64 and 0.90 million, respectively. 2.7 Degradation of Polystyrene and Polymethyl Methacrylate.-A 10 wt. % solution of polystyrene and a 5 wt. % solution of polymethyl methacrylate, both in a-methylnaphthalene, were sheared at 40" and 66,000 see.-'. The polysLyrene was an unfractionated material of abput 3 million M,. The polymethyl methacrylate was a top fraction of about_100J, from an unfractionated sample of about 5 million M,. Both of these polymers were supplied by Dr. L. A. Wall of the Polymer Structure Section of the National Bureau of Standards. The shear load curves for the shear degradation of these polymers are shown in Fig. 6. The shearing reduced the intrinsic viscosity, [77] in a-methylnaphthalene a t 30" from 4.64 to 4.28 dl./g. for the polystyrene and from 6.88 to 4.03 for the polymethyl methacrylate. These intrinsic viscosity values are not converted to molecular weights since the necessary constants are not established for the polymers in this solvent. The solvent was used because of its low vapor pressure since the viscometer used for degradation cannot be used with solvents having high vapor pressures. The scatter in the shear load curve for the polystyrene is reproducible as shown by comparing the two runs represented, but its cause is not known. The scatter during the early passes for the polymethyl methacrylate is often observed for especially high molecular weight polymers, and is probably associated with the nature of the degradation process for such polymers.
3. Discussion The unique dependence of the existence and extent of the observed degradation process on the occurrence and severity of shearing is demonstrated by the results of the runs with successive shearing a t different rates of shear and those with interruptions in the shearing process (see Figs. 1 and 2). That the observed degradation is not an artifact based on alteration of the solution condition during shearing is demonstrated by the absence of degradation effects during the shearing of the solution of the recovered, previously degraded polymer (see Fig. 3).
..
April, 1957
EVIDENCE FOR MECHANICAL SHEARDEGRADATION OF HIGHPOLYMERS
The insensitivity of the degradation process to the presence of hexane (see Fig. 4) shows that it is not caused by heating from adiabatic collapse of cavities from turbulence or any other source. Because of its relatively high vapor pressure, hexane in solution would increase any such heating. If the observed degradation were pyrolytic and dependent on the above type of heating, rather than mechanical, the presence of hexane would increase the severity of degradation. No cavitation is expected in the shearing reported and these results confirm its absence or lack of influence here. The degradation results on fractions from degraded samples (Fig. 5 ) and on fractions from undegraded polymer demonstrate's that it is the higher molecular weight individual molecules which are ruptured and that a minimum concentration of such molecules is necessary t o produce rupture in any. Apparently the fractionation of the degraded unfractionated polymer concentrated enough molecules of high individual molecular weight into the fractions so that they degraded further on subsequent shearing, although before fractionation their concentration was too low to support further degradation. For the fractions of undegraded polymer the molecular weight below which shear degradation does not occur under the reported conditions is apparently not far below two million. This limit previously was found to be somewhat below one million for the unfractionated, undegraded polymer i t ~ e l f . ~ The generally similar results for polystyrene and polymethyl methacrylate (Fig. 6) suggest that the degradation observed for the three different chemical compositions occurs by a common general mechanism. The concept of rupturing chemical bonds by concentrating the requisite activation energy into a single bond with the aid of entanglements between individual molecules is consistent with these results. 4. Conclusions The above discussed indications are all consistent with the proposed mechanical degradation process. No other type of degradation yet proposed is consistent with all the observations reported in this and earlier papers on the process. This is especially true of: (1) the unique dependence of the existence, rate and extent of the process on the severity of shearing, (2) the fact that a critical molecular weight exists, dependent on the
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WLYSTYRENE SOLUTION POLYMETHYLMETHACRYLATE
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Fig. 6.-Degradation
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severity of shearing, above which individual molecules in sufficient concentration suffer this degradation and below which they do not and (3) the fact that the presence of a highly volatile component in solution does not promote the degradation. Little doubt can remain that we are dealing here with mechanical shear degradation. Acknowledgments.-The assistance of Miss H. V. Belcher and of Maria Bestul in some of the experimental work and in the preparation of figures is gratefully acknowledged. The author expresses his appreciation to Dr. George M. Brown of the University of Maryland, who was in charge of the thesis from which parts of this paper were extracted. Thanks are offered to the Enjay Company, Inc., and to Dr. L. A. Wall of the National Bureau of Standards Polymer Structure Section for supplying the polymer samples investigated. Some of the experiments reported were suggested by colleagues. Dr. Philip Goodman, now a t the Corning Glass Works, Research Laboratory, Corning, New York, initiated one of the experiments.
