IRRADIATION FACTOR-DEPENDENCY ExploratorJv Studies, Irradiation Cycle, and Degassing E . F. D E G E R I N G , C H A R L E S M E R R I T T , J R . , M . B A Z I N E T , A N D G. J. C A L D A R E L L A Radiation Chemistry Laboratory, Pioneering Research Division, U. S. Army Natick Laboratories, Natick, M a s s .
Under the conditions employed in this laboratory, atmosphere (including small amounts of moisture or oxygen), dose rate, irradiation cycle or time interval between successive exposures, and temperature are significant variables or experimental parameters for the irradiation-induced polymerization of vinyl monomers by use of an electron accelerator operating a t 2 m.e.v. The molecular weight of the polymer may be predetermined within limits b y appropriate selection of variables. Small amounts, 1%, of another monomer added to styrene may have a pronounced effect on both conversion to polymer and on the molecular weight of the product. The molecular structure of the polymer, moreover, as evaluated by infrared spectroscopy, might vary as a function of atmosphere, including small amounts of moisture and oxygen.
T THE
INITIATION
of this program, the most important
A experimental variable in the useful applications of ionizing radiation was believed to be the amount of energy applied to a system. This laboratory and many others accepted the challenge, and currently there is a wide diversity of effort in numerous laboratories directed toward the determination of optimal experimental variables or irradiation parameters for various irradiation-induced reactions (2-4, 6, 7 7 , 74). The results presented here deal with experimental variables for specific systems which have not been reported elsewhere. Early in 1954, this laboratory was asked to explore the potential applications of ionizing radiation for the improvement of Quartermaster materials and items. The primary objective, accordingly, is the efficient utilization of radiation energy for peaceful uses such as the improvement per se, or through irradiation-induced graft polymerization, of the mechanical and physical properties of materials (70). Numerous early studies showed that the most obvious effect of electron bombardment of cotton and synthetic fabrics, leather, plastics, rubber, and similar materials is the deterioration of their mechanical and physical properties ( 9 ) . When nylon was irradiated in a n evacuated system, however, and leather was irradiated in a moist state, the extent of the deterioration was significantly reduced ( 9 ) . These experiments provided a definite indication of the possible importance of experimental conditions on irradiation-induced reactions. A program was planned, accordingly, to evaluate the effect of experimental variables on the irradiation-induced polymerization of vinyl monomers. I n the initial studies, atmosphere, the increment of dose, and temperature appeared to have a significant effect. A statistical study was made, accordingly, by use of a 4 X 4 Latinsquare type of experiment (76),from which it was determined that atmosphere, dose increment, and temperature are significant variables for the irradiation-induced polymerization of styrene, with respect to the amount and molecular weight of polymer obtained per unit of radiation energy (6, Rept. No. 1). Inasmuch as the increment of dose per pass under the scanning beam was found to be highly significant, it seemed probable that the time interval between successive exposures might also be important. Samples of vinyl monomers were irradiated, accordingly, on a shuttle system which allowed about 1 minute between exposures and on a continuous conveyor 114
I & E C P R O D U C T RESEARCH A N D DEVELOPMENT
which permitted a 20-minute interval. More polymer was obtained per unit of radiation energy by use of the longer interval between successive exposures. These results indicated the probability of a post-irradiation effect, which was next considered for both cotton duck and vinyl monomers. The samples were given a single exposure at 0.1 megarad, and then incubated along with controls a t 55’ C. with periodic withdrawals of samples for evaluation. The results of this study demonstrated that there is a post-irradiation effect for the samples studied ( 7 , 5, 8, 72, 75). The terms dose increment and dose level in this paper designate the energy delivered by the accelerator to the top of the test tube containing the sample. Dosimetry studies with 300 MSC light blue cellophane film have shown that the film “sees” about 50% more ionization on the inside than on the outside of the tube (6, Rept. No. 9). This is a consequence of the “forward-scattering” and “back-scattering” in the tube to give secondary ionization paths, which account for the bulk of most irradiation-induced chemical reactions. Irradiation Cycle
Studies were then made on the effect of the irradiation timecycle on the amount of polymer obtained from vinyl monomers. The procedure developed in this laboratory for the preparation of the monomer samples is as follows: the monomer is vacuum distilled, dried in an argon atmosphere for several days over anhydrous potassium carbonate a t about 5’ C., and siphoned as used into a n argon-filled 250-ml. glass-stoppered graduate, which is connected in turn to a n automaticleveling buret, through which a slow stream of argon flows to minimize uptake of air or moisture (6?Rept. No. 4). The buret is filled from the graduate by use of slight argon pressure. Borosilicate glass test tubes (1 X 6 inches), equipped with ground-glass male joints with a constriction in the tubing, are placed on a vacuum manifold, flamed-out twice a t 300 microns and once a t 5 microns to remove most of the adsorbed moisture, and then filled with argon. A 10-ml. portion of the monomer is transferred from the buret to each of the argon-filled tubes, which are replaced in turn on the manifold. Then Dewar flasks of an appropriate freezing agent such as acetone slush or liquid nitrogen are slowly raised (by use of jacks) in order to freeze the monomer. The stopcocks to the manifold are then
opened, the pressure in the system is reduced to 5 microns, and the tubes are sealed off a t the constriction and stored a t -20" C. until time for irradiation. The radiation facility used is a 2-m.e.v. electron accelerator.
1'1
ACRYLONITRILE
Inasmuch as both a time interval between exposures and an elevated temperature were indicated in various exploratory studies for increased polymerization from a given amount of vinyl monomer, the irradiation time-cycles as described in Figure 1 were selected. Incremental doses for curves A , B, and C were 1000, 10,000, and 100,000 rads per exposure, a t a conveyor speed of 46.5 inches per minute, to total dose levels, respectively, of 5000, 50,000, and 500,000 rads (Figure 1, top and bottom). Temperature changes during irradiation were less than 5' C. Data for the B and Ccurves (Figure 1) show that intermittent irradiation is more conducive to polymer formation than is continuous irradiation. T h e per cent polymerization was obtained from the amount of polymer isolated. The monomerpolymer mixture was placed in a tared aluminum-foil dish, reduced to a film on a hot plate a t 65" C., and then heated in a vacuum oven for 4 hours a t 85" C. to remove most of the residual monomer and the diluent used for the transfer. From the results on the controls and subsequent studies on the 45-minute cycle at 25' C . (Figure 4), it has been determined that approximately half of the spread between curves A and B is due to the time cycle and the rest due to the temperature effect. Relatively comparable results were obtained from other monomer systems such as styrene (6, Rept. No. 4), vinyl acetate, acrylonitrile-styrene (1 to 1, v./v.), butyl acrylatestyrene (1 to 1. v / v . ) , vinyl acetate-styrene (1 to 1, v./v.), 1,1,3-trihydroperfluoropropyl acrylate, and 1.1,7-trihydroperfluoroheptyl acrylate. In another series, four irradiation time-cycles were possible by timing of the shuttle a t speeds of 93.0, 55.8, 46.5, and 10.1 inches per minute. Each sample received five unit exposures of 1000 rads. The results for 1.1,3-trihydroperfluoropropyl acrylate are shown graphically by Figure 2. T h e highest conversion to polymer was obtained by the longest timr interval between successive exposures.
'
IA BUTYL ACRYLATE
I
r-
'"I
701
i
I
/ B
,a
oJ?' , 5 x 1 3 50x103
5O0x1O3 DOSE (RADS)
Figure 1 .
Comparable studies Lvith other vinyl monomer systems have confirmed the significance of the speed of the conveyor for irradiation-induced polymerization. In using a scanning beam with a width of '/d-inch and an 18-inch shuttle, moving a t the rate of 93.0 inches per minute, for example, any portion of the sample being irradiated is under the electron beam on a n average of only about 1/70th of the time required to complete each cycle of the shuttle. T h e time lapse between successive bombardments with elrctrons. accordingly, is a significant variable for most systems studied in this laboratory. but not for styrene a t changes of the above order (7).
Effect A. 8.
45-minute cycle a t 75' C. 1 - to 3-minute cycle a t 25" C. Continuous irradiation a t 25' C.
C.
T
of time cycle on polymerization
lA3- T R l HY DROPERFLUOROPROPYL
ACRYLATE
'.
Degassing of Samples (6, Rept. No. 5)
Inasmuch as atmosphere is a significant variable, a study was made of degassing as a function of increment of dose and dose level. The initial step in the preparation of the degassed samples of vinyl monomers for irradiation, is the same as the first portion of the procedure given under irradiation cycle. Then, Dewar flasks of an appropriate cooling agent such as acetone slush or liquid nitrogen are slowly raised (by use of jacks) in order to freeze the monomer. The stopcocks to the manifold are opened and the tubes evacuated to 5 microns, the stopcocks closed, the Dewar flasks lowered, and the monomer is allowed to melt and degas. T h e monomer is then frozen, the
0 ' 10
20
30
40
CONVEYER
50
60
70
80
90
100
SPEED (INCHES/MINUTE)
Figure 2. Effect of time cycle and temperature on polymerization, where bars show mean and deviation from the mean VOL. 2
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115
/ /
DOSE INCREMENT ( R A D S / P A S S ) 44
V ~ N Y LACETATE
1 _ -_ -- * - - E c I'
I'
/
,
/
/
/ /
0-
I
stopcocks are opened, and the system is again evacuated to 5 microns and the tubes sealed off and stored a t -20' C. until time for irradiation. The acrylonitrile samples for Figure 3 were irradiated on a 45-minute cycle a t 75' C. a t the indicated dose increments for five exposures with a conveyor speed of 46.5 inches per minute. After the induction period, the spread in the curves remains relatively constant in the range studied. There is, however, a significant difference during the induction period ivhich may be attributed to inhibition by oxygen in the system. Comparable results for vinyl acetate a t dose increments of 1000 rads per exposure, on a 30-minute time cycle a t 75' C.. with a conveyer speed of 46.5 inches per minute, and dose levels of 1000, 5000, and 10.000 rads: are given also in Figure 3. With this system also, the lines appear to become parallel after the dose level has been raised to a point where most of the oxygen in the system is consumed. The inhibition of the polymerization by oxygen appears, accordingly, to have a temporary effect. This degassing effect has been observed for seven other monomer systems Jrhich were studied, but the extent of the effect is dependent on the monomer system. The effect of variations in atmosphere and temperature on the irradiation-induced polymerization of a 1-to-1 mixture by volume of butyl acrylate and vin)-l acetate is shown by Figure 4. The dose increment \vas 35,000rads per exposure on a 45minute cycle a t 75' C. to a dose level of 125,000 rads. The lowest conversion was obtained in air at 75' C. with no flameout of the tubes and no degassing of the monomer, whereas the highest conversion \vas obtained with three flame-outs of the tubes and two or three degassings of the monomer systems. The second bar on the graph. which was obtained at 25' C . . represents less than half the conversion obtained under comparable conditions otherwise a t 75' C.: Ivhich is represented bv the last bar a t the right of the graph.
;'-___---_--i
---- - _
___
5r IO3 DOSE (RADS)
IlJC)
Figure 3.
-.--A 2xl:!
Effect of Degassing and Temperature on Viscosity
Effect of degassing
A.
Volues from nondegassed samples B. Values from degassed samples For acrylonitrile, dose increments, left to right, were: 1, 3, 5, 7.5,10, 15, 20, 25, 50, 75, 100, 500, 1000, and 1500 times 1 O 3 rods per exposure
BUTYL ACRYLATE-VINYL
ACETATE
The effect on relative viscosity of degassing the monomer prior to irradiation is indicated by Figure 5. which records intrinsic viscosities for polystyrene obtained in the presence of varying amounts of oxygen and moisture. The highest viscosity for polystyrene was obtained a t 25' C . in an evacuated system in tubes \rhich had been subjected to three flame-outs and from monomer lvhich had been given three degassings. and the next highest value was obtained at 75' C. in the presence of air Lvith no flame-out of the tubes and no degassing of the monomer. The lowest value was obtained in an evacuated system \vith no flame-out and no degassing. Each 0.1 unit on Figure 5 represents a change in molecular weight of about 40,000. Effect of a Vinyl Additive and Dose Increments on Molecular Weigh+
Atm6.a F O O D G
V
3 O
V 0 3
V
V
3
3
O
O
V 0
I
V
V
3 i
2
3 3
Figure 4. Effect of variations in atmosphere and temperature, where the mean and the deviation from the mean are indicated 116
l & E C P R O D U C T RESEARCH A N D DEVELOPMENT
In a study on the effect of 1% of a vinyl monomer on the irradiation-induced polymerization of styrene, the molecular weight varied with the additive and decreased rather consistently with an increase in dose increment and total dose. as shown by Figure 6. The three systems received the same irradiation treatment a t 75' C. and were given five exposures a t each dose increment on a 45-minute time cycle. .4lthough there is a marked difference in the effect of the monomers used as additives a t the lower dose increments and dose levels, the magnitude of this effect decreases with an increase in dose per pass and dose level. Also, there is in general a decrease in the molecular weight of the polystyrene with an increase in dose per pass and total dose.
Effect of Dose Increment, Total Dose, and Temperature
a - AIR
The composite effect of dose increments, dose level, and temperature on the relative efficiency of irradiation-induced polymerization of styrene and on molecular weight is shown for one set of experimental conditions on Figure 7. T h e upper graphs (left ordinate) show that there is a decrease in molecular weight with an increase in dose increment and dose level and with a decrease in temperature ( 6 , Rept. No. 1). T h e lower graphs show a n increase in efficiency, or the per cent polymerization per unit of radiation energy (right ordinate), with a decrease in dose increment and an increase in temperature.
V-VACUUM F O - F L A M E OUT DG-DEGASSING X-75'C. 0-25
..ATMS. V
Effect of Variables on Molecular Structure
Infrared evaluations of a n exploratory type on polymers obtained by irradiation-induced polymerization of styrene in the presence of varying atmospheres, as shown by Figures 8 and 9, indicate that molecular structure is dependent to some extent a t least, on the atmosphere in the system. The tracings of Figure 8, obtained from polystyrene produced by irradiation in the presence of argon and air, respectively, show significant variations, particularly a t the 5.18- and 5.83-p regions. T h e A-tracing of Figure 9 for the 5- to 6-,u region is for polystyrene obtained by emulsion polymerization or bulk peroxide polymerization (73), whereas the other five tracings are for polystyrenes obtained in this laboratory by use of irradiationinduced polymerization under different atmospheric conditions. Otherwise, all samples received the same treatment. T h e B-tracing is for samples of polystyrene obtained by irradiation-induced polymerization in the presence of air, with no flame-out of the tubes and no degassing of the styrene, whereas the C-tracing is for samples irradiated in nitrogen with three flame-outs and three degassings, samples for the D tracing were in nitrogen \vith no flame-out and no degassing, those for the E-tracing in a n evacuated system with three flame-outs and three degassings, and those for the F-tracing in a n evacuated system with no flame-out and no degassing. The films, from which the spectra of the polystyrene produced by irradiation-induced polymerization were obtained, were cast from a 2.07, solution of the polymer in toluene onto potassium bromide pellets in a desiccator and the toluene removed by prolonged evacuation a t about 60" C. The data in columns .4 and B of Table I give the relative
OC.
~
V
V
V
D G . 0
V i
2
3
F.O.O
3
3
0 3
V I
3
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Figure 5.
6{
STYRENE
0
!'
STYRENE
75.C
45.C
* ' M O L E C U L A R WEIGHT X I 0x % P O L Y M E R I Z A T I O N PER 0.I MEGARAO
(See Figure 9 ) A 5.18 to 5.83 p
Conventional, Plyler ( 73) Irradiated in air, no FO, no DG Irradiated in Nz, 3 FO, 3 DG Irradiated in vac., 3 FO, 3 DG Irradiated in N1, no FO, no DG Irradiated in vac, no FO, no DG
2.26 2.37 2 86 3.78 3.21 2.72
FO, Jame-outs of reaction tubes at 5 p.
3 3
Dose increment, 1 0 0 , 0 0 0 rads per pars. Dose, 500,000 rads. Conveyor, 4 6 . 5 inches per minute. Time cycle, 4 5 minutes a t 7 5 O c.
Relative Change in Absorbance Ratios of the A and B Bands
Experimental Conditions f o r Polymerization
V
0 0
Irradiation-polymerized polystyrene
.Table I.
I
a
B 5.78 to 6.00p
DG, outgassing
30.2 1.71
2.19 2 28 2.54
3.49 of
monomer
prior to irradiation. In the preparation of these samples, styrme was irradiated on a 7-hour cycle at 75' C., at a dose i m e m e n t of 700,000 radsferpass, and to a dose level of 500,000rads, with a 2 m.e.v. electron accelerator.
0025
005
01
02
04
08
20
DOSE I N C R E M E N T (RADS/PASS) Figure 7. Effect of dose increment, dose level, and temperature on molecular weight of polystyrene obtained by irradiation-induced polymerization (left ordinate, upper curves), and efficiency (right ordinate, lower curves) VOL. 2
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117
/A 5
6 m,cron*
7
5
6
rniCIO“,
7
5
Figure 8. Polystyrene infrared tracings obtained from polymerization A. 8. 1. 2.
In argon In air 5.18 p 5 . 8 3 fi
5
6
5
6
5
6
5
6
5
6
Figure 9. Polystyrene infrared tracings on a 45-minute cyd e at 75°C.with a conveyor speed of 93.0 inches per minUte a t 100,000 rads per exposure; dose, 500,000 rads Per cent increase of absorbance of the 5.1 8 fi (1 ) to the 5.83 p ( 2 ) bands over the control are, respectively: 8. 0, C. 22, D. 3 2 , E. 42, and F. 47, where A is the control according to Plyler ( 7 3).
significant changes in absorbance ratios of the 5.18-p to the 5.83-p bands and of the 5.18-p to the 6.00-g bands. These exploratory data indicate that the polystyrene obtained in this laboratory by irradiation-induced polymerization is structurally different from the polystyrene produced by conventional means, which was used by Plyler (73)for obtaining the standard infrared spectra as shown by the A-tracing of Figure 9. An atmosphere effect is clearly indicated also by contrasting the tracings of B, C, D,E, and F, which show significant variations. These tracings indicate that the molecular structure of the polymer obtained by irradiation-induced polymerization is a function of environmental factors such as oxygen and moisture. O n the basis of available data. some of the structural changes are attributed to variations in the amount of carbonyl groupings in the molecular structure. There is an indication, however, of other structural variations. Confirmation of these observations has been undertaken and the effect of these structural variations on the mechanical and physical properties of the polymer is being evaluated. There is an indication here that environmental conditions of irradiation may determine to some extent at least, the molecular structure of the polymer as well as molecular weight as previously shown.
Acknowledgment
The authors express their appreciation to: \’. Chapin,
F. E. Evans, M. Mancini, S. D. Gabelnick, E. F. Grey, S.Grib. T. Smith, and D. Whittington for their respective contributions; E. D. Black, J. E. Donnellan, Jr., C. E. Waring, and D. R. A. Wharton for suggestions about the manuscript; A. L. Bluhm and J. A. Sousa of the Spectroscopy Laboratory for assistance in obtaining and evaluating the infrared tracings; and W. H. Hall, who built the vacuum system and prepared the irradiation tubes.
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WAVELENGTH (MICRONS)
WAVELENGTH
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literature Cited (1) Ballantine, D. S., Am. SOC.Testing Materials, Spec. Tech. Publ. No. 276, 201 (1960). (2) Bovey, F. A., “The Effects of Ionizing Radiation on Natural and Synthetic High Polymers.” p. 41, Interscience, New York, 1958. (3) Chapiro, A,, “Radiation Chemistry,” High Polymer Series, No. 15, p. 159, Interscience (distributed by Wiley), New York, 1962. (4) Charlesby, A., “Atomic Radiation and Polymers,” Vol. I, p. 368, Pergamon Press, New York, 1960. (5) Currin, C. G., Am. SOC.Testing Materials, Spec. Tech. Publ. No. 276, 233 (1960). (6) Degering, E. F., et al., “Irradiation ‘Factor-Dependency’,” Radiation Chemistry Laboratory Series, Research Repts. Nos. 1-9, 1959 to 1960, Pioneering Research Division, U. S. Army Natick Laboratories, Natick, Mass. (7) Degering, E. F., Butler, S. R., Caldarella, G. J., Evans, F. E., Gabelnick, S. D., Mancini, M., “Irradiation ‘Factor-Dependency’ of Vinyl Monomers: Effects of Some Experimental Variables,” Proc. Intern. Symp. Radiation-Induced Polymerization and Graft Copolymerlzation, pp. 1-1 6, Columbus, Ohio, November 1962. (8) Degering, E. F., Caldarella, G. J.. Mancini, M.. Am. SOC. Testing Materials, Spec. Tech. Publ. No. 276, 244 (1960). (9) Degering, E. F., Merritt, Charles, Jr., Bazinet, M. L., Grey, E. F., “Irradiation ‘Factor-Dependency’ : Some Parameters for Vinyl Monomers,” P7OC. Third Syrnfi. on Electron Beam Technology, pp. 212-25, Boston, Mass., March 1961. (10) Degering, E. F., Weiner, L. I., Seligsberger, L., “Irradiation of Fabrics and Leather,” Radiation Effects Symposium, V O ~3,. paper 2, sponsored by Air Research and Development Command, U.S.A.F., Cincinnati, Ohio, September 1959. (11) Lind, S.C., Hochanadel, C. J., Ghormley. J. A , , “Radiation Chemistry of Gases,” p. 163, Reinhold. New York. 1961. (12) Parkinson, W. W., Binder, D., Am. SOC.Testing Materials, Spec. Tech. Publ. No. 276, 224 (1 960). (13) Plyler, E. K., Blaine, L. R., Nowak, M., J . Res. -l‘atl. Bur. Std. 5 8 , 195 (1957). (14) Swallow, J. A., “Radiation Chemistry of Organic Conpounds,” p. 61, Pergamon Press, New York, 1960. (15) Wall, L. A., Am. SOC.Testing Materials, Spec. Tech. Publ. No. 276, 208 (1960). (16) ,Youden, W. S., “Statistical Methods for Chemists,” p. 90, Wiley, New York, 1951. RECEIVED for review September 26, 1962 ACCEPTED January 30, 1963
