1784
I. HALLER AND P. WHITE
Vol. 67
POLYMERIZATION OF BUTADIENE GAS ON SURFACES UNDER LOW ENERGY ELECTRON BOMBARDMENT BYI. HALLER AND P. WHITE‘ International Business Machines Corporation, Thomas J . Watson Research Center, Yorktown Heights, N e w York Received January 31, 1965 The kinetics of the formation of butadiene polymer film has been investigated on a surface exposed t o 250-e.v. electrons. In the presence of butadiene vapors a t pressures between 3 x 10-4 and 1 0 - 6 torr, the rate of gro-rvth of the transparent film, insoluble in benzene, acetone, CCI,, or CSz, was found proportional to the square root of the current density. The rate increased with increasing pressure, becoming independent a t higher values of pressure. The kinetics has been interpreted to indicate that after the first layer of polymer has formed, subsequent layers grow by interaction of adsorbed monomer gas with active species, probably positive ions in the polymer. The measured lifetime of the active species is about 3 min. The apparent activation energy is - 6 f 2 kcal./mole.
Introduction Polymerization of residual gases by the electron beam was reported2 as a cause of deposit formation on anodes of electron microscopes. After a suggestion3 for the utilization of the process, Christy4 studied the rates of formation of films formed from silicone oil vapors. The results were consistent with a mechanisni in which the adsorbed silicone oil molecule was converted into a free radical by interaction with an electron beam, adjacent free radicals subsequently reacting to form the observed polymer. A similar mechanisni had also been suggested by Buck and Shouldersaand by Burkhard and Fotland.6 Kargin and co-~vorkers~~~ observed vinyl addition polymerizations on cold surfaces on which monomers and metals or salt catalysts were simultaneously condensed. The aim of the work reported in this paper was to examine the kinetics and to elucidate the mechanism of polymer formation cn surfaces under electron bombardment using a simple organic monomer. Butadiene appeared t o be a natural choice since its polymerization in various environments has been investigated extensively.8-12 A kinetic study of the polymerization of butadiene on surfaces exposed to ultraviolet light is reported elsewhere. The interaction between the adsorbed gas and the underlying substrate might be expected to have a profound effect on the rate of formation of the polymer. It should be pointed out, however, that after the first few monolayers of adsorbed gas have been converted to solid, subsequent layers will be adsorbed on a freshly formed polymer surface and the rate should be independent of the original substrate material. This fact establishes a connection between these experiments and the studies of radiation damage in polymers and of radiation induced graft polymerization which are relatively well explored fields.I4 (1) Standard Telecommunication Laboratories, Ltd., London Road, Harlow, Essex, England. (2) (a) K. M. Poole, Proc. Phzjs. SOC.(London), B66, 542 (1953); (b) A . E. Ennos, Brzt. J . Appl. Phys., 6 , 27 (1954). (3) D. A. Buck and K. R. Shoulders, “Proc. Eastern Joint Computer Conference,” Philadelphia, Penna., 1958, p. 55. (4) R. W. Christy, J . Appl. Phys., 31, 1680 (1960). (5) J. Burkhard and R. A. Fotland, AMC Techn. Report 7-857. (6) V. A. Kargin and V. A. Kabanov, J. Polymer Sca., 62, 71 (1961). (7) V. A. Kargin, V. A . Kabanov, V. P. Zubov, and I. M . Papisov, Vysokomolelcul. Soedin., 3, 426 (1961). (8) G. Gee, Trans. Faraday Soc., 34,712 (1938). (9) H . E. Gunning and E. W. R. Steacie, J . Chem. Phys., 12,484 (1944). (10) Y. Tabata, H. Sobue, and E. Oda, J . Phys. Chem., 65,1645 (1961). (11) H. Clasen, 2. Elektrochem., 60, 982 (1956). (12) D. ill. White, J . Am. Chem. Soc., 82, 5678 (1960). (13) P. White, J . P h y s . Chem., in press.
Experimental The experiments were performed in a flow system where the dynamic pressure of the polymerizing gas and the electron beam current were maintained a t a constant value. The rate of polymer formation was followed by use of a piezoelectric quartz crystal oscillator vacuum microbalance described in detail elsewhere.16 Small changes in the total vibrating mass cause a proportional change in the fundamental frequency of oscillation which can be measured accurately; this allowed measurement of the mass of polymer deposited on the surface of the crj‘stal exposed to the electron beam. The sensitivity of the 4.1-mc. crystals used was such that 8.7 X l o u 9g. deposited in an area of 0.172 cm.z caused a change in frequency of 1 C.P.S. Assuming a density of 1.05 for polybutadiene, the sensitivity can also be expressed as 4.8A./c.p.s. Gsing 10 sec. counting time the frequency readings vere reproducible to k 0 . 7 C.P.S. Figure 1 shows a diagramatic representation of the electron gun awembly used. A directly heated thoria coated iridium filam e n P was used as a cathode and the electrons were accelerated and focused by several electrodes to form a circular spot about 0.5 cm. in diameter on the gold electrode of the front face of the quartz crystal microbalance. The front face of the crystal was kept a t a potential +22 v. more than the next most positive element to recapture most of the secondary electrons. This permitted a meaningful measurement of the primary current. The intensity of the electron beam was adjusted by varying the potential on the grid, leaving all other voltages unchanged. The microbalance and the electron gun were enclosed in a high speed, all-glass mercury diffusion-pumped vacuum system. The ultimate pressure was in the 10-7-torr range. A Granville Phillips Type C all-metal valve connected the system t o a manifold from which butadiene, hydrogen, oxygen, or a mixture of these gases could be bled into the system. As vapors of stopcock grease can polymerize under electron bombardmenL2 there was no grease used in the low pressure part of the system. Apiezon W wax was used for mounting the removable parts. I n a typical experiment, the frequency of the oscillator was monitored until it showed no drift. The valve to the manifold was opened slightly (while continuously pumping on the system) until the required pressure of butadiene was flowing past the microbalance. The filament voltage and the accelerating voltages were turned on in that order. After 40 min. of bombardment, the voltages were turned off and the valve to the manifold was closed. The system was evacuated until the frequency readings showed no further drift. Successive experiments were performed without breaking the vacuum or removing the previously deposited polymer from the crystal surface. Hence, with the exception of the first experiment of each series, which was disregarded, the measured rates refer to polymerization on a previously deposited polymer surface and were not influenced by the underlying substrate. Butadiene pressures were measured with a Veeco ionization gage. The pressure readings relative to each other are believed to be accurate to better than 10%; their absolute values, however, should be regarded as approximate. (14) See, for example, -4. CharIesby, “dtomic Radiation and Polymers,” Pergamon Press, New York, N. Y., 1960. (15) I. Hailer and P. TThite, Rev. Scz.Instr., 84, 677 (1963). (16) Supplied by W. R. and W. Electron Corp., Port Washington, N. Y.
Sept., 1963
POLYMERIZATION OF BUTADIENC BY E~sc.raosBOMBAI~DMEST
The temperature of the crystal holder block was maintained constant to within about ~ I ~ 0 . 0by6 circulating ~ water through i t from a constant temperature bath. A thermocouple was placed in contact with the copper plug which served as an electricd connection to the front electrode of the crystal. It was observed that during electron bombardment the temperature of the copper plug rose rapidly to an equilibrium value above that of the crystal holding block. Since the copper plug was in intimate contact with the front surface of the quartz crystal and was in poor thermal contact with the cooling block, it was assumed that the temperature of the surface of the quartz crystal increased to a similar amount. The temperature rise amounted to 7.0" a t a beam current of 800 pa. and a t an accelerating voltage of 350 v. After the elertron bombardment was stopped, the temperature of the crystal stabilized (as indicated by the constancy of the frequency readings) in :bout 15 min. Butadiene was obtaincd from a cylinder (Mathieson Instrurnent Grade) and fractionally distilled into a 1-1. glass bulb, disc:trding the first and last fractions. It was stored a t room temperature in the dark as a gas a t a pressure of about 600 mm. Hydrogan and oxygen used were assayed reagent grade msnufarturcd by the Air Itduction Company.
Results The rate of polymerization was investigated as a function of beam current density,l7 butadiene pressure, and substrate temperature. KO deviation from linearity was fourid in the deposited mass vs. time plot within experimental error at constant beam current and gas pressure if the time of bombardment was longer than 10 min. Rates of the polymer formation were computed by dividing the mass change corresponding to the chaiige in the frequency readings before and after the electron bombardment by the time of the bombardment. The data presented are corrected for the rate change with temperature. The temperature rise per unit power input a t the reaction area was taken as that measured a t the copper plug. The temperature coefficient used was d In r/dT = 0.033 dcg.-' (see below) which corresponded to d In T/d(IV) = 1.15 watt-', where r is the rate of polymer formation, T the temperature in 'Ithat the average lifetime estimated from the inflection point on the curve is of the order of 3 min.
Discussion Polymerization reactions leading to the formation of a solid polymer filni on a surface can tatke place in the gas phase or on the surface. I’recipitation of polymer molecules formed in the gas phase is important in some photochemical polymerizations’ and when butadiene is subjected to a glow-disclinigc a t higher pressures.20 hssiuiniiig an ionization cross section of 6 x for butaditwe iii a path of .j cm. and a t a torr, ahout 17; of the clectroris would be pressure of absorbed in ioiiiziiig collisions which might result in fragments capable of initiating gas phase polymcrization. Abseiice of polynicr, howevcr, 011 parts of thr apparatus not exposed to the electron beam excludes a contribution of this type of rciaction to the over-all formation of polymer. Tlie ratc of a gas phase reaction is generally proportional to the number of molecules available for rwction, i c., tlie pressure in the gas phase, whereas a surface reaction may be dependent on the pressure only to the extent that the pressure controls the concentration of adsorbed gas taking part in the reaction. I n Fig. :3 it may be seen that, the reaction rate increases nith illcrease i r i prcssure up to a limit which appears to depend on the current. At 400 pa. the limit is approximately I x 10-5 torr; above this the exponent of the pressure decreases arid ewntually the rate becomes independent of pressure. A similar relation is obtained when the current is 800 p a . hit thc torr. The expressure limit is increased to 1 X perimental results presented 111 I’ig. 2 , 3, and -1 do riot appear to be consistent with the produetioii of polymer film by a gas phase reaction. If the reaction proceeds through a riicchaiiism involving an adsorbed gas, thc rute of reaction a t any particular value of current density must be depcndent on the number of inolecules in the adsorbed phasc. A t very low pressures thc niiniber of adsorbed gas molecules must be sinall, and if thcrc arc sufficient electrons to allow every adsorbed molecule to react, th(b reaction rate vi11 be proportional to the nuniber of adsorbed molecules. As the pressure is increased and the concentration of adsorbed molccules increases, a condition must aiise in which the number of artivc species created is dependent 011 the number of electrons striking the surface. ht this stage the increase in rate of polymer formation with increase in number of adsorbed molecules must saturate. If the mmi’her of electrons striking the surface is increased, thc pressure a t which the saturation occurs must incrcase This is
I
.60 ‘ * O L.
IO2
16‘
I
10 IO2 PERIOD ( s e d .
lo3
lo4
Fig. 5.--Thc, ratio of thc. ratc o f polymerization induced by the pulsed beam to th(1 rate of polyrnerization wit 11 a continuous beam of thr same intensity cs. the pulsing period.
son can be made between runs a t different current densities. It can be sren from the data that the points of limiting current on the constant pressure plots and of tlie limiting pressure on the constant current plots are consistent with each otlier. As will be discussed later, thew is good reason to believe that the polymerization takes place on the surface and proceeds by a chain reaction. To test the importance of free radical propagation arid chain transfer steps, rates of polymerization were measured using butadierieoxygen (4 :1) and butadiene-hydrogen (1 :4) mixtures. 90significant difference in rate could bc detected in comparison with pure butadiene samples. Figure 4 shows how the rate of polymerization as a function of pressure at constant current (800 pa.) varied with temperature. At all pressures studied the rate decreased with increase in temperature. Plots of log rate us. inverse temperature indicated an over-all activation energy of -6 + 2 kcal./mole over the whole pressure range studied. I t may be postulated that the polymerization proceeds by creation of active species which react with double boiids of neighboring molecules. The concentration of these actiw spccies mill be constant and controlled by the rates of formation and decay. The lifetime of the active specics taking part in the reaction was estimated using a pulsed electron beam technique. The principle of this is an extension of that described for use of intermittent illumination in photochemical reactions.Is
(18) W 4 N o i p s and P .\ Leipliton. “‘rlie I’hotoctieinisti\ of Cznsrs,” Noinhold Pub1 Coi11, Nee York, N Y , 1041, p 202 (19) 11 W Afel!~lle,PTOC K o y Sor (1,ondon). A163,511 (1917) (20) li RI. l h S i l v + prirate communication
Sept., 1963
POLYMERIZATIOK OF BUTADIEKE BY ELECTRON BOMBARDMENT
the result shown in Fig. 3. The relation between the concentration of adsorbed gas and the pressure in the gas phase should follow a normal adsorption isotherm. Thus continued increase in pressure will not be accompanied by a corresponding increase in the number of adsorbed molecules and thus must contribute to the saturation a t increased pressures. Consider now the results presented in Fig. 2. At the highest pressure, p = 1.6 X 10-4 torr, the reaction is proportional to the 0.6 power of current over the whole range examined. As the pressure is decreased, causing an accompanying decrease in concentration of adsorbed species, a limiting current appears, above which the rate becomes independent of current. Obviously, a t the lower pressures, on increasing the current a value is approached where the Concentration of adsorbed species becomes a limiting factor and the rate cannot increase because of the limiting number of molecules available for reaction. The lower the pressure, the lower this limiting current density becomes. This is in agreement with the argument proposed in the last paragraph and with the results shown in Fig. 2 and implies that the rate-determining step in this region is the formation of mme adsorbed butadiene species which are active in the polymerization process. The formation of this active adsorbed butadiene species must be preceded by a collision of a butadiene gas molecule with the surface. From the kinetic theoryz1 the number of gas molecules ( N ) striking unit area in unit time is given by N = p(2~mkT)-’/~ cm.-2 sec.-l, where p is the pressure in dynes/cm.2, m is the molecular weight of the gas, k is the Boltzmann constant, and T the temperature in OK. The ratio of the number of butadiene molecules incorporated into the polymer to the number of gas molecules striking the surface therefore will yield an effective sticking coefficient for production of the active adsorbed butadiene species. At a pressure of 1.3 X torr this ratio is about 1 X lom3. If it is assumed that the adsorbed butadiene molecules are the species that take part in the polymerization process, this implies a sticking coefficient of 1 X The primary step when an electron strikes the polymer surface is probably the same as in the case of high energy radiatio~n~~; that is, ionization or excitation of the polymer molecule followed by fragmentation resulting in active species which can propagate the polymerization reaction by addition of unactivated adsorbed monomer molecules. Adsorbed monomer molecules could undoubtedly be activated by electrons in a similar manner and this must indeed happen when the first layer is polymerized. The relative significance of this type of initiation on a polymer surface may be reduced as desorption of the activated monomer competes with the addition. The formation of polymer on surfaces bombarded by electrons in the presence of silicone oil vapors has been i n t e r ~ r e t e d as ~ . ~a recombination of free radicals created from adsorbed monomer molecules by impinging electrons. This type of mechanism cannot play an important role in the butadiene polymerization as it is inconsistent with the kinetics of the reaction and the observed long lifetime of the active species. (21) L. D. Landau and E. &I. Lifshitz, “Statistical Physics,” Pergamon Press, New York, N. Y., 1958, p. 115. (22) A. Chapiro, “Radiation Chemistry of Polymeric Systems,” Interscience. New York, N. Y.,1962, p. 37.
1787
The nature of the active species may be discussed in terms of the two possibilities: free radicals or ions. Observations23 of electron spin resonance in various hydrocarbons exposed to ionizing radiation have indicated the presence of free radicals in the solid which have lifetimes of minutes or longer. Conductivity with decay times ranging from seconds to hours has been observed in X-ray irradiated insulators. The results have been i n t e r ~ r e t e das~ ~due to electrons and corresponding holes created by the radiation, being separately trapped, and finally released from the traps with a rate corresponding to the observed decay times. If a similar model is invoked for the electron bombardment of the butadiene polymer, trapped electrons and holes correspond to localized positive and negative charges and those on the surface may propagate an ionic polymerization with the adsorbed monomer. A possible free radical mechanism fitting the observed kinetics in the case where the polymerization is rate determining may be expressed with eq. 1 through 3 P+e-+P-
+Ha + e
P M * f Bua
PM+l*
+ PN.+PM+N
Pair.
KJ
(1)
K Z[P*I [Bual (2)
K3[P*I2
(3) where P k l designates a polymer molecule on the surface composed of M butadiene units and Bu, an adsorbed butadiene molecule. Assuming that the size of the radical has no effect on its chemical properties and assuming a steady-state radical concentration, the rate of polymerization can be expressed as
The relation between [Bu,] and the pressure is determined by the adsorption isotherm which can have a shape similar to the observed pressure dependence curves. An ionic mechanism fitting the observed kinetics is presented in eq. 4 through 9. As in the case of the free radical mechanism, only those steps are considered which are necessary to obtain satisfactory kinetics. Other reactions, like cross linking and increase in unsaturation, presumably also take place but it is assumed that they have no effect on the rate of polymerization. P+e+P++ H - 2ec
+
e,
+8
-
e,
K4l
(4)
dV
Ks reccld,
(23) See, for example: (a) 4.Charlesby, D. Libby, and 14. G . Ormerod, Proc. 230% SOC.(London), A262, 207 (1961); (b) H.Kashiwabara,J. Phys. SOC.Japan, 16,2494 (1961); ( e ) C. H.Bamford and J. C . Ward, PoZpzeT, 2, 277 (1961). (24) J. F. Fowler, Proc. Row SOC.(London), 8286, 464 (1956).
1788
I. HALLER AKD P. WHITE
Here P+ denotes a polymer with a positive charge localized on one of the surface carbon atoms, e an impinging electron, e, an electron in the conduction band; eL denotes an electroii trapped in energy levels, L, which lie below the conduction band with an energy EL. Equation 8 signifies the transfer of a conduction electron to the metal substrate 8 under the influence of a potential gradient, dV/dx, resulting from excess charge. The concept of trapping levels in irradiated insulators was introduced by Fowlerz4 and may be invoked to account for long life of the active sites, P+. Assume that the resistivity of the polymer film is low (Le., step 8 is fast), then there will be no major charge separation in the polymer, i.e.
If it is assumed that a steady state exists and that step 7 is much faster than step 6, the rate of polymerization may be expressed as
Vol. 67
At a butadiene pressure of 1.6 X 10-4 torr and 200 pa. mole of butadiene is incorbeam current, 0.92 X porated per minute. This current corresponds to 1.15 X lo-’ mole of electrons per minute. Assuming that about half of the measured current results from electrons striking the polymer on the quartz crystal (the other half resulting from electrons hitting the connecting electrodes) the efficiency is 1/600. As the track range of 250 v. electrons is approximately 80 A.,14 no less than about one tenth of the impinging electrons should cause chemical changes in the surface layer. This means that even if the chain length of the propagation reaction is very low, in the limit 1, the initiation by electron impact is very inefficient. This can be due either to the fact that the majority of the species produced is not of the type capable of reacting with butadiene or to a cage effect where only in a small fraction of the events can the dissociation fragments separate. In the ionic mechanism the apparent activation energy, Ea, is given from (11) as
Ea
=
E5
+ ’/*(E4- E7 - EL)+ AHads = -6 kcal./mole
To determine the predominating mechanism, separate experiments were performed using in place of butadiene, 1:4 oxygen-butadiene, 4 : 1 hydrogenbutadiene mixtures and n-hexane. The presence of oxygen would be expected to retard or inhibit free radical polymerization.25 Hydrogen may increase the rate of migration of free radicals in polymersz6producing an increase in K , which would reduce the rate of free radical polymerization. Experimentally, however, the rates using these mixtures were found identical within experimental error with rates of runs with pure butadiene a t a pressure equal to its partial pressure in the mixtures. A saturated compound in place of butadiene could not react in step 2, whereas it may add to a carbonium ion with the elimination of a hydrogen molecule. 27 Experimeqtally , we have observed polymer growth in the presence of n-hexane in place of butadiene, but with a reduced rate. On the basis of these facts, we tentatively conclude that the ionic mechanism is predominant as the main path of the polymerization reaction. The efficiency of the polymer formation is very lorn. (25) P. J. Flory, “Principles of Polymer Chemistry,” Cornell University Press, Ithaoa, N. Y., 1953, p. 168. (26) M. Dole and F. Craoco, J . Phgs. Chem.. 66, 193 (1962). (27) W. P. Libby, J . Chem. Phys., 86, 1714 (1961).
where E4,E5, and E7 are activation energies of steps 4,5, and 7, respectively, and AHadsis the heat of adsorption of butadiene on the polymer surface. This last term enters because the temperature coefficient measurements were made at constant pressure rather than at constant surface coverage. Attempts were made to obtain an estimate of the activation energy of the propagation step for the ionic mechanism using the measured value of -6 kcal./mole for the over-all reaction and using reasonable values for the other steps in the reaction. Due to the large uncertainties in the latter, however, any numerical value thus obtained would have little significance. The experimental results presented are inconsistent with the formation of polymer by a gas-phase reaction. It has been shown that the reaction takes place predominantly on the surface of the growing polymer. Evidence is presented to indicate that active sites are produced on those parts of the surface struck by the electron beam and that the polymer film g r o w by interaction of adsorbed monomer with these active sites rather than by direct collision of gas phase molecules with these sites. It was tentatively concluded that the active sites propagating the polymerization are mainly ionic.
