- Email: [email protected]
Polymerisation by glow discharge electrolysis
EIectr~~himica
Acta
1973, Vol.
18, pp. 863-868.
POLYMERISATION
Pergamon
BY GLOW DISCHARGE A. R.
Department
of Chemistry,
Press. Printed in Great Britain
Liverpool
DENARO
and K. 0.
ELECTROLYSIS
HOUGH
Byrom Street, Liverpool
Polytechnic,
L3 3AF., England
(Received 22 March 1973) Abstract-A glow discharge passed from a metal anode to the surface of aqueous solutions of acrylamide produces radicals which initiate polymerisation. The yield of polymer together with its relative molecular mass has been examined as a function of monomer concentration and pressure in neutral solutions. The form of the variations can be explained in terms of a simplified reaction scheme. On the basis of the proposed mechanism there are two reaction zones. In the primary reaction zone no polymerisation occurs, the radical concentration being too high. About 0.2% of the radicals escape into the secondary reaction zone ta initiate polymerisation, the volume of this zone being abont 0.4 cm3. In alkaline solutions the yields and relative molecular masses of the polymers are very low and this is attributed to chain termination by oxygen which rises from the destruction of hydrogen peroxide in alkaline solution.
INTRODUCTION
In glow discharge electrolysis[l] (&e), a normal glow discharge is struck between a metal electrode and the surface of a solution. Previous work[l,2] has shown that in anodic &e where the metal electrode is the anode, positive ions bombarding the solution penetrate only a short distance into the solution and form a very shallow primary reaction zone containing high concentrations of radicals originating from the solvent. Most work in the past has been done with oxidisable inorganic substrates in solution but it was considered that a study of a polymerisation reaction might be helpful as unusual polymerisation kinetics result from high rates of initiation provided by high radical concentrations[3]. There is a patent[4] for the initiation of polymerisation by gde but this does not contribute to an understanding of the mechanism of gde. For these reasons the present work was undertaken, acrylamide being chosen as the substrate as it provides a water soluble monomer and polymer.
maintained through the cell during electrolysis. After electrolysis, polymerisation continued slowly but this effect was obviated by bubbling oxygen through the solutions immediately after electrolysis. The polymer formed by gde was precipitated by adding an aliquot of the polymer solution to a large volume of methanol. The precipitated polymer was filtered on a weighed sintered glass filter of porosity 4, washed with methanol and vacuum dried. The loss of monomer during gde was determined by bromination
of the residual
monomer
coupled
with
the
estimation of excess bromine with iodide and thiosulphate. The average relative molecular masses of the polymers were determined by viscometry using the equation established by Collinson et a1[5], to calculate the number average relative molecular mass. Hydrogen peroxide which was also a product of gde in neutral solutions was determined spectrophotometrically by the pertitanic acid method, the absorbances of the solutions being read at 405 nm.
EXPERIMENTAL RESULTS
The apparatus was the same as that described previously[l]. Solutions were made from the British Drug Houses
acrylamide dissolved in distilled water or O-lmol/dm3 NaOH aq. Subsidiary experiments showed that further purification of the material had no effect on the results. The solutions were saturated with nitrogen before use and then boiled out in the cell for 20 min. under reduced pressure at 15°C before electrolysis. After the boiling out period, the apparatus was filled with nitrogen before the pressure was reduced to the desired value which was maintained with a manostat. The discharge was usually operated at 50 Torr and 75 mA in nitrogen, EA
Vol.
18 No.
11-G
a continuous
flow of gas being
AND DISCUSSION
Under the experimental conditions employed, the cathode fall of the discharge remains constant and the amount of energy transferred to the solution depends only on the amount of charge passed. The results are expressed by plotting the amount of product against the amount of charge passed. The differential yield in mol/F is given by the slopes of the graphs and these rates are denoted by G. Neutral
solutions
The results of the gde of neutral acrylamide solutions of various concentrations at 75 mA and 50 Torr are shown in Fig. 1. It was not possible to precipitate 863
A. R. DENARO AND K. 0. HOUGK
15
c
.
Fig. 2. Variation of G and relative molecular mass with concentration. O,Gvaluesand~,relativemolecularmass. 1
35
Charge
passed/Fx
lO’4
Fig. 1. Glow discharge electrolysis of neutral acrylamide solutions. A, 0.25 moI/dm3; 0, 0.50 mol/dm3; A, O-75 mol/dm3; and 0, 130 mol/dm3. weighable quantities of polymer from solutions less concentrated than O-25 mol/dm3 acrylamide nor with quantities of electricity less than l-75 x 10h4 F. The ir spectra of the polymers obtained was virtually identical with that of a sampIe of polyacrylamide prepared by the method of Sorrensen and Campbell [61. With the possible exception of the lowest concentration all the rates might be considered to be acceleratory throughout although over those portions of the graphs which have been drawn as linear the rates are vir-tually constant. Further, the relative molecular masses of the polymers produced over these regions were also constant for a particular monomer concentration. The slopes of the linear portions of the graphs have been taken as the differential yields of polymerisation. The final more noticeable acceleratory phases at the three highest concentrations are associated with a marked increase in viscosity and presumably there is autoacceleration throughout the polymerisation process. The G(polymer) values obtained from Fig. 1 together with the relative molecular masses of the polymers produced are plotted against monomer concentration in Fig. 2. It can be seen that both the differential yield and degree of polymerisation are directly proportional to monomer concentration as is usually the case with radical initiated polymerisation. The G(polymer) values in Fig. 2 are expressed in terms of moles of monomer consumed in polymerisation. If, however, they are expressed in terms of moles of polymer produced, a value of about 10e2 mol/F is obtained. Assuming that two radicals are
incorporated in a polymer molecule, the implication is that 2 x 10e2 mol/F of radicals are used in the polymerisation reaction. Previous work[2] has shown that under the same experimental conditions as used here about 10 mol/F of OH radicals are produced in gde. Thus, even without considering the action of reducing radicals which must be produced it is evident that only a very small proportion of radicals is active in initiating polymerisation. This conclusion is at first surprising as it is known [l] that concentrations of solute of between 0.1 and O-2 mol/dm3 are usually sufficient to scavenge all the OH radicals produced in gde. The determination of residual monomer after gde, however, shows that in addition to the monomer consumed in polymerisation a further amount of monomer, to the extent of about 10 mol/F at each concentration is also consumed. It seems reasonable to conclude that all the radicals in the primary reaction zone are scavenged by acrylamide but that the concentration of radicals in this region is so high that no significant polymerisation occurs and effectively radicals are adding to the double bond in the acrylamide molecule. A few acrylamide radicals may escape this process, diffuse out of the primary reaction zone into the bulk of the solution and initiate conventional polymerisation there. The gde of aqueous solutions results in the production of OH radicals, I-I radicals and solvated electrons. For the purposes of the present discussion this initial process may be represented H,O
-VYA+ 2R’
rate = R.
where R ’ represents the primary radicals formed from the water. Denoting an a&amide molecule by M, the following reactions may then occur. R’
+R’
-
RZ
(1)
R’
+M
-
RM’
(2)
865
Polymerisation by glow discharge electrolysis
RM, + M RM,
+ RM,
RM,
+ R’
-
RM;+l
(3)
-
P.+,
(4)
-
P,.
(5)
In the present case it is suggested that the concentration of acrylamide is sufficiently high to scavenge all the primary radicals so that reactions (1) and (5) may be neglected. Further, there are two regions to be considered, the primary reaction zone containing a high concentration of primary radicals and the secondary reaction zone’corresponding to the bulk of the solution. It is proposed that no polymerisation occurs in the primary reaction zone, the only reaction being (2) and (4) where n = m = 1, viz,
RM;
R’ +M
-
RMi
(2)
+ RM;
-
R2M2.
(4)
If the rate of consumption r,,, , the above leads to
of monomer
is denoted
It is known that the system generates 10 mol/F OH and also that 10 mol/F of monomer is consumed by this process. One implication is that H radicals and solvated electrons are not reacting with acrylamide molecules even though the homogeneous rate constants of these reactions are comparable with that for the reaction of OH with acrylarnide. This lack of activity of reducing radicals is a feature of gde. It is further proposed that a few (O-2%) of the RM; radicals may escape from the primary reaction zone into the secondary reaction zone, the process being represented -
RMi(2)
rate = RI .
’
the relative molecular mass of the monomer Substituting from (6) and (7)
(9) being 71.
G = 96 500 V/C~R~~‘~[M]/Z~~“*,
(10)
M = 142 kJM]/R,1’2k.+1’Z.
(ll)
Equation (10) shows that a plot of G against monomer concentration should be linear provided that the volume of the secondary reaction zone is independent of monomer concentration. This is to be expected in a chain reaction where the concentration of polymeric radicah is independent of monomer concentration. From (10) and (11) it may be seen that the product of the slopes of the two graphs in Fig. 2 is
The values of k, and k4 are known[7] and hence a value of V may be calculated. With a current of 75 mA and taking ka2/k4 as 22*l/mol. dm3/s, V = O-4 cm3. This is a reasonable value compared with the volume of the secondary reaction zone of about 3 x 10M9 cm3 calculated for the case where sulphuric acid was the radical scavenger[2]. In the polymerisation where radicals can only disappear by combination reactions they would be expected to diffuse a much greater distance into the bulk of the solution. The quotient of the slopes of the graphs in Fig. 2
gives the quantity 96,500 I/R,/142 Z
These radicals then act as initiating radicals for polymerisation in the secondary reaction zone. Polymerisation proceeds according to reactions (3) and (4), the boundary of the secondary reaction zone being defined by the points at which all polymeric radicals have disappeared by recombination. The above scheme leads to the following expressions for the rate of polymerisation R,, and the degree of polymerisation, P, R, = ksR?*[M]/k.?‘*,
(6)
p = 2k,[M]/R~“‘k,“‘,
(7)
assuming that termination occurs exclusively by combination and that homogenous kinetics apply in the secondary reaction zone. The G(polymer) values and relative molecuIar masses, M,, plotted in Fig. 2 may be expressed in terms of R., and p by multiplying by the appropriate factors. Thus G = % SOOVRJZ,
M,=71F,
96,500 x 142 V/C~~/Z,L.
r JV-Ro
RM;(l)
where V is the effective volume of the secondary reaction zone and Z is the current. Also
(8)
so that knowing V, Ri may be calculated to be 4.2 x 1o-s mol/dm3/s. This implies that 1-7 x lo-* mol/s of initiating radicals escape from the primary reaction zone. Knowing that the differential yield of OH radicals in the primary reaction zone is 10 mol/F, ie, 7.8 x 10e6 mol/s under the conditions of the experiments, it shows that only about O-2 % of radicals escape recombination in the primary reaction zone. In the above considerations, chain transfer reactions to monomer and to hydrogen peroxide have been neglected as the rate constants for these reactions are[7] 0.22 and 9/mol. dm”/s respectively compared with a value of the propagation rate constant, k3, of 1.8 x 104/mol 9dm3/s. Hydrogen peroxide was formed at a virtually constant yield of about 0.2 mol/F. This indicates that the peroxide is formed, to some extent, by a route other than the dimerisation of OH radicals as these should all be scavenged by acrylamide at the concentrations used. This effect has been noted in one other case[S].
866
Efict
A.R.
DENAROAND
of pressure
or
Increasing the pressure of thedischarge has the effect of decreasing the area of the glow spot on the surface of the solution and hence of decreasing the volumes of the reaction zones. It is known[8] that the area of the glow spot is inversely proportional to pressure and hence variation of pressure has some effect on the kinetics[2]. Neutral solutions of 0.5 mol/dm” acrylamide have been examined over a range of pressures and the results are given in Table 1. Table 1. Variation of G and M, with pressure p Torr 50 75 100 150
G(polymer) mol monomer/F
M,
8.3 7.6 5.7 4.6
45,ooo 24,300 20,900 13,200
A simplified approach may be made to this situation on the basis of homogeneous kinetics in the secondary reaction zone which, as a first approximation may be considered as a cylinder. The area of the glow spot (base of the cylindrical reaction zone) is inversely proportional to pressure so that the concentration of initiating radicals entering the secondary reaction zone will be directly proportional to the pressure. The effective length of the cylindrical reaction zone will be defined by the distance travelled into the solution by the radicals before they disappear. (Theoretically this distance should be infinite but in practice there wiI1 be a finite distance at which the vast majority of radicals will have been consumed.) The distance travelled by the radicals will depend on their lifetime which, for a second order reaction, will be inversely proportional to their initial concentration where they enter the secondary reaction zone. The length of the cylinder is thus inversely proportional to pressure in addition to the area of the base of the cylinder being inversely proportional to pressure. Under these circumstances the volume of the cylinder will be inversely proportional to the square of the pressure or, V = a/p’,
where p As a dire&y assuming ing the
K. 0. HOUGH
(12)
is the pressure and a is a constant. consequence, the rate of initiation will be proportional to the square of the pressure that the number of initiating radicals entersecondary reaction zone is constant. Thus R, = bp2,
(13)
where b is a constant. Substituting from (12) and (13) into (10) and (11) G =96,500 k,ab”2[M]/lk~1’2p
G = AiMlIp
(14)
and M, = 142 k~[M]/k‘+“%“‘p or M, =
BbW.
(13
Equations (14) and (15) show that plots of G and M, against l/p should be linear through the origin. These plots are shown in Fig. 3 from which it may be
-4 -2 I5
IO
0.5 Reciprocal
pressure
g x 3
2-o
x IO2 torr
Fig. 3. Variation of G and relative molecular mass with reciprocal pressure. 0, G values and a, relative molecular mass. seen that the variation of M, with l/p agrees well with (15). The results for the G values are not quite so satisfactory and show rather more scatter about a line through the origin. Attention may be drawn to a further point about the graphs in Figs. 2 and 3. Denoting the slope of the graph of G against monomer concentration as S, and the slope of the graph of G against reciprocal pressure as S, , (14) shows that S.&
=PJMI,
,
where pr is the pressure (50 Tori-) at which the variation of G with concentration was examined and [Ml, is the monomer concentration (0.5 mol/dm3) at which the variation of G with pressure was studied. Further, denoting the slope of M. against monomer
Polymerisation by glow discharge electrolysis concentration shows that
as ??, and against pressure as %,,, (15) -S,/S, = pJM]p .
Hence the expected value of SJS, and 3,/s, is 25 Torr mol/dm- 3, The graphs in Figs. 2 and 3 yield the values
867
solution (2.0 mol/dm3 acrylamide) the relative molecular mass of the polymer was only in the region of 16 000 as compared with 100 000 in neutral solutions of half the monomer concentration. The G values obtained from Fig. 4 appear to be proportional to the square of the monomer concentration as shown in Fig. 5.
S,/S, = 34 * 6 Torr mol/dm3 - S,/S, = 21 Torr mol/dm3. The figure for 3,/s, is in reasonable agreement with the expected value but the value for S,/S, is less satisfactory and there could be a considerable error in the slope of the graph of G against 1/p. In view of the extent ofoversimplification in the treatment, however, it is doubtful whether better agreement could be expected. Alkaline solutions The results of the & of solutions of acrylamide in 0.1 mol/dmX NaOH at 75 mA and 50 Torr for various monomer concentrations are shown in Fig. 4. ( Concentration
of ocrylamide)‘/
mol’ dmm6
Fig. 5. Variation of G in alkaline solutions
with con-
centration.
Charge passed/
F x 10e4
Fig. 4. Glow discharge electrolysis of alkaline acrylamide solutions. A, @5 mol/dm3; 0, 1.0 mol/drn’; A, 1.5 mol/dm3 and 0, 2.0 mol/dm3. In contrast
to neutral solutions the differential yields of polymerisation remained constant but were much lower in alkaline solution. Furthermore the reIative molecular masses of the polymers were much smaller, so much so in fact that it was not possible to obtain
meaningful values. Even for the most concentrated
It would be expected[3] that the rate of polymerisation would be proportional to the square of the monomer concentration if all the growing polymer chains were terminated by primary radicals as in reaction (5). This cannot be the case in the present system which involves high monomer concentrations but it is known that hydrogen peroxide is not found in the gde of alkaline solutions. If hydrogen peroxide is initially formed and then destroyed it will generate oxygen which is an efficient chain terminator. It might be expected that such oxygen would be generated in proportion to the primary radicals and this could possibly lead to the dependence of G on monomer concentration depicted in Fig. 5. Certainly, if oxygen is being generated in the system the very low relative molecular masses of the polymers produced in alkaline solution can be understood. Unfortunately, confirmation of this situation could not be obtained from the dependence of relative molecular mass on monomer concentration owing to the inaccuracy of the figures obtained with such low values. REFERENCES 1. A. Hickling, Modern Aspects of Electrochemistry (Edited by J. O’M. Bockris and B. E. Conway), Vol. 6 p. 329. Butterworths, London (1971). 2. A. R. Denaro and K. 0. Hough, Electrochim. Acta 17, 549, (1972).
868
A. R. DENARO AND K. 0. HOUGH
3. A. Chapiro, High polymers, in Radiution Chemistry of Polymeric Systems, Vol. 15, p. 132. Interscience, New York (1962). 4. U.S. patent 2,632,729 (1953). 5. E. Collinson, F. S. Dainton and G. S. McNaughton, Trans. Faraday Sac. 53,489 (1957).
6. W. R. Sorrenson and T. W. Campbell, Preparative Metlbds of Polymer Chemistry. Interscience, New York (1961). I. F. S. Dainton and M. TordofT, Trans. Faraday Sot. 53,499 (1957). 8. A. R. Denaro and A. Hickling, J. efectrochem. Sot. 105, 265 (1958).
Acta
1973, Vol.
18, pp. 863-868.
POLYMERISATION
Pergamon
BY GLOW DISCHARGE A. R.
Department
of Chemistry,
Press. Printed in Great Britain
Liverpool
DENARO
and K. 0.
ELECTROLYSIS
HOUGH
Byrom Street, Liverpool
Polytechnic,
L3 3AF., England
(Received 22 March 1973) Abstract-A glow discharge passed from a metal anode to the surface of aqueous solutions of acrylamide produces radicals which initiate polymerisation. The yield of polymer together with its relative molecular mass has been examined as a function of monomer concentration and pressure in neutral solutions. The form of the variations can be explained in terms of a simplified reaction scheme. On the basis of the proposed mechanism there are two reaction zones. In the primary reaction zone no polymerisation occurs, the radical concentration being too high. About 0.2% of the radicals escape into the secondary reaction zone ta initiate polymerisation, the volume of this zone being abont 0.4 cm3. In alkaline solutions the yields and relative molecular masses of the polymers are very low and this is attributed to chain termination by oxygen which rises from the destruction of hydrogen peroxide in alkaline solution.
INTRODUCTION
In glow discharge electrolysis[l] (&e), a normal glow discharge is struck between a metal electrode and the surface of a solution. Previous work[l,2] has shown that in anodic &e where the metal electrode is the anode, positive ions bombarding the solution penetrate only a short distance into the solution and form a very shallow primary reaction zone containing high concentrations of radicals originating from the solvent. Most work in the past has been done with oxidisable inorganic substrates in solution but it was considered that a study of a polymerisation reaction might be helpful as unusual polymerisation kinetics result from high rates of initiation provided by high radical concentrations[3]. There is a patent[4] for the initiation of polymerisation by gde but this does not contribute to an understanding of the mechanism of gde. For these reasons the present work was undertaken, acrylamide being chosen as the substrate as it provides a water soluble monomer and polymer.
maintained through the cell during electrolysis. After electrolysis, polymerisation continued slowly but this effect was obviated by bubbling oxygen through the solutions immediately after electrolysis. The polymer formed by gde was precipitated by adding an aliquot of the polymer solution to a large volume of methanol. The precipitated polymer was filtered on a weighed sintered glass filter of porosity 4, washed with methanol and vacuum dried. The loss of monomer during gde was determined by bromination
of the residual
monomer
coupled
with
the
estimation of excess bromine with iodide and thiosulphate. The average relative molecular masses of the polymers were determined by viscometry using the equation established by Collinson et a1[5], to calculate the number average relative molecular mass. Hydrogen peroxide which was also a product of gde in neutral solutions was determined spectrophotometrically by the pertitanic acid method, the absorbances of the solutions being read at 405 nm.
EXPERIMENTAL RESULTS
The apparatus was the same as that described previously[l]. Solutions were made from the British Drug Houses
acrylamide dissolved in distilled water or O-lmol/dm3 NaOH aq. Subsidiary experiments showed that further purification of the material had no effect on the results. The solutions were saturated with nitrogen before use and then boiled out in the cell for 20 min. under reduced pressure at 15°C before electrolysis. After the boiling out period, the apparatus was filled with nitrogen before the pressure was reduced to the desired value which was maintained with a manostat. The discharge was usually operated at 50 Torr and 75 mA in nitrogen, EA
Vol.
18 No.
11-G
a continuous
flow of gas being
AND DISCUSSION
Under the experimental conditions employed, the cathode fall of the discharge remains constant and the amount of energy transferred to the solution depends only on the amount of charge passed. The results are expressed by plotting the amount of product against the amount of charge passed. The differential yield in mol/F is given by the slopes of the graphs and these rates are denoted by G. Neutral
solutions
The results of the gde of neutral acrylamide solutions of various concentrations at 75 mA and 50 Torr are shown in Fig. 1. It was not possible to precipitate 863
A. R. DENARO AND K. 0. HOUGK
15
c
.
Fig. 2. Variation of G and relative molecular mass with concentration. O,Gvaluesand~,relativemolecularmass. 1
35
Charge
passed/Fx
lO’4
Fig. 1. Glow discharge electrolysis of neutral acrylamide solutions. A, 0.25 moI/dm3; 0, 0.50 mol/dm3; A, O-75 mol/dm3; and 0, 130 mol/dm3. weighable quantities of polymer from solutions less concentrated than O-25 mol/dm3 acrylamide nor with quantities of electricity less than l-75 x 10h4 F. The ir spectra of the polymers obtained was virtually identical with that of a sampIe of polyacrylamide prepared by the method of Sorrensen and Campbell [61. With the possible exception of the lowest concentration all the rates might be considered to be acceleratory throughout although over those portions of the graphs which have been drawn as linear the rates are vir-tually constant. Further, the relative molecular masses of the polymers produced over these regions were also constant for a particular monomer concentration. The slopes of the linear portions of the graphs have been taken as the differential yields of polymerisation. The final more noticeable acceleratory phases at the three highest concentrations are associated with a marked increase in viscosity and presumably there is autoacceleration throughout the polymerisation process. The G(polymer) values obtained from Fig. 1 together with the relative molecular masses of the polymers produced are plotted against monomer concentration in Fig. 2. It can be seen that both the differential yield and degree of polymerisation are directly proportional to monomer concentration as is usually the case with radical initiated polymerisation. The G(polymer) values in Fig. 2 are expressed in terms of moles of monomer consumed in polymerisation. If, however, they are expressed in terms of moles of polymer produced, a value of about 10e2 mol/F is obtained. Assuming that two radicals are
incorporated in a polymer molecule, the implication is that 2 x 10e2 mol/F of radicals are used in the polymerisation reaction. Previous work[2] has shown that under the same experimental conditions as used here about 10 mol/F of OH radicals are produced in gde. Thus, even without considering the action of reducing radicals which must be produced it is evident that only a very small proportion of radicals is active in initiating polymerisation. This conclusion is at first surprising as it is known [l] that concentrations of solute of between 0.1 and O-2 mol/dm3 are usually sufficient to scavenge all the OH radicals produced in gde. The determination of residual monomer after gde, however, shows that in addition to the monomer consumed in polymerisation a further amount of monomer, to the extent of about 10 mol/F at each concentration is also consumed. It seems reasonable to conclude that all the radicals in the primary reaction zone are scavenged by acrylamide but that the concentration of radicals in this region is so high that no significant polymerisation occurs and effectively radicals are adding to the double bond in the acrylamide molecule. A few acrylamide radicals may escape this process, diffuse out of the primary reaction zone into the bulk of the solution and initiate conventional polymerisation there. The gde of aqueous solutions results in the production of OH radicals, I-I radicals and solvated electrons. For the purposes of the present discussion this initial process may be represented H,O
-VYA+ 2R’
rate = R.
where R ’ represents the primary radicals formed from the water. Denoting an a&amide molecule by M, the following reactions may then occur. R’
+R’
-
RZ
(1)
R’
+M
-
RM’
(2)
865
Polymerisation by glow discharge electrolysis
RM, + M RM,
+ RM,
RM,
+ R’
-
RM;+l
(3)
-
P.+,
(4)
-
P,.
(5)
In the present case it is suggested that the concentration of acrylamide is sufficiently high to scavenge all the primary radicals so that reactions (1) and (5) may be neglected. Further, there are two regions to be considered, the primary reaction zone containing a high concentration of primary radicals and the secondary reaction zone’corresponding to the bulk of the solution. It is proposed that no polymerisation occurs in the primary reaction zone, the only reaction being (2) and (4) where n = m = 1, viz,
RM;
R’ +M
-
RMi
(2)
+ RM;
-
R2M2.
(4)
If the rate of consumption r,,, , the above leads to
of monomer
is denoted
It is known that the system generates 10 mol/F OH and also that 10 mol/F of monomer is consumed by this process. One implication is that H radicals and solvated electrons are not reacting with acrylamide molecules even though the homogeneous rate constants of these reactions are comparable with that for the reaction of OH with acrylarnide. This lack of activity of reducing radicals is a feature of gde. It is further proposed that a few (O-2%) of the RM; radicals may escape from the primary reaction zone into the secondary reaction zone, the process being represented -
RMi(2)
rate = RI .
’
the relative molecular mass of the monomer Substituting from (6) and (7)
(9) being 71.
G = 96 500 V/C~R~~‘~[M]/Z~~“*,
(10)
M = 142 kJM]/R,1’2k.+1’Z.
(ll)
Equation (10) shows that a plot of G against monomer concentration should be linear provided that the volume of the secondary reaction zone is independent of monomer concentration. This is to be expected in a chain reaction where the concentration of polymeric radicah is independent of monomer concentration. From (10) and (11) it may be seen that the product of the slopes of the two graphs in Fig. 2 is
The values of k, and k4 are known[7] and hence a value of V may be calculated. With a current of 75 mA and taking ka2/k4 as 22*l/mol. dm3/s, V = O-4 cm3. This is a reasonable value compared with the volume of the secondary reaction zone of about 3 x 10M9 cm3 calculated for the case where sulphuric acid was the radical scavenger[2]. In the polymerisation where radicals can only disappear by combination reactions they would be expected to diffuse a much greater distance into the bulk of the solution. The quotient of the slopes of the graphs in Fig. 2
gives the quantity 96,500 I/R,/142 Z
These radicals then act as initiating radicals for polymerisation in the secondary reaction zone. Polymerisation proceeds according to reactions (3) and (4), the boundary of the secondary reaction zone being defined by the points at which all polymeric radicals have disappeared by recombination. The above scheme leads to the following expressions for the rate of polymerisation R,, and the degree of polymerisation, P, R, = ksR?*[M]/k.?‘*,
(6)
p = 2k,[M]/R~“‘k,“‘,
(7)
assuming that termination occurs exclusively by combination and that homogenous kinetics apply in the secondary reaction zone. The G(polymer) values and relative molecuIar masses, M,, plotted in Fig. 2 may be expressed in terms of R., and p by multiplying by the appropriate factors. Thus G = % SOOVRJZ,
M,=71F,
96,500 x 142 V/C~~/Z,L.
r JV-Ro
RM;(l)
where V is the effective volume of the secondary reaction zone and Z is the current. Also
(8)
so that knowing V, Ri may be calculated to be 4.2 x 1o-s mol/dm3/s. This implies that 1-7 x lo-* mol/s of initiating radicals escape from the primary reaction zone. Knowing that the differential yield of OH radicals in the primary reaction zone is 10 mol/F, ie, 7.8 x 10e6 mol/s under the conditions of the experiments, it shows that only about O-2 % of radicals escape recombination in the primary reaction zone. In the above considerations, chain transfer reactions to monomer and to hydrogen peroxide have been neglected as the rate constants for these reactions are[7] 0.22 and 9/mol. dm”/s respectively compared with a value of the propagation rate constant, k3, of 1.8 x 104/mol 9dm3/s. Hydrogen peroxide was formed at a virtually constant yield of about 0.2 mol/F. This indicates that the peroxide is formed, to some extent, by a route other than the dimerisation of OH radicals as these should all be scavenged by acrylamide at the concentrations used. This effect has been noted in one other case[S].
866
Efict
A.R.
DENAROAND
of pressure
or
Increasing the pressure of thedischarge has the effect of decreasing the area of the glow spot on the surface of the solution and hence of decreasing the volumes of the reaction zones. It is known[8] that the area of the glow spot is inversely proportional to pressure and hence variation of pressure has some effect on the kinetics[2]. Neutral solutions of 0.5 mol/dm” acrylamide have been examined over a range of pressures and the results are given in Table 1. Table 1. Variation of G and M, with pressure p Torr 50 75 100 150
G(polymer) mol monomer/F
M,
8.3 7.6 5.7 4.6
45,ooo 24,300 20,900 13,200
A simplified approach may be made to this situation on the basis of homogeneous kinetics in the secondary reaction zone which, as a first approximation may be considered as a cylinder. The area of the glow spot (base of the cylindrical reaction zone) is inversely proportional to pressure so that the concentration of initiating radicals entering the secondary reaction zone will be directly proportional to the pressure. The effective length of the cylindrical reaction zone will be defined by the distance travelled into the solution by the radicals before they disappear. (Theoretically this distance should be infinite but in practice there wiI1 be a finite distance at which the vast majority of radicals will have been consumed.) The distance travelled by the radicals will depend on their lifetime which, for a second order reaction, will be inversely proportional to their initial concentration where they enter the secondary reaction zone. The length of the cylinder is thus inversely proportional to pressure in addition to the area of the base of the cylinder being inversely proportional to pressure. Under these circumstances the volume of the cylinder will be inversely proportional to the square of the pressure or, V = a/p’,
where p As a dire&y assuming ing the
K. 0. HOUGH
(12)
is the pressure and a is a constant. consequence, the rate of initiation will be proportional to the square of the pressure that the number of initiating radicals entersecondary reaction zone is constant. Thus R, = bp2,
(13)
where b is a constant. Substituting from (12) and (13) into (10) and (11) G =96,500 k,ab”2[M]/lk~1’2p
G = AiMlIp
(14)
and M, = 142 k~[M]/k‘+“%“‘p or M, =
BbW.
(13
Equations (14) and (15) show that plots of G and M, against l/p should be linear through the origin. These plots are shown in Fig. 3 from which it may be
-4 -2 I5
IO
0.5 Reciprocal
pressure
g x 3
2-o
x IO2 torr
Fig. 3. Variation of G and relative molecular mass with reciprocal pressure. 0, G values and a, relative molecular mass. seen that the variation of M, with l/p agrees well with (15). The results for the G values are not quite so satisfactory and show rather more scatter about a line through the origin. Attention may be drawn to a further point about the graphs in Figs. 2 and 3. Denoting the slope of the graph of G against monomer concentration as S, and the slope of the graph of G against reciprocal pressure as S, , (14) shows that S.&
=PJMI,
,
where pr is the pressure (50 Tori-) at which the variation of G with concentration was examined and [Ml, is the monomer concentration (0.5 mol/dm3) at which the variation of G with pressure was studied. Further, denoting the slope of M. against monomer
Polymerisation by glow discharge electrolysis concentration shows that
as ??, and against pressure as %,,, (15) -S,/S, = pJM]p .
Hence the expected value of SJS, and 3,/s, is 25 Torr mol/dm- 3, The graphs in Figs. 2 and 3 yield the values
867
solution (2.0 mol/dm3 acrylamide) the relative molecular mass of the polymer was only in the region of 16 000 as compared with 100 000 in neutral solutions of half the monomer concentration. The G values obtained from Fig. 4 appear to be proportional to the square of the monomer concentration as shown in Fig. 5.
S,/S, = 34 * 6 Torr mol/dm3 - S,/S, = 21 Torr mol/dm3. The figure for 3,/s, is in reasonable agreement with the expected value but the value for S,/S, is less satisfactory and there could be a considerable error in the slope of the graph of G against 1/p. In view of the extent ofoversimplification in the treatment, however, it is doubtful whether better agreement could be expected. Alkaline solutions The results of the & of solutions of acrylamide in 0.1 mol/dmX NaOH at 75 mA and 50 Torr for various monomer concentrations are shown in Fig. 4. ( Concentration
of ocrylamide)‘/
mol’ dmm6
Fig. 5. Variation of G in alkaline solutions
with con-
centration.
Charge passed/
F x 10e4
Fig. 4. Glow discharge electrolysis of alkaline acrylamide solutions. A, @5 mol/dm3; 0, 1.0 mol/drn’; A, 1.5 mol/dm3 and 0, 2.0 mol/dm3. In contrast
to neutral solutions the differential yields of polymerisation remained constant but were much lower in alkaline solution. Furthermore the reIative molecular masses of the polymers were much smaller, so much so in fact that it was not possible to obtain
meaningful values. Even for the most concentrated
It would be expected[3] that the rate of polymerisation would be proportional to the square of the monomer concentration if all the growing polymer chains were terminated by primary radicals as in reaction (5). This cannot be the case in the present system which involves high monomer concentrations but it is known that hydrogen peroxide is not found in the gde of alkaline solutions. If hydrogen peroxide is initially formed and then destroyed it will generate oxygen which is an efficient chain terminator. It might be expected that such oxygen would be generated in proportion to the primary radicals and this could possibly lead to the dependence of G on monomer concentration depicted in Fig. 5. Certainly, if oxygen is being generated in the system the very low relative molecular masses of the polymers produced in alkaline solution can be understood. Unfortunately, confirmation of this situation could not be obtained from the dependence of relative molecular mass on monomer concentration owing to the inaccuracy of the figures obtained with such low values. REFERENCES 1. A. Hickling, Modern Aspects of Electrochemistry (Edited by J. O’M. Bockris and B. E. Conway), Vol. 6 p. 329. Butterworths, London (1971). 2. A. R. Denaro and K. 0. Hough, Electrochim. Acta 17, 549, (1972).
868
A. R. DENARO AND K. 0. HOUGH
3. A. Chapiro, High polymers, in Radiution Chemistry of Polymeric Systems, Vol. 15, p. 132. Interscience, New York (1962). 4. U.S. patent 2,632,729 (1953). 5. E. Collinson, F. S. Dainton and G. S. McNaughton, Trans. Faraday Sac. 53,489 (1957).
6. W. R. Sorrenson and T. W. Campbell, Preparative Metlbds of Polymer Chemistry. Interscience, New York (1961). I. F. S. Dainton and M. TordofT, Trans. Faraday Sot. 53,499 (1957). 8. A. R. Denaro and A. Hickling, J. efectrochem. Sot. 105, 265 (1958).