Effect of x-rays and γ-rays on synthetic polymers in aqueous solution

JOURNAL OF POLYMER SCIENCE VOL. XXIII, PAGES 355-375 (1937)

Effect of X-Rays and 7-Rays on Synthetic Polymers in Aqueous Solution

PETER ALEXANDER, Chester Beatty Research Institute, Royal Cancer Hospital, London, SW 3, England

ARTHUR CHARLESBY, Tube Investments Research Laboratories, Hinxton Hall, Cambridge, England

INTRODUCTION

In recent years, considerable research has been devoted to the changes produced in polymers when these are irradiated in the solid state. The major effects observed can be ascribed to the formation of crosslinks or to the fracture of the main chain, depending on the particular polymer studied.' Other changes of a less striking character, such as difTerences in unsaturation and fracture of side chains, are also found.2

A separate set of investigations intended for the interpretation of radiobiological phenomena has been carried out on the irradiation of aqueous solutions of long chain polymer^.^ In this case, the changes produced in the polymer can be of a more complex character and may result either from direct action where the energy from the incident radiation is captured directly by the polymer molecule or from indirect action where the energy captured by the water molecules causes chemical changes in these molecules and the products formed subsequently react with and modify the polymer molecules. The amount of energy taken up on exposure to ionizing radiation (such as fast electrons, x-rays, or gamma rays) depends almost entirely on the mass of the irradiated material and is practically independent of its chemical composition. In irradiated polymer solutions the solute and the water therefore absorb approximately equal amounts of energy on a weight basis and the fraction of the total absorbed energy which is taken up by the polymer directly is proportional to its concentration. Thus the average energy absorbed directly per polymer molecule is independent of concentration. This is not true for the indirect effect.

Distinction between direct and indirect is important in radiobiological studies and several methods may help to dzerentiate between these two processes, although their interpretation may not be conclusive. Additives or protectors added to the solution may react with the radiation products of the water and thereby protect the polymer molecule against the indirect effect, although protection can also occur in cases in which the action has been d i r e ~ t . ~ The decisive test is the influence of concentration. Where

355

356 P. ALEXANDER AND A. CHARLESBY

the radiation effect is direct, i.e., where the energy is initially taken up by the polymer molecules, there should be no change with concentration. (There may be an increased efficiency of crosslinking at higher concentrations, as the polymer molecules are closer together.) If the effect is indirect, the higher concentration of polymer will be less affected by a given radiation dose, there being more polymer molecules competing for the activated water products. The purpose of this work was to study the effect of polymer concentration on the changes produced by radiation and hence to derive information on the mechanism of direct and indirect action.

Radiolysis of Water

In its passage through matter, ionizing radiation (e.g., fast electrons, x-rays and y-rays, fast protons) produces ionized and excited atoms and free electrons. These products may then interact with one another and with neighboring molecules to produce free radicals and initiate chemical changes. These effects have been determined in detail in only a few cases, since the mechanism is understood only in general terms and it is not possible to predict the products of radiolysis except perhaps within a closely related series of organic compounds.

By far the greatest amount of research has been done on aqueous systems and chemical changes produced on irradiating water are reasonably well understood, although many points of detail remain to be elucidated.b In the absence of oxygen, water is decomposed into OH and probably H radicals (although there is no direct evidence for the formation of the latter), hydrogen gas and hydrogen peroxide also being formed. These molecular products can interact with the radicals to re-form water ; consequently, the concentration of Hz and HzOZ produced is affected by the presence of dissolved substances which can act as scavengers for the radicals. In pure (i.e., oxygen-free) water where all the radicals are available for reaction with the molecular products, the equilibrium concentration of Hz and HzOz produced by x-rays is too low to be detected. The distribution of the radicals formed will depend on their concentration and relative rates of diffu- sion and with densely ionizing radiation such as a-particles molecular products may build up even in pure water. Hydrogen atoms react very rapidly with oxygen to give HOZ radicals, which can then react with the solute. Consequently, radiochemical reactions are frequently entirely different in aerobic than in anaerobic conditions.

EXPERIMENTAL

Preparation of Polymers

Samples of acrylic acid, methacrylic acid, and acrylamide were poly- merized in aqueous solution by irradiation with 140 kv. x-rays or by using per sulfate as a catalyst. A 10% solution of monomer was completely deoxygenated (see below) and irradiated with a dose of lo4 r. at 324 r./min. Immediately after the irradiation, air was admitted and the solution

EFFECT OF RADIATION ON SYNTHETIC POLYMERS 357

dialyzed against distilled water till free from monomer. Alternatively 0.015% persulfate and 1.5% isopropyl alcohol were added to the 10% solution, which was then deoxygenated and raised to 6OOC. for two hours. Residual monomer and catalyst were removed by dialysis. Polymeth- acrylic acid set to a gel after irradiation and this was dissolved in alcohol and then dialyzed against water.

Polystyrene sulfonate was prepared by treatment of polystyrene with a mixture of concentrated and fuming sulfuric acid. Samples of polyvinyl and of polyvinyl pyrrolidone were kindly provided by Messrs. I.C.I. and May & Baker. Analysis of the polyvinyl alcohol obtained from I.C.I. failed to reveal the presence of any acetyl groups. The molecular weight as determined by light scattering was found to be 1.29 X l o6 but increased on standing due to aggregation.'

Viscosity Determination

The polymers obtained by x-irradiation and the polystyrene sulfonate were of high molecular weight and their solutions were markedly non- Newtonian. Their viscosities were determined at 20°C. in a flatbed viscometer' a t a number of shear rates. Values reported refer to a shear rate (p) of 50 sec. a t one concentration since extrapolation to zero concentration is very difficult under these conditions? The polymers prepared with the catalyst were often of lower molecular weight and their intrinsic viscosities [7] expressed in g./lOO ml. were obtained from measurements a t a number of concentrations in an Ubbelohde viscometer a t 25OC. The changes in viscosity referred to below are expressed in percentages as:

x 100 (viscosity before irradiation) - (viscosity after irradiation)

viscosity before irradiation

Changes in the weight-average molecular weight of polymethacrylic acid after irradiation were determined by Dr. K. A. Stacey by light scattering as described earlier.8

Irradiation Technique For low doses 140 kv. x-rays fdtered by copper were employed and for

higher doses the y-rays from a 400 curie cobalt 60 source in a position where the dose rate was 1000 r./min. Solutions were freed from oxygen by pro- longed bubbling of nitrogen which had been carefully freed from oxygen by passing over red hot copper and through a solution of sodium hydrosulfite and anthraquinone sulfonate.

The yield of radiochemical reactions is expressed as the number of reactions occurring for every 100 e.v. of energy deposited in the system by the radiations. This is known as the G value; it does not imply that the 100 e.v. is used exclusively in the reaction referred to. For example, when irradiating a polymer, there will be one G value for each type of change in the main chain, and one for the formation of each of the different gases evolved.


Gel Formation

After irradiation, many polymers become crosslinked into a gel structure which is swollen in the initial solvent. The transformation from a viscous fluid to a lightly crosslinked gel can be determined by visual inspection of the irradiated specimens.

BEHAVIOR OF DILUTE SOLUTIONS

Degradation of Polymethacrylic Acid

The first system to be studied was polymethacrylic acid3 in the concentration range 0.01 to O . l % , when it could be assumed that the changes observed would be due entirely to indirect action. The effect of x-rays was to degrade the molecule ( i e . , to reduce the average molecular weight by main chain fracture). In comparative experiments the degradation could be conveniently followed by determining the decrease in viscosity after irradiation measured a t a standard concentration. The magnitude of the effect observed depends on the concentration and initial molecular weight of polymer as well as on the radiation d o ~ e s . ~ , ~

Some results for a series of polymethacrylic acid solutions irradiated a t various dilute concentrations and for varying doses are shown in Table I.

TABLE I DEGRADATION OF POLYMETHACRYLIC ACID BY X-RAYS

(Initial Mw = 2.1. X 106)

Ionization Concentration degree (a)"

0.02576.. . . . . . . . 0 . 0

0.6 0.05 o/, . . . . . . . . . . 0.10/0. . . . . . . . . . . 0.025% . . . . . . . . .

Condition

In air In Nz In air In air In air In Nz In N, In Nr

Dose

1000 1'.

2000 r. 11000 r. 1000 r. lo4 r. 105 r.

De:rease

viscosity (%)a

1 0

45 4

67 62 64 3

16 85

Denotes the degree of ionization, i.e., the fraction of -COOH groups neutralized by the addition of NaOH; at a = 0.6, the pH of the solution is 7.1.

All viscosities measured at a concentration of 0.025% polyrnethacrylic acid; a = 0.6 and j3 = 50 sec.-l; viscosity before irradiation 14.2.

The doses and concentrations are so low that only indirect action can play any part in the observed changes. In the presence of air, the radiation dose needed to produce a given decrease in viscosity (ix., the same number of main chain fractures per molecule) is directly proportional to concentration. In other words, a given radiation dose will produce the same total number of main chain fractures in the polymer molecules independent of the concentration of these molecules over the range considered. This behavior is typical of the indirect effect.

To confirm that the observed decreases in viscosity are in fact due to a

EFFECT OF HADIATION ON SYNTHETIC POLYMERS 359

c75 Y

1.5

I25

I

075

0.5

0.25

U 500 1000 1200 2000 2 500 360 RADIATION D O Y IN RIDS

Fig. 1. Molecular weight changes in irradiated polymethacrylic acid. 0.355 % solution, ionized, x-ray irradiation, molecular weight changes from light scattering. (The number of main chain fractures is expressed as the number necessary to reduce a polymer of infinite weight to one having the M, used in the reciprocal on the left axis.)

fracture of the main chain, rather than to linking between monomers in the molecule resulting in a smaller swept volume, some absolute determinations using light scattering were carried out. 20 ml. of a 0.355% solution of polymethacrylic acid (60% neutralized) was irradiated with 140 k.v. x-rays and the molecular weight determined by light scattering. This was carried out at a number of different doses and the results plotted in Figure 1. These determinations give the weight-average molecular weights ; for a random molecular weight distribution there are twice the number averages, which are inversely proportional to the number of chain fractures. Figure 1 shows that the reciprocal weight-average molecular weight, and hence the number of main chain fractures,2 increases linearly with radiation dose. The G value for main chain fracture deduced from this plot is 1.7, in good agreement with the value (1.6) obtained by Alexander and Fox3 using viscosity measurements. In the absence of oxygen, very little degradation is observed (see Table I)

except when the doses are so high that sufficient hydrogen peroxide is produced as a “molecular product” to be equivalent to the amount of oxygen dissolved in aerated solutions. The accumulation of hydrogen peroxide makes it very difficult to maintain truly oxygen-free conditions when relatively high doses have to be used, and for this reason we have preferred to study high molecular weight polymers which show signifcant changes a t low doses when the quantity of hydrogen peroxide produced can be neg- lected.


Since oxygen must be present for degradation polymethacrylic acid, we were led to the conclusion that the main chains were broken by HOz radicals and that H and OH were ineffective for this purpose. An alternative mechanism, in which oxygen is required for production on the polymer of peroxides which subsequently decompose with main chain breakdown, was rejected in the case of polymethacrylic acid.3~9 Wall and Magatlo found oxygen to be necessary for the degradation of polymethyl methacrylate and polystyrene when dissolved in chloroform and the mechanism probably differs from that in aqueous solutions since HOz radicals cannot be formed.

When, more recently, we repeated these earlier experiments with polymethacrylic acid, only small differences between deoxygenated and aerated solutions were found; degradation was observed in completely oxygen free solutions (see Table 11). This unexpected behavior was traced to a small change in the method of preparation of the polymer. After x-ray polymerization in 10% solution, the polymethacrylic acid sets to a gel; in the earlier experiments, this gel was stirred in water kept a t 70-80°C. until it dissolved, a process taking several days. In the later experiments, the gel was dissolved rapidly at room temperature in ethyl alcohol, which was removed by dialysis. We found" that during the polymerization with x-rays peroxide groups were introduced into the polymer although the solution was quite free from oxygen. Heating of the polymer destroys the peroxide groups and makes the polymer resistant to irradiation in the absence of oxygen (see Table 11). It would appear that the peroxide groups repre- sent points of weakness at which rupture of the main chain can be produced by OH or H radicals in the absence of dissolved oxygen.

TABLE I1 OXYGEN EFFECT IN DEGRADATION OF POLYMETHACRYLIC ACID BEFORE

A N D AFTER HEATING TO REMOVE PEROXIDE GROUPS

(Irradiated at 0.025% concentration, 60% ionized, and irradiated with 1000 r.)

Treatment of polymer

Decrease

Radiation viscosity condition (%)a

In

None . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Air 63 N1 40.9 Air 58 Nt 10.5 Air 54.3 N2 5 . 7

0.1% soln. heated under air at 9OOC. for 48 hrs.. . . . . . . . . . .

0.1% soln. heated in vacuo at 90°C. for 48 hrs.. . . . . . . . . . . .

As in Table I.

Degradation of Polyacrylic Acid, Polyacrylamide, Polystyrene Sul- fonates, Pol yvinyIpyrrolidone, and Polyvinyl Alcohol

Preliminary experiments3 indicated that other water-soluble vinyl polymers, such as polyacrylic acid, were degraded in dilute solutions. An inorganic polymer sodium polyphosphate :


of high molecular weight (circa lo6) in dilute solution was completely resistant to indirect action when exposed to about l o 4 r.9 yet sensitive to direct action when irradiated in the dry state.

More detailed studies of poly acrylic acid, polyacrylamide, and polystyrene sulfonate have, however, revealed marked differences between these polymers and polymethacrylic acid, especially with regard to the oxygen effect. In aerated solutions, all these polymers degrade on irradiation with x-rays but there are no striking differences in the absence of oxygen. To

TABLE I11 DEGRADATION OF POLYACRYLIC ACID BY X-RAYS

Concentration

Ionization degree (4 Condition Dose

Decrease in viscosity (%)a

0 .08%. . . . . . . . . . . 0 . 0 Air 5000 r. 38.2 0 .08 yo . . . . . . . . . . . 0 . 0 N2 18.0 0.08 yo . . . . . . . . . . . 0 . 6 Air

0.025yo . . . . . . . . . . 0 . 0 N2 1000 r. 19 .6 0.025 yo . . . . . . . . . . 0 . 0 N2 2000 r. 42 .0 0.025%. . . . . . . . . . 0 . 0 N2 5000 r. 64 .6 0.025% . . . . . . . . . . 0 . 0 Nz 10,000 r. 74.7

. . . . . . . . . . . 40.9 0.25% 0 . 0 N2 90,000 r. 0 .4% . . . . . . . . . . . . 0 . 0 N2 140,000 r. Soln. has

48.0 0.08%. . . . . . . . . . . 0 . 6 N2 1 . 5

gelled . . . . . . . . . 43 0.025%. 0 . 6 Air 1,000 r.

0 . 5 0 . 6 Air 20,000 r. 58 5.5 0 . 6 Air 270,000 r. 83

0 Viscosities measured at 0.025% polyacrylic acid; (L = 0.6 and 0 = 50 sec. qsp before irradiation ranged from 8-10 in three different polymer samples used.

95 P

0 - - I x - - n

2 -

I .

250 5ao 7 s mo RATE OF SHEAP ( S d )

Fig. 2. Non-Newtonian behavior of irradiated polyacrylic acid solution. (11) 1000 r. x-rays given 0.025% solution, 60% neutralized. (I) Before irradiation.

under nitrogen. (111) 1000 r. x-rays given under air.

362 P. ALEXANDER AND A. CIIARLESHY

TABLE IV DEGRADATION OF POLYACRYLAMIDE BY X-RAYS

Concentration Irradiation Intrinsic

dose Condition viscosity

High molecular weight 0.0625 . . . . . . . . . . . . . . . . . . . . . . 500r.

1000 r . 2000 r. 2000 r.

0.259% . . . . . . . . . . . . . . . . . . . . . 10,000 20,000 30,000 10,000

Lower molecular weight

Air Air Air Nitrogen

Air

Nitrogen

8 . 0 7 .26 6 . 0 4 . 5 6 . 0 2 . 7 2.39 2 .00 1 . 9 0 2.52

eliminate the possibility of peroxide groups introducing spurious results, all these polymers were heated in carefully deaerated solution at 100°C. for 45 hrs., a treatment which was more than sufficient to destroy these groups (see Table 11).

In the absence of oxygen, polymethacrylic acid (either in the acid or in

k

CONCENTRATION (s/ 100 d) Fig. 3. Degradation of polyacrylamide. 0.0625 solution irradiated under air by

x-rays. ( X ) = 2000 r. ( 0 ) = 1000 r . (A) = 500 r . (0) = 0 r.

ISFFECT OF RADIATION ON SYNTHETIC POLYMERS 363

the ionized form) is not degraded by radiation; in aerated solutions the acid form is somewhat more resistant to radiation. Likewise ionized polyacrylic acid does not degrade in the absence of oxygen. Unionized polyacrylic acid, however, does degrade a t nearly the same rate as in air (Table 111, Fig. 2).

Polyacrylamide behaves like polyacrylic acid in unionized form-it degrades significantly in the absence of oxygen although less than in air. The results for two samples of different molecular weight are shown in Table IV. The higher molecular weight samples show significant changes in viscosity a t much lower radiation doses (see Fig. 3): This dzerence is not due to any inherent change in sensitivity but arises from the fact that the same number of main chain fractures in a given weight of polymer produces a greater relative change in molecular weight in a high molecular weight material.

Polystyrene sulfonate shows no oxygen effect a t all being equally readily degraded in aerated solution or under nitrogen (see Table V).

TABLE V DEGRADATION OF POLYSTYRENE SULFONATE BY X-RAYS

Concentration Condition Dose Viscosity

decrease. (%)

0.015% . . . . . . . . . . . . . . Air 1000 r . 44.7 N1 43.6 Air 200 r . 57.3 N2 54.8

Measured at 0.015% concentration in Ubbelohde viscometer. Specific viscosity be- In the absence of added salt, no extrapolation to c = 0 is possible fore irradiation 1.15.

because of the polyelectrolyte nature of the solution.

The position with regard to polyvinyl alcohol and polyvinylp yrrolidoiie is less clear. When irradiated in 0.1% solution, the viscosity of both polymers is decreased more in the presence than in the absence of oxygen. The molecular weights of the polymers studied were however low (light scattering gave 1.3 X lo5 for the sample of polyvinyl alcohol used.6 To produce measurable changes in each molecule, correspondingly high doses of radiation were necessary which introduced the complication of hydrogen peroxide formation. In any case, with these polymers, it is difficult to relate viscosity to molecular weight since they aggregate in solution.6

These experiments show that all the vinyl polymers examined when irradiated in dilute solution suffer main chain fracture and consequent degradation; the breaking of the C-C bond must be the result of reaction with free radicals formed in the water. A common mechanism does not, however, appear t o be operative since the influence of oxygen is difl'erent in Meren t polymers. A possible interpretation is that reaction with OH radicals can break main chains in only some of the polymers examined and that the others can be fractured by NOz radicals.

364 P. ALEXANDER AND A. CHARCESBY

CROSSLINKING IN SOLUTION

With the exception of polymethacrylic acid, all the water-soluble polymers examined (which degrade in dilute solution) crosslink when irradiated as solids (Le., when the action of the radiation is direct). It therefore became of interest to examine their behavior in more concentrated solutions where direct action can produce a significant contribution to the over-all effect. Unexpectedly we found that the following polymers when irradiated at concentration as low as 1% (when direct action cannot play an important role) were not degraded by irradiation but became crosslinked : polyvinyl alcohol, polyvinylpyrrolidone, polyacrylamide, polyacrylic acid (unionized). However, polyacrylic acid, when partially neutralized with sodium hy- droxide, and therefore carrying a charge, degrades a t all concentrations; a degree of ionization of 0.1 is already sufficient to inhibit crosslinking. Polymethacrylic acid both ionized and unionized degrades a t all concentrations; with solutions containing as much as 10% of polymer the degradation appears to be almost entirely due to indirect action since the amount of change for a given dose is inversely proportional to concentration as a t lower concentrations (see section on Behavior of Dilute Solutions).

TABLE VI CHANGES IN VISCOSITY OF POLYVINYL ALCOHOL AFTER IRRADIATION BY

Concentration

Sample 1: -. . . . . . . . . . . . . .

7% . . . . . . . . . . . . . 7% . . . . . . . . . . . . . 7% . . . . . . . . . . . . .

0 . 3 % . . . . . . . . . . . 0 . 3 % . . . . . . . . . . .

Sample 2: - 0.3%. . . . . . . . . . . 1% . . . . . . . . . . . . . 1%. . . . . . . . . . . . . 2%. . . . . . . . . . . . . 4%. . . . . . . . . . . . . 0 . 3 '% . . . . . . . . . . .

0.5% . . . . . . . . . . . 0.5% . . . . . . . . . . . 0.5% . . . . . . . . . . . 0.5% . . . . . . . . . . .

Sample 3: -

2% . . . . . . . . . . . . .

Y-RAYS

Dose

- 1 . 4 X 106r. 1 . 4 X 106r. 2 . 7 X 106r. 4 . 1 X 106r. 4 . 3 X 106r.

- 1 . 5 X 106r. 0.84 X lo6 r. 0 .92 X lo6 r. 0 . 9 0 X 106r. 1 . 2 X 106r. 1 . 5 X 106r.

0 . 9 X 10".

1 . 2 X 106r. 1 . 5 X 106r. 0 . 9 X 106r.

-

"

Condition

- Nz Air Air Air Air

- N2 Air Air Air Air Air

Air Nz Air Air Air

-

Intrinsic viscosity

0 .55 0.59 0.28 0 .57 1.18 Sets to

gel 0 .67 0 .70 2.84

1.69 1.30 0.40 0.76 0.76 1.42 0.77 0.85 1.15

gel

The occurrence of crosslinking may be recognized in the polymer solution by the fact that the viscosity does not fall but increases slightly (see Table VI) until at a well-defined dose the solution sets to a swollen gel which swells


further to very large volumes when placed into excess water after irradiation. As the irradiation is increased, so the swelling of the gel becomes progressively less until, a t a dose depending on the polymer and its original concentration, the gel pulls away from the walls of the vessel and floats freely in a solution which is essentially free from polymer (see Fig. 4).

Fig. 4. Gel formation in irradiated polyvinyl alcohol. (A) Unirradi- ated. (B) Gelling radiation dose. (C) Highly crosslinked gel shrinking away from walls.

So far we have confined our attention largely to the dose required to produce a gel and have not attempted to interpret the subsequent swelling behavior of the gels formed with higher doses. The general pattern is

WKIN r

Fig. 5. Increase in crosslinking with concentration and Polyvinylpyrrolidone, 7-radiation a t 1000 r./ radiation dose.

min.

shown in Figures 5 in which crosslinking is expressed as l/swelling. The rate a t which crosslinking proceeds depends on the initial concentration and except in very dilute solutions (i.e., near the changeover point from degradation to crosslinking) increases with increasing concentration.

366 P. ALEXANDER AND A. C€IARLESBY

01 0 5 10 15 20

PERCEWCE CONCEFsmATlON OF W P

Fig. 6. Radiation dose for gel formation in polyvinylpyrrolidone solutions. y-Radiation a t 1000 r./min.

A most remarkable feature of the crosslinking reaction in the very sharp changeover from degradation to efficient crosslinking. This is illustrated for polyvinylpyrrolidone in Figure 6 ; degradation occurs a t a polymer concentration of o.3yO while in a 1% solution the dose required to produce a gel has reached its minimum value. With polyacrylic acid of molecular weight 2.2 x lo5 the changeover from degradation to crosslinking occurs a t 0.4% concentration and the dose required to produce a gel is a t a minimum a t a concentration of approximately 1%. This system is now being studied in detail. We have only obtained preliminary results with poly-

TABLE VII CROSSLINKING OF POLYACRYLAMIDE~

Concentration Dose

1.08% . . . . . . . . . . . . . . . . . . 0 . 6 X 106r.

2.3% . . . . . . . . . . . . . . . . . . . 0.26 X 106r. 6.7% . . . . . . . . . . . . . . . . . . . 1 . 5 X 106r. 15%. . . . . . . . . . . . . . . . . . . . 0.8 X 106r. 30.4 % . . . . . . . . . . . . . . . . . . 1 . 0 X 106r.

Intrinsic viscosity of starting material is 2.7. Weight of water taken up by polymer/weight of polymer.

1.27 X 106r.

Swelling of gelb

72.5 62 171 38.5 38.0 42


acrylamide (see Table VII) but these are suflicient to show that crosslinking in solution follows the same general pattern.

MECHANISM OF CROSSLINKING

Any theory proposed for the behavior of polymers irradiated in solution must account for the following main observations, applicable to a number of polymers :

(1) Degradation a t low concentrations. (2) Crosslinking at higher concentrations. (3)

( 4 )

The following suggestions will be considered below : (1)

(2)

The sharp change from one reaction to the other in a very limited

The increase in the radiation dose for gelling with increasing con- range of concentration (about 0.4-0.6%).

centration above 1%.

Degradation is due to indirect action via water molecules, crosslinking results from direct action.

Internal linkages are formed between monomers in the same polymer molecule. The formation of such internal linkages competes with the formation of external linkages between separate molecules.

An alternative mechanism of gel formation relaying on main chain scission to produce a gel. [This last value was calculated as follows : the dose necessary to produce a detect- able amount of insoluble material on irradiating the dry polyacrylic acid (weight-average molecular weight 2.2 X lo5) used in these experiments is 2.3 X lo6 r. and this dose deposits 0.14 x loz1 e.v. in 1 g. of solid polymer. For gel formation to occur a minimum of one crosslinked monomer unit per weight average molecule is required on the average, so that the energy absorbed per crosslinked unit is 0.14 X 1021 X 2.2 X l o5 X 1.66 X or 52 e.v. There are two crosslinked units per crosslink so that a total energy of 104 e.v. is absorbed per crosslink formed in dry polyacrylic acid (G per crosslink = l).]

( 4 ) Unstable centers are formed in polymer molecules which can either lead to crosslinking or to degradation depending on the molecular environ- ment.

The hypothesis that degradation is due to indirect action and crosslinking due to direct action does not fit the observed facts for the following reasons :

Since the changeover occurs a t approximately 0.5% concentration direct action leading to crosslinking would have to be 200 times as effective as indirect leading to degradation. We have determined the ratio ex- perimentally and find that the G value for degradation of polyacrylic acid in extremely dilute solution is about 1 while that for crosslinking by irradiation of the dry polymer is likewise 1.

On this basis, the lowest concentration a t which crosslinking could occur would be about 50% of polymer but this would almost certainly be an under-

(3) This has been considered theoretically.12

(1) Direct uersus Indirect Action.

(1)


estimate since crosslinking in solution would be expected to be less efficient than in the solid. This is one hundred times greater than the observed concentration.

The proportion of polymer molecules activated directly by radiation is independent of concentration while linkages between activated polymer molecules are formed more readily a t the higher concentrations, when they are closer together. The radiation dose for gelling by direct action should decrease with increasing concentration, or, a t best, be constant. The observed results contradict this, since the radiation dose required for gelation increases with concentration once the minimum (at about 1%) has been passed.

We must therefore conclude that the free radicals produced in water can result in polymer degradation in dilute solution and crosslinking in more concentrated solution.

As a polymer solution is di- luted, the concentration of monomer units round a given unit does not tend to zero since other monomer units attached to the same molecule cannot be increasingly separated from it. The concentration of such monomer units is equivalent to a minimum concentration ci and, a t very low concentrations, c1 depends mostly on the degree of ionization and to a small extent on the molecular weight. At higher concentrations, ci will vary but slowly with the over-all polymer concentration c. Links formed by an activated monomer unit will therefore be shared as between internal and external links in the ratio ci:c.

For a radiation dose r, the energy absorbed per gram of solution is 0.6 X 1014 r. e.v. If e is the average energy absorbed directly or indirectly per activated monomer unit, then 0.6 x 1014 r / e activated units will be created, each capable of forming a crosslink (either inter- or intramolecular) with other monomer units. The number of intermolecular crosslinks, i.e., between separate polymer molecules (which can alone result in gel formation) is therefore 0.6 X 1014 ( r / e ) ( c / c + ci).

If the solution is so dilute that no interpenetration of polymer molecules is present, only internal linkages are possible, and with increasing radiation dose each polymer molecule will become more and more tightly linked within itself, thereby further reducing its chances of meeting a neighboring polymer molecule and forming a crosslink or eventually a gel. This picture would explain the transition from gel formation to apparent degradation (as shown by the reduction in viscosity) as the concentration is decreased. Ob- jections to this explanation are that: (a) the average molecular weight of the few polymers studied by light scattering does in fact decrease in dilute solutions, (b) the transition is sharp even in polymers of a very diffuse molecular weight distribution, and. (c) the value of the internal concentration c1 for most polymers lies well below the required transition value of

(2)

(2) Internal and External Linkages.

0.5%.


(3) Endlinking. It has generally been accepted that radiation induced linking between molecules and consequent gel formation arises from the fracture of bonds in the side chains, whereas degradation arises from fracture of main chain bonds. An alternative theory termed endlinking has however been investigated theoretically. l4 With this theory, it was shown

A

2

0

ac

Fig. 7. Theoretical relation for gelling dose at various concentrations.

that the formation of a gel structure could be equally accounted for by main chain fracture, the assumption being made that when a polymer molecule is fractured, the two fragments can attack and link themselves on to neighboring inert polymer molecules. At first, this induces branching and eventually a three-dimensional network may be formed. For these polymers whose fractured ends can become stabilized by a molecular rearrangement, such “endlinking” will occur less frequently and degradation may ensue. This theory therefore implies that the primary radiation act


may be similar for a range of polymers, but the subsequent effects (crosslinking or degradation) depend on the stability of the ensuring radicals.

Although this mathematical theory of endlinking fits the observed facts as well as the conventional crosslinking theory, no experimental evidence for it has been obtained. The equations derived have been applied by us13 to account for the concentration effect in polymer solutions. It is assumed that the polymer molecules irradiated in solution suffer fracture of the main chain. The two ends formed are unstable and eventually a molecular rearrangement occurs near the ends in order to stabilize them. This takes a certain time, during which either or both of these active ends may link themselves to other polymer molecules if these exist in the vicinity. The observed effects of main chain fracture will therefore depend on polymer concentration. If this is low, most main chain fractures result in degradation and reduced molecular weight. If it is high, branching and eventually gel formation will occur. It can be shown the~retically'~ that, for an initial random molecular weight distribution, gel formation by endlinking first occurs when there is an average of radiation induced main chain fracture per three molecules, the two fractured ends subsequently linking themselves to adjacent molecules (in other words, there must be a t least two active ends per six inactive ones for gel formation). In the case of polymer solutions the number of radiation-induced fractures will be 0.6 X 1014 r /e (see section (2 ) above) and of these a fraction a will result in endlinking. The parameter a will vary with polymer concentration in an unknown but smooth manner. The number of degrading fractures will be (1 - a) X 0.6 X 1014 r / e and these fractures will give an increased number of separate molecules to be linked together. Then for gel formation, the following equation must be satisfied:

0.6 a X l O I 4 r /e 2 l / 3 ( ( 1 - a) 0.6 X 1014 r / e + c N / M , )

where c N / M , is the number of polymer molecules initially present in 1 g. of solution. The minimum gelling radiation dose :

cNe 0.6 X 1014 rgez = - M , (4a - 1)

This formula gives the required sudden change from degradation (when a < 0.25) to gel formation (when a > 0.25). With a feasible variation of a with concentration, the observed effects can be e~p1ained.I~ The main objection to this theory arises on chemical grounds, as it is not easy to explain the chemical processes involved which would produce endlinking without a continued radical formation. No direct evidence for or against this theory is available.

(4) Simultaneous Crosslinking and Degradation. This theory assumes that a polymer molecule activated either directly, or via radicals

EFFECT 01' RADIATION ON SYNTHETIC POLYMERS 371

formed in the solvent, remains in this state for a time during which i t can link itself to a neighboring polymer molecule. If, during this time, no adjacent polymer molecule presents itself in a suitable orientation for crosslinking, the activated polymer molecule suffers a molecular rearrangement to stabilize itself in the course of which it s d e r s main chain fracture and degrades. The following simplifying assumptions will be made for mathematical simplification.

(1) No account is taken of internal linkages, i.e., linkages between monomer units in the same molecule.

Fig. 8. Gelling curve for polyvinylpyrrolidone: experimental; - theoretical curve a = 55.

( I )

(2) The energy required to activate the polymer molecule is the same, whether the effect be direct or indirect. Where this is not the case, a simple correction can be readily applied.

The initial molecular weight distribution is of the random (ex- ponential) type. The nonvalidity of this assumption would only alter the parameters defining the gelling curve to a minor extent. None of the main conclusions would be dec ted .

The probability of an activated monomer unit becoming linked to a neighboring unit depends on the polymer concentration, but cannot exceed unity. Fow low concentrations, when mutual exclusion effects are negli- ble, this probability is proportional to the polymer concentration c. The constant of proportionality, denoted by a, is related to the time available for an activated unit before i t rearranges itself into a stable degraded product. a is independent of concentration and of the molecular weight of the polymer. Its reciprocal represents the minimum polymer concentration which, but for exclusion and statistical fluctuations, would ensure that all activated units result in crosslinks. At very low concentrations, the ratio ac/(Z - ac) represents the number of crosslinked/degrading units. At higher concentrations, a more complicated expression obtains because of exclusion effects. This more general expression leads to the following equation for the gelling dose (derivation to be published)

(3)


Ne ac aM, 4 - 5 exp { -ac) = -

where N is Avogadro’s number, e is the energy absorbed per activated monomer unit, and M , is the initial number-average molecular weight. The relationship between r , and the concentration c is shown in Figure 7. No gel formation occurs a t concentrations such that ac < 0.223 and the minimum radiation dose for gel formation occurs at concentration some three times greater when rue2 = 0.46 Ne/aM,. The upward sweep of the gelling dose at higher concentrations arises from the absence of degradation at these concentrations and the increasingly higher number of polymer molecules available to absorb the same amount of radiation energy. Such an upward sweep is in fact observed, although it is not always so steep. A lower slope is obtained if less energy is required to activate a monomer unit by direct rather than by indirect action.

For many polymers, gel formation only occurs a t concentrations exceeding 0.4% so that ac = 0.223 at c = 4 x The minimum gelling dose is found at concentrations of about 1-2’%; the theory predicts a minimum at ac - 0.8 so that once again a -50. Theory gives a minimum value for the gelling dose of:

The only parameters in the theory are a and e.

and a is about 50.

(0.46Ne/aMn) e.v./g.

about 9 X lo7 e /M, roentgens. For polyacrylic acid, the minimum gelling doses lead to a value for e of 100 e.v. This compares well with the value for direct action, deduced above for dry polymer of 104 e.v.

Figure 8 shows the experimental gelling doses for polyvinylpyrrolidone, assuming a = 55. Good agreement with the theoretical curve is obtained if Mn/e-lO3 (which is the correct order of magnitude since, if e-100 e.v., Mn-106). In particular, the theoretical curve reproduces the sharp minimum and subsequent slow rise with increasing concentration. If energy absorbed directly on the polymer molecule is more effective than the indirect effect, the general shape of the curve is almost undected but the slope at higher concentrations is reduced.

Apart from mechanism (1) there are no decisive experiments to distinguish between the other three mechanisms described. The fact that polymers which crosslink at higher concentrations degrade at very low concentrations (see, for example, polyacrylic acid and polyacrylamide, Tables I11 and IV) is not decisive evidence against mechanism (2) since the existence of an oxygen effect greatly complicates the situation. One would expect that the influence of oxygen would be most marked in low polymer concentrations where it might be responsible for converting a labile point into a main chain break. At higher concentrations interaction with another molecule would be more likely so that the chance for oxygen to intervene might be less. The basic problem which remains to be solved is what are the chemical reactions which bring about the crosslinking?

(5 ) Conclusions.


Additives

The effect of radiation on polymer solution, as measured by the tend- ency to crosslink or degrade, can be profoundly affected by the presence of small amounts of certain additives, which may either reduce the radiation effect or shift its emphasis. Oxygen is one such additive, but a number of others such as thiourea and P-mercaptan have been studied. l3 In solutions such additives may act in one of several ways. They may react with the activated polymer molecule; if the compound formed is a stable one, then the additive has blocked the reactive point and prevented both crosslinking and degradation (ie., protection will have occurred). It is also possible that the additive combines with the active center to give a structure which can still degrade but which will no longer crosslink. For example, the effect whereby oxygen decreases crosslinking but promotes degradation could arise as follows : .

polymer radiation active center peroxide which which can can no longer degrade or crosslink but crosslink may stilidegrade

A detailed study of the effects produced by a number of additives will be published later.

This work has been supported by grants to the Chester Beatty Research Institute from the British Empire Cancer Campaign, the Jane Collin Childs Memorial Fund for Medical Research, the Anna Fuller Fund, and the National Cancer Institute of the National In- stitution of Health, U. S. Public Health Service. One of the authors (A. Charlesby) wishes to thank the Chairman of Tube Investments Ltd. for permission to publish this paper.

References

1. Charlesby, A., Proc. Roy. Soc. London, 1952, 215, 187; Nature, 1953, 171, 167.

2. Alexander, P., Charlesby, A., and Ross, M., Proc. Roy. Soc. London, 1954, A223, Dole,

3. Alexander, P., and Fox, M., Nature, 1952, 169, 572; Trans. Faraday Soc., 1954,

4. Alexander, P., and Charlesby, A., Nature, 1954, 173, 578. Alexander, P., Alexander, P.,

5. See, for example, Bacq, S. M., and Alexander, P., Fundamentals of Radiobiology,

6. Stacey, K. A., and Alexander, P., International Symposium on Macromolecular Distrib-

Lawton, E. J., Balwit, J. S., and Bueche, A. M., ibid., 1953,172,76.

392. M., Keeling, C. D., and Rosa, D. G., J. Am. Chem. Soc., 76,4304 (1954).

50, 605.

Charlesby, A., and Ross, M., Proc. Roy. Soc. London, 1954, A223, 392. and Charlesby, A., ibid., 1955, A230, 136.

Butterworth, London, 1955.

Chemistry, Milan-Torino, 1954. uted in the U. S. by Interscience, N. Y.

Alexander, P., Charlesby, A., and Black, R. M., ibid., 1955, A232, 31.

Published in Ricercia sci., 25, 889 (1955).

7. Alexander, P., and Hitch, S. F., Arch. Biochem 4 Biophys., 1952, 9, 229. 8. Alexander P., and Stacey, K. A., Trans. Faraday SOC., 1955,51,299. 9. Alexander, P., and Fox, M., J. chim. phys., 1953, 50, 415.

10. Magat, M., and Wall, L. A., ibid., 1953, 50, 308.


11. Alexander, P., and Fox, M., ibid., 1955, 52, 711. 12. Charlesby, A., Proc. Roy. SOC. London, 1954, A222, 542. 13. Charlesby, A., and Alexander, P., J . chim. phys., 1955, 52, 718. 14. Charlesby, A., Proc. Roy. Soc. London, 1955, A231, 521.

Synopsis

A study has been made of the effect of ionizing radiation (x- and y-rays) on aqueous solution of the following vinyl polymers: polymethacrylic acid, polyacrylic acid, polyacrylamide, polystyrene sulfonate, polyvinylpyrrolidone, and polyvinyl alcohol. In dilute solutions (<0.30/, for the samples of polymer used) all the polymers are degraded by main chain fracture although in some cases dissolved oxygen has to be present. In others, degradation occurs in carefully deoxygenated solution but is increased by the presence of oxygen. The breakdown of polystyrene sulfonate is independent of oxygen. At higher concentration, crosslinking becomes the dominant action for all the polymers studied with the exception of polymethacrylic acid and ionized polyacrylic acid, which degrade at all concentrations. The dose of radiation necessary t o produce a gel has been studied as a function of concentration and passes through a minimum. No gel formation occurs below a minimum polymer concentration of about 0.4%. The results cannot be explained on the basis that indirect action by the free radicals formed in the water leads to degradation while direct energy uptake by the polymer itself produces crosslinking. The manner by which added substances reduce the effectiveness of irradiation ( i .e . , protect) is discussed briefly.

Other possible mechanisms are examined.

R6sum6

Un etude B 6th faite sur l’effet des radiations ionisante (rayons-X et y) sur des solutions aqueuses des polymkres vinyliques suivants: l’acide polymCthacrylique, I’acide polyacrylique, la polyacrylamide, le sulfonate de polystyrhe, la polyvinylpyrrolidone I’alcool polyvinylique. En solution dilukes (infkrieures 1 0.3% des Cchantillons de polymBre utilisk) tous les polymbres sont dCgradCs par suite de la rupture de la chaine, principale, bien que dans certains cas l’oxyghe dissous semble 6tre indispensable. Dans d’autres, la dCgradation se passe en solution soigneusement dCsoxyg&Ce, mais elle s’accroit en prksence d’oxygike. La dCgradation du polystyr6ne sulfonC est indCpend- nat de I’oxygbne. Aux concentrations plus elevkes le pontage intermolkculaire devient prCpondkrant pour tous les polymbres CtudiCs sauf pour l’acide polymkthacrylique et pour I’acide polyacrylique ionisk qui dCgrade B toutes les concentrations. La dose de radiations nCcessaire 1 produite un gel a Ctk CtudiCe en fonction de la concentration; elle passe par un minimum. I1 n’y a pas de formation de gel en dessous d’une concentration en polymhre minimum de I’ordre de 0.4% On ne peut expliquer ces rCsultats en admettant une action indirecte des radicaux libres form& dans l’eau qui entrainerait une dkgradation, car I’absorption directe d’Cnergie de la part du polymbre lui-m6me en- traine le pontage. D’autres rnCcanismes possibles ont 6 th examinks. Le mkcanisme sui- vant lequel certaines substances ajoutCes reduisent I’efficacitC de ]’irradiation (par ex- ernple en les protkgCant) est brievement discutC.

Zusammenfassung

Es wurde eine Untersuchung der Wirkung ionisierender Strahlung (Rontgen- und 7-Strahlen) auf wassrige Lisungen der folgenden Vinylpolymere ausgefiihrt : Poly- methacrylslure, Polyacrylsiure, Polyacrylamid, Polystyrolsulfonat, Polyvinylpyrroli- don, Polyvinylakohol. In verdiinnten Losungen (<0,3% fur die verwendeten Poly- merproben) werden alle Polymere durch Hauptkettenbruch abgebaut, trotzdem in einigen Fallen aufgeliister Sauerstoff gegenwartig sein muss. In anderen tritt Abbau in sorgfaltig desoxygenierter Liisung auf, wird aber durch Gegenwart von Sauerstoff er- hoht. Der Abbau von Polystyrolsulfonat ist von Sauerstoff unabhangig. Bei hijheren


Konzentrationen wird Querbindung der vorherrschende Vorgang fur alle untersuchten Polymere mit Ausnahme von Polymethacrylsaure und ionisierter Polyacrylsaure, die bei allen Konzentrationen abgebaut werden. Die Bestrahlungsdosierung, die nijtig ist, um ein Gel zu bilden, wurde als Funktion der Konzentration untersucht und geht durch ein Minimum. Unterhalb einer minimalen Polymerkonzentration von ungefihr 0,4% tritt keine Gelbildung auf. Diese Tatsachen kijnnen nicht auf der Grundlage erklirt werden, dass indirekte Aktion durch die in Wasser gebildeten Freiradikale zum Abbau fiihrt, warend direkte Energieaufnahme durch das Polymer selbst Querbindung hervor- ruft. Andere mijgliche Keaktionsvorgange werden untersucht. Der Vorgang, durch welchen Zugabe von Substanzen die Wirksamkeit der Bestrahlung reduziert (d.h. beschutzt) wird kurz diskutiert.

Discussion

M. Magat (Paris): I1 parait difficile de dormer une interpretation antibrement satisfaisante des remarquables resultats de M. M. Alexander et Charlesby sur la reticulation de polymbres en solutions diluees, & moins qu’il ne s’agisse dans lous les cas de solutions micellaires. La concentration & partir de laquelle la reticulation devient possible correspondrait alors & la concentration d’apparition de micelles. Je voudrois savoir si Dr. Charlesby a envisage cette hypothese et sila structure exacte (molbculaire ou micel- laire) des solutions examinees est connue.

A. Charlesby: We first discussed crosslinking in two polymers, polyvinyl alcohol and polyvinylpyrrolidone, both of which gave evidence for aggregation in solution. This, we felt, might complicate interpretation and for this reason, all our recent work has been done with polyacrylic acid, which does not aggregate but which shows the same crosslinking behavior in solution as polyvinyl alcohol and polyvinylpyrrolidone. We feel confident, therefore, that aggregation of large micelles is not the cause of the crosslinking effect described. We have studied the behavior of polyvinyl alcohol in solution (P. Alexander and K. A. Stacey, International Symposium on Macromolecular Chemistry, Milan-Torino, 1954. Published in Ricercia xi., 25, 889 (1955). Distributed in the US. by Interscience, N. Y.) and have found it to be aggregated into large micelles at all concentrations. There is no evidence for a critical micelle point as suggested by Dr. Magat. Furthermore, a lightly irradiated polyvinyl alcohol solution continues to crosslink a t the same speed if the concentration of the solution is subsequently modified.

Le dernier mechanisme propose par M. Charlesby, a savoir une interaction entre une chaine de polymCre excitCe avec une chaine nonexcite, ne peut se justifier physiquement car on ne voit pas comment un tel processus pourrait conduire 1 un pont intermoleculaire. En effet, pour obtenir un tel pont il faut necessairement qu’il apparaisse une valence libre sur chacune des deux macromolecules et que ces deux macroradicaux se recombinent ensuite. Ces macroradicaux peuvent btre formbe soit par action “directe”, soit par transfert d’un hydrogbne du polymcre sur un radical libre forme a partir de I’eau.

En supposant que chaque pont exige deux radicaux libres formee independament, et que chacun de ces radicaux est instable, on trouverait un rapport de reticulation B degradation qui dependrait de I’intensitC du rayonnement. Or les resultats obtenus B ce jour semblent indiquer le contraire.

Light scattering and ultracentrifugation experiments show that polyvinyl alcohol is partially aggregated even at low concentration. This fact could perhaps explain some of the results obtained by Dr. Charlesby.

A. Charlesby: The aggregation of polyvinyl alcohol is mentioned in the paper, and we are now working with other polymers such as polyacrylic acid, which show the same general effect. In any case, protection effects due to thiourea are readily shown by polyvinyl alcohol.

A. Chapiro (Paris):

A. Charlesby: Je suis tout a fait d’accord avec les remarques de M. Chapiro.

V. Desreux (Liege):

Effect of x-rays and γ-rays on synthetic polymers in aqueous solution

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