I
JOHN J. COLLINS‘,
FRANK R. LITTERIO, and RICHARD L. MARKUS
Medical Research Department, White Laboratories, Inc., Kenilworth, N. J.
Degradation of Hydrophilic Cross-linked Resins Empirical Determination of O r d e r of Stability o f Sulfonated Styrene C o p o l y m e r s
b Stability of the cross-linking agent, its concentration in the copolymer, and particle size of the resin (mesh range) are directly related to the stability of the resin. b
Concentration of the catalyst i s inversely related to stability of the resin. Swelling of the resin in a specific swellant i s inversely related to its stability in that medium. ComLmmwzATioiv of styrene ot any other homopolymerizing monomer with a suitable diunsaturated cross-linking agent results in folmation of cross-linked polymers or resins. If the cross-linking agent-commercial divinylbenzene, for instance-is included in the monomeric mixture a t concentrations lower than 170, a marginal class of three-dimensional insoluble copolymers is obtained. These copolymers show highly swelling properties in suitable solvents, so-called swellants. This marginal class of loosely cross-linked resins, having definite and limited swelling properties, is clearly distinguishable from the soluble, nonPresent address, Armour Laboratories, Inc., Kankakee, Ill.
cross-linked, chain-type (linear or branched) polymers. Thus, the swellable loosely cross-linked resin network possesses an intermediate structure between the noncross-linked (chain) polymers and the densely cross-linked resins. Cross-linked copolymers of styrene and divinylbenzene, being of hydrophobic nature, may have a sulfonic acid group introduced into each aromatic nucleus of the polymer network by a conventional sulfonation procedure. By this procedure, the hydrophobic cross. linked polystyrene is transformed into hydrophilic sulfonated cross-linked polystyrene resin. Historically, this marginal class of loosely cross-linked resins has its origin in the work of Staudinger in the early 1930’s. When Staudinger polymerized styrene a t 100’ C. in the accidental presence of less than one tenth of divinylbenzene as a contaminant, he qbtained a considerable proportion of insoluble but highly swellable resin fraction in the resultant polymer mixture. These hydrophobic swellable fractions were evidently not of the soluble chain type, but loosely cross-linked in character, although they swelled in benzene, toluene, or monomeric styrene (9-72). Swelling may be defined as the maximum swollen volume in cubic centimeters attained by 1 gram of dry resin in an excess of an appropriate swellant a t the point of equilibrium. Effect of Concentration of Crosslinking Agent. Figure 1 shows what may
be expected if a large number of loosely
cross-linked polystyrene samples are prepared, with decreasing concentrations of the cross-linking agent. The solid curve is a succession of a large number of points, each representing the over-all swelling of one individual copolymer batch, prepared in the presence of one particular concentration of crosslinking agent. The dotted prolongation of the solid curve to the left indicates the area where noncross-linked soluble chain formation is to be expected. The dotted curves on the left illustrate the possible standard distribution of swelling in each individual copolymer. They are “tall and slender” or “flat,” depending on the efficiency of the method of copolymerization (whether the copolymers were prepared by the bulk, solution, or suspension copolymerization method). In the neighborhood of the extremely low concentration area of the cross-linking agent some fractions of the copolymer will consist of soluble chains. The method of copolymerization and the shape of the distribution curve of the resulting copolymer will cause different proportions of the polymer to consist of soluble chains; the flatter the distribution curve, the larger the proportion of the soluble fractions a t the same nominal concentration of cross-linking agent. Staudinger observed that when attempts were made to use less than 0.03Y0 of cross-linking agent in the copolymerization process, the resultant polymer was no longer truly cross-linked, but rather was soluble, noncross-linked VOL. 49, NO. 11
NOVEMBER 1957
1843
(linear or branched chain) in its structure. I n copolymerizations carried out with a concentration of 0.04y0 divinylbenzene (DVB), the process will still yield some noncross-linked chain polymers, depending on the efficiency of the method used. This situation is indicated by the assumed standard distribution curves in Figure 1. If the selected concentration of divinylbenzene was O.O2Yc or lower, the resulting copolymer mixture no longer contained any cross-linked resins, but consisted of soluble polymers of high molecular weight. After careful determination of the viscosity of these extremely loosely crosslinked polymers. Staudinger came to the conclusion that a t this extremely low range of divinylbenzene concentration, the linear polymer chains double. Apparently, the low concentration of the available divinylbenzene is no longer sufficient to form three-dimensional crosslinked netw0rIf.s. This phenomenon is illustrated on the extreme left-hand side of Figure 1. Since the early work of Staudinger, the theory and statistical treatment of cross-link formation and gelation have been worked out extensively ( 4 ) . Where the weight average distribution of the polymer chains in the corresponding homopolymers has been determined, the calculations of Flory may be of practical value. The calculated and round values usually agree where the concentration of the cross-linking agent is low (73). Unfortunately, calculations are complicated by environmental factors such as concentration during solution copolymerization or suspension copolymerization. At high conversions, however, the calculated values are of little use (75). After a number of samples of this interesting class of loosely cross-linked polymers, had been prepared they were sulfonated in a conventional manner and the ion exchange groups were converted into the potassium cycle. By using concentrated hydrochloric acid and a chlorinated hydrocarbon as a swellant (like perchloroethylene) at 90' C., the extent of the sulfone cross-link formation could be kept negligibly low. Influence of Sulfonation. Figure 2 shows the influence of sulfonation of the original hydrophobic resin on the extent of swelling in the appropriate swellant medium. The presence of the highly polar hydrophilic sulfonated groups increases the range of swelling by almost one order. The size and osmotic pressure of the cation in the exchange position contribute an additional increment to the swellability of the resin. This increment may be positive or negative, depending on the type of cation used (6). A typical highly swelling cross-linked polystyrene resin contains about 0.04% divinylbenzene incorporated in its struc-
1 844
ii
2.0 I
%.
% CROSS-LINKING AGENT
Figure 1. Swelling of polystyrene resins cross-linked with increasing amounts of divinylbenzene To left of dotted vertical partition line i s area of soluble chain polymers. With the same nominal concentration of cross-linking agent different methods of copolymerization give different standard distribution as illustrated b y dotted curves. The flatter the distribution curve, the larger the proportion of soluble resin fractions
ture. Statistically, this means a frequency of one cross-linking agent segmer to about 2500 styrene segmers in the copolymer. The authors were interested in the initiation and extent of breakdown of the three-dimensional polymer network into chain (water-soluble) polymer fractions upon oxidative degradation. The degraded polymer can be separated from the intact cross-linked resin by aqueous extraction. For quantitative measure of the extent of degradation, the extractable chain polymer content was determined before and after a standardized degradation procedure. I t became evident that the amount of water-soluble polymer formed increased with increasing degradation. Soluble chain polymer may be determined gravimetrically; in the specific case of sulfsnated polystyrene, an ultraviolet spectroscopic method may also be used. As soluble polystyrene exhibits a convenient peak at X=224 mp, the ultraviolet spectroscopic method was chosen. The extinction (or absorbance) is a linear function of the concentration and satisfies the Lambert-Beer law. The difference in molecular weight or chain length of the soluble chain polymer exerls a noticeable though not critical influence on the slope of the extinction curve. Degradation Experiments
In preliminary experiments, 0.500 gram of the hydrophilic sulfonated polystyrene resin was suspended in a n excess of 1000 ml. of pure water and allowed to
INDUSTRIAL AND ENGINEERING CHEMISTRY
distend to its highest swollen volume at room temperature. The authors were interested in changes produced by intimate contact with either moving air or an inert gas (carbon dioxide) in the presence or absence of small amounts of redox type of chemicals (such as ascorbic acid, ferrous sulfate, and cupric sulfate). The results shown in Figure 3 formed the basis of the degradation procedure later developed. As a result of contact with a large excess of moving air in the presence of ferrous sulfate: the entire cross-linked network will eventually be destroyed and reduced to soluble chain fragments. No significant degradation occurs in a n inert atmosphere. Surprisingly, the presence of 20% aqueous sulfuric acid as a medium also prevents degradation. This phenomenon became understandable only in the course of later studies. The 20y0 aqueous sulfuric acid causes a significant depression in the swelling of the resin due to coiling of the polyelectrolyte ; the lower the swelling, the higher the stability of swelling resins toward degradation. A number of points were determined during the first 750 minutes. This is the only experiment that shows such detailed information about the true shape of the degradation curve. For the experiment demonstrated in Figure 4, 0.500 gram of 602-P-453-1-K resin was suspended in 1000 ml. of water in four individual Erlenmeyer flasks. Air was bubbled through two of these resin suspensions for 105 minutes. Both flasks were charged with 50 mg. of ascorbic acid and 5 mg. of additional
E N G I N E E R I N G ASPECTS O F P O L Y M E R PROCESSES ferrous sulfate was added to one flask. Both of the air-treated suspensions showed extensive degradation after 105 minutes of contact with moving air. The two other Erlenmeyer flasks were treated with carbon dioxide gas and 50 mg. of ascorbic acid each. T o one flask was added 5 mg. of ferrous sulfate. After 105 minutes' contact with moving carbon dioxide gas, the sample containing ascorbic acid alone indicated a degradation by producing 26% soluble chain fragments, whereas the system with added ferrous sulfate showed a 15% degradation. CONCLUSION. Aqueous ascorbic acid in contact with air is one of the strongest degrading systems. From another batch of sulfonated polystyrene-divinylbenzene copolymer in the potassium cycle, 0.500 gram of 602-Q-30A-1-K-X resin (the parent copolymer of which was prepared by using 0.0201, benzoyl peroxide as an initiator) was suspended in 1000 ml. of water, 50 mg. of ascorbic acid was added, and air was bubbled through the system for 90 minutes. The soluble chain content indicated a degradation of 10% of the resin. After standing for one day, the resin was degraded further and formed 47% of chains. l h i s is considered significantly lower than the extent of degradation observed in the preceding experiment (Figure 4). CONCLUSION. This resin containing only one twelfth as much benzoyl peroxide as in the previous experiment (Figure 4) during its copolymerization shows a significantly greater stability against oxidative degradation. This finding is in agreement with recorded data (2). I n the next experiment, two samples
taining the resin copolymerized with benzoyl peroxide as an initiator showed a chain content of 60.7%, whereas the system containing the thermally initiated resin material showed only 4.23% of chains. CONCLUSION. The concentration of initiator used in the copolymerization significantly influences the stability of sulfonated polystyrene resins. I n two Erlenmeyer flasks, 0.500 gram of 602-4-1 8-A-1-K-X type of resin was suspended in 1000 ml. of water. This resin was copolymerized under thermal initiation (at 90' C.) in the absence of any catalyst and showed a relatively low amount of chain (12%) before sulfonation. Washing and drying were carried out under an inert (carbon dioxide) atmosphere. T o one system was given 1 mg. and to the other system 50 mg. of cupric sulfate. T o both systems was added 50 mg. ascorbic acid and contact was established with moving air for 90 minutes. The determination of the soluble chain polymers showed 4.8y0 for the experiment containing 1 mg. of cupric sulfate and 5.08y0 for the experiment containing 50 mg. of cupric sulfate. After standing for about 1000 minutes, the experiment containing 1 mg. of cupric sulfate showed 46.7y0 of chain; that containing 50 mg. of cupric sulfate contained only 10.3% of chain. CONCLUSION. The relative amount of cupric sulfate with respect to ascorbic acid present influences stability of the resin. i n another experiment (Figure 5), 0.500 gram of 622-A-2-K-X type of resin was suspended in each of two Erlenmeyer flasks that contained 1000 ml. of distilled water. This was a sulfonated
of 0.500 gram each of 602-P-453-1-K resin were suspended in 1000 ml. of water in two Erlenmeyer flasks. Ten milligrams of cupric sulfate pentahydrate added to both flasks and 50 mg. of ascorbic acid was added to one. After the system had been treated with moving air for 90 minutes, the chain content of the system that contained ascorbic acid was about 58%, whereas the other system showed less than 2% of the chain content. CONCLUSION. Small amounts of cupric sulfate do not inhibit the disruptive degradation of sulfonated polystyrene resins initiated by air in the presence of ascorbic acid. Two samples of 0.500 gram of 602-P453-1-K resin were suspended in 1000 ml. of water and to one system was added 1 mg. of cupric sulfate. T o the other system 10 mg. of hydroquinone was added. After 90 minutes of contact with moving air the system containing 1 mg. of cupric sulfate showed 1.63% chains, whereas the other system showed 2.1% chains. Upon standing a t room temperature for 22 hours, the system containing cupric sulfate showed an unchanged 1.53% chain content, whereas the hydroquinone-containing system increased its chain content to 8.73%. CONCLUSION. In the absence of ascorbic acid, even a small amount of cupric sulfate is a more potent inhibitor of the degradation of polymers than hydroquinone. I n a similar degradation experiment, 0.500 gram of 602-Q-18-A-1-K-X resin was suspended and compared with the 602-P-453-1-K resin. Each test was run in the presence of 50 mg. of ascorbic acid and was in contact with moving air for 90 minutes. The system con-
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Figure 3. Degradation of swollen polyelectrolytes in presence of air and ferrous sulfate Note relative stability in absence of air
VOL. 49, NO. 1 1
NOVEMBER 1957
1845
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polyvinyltoluene-divinylbenzene resin. initiated using 0.027, benzoyl peroxide as a catalyst. One milligram of cupric sulfate was added to one test. Both flasks were charged with 50 mg. of ascorbic acid. After contact with moving air for 90 minutes. the experiment containing curpic sulfate showed a chain content of 10.37, and the experiment with no cupric sulfate, 9.5%. After standing for 1000 minutes, the chain content of the test containing cupric sulfate was 15.037,, while that of the copper-free experiment was 40.57G. After standing for 5 days longer. the chain content of both tests increased to more than 507,. CONCLUSION. The inhibition of degradation by minute quantities of cupric sulfate was demonstrated in a sulfonated cross-linked polyvinyltoluene resin. When a commercially available, densely (about 10% divinylbenzene)
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Figure 5. Increase of stability in presence of small amounts of cuprous sulfate
cross-linked sulfonated polystyrene resin (Dowex 50) was tested in the presence of SO mg. of ascorbic acid and 50 mg. of ferrous sulfate, it showed no appreciable degradation either with or without 1 mg. of cupric sulfate. CONCLUSIOS. The degree of cross linking is roughly directly proportional to the stability of sulfonated cross-linked resins. In the next experiment, 0.500 gram of divinylbenzene cross-linked polyacrylic acid 363-A-3-Na resin was compared with 364-E-l-Na, a polyacr)-lic acid resin cross-linked by a different type of agent. To both tests was added SO mg. of ascorbic acid and, after contact with moving air for 90 minutes, the amount of chains formed was determined gravimetrically: 26.57, of soluble polymers in the former, 13.57, in the latter resin. O n standing for 1000 minutes, the results
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were 41 and 18%, respectively. These results were puzzling. until it became clear that the degree of swelling of the individual resin significantly influences the stability of its structure. The swelling values for the former resin were 640 cc. per gram and for the latter only 115 cc. per gram. This approximately five times greater swelling is a significant difference and will affect the stability of any type of resin. C o N c L r x l o N . This is the first instance in which the degree of swelling in water has been shown to be inversely proportional to the structural stability of hydrophilic resins (Figure 6). In Figure 7 the influence of mesh range on the stability of an acrylic type of resin is demonstrated. The resulrs show clearly that the larger the pariicle size, the greater the stability. CONCLPSION. Average diameter (par-
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Figure 7. Relation of particle size to stability in polyacrylic resins Figure 6.
1 846
Swelling vs. stability in polyacrylic resins
INDUSTRIAL AND ENGINEERING CHEMISTRY
Polyacrylic-divinylbenzene series. 500 grams 161-E-36 mi. of water. No ascorbic acid added
+ 37-Na in 1000
ENGINEERING ASPECTS O F POLYMER PROCESSES ticle size) is directly proportional to the stability of cross-linked resins. In the next experiment, two divinylbenzene cross-linked sulfonated polystyrene resins of different swelling in water were compared (Figure 8 ) . Again the resin having the higher swelling degraded more readily. I n this experiment the influence of mesh range was kept in mind and 40- to 60-mesh range was used throughout. I n a parallel experiment, the influence of subphysiological concentrations of sodium chloride was investigated. A hypotonic 0.01M sodium chloride (0.0585%) solution was used, which is substantially weaker than physiological saline (of 0.85y0 sodium chloride concentration). The stability of both resins was increased substantially in 0.01M sodium chloride solution as a medium. The swelling in pure water was 120 cc. per gram for one resin and 180 cc. for the other. (Resin 602-Q-37H-1-B-2-Mg-X is a divinylbenzene crosslinked polymer using 0.09% azobis' benzoyl perisobutyronitrile and 0.01% oxide as an initiator.) The increase in stability of both resins is undoubtedly related to the reduced swelling in the presence of a salt solution. In the last experiment, two sulfonated polystyrene resins in the magnesium cycle were selected (Figure 9), and to the higher swelling resin was added 0.023 mole of sodium chloride. The swellings were 602-P-454-Mg, 125 cc. per gram and 602-Q-52-B-3-A-l-Mg 300 cc. per gram. This latter resin was cross-linked with p,p '-diisopropenyldiphenyl (DID) and polymerized with 0.09% tetraphenylsuccinodinitrile (TPSDN) as an initiator. The swelling of the latter resin was depressed to 135 cc. per gram by compensating in a 0.023M sodium chloride solution. The experiment gave
the anticipated results. The resin of the higher initial swelling turned out to be the resin of greater stability, as it was prepared without benzoyl peroxide and contained only a low concentration of tetraphenylsuccinodinitrile as an initiator ( 5 ) . I t was cross-linked with an agent different from divinylbenzene (74). CONCLUSION. Resins of different preparational history may be compared, in that their swelling is compensated by addition of a neutral salt such as sodium chloride or sodium sulfate. Standard Degradation Procedure
After these runs, the stability tests were standardized as follows: A dry resin 60 (0.50 of the mesh range of -40 gram) was suspended in 1000 ml. of distilled water. Approxiqately 600 cc. per minute of filtered air was bubbled through the system. The exact rate of the air bubbling is not critical, as a t these rates the amount of air blown through in the actual tests was in a large molar excess. After 90 minutes of initial contact with air, the chain content of the resin was determined in an aliquot sample of 100 ml., either by ultraviolet spectrophotometry or a gravimetric method (Stage I). Ten milligrams of ascorbic acid was then introduced and the contact with moving air again established for 90 minutes. Immediately thereafter, the second aliquot sample of 100 ml. was withdrawn and the amount of chains determined (Stage 11). After the @stem had stood overnight, a third aliquot of 100 ml. was withdrawn for determination of the chain polymers (Stage 111). Then, 20 mg. of ascorbic acid was added to each system and contact with
+
moving air again established for 90 minutes. Samples were removed for the fourth time and the chain content again determined (Stage IV). After standing overnight, the fifth and final determination of the chain content was made (Stage V). This system of gradual degradation proved satisfactory for total degradation of most of the polymers prepared. The degradation method has limitations. The straight lines drawn between determined values are an oversimplification; they are nothing more than an indication of the rate of degradation. Determination of several points between stages actually would have established true curves rather than straight lines. At the time when these preliminary investigations were carried out, the amount of work necessary to follow up such curves more closely would have been prohibitive. Discussion
The equilibrium of forces in a swollen resin network may be expressed by: 7r
+
7r'
> = B
