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K. L. SMITH, A. E. WINSLOW, and D. E. PETERSEN Union Carbide Chemicals Co., South Charleston, W. Va.
Association Reactions for
...
Poly(alky1ene Oxides) and Polymeric Poly(carboxy1ic Acids) Tough, rubberlike, water-insoluble resins can be m a d e from water-soluble high polymer mixtures b y a n instantaneous room temperature reaction
INTERMOLECULAR The unique
interaction between similar polymers is \vel1 known (5, 6), but little has been reported about such linkages between dissimilar polymers, particularly when one is a nonelectrolyte. I n the work described here, molecular association between anionic polyelectrolytes and relatively nonpolar polymers is demonstrated. Insoluble Products from Soluble Components
Both poly(ethy1ene oxide) [molecular weights up to 8,000,000 i/. 2, 4, 7 ) ] and poly(acry1ic acid) are completely soluble in water, but when their water solutions are mixed together, immediate precipitation results. This is also true for polyethylene glycols of molecular weights 4000 and higher. The thick, gelatinous precipitate thus formed, when dried and subjected to heat and pressure, forms a clear, flexible, homogeneous film which absorbs water without appreciable dissolution. However, the precipitation step can be bypassed; adding an inhibitor gives a n aqueous, homogeneous solution which can be cast and insolubilized by drying. Thus, two water-soluble component polymers having low flexibility are transformed into a single, highly flexible, clear elastomer which is water-insoluble.
properties of the poly(ethy1ene oxide)-poly(acrylic acid) products probably result from hydrogen bonding between ether and carboxyl groups. Reasons for this conclusion are insolubility of the complex, unique characteristics of the stiffness-temperature and glass transition temperaturecomposition curves, and the abnormal changes in heat stability, as well as x-ray data which indicate strong intermolecular order resembling crystallinity. The property changes realized in these association products of dissimilar polymers approach in magnitude those obtainable by the well-established technique of copolymerization. Evidence for Association between Unlike Polymers
Thus far, infrared examination has not distinguished between the self-association of the polymeric poly(carboxy1ic acid) and the intermolecular ether-carboxyl
association. Hence, physical and chemical properties of the mixture must be compared with those of the individual components. Stiffness of highly crystalline polymers such as poly(ethy1ene oxide) drops rapidly a t the crystalline (first-order) transition temperature, with a distinct melting point at 66" C. (Figure 3 ) . Poly(acry1ic acid), a completely amorphous and atactic material, also gives an abrupt, but less severe drop. I n Figure 1, two transitions occur lor the mixture of 75 weight yopoly(ethy1ene oxide) and 25 weight yo poly(acry1ic acid). The first break, around 15" C . , is attributed to an intimate and compatible mixture of the two polymers. The new phase, an amorphous complex, has lower softening values than pure poly(acry1ic acid). The melting of an independent poly(ethy1ene oxide) crystalline phase which was detected by x-ray explains the second break. The amount of the complex increases with additional amorphous poly(acry1ic acid) in the mixture, as the curve of the mixture of equal polymer weights shows (Figure 1). The additional polymer acid has effectively eliminated the poly(ethylene oxide) crystalline phase, ostensibly leaving only a single, amorphous phase. VOL. 51, NO. 11
NOVEMBER 1959
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That this mixture is an amorphous complex or association product is supported by both compatibility in films and the water insolubility caused by interpolymer cross linkages. As the poly(ethy1ene oxide) content of the composition decreases, a series of similar stiffness-temperature curves approaching the position of the poly(acry1ic acid) curve results, one of which is drawn in Figure 2 (25y0PEO). Surprisingly, however, at still lower poly(ethy1ene
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oxide) concentrations, stiffness values are substantially above those for either component polymer, as witness the 10% poly(ethy1ene oxide) curve. At this same composition the glass transition temperature and the composition-dependent stiffness maxima are parallel. A maximum glass transition temperature (Figure 3) occurs at about 10% poly(ethy1ene oxide), as determined according to a method previously described ( 3 ) . This maximum cannot be
40
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TEMPERATURE, * C.
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Figure 1 Increase in temperature lowers stiffness according to composition-dependent variations
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Unrnodifled polymers of poly(acry1ic ac;d) and poly(ethy1ene oxide) (PEO) Product mixtures with designated weight percentages of poly(ethy1ene oxide)
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TEMPERATUAE,*C Figure 2. With low poly(ethy1ene oxide), stiffness-temperature curves indicate presence of intermolecular complexes
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INDUSTRIAL AND ENGINEERING CHEMISTRY
explained by the amorphous complex shown by the 50% composition, which was low in both stiffness and glass transition values. A more tenable explanation postulates an additional form of the complex. The new form must differ from the other by having a higher degree of inherent stiffness and a higher interchain bonding energy. The amounts of the new form would be limited by the relatively small percentage of poly(ethy1ene oxide) present. The x-ray patterns of the polymer mixtures from 10 to joy0poly(ethy1ene oxide) support the concept of such a highly ordered product. Known poly(ethylene oxide) patterns do not explain the strong intermolecular order which is observed. This order (of the complex) persists above 250" C., at which temperature the mixture decomposes. The complex's crystal binding energy seems surprisingly high, considering the 66" C. poly(ethy1ene oxide) melting temperature and the 75" C. poly(acry1ic acid) glass transition temperature. The stiffness and glass transition temperature maxima of the 10% poly(ethy1ene oxide) composition probably are produced by the similar bonding characteristics present in the coexisting, less-ordered portions of the complex. The minimum on the glass transition curve (Figure 3), occurring at the 50/50 ratio, is the lowest glass transition temperature shown by the amorphous complex. It coincides in respect to composition with the minimum stiffness and minimum water-solubility. The glass transition then rises slightly with increasing poly(ethy1ene oxide) content. This rise is not readily explained, but perhaps the crystalline poly(ethy1ene oxide) present in this composition range has indirectly affected the bonding efficiency in the amorphous phase. Heat stabilities of these polymer mixtures also serve as further evidence for association. In Figure 4 the heat stabilities are plotted as a function of composition, for film samples heated in air at 300" C. for 1 hour. Predicted weight retentions for mechanical mixtures of the two polymers are shown by the straight line which traces a gradual increase in stability, assuming that each polymer contributes its proportional share (on a weight basis) to the heat stability of the mixtures. The upper curve shows that the actual observed heat stability of intimate polymer mixtures rises rapidly with poly(acry1ic acid) content, reaching a maximum which levels off as a short plateau for polymer acid concentrations of 30 to 45y0. Interestingly, the maximum value of 7970 weight retention reached at this point is higher than for either component alone. Minor poly(acrylic acid) additions, therefore, give heat stabilities as good as or better than those obtained with very substantial
POLYMER A S S O C I A T I O N PRODUCTS
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40 60 80 POLY(ETHYLENE 0x1DE),% BY WE1 GHT 80 60 40 20 POLY(A6RYLlC ACID), %BY WElGHT
additions. The magnitude of the difference between these two curves is believed indicative of intermolecular association. Stress-strain curves for the poly(ethylene oxide)-poly(acry1ic acid) mixtures also indicate mild cross linkages (Figure 5). As a composition of equal polymer weights is stretched to 1% elongation at -40' C., an increasing slope or inverse curvature is observed. Stress is increasing faster than strain and probably reflects the presence of mild cross linkages (hydrogen bonds) in the sample. Upon stretching, they either exhibit a higher energy of interaction or more linkages are formed, resulting in a higher degree of order. Crystallization of rubber, upon stretching, will give this same effect, of course, although usually at much higher elongations. Nature of Association Forces
Synergistic changes in physical properties constitute the principal evidence for the associations discussed here. Association may be further explained by study of molecular models.
Figure 3. Maximum glass transition temperature pinpoints composiiion of unique intermolecular bonding
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Fisher-Taylor-Hirshfelder scale models of polymer segments can be closcly aligned together so that the carboxylic hydrogen atoms of poly(acry1ic acid) touch consecutive etheric oxygen atoms of poly(ethy1ene oxide), even when different polymer configurations are assumed. Because space relations were favorable for hydrogen bonding in this test, space limitations should not prevent the formation of such bonds in largely amorphous compositions containing a large distribution of chain segment configurations. Direct physicochemical measurement of the amounts and types of hydrogen bonds in these systems has not been made. Analogy with conventional cross-linked polymers, however, shows that substantially less than the theoretical maximum amount of hydrogen bonding would be sufficient to accomplish major property changes. Low bonding efficiency, together with low steric requirements, result in the observed high speed of formation of the amorphous, solvated poly(ethy1ene oxide)-poly(acry1ic acid) precipitate. I n
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a curve representing precipitate composition b s . charge composition (Figure 6), the precipitate composition rises most rapidl)? in the curve's mid-section, while both end portions are nearly straight and have much lower slopes. Fluctuation of precipitate composition from 30 to 52% poly(ethy1ene oxide) over the total range of charge composition points up the degree of disorder tolerated in the initial precipitate ; further variation of its composition is prevented by sufficient amounts and proportions of the original polymers remaining in solution. Preparation of the product in quantity will be facilitated by the observed matching of precipitate with charge compositions at 40 weight % poly(ethy1ene oxide). Precipitation inhibitors were mentioned earlier for use in preparing these water-insoluble products. These inhibitors are usually hydrogen-bonding solvents, such as acetone, dioxane, ethylene glycol monoethyl ether, methyl ethyl ketone, and acetic acid. A logical explanation for their action is that they hinder precipitation by competing for the hydrogen bonding sites of the respective polymers. Thus, sufficient acetone in a polymer ether-polymer acidJvater system can either block precipitation or redissolve an existing precipitate by its probable solvation of the carboxyl groups of the polymer acid. Neutralization of a small portion of the carboxyl groups with inorganic or organic bases also prevents precipitation.
Use Properties of Association Products
The water-insoluble association complexes described in this article dissolve
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POLY(ACRYLIC ACID), X BY WEIGHT
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80 60 40 20 POLY(E1HYLENE OXIDE) ,*I. BY WE1 GHT
Calculated h e a t stability of mechanical mixture Experimentally determined h e a t stability
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Figure 4 . Polymer blends exhibit synergistic stabilization to elevated tern peratures
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STRAIN, PER CENT ELONGATION Figure 5. Strain responding less than proportionally to increased stress may indicate stretch-induced interaction VOL. 51, NO. 1 1
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Figure 6. Charge ratio extremes give restricted variation in association product composition
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80 100 POLY(ETHYLENE OXIDE) CHARGED, % B Y WEIGHT 100 80 60 40 20 0 POLY (ACRYLIC ACID) CHARGED, % B Y WEIGHT 40
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in dilute alkali and reprecipitate on addition of mineral or strong organic acids. After drying they have good solvent resistance to acetone, acetonitrile, isopropyl alcohol, and aromatic and aliphatic hydrocarbons. Water extractability of air-dried prcducts amounts to about 10% in a 48-hour test at room temperature, using 100 parts of water per unit weight of sample. Soxhlet extraction with boiling water increases extraction values of air-dried products an additional 2 to 501,. Considerable swelling accompanies this extraction, but the original properties of the dry material return after redrying. Prior baking at 150” C. for periods up to 2 hours gradually decreases water extractability at room temperature from 10% down to a range of 3 to 570 and produces a similar change in boiling water extraction values. Tensile strengths and ultimate elongations for the poly(ethy1ene oxide)poly(acry1ic acid) products are moderately lower than for poly(ethy1ene oxide) alone. Typical association product properties and corresponding values for poly(ethy1ene oxide) are summarized in the table.
Scope
of Associating Polymer Pairs
The poly(ethy1ene oxide)-poly(acry1ic acid) example is only one of some 30 or 40 examined, wherein a polymer containing a multiplicity of ether groups associates with a polymer containing a multiplicity of carboxyl groups. Polymer ethers which associate include poly(viny1 ethers), cellulose ethers, and a large variety of hydroxyethylated compounds and polymers. Useful polymeric poly(carboxy1ic acids) include maleic acid copolymers, acrylic acid homo- and copolymers, and carboxyl derivatives of cellulose and silicones. Among the other polymer ethers useful in this association are the branched polymers normally obtained by hydroxyalkylation of active hydrogen compounds, such as triols and higher polyols, where three or more reactive hydrogen atoms serve as initiators for chain growth. Examples of such polyfunctional “starters” are glycosides, starch, cellulose, vegetable gums, polyfunctional amines, and amides. The association reaction takes place independently of other functional groups that may occur as relatively minor com-
ponents, although possibly as repetitive units, either attached to or within the backbone polymer ether chains, such as carboxylic ester, inorganic ester, acetal, hemiacetal, and amide. Permissible end groups are almost unlimited and include hydroxyl, aldehyde, carboxyl, halogen, and alkoxy. Product may be formed by mechanical blending of powdered components on heated two-roll mills, or from formulations giving aqueous dispersions, in addition to the already cited casting and molding procedures. The two-roll mill produces a semiflexible, translucent sheet which retains flexibility and insolubilizes upon baking. Molding of powder mixtures also produces flexible, water-insoluble products. In a watermixed system the use of special surfactants limits the size of product particles so that dispersions result. Additional dispersants undergoing study enhance film properties and simplify high solids formulations. Association products may also be formed from organic solutions, with or without water present. Although association of water-soluble polymers provides some spectacular examples, nonwater-soluble polymers may also be used, such as poly(propy1ene oxide), the higher poly(viny1 alkyl ethers), and the free acid forms of carboxymethylcellulose and silicones. A minimum of about 0.3 carboxylic acid group and about 0.5 ether group per 100 molecular weight must be present in the respective polymers to permit substantial association. Furthermore, the molecular weights of the combining polymers must be of the order of 1000 or more. Acknowledgment
The authors are indebted to F. P. Reding for numerous helpful suggestions, to E. R. Walter for x-ray diffraction analysis, and to various members of the research staff for help in preparation of the manuscript. literature Cited
Representative Properties of Poly(ethy1ene Oxide) and Its Association Product with Poly(acry1ic Acid)
Property Stiffness, secant modulus (25’ C . ) ,p.s.i. Glass transition temperature, O C. Heat stability, wt. retention in 1 hour at 300’ C., % Water extraction resistance, yo insoluble at room temperature Shore hardness, D scale Tensile strength, p.s.i. Ultimate elongation, yo a
Association Producto
Poly(et,hylene Oxide)
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77 87-93 20 800-2100 400-800
None 52 1800-2400 700-1200
Properties apply to products containing 40 to 50% poly(ethy1ene oxide).
(1) Bailey, F. E., Jr., Kucera, J. L., Imhof, L. G.: J . Polyrnet Sci. 32, 517-18 (1958). (2) Bailey. F. E., Jr., Powell, G. M., Smith. K. L.. IND.ENG. CHEM.50. 8-11 (1958). ’ (3) Brown, A., Textile Research J. 25, 891-901 (1955). (4) Hill, F. N., Bailey, F. E., Jr., Fitzpatrick, J. T., IND. ENG. CHEM.5 0 , 5-7 (1958). ( 5 ) Huggins, M. L., J . Chern. Educ. 34, 480-8 (1957). (6) Robinson, C., Bott, M. J., Nature 168, 325 (1951). (7) Smith, K. L., Van Cleve, R., IND. ENG.CHEM.50,12-16 (1958).
RECEIVED for review March 9, 1959 ACCEPTED July 27, 1959 Division of Polymer Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958.
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