Microgel, A New Macromolecule Relation to Sol and Gel as Structural Elements of Synthetic Rubber W. 0. BAKER Bell Telephone Laboratories, Murray Hill, N.J .
I
N CONNECTION with the rubber crisis of the a a r , useful fundamental controls and improvements of quality of synthetic rubber were urgently needed, which would fit in with current engineering, Variations in the solubility of butadiene polymers and copolymers had been noted by many rubber researchers seemingly to reflect important and controllable variables in the polymerization process, such as “modification” and degree of conversion. Solubility was also thought by some workers to bear on processability. Agreement was neither definite nor general on these phenomena. Solubility behavior and gel fraction had been casually observed for 30 years in natural and synthetic rubbers studied in Europe. No record of engineering application was found. After the organization of the polymer research program in the Office of the Rubber Director, more intensive examination of the significance of sol-gel in controlling the uniformity of GR-S was begun. A proposal new to polymer technology was made: that the gel fraction of synthetic rubber comprised a characteristic structure in the raw polymer which affected the physical properties and quality (as in tire performance) of the final vulcanizate. This idea is to be distinguished from those of Spence and Ferry (68). Here, extensive vulcanization of natural rubber by air, sulfur, and other “catalysts” was measured by solubility and swelling volume in benzene. Then, the results were interpreted in connection with the stress-strain properties of the given rubber samples after compounding and further vulcanization in the usual way. The conclusions on quality were, quite properly, that Hevea rubber containing gel resembled in many ways a good vulcanized stock or a t least fine, air-dried Para rubber, and yielded faster curing stocks. Similar results on the effect of gel in Hevea had been found by Smith and Holt (66) and Smith and Saylor (67). However, the present work concerns the more local structure of the gels of emulsion polymers. These are, indeed, units causing certain intrinsic properties of the rubber other than the degree of cure. Likewise, it was indicated that certain elements of processing, as well as the initial synthesis, could affect this structure in synthetic rubber, and thereby obscure vital relations between polymerization and tire quality, as well as themselves reduce that quality. The present report concerns the basic concepts of gel as compaled to sol, particularly in emulsion polymers. Other reports will describe the methods of identifying so1 and gel, and their significance in the synthesis and processing of synthetic rubber. Indeed, important applications of these concepts and techniques have already been described (66, 81). CHRONICLE
Thomas Graham and other early philosophers recognized the network properties of gels (37, 40, 6 1 ) , although frequently they meant secondary valence networks, with which we are not now concerned. Nevertheless, the swelling of vulcanized rubber (83) and of a portion of raw rubber found benzene-insoluble after 15 years of aging (10) furnished behavior that forecast the present interpretation. At an early symposium on colloids, it was said, “the jelly would tend to swell to infinity and ultimately dissolve; and since it does not do so, we must assume some opposing force,
and the only possible one appears to be the elastic cohesion of the jelly, which implies some sort of connected structure” (59). Now, taken with the contribution of Staudinger (70)in establishing the beautiful generality of macromolecules, or huge primary valence bonded units (as opposed to vague theories of cemented micelles), this provides most of the basis for interpreting the gel phase in cross-linked polymers. There have been long argument and uncertainty, however, as t o how such networks were formed chemically, as during polymerization, and thus as to why some polymers were all sols (in suitable solvents), while others were largely gels (in all solvents). Staudinger, with his bold thesis of primary valence chains and networks, showed how certain organic units could lead to network gelation (60, 7 1 ) ; a similar approach was clarified and systematized by Carothers (17) and Kienle (46). Then, specific mechanisms for gelation in polyhutadiene, natural rubber, etc., with hydrogenation experiments to examine the nature of branched fragments (73) were followed by data on polyester resins and drying oils ( I S , 14, 46). It is known now that the chemical reactivity of natural rubber often prevented earlier consistent isolation of its sol and gel components (56, 67). This is especially because the isoprene residue is liable predominantly to oxidative sciBsion, in contrast, for example, to the butadiene residue, which is readily cross-linked. Finally, a theory for statistical interpretation of polymer gelation, as infinite network formation, has been constructed ( 2 6 ) . Generalizations (76-78) have further affirmed the reality of the molecular network model. Likewise, recent experiments with various monomers, each of known functionality, have agreed with gelation concepts. Examples include divinyl ether (11), hydroquinone diacrylate (44), other unsaturated esters (79), and partially methylated cellulose cross-linked with oxalyl chloride (65). On the other hand, the true solubility of essentially linear polymer molecules has been widely established following Staudinger’s early concepts. A model for thermodynamic understanding of such solutions, proposed by Meyer ( 3 4 6 5 )has been applied in the well-known contemporary theories (28, 43) of high polymer solutions, whose agreement with experimenb (56)is st least indicative of a basically valid idea. This is, that above a critical temperature (16)chain molecules are ‘nfinitely miscible with some solvent whose heat of mixing with the polymer is tolerable; GR-S sol with benzene seems to be an example. However in partial solvents, or in solvent-nonsolvent mixtures, even strictly linear molecules show a solubility or phase separation dependent on molecular weight ( I , 69). This is the old scheme for polymer fractionation. Insolubility caused in this way is not to be confused with the insolubility of networks or gel. The solubility of linear macromolecules was attributed by Hronsted (15) and Schulz (63) to a favorable potential energy state for the dissolved macromolecule contrasted to its solid state of aggregation. Previously, however, Meyer had emphasized a high entropy of mixing for the formation of polymer solutions. Schulz’s theory of potential energy change on solution was especially criticized as neglecting the predominant entropy change ( 1 ) . The recent studies noted take care of this (28, @’), and entropy gain is probably the chief cause of dissolution of most hydrocarbon polymers (36). However, the heat or “solvation”
512
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY R A D I C A L IS INITIATED IN GROWN C H A I N A N D A BRANCH S T A R T E D \
Vol. 41, No. 3
(8)., Here, a small volume of latex is dissolved in a xylene (75%)pyridine (25y0)or siniila,r solvent mixture.
A similar scheme of‘ converting the polynicr in h t u x (lyopiiobic system) into a solution or disl)crsion of the polymer (lyophilic system) on which mea.ningfu1 viscosities, etc., may be run was apparently worked out. in Germany (62). ITowever, chlorobenzene and like sol~7ent.swhich “extracted” but did not, \\-holly take up the latex were used by boiling the water offl and hnncv some coagulation may have preceded dispersion.
CHOWN C H A l
CROW IPIC (PR05ABLY S U ACTIVAT
MICROGEL
Figure 1. Schematic Diagi*am of hlicuogel Formation in Pols butadiene
faelor lias also been evaluat e j for hydrocarbons like natural rubber (%), arid conditions for dissolving polymers can IIOW be defined. This is the background from which study of sol-gel in GK-S and other butadiene polymers has been proposed to reveal essentially distinct and significitnt, alt8hough chemically int,erconvertible, elements of structure.
FTith n polymer (miitailring microgrl, this dispersion TP~UIJ be a true solution of the sol fraction :md also of the microgcl particles, uiiless they happen to be large enough to sedimerlt s])orit.aueously. (In the latter case, sedinientatioii equilibrium m:Ly COIIIC! so slowly that a colloidal dispersion of the lnige microgel, rather t h m a true solution, will exist.) A similar system mould result if a p l p w containing microgel were coagulated from the latex, arid tiion, without any reaction wit,li oxygen, el e., t o iiit r d u c e macrogd, were redispersed in a solvent,, say benzene. This rcdispersiori could occur from agitation, and again, the system would IJC a “true” solution of polymer including microgel, cr, for w r y lwgc particles of microgc.1, a “cullotlial” solution of microgel. This distinction bet ween true and colloidal solution of course deponrls entirely on the size ol‘ the microgel. This lat,kr casc, it-here a coaguluin protected from osy-geii is re~lirperscci,is i:spi:ciall,v familiar in GR-S studics. Such a disperfioii in henzena \vas rriatlc froin a form of ‘211-S brought t,o %yoconversion anti containing 577* iiiicrogel of sn-olling volunic~ 36. This sivelling volume (approximatr:ly t,he multiplicwtio11
A
B
MICROGEL
Association of sol-gel struct tires with quality control of GIt-S first required separation of the gel phase. All the earlier considerations and experiments, surveyed above, were with gelations in which the infinite network essentially spread through the whole containing vessel, solution, or (as in vulcanization) mold. Coherent, readily separable, agglomerates of gel, from which most of the sol was extract,ed, were formed in phenolic resins, alkyds, vulcanized rubber, mass-polymerized Uuna, etc. Emulsion polymerization, hon-ever, sharply localizes gelation. For the reaction unit is here the latex particle (or soap-monomer aggregate preceding it). o K i t h regular GR-S, its average diameter of 1000 t o 2000 A. or volume of 0.5 to 4.5 X 10-16 ml. means that the largest gel network is still only of superinolecular size and weight (with M around BO X 100, or only 30 times that of high molecular weight polystyrene). This unit \Tas called microgel ( 4 , 24). It ea11 be separated only under static conditions, which lead to a particular, possibly of tc:n chemical, ngglomeratioii. This is treated in the report, on techniques. Whether microgel sometimes occupies a whole 14 tex particle, or is found in particular typos of latex particles, is not yet :vel1 known. It may be especially common in smaller latex particles (68). These and related problems involve the detailed media,nisrn of emulsion polymerization (41). But microgel is the form of primary reaction gel in GR-S while that coining frvin drying, aging, vulcanizing, etc., is macrogel ( 4 ) . Microgel is probably formed in polydieries by the rciiction illustrated in Figure 1. Evidently, such network forniat,ion will be governed by accessibility and concentration of catalyst, modifier, monomers, etc. Hence, in Figure 1 the reactions shon-n are probably affected by surface to volume ratio-e.$., latex particle size.
l: .
H. OBSElWATION OF MICROGEL
Conteiits of emulsion polymerization latex particles may be uniformly disperbed in lyophilic liquids by the vistex techirique
C’ I).
polymer. 25,600 X I’ol~-t~ctitdiene nncrozel d e p o 4 r r d from 0.15% eoliition in cyclohexane. Cirainlikc struoture i- probably preludr t o agglomeration. 68,600, X l i i c m # d from methyl iii(~thact.~latP-et~,ylene diacrylate elnuisloxi i,oiyiiier of 84.45%conversion, (i.8 weight 70of cross-linking agent
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March 1949
513
Other electron micrographs of microgel in Figure 2 confirm the
TABLEI. DIAMETERSOF MICROGELPARTICLES DEPOSITED basic concepts. AFTER DISPERSION OF ORIGINAL WHOLEPOLYMER IN BENZENE I n B appear the largely discrete macromolecules in a n under
