PHYSICAL CHEJfIC.1L INVESTIGATIOSS O F G O L D E S R O D RUBBER.
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PHYSICAL CHEMICAL IX\’VESTIGAATIOXSOF GOLDEXROD RUBBER. I “PRECIPITATION
T’ALUE’’
A S D ITS
SIGAIFICASCE
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CHARACTERIZATIOS
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EVALUSTIOK OF YATURAL RUBBERS EVALD L. SIIAU, WILLIAM J. RUXCICEL, FLORENCE B. KREEGER, MARS- -1.SULLIT*1S
AND
Southern Regional Research Laboratory,‘ S e w Orleans, Louisiana Received Januarv SO, 1946
The solubility of a polymer is a function of the degree of polymerization; in general, i t decreases with an increase in molecular n-eight. The direct determination of solubility for such a substance is usually very difficult and cumbersome (2). When a non-soh-ent is added gradually with thorough mixing to a dilute solution of a polymer. a mixture of non-solvent and solvent in definite proportions is eventually obtained in n-hich the polymer is no longer completely soluble. ;it this point a turbidity appears, o n ing to incipient precipitation. Jirgenqons ( 2 ) used this simple method of titrating to the cloud point in order to determine ielative degrees of polymerization in such substances as njtrocellulose, starch triacetates, and polystyrene polymers. In the case of rubber, Alidgley rt aZ. (3, 4) distinguished between fractions of different molecular weight by means of their solubilities in mixed solvents and in thii connection used the “standard precipitation point” as a characterizing property. This point n-as defined as the temperature at n hich a slonly cooled mixture of 0.85 per cent of the rubber. 28.55 per cent of absolute alcohol, and iO.(iO per cent of benzene S ~ O T T aS sudden increase of turbidity. In the course of the evaluation or characterization of rubber recovered on a pilot-plant vale from the leaves of goldenrod (Solidago leauenzuorthii) in this laboratory by a tn-o-stage extraction process in Tyhich the leaves are extracted n i t h acetone prior to extraction of the rubber with benzene, a simple titration method hai been developed for estimating the relative solubilities or “precipitation valiie” of rubber samples. This method might be modified for use in characterizing polymeis other than those of isoprene. The “precipitation value’’ as used for rubber is defined as the number of milliliters of absolute ethyl alcohol required to produce a cloud point in 10 ml. of a clarified benzene solution containing 0.01’75 g. of sample. The larger the volume of alcohol required, the greater is the Folubility of the rubber. I-nless stated to be otherwise, all precipitation values are corrected to 25°C. Evperiments have been performed n-hich show that, in general, treatments such as ma.tication, heating, and exposure to light, which are known t o cause a decrea.e in the viscosity (average molecular weight) of rubber, cause an increase in the precipitation value. Information has been obtained concerning the One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, United States Department of .igriculture.
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E . L. SKAU, IT. J . IITSCIiEL, F. B. KHEEGER, A S D M . -1.SULLIY.1S
significance of the precipitation value and therefore of solubility data, and concerning the limits of their applicability in the characterization of ruhber. EXPERIMESTAL NETHODS
Prepayatton of the rubber solution One gram of the rubber sample is dissolved in 200 ml. of reagent-g;iiJe Iieiizene in a stoppered flask by alloiring it to stand in the dark overnight or until -oliition is complete. To clarify the solution 1 g. of llerck’q activated charcoal i- added and the solution is slurried and alloued to settle a t least 15 min. The -1urry is then filtered through a filter paper on a Buchner funnel previou-iy prepared by filtering a small benzene slurry of activated charcoal and a filter. aid tHyflo Super-cel) and mshing n-ith benzene. The rubber solution is not r i d d e d until the benzene comes through clear. S o n-ashing is necessary. Total solids are determined on tlie clarified solutions by evaporating ;.n diquot in an impinging stream of nitrogen under a bell jar. The solution i- then tliluted with benzene to such a concentration that 100 ml. contains 0.17.5 p. of total solids. Other methods were tried for making tlie solutions clear enough tcl hi. Titr:ited. Aluminum oxide columns similar to chromatographic columns were ~~rnployed , though effectii-e, they u-ere too slon- to be practicable. The precipi:r,tion mlue obtained seems to be independent of the method of clarification c
