No abstract

and 1.15, for increasing molecular weight fractions, respectively. Furthermore, the reported uncertainty in individual Mw and Mn values is such that, in all cases but one, Mw/Mn could be as small as unity, giving a monodisperse fraction, with no violation of the stated precision. The one exception is the high molecular weight fraction, Mn = 735,000, for which the polydispersity effect well exceeds our experimental results at either extreme of the Ew/Mn ratio. This could possibly be explained if there were a high molecular weight tail to the distribution curve which might become lost in the base line without contributing to the apparent plate height. Further study is being conducted on this and related questions.

No abstract

The procedure described attains its accuracy and precision by the refinement and improvement of conventional simple operations over a periods of years. The trend in analysis recently is, of course, toward the use of rapid physical methods. In a great many instances these involve relative measurements requiring the initial establishment of reference materials with compositions determined by methods such as the one here described, which bases the numbers derived solely on readings of an analytical balance. In fact, as already mentioned, one of the principal applications of the present method has been in the establishment of the relation between refractive index and styrene content for SBR polymers so that refractive index measurements can be used in routine determinations of bound styrene content.

A d etailed d esc ription is given of t he method of combustion a na lysis for carbon and hydrogen to determine the co mpos ition of a copoly me r from its carbo ll-hyd roge n ratio. Th e ap paratus a nd procedu res have been d eveloped at the Nationa l Bureau of Stand a rd s over a period o f years. The method has been app lied chiefly to determine what fraction of a styre n e~but adien e co p o l ym~r is d eriv ed [rom styrene. l\1inor in g l·edi nts are r emoved by extraction, with t he exception of the bound mercapta n re idue for whic h correcLion mu t be made. The amo un t of bound mercap tan res idu e is determin ed from measure me nts of t ile sulfur content of t he copolymer by t he Carius method. Meas urement a re made of t he oxygen con Lent a nd Lhe ash co ntent, a lLho ugh the e do not enter in to the calculations. The ~tan dard deviation of a meas urement of carbon-hydrogen ratio is a pproximately 0.001 0 and IS Inde pendent o f sLy rene contenL. This corresponds to a stand ard deviation of about 0.036-percent bound styrene for pol y mers of low styrene co ntent a nd 0.018 percent for polyme rs o f hi g h styre ne conten t. Th e acc uracy of t he meL hod is d emonstrated bv t he fact thaL observ ,tLions o f carbon-hydrogen ratio for fou r out of fiv e samples of po lyb utadiene differed by less Lhan one standard devi ation from t he theoretical valu e.