Determination of the Molecular Weight of Nylon J . E. WALTZ, Technical Division, Rayon Department, A h D G1,Y B. TAYLOR, ChPniicnl D e p a r t m e n t , E . I . dit P n n t de .%‘emoursand Company, Tilmington, Del.
3Xethods for the determination of the molecular weight of nylons are described. The active end groups of alcohol-soluble nylona are determined either potentiometrically or conductometrically. The amine end groups of alcohol-insoluble, phenol-soluble nylons may be determined by conductometric titration in the solvent phenol-alcohol-water. Carboxyl end groups of the latter nylons are determined by dissolting the polymer in benzyl alcohol at 175’ C. and titrating H ith potassium hydroxide, using phenolphthalein as indicator.
D
E T E K M S A T I O S of the n m l r d a r weight of nylon has proved singularly difficult because of a d w r s e solubility characteristics, and, in these lahoratorics, only the method of endgroup determination has given unequivocal results. Sichols (1 ) has made osmotic pressure nieasurement,s on nylon in mixed solvents for a number of samples, and has calculated number average molecular weights ranging from 3000 t o 20,000. Staudinger and .Jordcr ( 7 )were unahlr t o determine the molecular weight of nylon osmotically because of lon- solubility in organic solvents. They measured the intrinsic viscosity of nylon in nr-cresol and estimated the Staudinger viscosity constant, K , from osmotic mrasurements of more soluble nylons prepared from srcondary diamines and dicarboxylic acids. This procedure may be open to question (3) as may that of Matthes ( 6 ) ,who determined the miilecular weight of poly-e-caproamide by a viscometric method, using end-group determination of 1on-t.r homologs for standardization. I n this work methods are presented for determining t h r r i u i n h ~ i ~ average molecular weight of nylon by end-group titration. Nylon Types. Xylon can be prepared ( 2 ) by condensation polymerization (1) of diamines and dicarboxylic acids or (2) of aminocarboxylic acids, or by copolymerization of mixturw of 1 and 2. These have the structural arrangenimts:
’--SH) ,,H
HO (OC-R-CO-XH--K
(1,
and
(2)
HO(OC-It-SH)nH
Those in class I11 are crowlinked or gelled, are of very high or “infinite” molecular weight, and are not included in this discussion. Thosein classes I and I1 are heliwed to be xtrirtly linear and \vel1 defined structurally. I n order to simplify discussion of polyamides a notational system is used in which the components are represented by numbers, the first digit referring to the number of carbon atoms in the diamine and the second to the number of carbon atoms in the diacid. Thus nylon 66 (pronounced six-six) is polyhesamethylene ndipamide. ANALYTICAL METHODS
Solubility Class I. The nylons ivhich fall in Class I may b t . prepared by copolymerization of suitable proportions of variouh diamines, dicarboxylic acids, and aminocarboxylic acids. For example, a polymer prepared by the copolymrrization of 40 parts of 66 salt (hexamrthylenedianinioniuni adipatr), 30 parts of 610 salt ~hexttmethylenetiianimoniunisebacatel, and 30 parts of 6 it-aminocaprolactani) is soluble to the extent of about 15:; in the l o w r alcohols in the presence of water, and thus is amenable to determination of molecular w i g h t by titration- of tvid groups. The following procedure may h~~ w d : Samples of 2.00 grams of this nylon are dissolved by refluxing in 100-ml. portions of a 72y0by weight aqueous solution of ethanol. The samples are cooled to room temperature, stirred continuously a t slow speed, and titrated potentiomet‘rically with 0.1 *V sodium hydroxide or hydrochloric acid solutions, added from a microburet. p H measurements are made with a Beckman Model G hydrogen-ion meter with long-style glass and calomel electrodes. Special $1 electrodes for high pH ranges were not used.
\vherc n depends upon the molecular weight, and R and R’ may he alkylene chains containing difftsrent numbers of methylrms groups. If only a n amidation reaction is involved, estimation of the free 12 amino and acidic groups on the ends of the chains will provide a means of 10 dekrmining the number average molecular weight. T h e practicability of de- 8 termining the end groups G by titration drpends upon the solubility character6 istics of the polymer, which ran be made to 4 fall into three general classes:
-
I. Soluble in the lower alcohols, particularly in the presence of water. 11. Soluble a t room temperature in the phenols, formic acid, and halogen-substituted lower
fatty acids only. Strong mineial acids act as solvents hut degrade the polymer. 111. Insoluble and infusible.
0.0 0.8 1.6 i r i . of 0.1 A- IiaOH
Figure 1. Titration of 0 . O O O l .Y Acetic Acid 0 I n water
0 In 72% ethanol c 72% ethanol alone
448
T h e influence of the alcohol used iri these experiments on the position of the inflection point’inthe tit’rationof 0.0001S acetic acid (the approximate normality of the polymer solution) is shown in Figure 1. T h e inflect’ion point was found to be displaced in the alcoh’ol solution from the true equivalence point, as shown by aqueous titration, by a n amount equal to the quantit’y of base required to bring the solvent alone to the p H of the inflection point. I n all probability, by subtracting the blank titer, the true equivalence point can be found within the limits of determining the inflection point. I n any case, considering the reproducibility among repeat runs, the accuracy of molecular weight determination with this method is believed t o be equal to that of any othcr technique for determining the molecular weight of high polymers. -1typical titration curve for this polymer is shown in Figure 2. The corrected inflection points correspond to 0.112 niilliequivalrnt of carboxyl and 0.044 milliequivalent, of amine ends. The number of gram equivalents of end groups per gram of polymer are thereforc 56 X 10-6 COOH and 22 X 10-6 XH,, from which the
IULY
449
1947
nuintitar average molecular w i g h t is 2/(78 X 10-8) or 25,600 grams per mole. Two objections may be raised to this method of determining molecular weight-viz., (1) the procedure does not distinguish hetxeen terminal acidic and basic groups and any other acid or bast. in the sample, and (2) all molecules are assunicd t o tsrniinate in aridir or basic groups. Since a t present, no othrr method of suific.ient accuracy for determining niolecular \wight has becxn velopecl, t,hese objections cannot' be wholly met. Hoivever, thew is good reproducibility of titrations among samples from a sin&, lilend of polymer, and a linear relatioriship was found to exist b ( 3 t i w r n molecular weight determined by this method and the logarithm of the relative viscosity of the polymer solution.*. (Viscosity determined with a pipet-style viscometer a t a conwntration of 8.4c; in 9 0 5 aqueous formic acid. Such concentrated solutions were used to take advantage of their grc,atclr sensitivity to molpcular weight changes.) T t n ~ ~ uthus l d s w m likely that t h t w :I r~ t rui' inolt~t*ulxr IvtAight., . &%-
10
0.0
1.0
metrically measures all carboxyl in tht. polymer. Similarly, addition of hydrochloric acid solution suppressrs ionization of the --COO- ends, a n d the amine ends are quantitatively determined a t p H 4.5. If this is true, tht, distance t)titnec,n the two breaks on the carboxyl side should be a fair nitlasure of the number of amine ends. Examination of all curvt's showing two wll-defined hreaks shoived that such \vas the case. Solubility Class 11. The application of a similar method to the determination of the molrcular weight of the nylons in Class I1 deprnded upon finding a suitable solvent. Aitypical nylon in Class I1 is 66 (polyhexamethylene adipamide), which may have added to it some monofunctional group, such as acetic acid, as a inolwular w i g h t stabilizer. Since no evidence of loss of acetic acid during polymerization has been found, it is assumed that the number of acetylated amine rnds in the polymer can be estimated satisfactorily by the amount of acid addc,d before polymerization. Thus only terminal amino a n d carboxyl groups need be determined by titration. Several solvents for 66 nylon arr acidic in reaction and might permit, a sudden increase of hydrogen-ion roncrntration a t the equivalence point of the amine end-group titration, but the most satisfactory solvent which is not highly acidic is phenol. It was desired, although not rrquirrd, to dilutc. the solvent phenol considerably before titration, SO as t o provide less chance for ambiguity in t,he interpretation of the titration c u r v c s T o this end, it was found that if 2 grams of polymer ~ v e r etiissolvrd in 50 ml. of 885, phenol, the polymer was retained in a single-phase solution upon additiop of 25 ml. of 957( ethanol and 25 ml. of water. Commercially available phenol contains a n acid impurity which can be removed by treating with potassium cxrhonatt. anti sutisequent distillation. Curve B in Figure 4 shows the titration of 100 nil. of a purified phenol-alcohol-water solution with 0.1 .Y hydrochloric acid solut,ion. Curve A in Figure 4 shows the same titration when the solution contains 2.00 grams of a 66 nylon polymcr. T h e equiva-
2.0
A' NaOH Figure 2. Titration of Alcohol-Soluble Nylon in Alcohol-Water M I . of 0.1 A' HC1
hI1. of 0.1
Solvent alone
.in interest,ingfeature of the (wrve sho\vn in Figure 2 is the appearance of two inflections on the rarhoxyl side of the curve, suggesting the presence of two acids of differing s h n g t h . Although the carboxyl groups in the polymer originate from acids of differing strengt,h, after polymerization the carboxyl groups differ only hy whet,tier they are attached through methylene groups t o the group. Since innitrogen or the, carbon atom of a -COXHtwnally transmitted polar effects beconie negligibly after propxgation through two or three saturatrd carbon atoms ($) this situation n-ould not cause such a n effrct. I t was not,ed that when polymers ivtlre prepartd containing cxw s s diamine the two breaks n-ertL found on the a m i m s i d e of the curve, and therefore the hrraks appcwed to be associated wit,h an excess of one reactant, over the othrr. On the hypothesis t h a t salt formation was responsible for the two breaks, samples of polyiner were titrated for carboxyl ends with and without the addition o f formaldehyde to react with the amine ends to form methy1tLnr.imino groups, as in the Sijrenson method for titrating amino :wids. The result was thc. curves shown in Figure 3. IVith amine groups blocked off (curve 13) only one break \vas obrainrd. I t thus appears that if carboxyl groups are present in es('ess all of the amine ends present react, with some of the carboxyls t o form a salt. Cpon addition of sodium hydroxide solution t h e --C:OOH ends are titrated and this reaction is complete at, about pH 8.5. Further addition of base suppresses ionization of the -SH3ion, and this reaction is complete at pII 11 and stoichio-
li
00
10
0.0
1 IJ
2.0
M I . of 0 1 S S a O H
Figure 3. Titration of Carboxyl Groups of an Alcohol-Soluble Nylon Formaldehyde added to sample B
lence point, C, is found by subtractingfroni curve h the amount of titrant required to bring the solvent alone t o the p H of curve .1at the inflection point. I t was demonstrated hy titration of a knoa-n standard (66 salt) that this method accurately determined amine end groups, but for routine analysis a some^ hat simpler method ryas desired. Preliminary mrawrements of the conductance of the solution during titration showed a pronounced increase in conductance after the equivalence point was passed, and the following procedure was therefore adopted:
'
VOLUME
450 h 2.00-gram sample of dry nylon 66 is disolved in 50 ml. of purified phenol by shaking, 25 ml. of 95YC ethanol and 25 ml. of water are added, and, with constant slow stirring, the solution is titrated conductometrically with 0.1 S hydrochloric acid solution added from a microburet. The conductance cell consists of two platinum electrodes about 3 sq. cm. in area and 1 cm. apart immer:ed in the solution. Resistance is measured by a Leeds & Soitliruii re i-tance bridge using 110-volt, 60-cycle alteriiating curre..t and ail alternating current galvanometer as null-poi it instrume..t. The re istance is measured after successive 0.2-ml. additions of the acid, until t,lie equivaleiice point is passed 1)y 0.G to 0.8 ml. The reciprocal of resistance is plotted versus the milliliters of titrant, and the points lie o n two straight lines, the intersection of which estab!ishes the equivalence point. some typical titration curves are s h o m in Figure 5. Curre A is tlie titration of 0.05 millimole of 66 salt, curve 13 the titration of 2.00 grams of an alcohol-soluble polymer shown by potentiometric titration t o contain 36 x 10-6 gram equivalents of amine end groups per gram of polymer, :tnd curve C shows a typical titratioii of 66 nylon polymer. Tile agreement of curves .i and B n-ith t!ie known co:iceiltr:ition of amine is escc1lc:it.
,
The theory behind the titration is simple and sn:iighlforivard. .ipparently the niobility of t,he polymer molecules is so lon. that they need not be considered. Until the equivalence point is reached the addition of hydrochloric acid in solution adds the modwately mobile chloride ion t,o the solution, and tllc conductaric~,increases slightly in a linear fashion. Xftcr the cquivnlence point has been passed, addit,ion of hydrochloric :acid prouitlcn both the chloride ion :tnd the highly mobile hydrogen ion t o the! solution, so the conductance increases sharply in a lincvir fashion. The int,ersection of the t,wo straight lines, arising from :I plot of these points, is the equivalence point of the reaction. I n direct contrast to the potentiometric method in which much weight is placed on points near the equivalence point, in thib titration such points are given no weight, ( 5 ) . I n general, seven or eight measurements suffice to determine t,hetwo lines. T h e conductometric method was so simple and accurate t h a t it was subsequently applied with excellent results t o the titration of both amine and carboxyl groups cf alcohol-drtble polymers. a solvent for the dePhenol was, of course, not satisfactor terminat,iori of carboxyl ends in 66 nylon for, after t h r tquivalence point was passed, reaction of the h a w v i t h phthnol pic-vented any sudden increase in hydrosyl-ion conccntration. Efforts to detect the appearance of phrnolate ions by conductance nieasuremcnts were likewise unfruitful. S o other room temperature solvents 4 were found which permitted unambiguous 3 titration of carboxyl groups. Thesearch was then directed toward the 2 higher alcohols, some ' of which were known to dissolve nylon a t high temperature. 0.0 0.b l.(i MI. of 0.1 N HC1 The most promising solvent uncovered was Figure 4. Titration of 66 Nylon benzyl alcohol, which in Phenol-Alcohol-Water dissolves 66 nylon to B. Titration of s o l v e n t alone the extent, of about 10% a t 175" C. Although benzyl alcohol distills readily a t its boiling point without decomposition, no method has bcen found for preparing the completely neutral alcohol. Repcated distillations have given slightly acid products. I n addition the alcohol develops acidity on heating a t 175' C . at the rate of 0.005 t o 0.02 milliequivalent' per hour per 100 ml. This m w n i that a corrcction, which is ob-
-
L
1
0
2
0
2
1
NO. 7
19,
0
1
2
11' of 0 1 S HC1
Figure 5 . Conductometric Titration for Amine Dissol\ed i n Phenol- ilcohol-B'ater
tained tiy heating alcohol alone for the same length of time as the solution, is roquired for all titrations. The hot solvent is a satisfactory titration mcdium for carboxyl end groups, as is shown by the data in Tablc' I . T h e titrant was 0.1 S potassium hydroxide solution in 10TCniethanol-90yc benzyl alcohol and phenolphthalein \\-as uqcd as indicator.
Table I.
Titration of icids in Benzyl Alcohol at 17.5' C. \\eight, Grams
l\laterial Adipic acid 66 salt 66 s a l t Aniino-stearic acid Alcohol-soluble nylon 0
0.0182 0 0365 0 0830 0 060 3 00 Frorii t i t r a t i o n in ethaiiul a-titer
l l i l l i e q u i \ alent of Carboxyl Found Calculated (corr ) 0,250 0 2i8 0 683 0,200 0.159u
0 249 0 274 0.635 0 198 0.160
Tlic, ugrccmc~ntof obscrved va1uc.s with calculated i: good. For these tests and in subsequent work benzyl alcohol was kept at about 175' C. by keeping the solvent beaker in contact with the v:tpor of boiling p-cymene. It, is interesting to note t,hat in this system also titrations may be made conductometrically. The conductance is considerably lo\wr than for the phenol-alcohol-water solvent, and the break not so sharp, but good agreement of end point with phenolphthalein color change was found. SUMMARY
Met,hods are described for determining the number average molecular weight of alcohol-soluble nylons and alcohol-insoluble, phenol-soluble nylons. LITERATURE CITED
(1) Alexander, J., "Colloid Chemistry," Yol. 6, Chap. 60 (by J. B. Nichols), New York, Keinhold Publishing Corp., 1946. (2) Carothers, W. H. ( t o E. I. du Pont de Nemours and Co., Inc.), U.S.Patents 2,071,250 and 2,071,253 (Feb. 16, 1937). (3) Flory, P. J., J . Am. Chenl. Soc., 65, 372 (1943). (4) Gane, R . , and Ingold, C . K., J . Chem. SOC..1931,2153. (51 MacInnrs, D A , . "Principles of Electrochemistry," p. 384, New York, Keinhold Publibhing Corp., 1039. (6) Matthes, h., J . p r a k t . Chem., 162, 245 (1943). (7) Staudinger, H.. and Jiirder. H . , Kunsfseide u. Zellwolle, 24, 88
EERBISHURW3M W,
(1942). COSTRIBCTION
211;
