1016
INDUSTRIAL AND ENGINEERING CHEMISTRY
The stiffening effect of pigment in the polymer is sufficient to permit the storage and shipment of bales without packaging in conventional paper bags.
Vol. 38, No. 10
cooperation and interest of Rubber Reserve in the work and the permission to present this paper are greatly appreciated. LlTERATURE CITED
ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of their associates in this work, H. J. E. Segrave, R. J. Meyer, H. Z. Hurlburt, F. Price. The proportioning equipment was designed and installed under the direction of C. G. StronTe. The funds for carrying Out this program were appropriated the Office Of .. . Rubber- Reserve, keconstruction Finance Corporation. The
(1) Garvey, Whitlock, and Freese, IND. ENQ.CHEM.,34,1309 (1942). (2) Rongone, Frost, and Swart, Rubber A g e (N. Y . ) , 55, 577-82 (1944); Rubber Chem. Tech., 18, 130-40 (1945). (3) Smothers and Herold, Univ. ilfo. School Mines Minerd. Pub., 15,N o . 3 (Sept., 1944). PRESENTED before the Division of Rubber Chemistry at the 109th Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J.
Melting Points of N-Substituted Polyamides
B. S. BIGGS, C. J. FROSCH, AND R. H. ERICKSON
J
Bell Teleph,one Laboratories, Murray Hill, N . J .
L
INEAR polyamides, it is generally known, may be made by
condensation of a dibasic acid with a diprimary diamine (6). If a portion of the diprimary diamine is replaced by a primary-secondary diamine or a disecondary diamine, the resulting product is a partially substituted polyamide. Unsubstituted and partially substituted chains are compared in formulas 1 and 2. I n this paper the preparation of a number of such substituted polyamides is reported, and the correlation of their melting points with the degree and type of substitution is discussed. Unmodified polyamides, whether of the 6-6, 6-10, or 10-10 series, are high melting crystalline compounds insoluble in the usual solvents; when allowed to cool slowly from a melt, they are very brittle. As the hydrogen atoms of the amide groups are progressivrly replaced by alkyl radicals, these original properties are modified in the direction of lower melting point, greater flexibility, and greater solubility in ordinary solvents. I n general i t has been noted that, in a slowly cooled sample of a n N-substituted polyamide, there is good correlation between flexibility and melting point, As a m a t t e r of fact, in any given series the melting point is a better criterion of how much flexibility is to be expected than is the percentage substitution, since it makes a difference whether the substitution is furnished by a primary-
0
0
0
--N-(CH,)
I
H
/I
,-N-c-(CH,)
I
CHs
0
0
0
/I
Ii
n-C-A%--(~~2) ,--N--c(cH,)
I
H
I
H
Melting point curvee are presented for several families of polyamides in which the amide hydrogen atoms are progressively replaced with alkyl substituents. Lowered melting point, increased solubility, and greater flexibility are correlated with increased substitution, and it is suggested that these effects are due to a large extent to elimination of the hydrogen bonds between the chains.
secondary diamine or a secondary-secondary diamine, or both. For example, a 10-10 polyamide of 35Yc methylation prepared from a mixture of primary-primary diamine and secondarysecondary diamine melts a t about 170' C., whereas one of equal degree of methylation prepared from a mixture of primaryprimary and primary-secondary diamines melts a t about 133" C. These differences in structure are shown by formulas 3 and 4. I n a sample of the type of formula 4 only 17V0 methylation would be required to lower the melting point to 170" C. It thus appears that, for a given number of substituents, the maximum effect on the properties of the polymer is obtained when the substituents are as far apart as possible. The differences in melting point are well illustrated by the melting p o i n t-com p o s i t i o n curves of Figure 1. The area bounded by these curves includes all the melting- -points possible for substituted polyamides of the 10-10 series in which the substituent.is methyl. The boundary to the right (curve 1) is the melting pointcomposition curve for sebacamides prepared from mixtures of decamethylene diamine with pure N,N'-dimethyldecamethylene diamine. The boundary to the left (curve 4) is the analogous curve for mixtures of decamethylene diamine with N0 methyldecamethylene diamine. I1 The straight line a t the bottom (4) .-C-X-( CHJ,,-N(curve 5 ) is the curve for all mixi I turesof N-methyl decameth ylene H CHa
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1946 1 -PP-SS 2-PP-PS-SS
BY A D D I N G P P MIXTURE OF MAXIMUM METHYLATION, 3-PP-PS-SS ( a 2 , 2 a b , b 2 ) 4-PP-PS 5- PS-ss
1-PP-ss
TO
2 - p p - p s - s ~ e y ADDING PP TO
a2,2ab,b2
2001
1017
a2,2ab,b2
M I X T U R E OF MAX I MUM METHYL AT ION. 3 - p p - p s - s ~ (a2,zab,b2) 4- P P - PS 5-PS-ss
I
PERCENT M E T H Y L A T I O N
Figure 1. Composition-Melting Point Curves for hlethylated 10-10 Series diamine with NJ’-dimethyl decamethylene diamine. Mixtures containing all three of these compounds fall in the area enclosed. It will be noted t h a t the lowest melting point obtainable by methylation in the 10-10 series is about 63’ C. The Scurve connecting the high and low corners of the area, curve 3, represents the approximate melting points of all polyamides made from mixtures in which the primary groups and secondary groups are distributed in a perfectly random manner between primary-primnry, primary-secondary, and secondary-secondary compounds-that is, the relative proportions of these three compounds are a 2 ,Zah, and b*, respectirely, where a is the mole fraction of primary groups and b is the mole fraction of secondary groups (3). These are the mixtures which are obtained directly by the methylamine-ammonia hydrogenation process and therefore can be made economically. Curve 2 of Figure 1 represents all the mixtures obtainable by adding primary-primary compound t o an u2, 2ab, b 2 mixture of 62y0methyhtion, which is about the maximum obtainable under the hydrogenation conditions used, If a mixture of less thac 62% methylation is thus diluted with primary-primary diamine, the melting points will lie in the shaded area to the left of curve 2. The shaded aren includes all such mixtures possible. A similar melting point-composition diagram for the 6-6 series is shown in Figure 2. Since the highly methylated products are amorphous a t room temperature, the curves are necessarily less complete. Again, however, the area of practical importance lies i n the narrow shaded band. I n Figure 3 a single curve for ethyl and longer substituents (up to amyl) in the &6 series is shown with a n analogous methyl curve. I n these samples the substituent was furnished largely by random mixtures. Whereas an ethyl group has a greater effect on t h e
PERCENT METHYLATION Figure 2.
Composition-Melting Point Curves for RIethylated 6-6 Series
melting point than methyl, groups longer than ethyl have substantially the same effect as ethyl; this suggests that long substituents tend t o lie along the chain in the crystal. The same condition holds for the 10-10 series (Figure 43. Whether a group long enough to overlap the next polar group will exert a more pronounced effect has not been determined. Figure 5 shows representative c u n w for the 6-10 series t o indicate the general possibilities. These samples were made from random mixtures and hence fall in the practical arens. EXPERIMENTAL PROCEDUKES
PREPARATIOV OF DIaafIhEs.
The diamines used were ( a ) pure hexamethylene and decamethylene diamines prepared by hydrogenation of the corresponding dinitr~lesin the presence of liquid ammonia, ( b ) pure il’,N’-dimethyl hexamethylene and decamethylene diamines prepared by the reaction of the corresponding dibromides with a large excc‘ss of methylamine, and (c) mix-
1018
INDUSTRIAL AND ENGINEERING CHEMISTRY
1 -PP-PS-SS METHYL 2 - PP - PS -SS ETHYL-PROPY L - A M Y L
PERCENT ALKYLATION Figure 3. Composition-~Ieltin& l'oiiit Cunei for Alethylated and Higher .ilkylated 6-6 Series
tures of methylated diamines obtained by hydroi.erintion of the correspontlinrr dinitriles in methy1:rmine or methyhmiiie :ind ammonia ( 3 ) . The: mixtures cont:iining only S-metliy1dec:rmethylene dinminc and S , . ~ ' - d i m c t i i y l d e c : i ~ e t h y l ~diamine, ~ie such as iited in curve 5 of Figure 1,u-ere o1it:iiwd by removing nr?- derivative from a random mixture as tlic sebao:ite salt whicii is insoluble in ethyl alrohol. From the rccoveied seconti:iry amine mixture, a solid fr:iction rich in the diwcondary compound :ind a liquid fraction rich i n the primary-secontlnr: compound can be isolated. -4 pure sample of the primarysecondnry compound \vas not obtained; hcnce tlie dotted c u r w 4 of Figu1.e 1 is not a n experimental curve but merely repi a reasonable guess as t o the 1oc:ition of the primary-secondary curve. 'I'hc. anchor point is approximately fixed by the intcrsection of thc straight line 5 n-ith vertical line of 50% methylation, which must necessarily contain the product made from pure primary-tecondary diamine. The composition of all mixtures w:is determined hy the modified Van Slyke analysis. PREPAILITIOX O F POLY.UIIDES. The usual procedure for making polyamides consists of heating the salt of the dibasic acid and diamine in a n inert atmosphere either a t atmospheric pressure
Vol. 38, No. 18
or in a vacuum until a high polymer is obtained. This is not usually fensitile n-ith the substituted polyamitlcs, since the salts are not c a d y obt:iined in good crystalline form and do not in all cases represent stoichiometric proportions-that is, the acid s:tlts :ire in some cases more stable or more insoluble in normal solvents than arc the neutral sslts. In general, therefore, the substituted polynmidcs were prepared by heating together correctly weighed proportions of dibasic acid and diamine mixture, taking arcount of the fact that the molecular \veight of the diamine wries Jvith the per cent substitution. Reaction is slower with secondxy amines than with primary, and hencc it w:is usur y to react them overnight a t atmospheric prrwure ying vacuum, since otherwise diamine would be lost and thc proportions changed. Even a t best some serondary amino is lost because of the sloiv stream of hydrogen tvhich is passed through the sample to preserve a n inert atmosphere; hence tlie quoted per cent methylation can be in error by per cent. The exact composition of the finished polymer can be obtained by hydrolyzing it in strong hydrochloric acid ant1 reanalyzing the dinmines by the Van Slyke method. In cases wherc the d e m c of methylation is high, it is desirable to carry out the re:irtion in n sealed, evacuated tube in the early stages and then transfer the product to the usunlappnrntus. One might think that n loss of diamine would prevent further condcnsntion from taking place, antl this is true if the loss is large: but if 50 molecules of dibasic acid are tied u p in the final polymer (a good molecular weight for polyamides), the amount of din mint^ bound can w r y from 51 to 49 molecules, a maximum variation of about 4 7 on :L m i g h t basis. I n order t o assure retention of enough diamine to r e x h a rensonnbly high molecular neipht, it is convenient to start with 1 to 2Q" excess diamine. The tcmper:iture of reactionis from 200" to250°C., depending on the composition. I n the nppnr:itus used, 6-6 polymers tend t o discolor a t thc high temperaturc more than do 10-10 polymers. A conrenicnt visual criterion of the prorrreis of the reaction is the speed with which a bubble of hydroCen rises through the mixture. The reaction is stopped when tlie viscosity of the polymer is so great that the channel made by a bubble from a regulated slow flow of &is fails t o clox before t h r x nc'xt bubble rises. TION O F ~ I E L T I XPoIsTs. G Polyamides are in illine materi:ds: the crystallinity manifests itsclf in ien the polymer is melted it loses its opacity (or translucencci and its rigidity. and it becomes a viscous liquid. The substituted polyamides are not only lo\T-er melting tlian the unsubstituted, but : i h i tent1 to be less crystalline below the melting point: lic~ncctlie tlc,terniination of an accurate melting point is quite difficult. Ttic method used in this work is as follows: Fibers or thrc,:id.: ot' tile polymer about inch in diiinietcr and 1 inch lung were c.l:imped in a bath of clew mineral oil ne:ir the bulb of :I thermometer. The oil n-as stirred rapidly as it was heated antl, \Then the tcmIieratui-e of the b:ith reachecl the melting tc.mpi.i.:iturc of t h e polymer, the thread lost shape and was i n the oil stream. Careful observation also r e v d e d in transparency as the melting point is reached. I n t h e h:inds of a n experienced operator the values are roproducible \T-ithin about * Z o C. DISCUSSION
The 6-6 and 10-10 polyamides are much higher melting and much harder tlian the corresponding crystalline polyesters ( 8 ); this is thought to be due t o the high degree of hydrogen bonding in the polyamides ( I , 10, 1 8 ) . The replacement of an amide hydrogen by an alkyl group prevents the formation of this bond, and in the case of methyl, a t least, this is probably the principal effect of alkylation. There is in addition a steric factor derived from the irreguhrities presented by the alkyl groups and analogous t o that encountered in polyesters prepared from substituted glycols or substituted dibasic acids ( 4 ) . This steric effect is
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1946
1- PP- PS - 5 5 M E T H Y L 2 - P P - P S - S S ETHYL-BUTYL-AMYL
601 0
'
10
I
I
0
20 30 40 50 60 PERCENT ALKYLATION
70
80
Figure 4. Composition->felting Point Curves for Alkylated 10-10 Series
. . . 1-PP-PS-SS M E T H Y L 2-PP-PS-SS
ETHYL-PROPYL-AMYL
230
1019
well illustrated in the 6-6 polyamides derived from P-methyladipic acid with hexamethylene diamines, and adipic acid with 3-methylhexamethylenediamine. Here the methyl groups do not replace bonding hydrogen atoms, although they do undoubtedly weaken the bonds t o some extent. The melting point of the former is variously given as 190' C. and 216' C., whereas that of the latter is said to be 1SO" C. (5, 6, 9). Even though these depressions from the melting point of the unsubstituted 6-6 polymer are quite large, they are considerably less than that caused by the S- methyl group of the polyamide derived from adipic acid and N methyl hexamethylene diamine, which has the same number and distribution of methyl groups but completely loses one hydrogen bond for every methyl group. The melting point of this polymer is estimated in Figure 2 to be about 145' C. K i t h ethyl groups the steric factor is somewhat more important, but increases in chain lencth of the substituent, a t least up t o amyl, show no greater effects than ethyl. These polymers differ from unsubstitutcd polyamides which have been made flexible by quenching. Quenching freezes the chains in a n amorphous condition, and hence the product is flexible; but the N-hydrogen atoms are still present and capable of bonding to carbonyls of adjacent chains. Thus the melting point and solubility are unchanged, and the flcxibility is lost when the polymers are annealed a t elevated temperatures. The substituted polyarnides may also be quenched, in which case there is superimposed on their inherent amorphous content a n amorphousness resulting from freezing the chains in a liquid state; but the flexibility mentioned is t h a t Jvhich the polymer retains even after proloneed heat treatment below the melting point (when crystallization has progressed as far a s possible), and is thus a permanent or built-in effect. The polymer is internally plasticized in the manner of cellulose acetate butyrate (11). It is interesting to compare the melting point curves of the substituted po1y:imides lyith those of the copolyamides or interpolyamides such as are derived from reacting mixtures of two diprimary diamines with two dibasic acids ( 7 ) . I n the latter cases the melting point curvcs are concave upn-ard; this indicates that, in the middle regions, hydropen bonding is r e h c t e d by the staggering of the amide groups along the polymer chain. The substituted polyamitles described in this paper are not copolyamides in that sense, because the repeating unit3 have not been disturbed and the melting point falls steadily from that of the pure primary-primary polymer to that of the pure secondnly-becoridai y polymer; .ilthough the flexibility mid toughness of these polymers are desirable in themselves, they are gained a t the expense of certain other properties, since the same mobility and openness of structure which produce them also tend t o increase the dielectric constant and moisture absorption (Z), a fact IThich vvould limit their usefulness for many applications. LITERATURE CITED
(1) Baker, IT. O., and Fuller, C.
Figure 5. Composition-3Ielting Point Curver for Rlethylated and Higher Alkylated 6-10 Series
S.,J . Am. Chem. SOC.,64, 2399 (1942). (2) Baker, W.O., a n d Yager, W.A , , I b i d . , 64,2171 (1942). (3) Biggs, B. 3., and Bishop, V. S . , IND.ENG.Crimf., 38, 1084 (1946). (4) Bigge, B. S.,and Fuller, C. S..Chem. End. .Vews. 21, 962 (1943). ( 5 ) Carothers. W. H., U. S.Patent 2,130,948 (Sept. 20, 1938). (6) I bid., 2,190,770 (Feb. 20, 1940). (7) Ibid., 2,252,554 (Aug. 12, 1941). J . 4 m . Chem. SOC.,54, 1559 (8) Carothers, W. H., and Hill, J. IT., (1932). (9) E.I. du Pont de Kernours & Co., Brit. Patent 503,376 (April 5 , 1939). (10) Huggins, 51. L., J . Org. Chem., 1,440 (1936). (11) hlalm, C. J., Fordyce, C. R., and Tanner, H. A . , IND. ENO. CHEY.,34,430 (1942). (12) Zellhoefer, G . F.. Copley, ?*I.J., and Marvel, C. S., J . Am. Chem. Soc., 60, 1357 (1938).
