DIELECTRIC PROPERTIES AND STRUCTURE OF LINEAR POLYAMIDES
Sept., 1942
2171
120' temperature range for one) for the frequency with maxima at low temperatures. This supports interval 1 kc. to 75 mc. These macromolecular the concept of small oscillating units in the chains, chain polymers are of known structure and com- in agreement with the observed activation energy position, so that the observed polarization and dis- of orientation. persion were related to the orientation and conThe packing of the chains is strongly influenced centration of polar groups in the chains and to by the dipoles and the formation of dipole layers their relative positions in adjacent chains. The makes the interaction, which contributes to didielectric constants, E', of all of the polymers ex- electric absorption, largely independent of polar ceed the refraction value even a t room tempera- group concentration. ture. The oscillation of individual dipole groups The dielectric results reveal thermal motion in (ester linkages) contributes the orientation polari- the polymers. These chain oscillations are supzation. These dipoles interact somewhat along a posed to account for mechanical properties such given chain, but chiefly between chains, to cause as thermal retraction associated with chain kinkthe observed broad dispersion. The polyesters ing in long chain molecules. exhibit principally high frequency absorption, MURRAYHILL,N. J. RECEIVED NOVEMBER 13, 1941
[CONTRIBUTION FROM BELLTELEPHONE LABORATORIES, INC.]
The Relation of Dielectric Properties to Structure of Crystalline Polymers. 11. Linear Polyamides BY W. 0. BAKERAND W. A. PAGER Linear condensation polymers have been selected as appropriate for study of the effects of molecular structure and molecular order on the dielectric properties of solids.' They represent an important class of structural and insulating plastics. Polyesters have been treated previously and the general implications of such an investigation have been reviewed. The present report includes preliminary results on the polyamides, in which the -0- of the ester linkage has been replaced by the -NHgroup. conditions for extensive hydrogen bonding have thus been introduced, and the whole structure resembles that of polypeptides and proteins. The dielectric measurements indicate great mobility in the alternating field of some atomic group, possibly a charged hydrogen. The close analogy of the polyamides to the polyesters allows assignment of the observed differences to the NH group. Certain of the results may be examined for direct evidence of isomerism in the amido linkage. Thus, the linear polyamides may assist, as models, in elucidating the structure of proteins. Experimental Materials.-The polyamides were obtained commercially (du Pont Company) or from the procedures of Carothem2 They were pure, white (1) Yager and Baker, Tars (2) Carothers,
U
JOURNAL, 64, 2164 (1942). S Patents 2,071,250 (1937) and 2,1~0,3!43(18M).
polymers with weight average molecular weights greater than 10,000. All were carefully protected from degradation during preparation of the test discs.l The co-polyamide was of the 50-50 composition noted in the patent describing it.8 All samples were annealed to states of maximum ~rystallinity,~ unless otherwise noted. The sample discs were molded and machined to a two-inch diameter and 50-mil thickness, were thoroughly dried over phosphorus pentoxide, or conditioned as noted, and equipped with electrodes as previously described.6 Dielectric Measurements.-The bridge and Q-meter apparatus and technique used for the polyesters were again employed. The dielectric loss of polyhexamethylene sebacamide a t and above looo, and of polyhexamethylene adipamide a t 100% relative humidity was too high for direct measurement on the Qmeter. Consequently, the top electrode of the test cell was raised above the test specimen, thus introducing a series air gap. The effective capacity, C , and loss, tan .,6 of this two-layer arrangement were determined on the Q-meter and the true capacity, C,, and loss, tan ,a, calculated from the equations (3) Carothers, U. S. Patent 2,191,367 (1940). (4) FulIer, Baker and Parre, THIS J O ~ N A L , 62, 3275 (1940); see a180 Baker and Fuller. ibid.. 64. October (1942). (6) Yagu, Trem. Ebclrochrm. Soc , 74, 118 (19;s)
2 172
LV. 0 . RAKER AND W. A . YAGER
VOl. 64
parison. Apparently an appreciable orientation polarization contributed by the motion of the c. cin 1 Cx=-------. polar groups is present in all of the solids. The C. - Cm 1 + tanz 6, polyester has the same concentration of polar links where Ca represents the series air capacity. C, in the chain as the polydecamethylene sebacamide was calculated from the above equations by meas- of curve C (9 methylene groups per linkage). uring C, and tan ,6 and then the corresponding The polyamide exhibits significant dispersion a t C, and tan 6, directly on the capacitance and room temperature. The restraint of orientation conductance bridge. found in the polyesters' appears much increased in The direct current conductivity of the poly- the polyamides, in accord with their higher meltamides was determined after the application of 100 ing points and mole cohesion. The sequence of E ' volts for one minute, by use of a k e d s and values in Fig. 2 also shows expected increased Northrup H.S. type galvanometer. polarization in the polyamides with increasing concentration of polar groups in the chain. Curve Results and Discussion C, for polydecamethylene sebacamide, with 9 CHo The structural formulas of tw70 typical polygroups per polar linkage is followed by curve B amides appear in Fig. 1. Evidently, the concen(polyhexamethylene sebacamide) with 7 and tration of polar groups may readily be varied Curve E, for polyhexamethylene adipamide, with methylene "spacers," as in the polyesters.' with 5. Evidently polyhexamethylene adipamide possesses a broad maximum in a" a t room temperature. This breadth and height of maximum indicate a distribution of relaxation times, generally found L in solids. The E'' values for the polyamides have been corrected for d. c. conductivity, which, a s POLYHEXAMETHYLENE SEBACAMIDE noted later, is appreciable for these pure compounds. The E " curves in Fig. 2 do not follow the order of the E' for increasing values of the ordinates. This is related to an important aspect of the dielectric behavior of linear polymers. The concenPOLY HEXAMETHYLENE ADlPAMlDE tration and resultant moments of the polar groups Fig. 1.-Structural formulas of typical linear polyamides. in the chain determine the essential polarity of Likewise, the polar groups in adjacent chains occur the polymer, but it has been emphasized that the in layersJ4and their interaction includes hydro- interaction with neighboring chains chiefly governs gen bonding6as well as dipole and dispersion forces. the observed loss. The following discussion inThe presence of a bonding hydrogen profoundly dicates some factors causing a variation in interalters the properties of the polymers. The pri- action and hence in loss. In the melts of these mary valence chains wander in kinked paths polymers, strong dipole association facilitates the through the crystalline and intercrystalline mat- formation of the dipole layers observed in the t e ~ -but , ~ in addition a network of secondary forces solid. This layering is distorted, however, in the (hydrogen bonds) obtains. Thus, the interchain portions of the polymer which are imperfectly or forces, and hence the restraints on dipole orienta- hardly crystalline, and hence the dipole interaction, should exceed those for the polyesters, as tion would be expected to vary with the degree of confirmed below. Such predictions of polymer molecular organization present in the solid studied. properties from structural formulas have been This degree of order controls considerably many physical properties of the plastics, such as elastic possible only recently. ~ some correlation between phyFigure 2 shows the dielectric behavior of several m o d ~ l u s . Hence, sical and mechanical properties and dielectric bepolyamides a t 25', with the curves A for polyhavior is a necessary, and observed, consequence decamethylene sebacate included for direct comof solid structure. The variety of ordered states (6) Pauling, "The Nature of the Chemical Bond Cornell Uniwhich may be produced in natural products like versity Press, Ithaca, N Y , 1939, Chap I X "
Sept., 1942
DIELECTRIC PROPERTIES AND STRUCTURE OF LINEARPOLYAMIDES
2173
cellulose derivatives7 should likewise 4.2 be reflected in their dielectric values ; the &ect is general. Clearly, the 3.8 dipoles may be disposed differently 3.4 in adjacent chains either by shifting along the chain, or rotation about its 3.0 axis. Both of these conditions are present in the non-crystalline regions 0.30 of linear polymers, as diffraction studies ~ h o w . ~ JIf the argument above that the longer relaxation * 0.20 time of the polyamides compared to w 0.10 the polyesters is caused by strong intergroup action, principally hy0 drogen bonding, is correct, then al103 104 106 106 107 108 tered relative positions of the groups Frequency in cycles per second. should alter the dielectric loss. This Fig. 2.-Frequency variation of dielectric constant, e’, and loss, e’, of is shown by Curves C and D of Fig. polyamides at 25”: A, polydecamethylene sebacate; B, polyhexamethylene 2. CUNe C is for annealed poly- sebacamide; C, polydecamethylene sebacamide, annealed; D, polydecadecamethylene sebacamide, in which methylene sebacamide, quenched; E, polyhexamethylene adipamide; F, polyhexamethylene-decamethylene adipamide-sebacamide. the polar groups are as efficiently packed and as extensively hydrogen bonded as the adjacent chains will be unable to coincide to form lattice-forming deficiencies of long chains permit.7 a layer, and will be found surrounded by the The sample of curve D, however, was the same hydrocarbon portions of the m01ecules.’~ The polyamide rapidly quenched from the melt. It compound of curve F was thus obtained from hexacontains relatively few crystallites, and the chain methylenediamine, decamethylenediamine, adipic segments are rotated so that the hydrogen bonds acid and sebacic acid.3 Its average polar group are, on the average, less efficiently directed and concentration is one per 7 methylene groups, so formed than in the annealed compound. They that it should compare with curve B, yet it shows resemble the hydrogen bonding in a liquid rather markedly increased dielectric constant and loss. than in a well-ordered solid. Still, they are strong The average composition of the copolyamide enough to stabilize the chains in their disordered chains is the same as that of polyhexamethylene configurations of higher free energy (compared to sebacamide, but the irregular spacing between the crystal) for long periods, as shown by the rate linkages in the molecule reduces the hydrogen of crystallization of polyamides in the solid state.4 bonding and other interaction in the solid so that However, the reduced forces in the quenched ma- e‘ and E “ are much enhanced. This is further terial permit augmented though still hindered di- emphatic indication that the chains do not act as pole freedom, and hence curve D lies clearly above units, but that each individual dipole is sensitive curve C, for e‘ and E ” . These differences were to its own environment. Also, the intrachain caused not by a change in the composition of the coupling would be little different in the copolychains, but only in their relative positions. amides than in the normal polyamides, so seemThe structural variations possible in condensa- ingly the interchain action alters the dielectric tion polymers allow another test of the hy- properties. The effect of these changes in interpothesis that interaction is modified by relative action in the quenched and co-polyamides on the positions and hydrogen bonding of amido links. distribution of relaxation times in the polymer If a polyamide is prepared by the intercondensa- alone forms an extensive study. tion of two different dibasic acids and two differThe orientation of polar groups is probably not ent diamines, say in equimolar ratios, some of the the sole source of high polarization in polyamides, polar groups will associate as usual, but, along a especially a t elevated temperatures. A new chain, the “repeating units” are quite random in mechanism which we associate with the isomerism composition, so that many of the dipole groups in of the amido linkage seems necessary to explain the YI
(7) W.0. Baker, C. S. Fuller and N. R. Pape, THISJOURNAL, 64, 776 (1’242).
(7a) W.0. Baker and C. S. Fuller, “Conference on Physics of the Solid State,” N. Y.Academy of Sciences, February, 1942.
w. 0. BADR AND w. A. YAGER
2174
VOl. 64
within and between folded polypeptide molecules,sa and between the 30 polyamide molecules here treated. Hugginsg has proposed that the hy25 drogen in the bridge i s virtually ionic. At least one may regard it as in one 20 of two potential minima, one provided by the oxygen arid the other by its 15 ''own" nitrogen. These two minima are separated by a hump, over which 10 charged hydrogens with requisite energy may oscillate.'0 The dielectric results are consistent with a concept 5 embodying mobile, charged atoms 0 which can contribute an atomic po1 larization and anomalous dispersion Frequency in cycles per second. by moving translationally (vibrationFig. 3 -Frequency variation of e' of polyhexamethylene sebacamide, 27.6 ally) from one potential minimum to 146'. to another in the dipole layers. This succeeding results, as in Figs. 3 and 4, for poly- motion should be strongly temperature-dependhexamethylene sebacamide, which 27 are typical of the series. Some repre24 sentative values are also given in 21 Table I. At 1 kc. E' has a value at 18 146' (70' below the melting or soften15 ing point of the polymer) of about 12 48, and e'' is about 27. In addition, 9 as is seen from its temperature varia6 tion in Fig. 7, there is a relatively high d. c. conductivity in the compound. The galvanometer behavior indicated that this was not from a space charge. These compounds are likewise free from ionic impurities, as determined by ashing, extraction and quantitative analysis. 103 104 105 108 107 108 Frequency in cycles per second. The isomerism of the amido linkage has been chiefly considered in the Fig 4.-Frequency variation of e' of polyhexamethylene sebacamide 27 6 to 146'. structure of polypeptides and proteins. Hydrogen bonding is important in the ent, which is supported by the data of Figs. 3, 4 molecular configuration of these compounds.s A and 5. The high frequency E" maxima of Fig. group of the type shown here probably occurs 4 represent a second mechanism and seemingly Ho . correspond to the maxima observed a t lower temperatures for the polyesters. They occur a t eleII vated temperatures presumably because the os0 cillations of, say, the carbonyl groups are inhibited H by hydrogen bridges of the sort shown in the diagram above. From curve A of Fig. 6, obtained \c/N\c/ as noted before for the polyester' (curve B), E = II 35
/cbP\
0
(8) For a revicw und discussioir day sot.,
as, 871 (1040).
bee
W. T. A i t L i r y . 7 ' 1 o ) r ~E.
d ~ u -
(8a) Frank, Nalurc, 198, 242 (1936). (9) M.L.Huggins, J . O v g . Chcm., 1, 407 (1937). (10) This id& w8b forwarded by M. 1. Huggins in a discu*iuu iu Apd,
1889.
DIELECTRIC PROPERTIESAND STRUCTURE OF LINEAR POLYAMIDES
Sept., 1942
217.5
109 5
108 ; I
5
4
107 5
106 22
3.0 3.4 3 8 4.2 4.6 1 / T x 1000. Fig. 6.-Temperature dependence of the relaxation time of the dielectric, 7 6 , for polyhexamethylene sebacamide (curve A) and polyethylene sebacate (curve B). 2.6
bridge hydrogen. It may pass over to one or the other of the positions near the 0 or N but possibly it can also acquire even more freedom and act like a migrant ion over very short distances, thus caus. ing the high d. c. conductance. The temperature coefficient of this conductance appears in Fig. 7, 20
I
,
1
#
,
1
I
,
,
/:
12 yl
8 4
,
2.4 I
2.2
60 80 100 120 140 160 Temperature in "C. Fig. 5.-Temperature variation of e' and e " of polyhexamethylene sebacamide, 1 kc. to 75 mc. 20
40
Further, the activation process apparently produces another sort of enhanced freedom for the
2.6 3.0 l / T X 1000. Fig.;.-Temperature dependence of the d. c. conductivity, r, of polyhexamethylene sebacamide.
and corresponds to an activation energy of 29,700 cal. per mole of conducting particle. This value is several fold the clissouation energy for a hydro-
u'.0. RAKER
2117(i
AND
111' 1(P 106 10 Frequency it1 cycles per second. Fig. 8 -Effect of sorbed water on frequency dependence of e' and e'' of polyhexarnethylene adipamide a t 25' : dry (curves A), 60% R H (ciirves PI) and 100% R. E1 (curves C) 103
gen bond," and may well reflect the energy necessary to liberate charged hydrogens from restraining negative force fields around a given N or 0 atom. It is less than half the energy of the N-H bond, but one would expect this link to be perturbed and weakened by the hydrogen bridges it1 the solid polyamides and by its adjacent carbonyl. i It should be emphasized that the atomic or nuclei motion cannot contribute, however, to the resonance in the amide linkage.) The d. c. conductance and low frequency loss may thus represent different degrees of the same sort of hydrogen motion. They are not t o be regarded as measuring \rery different phenomena. The low frequency loss may not exhibit a maximum. Xolten polyamides ,ind the solids at higher temperatures thus are ,ipparently semi-conducting materials. I t is a striking contrast that the relaxation time the dielectric is 5 X lO-'sec. for polyethylene irbacate a t - 1'3" and for polyhexamethylerie seba ratntde a t 146'. This large difference may be regarded chiefly ai a consequence of the effect of molecular structure on the solid state. The isolated molecules would show nu such different kinetic characteristics, nor would they be expected to show diverse intrachain coupling. On the other hand, the polyester melts a t 74") the polyamide at 'l.Jc, a 141' interval emphasizes the different forces. The relaxation times reflect thermal motion present in the polymers irrespective of the applied field, and hence are related to the elastic and plastic properties of the solids. The explanation of hydrogen bridging and charged hydrogen motion in these bridges as the Source of the extraordinary dielectric properties of i);
( 1 1 ) b e e review in J \oc
36, 807 ( 1 9 4 0 ~
J 1.nL i r i d A E
W. A. YAGEK
Vol. ti4
typical linear polyamides suggests that strong hydrogen bonding agents added to the polymers a t ordinary temperatures should produce the same effectas elevated temperatures alone. This is confirmed in Fig. 8 for water, whose sorption is of technical importance. Curves A were obtained from polyhexamethylene adipamide dried two weeks over calcium chloride, curves B, for the sub-stance equilibrated (fifty-six days) a t 60% relative humidity, and curves C, after immersion in distilled water, all a t 25". Evidently the coinbination of bound water (about 10% for saturation at room temperature) and the amido groups causes a high dielectric constant and appreciable dispersion. e'' is again corrected for d. c. conductance. Water may be here considered as a loosening (plasticizing) or pseudo-ionizing medium, which substitutes for intermolecular polyamide bonds, and facilitates motion of the whole system.; Thus, if the dielectric results are admitted t o show enhanced interchain motion directly, the mechanism discussed above could apply to the dependence on PH and degree of swelling of the physical properties of polypeptides and proteins. For example, the molecular motion in muscular contraction would be controlled by the degree of interchain force reduction, and hydrogen bonding suppression, as indicated by many other researches. The present results also cause inquiry of how much wandering of hydrogen atoms among peptide linkages, and hence energy exchange, occurs in proteins swollen or thermally agitated. This latter is related to the hypothesis of energy transfer recently proposed by Szent-Gyorgyi." We are grateful to Dr. I3. S. Biggs and Mr. W. S . Bishop for providing certain of the polyamides studied.
summary 'The dielectric constant and loss have been determined over a 120' temperature interval and over the frequency range 1 kc. to 7
