FURTHER IXVESTIGATIOS O F THE CHAIN STRUCTURE O F LIXEAR POLYESTERS C. S. FI'IJI,i3R
AND
C. .J. FROSCH
Bell Telephone Laboralories, Inc., New Y o r k , New Y o r k
Received
Jiilg
20, 1938
INTRODUCTION
The crystalline or pseudocrystalline riatme of high polymcrs is of profound importance in determining the physical properties of these substances. Because of their polycrystallinity, however, the inforinntion that can be deduced concerning their crystalline structure from x-ray studies is limited. Lack of definite information regarding the chemical structures of many of the compounds heretofore investigated has further complicated this problem. By the examination of a number of closely related compounds of known chemical structure this latter difficulty is avoided, and at the same time an added advantage is provided by thr progressive changes that are known to occur in homologous series. In a previous paper (5) certain general features of the x-ray diffraction patterns of a number of linear polyesters of known chemical constitution were considered. It was shown that oriented fibers of these compo~unds produce sharp x-ray fiber patterns, which are characteristic of the chrmical repeating unit present. It was also pointed out that the long-chain molcrules in some of these compounds conform closely to a planar zigzag arrangement of the chain atoms, whereas in others a helical or folded form of chain was required in order to explain the experimental results. I t is the aim of the present paper to present systematically the data now available on the ethylene series of polyesters and to reinterpret the results on the basis of new information. In doing this, use is made of the older data as well as of recent results on polyethylene suberate. Data on the. polymeric self-polyester of o-hydroxydecanoic acid are also considcrcd with the results on the ethylene compounds. This ester possesses n particularly simple chain constitution and is most conveniently treatrtl at this time. The methods employed for preparing the compounds and the x-ray technique employed will not be given here, since details are availablr in
' Presented a t the Fifteenth Colloid Symposium. held a t Camtiridge. Alassachusetts, June 9-11, 1938. 323
D
C
F
E
FIG.1
(see facing page for legend)
324
CHAIN STRUCTURE OF LINEAR POLYESTERS
325
other publications (2, 5 ) . It should be mentione”, however, that in the present work efforts have been made to improve the accuracy of measurement of the specimen-to-plate distance by automatic standardization with sodium chloride. This was done by dusting the fiber bundles with the finely divided salt (1). The results so obtained were found to agree with those previously reported to within the experimental error (approximately 1 per cent). COMPOUNDS STUDIED
In order to secure data on other members of the ethylene series than those which were studied in the previous work, attempts were made to prepare polyethylene oxalate, malonate, glutarate, and suberate. Only in the last case, however, was i t found possible to secure a product suitable for x-ray investigation. The polymeric self-ester of w-hydroxydecanoic acid was prepared from the pure acid2 by heating for several days in a stream of pure nitrogen gas a t 200OC. Both of the polyesters had average molecular weights greater than 15,000, as measured by viscosity methods, and could be cold drawn readily into highly oriented fibers suitable for x-ray investigation (2). RESULTS
The x-ray fiber patterns corresponding to the ethylene polyesters that have been prepared to date are given in figure 1 (A to E). Figure 1F shows the pattern obtained from the self-ester of w-hydroxydecanoic acid. Under each photograph is given the formula of the chemical repeating unit, together with the observed identity period as obtained from the fiber diagram and the value of this period calculated on the assumption of a planar zigzag chain. The identity periods for the succinic, adipic, FIG. 1. Fiber patterns of the polyesters A. Polyethylene succinate B. Polyethylene adipate -O(CHz)20CO (CH2)zCOZ (calculated) 9.70b. Z (observed) 8.32b.
-O(CHz)zOCO(CH1)rCOI (calculated) = 12.21 A. I (observed) = 11.71 A.
C. Polyethylene suberate -O(CH,),OCO(CHz)&OZ (calculated) = 14.73 Z (observed) = 14.1 A.
D. Polyethylene azelate -O(CHz)zOCO(CHz)rOI (calculated) = 32.0A. I (observed) = 31.5b.
E. Polyethylene sebacate -0 (CHz)zOCO(CH2)sC0I (calculated) = 17.24 A. I (observed) = 16.83 A.
F. Poly-w-hydroxydecanoate -0 (CHZ)oC0I (calculated) = 27.30 A. I (observed) = 27.1 d.
E
E
4.
* The authors are indebted to Mr. W. S. Bishop for the preparation of the pure acid.
326
C. S. FULLER AND C. J. FROSCH
TABLE 1 Interplanar spacings and fiber periods o j polyesters POLYETHYLENE IUBEBATE
U-HYDBOXYDECANOIC EBTEB
I'
I
vs
4.11 3.61 2.93 2.49 2.38 2.17 2.06
VS S M
w S
W 13.8 13.8 (14.8)
6.45 4.08 2.93 2.43 4.27 3.68 2.78 2.51
14.2 13.8 14.1 14.1 14.1 14.2 13.8
IV
3.19 2.53
14.2 14.1
hl W
2.57 2.35 2.21
14.1 14.1 14.1
M M
(2.28)? 2.12 2.00 1.85
J3.7) 14.2 14.2 14.1
M VS
* I = identity
Intensity
vs vs
4.17 3.73 2.99 2.49 2.36 2.22 2.08
M M
vw S M
VS
12.60 6.30 2.66
Mean fiber period
t
Intsnsityt
w
VW 13.40
26.8
vs
6.65
(26.6)
vw
4.47
26.8
W
3.33
(26.6)
2.21 2.11 2.06 1.92 1.75
26.9 27.3 27.2 27.2 27.4
S
w VW VW W S
w
W
M hl
14.1 f 0 . 0 5
period. V = very; iV = weak; S = strong; ILI = moderate
27.1 f O . l
vw vs M M M W
CHAIN STRUCTURE O F LINEAR POLYESTERS
327
and azelaic esters are those previously published ( 5 ) . For the sebacic ester recent measurements favor the value 16.83 f 0.05 d.,which is somewhat higher than that previously reported. The results of the measurements on the suberic and w-hydroxydecanoic esters are given in table 1, in which the conventional way of designating the reflections has been followed. The corresponding measurements on the other esters have been given previously ( 5 ) . INTERPRETATION O F RESULTS
A . The fiber periods From figure 1 (A to F) it is evident that the succinate ester, the lowest member of the series which has been prepared in suitable form for study, shows a decidedly different pattern from the succeeding esters. This has been interpreted (5) as indicative of a coiled or helical form of chain molecule, since the observed identity period in the fiber direction is much too short to be explained in any other way. The other esters in figure 1 agree well with a planar zigzag form of chain. As the comparative values below each photograph show, however, the observed periods appear to be defionitely shorter than those calculated on the assumption of C-C = 1.54 A., G O = 1.43 A., and the tetrahedral angle (13). Seglecting the possibility that a constant error may be present,3 we must assume either that the chains deviate slightly from the zigzag arrangement or that the bond distances or angles deviate from those employed in the calculation. Some additional information on this point is furnished by figure 2, which is a plot of the calculated and observed lengths of the chemical repeating units against the number of atoms in these units. Since the observed increase in length per chain atom as calculated from the slope f! the line drawn through the esters other than the succinate is 1.27 A. per added CH2 group, there is good evidence that the hydrocarbon portions of the acids conform to the planar zigzag arrangement (8). This would require that any difference occur in the glycol portion of the ester either as a deviation from the planar form or as a deviation in the oxygen bond angle. A bond angle of approximately 70' would account for the observed differences. In this connection it is interesting to note that the polyester of w-hydroxydecanoic acid agrees with the calculated value within the allowable experimental error, indicating that the former explanation of the shortening is the more reasonable one. Figure 2 likewise furnishes confirmation of the idea that the chains of the polyesters lie parallel to the axis of the fibers drawn from them. This Electron diffraction measurements on the adipate and sebacate esters by K. H. Storks (12) also show lower values of the identity period than those calculated.
328
C. 8. FULLER
AND C. J. FROSCH
follows from the fact that the observed increase in chain length per CHa group in this direction is in almost exact agreement with that calculated.
B . Cystalline structure of the polyesters The sharpness of the fiber patterns for the esters (figure 1) shows that in spite of their high molecular and polydisperse nature they are to be regarded as highly crystalline substances. Furthermore, the agreement of the observed fiber periods with the lengths of the chemical repeating units as calculated from their structures proves that it is the latter which
FIQ.2. Lengths of the chemical repeating units plotted against number of chain atoms for the ethylene polyesters
function as the units of packing. This is in contrast to the lower molecular esters (lo), acids (ll),and paraffis (S), in which the molecule is the packing unit. There is a striking similarity, however, between the x-ray diffraction patterns of high and low molecular chain compounds, as will be apparent from what follows. If the x-ray measurements on the polyesters (excluding the succinate) are compared with those deduced by Muller (8) for the paraffi CNHBO, a striking resemblance is apparent. Table 2 shows the interplanar spacings of these esters for planes parallel to the fiber axis, along with the
329
CHAIN STRUCTURE OF LINEAR POLYESTERS
spacings calculated from Muller's model. The recent results of Kohlhaas (7) on cetyl palmitate have also been included. It is evident from the agreement that the same type of packing of the chains is present in all of theae long-chain compounds. It appears, therefore, that, insofar as the reflections from these planes are concerned, i t makes little difference whether the crystal units are joined by primary valences or simply by association forces. It has been found possible to account for all of the reflections observed in the fiber patterns of polyethylene azelate and in poly-w-hydroxydecanoate on the basis of the orthorhombic cell proposed by Muller (8), where the observed fiber periods are substituted for c in each case." For these esters the dimensions of the orthorhombic cells are a = 7.45 A., b = 4.97 b.,and c = 31.2 A. or 27.1 A. for the azelate and decanoate, TABLE 2 Comparison of interplanar spacings i n the zone of chain axis SPACINO IN iNG8TROY UNITS COXPOUND
A,
Polyesters: w-Hydroxydecanoate . . . . . . . . . . Ethylene adipate, . . . . . . . . . . . . . Ethylene suberate . . . . . . . . . . . . . Ethylene azelate. . . . . . . . . . . . . . Ethylene sebacate . . . . . . . . . . . . Paraffin : ClpHao (Muller). . . . . . . . . . . . . . . . Ester: Cetyl palmitate (Kohlhaas). . . .
4.17 4.13 4.11 4.17 4.17
3.73 ' 2.99 I 2 3.62 2 94 2 3.61 2 93 2 2 97 1 2 3.70 3.70
4.13
3.73
1
49 52 49 49
~
2.36 2 35 2 38 2 36
2.22
2.07
4.099 3.711
respectively. Simple calculation also shows that the number of chemical repeating units per unit cell on the basis of the observed densities of 1.172 and 1.064, respectively, is 3.84 and 3.80, which is in close agreement with the expected value of 4.0. It is not possible, however, to account for all of the observed reflections in the case of the other polyesters on this same basis. Thus, in the adipate, suberate, and sebacate polyesters there are present pyramidal planes of large spacings, which require unit cells considerably larger than that of Muller. In order to obtain a truer picture of the situation, let us examine the fiber patterns of figure 1 more closely. Several important facts are immediately evident: (1) The polyesters that contain an odd number of chain atoms possess This agreement, of course, does not prove that these esters necessarily have this structure.
330
C. S. FULLER AND C. J. FROSCH
a twofold screw axis on the basis of a planar zigzag structure. The presence of strong meridian reflections on the second layer-lines of the fiber patterns of these esters (figure 1, D and F) proves that the screw axis of the chain is communicated to the crystal lattice itself, and that the repeating units line up with the terminating and median C=O groups in planes which are perpendicular to the fiber axis. These planes are represented by the horizontal lines in figure 3b. For the even esters, on the other hand, the strong reflections due to the fiber periods occur on the layer-lines corresponding to the number of chain atoms in the chemical repeating units in each case.s In order to explain this result we are forced to assume that, although the carbon atoms of the various chains in the PROJECTION PERPENDICULAR TO FIBER D I R E C T I O N
ETHYLENE SUBERATE
ETHYLENE A Z E L A T E
FIG. 3. Diagrammatic projection taken perpendicular to the chain direction for the even (a) and odd (b) polyesters
crystals of these esters fall in horizontal planes, the corresponding C=O groups of successive chains must be displaced along the chain direction with respect to one another so as to fall in planes which are inclined to the fiber direction. This is verified for the even esters by the appearance of strong first-order reflections from pyramidal planes. These reflections are apparent in B, C, and E of figure 1 near the primary beam. ( 2 ) Of particular interest is the strong reflection that occurs just off the meridian line on all of the patterns of figure 1 with the exception of the succinate. This reflection is present whether the ester under consideration is even or odd and always falls on the layer-line corresponding to one6 This is not evident from the x-ray patterns, but is shown by electron diffraction patterns taken by K. H. Storks (12).
CHAIN STRUCTURE OF LINEAR POLYESTERS
331
half the number of chain atoms present in the x-ray repeating unit. This immediately suggests that the plane corresponding to this reflection makes an angle with the fiber axis such that two atoms of each chain are included (or nearly included) in it. This plane is represented by the inclined parallel lines in figure 3a. The fact that the common spacing (2.13 A.) of these planes is greater than the C-C bond distance proves that they lie a t an angle to the fiber axis which is greater than that formed by the C-C bond with this axis (35' 15'). As mentioned above in the case of the adipate, suberate, and sebacate esters (figure 1 B, C, and E), koth the reflection under discussion corresponding to. the spacing 2.13 A. and those corresponding to low-order reflections from pyramidal planes of large spacings are observed. A consideration of these various spacings shows that the former may be regarded as the N / 2 orders of the latter reflections, where N represents the number of chain atoms in the x-ray repeating unit in each case. This suggests not only that the chain carbon atoms fall in planes inclined to the fiber axes but also that the carbonyl oxygen atoms fall in these planes. As stated previously, high intensities of the first-order reflections from these planw are in agreement with an arrangement shown diagrammatically in figure 3a, in which the carbonyl groups of adjacent chain molecules are shifted uniformly an integral number of chain atoms with respect to one another. The amount of this shift has not been deiinitely determined and is not necessarily two carbon atoms as shown in figure 3a. The absence of lower orders of reflection from the meridian planes is in further support of this view. A more quantitative treatment of the intensities of the various fiber diagrams of these compounds must await the investigation of other members of the polyester class. (3) The occurrence of the first-order reflection of these pyramidal planes has another important result. It introduces large spacings which it is impossible to index on the basis of the simple model of Muller, which served for the odd esters. Since the equality of the spacings of theplanes of the zone of the chain axis indicates that the chains must be packed together laterally in the same manner as for the paraffins, the shift of the molecules along their lengths offers a convenient way of accounting for the increase in the cell dimensions without the necessity of changing the orientation of the chains in azimuth. It has been found by the application of graphical methods that the crystal system of the even esters of figure 1 is in all probability monoclinic, in which the chain axes are perpendicular to the mdnoclinic base. The cell dimensions, however, are not those proposed previously by Fuller and Erickson ( 5 ) , since the present results indicate that the cell increases prsgressively in two dimensions &s we ascend the series. Furthermore, the type of cell found to apply to cetyl palmitate by Kohlhaas (7) and to stearic acid by Germer and Storks
332
C. 8. FULLER AND C. J. FROBCH
(6) does not apply here, since the chain molecules in these cases do not lie perpendicular to the cell base. We shall not attempt to go into the details of the molecular arrangement at this time, since a later publication will treat this subject. The general features of the crystalline structure of the polyesters, however, will be apparent from the above discussion.
C. Polymorphism in the polyesters The x-ray results so far reported on the polyesters have been obtained a t room temperature. In view of the transitions that occur in the crystalline structures of the parafEns (9) and stearic acid (3) under certain conditions, it might be expected that such transitions would be observed also in the case of the polyesters. That this is the case is shown by the occurrence of two sets of equatorial reflections in one photograph obtained on the ethylene succinate polyester. The occurrence of the two forms has TABLE 3 Measurements of the equatorial spacings of polymorphic formsof polyethylene succinate BPACINQ M
LNQBTE~MUNITS
FORM
AI
Ai
--__--A-form.. . B-form. . .
. ........................ 5.37 . . . . . . . . . . . . . . . . . . . . . . . . . 4.12
~
::ti
~
Ai
3.85 2.92
~
32:
1 I O :2
A8
1.92
not been observed in any of the other fiber diagrams that have been taken. This erratic behavior has been observed by Germer and Storks (6) in the case of stearic acid and by Fuller (4) in the case of gutta-percha. To the authors’ knowledge, this is the first time pflymorphy has been observed in a synthetic polymeric compound. Table 3 gives the results of the measurements of the equatorial spacings. It is evident from this table and from table 2 that the second form of the succinate (B-form in table 3) corresponds to the lattice spacing that is characteristic of the other ethylene esters. It is therefore logical t o conclude that the new crystal form represents a transition from the helical to the planar (or nearly planar) zigzag type of chain. Unfortunately, the new form is too weak in intensity to allow observation of any of the layerline reflections from this modification, so that i t cannot be determined whether or not the expected expansion in the fiber direction has occurred. Further study of the phenomenon will be necessary before definite conclusions in this regard can be drawn.
CHAIN BTRUCTURE OF LINEAR POLYESTERS
333
SUMMARY
1. From a study of the fiber patterns obtained on oriented fibers of the ethylene polyesters of succinic, adipic, suberic, azelaic, and sebacic acids and the self-polyester of w-hydroxydecanoic acid i t has been shown that, with the exception of the succinate ester, the polyesters conform essentially to a planar zigzag type of chain structure in which the chains lie along the fiber axis. A comparison of the fiber periods calculated on this basis with those observed indicates that a slight shortening from the planar form of chain exists. Evidence favors the view that such shortening occurs in the glycol portions of the esters, either because of a deviation from the planar configuration or because of a decrease in the oxygen valence angle. In the case of the succinate ester it has been found that two crystalline forms are possible. In one form (the usual one) the molecules appear to possess a helical configuration. The second form corresponds to that observed in the case of the other esters, in which the chain molecules are essentially planar zigzag in structure. 2. The crystals of the polyesters that correspond to the planar zigzag chain configuration have been found to have many points in common with the low molecular long-chain compounds. In particular, the polyesters containing an odd number of chain atoms in the repeating unit are found to conform to the same unit cell as has been found for C29H~0, except for the c dimension. For these odd polyesters the ends of the chemical repeating units of adjacent chains are arranged in planes perpendicular to the fiber axis. 3. In the case of the polyesters that contain an even number of chain atoms in the repeating unit, the simple orthorhombic cell above is not satisfactory. The appearance of reflections from pyramidal planes of large spacing in the fiber patterns of these even esters favors an arrangement in which the repeating units in neighboring chains are shifted along the fiber direction with respect to one another so that the terminating C=O groups fall in these planes. 4. The appearance of the first-order reflections from the pyramidal planes mentioned in the preceding paragraph requires considerably larger unit cells for the even esters. Present evidence favors monoclinic cells for these esters, in which the chain molecules lie along the orthogonal axis of the cell. The cell previously suggested by Fuller and Erickson, however, is not correct, since the different esters require cells which are of different size in the plane perpendicular to the fiber axis. REFERENCES
(1) CLARK,WALTHIUS, AND SMITH: Rubber Age 4, 35 (19371. (2) CAROTHERS AND HILL:J. Am. Chem. SOC. 64, 1579 (1932).
334
C. S. FULLER
AND C. J. FROSCH
(3) DUPRBLA TOUR,F. : J. phys. radium 8, 125 (1937). (4) FULLER,C. S.: Ind. Eng. Chem. 28, 907 (1936). (5) FULLER AND ERICKSON: J. Am. Chem. SOC. 69, 344 (1937). (6)GERMER AND STORKS: J . Chem. Phys. 6, 280 (1938). (7) KOHLHAAS, R.: Z. Krist. 98, 419 (1938). (8)MULLER,A.: Proc. Roy. SOC. (London) l2OA, 437 (1928). (9) MULLER,A . : Proc. Roy. SOC.(London) 138A, 514 (1932). (10) MULLERAND SHEARER: J. Chem. SOC.123, 3156 (1923). (11) PIPER,S. H.: J. Chem. SOC.1929, 231. (12) STORKS,K. H.: J. Am. Chem. SOC.Bo, 1753 (1938). (13) SUTTONAND BROCKWAY: J. Am. Chem. Sob. 67, 473 (1935).
