OPTICAL ANISOTROPY AND T H E STRUCTURE OF CELLULOSIC SHEET MATERIALS' JOHN SPENCE Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York Received November 1 , 1938
In this communication the approach to the optical anisotropy of xerogel sheets described by McNally and Sheppard (4) is extended to give a more complete representation of their birefringence. This property is now expressed in quantitative rather than in graphical form, the directions of the indices are defined, and, whenever profitable, the structural interpretation is supplemented with x-ray diffraction evidence. METEOD
The birefringence is evaluated by observation of the birefringence a t known angles of refraction as the refracted beam is swept, by rotation of the specimen, through as large an angle as possible in two of the three principal sections of the biaxial sheet. This, with suitable graphical extrapolation, allows the appraisal of the differences between the refractive indices. The uniaxial sheet for which only one rotation is required is considered the special case. At any given angle of refraction in the principal section, d 6 = -(721
cos T
- 121)
where 6 is the observed birefringence, d, the thickness of the sheet, nl and
m,the refracted indices of the ray a t the angle T , and T , the angle of refraction, is defined by the relation PI sin
e
= p2 sin
r
(2)
where fil and kz are the indices of refraction of the medium and specimen, respectively. The curves obtained by plotting 6 cos r / d against T O are shown in figure 1: the curve which crosses the zero birefringence axis identifies the section containing the optic axes; the other is readily recognized. Even with the extended curves now available, accurate extrapolation is not possible in their present form, but this is easily accomplished when 6 cos r / d is expressed as a straight-line function of the angle of re-
' Communication No. 700 from the Kodak Research Laboratories. 865
866
JOHN SPENCE
fraction. This has been done from the following considerations: In each wave surface section, the birefringence is in every case the difference between a constant and varying index, i.e., 7 ~ 1- %. The general relation between the angle of refraction and the birefringence in each section must have the form
m
= I(n3
- ni
(3) In the section the constant index 7tz is reciprocally associated with the circle radius. The variation of the other index, nl,is again dependent 0
.I
. coe'r* .a I
x
3 .6
9TRETCYED ACSTATL
.s
1.0
8UEET
FIG. 2 FIG. 1. Birefringence curve with angle of refraction FIG. 2. Birefringence curve with cos' of angle of refraction
reciprocally on the normal to the tangent to the wave front at the point of emergence of the ray. Thus,
where ON is normal to the tangent to OR, the ray direction at R. The 1 variation of - with the angle of refraction may be computed, when a ON and b, the semiaxes of the elliptic section whose reciprocals correspond to
867
OPTICAL ANISOTROPY
the maximum and minimum refractive indices, do not differ to a great extent, as is the case with sheet materials
n l - n : = - -l- c oas b b where 1 cos2 7 .
+ p = K and K
=
2
r-%
a/b. Thus,Itl
Obviously, if r is 90°, equation 4 becomes
PLATE -COATED CCLLULOSS T R I A C C T A T I
1
-
(4)
is a linear function of
- %.
PLATE'- COATED CELLULOOL TRIACETATE
FIQ.3 FIQ.4 FIG.3. Birefringence curve with angle of refraction FIQ.4. Birefringence curve with cos* of angle of refraction
It can be shown also that the expression is valid, within the limits of the approximation, if r is 0". Then equation 4 becomes
1 i; (1 -a) - m and
For this expression, which makes possible accurate extrapoletion of the birefringence curves, thanks are due to Dr. Hersberger of these Laboratories. This transformation of the curves of figure 1 is shown in figure 2, and a similar transformation in the case of a negative uniaxial sheet of cellulose triacetate is shown in figures 3 and 4. In upwards of three hundred cellulosic sheets examined in these Laboratories, this relation of 6 cos r/d and cos2 r has been found sufficiently valid.
868
JOHN BPENCE
It is thus possible to present the results, not, as before (4), in graphical, but in quantitative form. This, together with the location of the index ellipse in the film with reference to stresses applied, etc., provides the requisite data for the fullest structural interpretation. This is demonstrable with two general cases without reference to the position of the optic axes.
I1
I
TABLE 1 Values obtained from the two rotations when eztrapolated to 00' (4)
(3) (a)
~
CABE
PLANE
OPIRmAmoN
BIREBRINQENCE AT 1.
= 0"
-
-
BIRETRINQENCE AT 1.
90.
NO.
I..............
Ya
6 - a
Y - 8
11, . . . . . . . . . . . . 11. . . . . . . . . . . . . 111, . . . . . . . . . . .
Y8
Y
ya
8 - a Y - 8
8-a
3
a8
r-8
7 - a
4
-0
1 2
869
OPTICAL ANISOTROPY
of 60 X and greater; for lower values, it ranges from 10 to 20 per cent. The estimation of the optic axial angle may show a 20 per cent variation at angles of less than 15”. The apparatus used in these experiments consists essentially of a unit provided with three axes of rotation for the sample under examination. Since its distinctive feature allows for immersion of the sample during its rotations about the extinction position, it is possible, therefore, by a suitable choice of immersion media to obtain from equation 2 a reasonable number of values of r A necessary precaution is the avoidance of any swelling or solvent action by the medium,-an eventuality which may be detected by change of birefringence with time. The apparatus, which is
-
TABLE 2 Preliminary examination of &etched acetate sheeting
I
1
CONOSCOPIC OBSERVATION
Interference pattern
FILM
Vibration direction of slow ray in film Axial section
-1 Stretched acetate
1 Biaxial
_-
-
~
~
Acute bisectrix
Direction of stretch
I
PARALLEL L I Q E T
’lane’ red plate
i Direction I
of
stretch
TABLE 3 Complete representation o j stretched acetate sheeting
Stretched acetate
1
1 1 1 m/
~ - - - _ _ _ _ _ _
Biaxial
~
-
~
66” 110
238
~
348
+Stretch
not unique, is outlined in figure 5. Such an arrangement facilitates change and control of the immersion medium. A Polaroid filter is employed as polarizer, and, while a choice of compensators is possible, the type most frequently used in these measurements is the double quartz wedge whose components have their “slow” vibration directions a t right angles (4). This instrument when mounted with analyzer allows a rapid measurement. For certain purposes the Berek compensator may be used; it is more sensitive with low birefringence values than the former and may be used to determine “slow ray” vibration directions, etc. Unlike the wedge type, however, which employs a direct scale reading, the latter must be adjusted to compensation for each reading, and with a number of films to be examined the time involved may be a consideration. The source of
870
JOHN BPENCE
light is the type H3 mercury lamp supplied by the General Electric Com; pany; with ;he requisite filters, a good range and intensity of lines are given. Such a source is preferable to a monochromatic one which does not allow rapid recognition of the influence of dispersion on the birefringence, an influence which is frequently appreciable in plasticized sheets. The filters employable are listed in table 4.
FIG.5. The apparatus TABLE 4 Description of $lters LINE
PILTEB
A. 4360 4916 5416 5790 6900
Corning No. 71 2X Wratten 75
+ +
Wratten 77A No. 58 Wratten 22 Monochromat Wratten 70 No. 16
Most of the measurements are made with X 5790 A. A desirable precaution in such an assembly is the placing of gelatin filters behind the Polaroid filter; the gelatin sheet itself is birefringent-uniaxial with the optic axis normal to the sheet, and if placed in front of the polarizer may introduce an appreciable error if accidentally tilted from the vibration plane of the system. To facilitate computation graphs of the ratio p ~ / p n against cos r and cos*r for selected values of e, the angle of incidence may be constructed from equation 2.
871
OPTICAL ANP30TROP~
44.6 40.6 39.3 35.0 27.0
-66
-246 -580 -620 -700
thickness was 0.13 mm. are shown in table 5. The index ellipse for these sheets is placed in the thickness of the film:
Thus, the increase in negative birefringence shown in tab18 5 is due to an increase in no,whose vibration direction is the plane of the sheet. An effect of this magnitude must be chiefly the result of molecular changes and not of any changes in micellar orientation. With completely esterified products it is assumed that the anisotropy of the orientated crystallites or micelles may be interpreted in terms of a similar orientation of molecules. This is permissible only if the aliphatic side chains of the crystallite are perpendicular to the glucopyranose residues not only on the same but also on neighboring parallel molecules. There
872
JOHN' '&PENCE
is good experimental support for acceptanae of this assumption from the x-ray diffraction studies of J. J. Trillat (7) on a homologous series of cellulose esters. In general, while a weakly anisotropic molecule can never give a strongly birefringent crystal, a strongly anisotropic molecule may give either a strongly or weakly birefringent crystal, according to the crystalline molecular arrangement. There may be cases, therefore, when x-ray diffraction evidence is absent, in which the aforementioned assumption cannot be made. X-ray evidence is fairly conclusive in the identification of film crystallites as similar to, or derived from, their fibrous counterparts (5). Birefringence of the sheet, however, results from the orientation of such units in the aggregate, and in its interpretation consideration is given to the resultant molecular orientation. With such cellulosic sheets the index ellipse is placed normal to the plane of the sheet without serious error, and the birefringence interpreted in terms of the molecular architecture which the micellar orientation determines. TYPES OF ORIENTATION IN XEROQELS
There are three classes of micellar or crystallite orientation which are idealized for descriptive purposes : uiz., (a) uniplanar Orientation, ( b ) selective uniplanar orientation, and ( c ) directional orientation. The first two terms are taken from Sisson's description (6), but it is felt that his description of class c as uniaxial orientation is, in this application, an unfortunate choice since, in many instances, the result is optically biaxial. Uniplanar orientation involves a completely random orientation except for the restriction that the principal valence chains lie in one plane,in general, the plane of the film. The result is optically uniaxial. Selective uniplanar orientation places the added restriction on uniplanar that the side chains are not at random but are in a k e d direction. This result is again uniaxial. Directional orientation restricts the direction of the principal valence chains to one direction; the result is usually optically biaxial, but complete directional orientation in one direction should in the absence of the other type give a uniaxial film with the optic axis in this direction. Uniplanar orientation While the three types of orientation cannot always be separated, the increasing birefringence with diminishing sheet thickness is clearly the result of increased uniplanar orientation. This effect is particularly evident with slightly hydrolyzed esten, 9a. is shown by such an acetate propionate (see table 6).
Selective uniplanar orientation This type of orientation is more evident with fully esterified sheets than in those containing a proportion of hydroxyl groups. The side-chain
873
OPTICAL ANIBOTROPY
spacings, identifiable from Trillat's results, show marked fibering with the x-ray beam in the plane of the sheet. As a consequence of this arrangement, the directional influence of the side chains on the refractive index must be considered, as shown in table 7. The introduction of side chains longer than acetate, as in acetates butyrate and propionate, increases the index normal to the sheet to the extent of a reversal of sign. Cellulose caprylate, on the other hand, possesses a relatively high negative birefringence,-a result that suggests a,proportion of side chains in the plane of the sheet. TABLE 6 Increasing birefringence rm'th diminishing sheet thickness
I
WCKNW
BIREFMNQENCE
n.
- no X 10-6
mm.
203 190 150 140 132
0.010 0.028
0.051
0.076 0.127
TABLE 7 Directional in-fluence of the side chains on the birefrinaence ACBPATE
Acetate butyrate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Butyrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caproate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heptoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...........................
BIBWFRINQENCE
j
ne
- no X 10-6 +25
- 14 -62 -76
Directional orientation This type results from unidirectional stresses. If complete, the long chains would be aligned in the direction of stress in a similar manner to those of the fiber. Its interpretation in terms of micellar orientation from birefringence measurements necessitates the elimination of any contribution to the double refraction from deformation of valence bonds, and? in the present observations, this was accomplished by the application of tensions to the incompletely cured sheet. I n this state an appreciable amount of solvent is retained, the sheet possessing the elastic properties of a swollen gel. Table 8 shows the difference in index in and normal to the plane of the sheet of plate-coated material and those stretched while still retaining solvent. Since the sheets on extension become biaxial, an
874
JOHN SPENCE
effective comparison can only be made by noting the difference between the greater index n; in and that normal n: to the plane of the sheet. When the latter has the greater index, the birefringence is preceded with a negative sign. In column 3 the difference in indices, n; - nil in the plane of the sheet is given. The nomenclature n:, nil and n: is used only for comparative purposes, and is not always identifiable with the customary n., na, and n?. Comparison of columns 1 and 2 shows that materials that give negative plate-coated sheets Show a reduction of this birefringence in the stretched sample. With positive sheets there is a slight increase or little change. In evaluating this comparison it is necessary to consider the influence of extension on this index in the plane of the sheet. It appears reasonable to assume that the “greater index” n: in the plane of the sheet is larger than the ordinary index in the plate-coated sheets. Any alignment of the principal valence chains or a greater orientation of TABLE 8 Differences i n birefringence between plate-coated and stretched sheets
1
Triacetate. . . . . . . . . . . . . . . Acetate propionate. ...... Acetate butyrate. . . . . . . . . Butyrate. . . . . . . . . . . . . . . . . Heptoate . . . . . . . . . . . . . . . . Caprylate... . . . . . . . . . . . . .
-66 46 25 14 76 -400
-
(a) nh
- n’,
(8) BTTBETCEaD EEIJam
X 101
-21 50 25 16
-50
-200
5 11 4
13 130 2
glucopyranose rings in the plane of the sheet would tend to increase the index in this plane. X-ray diffraction evidence, in general, supports this view; the contrast in ring fibering between plate-coated and stretched sheets of cellulose butyrate is shown in figure 6. Triacetates show little difference, but the increased ring fibering is clearly shown in the diagrams of acetate propionate and acetate butyrate. Comparison diagrams of sheets stretched to the same extent, 15 to 20 per cent, when the solvent has been removed as far as possible, show much less fibering than the two aforementioned types. On this view the birefringence results are readily explained, for, if extension results in an increase in the index n; in the plane of the sheet, a reduction in negative and little change or an inlcrease in positive birefringence can only be explained by an increase in n,, which, in turn, is attributable to increased selective uniplanar orientation. It is concluded that a second influence of stresses on such open structure sheets is the forcing of side chains perpendicular to the direction of stress.
875
OPTICAI, ANlSOTROPY
A similar comparison of thc "slow my" vibration direat,ion in the film B provocative result. With plate-coated sheets such ii slight orientation may not alu-ays hc evident, even rrtrcm t h c Rcwck compensator or first-order red plate is used, arid, as far as thr quantitative cxprcssion is concerned, they are all considered uniaxial. Thc roat,irig direction in prodores
X-Ray %ant in Plane of Skeet
Acetate propiannto. . . . . . . . . . Butyrate. . . . . . . . . . . . . . ..... Heptoate. . . . . . . . . . . . . . . . . . Caprylate. . . . . . . . . . . . . . . . .
.L
l Li l
L
876
JOHN SPENCE
coating knife edge; it is only at considerable elongations that the triacetate shows a slow vibration in the direction of stretch. DISCUSSION AND CONCLUSIONS
The appreciable increase in negative birefringence occasioned by the introduction of hydroxyl groups is in marked contrast to the results obtained by Kanamaru (3) on a series of progressively acetylated fibers. His observations, made by means of the immersion method, show negligible change over a wide range of acetyl contents-15 to 45 per cent-and are attributed to the topochemical nature of the acetylation process, giving a completely acetylated micellar surface, which, by this experimental method, determines the birefringence. It should be noted that these acetylated ramie fibers, obtained by a progressive increase in the time of reaction, show, in comparison with acetate sheets, three relevant differences: Firstly, the sheets are obtained b y partial hydrolysis of the fully esterified product, which results in not only a greater proportion of primary alcoholic groups but also the presence of hydroxyl groups on the micellar surface. Secondly, these sheets are obtained by at least partial dissolution and subsequent regeneration of the crystallites with a measure of intracrystallite change (5). Thirdly, and significantly, the presence in sheets of planar orientation, implying the presence of a side-chain orientation in comparison with a more random orientation in fibers, makes quantitative comparison impossible. Bernal and Crowfoot (1) suggest that, in testosterone and androsterone, the presence of hydroxyl groups perpendicular to the planar system may be responsible for the increased index in that direction. On this view it is possible to speculate that the relatively high negative birefringence promoted by partial hydrolysis of the ester groups indicates the presence of a proportion of hydroxyl groups in the plane of the sheet. The evidence available suggests, but does not definitely prove, that the presence of a small proportion of hydroxyl groups diminishes the selective uniplanar orientation. X-ray diffraction diagrams show that, in comparison with the introduction of a similar proportion of propionyl or butyryl groups, the hydroxyl group produces much more intracrystallite lattice disorganization. Ring fiber structure is much less in evidence, but this does not exclude its presence. One may suggest that the relatively great disorganizing effect on the structural manifestations occasioned by such an introduction of a proportion of hydroxyl groups may be associated Tvith hydrogen bridge formation by hydroxyl groups in proximity. The significance of this type of bonding in cellulose and other long-chain molecules has been pointed out by Huggins (2), and it is easy to visualize inter- and intracrystallite bridging promoting the less organized arrangement. On this view the effect of partial hydrolysis of a cellulose ester gives rise to t n o stcrcochemically opposing tendencies : namely, the align-
OPTICAL ANISOTROPY
877
ment of the aliphatic side chains and the relatively powerful bridging of the hydroxyl groups. The influence of thickness on the birefringence of sheets indicates also that, with increasing thickness, the planar orientation decreases. Thus, thicker sheets are biaxial where the orientating influence of the spreading knife edge becomes more effective and the rate of coating becomes increasingly influential on the birefringence (5). If, in the greater thicknesses, symmetrical distribution of “dope” were possible, an isotropic material would result. It is this dimensional influence on the planar orientation that invalidates comparisons with the optical anisotropic properties of the bulkier samples employed for photoelastic measurements. The introduction of side chains longer than acetyl increases the index normal to the sheet, and with increasing length, as in caproate and heptoate, a tendency to an increase in the index in the sheet occurs. The first effect is paralleled by the observations of Bernal and Crowfoot (1) on the influence on the birefringence of side chains attached to a saturated ring system of a series of sex hormones. They suggested that the introduction of methyl groups perpendicular to the plane of a ring system produced weak positive birefringence. There is no definite x-ray diffraction indication of differences in a selective uniplanar orientation between triacetate, acetate propionate, and acetate butyrate, and the differences in optical anisotropy may be attributed to the difference in side chains attached to each molecule. The high negative birefringence of cellulose caprylate and the reduction thereof on extension finds some explanation from the results of Trillat’s investigations on the structures of a series of aliphatic esters of cellulose. From these studies Trillat identified the side-chain spacing, firstly, by its increasing distention with increasing length of aliphatic chain and, secondly, by noting similar spacing increases of the series of free aliphatic acids and their lead salts. He also demonstrated that the “crystallinity” diminished rapidly beyond propionate; this he attributed to the diminishing cohesive forces between the principal valence chains, owing to their being pushed apart by the aliphatic side chains. Furthermore, when cellulose caprylate film is stretched to 100 per cent of its original length, a marked discontinuity in the side-chain spacing is noted when the plane of the film is perpendicular to that of the diagram. This suggests that the reduction of the negative birefringence on extension is due to the forcing of the side chains perpendicular to the plane of the film. On the other hand, the high negative value of the platecoated material a t least indicates that the aliphatic side chains are in the plane of the sheet. Any other orientation, such as normal to the plane of the sheet, would tend to develop, first, diminishing negative and then possibly positive birefringence. It may be suggested that this tendency,
878
JOHN SPENCE
involving a tilting of the glucopyranose rings of the long chains, may idso explain the birefringence values of the caproate and heptoate. The influence of unidirectional stresses on the sheet structure produces a more marked selective uniplanar orientation when the sheet is in the swollen condition. A somewhat similar observation has been made by Sisson (6), who noted the development of “Selective uniaxial orientation” as stretched synthetic cellulose membranes approached the film structure. Similar x-ray diffraction evidence has been given by Trillat (8) on cellulose acetate. Presumably, the application of tensions to the open structure of the solvent-retaining sheet produces the more organized structure because of the greater freedom to orientate. The result that the “slow ray” vibration direction in the plane of the film of the fully esterified stretched sheets is normal to the direction of extension may be attributed to at least three factors: (1) The remaining side chains in the plane of the sheet may be sufficient to contribute to the greater polarizability in this direction. Some support for this view may be gathered from the result that cellulose triacetate fibers are negative; the vibration of the ordinary index in the plane of the side chains is that of the slow ray. ( 2 ) The long chains may be, in part, aligned normal to the direction of stretch,-a condition that may well be predetermined by the orientating influence of the spreading knife edge. (3) There may be marked discontinuities in the orientation throughout the thickness of the film; it is probable that a highly organized structure exists near the sheet surfaces approaching a more random structure in the center. The nature of the orientated region may determine the slow ray direction. It may be noted that isolated direct quantitative comparisons of the birefringence of chemically different sheets for the purposes of structural interpretation are spurious. Thus, despite the greater birefringence of the partially hydrolyzed esters, there is reason to believe that the fully esterified, though less birefringent, materials possess the higher degree of crystallite organization. Diminishing birefringence of negative sheets may be the result of increased disorganization in the plane of the sheet or of increasing index normal to this plane, due to increasing organization. X-ray diffraction and subsidiary optical evidence generally permit a decision on this alternative. It is also clear from these results that, with an appropriate introduction of side chains, the sheet may appear optically isotropic if selective uniplanar orientation is present. Such a sheet would be well organized despite the absence of birefringence arising from the balanced polarizabilities in and normal to the plane of the sheet. Cellulose butyrate illustrates the possibilities in this direction. I n general, however, these sheets should be regarded as anisotropic because of their intrinsic property of planar orientation. It is possible to produce a quasi-isotropy by
OPTICAL ANIBOTROPY
879
introduction of a second component, e. g., camphor in cellulose nitrate, but differences in optical dispersion preclude the absence of birefringence over a range of visible wavelengths. The chief advantage of such birefringence measurements is a practical one. They may be made whether the sheet gives instructive x-ray diffraction diagrams or not, and the latter alternative is a frequent occurrence with cellulose ester sheets. Sisson's (6) studies on synthetic cellulose membranes exemplify the x-ray method at its best; the diagrams are well resolved with the different types of orientation clearly shown, and for a structural interpretation birefringence measurements, apart from a theoretical interest, would be superfluous. It is hoped to deal with the influence of solvents and plasticizers in a later publication. These influences, while real, do not alter the general conclusions of this paper. SUMMARY
The evaluation of the birefringence of sheet materials is described and the general considerations in its structural interpretation are discussed and demonstrated. The writer desires to express his sincere thanks to Dr. S. E. Sheppard for his experienced and encouraging counsel during these investigations, to Mr. S. S. Sweet for valuable assistance in the coating of diverse sheet materials, and to Dr. P. T. Newsome for helpful discussion of the subject matter. REFERENCES (1) BERNAL, J. D., AND CROWFOOT, D.: 2.Krist. 93,464 (1936). (2) HUOOINS, M.L.: J. Org. Chem. 1,407 (1936). (3) KANAMARU, Krson: Helv. Chim. Acta 17, 1429 (1934). (4) MCNALLY, J. G., AND SEEPPARD, S. E.: J. Phys. Chem. 34,165 (1930). (6) SEEPPARD, S.E., AND NEWSOME, P. T.: J. SOC. Chem. Ind. 66,2561' (1937). (6) SISBON, WAYNEA.: J. Phys. Chem. 40,343 (1936). (7) TRILLAT, J. J.: J. phys. radium [71 6, 207 (1934). (8) TRILLAT, J. J.: J. phys. radium [6110,370 (1939).
