mobility would be most pronounced in the terminal fractions of a run or in the fractionation of high molecular weight polymers. The difficulties experienced in the fractionation of polypropylene, mentioned earlier, as well as reported difficulties with higher molecular weight poly(methy1methacrylate) samples @ I ) , indicate that polymer mobility may sometimes be a critical problem. On the basis of these considerations and the observation that the polystyrene precipitate is 60% more swollen in benzene than in methyl ethyl ketone ( l o ) , it appears that the difference in efficiency of these two solvents may arise from the greater swelling and, therefore, greater mobility of the polymer in benzene. This interpretation, if correct, indicates the desirability of using poor solvents to limit the swelling of the sample. A second difference from batch methods is due to the possible adsorption of polymer on the support. The occurrence of a reversible adsorption of polystyrene on sand has been shown by its effect on the solubility of the polymer ( l l ) ,but it is not certain what role this might play in the fractionation. On Celite there was a pronounced irreversible adsorption of the higher molecular weight portion of the same sample. This would appear to rule out t h e use of Celite for small samples, except in special cases such as that described by M c h o d and Hulme (la), where the sample was deposited on Celite to eliminate plugging of the column with rubbery polymers such as polyisobutylene, the remainder of the column being filled with the usual 0.1mm.-diameter glass beads. However, even in this case it was necessary to pretreat the Celite with a high molecular weight sample to minimize adsorption of the sample being analyzed.
LITERATURE CITED
CONCLUSIONS
The column methods of fractionation have proved their value for the routine fractionation of a variety of polymers, giving sufficient detail, in many cases, to permit a n estimation of the molecular weight distribution curve. Although it is not certain that all polymers which yield t o batch fractionation can be handled equally well on a column, the increasing variety of polymers that are being fractionated in this way suggests that the column methods are applicable to most samples of moderate molecular weight and polydispersity. However, i t must be admitted that unsolved difficulties persist in the fractionation of polymers of very narrow molecular weight distribution or of very high molecular weight or in attempting t o define the terminal portion of the sample (above 95% cumulative weight). In regard to the thermal gradient method, the efficiency of the thermal gradient needs to be evaluated more critically and it would be useful to have a careful comparison of the results from this method with those from the elution method employing selective deposition. With regard to the latter method, i t is important to determine whether selective deposition is equally effective with amorphous polymers and, in particular, to learn whether Henry’s method of scaling up the fractionation using Celite offers a general approach to preparative scale fractionation on a column. In any case, t o take full advantage of these methods requires a more complete understanding of the processes that control fractionation. This, in turn, depends on a more basic and varied approach which goes beyond the goal of finding a single set of conditions which give acceptable fractionation for a particular sample.
(1) Baker, C. A., Williams, R. J. P., J. Chem. SOC.1956, 2352. (2) Desreux, V., Spiegele M. C., Bull. SOC. chim. Belges 59,476(1950). (3)Flory, P. J., “Principles of Polymer
Chemistry,” p. 344, Cornell Universlty Press, Ithaca, N. Y., 1953. (4)Ibid., p. 568. (5) Francis, P. S., Cooke, R.. C., Jr., Elliott. J. H.. J . Polttmer Sea. 31, 453 (1958): (6) Guillet J. E., Combs, R.L., Slonaker, D. F., t)oover, H. W., Ibid., 47, 307 (1960).
(7) Hall, W., in “Techni ues of Polymer Allen, ed., Characterization,” P. Academic Press, New York, 1959. (8) Henry, P. M., J. Polymer Sci. 36, 3
%.
(1951). (9) Jungnickel, J. L., Weiss, F. T., Zbid., 49,437 (1961). (10)Kenyon, A. S , Salyer, I. O., Zbid., 43,427 (1960). (11) Krigbsum, W. R., Kurz, J. E., Ibid., 41,275(1959). (12) McLeod, L. A,, Hulme, J. M., Tenth Canadian High Polymer Forum, Ste. Marguerite, Quebec, September 1960. (13) Pepper, P. C., Rutherford, P. P., J. Appl. Polymer Sci. 2,100(1959). (14).PFpe, M. T.,Weakley, T. J. R., Williams. R. J. P., J . Chem. 8oc. 1939, 3442. (15) Richards: R. B., Trans. Faraday SOC.42, 10 (1946). (16) Schneider, N. S., Holmes, L. G., Mijal, C. F., Loconti, J. D., J. Polymer Sci. 37, 551 (1959). (17) Schneider, N. S., Loconti, J. D., Holmes, L. G., J. A p p l . Polymer Sci. 3,251 (1960). (18)Zbid., 5, 354 (1961). (19) Schneider N. S., Loconti, J. D., Holmes, L. d.,unpublished results. (20) Shyluk, S., 2, 133 (1960); Preprint, Papers Presented t o Division of Polymer Chemistry, 138th Meeting, ACS, New York, N. Y., September 1960. (21) Weakley, T.J. R., Williams, R . J. P., R’ilson, J. D., J. Chem. Soc. 1960,3963. RECEXVED for review July 10, 1961. Accepted August 24, 1961. Division of Analytical Chemktry, 139th Meeting, ACS, St. Louis, Missouri, March 1961.
etermination of in Polymer Films J. A. GAILEY Research Center, Hercules Powder Co., Wilmingfon, Del.
lb An infrared absorption method has been developed which provides a relative measure of the three-dimensional molecular orientation in high polymer films. The method requires little special equipment, and the complete analysis, including interpretation, can be performed in about 15 minutes per sample.
T
orientation of the molecular chains in a high polymer film has an important effect on the level of properties such as tensile strength and permeability. X-ray diffraction analysis measures orientation, but is very timeconsuming, especially if more than qualitative or the simplest quantitative interpretation is desired. The infrared HE
method described below was developed for speed of determination and simplicity of interpretation. For these m e reasons i t is best used empirically. The orientation must be measured in all three dimensions of the fih. I n x-ray diffraction analysis the film is stacked t o provide m area large enough for direct irradiation of the edges by VOL 33, NO. 13, DECWBER ’1961
1831
the x-ray beam. This approach is unsuitable for infrared measurements, however, because it is impractical to stack films whose width is less than 0.25 mm., which is the maximum path length for infrared analysis of most substances even when bands of relatively low intensity are used. The infrared procedure described below determines, on a single thickness of film, the average direction of the polymer chain backbone with respect to an arbitrary axis in the plane of the film and the plane of the film itself. This information provides a measure of the molecular orientation in three dimensions.
IA
D.R. 1.0
18
IC
P.P.R.
3 !
P.P.R.I.
I I
BASIC PRINCIPLES AND DEFINITIONS OF ORIENTATION
Molecular vibrations involving atoms which form electrically polarized bonds produce an oscillating dipole which is called a transition moment (a). The transition moment of a given vibration bears a definite directional relationship to the rest of the molecule-for example, in a high polymer i t can be either parallel or perpendicular to the main chain backbone. When the frequency of the infrared radiation coincides with that of the molecular vibration, energy exchange (absorption) can occur. The magnitude of the interaction is a function of the square of the cosine of the angle formed by the transition moment of the molecular vibration and the electric vector of the radiation. Thus, maximum absorption occurs when the two vectors are parallel. Three kinds of simple orientation will be considered. The backbone of the stretched polymer chain will be the polymer reference axis; thus the definitions fit amorphous as well as crystalline polymers. Random orientation is equivalent to no orientation. I n axial orientation, the chain axis is parallel to an arbitrary reference axis. In planar orientation, the chain axis is parallel to an arbitrary referenee plane. In this paper, the reference plane is the plane of the polymer film. Although it is convenient to think in terms of orientation of individual chains, actually the crystallites are oriented in crystalline polymers. MEASUREMENT OF AXIAL ORIENTATION
The molccular chains in a stretched high polymer film tend to line up in the direction of draw ( I , 5). Thus, one can use infrared radiation which has been plane-polarized in a known direction t o determine which absorption bands have their transition moments preferentially parallel to the chain axis and which are preferentially perpendicular t o it. Such bands are called, respectively, parallel and perpendicular bands. With this information, one can determine the direction of avial e
ANALYTICAL CHEMISTRY
Figure 1. Effect of orientation on 842-cm.-' parallel band and 8 1 O-crn.-l perpendicular band of isotactic polypropylene 1. Film extruded at 260' C . and coaled 1. Same film as 1, cold drawn uniaxially 3. Same fllm as 2, cold drawn further, rnul*iaxially A. Electrlc vector parallel to original direction of draw B. Electric vector at right angles to original direction of draw C. Film rotated at 1800 r.p.m. Cold drawing alters the film thickness. For ease af comparison the urdinate scale has been adjusted electronically to put the band intensities in the same range. The dichroic ratios (D.R.) refer to the 842-cm.-' band. The P.P.R. of randam oriented film used in calculating the P.P.R. index (P.P.RI.) Is 2.8
orientation in an unknown film of the same material. In this paper the dichroic ratio is defincd as the ratio of the absorbances of an absorption band when the electric vector of the plane-polarized infrared beam a t normal incidence is first parallel and then perpendicular to an arbitrary reference direction. It is convenient to use the direction which gives the greater band intensity as the reference direction. The dichroic ratio is then never less than 1,and comparison of the degrees of orientation of different films is immediately more meaningful. The dichroic ratio for a fixed set of instrumental parameters is a measure of the relative degree of axial orientation. Thus, one can measure both the direction and the relative amount of axial orientation in the plane of an unknown film. The concepts just applied to axial orientation in one direction can be applied to a film which has been stretched in more than one direction, but only bhe net orientation in the xy planc is measured, and this value can be misleading. (For convenience in referring to the orientation of the sample film with respect to the polarized analyzing beam, let the x and y axes in a system of rcctangular coordinates lie in the
plane of the film with the z avis porpendicular to it.) Consider, for example, a film which is drawn such that the orientation in the x direction is exactly balanced by that in the y dircction. The dichroic ratio is 1-for example, see Figure 1, row 3. This is the same dichroic ratio that a random oriented film would have, yet the physical properties are quite different! In this simple example the explanation i s that orientation has also occurred in the xz and yz planes. MEASUREMENT OF PLANAR ORiENTATlQN
When a polymer film is drawn i i i either the x or the y direction the tcndency of the moiecular chains to line u p in the direction of draw includes those molecules which were parallel to t h e z axis and which now tend to become parallel to the ry plane. This plauar orientation is undetected by planepolarized radiation a t normal incidence. How can we measure it? Slanted Film Technique. Irradiation normal to the zz and yz planes can be approximated by irradiating with the sample film inclined in the analyzing beam. The angle of inclination is limited to about 70" from normal incidence in most instruments by t h e
size of the sample compartment and the height of the beam. This would appear a t first glance to be a fair approximation t o irradiating the film edgewise, but refraction a t the air-film interface causes the beam to bend back closer t o the normal as it passes through the sample. If the refractive index is 1.5, for example, the path of the beam in the sample is actually inclined only about 39” instead of 70”. The small changes in dichroic ratio which result are difficult to measure with precision. The use of a liquid medium to eliminate the air-film interface is ruled out because no liquid is available with enough infrared transparency a t the long path lengths required. A further complication arises at these high angles of inclination There Brewster’s angle is approached. -4t a given angle of slant, reflection losses are a function of sample rotation with respect to the polarized beam. Reflection becomes severe a t certain angles. Parallel - Perpendicular Ratio Method. The parallel-perpendicular ratio (P.P.R.) is defined as t h e ratio of the intensity of a parallel band t o t h a t of a perpendicular band. It measures the degree of planar orientation. The method is based on the fact t h a t electromagnetic radiation has no electric vector in the direction of propagation, I n this sense, the radiation is perfectly polarized. Because the radiation in a spectrophotometer is always partially plane-polarized by selective reflections from prisms or gratings, the system is simplified by rapidly rotating the sample film around the z axis. This averages out the effects of plane polarization in the zy plane. Thus, in a polymer film a t perpendicular incidence to the analyzing beam, only those transition moments which haye a component in the zy plane will absorb, and the absorption will be independent of orientation in the z and g directions. Sow, consider the effects of molecular orientation with respect to this type of polarizstion. hlolecules which have their chain axes parallel to the z axis necessarily have the transition moments of their parallel absorption bands in the same direction. Since there is no electric vector in this direction. no absorption [Till occur. These same chains, however, have all their perpendicular vibrations parallel to the zy plane, and absorption does occur. Molecules which have their chain axes parallel to the ay plane have all their parallel vibrations in the same plane and mill all absorb, but only part of their perpendicular vibrations will be in the absorbing orientation. It is now apparent that as planar orientation occurs, the intensities of parallel bands will increase and those of perpendicular ones will
decrease. Thus the P.P.R. is a measure of planar orientation. This treatment assumes random orientation about the chain axis. INTERPRETATION OF DICHROIC AND PARALLEL-PERPENDICULAR RATIOS
The value of the dichroic ratio, as defined above, theoretically can vary from 1 to infinity. Under usual conditions, the value is seldom greater than 10. The direction of orientation in the xy plane is determined from a knowledge of the polarization of the analyzing beam and of whether the band being measured is parallel or perpendicular. For example, a parallel band will have greater intensity when the chain axis is parallel to the direction of polarization. The P.P.R. becomes meaningful when it is compared with the same ratio in a film which is known to be randomly oriented. The ratio of the P.P.R. of an unknown film to the P.P.R. of a random oriented film is called the P.P.R. index. It can vary from 1 in a nonoriented film to 1.75in a film having perfect planar orientation. Its value indicates how nearly flat the chains lie in the plane of the film. Used together, the dichroic ratio and the P.P.R. index provide a simple means of comparing orientation in different films, the dichroic ratio measuring the net direction and relative degree of orientation in the ay plane, and the P.P.R. index measuring the degree of “flatness” within the plane. There are two important sources of confusion which may arise in the interpretation of the data. They are unavoidable and emphasize the need for an empirical approach, the use of relative values. Some semicrystalline polymers exhibit “double orientation’’ upon stretching-that is, two of the crystal axes orient themselves with respect to some axis or plane of reference. When double orientation occurs, the assumption of random orientation about the chain axis is false and the value of the P.P.R. index may range either above or below the calculated limits of 1 to 1.75. This type of behavior is shown by the polypropylene example in Figure 1. However, this effect is likely to be reproducible, so that relative values still are meaningful. Crystalline high polymers sometimes undergo unpredictable changes in morphology when subjected to mechanical or thermal strains. These changes are often signaled by changes in band shape or position. CHOICE OF ABSORPTION BANDS
Many polymers of interest will exist in more than one phase, usually one amorphous and one or more crystalline. It is probable that there will be dif-
ferences in the degree of orientation of different phases subjected to a given stress. Therefore, the choice of absorption bands should depend on which phase is most important in producing the film properties desired. Occasionally a band can be found which is relatively indifferent to changes of phase. Two other considerations in the choice of bands for measurement are that they be of a readily measurable intensity in the range of film thickness of interest, and that they exhibit sufficient dichroism to give measurable changes over the range of orientation of interest. When choosing two bands for use in measuring the parallel-perpendicular ratio, it is especially important that both originate in the same phase or that both be phase-independent. Otherwise, changes in the phase composition will affect the result. EXPERIMENTAL
The dichroic ratios of moderately to highly dichroic bands can be measured with only the inherent polarization of the spectrophotometer. This procedure has the advantages of convenience and a highly reproducible degree of polarization. The rotating sample holder consists of a 13/s-inch i.d. precision ball bearing. The inner race is attached to a standard cell mounting plate by means of a pressed-in bushing. The sample film is fastened to a steel plate which can be clamped to the outer race of the bearing, which in turn is rotated a t 1800 r.p.m. by a friction drive from a small synchronous motor. Interference fringes which frequently hinder infrared analyeis of thin films have been eliminated by using a sample holder plate which has a piece of optical grade silver chloride sheet pressed into the circular aperture which passes the analyzing beam. The sample film is stuck t o this window with an appropriate grease -for example, Apiezon N grease is useful over a wide range of wave lengths.
A randomly oriented reference film for calculating the P.P.R. index is sometimes difficult to prepare. The most satisfactory methods involve casting from solution or molding from finely divided polymer in such a way that a minimum of flow occurs. Solution-cast films mag differ morphologically from molded or extruded ones; thus it is usually advisable to fuse a cast film before use. X-ray analysis can be used as a check on the appropriateness of the reference film. However, since relative values are used, perfect random orientation is not too important. The polypropylene film used as an example in Figure 1 was cold-drawn by hand. A Beckman IR-7 spectroVOL. 43, NO. 13, DECEMBER 1961
e
1833
photometer was used without an auxiliary polarizer. PRECISION
High precision in measuring absorbance is usually not necessary. I n fact, slight variations in film thickness often make high precision impossible. The measurement of dichroic ratio is especially sensitive to thickness variations, whereas sample rotation during
emic F. E. CRITCHFIELD and
measurement of the parallel-perpendicular ratio tends to minimize their effect. Two significant figures in the P.P.R.I. and dichroic ratio are sufficient for most work.
LITERATURE CITED
w.,
MCOCk, , T. c., T ~ Faraday SOC.41, 317 (1945). (2) Hemberg, G., “Molecular Spectra and Molecular Structure,” Vol. TI, p. 414, Van Nostrand, Princeton, N. J., (1) B
~ c. ~
1945.
(3) Krimm, S., ACKNOWLEDGMENT
The author thanks W. E. Davis for his derivation of the equations for the theoretical value of the P.P.R. index.
(1954).
J. Chem. Phys.
22, 567
for review 10, 1961. Accepted September 25, 1961. Division of halytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961. &X2EIVED
nalysis of P D. P. JOHNSON
Development Department, Technical Center, Union Carbide Chemicals Co., Soufh Charleston, W. Va. Application of chemical methods to alyses ob polymers is made difficult b y their limited solubility and chemical resistance. However, chemicat methods often can be used for determining trace concentrations OS impurities such as metals, monomer ratios of copolymers, trace concentrations of polymers, and end groups in polymers. The method selected for the preparation of the sample usually governs the success of the analysis for metals in polymers. Procedures used include dry and wet aohing and solution techniques. Chemical methods can be used to determine the monomer ratio of copolymers, if solubility difficulties can be overcome and the functionality being determined is not chemically resistant to the reagent. The copolymer of ethylene and ethyl acrylate can b e analyzed by a chemical method. The recent problem of determining trace concentrations of polymers is motivated by the increased use of polymers for packaging foods. The determination of low concentrations of polyethylene in liquid fats is one example of a method of this type. The sxcessful application of chemical methods to the determination of end groups in polymers is usually dependent upon the molecular weight of the polymer, the sensitivity of the method and the reactivity of the end group. End groups that have been determined include hydroxyl and epoxy. HE application of chemical methods to analytical problems associated with polymers is difficult, principally because of low solubility and chemical resistance. To analyze a polymer by chemical methods, the component or functionality being determined must be brought into intimate contact with the reagent. In contrast to more simple molecules, this is usually difficult.
4
o
ANALYTICAL CHEMISTRY
Depending upon the analysis being performed, contact with the reagent is brought about by ashing or extraction of the polymer or via solution techniques. Even if the polymer can be dissolved in a suitable solvent, it may have no functionality of sufficient reactivity to allow chemical analysis. I n spite of these difficulties, chemical methods can be used for the solution of the following types of problems: determination of trace concentrations of metals; determination of the monomer ratio of copolymers; determination of trace concentrations of polymers; and determination of the end groups of polymers. DETERMINATION OF TRACE METALS IN POLYMERS
Trace (low p.p.m.) Concentrations of metals in polymers may be detrimental to the stability of the polymer, affect its physical properties, or prohibit its use in food packaging applications. For any or all of these reasons, the determination of trace metals in polymers is a common problem. The success of any method for determining metals in polymers is usually governed by the procedure selected for the preparation of the sample prior to the actual determination, Methods commonly used include dry and wet ashing, extraction, and solution techniques. Extraction of metals is usually not satisfactory because of the inability of the extractant to penetrate the polymer completely. Solution techniques are valuable when they can be applied, because of their simplicity. However, satisfactory solvents cannot always be found and, even if they can be, interferences may obviate the technique. In addition, the metal may be in a form that is insoluble in the solvent for the polymer.
Because of the limitations of the above techniques, wet- or dry-ashing procedures are generalIy used. Each of these techniques has its advocates and, therefore, considerable controversy has existed about their relative merits. Recently, a n excellent comparison of the procedures was reported, in which the various sources of errors were ascertained by radioactive tracers (8). A summary of this study is presented in Table I. Wet-ashing techniques can be used for all of the elements studied. Only with mercury is recovery Ion-, the loss being due to volatilization. Although losses are negligible in wet-ashing techniques, the procedures cannot always be used because of trace metal contaminants in the large quantities o f reagents used for the oxidation, For this reason, dry-ashing procedures are advocated by many workers. Besides being lengthy, the major objections to dry ashing are losses due to the volatility of certain metals and to fusion of the metal with materials in the crucibles. Each of theqe losses can be inhibited b y the use of sulfuric acid as an ashing acid; however, as shown in Table I, serious losses can occur even under these conditions. Mercury and selenium are particularly bad in this respect and cannot be determined readily. Losses due to volatility can also occur with other elements, particularly lead, zinc, and iron, in the presence of high concentrations of chloride ions or organically bound chlorine as in poly(viny1 chloride) polymers. Even when sulfuric acid is present, care must be taken t o prevent this loss by maintaining the temperature of the oxidation below 500” C. I n summary, wet-ashing techniques are relatively free from losses, but large amounts of metals in the reagents may produce excessively high blanks. Low
~
~
~
