spectra as Figure 1, but the oscillograph chart speed was 0.125 i p s . At this speed a large number of adjacent spectra may be examined for detection of differences. Figure 3 illustrates the use of this technique for closer inspection of the m l e 57 to 99 region of the same spectra at a chart speed of 8 i.p.s. The bean rate for all spectra was 6 see. for a range of masses from m / e 12 to 200. The reference timing signal seen a t the bottom of each record was generated by a General Radio Type 1217-B pulse generator with a repetition rate of 250 p.1, .s.
A chromatogram may be displayed either by recording total ionization on one tape track or by playing back the
tape into a slow response servo-recorder in parallel with the oscillograph as described by Dorsey, Hunt, and O'Neal (1). The latter method provides greater economy of tape track utilization without loss of data. Magnetic tape provides a simple versatile means for recording large volumes of analog data which must be examined in detail a t some time after recording. When the data have been transferred to graphic form or digitized, the tape may be re-used or stored for permanent reference. -4more detailed description of a completely automated data handling system for fast scanning mass spectrometers will be the subject of a forthcoming manuscript.
ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance and advice of Robert Olweiler and Edward Mortenson of Consolidated Electrodynamics Corp.
LITERATURE CITED
(1) Dorsey, J. A., Hunt, R. H., O'iYeal, nf. J., ANAL.CHEM. 35,511 (1963). ( 2 ) hlerritt, C., Jr., Paper presented before 3rd Annual bleeting of ASTlI
Committee E-19, Houston, Texas, October 1964. ( 3 ) Merritt, C., Jr., Issenberg, P., Bazinet, M. L., Green, B. X.,Merron, T. O., Murray, J. G., ANAL.CHEM.37, 1037 (1965).
X-Ray Crystallinity of Polymer Emulsions Bert H. Clampitt and Charles
E. Walker,
vc
Thile t,he emulsion polymerization of polyethylene has been known for some time, the characterization of t,he emulsion per se has received only limited study. The analysis of the emulsion is usually limited to the determination of particle size by light scattering or by soap titration. Properties of the polyethylene such as molecular weight, density, crystallinity, glass transit,ions, etc., are usually determined on the polyethylene after it has been isolated from the dispersing medium and these properties may or may not be t'he same as the polymer dispersed in the emulsion. Of particular interest in the present study is the assumption that the crystallinity of the polymer in the emulsion is the same as that of bulk polyethylene. This assumption is usually made in the calculation of particle size from light scattering data, where the refract'ive index of the polyethylene particles is taken as 1.52. If, however, the emulsion polymerization process were to produce an amorphous polyethylene emulsion, then the value to use for the refractive index of the dispersed particles would be 1.49. If the Rayleigh equation (6) is used to calculate particle size from the light scattering data, then assuming an amorphous polymer leads to particle volumes 18% larger than if one assumes the polyethylene to be partially crystalline. Then, too, one is often curious to know if a different crystal form exists in the latex particles, especially if the material under study happens to be polymorphic. EXPERIMENTAL
Materials. Two polyethylene emulsions, a polystyrene emulsion, and a molding grade polyethylene were investigated. Specifically an anionic 1076
ANALYTICAL CHEMISTRY
Spencer Chemical Division,
Gulf Oil
Corporation, Merriam, Kan.
(Poly-Em 10) and a nonionic (PolyE m 40) polyethylene emulsion were studied. These, as well as t h e molding grade polyethylene (PE-1009), are manufactured by the Spencer Chemical Division of Gulf Oil Corp. The polystyrene emulsion (RWL-112) is manufactured by the Morton Chemical Co. -411 of the emulsions were 40YG total solids and they possessed particle sizes in the range of 500-1000 angstroms. S o t only were the emulsions per se studied, but the two polyethylene latexes were further characterized by determining the x-ray diffraction patterns on solids derived from the emulsions. Two methods were used to prepare solids from the emulsion. The first method was to freeze-dry the emulsion which results in a fine powder which could be analyzed directly by conventional x-ray techniques. The second method consisted of coagulating the latex with acetone, filtering, drying, and making a compression-molded specimen for x-ray analysis. Procedure. A standard aluminum specimen mount for the Norelco diffractometer is prepared by painting a thin layer of P E R M A - T I T E (Permatite hffg. Co., Minneapolis 13, hlinn.) liquid rubber adhesive around t h e opening in the mount and on the upper side. When the solvent has evaporated from this material it becomes a pressure-sensitive adhesive and is used to hold the very thin collodion window firmly in place. The collodion window which is from 10 to 20 microns thick is prepared in the following manner: 2 or 3 ml. of collodion is placed on a flat glass plate using an eye dropper. & i napplicator, made by wrapping two or three turns of cellophane tape at two different places about 4 inches apart on a glass stirring rod, is used to draw the collodion into a thin sheet. b'hen enough of the solvent has evaporated to leave a solid collodion film which is still firmly adhered to the glass plate,
the aluminum specimen mount is pressed, tacky side down, on a clearlooking area of the collodion film. Upon thorough drying, the film will begin to pull away from the glass plate. At this time, the specimen mount is carefully lifted up with the collodion adhering t'o the adhesive side. The excess is trimmed away from the outside of the specimen mount and the film is pressed into good contact with the adhesive on the mount. With the specimen mount held collodion side down, an eye dropper is used to fill the cavity in the mount with the liquid sample. When the cavity is slightly more than full due to surface tension of the liquid, a microscope cover glass that has been trimmed to the proper size is carefully placed on the surface of the liquid. Capillary attraction will pull the cover glass down firmly on the bottom of the specimen mount, holding the liquid without leaking in the cavity. X-Ray Equipment. The equipment used in this investigation was the standard Norelco diffractometer manufactured by Philips Electronics Instruments. A copper target x-ray tube was used, operated a t 35 kv., 20 ma. 4 xenon proportional counter was used on the wide range goniometer, and t,he angular 28 range transversed by the goniometer was from 9" to 36" a t a rate of 0.5" per minute. RESULTS A N D DISCUSSION
I n order to investigate t'he effect of the collodion window, a sample of PolyEm 40 coagulated solids was pressed into a solid block in one of the aluminum specimen mounts and analyzed with and without one of the collodion films. The results showed that although about 15y0 reduction in overall intensity resulted, the calculated x-ray crystallinity (IT-,) is essentially unaffected, the numbers being within experimental precision.
- Poly.Em 10 (anionic)
90-
-
full 6coIe
240CIS
4 0 L a t e x tull scale = 400 C/S
-Poly-Em
00-
---PE
L
1009 Molding Resin full scole , 2 5 6 0 C I S
Z70-
E 6050-
-
= 40.----_______.,' t 304
--._. ----
2010-
Figure 2.
The crystallite size (Illlo) which is the crystallite dimension in the direction perpendicular to the (110) planes is also relatively unaffected. Figure 1 shows diffraction patterns of the two polyethylene emulsions PolyEm 10 and Poly-Em 40. The two materials appear to be essentially similar except for an overall lower diffracted intensity for the anionic Poly-Em 10. A possible reason for this difference is provided by the double layer theory. A colloidal particle emulsified with an anionic detergent is surrounded by the negative hydrophylic groups of the surface active agent. This then attracts a shield of positive ions. In the case of Poly-Em 10, these ions are potassium (3) and they partially shield the polymer particle from the incident x-rays. The result is a high random scatter and less diffracted intensity. This situation does not exist in the nonionic Poly-Em 40, it having a carbon-hydrogen-oxygen group as the hydrophilic agent on the surface of the particle. A comparison of crystallinitj. between emulsion and solid polyethylene is shown in Figure 2. The two materials give very similar diffraction patterns except for the expected lower intensity of the emulsion. (Again, note the differences in sensitivity.) The slope of the background is somewhat different probably due to water. In order to give quantitative comparison between the crystallinity and crystallite size of polyethylene in the various
Table
I.
Comparison of emulsion and solid polyethylene
Effect of Sample Form on X-Ray Properties
X-ray crystallinity
Sample Poly-Em 10 Poly-Em 40 Poly-Em 1009
latex freeze-dried coagulated latex freeze-dried coagulated powder molded solid
states, the following methods of calculation were used. Figure 3 illustrates the method used in determining x-ray crystallinity from the diffraction pattern ( I , 2 ) . A baseline is drawn from 10'27'. A line is drawn from the baseline a t 25' to the diffracted line a t 19.6". This arbitrarily defines the area of the so-called amorphous peak. The areas of the amorphous, the (110) and the (200) peaks are measured with a planimeter and are multiplied by the appropriate instrumental correction factors to determine the intensities of each area. These can be expressed as total number of counts when the count rate is known, or can be simply the area times the factor to simplify calculations. The x-ray crystallinity is a straightforward percentage calculation based on the integrated intensities of these three areas. This number is not to be treated
Crystallite size
WC
Dl10
48.5'30 57.1 53.5 57.7 52.3 56.8 58.2 56.0
132 A. 85 154 202 131 159 82 144
as an absolute figure because there are other smaller diffraction lines at higher Bragg angles and these have an appreciable effect on the crystallinity when the proper correction factors are applied and they are figured into the equation. For all practical purposes, however, these two diffraction lines and the amorphous area can be used to compare one sample of polyethylene with another with respect to crystallinity. The calculation of crystallite size [actually in this case, the dimension of the crystallite in the direction perpendicular to the (110) planes] is done via the well known Scherrer equation ( 5 ) . A summary of crystallinity and crystallite size data for the various systems which mere investigated is shown in Table I. X-ray crystallinity and crystallite size are shown for Poly-Em 10 and Poly-Em 40 for the three forms of each sample; latex, freeze-dried powder, and coagulated solid. Data are also
I
-
- Poly.Em 4 0
eo
Woter
f u l l scale 4 0 0 C/S f u l l scolc 2 0 0 CIS full scale = 4 0 C I S
whare: 1(11o).A(~~o~Xl 00
4200)**(2003( 1.46 1, =A, Y O 75
Figure 3.
Calculation of x-ray crystallinity
*
2'5
'
I
'
'
20 ,
'
-2 ' 9'
'
15 '
'
'
'
'
1'0
'
Fiuure 4
Figure 4.
X-ray
diffraction patterns of three liquids VOL. 37,
NO. a,
JULY 1965
1077
included for both powdered and molded PE-1009. The crystallinity figures in Table I appear reasonably close to one another especially if one considers the experimental difficulties. The data are also similar in magnitude to that for the molding grade resin PE-1009. The crystallinit'y of the Poly-Em 10 latex appears to be a little low but this material may not be a t all typical due to the shielding effect of the potassium ions. This effect, is removed akogether in the coagulated materials as it is washed out during the process. Considerable discrepancies are seen in the crystallhe size figures in Table I. The crystallite size of the freeze-dried powder samples appear extremely low with respect to the other forms. A sample of PE-1009 powder was run for comparison purposes and it is apparent that the sample state has some effect on this measurement. A possible explanation for this effect is that the bulk density of the powder is about half that of the molded solid. This leads t'o a lower absorptivity for the powder. The x-ray beam will, therefore, penetrate about twice as far into the powdered sample and results in the so-called transparent sample effect ( 4 ) . The x-rays which are re-
flected from deep in the sample may appear a t some place on the small angle side of the detector. The result is an anomalous broadening of the band and probably a lower intensity a t the center of the peak. In fact, calculations of effective sample thickness indicate that the determination of crystallite size by this technique may not be a t all accurate for organic materials of this type as the thickness "seen" by the radiation is about 10 to 20 times that of the silicon powder sample that n a s used as a standard for this method. Figure 4 illustrates the comparison between three liquid systems; the first, a crystalline polymer emulsion; the second, an amorphous polymer emulsion; and the third, water. It is interesting to note that the polystyrene latex has a broad reflection a t approximately the same angle as the amorphous region in polyethylene. This is logical since amorphous phases of each polymer are essentially a disordered carbon-hydrogen matrix. CONCLUSION
It is felt that a technique has been developed which offers a means for evaluating the crystallinity of polymers in an emulsion or fluid system. Calcu-
lations on the systems studied indicate that the degree and type of crystallinity of the polyethylene particles in emulsion is very similar to that of the isolated materials and also similar to that of a standard solid molding grade polyethylene. It will be interesting to extend this type of measurement to other crystalline polymer emulsion system.;. ACKNOWLEDGMENT
The authors express their appreciation to Sam Baker for carrying out many of the experimental measurements. LITERATURE CITED
(1) Aggarwal, S. L., Tilley, G. P., J . Polymer Sei. 18, 17 (1955). (2) Bunn, C. W., Trans. Faraday SOC.35,
482 (1939).
( 3 ) Helin, A. F., Mantell, G. J., Stryker, H. K., J . A p p l . Polymer Sci. in press. (4)Parrish, W., "Advances in X-Ray
Diffraqtometry and X-Ray Spectrography, p. 63, Centrex Publishing Co., Eindhoven. 1962. (5) Scherrer,' P., Sachr. Ges. Was. Gottingen 1918, 96-100. (6) Taylor, H. S., Glasstone, S., "Treatise on Physical Chemistry," Vol. 2, 3rd Ed., D. 549. Tan Kostrand. Yew York. 1951.'
Sixteenth hlidwest Chemistry Conference, Kansas City, hIo., Sovember 20, 1964.
Improved Method for Microtitration of Fatty Acids Thomas F. Kelley, Bio-Research Institute and Bio-Research Consultants, Inc., Cambridge, Mass.
icrotitration of fatty acids released
M by the action of enzymes, such as
lipases, is frequently used as an assay method. It is a more accurate and direct measure of enzyme activit'y than turbidity measurement or glycerol assay. Microtitration is, however, a time consuming and tedious technique. I n the procedure generally employed, fatty acids are extracted into an organic solvent to exclude water soluble acids ( I , 2 ) , and titrated with dilute aqueous NaOH. Indicator dissolved in 95% ethanol is added. This two-phase system is mixed during the titration by bubbling prepurified nitrogen through it which also acts to exclude atmospheric carbon dioxide. The mixing of the two phases results in a translucent emulsion which acts to mask the end-point color. Thus, t,he titrat,ion must be stopped frequently to check progress toward the end point. Overtitration is a constant danger because of the long time lag in the reaction due to the existence of two phases. &tempts to increase t'he rate of react,ion by accelerating the nitrogen bubbling only result in rapid evaporation of the solvent, and the consequent cooling results in the coating of the tube with a layer of condensed water. The 1078
ANALYTICAL CHEMISTRY
whole procedure is, therefore, very tedious. A microtitration system is described below which, because it operates in a single-phase system, Overcomes these difficulties and decreases the time required for a set of titrations to less than one fourth. EXPERIMENTAL
Reagents. T l T R A N T . ii stock titrant solution is prepared by diluting 15 ml. of tetrabutylammonium hydroxide titrant in methanol (Eastman No. 7774) to 100 ml. with methanol. Fresh working solution (about 0.01N) is prepared daily by diluting the stock solution 1 to 10 with methanol. INDICATOR SOLETIOX. Phenol red indicator solution is prepared in a manner similar to that described by Mosinger ( 3 ) . One milliliter of a 1% aqueous phenol red solution is diluted with 99 ml. of absolute ethanol and then added to 200 ml. of heptane. A concentration curve was plotted for this solution titrated to end point. h quantity of 0.4 ml. of the indicator solution lay in the flat part of the titration curve, and this concentration was chosen to minimize pipetting errors.
02 141
For ease in manipulation, the stock solution is diluted 2.5-fold with heptane so that 1-ml. portions can be used. FATTY A C I D STANDARD. Ninety-two milligrams of oleic acid (99% pure by gas chromatography) are dissolved in 10 ml. of ether. This solution is then added to 50 ml. of 95% ethanol. The alcohol-ether solution is rapidly mixed with about 250 ml. of water, to yield an emulsion which is stable for four to six weeks under refrigeration. The final volume is adjusted to 325 ml. with water t o yield a solution which contains 1 peq./ml. Procedure. F a t t y acids from both enzyme reaction mixtures and samples of standard solution are extracted with isopropyl alcohol, heptane, l N H2S04 (40: 10: 1 v/v) according to Dole ( I ) . Three milliliters each of water and heptane are then added to separate the mixture into two phases. Three milliliters of the top layer are transferred to 18- X 150-mm. tubes. Indicator solution (1.0 ml.) is added, and the solution is titrated to a purple end point with O.Olhr tetrabutylammonium hydroxide. The color changes from a lemon yellow to an orchid (purple). For manual microtitration, a Rehberg microburet of 0.125-ml. capacity with 1-pl. divisions is used. The solution is stirred by bubbling NZ through it.
