to other ions. Evidently the tailing a t high acid concentrations is a property of magnesium rather than of the resin. Possibly AIg(II), to a greater extent than the other ions studied, remains in the aquo form in solutions of high hydrochloric acid concentration. This hydrated form would be likely to have a larger effective radius than the chloro complexes of the other ions. and therefore to be more strongly affected by diminution of resin pore size in solutions of highionic strength. Conditions for Separation. To estimate optimum conditions for separations, the data given in Table I have been used t o calculate the minimum bed depth necessary for 99% separation as described above. The ions were taken two at a time in all possible combinations. I n order to summarize briefly the collection of figures obtained the folloning criteria for practicable separation have been assumed: calculated bed depth not greater than 5 em., and elution time to remove the first ion not greater than 1 hour a t a flow rate of 0.5 ml./sq. cm./minute. The results of experiments with 1Ji hydrochloric acid and resins of varied cross linking may be considered separately. In almost all cases in which there is difference in behavior with change in cross linking 127, DVB resins would give the most efficient sep-
arations. On the basis of the discussion of effect of changing degree of cross linking, it seems reasonable to expect that this would generally be true a t acid concentrations other than 121. Calculations based on results of esperiments involving elution n ith various acid concentrations from lay0 DVB resin indicate that Hg(I1) can be separated from all other ions studied by elution with 0.5J1 hydrochloric acid. Hg(I1) passes through the bed without apparent interaction. Similarly, Cd (11) can be separated from all ions studied except Hg(1Ij by elution v i t h lJ1 hydrochloric acid. Zn(I1) is eluted preferentially from X ( I I ) , l I n ( I I ) , Fe(II), and ilIg(I1) by 2 to 431 hydrochloric acid. The separation of Zn(I1) from Fe(I1Ij is a borderline case according t o the criteria given above. With 1.11 hydrochloric acid, the calculated bed depth is approuimately 4 cni. with approximately 2 hours to remove zinc from the bed. If 2J1 acid is used, the elution goes rapidly, but the calculated bed depth is 7 em. Zn(I1) is not separated from Cu(I1). Cu(II), Si(II), Cd(lI), and Hg(I1) can each be separated from Fe(II1) by elution with 1231 hydrochloric acid. Otherwise, Cu(II), Ki(II), lIn(II), lIg(II), Fe(II), and Fe(II1) are not readily separated by hydrochloric acid elution from Dowex 50 resin.
The foregoing pertains to separations involving equal amounts of components, with loading amounting t o approximately lyo of resin capacity. For small d u e s of distribution coefficient, changes in loading have little effect. K i t h distribution coefficients larger than 10, the coefficient increases appreciably nith decrease in loading ( 5 ) . Thus some separations, especially those involving separation in concentrated hydrochloric acid of metallic ions from Fe(III), iyould be more effective with smaller amounts of iron. ACKNOWLEDGMENT
The authors are indebted to the Research Corp., Kew York, for financial support. LITERATURE CITED
(1) Bonner, 0. D., Jumper, C. F., Rogers, 0. C., J . Phys. Chem. 62,250 (1958).
12) . , Gleukauf, E., “Ion Exchange and Its Applications,” p. 34, Society %f Chemi-
cal Industry, London, 1955. (3) Kraus, K. A,, Michelson, D. C., Kelson, F., J . Am. Chem. Sac. 81, 3204 (1959). (4) hIann, C. K., ANAL. C H E ~ I . : ~1385 ~, (1957).
(5) AIann, C. K., I b i d . , 32, 67 (1960). (6) Strelow, F. W.E., I b i d . , p. 1185. RECEIVEDfor review August 9, 1960. Accepted October 31, 1960.
Measuring Melting Points and Rates of Crysta IIizatio n . of Polymers by Recording Changes in Birefringence CHARLES
W. HOCK and JOSEPH F. ARBOGASTI
Research Center, Hercules Powder Co., Wilmington, Del.
b This paper describes a rapid instrumental melting point test based on the disappearance of birefringence from crystalline polymers on heating. With the same versatile equipment, rates of crystal1izat;on can b e measured also, Both tests use instruments to measure and record simultaneously and automatically on a moving strip chart the brightness between crossed Nicols and also the temperature of the specimens. The test is applicable to polymer in the form of powder, film, or molded piece. Under certain conditions the melting point values are reproducible to better than ~ k 0 . 5 ’C. owing to their crystallinity, appear bright when viewed between crossed Nicol prisms. With these polymers, depolarization of ANY POLYMERS,
462
ANALYTICAL CHEMISTRY
the transmitted light as a function of temperature is one of the most convenient, as well as most meaningful, ways of getting a melting point. The simplest way to do this involves placing a thin piece of the polymer on a hot stage on a polarizing microscope and then reading on a mercury bulb thermometer the temperature a t which the last trace of birefringence disappears as the specimen melts. Visual observations of loss of birefringence are fairly easy to make but the operation beconies tedious when many samples must be tested. By using instruments to record the basic observations, the test can be made faster and more precise, while providing relief for the operator. This paper describes a n instrumental melting point test based on the disappearance of birefringence at the melting point. Es-
sentially the same equipment can be used to measure rates of crystallization, also. The amount of depolarization is a complicated function of the content of crystalline material, the size of the crystalline regions, the orientation of these regions with respect to their neighbors, and the orientation of the specimen as a whole. For these reasons the instrumental response cannot be used to determine directly the degree of crystallinity of specimens. These factors do not limit, hoivever, the usefulness of the instrument for determining the point of disappearance of the last-melting crystal or for determining the rate of crystallization when this occurs in a definite front spreading from centers of nucleation. 1 Present address, Bacchus Works, Hercules Powder Co., Magna, Utah.
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Diagram of melting point apparatus
A. 6.
Polarizing microscope (Bausch & tomb Research Model) Hot stage (custom made) C. Specimen-holding slide (custom mode) D. Demonstration eyepiece (Bausch & tomb) Multiplier phototube housing (custom made) E. F. Amplifier (Aminco Photomultiplier Microphotometer, No. 1 0 - 2 1 0) G. Two-pen recorder (Brown Strip Chart, Class 15) H. Recorder-controller for hot stage (Brown Circular Chart, Class 15) 1. Recorder-controller for oven (Brown Circular Chart, Class 15) J. Pretreating oven (custom made) K. Hot plate (Fisher Scientific Co., modified) I. Indicating and controlling pyrometer (West Instruments, Model J) M. Lamp (American Optical Co., No. 3 7 0 8 ) N. Air jet for cooling 40-mm. objective 0. Air-controlled voriable transformer (Conoflow Model EB-73N12) P. Air-controlled variable transformer (Conoflow Model EB-73N12) -11-11- Air lines -Thermocouple wires Electrical leads
---
INSTRUMENTAL SETUP
The instrumental setup is diagrammed in Figure l . A polarizing microscope, A, is equipped with a hot stage, B , into which a specimen-holding slide, C, is inserted. The hot stage consists eqsentially of a block of Duralumin covered with Transite. The specimen-holding slide is made of silver (Figure 2 ) . h demonstration eyepiece, D, situated in the top of the microscope tube divides the light from the objective. The axial ocular of this eyepiece permits visual observations of the polymer specimen. d multiplier phototube housing, E , is attached to the side a r m of the eyepiecr. The intensity of the light which reaches the tube housing is measured by a multiplier phototube and amplifier, F , nith a meter for direct reading. At the same time, leads from the amplifier are inserted into the circuit of a tn-o-pen, strip-chart recordei G, where the signal is recorded, in red ink, on the chart paper. Thermocouple leads from the specimen-holding slide, C , run to the same recorder, G, where the signal is recorded in black ink by the second pen. A recorder-controller, If, whose signal actuates a n air-controlled variable trans~
f Figure 2. slide
former, 0, regulates the temperature of the hot stage. The controller can be set either to heat the stage a t a constant rate of rise of temperature of 0.4" C. per minute, by a motor-driven set point, or to maintain continuously a constant temperature. If desired, the controller, H , can be switched t o manual control, to allow the stage to $at at a n average rate of about 10 C. per minute. Another pneumatic recorder-controller, I . with motor-driven set point and aircontrolled variable transformer, P, regulates the rate of cooling of a small specimen pretreating oven, J. The oven, which is made of Duralumin, is diagrammed in Figure 3. With this oven, 12 specimens can be held simultaneously for a specified length of time a t a preselected temperature above the melting point and then cooled a t a constant rate of 0.3" C. per minute, during which time recrystallization occurs. I n measuring rates of crystallization, the specimen is first fused by inserting the specimen-holding slide, C, containing the test specimen into the slot on top of the hot plate, IC,whose temperature is regulated by a pyrometer, L , After fusion, the specimen-holding slide is inserted into the hot stage maintained a t the temperature below the melting
point a t which crystallinity is to be measured. During operation, the microscope is covered by a black velvet cloth, supported by a wire frame. to cut down extraneous light and air drafts. TEST PROCEDURES
Measuring t h e Melting Point. Polynier in the form of powder, film, or molded piece can be tested. A prime requiiement is, of couise, t h a t the specimen be ti anspa rent or nearly so. K h e r e thtl polymer is in the form of poijder or flake, a suitable specimen can be made by pounding a n approximately 30-mg. mound of the powder between t n o metal plates. This. in effect, forms a thin m f e r or disk. Films are u-ually thin enough as received so that pieces of them can be used directly. ;\loldcd objects must ions. The dianieter of a specimen inay rang' between about 5 and 10 mni. Each specimen is placed brtneen 12-mm. round cover glasses for use in either the spmrnenholding slide or the pretreating oven. K i t h the instrummtal setup described here, inrxlting points can be ineasured in either of t n o general waj s, the principal differcnce bet\\-em them being the rate a t nliich specimens arc heated and cooled. I n both methods the specinlens are blanketcd n ith nitrogen during teating. In method I the rate of hcating and of cooling the polymer is rapid. Specifically, it i. heated at an avrrage rate of about 10" C. per minute and cooled a t an average rate of about 3" C. per minute. This method is esperially useful for testing the melting point of uncharacterized experimental polymt'rs whose approxiniate range of melting is not known beforehand. In practice, the specimen is heated, then cooled, and reVOL. 33,
NO. 3,
MARCH 1961
* 463
VIEW
SIDE
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Figure 4. Melting point curve according to method I
Figure 3.
several degrees higher than that determined even by careful experiment (1,s). Method I1 is more accurate and more precise than method I. iinother advantage of this method is the shortening of the test time per sample. I t is reduced to about one third the time required by method I. Method I1 is especially useful when looking for relatively small melting point differences in a large number of polymer samples of the same generic type.
\
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NITROGEN INLET
Diagram of pretreating oven
heated (Figure 4). During initial heating the temperature a t which the last trace of birefringence disappears is sometimes difficult to establish, owing to noise in the brightness curve. This is caused largely by movement of the specimen as it melts. During the initial fusion and subsequent recrystallization the specimen settles down. On heating a second time, the temperature a t which the last trace of birefringence disappears is defined sharply. The first two steps in the fusion-recrystallization-fusion cycle can, in a sense, be considered pretreatment. The reproducibility of melting point and recrystallization temperatures measured by this method is about &lo C., depending largely on homogeneity of the polymer. In method I1 the rate of heating and of cooling is comparatively slow. Oncefused specimens are cooled a t a constant rate of 0.3' C. per minute to permit a reasonably high degree of recrystallization. The recrystallized specimens are then heated a t a constant rate of 0.4" C. per minute (Figure 5 ) . These rates are a reasonable compromise for the specific case of polypropylene. Faster rates
of polyethylene tested
are tolerable for polyethylene. Materials such as polystyrene require long times a t a critical temperature to crystallize satisfactorily. Melting points measured by method I1 are reproducible to better than 10.5' C. Twelve specimens a t a time are pretreated (under nitrogen and a t a slight negative pressure) in the small specially designed oven (Figure 3). The pretreatment consists of holding them for 10 minutes a t a temperature above the melting point. The fusion temperature must be sufficiently high to allow for molecular randomization and relaxation of most of any orientation in the original sample. Then the specimens are cooled slowly a t 0.3" C. per minute to room temperature. This basic cycle can be modified to meet particular requirements but, in any case, the pretreated specimens are finally run, one a t a time, on the hot stage of the microscope, where the heating rate is 0.4" C. per minute. The slow heating and cooling used in this method favor obtaining higher melting point values than when the same polymers are run according to method I. The true equilibrium melting point may, however, be
Measuring Rates
of Crystallization.
To measure rates of crystallization, specimens just like those used for the melting point determination are required. The specimen, in the specimen-holding slide, is first fused in the slot of the modified hot plate ( K in Figure 1) where it is held for the desired length of time a t any specified temperature above the melting point. Then it is transferred quickly to the hot stage already maintained at the desired recrystallization temperature. Throughout these manipulations the temperature of the specimen is recorded on the strip chart (Figure 6). The length of time required for the fused polymer to reach thermal equilibrium a t the new and lower stage temperature can be shortened by placing the slide momentarily in a cooled metal block, during transfer from the hot plate to the hot stage. The time which elapses before crystallization starts (shown on the chart by a sharp rise in brightness) is called the induction time ( 2 ) . When
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464
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Figure 6. Crystallization curve of polypropylene fused a t 200' C. and then allowed to crystallize a t
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the brightness curve levels off, crystallization is presumed to be complete. Whether measuring melting points or rates of crystallization the setup permits visual observation of the progress of the tests, as well as automatic recording. This is highly advantageous. The ability to see the specimen and to recognize its morphology and any visible contaminants it may contain is a valuable adjunct. Also, although the test specimens are small, they can be recovered after melt testing and additional measurements such as viscosity and x-ray diffraction can be made. This, too, may be a source of valuable information about the behavior and properties of the polymers. Calibration. As shown in the dia-
gram of the specimen-holding slide (Figure 2), the thermocouple is adjacent to, rather than actually in, the polymer whose temperature is to be measured. This makes i t necessary t o calibrate internally. For this purpose heat-stable birefringent crystalline materials, whose freezing or melting temperatures have been measured very accurately by other methods, are used. The melting points of the calibrating substances are then measured by the birefringence method, under conditions identical to those used for testing the polymers. ilny difference in the melting points is a correction applied to subsequent test values. The magnitude of the correction varies a t different temperature levels but sel-
dom exceeds l’ C. I n some instances no correction is required. ACKNOWLEDGMENT
We are grateful to J. J. Dolan and J. C. Bowers, for their efficient and willing help. LITERATURE CITED
(1) Mandelkern, L., J . Engineers 15, 63 (1959).
SOC. Plaslics
(2) Mandelkern, L., Quinn, F. A., Flory, P. J., J. A p p l . Phys. 25,830 (1954). (3) Wood, L. A,, Bekkedahl, N., Ibid.,
14,362 (1946).
RECEIVEDfor review June 1, 1960. Accepted ZITovember25, 1960.
Determination of the Distribution of Nonionic Surface Active Agents between Water and Iso-octane H. L. GREENWALD, E. B. KICE,’ M. KENLY, and JOSEPH KELLY2 Rohm & Haas Co., 5000 Richmond St., Philadelphia, Pa.
b The distribution coefficient of a surface active agent between water and a paraffin i s a measure of the relative hydrophilic-lipophilic nature of the surface active agent, if the measurement i s performed at low concentration. A procedure for carrying out the determination with iso-octane as the paraffin i s outlined. The effect of concentration on the distribution i s investigated over a 3000fold (aqueous) concentration range for one surface active agent, p-tert,tertoctylphenoxypoly (8.7)oxyethyleneethyl alcohol (OPE9.7). The distribution of the surface active agent remains constant for a considerable concentration range below the critical micelle concentration (CMC). Distribution coefficients, at a concentration below the CMC, were determined for five “compounds” in the range OPE5 to OPE20. The coefficient i s 1 when the number of ether units i s about 10. This compound corresponds to the member of the series b y far the most widely used.
S
(SAA) have usually been classified on the basis of use, chemical structure, and solubility in water and oil. The oil- and water-solubility behavior of a substance is a measure of its relative hydrophilelipophile character. Unfortunately, i t URFACE ACTIVE AGENTS
Present address, Columbia, S. C. Present address, Foxboro Go., Foxboro, Mass.
is a poor and erratic measure, as one solubility is not necessarily an inverse function of the other, and the water solutions of surfactants will convert to micellar solutions a t high concentrations. A previous paper from this laboratory (5) describes several methods, all empirical, used to determine the relative hydrophilic and lipophilic tendencies, and adds a new one to the list. Weeks, Lewis, and Ginn (15) have since suggested improvements in that method. A scale for the measurement of relative hydrophilic and lipophilic tendencies, which is amenable to physical measurement and some measure of theoretical interpretation, is proposed. The scale is the distribution coefficient of the substance between water and a paraffin oil as the concentration of the substance goes to zero. Extrapolation to zero concentration is necessary to avoid errors because of the formation of micelles and, in the case of ionic surface active agents, because of changing ionic strength. This scale may prove deficient when applied to molecules whose hydrophobic end is not hydrocarbon. I n these cases, such as fluorocarbon hydrophobes, it may be better to replace the paraffin by some other liquid, or to work with two other immiscible liquids in cases where surface activity in nonaqueous solutions is of interest. The distribution of a surfactant can then be determined in practice by a procedure such as the following:
Make up nater solutions of thc SUIfactant a t several concentrations below the critical micelle concentration (CMC). Make up iso-octane solutions of the surfactant a t several concentrations. I n bottles, layer pure iso-octane over the water solutions from the first step, and in other bottles layer the solutions from the second step over pure water. After allowing the bottles to stand for some time, analyze both layers in each bottle. I n considering possible water-immiscible solvents, it was assumed that if iso-octane or some other pure paraffin were used as the lipoid, chemical specificities such as hydrogen bonding or large van der Waals forces would be minimized. The distribution coefficient of a substance can be defined as:
D
=
CJC,
(1)
where C,
total concentration of the substance in the water phase a t equilibrium C, = corresponding concentration in the oil phase =
Thus, over the range of variables where the ratio of the activity coefficients remains constant, the distribution coefficient will remain constant. As a function of concentration there will usually be two factors having a marked influence on the activity coefficient ratio: the degree of dissociation VOL. 33, NO. 3, MARCH 1961
465
