The suggested simplification for first-order decompositions was tested by repetition of the 114 “C rate curve of N-phenylN’-tosyloxydi-imide N-oxide (cupferron tosylate), reported by Dorko et al. ( 2 ) , and the 271 “C rate constant for “cyclotetramethylenetetranitramine” (octahydro-1,3,5,7-tetranitro1,3,5,7-tetrazocine, HMX), reported by Robertson (5). EXPERIMENTAL Decompositions were run in a Perkin-Elmer Model DSC-1B differential scanning calorimeter. Samples were sealed in aluminum cells, Perkin-Elmer Part Number 219-0062, which were perforated with a single hole approximately 0.15 mm in diameter. Both the differential and average temperature calibrations of the DSC-1B were carefully checked before the runs. The sample of cupferron tosylate was prepared and purified by M. D. Coburn of this Laboratory. The HMX sample was crystallized from a filtered acetone solution of 99.9+ pure HMX, provided by the United Kingdom Atomic Weapons Research Establishment, Aldermaston.
z
RESULTS AND DISCUSSION A plot of In deflection 6s. time for what Dorko et al. called the “decay” portion of the 114 “C rate curve of cupferron tosylate is shown in Figure 1. The linearity of the plot shows that the decomposition is first order. The least-squares slope
of the line is 0.0052 sec-1, which compares extremely well with a rate constant of 0.0051 sec-1 calculated from a least-squares ) by Dorko et al. treatment of the rate constants ( k ~presented (2). A later publication by Crossley, Dorko, and Diggs (6) lists a rate constant of 0.00578 sec-l as an experimental point at 114 “C. The average error of the present determination was found to be slightly better than 2 X sec-l; therefore results are reported to two significant figures only. Dorko did not report on the precision of either of his sets of data. A similar treatment was made for HMX at 271 “C. This temperature was chosen, because it was the only point on Robertson’s plot (5) that could be read accurately. Robertson’s rate constant was 0.0013 sec-’; the value obtained by the simplified method was 0.0015 sec-’. It appears that the simplified method makes it possible to obtain rate constants very quickly with little or no decrease in accuracy. The reduction in the number of manipulations of the data required may actually make it possible to improve accuracy and precision in isothermal measurements. RECEIVED for review October 6, 1971. Accepted December 21, 1971. This work was performed under the auspices of the United States Atomic Energy Commission. (6) R. W. Crossley, E. A. Dorko, and R. L. Diggs, in “Analytical Calorimetry,” Vol. 2, R. S. Porter and J. F. Johnson, Ed., Plenum Press, New York, N.Y., 1970, p 429.
( 5 ) A. J. B. Robertson, Trans. Faraday SOC.,45,85 (1949).
Application of Metallic Membrane Filters in the Clarification of Styrene-Butadiene Rubber Solutions for Gel Permeation Chromatography M. D. Baijal Research and DeGelopment Dicision, Beech-Nut, Inc., Port Chester, N . Y. 10573
IN GEL PERMEATION CHROMATOGRAPHY (GPC) of materials, it is important to clarify sample solutions properly before injection to avoid plugging of the G P C columns. Normally this clarification procedure calls for filtration of the sample solution using Waters Associates ( I ) pressure filtration assembly loaded with Krueger asbestos filter. The asbestos filter, though suitable for a variety of polymer structures, was inadequate for the clarification of certain Styrene-Butadiene Rubber (SBR) solutions. Even very dilute solutions of these SBR products prepared in toluene and filtered through asbestos filter were found to plug the G P C columns. The cause for this plugging phenomenon was traced to the gel structures (or supermolecular structures) present in high proportion in the SBR products analyzed. In this study it was found that proper clarification (and degelling) of such SBR solutions for room temperature operation of G P C can be carried out by filtering first through an asbestos filter-metallic membrane filter combination, and finally through filtration with another metallic membrane filter at the injection point. EXPERIMENTAL Materials. Six SBR samples (SBR-I to SBR-VI) were obtained as type 2006 latices from Uniroyal Chemicals. These latices were emulsion copolymers of 25 styrene-75
z
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(1) Waters Associates, Inc., 61 Fountain Street, Framingham, Mass. 01701.
butadiene having a number-average molecular weight ( E n ) of 55,000 i lo%, and a total gel content (G) of 50 f 15% (2). Both E n and G data was determined on coagulated latices. Analytical grade mercaptoacetic acid was obtained from Eastman Kodak Company. Tetrahydrofuran (THF) and toluene were obtained as reagent grade solvents from E. I. du Pont de Nemours and Company and Fisher Scientific Company, respectively. Methods. Both T H F and toluene were filtered prior to their use. The SBR latices were coagulated according to a modified ASTM D 1417 procedure using a drying time of 15 minutes. The 0 . 2 5 z and 0.125% THF solutions of the coagulated mass for GPC were made by swelling the rubber for 2 hours in the solvent at 50 “C. The 6z T H F in toluene (vol/vol) was made at 25 “ C for G P C column plate count. The calibration curve was constructed in the usual manner using polystyrene standards obtained from Waters Associates. A 10% T H F solution of mercaptoacetic acid-toluene mixture (1 :1) was made at 25 OC to unplug GPC columns. Three types of filtration procedures were used to clarify the T H F solutions so prepared. Single filtrations (S) in(2) Polymer type was suppliers data. f i n and G were authors -
unpublished results. Mn was determined using HewlettPackard Membrane Osmometer Model 502, operating at 35 “C with toluene and SS 0-8 membrane. G was determined according to J . AppI. Polym. Sei., 8, 1306 (1970).
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Table I. GPC Columns Pressure as a Function of SBR Injections Final pressure, psi Filtration of Filtration of Initial Sample pressure, 0.125z solution 0 . 2 5 z solution psi S D T S D T type SBR-I 145 200 185 145 220 200 145 SBR-I1 SBR-111 SBR-IV SBR-V SBR-VI
145 145 145 145 145
190 190 205 200 200
180 180 180 180 180
145 145 145 145 145
220 210 220 220 220
200 145 200 145 200 145 200 145 205 145
volved filtration using Waters Associates pressure filtration assembly loaded with asbestos filter operating with 20 psi N f at 25 "C. Double filtration (D) consisted of filtration using Waters Associates pressure filtration assembly loaded with a n asbestos filter-FM-41-0.2 metal membrane filter combination operating with 20 psi NPat 25 "C. Triple filtration (T) amounted to refiltration of the doubly filtered solution (according to filtration D) prior to injection at the GPC columns. This was accomplished using a G P C 10-ml hypodermic syringe installed with a Swinney type hypodermic adapter loaded with a FM-13-0.2 metal membrane filter. The metal membrane filters used were obtained from Selas Flotronics ( 3 ) in 41 mm (for pressure filtration) and 13 mm (for Swinney adapter) sizes having a maximum pore diameter of 0.2 micron. The Swinney type hypodermic adapter was obtained from Gelman Instrument Company. SBR solutions were filtered using filtration procedures S, D, and T and the other solutions were filtered using only filtration procedure S. Gel permeation chromatography was carried out using Waters Associates GPC-301 installed with a four-columns bank having the following permeabilities (A"): 5(10)67(10)5, 7(10)s-1.5(10)5, 5(10)4-1.5 (lO)l, and 2(10)3-7(10)2. The following conditions prevailed in the G P C analysis: carrier solvent, T H F ; temperature, 25 "C; flow rate, 2 ml/min; injection time, 1 min, and sensitivity, 4 X . The unplugging of the plugged GPC columns (caused as a result of SBR injections) was accomplished by 4 to 6 injections of mercaptoacetic acid-toluene 10% T H F solutions (4). RESULTS AND DISCUSSION
The plugging of the G P C columns encountered as a result of SBR injections can be readily seen from Table I. As test cases, two injections of SBR-I1 were made as the 0.063z toluene solutions after having been clarified utilizing filtration procedure D. Both these injections resulted in a pressure rise to 180 psi indicating that even very dilute injections made (3) Selas Flotronics, Box 300, Spring House, Pa. 19477. (4) J. Sarruda, Waters Associates, Inc., 61 Fountain St., Framingham, Mass. 01701, personal communication, 1971.
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in thermodynamically good solvent were incapable of avoiding column plugging. (GPC of SBR is normally carried out in toluene, but when UV and RI detectors are coupled, the solvent of choice seems to be THF because toluene absorbs the UV radiation.) The variation of pressure rise as a function of the concentration of injection can be understood in terms of the effect of viscous trapping and polymer structure on the column support matrix. A temporary pressure climb of 20 over initial pressure is not an uncommon phenomenon, but a higher pressure climb with stability suggests plugging of the columns. The latter was exactly the case with SBR injections clarified through filtration procedures S and D. Shorter injection times and reversing the column arrangement did not avoid column plugging when the injections made were clarified according to filtration procedures S and D. Elevated temperatures, though helpful in reducing the viscosity and increasing the mobility of the molecules through the column support, were not tried because the analysis was aimed for 25 "C. It appears from this study (Table I) that for G P C of SBR (structures studied) at 25 "C, one must clarify injection solutions by filtration procedure T to avoid plugging of the columns. Triple filtration procedure T essentially reflects the incapability of Krueger asbestos filter (0.01 p ) in removing gels from SBR solutions, which are responsible for plugging of the columns. The structure of SBR products analyzed contains both microgel and normal gel, the sum of which is defined as the total gel content, in very high proportions, and these gel particles are strong contributors to the physical properties of the rubber. The gel particles could be branched and/or linear cross-linked molecules. Both the type and amount of gel for the same percentage of styrene content in the block copolymer can vary with the variation in the synthetic conditions employed in the synthesis of SBR polymers. The GPC chromatograms of SBR structures analyzed consisted of three components, all accompanied by UV absorptions. As far as the integrity of the columns is concerned, it did not change as a result of sequential plugging and unplugging because both the calibration curve and the initial and final plate count remained unaltered. Based on this study, it is concluded that proper clarification of high gel SBR structures for G P C at 25 "C can be carried out by using metallic membrane filters in conjunction with the Krueger asbestos filter. ACKNOWLEDGMENT
The author thanks D. Harmon and M. Ezrin for a discussion of this problem, Miss A. Manz and P. Heinlein for assistance in the experimental work, and Beech-Nut, Inc., for permission to publish the results. RECEIVED for review October 12, 1971. Accepted December 13,1971.
