Analysis for Surface Peroxides Arthur Bradley and T. R. Heagney Surface Activation Corporation, 1150 Shames Drive, Westbury, N . Y . 11590
Oxidizing groups on or near the surface of polyester film are determined iodometrically. A substantial increase in oxidizing power observed after exposure to a gas discharge ion plasma under specific conditions, coupled with greatly enhanced reactivity for initiation of raft copolymerization, i s attributed to the peroxide unction. Results indicate that approximately one peroxide linkage (presumably preceded by one free radical site) is formed on each polyester monomer unit that lies close enough to the film/ plasma interface to be excited by ion bombardment or the release of ionization energy. A paratus and procedure for determinations of sur ace peroxide levels of the order of 1014 to 1015 molecules/cmz are described. Confirming evidence of the peroxide character of the discharge-treated film surface is . derived using this analytical method.
7
P
FORSEVERAL YEARS this laboratory has conducted studies on the modification of the surface properties of polymers, including plastic films. One approach to rendering a film such as polypropylene or polyester permanently wettable is to graft a thin coating of polyacrylic acid on the surface. Characteristically, this procedure is conducted in two steps: (a) exposure to an ionized plasma in a gas discharge apparatus to create reactive sites, followed by (b) immersion in warm monomer, purged free of air, until the desired degree of grafting has occurred ( I ) . The time required for an electric discharge to flood a typical film surface with graft-initiation sites (a) is of the order of one second. A typical grafting time at 60 to 80 “C (b) might be 20 minutes. It is of particular interest, however, that there is virtually no limit to the lifetime of the surface activation created by step (a). Film samples retained for months in air without precautions in handling have yielded excellent grafts. The properties of indefinite stability under ambient conditions and initiation of graft polymerization at slightly elevated temperatures suggested that the reactive sites were peroxide or hydroperoxide groups, particularly since aqueous potassium iodide virtually neutralized the effect of discharge activation as a precursor to grafting. The work reported here was initiated both to confirm the presence of the peroxide linkage on the surface of plasma-activated substrates and to provide a means of analysis that could be used to evaluate the effectiveness of various modes and conditions of discharge treatment. The use of iodometry for trace analysis was complicated by the sensitivity of the reagent (acidified alcoholic iodide) to atmospheric oxygen. Moreover, the procedure had to be designed to mount a sample of maximum area in a minimumsized vessel to limit the volume of reagent required. As an aid to reproducibility, the apparatus was constructed in duplicate, with the reduction procedure for both a discharged sample and a control conducted simultaneously. The peroxide assay was taken as the result for the discharged sample less that of the control. A repeat determination on any film specimen in a clean apparatus with fresh reagents showed that essentially all the oxidizing function was consumed in the first test. The results indicate that between 3 X 1014and 7 X 1014 peroxide sites per square centimeter can be found on dis(1) J. H. Coleman, U. S. Patent application filed 11 March 1966.
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
charged film. This is approximately equivalent to the number of monomer units exposed on the polyester film surface (typically mol wt 192, density of unity). It is reasonable to assume that no more than one reactive site leading to a peroxide was formed on any one glycol terephthalate unit, and that the discharge conditions tested here led to saturation of all available site locations. Iodometry is discussed in many texts (2, 3). The method developed in this Laboratory was based on a procedure described by Mair and Graupner (4, adapted for trace analysis and incorporating a novel application of the Fisher Titrimeter (5). The analysis procedure may be summarized as follows: Sodium iodide is dissolved in boiling isopropyl alcohol and introduced while hot into the loading flasks (Figure 1) and purged with nitrogen. Samples of polyester film, wound onto photographers’ stainless steel spiral reels, are inserted in the reactor flasks and refluxed for five minutes under nitrogen in a mixture of isopropyl alcohol and acetic acid. The sodium iodide solutions are then forced into the reactor vessels by nitrogen pressure and the total contents, about 300 cc, heated to a gentle boil for 30 minutes. The spiral reels keep each surface of the long thin strip of film free to contact the iodide reagent. Any oxidizing agent on the film will tend to release free iodine in the reagent solution, determined by potentiometric titration with 0.001Nsodium thiosulfate dispensed from a microburet. The dimensions of the various pieces of apparatus and exact details of procedure are given in the Experimental section. EXPERIMENTAL
Materials. Assays for peroxide were conducted on polyester film samples obtained from three different sources, identified in the data sheets as Mylar I, 11, and 111. Visually they were indistinguishable. The suppliers and thicknesses were as follows: Mylar I IBM Corp., San Jose, Calif. 3 . 5 mils Mylar I1 Eastman Kodak, Rochester, N. Y. 4 . 2 mils Mylar I11 Brownell Corp., New York City 3 . 0 mils Sodium iodide, glacial acetic acid, and isopropyl alcohol (IPA) were reagent grade. Stock solutions of 0.1N sodium thiosulfate were prepared from an appropriate concentrate supplied by the Fisher Scientific Co. A portion of this stock was diluted 1 :lo0 before each series of titrations. Its concentration was confirmed at intervals by titration against a freshly prepared solution of 1.27 grams of iodine crystals and 4 grams of potassium iodide in 50.0 ml of water (0.10M). All aqueous solutions and dilutions were made with a commercial grade of distilled water that was boiled with an immersion heater inserted into the 5-gallon jar and stored (2) I. M. Kolthoff and E. B. Sandell, “Textbook of Quantitative Inorganic Analysis,” Macmillan, New York, N. Y.,1952, pp 585-605.
(3) W. E. Pierce, D. T. Sawyer, and E. L. Haenisch, “Quantitative Analysis,” Wiley, New York, N. Y., 1959, pp 216-217, 291-299. (4) R. D. Mair and A. U. Graupner, ANAL.CHEM., 36,194 (164). (5) “Fisher Titrimeter” Instruction Manual, Fisher Scientific Co., Springfield, N. J.
a
n
Figure 1. Peroxide-iodide reactor and loading flask, half of tandem operation (1) water inlet, (2) water outlet, (3)adapter,(4) rubbercap,(5) common T joint, (6) thermometer, (7) condenser, (8) vent, (9) outer stopcock, (10)inner stopcock, (11) rubber seal, (12)Teflon adapter, (13) rubber gasket, (14) clamp, (15) delivery tube, (16) common T joint, (17) resin flask, (18)delivery tube, (19) steel spool, (20) stand, (21) loading flask, (22) electric heater
under nitrogen. The latter gas, also used for purging the apparatus of air and transferring solutions in and out of the reaction flasks, was a welding grade obtained from a local supplier. Procedure. SAMPLE AND APPARATUS. Mylar film is precut to the dimensions of 35 mm by 120 cm, for a total area of 840 cm2. A “control” and a “discharged film” are inserted into separate stainless steel spiral reels, each 3.5 cm high and 7.6 cm in diameter, and placed into “reactors,” as described below. If the control film goes into reactor “A” and discharged film to reactor “B” at first, in the following experiment they are interchanged. Reactors A and B are constructed to be identical in every respect. Each reactor is essentially a 500-cc resin flask fitted with a wide ground glass cover having four 24/40 ground glass necks. To these necks are affixed a water-cooled reflux condenser, a thermometer, a rim stopcock adaptor, and a central stopcock adaptor with an inner glass tube extending down through the center of the spiral reel to the bottom of the flask. The outlet of this center stopcock connects to a short rubber hose with a T insert for venting to air and connection to the glass tubing that reaches the “loading flasks.” All reagents enter and leave the reactors through this center stopcock. A film reel is placed in each resin flask and the reactors are assembled, a rubber gasket coated with silicone grease ensuring an air-tight seal, To the mouth of each condenser is attached a spray-trap adaptor that is fed by one common nitrogen gas supply. The central outlet of each reactor is connected by rubber hose and glass tubing to a 500-cc Erlen-
meyer flask, the “loading flasks.” Auxiliary lines from each of these flasks are brought to a common point by a T connection. A diagram of one of the reactor flask assemblies is presented in Figure 1. EVACUATION AND PURGING.A tube attached to the loading flasks’ auxiliary line is connected in series with a pressure gauge and a vacuum pump, which is used to reduce the time required for purging the system with nitrogen. The pressure is gradually brought back to atmospheric over a period of 15 to 20 min, and the purge outlet to air is restored by removing the rubber caps from the condenser adaptors. PREPARATION OF REAGENTS AND PRELlMINARY LOADING. To each of the two 500-cc Erlenmeyer “loading” flasks, 240 cc of dry reagent grade isopropyl alcohol (IPA) is added along with 10 cc of acetic acid (HOAc). A nitrogen cylinder line (independent of reactor purge described above) is now attached to the joint common to both loading flasks and carefully opened. The IPA-HOAc mixture is gently forced into each reactor by nitrogen pressure. When the loading flasks are empty, the pressure is discontinued, and the stopcocks leading into each reactor are closed off. The contents of each reactor are warmed to the boiling point and maintained for 5 min at reflux temperature by the hot plates mounted beneath them. In the meantime the two 500-cc loading flasks are again purged with nitrogen from their common T connection. The pinch clamps on each reactor (vent No. 8, Figure 1) are cracked open to allow gas to escape. Also, at the same time, the IPA-NAI solutions are prepared. Into each of two clean 125-ml Erlenmeyer flasks are placed 10.0 grams of sodium iodide (NaI) and 75 cc of dry IPA. These two flasks are placed on a hot plate and the alcohol is brought to a boiling point and kept there until all NaI has been dissolved. FINALLOADINGAND REACTION.After the IPA-HOAc mixture in each reactor has been maintained at its boiling point for 5 min, the heat is reduced. When most of the reflux action has subsided, the nitrogen supply to the 500-cc loading flasks is cut off while each is opened and charged with approximately 60 cc of NaI-IPA. Once loaded, all connections are again made fast and the nitrogen pressure resumed at the T joint common to both 500-cc loading flasks to force the IPA-NaI solution into each reactor. The total charge now consists of approximately 300 cc of reacting solution. As soon as both of the NaI-IPA solutions have been transferred to their respective reactors, the stopcocks are closed and the total contents in each reactor are heated to boiling. UNLOADING.After 30 min at reflux, the contents are transferred to stoppered flasks and stored for titration with standard thiosulfate solution. To pump the solutions from the reactors, rubber caps are placed over the adaptors on each condenser, and the outside stopcocks located on each of the reactor covers are opened. The glass angle tubing is again attached to the central stopcock and the reactor contents forced into 500-cc ground glass stoppered bottles or flasks by gentle nitrogen pressure. If the analysis can be completed immediately, the storage flasks may be left open with a nitrogen purge at the neck. DETERMINATION OF FREEIODINE. The equipment employed for this phase of the operation is a Fisher Titrimeter (Model No. 35), used in conjunction with a platinum electrode and a saturated calomel electrode. The Fisher stirrer base of the titrimeter was replaced with a Thermolyn heatermagnetic stirrer combination. Thiosulfate (0.001N) was dispensed by a special microburet with reservoirs for standard solution and attachment for nitrogen purge and flow pressure. The reagent storage/dispensing system and titration setup is illustrated by the diagram in Figure 2. Of the five stopcocks, only No. 1 at the nitrogen inlet is bifunctional. It was convenient to include a nitrogen ballast flask as an aid to refilling the 1.00-ml microburet. The manipulation of the various stopcocks to purge the apparatus and apply cautious ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
0
895
molecules/cm2. For consistent results it is important to conduct all titrations in a single series at the same temperature. A fixed “reaction time” of about 30 to 60 sec between bureting in standard thiosulfate solution and balancing the instrument to the null point is observed. Slow changes in end point that may occur on long standing are disregarded. RESULTS AND DISCUSSION
A sample calculation for a set of titrations consisting of a polyester control, a discharge-activated polyester, a reagent blank, and a repeat assay of the discharged film is presented herewith. These were among the first titrations attempted, and the control result was high compared to later experiments. Calculation step (expts 3ab, 4ab) Reactor
Figure 2. Thiosulfate storage and dispensing system, clamps and supports not shown (1) two-directional stopcock, (2a) (2b) (3a) and (3b) one-directional stopcocks, (4) microburet (5) thiosulfate storage flask, (6) nitrogen storage flask, (7) titation beaker with electrodes, (8) mag-
netic stirrer
nitrogen pressure to force reagent into the buret should require no elaboration. Briefly, with nitrogen flowing through No. 1 into the solution reservoir and out through No. 3b, the solution reservoir flask (left, Figure 2) is filled to working capacity with thiosulfate reagent. To fill the buret, open Nos. 2a, 3a, and 3b and begin to close off on No. 2b until liquid rises in the delivery tube on its way to the buret. When the buret is filled slightly beyond capacity, open No. 2b to release pressure and close No. 3b. Excess solution will fall back into the reservoir. Then open the buret stopcock to fill its tip, drain excess solution, and bring the meniscus to the zero graduation mark. Stopcock No. 2b is closed, No. 1 switched over to the nitrogen reservoir, and Nos. 3a and 3b are opened. The system is now ready for the first titration. The titration reaction vessel is a cut-down 250-ml borosilicate glass beaker provided with a close-fitting Teflon (Du Pont) cover plate with openings for a platinum electrode, a saturated calomel electrode, a nitrogen inlet probe, and the tip of the microburet. Prior to titration a Teflon-coated bar magnet stirrer is placed in the beaker, the cap is affixed, and the Fisher electrodes are lowered into place. After 1 min of nitrogen gas purge, the test solution consisting of a 50-ml aliquot from the storage flask is pipetted into the beaker. Immediately before titration, a portion of 10 cc of boiled distilled water is added to the 50 cc of test solution and the magnetic stirrer is activated. After about 30 sec, the potentiometer is brought to its null position to observe the starting voltage corresponding to 0 ml of thiosulfate. The reagent is then added dropwise and the potential recorded at 0.05-ml intervals. The end point is best identified as the maximum rate of change of potential with increments of reagent after plotting the titration data. A minimum of three 50-ml aliquots of the original 300-ml sample are assayed for each reactor solution and the results averaged, converted into peroxide equivalent and eventually peroxide 896
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
Control B
DisReagent Repeat charged blank sample A B A
End point, ml 0.001N thiosulfate 0.56 0.69 Oxidant (as peroxide) in test solution pmoles/50 ml 0.28 0.345 Oxidant (as peroxide) pmoles in 300 ml (840cm20ffilm) 1.68 2.07 Oxidant, moles/cm2 x 109 2.01 2.48 Oxidant, molecules/ 12.1 14.9 cm2 X 10-14 Peroxide molecules/ cm2 . . . 2 . 8 X lo1*
0.12
0.14
0.06
0.07
0.36
0.42
0.43
0.50
2.6
3.0
...
...
A total of 17 pairs of determinations were carried out, of which 12 pairs specifically matched control and discharged samples of Mylar I, Mylar 11, and Mylar 111. A final two sets compared the effects of heating in air and vacuum on the peroxide content of Mylar 111. In the discussion to follow it will be convenient to omit the coefficient ( X 1014) in referring to oxidizing sites per square centimeter. According to the results summarized in Table I, the net peroxide assays of the Mylars I, 11,and I11 (3.5 =k 4,5.6 f 0.2, and 7.0 i 0.4, respectively) appear to differ significantly. However, these were all exhaustively discharge-treated substrates and the peroxide content should be essentially that of saturation: one peroxide bond for each molecular unit of polyester available at the surface. It would be difficult to rationalize any difference in saturation surface oxidizing power between the three substrates. A closer look at the data reveals that the discrepancy can be explained as a gradual development of the technique. The control values for oxidizing power of untreated substrate were highest with Mylar I, lowest with Mylar 111, as the procedure was expanded to include such improvements as prerinsing of samples with IPA and longer purge times. Experiments 16 and 17 were conducted with Mylar I11 under conditions comparable to those that gave a net peroxide assay of 7.0 for this substrate. They were carried out to illustrate how this analytical method can be used as a research tool. The polyester film samples were heated in vacuum (Experiment 16) and in air (Experiment 17), but only in the latter case does it appear that the peroxide sites remained intact. In Experiment 16 the peroxide assay fell from 7 to 1.5 ; in Experiment 17 it increased slightly to 9. This is consistent with acrylic acid-grafting results that showed a reduction in initia-
Expt lb 2a 2b 5a 5b 9a 9b 13a 13b
Table I. Typical Results of lodometric Titration Analyses Oxidant, End point as peroxide, (av of 3) molecules/ Sample Reactor ml OOlN SOg2- cm2 X 10-14 Mylar I, control B 0.44 9.5 Reagent blank A 0.15 3.3 Repeat Mylar 1b B 0.17 3.7 A 0.48 10.4O I, control I, discharged B 0.69 14.9 Mylar I, average of three determinations Mylar 11, control 11, discharged Mylar 111, control 111 discharged
Net peroxide assay, molecules/cm2 x 10-14
4.5 3.5 f 0.4
A
0.38 8.3 0.65 14.1 Mylar 11, average of four determinations
5.8
B 0.31 6.7 A 0.66 14.2 Mylar 111, average of four determinations
7.5
B
A 0.83 18.0 16a 111 controlb B 0.90 19.5 16b I11 dischargedb A 0.55 11.9 17a I11 controlc 17b I11 discharged" B 0.98 21.2 a Preliminary rinsing of samples on spiral reels with hot IPA began with Experiment 6. b Heated 4 hours in vacuum at 100 "C. c Heated 4 hours in air at 100 "C.
tion sites when film substrates were prebaked in vacuum but not when they were heated to the same degree in air. Grafting results could not show, however, that both control and discharged samples gained considerably in nonsurface peroxide oxidizing power as a result of the baking step. This suggests that the background noise in the peroxide analysis was due to thermal dissociation of the polyester molecular structure not confined to the surface. It may well be found that heated films have peroxide (and other) functional groups that originated as radical sites distributed within their bulk, accessible to iodide but too well shielded to serve as initiation sites for grafting. On the other hand, the peroxide sites which flood a film surface exposed to an ionized plasma are largely destroyed by
5.6 f 0.2
7.0 f 0.4
1.5 9.3
heating in vacuum by a mechanism which is essentially the reverse route of the formation of peroxide from discharge sites. In the absence of oxygen, the surface radicals have time to find each other and recombine forming a cross-link bond, thus reducing the oxidizing power of the surface. In air they tend to regain oxygen and at some sites there may result two hydroperoxides where one peroxide dissociated. This, of course, would lead to a net increase in surface oxidizing groups. R-O-O-R'
+ R-0 * heat
+ R'-O 2 *
R-O-OH RECEIVED for review March 11,1970.
+ R'-O-OH
Accepted May 8,1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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