Articles Anal. Chem. 1995, 67, 1955-1 962
Characterization of a Perfluorotetradecahydrophenanthrene Oligomer William H. Tuminello,* loannis V. Bletsos, Fredric Davidson, and Frank J. Weigert The Dupont Company, Experimental Station, P.0.Box 80356, Wilmington, Delaware 19880-0356 Nigel Simpsont and David Slinn* RhOne-Poulenc Chemicals, P.O. Box 46, St. Andrews Road, Bristol BS11 9YF, United Kingdom
A complex mixture of oligomeric perfluorocarbons was characterized primarily by nuclear magnetic resonance spectroscopy, electron spectroscopyfor chemical &is, and tixne-of-flight secondary ion mass spectrometry. Supporting tools were infrared and ultraviolet/visible spectrometry, plus elemental analysis and conventional gas chromatography/mass spectrometry. The following distribution of carbon and fluorine groups (expressed as the percentage of total carbons) was determined: olefinic carbon, 8%;CF3,8%,CF2,51%,and CF, 33%. Ageneral structural framework is proposed with the chemical formula C14F23(C14F~~)~C14F23, where n = 0, 1, and 2 for dimer, trimer, and tetramer, respectively. The monomer has the structure of perfluorotetmdecahydmphenanthrene. A wide diversityof structureswas created during synthesis by (1) loss of fluorine atom pairs to create further ring closure or unsaturatedcarbon-carbon bonds and (2) ring opening to obtained perfluoroalkyl side chains. Perfluorocarbon oligomers are byproducts in the commercial synthesis of perfluorotetradecahydrophenanthrene(C14F2.4). The synthesis involves the fluorination of phenanthrene via cofeeding elemental fluorine and the hydrocarbon in the gaseous state over a bed of cobalt(I1D fluoride.’ c14F24 manufactured by this route is sold commercially as Flutec PP11, primarily used for vapor phase soldering of printed circuit boards. Formerly this was carried out by RhBne-Poulenc, but the manufacture and sales have now been taken over by BNFL Fluorochemicals Ltd. Part of the manufacturing process requires separation of the Flutec PPll from the oligomeric byproducts by fractional distillation. The oligomers are the “still bottoms”. It was recently reported2 that the oligomeric byproducts are excellent atmospheric pressure solvents for PTFE, which opens the door to unique new characterization and processing possibilities for this polymer. Our objective in characterizing this material Current address: Fluoro Systems Ltd., Honeysuckle Cottage, Tidenham Chase, Nr. Chepstow, Gwent NP6 7JW,U.K. $ Current address: Fluoro Systems Lid., Cotswold House, Manor Road, Abbots Leigh, Bristol BS8 3RP, U.K (1)Joyner, B. D. J. Fluon’ne Chem. 1986,33, 337. (2) Tuminello, W. H.; Dee, G. T. Macromolecules 1994,27, 669. +
0003-2700/95/0367-1955$9.00/0 0 1995 American Chemical Society
was to identify it suf6ciently for government cert&ation, strengthening of any patents referring to the oligomer, formulation of a basis for manufacturing specifications, and a deeper understanding in further research efforts. EXPERIMENTAL SECTION It was believed that the as-synthesized oligomers contain small amounts of hydrogen and olefinic carbon (by olefinic carbon we mean trigonal sp2). One or more fluorines might be attached to this olefinic carbon. Posffluorination was used to reduce the unsaturation and hydrogen content. The material we are characterizing is one of these posffluorinated substances. During the postfluorination process, the “crude”oligomer was stirred at -200 “C in a vessel while free elemental fluorine, diluted 3:l with nitrogen, was bubbled through the liquid for several hours. The process was continued until about 1.5 mol of fluorine/mol of oligomer had passed through the reactor. The oligomers have never been a controlled product, so there is expected to be variability from lot to lot. This particular oligomer sample has an atmospheric pressure boiling point of about 322 “C. Its components have boiling ranges from about 280 to 400 “C. The whole oligomer is an exceedingly viscous, transparent, colorless liquid at room temperature which flows easily at about 60 “C. Higher boiling fractions are more viscous. The highest boiling fractions are brittle glasses at room temperature. All fractions observed thus far are transparent and nearly colorless. Discoloration occurred after several hours above 330
“C. The following solvents were used as received. HPLC grade 99.9%pure Freon 113 (1,1,2-trichlorob;inuoroethane),HPLC grade 99.9+% carbon tetrachloride, 99.8% deuterated chloroform, and 99+% Freon 11 (fluorotrichloromethane) were all obtained from Aldrich. The chloroform, obtained from EM Science, was glass distilled, hydrocarbon stabilized, and 99.97%pure. l9F NMR. I9FNMR spectra were obtained on a Nicolet NT200 IT-NMR instrument. Both deuteriochloroform and carbon tetrachloride were used as solvents. The oligomer was only marginally soluble in deuteriochloroform. Solubility was improved in carbon tetrachloride but was still limited to about 1%.Since deuteriochloroform was used as a “lock, it was added (about 10%) Analytical Chemistry, Vol. 67, No. 73, July 7 , 7995 1955
to the CCb solutions. Freon 11was used as a reference. About 5 cm3 of Freon 11 vapor was added to the NMR tube and mixed with the sample by shaking. lgF-Decoupled13C NMR. Two types of experiments were performed on a GE Omega 300 NMR instrument. A quantitative inverse-gated I3C{19F}experiment using GARP decoupling was performed to determine the relative concentrations of the different types of carbons present. The 13C APT-like, lgF-decoupled experiment allowed the assignment of the number of fluorines attached to each type of carbon. (Although APT is the acronym for "attached proton test", in our case we were using fluorine in place of protons.) Solutions were prepared by using very small amounts of Freon 113 to make the oligomer mobile enough to be added to an NMR tube. A smaller bore tube containing dimethyl sulfoxide (DMSO) was placed inside the tube containing the plasticized oligomer and acted as the NMR deuterium lock. 'H NMR. Proton NMR spectra were obtained to quantitatively determine the hydrogen content. A GE QE 300 MHz NMR instrument was used. The oligomer was dissolved in Freon 113 (23 wt?? oligomer), and a known amount (-0.01 wt %) of chloroform was added as an internal proton reference. The relative amounts of protons in the chloroform were compared to those in the oligomer by spectral integration. TOF-SIMS. Very thin films (-100 nm) of the oligomer were prepared by rubbing a drop of the heat-softened material (with a Kimwipe tissue) onto a silver foil etched with nitric acid (20%v/v). These coated silver samples were placed in a reflectron time-offlight selected ion mass spectrometry (TOF-SIMS) instrument (IX23S) manufactured by Fisons, Inc. It has been ~ h o w nthat ~.~ molecular and fragment ions of considerable size (up to m/z = 10 000) are desorbed and separated by mass. In this way, structural characterization and molecular weight distributions can be obtained. We operated in the static mode at a mass resolution of m / A m = 1000. Positive and negative ion spectra were obtained without charge compensation. In this report, we present the negative ion spectra, which are the most revealing in terms of the structures of the oligomers and their molecular weight distributions. ESCA. Very thin films (-100 nm) of oligomer were prepared by rubbing a drop of heat-softened material (with a Kimwipe tissue) onto a clean indium foil. The oligomer film thus obtained was continuous and thick enough to prevent indium from being detected by electron spectroscopy for chemical analysis (ESCA) . The analysis was performed using a Perkin-Elmer Physical Electronics 5000 LS ESCA instrument (Mg Ka X-rays at 1253.6 eV and 45" takeoff angle). Charge compensation was not used; therefore, the peaks in the ESCA spectrum were shifted due to charging. Peak assignments were based on binding energy differences between peaks and referencing to the peak at the highest binding energy in the carbon 1s spectrum, assuming -cF, is at 293.7 eV.5,6 The absence of an aliphatic hydrocarbon peak did not allow the usual referencing to C-C at 284.6 eV. Verification of structural group assignments (based on binding energy differences) were made using Flutec P P l l and a Fisons (3) Bletsos, I. V.; Hercules, D. M.: Fowler, D.; vanbyen, D.; Benninghoven, A. Anal. Chem. 1990,62,1275. (4) Bletsos, I. V.: Hercules, D. M.: vanleyen, D.: Hagenhoff. B.; Niehuis, E.; Benninghoven, A.Anal. Chem. 1991,63, 1953. (5) Clark, D. T. In Structural Studies ofMucromolecules by Spectroscopic Methods; Ivin, K. J., Ed.; John Wiley and Sons: London, 1976; pp 111-179. (6) Chambers, R D.; Clark, D. T.; Kilcast, D.; Partington, S. J. Polym. Sci., Polym. Chem. Ed. 1974,12, 1647.
1956
Analytical Chemistry, Vol. 67,No. 73, July 7, 7995
ESCA LAB MKII instrument equipped with a cold probe. The P P l l was kept at liquid nitrogen temperatures to minimize evaporation in the ultrahigh vacuum of the instrument. GC/MS. Freon 113 solutions were injected into a HewlettPackard Model 5890 gas chromatograph, and the effluent was directly fed into a Fisons Model VG-70SE mass spectrometer. A DB-1 capillary was used in the gas chromatograph programmed to heat from 60 to 280 "C at 10 deg/min. The mass spectrometry was done in the negative ion mode using electron capture mass spectrometry. Argon gas was used to thermalize the electrons. It might seem odd that our gas chromatograph was programmed to heat to only 280 "C when we have already stated that some components of the oligomer mixture boil as high as 400 "C. However, we have found that perfluorocarbons are extremely volatile at temperatures far below their boiling points. For example, all of these oligomers will evaporate completely at temperatures below 300 "C. This is due to the very low intermolecular forces acting in these systems. In short, at 280 "C, all the oligomer had evaporated. IR. IR spectra were obtained on samples of both Flutec P P l l and the oligomer as a film between potassium bromide plates and in a calcium fluoride liquid cell. The instrument was a Nicolet 20SX FT-IR spectrophotometer run at 2 cm-l resolution with 100 scans coadded. Wflis. Measurements were made on the neat liquid in a 1.0 mm length cell using a Cary 2300 spectrophotometer. Elemental Analysis. The sample was sent to a contract lab, Micro-Analysis, Inc., for the non-fluorine analyses. The fluorine analysis was done using a Wickbold torch ion selective electrode. Vapor Pressure Osmometry. A Perkin-Elmer 115 vapor pressure osmometer was used to determine the number average molecular weight, of the oligomer. Measurements were taken in Freon 113 at 39.5 "C. Benzil was used as a standard. Considerable scatter was observed in the measurements which we attribute to the high vapor pressure of the solvent at the experimental conditions. We estimate the error to be -5%.
a,,,
RESULTS
IR, GUMS, and W/vis measurements confrm that the oligomer is a complex mixture of many components containing a low level of unsaturation and possibly some hydrogen. The GC trace in Figure 1indicates a complex mixture of components. We were not able to find conditions permitting the resolution of any peaks, nor could we make any structural determinations from the mass spectra. One can, however, decipher three to four broad bands of intensity suggesting the presence of dimers, trimers, and tetramers. On first reflection, one might consider it odd that this chromatogram in Figure 1is so complex. However, the building block of these oligomers, pure perfluorotetradecahydrophenanthrene, has six structurally distinct isomers. Each isomer has at least 12 distinct fluorines. Thus hundreds of dimers can result from coupling two monomers. Hundreds of dimers imply thousands of trimers. These counts do not even consider the possibility of anthracene ring connection isomers, ring opening, and ring contraction. Even if only a fraction of these isomers are actually present, the gas chromatograph can be complex and unresolved. IR spectra for Flutec P P l l and the oligomer are compared in Figure 2 for a limited portion of the IR range. As is true for the entire spectral range, all major bands roughly correspond in both materials. The lack of sharp bands for the oligomer is consistent
\
100
Figure 3. UV/vis spectra for water, the chlorotrifluoroethylene oligomer, and the Flutec PP11 oligomer.
Temperature 1 4 Time 1mlns.l
Figure 1. Gas chromatograph of the oligomer.
I
/Oligomal
. I
-100 Ch.mlc.l ShHt
_____.... .. I -.-...._....... .....___... 1 , / 3600
3200
ZBOO
w.vnumb.r
I
2100
-2w
Figure 4. 19F NMR spectrum for the Flutec PP11 oligomer in deuteriochloroform with CFC13 as a reference.
~
bO0Q
-150
2wO
WOO
IC"I
Figure 2. Comparison of the IR spectra for perfluorotetradecahydrophenanthrene and the oligomer, showing the regions where absorptions due to the presence of C-H and C=C bonds are expected in fluoropolymers.
with a mixture of similar compounds with overlapping bands. As is illustrated in Figure 2, there is no significant absorption in the C-H stretch region for either material. There are some weak bands in the 1650-1900 cm-l region which may be due to unsaturation. More evidence of unsaturation was obtained with W/vis spectrometry. Figure 3 shows much more absorption at lower wavelengths for the oligomer than for the chlorotrifluoroethylene oligomer (trade name Fluorolube) and water. Neither of the latter materials has unsaturation. Elemental analysis (amounts given in wt %)indicates the presence of carbon (26.3), hydrogen (0.25), nitrogen (4.0), and fluorine (67.9). (More will be said of this later.) The 'H NMR analyses revealed that hydrogen was present in only trace quantities (-2 x 10-7 mol of hydrogen/g of oligomer). More detailed reporting using the remaining techniques follows. 19FNMR. The 19FNMR spectrum in deuteriochloroform is shown in Figure 4. The reference peak (0 ppm chemical shift) represents the single fluorine in CFCl3. Although there are few sharp resonances, there is considerable s h c t u r a l information in this spectrum. Clear regions of resonance attributable to CF, CF2, and CF3were identified. These are summarized in Table 1. The
Table 1. Structural Assignments for ISFNMR Spectrum of Oligomer in CDCIJ'
chemical shift (ppm) -68.5
to -74.0
-81 -106 to -134
-134 to -144 -168 to -192
description broad sham
broad moderately broad broad
structural assignmenp
CF3CF C=C< and CF3, respectively. Sharper peaks at 293.6 and 295.5 eV were assigned to CF and CF2, respectively. Nitrogen was not detected (detection limit, -0.5 atom %). Oxygen was detected at the detection limit but could be the result of adsorption. Hydrocarbons were not detected. Atom % abundances based on total carbon are listed in Table 4 and compared with the values obtained by decoupled NMR The differences are within experimental error. Atom % calculation errors are probably of the order of 10-20%, whereas for the broader peaks, greater errors (-20%) are expected. The precision in determining the degree of unsaturation is so much poorer by NMR that the ESCA figure will be used as the best number available. Using the cold probe ESCA did not give significantly different results. Double- and triple-distilled Flutec P P l l samples were analyzed with the cold probe ESCA. CF, CF2, and CF3 were detected in the following respective abundances (carbon atom %): 25-27%, 68-71%, and 3 4 % . Carbon to fluorine ratios of 32:68 were calculated. If Flutec P P l l consists of pure perfluorotetradecahydrophenanthrene, only CF and CF2 should be detected at concentrations of 29 and 71%and C/F ratios of 3753. However, P. Coe of Birmingham University has deduced that 6-10% of C14F26 is present on the basis of mass spectrographic analyses? Coe proposed that this corresponds with one of the rings opening by the addition of two fluorines and thus at least one CF3 group per C14F26 molecule. This material should be very difiicult to separate from the c#24 by fractional distillation, accounting for its presence in the doubly and triply distilled samples. Also, traces of oligomer were detected by TOF-SIMS in these fractions. After correction for the CF3, the composition of the P P l l sample becomes identical to the theoretical. TOF-SIMS. Earlier in this section, we reported that ESCA and NMR determined that the oligomers consist of > C=C Cd
CF30
CFf
CF'
E +A6
exptlc
8 7 10 14
8 7 7 7
51 50 48 43
33 36 36 36
17 18 16 16
I I1 I11
Carbon atom %. Fluorine atom % of equatorial and axial fluorines in &memberedrings. Average of fluorinedecoupled NMR and ESCA results. (I
ESCA at the limit of detection (-0.5 atom %),but this is too low a concentration to account for the missing material. We must assume that the fluorine analysis was in error, although fluorine recovery should be above 97%: Also, as mentioned earlier, the IR spectrum substantiates the ESCA and 'H NMR results that no signiicant amounts of hydrogen are present. We would now like to make structural assignments consistent with the ESCA, NMR, and TOF-SIMS results. The results from NMR and ESCA summarized in Tables 2 and 4 indicate the following mole ratios of carbons: 8%unsaturated, 8%C S , 51% CF2, and 33%CF. In addition, about 17 atom % of the total fluorine is of the equatorial and axial type in &membered rings. Also, half of the CF3 groups are attached to CF2 groups. The general structural framework we described in the TOFSIMS Results section is intuitively sensible because (1) it is based on C14F21 building blocks and (2) one can imagine the rings connecting and opening in the suggested ways during the harsh fluorination conditions. There is precedence for polymerization in the way we suggest. Laatsch et al.'O reported the trimerization of anthracene 1,4quinones. When considered in conjunction with the known dimerization of hexafluorobenzene in the presence of elemental fluorine,'l polymerization of the phenanthrene seems plausible. The structures I, 11, and I11 (for dimer, trimer, and tetramer, respectively) drawn in this report are only a few of the myriad of possibilities. A comparison of these suggested structures with the experimental results for the groups detected is given in Table 8. It is most important that the dimer structure compares favorably (and it does) with the experimental results since it is present in the largest quantities. Also, one can easily imagine dimer structures on either side of the MW of structure I which would give the proper balance of group concentrations when averaged together. Although they do not affect the average concentration of groups as much as the dimer, the trimer and tetramer structures suggested are reasonably close to the experimental findings as well. The unsaturation suggested in structures I, 11, and 111is always "buried" in the molecule and of the form (RF1)(RF2) C-C (RF3)(RF4). We feel that any unsaturation not sterically protected in this way would easily have been fluorinated either by the initial CoF3 reaction or by the subsequent reaction with elemental fluorine. There are two remaining questions concerning the TOF-SIMS experiment: (1)Are the masses of the observed peaks near those of the parent ions (or are they predominantly much lower mass fragments)? (2) Is the TOF-SIMS distribution skewed by loss of (10)Laatsch, H.; Beck, H.; Egert, E. Liebigs Ann. Chem. 1992, 11, 1125. (11) Hotchkiss, I. J.; Stephens, R; Tatlow, J. C.J. Fluorine Chem. 1975, 6, 135.
Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
1961
Table 9. Molecular Weight Calculation for TOF-SlMS Data*
M,
Ni
Mz
Ni
M,
Ni
1001 1039 1058 1077 1096 1115 1134 1153 1172 1191 1210 1229 1267 1550
170 516 318 1789 1437 3117 2771 4926 1004 3776 241 1239 428 130
1569 1588 1607 1626 1645 1664 1683 1702 1721 1740 1759 1778 1797
310 340 752 663 1074 741 1098 623 667 500 326 274 100
1816 2079 2098 2117 2136 2155 2174 2193 2212 2231 2250 2269 2288
115 77 62 80 65 155 79 149 62 147 48 72 48
M, = 1313. Molecular mass of the TOF-SIMS peak. Intensity of the TOF-SIMS peak.
lower mass volatiles? Two sets of experiments have shown that the peaks are predominantly due to oligomer ions. First, we obtained TOF-SIMS spectra of C14F24 the same way as we did for the oligomers. Even though this material is quite volatile under the high-vacuum conditions of the experiment, apparently enough remained on the silver foil to allow us to successfully make the measurements. In the negative ion TOF-SIMS spectrum of this material, the principal peak was obtained at m/z = 624 and was due to the parent ion C14F24-. The peak at m/z = 586, only about 15%the intensity of the parent ion peak, was due to the fragment C14F22-. We also compared the findetermined by TOF-SIMS with that obtained by vapor pressure osmometry (VPO). The intensity (NJ and molecular mass (Mi) of each peak used in the calculation are listed in Table 9. fincalculated from the TOF-SIMS data is 1313. This is considerably higher than the value of 869 determined by VPO. It must be remembered, however, that the TOF-SIMS
1962 Analytical Chemistty, Vol. 67, No. 13, July 1, 1995
experiment is performed under a vacuum of Torr. Under these conditions, the lower MW oligomer fractions would probably vaporize and not be recorded, thus biasing the molecular weight distribution to the high end. If this is true, one would expect the VPO analysis to give a lower MW. This is also consistent with the species observed by ToF-SIMS being molecular ions or nearly so. If the peaks used in the M, calculation were due to fragments of substantially lower mass than molecular ions, the calculated Mnwould have been lower than the VPO result. Although the discrepancies between the ESCA and NMR results were within experimental error, one could expect differences if the lower MW components were structurally different from the higher MW components and they vaporized during the ESCA experiment. Like TOF-SIMS, ESCA was performed at Torr, possibily rendering the lower boilers volatile. ACKNOWLEDGMENT The authors gratefully acknowledge the skilled assistance of R. McKay under the guidance of Dr. F. Kitson and Dr. C. N. Mc Ewen for the G U M S analysis, C. Bowers under the guidance of Dr. E. Kissa for the elemental analysis, S. Krakowski for the 'H NMR spectra, K. M e 1 1 for the 19FNMR spectra, and B. Bennett under the guidance of R Fuller for the VPO analysis. We would also like to acknowledge the skilled assistance of W. Justison under the guidance of Dr. K. G. Lloyd for the TOF-SIMS. We also thank Dr. E. Matthews for the W/vis and Dr. D. Kasprzak for the FT-IR analyses. D. Davidson and J. Wyre should be thanked, as well, for their skilled assistance in performing the ESCA experiments. Parts of the TOF-SIMS analysis reported herein were reported previously at the SIMS M Conference, Yokohama, Japan, November 7-12, 1993. Received for review September 15, 1994. Accepted March
30, 1995.@ AC940923A Abstract published in Advance ACS Abstracts, May 15, 1995
