Anal. #em. 1990, 62,2088-2092
2088
Determination of Molecular Weight and Composition of a Perfluorinated Polymer from Fragment Intensities in Time-of-Flight Secondary Ion Mass Spectrometry David E. Fowler* and Robert D. Johnson IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 Dieter vanLeyen a n d Alfred Benninghoven
Physikalisches Institut, Universitat Miinster, 0-4400 Munster, Federal Republic of Germany
Analysis of wknicrofjram quantttles of a perfluorinated poiyether (PFPE) homopofymer on different surfaces has been done by fkrdhg empirlccli, quantltatlve reiatkrWMp0 between the intendth of 8peclfic peaks in the high mass fragmentatkn spectra of ttneof-tllght )Bcondary bn mass spectrometry (TOF-SIMS)a d the canpoe#knof the PFPE a d the nmber average molecular weight of the polymer samples. These relationshipsare explained in terms of a dlrecl proporHonaHy of the appropriate high mass fragment ion yields to these different properties of the PFPE material. Since both the posltive- and negative-ion, hlgh mass fragments of the PFPE are formed without any observable cationization from the substrate specks, these quantitative relatlonsMps are found to be relatively Idupetadent of the substrate used to support the liquid PFPE films. This study represents one of the first attempts to make quantitative use of the hlgh ma88 fragment ion intenrltles In SIMS studles of polymers. This is an important advance for the gefterai case of q~tanutauveanalysis of polymers of up to 10000 amu on nonspecific substrates and H complements the powerful, but le88 general, technique of cationized molecular Ion SIMS, which requires special sample treatment and/or special substrates.
INTRODUCTION Detailed quantitative analysis of submicrogram quantities of polymeric materials, such as perfluorinated polyethers (PFPE), on surfaces is an extremely challenging analytical problem. Surface analytical tools, such as X-ray photoelectron spectroscopy (XPS) ( l ) ,are excellent methods for obtaining information about the chemical functionality and general atomic composition of a polymer. Yet, these standard surface sensitive techniques and other tools for in situ analysis are relatively insensitive to the molecular weight (MW) of a polymer, to the molecular weight distribution (MWD), and to changes in only a few chemical bonds in polymers containing more than 100 atoms, e.g., limited cross-linking, a single or a few chain cleavages, or changes in the end group chemistry of some fraction of a polymer sample. Nuclear magnetic resonance (NMR) is, perhaps, the best method for examining the detailed structure of bulk samples of polymers and for giving the number averaged molecular weight, M,, but the necessary NMR sensitivity has not generally been realized for analyzing submicrogram quantities of polymers on general surfaces. Mass spectrometry coupled with methods to desorb nonvolatile species on surfaces has the potential of providing extensive indirect information about polymer molecules through the interpretation of the spectral intensity and mass of the fragment ions and molecular ions generated in a
* To whom correspondence should be addressed. 0003-2700/90/0362-2068$02.50/0
measurement. One particularly promising mass spectroscopy for analyzing such materials is time-of-flight secondary ion mass spectrometry (TOF-SIMS). SIMS has been used successfully to catalog and fingerprint many organic materials and polymers (2-4). More recently, changes in the molecular structure a t polymer surfaces have been examined with TOF-SIMS (5). In addition, parent molecules and high mass molecular fragments have been measured and identified through the use of Ag+ or alkali cationization (6,7).This latter ionization method to prepare molecular ions has provided the possibility for quantitatively measuring the MWD of polymers with MW's in an appropriate range. Estimates of M , determined from the TOF-SIMS measured MWD of highly dispersed polymers on Ag surfaces were in good quantitative agreement with values from other methods on bulk samples (6,7). Much less well documented is the ability to obtain a quantitative analysis of polymers on or at an unspecified surface where cationization is unlikely. Although TOF-SIMS, using Ag+ cationization, is an extremely sensitive and powerful technique for analysis, it is not generally applicable to many important situations where an in situ analysis of a polymer sample, in thin film form or highly dispersed on a nonspecific substrate, is desired. The use of PFPE as a surface lubricant is a good example of this. An excellent candidate technique quantitatively analyzing such polymers on nonspecific surfaces is the interpretation of the intensities of mass peaks in their high mass fragmentation SIMS spectrum. I t will be demonstrated in this paper that a good quantitative determination of the M , of a P F P E sample and its composition can be derived directly from specific ions in the TOF-SIMS fragmentation spectra. Previously, such information for polymers on surfaces was not available, except under the specialized conditions required for cationization. The present results are shown to be relatively insensitive to the substrate used to support the PFPE's. EXPERIMENTAL SECTION The specific perfluorinated polyether used in the present study was a homopolymer F -CF -CF2 -O-(CF-C
I
CF3
I
F 2 - 0 ) ~ --R,
CF3 1
where R1= CF2-CF3, is normally the principal end group. This material has the registered tradename Krytox and is manufactured by E. I. du Pont de Nemours and Co. (Wilmington, DE). Krytox samples were obtained where the mean value of N , the degree of polymerization, varied from 10 to 66. Table I gives the measured average degree of polymerization as determined by NMR. Data supplied by Du Pont show that the vapor pressure of samples Torr at 20 "C. This makes in this range varies from lod to the samples generally suitable for measurement in the vacuum of the TOF-SIMS spectrometer. K r y b x material actually contains 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
Table I. Number Average Molecular Weight of Krytox Samples' sample
Mn
sample
Mn
NIOb N12 AA N24
9.0 11.3 15.5 22.6
N36 N48
28.6 33.1 37.2 43.3
AD N66
x105
"1
Values are average degree of polymerization = N + 1, where N is in monomer units, as determined by NMR. * N10 is a monomolecular weight sample. several components which are composed of the above homopolymer, but which have end groups different from the principal termination at the R, position in structure 1. These different components result from the chemistry involved in the termination and capping of the polymerization at the Ri position (8). One of these additional components has -CHF-CFS as the terminating end group, which we have labeled R2. Fragment mass peaks related to this component and the R1 component will be used, below, to determine their fractional content in the PFPE samples. To probe the sensitivity to sample preparation and the sensitivity to substrate type (1)highly dispersed films of 11molecular layer (
