Langmuir 1996,11,2761-2767
2761
Time-of-FlightSecondary Ion Mass Spectrometric Measurements of Molecular Weight Distributions for Functionally Terminated Oligomers and Transferred Langmuir-Blodgett-Kuhn Monolayers James F. Elman,? Daniel H.-T. Lee,* and Jeffrey T. Koberstein” Department of Chemical Engineering a n d Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136
Patrick M. Thompson Eastman Kodak Company, Rochester, New York 14650-2132 Received May 11, 1994. I n Final Form: May 5, 1995@ We report the first direct measurements of the molecular weight distribution in transferred condensed monolayers of amphiphilic oligomers determined by time-of-flight secondary ion mass spectrometry (TOFSIMS). Two a,w-functional oligomers of poly(dimethylsi1oxane) are investigated, comprising pentylamine and propylcarboxyend groups. Measurements are performed directly on condensed monolayer films transferred to silver-coated substrates using the Langmuir-Blodgett-Kuhn (LBK)technique. These results are compared to the original distributions measured on submonolayer thin films of the original oligomers prepared by spin coating onto silver-coated substrates and to molecular weight determinations provided by end group titration and size exclusion chromatography analyses. Different families of ions are found for the two different thin film preparation methods. A number of tentative assignments are proposed for these masses, based upon consideration of the effects of the LBK film deposition process and the influence of interactions between the end groups and the substrate. The molecular weight distributions for LBK films of both oligomers is found to be narrower and shifted to higher molecular weights than are those for the corresponding spin-coated films. The changes in molecular weight distribution observed are attributed to dissolution of lower molecular weight species into the aqueous subphase during the LBK film deposition process.
Introduction The measurement of the average molecular weight and its distribution (MWD) for end-functional low molecular weight oligomers and polymers is often a daunting task fraught with many difficulties. Ideally, one would like to have a characterization technique that is capable of independently determining the functionality, the molecular weight, and the molecular weight distribution on an absolute basis without the requirement of limiting assumptions regarding the chemical precision. Traditional approaches are most indirect and require restrictive assumptions in the subsequent data analysis. For example, it is common to analyze for the end groups by either end group titration or NMR.l The number average molecular weight is then directly calculated from the knowledge of the number of end groups per unit mass. The disadvantage of this method is the necessity to know precisely the functionality lof the molecule. With many chemistries, the functionality is difficult to control and therefore is usually not known precisely. The resultant molecular weight measured by the method is therefore dependent on the functionality one assumes in the data analysis.2 Even when the assumptions are correct, these methods supply only a value for the number average molecular weight (Mn). Vapor phase osmometry (VP0)l
* To whom correspondence
should be addressed. Present address: Eastman Kodak Company, Rochester, NY 14650-2132. Present address: Industrial Technology Research Institute, Hsinchu, Taiwan. @Abstractpublished in Advance ACS Abstracts, July 1, 1995. (1)As a general reference, see for example: Polymer Characterization Schroder E., Muller G., Arnt K.; Eds.; Hanser: Munich, 1989;p 18. (2)Fleischer, C. A. Ph.D. Dissertation, University of Connecticut, 1992.
*
0743-7463/95/2411-2761$09.00/0
is another frequently used method for the characterization of low molecular weight materials. While it is a n absolute method, it again provides only the number average molecular weight. Size exclusion chromatography (SEC) coupled with a light scattering3 andor viscosity detection system is a very powerful method that can provide absolute molecular weight information. The method is most applicable to high molecular weight materials however, and the relatively low molecular weights being presently considered preclude the use of such absolute molecular weight detectors (i.e., relatively little scattering is produced). Practical SEC for low molecular weight functional oligomers is a relative technique requiring construction of a calibration curve. A series of standards for the calibration curve is usually not available for end-functional polymers. Furthermore, problems with functional materials adsorbing in the columns are often encountered and the hydrodynamic volume of the oligomer will normally be dependent on the functionality. Supercritical fluid chromatography (SFC)is capable of separating individual n - m e r ~ .If~one can assign a value of ‘‘n”for one species, then all the masses in a homologous series are known and the MWD can be calculated. The measurement can be compromised however if the material is not homologous (i.e., if it contains nonfunctional linear or cyclic analogs andor linear monofunctional n-mers a n d or a distribution of functionalities) since retention is dependent on both molecular size and functionality. The case of interest in our own work, that is, the molecular weight distribution of transferred Langmuir(3) Mourey, T. H.; Coll, H. Proc. ACS Div. Polym. Mat.: Sci. Eng. 1993,69,217. (4)Schmitz, F. P.; Klesper, E.J Supercritical Fluids 1990,3, 29.
0 1995 American Chemical Society
Elman et al.
2762 Langmuir, Vol. 11, No. 7, 1995
Blodgett-Kuhn (LBK) films, presents additional challenges due to the small amount of sample involved and the necessity of redissolving the LBK film for traditional methods requiring solutions for analysis. LBK films have been the focus of active research for many years and a wide variety of techniques have been employed to elucidate the morphology, anisotropy, and potential applications of these material^.^ Most of these studies have involved more or less pure materials that have been carefully cleaned and present a single molecular structure. More recently, the nature of LBK materials has been broadened considerably to include oligomeric and polymeric materials which cannot be easily fractionated and, thus, have a n intrinsic distribution in molecular weight. The MWD is therefore a n important addition to the list of LBK film characteristics that need to be determined. Functionally terminated oligomers (FTO) are one class of materials that fall into this new category of LBK film candidates. These materials are polydisperse in nature and may not have well-defined functionality but exhibit interesting monolayer In these polydisperse systems it is possible that low molecular weight materials, or materials with higher functionality, dissolve into the subphase. In order to fully understand the behavior of Langmuir films of these materials, it is necessary to know the average molecular weight and MWD of the actual floating monolayer film and how it might differ from that originally spread onto the subphase. Static scondary ion mass spectrometry (SIMS) is a relatively new surface spectroscopy technique that has been used to study a wide variety of organic and inorganic materials including LB films. It has been used in a quantitative fashion to study the SIMS cationization process in fatty acid LBK m o n o l a y e r ~ , ~multilayers,11J2 J~ and self-assembled 1 a ~ e r s . lQuantitative ~ information such as MWD of polymers or oligomers and qualitative insights such as the presence of contaminants are accessible with this technique, particulary if the instrument has a time-of-flight mass analyzer (TOFSIMS). Recently,14the synthetic outcome of various chain termination chemistries was probed using static TOFSIMS. Previous workers have used both TOFSIMS and laser desorption mass spectrometry to determine apparent molecular weight distributions and compare these to MWD's obtained from SEC15J6and SFC.17 The materials studied so far have been, for the most part, films spin coated from dilute solutions of monomodal narrow MWD polymers. The one paperls that did consider TOFSIMS of polymeric LBK films dealt with materials beyond the (5)As a general reference,see for example: Lmgmuir-Blodgett Films; Roberts, G., Ed.; Plenum: New York, 1990;Chapter 4. (6)Lee, D. H.-T. Ph.D Dissertation, UniversityofConnecticut, 1990. (7)Elman, J.F.Ph.D. Dissertation, University of Connecticut,1993. (8)Lenk, T. J.;Lee, D. H.-T.; Koberstein, J. T. Langmuir 1994,10, 1857. (9)Wandass, J.H.; Gardella, J. A. J . Am. Chem. SOC.1986,107(22),
CH3
CH3
I
I
H2N(CH2),-( Si-O)n-Si-( CH2),NH2
I
I
CH3
CH3
Dipentylamine-TerminatedPDMS
CH3
CH3
I
I
HOOC(CH2)5-( Si-O)n-Si-(CH2)5COOH
I
I
CH3
CH3
Dipropylcarboxy-Terminated PDMS Figure 1. Schematic structures of a,w-functional PDMS oligomers.
CH3 CH3
I
I
+
R-Si-0-Si-R
I
CH3
I -(Sio)s-
1
=>
I
CH3 CH3
CH3 CH3
CH3
I
I
R-(Si-O),-Si-R
I
I
CH3
CH3 plus residua1 cyclic tetramer, trimer Figure 2. Synthesis scheme for a,w-functional PDMS oligomers.
range of present instrumentation ( > 15 000 amu) and concerned itself with the technical aspects of TOFSIMS (sampling depth and imaging capabilities). Previous studies generally employed polymers with chain termini that did not differ significantly in polarity from the backbone. The single inve~tigator'~ to study a n endfunctional polymer noted difficulties in achieving reasonable MWD data from diacid terminated poly(1,2-butadienes). In this study,lg TOFSIMS is used to characterize the molecular weight distribution of polydisperse, endfunctional poly(dimethylsi1oxanes) (PDMS), both before and after spreading onto a Langmuir trough and subsequent transfer to a substrate. These materials have polar, functional termini (Figure 1)that can interact with the substrate. It is these functional termini that allow the materials to form condensed LBK monolayers.6-8
6192.
(10)Bolbach, G.;Blais, J. C. In Secondary Ion Mass Spectrometry, Proceedings of the 6th International Conference; Benninghoven, A., Huber, A. M., Werner, H. W., Eds.; Wiley: Chichester,UK, 1988;p 655. (11)Cornelio,P. A,; Gardella, J. A. JVac. Sci. TechnoZ. 1990,A8(3), 2283. (12)Cornelio-Clark, P. A,; Gardella, J. A.Lungmuir 1991,7,2279. (13)Hagenhoff, B.; Benninghoven, A.; Spinke, J.; Liley, M.; Knoll, W. Langmuir 1993,9, 1622. (14)Belu, A.M.;Hunt, M. 0.;DeSimone, J. M.; Linton, R. W. Polym. Prepr. 1993,34 (21,376. (15)Cotter, R. J.; Hanovich, J. P.; Olthoff, J. K.; Lattimer, R. P. Macromolecules 1986,19, 2996. (16)van Leyen, D.; Hagenhoff, B.; Niehuis, E.; Benninghoven,A,; Bletsos I. V.;Hercules, D. M. J . Vac. Sci. Technol. 1989,A7, 1790. (17)Hagenhoff, B.; Benninghoven,A.; Barthel, H.; Zoller, W. Anal. Chem. 1991,63,2466. (18)Hagenhoff,B.; Deimel, M.; Benninghoven,A.; Siegmund,H.-U.; Holtkamp, D. J.Phys. D : AppZ.Phys. 1992,25,818.
Experimental Section Synthesis and Sample Preparation. Two end-functional poly(dimethylsi1oxanes) (PDMS) (Figure 1) were synthesized (Figure 2) from cyclic monomers and a difunctional precursor using previously reported methods.20 The synthesis procedure ensures a functionality of two for all polymerized species but leads to a broad distribution in molecular weight. The dipentylamine- and dipropylcarboxy-terminatedmaterials were initially reported to haveM,'s of 960 and 2020, respectively. These values were measured by end group titration using, in the case (19)A preliminary report on this subjectcan be found in Kobserstein, J. T.; Lee, D. H.-T.; Elman, J. F.; Thompson, P. M.; Yilgor, I. Polym. Prepr. 1991,32 (l), 265. (20)Yilgor, I.; McGrath, J. E. Adu. Polym. Sci. 1988,86,1.
Molecular Weight Distribution in Monolayers
2i
O ?.O ,(l
d .
2 1 00
'&
B I
i ,000 2.00
3.00
4.00
Log (Molecular Weight)
Log (Molecular Weight)
Figure 3. Size exclusion chromatograms for a,w-functional PDMS oligomers: (a)dipentylamine-terminated;(b) dipropylcarboxy-terminated.
CH3 CH3 1.
H~N(CH~)S-(S~O),-S~(CH~)~NH~.(Ag)+
CH3 (333 Mass = n(74) + 338
2.
CH3 CH3 H2N(CH2)j-(SiO),-Si + CH3 CH3 Mass = n(74) + 144
Langmuir, Vol. 11, No. 7, 1995 2763 molecular weight but are on the order of 30 A. Control samples were prepared by spin coating (Headway Research) onto the silver-coated slides from dilute toluene solutions using the suggested concentration based upon the repeat unit molecular weightz2(about 0.3%(w/v) solution). Time-of-FlightStatic Secondary Ion Mass Spectrometry (TOFSIMS).The Poschenrieder time-of-flight instrument manufactured by VGScientific was previously described in detai1.23 The primary ion source was operated at 30 keV and produced 2 nA of continuous 69Ga+ current. During acquisition of each spectrum, the primary beam was pulsed at 10kHz with a 5 . 6 ; ~ ~ pulse width for a total of 3.8 x lo7pulses. The sampled area was which gave a total ion dose of 3.6 x 10l2ions/ about 0.09 "2, cm2,within the accepted value of ions/cm2. This ion beam impinges on a thin organic film deposited on a silver coated substrate. The composite film is essentially conductive to electrons and thus no charge compensation is required. Nitric acid etched Ag substrates were chosen to facilitate the desorption and ionization of high mass ions (Le., [M + Agl+), where M represents the intact oligomer chain) as demonstrated by Benninghoven et al.17 While the primary ion beam is sufficiently energetic to damage organic materials, these static conditions do not statistically sample the same area twice. Thus one observes molecular ions that result from the initial, pristine interaction between an energetic 69Ga+ion and the polymer/Ag matrix. Fragmentation does occur, but the process is sufficiently soft so that one observes masses that correspondto intact polymer chains. Measurement of the MWD. The area and mass of each ion in the spectrum were individually recorded followed by subtraction of the underlying background. Various families of ions can be distinguished (see, e.g., Figure 41, by their repeating intervals of mass 74 (the mass of one PDMS repeat unit). This database was subsequently analyzed using the standard definitions for the number and weight average molecular weights, and the polydispersity index (P.I.): M, =C m i n i p i i
CH3 CH3 3.
H ~ N ( C H ~ ) ~ ( S ~ O ) , - S ~ ( C H ~+) S N H ~
M, =C m ? n i F i n i
CH3 CH3
4.
4 b.
CH3 CH3 CH3(SiO), Si + CH3 CH3
The cyclic andog of 4.
Mass = n(74)+133 (for n=O,l,2,3,4) Figure 4. Structural assignments and mass formulas for
TOFSIMS molecular positive ion peak families for dipentylamine-terminated PDMS. The third family of peaks was found only in the LBK film. ofthe dipentylamine PDMS,bromophenol blue as the indicator.21 Sizeexclusion chromatography (SEC)was performed on a Waters system using toluene as the solvent and three 10-pm Polymer Laboratories mixed-bed columns that had been calibrated with polystyrene standards (Figure 3). LBK films were formed at room temperature on a Langmuir trough constructed in house6 at a compression rate of 5 mdmin. The subphase was water produced from a Milli-Q system. Films were transferred at 25 dydcm surface pressure onto silver-coatedglass slides by a single down and up deposition cycle. Under these conditions (transfer condensed anisotropic films are formed, wherein the ratio d), chain backbones are oriented more or less perpendicular to the film Film thicknesses vary depending on material (21) Fleischer, C.A.; Koberstein, J. T.; Krukonis, V.; Wetmore, P. A. Macromolecules 1993,26,4172.
i
(2)
i
P.I. = MJM,, (3) where mi is the mass of a particular species adjusted for its structure (i.e., if the [M Ag]+family of ions was used, then the mass of Ag was subtracted), and ni is the area of that peak. Also note that mini can be referred to as the weight fraction (wi).
+
Results Dipentylamine-TerminatedPDMS. Two homologous families are apparent in the TOFSIMS spectrum (Figure 5) of the spin-coated material: one series corresponds to [M-pentylamine]+ species where a fragment containing the pentylamine chain end has been removed in the ionization process; the other more intense series can be assigned to the intact oligomer chains cationized by one silver ion. This pattern is reminiscent of the TOFSIMS spectrum of methyl-terminated PDMS22which also has two homologous series. Their origins were attributed initially to the more intense [M Ag]+ series and [M- CH2 Agl+ for the less intense series. A later interpretation reassigned the less intense family to [M methyl]+ species resulting from the loss of a fragment containing the methyl chain end. The latter assignment is more consistent with our data and provides a reasonable assignment protocol for PDMS materials: the intense series arises from Ag cationization of intact chains, and the weaker ensemble(s) arise from chains cationized by a fragmentation process involving loss of a species containing the end group.
+
+
(22) Bletsos, I. V.; Hercules, D. M.; van Leyen, D.; Benninghoven, A. Macromolecules 1987,20,407. (23) Thompson, P. M. Anal. Chem. 1991,63,2447.
Elman et al.
2764 Langmuir, Vol. 11, No. 7,1995
Table 1. Molecular Weight Characterization Results for Dipentylamine-TerminatedPDMS Films method M,, M, polydispersity index 960 n.a. n.a. end group titration
SEC TOFSIMS (spin coated) TOFSIMS (LBK)
300 0' 1000
1050
1100
1150
1200 AtOmlS
1250 1200 H. Unit.
11SO
1400
200
I
1450
%OD0
Figure 5. Expanded TOFSIMS spectrum for the dipentyl-
1.72 1.50 1.10
LE Film
4
n
800
m
-
700
s,
600
m
.
o ~
0
500
1000 1500 2000 2500 3000 3500
I/)
gl
W so0
,
c
815 1132 1403
1 aa
amine-terminated PDMS spin-coated film.
c
473 756 1277
roo 300
400
1
I
I
1
300
200 100
'
I roo0
$050
1100
b160
1200 12SO 1300 A t o m i c Ma.. Unit.
1310
1400
1450
1
c
200
1
1500
!
Figure 6. Expanded TOFSIMs spectrum for the dipentyl-
amine-terminated PDMS LBK film.
Below 500 amu, the spectrum exhibits two series of ions that are characteristic of methyl-terminated PDMS. These are usually assigned as linear and cyclic fragmentation products of PDMS.24 It is possible that these arise from the loss of both pentylamine groups and subsequent recombination, but they are more likely associated with the nonfunctional cyclic trimer and tetramer involved in the synthesis of these materials.20 The spectrum for the LBK (Figure 6) film shows a n additional series of [M proton]' species superimposed on the [M Agl+ and [M-pentylamine]+ series found in the spin-coated film (Figure 5). This new family is less intense than the silver-cationized series contained in the spectrum, which is consistent with the assignment of these new ions as a non-silver-cationized species. It also appears that, during the LBK process, some of the dipentylamineterminated PDMS chains may be protonated due to exposure to the water subphase (pH = 6.2). The TOFSIMS assignments for both dipentylamine-terminated films are summarized in Figure 4. The MWD (using the [M Agl+ intensities) obtained for the dipentylamine PDMS spin-coated compares favorably with that determined by size exclusion chromatography (Figure 3a). Both techniques show the distribution to be bimodal (with a large lower MW mode and a small higher MW mode). TOFSIMS (Table 1) yields higher average molecular weight values and a narrower MWD than does SEC. These quantitative differences are understandable given the limitations of the SEC analysis, that is, that results are based upon polystyrene equivalents and also include the effects of low molecular weight nonfunctional cyclics that are not included in the TOFSIMS analysis. The TOFSIMS M , is in reasonable agreement with the value measured by end group titration
+
Spun Cast Film
+
+
(24) Briggs, D.; Brown, A.; Vickerman, J. C. Handbook of Static Secondary Mass Spectroscopy (SIMSI; Wiley and Sons: New York, 1989.
100
0
1
i500
0
11
1000 1500 2000 2500 3000 3500
Molecular Weight Figure 7. Comparison of the TOFSIMS molecular weight distribution functions for spin-coated (lower graph) and LBK (upper graph)films of dipentylamine-terminated PDMS. These
+
were calculated using the [M Ag]+peaks with masses shifted according to M = 74n + 230.
(756 vs 960, see Table 1). The TOFSIMSM, may be biased toward low molecular weights and the polydispersity may be underestimated, because these distributions could not be measured above 2500-3000 amu (due to low signal to noise above that region). The truly significant finding of this study is that a pronounced fractionation occurs during the LBK film preparation and transfer process (MWDalso derived from the [M Agl+ series). This can be seen by direct comparison of the data for the spin-coated and LBK films (Figure 7). The low molecular weight mode of the MWD is virtually eliminated during the LBK film preparation process, apparently due to dissolution of these species into the aqueous subphase. I t is interesting to note that the LBKmass spectrum still contains ions associated with cyclic trimer and tetramer. Whether these secondary ions arise from SIMS fragmentatiodrecombination or were contained in the LBK film cannot be differentiated a t this point. Dipropylcarboxy-TerminatedPDMS. Several series of homologous species are observed in the spectrum for the spin-coated diacid-terminated PDMS film, and tentative assignments are given in Figure 8. Unfortunately higher mass resolution spectra were not obtained on this sample. Therefore not all the species could be unambiguously assigned (Figure 9). Given these limita-
+
Langmuir, Vol. 11, No. 7,1995 2765
Molecular Weight Distribution in Monolayers CH3 CH3 1.
la0
Ag OOC(CH2)3-(SiO)~-Si(CH2)3COOAg.,(Ag)+
160
CH3 CH3 140
Mass = n(74) + 554 (spin coatedfllm, n=5 to 37)
CH3 CH3 HOOC(CH2)3SiOSiOCH3 CH3
2.
120
*
100
0
80
Y
so + 40
20
Mass = n(74) + 205 (spin coatedjilm, n=O to SI)
O
1000
1500
PO00
woo AtOllS Y ..
3.
3100
4000
Figure 10. High mass region of the TOFSIMS spectrum for the dipropylcarboxy-terminated PDMS LBK film.
CH3 CH3 HOOC(CH2)3-(SiO)n-Si(CH2)3COOH...OI)+ CH3 CH3 Mass = n(74) + 233 (only LBKfilm, ne14 to 38)
1000
CH3 CH3 CH3(SiO),, - Si + CH3 CH3
4.
3000
lmlt.
1
LB Film
(M
+
H)+
Mass = n(74) + 73 Vor n= 1,2,3,4)
4 b.
0
m
The cyclic analog of 4.
500 1000 1500 2000 2500 3000 3500 4000
W
Mass= n(74)+133 Vor n=0,1,2,3,4)
Figure 8. Structural assignments and mass formulas for
I
TOFSIMs molecular positive ion peak families for dipropylcarboxy-terminated PDMS.
1
Spun Cast Film
i
1
500
3000 2000 1000 0
0
500 1000 1500 2000 2500 3000 3500 4000
Molecular Weight 1000
I050
I100
1150
1200 1250 1300 A t o m i c U.S. Unit.
13%
1400
1450
ll00
Figure 9. Expanded TOFSIMS spectrum for the dipropyl-
carboxy-terminated PDMS spin coated film.
Figure 11. Comparison of the TOFSIMS molecular weight distribution functions for spin-coated (lower graph) and LBK (upper graph) films of dipropylcarboxy-terminated PDMS. These were calculated using different families of peaks (see text) with masses shifted according to M = 74n 232.
+
tions the following tentative assignments have been made for two families: [M - 2H 3Agl+, and [M - propylcarboxy(cyclic)l+. In dramatic contrast, the spectrum for the LBK film (Figure 10)contains only a single family corresponding to [M + HI+. This differs markedly from the behavior found for the dipentylamine PDMS where the species originally found in the spin-coated are retained in the LBK film spectrum. The introduction of diacid groups appears to radically change the interaction between the LBK film and the substrate. Direct comparison of the MWD's for both samples is compromised by the lack of a common ion family. The [M - propylcarboxy (cyclic]+series was chosen to determine the MWD of the spin coated film, while the [M HI+
+
+
series was used for the LBK film calculations. As with the diamine system, the LBK film preparation and deposition process shifts the MWD to a higher and narrower range (Figure 11);however, the MWD of the spin-coated diacid film bears little resemblance to the SEC chromatogram (Figure 3))and the calculated M,, does not correspond to the value determined by titration (Table 2). The TOFSIMS M , of 1900 is reasonably close to the titration value, but given the low count rate and strong apparent substrate interactions, this may be fortuitous. Negative TOFSIMS spectra of the dipropylcarboxy material were acquired in the hopes that this PDMS might form anions; however no significant secondary ions could be detected.
Elman et al.
2766 Langmuir, Vol. 11, No. 7, 1995 Table 2. Molecular Weight Characterization Results for Dipropylcarboxy-TerminatedPDMS Films method M, M, polydispersity index end group titration 2020 n.a. n.a.
SEC TOFSIMS (spin coated) TOFSIMS (LBK)
1340 1357 1900
3170 1552 2017
2.37 1.14
1.06
Discussion The use of a,o-functional PDMS oligomers is a near optimal choice for this first TOFSIMS study of MWD in functional oligomers and transferred monolayer thin films. The functionality of these oligomers, excepting the cyclics, is virtually guaranteed by the synthetic procedure to be exactly two,20and the facile secondary ion formation that is characteristic of this backbone22 provides reasonable statistics even if optimal sample preparation is not achieved. Due to our limited access to the instrument, these initial data do not represent optimal experimental conditions, yet the precision and reproducibility based on a few replicates were sufficient to show clearly that the MWD of transferred PDMS monolayers differs markedly from that of the initial spreading solution. Dipentylamine-TerminatedPDMS. Both the LBK and spin-coated films yield [M + Agl+ and [M - pentylamine]+ species, while the addition of the [M HI+family differentiates the LBK from the spin-coated film. The MWD’s for these materials are easily determined by this method, allowing for the first time a comparison between the molecular weight distribution of a n LBKfilm and how this distribution differs from that of the startingmaterial. Dipropylcarboxy-TerminatedPDMS. In hindsight it is difficult to imagine that this material would produce a series of intact [M Ag]+ for a reasonable MWD determination. To create such species from this molecule, three positive charges have to be introduced: two to counter both carboxy end groups and one more to bring the chain to a formal +1 charge. A solution to this experimental difficulty is to apply the same technique employed in SEC studies of functionally terminated polymers,2 that is, esterification of the end groups. A similar approach7has been found to be quite effective for field desorption mass spectrometry (FDMS)of dicarboxyterminated poly(1-butene). The native dicarboxy polymer produces a weak spectrum that is biased toward low molecular weight. By esterification, the count rate is improved since the polarity of the substratelchain end interactions is reduced, and the MWD is much more favorably reproduced. Future studies of carboxy-terminated LBK materials should consider end group esterification to be a n appropriate methodology. The esterification can be performed before spin casting for the TOFSIMS control, and in the LBK case esterification of the end groups can be accomplished after film transfer, for example, by exposure to diazomethane vapors. Although the TOFSIMS spectrum of the diacidterminated PDMS did not capture the bimodality of the MWD as observed by SEC or agree with the end group titration Mn,a comparison between the spectra for the LBK and spin-coated films is still instructive, because the TOFSIMS fragmentation behavior is very sensitive to sample morphology. In the case of spin-coated films a random orientation of PDMS chains yields a rich assortment of TOFSIMS species with reasonable statistics. This contrasts markedly with the single homologous series found in the oriented LBK film which had rather poor count rate. The loss of intensity is perhaps related to the greater coherence of this LBK film to the substrate, thus discouraging the release of secondary ions. This contrasts with the diamine material which showed no such dramatic
+
+
change between LBK and spin-coated films. Thus it appears that TOFSIMS may give insights into the interaction between the polymer end group and substrate, which will depend upon the thin film preparation technique. The effect of layer thickness was not measured in this study. In hindsight it would have been instructive to measure each layers’s thickness with ellipsometry as layer thickness can affect SIMS data.1° The LBK layers produced in this study cannot be greater than 40 A as they were produced by a single dipping cycles while the spin-coated layers were of unknown thickness. For the two materials studied TOFSIMS indicates that LBK films have higher and narrower MWDs than the material that is initially spread. This seems to be a general phenomenon for lower weight diterminated materials that is driven by the solubility of the low molecular weight side of the distribution and magnified by the high surface to volume ratio inherent in the LBK process. During the TOFSIMS experiment, a certain amount of fragmentation occurs, even at the “static”,or low exposure rates employed in our study. The fragmentation mechanism is not well understood a t present, but its occurrence is clearly indicated by the detection of species of the type [M - end group]+. It is not possible, from analysis of this mass family, to unequivocally determine the MWD of the intact oligomers, since identical fragments can result from fragmentation of different molecular weight oligomers. If fragmentation occurs directly a t the end group site, then the MWD will accurately represent the true MWD of the oligomers. If however, fragmentation is a more random process occurring anywhere along the chain backbone, the distribution function will be shifted to lower molecular weights, since the fragmentation of a difunctional oligomer then necessarily leads to the formation of two shorter chains, each having only one functional end group. For this reason, with the exception of the spin-coated propylcarboxy-terminated film, we have used families of intact chains for the calculations of the MWD. In principle, the fragmentation process may be investigated by comparing the MWD results measured from a series of intact chains to one determined from analysis of masses corresponding to fragmented chains. We have not attempted this procedure in the present paper due to the limited precision of our data.
Conclusions 1. The TOFSIMS technique is shown to be capable of measuring the molecular weight distribution of functional oligomer monolayer and submonolayer ultrathin films prepared either by spin coating or by the LangmuirBlodgett-Kuhn technique. 2. The results demonstrate that the molecular weight distribution of a transferred monolayer of a,w-functional PDMS oligomers differs markedly from that in the initial spreading solution. The low molecular weight side of the distribution is absent in the LBK films due to dissolution of the shortest oligomers ( n 5 about 6) into the water subphase. This effect narrows the MWD and shifts the M,, to higher molecular weights for the LBK films. 3. The mass spectrum for a dipentylamine-terminated PDMS spin-coated film consists of two families attributable to cationized [M + Ag]+ and [M - pentylamine]+ species. This behavior is similar to that observed previously for methyl-terminated PDMS and is consistent with the mechanisms of cationization of intact chains and loss of a chain fragment containing a n end group, respectively. Cyclic, nonfunctional oligomeric ions are detected and can be attributed either to fragmentation products induced by the SIMS technique o r to cyclic residual trimer and tetramer reactants that are not pulled into the subphase.
Langmuir, Vol. 11, No. 7, 1995 2767
Molecular Weight Distribution in Monolayers 4. The TOFSIMS spectrum for the dipentylamineterminated PDMS LBK film shows a n additional family associatedwith proton cationized [M HIf species possibly formed as a result of contact with the water subphase during the LBK process. Layer thickness differences between the two methods of preparation while small could also explain this observation. 5. The TOFSIMS spectra for a spin-coated dipropylcarboxy-terminated PDMS film exhibits several families of ions, two of which can tentatively be assigned to [M 2H 3Aglf, and [M - propylcarboxy (cyclic)l+species. The spectra cannot be fully assigned, possibly due to the interactions between the substrate and the functional end group. The TOFSIMS spectra for an LBK dipropylcarboxy-terminated PDMS monolayer exhibits only one
+
+
+
relatively weak family assigned to [M HI+ species, due to the interaction between the carboxylic acid end group and the silver substrate. The same layer thickness concerns as in the previous conclusion may also be in action here.
Acknowledgment. This work was supported in part by grants from the U.S.Army Research Office and the Connecticut Department of Higher Education (#631303). Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views ofthe Connecticut Department of Higher Education. LA9403892
