Spontaneous Polymer Thin Film Assembly and Organization
J. Am. Chem. Soc., Vol. 118, No. 8, 1996 1859
KE),20 the photoelectron escape depth was assumed to vary as KE
0.7 20
,
Table 2. Polymer Ultrathin Film Properties before and after
(
2
1
Treatment at Elevated Temperature in Trichlorobenzenea
and Scofield’s photoionization cross sections were used.
Angle-dependent XPS data were collected at nominal photoelectron
take-off angles of 0°, 55°, and 80°. The take-off angle was defined as
the angle between the surface normal and the axis of the analyzer lens
system. Using mean free paths calculated from the equations given
by Seah and Dench, the sampling depth (three times the mean free
path) should decrease from 90 to 15 Å as take-off angle increases from
film
thickness
(Å)
aqueous
contact
angle (deg)
monolayer film
octadecanethiol (initially)
octadecanethiol (after 30 min,
23
7
97
63
2
2
9
0 °C in solvent)
0
° to 80°.
polymer A (initially)
polymer B (initially)
polymer C (initially)
polymer C (after 60 min, 90 °C in solvent)
polymer C (after 120 min, 120 °C in solvent)
polymer C (after 60 min, 150 °C in solvent)
22
28
42
28
18
4
89
92
107
107
102
92
Quadrupole Static Secondary Ion Mass Spectrometry. Static
secondary ion mass spectrometry (SIMS) experiments were performed
with a Physical Electronics 3700 SIMS system (PHI Electronics, Eden
Prairie, MN) mounted on a custom ultra-high-vacuum (UHV) system
(see supporting information for details). The ion beam was rastered
over a 5 × 5 mm area, and the total exposure time of the sample to the
a
Monolayers on gold substrates; solvent ) 1,2,3-trichlorobenzene.
ion beam, including setup and data acquisition, was less than 7 min.
1
2
2
Corresponding total ion doses per sample (<5 × 10 ions/cm ) are
within the generally accepted limit for static SIMS conditions for
tion times (>72 h). Multilayers presumably result from side
chain interlayer associations which increase polymer adsorption
and deposition.
23-25
organic surfaces.
Both positive and negative secondary ions were
collected over a m/z range of 0-300 with a nominal mass resolution
of unity. Data acquisition and control of the energy filter and
quadrupole used the Physical Electronics SIMS software package.
Sample Imaging Using Time-of-Flight Secondary Ion Mass
Spectrometry (ToF-SIMS). Imaging of patterned polymer thin films
was performed on a Model 7200 Physical Electronics ToF-SIMS
instrument (PHI Electronics, Eden Prairie, MN). A gallium liquid metal
ion source (LMI Ga), operating at a beam energy of 25 keV, was utilized
for all analysis presented in this study (see supporting information for
details).
The aqueous contact angle shows that the fluorocarbon side
chains in the polymers increase film hydrophobicity compared
to that of the pure dithioalkyl side chain analog (polymer A)
and that surface hydrophobicity increases with increasing the
perfluoroalkyl side chain content in the polymer (Table 2). This
is attributed to the relative enrichment of fluorocarbon chains
in the outermost regions of the monolayer (see XPS film
characterization section below). However, compared to other
fluorinated surfaces such as Teflon, plasma-deposited perfluo-
ropropane films, and self-assembled monolayers from organic
perfluoro compounds,
monolayers derived from spontaneous adsorption are 7-20°
lower, suggesting incomplete coverage of the underlying silox-
ane backbone and dithio-anchoring side chains by the overlaying
perfluoroalkyl chains (Figure 2).
Electrochemical Characterization of Polymer Films on
Gold Electrodes. Soluble redox probes, including Ru(NH3)6
Fe(CN)6
used for electrochemical measurements on organothiol mono-
layers in aqueous electrolyte.
function of a number of variables including alkanethiol length,
probe chemistry, and film defects (pinholes, grain boundaries).
Generally, little Faradaic current is observed through alkanethiol
monolayers of dodecanethiol and longer alkyl lengths when
Results and Discussion
Three-component, grafted polysiloxanes bearing both per-
fluoroalkyl and alkyl disulfide chains were synthesized in
various compositions, with three examples shown in Table 1:
polymer A bearing only alkyl disulfide chains; polymer B
bearing 19 mol % perfluoroalkyl chains and 64 mol % alkyl
disulfide side chains; and polymer C bearing 41 mol %
perfluoroalkyl and 21 mol % alkyl disulfide side chains.
Polymer Film Thickness and Wettability on Gold Sub-
strates. Perfluoroalkyl and dithioalkyl co-hydrosilylated poly-
siloxanes were self-adsorbed onto fresh gold-deposited wafers
from dilute chloroform solution. Ellipsometric thicknesses of
the fabricated polymer films range from 22 to 42 Å (Table 2),
consistent with measurements of alkyl disulfide side chain
15,26,27
contact angles for the polymeric
3+/2+
,
3-/4-
2+/+/0
, and methylviologen (MV
) have often been
28-31
Current attenuation is a
12
monolayers alone (14 Å), plus the addition of a siloxane layer
4 Å) and a perfluoroalkyl layer (14 Å). Film thicknesses are
(
28
assembled on carefully prepared gold substrates.
Cyclic voltammograms for Fe(CN)6 on clean, bare gold
electrodes show the well-characterized, reversible, one-electron
transfer process (data not shown, ∆Ep ) 60 mV, E1/2 ) 190 (
mV).
on gold electrodes are able to completely attenuate electron
transfer under the same conditions (not shown). Thinner
multilayer films of polymer C on gold electrodes (64 Å) are
also very effective at attenuating electron transfer to/from the
consistent for each polymer from “batch to batch” after 24 h of
dilute solution adsorption (standard deviations of (4 Å) and
do not increase with extended adsorption times up to 1 week,
demonstrating only stable monolayer formation and absence of
any significant interchain cross-linking by disulfide reduction.
Given the uncertainty in the polysiloxane contribution film,
thickness differences between films of polymers A and B are
not significant. Thicker multilayer films can be prepared from
concentrated polymer solutions (20 mM) and extended adsorp-
3-
28-30
5
Thick, spin-cast films of polymer C (330 Å thick)
(26) (a) Chau, L.-K.; Porter, M. D. Chem. Phys. Lett. 1990, 167, 198.
(
20) Application note from Surface Science Instruments, Mountain View,
(b) Arduengo, A. J.; Moran, J. R.; Rodriguez-Parada, J. R.; Ward, M. D. J.
Am. Chem. Soc. 1990, 112, 6153. (c) Chidsey, C. E. D.; Loiacono, D.
Langmuir 1990, 6, 682. (d) Alves, C. A.; Porter, M. D. Langmuir 1993, 9,
3507. (e) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.;
Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610.
(27) (a) Haque, Y.; Ratner, B. D. J. Appl. Polym. Sci. 1986, 32, 4369.
(b) Dann, J. R. J. Colloid Interface Sci. 1970, 32, 302.
(28) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J.
Am. Chem. Soc. 1987 109, 3559. (b) Fabinowski, W.; Coyle, L. C.; Weber,
B. A.; Granata, R. D.; Castner, D. G.; Sadownik, A.; Regen, S. L. Langmuir
1989 5, 35.
CA (1987).
(
(
(
(
21) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129.
22) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2.
23) Briggs, D.; Hearn, M. J. Vacuum 1986, 36, 1005.
24) We have also studied hydrosilylation of PHMS by (1H,1H,2H-
perfluoroalkyl)olefins (C10 and C6) and found in both circumstances an
unexpected transfer of fluorine from the -CF2- group in the R-position to
the olefin to the polymer backbone Si atoms. Resultant polymers had less-
than-desired side chain density (unpublished result). This undesirable
reaction was not observed with compound 5, where the terminal olefin is
moved away from the perfluoroalkyl moieties.
(29) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990,
112, 558.
(
25) A model study conducted under similar reaction conditions where
n-pentyl disulfide was used as a “poisoning” additive (replacing dithioalkyl
(30) Niwa, M.; Mori, T.; Higashi, N. J. Mater. Chem. 1992, 2, 245;
Macromolecules 1993, 26, 1936.
(31) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884.
allyl ether) in hydrosilylation of PHMS with 1-octene shows over 90-
1
9
5% coupling conversion proven by both H-NMR and FTIR.