Supplementary Material for
Polymer-Brush Lubrication in Oil: Sliding Beyond the Stribeck Curve
Robert M. Bielecki, Maura Crobu, Nicholas D. Spencer*
ETH Zurich, Department of Materials, Laboratory for Surface Science and Technology,
Wolfgang-Pauli-Strasse 10, HCI G517, 8093 Zurich, Switzerland
*Corresponding author.
Phone: +41-44-632-5850 Fax: +41-44-633-1027
E-mail address: nspencer@ethz.ch
This supplementary material contains X-Ray Photoelectron Spectroscopy (XPS) characterization
of the initiator-functionalised surfaces, complementary tribological data illustrating data
reproducibility and some additional information concerning the coefficient-of-friction maps
(triboscopy).
1. XPS characterisation of initiator-functionalised surfaces
The aim of the supplementary XPS measurements was to analyse and compare the chemical
composition of the initiator-functionalized surfaces, prior to synthesis of the polymers. By means
of XPS it was possible to estimate the packing densities of the initiator layers formed on iron
oxide substrates.
1.1.
Experimental
The XPS analysis was performed using a VG Theta Probe (Thermo Fisher Scientific, East
Grinstead, UK) spectrometer (a detailed description can be found in [1]). The spectra were
acquired using a monochromatic Al Kα source with a beam size of 300 μm in the constantanalyzer-energy (CAE) mode. A pass energy (PE) of 100 eV and a step size of 0.1 eV (FWHM for
silver Ag 3d5/2 = 0.88 eV) was used to acquire the high-resolution spectra. A PE of 200 eV with a
step size of 1 eV was used for the survey spectra.
The angle-resolved mode spectra (16 emission angles) were acquired using a PE of 150 eV
(FWHM for silver Ag 3d5/2 = 1.15 eV). The overlayer thickness of the iron oxide samples has
been calculated considering the attenuation of the iron Fe 2p3/2 signal at two different emission
angles (25˚ and 66˚). The inelastic mean free path, λ, was calculated using the Seah and Dench
formula [2].
Two sample types for Si-based substrates were analysed (bare and BPCS-initiator functionalized)
and three samples for Fe-bases substrates (bare, with nitrodopamine and with nitrodopamine
coupled to an initiator group) were analyzed in standard (3 points per sample) and angle-resolved
mode (1 point per sample). Two independent series of samples (number 1 and 2) were analyzed
for a total of 10 samples
1.2.
Results and discussion
The typical survey spectra of a bare silicon wafer and a silicon wafer functionalised with BPCS
initiator are reported in Figure 1. C, O and Si signals were detected on the bare silicon wafers
while C, O, Si and Br were present on the silicon wafers functionalised with the BPCS initiator.
Traces of Pt were also found on the initiator-functionalised silicon wafers and can be assigned to
the remaining Pt catalyst used for the synthesis the BPCS molecules. The Br 3d5/2 signal (see
Figure 1) was found at the binding energy (BE) of 70.7±0.2 eV.
Figure 1 XPS survey spectra of a bare silicon wafer and a silicon wafer functionalised with BPCS initiator. The
high-resolution spectrum of the Br 3d signal is reported as well, at the top-right of the graph.
Knowing the initiator layer thickness from the ellipsometric measurements (d=1.8 nm), and the
molecular weight of this molecule (Mw=413.9 g/mol), a simple equation (Eq. 1) was used to
calculate the packing density of the BPCS initiator molecules on the silicon oxide surface σ=2.9
molecules/nm2 (assuming density ρ=1.1 g/cm3).
(Eq. 1)
The bare iron oxide samples showed the presence of Fe, O and C (Figure 2), while Fe, O, C and N
were detected in the spectra of the samples functionalised with nitrodopamine. Traces of sulphur
on those surfaces can be explained by the usage of nitrodopamine sulphate in surfacefunctionalisation protocols.
The “iron oxide + catecholamine + initiator” samples showed the presence of Fe, O, C, N, and Br.
In both samples, the Br is present in two different oxidation states (the two components of the Br
3d5/2 signal were found at 70.4±0.2 eV assigned to Br bound to a tertiary carbon in the form of an
initiator, and 68.8±0.2 eV that could be assigned to bromide salts)
Survey
2000
Br 3d
1950
1900
1850
1800
60000
1750
Fe2O3 + nitrodopamine + initiator
1700
O 1s
Fe 2p
40000
1650
1600
77
N 1s
0
CPS
73
71
69
67
65
BE [eV]
20000
60000
75
C 1s
Br 3p
Fe 3s
Fe2O3 + nitrodopamine
40000
O 1s
Fe 2p
20000
N 1s
C 1s
Fe 3s
0
60000
Bare Fe2O3
O 1s
Fe 2p
40000
20000
Fe 3s
C 1s
0
1200
1100
1000
900
800
700
600
BE [eV]
500
400
300
200
100
0
Figure 2: XPS survey spectra of a bare Fe2O3, a nitrodopamine functionalised Fe2O3 and a Fe2O3 functionalised
with nitrodopamine and initiator.
From the stoichiometry of the molecule, the ratio of Br to N should be 1:2. However, the
measured, corrected intensity ratio calculated from the XPS data appears to be roughly 1:3,
indicating a yield of ca 60 % for the coupling reaction of the initiator molecules with the amines
present on the substrate.
Further, from the angle-resolved data it was possible to calculate the overlayer thickness of 1.4 nm
for the “catecholamine+initator” substrates on the iron oxide samples. Using the same calculation
principle as for the BPCS molecules on silicon, and taking the molecular weight of a molecule
where each amine is coupled with one molecule of α-bromoisobutyryl bromide – 346 g/mol (and
assuming again density of 1.1 g/cm3) the packing density of the nitrodopamine based molecules on
iron oxide was estimated to be 2.7 molecules/nm2. This calculation was performed, without taking
into account a possible layer of contamination, and assuming a perfect yield of the amide-coupling
reaction, both of which may lead to overestimation of the values.
Even if the actual initiator packing density is as low as 2.0 molecules/nm2, the measured
differences between the packing density of Br-initiating groups on silicon oxide and iron oxide
ensure a massive excess of the initiating groups, considering typical polymer grafting densities
(approx. 0.2-0.3 chains/nm2, or using only around 10% of initiating groups).
2. Complementary data displaying the tribological data reproducibility
Figure 3 As supplementary data to Figure 3 in the main manuscript obtained in PF350 oil, a total of 4 series of
experiments displaying data reproducibility in the sliding speed - COF plot.
Figure 4 As supplementary data to Figure 3 in the main manuscript obtained in EO500 oil, a total of 4 series of
experiments displaying data reproducibility in the sliding speed - COF plot.
Figure 5 Typical COF evolution with sliding distance, obtained for brush-brush contact at 0.1 cm/s at 20 mN
and studied in hexadecane (140 nm dry P12MA thickness, data supplementary to Figure 5 in the manuscript).
Figure 6 Supplementary experiments displaying the coating-stability dependence on the sliding speed.
Coating thickness 120 nm, brush-brush configuration. Data obtained in a linear module with
reciprocating sliding (distance of 4 mm per cycle), as supplementary material to Figure 5 in the
manuscript, in which the curves obtained at higher speeds (0.5, 1 and 5 cm/s, in hexadecane) are more
susceptible to differences among samples. In this additional figure a logical trend can be observed, in
which the coating stability is higher at higher sliding speeds, supporting the information contained in
Figure 5 in the main text (In this case the effect on stability is even more profound since coating failure
was achieved for all samples).
3. Triboscopy of poly(alkyl methacrylates)
Coefficient-of-friction maps plotted as a function of position along the wear-track arc and
sliding distance can be useful to identify the degradation mechanism of the polymer coating.
This type of “triboscopy” diagram has been demonstrated by Belin as a useful tool to monitor
degradation of thin coatings [3]. The images below display COF values that are an average of
the values measured during the clockwise and counterclockwise movement at the given point
on the sample.
Figure 7 Acquisition of triboscopic data: a) with the speed varying as a sinusoidal function as f(t) along a
sliding arc of radius R and angular range a. At the turning points the speed is 0 and in the center (angle 0°) the
speed reaches its maximum. b) The two possible coefficient-of-friction representations: COF plotting against
an angular position or against sliding speed.
Figure 8 Triboscopy of a 70 nm thick P12MA coating studied in PF350 oil under 20 mN load and with maximal
speed of 0.1 cm/s. The COF during the initial cycles was measured as approx. 0.08 for the majority of the
sliding track length (ca 75 %). Afterwards it decreased to ca 0.03, later increasing slightly to ca 0.04. The arrow
indicates time evolution.
Figure 9 Triboscopy of P12MA 140 nm thick coating studied in hexadecane under 20 mN load and with a
maximal speed of 0.5 cm/s. Initially coefficient-of-friction values start at a relatively uniform value along the
entire length of the sliding track at ca 0.08, followed by steady sliding, with COF values below 0.05 at positions
on the wear track where the speed varied between ca 0.5 cm/s and 0.25 cm/s, and at slightly higher values over
the remaining part of the sliding track. Ultimately the coating fails, reaching COF values above 0.15,
preserving only a narrow area in the center of the sliding track (highest sliding speed), where COF remained
below 0.1. The arrow indicates time evolution.
References
[1]
Mangolini F., Rossi A., and Spencer N. D.: Chemical Reactivity of Triphenyl
Phosphorothionate (TPPT) with Iron: An ATR/FT-IR and XPS Investigation. J. Phys.
Chem. C 115, 1339-1354 (2011).
[2]
M. P. Seah W. A. D.: Quantitative electron spectroscopy of surfaces: A standard data base
for electron inelastic mean free paths in solids. Surf. Interface Anal. 1, 2-11 (1979).
[3]
Belin, M., Lopez, J., Martin, J.M.: Triboscopy, a quantitative tool for the study of the wear
of a coated material. Surface and Coatings Technology 70(1), 27-31 (1994).