Toward a Force Spectroscopy of Polymer Surfaces
Kirill Feldman, Theo Tervoort, Paul Smith, and Nicholas D. Spencer*
Department of Materials, Swiss Federal Institute of Technology,
ETH-Zu ¨ rich, CH-8092 Zu ¨ rich, Switzerland
Received March 29, 1997. In Final Form: October 7, 1997
The adhesional forces between a series of polymer film surfaces and chemically well-defined atomic force
microscopy tips have been measured and found to depend strongly on the chemical nature of both probe
and sample surfaces. For a given series of polymers, the ranking in adhesion strength was markedly
different for polar and nonpolar probes, irrespective of the precise chemical composition of those probes.
In the case of nonpolar polymers, a correlation of adhesion force with calculations based on the Lifshitz
theory of Van der Waals interactions was found. In the case of polar polymers, a reasonable correlation
with water-contact angle was observed. The adhesional differences between different probe tips translate
into reversals of chemical contrast in high-spatial-resolution lateral force images, when examining polymer
blends using chemically different tips, demonstrating the potential of this approach for the nanometer-
scale, friction-mediated surface-chemical imaging of polymers. Central to these experiments has been the
use of perfluorodecalin as a medium for measuring interactions. Employment of this liquid greatly facilitates
measurement of the forces between the probe tip and the polymer surface.
Introduction
Atomic and lateral force microscopy techniques (AFM
and LFM) have, since their development in the 1980s,
1,2
shown considerable promise as methods for nanometer-
scale, surface-chemical analysis, since they can provide
quantitative, spatially resolved, chemically dependent
information on interactions between the scanning probe
and sample surfaces. This feature has been exploited by
many researchers, using approaches such as chemical
modification of probe tips for the recognition of specific
surface groups
3-8
or monitoring the pH dependence of the
tip-surface interaction.
9-11
The majority of such studies
have involved self-assembled monolayers (SAMs) on flat
gold surfaces,
12
which provide an idealized test surface,
presenting a well-ordered, morphology-free, highly con-
centrated plane of functionality. The usefulness of SAMs
as models for polymer surfaces is limited, however, since
issues such as complex surface morphology, disorder,
mechanical properties, and solvent interactions signifi-
cantly complicate the issue with real polymers, making
chemical imaging extremely challenging.
6,13,14
Force-distance measurements with conventional (non-
vacuum) scanning probe microscopes are often performed
in a liquid environment in order to eliminate the contri-
bution of capillary forces resulting from water adsorption
from the air.
15
Moreover, the liquid environment can be
used to tune the Van der Waals forces between the probe
and the surface.
16a
This is a valuable approach that others
have used for DNA imaging,
16b
for example. An important
consideration here is the makeup of the van der Waals
interaction, which can be calculated from the nonretarded
Hamaker constant, A
total
, and which, in turn, consists of
the two terms A
v)0
and A
v>0
, corresponding to the dipole-
dipole and dipole-induced-dipole contributions and the
dispersion (London) contributions, respectively, to the van
der Waals interaction. According to Israelachvili’s sim-
plification
17
of the Lifshitz theory,
18
these contributions
can be calculated for a system where two macroscopic
phases interact across a third phase, from the respective
static dielectric constants (ǫ
1
, ǫ
2
, ǫ
3
) and optical refractive
indexes (n
1
, n
2
, n
3
) as follows:
where the electronic absorption frequency, ν
e
, is assumed
to be equal for all three components (ν
e
) 3 × 10
15
Hz).
The consequence of this relationship is that a close match
between the dielectric constants of the tip, the sample,
and the medium leads to a suppression of the first term,
with the result that dispersion forces (determined by the
optical refractive index) play the dominant role in
determining the tip-sample adhesion. In fact, if the
refractive index of the intervening medium is intermediate
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A
total
) A
ν)0
+ A
ν>0
≈
3
4
kT
(
ǫ
1
- ǫ
3
ǫ
1
+ ǫ
3
29(
ǫ
2
- ǫ
3
ǫ
2
+ ǫ
3
29
+
3hν
e
8 2
×
(n
1
2
- n
3
2
)(n
2
2
- n
3
2
)
(n
1
2
+ n
3
2
)
1/2
(n
2
2
- n
3
2
)
1/2
{(n
1
2
+ n
3
2
)
1/2
+ (n
2
2
+ n
3
2
)
1/2
}
(1)
372 Langmuir 1998, 14, 372-378
S0743-7463(97)00335-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/01/1998