Rubbed Polyimide Surface Studied by Sum-Frequency Vibrational
Spectroscopy
Doseok Kim*
Department of Physics, Sogang University, Seoul, 121-742 Korea
Masahito Oh-e
†
and Y. R. Shen
Department of Physics, University of California at Berkeley, Berkeley, California 94720
Received January 17, 2001; Revised Manuscript Received July 31, 2001
ABSTRACT: Surface-specific sum-frequency vibrational spectroscopy was used to study the structure of
a rubbed polyimide surface. The spectra showed that the polymer backbones were well aligned by rubbing
along the rubbing direction, and the imide cores were inclined toward the surface plane with a broad
distribution. Quantitative analysis yields an approximate orientational distribution function for the aligned
imide cores and the backbones.
Introduction
Mechanical rubbing of polymer-coated substrates is
commonly used to homogeneously align liquid crystal
(LC) films in the LC display industry.
1
Presumably,
rubbing aligns the surface polymer chains, which in turn
aligns the LC monolayer adsorbed on the polymer
surface. The latter then determines the LC orientation
and alignment throughout the bulk film through cor-
relation of LC molecules.
2
Detailed understanding of the
alignment mechanism would help the design of LC
display cells. Thus many studies of the problem using
various techniques such as infrared spectroscopy,
3-5
optical retardation,
1,6
ellipsometry,
7
scanning force
microscopy,
8-10
X-ray scattering,
11
and X-ray spectros-
copy
12-15
have been reported in the literature. Recently
infrared-visible sum-frequency vibrational spectroscopy
has been demonstrated to be an effective probe for
polymer surfaces.
16-18
It has the advantage of being
surface-specific and sensitive to the surface monolayer.
Application of the technique to a rubbed surface of poly-
(vinyl alcohol) (PVA) was able to yield quantitative
information about the orientation and alignment of PVA
chains at the surface.
17
In the LC display industry,
however, polyimide (PI) is the preferred polymeric
material for LC alignment. It is therefore important to
carry out a similar study on PI. We present here the
results of such a study.
There exist already many reports on investigation of
rubbed PI.
3-15
In particular, surface second harmonic
generation (SHG) which is a special case of sum-
frequency generation (SFG) has been used to study side-
chain PI.
19,20
In these experiments, however, SHG is
mainly generated from the side chains of the polymer,
and does not yield information about rubbing-induced
alignment of the main chains. Yet it is the latter that
is responsible for the surface-induced homogeneous
alignment of LC films. With SFG spectroscopy, we were
able to measure the anisotropy induced by rubbing in
the vibrational spectrum of CO groups associated with
the imide rings of PI at the surface. The results allowed
us to deduce an approximate orientational distribution
of the imide rings, and hence of the PI main chains, at
the rubbed surface.
Experimental Section
The polymide used in our experiment was poly(n-alkylpy-
romelitic imide) [-N-(CO)2-C6H2-(CO)2-N-(CH2)n], with n
) 6 (P6). Its chemical structure is shown in Figure 1a. The
polymer film was prepared by spin-coating the polyamic
precursor onto a fused silica plate and letting the solvent
evaporate for 1 h at 60 °C. Subsequently, the sample was
baked at 200 °C for 2 h and cured at 300 °C for 5 h. The film
thickness determined from atomic force microscopic measure-
ment was about 40 nm. The PI film was then rubbed with
velvet cloth. The rubbing strength used for the sample was at
the saturation level, such that further rubbing would not
enhance the observed anisotropy in the SFG spectra.
The experimental setup for SFG has been described else-
where.
21
In brief, a pulsed Nd:YAG laser system was used to
generate a visible beam at 532 nm and a tunable IR beam
around 5.9 µm, both having a 15 ps pulse width and a 20 Hz
repetition rate. The two beams coming in from the air side
overlapped at the sample surface, and the SFG output was
detected in the reflection direction.
Theory
Theoretically, the surface SFG signal intensity gener-
ated by the input fields with intensities I
1
(ω
vis
) and
I
2
(ω
ir
) can be expressed by the following equation
17
where
SF
is the angle of the sum-frequency output with
respect to the surface normal. The effective nonlinearity
eff
(2)
takes the form of
with e ˆ(ω) being the unit polarization vector and L 6(ω)
the tensorial Fresnel factor at frequency ω.
21
The
nonlinear susceptibility 6
(2)
can be written as
†
Current Address: Hitachi, Ltd., Displays, 3300 Hayano,
Mobara-shi, Chiba-ken, 297-8622 Japan.
I(ω
SF
) )
8π
3
ω
SF
2
sec
2
SF
c
3
|
eff
(2)
|
2
I
1
(ω
vis
)I
2
(ω
ir
) (1)
eff
(2)
) [e ˆ(ω
SF
)‚L 6(ω
SF
)]‚ 6
(2)
:
[e ˆ(ω
vis
)‚L 6(ω
vis
)][e ˆ(ω
ir
)‚L 6(ω
ir
)] (2)
6
(2)
) 6
NR
(2)
+ N
s
∫
R6
(2)
(Ω)f(Ω)dΩ (3)
9125 Macromolecules 2001, 34, 9125-9129
10.1021/ma0100908 CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/22/2001