Rubbing-induced molecular reorientation on an alignment surface of an aromatic polyimide containing cyanobiphenyl side chains

Article abstract of DOI:10.1021/ja0042682

Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical second harmonic generation (SHG) experiments have been utilized to study the molecular orientation and conformation changes at a rubbed polyimide alignment-layer surface. This aromatic polyimide containing pendent cyanobiphenyl mesogens was synthesized via a polycondensation of 2,2′-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) with bis{ω-[4-(4-cyanophenyl)phenoxy]hexyl} 4,4′-diamino-2,2′-biphenyldicarboxylate (nCBBP, n = 6), abbreviated as 6FDA-6CBBP. Uniform alignment layers, possessing high pretilt angles ranging from 39° to 43°, have been achieved after mechanical rubbing of the polyimide thin film surface at room temperature and subsequent annealing. This is the first time that high pretilt angles have been detected to possess a negative angle (-θ_c) with respect to the rubbing direction (i.e., opposite to the rubbing direction), considerably different from the conventional pretilt angle (θ_c) observed along the rubbing direction. This observation is confirmed using magnetic null and SHG methods. Combined polyethylene (PE) SLD and atomic force microscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-on PE lamellar crystals is oriented normal to the rubbing direction, indicating that the PE chains are aligned parallel to the rubbing direction. This SLD technique probes the anisotropic surface orientation of the outermost molecules of the rubbed polyimide layer. The SERS results show that prior to rubbing the surface, both the pendent cyanobiphenyls in the side chains and backbones possess nearly planar chain conformations at the polyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylene structure, but also significant conformational rearrangements of the pendent side chains at the surfaces. The molecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms a uniformly tilted conformation on the rubbed surface, and the polar cyano groups point down toward the layer surface deduced from SHG phase measurements. This conformational rearrangement of the side chains results in the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenyls having the -θ_c pretilt angle with respect to the rubbing direction.

Full text of DOI:10.1021/ja0042682

5768
J. Am. Chem. Soc. 2001, 123, 5768-5776
Rubbing-Induced Molecular Reorientation on an Alignment Surface
of an Aromatic Polyimide Containing Cyanobiphenyl Side Chains
Jason J. Ge,† Christopher Y. Li,† Gi Xue,†,§ Ian K. Mann,† Dong Zhang,† Shyh-Yeu Wang,†
Frank W. Harris,† Stephen Z. D. Cheng,*,† Seok-Cheol Hong,‡ Xiaowei Zhuang,‡ and
Y. R. Shen‡
Contribution from the The Maurice Morton Institute and Department of Polymer Science, The UniVersity
of Akron, Akron, Ohio 44325-3909, and Department of Physics, UniVersity of California Berkeley,
Berkeley, California 94720-7300
ReceiVed December 15, 2000
Abstract: Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical second
harmonic generation (SHG) experiments have been utilized to study the molecular orientation and conformation
changes at a rubbed polyimide alignment-layer surface. This aromatic polyimide containing pendent
cyanobiphenyl mesogens was synthesized via a polycondensation of 2,2′-bis(3,4-dicarboxy-phenyl)hexafluo-
ropropane dianhydride (6FDA) with bis{ω-[4-(4-cyanophenyl)phenoxy]hexyl} 4,4′-diamino-2,2′-biphenyldi-
carboxylate (nCBBP, n ) 6), abbreviated as 6FDA-6CBBP. Uniform alignment layers, possessing high pretilt
angles ranging from 39° to 43°, have been achieved after mechanical rubbing of the polyimide thin film surface
at room temperature and subsequent annealing. This is the first time that high pretilt angles have been detected
to possess a negative angle (-θc) with respect to the rubbing direction (i.e., opposite to the rubbing direction),
considerably different from the conventional pretilt angle (θc) observed along the rubbing direction. This
observation is confirmed using magnetic null and SHG methods. Combined polyethylene (PE) SLD and atomic
force microscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-on
PE lamellar crystals is oriented normal to the rubbing direction, indicating that the PE chains are aligned
parallel to the rubbing direction. This SLD technique probes the anisotropic surface orientation of the outermost
molecules of the rubbed polyimide layer. The SERS results show that prior to rubbing the surface, both the
pendent cyanobiphenyls in the side chains and backbones possess nearly planar chain conformations at the
polyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylene
structure, but also significant conformational rearrangements of the pendent side chains at the surfaces. The
molecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms a
uniformly tilted conformation on the rubbed surface, and the polar cyano groups point down toward the layer
surface deduced from SHG phase measurements. This conformational rearrangement of the side chains results
in the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenyls
having the -θc pretilt angle with respect to the rubbing direction.
Introduction
layers are essential to inhibit the occurrence of the twist nematic
(TN) reverse-tilt disclination and the appearance of the STN
two-dimensional striped distortion, in addition to reducing the
operational voltage.2 Linearly polarized ultraviolet (UV) pho-
toalignment techniques have also been reported to form aniso-
tropic LC alignment.3-5
To achieve a reasonable gray scale for displays, a sufficiently
high pretilt angle is required. One of the most significant
scientific ideas is whether high pretilt angle alignment layers
can be obtained after rubbing by introducing side chains on
polyimide backbones for adjusting the structure and chemical
properties. Mechanisms of rubbing-induced LC bulk alignment
and pretilt angle in LC cells have been actively discussed over
the past decade.6-14 Two major points of view are that first,
High-resolution liquid crystal displays (LCDs), including full
color addressed passive matrix super-twisted nematic (STN)
LCDs and active matrix thin-film-transistor (TFT) LCDs, are
widely used in broadcasting, telecommunications, laptop/desktop
computers, and other fields of use.1 In the mass production of
LCDs, polyimide alignment layers play a critical role, via
mechanical rubbing, in controlling the bulk orientation of liquid
crystals (LC) in a preferential direction. These layers are
conventionally spin-coated or rotary-printed on indium tin-oxide
glass substrates with a thickness between 50 and 200 nm. It
has been speculated that upon mechanical rubbing, the molecular
alignment on the layer surface can substantially differ from the
bulk. It is important to recognize that high pretilt alignment
* To whom all correspondence should be addressed. E-mail: cheng@
polymer.uakron.edu.
(2) Bos, P. J.; Fredley, D.; Li, J.; Rahman, J. In Liquid Crystals in
Complex Geometries; Crawford, G. P., Zumer, S., Eds.; Taylor and
Francis: Great Britain, 1996; pp. 281-289.
† The University of Akron.
‡ University of California Berkeley.
(3) Gibbons, W. M.; Shannon, S. T.; Sun, S. T.; Swetlin, B. J. Nature
1991, 351, 49-50.
§ Current address: Department of Macromolecular Science, Nanjing
University, Nanjing, China.
(4) Shannon, P. J.; Gibbons, W. M.; Sun, S. T. Nature 1994, 368, 532-
(1) Morozumi, S. In Liquid Crystals; Bahadur, B., Ed.; World Scientific
Publishing: Singapore, 1990; Vol. 1, pp 171-194.
533.
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10.1021/ja0042682 CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/25/2001

Rubbing-Induced Molecular Reorientation
J. Am. Chem. Soc., Vol. 123, No. 24, 2001 5769
periodic microgrooves (or microtrenches), generated by rubbing,
may force the LC molecules at the alignment-layer surface to
be parallel to the rubbing direction (topological alignment), due
to geometric restrictions between surface microgrooves and LC
molecules.6 Second, van der Waals physical interactions, at the
interface between LCs and oriented polymers at alignment-layer
surfaces, may play the predominant role in aligning LC
molecules.7 To further explore the molecular mechanism
responsible for this surface-induced LC alignment and design
new polymer architectures to achieve desirable alignment
properties, it requires a deep understanding of the surface
chemical and physical structures.
Scheme 1. Synthesis of the Side Chain Polyimide
Containing Pendent Cyanobiphenyl Mesogens
Among the experimental techniques in surface characteriza-
tion, the optical second harmonic generation (SHG) and sum
frequency generation (SFG) vibration spectroscopy are intrinsi-
cally surface-sensitive.8-10 In the second-order nonlinear optical
process, it is forbidden in a medium with inversion symmetry,
but allowed at surfaces where such symmetry is necessarily
broken. Consequently, the polarization dependence of SHG and
SFG permits a quantitative determination of surface molecular
orientation. Grazing-incidence X-ray scattering experiments have
been applied to study the rubbing-induced orientation near the
polyimide surface of a semicrystalline polyimide (such as
BPDA-PDA), which detects the crystal-orientation profiles
along different directions with respect to the rubbing direction.11
Near-edge X-ray absorption fine structure spectroscopy probes
different orientation order parameters of the rubbed polyimide
surface (such as PMDA-ODA), deduced from the changes in
absorption energies.12 X-ray photoelectron spectroscopy has
probed the surface with binding energy changes at rubbed
polymer surfaces in a typical range of 1-5 nm after rubbing.13,14
Scanning probe microscopy techniques, such as atomic force
microscopy (AFM), have demonstrated surface topologies down
to an atomic level in static single crystals.15,16 However, in
almost all the cases of alignment-layer studies, AFM only
provides aligned physical scratches on the rubbed surface on a
length scale of a few hundred nanometers to micrometers. No
individually oriented chain has been clearly observed on a
rubbed surface.17,18 This suggests that we need an effective
anisotropic probe to detect chain orientation on the noncrystal-
line alignment surface when using AFM.
resulting in intense Raman scattering signals from molecules
adjacent to a rough silver substrate.19,20 The enhancement
decreases rapidly as a function of distance from the molecular
surface, making SERS particularly surface-sensitive. In recent
years, SERS techniques have been used to probe single
molecular chains and single nanoparticles.21,22
In this study, we focus on a specifically designed aromatic
polyimide containing pendent cyanobiphenyl mesogens as an
alignment layer having a high pretilt angle. The introduction
of cyanobiphenyls in the side chains generates high pretilt angles
with a polar interaction to align bulk LC molecules in LCDs.
Using several surface-sensitive methods, we report our experi-
mental results on surface chemical and physical structures of
this polyimide alignment layer, and provide a new understanding
of molecular orientation and conformation changes at the
outermost surface after rubbing.
Generally speaking, conventional infrared (IR) and Raman
spectroscopy are not surface-sensitive. However, surface-
enhanced Raman scattering (SERS) is a unique technique
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Experimental Section
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Lett. 1999, 82, 4256-4259.
Materials and Samples. A series of side-chain polyimides contain-
ing pendent cyanobiphenyl mesogens was synthesized. One of the
polyimides in this series, having six methylene units in the side chains,
was chosen for this study (Scheme 1). The key monomer for this
polymerization was the new diamine containing pendent cyanobiphenyl
mesogens of bis{ω-[4-(4-cyanophenyl)phenoxy]hexyl} 4,4′-diamino-
(11) Toney, M. F.; Russel, T. P.; Logan, J. A.; Kikuchi, H.; Sands, J.
M.; Kumar, S. K. Nature 1995, 374, 709-711.
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M.; Kumar, S. K. Macromolecules 1996, 29, 8334-8342.
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61, 2392-2394.
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Macronolecules 1996, 29, 8894-8899.
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175-221.
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Spectroscopy; Ellis Horwood: England, 1991; pp 228-259.
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Dasari, R. R.; Feld, M. S.; Phys. ReV. Lett. 1997, 78, 1667-1670.
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H. Appl. Phys. Lett. 1994, 65, 49-51.
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Buivydas, M.; Rudquist, P. Mol. Cryst. Liq. Cryst. 1995, 262, 89-98.

5770 J. Am. Chem. Soc., Vol. 123, No. 24, 2001
Ge et al.
Scheme 2. Synthesis of the Key Diamine Monomer of Bis{ω-[4-(4-cyanophenyl)phenoxy]hexyl}
4,4′-Diamino-2,2′-biphenyldicarboxylate
2,2′-biphenyldicarboxylate (5) synthesized by five steps as described
in Scheme 2 (see Supporting Information).23
pretilt angles (>10°) based on the null of the LC optical phase
retardation with a continuous increase of an external magnetic field.
The basic principle of this method was described in ref 24. In brief, a
polarized He:Ne incident laser beam passed through a front polarizer,
then a LC cell, followed by a rear polarizer (or an analyzer), and was
finally detected by a position-sensitive detector. When an LC cell was
rotated such that the LC molecules were oriented parallel to the external
magnetic field, the light transmission was independent of the strength
of the external magnetic field. The pretilt angle of alignment layers
was derived from this position with a minimum optical retardation
change.
Polyimide thin films for LCD alignment layers were processed by
spin-coating polyimide solutions with a concentration of 2 wt % in
cyclopentanone at 2000 rpm for 45 s on clean glass substrates. The
films were subsequently baked at 100 °C (primary baking temperature)
for 1 h and then at high temperatures between 150 and 250 °C (post-
baking temperature), for an additional hour. The film thickness was
controlled in the range of 50-200 nm. Thicker films (bulk samples)
with thickness of 10-50 µm were also obtained by casting concentrated
solutions, which were used for conventional FT-Raman measurements.
An AFM (Digital Instruments Nanoscope IIIa) was utilized in the
tapping mode with phase and amplitude imaging to study topological
feature changes on the polyimide surface after rubbing. The two-
dimensional (2D) spectrum was converted from the height or amplitude
image using the fast FT software supplied with the AFM. To observe
the molecular orientation at a thin film surface on a nanometer scale,
the polyethylene (PE) surface lamellar decoration (SLD) technique25,26
Polymerization was carried out in a one-step chemical imidiazation
via polycondensation of 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropro-
pane dianhydride (6FDA) and bis{ω-[4-(4-cyanophenyl)phenoxy]-
hexyl}-4,4′-diamino-2,2′-biphenyldicarboxylate (6CBBP). An equal
molar ratio of the dianhydride and diamine was used in the reaction.
The corresponding polymerization was conducted in a three-neck flask
refluxing in a solution of m-cresol under a dry N2 atmosphere with a
tertiary amine catalyst (isoquinonline) at elevated temperatures for 24
h (Scheme 1). The imidiazation process was detected by Fourier
transformation (FT)-IR and proton nuclear magnetic resonance spec-
troscopy. The intrinsic viscosity of the resulting 6FDA-6CBBP was
0.76 dL/g measured in chloroform at 30 °C. The glass transition
temperature of 6FDA-6CBBP was 136 °C as measured using a Perkin-
Elmer differential scanning calorimeter DSC-7 under the nitrogen
atmosphere. The decomposition temperature of 2% and 5% weight loss
were 326 and 358 °C, respectively, at a heating rate of 10 °C/min in
dry nitrogen.
Equipment and Experiments. Rubbing was conducted on the
polyimide alignment layers using a rotating drum wrapped with velvet
at room temperature. The LC material ZLI-2293 (Merck Co.) was filled
into the empty cells (with 10 µm spacers) in which the alignment layers
were assembled into an antiparallel rubbing configuration. The pretilt
angle of the alignment layers was characterized in the LC cells by the
magnetic null (MN) method constructed in our laboratory. The MN
method was a precise and reliable measurement for determining high
(23) Wang, S.-Y. Ph.D. Dissertation, Department of Polymer, Science,
The University of Akron, Akron, OH 44325-3909.
(24) Scheffer, T. J.; Nehring, J. J. Appl. Phys. Lett. 1984, 45, 1021-
1023.

Rubbing-Induced Molecular Reorientation
J. Am. Chem. Soc., Vol. 123, No. 24, 2001 5771
Figure 1. Pretilt angles of 6FDA-6CBBP alignment layers as a function of the post baking temperatures as well as pretilt angles of 6FDA-PFMB
(the chemical structure is also included) (a); and a schematic drawing of the structure of a typical LCD cell in an antiparallel assembly (b).
was used for the first time on a rubbed film surface. In brief, a linear
The rubbed aromatic polyimide 6FDA-PFMB (the chemical
PE was used as the decoration material (Mn ) 17 300 g/mol and
polydispersity ) 1.11, from Phillips Petroleum). The decoration was
conducted in a vacuum evaporator. The PE was degraded, evaporated,
and deposited onto the alignment-layer surface. After the PE oligomer
molecules were crystallized, they were used to probe the anisotropic
surfaces.
SERS-active substrates were produced using precipitation of silver
colloidal aggregates through a chemical reduction of AgNO3 dissolved
in deionized water. An ammonia solution and dilute formaldehyde was
subsequently added into the AgNO3 solution to form silver colloidal
aggregates on clean glass substrates. Rough silver colloids were then
deposited onto the unrubbed and rubbed surfaces of the polyimide thin
films.27 The SERS measurements were conducted in a backscattering
geometry on a Bruker IFS 100 FT Raman spectrometer equipped with
a light source from an air-cooled Nd:YAG laser (1.064 µm) and a liquid
nitrogen-cooled Ge detector. Generally, 200 scans were averaged to
obtain optimum signal-to-noise ratio.
structure is included in Figure 1a), which is chemically identical
to the backbones of 6FDA-6CBBP, results in a low pretilt angle
(1.3-1.5°), and it is independent of the post-baking temperature
(also included in Figure 1a). This indicates that the pretilt angles
of bulk LCs are unambiguously associated with the intrinsic
polyimide chemical architectures, in which the side chains must
play a vital role in generating the high pretilt angles of bulk
LCs. It should be pointed out that in the past, alignment layers
with high pretilt angles (>10°) have not been achieved via
mechanical rubbing. It was known that the alignment layers
having high pretilt angles could only be obtained via the oblique
vapor deposition of inorganic SiOx materials.
Furthermore, the pretilt angle of 6FDA-6CBBP alignment
layers is uniformly oriented in a -θc direction, which is opposite
to the conventional pretilt angle θc of bulk LC molecules with
respect to the rubbing direction as shown in Figure 1b (a
schematic drawing of a typical LC cell structure).6-14 This
unexpected observation must be associated with the specific
conformation and orientation of the side chains in this polyimide
at the film surface after rubbing (see below).
Surface Lamellar Decoration on Rubbed 6FDA-6CBBP
Alignment Layers. To understand the molecular orientation
distribution at the rubbed surface of this noncrystalline polyimide
alignment layer, the SLD experiment was conducted. If the
rubbing process can increase the conformation and orientation
regularity at the surface, the PE oligomers may have opportuni-
ties to be aligned along the rubbing direction on the surface
through short-range van der Waals interactions with 6FDA-
6CBBP. As the oriented PE oligomers nucleate and crystallize
at the surface, PE lamellar crystals may act as anisotropic probes
to manifest the surface molecular orientation.
In the SHG experiment, a frequency-doubled Q-switched mode-
locked Nd:YAG laser at 532 nm was used as the fundamental input
beam, and the second harmonic field was detected in the reflection
direction at 266 nm after proper filtering. The incidence angle was 67°
with respect to the normal substrate plane on the rotation stage. The
polarization dependence of SHG allowed the determination of surface
molecular orientation.8,9 The SHG process was governed by a surface
nonlinear susceptibility tensor 6X(2) ) N R5(2) , where N was the surface
density, R5(2) was the hyperpolarizability tensor, and
denoted the
average over the orientational distribution of the molecules. From the
measurement of 6X(2) in the SHG, the approximate orientational
distribution was determined assuming R5(2) was known. In rodlike
4-cyanobiphenyls in the side chains, R5(2) was dominated by a single
element R(ê2ê)ê, with ê along the long axis, and thus, the orientation
distribution was directly measured from 6X(2) 8-10
.
Results and Discussion
6FDA-6CBBP Polyimide Alignment Layers Having High
Pretilt Angles. Figure 1a shows a relationship between the LC
pretilt angle and post-baking temperature for 6FDA-6CBBP
alignment layers. When the temperature is increased from 150
to 250 °C, the pretilt angle gradually increases from 39° to 43°.
An amplitude image, obtained in the tapping mode AFM in
Figure 2a, indeed shows a direct visualization of the orientation
of PE lamellar crystals on the rubbed 6FDA-6CBBP alignment-
layer surface after the SLD (on the scan area of 7.0 × 7.0 µm2).
The crystalline lamellae are in an edge-on arrangement, and
the interlamellar spacing is approximately 60 nm along the
rubbing direction (as indicated by the arrow in Figure 2a).
Therefore, the c-axes of the PE lamellar crystals (the chain
direction) are parallel to the short axis of the PE crystals (the
rubbing direction). The long axis of the PE crystals is roughly
500 nm, which is associated with the lateral aggregation of the
(25) Wittmann, J. C.; Lotz, B. Makromol. Rapid Commun. 1982, 3, 733-
738.
(26) Wittmann, J. C.; Lotz, B. J. Polym. Sci., Polym. Phys. Ed. 1985,
23, 205-226.
(27) Ge, J. J.; Xue, G.; Li, F.; McCreight, K. W.; Wang, S.-Y.; Harris,
F. W.; Cheng, S. Z. D Zhuang, X.; Hong, S.-C.; Shen Y. R. Macromol.
Rapid Commun. 1998, 19, 619-623.

5772 J. Am. Chem. Soc., Vol. 123, No. 24, 2001
Ge et al.
Figure 3. A schematic model of orientation epitaxy of the edge-on
PE lamellar crystals on the rubbed surface of 6FDA-6CBBP (the
rubbing direction is indicated by an arrow) (a); a lateral dimension of
the orientation epitaxy of the PE crystals (b).
Figure 4. Normal FT-Raman spectrum of 6FDA-6 CBBP of the bulk
sample (the film thickness is approximately 50 µm without rubbing).
along the rubbing direction, which is attributed to the inter-
lamellar spacing of the edge-on PE lamellar crystals. A
schematic representation of the PE lamellar crystals and their
interlamellar arrangement on the rubbed polyimide surface is
illustrated in Figure 3a. Each PE oligomer is represented as a
rod with a length of 10-20 nm.26 Many uniaxially oriented PE
oligomers aggregate to form edge-on lamellar crystals, and the
height difference of the interlamellar crystal surface is ap-
proximately 3-5 nm (see Figure 3b). Furthermore, it needs to
be noted that at single-crystal surfaces of polymers, such as
PE, polypropylene,26 poly(ethylene oxide)31 and others, the SLD
observations reflect the existence of regular conformational
structures on the surface, such as oriented chain folds, loops,
and uniaxially extended chains, rather than dangling chains.26
We have also used the SLD on unrubbed polyimide thin film
surface; only random orientation of the PE lamellar crystals can
be observed, Therefore, it is interesting to ask what kinds of
the chain arrangements at the rubbed surface of 6FDA-6CBBP
can lead to the observation of this uniaxial PE lamellar crystal
orientation (see below).
Figure 2. Tapping mode AFM morphologies of the rubbed surface of
6FDA-6CBBP after the PE lamellar crystal decoration: (a) an
amplitude image at 7 × 7 µm2 scanning area, (b) a fast FT-2D pattern
from the height image area (the rubbing direction is indicated by an
arrow).
PE molecules in forming the lamellar crystals. On the other
hand, the depth resolution of this SLD is less than 1 nm due to
the limitation of short-range physical interactions.25,26 This
process does not require a lattice match between the PE crystal
and the alignment-layer surface, since the rubbed surface is at
most oriented amorphous. Therefore, this kind of SLD can be
recognized as a “soft” orientation epitaxy, which only possesses
a one-dimensional (1D) orientation correlation. This observation
is different from the lamellar self-decoration for observing
defects at the surface of LC polymers,28 and the two-dimensional
(2D) crystal epitaxy on the oriented polymer surface which
requires crystal lattice matching.29,30 It is presumable that the
formation of this anisotropic surface decoration is due to the
1D uniaxial and regular conformational arrangements at the
rubbed polyimide surface.
Bulk Raman Spectra of 6FDA-6CBBP. The bulk FT-
Raman spectrum recorded from a 6FDA-6CBBP thick film is
shown in Figure 4. The characteristic Raman bands of the
polyimide containing pendent cyanobiphenyls involve two major
components: the side chains and the aromatic polyimide
backbones. The characteristic bands of the imide backbones at
1786, 1379, and 1115 cm-1 are associated with the CdO
Figure 2b shows a 2D fast FT pattern from the AFM height
image of the alignment-layer surface after the SLD. A spacing
value of approximately 60 nm is obtained from the pair of halos
(28) Wood, A. B.; Thomas, E. L. Nature 1986, 324, 656-657.
(29) Wittmann, J. C.; Smith, P. Nature 1990, 352, 414-417.
(30) Damman, P.; Dosire, M.; Brunel, M.; Wittmann, J. C. J. Am. Chem.
Soc. 1997, 119, 4333-4639.
(31) Chen, J.; Cheng, S. Z. D.; Wu, S. S.; Lotz, B.; Wittmann, J. C. J.
Polym. Sci., Polym. Phys. Ed. 1995, 33, 1851-1855.

Rubbing-Induced Molecular Reorientation
J. Am. Chem. Soc., Vol. 123, No. 24, 2001 5773
Figure 5. Surface enhanced Raman scattering spectrum of 6FDA-
6CBBP from an unrubbed thin film surface contacted with a metallic
layer.
Figure 6. Surface enhanced Raman scattering spectrum of 6FDA-
6CBBP from a rubbed thin film surface contacted with a metallic layer.
-C6H4 components) are weakened. The aromatic-cyano stretch-
ing vibration at 2227 cm-1 is absent in the SERS spectrum. As
compared to the bulk FT-Raman spectrum in Figure 4, this
implies that the long axis of the pendent cyanobiphenyls lies
down parallel to the substrate surface. For the polyimide
backbones, the CdO stretching at 1786 cm-1 is not detected,
indicating that the imide-phenyl conjugated structure in the
backbones also adopts a nearly planar conformation on the
surface. In addition, the enhanced intensity of the axial C-N-C
stretching in the backbones shifts to 1400 cm-1 away from 1379
cm-1 in the bulk. This shift and enhancement reflect that the
presence of the side chains in the diamine moiety distorts the
orientation of the axial C-N-C imide stretching, causing the
vibration moment toward the direction perpendicular to the
stretching, the axial, and transverse C-N-C stretching, respec-
tively.32,33 The aromatic tangential ring stretching in the
backbones at 1621 cm-1 is partially overlapped with a stronger
biphenyl tangential ring stretching at 1606 cm-1 of the side
chains based on the model compound and peak deconvolution
analyses. For the pendent cyanobiphenyls, the characteristic
features arise from the aromatic C-H stretching at 3076 cm-1
,
the aromatic-cyano stretching at 2227 cm-1, the aromatic ring
stretching at 1526 cm-1, the C-C bridge stretching at 1290
cm-1 and the C-H in-plane bending (-C6H4) at 1183 cm-1
.
In the n-alkyl side chains, the characteristic bands are identified
at 2900 and 2856 cm-1 (the asymmetric and symmetric CH2
stretching) and 1439 cm-1 (the CH2 in-plane deformation),
respectively.34 When the polyimide films are thin enough
(thickness < 1 µm), unenhanced FT-Raman spectroscopy is not
sufficiently sensitive to collect any meaningful spectra due to a
poor signal-to-noise ratio. On the other hand, unenhanced FT-
Raman spectra are also not sensitive to identify subtle differ-
ences in the thick polyimide film before and after rubbing the
surface, indicating that mechanically rubbing cannot substan-
tially affect the bulk conformation and orientation changes.
Molecular Orientation at the Unrubbed 6FDA-6CBBP
Thin Film Surface via SERS Measurements. After silver
colloid aggregates are directly deposited onto the unrubbed
6FDA-6CBBP thin film surface, strongly enhanced Raman
signals of the 6FDA-6CBBP thin films are detected as shown
in Figure 5. Note that the film thickness is only around 100
nm, and therefore, the intense signals do arise from the
molecules at the thin film surface adjacent to silver colloids.
According to the surface selection rules of SERS for molecules
adsorbed on metal surfaces, the signals of molecular vibrations
with moments perpendicular to the substrate surface are intensi-
fied, while those with moments parallel to the surface are
weakened.35 This rule makes SERS surface-specific and capable
of detecting the chain conformation and orientation at surfaces.
In the side chains of the polyimide, the band intensities at 1606
cm-1 (the aromatic tangential ring stretching), 1296 cm-1 (the
aromatic C-C bridge stretching from the C6H4-C6H4 compo-
nents), and 1183 cm-1 (the C-H in-plane bending from the
substrate surface. The medium enhancement of 1643 cm-1
,
which can be assigned as biphenyl stretching in diamines,
implies that 2,2′-substituted biphenylenes are twisted to a certain
degree. Furthermore, the CH2 asymmetric stretching at 2926
cm-1 (shift 26 cm-1 compared to the bulk spectrum) is
enhanced, along with a shoulder band of the CH2 symmetric
stretching at 2860 cm-1. It is probably due to the close contact
of aliphatic side chains with the silver substrate.
On the basis of the SERS observations, we can qualitatively
illustrate both conformations and orientations of the backbones
and side chains at the surface before rubbing. The pendent
cyanobiphenyls and the imide-phenylenes adopt nearly planar
conformations at the surface, and the aliphatic side chains are
close to the surface. This anisotropic in-plane orientation of the
backbones is similar to many aromatic polyimide thin films in
which the polyimide chains are oriented parallel to the substrate
surface, leading to a uniaxial negative birefringence.36,37 This
study presents additional evidence that chain conformations at
the surface are considerably different from those in the bulk.
Molecular Reorientation at the 6FDA-6CBBP Rubbed
Thin Film Surface via SERS Measurements. Figure 6 shows
the characteristic features of the SERS spectrum from the rubbed
thin film surface of 6FDA-6CBBP. In this figure, the band at
1606 cm-1, representing the aromatic tangential ring stretching
from the cyanobiphenyls, is substantially enhanced as is the
aromatic-cyano stretching band at 2227 cm-1. It is deduced that
(32) Young, J. T., Tasi, W. H.; Boerio, F. J. Macromolecules 1992, 25,
887-894.
(36) Cheng, S. Z. D.; Arnold, F. E., Jr.; Zhang, A.; Hsu, S. L.; Harris,
F. W. Macromolecules 1991, 24, 5856-5862.
(37) Li, F.-M.; Kim, K.-H.; Savitski, E. P.; Chen, J.-C.; Yoon, Y.; Harris,
F. W.; Cheng, S. Z. D. In Photonic and Optoelectronic Polymers; ACS
Symposium Series; American Chemical Society: Washington, DC, 1997;
Vol. 672, pp 1-14, 1997.
(33) Perez, M. A.; Ren, Y.; Farris, R. J.; Hsu, S. L. Macromolecules
1994, 27, 6740-6745.
(34) Hallmark, V. M.; Zimba, C. G.; Sooriyakuamaran, Miller, R. D.;
Rabolt, J. F. Macromolecules 1990, 23, 2346-2350.
(35) Moskovits, M. J. Chem. Phys. 1982, 77, 4408-4416.

5774 J. Am. Chem. Soc., Vol. 123, No. 24, 2001
Ge et al.
interactions with the backbones at the layer surface, the fold-
like bent conformation in Figure 7a should be more stable and
favorable. Furthermore, the dangling chain ends on the surface
do not generate a stable high pretilt angle with a uniaxial
orientation, and no clear regular PE decoration image can be
probed on the surface.26 Therefore, the SDL results also support
the existence of the unique and stable fold-like bent conforma-
tion, which allows us to fabricate uniform LC cells. However,
this conformational arrangement has to be determined by
examining the cyanobiphenyl orientation at the alignment-layer
surface.
Figure 7. A schematic model of the molecular orientation of pendent
cyanobiphenyls on the rubbed side chain polyimide surface of 6FDA-
6CBBP: (a) a stable conformation and (b) a less stable conformation.
Both of the long axis orientation of the tilt cyanobiphenyls is opposite
to the rubbing direction.
Cyanobiphenyl Orientation at the Rubbed Thin Film
Surface via SHG Measurements. The surface SHG results
further demonstrate that, after rubbing the 6FDA-6CBBP thin
film, those pendent cyanobiphenyls in the side chains possess
a significantly anisotropic azimuthal distribution at the align-
ment-layer surface along the -θc direction. Since the cyano-
the cyanobiphenyls in the side chains adopt a tilted conformation
with respect to the surface substrate after rubbing, significantly
different from the nearly planar conformation before rubbing
(see Figure 5). Note that the long axis of cyanobiphenyls is
tilted approximately 41 ( 2° from the layer surface, in the
direction opposite that of the rubbing, as determined by the MN
method via LC orientation epitaxy at the surface. Further
supporting evidence comes from the drastic increase in intensi-
ties of the bands at 1291 cm-1 (the aromatic C-C bridge
stretching of the C6H4-C6H4 components) and 1183 cm-1 (the
CH in-plane bending of the -C6H4 components). The axial
C-N-C stretching band at 1397 cm-1 in the linkage between
the dianhydride and diamaine is also enhanced and shifts 18
cm-1 from 1379 cm-1 in the bulk (Figure 4). This observation
confirms the existence of the nonplanar conformation in the
linkage. The appearance of the carbonyl band at 1788 cm-1
suggests that the rubbing process also causes the rigid imide-
phenyl structure or 2,2′-substituted biphenylene of the backbones
to slightly tilt from the substrate, leading to a small deviation
of the polyimide backbones from the planar arrangement. A
new, small band at 1126 cm-1 of the transverse C-N-C
stretching mode appears and shifts from the 1122 cm-1 band
in the bulk. This may imply an increase in ordering of the imide
ring packing at the surface after rubbing.33 In the aliphatic side
chains, the CH2 in-plane deformation shifts to 1445 cm-1 from
the original surface band at 1449 cm-1, while the CH2 symmetric
stretching shifts toward 2854 cm-1 on the surface from the
original 2860 cm-1. Note that these frequency shifts are probably
associated with changes in aliphatic chain conformations
interacting with the silver substrate. Since the aliphatic side
chains connect the pendent cyanobiphenyls with the backbones,
these chains must act as linkages to support the cyanobiphenyl
to form a regular and stable conformation on the rubbed surface
(see below).
biphenyls are chromophores, which have a hyper-polarizability
(2)
tensor R5(2) dominated by a single element R , its orientation
êêê
can be deduced from the nonlinear susceptibility tensor 6X(2)
determined by measuring an azimuthal variation of SHG with
four input/output polarization combinations,8-10 as shown in
Figure 8. The insert in this figure provides the definition of the
polar orientation angle θ, which utilizes the angle between the
cyanobiphenyl long axis and the z-axis. The azimuthal angle
(φ) is constructed between the projection of this long axis and
the x-axis in the xy plane. These two parameters determine the
average direction of the cyanobiphenyl long axis, and thus, the
pretilt angle orientation at the rubbed surface in three-
dimensional real space. For the cyanobiphenyl long axis at the
6FDA-6CBBP alignment-layer surface having an orientation
distribution of f(θ,φ), the surface nonlinear susceptibility can
be obtained as a function of f(θ,φ). Since the cyanobiphenyl
long axis at the surface possesses C1ν symmetry, five parameters
of θ0, σ, d1, d2, and d3 can be deduced from the nonlinear
susceptibility measurements based on the approximate distribu-
tion function equation of
f(θ,φ) ) A exp[-(θ - θ0)2/2σ2](1 + d1 cos φ +
d2 cos 2φ + d3 cos 3φ)
Note that for the isotropic samples the SHG signals with an
s-polarized output are forbidden due to inversion symmetry.
However, the SHG intensity from the rubbed surface is
considerably anisotropic, indicating that the rubbing process
induces a significant rearrangement of the cyanobiphenyl
orientation. Detailed data fitting (the solid lines in Figure 8 a-d)
allows us to determine six independent nonvanishing elements
of 6X(2). Their corresponding values
The significantly enhanced signals of the cyanobiphenyls in
the side chains indicate that the cyanobiphenyl units must play
a critical role in generating the pretilt angle and the tilting
direction on the surface. Considering that the orientation di-
rection of LCs is opposite to the rubbing direction, the tilted
cyanobiphenyl conformation at the surface must be self-
assembled into one of two possible arrangements as shown in
Figure 7 a and b. In the case of Figure 7a, the cyanobiphenyls
are tilted from the surface with the cyano groups pointing down
toward the layer surface, which is similar to the chain folding
at polymer crystal-basal surfaces. In the other case of Figure
7b, the cyanobiphenyls are tilted from the surface with the cyano
groups pointing up with respect to the surface. This is close to
the case of dangling chain ends on the crystal surface. Since
the cyanobiphenyls of the side chains can form attractive
ø(x2x)x : ø(x2y)x : ø(x2z)z : ø(z2xx) : øz(2yy) : ø(2) ) 1: 0.22: 0.031:
zzz
-0.12: -0.072: -0.041
and their errors are (5, (20, (10, (20, and (25%, respec-
tively. Even though 6X(2) for 6FDA-6CBBP originates mainly
from the cyanobiphenyls in the side chains, it has also been
found in a polyimide of 6FDA-PFMB (see the chemical struc-
ture in Figure 1a), which has identical backbones but no side
chains, the field discontinuity at the surface along z contributes
(2)
a value of ø (6FDA-PFMB) ) 0.012.38 This value is big
zzz
(2)
zzz
enough when compared to the value of ø ) -0.041 at the
surface of 6FDA-6CBBP, which attributes to both contributions
(38) Hong, S.-C.; Oh, M.; Zhuang, X.; Shen, Y. R.; Ge, J. J.; Harris, F.
W.; Cheng, S. Z. D. Phys. ReV. E 2001. In press.

Rubbing-Induced Molecular Reorientation
J. Am. Chem. Soc., Vol. 123, No. 24, 2001 5775
Figure 8. Anisotropic optical SHG results from 6FDA-6CBBP rubbed thin layer surfaces. They are s-input and s-output (a); s-input and p-output
(b); p-input and s-output (c); and p-input and p-output (d). Circles are experimental data and lines which are fitted using the equation in the text.
The insert is the geometry definition of the rubbed surface.
of the backbones and cyanobiphenyls in the side chains.
Therefore, the difference between these two values is the true
contribution of the cyanobiphenyls at the surface of the 6FDA-
6CBBP. Contributions of the field discontinuity of the other
elements of 6X(2) (except for øz(2zz)) are much weaker and can be
considered negligible. The deduced parameters can thus be
calculated as: θï ) 84.6, σ ) 12.0, d1 ) -1.03, d2 ) 0.50,
and d3 ) -0.29. Note that the d1 value is negative, implying
that that cyanobiphenyls in the side chains are aligned opposite
to the rubbing direction.
However, the SHG intensity measurements cannot directly
distinguish those two different possible cyanobiphenyl orienta-
tions, as shown in Figure 7 a and b. To search for the definitive
answer, an SHG phase measurement is introduced. The cyano-
biphenyls in the side chains with opposite orientations (in Figure
7 a and b) should cause a 180°-phase difference in the element
Figure 9. Schematics describing the orientation of 5CB LC monolayer
absorbed on a commercial polyimide (P6) alignment-layer surface. The
cyano groups point down toward the layer surface and the pretilt angle
is θc with respect to the rubbing direction. The arrow represents the
(2)
ø . On the other hand, a well-known example of cyanobi-
zzz
phenyl groups in 5CB monolayer aligned on a commercially
available polyimide alignment-layer surface (P6 which does not
contain side chains) shows that the cyanobiphenyls point down
toward the polyimide-layer surface having a pretilt angle of θc
as shown in Figure 9a.38 Since the SHG phase measurements
are critically associated with the polar vector along the cyano-
biphenyl long axis, the arrangements between Figures 7a and
rubbing direction (a). The experimental data points and the fitting curves
(2)
xxx
in (b) and (c) represent the results of SHG phase measurements of ø
on the surface of the rubbed 6FDA-6CBBP and 5CB monolayer on
the surface of the rubbed P6 alignment layer.
9a should also lead to a 180°-phase difference in the element
chains at the rubbed surface. It is also expected that the high
pretilt angle is controllable by varying the number of methylene
units and chemical linkages of the side chains in this series of
polyimides.
(2)
ø . However, if the cyanobiphenyls are arranged as shown in
zzz
(2)
zzz
Figure 7b, there should be no phase difference in ø com-
pared with that in Figure 9a. As seen from the measured in-
terference patterns for both of the cases in Figure 9 b and c, the
Conclusions
(2)
measured SHG X phase of the cyanobiphenyls in 6FDA-
zzz
6CBBP (in Figure 9b) is opposite to that of 5CB monolayer
absorbed on the polyimide alignment layer P6 (in Figure 9c).
Therefore, it can be concluded that the orientation of the
cyanobiphenyls in the side chains of 6FDA-6CBBP is as
described in Figure 7a. This leads to a fold-like bent confor-
mational structure. The molecular mechanism of observing a
negative pretilt angle in the LC bulk alignment is thus the
specific orientation of the pendent cyanobiphenyls in the side
The aromatic polyimide 6FDA-6CBBP which contains
pendent cyanobiphenyls in the side chains has been designed
and synthesized for achieving high pretilt angle alignment layers.
After mechanical rubbing at room temperature, high pretilt
angles are aligned in the -θc direction opposite to the rubbing
direction as detected by both MN and SHG experiments. In the
SLD experiments, when PE oligomers are nucleated and
crystallized at the rubbed alignment surface, edge-on PE lamellar

5776 J. Am. Chem. Soc., Vol. 123, No. 24, 2001
Ge et al.
crystals directly probe the anisotropic molecular orientation at
the surface observed in AFM. The PE chains in the crystals are
oriented parallel to the rubbing direction. A 2D fast FT pattern
from the AFM height image verifies that the anisotroptic and
preferential orientation of the PE crystals is induced on the
rubbed polymer surface. Nearly planar molecular conformations
in both the backbones and the pendent mesogens (in-plane
orientation) at the original polyimide surface can be identified
using SERS. After rubbing, the uniform reorientation and
rearrangement of the pendent cyanobiphenyl conformations at
the rubbed surface are the major cause of generating the high
pretilt angles opposite to the rubbing direction. The alignment
mechanism of this rubbed 6FDA-6CBBP can be further de-
duced to attribute to the specific orientation of the cyanobi-
phenyls in the side chains. The long axis of the cyanobiphenyls
are anisotropically tilted from the surface by rubbing, and the
cyano groups prefer pointing down toward the polyimide-layer
surface to form a stable, fold-like bent conformation.
Acknowledgment. This work was supported by the NSF
Science and Technology Center of Advanced Liquid Crystalline
Optical Materials (ALCOM, DMR-91-57738), Nitto Denko
Corporation, and NSF DMR-96-17030.
Supporting Information Available: Detailed experimental
routes for the syntheses of monomers (1), (2), (3), (4) and (5)
in Scheme 1 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA0042682