SELF-ASSEMBLED POLYCARBONATE FILMS DEPOSITED ON
ORGANIC AND NON-ORGANIC SUBSTRATES, THEIR
STRUCTURE, PROPERTIES AND APPLICATIONS
Edward Bormashenko*, Roman Pogreb, Oleg Stanevsky, Ye. Socol, Yaniv Biton,
Yelena Bormashenko
The College of Judea and Samaria, The Laboratory of Polymer Materials, Ariel,
44837, Israel.
O.V.Gendelman
Technion, Technion City 32000, Faculty of Mechanical Engineering, Haifa, Israel.
E-Mail: edward@ycariel.yosh.ac.il
ABSTRACT. Self-assembled honeycomb polycarbonate films were deposited on
organic and non-organic substrates under conditions of fast dip-coating. Selfassembled patterns were revealed by optical and SEM microscopy on two scales:
mesoscopic and submicroscopic. Close-packed hexagonal 2D submicrometric
structures were formed on the polymer piezoelectric (poled PVDF) substrates. IR
spectra and diffraction properties of self-assembled films were studied. The infrared
transmission spectrum of the films deposited on the poled PVDF substrates exhibits a
bandgap in the near IR band, thus allowing electrooptical applications of obtained
films as 2D tunable photonic crystals. Semi-quantitative models explaining the
phenomenon of self-assembly on both scales are proposed.
Keywords: self-assembling, polymer films, mesoscopic, nanometric, photonic
bandgap, instability.
1. Introduction
Processes of self-assembling and self-organization in thin polymer films attracted
considerable interest recently [1-6]. It was shown that self-assembling brings into
existence strictly ordered, periodic patterns both on micrometric (i.e., mesoscopic)
and nanometric scales, thus allowing various optical applications, including photonic
molecules and photonic crystals - a new class of dielectric media that can provide
novel ways to manipulate and control light.
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Photonic molecules are mesoscopic hierarchical structures, constructed from units
with typical dimensions of 1-10 μm, which function as coupled optical resonators [7].
Mesoscopic periodic structures have also a potential as long-period optical gratings
(LPG) [8]. LPGs with a grating period of 50-1000 μm have been effectively employed
as spectral-selective filters in various optical devices [8].
Usually a polymer-based photonic crystal is formed as a nanoscaled periodic
polymer lattice including tiny air holes. Photonic crystals are characterized by a
bandgap that blocks the propagation of the light in a certain frequency range, thereby
making integrated optics devices possible [9-13]. One of the most attractive methods
of fabrication of 2D honeycomb patterns in polymers uses evaporation induced selfassembly, which brings into existence ordered patterns of air bubbles embedded a in
polymer matrix. This method is not connected with production of microparticles or
micromolds; however, it allows effective control of the pore size distribution. At the
same time, the mechanism of the evaporation-induced self-assembly is still not clearly
understood. Srinivasarao, Shimomura and Pitois have related the phenomenon to
atmospheric humidity, which can favor the formation of the honeycomb films [1418]. Mesoscopic self-assembling in thin polymer films was studied by De Gennes
recently [20-21]. De Gennes related mesoscopic self-organization to BénardMarangoni instabilities formed under intensive evaporation of polymer solutions [22].
Srinivasarao suggested that the microscopic self-assembling could be explained in the
framework of the Bragg-Nay-Geguzin model of crystal structure [14, 23-24].
We developed a very simple process (we call it fast dip-coating) which facilitates
formation of self-organized structures demonstrating periodicity on both mesoscopic
and submicrometric/nanometric scales. The proposed process allows formation of
self-assembled mesoscopic and submicrometric structures on both non-organic and
organic substrates, including the poled polyvinylidene fluoride (PVDF) substrates.
Poled PVDF is a semi-crystalline polymer distinguished by its strong piezoelectric
properties [25-26]. Zhang has shown recently that unusually great strains, as high as
3-4%, are available in the copolymers of polyvinylidene fluoride, allowing effective
tunability of photonic bandgap structure [27]. Deposition of self-assembled structures
on the polymer piezoelectric substrates allows realization of the tunable photonic
bandgap structures, which is a subject of extensive recent theoretical and experimental
investigations [28-29]. Tuning of photonic crystals is achieved either by distorting the
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symmetry of photonic crystals or by changing the dielectric constant of one of two
constituents. The band structures of the tunable photonic crystals were found to be
highly anisotropic, giving rise to optical switching and modulation applications [30].
2. Experimental
Polycarbonate PC Lexan 141 (supplied by GE Plastics) was dissolved in
dichloromethane CH2Cl2 pure for analysis supplied by Karlo Erba Reagenti. The
concentration of solution was 5 % wt. Four types of substrates (quartz glass, chrome
steel, polypropylene (PP) and poled polyvinylidene fluoride (PVDF)) were coated
under conditions of the fast dip-coating process. The thickness of quartz glass and
polypropylene substrates was varied in the range 40-4000 μm. The thickness of the
chrome steel substrates was varied in the range 10-350 μm. Poled PVDF substrates
with a thickness of 25 μm were supplied by Precision Acoustics Ltd. When the
substrate is dip-coated, the liquid film runs out from the polymer solution, adheres to
the substrate surface and solidifies during the evaporation of the solvent (see Fig. 1).
Dip-coating was carried out with an unusually high pulling speed V up to 47 cm/min (
the traditional speed of the dip-coating process is about 0.1-1 cm/min). Films were
immediately dried with an air current or dried nitrogen ( v  3m / s ), under isothermal

conditions. The drying temperatures were varied in the range t d  18  46 C . The

temperature of the solution was kept constant: t s  9  20 C.
The structure of the dry film was studied by means of optical and scanning
electron microscopy. The diffraction properties of the patterns were studied with a
normally incident beam produced by He-Ne laser (λ=633 nm). A narrow laser beam
was obtained while using a small diaphragm with a diameter of 25 μm. Transmission
visible and near-IR spectra of the films were studied by use of a Bruker Equinox 55
FTIR spectrometer. Middle and far-IR spectra were studied with a Bruker 22 FTIR
spectrophotometer in the range 1.5-25 μm.
3. Results and discussion.
3.1. Optical microscopy and SEM study of the structure of self-assembled
films.
The structure formation in our “fast” dip-coating process relies on the balance
between solvent evaporation and polymer consolidation occurring in the vertical flow
of the solvent.We revealed that both drying temperature and temperature of the
solution have an influence on the process of self-assembling. Drying temperatures
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which were within 18-25 °C, and temperatures of solution limited within 9-11°C led
to formation of the ordered structures both on micrometric (i.e., mesoscopic) and
nanometric (submicrometric) scales. Figure 2 depicts optical microscopy and SEM
images of polycarbonate films applied to quartz-glass substrates under conditions of
fast dip-coating. Ordering on two scales - mesoscopic and nanometric - is recognized.
The orientation of the mesoscopic structure with a period of 50 μm is vertical and is
obviously due to gravity.
The SEM image displayed in Fig. 2B presents PC domains separated by
highly porous areas. Such mesoscopic structures have a potential as long-period
gratings (LPGs). However, the honeycomb structure of the porous area presented in
Fig. 2C seems to be of much more interest in the light of photonic bandgap
applications. The average size of pores is 2 μm, allowing near IR optics applications
of obtained structures.
The structure of obtained films depends strongly on the substrate type. PP
substrates bring into existence ordered self-organized structures as presented in Fig. 3.
Strictly ordered mesoscopic structure is formed on the PP substrates, characterized by
large domains of PP separated by the rows of nanoscaled pores. The average size of
the PC domains is larger compared to those formed on the quartz glass substrates
under the same conditions. Use of PP substrates, which are highly transparent in the
broad IR band (from 1 up to 25 μm wavelength, with the exception of the narrow
absorption band located close to 3 μm), allows an extended area of application of selfassembled structures to middle and far IR bands.
Self-assembling on the mesoscopic scale was observed when PVDF substrates
were coated under the same condtitions. However, the structure of mesoscopic
patterns is quite different from those obtained with PP and quartz glass substrates (see
Fig. 4). When the drying temperature was increased up to 30 °C, the mesoscopic order
was destroyed on all substrates. At the same time, 2D regular nanoscaled structures
were formed on the poled PVDF and PP films under higher drying temperatures:

t d  30  46 C . The average size of the pores was 400-1000 nm, depending on the
drying temperature. Thus the structures under discussion could be related to both
micrometric and nanometric scales. The higher drying temperature led to formation of
smaller holes. The hole size distribution for a fixed drying temperature is narrow. Fig.
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5 presents a nanoscaled self-assembled structure based on the hexagonal elementary

cell, formed on the poled PVDF substrates under the drying temperature t  36 C .
3.2. Impact of the atmospheric humidity on the process of patterning.
Srinivasarao, Shimomura and Pitois [14-16, 18-19] related the formation of
honeycomb structures to the atmospheric humidity; therefore it was important to
control this parameter in our experiments. We varied the atmospheric humidity in our
experiments over a wide range and didn’t observe any evidence of the humidity
impact on pattern’ formation. We performed a series of experiments in which hot
compressed nitrogen was used for the drying procedure, and porous self-assembled
structures were still formed in the very same manner, as it was when hot air was used
for drying. Thus we finally ruled out any crucial influence of the humidity on the selforganization process in our experiment.
We want to emphasize that we deposited our films on vertical plates, whereas other
groups worked with horizontal substrates; hence, the sinking of water droplets,
discussed by Srinivasarao1 et al, cannot be invoked for the explanation of the
phenomenon.
3. 3. The impact of the substrate thickness on pattern formation.
The most surprising results were obtained when both organic and non organic
substrates of various thicknesses (varied from 40 to 4000 μm) were coated under the
fast dip-coating process. A strong impact of the substrate thickness on the mesoscopic
self-assembling was revealed. Self-assembling was observed exclusively on relatively
thin quartz glass and PP substrates, with a thickness less than 150 μm. We deposited
PC films on the chromium steel substrates with thicknesses of 10, 20, 30, 40, 60, 70,
80, 100, 150 and 350 μm. Mesoscopic self-organization was revealed on the metal
substrates with a thickness less than 100 μm only (see Fig. 6).
3. 4. Diffraction properties of the self-assembled films.
Diffraction properties of the self-assembled films were studied with a narrow laser
beam with a diameter of 25 μm. When a narrow laser beam was directed normally to
the samples, diffraction spots were observed. Very distinct diffraction pictures were
obtained when self-assembled structures deposited on PVDF substrates (such as
depicted at Fig.5) were irradiated, indicating the hexagonal structure of the selfassembled area (see Fig. 7). Similar diffraction properties of self-assembled arrays of
large latex particles were reported by Goldenberg [2].
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3 .5. Transmission IR spectra of self-assembled films.
Study of middle and far-IR transmission spectra of the self-organized patterns
supplied valuable information about their makeup and properties. Fig. 8 depicts the
middle-IR transmittance spectrum of the films under discussion compared to the
spectrum of a PVDF substrate. First of all it should be emphasized that there is no
evidence of water absorption band located between 2.8 and 3.2 μm; hence it could be
concluded that bubbles don’t contain traces of water vapor. Unfortunately, the
presence of solvent vapor traces in the bubbles could not be revealed from the spectral
data. Both the solvent and polycarbonate demonstrate strong absorption in the
fingerprint region at the wavelength 13.5 μm. Strong absorption at the 5.6 μm
wavelength is inherent to PC molecules (Fig. 8).
The spectrum demonstrates a rise of transmittance when wavelength increases
within 1.25-3.25 μm. This phenomenon is related to the scattering of IR light by PC
film. Very similar spectral behavior of polymer-based composite film has been treated
by the authors already [31]. Antireflection properties of PC films in the band 3.5-5.5
μm could be recognized from the spectral data as well.
The visible and near-IR transmission spectrum of nanoscaled PC films deposited
on the poled PVDF substrates is of ultimate interest for tunable photonic bandgap
applications of self-assembled films. Fig. 9 displays visible and near-IR spectra of the
self-assembled PC film, such as depicted in Fig. 5. A distinct photonic bandgap could
be recognized, located in the vicinity of 0.95 μm wavelength. It has to be emphasized
that both components (PVDF and PC) don’t display any absorption peak in this
spectral band; hence the bandgap is related to the 2D photonic crystal-like structure
deposited on the PVDF substrate [12].
3.6. Mechanism of self-assembling
Experimental data led us to the conclusion that atmospheric humidity doesn’t play
a considerable role in the pattern formation. We suggest that evaporation of the
solvents plays a decisive role in the simultaneous formation of bubbles in the
evaporated film’s bulk and in mesoscopic pattern formation. The low boiling
temperature of dichloromethane, which is 39.6 °C is noteworthy; hence the drying
takes place at a temperature which is close to this of boiling point. Polycarbonate
dissolved in the dichloromethane promotes bubble formation, which in turn tends to
form the submicrometric (nanoscaled) structures described above.
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Formation of the mesoscopic structures may be described with the help of the
considerations presented below. Due to fast external drying, all solvent which leaves
the solution is immediately removed from the system. Then, at an early stage of the
evaporation process, a polymer – rich layer is formed [21]. This boundary layer has
essentially lower diffusion coefficient than the bulk solution; therefore, the
evaporation of the solvent is primarily governed by diffusion through this layer. The
solvent flux through the boundary layer is determined by the equation
J D
c
s
(1)
where J is the volume flux per second through unit area (number of molecules x cube
of characteristic solvent molecule size), s is the boundary layer thickness, ∆c is the
dimensionless concentration jump at the boundary layer and D is characteristic
diffusion coefficient (see Fig. 10). Due to the evaporation, the upper boundary of the
system moves. Rather roughly, it is possible to estimate the velocity of motion of the
upper boundary as
v0   J  D
c
s
(2)
Estimation (2) describes the steady evaporation process with almost constant
velocity. Still, similarly to other hydrodynamic instabilities [32], there exists a
possibility for instability of a fixed – thickness boundary layer. The physical
mechanism of this instability is based on the observation that the local trough from
outside the boundary layer has a tendency to grow as the reduced thickness facilitates
the diffusion of the solvent in this place. Consequently, a local crest outside the
boundary layer has a trend to grow as it suppresses the diffusion. Small – wavelength
perturbations of this sort are suppressed by surface tension, and therefore there exists
some critical scale for development of the layer instabilities.
In order to evaluate this critical scale, we pass to the frame system moving
with constant velocity v0 together with the upper boundary of the layer. χ(x,y,z,t) is a
local coordinate of the upper layer boundary. The thickening of the layer leads to a
decrease of the local velocity:
v  v0  D
c
s
(3)
Consequently, the local perturbation equation for the χ variable with respect to
viscosity and surface tension [3] may be written as follows:
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2   2 2   2 2
( Dc) 2
~
(

)

(

)


 t x 2 y 2
 z x 2 y 2
t 2
s4
(4)
where η is the viscosity coefficient, ρ is the density of the solution, σ is the
coefficient of the surface tension. Standard linear stability analysis invokes the
following characteristic shape of the perturbation:
   (t ) exp(kz ) sin( kx) sin( ky )
(5)
Substituting (5) to (4), one gets
d 2
2 2 d
( Dc) 2 2 3
~
k
 (

k )
2

dt

dt
s4
(6)
The critical wavenumber for stability thus may be estimated as
kcrit ~ 3
( Dc) 2 
s 4
(7)
For the sake of numeric evaluation we take s~70 nm [21], ρ~1.4 g/cm3,
σ~27·10-3 J/m2 [34], Δc~1, D~3.9·10-9 m2/s [34], one gets kcrit~6·105m-1. This
estimation corresponds to the typical scale for the loss of stability equal to 2π/kcrit~10
μm. This figure at least qualitatively coincides with experimental findings. It should
be mentioned that the above approximation is very crude. No attention has been paid
to interaction with the underlying surface in the final stages of the evaporation process
and other important issues.
Let us discuss the influence of the substrate thickness. We have shown
experimentally that heat transfer in the substrate plays a decisive role in the process of
self-organization. It is necessary to compare the typical drying time τd (determined
experimentally as τd ≈ 10 s) with the characteristic time necessary for the
establishment of thermal equilibrium in the substrate τ*, given by τ* ≈ Δ2/α, where Δ
and α are the thickness and thermal diffusivity of the substrate respectively (see Table
1).
We observed mesoscopic patterning at all substrates only if condition τd >> τ*
was fulfilled. In other terms, a cooling due to the evaporation has been rapidly
compensated by the heat flux from outside the system. It is possible to suggest that in
absence of such compensation the evaporation cooling would lead to decrease of the
diffusion coefficient and increase of the viscosity. Both these factors are able to
prevent formation of nontrivial mesoscopic structures.
Formation of nanoscaled self-assembled structures as depicted in Fig. 5 could be
explained on the basis of the Bragg-Nay model of crystal structure [23-24]. Bragg and
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Nay have shown that the crystal structure of metals could be represented by an
assemblage of bubbles floating on the surface of a soap solution. The bubbles blown
from a fine orifice beneath the surface of a soap solution formed large-area 2D
hexagonal close-packed structures. The phenomenon was explained by Geguzin, who
suggested that between bubbles are attractive and repulsive forces at work. Attractive
force is due to surface tension, its rise illustrated by Fig. 11. Drawing of two bubbles
together is followed by a reduction in surface tension energy. Geguzin has
demonstrated this attractive force F 
1
, where d is the distance between the
2
d
bubbles’ centers. Repulsive force is due to the pressure of the gas filling the bubble
(in our case a mixture of air and solvent vapor). It acts when bubbles come into
contact; the balance of repulsive and attractive forces brings into existence the HCP
2D structure.
Conclusions
The process of self-assemlbling of thin polycarbonate films deposited on organic
and non-organic substrates under conditions of fast dip-coating was studied first. The
phenomenon of self-assembling was observed on two scales: mesoscopic and
submicroscopic. Hexagonal close-packed 2D submicrometric structures were formed
on the polymer piezoelectric (poled PVDF) substrates. Diffraction properties of the
self-assembled films were studied. Semi-quantitative models explaining the
phenomenon of self-assembling on both scales are proposed. The transmission
spectrum of the films deposited on the poled PVDF substrates exhibits a bandgap in
the near-IR band, thus allowing electro-optical applications of obtained films as 2D
tunable photonic crystals.
Acknowledgements
The authors are grateful to Professor M. Zinigrad and Professor Dan Davidov for their
continuous support of our research activity. The authors are thankful to Professor De
Gennes, Professor A. Nepomnyashchy, Professor A. Voronel, Mr. A. Sheshnev for
fruitful discussions. We thank Mrs. N. Litvak and Mr. Al. Shulzinger for SEM
imaging of the samples. We thank Mr. Roy Ziblat for his generous help in spectral
measurements and Mrs. Albina Musin for her help in the preparing of the paper. The
4 - 73
work has been supported by the Israel Ministry of Absorption. O. V. Gendelman is
grateful to the Taub and Shalom Foundations for their financial support.
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Table 1. Thermal diffusivity α of the substrates used in the experiment.
Substrate
α (10–7 m2/s)
Polypropylene
0.95
Polyvinylidene fluoride
0.7
Quartz glass
4.4
Chrome steel
40
4 - 76
V
air or nitrogen
solid film
substrate
polymer solution
Fig. 1.
4 - 77
Fig. 2.
4 - 78
Fig. 3.
4 - 79
A
B
Pp
pvdf
Fig. 4.
4 - 80
A
B
Fig. 5.
4 - 81
100 μm
1000 μm
A
B
Fig. 6.
4 - 82
Fig. 7.
4 - 83
1
0.9
Transmittance, a.u.
0.8
0.7
0.6
0.5
0.4
A
0.3
B
0.2
0.1
0
1.25
1.75
2.25
2.75
3.25
3.75
4.25
Wavelength, m
Fig. 8.
4 - 84
4.75
5.25
5.75
0.5
Transmittance, a.u.
0.4
0.3
0.2
0.1
0
0.65 0.75 0.85 0.95 1.05 1.15 1.25 1.35 1.45 1.55 1.65 1.75 1.85 1.95
Wavelength, m
Fig. 9.
4 - 85
z
y
x
Boundary layer
Liquid solution
Substrate
Fig. 10.
4 - 86
d
d
Fig. 11
4 - 87
Fig. 1. Scheme of the dip-coating process.
Fig. 2. Optical microscopy and SEM images of self-organized mesoscopic and
nanoscaled structures obtained under fast dip-coating deposition of PC films on the
quartz glass substrates. The drying temperature – 25°C. A – Optical microscopy
reveals ordered mesoscopic structure. B – SEM image of the same sample shows
domains of PC separated by porous areas. C - Magnified SEM image of the porous
area demonstrates honeycomb nanoscaled structure.
Fig. 3. Optical microscopy and SEM images of self-organized mesoscopic and
nanoscaled structures obtained on PP substrates. The drying temperature – 25°C. A
and B – Optical microscopy images of the ordered mesoscopic structure. C – SEM
image of the porous boundary.
Fig. 4. Optical microscopy images of the mesoscopic ordered structures formed on PP
(A) and PVDF substrates (B).
Fig. 5. SEM images of self-assembled nanoscaled structures obtained under fast dipcoating deposition of PC films on the poled PVDF substrates. The drying temperature
– 36°C.
Fig. 6. Optical (A) and SEM (B) microscopy images of the mesoscopic ordered
structures formed on chrome steel substrate (thickness of the substrate 60 μm).
Fig. 7. Photograph of the diffraction picture obtained on the screen under irradiation
of self-assembled structures deposited on the poled PVDF substrates with normally
incident laser beam (633 nm).
Fig. 8. Middle IR spectra of the poled PVDF substrate (curve A) and PC selfassembled film deposited on the poled PVDF substrate (curve B).
Fig. 9. Visible and near-IR spectrum of the self-assembled film deposited on the poled
PVDF substrate (average diameter of holes – 1400 nm).
Fig. 10. Scheme of the evaporation process.
Fig. 11. Interaction between two bubbles floating on the liquid surface.
4 - 88