MESOSCOPIC AND SUBMICROMETRIC ORDERING IN
EVAPORATED POLYMER FILMS: INFLUENCE OF THE
SURFACE DEFECTS
Ed. Bormashenko1*, R. Pogreb1, O. Stanevsky1, Ye. Bormashenko1, T. Stein1, N.
Litvak1, A. Shulzinger1, O. Gendelman2
1
2
The College of Judea and Samaria, The Laboratory of Polymer Materials, Ariel,
44837, Israel, E-Mail: edward@ycariel.yosh.ac.il
Faculty of Mechanical Engineering, Technion, Technion City 32000, Haifa, Israel
Abstract: The influence of surface defects on the patterning of evaporated polymer
films, deposited on solid substrates by a dip-coating process was investigated. It was
shown that surface defects favor formation of mesoscopically ordered patterns. The
characteristic dimension of the mesoscopic cell is about 50 μm. The characteristic
scale of the mesoscopically patterned area is about an order of magnitude more than
the size of the defect (about 100 μm) itself. The structure of mesoscopically scaled
cells was studied by means of optical and SEM microscopy. A semi-quantitative
model explaining the phenomenon of mesoscopic cell formation is proposed.
INTRODUCTION
Evaporation-induced submicrometric and mesoscopic ordering in organic and nonorganic liquids was under intensive investigation recently [1–11]. Nanoscaled selfassembling, allowing formation of strictly ordered honeycomb structures comprised
of air holes dispersed in the solid matrix was explained satisfactorily in the framework
of approach, developed by Kralchevsky and Nagayama [1, 12]. However evaporationinduced mesoscopic patterns formation remains mysterious, in spite of much
theoretical effort devoted to understanding of the mechanisms responsible for
mesoscopic structuring. Two main approaches could be recognized: the first treats
mesoscopic structuring as a process of cracking under solvent evaporation [6, 13–15];
and the second relates mesoscopic self-assembling to various kinds of surface
instabilities, such as Marangoni surface tension instability [8–9, 16–22]. It has to be
emphasized that describing physical processes occurring in evaporating films turns
out to be very challenging, since couplings between the microstructure and
hydrodynamic flow are highly non-linear [18, 23–25].
We have shown already that evaporation of the same polymer solutions could bring
into existence patterns ordered on mesoscopic and submicrometric scales, depending
on the drying conditions [11, 26]. This work is devoted to one more factor, which (as
it will be demonstrated) has a decisive impact on the patterning: namely, the impact of
the surface defects.
EXPERIMENTAL
Polystyrene (PS) 143 E supplied by BASF, was dissolved in a mixture of
dichloromethane CH2Cl2 (90% wt.) and chloroform CHCl3 (10% wt.). Solvents were
supplied by Karlo Erba Reagenti, the concentration of PS in the solutions was 5% wt.
Polypropylene substrates were coated under conditions of a fast dip-coating process.
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Substrates were cleaned thoroughly with acetone and ethyl alcohol and rinsed with a
large amount of distilled water. Substrates were perforated in a way described in
Figure 1. Diameter of the holes was 100 μm, separations between holes were: L = 10
mm, H = 20 mm (see Figure 1). Large distances between holes avoided their
reciprocal interference during the pattern formation.
L
H
d
Fig.1. Perforated polypropylene substrate used for dip-coating process.
V
hot air
solid film
substrate
polymer solution
Fig. 2. Scheme of dip-coating process.
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 Figure 2). Dip-coating was carried out with a high pulling speed V = 47
cm/min. Pulling direction was vertical. Films were immediately dried with a hot air
current (v = 5 m/s), under isothermal conditions. Gas current velocity was measured
with anemometer PROVA AVM 03. Gas stream was stabilized with 0.5 m length
tube. The drying temperature was td = 65ºC. We have demonstrated already that the
above-noted mixture of chlorinated solvents components, when dried under the
mentioned conditions, promotes formation of submicrometric honeycomb structures
[27]. The structure of the dry film was studied by means of optical and scanning
electron microscopy. Images were processed with SIAMS 600 software.
RESULTS AND DISCUSSION
2
We established that surface defects exert a decisive impact on the patterns
formation. “Brain-like” mesoscopic patterns were formed in the vicinity of surface
defects (see Figure 3). In specific cases polymer film covered the hole (Figure 3b-c),
and in some cases the hole remained unfilled (Figure 3a), however in all situations the
hole promoted formation of mesoscopic patterns. The characteristic dimension of the
mesoscopic cell was ~50 μm. The size of the mesoscopically patterned area could be
estimated as 800–900 μm. Areas which were away from the surface defects were
coated with the submicrometric structure, as reported previously by the authors [11]
and depicted in Figure 4a. We relate formation of these pores to the solvent
evaporation under drying. It has to be emphasized that drying procedure was
performed under the temperature (65°C), which is higher than the solvents boiling
points (60°C for chloroform and 39.5°C for dichloromethane).
I
b
a
II
flow direction
boundaries,
separating cells
c
Fig. 3. a-b: typical brain-like mesoscopic structures observed in the vicinity of the
surface defects, c: magnified optical microscopy image of the mesoscopic structure
(area II in Fig. 3 b).
Interesting results were obtained when images, depicting submicrometric
structures were processed with SIAMS 600 software, intended for processing of
speckled images. Elements of broken hexagonal symmetry could be recognized (see
Figure 4b, which displays segments connecting geometric centers of pores). The
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average distance between pores was established with SIAMS 600 as 960 nm; the
average diameter of the pore was 360 nm.
a
4 μm
b
Fig. 4. a: SEM image of the submicrometric scaled porous area (area I in Figure 3a),
solvent mixture of dichloromethane (90%) and chloroform (10%), drying temperature
– 65°C, b: the same image processed with SIAMS 600 software.
A closer inspection of the mesoscopic cells, displayed in Figure 3, was carried
out with a scanning electron microscope. SEM study revealed that mesoscopic cells
are separated with porous boundaries presented in Figures 3c, 5–6. Asymmetry in the
shape of the patterns near the pore may be attributed to the overall flow of the
polymer solution while drying takes place. Diameters of pores, forming the boundary,
were estimated as 1-2 μm. The internal area of the mesoscopic cell contains very
small pores of 100–300 nm in diameter presented in Figure 7. However, the surface
density of these pores is less, comparatively to those of pores distant from the
mesoscopically patterned area (such as depicted in Figure 4); the average distance
between pores was estimated as 1800 nm.
Now let us discuss mechanisms responsible for mesoscopic patterning. It
should be noted that classical Bénard or Bénard-Marangoni instabilities driven by the
buoyancy or the surface tension hardly may be responsible for the pattern formation in
our case [16–22]. The reason is that polymer film is very thin (~ 1-3 μm) and the
characteristic time of the thermal conduction was estimated already as 10-2 s [26].
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100 μm
Fig. 5. SEM image of mesoscopically patterned structure.
50 μm
50 μm
Fig. 6. SEM image of typical boundaries separating mesoscopic cells formed in the
mesoscopic “brain-like” structure.
5 μm
Fig. 7. SEM image of the internal area of the mesoscopic cell.
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Thus the whole process may be considered as isothermal, and Bénard or
Bénard-Marangoni instabilities could not be invoked for the explanation of the
phenomenon.
We already proposed another kind of mass transport instability, which is
perhaps responsible for the mesoscopic cells formation [26]. De Gennes supposed that
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 [14]. According to earlier results, the characteristic
thickness of such a layer may be on the order of 50-70 nm, about 20 times less than
the characteristic thickness of the film by the end of drying. This boundary layer has
an essentially lower diffusion coefficient than the bulk solution; therefore, the
evaporation of the solvent is primarily governed by diffusion through this layer.
Now let us discuss the physical mechanism of diffusion instability: it 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 tends to grow as it suppresses
the diffusion. Small-wavelength perturbations of this sort are suppressed by surface
tension, and therefore there exists a critical scale for development of the layer
instabilities. The critical wavenumber for stability was thus estimated as [26]:
( Dc) 2 
,
s 4
where D is the characteristic diffusion coefficient, Δc – is the dimensionless
concentration jump at the boundary layer, σ and ρ – surface tension and density of the
solution respectively, s – the boundary layer thickness. For the sake of numeric
evaluation we take s~70 nm (this estimation for the thickness of the boundary layer
has been obtained by De Gennes in paper [14]) and the physical parameters of the
solvents established experimentally [28–30]. Our estimation results on 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 results
presented in Figures [3, 5]. Actually, mesoscopic cells characteristic dimensions
demonstrated a considerable dispersion: 10–100 μm, and it has to be stressed that
mentioned above theoretical estimation is very rough and complete understanding of
the mesoscopic patterning is not achieved still. However, it seems to be reasonable to
conclude that surface defects promote the progress of the diffusion instability.
The results presented demonstrate that mesoscopic structural surface defects
strongly affect the pattern formation process. Somewhat surprisingly, the
characteristic scale of this influence is about an order of magnitude more than the size
of the defect itself. Such long-range influence may be explained as related to the
relatively long time necessary for full drying and complete development of the
structure. In other terms, the surface defect (pore) influences formation of a relatively
large number of cells in the course of the solution flow.
k crit ~ 3
CONCLUSIONS
Processes of self-organization in evaporated polymer films were investigated.
Polystyrene films were deposited on specially prepared polypropylene substrates that
included surface defects. It was demonstrated that surface defects exert decisive
influence on the patterns formation. Mesoscopically scaled patterns were observed in
the vicinity of the surface defects. The characteristic scale of the mesoscopically
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patterned area is about an order of magnitude more than the size of the defect. SEM
microscopy revealed that mesoscopic cells are separated with porous boundaries.
Diameters of pores, forming the boundary, were estimated as 1-2 μm. The internal
area of the mesoscopic cell is comprised of very small pores of 100–300 nm in
diameter. The mechanism of mesoscopic patterning is discussed.
ACKNOWLEDGEMENTS
The work has been supported by the Israel Ministry of Absorption. The
authors are grateful to Professor M. Zinigrad, for his continuous support of our
research activity. Oleg Gendelman is grateful to the Taub and Shalom Foundations for
financial support. We thank Mrs. Albina Musin for her help in preparing the paper.
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