SWITCHABLE REVERSIBLE WETTING OF MICROMETRICALLY
SCALED HONEYCOMB POLYARYLSULFONE RELIEFS
OBTAINED WITH BREATH-FIGURES SELF-ASSEMBLY
Edward Bormashenko*, Sagi Balter, Roman Pogreb, Doron Aurbach
*
Ariel University Center of Samaria, the Research Institute, Applied Physics Faculty,
Department of Chemical Engineering and Biotechnology, 40700, Ariel, Israel.
E-mail: edward@ariel.ac.il
Department of Chemistry, Bar-Ilan University, 52900 Ramat-Gan, Israel
E-mail: aurbach@mail.biu.ac.il
Abstract.
Organic and non-organic surfaces demonstrating switchable wetting properties
attracted significant attention of investigators during the past decade.[1-13] Change in
the wettability could be achieved by application of an electromagnetic field,
temperature, solvent, pH jump, UV irradiation or other external stimuli.[1-4,14-17]
Various techniques were applied for controlled wetting of solid substrates, including
the use of polymer brushes[8-10], introduction of photoresponsive spiropyran
molecules[11], application of noncovalently assembled multilayered films[12], surface
modification with silanes[13] and other techniques. Smart interfaces with tunable
wettability demonstrate promising applications as biosensors, microfluidic devices,
intelligent membranes and also substrates for cell engineering.[2,7,18-19] We already
reported in our previous work the possibility to tune reversibly interface properties of
aromatic polyarylsulfones by UV irradiation.[20] Our present communication
concentrates on strengthening the effect by producing a micro-scaled honeycomb
relief on the surface of polyarylsulfone films, where wetting properties are modified
by the roughness of the substrate.[5-6, 21]
We manufactured highly developed reliefs with breath-figures (water assisted)
self-assembly, allowing formation of micrometrically scaled honeycomb structures.[2238]
The micro-patterns obtained with the breath figures technique are related to
condensation of water droplets at the solution/air interface cooled as a result of
evaporation.[22-32] Traditional "breath-figures" self-assembly is based on the
evaporation of a thin layer of polymer solution deposited on the organic or nonorganic substrate.[22-32] We report first the modification of the "breath-figures"
method. We immersed 180 µm polysulfone (PSF) and 130 μm polyethersulfone
(PESF) films in the mixture of chlorinated solvents (chloroform CHCl3 (8 wt.%) and
dichloromethane CH2Cl2 (92 wt.%) as described in our previous papers.[33-37]
Polyarylsulfones are soluble in chlorinated solvents; however, the dissolving is
relatively slow (it takes several minutes for dissolving 100 µm films at room
temperature). Thus, the immersed film was surrounded by the liquid layer enriched
with the dissolved molecules of polyarylsulfone (see Scheme 1). The immersed
polyarylsulfone film was immediately pulled out from the solution under a fast dipcoating process.[33-37] 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. In our case, the substrate was the polyarylsulfone film
109
coated with a solution of the same polyarylsulfone in the mixture of chlorinated
solvents. The rapid evaporation of the solvent accompanied by water condensation
gave rise to honeycomb reliefs depicted in Figure 1. Ideal affinity of the honeycomb
relief to the substrate (both are built from the same polyarylsulfone molecules) has to
be emphasized. The pattern obtained in our experiments comprised irregularly shaped
micro-pores with an average diameter of 1µm, shown in Figure 1. It is noteworthy,
that the pores demonstrate neither long-range nor short-range order, contrasting the
well-ordered patterns reported in Ref. 22-27.
The reliefs presented in Figure 2 demonstrate reversible tunable wettability.
The initial contact angles were established as  * PSF  107±1.5° and  * PESF  96±1.5°
for the PSF and PESF reliefes respectively (see Figure 3),  * is so called apparent
contact angle. [39-40 These angles are larger than those established on the flat extruded
PSF and PESF substrates, which are  PSF  84±1° for PSF and  PESF  79±1° for
PESF. It could be recognized that inherently hydrophilic surfaces (   90 0 ) became
hydrophobic (  *  900 ), when textured. This observation means that the CassieBaxter wetting regime occurs; i.e., air is trapped by the honeycomb relief and water
droplets sit at least partially on air pockets.[21, 39-41] We already reported that the
Cassie wetting regime is typical for polymer honeycomb reliefs.[36, 41] The apparent
contact angle  * predicted by the Cassie-Baxter formula is given by Equation (1):
cos *  1   S (cos   1)
(1)
In Equation (1)  S is the fraction of the liquid/polymer interface. Treatment of the
SEM images supplied  PSF S  0.73,  PESF S  0.82. Thus, Equation (1) yields for the
calculated apparent contact angle  * PSF,calc  101° and  * PESF,calc  91°; the satisfactory
correspondence of Cassie angles with experimental ones is recognized.
~
After irradiation with a UV lamp the apparent contact angles  of the
~
~
honeycomb reliefs changed dramatically:  PSF  63°,  PESF  14° (see Figure 2). This
means that the Cassie-Wenzel wetting transition occurred.[42-46] The Cassie-Wenzel
wetting transition is well-known to be irreversible.[40, 46] However the initial
hydrophobic wetting properties could be restored by external stimuli.[20] We
established that heating of the UV-irradiated honeycomb PSF and PESF reliefs
restores their initial obtuse apparent contact angles. Several cycles of UV irradiationheating demonstrated the reversibility of the effect, illustrated with Figure 3.
We conclude that micrometrically scaled honeycomb polysulfone and
polyethersulfone films demonstrate reversibly switchable wetting when subjected to
UV irradiation/heating exposure, indicating that the Cassie-Wenzel-Cassie transition
is occurring under external stimuli. Polyarylsulfones feature excellent mechanical
properties and high thermal stability (the heat deflection temperature of PSF is 174ºC,
and of PESF is 204ºC), allowing diverse technological applications of the reported
effect.
Experimental
Extruded PSF (Thermalux) and PESF films were supplied by Westlake Plastics, Inc.
Honeycomb reliefs were manufactured as described in Scheme 1 with a fast dipcoating process under ambient temperature. The pulling speed was 43 cm/min,
humidity was 40-50%.
110
Reliefs were irradiated with a UV lamp at a wavelength of 254 nm with the intensity
of 1 mW/cm2 for one hour. The distance between the polymer film and the UV lamp
was 1.5 cm.
The restoration of wetting properties was achieved by heating at the temperature of
130°C for 60 minutes.
Droplets of 10 μL volume were deposited carefully on the freshly irradiated substrate
with a microdosing syringe. Contact angles were established by a homemade
goniometer and image-processing technique. A horizontal laser beam illuminated the
entire droplet profile and gave an enlarged image on a screen using a system of lenses.
Measurements were made on both sides of the droplet and averaged for 10 droplets.
Acknowledgments
The authors are grateful to Professor M. Zinigrad for his generous support of their
experimental activity. The authors are thankful to Mrs. Y. Bormashenko for her help
in preparing the paper.
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polyarylsulfone
substrate
evaporation
of the
solution
V
V
molecules of
polyarylsulfone
solvent
B
A
Scheme 1. Scheme of the manufacturing of honeycomb mocro-reliefs.
A
B
Figure 1. SEM images of polysulfone (A) and polyethersulfone (B) reliefs.
113
A
B
C
D
Figure 2. Water droplets deposited on the honeycomb PSF relief before (A) and after
(B) UV-irradiation; water droplets deposited on the honeycomb PESF relief
before (C) and after (D) UV-irradiation.
114
Figure 3A. Apparent contact angle vs. number of irradiation-heating cycles for PSF
films.
Figure 3B. Apparent contact angle vs. number of irradiation-heating cycles for PESF
films.
115
Cover picture
Table of Contents Text
Micrometrically scaled honeycomb polymer surfaces demonstrating reversibly
tunable wetting properties are reported. Polyarylsulfone based micro-reliefs obtained
with a modification of breath-figures method demonstrated reversible changes in their
wettability under UV-irradiation/heating cycles.
Surfaces demonstrating controlled wetting properties are important for various
technological applications, including biosensors, microfluidic devices, intelligent
membranes and cell engineering. Micro-scaled honeycomb polyarylsulfone reliefs
feature reversible change in their wetting properties when reliefs are exposed to UVirradiation/heating cycles. Micro-pores are important for constituting the CassieBaxter wetting regime and strengthening the jump in the apparent contact angle. A
novel technique allowing formation of micro-scaled patterns is reported. We suggest,
that the aforementioned arguments justify publication of our communication in Small
Journal.
116