Instability of polymer thin films under water at room temperature
Dan Liu,a* Tao Wang,a§ Michele Sferrazza,b and Joseph L. Keddiea+
a. Department of Physics and Surrey Materials Institute, University of Surrey,
Guildford, Surrey, United Kingdom GU2 7XH
b. Département de Physique, Faculté des Sciences, Université Libre de Bruxelles,
Boulevard du Triomphe, CP223, B-1050 Bruxelles, Belgium
+
Corresponding author: Joseph L. Keddie
Tel: +44 1483 686803
Fax: +44 1483 686781
e-mail: j.keddie@surrey.ac.uk
*Present address: School of Physics and Astronomy, University of Leeds, Leeds,
United Kingdom LS2 9JT
§
Present address: Department of Physics and Astronomy, University of Sheffield,
Sheffield, United Kingdom S3 7RH
1
Abstract
Thin films of viscoelastic polymers, such as polyisoprene (PI) at room temperature,
have applications in resisting protein adhesion and in protein nanopatterning. In this
work, the instability of a PI thin film on a silicon substrate at room temperature in an
aqueous environment was observed by atomic force microscopy and optical
microscopy. The dewetting proceeds via four successive stages of (1) hole nucleation
and growth, (2) hole coalescence, (3) cellular pattern formation, and (4) droplet
formation. At a given time, the dewetting pattern was found to depend on the film
thickness: for thinner films a cellular pattern is observed, whereas thicker films are in
the hole nucleation stage. As the film thickness is increased, there is a slowing down
in the rate of the PI dewetting process and an increase in the pattern size. This study
of dewetting in a liquid at room temperature differs from previous studies of thermal
dewetting in air and solvent-induced dewetting.
2
1. Introduction
Thin polymer films play an important role in modern technology. For
applications as photoresists1, photovoltaic devices2, anti-reflection coatings3, or antifouling coatings4, thin films must be stable over time. The past several years have
seen a burgeoning of activity on the dewetting of thin films of polymer melts on solid
substrates in air at temperature above the glass transition, while very little research
has been reported in any different environmental conditions. Thin film stability is, in
general, a crucial issue both from the fundamental point-of-view of phase stability in
confined systems and from the viewpoint of applications. For example, it has been
found that the adsorption of the protein bovine serum albumin is very low on surfaces
of polyisoprene (PI),5 allowing the creation of protein nanopatterns on the surface of a
microphase-separated copolymer containing PI as one block. Furthermore, the
patterning of adhesion proteins on PI-copolymer surfaces has been shown to influence
biological cell attachment and spreading.6 Patterning of biological molecules is also
important for sensor applications. For any applications that use polymers to create
patterns or to influence adsorption, the stability of the soft substrate layer is crucial.
Considering the cited case of protein nanopatterning, the stability or instability of PI
in an aqueous environment is an important issue, and it has motivated this current
investigation of PI film stability in water compared to air.
In the past several decades, there have been numerous experimental and
theoretical studies of the dewetting of polymer thin films on substrates.7 By far, the
most commonly studied system has been polystyrene (PS) on a solid substrate in
air8,9,10,11,12,13,14,15,16,17,18 at temperatures above its glass transition temperature (Tg).
Most studies of polymer dewetting have considered thermal dewetting processes in
3
which the polymer is heated to temperatures above its Tg so that it is in its rubbery
state. These studies find that the dewetting of thin films of molten polymers, such as
polystyrene, on non-wetting surfaces under thermal annealing consists of four stages:
(1) hole nucleation and growth, (2) hole coalescence, (3) cellular pattern formation,
and (4) droplet formation. In related work, solvent-induced dewetting has also
received attention.19,20,21,22 In this mechanism, a glassy polymer is plasticized through
an exposure to a solvent, and it then de-wets its substrate. Several recent studies
demonstrated that polyelectrolyte films can undergo large-scale changes in their
internal structure and surface morphology upon exposure to an aqueous media.23,24
The patterns and structures generated during this transformation follow the same four
stages seen in thermal annealing.
There have also been reports of the dewetting of glassy thin films being
induced by a non-solvent. Specifically, the dewetting of thin, glassy polystyrene films
exposed to an oversaturated water vapor at room temperature has been studied.25,26
The mechanism of nonsolvent-induced dewetting was found to differ from that of
thermal annealing and solvent-induced dewetting. It was found to proceed through a
sequence of penetration, replacement, and coalescence.
Here, we have followed the dewetting of films of PI, which is in the melt state
at room temperature, in a non-solvent (water), and to our best knowledge it constitutes
the first report of a viscous polymer film dewetting a substrate in a non-solvent at
room temperature. We are aware of no reports of the dewetting of molten (i.e. rubbery)
polymer films in non-solvents.
4
2. Experimental Details
2.1 PI thin film deposition
A 1 wt.% solution of PI (number-average molecular weight Mn = 100 kg/mol,
obtained from Sigma-Aldrich) in toluene was spin-cast at a rate of 2000 r.p.m. onto
(100) polished Si wafer with a 2.5 nm native silicon oxide layer. Si wafers were used
in their as-received state, without any surface treatment. The native layer of silicon
dioxide on the Si wafer will be referred to as “silicon” throughout this manuscript for
simplicity. The thickness of the films was determined to be ca. 50 nm by variableangle spectroscopic ellipsometry using a commercial instrument with a rotating
analyzer (J.A. Woollam Co., Inc., Lincoln, NE, USA). Ellipsometry scans were made
from 400 to 800 nm. The data analysis considered the effects of the native oxide.
After film formation, the PI films were stored in a desiccator with silica gel at room
temperature. After 35 days of storage, the film morphologies were examined by
optical microscopy.
2.2 Contact angle analysis
The relative substrate surface energies were compared through water contact
angle analysis (WCAA) using the sessile drop method (Easy Drop, Krüss GmbH,
Germany). 5 L of de-ionized (DI) water was deposited onto each sample surface.
After capturing the drop images with a camera, the angles were measured using the
commercial image analysis software. For every sample, the average value was
obtained from measurements of three drops, which were deposited at different spots
on the same surface.
5
2.3 Atomic force microscopy of thin film dewetting dynamics
PI films with thickness of ca. 50 nm on Si were incubated in DI water for
varying lengths of time (1 min, 5 min, 15 min and 20 min) at a temperature of 20 C.
After taking the samples out of the DI water and allowing them to dry, the film
morphologies were determined by an atomic force microscope (AFM). A commercial
microscope (NTEGRA, NT-MDT, Moscow, Russia) was used in the intermittentcontact mode to determine the patterns of the PI films after incubation in DI water.
Silicon cantilevers with a gold coating (NT-MDT, Moscow, Russia) were used in the
measurements. The nominal spring constant is 5 N/m and the nominal resonant
frequency is 130 KHz.
In other experiments, solutions of PI in toluene, at various concentrations,
were spin-coated onto as-received Si wafers to deposit thin films with thicknesses of
23 nm, 46 nm, 85 nm and 182 nm, as were determined by ellipsometry. All films were
incubated in DI water for 15 min. After drying, the films were examined by optical
microscopy and AFM, as already described.
3. Results and Discussion
PI has a glass transition temperature of -70 C,27 and hence it is in the melt
state at room temperature. We found that PI thin films (with a thickness of 50 nm)
were stable on Si substrates in air at room temperature. Examination of PI films that
were more than one month old, using optical microscopy and atomic force
microscopy, found no evidence for de-wetting; the surface was quite smooth, with a
roughness of less than 1 nm. The spreading coefficient, S, is defined as a function of
the interfacial energy, , of the three relevant interfaces as:
6
S = SA – SP – PA,
(1)
where the subscript SA refers to the substrate/air interface, SP refers to the
substrate/polymer interface, and PA refers to the polymer/air interface. 28 Considering
that the PI film is stable in air, we set S > 0, which is the condition for a stable film.
Taking a literature value of PA = 30.5 mN/m for PI in air,27 we then see that
SP < SA – 30.5 mN/m.
(2)
We measured the contact angle , of a sessile drop of DI water on a PI surface to be
100.5, indicating that the polymer is hydrophobic. The Young equation29 provides a
relationship between  of the water sessile drop and the interfacial energies between
the liquid (W), polymer, and air phases as

PA = WA cos + PW.
(3)
Using the measured  along with a literature value30 of WA = 72 mN/m, we use Eq. 3
to calculate PW to be 43.6 mN/m. The contact angle of water on the Si surface was
measured to be  = 30, indicating its hydrophilic nature. Using the Young equation
for water on Si in air, we predict that
SW = SA – 62.4 mN/m.
(4)
We are now in a position to estimate the spreading coefficient for a liquid-like PI drop
on a Si substrate immersed in water, which is written as
S = SW – SP – PW.
(5)
Using Eq. 4 for SW and Eq. 2 for SP, along with our calculated value for PW, we find
that a lower limit for S is –75.5 mN/m; S is most likely negative for a PI film under
7
water. It is therefore predicted by this simple argument that a PI film will dewet Si in
water, even though the film is stable in air.
3.1 Dewetting dynamics of PI thin films under water
The dewetting dynamics of PI films on Si substrates after incubation in water
were observed by atomic force microscopy. Fig. 1 presents the AFM height and phase
images of PI films (ca. 50 nm thick) dewetted after various times of immersion in
water. Height images are in the left column, and phase images are in the far right
column. In the phase images, when the AFM tip dissipates less energy and hence has
a lower phase angle, the image appears bright.
The AFM height images reveal that holes were nucleated in the PI film (Fig.
1a), and then grew in size (Fig. 1b). The film was pushed into the boundary regions
between the holes, thereby creating polygon patterns (Fig. 1c). Finally, the boundaries
broke up into a droplet pattern (Fig. 1d). This process is similar to the conventional
thermal dewetting process, such has been found for PS on Si annealed above its Tg.
The process is very fast, as there are examples of holes being large enough to make
contact with their neighbours after only 1 min.
Analysis of the cross-sectional profile obtained after 1 min. shows a 5 m
wide hole, 50 nm deep with a 23 nm high rim around it. The depth of 50 nm is
comparable to the original PI film thickness. We deduce that the PI film ruptures to
form holes that reach the Si substrate. In the figure, the rim decreases via a “trough”
on the wet side of the rim (i.e. the side of the unperturbed film). This feature was
previously observed by other authors, and it is related to the fact that a disturbance of
a Newtonian liquid (in the case the hole rim) should decay via an undulation.31
8
The AFM tip dissipates less energy when interacting with the hole region
compared with when it is scanning the PI film. Hence, the holes appear as bright spots
in the phase images.
After 5 min. of incubation, the holes expand laterally until they touch each
other and form thin strips between every neighbouring hole. At this point, the average
hole has widened to be 6.5 m wide and 60 nm deep. After 15 min. of incubation,
hole coalescence is observed (Fig. 1c). The rims of neighbouring holes overlap and
form a cylindrical ribbon. As the adjacent holes have coalesced, a characteristic
polygonal cellular pattern is created. The height of the pattern is of the order of 120
nm, which is more than twice the initial film thickness.
In the final stage, the cylindrical ribbons are unstable and break up to create
drops. A polygonal network of droplets is thereby created, as is illustrated in Fig. 1d.
AFM scans reveal that the droplets exhibit the shape of a spherical cap, with a drop
height of about 280 nm and a diameter on the order of 3 m.
9
a
5 m
23 nm
50 nm
b
23 nm 6.5 m
60 nm
c
120 nm
d
280 nm
Figure 1. Atomic force microscopy of the structures in PI films (with initial thickness
of ca. 50 nm) after being incubated in DI water for (a) 1 min., (b) 5 min., (c) 15 min.,
10
and (d) 20 min.. The left column shows the AFM height images, the middle column
shows the cross-sectional profiles of the traces marked on the height image, and the
right column shows the AFM phase images. Image sizes are 50 m ×50 m. These
processes are identified as: (a) hole nucleation and growth; (b) hole coalescence; (c)
cellular pattern formation; (d) droplet formation.
Figure 2a shows the dewetted hole structure obtained from hole nucleation and
polymer displacement (after a time of 1 min.), while a 3-D image of the surface is
presented in Figure 2b. Bright rings around the holes in the height image are clearly
observed corresponding to the accumulation of material displaced from the hole to the
surrounding rim. The height profiles of the rim are typically asymmetric, as has also
been observed in other studies. That is, on the interior side of the hole there is a
higher slope than on the outer side of the profile, where it meets the undisturbed film.
A magnified height image of area identified using a green square in Figure 2a
is shown in Figure 2c. Clearly the polymer film structure is not homogeneous on this
shorter length scale between the holes: a fine cellular structure is visible with an
average height of the structure of around 6 nm, well below the total thickness of the
film of around 50 nm. The lateral scale of the cellular structure is of the order of 0.10.3 m. Thus, we observe the coexistence of two patterns of two different lengths; the
hole growth from the silicon surface is of the order of a few m in diameter, whereas
the PI surface presents a cellular size of much shorter scale and extending only a few
nm from the film surface. This latter structure may have an origin in the melted state
of the polymer at room temperature (see section below). It was likewise observed in
films with different thicknesses.
11
a
b
c
Figure 2. AFM images show that holes are nucleated in the PI film after 1 min. in
water. (a) Height image of a large area: 50 m×50 m (b) and the corresponding 3D image. (c) Magnified height image of the 2 m×2 m area identified with a green
square in part a.
3.2 Dewetting mechanism of PI thin films
It could be expected that water has an influence on the wetting of PI because
the less polar PI could be replaced by polar water at the hydrophilic SiOx surface. The
scheme in Fig. 3 illustrates the possible PI dewetting process in a water environment.
First of all, water molecules penetrate to the silicon oxide surface, possibly through
small film defects, and nucleate small droplets. Nucleation happens at roughly the
same time at a relatively few nucleation sites. When water molecules reach the
substrate, they will replace the nonpolar PI from the substrate, because water wets
12
SiOx better than PI.28 The strong capillary force drives the formation and the growth
of holes. The water droplets grow and spread, pushing the PI in a radial direction to
the periphery. Then the impinging holes coalesce to form the polygon structure.
Finally, the polygonal PI edges decay into spherical droplets. It is noteworthy that the
water droplets increase in size and displace the PI film. The remaining polymer film,
being displaced by the water, becomes thicker.
Polyisoprene
Si wafer
Figure 3. Scheme of PI pattern formation through dewetting on silicon oxide surface
under water incubation (not drawn to scale).
Water nucleates holes that spread
laterally to push material into the thicker rim. The holes continue to spread until there
are thin ribbons at the boundaries between the holes.
In previous research26 on the dewetting of thin, glassy PS films in water, an
irregular structure appears on the PS film in the process of hole coalescence. The
process differed from the usual thermal and solvent-induced dewetting process. This
result may be explained by the PS being glassy and not in its viscous melt state under
13
the reported experimental conditions. It was concluded that the mobility of polymer
chains is an important factor in influencing the morphology of the film in the
nonsolvent-induced dewetting process. In our experiments, the PI film is viscous at
room temperature. The high chain mobility of PI molecules should make water more
able to displace the PI. Hence, the PI dewetting patterns in each of the stages follow
what has been observed for the thermal dewetting of a thin polymer film in air.13,18
3.3 Thickness dependence of the dewetting of PI thin films
A previous study of PS film dewetting under thermal annealing conditions
concluded that an increase in the film thickness decreases the number density of initial
holes, increases the polygon diameter, and increases the drop diameter.18 We
investigated whether PI films dewetting in water followed this same trend. Fig. 4
shows the PI film patterns with four different original film thicknesses incubated in DI
water for a fixed time of 15 min. The thinnest PI film (23 nm) shows a broken-up
polygonal cellular structure (Fig. 4a). A similar pattern, but with a larger characteristic
length (by approximately a factor of two), is found in a thicker film of 46 nm, as is
shown in Figure 4b. The cellular pattern seems to follow the mechanisms of spinodal
dewetting, with a characteristic length that depends on the initial thickness of the
film.32,33 However, when the thickness is further increased (85 nm), the PI exhibits
hole nucleation (Figure 4c), with diameters of holes being about 5 m. Some holes
have spread and impinged on each other to form ribbons. Finally, the thickest PI film
(182 nm) has only passed through the first step of hole formation. The hole density is
less than what was found in the thinner 85 nm film, and the holes have not yet made
contact with their neighbours. Thus, after 15 min. of incubation, the thicker films are
at an earlier stage of dewetting than are the thinner films. This experimental result
14
agrees with results from PS films dewetting under thermal annealing in air, where it
was found that thicker films dewet more slowly.18
There are clear differences, however, between the present experiments and
these previous experiments on PS.
PI dewets under water incubation at room
temperature, but PS dewets under thermal annealing in air. We propose that PI
dewetting initiates by the water molecule penetrating and nucleating, but PS
dewetting nucleates from a spinodal decomposition mechanism or from defects in
films.
15
a
b
c
d
Figure 4. PI film dewetting patterns after incubating in water for 15 minutes. Film
thicknesses are (a) 23 nm, (b) 46 nm, (c) 85 nm, and (d) 182 nm. The left column
16
shows the optical images; the scale bar is 10 m. The right column shows the AFM
height images; image sizes are 50 m x 50 m.
4. Conclusion
Whereas there are numerous studies of the dewetting of polymer films in air
(or vacuum) at elevated temperatures, there are fewer – if any – studies of nonsolvent-induced dewetting of polymer films at room temperature. Spin-coated PI films
on Si wafers were found to dewet under incubation in water. The dewetting process
consists of nucleation and growth of holes, coalescence of holes, formation of celltype structures, and the subsequent breakup of these features into droplets. This
process is very similar to the conventional thermal-dewetting process of PS films. The
mechanism is proposed to be same with the conventional PS dewetting over its Tg
except that the polar water contributes an important stimulus. Where the water
molecules penetrate to the polar silicon oxide surface and nucleate water beads. As
the water spreads, PI holes coalesce and form ribbons. It was found that the PI film
dewetting patterns are dependent on the original film thickness. Larger patterns are
found in thicker films. Increasing the film thickness slows down the PI dewetting
process.
Acknowledgements
D.L. acknowledges scholarships from the Kwan Trust (University of Surrey) and
funding from Angiotech BioCoatings. T.W. acknowledges an Overseas Research
Student scholarship from Universities UK and a University Research Scholarship
from the University of Surrey. We have benefitted from useful discussions with Dr.
17
James Sharp (University of Nottingham). We acknowledge the help of Mrs. Violeta
Doukova, who provided general laboratory assistance.
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