Inkjet Printing of Well-Defined Polymer Dots and Arrays

Inkjet Printing of Well-Defined Polymer Dots and Arrays
Berend-Jan de Gans and Ulrich S. Schubert*
Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of
Technology and Dutch Polymer Institute (DPI), P.O. Box 513,
5600 MB Eindhoven, The Netherlands
Received March 1, 2004. In Final Form: June 9, 2004
Inkjet printing represents a highly promising polymer deposition method, which is used for, for example,
the fabrication of multicolor polyLED displays and polymer-based electronics parts. The challenge is to
print well-defined polymer structures from dilute solution. We have eliminated the formation of ring stains
by printing nonvolatile acetophenone-based inks on a perfluorinated substrate using different polymers.
(De)pinning of the contact line of the printed droplet, as related to the choice of solvent, is identified as
the key factor that determines the shape of the deposit, whereas the choice of polymer is of minor importance.
Adding 10 wt % or more of acetophenone to a volatile solvent (ethyl acetate)-based polymer solution
changes the shape of the deposit from ring-like to dot-like, which may be due to the establishment of a
solvent composition gradient. Arrays of closely spaced dots have also been printed. The size of the dots
is considerably smaller than the nozzle diameter. This may prove a potential strategy for the inkjet printing
of submicrometer structures.
Introduction
Inkjet printing is considered one of the most promising
methods for controlled deposition of polymers and functional
materials, 1,2 especially in relation to the fabrication
of multicolor polyLED displays and polymer-based electronics
parts. Such devices cannot be prepared by conventional
spin coating. Instead, a microscopic patterning
technique is needed. Inkjet printing has the advantage of
simplicity, low cost, flexibility, and maturity. Outstanding
examples include inkjet printed all-polymer transistors3-5 and 130 ppi RGB polyLED displays. 6,7 However, the
potential of inkjet printing is huge and goes far beyond
what has been realized up to now. Future areas of
application may include the fabrication of sensors, polymerbased
solar cells, or microfluidics devices, all based on
polymers as matrix or functional materials. In the field
of combinatorial materials science, inkjet printed libraries
of polymer dots could bridge the gap between parallel
synthesis and property characterization.
As inkjet printing requires low viscosities (typically
1-10 mPa s), polymeric materials can only be deposited
from dilute solution. 8 However, a drying droplet usually
deposits its solute as a ring stain that marks the perimeter
of the droplet before drying. Ring formation, commonly
known as the “coffee drop effect”, is due to the combined
* Corresponding author. Fax: 0031-40-247-4186. E-mail:
u.s.schubert@tue.nl.
(1) De Gans, B. J.; Duineveld, P. C.; Schubert, U. S. Adv. Mater.
2004, 16, 203.
(2) Calvert, P. Chem. Mater. 2001, 13, 3299.
(3) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran,
M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123.
(4) Stutzmann, N.; Friend, R. H.; Sirringhaus, H. Science 2003, 299,
1881.
(5) Paul, K. E.; Wong, W. S.; Ready, S. E.; Street, R. A. Appl. Phys.
Lett. 2003, 83, 2070.
(6) Funamoto, T.; Matsueda, Y.; Yokoyama, O.; Tsuda, A.; Takeshita,
H.; Miyashita, S. Proc. 22 nd Int. Display Res. Conf., Boston 2002; Society
for Information Display: San Jose, 2002; p 899.
(7) Giraldo, A.; Duineveld, P. C.; Johnson, M. T.; Lifka, H.; Rubingh,
J. E. J. M.; Childs, M. J.; Fish, D. A.; George, D. S.; Godfrey, S. D.;
McCulloch, D. J.; Steer, W. A.; Trainor, M.; Young, N. D.; Hunter, I. M.
Proc. 7 th Asian Symp. Information Display (ASID2002), Singapore 2002;
Society for Information Display: San Jose, 2002; p 43.
(8) De Gans, B. J.; Kazancioglu, E.; Schubert, U. S. Macromol. Rapid
Commun. 2004, 25, 292.
Langmuir 2004, 20, 7789-7793
7789
action of an increased evaporation rate at the droplet edge,
and contact line pinning due to surface irregularities and
solute deposition (“self-pinning”). 9-11 A capillary-driven
flow from the droplet center toward the edge compensates
for evaporation losses and transports most of the solute
toward the contact line. All previously mentioned applications
of inkjet printing require well-defined, often
dot-like deposits. Therefore, the elimination of ring
formation is of great practical interest.
To improve the homogeneity of the deposits, mixtures
of low-boiling good solvents and high-boiling bad solvents
for the polymer under consideration were used. During
evaporation, the solvent quality decreases, and the
polymer will precipitate. 12 A disadvantage is the poor
(mechanical) properties of the deposit formed. It was
shown that when a single solvent was used, the homogeneity
increases with decreasing rate of evaporation. 13
This could be due to the time scale for depinning of the
contact line matching the time scale of evaporation.
Polyvinylcarbazole dots having a Gaussian shape were
inkjet printed, although the optical micrograph provided
clearly shows irregularities, probably due to stick-slip
(de)pinning of the contact line during evaporation. 14 ITO
was used as the substrate. The velocity of the print head
and the evaporation conditions are also known to affect
the shape of the deposit. 15
Casting experiments with macroscopic, aqueous drops
have shown that ring formation can be (partially) eliminated
by the use of a hydrophobic substrate, that is, a film
of castor oil on glass. 16 In the field of inkjet printing, use
(9) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S.
R.; Witten, T. A. Nature 1997, 389, 827.
(10) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S.
R.; Witten, T. A. Phys. Rev. E 2000, 62, 756.
(11) Deegan, R. D. Phys. Rev. E 2000, 61, 475.
(12) Lyon, P. J.; Carter, J. C.; Bright, J. C.; Cacheiro, M. WO Patent
02/069119 A1, 2002.
(13) Madigan, C. F.; Hebner, T. R.; Sturm, J. C.; Register, R. A.;
Troian, S. MRS Symp. Proc. Vol. 625; Materials Research Society:
Warrendale, 2000; p 123.
(14) Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Sturm, J. C. Appl.
Phys. Lett. 1998, 72, 519.
(15) Shimoda, T.; Morii, K.; Seki, S.; Kiguchi, H. MRS Bull. 2003,
28, 821.
(16) Takhistov, P.; Chang, H.-C. Ind. Eng. Chem. Res. 2002, 41, 6256.
10.1021/la049469o CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/30/2004

7790 Langmuir, Vol. 20, No. 18, 2004 de Gans and Schubert
Table 1. Solvent Properties at Room Temperature
liquid Tb (°C) Pv (mmHg) θc (deg) γLV (mN m -1 )
ethyl acetate 77.1 76 37.4 ( 2.2 23.39
anisole 153.7 3.51 57.2 ( 1.3 35.10
acetophenone 202 0.35 69.1 ( 1.4 39.04
was made of hydrophobic/hydrophilic patterns to control
the position of inkjet printed droplets, 1,15,17 but not yet for
influencing the shape of the deposit. In this contribution,
we will demonstrate how homogeneous deposits of polymeric
material can be obtained via use of a hydrophobic,
perfluorinated surface and a single solvent rather than
a mixture of a good and a bad solvent. In addition, we will
show that the use of a mixture of two good solvents for the
polymer allows even further control over the shape of the
deposit and that arrays of dots can be inkjet printed.
Experimental Section
Materials. Polydisperse polystyrene (N5000, Shell Nederland,
Den Haag, The Netherlands; Mn 80 kD, Mw 282 kD) was used.
Poly(methyl methacrylate) was purchased from Sigma-Aldrich
(Steinheim, Germany; Mn 130 kD, Mw 240 kD). Monodisperse
polystyrenes were purchased from Polymer Standards Service
(Mainz, Germany; Mn/Mw 17/17.5, 30/34, 60/64, 120/125, 214/
226 kD). Ethyl acetate (Biosolve, Valkenswaard, The Netherlands),
anisole (Acros Organics, Geel, Belgium), and acetophenone
(Sigma-Aldrich, Steinheim, Germany) were used as solvents, or
mixtures thereof. Solvent properties are listed in Table 1.
Solutions used for printing contained 1.0% polymer by weight.
Viscosities were measured with an Ubbelohde viscometer at
20.0 °C. Despite the high molecular weight, these solutions could
easily be printed without persistent filament formation. 8
Substrate Preparation. Glass slides (Marienfeld, Lauda-
Königshofen, Germany) were ultrasonicated in acetone for 5 min,
rubbed with sodium dodecyl sulfate solution, ultrasonicated in
sodium dodecyl sulfate solution for 5 min, flushed with demineralized
water to remove the soap, treated with 2-propanol
vapor to remove the water in a reflux setup, dried with a flow
of nitrogen, and subsequently treated in a UV-ozone photoreactor
(PR-100, UVP, Upland, CA) for 30 min to remove any remaining
organic contamination. Substrates were used immediately after
cleaning. Alternatively, glass slides were coated with (tridecafluoro-1,1,2,2-tetrahydro-octyl)trichlorosilane
(ABCR, Karlsruhe,
Germany), 18 resulting in strongly hydrophobic surfaces. Contact
angles were measured with an OCA30 device from Dataphysics
(Filderstadt, Germany). The results are listed in Table 1.
Inkjet Printer. A microdrop Autodrop device (Norderstedt,
Germany) was used, consisting of an automated XYZ-stage in
combination with a AD-K-501 micropipet print head. 19 The
printer offers a workspace of 200 × 200 × 80 mm. The positioning
accuracy of the print head is 3 µm. The diameter of the micropipet
nozzle is 70 µm.
The Autodrop is a piezoelectric inkjet printer. Droplet ejection
occurs via contraction of a piezoelectric actuator that generates
an acoustic wave. The signal that drives the actuator is a
rectangular pulse, with an amplitude of 80 V, and a width of 29
µs. The ejection frequency is 200 Hz, and the print head velocity
is5mms-1 . Variable amounts of material can be deposited by
changing the number of droplets that are printed at a spot. The
inkjet printer produces highly uniform droplets with a typical
radius error of less than 1%. Unless stated otherwise, all samples
were printed as a 2 × 10 array, with 1.00 mm spacing between
the dots. Each dot is the result of five drops that coalesce on the
surface to form one large drop.
Surface Topography. A white light confocal microscope
(Nanofocus µSurf, Duisburg, Germany) was used to determine
(17) Wang, J. Z.; Zheng, Z. H.; Li, H. W.; Huck, W. T. S.; Sirringhaus,
H. Nat. Mater. 2004, 3, 171.
(18) Trimbach, D.; Feldman, K.; Spencer, N. D.; Broer, D. J.;
Bastiaansen, C. W. M. Langmuir 2003, 19, 10957.
(19) microdrop GmbH, Norderstedt, Germany; http://
www.microdrop.de.
Figure 1. Drying pattern of an inkjet printed droplet of a
1 wt % solution of polystyrene in acetophenone on clean glass.
the surface topography. The setup uses an external 100 W xenon
cold light source and a 20× objective.
Results and Discussion
Figure 1 shows a typical drying pattern of a drop (i.e.,
consisting of 5 droplets printed on top of each other) of a
1.0 wt % solution of polystyrene (Mn 80 kD; Mw 282 kD)
in acetophenone on an uncoated, cleaned glass substrate.
The viscosity of the solution is 3.5 mPa s. Despite its high
boiling point, the droplet dries within a few seconds, due
to the high surface-to-volume ratio. The pattern consists
of a series of rings that originate from the increase in
surface energy when drying with the contact line fixed.
Eventually, the droplet will contract in a stick-slip manner,
producing the pattern. 11 Changing the solvent does not
improve homogeneity.
Figure 2a shows the deposit that is left by the same
solution of polystyrene in acetophenone on a perfluorinated
glass slide. After being dried, a regular polystyrene dot
is formed. Figure 2b shows cross-sections of the dot in the
x- and y-directions. Its diameter is 86 µm, and its height
3.0 µm. The dot is shaped as a volcano; that is, it has a
small crater on top. The experiment was repeated using
a 1.0 wt % solution of poly(methyl methacrylate) (Mn 130
kD; Mw 240 kD) in acetophenone, which had a viscosity
of 3.9 mPa s. In this case, the diameter of the dot is 75 µm,
and its height 4.1 µm; that is, the dimensions of the
polystyrene and the poly(methyl methacrylate) deposits
and their shape are similar. This implies that the dominant
factor that determines the deposition process is the solvent
and its physical properties, rather than the solute.
To check this assumption, polystyrene solutions were
printed using anisole as a solvent. Anisole (methoxybenzene)
has a boiling point and a vapor pressure that are
considerably lower and higher, respectively, than those
of acetophenone. The anisole dot is similar to the acetophenone
dot, although the crater is more prominent.
The shape of the deposit changes completely when using
a low-boiling solvent like ethyl acetate. Figure 3a shows
the deposit that is left by a drop (i.e., 5 printed droplets)
ofa1wt%solution of polystyrene in ethyl acetate. This
solution has a viscosity of 0.68 mPa s. A ring stain that
marks the perimeter of the original drop is formed. The
cross-section displayed in Figure 3b shows that indeed
most of the polymeric material is present at the ring rather
than at the center. In principle, this effect can be used to

Inkjet Printing of Well-Defined Polymer Dots Langmuir, Vol. 20, No. 18, 2004 7791
Figure 2. (a) Volcano-shaped polymer dot, formed by an inkjet
printed droplet ofa1wt%solution of polystyrene in acetophenone
on perfluorinated glass; (b) cross-sections in the x- and
y-directions.
Figure 3. (a) Polymer dot, formed by a droplet of a1wt%
solution of polystyrene in ethyl acetate on perfluorinated glass;
(b) cross-sections in the x- and y-directions.
print thin rings or lines with a resolution better than the
limit that is set by the nozzle diameter. 20
Drop-casting experiments showed that on the perfluorinated
surface the contact line of a droplet of this solution
of polystyrene in ethyl acetate is pinned, whereas the
contact lines of acetophenone and anisole solution droplets
are at least initially unpinned. This may be due to self-
(20) Cuk, T.; Troian, S. M.; Hong, C. M.; Wagner, S. Appl. Phys. Lett.
2000, 77, 2063.
Figure 4. (a) Polymer dot, formed by a droplet of a1wt%
solution of polystyrene in an 80/20 wt % ethyl acetate/
acetophenone mixture on perfluorinated glass; (b) cross-sections
in the x- and y-directions.
pinning of the ethyl acetate droplet: The vapor pressure
Pv of ethyl acetate, to which the rate of evaporation is
proportional, is 1 order of magnitude larger than the vapor
pressure of anisole, which is again 1 order of magnitude
larger than the vapor pressure of acetophenone (see Table
1). A ring-like deposit that pins the contact line is therefore
more easily formed. In addition, the contact angle θc of
ethyl acetate is smaller than the contact angle of anisole,
which is again smaller than the contact angle of acetophenone.
Near the contact line, the evaporation flux
J is proportional to (R-r) -λ , with λ ) (π - 2θc)/(2π - 2θc),
where r is the distance from the edge toward the center
of the droplet and R is its radius. λ increases with
decreasing θc, and therefore the flux. 9 Viscosity effects
cannot explain the observed behavior. The viscosity of the
acetophenone solution is considerably higher than the
viscosity of the ethyl acetate-based solution. A large
acetophenone dot as compared to ethyl acetate is therefore
expected, but the opposite is found.
The effect of mixing the two solvents is particularly
interesting. A typical result, obtained with a mixture of
80% ethyl acetate and 20% acetophenone by weight, is
shown in Figure 4a. Figure 4b displays the corresponding
cross-sections. Again, a regular polystyrene dot is formed,
with a height of 6.2 µm, that is, twice as high as the pure
acetophenone dot, and with a diameter of 68 µm. Assuming
a cylindrical shape, it follows that the total amount of
deposited material must be equal in both cases, as it
should, as both concentration and number of deposited
droplets are equal in both cases. A striking difference
between the pure acetophenone deposit and the deposit
left by the mixture is the absence of a crater on top in the
case of the latter. Perhaps most surprising is the fact that
a small amount of acetophenone already suffices to change
the shape of the deposit from ring-like to dot-like, that is,
to depin the contact line. Dots, identical to that shown in
Figure 4a, were already obtained with mixtures containing
10% acetophenone by weight.

7792 Langmuir, Vol. 20, No. 18, 2004 de Gans and Schubert
Figure 5. Dot height as a function of polymer molecular weight,
using monodisperse polystyrene standards. The inset shows
the droplet mass (0) and the specific viscosity (2) as a function
of molecular weight. A power law was fitted to the viscosity
data, the exponent of which is 0.77.
To understand the origin of this phenomenon, the
contact angle of the 80/20% w/w ethyl acetate/acetophenone
solution mixture on the perfluorinated surface was
measured. It was found that the (initial) contact angle θc
(t ) 0) is 41.5 ( 1.0°, which is very close to the value of
pure ethyl acetate, that is, 37.4°. This proves that the
results cannot be explained by a change in contact angle.
We then realized that the process of droplet evaporation
and solute deposition depends on the composition of the
droplet at the contact line rather than the overall
composition of the droplet. When using a mixture of a
low- and a high-boiling solvent, the composition at the
contact line will shift toward a higher fraction of highboiling
solvent than in the bulk, due to the increased rate
of evaporation at the edge. Therefore, the rate of evaporation
at the contact line decreases, and a surface tension
gradient is established. A flow will be induced from regions
with low to regions with high surface tension when the
Marangoni number M ) ∆γL/ηD is sufficiently large. 21
Here, ∆γ denotes the surface tension difference, L is the
length scale involved, η is the viscosity, and D is the
diffusion coefficient. Taking for ∆γ the difference between
the bulk values of ethyl acetate and acetophenone, and
using typical values for L, η, and D, it follows that M is
of the order 10 6 . This result shows that even a very small
concentration gradient will be sufficient to cause a
Marangoni flow, which will homogenize the liquid droplet
and reduce the concentration gradient between the contact
line and the bulk. 16
To gain insight into the process of dot formation, we
studied the height of the dots as a function of molecular
weight of the dissolved polymer, using solutions containing
1.0% monodisperse polystyrene by weight in acetophenone.
One single substrate was used on which all solutions
were printed as 1 × 10 arrays. In addition, the reduced
viscosities of the polymer solutions were measured, and
their contact angles on the fluorinated substrate were
used. The mass of the droplets as generated by the inkjet
printer was measured as follows: The total amount of
printed material was collected during a certain period of
time, weighed, and divided by the number of droplets.
The results are shown in Figure 5. It should be emphasized
that, although the errors in the dot height are generally
small (1-3.5%), the errors between different surfaces are
much larger (up to 30%). As for the three highest molecular
weight samples, the dot height shows very little variation
(21) Pesach, D.; Marmur, A. Langmuir 1987, 3, 519.
Figure 6. (a) Array of polymer dots printed at a mutual distance
of 150 µm, formed by droplets ofa1wt%solution of polystyrene
in acetophenone. The typical size of the dots shown here is
29 ( 2 µm, which is smaller than the nozzle diameter; (b) crosssection
in the x-direction, corresponding to the line in
Figure 6a.
with molecular weight. However, a jump in dot height
occurs between 34 and 64 kD. The inset of Figure 5 shows
that the mass of the droplets decreases with molecular
weight. The specific viscosity, also shown in the inset,
increases with molecular weight. A power law, fitted to
these data, yields [η]c ∝ Mw 0.77 , in perfect agreement with
the prediction of Zimm for polymer chains in a good
solvent. 22 Finally, it was found that within the experimental
error the presence and the molecular weight of
the polymer do not affect the contact angle. It is expected
that dot height decreases with viscosity as the dot diameter
increases. Furthermore, it is expected that the dot height
scales linearly with droplet size, that is, with the cube
root of the droplet mass, which also decreases with
molecular weight. However, the reason for the discontinuous
decrease of droplet height with molecular weight
remains unclear.
For most applications, the formation of dot arrays will
be of interest. Figure 6a shows a dot array that was
obtained by printing a rectangular array of single,
acetophenone-based droplets at a mutual distance of 150
µm on a perfluorinated surface. Figure 6b displays a
corresponding cross-section. Here, the average dot diameter
is 29 µm, and no craters can be observed. This could
be due to different evaporation conditions when printing
an array of closely spaced droplets. Similar results were
obtained by Shimoda et al., who studied shape differences
of polyLED dots: Crater formation decreased with increasing
velocities. 15 The effect was attributed to partial
drying before the droplet hits the surface. However,
Duineveld et al. have shown that droplet mass reduction
during flight is negligible, assuming typical values for
traveling distance, droplet size, and solvent vapor pressure.
23 Both our and Shimoda’s results can be explained
(22) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford
University Press: Oxford, 1986; p 144.

Inkjet Printing of Well-Defined Polymer Dots Langmuir, Vol. 20, No. 18, 2004 7793
assuming a correlation between rate of evaporation and
dot shape: Slow evaporation occurs when the time for
printing is small as compared to drying times (i.e., at high
printing velocities), as the droplets evaporate in an
atmosphere with a high partial solvent pressure, due to
evaporation of neighboring droplets. This prevents crater
formation.
Minimum spacing at which dots can be printed without
coalescence of the droplets on the surface is at present
about 125 µm. It is seen from Figure 6a that the deposition
accuracy is far worse than its theoretical limit, which is
set by the positioning accuracy of the print head, that is,
3 µm. This is probably due to random motion of the droplets
during evaporation. Confinement of the droplets, either
mechanically or by surface energy patterning, will be
necessary to produce regular arrays. Finally, it is interesting
to note that, by using dilute solutions, structures
can be created of size smaller than the diameter of the
nozzle (29 vs 70 µm). This may prove to be a potential
strategy for the inkjet printing of defined submicrometer
structures.
Conclusions
We have inkjet printed well-defined (arrays of) polymer
dots on a hydrophobic, perfluorinated substrate using a
nonvolatile solvent such as acetophenone. Different
polymers yield qualitatively identical deposits, suggesting
that the choice of the solvent may be the key factor
determining the deposition process. When a volatile
solvent such as ethyl acetate is used, ring-like deposits
are formed. Drop-casting experiments show that the
(23) Duineveld, P. C.; De Kok, M. M.; Buechel, M.; Sempel, A. H.;
Mutsaers, K. A. H.; Van de Weijer, P.; Camps, I. G. J.; Van den Biggelaar,
T. J. M.; Rubingh, J.-E. J. M.; Haskal, E. I. Proc. SPIE Vol. 4464; SPIE:
Bellingham, 2002; p 59.
contact line of acetophenone droplets on the surface used
is unpinned, whereas the contact line of ethyl acetate
droplets is not, which may explain the results. Mixing a
small amount of acetophenone, that is, 10 wt % or more,
with ethyl acetate changes the shape of the deposit from
ring-like to dot-like, probably due to a local increase of the
acetophenone concentration at the contact line. Investigation
of the dot height as a function of molecular weight
using monodisperse polystyrene samples showed a discontinuous
decrease with molecular weight. Simultaneously,
the mass of the ejected droplets decreases with
molecular weight, whereas the solution viscosity increases
and the contact angle of the droplets remains constant.
Thus, none of these three factors provides an explanation
for the observed behavior. Finally, arrays of dots were
printed. The size of the dots that are printed is considerably
smaller than the nozzle diameter. This may represent a
potential strategy for the inkjet printing of submicrometer
structures.
Acknowledgment. We thank Carlos Sanchez for his
help in preparing perfluorinated substrates, Caroline
Abeln for the GPC measurements, Prof. Bert de With
(Laboratory of Coatings Technology, TU/e) for the use of
the confocal scanning microscope, Niek Lousberg and
Alper Tiftikci for sharing their expertise, and Wilhelm
Meyer from Microdrop for fruitful collaboration. This work
is part of the Dutch Polymer Institute (DPI) research
program (Project 400).
Supporting Information Available: Scanning confocal
micrograph of poly(methyl methacrylate) dot and corresponding
cross-sections. Scanning confocal micrograph and corresponding
cross-sections of polystyrene dot, printed from anisole solution.
This material is available free of charge via the Internet at
http://pubs.acs.org.
LA049469O