Alberda van Ekenstein, Gert

Ikkala, O

Polushkin, E

Poluslikin, E.

ten Brinke, G

van Ekenstein, GA

van Zoelen, W

University of Groningen Research Database

  
 University of Groningen
Nanorod engineering by reinforcing hexagonally self-assembled PS-b-P4VP(DDP) with PPE
van Zoelen, W; van Ekenstein, GA; Polushkin, Evgeny; Ikkala, O; ten Brinke, G; Alberda van
Ekenstein, Gert; Poluslikin, E.
Published in:
Soft Matter
DOI:
10.1039/b508120c
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2005
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Citation for published version (APA):
van Zoelen, W., van Ekenstein, G. A., Polushkin, E., Ikkala, O., ten Brinke, G., Alberda van Ekenstein, G.,
& Poluslikin, E. (2005). Nanorod engineering by reinforcing hexagonally self-assembled PS-b-P4VP(DDP)
with PPE. Soft Matter, 1(4), 280-283. DOI: 10.1039/b508120c
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Nanorod engineering by reinforcing hexagonally self-assembled
PS–b–P4VP(DDP) with PPE
Wendy van Zoelen,a Gert Alberda van Ekenstein,a Evgeny Polushkin,a Olli Ikkala*b and Gerrit ten Brinke*a
Received 9th June 2005, Accepted 13th July 2005
First published as an Advance Article on the web 17th August 2005
DOI: 10.1039/b508120c
Nanorods consisting of a polystyrene core and a poly(4-vinylpyridine) shell produced via the
self-assembly route of comb-shaped supramolecules exhibit very poor mechanical properties.
Adding a sufficient amount of poly(2,6-dimethyl-1,4-diphenyl oxide) introduces entanglements to
the PS-core resulting in nanorods with much better properties, which can be used as templates for
e.g. transition metal oxide tubes.
Introduction
Self-assembling block copolymers are of great interest as
building blocks for various nanotechnology applications.1–5
Simple examples include nanoporous membranes6,7 and
nanorods.8 If block copolymers are combined with supra-
molecular concepts to form e.g. comb-shaped supramolecules,
additional possibilities arise and the procedures to form e.g.
nanoporous membranes and nanorods are in some respects
even simpler.9–11 Typically, the nanorods synthesised via the
comb-shaped supramolecules route consist of diblock copoly-
mers, with one block forming the core and the other forming
the corona. The characteristic element of the supramolecular
route, setting it apart from nanorods prepared via the tradi-
tional pure block copolymer approach, is the fact that for a
given diameter of the rods, the corona may be considerably
thinner than for pure block-copolymer systems. No longer
does the core block have to be the minority block as becomes
clear from the following example.
To prepare comb-shaped supramolecules, a block copoly-
mer of polystyrene (PS) and poly-4-vinylpyridine (P4VP),
PS–b–P4VP, is hydrogen bonded with alkylphenols with one
alkylphenol molecule per pyridine group. The block lengths of
the block copolymer are selected in such a way that the self-
assembly gives rise to hexagonally ordered PS cylinders in a
P4VP(alkylphenol) matrix. To arrive at this structure the
volume fraction of the P4VP complex has to be of the order of
0.7–0.8. Since, for the alkylphenols used, e.g. dodecylphenol
(DDP) or pentadecylphenol (PDP), the alkylphenol moiety
corresponds to 70–75 w/w% of the complex, this can even be
achieved if the P4VP block has a considerably lower molar
mass than the PS block. From the hexagonally self-assembled
structure it is possible to produce nanorods with a polystyrene
core and a poly(4-vinylpyridine) corona by simply removing
the alkylphenol molecules by dialysis with ethanol. The
nanorods produced in this way generally have a length not
exceeding 1 mm. Substantially longer nanorods, in the order of
10 mm or more, may be obtained if the cylindrical structure is
first aligned by large amplitude oscillatory shear.12
The use of polymeric nanorods as templates for the
production of polymer, metal, and hybrid nanotubes was
discussed by Greiner and co-workers.13 They coined the phrase
‘‘TUFT’’ (tubes by fiber templates) for the concept of coating
degradable template polymer fibers with the desired wall
materials using various deposition techniques. Nanotubes
are subsequently obtained by removal of the core material.
In our case, the P4VP corona makes the core–shell nanorods
potentially very interesting as templates for e.g. transition
metal oxide nanotubes.14 Such applications require the
nanorods to possess sufficient mechanic properties to allow
handling and manipulation. However, due to the specific
orientation of the copolymer blocks away from the interface,
the above procedure in general leads to a core–shell structure
without entanglements between the molecules. Hence, very
poor mechanical properties are expected. It is the objective of
this study to show that this is indeed the case and to introduce
a simple procedure to remedy this drawback.
Experimental
Polishing of alumina membranes
As substrates for the AFM measurements, alumina (aluminum
oxide) ultrafiltration membranes (Whatman Anodisc, 200 nm
pores) were used. Before use, the membranes were carefully
polished for 70 min on a Kent polishing machine. The slowest
rotation speed was used with a soft polishing pad (Kemet Int.
Limited, MBL, 150 mm) and Buehler Masterpolish1 2 polishing
medium as the polishing slurry. The membranes were attached
to the wafer carrier with beeswax as the carrier film.
Nanorod preparation
A diblock copolymer of polystyrene and poly(4-vinyl pyridine)
(PS–b–P4VP) was used (Mn (PS) 5 21 400, Mn (P4VP) 5
20 700 and Mw/Mn5 1.13, Polymer Source, Inc.). To reinforce
the nanorods, homopolymer poly(2,6-dimethyl-1,4-diphenyl
oxide) (PPE) was used (Mw5 25 700 g mol
21, Mw/Mn5 1.37),
also acquired from Polymer Source, Inc. The polymers were
aLaboratory of Polymer Chemistry, Materials Science Centre,
University of Groningen, Nijenborgh 4, 9747AG, Groningen, The
Netherlands. E-mail: g.ten.brinke@rug.nl
bDepartment of Engineering Physics and Mathematics and Center for
New Materials, Helsinki University of Technology, P.O. Box 2200,
FIN-02015, HUT Espoo, Finland. E-mail: olli.ikkala@hut.fi
PAPER www.rsc.org/softmatter | Soft Matter
280 | Soft Matter, 2005, 1, 280–283 This journal is  The Royal Society of Chemistry 2005
used without further purification. 4-Dodecylphenol (DDP)
was purchased from Aldrich as a mixture of isomers and was
used as received.
To obtain the comb-shaped PS–b–P4VP(DDP) supramole-
cules, about 0.2 g of PS–b–P4VP was hydrogen bonded with a
stoichiometric (with respect to the number of pyridine groups)
amount of DDP. The samples were prepared by mixing PS–b–
P4VP and the DDP in analysis grade chloroform, keeping the
concentration below 2 wt% to ensure homogeneous complex
formation. After stirring for 2 to 3 h, the solution was poured
into a petri dish and the solvent was allowed to evaporate
slowly overnight. After this, the sample was further dried in a
vacuum oven at 40 uC for at least 48 h. For the reinforced
polymers, different amounts of PPE were added to the
mixture. All samples used are listed in Table 1, and on the
basis of the comb weight fraction all fall within the cylindrical
morphology range as further verified with SAXS. Subscripts
denote the weight fraction of PPE in the PS domains of the
self-assembled PS–b–P4VP(DDP) supramolecules.
Shearing was carried out with an AR 1000N rheometer (TA
Instruments) in oscillatory mode with a cone–plate geometry
(4u cone, 20 mm diameter). The oscillatory shear was per-
formed with constant shear frequency of 1 Hz and strain
amplitude of 50%. The samples were sheared for 2 h at 130 uC.
After the alignment procedure part of the sample was placed
in a dialysis tube of 29 mm diameter (SERVAPOR, cutoff
M 5 12 000, Serva) filled with ethanol and dialyzed against
ethanol for about 2 weeks. After 1 week, the solvent was
replaced and after 2 weeks, the nanorod suspension in ethanol
was recovered from the dialysis tube.
AFM measurements
For the AFM measurements a small drop of a nanorod
suspension was cast on a piece of polished ultrafiltration
membrane and the ethanol was allowed to evaporate for
several minutes before measuring. Tapping mode AFM
measurements were carried out on a Digital Instruments
MultiMode2 AFM equipped with a Nanoscope IIIa con-
troller. Etched silicon cantilevers (Veeco, model TESP)
were used.
SAXS measurements
SAXS measurements were carried out at room temperature
using a NanoStar camera (Bruker and Anton Paar). A ceramic
fine-focus X-ray tube, powered with a Kristallflex K760
generator at 35 kV and 40 mA, has been used in point focus
mode. The primary X-ray flux is collimated using cross-
coupled Go¨bel mirrors and a pinhole of 0.1 mm in diameter
providing a Cu Ka radiation beam with a full width at half-
maximum of about 0.2 mm at the sample position. The
sample-detector distance was 1.04 m. The scattering intensity
was registered by a Hi-Star position-sensitive area detector
(Siemens AXS) in the q range of 0.1–2.0 nm21. The scattering
vector q is defined as q 5 (4p/l) sin (h/2), where l 5 0.1542 nm
and h is the scattering angle. The measuring time for most
samples was 1 h.
Results and discussion
The following cartoon (Fig. 1) illustrates the procedure to
produce core–shell nanorods via the comb-shaped supramole-
cules route using suitable PS–b–P4VP diblock copolymers
together with pentadecylphenol (PDP). Apart from the
hexagonally ordered PS-cylinders, the P4VP(PDP) matrix is
further self-assembled in a lamellar morphology below ca.
60 uC.15 The shear itself takes place at elevated temperatures
(e.g. 120 uC) where the P4VP(PDP) matrix is still in a
disordered state. In the case of dodecylphenol (DDP) the alkyl
tail is too small to give rise to self-assembly of the P4VP(DDP)
matrix even at temperatures as low as room temperature.
In the present study we focus on DDP but similar experi-
ments have been performed using PDP. The PS–b–P4VP
diblock copolymer used in our experiments has a molar mass
of Mn (PS)5 21 400, Mn (P4VP)5 20 700 and Mw/Mn5 1.13.
For bulk polystyrene, the molar mass between entanglements
equals Me (PS) 5 19 100.
16 However, despite the fact that the
PS-block has a slightly larger molar mass, due to specific
Table 1 Systems investigated
Sample Comb weight fraction
PS–b–P4VP(DDP) 0.772
PS–b–P4VP(DDP)/PPE0.09 0.757
PS–b–P4VP(DDP)/PPE0.17 0.743
PS–b–P4VP(DDP)/PPE0.23 0.733
PS–b–P4VP(DDP)/PPE0.28 0.726
Fig. 1 Nanorod preparation from hexagonally self-assembled PS–b–P4VP(PDP).9,12 The same principle applies in the case of DDP except that the
lamellar morphology of the matrix is not present.
This journal is  The Royal Society of Chemistry 2005 Soft Matter, 2005, 1, 280–283 | 281
orientation of the copolymer blocks away from the interface,
no entanglements between the molecules should be expected.
This results in very poor mechanical properties as can be easily
demonstrated in the following way. First, the nanorods were
prepared according to the procedure described above. SAXS
of a sheared PS–b–P4VP(DDP) sample taken with the X-ray
beam parallel to the shear direction, demonstrated a good
hexagonal alignment of self-assembled PS-cylinders. The first
order peaks are at q* 5 0.221 nm21 and the PS–b–P4VP
nanorods obtained after removing the DDP are estimated to
have a diameter of d 5 23 nm. A droplet of the PS–b–P4VP
nanorods dispersed in ethanol was put on a nanoporous
alumina membrane with Fig. 2 showing a characteristic AFM
picture of the outcome. Invariably, those parts of the rods on
top of the pores simply disappeared inside the pores.
Scanning electron microscopy confirmed the absence of
rods bridging the pores. Hence, it is not due to the AFM
tip breaking the nanorods, but rather the result of the
capillary forces arising during the evaporation of the ethanol,
illustrated in Fig. 3.17
In order to improve the mechanical properties, homo-
polymer poly(2,6-dimethyl-1,4-diphenyl oxide), PPE, with
molar mass Mw 5 25 700 g mol
21, was added to the system.
PPE is well known for its excellent thermodynamic miscibility
with polystyrene.18 Therefore, when combined with PS–b–
P4VP(DDP), PPE will segregate into the PS-cylinders even if
its molar mass is larger than that of the PS-block. Since the
molar mass between entanglements Me of pure PPE is only
4300,16 the PPE molecules, being diluted by the PS blocks, will
form entanglements above a critical concentration. To what
extent the PPE mixes with the PS blocks protruding from the
interface is not known yet. At any rate, the presence of the PS
blocks will lead to a larger molar mass between entanglements
compared to pure PPE. If we assume homogeneous mixing,
an upperbound for the molar mass between entanglements
can be simply estimated using Me(x) $ Me(x5 1.0)/x, where x
is the weight fraction PPE in the core.16 Since a PPE with
Mw 5 25.700 g mol
21 was used, this relation implies that a
weight fraction of x $ 0.17 is sufficient to introduce
entanglements. Experiments using x 5 0.28, 0.23, 0.17 and
0.09 were performed to test this prediction. In all four cases,
the presence of the hexagonally self-assembled morphology
was confirmed by SAXS. For the highest amount of PPE
used, the first order peaks were at q* 5 0.192 nm21 and after
removing the DDP, the rods are estimated to have a diameter
of d 5 28 nm, which is about 5 nm more than in the absence of
PPE. AFM experiments (Fig. 4) showed that the reinforcement
effect of the added PPE indeed resulted in rods spanning the
pores for the highest amount of PPE used, i.e. when the weight
fraction of PPE was 0.28. For smaller amounts the rods
essentially behaved as those without PPE (cf. Fig. 2), being
apparently still too weak to span pores of ca. 200 nm in size.
The effect is not simply due to the larger diameter of the
nanorods. Using homopolymer polystyrene (Mn 5 20 800,
Mw/Mn 5 1.07) instead of PPE, nanorods with similar
diameter were prepared, which, however, were too weak to
span the pores.
In conclusion, we have demonstrated that the mechanical
properties of core–shell nanorods, obtained via self-assembly
of comb-shaped supramolecules, were initially quite poor. To
improve the properties, homopolymer PPE was added and
nanorods with a PS/PPE-core and P4VP-corona were obtained
that, for the highest amount of PPE used, withstood the
capillary forces when a droplet of a nanorod ethanol
suspension was put on the alumina ultrafiltration membranes.
This PPE-reinforcement may have important implications
for the use of these kind of materials, e.g. as templates for
oxidic nanotubes.
Acknowledgements
Prof. Ulli Steiner is acknowledged for useful discussions
during the course of this work. Henk Bron is acknowledged
for his assistance in the polishing of the alumina ultrafiltration
membranes.
Fig. 2 AFM of PS–b–P4VP nanorods on a polished alumina
ultrafiltration membrane. Cartoon presents PS–b–P4VP nanorod cross
section with PS core and P4VP corona.
Fig. 3 Illustration of capillary forces working on the PS–b–P4VP
nanorods during the evaporation of ethanol.
Fig. 4 AFM pictures of suspended PS/PPE–b–P4VP nanorods on a
polished alumina ultrafiltration membrane. The rods are deflected
about 40 nm in the middle of the pore probably due to the force
exerted by the AFM tip. Cartoon illustrates the presence of additional
homopolymer (PPE) in the core of the PS–b–P4VP nanorod.
282 | Soft Matter, 2005, 1, 280–283 This journal is  The Royal Society of Chemistry 2005
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Nanorod engineering by reinforcing hexagonally self-assembled PS-b-P4VP(DDP) with PPE

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Nanorod engineering by reinforcing hexagonally self-assembled PS-b-P4VP(DDP) with PPE

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