Nanostructured polymers with embedded self-assembled reactive gel networks

Article abstract of DOI:10.1039/b809077g

Generating polymers in the presence of a self-assembling gelator with terminal double bonds yields polymeric materials with embedded reactive nano-skeletons-subsequent washing gives nanoscale imprinted materials with fibrillar architectures. The Royal Society of Chemistry.

Full text of DOI:10.1039/b809077g

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Nanostructured polymers with embedded self-assembled reactive gel
networksw
Jamie R. Moffat,a Gordon J. Seeley,b Jeff T. Carter,b Andrew Burgessb and
David K. Smith*a
Received (in Cambridge, UK) 28th May 2008, Accepted 20th June 2008
First published as an Advance Article on the web 1st August 2008
DOI: 10.1039/b809077g
Generating polymers in the presence of a self-assembling gelator
with terminal double bonds yields polymeric materials with
embedded reactive nano-skeletons—subsequent washing gives
nanoscale imprinted materials with fibrillar architectures.
It seems increasingly likely that nanochemistry will underpin
manufacturing methods of the 21st century, with the self-
assembly of molecular-scale building blocks providing a
simple method of generating nanoscale objects.1 Gel-phase
soft materials assembled from low molecular weight building
blocks provide an excellent example of the bottom-up assem-
bly of nanomaterials.2 However, these materials are generally
Fig. 1 Gelator 1.
relatively weak in terms of their macroscopic rheological
properties, being held together by non-covalent interactions.
testing proves the embedded nano-scaffold has a profound
effect on materials’ behaviour. The embedded network can be
A number of attempts have been made to ‘capture’ self-
washed out to yield ‘nano-imprinted’ materials.
Gelator 1 (Fig. 1) was synthesised using a combination of
assembled nanostructures. Gel-phase organic nanostructures
have been transcribed into inorganic materials such as silica,3
our lysine based methodology10 and chemistry developed by
cadmium sulfide4 and zinc oxide.5 Polymerisable groups have
been incorporated into gelators, and the gel fibres poly-
Wendland and Zimmerman11 (see ESIw). This gelator should
merised.6 Alternatively monomeric solvents which can sub-
give rise to reactive alkene-functionalised gel fibres within the
nanostructured polymer. Compound 1 was demonstrated to
sequently be polymerised (e.g. styrene or methyl methacrylate)
form effective gels in styrene–divinylbenzene (DVB) (9 : 1)
have also been gelated, with polymerisation capturing an
using a simple tube inversion methodology.12 Using this
embedded nanoscale gel-phase network (Scheme 1).7 Stupp
approach, it was possible to assess the thermal stability of
and co-workers reported changes in the materials properties of
nanostructured polymers.8 An alternative approach to
the gel. Fig. 2 illustrates the effect of concentration on the
gel–sol phase boundary. At low concentrations, relatively poor
gelator-modified polymers assembles gel fibres within a
pre-formed polymer.9 Embedded self-assembled structures
gels were formed, but above 20 mM gelation was more
can subsequently be ‘washed out’ to yield nano-imprints,7 a
effective.
In order to assess the nanostructure of the self-assembled
general approach with great untapped potential.
gel, a sample was allowed to dry, and was imaged using field
In this communication, we introduce the concept of incor-
emission gun scanning electron microscopy (FEGSEM). As
porating reactive nanoscale self-assemblies within polymeric
materials. Gelator 1, assembles nanoscale morphologies with
reactive peripheral alkene groups. Preliminary mechanical
Scheme 1 Use of self-assembling low molecular weight gelators to
nanofabricate polymers.
a Department of Chemistry, University of York, Heslington, York, UK
YO10 5DD. E-mail: dks3@york.ac.uk; Fax: +44 (0)1904 432516
b ICI Technology, PO Box 90, Wilton Centre, Middlesbrough,
Cleveland, UK TS90 8JE
w Electronic supplementary information (ESI) available: Characteri-
sation data and synthetic method for 1. See DOI: 10.1039/b809077g
Fig. 2 Thermal stability of gelator 1 in styrene–divinylbenzene (9 : 1).
Chem. Commun., 2008, 4601–4603 | 4601
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Fig. 4 TEM image of cross section of the poly(styrene–DVB) wafer
created in the presence of gelator 1 (20 mM). Reactive staining agent:
OsO4. Scale bar: 200 nm.
Fig. 3 FEGSEM image of gelator 1 (scale bar = 200 nm). Gel
formed in styrene–DVB (9 : 1) at a concentration of 20 mM, and then
allowed to dry under ambient conditions.
expected, the gel had a fibrillar morphology (Fig. 3)—indeed
fibres with very small diameters (o10 nm) were observed.
Polymerisation of the styrene–DVB solvent in this gel was
then achieved using UV initiation, at wavelengths 4300 nm,
with 4,40-bis (dimethylamino)benzophenone as sensitizer. It
should be noted that it was possible that the double bonds in
gelator 1 could also have reacted/polymerised under these
conditions, however, this was not the case (evidence below).
The polymerisation was performed in a flat glass mould under
nitrogen to produce polymer wafers with dimensions of ca.
5 cm ꢁ 5 cm ꢁ 200 mm. The mould was water-cooled in order
to prevent UV-induced heating effects from destroying the
self-assembled gel-phase network during polymerisation. In
the absence of gelator, the polymerisation of styrene–DVB
(9 : 1) yielded a polymer standard—a homogeneous material
which gave featureless electron microscopy images.
should selectively react with the alkene groups on the self-
assembled gelator (assuming that the double bonds in gelator
1 remained unreacted). Fig. 4 shows the imaging of the
nanoscale fibres embedded within the polystyrene–DVB sam-
ple. Extended fibres can be observed, whilst the dark spots
probably represent fibres viewed in cross-section after fractur-
ing the polymer. This image is perhaps the clearest visualisa-
tion of self-assembled nanofibres embedded within a polymer
and demonstrates that many of the alkene groups on gelator 1
maintain their reactivity.
We then performed preliminary dynamic mechanical ther-
mal analysis (DMTA) on the standard polymer (Fig. 5 left),
and the polymer formed in the presence of gelator 1 (Fig. 5
right). In the presence of the fibrillar network, the Tg value
(glass transition temperature) of the polymer increased from
ca. 80 1C to 100 1C. Furthermore, the degree of damping at the
Tg value, as given by tan d, decreased from 0.285 to 0.235. The
storage modulus (G0) of the standard polymer at 20 1C was
4.0 ꢁ 109 Pa, whilst in the presence of embedded gelator 1, G0
increased by almost an order of magnitude to 1.95 ꢁ 1010 Pa.
Therefore, only 3% wt/vol of gelator induced significant
modification of the materials behaviour. We propose this is
a consequence of the presence of the embedded nanostruc-
tured network, which can dissipate the stress exerted on the
polymer. This is in analogy with the effects of other polymer
additives used for toughening, such as rubber particles used in
Variable depth infrared spectroscopy studies on the polymer
wafer formed in the presence of gelator 1 indicated that the
gelator was distributed evenly through the polymer, with
bands associated with the gelator (e.g. N–H, CQO etc.) being
observed throughout the material (data not shown). By
FEGSEM imaging, however, the polymer wafer appeared to
be relatively featureless and homogeneous—fibrillar assem-
blies were not observed. We therefore used transmission
electron microscopy (TEM) in an attempt to image the
nanoscale fibres. We employed a reactive stain (OsO4), which
Fig. 5 DMTA data for (left) standard polymer, (right) polymer formed in the presence of gelator 1 (20 mM). Modulus is the storage modulus, G0.
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Simple solvent treatment yields ‘washed-out’ nanoimprints.
Importantly, the gelator has been endowed with reactive
double bonds, the reactivity of the assembled nanostructures
has been demonstrated using OsO4 as reagent and TEM for
imaging. This approach holds out the prospect of using self-
assembly to fabricate functional materials with embedded
reactive nano-scaffolding, an approach which could give rise
to a new generation of advanced functional materials.
Notes and references
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Fig. 6 (A) TEM image of cross section of the poly(styrene–DVB)
wafer created in the presence of gelator 1 (20 mM) after washing with
MeOH–THF. Reactive staining agent: OsO4. Scale bar: 200 nm. (B)
FEGSEM image of cross section of the same wafer—showing fibrillar
nano-imprinting (scale bar = 20 nm).
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MeOH–THF. This mixture was chosen because gelator 1 has
a high solubility in MeOH, whilst THF provides good com-
patibility with the polymer film. Washing removed the
majority of the gelator from the polymer, as shown by the
loss of nitrogen in the elemental analysis data (see ESIw).
Once again, the fact the gelator can be washed from the
polymer wafer proves that the majority of double bonds on
the periphery of gelator 1 had not reacted during the poly-
merisation of styrene–DVB. TEM imaging of the washed
polymer using reactive OsO4 stain supported this proposal,
as fibrillar staining was no longer observed (Fig. 6A). Further-
more, mass spectrometric data (not shown) indicated that the
gelator was washed out in monomeric form. This lack of
double bond reactivity under polymerisation conditions can
be rationalised, as the double bonds on gelator 1 provide much
less stable sites for radical propagation than those in styrene–
DVB. Interestingly, the washed material maintains its
nanoscale ordering which after washing could be imaged by
FEGSEM (Fig. 6B), with the polymer containing ‘washed out’
(imprinted) nano-fibrillar architectures.
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In summary, this communication reports a gelator incor-
porated into nanostructured polymers, with the resultant
materials having significantly modified materials behaviour.
13 See for example: D. Mathur and E. B. Nauman, J. Appl. Polym.
Sci., 1999, 72, 1151–1164.
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