Molecular recognition in bisurea thermoplastic elastomers studied with pyrene-based fluorescent probes and atomic force microscopy

Article abstract of DOI:10.1039/b804457k

Fluorescence spectroscopy and atomic force microscopy (AFM) measurements using bisurea-pyrene probes show that they are randomly dispersed in the hard blocks of thermoplastic elastomers with matching bisurea groups, whereas they phase separate from polymers with non-matching or no bisurea groups. The Royal Society of Chemistry.

Full text of DOI:10.1039/b804457k

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Molecular recognition in bisurea thermoplastic elastomers studied with
pyrene-based fluorescent probes and atomic force microscopyw
Nicole E. Botterhuis, S. Karthikeyan, Dirk Veldman, Stefan C. J. Meskers and
Rint P. Sijbesma*
Received (in Cambridge, UK) 14th March 2008, Accepted 22nd May 2008
First published as an Advance Article on the web 2nd July 2008
DOI: 10.1039/b804457k
Fluorescence spectroscopy and atomic force microscopy (AFM)
measurements using bisurea-pyrene probes show that they are
randomly dispersed in the hard blocks of thermoplastic elasto-
mers with matching bisurea groups, whereas they phase separate
from polymers with non-matching or no bisurea groups.
Molecular recognition via hydrogen bonding is a useful tool to
functionalize materials in a noncovalent manner. A wide range of
hydrogen bonding motifs has been investigated to perform this
function, including the diamidopyridine–thymine couple,1 urei-
dopyrimidinone2 and the bisurea motif. The latter strong and self-
complementary motif is based on the formation of bifurcated
Scheme 1 Set of molecules used for the mixing experiments.
hydrogen bonds between urea groups,3 and has been used in
organo-gelators,4 hydrogelators,5 templates for crystallization,6
DNA-based coatings,7 as a patterning tool in self-assembled
monolayers8 and in micelles.9 Bisurea segments have also been
used in thermoplastic elastomers (TPEs).10–12 These TPEs derive
their elastic properties from the microphase separation of the
bisurea segments in fibrous hard blocks, consisting of a few layers
of polymeric ribbons of linearly aggregated bisureas.11 The small
size of the hard blocks results in highly transparent, elastic
materials. When a small amount of a molecule with a matching
bisurea motif is mixed into a bisurea TPE, the guest molecules are
integrated in the hard block, and increase the Young’s modulus of
the material without a reduction in tensile strength or strain at
break.12 It has also been shown with extraction experiments that
dye molecules with a bisurea unit are strongly bound to the
bisurea TPE if the alkyl spacing between the two urea groups is
the same (i.e. matching) for the TPE and dye, whereas dyes with
non-matching bisurea moieties were rapidly released from the
matrix.13 Similar behaviour was observed when a peptide with a
bisurea unit was used to functionalize a polycaprolactone bisurea
TPE to obtain a biofunctional material.14
poration in micellar bisurea hosts.9 Pyrene is known to form
excited state dimers (excimers), which fluoresce at a longer
wavelength (400–600 nm) than the monomers, which emit be-
tween 370 and 450 nm.15 If guest 5 is randomly incorporated in a
host fiber, few excimers will be formed at a low concentration
(Fig. 1a). However, at higher concentrations the molecules may
form intermolecular excimers as part of the hard block of the host
(Fig. 1b). In a non-matching polymer, the probe molecules may
form phase separated stacks (Fig. 1c). It has also been shown that
freely dissolved 5 forms intramolecular excimers.9 Therefore, the
fluorescence of molecule 5 can be used to probe incorporation of
bisurea guests in hard blocks in the bulk of host polymers, while
incorporation of the molecules in fibers at the surface can be
probed with AFM.
Thin polymer films were prepared containing different amounts
of 5 in segmented polytetrahydrofuran (pTHF) block copolymers
with matching (1) or non-matching (2, 3) bisurea blocks. pTHF
without bisurea groups (4) with a Mn comparable to 1–3 was used
as a reference host. Mixtures of 5 (stock solution of 20 mg mlꢀ1 in
15% TFA in CHCl3) with 1–4 (stock solution of 20 mg mlꢀ1 in
10% MeOH in CHCl3) with a final polymer concentration of 12
mg mlꢀ1 were spincoated on thoroughly cleaned quartz platesz at
1500 rpm for 2 min. In the solutions used for spin coating,
hydrogen bonding between bisurea compounds is suppressed by
the use of TFA and methanol. Investigation of the spin coated
films with AFM showed that in annealed films of pristine 1,11
Because of their role as a molecular reinforcer and their use in
noncovalent functionalization of TPEs, the details of guest
incorporation in bisurea TPEs deserve detailed investigation with
bulk and surface techniques. We therefore decided to use bisurea
molecule 5 (Scheme 1) as a guest molecule for characterization
with AFM and optical spectroscopy. Bisurea 5 has 2 fluorescent
pyrene moieties and was used previously to study guest incor-
Eindhoven University of Technology, Department of Chemical
Engineering, Laboratory of Macromolecular and Organic Chemistry,
PO Box 513, 5600 MB Eindhoven, The Netherlands. E-mail:
r.p.sijbesma@tue.nl; Fax: +31402451036; Tel: +31402473111
w Electronic supplementary information (ESI) available: Synthetic
procedures, UV spectra and data for monoPyrene 6. See DOI:
10.1039/b804457k
Fig. 1 Bisurea stacking. (a) 5 at low concentration in 1. (b) 5 at high
concentration in 1. (c) Self-assembled, phase-separated stack of 5.
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Fig. 2 Intermittent contact mode AFM phase images. Top: thin films of 1 mol% 5 in 1 (a), 2 (b), 3 (c) and 4 (d). Bottom: thin films of 0 (e), 10 (f),
20 (g) and 30 (h) mol% 5 in 1. Samples in a and e–h were annealed for 30 min at 110 1C prior to AFM imaging. The scale bar is 200 nm in all
images.
small, approximately 10 nm wide fibers are present (Fig. 2e).
Without annealing, these pTHF-bisurea fibers are less defined
(Fig. S1, ESIw). The films of matching polymer 1 with 1 mol%
(relative to the bisurea segments in 1) of 5, showed no changes in
surface morphology (Fig. 2a). However, when 1 mol% of 5 was
present in films of polymers with non-matching (2–3) or no (4)
bisurea segments,y hard, needle-like features were observed on the
surface of the polymer films, indicative of phase separation of 5
(Fig. 2b–d). These features increased in abundance with decreas-
ing match between guest and host. When higher mole percentages
of 5 are mixed in films of matching polymer 1, phase separation of
5 is observed clearly above 20 mol% (Fig. 2g and h).
from 1 to 30 mol% in polymer 1 on the emission spectra is shown
in Fig. 3b. Distinct excimer bands were only present in the
emission spectra of films with higher (more than 3 mol%) guest
concentrations. Excitation spectra of the different films give
additional information on the interaction of 5 with the host
polymers. Excitation spectra recorded at the monomeric emission
wavelength of 377 nm are similar for all samples (Fig. S2, ESIw).
However, excitation spectra of the non-matching polymer mix-
tures recorded at the emission band of the excimer (487 nm,
Fig. 3c) are broader and have a higher ratio of intensities I330/I347
than the excitation spectrum of 1 mol% of 5 in 1. This supports
the conclusion that the guest molecules are highly aggregated in
the non-matching polymers, while in the matching host they are
dispersed in the fibers. Interestingly, the excitation spectra of the
The experiments confirm previous results on incorporation of
various bisurea guests in polyester based TPEs, which indicated a
high specificity for the bisurea molecular recognition and phase
separation of a bisurea guest above 23 mol%.12,14 However, the
AFM experiments are not able to probe the behavior of guest
molecules below the surface of the film and they cannot shed light
on the molecular details of guest incorporation. Therefore, films
of the polymers containing increasing amounts of 5 were studied
using fluorescence spectroscopy. The fluorescence emission spec-
trum of a film of 1 containing 3 mol% of 5, excited at 330 nm, is
shown in Fig. 3a, solid line. This spectrum resembles that of
molecularly dissolved pyrene and there is only minimal emission
between 450 and 600 nm that is characteristic for aggregated
pyrene. However, if 5 was incorporated in bisurea polymers with
non-matching bisurea groups (2 or 3), an excimeric emission band
was clearly observed (Fig. 3a), indicating that 5 is not fully
molecularly separated by the bisurea segments of these polymers.
If no bisurea moiety was present in the polymer (4), the excimer
band was of even higher intensity relative to the monomeric
pyrene emission. Therefore we conclude that the guests are phase-
separated in the non-matching hosts, while in the matching
polymer (1) they are more or less randomly dispersed in the hard
segment fibrils. The effect of increasing the guest concentration
Fig. 3 Fluorescence emission (a–b, lexc = 330 nm) and excitation (c–d,
lem = 487 nm) spectra of 3 mol% of 5 mixed with different polymers (a
and c) or different mol% of 5 in the matching polymer 1 (b and d). The
emission (excitation) spectra were normalized to the peak at 377 (347) nm.
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bisurea segments of the host and only form excimers with
neighboring pyrene guest molecules in the fibers (Fig. 1b).
In conclusion, we have shown that probe 5 is randomly
dispersed in the hard blocks of TPEs with matching bisurea
groups, but phase separates when the bisurea hard blocks have
a different alkyl spacer length between the urea groups. We are
currently investigating application of the observed selectivity
for colocalization and separation of multiple functional guests
in TPEs with more than one type of bisurea hard block.
This research was supported by NanoNed, a national
nanotechnology program coordinated by the Dutch Ministry
of Economic Affairs (project EPC.6338) and by the EU
Integrated Project NAIMO (No NMP4-CT-2004-500355).
Fig. 4 Fluorescence spectra (lexc = 358 nm) of thin films containing
1 and 10 mol% of 5 in 1 and 1 mol% of 5 in 4 at 10–30 ns delay time
after the excitation pulse (a) and decay of the emission between 400
and 700 nm of these films after the first 10 ns (b).
excimers in the films containing 3 and 10 mol% show almost no
broadening compared to the spectrum with 1 mol% of 5,
indicating that most guest molecules are dispersed up to at least
10 mol% (Fig. 3d). At much higher concentrations (20 and 30
mol%), the excitation spectra recorded at the emission wave-
length of 487 nm are broadened, however to a much lesser extent
than any of the non-matching excitation spectra containing only 1
mol% of 5. Also with UV/vis spectroscopy, the broadening of
spectra was only observed in the non-matching systems and at
very high concentrations in the matching system (Fig. S3, ESIw).
These observations establish that the needles observed by AFM in
these films are not an exclusive surface phenomenon, and that
they are in fact the phase separated pyrene molecules. Phase
separation of bisurea guests was already studied with DSC
measurements by Wisse et al.12
Notes and references
z The cleaning procedure comprises sonication in acetone for 15 min,
rubbing briefly with SDS soap solution, sonication in SDS soap
solution for 10 min, rinsing in a stream of demi water for 15 min
and finally sonication in isopropanol for 10 min.
y For polymer 4, a concentration of 1 mol% relative to the amount of
bisurea segments is not possible, therefore the weight concentration of
5 in 4 is kept the same as for 5 in 1.
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tion, a more soluble probe with a single pyrene moiety was
synthesized (6). When this molecule was mixed with the
matching polymer 1, the excimer band remained low even at
30 mol% incorporation, and no broadening was observed in
the excitation spectrum recorded at 487 nm (Fig. S4, ESIw).
Furthermore AFM showed that hard needle-like structures
were absent, which indicates that 6 is not phase separated from
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