Morphological control and molecular recognition by bis-urea hydrogen bonding in micelles of amphiphilic tri-block copolymers

Article abstract of DOI:10.1039/b507171b

Hydrogen bonding between urea groups of amphiphilic triblock copolymers considerably affects their self-assembly in water, which results in a strong modification of morphology and viscosity of aqueous solutions; the hydrogen bonding motif in these amphiphilic copolymers allows molecular recognition of small molecules with complementary hydrogen bonding units. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b507171b

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Morphological control and molecular recognition by bis-urea hydrogen
bonding in micelles of amphiphilic tri-block copolymers{
Natalia Chebotareva,a Paul H. H. Bomans,b Peter M. Frederik,b Nico A. J. M. Sommerdijka and
Rint P. Sijbesma*a
Received (in Cambridge, UK) 24th May 2005, Accepted 9th August 2005
First published as an Advance Article on the web 8th September 2005
DOI: 10.1039/b507171b
the bis-urea groups were placed at the centre of the apolar block,
and were separated from a PEO block by an aliphatic spacer. For
comparison, a reference block copolymer (2) was prepared, which
lacks the urea groups but has exactly the same hydrophilic :
hydrophobic ratio (weight fraction of PEO (wpeo) 5 0.55).
Synthesis of tri-block copolymers 1 and 2 was performed starting
Hydrogen bonding between urea groups of amphiphilic tri-
block copolymers considerably affects their self-assembly in
water, which results in a strong modification of morphology
and viscosity of aqueous solutions; the hydrogen bonding
motif in these amphiphilic copolymers allows molecular
recognition of small molecules with complementary hydrogen
bonding units.
from commercially available polydisperse PEO with Mn
5
350 g mol21 using a multistep synthetic strategy and were fully
characterized (see ESI{). Both compounds were dissolved in water
to form micelles. Critical micelle concentrations (CMC) were
determined by using pyrene as a fluorescent probe.9 From a plot of
the ratio of band intensities I1 : I3 in the fluorescence spectrum vs.
the concentration of block copolymer, the CMC was taken as the
inflection point on the curve. CMC values were determined to be
1.9 6 1026 M for 1 and 2.5 6 1026 M for 2.{
Molecular recognition via hydrogen bonding in water is a topic
with high relevance to biological systems, however it has been
investigated rather infrequently in synthetic systems.1 The urea
group is a strong hydrogen bonding unit and the cooperativity of
two urea groups is a powerful motif in supramolecular chemistry.
Bis-urea groups have been used as ‘‘hard blocks’’ in thermoplastic
elastomers,2–4 furthermore they also may act as hosts for small
molecules which contain complementary bis-urea units.5 Bis-ureas
have also been shown to form gels in organic solvents6 or even
in water.7
The hydrogen bonding between urea groups of 1 in water was
established with FT-IR on 1 wt% aqueous solutions (Fig. 1). The
presence of strong, narrow bands in the spectrum of 1 at 3338 cm21
(N–H), 1608 cm21 (amide I) and 1578 cm21 (amide II) indicated
that urea groups in 1 are strongly hydrogen bonded.8,10
Even though the critical micelle concentrations of both
copolymers were comparable, the hydrogen bonds of urea groups
strongly affect the ability of the material to be solubilized in water
and the morphology of the resulting aggregates. Whereas homo-
geneous solutions of 2 were readily obtained by stirring, sonication
was required for solubilization of urea-containing polymer 1. The
Here we combine the concepts of hydrogen bonding in water
and molecular recognition using the bis-urea motif’s hydrogen
bonds. For this purpose we designed and synthesized amphiphilic
tri-block copolymer 1. In order to minimize the competing
interactions of urea with water and polyethylene oxide (PEO),8
aEindhoven University of Technology, Department of Chemical
Engineering, Macromolecular and Organic Chemistry, PO 513,
5600 MB, Eindhoven, The Netherlands. E-mail: r.p.sijbesma@tue.nl;
Fax: +31 40245 1036; Tel: +31 40247 3131
bUniversiteit Maastricht, EM unit, Pathologie, Universiteitssingel 50,
6229 ER, Maastricht, The Netherlands.
E-mail: peter.frederik@elmi.unimaas.nl; Fax: +31 43367 1040;
Tel: +31 43388 1281
{ Electronic supplementary information (ESI) available: Synthetic proce-
dures, cryo-TEM procedure, CMC determination. See http://dx.doi.org/
10.1039/b507171b
Fig. 1 FT-IR spectra of 1 wt% H2O solution of 1 and 2.
Chem. Commun., 2005, 4967–4969 | 4967
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morphology of the micellar aggregates of both compounds
was investigated by cryo-TEM on 1 wt% solutions. For a fair
comparison, both samples were sonicated for 1 h and equilibrated
for 24 h prior to the measurements. As shown in Fig. 2a and 2b
both 1 and 2 form cylindrical micelles in water, however the length
of the cylinders is noticeably different. Micelles of 1 are short
and straight (dcore 5 9 nm, , 5 100 nm), whereas the micelles of
2 are narrower, considerably longer (dcore 5 7 nm, , . 1 mm) and
show some curvature.
Remarkably, despite the much shorter micelles, the viscosity of a
1 wt% solution of 1 is much higher than for a similar solution of 2.
Measured 1 h after sonication, the specific viscosities of 1 and 2
were determined to be 6.41 and 0.21, respectively. Further
observation of 1 shows that the viscosity slowly increases with
time after sonication, whereas the viscosity is constant for 2.
Unfortunately, attempts to quantitatively determine the
viscosity of solutions of 1 as a function of time failed due to
the thixotropy of the sample, indicating significant interaction
of the rods11 combined with slow kinetics of micellar scission/
recombination.12 The viscosity of 2 was constant and not
dependent on shearing history, in line with considerably faster
kinetics.
To investigate the effect of sonication on the rod-like aggregates
we conducted dynamic light scattering (DLS) measurements. DLS
of a 0.5 wt % sample of 1, which was sonicated for 60 min, stored
for 24 h, and subsequently diluted to 0.05 wt% under vigorous
stirring showed the presence of polydisperse large aggregates
(Rh 5 20 nm–10 mm). However, after sonication for 10 min., the
aggregates were smaller and less polydisperse (Rh 5 15–400 nm).
This shows that sonication is indeed able to disrupt the
aggregation of rods, however, the system returns to its original
aggregated state on a time scale of days due to slow dynamics.
Similar treatment of solutions of 2 did not lead to a large reduction
in aggregate size. Above 37 uC both compounds precipitate from
solution.13 However, 2 dissolves and forms a clear micellar
solution upon recooling, whereas 1 requires sonication, confirming
the strong differences in the dynamics of both compounds. At
concentrations above 3 wt%, freshly sonicated aqueous solutions
of 1 have extremely high viscosities, whereas solutions of 2 easily
flow even at 10 wt%. Comparison of 1 and 3 wt% solutions of 1
(Fig. 2a and 2c, respectively) shows that apart from the higher
density of rods at 3 wt%, there is no change in morphology,
suggesting that the high viscosity is due to overlap of rods{ rather
than from formation of physical crosslinks.
Fig. 2 a, b: Cryo-TEM images of 1 wt% micellar solutions of 1 and 2
respectively; c: cryo-TEM images of a 3 wt% solution of 1.
Having shown that in the aggregates of 1 hydrogen bonds
between bis-urea groups effectively modify micellar properties, we
set out to use this system for the investigation of molecular
recognition by hydrogen bonding in water. To probe the self-
complementarity of the hydrogen bonding motif within the
micellar aggregates, we designed and synthesized molecule 3.{
Due to hydrogen bonding, 3 is strongly aggregated in organic
solvents. As a result, a strong excimer band at 480 nm is observed
in the fluorescence spectrum. The intensity of the excimer emission
depends on the ability of the solvent to compete with intra- and
intermolecular hydrogen bonding to separate the pyrene groups.{
However, if urea–urea hydrogen bonds are absent, the intensity of
excimer emission is a measure of the microviscosity of the
environment, since it depends on the probability that pyrene
groups of a single molecule of 3 come together during the lifetime
of the excited state.14 Compound 3 was brought to a molecularly
dissolved state by titrating a chloroform solution with trifluoro-
acetic acid (TFA) until the ratio between excimer and monomer
emission was constant (IE/IM 5 0.18; 15% TFA) (Fig. 3). These
conditions were used to premix 3 with polymer 1 or 2. After
premixing and slow drying, a micellar solution was formed by
adding water to the dry mixture followed by sonication. Cryo-
TEM did not reveal any changes in micellar morphology upon
addition of 3. Fig. 3 shows the fluorescence spectra of 3 in 1 wt%
aqueous solutions of 1 and 2 as well as in 15% TFA in chloroform.
Excimer fluorescence was completely absent in micelles of 1
whereas a clear band with relative intensity IE/IM 5 0.34 was
observed in the micellar solution of 2. Due to the strong tendency
of 3 to aggregate, excimers can be formed either intramolecularly
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Fig. 4 a, b: Schematic representation of micelle formation in 1 and 2
respectively, indicating steric hindrance by PEO chains to the growth of
the rod-like micelles.
In conclusion, by introducing the urea hydrogen bonding
motif as a highly specific supramolecular synthon into amphiphilic
tri-block copolymers, the nanoscopic structure and self-assembly
kinetics of the micelles are strongly modified. Moreover, it was
shown that the self-complementarity of bis-urea hydrogen bonding
may be used for non-covalent functionalization of the micelles. To
our knowledge this is the first example of non-covalent micellar
functionalization in water using bis-urea groups, thereby creating
opportunities for future applications in such diverse fields as target
specific drug delivery16 or catalyst immobilization.17
Fig. 3 Fluorescence spectra of 1026 M solution of pyrene clicker 3 in
CHCl3/15% TFA, 1 wt% H2O solution of 1 and 2, lex 5 337 nm.
or intermolecularly. While it is not possible to distinguish between
these types of excimers in micelles of 2, the complete absence of
excimer fluorescence in micelles of 1 is clear evidence that the
pyrene moieties are held apart from each other and cannot come
together during the lifetime of the excited state. Since the block
copolymers 1 and 2 differ only in the presence of bis-urea groups,
the distinct difference in excimer emission is attributed to the effect
of bis-urea hydrogen bonding. We believe that 3 is incorporated
into the hydrogen bonded urea stacks of 1 in an unfolded
conformation, and that it hydrogen bonds with the urea groups of
1 keeping the pyrene moieties apart (Fig. 3, inset).
We thank Dr Aissa Ramzi, University Utrecht, for help with
DLS.
Notes and references
{ A very rough estimate of the volume fraction (Q) of rods at which they
start to overlap is: Q 5 (d/,)2, corresponding to a 4% solution for d 5 20 nm
(core + corona) and , 5 100 nm.
The results outlined above indicate that the urea groups have a
strong effect on nearly every aspect of amphiphile assembly in
water. The morphology, rheology and host–guest chemistry of 1
are completely different from its non-hydrogen bonding analogue
2. Although direct structural information is lacking at the moment,
the observations allow us to construct a general picture of the
aggregates, and of the way hydrogen bonding affects their
behavior. Hydrogen bonding between urea groups constrains the
packing of the hydrophobic part of the molecules (they become
more stretched), resulting in an increase in the core diameter
from 7 to 9 nm. As a consequence, the interfacial area per
chain is reduced.11 The PEO chains of 1 are therefore also forced
to stretch, and accumulation of steric strain limits the length of
the micelles (Fig. 4).
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A similar effect was observed by Stupp et al. in mushroom
shaped aggregates of asymmetric tri-block copolymers.15 Partial
dehydration of PEO may accompany stretching, which results in
the observed lower solubility of 1. The thixotropy and slow
recovery of viscosity after sonication of 1 indicate that the kinetics
of self-assembly of 1 in water are much slower than either self-
assembly of 2, or of bis-urea derivatives in less polar media.5 Even
though the kinetics of self-assembly of 1 in water are not
understood completely, these observations point to a cooperative
role of hydrogen bonding, hydrophobic effects and steric shielding
by PEO in slowing down self-assembly kinetics.
This journal is ß The Royal Society of Chemistry 2005
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