Dendrimer-based transient supramolecular networks

Article abstract of DOI:10.1021/ja051775g

Association of a 16-fold excess of a monodisperse telechelic oligo(THF) (Mw = 1270 g/mol) containing two end groups that selectively bind to the 32 binding sites of a fifth generation dendritic host (Mw = 18511 g/mol and radius R _h = 2.4 nm) re

Full text of DOI:10.1021/ja051775g

Published on Web 09/17/2005
Dendrimer-Based Transient Supramolecular Networks
Ron M. Versteegen,† D. J. M. van Beek,†,‡ Rint P. Sijbesma,†
Dimitris Vlassopoulos,‡ George Fytas,‡,§ and E. W. Meijer*,†
Contribution from the Laboratory of Macromolecular and Organic Chemistry, EindhoVen
UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, Department of
Materials Science and Technology, UniVersity of Crete and IESL/FORTH, P.O. Box 1527,
71110 Heraklion, Greece, and Max-Planck Institute for Polymer Research, P.O. Box 3148,
55128 Mainz, Germany
Received March 21, 2005; Revised Manuscript Received August 15, 2005; E-mail: e.w.meijer@tue.nl
Abstract: Association of a 16-fold excess of a monodisperse telechelic oligo(THF) (Mw ) 1270 g/mol)
containing two end groups that selectively bind to the 32 binding sites of a fifth generation dendritic host
(Mw ) 18 511 g/mol and radius Rh ) 2.4 nm) results in the formation of reversible and dynamic
supramolecular complexes. The structure of these complexes in solution depends strongly on the
concentration. At low concentration, the two end groups of one guest are proposed to complex to the
same host, and flowerlike structures are formed with a radius of Rh ) 3.7 nm. At higher concentrations,
both end groups of one guest are complexed to different hosts, forming a bridge between them. This gives
rise to the formation of larger associates, and eventually to a transient supramolecular network. Dynamic
light scattering unequivocally showed that three distinct relaxation processes, associated with the proposed
structures, are present in this system. In addition to the dynamics related to the flowerlike (fast) and the
transient network structures (slow), an intermediate dynamic process is attributed to the cooperative motion
of a few (∼6) connected flowerlike structures. Rheological data elucidate the nature of the intermittent
network responsible for the slowest process. A monofunctional guest, not capable of forming a network
structure, was used as a reference, and starlike supramolecular structures are formed at all concentrations,
indeed.
Introduction
forcing the hydrophobic chain ends to aggregate into micelles.
These systems show highly interesting rheological properties4e,f,5
Dissolution of triblock copolymers in a solvent that selectively
dissolves the middle block leads to micellization of the end
blocks. A well-known example consists of the polystyrene/
polybutadiene/polystyrene (SBS) triblock copolymers.1 At high
concentrations, both polystyrene blocks are part of different
micelles, and the polybutadiene block forms a bridge between
neighboring micelles, giving rise to the formation of a percolated
network (gel). On the other hand, under dilute conditions, the
two polystyrene blocks of one polymer chain are aggregated
within one micelle, forcing the polybutadiene block to make a
loop, so that flowerlike structure are formed. Systems that show
very similar behavior include the hydrophobically end-capped
urethane-coupled poly(ethylene oxide)s (HEURs)2 and telechelic
ionomers.3 Especially the HEURs have been studied exten-
sively,4 with water as the selective solvent for the PEO block,
and are employed as associative thickeners in coatings,2b,4a as
sieving media for DNA sequencing,6 and as gels for controlled
drug release.7 The association of the chain ends in these ABA-
triblock copolymers is based on phase separation, and the
aggregation is a supramolecular process of polymers that have
their blocks covalently linked to each other.
Recently, supramolecular polymers have been introduced.
There, the repeating units are held together by reversible
secondary interactions (e.g., hydrogen bonding, electrostatic
interactions, or metal-ligand complexation).8 The association
of molecules or functional groups is highly directional and
specific. This class of supramolecular polymers holds promise
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1998, 31, 4149-4159. (f) Pham, Q. T.; Russel, W. B.; Thibeault, J. C.;
Lau, W. Macromolecules 1999, 32, 5139-5146.
† Eindhoven University of Technology.
‡ University of Crete and IESL/FORTH.
§ Max-Planck Institute for Polymer Research.
(1) (a) Quintana, J. R.; Janez, M. D.; Katime, I. Polymer 1998, 39, 2111-
2117. (b) Villacampa, M.; Quintana, J. R.; Salazar, R.; Katime, I.
Macromolecules 1995, 28, 1025-1031. (c) Hadjichristides, N.; Pispas, S.;
Floudas, G. Block Copolymers; John Wiley & Sons: New Jersey, 2003.
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Prochazka, K. Polymer 1990, 31, 2118-2124.
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17, 1451-1458. (b) Menchen, S.; Johnson, B.; Winnik, M. A.; Xu, B.
Chem. Mater. 1996, 8, 2205-2208.
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J. A.; Hubbell, J. A.; Lal, J. Macromolecules 2002, 35, 4448-4457.
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J. AM. CHEM. SOC. 2005, 127, 13862-13868
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Dendrimer-Based Transient Supramolecular Networks
A R T I C L E S
Figure 1. Proposed scheme for the complexation of a dendritic host with a ureido-acetic acid modified guest molecule (1 + guest) and the two guest
molecules (2 and 3) used in this study.
as a unique class of novel materials, because they combine many
of the attractive features of conventional macromolecular
polymers (mechanical strength) with properties that result from
the reversibility of the bonds between the monomer units. Hence,
materials are obtained that are able to respond to external stimuli
(smart materials) and that are typically in their thermodynami-
cally most stable state. Here, we introduce a new concept of
reversible transient networks with two building blocks that are
brought together with a directional supramolecular interaction.
This concept is based on supramolecular interactions with a
dendrimer host, a topic of increasing interest.9 We recently
introduced a mechanism to modify the periphery of adaman-
tylurea-functionalized poly(propylene imine) dendrimers with
guest molecules containing a ureido-acetic acid unit, exhibiting
a Kass ) 104 M-1 in chloroform, giving rise to reasonably stable
binding (Figure 1).10 The multivalent dendritic host with 64 end
groups complexes 32 guests through strong and directional
multiple interactions: both an electrostatic interaction between
the protonated tertiary amine and the carboxylate moiety as well
as hydrogen bonding between the urea groups of host and guest
are proposed to be responsible for this.
In this paper, we introduce transient supramolecular networks,
with reversible, supramolecular interactions between a well-
defined number of anchoring points per core (or cross-linking
site in the dendritic host) and a macromer with two of these
ureido-acetic acid end groups. Both flowerlike structures and
transient networks are analyzed in detail by light scattering,
showing the presence of an intermediate process of a few (∼6)
connected flowerlike structures.
Design and Synthesis of the Modules
The formation of flowerlike or transient-network structures
is based on the reversibility of the interaction between a dendritic
core module and a linear chain module. To eliminate the
difficulties of polydispersity in the evaluation of the systems
under study, we have designed a supramolecular network that
consists of truly monodisperse components only. The dendrimer
multivalent component, as well as the telechelic oligo(tetrahy-
drofuran), oligo(THF), are synthesized in a repetitive synthesis.
Their properties are investigated with a variety of techniques,
and detailed SAXS measurements disclose a number of new
features of this novel dynamic system.
The adamantylurea-functionalized poly(propylene imine) den-
drimers 1 are synthesized following the reported sequence. To
obtain well-defined aggregates, not only the cross-linking core
was aimed to be monodisperse, also a new synthesis for oligo-
(THF) with exactly 13 repeating units and ureido-acetic acid
end groups was followed. Scheme 1 shows our synthetic
sequence for the synthesis of monodisperse oligo(THF). First,
the trimer was prepared by reaction of excess 1,4-butanediol
with 1,4-dibromobutane. Benzyl 4-bromobutyl ether was then
coupled to this trimer via the Williamson synthesis in THF using
15-crown-5 to enhance the reactivity of the alkoxide formed.
An excess of 4-bromobutyl ether was used to compensate for
the amount that will be lost due to elimination. After the reaction
was completed, the reaction mixture contained next to desired
(8) (a) Lehn, J.-M. In Supramolecular Polymers; Ciferri, A., Ed.; CRC Press:
Boca Raton, FL, 2005; pp 3-27. (b) Folmer, B. J. B.; Brunsveld, L.; Meijer,
E. W.; Sijbesma, R. P. Chem. ReV. 2001, 101, 4071-4097. (c) Corbin, P.
S.; Zimmerman, S. C. In Supramolecular Polymers; Ciferri, A., Ed.; CRC
Press: Boca Raton, FL, 2005; pp 153-185. (d) Schubert, U. S.; Esch-
baumer, C. Angew. Chem., Int. Ed. 2002, 41, 2892-2926.
(9) (a) Zimmerman, S. C.; Zeng, F.; Reichert, D. E. C.; Kolotuchin, S. V.
Science 1996, 271, 1095-1098. (b) Daniel, M.-C.; Ruiz, J.; Astruc, D. J.
Am. Chem. Soc. 2003, 125, 1150-1151. (c) Hirst, A. R.; Smith, D. K.;
Feiters, M. C.; Geurts, H. P. M. Langmuir 2004, 20, 7070-7077.
(10) (a) Baars, M. W. P. L.; Karlsson, A. J.; Sorokin, V.; de Waal, B. F. W.;
Meijer, E. W. Angew. Chem., Int. Ed. 2000, 39, 4262-4265. (b) Boas, U.;
Karlsson, A. J.; de Waal, B. F. M.; Meijer, E. W. J. Org. Chem. 2001, 66,
2136-2145. (c) Boas, U.; Sontjens, S. H. M.; Jensen, K. J.; Christensen,
J. B.; Meijer, E. W. ChemBioChem 2002, 3, 433-439. (d) Baars, M. W.
P. L.; Meijer, E. W. Polym. Mater. Sci. Eng. 1997, 77, 149-150. (e)
Broeren, M. A. C.; de Waal, B. F. M.; van Genderen, M. H. P.; Sanders,
H. M. H. F.; Fytas, G.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 10334-
10343.
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Versteegen et al.
Scheme 1. The Synthetic Scheme for the Synthesis of Monodisperse Guest 2, x ) 3, 5, 7, 9, 11, and 13
product also the excess of reagent and elimination products.
Subsequently, the benzyl groups of all products in the reaction
mixture were cleaved by hydrogenation. In this reaction, the
double bonds of the elimination products were hydrogenated
as well. After the new byproduct 4-bromo-1-butanol was
converted to THF by cyclization at alkaline conditions, all
byproducts were easily removed by evaporation. The pentameric
oligo(THF) product with two more units than the starting trimer
was subsequently purified by crystallization. The same sequence
was used to obtain the heptamer, nonamer, undecamer, and
finally the tridecamer. All oligomers were pure on SEC and
NMR. Similar to dendrimers, mass spectrometry is unique in
analyzing the exact structural purity. The MALDI-TOF spectrum
of the tridecamer showed a polydispersity of 1.004; this is quite
similar to the 1.002 for the fifth generation dendrimer. The
tridecamer was transformed into guest molecule 2 (Scheme 1)
by a double Michael addition with acrylonitrile, followed by
reduction with BH3 in THF to afford the bis-amine. Pure 2 was
obtained after both amine end groups were reacted with ethyl
isocyanatoacetate followed by careful hydrolysis to prevent
hydantoin formation. The molecular weight of the module used
in this study is 1270 g/mol.
the ureido-acetic acid unit and a host of a adamantylurea
functionalized poly(propylene imine) dendrimer of the fifth
generation: for example, H NMR, FT-IR, isothermal micro-
1
calorimetry, and NOESY NMR.10a Two solutions (between 30
and 100 mg/mL) in chloroform were prepared, the first
containing the dendritic host 1 and the second containing the
bifunctional guest 2. 1H NMR and FT-IR spectra were taken of
these solutions. Subsequently, the two solutions were mixed in
a 1:16 molar ratio, making sure that a stoichiometric ratio of
both complexing groups was achieved. The solution remained
completely clear and homogeneous, and an increase of the
viscosity was observed. Again, 1H NMR and FT-IR spectra were
taken of this solution and compared with the spectra of the host
and guest solutions. The effect of complexation is evident from
the FT-IR spectrum in the carbonyl region (1800-1600 cm-1).
Upon complexation, the 1732 cm-1 band of the acid disappears
due to protonation of the tertiary amine of the dendrimer. The
urea bands of guest 2 (at 1664 cm-1) and host 1 (1637 cm-1
)
both move to 1630 cm-1 in agreement with a strong hydrogen-
bonded state in the complex. Even the host shows an increase
in hydrogen bonding upon complexation.11 No shifts in the
infrared spectra are observed upon diluting the solution,
demonstrating the robustness of the supramolecular complex.
1H NMR spectra of bifunctional guest, host, and complex
show features typical for the kind of interactions indicated in
Figure 1. Upon addition of the guest to the host, the peak at
2.4 ppm, corresponding to the methylene groups adjacent to
tertiary amines of the host, broadens considerably and shifts
slightly downfield. This is caused by protonation of these tertiary
amines by the carboxylic acid functionalities of the guest.
Furthermore, the peaks between 5.4 and 6.2 ppm, corresponding
to the urea groups of both host and guest, shift slightly downfield
Monofunctional oligo(THF) guest 3 was prepared as a
reference compound starting with a living cationic ring-opening
polymerization of THF followed by end-group modification
similar to the procedure of 2. Product 3 was well soluble in
chloroform and was successfully characterized by 1H NMR and
FT-IR. Due to some fractionation of the polymer during workup,
the number-averaged molecular weight was finally 1650 g/mol.
Molecular Characterization of the Supramolecular
Complex
Several techniques have been used to study the molecular
structure of the guest-host system consisting of guests with
(11) Baars, M. W. P. L.; Meijer, E. W. Polym. Mater. Sci. Eng. 1997, 77, 149-
150.
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A R T I C L E S
Figure 2. Schematic illustration of the supramolecular complexes, ranging from flowerlike structures (blue area), via the aggregates of flowerlike structures
(green area), to the supramolecular transient network (gray area).
and broaden as well. This is an indication that hydrogen bonding
between the dendritic host and the encapsulated guest is taking
place. The broadening is the result of the rigidification of the
periphery at the site of hydrogen bonding and of an increase in
(apparent) molecular weight upon complexation.
This corresponds to probing lengths of 180-1200 nm using
this technique.
From the single translational diffusion coefficient D0 ) kTT/
(6πηsRh) (kT is the Boltzmann’s constant, T is the absolute
temperature, and ηs is the solvent viscosity) of the 1:16 guest-
host complex at dilute concentrations (<3 g/L), the computed
hydrodynamic size of Rh ) 3.7 nm is determined. This Rh of
the complex exceeds the corresponding size (2.4 nm) of the
bare dendritic host and is smaller than the 1:32 complex of 1
and 3, exhibiting a computed hydrodynamic size of Rh ) 4.0
nm. As expected from the size (qRh < 1) of the complexes, the
time average intensity P(q) is found to be independent of q.
From the experimental P(0) and the refractive index contrast
(dn/dc ) 0.070 cm3/g in chloroform), the obtained molecular
mass M of the complex of 1 and 2 amounts to 36.5 ( 1 kg/
mol, being very close to the proposed ideal 1:16 supramolecular
flowerlike structure, which is calculated to be 38.8 kg/mol. Note
that the solution of the bare host in chloroform exhibits a
different value for dn/dc ) 0.103 cm3/g, supporting a surface
modification of the dendritic host.
The availability of designed stations in the dendritic host and
the directionality of the host-guest interactions is the basis of
the proposed supramolecular complex of Figure 2, which is
explained below. Although molecular spectroscopic tools are
all in agreement with the supramolecular structures in dilute
solutions, a confirmation of the mechanism and the emerged
supramolecular structures at dense conditions is needed. A
stoichiometrically 1:16 complex of the supramolecular system
with the monodisperse dendrimer 1 and the monodisperse guest
2 (in dilute solutions of c < 20 g/L) is revealed by its molecular
mass determined by dynamic light scattering (see below). With
increasing concentration, the inherently oligomeric nature of the
supramolecular complex is anticipated to fold into intermediate
larger structures, which eventually drive the solution into a
transient transparent supramolecular network (Figure 2). A
proper characterization of this dynamic system over the full
concentration range, being a challenging task, is described
below.
A complex is therefore formed via stoichiometric mixing (1:
16) host and guest with a flowerlike structure that is well
represented by Figure 2 (blue area). This type of effectively
grafted particle is distinct from the one-component triblock
copolymer micelles. Hence, the primary structure in the dilute
regime should be rather robust as indicated by the constant Rh
up to the boiling temperature of the solvent, that is, 61 °C. In
fact, D0 displays an activation energy EA ) 1.6 kcal/mol that is
similar to solvent viscosity. Yet, the designed new supramo-
lecular structure exhibits distinct material properties when it is
transformed into a network (see below).
The Dynamics of the Supramolecular Structures
We employed photon correlation spectroscopy, a laser light
scattering technique operating in the 10-7-103 s range, to probe
the dynamics of the guest-host structures in solution over a
broad range of concentrations. The amplitude and the time scale
of the fluctuations in the intensity of the scattered light (due to
the thermal molecular motion) characterize the scattering
moieties. In the absence of thermodynamic interactions at low
concentrations, the measured intensity autocorrelation function
yields the dynamic form factor C(q,t) ) P(q) exp(-D0q2t) at
To appreciate the revealed structural and dynamic changes
in nondilute solutions, we investigated the behavior of the
individual partners, host, and guest molecules as well and
different wave vectors q in the range 5 × 10-3 to 0.034 nm-1
.
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Versteegen et al.
counterintuitive slower diffusion at short distances than at long
distances, as shown in the normalized Deff(q)/D(q ) 0) versus
q plot in the inset of Figure 3 for two temperatures, is rarely
observed.14 Triggered by a theoretical analysis15 of rigid rod
diffusion at concentrations near the lyotropic transition, we have
parametrized this variation (intermediate between q2 and q0) by
Deff(q) ) D(0)(1 + Bq2 + B′q4) (dotted line in the inset of Figure
3). The negative value of the dynamic virial coefficient B (<0)
reflects short-range driving forces in contrast to the common
osmotic pressure measured in the low-q macroscopic limit. This
might arise from attractive local interactions between the primary
rigid guest-host structures (blue area in Figure 2).
This unusual behavior can be rationalized if we realize that
above about c′(∼20-25 g/L) the average distance between the
supramolecular complex matches the contour length (∼14 nm)
of the monodisperse guest. Hence, a secondary structure (Figure
2 green area) via bridging of the primary flowerlike structures
is feasible. Based on the moderate activation energy EA ≈ 6
kcal/mol obtained from the temperature dependence of Deff(q
) 0), this guest-host exchange occurs very fast so that the
effective diffusion describes the translational motion of this
quasi-supramolecular assembly. The latter consists of about six
primary objects as obtained from the comparison of the
corresponding intensities Is/c and I(c ) 0)/c. The slower motion
at high q’s, ascribed15 to the distance-dependent interactions
(Bq2), apparently reflects delayed internal rearrangements of the
secondary structure. This assembly undergoes diffusive transla-
tion at long distances and long times as compared to the motion
of the single bulk particle of the same size. However, when
this assembly is probed at short distances and respective short
times, it exhibits a slowing down. This new finding reflects
interactions between the constituent (∼6) primary objects,
bringing analogies to cage dynamics.16
Figure 3. Dynamic structure factor C(q,t) of the 1:16 stoichiometric mixture
of the dendritic host and monodisperse telechelic guest for 71 g/L in
chloroform at a given probing length (∼190 nm) and the two indicated
temperatures. The solid lines indicate a bimodal distribution function fit to
C(q,t). Inset: Normalized Deff(q)/D(0) for the slow process of C(q,t), using
D(0) ) 2.95 × 10-8 (20 °C) and 8.25 × 10-8 cm2/s (45 °C). The dotted
lines represent the polynomial fit discussed in the text.
summarize it here. As expected,12 the dendritic host without
guest behaves like a hard particle up to the overlap concentration
c* ) M/(NAV) ≈ 500 g/L (NA is the Avogadro number, and V
) (4/3)πRh is the volume of the dendrimer). Furthermore, it
3
forms an ideal solution in chloroform, that is, the interaction
parameter A2 ≈ 0. Above this concentration, first clusters are
formed and then the solution vitrifies above about 800 g/L at
room temperature. On the other hand, the monodisperse
telechelic guest alone exhibits unfavorable solute-solvent
interactions (A2 < 0) in the same solvent, and hence aggregates
are observed at lower concentrations (c ≈ 100 g/L) due to
H-bonding chain association. The solution remains in the (highly
viscous) liquid state for concentrations as high as 95 wt %.
The flowerlike complex exhibits a very narrow dilute regime
(c < c′ ≈ 20 g/L) with unfavorable (A2 < 0) solute-solvent
interactions as the result of its coated structure (Figure 2, blue
area). This finding is surprising given its nanometer size with
an anticipated broad dilute regime (estimated c* ) 300 g/L).
Above this transition concentration c′, the dynamic structure
factor C(q,t) changes from the single-exponential shape in the
dilute regime to a complex shape revealing a crossover to a
second regime. Figure 3 shows the C(q,t) of the 1:16 supramo-
lecular complex at 71 g/L for two temperatures at a constant q
()0.033 nm-1), which corresponds to a diffusion length of 190
nm. The broad two-step-decay shape of C(q,t) at 20 °C
extending over four decades in time is best represented by a
bimodal distribution of relaxation times obtained by standard
analysis,13 which yields the rate Γi(q) and the intensity Ii(q) of
the I-th (i ) f,s) process at a given q. The fast process with Γf
) Dfq2 and q-independent intensity If resembles the single-
exponential process in the dilute regime and is therefore assigned
to the diffusion of the flowerlike structures; in fact, Df ≈ D0.
For the new second process, unexpected at these very low
concentrations, Is is also q-independent, compatible with sizes
less than about 20 nm, but the relaxation rate Γs shows a peculiar
q-dependence. Unlike Df, the slower effective Deff ≡ Γs/q2 is
not a constant but is found to decrease with increasing q. This
Because the second process in C(q,t), associated with the
secondary dynamic structure, is absent in the dendrimer solution,
it is reasonable to relate it with the bridging of the flowerlike
structure through the supramolecular interactions, and hence
temperature should matter. As shown in the inset of Figure 3,
the pertinent variation Deff(q) at 45 °C is clearly weaker
1
described by B(45 °C) ≈ /6B(20 °C). Further, the reduction of
the intensity Is for the slow process at 45 °C suggests a decrease
for the population of the interconnected flowerlike structures
(Figure 2, green area). With increasing concentration at ambient
temperatures, the solution of the supramolecular guest-host
complex should undergo an additional transformation to a
transient network as sketched in Figure 2 (gray area).
Indeed, above another transition concentration of about c′′
) 100 g/L, the dynamic structure factor C(q,t) of Figure 4a
displays a third slower mode associated with big clusters, evident
from the strong q-dependent intensity (Figure 4a). The displayed
characteristics of the ergodic (i.e., nontrapped configurations)
C(q,t) are typical for weak gellike systems.17 This notion is
further corroborated by the strong increase of the solution
viscosity and the activation energy EA ≈ 18 kcal/mol of the
respective slow diffusion. The nature of these transient as-
semblies can be further elucidated by means of linear rheological
(12) Topp, A.; Bauer, B. J.; Prosa, T. J.; Scherrenberg, R.; Amis, E. J.
Macromolecules 1999, 32, 8923-8931.
(13) Fytas, G. In Light scattering from dense polymer systems. Scattering
Encyclopedia; Pike, P., Sabatier, P., Eds.; Academic Press: New York,
2003; Vol. 35, Chapter 2, pp 849-863.
(14) Delong, M.; Russo, S. P. Macromolecules 1991, 24, 6139-6155.
(15) Doi, M.; Shimada, T.; Okano, K. J. Chem. Phys. 1988, 88, 4070-4074.
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Figure 5. “Phase state diagram” in terms of the total reduced intensity c/I
(*) in (a) and the diffusion constant D in (b) for the three revealed structures
versus the concentration c of the 1:16 complex at 20 °C. Regions I, II, and
III indicate the three different regimes. In (a), the circles are for the slow
gellike process at q ) 8.5 × 10-3 nm-1, and in (a) and (b), the open symbols
refer to the complex formed at c ) 48 g/L (>c′). In (b), the colors of the
symbols correspond to the areas of Figure 2.
Figure 4. (a) Dynamic structure factor of the 1:16 stoichiometric host-
guest mixture at 128 g/L, 20 °C, and two magnifications given by the values
of q. Inset: The intensity I(q) for the intermediate and slow process. The
dashed line has a slope -4. (b) Dynamic shear moduli G′ (4), G′′ (0), and
the complex viscosity (]) as a function of the deformation frequency for
the same system at a concentration of about 90 wt % at ambient
temperatures. The solid lines indicate the limiting scaling laws for flow.
unfavorable interactions reflected in the decrease of c/I and D(c).
The crossover into regime II is marked by the turn-over in the
c-dependence of c/I and the bifurcation of D(c) due to the
coexistence of the primary and secondary supramolecular
objects. With increasing crowding, the solution becomes more
homogeneous with weaker fluctuations (I/c decreases) via the
overlap of the interconnected complexes, and progressively the
solution becomes more viscous. Regime II extends up to c′′ ≈
100 g/L marked by the appearance of a third slow mode in
regime III related to the formation of big clusters arranged in a
lose network-like fashion (Figure 5). It is remarkable that all
three structures of Figure 2 are present in regime III and the
primary flowerlike supramolecular complex keeps its initial fast
diffusion coefficient. The presence of the flowerlike complexes
even at very high concentration is similar to the presence of
cyclic structures in linear polymers (e.g., ring-chain equilibrium
in Nylon-6 and caprolactam) and suggests equilibrium of large
and small supramolecular objects.17 The strong increase of the
viscosity, primarily attributed to the slow process, precludes
dynamic light scattering measurements at even higher concen-
trations. It is worth mentioning that telechelic macro-ionomers
lacking the present directional interactions show polymer-like
cooperative diffusion driven by osmotic pressure and an ill-
defined gellike process.3
measurements. The dynamic shear moduli are shown in Figure
4b for a concentration of about 90 wt %. Both the elastic (G′)
and the viscous (G′′) moduli at low frequencies have reached
the limiting flow scaling relations against frequency, 2 and 1,
respectively, whereas the zero shear viscosity, η, has achieved
high (∼6 × 105 Poises) values. Based on these rheological data
and the existence of big clusters manifested in Figure 4a, it is
evident that there is no permanent interconnected percolated
network, which would preclude flow yielding frequency inde-
pendent G′ > G′′. Instead, the high η can be rationalized by
the presence and ergodic response of the big clusters, which
can slide against each other, allowing for slow flow. The lack
of a plateau in G′ is also compatible with this proposed structure
of Figure 2, because the short chain guests are unentangled.
Evaporation of the solution to complete dryness gave a
transparent film with a completely different appearance than
the starting host and guest, which are an amorphous powder
and a waxy solid, respectively. The film possesses appealing
mechanical properties, such as elasticity, although it is slightly
sticky.
The Three Supramolecular Structures
The stability of the physical state of the 1:16 stoichiometric
host-guest mixture was examined by preparing two different
initial concentrations at 3.2 g/L (<c′) and 48 g/L (>c′ and <c′′)
to create higher concentrations by slow solvent evaporation
indicated by open and solid symbols in Figure 5. Despite the
possibility of thermodynamic metastability when the stoichio-
metric host-guest mixing occurs at c > c′, the two experimental
The physical state of the solution is mapped in the “phase
diagram” given in terms of the total I/c (Figure 5a) and Di(q )
0) (Figure 5b) of the various processes versus the supramolecular
complex concentration at 20 °C. Three regimes are consistently
revealed in both plots. Dilute regime I is characterized by the
single process for the flowerlike complex (Figure 2), having
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A R T I C L E S
Versteegen et al.
Experimental Section
protocols lead to similar structures, unique for this system
designed. Alternatively, a stoichiometric mixture (1:32) of the
same dendrimer with chemically similar but mono-functional
guest 3 in chloroform behaves distinctly different, for example,
low viscosity, and moreover phase separates above 120 g/L at
20 °C.
The synthesis and characterization of the materials used are given
in the Supporting Information.
Photon Correlation Spectroscopy (PCS). The solution of dendrimer
in CHCl3 (amylene stabilized) at a concentration of 3.2 g/L was filtered
through a 1 µm Teflon Millipore filter into the dust free optical cell
(i.d. 8 mm). The guest/CHCl3 solution at the desired stoichiometric
composition was then filtered into the dendritic host solution in the
light scattering cell. Preparation of higher concentrations was ac-
complished by slow solvent evaporation. The desired dynamic structure
factor C(q,t) ) [(G(q,t) - 1)/f*] was computed from the measured light
scattering intensity It(q,t) (using ALV5000 digital correlation) and the
autocorrelation function G(q,t) over a broad time range (0.1 µs to 100
s) at a given scattering wave vector |q| ) (4πn/λ) sin(θ/2), where n, λ,
and θ are the refractive index of the medium, laser wavelength in a
vacuum, and scattering angle, respectively, and f* is an instrumental
contrast factor; q varies between (0.004-0.034) nm-1. The amplitude
R of C(q,t) at short times(<0.1 µs) yields the intensity I(q) ) RIt(q)
associated with the individual moieties, that is, complex or their
interactions in nondilute solutions. Further, the origin of the C(q,t) is
reflected in its temporal decay at a given q. In the absence of interactions
(dilute solutions), its exponential decay rate Γ(q) ) Dq2 relates to the
translational diffusion coefficient D of the probed species and hence
their hydrodynamic size. In the latter case, I(q ) 0) yields the molecular
weight of the diffusing object.
Conclusions and Outlook
A new approach has been presented to create flowerlike and
transient supramolecular network structures. In contrast to the
block-copolymers studied so far, the two physically different
blocks are held together by supramolecular secondary interac-
tions that are highly directional and reversible. This allows the
dynamic system to be under thermodynamic equilibrium. The
association constant of the supramolecular binding of about Kass
) 103-104 M-1 gives rise to a reasonable stable binding
between the two components, while at the same time the
reversible nature is ensured. The dendritic structure of the cross-
linking unit or multivalent associate creates the additional
stability of the supramolecular complexes formed. The mono-
disperse character, as a result of unique synthetic pathways, of
both components allows detailed analysis, and dynamic light
scattering has been used to elucidate the different supramolecular
structures formed. The observed collective process of roughly
six flowerlike structures exhibits unique temperature depen-
dence. The resulting phase state in a range of concentrations
from the dilute up to the melt reveals strong interactions well
below the nominal overlapping concentration and a highly
viscous fluid that does not bear permanent network feature.
With this new approach of supramolecularly bringing together
components that can form well-defined superstructures, a new
avenue is created. It is anticipated that it should be possible to
perform this approach in water and that some of the supramo-
lecular units of the dendrimer can be used to anchor (bio-)-
functional units.
Acknowledgment. We are grateful to Mr. Bas de Waal (TU/
e) and Mrs. Antje Larsen (FORTH) for technical assistance,
and we acknowledge the financial support of the EU (HPRN-
CT-2000-2003) and the Council for Chemical Sciences of The
Netherlands Organization for Scientific Research (CW-NWO).
Supporting Information Available: Additional information
about the synthesis and characterization of the molecules used.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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