Self-sorting of guests and hard blocks in bisurea-based thermoplastic elastomers

Article abstract of DOI:10.1021/ma902585w

Self-sorting in thermoplastic elastomers was studied using bisurea-based thermoplastic elastomers (TPEs) which are known to form hard blocks via hierarchical aggregation of bisurea segments into ribbons and of ribbons into fibers. Self-sorting of different bisurea hard blocks in mixtures of polymers to give separate ribbons was established by studying exciplex formation between fluorescent pyrene and dimethylaniline (DMA) functionalized bisurea probes. If both probes had the same spacing between the bisurea units as the matrix hard segments, exciplex bands were observed in the fluorescence emission spectra, whereas exciplex intensities were strongly reduced when DMA and pyrene probes with, different spacer lengths were incorporated in a mixed matrix that provided matching host segments for each guest separately. Probes with butylene spaced bisurea groups showed the highest degree of self-sorting. Self-sorting was further improved by annealing of the polymer films leading to a degree of self-sorting of more than 95%. Self-sorting at the fiber level of aggregation was studied by fluorescence resonance energy transfer (FRET) between naphthalene (donor) and pyrene (acceptor) bisurea guests. Large differences in FRET behavior for matching and nonmatching probes showed that significant self-sorting takes places between ribbons to give separate fibers.

Full text of DOI:10.1021/ma902585w

Macromolecules 2010, 43, 745–751 745
DOI: 10.1021/ma902585w
Self-Sorting of Guests and Hard Blocks in Bisurea-Based Thermoplastic
Elastomers
Nicole E. Botterhuis, S. Karthikeyan, A. J. H. Spiering, and Rint P. Sijbesma*
Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
Received November 23, 2009; Revised Manuscript Received December 3, 2009
ABSTRACT: Self-sorting in thermoplastic elastomers was studied using bisurea-based thermoplastic
elastomers (TPEs) which are known to form hard blocks via hierarchical aggregation of bisurea segments
into ribbons and of ribbons into fibers. Self-sorting of different bisurea hard blocks in mixtures of polymers to
give separate ribbons was established by studying exciplex formation between fluorescent pyrene and
dimethylaniline (DMA) functionalized bisurea probes. If both probes had the same spacing between the
bisurea units as the matrix hard segments, exciplex bands were observed in the fluorescence emission spectra,
whereas exciplex intensities were strongly reduced when DMA and pyrene probes with different spacer
lengths were incorporated in a mixed matrix that provided matching host segments for each guest separately.
Probes with butylene spaced bisurea groups showed the highest degree of self-sorting. Self-sorting was further
improved by annealing of the polymer films leading to a degree of self-sorting of more than 95%. Self-sorting
at the fiber level of aggregation was studied by fluorescence resonance energy transfer (FRET) between
naphthalene (donor) and pyrene (acceptor) bisurea guests. Large differences in FRET behavior for matching
and nonmatching probes showed that significant self-sorting takes places between ribbons to give separate
fibers.
Introduction
bisurea molecules selectively formed 2D crystals with bisurea
molecules having identical spacer length.17 Recently, Khan et al.
Self-sorting is the ability of an entity to distinguish between self
and nonself. This principle is found in nature in the replication of
DNA and in the crystallization of racemates into conglomerates
(mixtures of enantiomerically pure crystals of one enantiomer
and its opposite), but also in phase separation of oil and water.
On the molecular level, Isaacs and co-workers have investigated
the self-sorting capacity of several synthetic host-guest com-
plexes in organic solvents and water.1-5 They showed that each
guest molecule in these studies can bind to its own host in a
complex mixture of hosts and guests. Others have extended this
approach to self-sorting in (block) copolymers6-14 and on
surfaces.15 Weck et al. synthesized polynorborenes with host
groups that can complex different guest molecules based on
metal-ligand interactions, hydrogen bonding, and ion-dipole
interactions. Association was observed to occur preferentially
between the established host-guest complexes in a mixture
having different hosts and guests. Also, functionalization of
polynorbornene with two similar hydrogen-bonding hosts led
to selective binding of the guest molecules to the best matching
hosts.11 Schalley and co-workers have reported self-sorting
behavior based on 11 tetraurea-substituted calix[4]arenes having
wide rim substitutents.16 Dimerization of equimolar mixtures of
these 11 calix[4]arenes in nonpolar solvents exclusively led to the
formation of six structures out of 35 possible different homo- and
heterodimers. In a simple four-component self-sorting system
based on crown ethers and ammonium ions, using ESI-MS and
tandem mass spectrometry, they were able to study distinctly
different pseudorotaxane assemblies formed upon mixing.
Self-sorting has also been studied on surfaces by assembling
molecules of different bisurea spacings. In these studies, the
showed with a series of biscarbamates that length of the total
molecule also plays a role in the self-sorting of small molecules.18
The degree of self-sorting in any system is limited by mixing
entropy and will only proceed up to the point where this term is
compensated by the difference between the ΔGself-recognition and
ΔGrecognition (ΔΔG). Hence, self-sorting is optimal when a high
value of ΔGself-recognition and a low value for ΔGrecognition are
combined. Urea groups are known to self-associate via hydrogen
bonding, and this strong association has been used in the
development of gelating agents.19-26 Segmented poly-
(THF)-bisurea polymers have been shown to be thermoplastic
elastomers with excellent mechanical properties due to the high
degree of phase separation between the soft poly(THF) and the
hard bisurea segments.27a The bisurea segments aggregate via the
well-known hydrogen-bonding motif of urea groups to form
ribbons (Figure 1a), which further aggregate in a hierarchical
fashion to form fibers consisting of stacks that are estimated to
contain 4-6 ribbons (Figure 1b). The segmented copolymers
1a-1c (Scheme 1) used here as matrix in the self-sorting study are
highly elastic materials, which show Hookean behavior upon
elongation up to 100%, with plastic deformation at higher strains
and a strain at break of 1000%.27b The selectivity of the binding
of bisurea guests in a thermoplastic elastomer with bisurea hard
blocks has been studied extensively in our group because it
enables an easy, modular approach for the functionalization of
these polymers leading to materials with useful properties.28-30
Previously, we have shown that the amount of bisurea-
functionalized dyes that was extracted by a detergent solution
from bisurea-poly(tetrahydrofuran) polymers is much lower for
dyes that match the spacing between urea units of the matrix than
for dyes that are mismatched by a single methylene unit. Further-
more, upon elongation of elastomer films, strong differences in
*Corresponding author. E-mail: r.p.sijbesma@tue.nl.
r 2009 American Chemical Society
Published on Web 12/09/2009
pubs.acs.org/Macromolecules

746 Macromolecules, Vol. 43, No. 2, 2010
Botterhuis et al.
alignability of matching and nonmatching dyes were observed.28
We have also shown that pyrene-based fluorescent probe mole-
cules provided with “matching” bisurea moieties, which have the
same number of methylene units between urea groups as the host,
are specifically bound to the fibrous hard blocks of these poly-
mers, while probe molecules with “nonmatching” bisurea units
are not incorporated in the hard blocks of the host.30-32 The
observed selectivity of guest binding in these matrices provides
opportunities to form interpenetrating networks of fibrous hard
blocks with multiple types of functionality in separate domains.
Such features are useful when one type of functionality has to be
kept spatially separate from another within a single matrix.
Microphase separation in tri- and multiblock copolymers is
known to give rise to materials with multiple phases and can be
considered as a form of self-sorting.33 However, self-sorting of
hard blocks in segmented copolymers that have much smaller
hard segment dimensions of around 1 nm requires strong and
directional interactions to prevent mixing. Here, we describe the
result of self-sorting experiments in mixed thermoplastic matrices
with bisurea polymers having minimal structural differences;
variation of the spacer length by one or a few methylene units
between the urea groups. Self-sorting in blends of these polymers
is probed with fluorescent bisurea guests. With these experiments
we obtain an insight into the following questions: Do hard
segments with different bisurea spacings aggregate into separate
ribbons in a mixture of polymers (Figure 2)? Do these ribbons
aggregate into mixed fibers, or do fibers contain a single type of
hard segment? Do bisurea guest molecules selectively bind to the
fibers of the matching hard segment, so-called “narcissistic” self-
sorting (Figure 3a)? Self-sorting of bisurea guest molecules in
pTHF-bisurea polymer matrix was studied using fluorescent
probe molecules based on pyrene and dimethylaniline (DMA)
which form exciplex upon excitation if they are in direct contact.
The information obtained from these studies was utilized to
investigate whether the guest molecules reside in the same
type of ribbon. Fluorescence resonance energy transfer (FRET)
Figure 1. (a) Aggregation motif of bisurea as displayed in the X-ray
structure of a model compound.28 (b) Aggregation of bisurea hard
segments into ribbons (left) and subsequent stacking of ribbons to form
fibers (right).
Figure 2. Cartoon of perfect self-sorting in a bisurea thermoplastic
elastomer. In a mixture of pTHF-bisurea segmented copolymers with
varying spacer length between urea groups, the hard blocks phase
separate.
Scheme 1. (a) Segmented Block Copolymers with Bisurea Hard Blocks and pTHF as a Soft Block (1a-1c) Used for Self-Sorting Experiments and (b)
Synthesis of Bisurea-Based Guest Molecules (6a-6c, 7a-7c, and 8a,8b)

Article
Macromolecules, Vol. 43, No. 2, 2010 747
Figure 3. Proposed inclusion behavior of fluorescent guest molecules in the poly(tetrahydrofuran) bisurea-based polymer matrix. The poly-
(tetrahydrofuran) soft blocks are omitted, and only hard blocks of the polymer matrix are shown for clarity. (a) Poly(tetrahydrofuran)-bisurea
polymer matrix and bisurea probe molecules all have identical spacing (C6) between the bisurea group resulting in molecular contacts between
dimethylaniline and pyrene moieties. (b) Poly(tetrahydrofuran)-bisurea polymer matrix and fluorescent probe molecules dimethylaniline and pyrene
with different spacing (C4 and C7) between the bisurea groups are bound to separate hard blocks with little molecular contact of the chromophores.
between chromophores typically occurs up to distances of several
nanometers. FRET between naphthalene- and pyrene-labeled
bisurea probes was used to investigate self-sorting at the level of
aggregation of ribbons into fibers in order to establish whether
indeed separate networks of fibers consisting of one type of ribbons
are formed in pTHF-bisurea polymers, as depicted in Figure 2.
unit of 1400 g/mol for all polymers. The stock solutions were
freshly prepared prior to spin-coating, and the pyrene stock
solutions were used within 30 min to prevent degradation of the
compounds by the TFA. The solutions were spin-coated at 1500
rpm for 2 min.
Fluorescence Measurements. Fluorescence spectra were re-
corded on an Edinburgh Instrument FS920 double-monochro-
mator spectrometer with
a
Peltier-cooled red-sensitive
Experimental Section
photomultiplier. The dwell time was set to 0.2, the number of
scans to 3, and the step size to 1 nm. The emission and excitation
slit sizes were set to 4 nm. Emission spectra were recorded after
excitation at 290 nm (FRET) or 347 nm (exciplex). Excitation
spectra were recorded at emission wavelengths of 377 and
487 nm.
Materials and Methods. Polymers 1a-1c were synthesized as
reported in the literature.27a Solvents used in the synthesis were
reagent grade. The reagents 1,4-diisocyanatobutane, 1,6-diisocya-
natohexane, 1-pyrenemethylamine hydrochloride, 2-naphthalene-
methylamine, 4-(dimethylamino)benzylamine dihydrochloride,
N-Boc-1,4-butanediamine, N-Boc-1,6-hexanediamine, 2-ethyl-
hexyl isocyanate, and bis(3-aminopropyl)poly(tetrahydrofuran),
Mn=1100 g/mol, were purchased from Aldrich and used without
additional purification. The NMR spectra were acquired on a
400 MHz Varian Mercury Vx (400 MHz for 1H NMR, 100 MHz
for 13C NMR) or a 300 MHz Varian Gemini-2000 (300 MHz for
1H NMR, 75 MHz for 13C NMR) spectrometer. Proton and
carbon chemical shifts are reported in ppm downfield of tetra-
methylsilane using the resonance of the deuterated solvent as
internal standard. Splitting patterns are designated as singlet (s),
doublet (d), triplet (t), and multiplet (m). Infrared spectra were
recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer
with a Universal ATR sampling accessory. MALDI-TOF was
performed on a Perseptive DE PRO Voyager MALDI-TOF mass
spectrometer using R-cyano-4-hydroxycinnamic acid as the cali-
bration matrix. UV/vis spectra were recorded on a Perkin-Elmer
Lambda 900. Fluorescence spectra were recorded on an
Edinburgh Instrument FS920 double-monochromator spectro-
meter with a Peltier-cooled red-sensitive photomultiplier.
Preparation of Polymer Films for Fluorescence Measurements.
Quartz substrates (3 ꢀ 3 cm) were cleaned by ultrasonic treat-
ment in acetone (10 min), rubbing with SDS soap solution,
sonication in SDS soap solution (10 min), rinsing in a stream of
demineralized water (15 min), sonication in isopropranol
(5 min), and finally UV/ozone treatment (30 min). Solutions
for spin-coating were freshly prepared by mixing different stock
solutions, reaching a final polymer concentration of 12 mg/mL.
Stock solutions for pTHF-bisureas were always 20 mg/mL in
MeOH/CHCl3 (1:9 v/v). Stock solutions for pyrene-bisurea
guests were always 20 mg/mL in TFA/CHCl3 (15:85 v/v). Stock
solutions for DMA-bisurea guests were 10 or 20 mg/mL in
MeOH/CHCl3 (1:9 v/v). The mol % of bisurea guest was
calculated with respect to the number of bisurea groups in the
polymer, assuming a molecular weight of a polymer repeating
Results and Discussion
Synthesis of Bisurea-Based Guest Molecules. The bisurea-
poly(tetrahydrofuran) polymers (1a-1c) and bisurea-based
fluorescent guest molecules (6a-6c, 7a-7c, and 8a,8b)
used for the self-sorting experiments are depicted in
Scheme 1. The synthesis of the guest molecules (6a-6c,
7a-7c, and 8a,8b) included the following steps: (a) monoBoc
protection of 1,7-diaminoheptane by reaction with di-tert-
butyl dicarbonate followed by (b) reaction of monoBoc-
protected diaminobutane (3a), diaminohexane (3b), and
diaminoheptane (3c) with 2-ethylhexyl isocyanate, yielding
intermediates with one urea group (4a-4c). In the third step,
the Boc protecting group was removed using trifluoroacetic
acid (TFA) to yield the free amine groups (5a-5c) which
were coupled in the last step to isocyanate-functionalized
probe molecules, resulting in the formation of monofunctio-
nalized bisurea molecules (6a-6c, 7a-7c, and 8a,8b). All
compounds were purified via column chromatography. The
detailed synthetic procedure and characterization of the
intermediates and final products are provided in the Sup-
porting Information.
Self-Sorting Monitored via Exciplex Formation between
Pyrene and Dimethylaniline. Fluorescence emission of pyrene
is quenched by the complexation with dimethylaniline
(DMA) in the excited state in a so-called exciplex with a
band in the fluorescence emission spectrum at 487 nm.34-37
The formation of the exciplex requires molecular contact
between pyrene and DMA. Therefore, observation of exci-
plex emission from a film containing a mixture of pyrene and
DMA-bisurea guests indicates that the guest molecules are
assembled in the same fiber (Figure 3a). However, if the two

748 Macromolecules, Vol. 43, No. 2, 2010
Botterhuis et al.
Figure 5. Fluorescence emission spectra of polymer films containing
bisurea guest molecules Py-U7U (6c) and DMA-U6U (7b) or
DMA-U7U (7c) in the polymer matrix pTHF-U6U (1b) and
pTHF-U7U (1c). The excitation wavelength used was 347 nm, and
spectra are normalized to the peak at 377 nm for comparison. Insets
show magnifications of the spectrum between 440 and 600 nm, all
drawn to the same intensity scale.
Py-U6U (6b) and DMA-U6U (7b) indeed formed exci-
plexes with an increase in the intensity of exciplex band upon
increasing concentration of the guest molecule DMA-U6U
(7b). On the other hand, polymer films prepared using
nonmatching guests Py-U7U (6c) and DMA-U4U (7a)
in a mixed matrix of 1a and 1c did not show exciplex emission
(Figure 4b). In this mixture, exciplex intensity is reduced by
more than 90%, indicating a high degree of separation
between the two kinds of hard segments. A sample measured
twice with a time interval of 1 day yielded two identical
emission spectra, indicating that the degree of self-sorting in
the films is constant over time.
The results obtained with polymer films spin-coated from
pTHF-U6U (1b) and pTHF-U7U (1c) containing 1 mol %
Py-U7U (6c) and 5 or 10 mol % DMA-U6U (7b) illustrate
the extent of self-sorting that can be achieved with a minimal
difference in structure of the hard segments. The bisurea
units in this system differ by just a single methylene unit. The
fluorescence emission spectrum of these polymer films is
shown in Figure 5. In these samples, the exciplex band is
reduced by 60% compared to the reference system contain-
ing matching guests (DMA-U7U (7c) and Py-U7U (6c)),
indicating that although specificity is lower than when the
bisurea units differ more in size, significant self-sorting still
takes place with a difference in structure of just a single
methylene unit. Since annealing of thermoplastic elastomer
films leads to thermodynamically more stable fibers,38 the
polymer films described in Figure 5 were annealed for 1 h at
130 °C and cooled down at a rate of 20 °C/h, and the
fluorescence emission spectra were recorded again. Anneal-
ing led to lowering of the intensity of exciplex band by 88%
compared to the reference system, indicating an increase in
self-sorting.
Figure 4. Fluorescence emission spectra of polymer films containing
(a) bisurea guest molecules Py-U6U (6b) and DMA-U6U (7b) that
match with the polymer matrix pTHF-U6U (1b) and (b) bisurea guest
molecules Py-U7U (6c) and DMA-U4U (7a) in a mixed polymer
matrix pTHF-U4U (1a) and pTHFU7U (1c). The excitation wave-
length used was 347 nm. Spectra are normalized to the peak at 377 nm
for comparison. Insets show magnifications of the spectrum between
440 and 600 nm, all drawn to the same intensity scale.
guests are assembled in different fibers, molecular contact
between pyrene and DMA is not possible, and no exciplex
will be observed (Figure 3b). Exciplex emission intensity is
therefore a good measure for the degree of self-sorting of
these bisurea compounds into their matching bisurea rib-
bons. The fact that monofunctional pyrene-bisurea guests
(6a-6c) are less prone to aggregate into fibrous structures
than the bisfunctional pyrene-bisurea guests prompted us
to use monofunctional guest molecules for our study.32
Furthermore, in order to prevent the formation of pyrene
excimer, a very low concentration (1 mol %) of guest
molecule (Py-U6U (6b)) was used.
Atomic force microscopy (AFM) performed on films with
the highest concentration of the guest molecule (10 mol % of
DMA-U6U (7b)) indicated normal pTHF-bisurea fiber
morphology with no phase separation. Furthermore, fluore-
scence studies of the solutions used for spin-coating of the
films showed that self-sorting does not take place in solution
(see Supporting Information). Fluorescence spectra of poly-
mer films spin-coated from polymer matrix pTHF-U6U
(1b) and matching guests (1 mol % pyrene-U6U (6b) with 0,
1.5, and 5 mol % DMA-U6U (7b)) and 1:1 ratio of
pTHFU4U (1a) and pTHF-U7U (1c) polymer matrices
and guests molecules (1 mol % Py-U7U (6c) with 0, 1.5,
and 5 mol % DMA-U4U (7a)) were recorded (Figure 4a,b).
The spectra were normalized to the emission at 377 nm to
enable comparison. As can be seen from Figure 4a, the
polymer films prepared using matching guest molecules
Self-Sorting of DMA and Pyrene Guests in Mixtures of
Three Polymers. After having demonstrated the self-sorting
behavior in mixtures of two polymers, we wanted to extend
the principle of self-sorting to more complex mixtures.
Therefore, we investigated self-sorting in a mixture of three
polymers, with three different guest molecules. Nine possible
combinations of polymer films were spin-coated from solu-
tion containing equal quantities of polymer matrices
(pTHF-U4U (1a), pTHF-U6U (1b), and pTHF-U7U
(1c)) along with 1 mol % pyrene-bisurea guest containing

Article
Macromolecules, Vol. 43, No. 2, 2010 749
Figure 7. Comparison of the relative intensity of exciplex bands before
and after annealing in the polymer films containing matching and
nonmatching DMA and pyrene-bisurea guest molecules in the poly-
mer matrix pTHF-U4U (1a), pTHF-U6U (1b), and pTHF-U7U
(1c) . Blank indicates absence of DMA guest molecules in these samples.
Nonmatching indicates presence of DMA guest molecule having a
different alkyl spacer as compared to the pyrene guest molecule.
Matching indicates both the pyrene and DMA guest molecules have
the identical alkyl spacer between them.
DMA-U4U 7a), indicating that in the matching systems
having six methylene units (U6U system, Py-U6U 6b and
DMA-U6U 7b) and seven methylene units (U7U system,
Py-U7U 6c and DMA-U7U 7c) some guests molecules
probably are not incorporated in the bisurea fibers. Anneal-
ing (130 °C for 1 h) of the samples as described earlier had a
pronounced effect in the self-sorting behavior, resulting in an
increase of the estimated degree of self-sorting to 95% for all
the polymer films having nonmatching guest molecules
(Figure 6b). An overview of the relative intensity of the
exciplex bands for all samples before and after annealing is
shown in Figure 7. The influence of annealing is clearly
visible as well as the differences between the self-sorting
capacities of the C4, C6, and C7 spaced bisurea systems.
These results clearly show that in a mixture of three polymers
with different recognition units the bisurea units are able to
find their analogues leading to self-sorting.
FRET as a Probe To Analyze the Stacking of Ribbons into
Fibers. The hard blocks in bisurea-based thermoplastic
elastomers are known to consist of fibers that consist of
stacks of several hydrogen-bonded bisurea ribbons.27-29,39
Although the exciplex formation between DMA and pyrene
probed self-sorting behavior within ribbons, investigating
self-sorting of different bisurea segments into separate fibers
requires probes that allow studying interactions between
bisurea guest molecules over a larger distance. For this
purpose, we used the fluorescence resonance energy transfer
(FRET) pair of naphthalene (as donor) and pyrene (as
acceptor).40-44 The structures of the FRET-based probe
molecules (6a-6c, 8a, and 8b) used in our study are shown
in Scheme 1. The distance dependence of the efficiency of
Figure 6. Fluorescence emission spectra of polymer films containing
different combinations of DMA and pyrene-bisurea guest molecules in
the polymer matrix pTHF-U4U (1a), pTHF-U6U (1b), and
pTHF-U7U (1c) before (a) and after (b) annealing for 1 h at 130 °C.
The excitation wavelength used was 347 nm, and spectra are normalized
to the peak at 377 nm for comparison. Insets show magnifications of the
spectrum between 440 and 600 nm, all drawn to the same intensity scale.
either no DMA-bisurea guests, 10 mol % of the matching
DMA-bisurea guests, or 5 mol % of nonmatching
DMA-bisurea guests. Figure 6a,b shows two representative
fluorescence emission spectra before and after annealing (see
Supporting Information for comparative emission spectra of
all the prepared films). A large difference in the intensity of
the exciplex band was observed for matching and nonmatch-
ing systems, clearly indicating the presence of self-sorting in
mixtures containing three polymers. The polymer films
prepared using the mixture of Py-U4U (6a), DMA-U6U
(7b), and DMA-U7U (7c) showed the best self-sorting
behavior (least intensity of exciplex band), while the match-
ing combination of Py-U4U (6a) and DMA-U4U (7a) as
expected showed the highest intensity of exciplex band. The
estimated degree of self-sorting was 94% for the mixture with
Py-U4U (6a), DMA-U6U (7b), and DMA-U7U (7c) and
83% for the polymer films prepared using Py-U6U (6b),
DMA-U4U (7a), DMA-U7U (7c), Py-U7U (6c),
DMA-U4U (7a), and DMA-U6U (7b). The fact that the
polymer films prepared from the mixture of Py-U4U (6a),
DMA-U6U (7b), and DMA-U7U (7c) yielded the stron-
gest reduction in exciplex intensity (94%) could be attributed
to the largest difference in the spacer lengths between the
pyrene and DMA guest compared to the other combinations
of guest molecules used. Furthermore, the highest exciplex
band is formed for the matching system having four methyl-
ene units between them (U4U system, Py-U4U 6a and
€
FRET in a donor-acceptor pair is given by the Forster
6
6
equation E=R0 /(R0 þ r6) in which E is the efficiency of
€
energy transfer, R0 is the Forster radius, and r is the distance
between donor and acceptor. For the naphthalene-pyrene
FRET pair, the value of R0 is reported as 2.86 nm.45 Because
of the strong distance dependence, FRET is generally only
observable when the donor and acceptor are separated by a
distance of less than approximately 1.5R0 (4.3 nm). Because
the average distance between two pTHF-bisurea fibers in
polymers 1 is larger than 5.5 nm,27 a very low (less than 2%)
efficiency of FRET is expected for nonmatching donor and
acceptor probes if the different ribbons aggregate in separate
stacks. On the other hand, the high intensity of a FRET band
in a single polymer matrix with matching donors and accep-
tors at low concentration may be taken as corroboration of
aggregation of ribbons into fibers.

750 Macromolecules, Vol. 43, No. 2, 2010
Botterhuis et al.
Figure 9. A comparison of the ratios of integrated intensities between
375 and 430 nm and 320-345 nm before and after annealing of the
polymer films containing matching and nonmatching naphthalene and
pyrene-bisurea guest molecules.
Moreover, the intensity of characteristic naphthalene fluore-
scence in the wavelength region 320-345 nm is higher in the
nonmatching system than in the matching system, which is
also indicative of considerably lower amount of energy
transfer in the nonmatching system. Annealing of the
samples at 130 °C for 1 h followed by slow cooling down
(20 °C/h) had a pronounced effect on the FRET ratio.
For the polymer films with nonmatching probe molecules,
annealing led to a significant decrease of FRET, whereas for
the matching samples, the decrease was much less. An over-
view of the comparison of FRET ratios (I375-430/I320-345) for
the matching and nonmatching FRET pairs is shown in
Figure 9. Differences in FRET ratio between matching and
nonmatching systems shows that, in addition to self-sorting
within ribbons, there is significant self-sorting among ribbons
to give fibers containing predominantly a single type of
bisurea hard segment. The FRET ratio is nearly independent
of naphthalene concentration because the average distance
of a naphthalene donor unit to the nearest pyrene acceptor
moiety is dependent on the pyrene concentration, which is
the same in all samples. However, when the concentration of
both pyrene and naphthalene was lowered to 0.2 mol %, the
FRET ratio of the matching system dropped to a value of
0.3, approximately the same value observed for the non-
matching systems. In principle, the number of ribbons in a
fiber can be estimated from the FRET efficiency because at a
constant acceptor concentration, the average number of
Figure 8. Fluorescence emission spectra of polymer films before an-
nealing. (a) Matching donor-Naph- and acceptor-pyrene-bisurea
guest molecules and (b) nonmatching donor-Naph- and acceptor-
pyrene-bisurea guest molecules. The excitation wavelength used was
290 nm. The spectra were normalized to the amount of pyrene
calculated from the emission spectra recorded at 347 nm and were
corrected for direct pyrene excitation at 290 nm.
Polymer films were spin-coated from polymer solution
pTHF-U6U (1b) containing 1 mol % Py-U6U (6b) and 0,
1, 5, or 10 mol % Naph-U6U (8b) in the case of matching
pairs or from a solution containing a 1:1 mixture of 1a and
1c, 1 mol % of Py-U7U (6c), and 0, 1, 5, or 10 mol %
Naph-U4U (8a). Analysis of the polymer films by AFM
showed Naph-U4U (8a) and Naph-U6U (8b) did not
phase separate from the hard blocks, even at the highest
concentrations of 10 mol %. Fluorescence emission spectra
of the polymer films were recorded at the excitation wave-
length of 290 nm where the donor naphthalene absorbs. To
compensate for direct pyrene excitation at this excitation
wavelength, the fluorescence spectrum of a polymer film
with 1 mol % pyrene probe without naphthalene was re-
corded at the excitation wavelength of 290 nm and sub-
tracted from all fluorescence spectra recorded for the
polymer films containing pyrene and naphthalene. The
results are provided in Figure 8a,b. Figure 8a shows that
for the matching system (Py-U6U (6b) and Naph-U6U
(8b)) the intensity of characteristic pyrene fluorescence in the
wavelength region 375-430 nm increases with the amount of
Naph-U6U (8b). Increased energy transfer is attributed to
the increased fraction of pyrene that is within a few nano-
meters of naphthalene. In contrast to this, only a minimal
increase in intensity of pyrene emission with increasing
concentration of naphthalene donor occurs in the nonmatch-
ing system Py-U7U (6c) þ Naph-U4U (8a) (Figure 8b).
€
acceptor molecules within Forster distance of a donor in-
creases with the number of ribbons per fiber. However, the
current experiments do not allow us to extract quantitative
information from the data.40-42
Conclusions
In the present article, we have successfully demonstrated self-
sorting in elastomeric matrices by using bisurea fluorescent probe
molecules that bind to the hard segments of the pTHF-bisurea
segmented copolymer. Since no self-sorting was observed in
solution, it must be a remarkably fast process, occurring during
spin-coating of the films. Annealing of the polymeric films led to
improved self-sorting in all nonmatching systems and also led to
an increase in exciplex ratio in the matching systems. The latter
observation suggests that before annealing some of the guest
molecules are present in the soft matrix and that the fraction of
guest molecules present in the fibers increases upon annealing.
The experiments with U6U and U7U probes demonstrate a high
degree of self-sorting even when the bisurea segments differ by
just a single methylene unit. FRET measurements show that
ribbons aggregate predominantly into fibers containing a single
type of bisurea unit. Thus, self-sorting in bisurea thermoplastic
elastomers occurs at both levels of the hierarchical self-assembly

Article
Macromolecules, Vol. 43, No. 2, 2010 751
process that brings the hard segments together in ribbons that
consecutively aggregate to form fibers.
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Acknowledgment. The authors thank Dr. S. C. J. Meskers
and Dr. D. Veldman for their kind help with fluorescence
measurements, Ir. Marko Nieuwenhuizen for comments, and
NWO for funding. This research is supported by NanoNed, a
national nanotechnology program coordinated by the Dutch
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Supporting Information Available: A detailed description of
the synthesis and characterization of probe molecules (6a-6c,
7a-7c, and 8a-8b), their synthetic intermediates, and addition
fluorescence spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
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