Molecular recognition in a thermoplastic elastomer

Article abstract of DOI:10.1021/ja0451160

Selective incorporation of bisurea guests in thermoplastic elastomers with poly(tetrahydrofuran) soft blocks and bisurea containing hard blocks is observed when the distances between the urea groups of host and guest match. The incorporation leads to significant modulation of mechanical properties. With bisurea-functionalized dyes as guests, a strong difference in extractability by detergent solution was shown between dyes differing by just one methylene unit between urea groups. Upon elongation of elastomer films, strong differences in alignability of matching and nonmatching dyes were observed.

Full text of DOI:10.1021/ja0451160

Published on Web 02/10/2005
Molecular Recognition in a Thermoplastic Elastomer
Rolf A. Koevoets,† Ron M. Versteegen,† Huub Kooijman,‡ Anthony L. Spek,‡
Rint P. Sijbesma,*,† 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, and BijVoet
Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht UniVersity,
Padualaan 8, 3584 CH, Utrecht, The Netherlands
Received August 13, 2004; Revised Manuscript Received December 23, 2004; E-mail: r.p.sijbesma@tue.nl
Abstract: Selective incorporation of bisurea guests in thermoplastic elastomers with poly(tetrahydrofuran)
soft blocks and bisurea containing hard blocks is observed when the distances between the urea groups
of host and guest match. The incorporation leads to significant modulation of mechanical properties. With
bisurea-functionalized dyes as guests, a strong difference in extractability by detergent solution was shown
between dyes differing by just one methylene unit between urea groups. Upon elongation of elastomer
films, strong differences in alignability of matching and nonmatching dyes were observed.
Introduction
the strong association between low molecular weight compounds
containing bisurea groups to obtain gelling agents.10 Thermoplas-
Molecular recognition in polymers is used extensively to
selectively bind small molecules or ions, and it provides a
powerful tool for the modulation of materials properties.1-4 In
elastomeric polymers, reversible alignment of, for example,
covalently attached azobenzenes molecules5 or noncovalently
embedded fluorescent dyes6 by mechanical deformation of the
polymeric matrix has been used to tune the optical properties
of the resulting materials. Smith et al. studied the orientation
of dyes and π-conjugated polymers that were blended with
polyolefins.7 Subsequently drawing the films resulted in highly
oriented chromophores, with dichroic ratios exceeding 20.
Here, we demonstrate the design of thermoplastic elastomeric
hosts bearing uniform bisurea recognition units8 and size-
selective guests that self-assemble into supramolecular ribbons
(Figure 2) to form functional materials.
tic elastomers (TPEs) containing urea groups have also been syn-
thesized before.11 There are, however, only a few examples of
TPEs possessing hard blocks comprising solely urea groups.12
Block copoly(ether)ureas 1 and 2 (Chart 1) are a new class of
thermoplastic elastomers containing urea groups. They consist
of poly(tetrahydrofuran) (pTHF) soft segments (Mn ) 1100) and
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Urea groups are known to associate strongly via bifurcated
hydrogen bonds,9 whose strength exceeds that of amide and
urethane hydrogen bonds. Several research groups have utilized
† Eindhoven University of Technology.
‡ Utrecht University.
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J. AM. CHEM. SOC. 2005, 127, 2999-3003
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A R T I C L E S
Koevoets et al.
Figure 1. Single-crystal X-ray structure of 3 (a) and 4 (b).
Chart 1. Compounds 1-7
Figure 2. AFM images (1 µm2) of thermoplastic elastomer 1; height (left)
and phase contrast (right).
uniform bisureido-butylene or bisureido-pentamethylene hard
segments for 1 and 2, respectively. Association of the hard seg-
ment domains via hydrogen bonding results in the formation of re-
versible cross-links embedded in a soft, rubbery, amorphous pTHF
matrix. The design of thermoplastic elastomers with uniform
bisurea hard blocks allows for the incorporation of guest
molecules bearing a bisurea moiety via a “perfect-fit” principle.
In this way, modification of the polymer properties, or functional-
ization of the material, can be effectuated in a modular approach
by simply mixing the complementary guests (Chart 1) with the
polymers. Furthermore, we show that the optomechanical prop-
erties of the host-guest material are modulated in a selective
manner.
Results and Discussion
Design of Selective Host-Guest System. To gain more
insight into the way bisurea units associate via hydrogen bond-
ing, crystals were grown from two model compounds (Figure
1) containing an “even” bisureido-butylene (3) and an “odd”
bisureido-heptylene unit (4). After synthesis, single crystals
suitable for X-ray analysis were obtained by slow diffusion of
water vapor into a 50 g/L solution of 3 or 4 in DMSO.
The all-trans conformation of the “even” butylene spacer leads
to a transoid arrangement of the two urea groups, whereas the
all-cis conformation of the “odd” heptylene spacer leads to a
cisoid arrangement of the two urea groups. The urea groups
and the spacer are virtually coplanar. The angles between the
least-squares planes through the urea group and through the
carbon atoms or the spacer are 7.84(10)° for structure 3 and
2.78(7)° for structure 4. Bifurcated hydrogen bonding between
adjacent urea groups gives rise to the formation of an infinite
stack of hydrogen bonds, with a spacing between the two
hydrogen-bonded urea groups of 4.64(8) Å for the “even”
butylene spacer and 4.63(10) Å for the “odd” heptylene spacer.
This is comparable to the distance in crystal structures of
substituted urea groups described in the literature.8b,9,10b,e,f
To see whether block copoly(ether)ureas 1 and 2 with a
uniform bisurea hard block would form similar long stacks via
hydrogen bonding of the urea groups as described above, the
morphology of 1 was studied with AFM in the tapping mode.13
Figure 2 shows the resulting AFM images, in which the hard
phase appears lighter than the soft phase.
(13) Versteegen, R. M. Ph.D. Thesis, Eindhoven University of Technology,
Eindhoven, The Netherlands, 2003.
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Molecular Recognition in a Thermoplastic Elastomer
A R T I C L E S
optical microscopy, is shown in Figure 3. An increase of the
flow temperature of 1 from 140 to 175 °C was observed upon
addition of 20 wt % of filler 5, whereas the flow temperature
only increased to 150 °C upon addition of the same amount of
6, strongly suggesting preferred incorporation of 5 in the hard
blocks of 1.
A similar trend is observed for mixtures of 2 and nonmatching
and matching additives 5 and 6, supporting the preferred
incorporation. The flow temperature of 2 increased from 105
to 137 °C upon addition of 17 wt % of 6, whereas it only
increased to 113 °C upon addition of the same amount of 5.
Selective Extraction of Dyes. To further study molecular
recognition within block copoly(ether urea)s, 0.25 wt % of
bisurea-functionalized azobenzene dyes 8 or 9, having four and
five methylenes in the spacer between the urea groups, were
incorporated in films of thermoplastic elastomers 1 and 2.
Combining these compounds gave two matching polymer-guest
pairs (1 + 8 and 2 + 9) and two nonmatching pairs (1 + 9 and
2 + 8). In none of the films was phase separation observed
with optical microscopy. Four similar-sized pieces were cut from
these films, and they were individually stirred in 0.1 M sodium
dodecyl sulfate solutions at 60 °C. Extraction of the dyes was
followed with UV-vis spectroscopy of the solutions. The results
depicted in Figure 4 show that the dyes containing bisurea units
that match the hard block of the elastomer are extracted more
slowly and to a lesser extent than the nonmatching dyes. After
60 h, only 38% of the complementary dyes was extracted from
the film compared to approximately 80% of noncomplementary
dyes, demonstrating the size selectivity of the dye incorporation.
The consistently higher extractability of nonmatching dyes
excludes differences in solubility between the dyes as a possible
cause for the effect.
Differential dye extraction was also studied in thin fibers of
1 and 2 produced by electrospinning, a technique that produces
fibers of nanometer to micrometer size in diameter. A polymer
solution is slowly pumped through a millimeter size nozzle that
is connected to a high-voltage supply (12 kV). Under the applied
electrostatic force, the polymer is ejected from the nozzle as a
fiber whose diameter is reduced significantly as it is transported
and deposited on a grounded template.16 In this way, materials
with a high surface-to-volume ratio can be obtained, with inter-
esting applications ranging from nanofiber-reinforced composite
materials17 to supports for enzymes18 and catalyst19 to sensors.20
Figure 3. Flow temperature of 1 (a) and 2 (b) as a function of the amount
of reinforcement filler 5 or 6. Lines are used as a guide for the eyes.
The AFM images show long fibers with a length of up to
500 nm and an apparent width of 10 nm. Smaller details cannot
be discerned since the dimensions of the AFM tip limit the
resolution. The fibers most probably consist of stacks of bisurea
hard blocks embedded in the soft matrix. The stacks were used
to intercalate guest molecules functionalized with a bisurea
recognition unit.
Modification of Polymer Properties. Polymers 1 and 2,
differing from each other by just one methylene group between
urea groups, are rubberlike materials with flow points of 140
and 105 °C, respectively. The molecular weights Mn (as
determined with GPC) of 1 and 2 are 45 × 103 and 50 × 103
g/mol, respectively. They have an elastic modulus of 95.8 and
93.0 MPa, respectively, plastic deformation sets in at λ ) 100%,
and the elongation at break is approximately 1060%.13,14 The
relative orientations of the urea groups in the hard blocks of
these TPEs are expected to be transoid and cisoid based on the
odd-even effect (four and five methylene units, respectively)
observed in the crystal structures of model compounds 3 and 4
(four and seven methylene units). The effects of 5 and 6 as
additives15 (four and five methylene units between urea groups)
on the thermomechanical properties of 1 and 2 are remarkably
different. Solution cast films of 1 containing 5, a low molecular
weight bisurea compound matching the hard block of 1, were
transparent up to 12 wt %. In contrast to this, films of 1
containing 6, which has a nonmatching pentamethylene spacer
between its two urea groups, were turbid because of phase
separation even when small amounts (1 wt %) of the additive
were added. The effect of the presence of different amounts of
5 or 6 on the flow temperature of 1 and 2, as determined with
Figure 4. (a) Table showing combinations of TPEs and dyes used in extraction experiments. (b) Percentage of dyes 8 and 9 extracted from films of 1 and
2 by aqueous SDS solution as determined with UV-vis spectroscopy.
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A R T I C L E S
Koevoets et al.
Figure 5. SEM image of electrospun sample of 2 containing 0.25 wt % of
9.
Figure 7. Dichroic ratio as a function of the elongation of dyed films of
1 (a) and 2 (b).
a technique that has often been used to study the orientation of
functional groups within polymers.21 This technique has shown
strain-induced orientation of hard segments in polyether urethane
ureas to be perpendicular to the deformation axis at strains
beyond the yield point.22 In films of 1, the dichroic ratio of
absorption bands due to stretch vibrations was measured with
polarized infrared light parallel and perpendicular to the
deformation axis. Significant differences were observed between
dichroic ratios of bands arising from the hard segment at ν )
3325 cm-1 (N-H stretching vibration) and 1615 cm-1 (CdO
stretching vibration) and from the soft segment at ν ) 1110
cm-1 (C-O-C stretching vibration). The dichroic ratio of the
urea groups in the hard segments reaches values of close to 5
at high strains, indicating that the chain axis of the hard segment
orients parallel to the deformation axis, while the dichroic ratio
of the soft pTHF blocks remains below 1.5.
Figure 6. Percentage of 9 extracted from dyed electrospun fibers of 1 and
2 as determined with UV-vis spectroscopy.
Fibers containing 0.25 wt % dye 9 were spun and analyzed
with SEM, showing very uniform fibers with a diameter of
approximately 5 µm (Figure 5).
Orientation of dye chromophores was studied by UV spec-
troscopy in films of 1 and 2 containing small amounts (0.25 wt
%) of dye molecules 7, 8, or 9 prepared by dissolving both
components in a chloroform/methanol mixture and casting the
solution in a Petri dish. UV-vis spectra were measured at 0°
and 90° polarization angles with respect to the deformation axis,
Extraction of the dyes, as described above for films of 1 and
2, was repeated with dye 9 incorporated in electrospun fibers
of noncomplementary polymer 1 and complementary polymer
2. The extraction kinetics were much faster in the electrospun
fibers because of the increased surface area-to-volume ratio, and
furthermore, slightly more of the dye could be extracted:
whereas extraction of films took several hours, extraction
reached its final value of 90 and 50%, respectively, after 20
min for the electrospun fibers (Figure 6).
Dye Orientation. Orientation and alignment of dyes by
mechanical deformation is a useful way to modulate optical
properties of polymer films.6,7 The present system offers
interesting possibilities to perform mechanical alignment in a
guest-selective manner. To study guest-selective orientation, we
first investigated mechanical orientation of urea blocks of host
polymers 1 and 2 with infrared linear dichroism spectroscopy,13
and the dichroic ratio was determined at λmax
.
Upon elongating films of 1 containing 0.25 wt % of
complementary 8, the dichroic ratio increased rapidly, reaching
values of 11 for λ ) 500% (Figure 7a), indicating that the
chromophore of the dye is oriented parallel to the deformation
axis upon stretching the film. On the other hand, the dichroic
ratio for noncomplementary dye 9 does not exceed a value of
approximately 3, while for monourea-functionalized dye 7 the
dichroic ratio remains below 2, indicating that both 7 and 9 do
not orient as strongly as 8. Similar trends were observed for
films of 2 to which were added 0.25 wt % of matching dye 9
or nonmatching dyes 7 and 8 (Figure 7b). After relaxation of
samples of 1, which were strained to 500%, residual strain was
220%, indicating significant plastic deformation. The dichroic
ratio of 8 decreased from 11 to 4.7 after releasing the stress,
while for 9 as guest the dichroic ratio decreased from 3 to 1.5.
The high dichroic ratio observed in elongated films of match-
ing dye and TPE indicates specific incorporation of urea-func-
tionalized dyes in the hard domain of the elastomer resulting in
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Molecular Recognition in a Thermoplastic Elastomer
A R T I C L E S
elongating the films above 300%, whereas no shift was observed
for the nonmatching pairs. Absorption of light with this
polarization is mainly caused by nonoriented dye molecules.
This is a strong indication that also the matching dyes are partly
present in the pTHF soft block. Thus, both dye extraction and
dye orientation experiments allow the conclusion that matching
dyes are taken up preferentially in the hard blocks, while
nonmatching dyes are predominantly present in the soft block.
Conclusions
We have shown that length-dependent molecular recognition
between bisurea units in an elastomeric host results in selective
modulation of mechanical properties. Strong discrimination
between guests differing in size by a single methylene group
was observed. Incorporation of bisurea containing dye molecules
in the hard blocks of the polymer is also selective, resulting in
different extractabilities of the dyes by detergent solutions. The
guest selectivity of the elastomeric material can be used to
selectively orient dyes upon elongation of thin films. With the
possibility to draw functionalized fibers by electrospinning, we
introduce a modular approach for thermoplastic meshes with a
variety of applications. Work along this line is in progress.
Figure 8. Schematic representation of dye 8 in the hard block of an
elongated film of 1.
orientation of the transition dipole moment of the dye parallel to
the deformation axis. This is schematically depicted in Figure 8.
The much lower dichroic ratio of nonmatching dyes is in line
with the predominant location in the more weakly oriented
amorphous phase.
Further support for the different locations of matching and
nonmatching dyes comes from the positions of λmax for matching
pairs of dye and elastomer (λmax ) 406 nm) and nonmatching
pairs of dye and elastomer (λmax ) 420 nm). The latter λmax is
close to the value of 423 nm found for both dyes in amine-
terminated pTHF prepolymer 11, the molecule constituting the
soft phase of the TPEs. However, the fact that the shape of the
absorption band does not change upon going from a matching
to a nonmatching host is a good indication that the dye
molecules are not self-aggregated in either of these host phases.
Acknowledgment. This work was supported in part (A.L.S.)
by the Council for the Chemical Sciences of The Netherlands
Organization for Scientific Research (CW-NWO). We thank R.
G. H. Lammertink for AFM measurements, C. M. Vaz for help
with electrospinning, and D. C. Popescu for SEM measurements.
Supporting Information Available: Synthesis and charac-
terization of compounds 3-9, experimental procedures, dis-
placement ellipsoid plots for compounds 3 and 4 (PDF), and
details of the structure determinations of compounds 3 and 4,
including atomic coordinates and displacement parameters (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
For the matching pairs of dye and elastomer, the perpen-
dicularly polarized λmax shifted from 406 to 414 nm upon
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