Supramolecular materials from benzene-1,3,5-tricarboxamide-based nanorods

Article abstract of DOI:10.1021/ja0774764

Telechelic polymers end-capped or copolymerized with the benzene-1,3,5-tricarboxamide (BTA) moiety lead to supramolecular materials. The intrinsic phase segregation of BTA nanorods with an amorphous polymer such as poly(ethylene butylene) results in therm

Full text of DOI:10.1021/ja0774764

Published on Web 01/10/2008
Supramolecular Materials from Benzene-1,3,5-tricarboxamide-Based
Nanorods
Jorg Roosma, Tristan Mes, Philippe Lecle`re, Anja R. A. Palmans, and E. W. Meijer*
Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology,
P.O. Box 513, 5600 MB EindhoVen, The Netherlands
Received September 28, 2007; E-mail: e.w.meijer@tue.nl
Scheme 1
Motifs exhibiting directional and reversible interactions have been
the foundation for the development of supramolecular polymers,
i.e., polymers exhibiting macroscopic material properties as a result
of noncovalent interactions between the monomers or macromers.1-3
Not only is the strength of the noncovalent interactions important
to obtain polymer properties but also is the propensity of the motif
to phase segregate, as convincingly shown by Stadler and Rowan.4,5
This phase segregation results in strong multiple cross-linking points
with order in two or three dimensions within the soft polymer
matrix, leading to materials with thermoplastic elastomeric proper-
ties. Bouteiller recently proposed a class of materials that would
physically cross-link by the self-association of functional groups
forming long supramolecular chains in one dimension (1D).6 Such
well-defined nanorods embedded in an amorphous polymer matrix
would be an interesting addition to the field of supramolecular
polymer chemistry since they would combine high strength and
spatial control with reversibility. Our group recently showed the
presence of 1D columnar structures in telechelic poly(ethylene
butylene) (PEB) and poly(ꢀ-caprolactone) functionalized with both
lateral and end-to-end hydrogen-bonding functionalities by the
introduction of both the ureidopyrimidone (Upy) and urea group.7,8
We here propose the benzene-1,3,5-tricarboxamide (BTA) motif
for supramolecular materials based on phase segregated nanorods
in a polymer matrix.9 The BTA motif forms a 1D polymer in
solution and in the solid state as a result of the three-fold R-helix-
type arrangement of the intermolecular hydrogen bonds.10,11 CD
spectroscopy revealed that the introduction of chiral side chains
results in columnar structures with a preferred helicity in a highly
cooperative fashion.11 Since the lateral interactions in BTAs suffice
to form a columnar structure, we anticipated that end-capping or
copolymerizing low-molecular weight (MW) telechelic polymers
with BTAs may result in supramolecular materials. With the BTA
motif inducing physical cross-linking via the formation of nanorods,
elastomeric properties can be expected in these supramolecular
materials.
polymeric BTAs. Chiral 1a was dissolved in dodecane (c ) 8 ×
10-6 M), and its UV and CD spectra were compared to those of
chiral 3 measured in heptane (c ) 2.1 × 10-5 M) (Figure 1). In
the UV spectrum of dumbbell-shaped 1a in dodecane (see SI), the
λ
max at 208 nm (typical for molecularly dissolved BTA species)11b
was absent, indicating that the BTA units were aggregated.
Moreover, the CD spectrum of 1a was similar in shape and
amplitude to the CD spectrum of 3 suggesting that the helical
columnar order present in the disk-shaped BTAs is retained in the
dumbbell-shaped BTAs in solution.
Copolymer 2a could not be measured in alkane solution as a
result of its poor solubility. Since 2a is an amorphous polymer (see
below), we spin-coated films with a thickness of 274 nm from a
CHCl3 solution on quartz plates and measured CD and UV of the
films. A clear CD effect was observed for 2a, similar in shape as
observed for 1a and 3 (Figure 1, inset). The CD effect was
independent of the orientation of the quartz slide with respect to
Amine telechelic poly(ethylene butylene) (PEB) (Mn ) 3.5 kDa,
polydispersity ) 1.08) was converted into dumbbell-shaped mol-
ecules 1a,b and into copolymers 2a,b (Scheme 1) via a straight-
forward synthetic approach (see SI for details). The copolymers
2a,b demonstrated molecular masses in the range of 27-33 kDa,
corresponding to degrees of polymerization of around 5, and
polydispersities of around 1.7. As a reference, the chiral disk-shaped
BTA 3 encompassing three (R)-3,7-dimethyloctyl side chains was
included (Scheme 1). We selected chiral derivatives in this study
to evaluate the presence of helicity in the columnar structure in
solution and in the solid state by using CD spectroscopy. The latter
offers an additional tool to investigate the dynamics of the system.
The chiral derivatives 1a, 2a, and 3 were studied in solution
with UV and CD spectroscopy to verify the columnar helical
structure found for 3 in solution for dumbbell-shaped BTAs or even
Figure 1. CD spectra of 1a (c ) 8 × 10-6 M in dodecane) and 3 (c ) 6.5
× 10-5 M in heptane). The concentration independent molar ellipticity ∆ꢀ
was used on the y-axis. (Inset) CD spectrum of 2a spin-coated on a quartz
slide with a film thickness of 274 nm.
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10.1021/ja0774764 CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
of copolymer 2b shows a less developed morphology than the one
observed for 1a (Figure 3B). Although phase segregation is clearly
present in the morphology of 2b, a more rodlike structure was
observed with a diameter of 6 nm, but with much shorter lengths
(20-30 nm), indicating that the aspect ratio is significantly lower
than the aspect ratio observed for 1a.
Although dumbbell-shaped BTAs 1a,b appeared as elastic solids,
the material properties were not sufficient to allow tensile testing
to be performed. Gratifyingly, the elasticity in copolymers 2a,b
was more pronounced than the elasticity (qualitatively) observed
for 1a,b. Therefore, we performed tensile tests on 2b (see SI). The
stress-strain curve was typical for a thermoplastic elastomer. The
E-modulus was 3.4 ( 0.2 MPa, indicative for a soft rubber. The
yield stress and the strain at break were 1.42 ( 0.07 MPa and 497
( 43%, respectively. These values compare well with those of Upy-
urea functionalized PEB,8 poly(urea EB)12 analogues, and bis-urea
grafted poly(dimethylsiloxane).6
Figure 2. Amine telechelic PEB before (left) and after (right, compound
1b) functionalization with a BTA moiety.
In conclusion, we have shown for the first time that telechelic
polymers end-capped or copolymerized with BTA’s lead to
supramolecular materials. The intrinsic phase segregation of BTA
nanorods with an amorphous polymer such as PEB results in
thermoplastic elastomeric behavior. Our current research is devoted
to apply this principle to other classes of polymers and to
incorporate functional BTAs into the columnar aggregates.
Figure 3. AFM phase image of (A) 1a and (B) 2b. The scale bar is
100 nm.
the beam, and no linear dichroism effects were present. This
suggests that the columnar helical order present in 3 in solution is
retained in 1a and 2a.
Acknowledgment. Ph.L. is “chercheur qualifie´” from FNRS
(Belgium). The work of J.R. forms part of the research programme
of the Dutch Polymer Institute project #475. NWO is acknowledged
for funding.
The physical properties of dumbbell-shaped BTAs 1 and
copolymers 2 were then studied in the solid state. While amine
telechelic PEB is a liquid at room temperature, 1a,b and 2a,b were
obtained as transparent, elastic solids. As an example, the dramatic
visual change in properties of PEB and BTA end-capped PEB
(compound 1b) is shown in Figure 2. Differential scanning
calorimetry (DSC) of 1a showed a Tg at -60 °C and a small
transition around 195-215 °C (∆H ) 2.63 J/g for 1a and ∆H )
2.79 J/g for 1b). With polarization optical microscopy (POM), we
observed a mobile, birefringent texture typical for a nematic phase
starting from 60 °C up to the clearing temperature around 200 °C.
Infrared (IR) spectra in the solid state of 1a,b showed vibrations at
positions typical for amides involved in three-fold R-helical-type
hydrogen bonding as evidenced by comparison with compound 3
(ν(NH) ) 3240 cm-1, ν(CdO) ) 1642 cm-1 and ν(C-N) ) 1562
cm-1 for 1a,b and (νNH) ) 3226 cm-1, ν(CdO) ) 1637 cm-1
and ν(C-N) ) 1563 cm-1 for 3). Variable-temperature IR (see
SI) revealed a gradual shifting of the characteristic vibrations
starting from 75 °C and allowed us to attribute the transition around
200 °C to the loss of the intermolecular hydrogen bonds. The NH,
CdO, and C-N vibrations in the IR spectra of the copolymers
2a,b were at similar positions as observed for dumbbell-shaped
compounds 1a,b and typical of intermolecularly hydrogen-bonded
amides. DSC traces for the copolymers 2a,b revealed a Tg at -57
°C and a small transition at around 185 °C (∆H ) 1.87 J/g for 2a
and 2.12 J/g for 2b). In this case, no liquid crystalline phase was
observed with POM.
Supporting Information Available: Experimental procedures and
analysis of 1-3, tensile data, AFM images of 1b, variable temperature
IR, UV, and CD spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
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