Liquid-crystalline polymers from cationic dendronized polymer-anionic lipid complexes

Article abstract of DOI:10.1021/ja065113i

The use of cationic dendronized polymers as a polyelectrolytic system for templating thermotropic liquid-crystalline phases (LC) via complexation and self-assembly with counter-charged ionic lipids is described. The topology of the LC phases resulting fro

Full text of DOI:10.1021/ja065113i

Published on Web 10/06/2006
Liquid-Crystalline Polymers from Cationic Dendronized Polymer-Anionic
Lipid Complexes
Nadia Canilho,† Edis Kase¨mi,‡ Raffaele Mezzenga,*,†,§ and A. Dieter Schlu¨ter‡
Department of Physics and Fribourg Center for Nanomaterials, UniVersity of Fribourg, Ch. du. Muse´e 3,
CH-1700 Fribourg, Switzerland, Laboratory for Polymer Chemistry, Swiss Federal Institute of Technology,
Department of Materials, Institute of Polymers, ETH-Zurich, HCI J 541, CH-8093 Zurich, Switzerland, and
Nestle´ Research Center, Vers-Chez-les-Blancs, 1000, Lausanne 26, Switzerland
Received July 18, 2006; E-mail: raffaele.mezzenga@unifr.ch
Scheme 1. Complexes of Dendronized Polymers and Surfactants
Used
During the past decade, ionic complexation of polyelectrolytes
and oppositely charged low molecular mesogenic units has been
gaining increasing attention as a viable route to design liquid-
crystalline (LC) polymers with long-range order.1,2 The main
advantage of this strategy over other self-assembly methods is the
simplicity with which complex architectures can be accessed by
putting together simple constituents from a “toolbox”. At the same
time, the liquid-crystalline phases obtained for the ionic supra-
molecular complexes are comparable to those obtained in systems
where mesogenic units are covalently attached to macromolecules.3-5
Various molecular architectures have been investigated for
cationic and anionic polymers, including linear,6 dendritic,7 and
hyperbranched polyelectrolytes.8 Hydrocarbon-based cationic and
anionic surfactants, which have been the most widely investigated
class of mesogenic units, have also been shown to directly control
the period and the structure of the complex mesophases.9
Here we report for the first time the use of cationic dendronized
polymers10 and anionic lipids as a model system in which the
molecular architecture can be rationally controlled by two factors,
the generation of the dendrons attached to the polymer backbone
and the length of the lipid tail. Scheme 1 shows the molecular
structures of the first (PG1) and second (PG2) generation den-
dronized polymers used in this study. They have two and four
ammonium triflate end groups, respectively. The syntheses (see
Supporting Information) follow closely the lines given for a similar
PG2 polymer.11 The polymers were isolated on the multigram scale
in their tert-butyloxycarbonyl (Boc)-protected form. The molar
masses were determined by gel permeation chromatography (in
DMF, 1% LiCl, 80 °C, referenced to PMMA standards using two-
angle light scattering, refractive index, and viscometry detectors):
PG1 (Mw ) 1.5 × 106), PG2 ) (Mw ) 2.5 × 106).12 The
deprotection was achieved quantitatively by treatment with neat
trifluoroacetic acid.
Complexation with the lipids was carried out by mixing polymers
and lipids at a stoichiometric ratio in water at pH ) 3, so that the
dendrons’ primary amines and the lipids’ sulfonic groups are
positively and negatively charged, respectively. Upon complexation,
the comb-like supramolecule precipitates in water. After its
re-precipitation into water from an ethanol/butanol mixture as
solvent, drying, and annealing at 50 °C under ultrahigh vacuum
(10-8 mBar), the complete complexation of NH3+/SO4- in the solid
polymer/lipid complex was demonstrated by the appearance of the
FTIR band at 1074 cm-1 corresponding to the bound sulfonic
groups. All the comb-like complexes obtained as described above
showed liquid-crystalline behavior as indicated by birefringency
in cross-polarized optical microscopy.
Small-angle X-ray scattering allowed us to determine the specific
group space of the LC complexes as a function of the lipid tail
length and dendron generation. The effect of the alkyl chains can
be understood by comparing, in Figure 1, SAXS diffractograms of
the same PG1 complexed with C8, C12, and C18 sulfonated lipids
(lowest three curves). The C8 lipid is too short to induce segregation
of the backbone and alkyl chain, and the corresponding spectrum
shows a single broad peak indicative of an amorphous structure.
By increasing the alkyl chain length, however, segregation of the
polymer backbone and lipid chains takes place. This is reflected in
the SAXS spectrum of the PG1-C12 complex, where a first sharp
peak centered at q1 ) 1.9 nm-1 appears together with a second
reflection at q2 ) 3.8 nm-1. The spacing ratio of 2 for q2/q1indicates
a lamellar structure with periodicity d ) 2π/q1 ) 3.3 nm. By further
increasing the length of the alkyl chain to C18, increased segregation
drives the system into a lamellar phase with very long range order,
as revealed by the respective SAXS diffractogram showing as much
as four consecutive reflections, q1:q2:q3:q4 spaced as 1:2:3:4. The
first peak, centered at q1 ) 1.52 nm-1, reveals a period of 4.13 nm
for the C18-dendronized polymer complex lamellar phase, which
is 0.8 nm larger compared to the lamellar phase obtained for the
C12-dendronized polymer complex, consistent with the larger size
of the alkyl chain.
The lamellar phases are made up of two segregated domains:
the dendronized polymer and the lipid tails. Yet, the alkyl chains
can form both a bilayer of facing lipid chains or a monolayer of
intercalated lipid chains. Because the three lowest diffractograms
† University of Fribourg.
‡ ETH-Zurich.
§ Nestle´ Research Center.
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10.1021/ja065113i CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Schematic representation of (A) tetragonal columnar phase and
(B) lamellar phase obtained for PG2-C12 and PG1-C12 dendronized
polymer-surfactant complexes, respectively (see Supporting Information).
The present results demonstrate that self-assembly of dendronized
polymers decorated by alkyl chains ionically attached onto the
dendrons’ “surfaces” is a viable route to design liquid-crystalline
mesophases where both structure and period can be rationally tuned
by either the dendron generation (and thus the cross-section of the
polymers) or by the length of the alkyl chains. Owing to the
reversible nature of the ionic complexation, this process proves high
relevance for nanoporous channels, biomimetic, transport, and
nanotemplating applications.
Figure 1. SAXS diffractograms for PG1 (three lowest curves) and PG2
(upper and inset curves) dendronized polymers complexed with C8, C12,
and C18. Multiple reflections are indexed by arrows. The PG2-C12
spectrum (zoomed in the inset in the 2.5-5 nm-1 region) is given in I × q2.
Note Added after ASAP Publication. After this paper was
published ASAP on October 6, 2006, production errors in Scheme
1 were corrected on the same day.
in Figure 1 refer to complexes with the same PG1 polymer but
different alkyl tails, the difference in period of the lamellar phase
can only arise from the difference in lengths of sulfonated C12
and C18 lipids. By considering fully stretched lipid chains, the
difference in length between C18 and C12 chains is 0.65 nm.
Because a bilayer model for lipids would then lead to an increase
of 1.3 nm when going from C12 to C18, calculations are consistent
with an intercalated alkyl tails model or a bilayer of tilted alkyl
tails. Moreover, a fully stretched intercalated lipid chain model
combined with SAXS period also yields the thickness of the
dendron phase, calculated at 1.63 nm for PG1, indicating high space
filling efficiency of dendronized polymers.13
Figure 1 also highlights the effect of generation of dendronized
polymer when the surfactant is kept at fixed length, if one considers
the PG1-C12 and PG2-C12 SAXS spectra. As discussed previ-
ously, when C12 is complexed to a PG1-dendronized polymer, the
liquid-crystalline phase is lamellar with 3.3 nm period. When a
PG2 polymer is complexed with the same C12 alkyl chain,
however, the diffractogram changes into four q1:q2:q3:q4 reflections
spaced as 1:x2:2:3 (top curve and inset), which is the signature of
a rarely observed and well-organized columnar tetragonal phase,
with period 3.93 nm. The larger period of the tetragonal phase,
whose existence has been reported in the literature for phthalo-
cyanine derivatives14-16 and dendrimer-lipid complexes,9 is con-
sistent with the increase of generation of the dendronized polymer.
The change in structure is, however, more interesting. Presumably,
in PG1 polymers, the dendrons are still small enough to accept
alkyl chains in a lamellar arrangement, similarly as linear polymer-
lipid complexes.1 With increasing generation, however, steric
hindrance between dendrons starts to play an important role, further
enhanced by the volume occupancy of alkyl chains. Thus, to
optimize space-filling requirements and reduce lipid crowding, the
dendron-lipid bulky moieties will have to twist each other with
respect to the polymer backbone, leading to a columnar rather than
a lamellar phase, thus undergoing a transition similar to that reported
in dendrimer-based LC.17
Supporting Information Available: Synthesis and molecular
packing data. This material is available free of charge via the Internet
at http://pubs.acs.org.
References
(1) (a) Antonietti, M.; Conrad, J.; Thuenemann, A. Macromolecules 1994,
27, 6007. (b) Kato, T. Science 2002, 295, 2414.
(2) (a) Binnemans, K. Chem. ReV. 2005, 105, 4148. (b) Paleos, C. M. Mol.
Cryst. Liq. Cryst. 1994, 243, 159. (c) Tschierske, C. Prog. Polym. Sci.
1996, 21, 775.
(3) (a) Kim, Y. H. J. Am. Chem. Soc. 1992, 114, 4947. (b) Bauer, S.; Fischer,
H.; Ringsdorf, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 1589. (c) Park,
C.; Choi, K. S.; Jeon, H. J.; Song, H. H.; Chang, J. Y.; Kim, C. Langmuir
2006, 22, 3812.
(4) (a) Percec, V.; Cho, W. D.; Moeller, M.; Prokhorova, S. A.; Ungar, G.;
Yeardley, D. J. P. J. Am. Chem. Soc. 2000, 122, 4249. (b) Zeng, X.; Ungar,
G.; Liu, Y.; Percec, V.; Dulcey, A. E.; Hobbs, J. K. Nature 2004, 428,
157.
(5) (a) Lorentz, K.; Holter, D.; Stuhn, B.; Mu¨lhaupt, R.; Frey, H. AdV. Mater.
1996, 8, 414. (b) Pesak, D. J.; Moore, J. S. Angew. Chem., Int. Ed. Engl.
1997, 36, 1636.
(6) (a) Antonietti, M.; Kaul, A.; Thuenemann, A. Langmuir 1995, 11, 2633.
(b) Ponomarenko, E. A.; Tirrell, D. A.; MacKnight, W. J. Macromolecules
1996, 29, 8751. (c) General, S.; Antonietti, M. Angew. Chem., Int. Ed.
2002, 41, 2957. (d) McManus, J. J.; Radler, J. O.; Dawson, K. A. J. Am.
Chem. Soc. 2004, 126, 15966. (e) Franke, D.; Vos, M.; Antonietti, M.;
Sommerdijk, Faul, C. F. J. Chem. Mater. 2006, 18, 1839.
(7) Marcos, M.; Martin-Rapun, R.; Omenat, A.; Barbera, J.; Romero, P.;
Serrano, J. L. Chem. Mater. 2006, 18, 1206.
(8) Chen, Y.; Shen, Z.; Gehringer, L.; Frey, H.; Stiriba, S. E. Macromol.
Rapid Commun. 2006, 27, 69.
(9) Martin-Rapun, R.; Marcos, M.; Omenat, A.; Barbera, J.; Romero, P.;
Serrano, J. L. J. Am. Chem. Soc. 2005, 127, 7397.
(10) (a) Schlu¨ter, A. D.; Rabe, J. P. Angew. Chem., Int. Ed. 2000, 39, 864. (b)
Schlu¨ter, A. D. Top. Curr. Chem. 2005, 245, 151.
(11) Kase¨mi, E.; Zhuang, W.; Rabe, J. P.; Fischer, K.; Schmidt, M.; Colussi,
M.; Keul, H.; Yi, D.; Co¨lfen, H.; Schlu¨ter, A. D. J. Am. Chem. Soc. 2006,
128, 5091.
(12) Zhang, A.; Zhang, B.; Wa¨chtersbach, E.; Schmidt, M.; Schlu¨ter, A. D.
Chem.sEur. J. 2003, 9, 6083.
(13) Zhang, A.; Okrasa, L.; Pakula, T.; Schlu¨ter, A. D. J. Am. Chem. Soc.
2004, 126, 6658.
(14) Belarbi, Z.; Sirlin, C.; Simon, J.; Andre, J. J. J. Phys. Chem. 1989, 93,
8105.
(15) Lelievre, D.; Petit, M. A.; Simon, J. Liq. Cryst. 1989, 4, 707.
(16) Komatsu, T.; Ohta, K.; Watanabe, T.; Ikemoto, H.; Fujimoto, T.;
Yamamoto, I. J. Mater. Chem. 1994, 4, 537.
(17) Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P.; Richardson, R. M.
Whitehouse, I. J.; Rebrov, E. A.; Muzafarov, A. Z. Macromolecules 2000,
33, 5549.
On the basis of SAXS data and molecular calculations on the
size of alkyl tails, we propose the molecular organization model
sketched in Figure 2 for the LC polymers based on PG1 and PG2
complexed with sulfonated C8, C12, C18 lipids.
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