Self-assembling behavior of amphiphilic dendron coils in the bulk crystalline and liquid crystalline states

Article abstract of DOI:10.1021/ja801163m

We prepared a series of amphiphilic dendron coils (1-3) containing aliphatic polyether dendrons with octadecyl peripheries and a poly(ethylene oxide) (PEO) coil (DP = 44). The molecular design in this study is focused on the variation of dendron generation (from first to third) with a fixed linear coil, upon which the thermal and self-assembling behavior of the dendron coils was investigated in the bulk. All the dendron coils exhibit two crystalline phases designated as k₁ (both crystalline octadecyl chains and PEO) and k₂ states (crystalline octadecyl chains and molten PEO). Crystallinities for both octadecyl peripheries and the PEO decrease as generation increases. In particular, the dendron coil (3) containing third generation shows a drastic reduction of the PEO crystallinity, which is attributed to the considerable chain folding and plasticization effects by the largest hydrophilic dendritic core segment. All the crystalline phases are bilayered lamellar morphologies. On going from k₁ to k₂, the periodic lamellar thickness decreases in the dendron coil (1) with first generation, but interestingly increases in 3. After melting of octadecyl peripheries, 1 shows no mesophase (i.e., liquid crystalline phase). Additionally, dendron coil 2 (3) displays a network cubic mesophase with la3d symmetry (micellar cubic with Pm3n) which is transformed into a lamellar (hexagonal columnar) mesophase upon heating. Remarkably, the temperature-dependent mesomorphic behavior in 2 and 3 is a completely reverse pattern in comparison with conventional linear-linear block copolymers. The unusual bulk morphological phenomena in the crystalline and liquid crystalline phases can be elucidated by the dendron coil architecture and the associated coil conformational energy.

Full text of DOI:10.1021/ja801163m

Published on Web 05/03/2008
Self-Assembling Behavior of Amphiphilic Dendron Coils in
the Bulk Crystalline and Liquid Crystalline States
Yeon-Wook Chung,† Jeong-Kyu Lee,‡ Wang-Cheol Zin,‡ and Byoung-Ki Cho*,†
Department of Chemistry and Institute of Nanosensor and Biotechnology, Dankook UniVersity,
Gyeonggi-Do, 448-701, Korea, and Department of Materials Science and Engineering,
Pohang UniVersity of Science and Technology, Pohang 790-784, Korea
Received February 23, 2008; E-mail: chobk@dankook.ac.kr
Abstract: We prepared a series of amphiphilic dendron coils (1-3) containing aliphatic polyether dendrons
with octadecyl peripheries and a poly(ethylene oxide) (PEO) coil (DP ) 44). The molecular design in this
study is focused on the variation of dendron generation (from first to third) with a fixed linear coil, upon
which the thermal and self-assembling behavior of the dendron coils was investigated in the bulk. All the
dendron coils exhibit two crystalline phases designated as k1 (both crystalline octadecyl chains and PEO)
and k2 states (crystalline octadecyl chains and molten PEO). Crystallinities for both octadecyl peripheries
and the PEO decrease as generation increases. In particular, the dendron coil (3) containing third generation
shows a drastic reduction of the PEO crystallinity, which is attributed to the considerable chain folding and
plasticization effects by the largest hydrophilic dendritic core segment. All the crystalline phases are bilayered
lamellar morphologies. On going from k1 to k2, the periodic lamellar thickness decreases in the dendron
coil (1) with first generation, but interestingly increases in 3. After melting of octadecyl peripheries, 1 shows
no mesophase (i.e., liquid crystalline phase). Additionally, dendron coil 2 (3) displays a network cubic
mesophase with Ia3d symmetry (micellar cubic with Pm3n) which is transformed into a lamellar (hexagonal
columnar) mesophase upon heating. Remarkably, the temperature-dependent mesomorphic behavior in 2
and 3 is a completely reverse pattern in comparison with conventional linear-linear block copolymers.
The unusual bulk morphological phenomena in the crystalline and liquid crystalline phases can be elucidated
by the dendron coil architecture and the associated coil conformational energy.
Introduction
drimers/dendrons have displayed self-assembled micelles and
cylinders in the bulk because of their typical cone or tapered
A great deal of attention has been paid to the generation of
supramolecular nanostructures by self-assembling molecules due
to the tremendous potentials for the development of functional
materials.1 It has been well established that the morphological
feature of each nanostructure is entirely dependent upon the
identity of organizing building blocks and can be fine-tuned by
the elaborate engineering of related molecular parameters such
as polarity, stiffness, and architecture.2 Therefore, a hybridization
approach of architecturally distinct building blocks with their
own self-assembling characteristics may become a powerful tool
to develope a new class of self-assemblers.
Dendrimers/dendrons are monodisperse macromolecules with
regular and highly branched architectures consisting of dendritic
cores and peripheral sites. Such a peculiar structural feature
provides a variety of possible applications in the field of
host-guest, medicinal, and catalytic chemistries.3 In addition,
the dendrimers/dendrons have been used as a fascinating self-
assembling building block.4 The compartmentalization of
chemically distinct peripheries and the dendritic core can afford
to microphase-separated structures.5 In most cases, the den-
molecular shape.
These aesthetic dendrimers/dendrons have been combined
with other molecular building blocks such as rods,6 discs,7 or
linear coils into hybrid macromolecules for the creation of new
functional materials. In particular, linear coils have been
compared as a counterpart with dendrimers/dendrons because
of their extremely different molecular shapes, by which they
reveal distinct physical properties in terms of viscosity, hydro-
dynamic volume, glass transition temperature, etc.3c,8 In the last
15 years or so, because of the rapid advances in synthetic
technology, an effort has been made to combine these two
molecular extremes, in order to explore the role of chain
topology on the physical and chemical properties. In the early
stages, the aim was mostly focused on the preparation of
dendritic-coil macromolecular structures by using a variety of
(3) (a) Tomalia, D. A.; Baker, H.; Dewald, J. R.; Hall, M.; Kallos, G.;
Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117–
132. (b) Zeng, F. W.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681–
1712. (c) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV.
1999, 99, 1665–1688. (d) Newkome, G. R.; He, E.; Moorefield, C. N.
Chem. ReV. 1999, 99, 1689–1746.
† Dankook University.
(4) (a) Grayson, S. M.; Fre´chet, J. M. J. Chem. ReV. 2001, 101, 3819–
3867. (b) Hudson, H. D.; Jung, H.-T.; Percec, V.; Cho, W.-D.;
Johansson, G.; Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278,
449–452. (c) Gehringer, L.; Bourgogne, C.; Guillon, D.; Donnio, B.
J. Am. Chem. Soc. 2004, 126, 3856–3867. (d) Fisher, M.; Vo¨gtle, F.
Angew. Chem., Int. Ed. 1999, 38, 884–905.
‡ Pohang University of Science and Technology.
(1) (a) Lehn, J. M. Supramolecular Chemistry; VCH: Weinheim Germany,
1995. (b) Ciferri, A. Supramolecular Polymers, 2nd ed.; Taylor &
Francis: New York, 2005. (c) Kato, T. Science 2002, 295, 2414–2418.
(2) Tschierske, C. J. Mater. Chem. 2001, 11, 2647–2671.
9
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2008 American Chemical Society
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A R T I C L E S
Chung et al.
synthetic strategies such as anionic/living radical/ring opening
polymerizations, convergent/divergent synthetic approaches, and
coupling reactions.9 More recent interest has shifted to su-
pramolecular assembling systems applicable to material func-
tionalities. So far, many dendritic-coil systems that self-order
into nanometer-sized aggregates in a selective solvent have been
reported,10 while several examples11 have been shown to
organize into microphase-separated morphologies in the bulk
state (mostly in the solid state). Among them, Meijer and co-
workers reported the self-assembling behavior of dendritic-linear
hybrid copolymers consisting of poly(propylene imine) dendrons
functionalized with polar carboxylic acid groups and a hydro-
phobic linear polystyrene. They demonstrated that classical
hexagonal columnar and lamellar morphologies in the solid state
can be controlled as a function of dendron generation.11a
(5) (a) Cho, B.-K.; Jain, A.; Nieberle, J.; Mahajan, S.; Wiesner, U.; Gruner,
S. M.; Tu¨rk, S.; Ra¨der, H. J. Macromolecules 2004, 37, 4227–4234.
(b) Cameron, J. H.; Facher, A.; Lattermann, G.; Diele, S. AdV. Mater.
1997, 9, 398–403. (c) Percec, V.; Cho, W.-D.; Mosier, P. E.; Ungar,
G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120, 11061–11070.
(d) Percec, V.; Cho, W. D.; Ungar, G.; Yeardley, D. J. P. Angew.
Chem., Int. Ed. 2000, 39, 1597–1602. (e) Percec, V.; Glodde, M.;
Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balaguru-
samy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.;
Hudson, S. D.; Duan, H. Nature (London) 2002, 419, 384–387. (f)
Ungar, G.; Liu, Y.; Zeng, X.; Percec, V.; Cho, W. D. Science 2003,
299, 1208–1211. (g) Baars, M. W.; So¨ntjens, S. H. M.; Fisher, H. M.;
Peerlings, H. W. I.; Meijer, E. W. Chem.-Eur. J. 1998, 4, 2456–
2466. (h) Park, C.; Lee, I. H.; Lee, S.; Song, Y.; Rhue, M.; Kim, C.
Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1199–1203. (i) Kim, C.; Kim,
K. T.; Chang, Y.; Song, H. H.; Cho, T.-Y.; Jeon, H.-J. J. Am. Chem.
Soc. 2001, 123, 5586–5587. (j) Ponomarenko, S. A.; Boiko, N. I.;
Shibaev, V. P.; Richardson, R. M.; Whitehouse, I. J.; Rebrov, E. A.;
Muzafarov, A. M. Macromolecules 2000, 33, 5549–5558.
Figure 1. Molecular models of dendron coils 1-3.
Hammond and co-workers used stearate-terminated poly(ami-
doamine) (PAMAM) dendrons as a hydrophobic head and a
poly(ethylene oxide) (PEO) coil as a linear hydrophilic tail. They
observed that all the copolymers exhibited lamellar structures
at room temperature due to the crystallization force of the PEO
coil.11b Nevertheless, only a few examples have displayed
ordered morphologies in the melt (i.e., liquid crystalline state)
as well as solid state.12 As a representative example, Cho et al.
devised amphiphilic dendron coil molecules based on third
generation dendron with hydrophobic docosyl peripheries and
hydrophilic linear PEO coils.12a They demonstrated that unique
mesophase structure-mechanical and ionic transport correlations
are observed by keeping the dendron generation constant while
varying the length of the PEO coil.
For the engineering of crystalline and liquid crystalline
morphologies in the dendron coil hybrid system, an alternative
strategy is proposed utilizing variation of dendron generation
at a constant linear coil length. To this end, we prepared a series
of dendron coil hybrid molecules (1-3) which consist of
aliphatic polyether dendrons with hydrophobic octadecyl pe-
ripheries from first to third generation and a fixed linear PEO
coil with DP of 44 (Figure 1 and Scheme 1). Although the
molecular design concept seems to be similar to a previous
example,11a the enhanced incompatibility between hydrophilic
and hydrophobic moieties, due to the selection of the chemically
distinct molecular compositions, was capable of the study on
(6) (a) Lee, M.; Jeong, Y.-S.; Cho, B.-K.; Oh, N.-K.; Zin, W.-C. Chem.-
Eur. J. 2002, 8, 876–883. (b) Ryu, J.-H.; Kim, H.-J.; Huang, Z.; Lee,
E.; Lee, M. Angew. Chem., Int. Ed. 2006, 45, 5304–5307. (c) Zubarev,
E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2001,
123, 4105–4106. (d) Zubarev, E. R.; Sone, E. D.; Stupp, S. I. Chem.-
Eur. J. 2006, 12, 7313–7327.
(7) Choi, M.-S.; Aida, T.; Luo, H.; Araki, Y.; Ito, O. Angew. Chem., Int.
Ed. 2003, 42, 4060–4063.
(8) Hawker, C. J.; Malmstro¨m, E. E.; Frank, C. W.; Kampf, J. P. J. Am.
Chem. Soc. 1997, 119, 9903–9904.
(9) (a) Gitsov, I.; Wooley, K. L.; Hawker, C. J.; Ivanova, P. T.; Fre´chet,
J. M. J. Macromolecules 1993, 26, 5621–5627. (b) Gitsov, I.; Fre´chet,
J. M. J. Macromolecules 1994, 27, 7309–7315. (c) Leduc, M. R.;
Hawker, C. J.; Dao, J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1996, 118,
11111–11118. (d) Zhu, L.; Tong, X.; Li, M.; Wang, E. J. Polym. Sci.,
Part A: Polym. Chem. 2000, 38, 4282–4288. (e) Zhao, Y.-L.; Jiang,
J.; Liu, H.-W.; Chen, C.-F.; Xi, F. J. Polym. Sci., Part A: Polym. Chem.
2001, 39, 3960–3966. (f) Mecerreyes, D.; Dubois, P. H.; Je´roˆme, R.;
Hendrick, J. L.; Hawker, C. J. J. Polym. Sci., Part A: Polym. Chem.
1999, 37, 1923–1930. (g) Connal, L. A.; Vestberg, R.; Hawker, C. J.;
Qiao, G. G. Macromolecules 2007, 40, 7855–7863. (h) Emrick, T.;
Hayes, W.; Fre´chet, J. M. J. J. Polym. Sci., Part A: Polym. Chem.
1999, 37, 3748–3755. (i) Al-Muallem, H. A.; Knauss, D. M. J. Polym.
Sci., Part A: Polym. Chem. 2001, 39, 152–161. (j) Froimowicz, P.;
Paez, J.; Gandini, A.; Belgacem, N.; Strumia, M. Macromol. Symp.
2006, 245-246, 51–60.
(11) (a) Roma´n, C.; Fischer, H. R.; Meijer, E. W. Macromolecules 1999,
32, 5525–5531. (b) Johnson, M. A.; Iyer, J.; Hammond, P. T.
Macromolecules 2004, 37, 2490–2501. (c) Mackay, M. E.; Hong, Y.;
Jeong, M.; Tande, B. M.; Wagner, N. J.; Hong, S.; Gido, S. P.;
Vestberg, R.; Hawker, C. J. Macromolecules 2002, 35, 8391–8399.
(d) Pochan, D. J.; Pakstis, L.; Huang, E.; Hawker, C.; Vestberg, R.;
Pople, J. Macromolecules 2002, 35, 9239–9242. (e) Magbitang, T.;
Lee, V. Y.; Cha, J. N.; Wang, H.-L.; Chung, W. R.; Miller, R. D.;
Dubois, G.; Volksen, W.; Kim, H.-C.; Hedrick, J. L. Angew. Chem.,
Int. Ed. 2005, 44, 7574–7580. (f) Duan, X.; Yuan, F.; Wen, X.; Yang,
M.; He, B.; Wang, W. Macromol. Chem. Phys. 2004, 205, 1410–
1417. (g) Gao, Y.; Zhang, X.; Yang, M.; Zhang, X.; Wang, W.;
Wegner, G.; Burger, C. Macromolecules 2007, 40, 2606–2612.
(12) (a) Cho, B.-K.; Jain, A.; Gruner, S. M.; Wiesner, U. Science 2004,
305, 1598–1601. (b) Cho, B.-K.; Jain, A.; Gruner, S. M.; Wiesner, U.
Chem. Commun. 2005, 2143–2145. (c) Cho, B.-K.; Jain, A.; Gruner,
S. M.; Wiesner, U. Chem. Mater. 2007, 19, 3611–3614. (d) Kim, J.-
K.; Hong, M.-K.; Ahn, J.-H.; Lee, M. Angew. Chem., Int. Ed. 2005,
44, 328–332.
(10) (a) Chapman, T. M.; Hillyer, G. L.; Mahan, E. J.; Shaffer, K. A. J. Am.
Chem. Soc. 1994, 116, 11195–11196. (b) van Hest, J. C. M.; Delnoye,
D. A. P.; Baars, M. W. P. L.; Van Genderen, M. H. P.; Meijer, E. W.
Science 1995, 268, 1592–1595. (c) van Hest, J. C. M.; Baars,
M. W. P. L.; Elissen-Roma´n, C.; van Genderen, M. H. P.; Meijer,
E. W. Macromolecules 1995, 28, 6689–6691. (d) Iyer, J.; Fleming,
K.; Hammond, P. T. Macromolecules 1998, 31, 8757–8765. (e) Iyer,
J.; Hammond, P. T. Langmuir 1999, 15, 1209–1306. (f) Chang, Y.;
Kwon, Y. C.; Lee, S. C.; Kim, C. Macromolecules 2000, 33, 4496–
4500. (g) Aoi, K.; Motoda, A.; Okada, M.; Imae, T. Macromol. Rapid
Commun. 1997, 18, 945–952. (h) Barriau, E.; Marcos, A. G.; Kautz,
H.; Frey, H. Macromol. Rapid Commun. 2005, 26, 862–867. (i) Ge,
Z.; Luo, S.; Liu, S. J. Polym. Sci., Part A: Polym. Chem. 2006, 44,
1357–1371. (j) Gillies, E. R.; Jonsson, T. B.; Fre´chet, J. M. J. J. Am.
Chem. Soc. 2004, 126, 11936–11943. (k) Tian, L.; Hammond, P. T.
Chem. Mater. 2006, 18, 3976–3984.
(13) Chung, Y.-W.; Lee, B.-I.; Kim, H.-Y; Wiesner, U.; Cho, B.-K. J.
Polym. Sci. Part A: Polym. Chem. 2007, 45, 4988–4994.
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Self-Assembling Behavior of Dendron Coils
A R T I C L E S
Scheme 1. Synthesis of Dendron Coils 1-3
Table 1. Characterization of Dendron Coils 1-3
Mn (g/mol)
crystallinityh
hydrophilic
volume fraction
(fv)
dendron
coil
PDI
(Mw/Mn)
phase transitions (°C) and corresponding
enthalpy changes (J/g)
1H NMR
MALDI-TOF MS
PEO
octadecyl
1
2
3
2659.84
3340.51
4701.85
2512.20
3249.13
4591.16
1.01a-1.04b
1.00a-1.05b
1.00a-1.04b
0.75c-0.77d
0.62c-0.64d
0.49c-0.51d
k1 51.8 (104.3)e k2 73.2 (52.1)e i
0.75
0.70
0.35
0.93
0.49
0.46
k1 45.0 (76.8)e k2 52.6 (43.1)e nc 62.0f 1am 92.3g
k1 23.9 (27.4)e k2 49.5 (57.9)e mc 77.0f col 102.1g
i
i
a From MALDI-TOF MS. b From GPC. c k1 phase. d Melt state. e Second heating data from DSC. f From SAXS. g From DMS. h Crystallinity )
experimental heat of fusion of a sigle PEO (octadecyl group)/heat of fusion of a perfect crystalline PEO (octadecyl group). Experimental heat of fusion
of a PEO(octadecyl group) (kJ/mol) ) heat of fusion (J/g) from DSC × mass of a PEO (octadecyl group)/weight fraction. The heat of fusion of a single
perfect crystalline PEO (octadecyl group) was calculated on the basis of the heat of fusion of a repeat unit in the perfect crystalline of 8.4 kJ/mol (4.11
kJ/mol). k ) crystalline, nc ) network cubic, lam ) lamellar, mc ) micellar cubic, col ) hexagonal columnar, i ) isotropic liquid.
the self-assembling behavior in both liquid crystalline (i.e.,
mesophase) and solid phases.
molecular weight of 2000 g/mol from Aldrich Corp. As
measured by H NMR spectroscopy, the average number of
1
In this paper, we report on the details of synthesis, and the
thermal and self-assembling properties of dendron coils 1-3,
providing insight on the correlation between chain topology and
physical properties, and a molecular design concept for the
creation of a new class of self-assembling soft materials. In
particular, unprecedented findings in crystalline and liquid
crystalline assemblies as a function of dendron generation or
temperature, for example, crystalline lamellar dimensional
variation and temperature-dependent phase transition, are
described.
repeating units (DP ) 44) in the PEO of the dendron coils
determined from the ratio of the four protons next to the
carbonyl groups to the ethylene protons of the PEO backbone,
was almost identical to the value of DP ) 45 assigned by the
chemical supplier. The number average molecular weights (Mn)
in Table 1 were calculated on the basis of the NMR measure-
ments. The hydroxyl ends of both the dendron and the PEO
coil were converted into amine groups by the following reaction
sequence; (i) tosylation by tosyl chloride, (ii) azidation by
sodium azide, and (iii) reduction by lithium aluminum hydride.
Most of the reaction steps were performed in good yields above
70%. For the coupling of the dendron and coil segments, each
dendron generation was functionalized with a carboxylic acid
end group by reacting with excess succinic anhydride. The final
coupling reaction of carboxylic acid terminated dendrons (n-
Results and Discussion
Synthesis. The synthetic procedure is outlined in Scheme 1.
The synthesis of the dendrons (n-18) from first to third
generation containing aliphatic polyether dendritic cores and
octadecyl peripheries was performed via a convergent synthetic
method consisting of a Williamson etherification and hydrobo-
ration/oxidation reactions.13,14 As a fixed hydrophilic coil, we
employed a poly(ethylene glycol) methyl ether with average
(14) Jayaraman, M.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1998, 120, 12996–
12997.
(15) Grason, G. M.; DiDonna, B. A.; Kamien, R. D. Phys. ReV. Lett. 2003,
91, 58304.
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A R T I C L E S
Chung et al.
Figure 2. (a) GPC curves and (b) MALDI-TOF mass spectra of 1-3.
Figure 3. DSC thermograms of 1-3 from (a) second heating and (b) first cooling scans.
18-acid) and amine terminated PEO was performed using N,N′-
diisopropylcarbodiimide in the presence of 4-dimethylaminopy-
ridine.
Thermal Characterization. The thermal behavior of 1-3 was
studied by differential scanning calorimetry (DSC), temperature
dependent small-angle X-ray scattering (SAXS), and dynamic
mechanical spectroscopy (DMS). The results are summarized
in Table 1. As shown in Figure 3, the DSC thermograms of
1-3 display two melting transitions which can be observed
reversibly on both heating and cooling scans. Considering the
molecular compositions, the melting transitions observed at the
lower and higher temperatures correspond to the meltings of
the crystalline PEO coil and octadecyl peripheral chains,
respectively. Both melting transition temperatures decrease on
going from 1 to 3. This phenomenon might be attributed to the
reduction of crystalline parts in both alkyl and PEO regions as
the dendron generation increases. Table 1 presents the degrees
of crystallinity in both melting transitions for 1-3. The values
can be determined by comparing the heats of fusion on the
second heating to that of a perfect crystalline octadecyl chain
(PEO coil). For octadecyl peripheries, the crystallinity decreases
The resulting dendron coil molecules (1-3) were character-
ized by 1H NMR and matrix-assisted laser desorption ionization
time-of-flight mass spectrometry (MALDI-TOF MS), elemental
analysis (EA) and gel permeation chromatography (GPC). All
dendron coils show narrow polydispersities (Mw/Mn) of less than
1.05 in GPC (Figure 2a and Table 1), and exhibit signals for
the R-protons to the carbonyl groups at 2.56 ppm. Elemental
analysis data of 1-3 are also in good agreement with the
theoretical values.
Figure 2b is the MALDI-TOF mass spectra of 1-3. The Mn
values of 1-3 were estimated to be 2512, 3249, and 4591 g/mol,
respectively. Theses molecular weights are consistent with the
Mn values determined from the NMR measurement within a
5% error range. In addition, their polydispersities from MALDI-
TOF MS are less than 1.01, indicative of high purity (Table 1).
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Self-Assembling Behavior of Dendron Coils
A R T I C L E S
Figure 4. Elastic modulus G′ as a function of temperature for dendron coils 1-3.
from 93, 49, to 46% with increasing dendron generation. We
conjecture that the reduction is strongly dependent on the
variation of intrinsic curvature with each generation. The
interface between peripheral groups and dendritic core would
be more curved with increasing generation number because of
the starburst molecular shape, which decreases the packing area
of octadecyl peripheries, resulting in the reduction of crystal-
linity. Also, the PEO crystallinity decreases as generation
increases. Particularly, a significant reduction from 70 to 35%
occurs between 2 and 3. This might be predominantly caused
by the degree of chain folding in PEO crystals depending on
dendron size. Because the dendron cross section of 3 is the
largest, the PEO crystals formed by 3 may contain the largest
folding defect, resulting in the reduction of crystallinity. In
addition, a plasticization of PEO coils by amorphous dendritic
cores can influence the crystallinity to some extent.9a Since the
basic repeating unit of aliphatic polyether dendritic cores, that
is, CH2CH(CH2O)2, is almost identical to a doubled poly(eth-
ylene oxide) structural unit: 2 × (CH2CH2O), it is probable that
the dendritic cores plasticize the crystalline linear PEO coils.
The size of aliphatic polyether dendritic cores increases by a
factor of 2 with each generation, therefore both chain folding
and plasticization effects are expected to be maximized in 3,
rationalizing the precipitous reduction of PEO crystallinity in
3.
Meanwhile, phase transitions in the melt were difficult to
observe in the DSC data. Our dendron coils consist of fully
aliphatic chains, thus the associated energy to phase transitions
such as mesophase-to-mesophase/mesophase-to-liquid, might be
too small to be identified by DSC. However, the phase
transitions are accompanied by detectable mechanical change,
for example, elastic modulus (G′). Therefore, the transitions were
able to be monitored by DMS measurements (Figure 4). As
determined by combination of temperature variable SAXS data,
the phase transitions in the melt are summarized in Table 1.
Self-Assembly in the Crystalline k1 and k2 Phases. To
investigate the microstructures in the crystalline and liquid
crystalline phases, SAXS experiments have been performed with
the dendron coils at various temperatures. The SAXS patterns
of all the dendron coils in k1 and k2 phases display multiple
reflections with q-spacing ratios of 1:2:3..., indicative of lamellar
structures (Figure 5). From the observed primary peak, d-
Figure 5. SAXS patterns of 1-3 in k1 and k2 phases.
Table 2. Characterization of Dendron Coils 1-3 by Small-Angle
X-Ray Scatteringa
crystalline
liquid crystalline
k1 - lamellar
d-spacing
(nm)
k2 - lamellar
d-spacing
(nm)
lamellar
d-spacing
(nm)
hexagonal
columnar
c (nm)
dendron
coil
ia3d
a (nm)
Pm3n
b (nm)
1
2
3
17.03
12.90
9.78
16.38
12.90
10.01
21.32
7.47
20.49
9.25
a a, b, and c are the lattice constants of the structures.
spacings in the crystalline phases were estimated and sum-
marized in Table 2.
For the details of packing structure, we can assume that the
molecular section of a single molecule is a square column in
the lamellar structure. On the basis of that assumption and the
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Chung et al.
Figure 6. Molecular sections of (a) 1, (b) 2, and (c) 3 in the bilayered crystalline phases; the blue and red colors represent the hydrophobic and hydrophilic
parts, respectively.
fully stretched octadecyl chain length of 2.34 nm from CPK
model, the lengths of the molecular sections were calculated to
be 8.79, 5.83, and 4.33 nm for 1-3, respectively. The decrease
of the calculated molecular sectional length from 1 to 3 is quite
reasonable because the cross sectional area of the molecular
section increases proportionally with dendron generation. When
the molecular sectional length is compared with the lamellar
d-spacing, the latter is approximately twice of the former.
Therefore, it can be concluded that the dendron coils form
bilayered lamellar structures in k1 and k2 crystalline phases
(Figure 6).
Additionally, it is interesting to note the subtle d-spacing
change between k1 and k2 in 1-3. When comparing between
k1 and k2, the lamellar d-spacing slightly decreases in 1, but
somewhat increases in 3. We speculate that this reverse trend
between 1 and 3 is attributed to the variation of conformation
energy and density of PEO coils upon melting of PEO coils. In
contrast to k1 where PEO coils are crystalline, PEO coils in k2
are molten. Therefore, the conformational energy of PEO coils
in k2 is closely dependent upon the shape of the space occupied
by them in the bilayered lamellar structure. As shown in Figure
6, the PEO space becomes isotropic as the dendron generation
increases, thus we can expect that the PEO conformational
energy (i.e., stretching penalty) would be the highest in 1 and
the lowest in 3. In this regard, one way to reduce the PEO
conformational energy in 1 might be a slight lateral expansion
Figure 7. SAXS patterns of 2 and 3 in the liquid crystalline phases.
of linear coil conformation, although at the present moment we
are not aware of how to change the octadecyl packing.
Characterization of the Liquid Crystalline Phases of 2 and
3. As supported by the DMS data, upon melting of the octadecyl
peripheries dendron coils 2 and 3 show two liquid crystalline
phases (mesophases) before disordering into an isotropic liquid
at 92.3 and 102.1 °C, respectively. The microstructures were
characterized by a temperature-dependent SAXS technique. In
Figure 7a, the SAXS pattern of 2 at 55 °C exhibits multiple
reflections with q-spacing ratios of ꢀ6:ꢀ8:ꢀ14:ꢀ16:ꢀ20:ꢀ22.
Consequently, the overall d-spacing in k2 may be estimated to
be slightly smaller than that in k1. On the contrary, the PEO
section of 3 in k1 would be relatively isotropic to adopt more
random coil-like conformations. In this situation, the increased
PEO volume upon melting of the PEOs might lead to the slight
increase of the overall d-spacing. In intermediate dendron coil
2, we assume that no change in the d-spacing was observed as
a result of the compensation of these two factors.
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Figure 8. Schematic sketches of (a) network cubic (Ia3d symmetry), (b) lamellar, (c) micellar cubic (Pm3n symmetry), and (d) hexagonal columnar mesophases
of dendron coils 2 (a,b) and 3 (c,d). The blue and red colors represent the hydrophobic and hydrophilic regions, respectively. For reasons of clarity, the
hydrophobic matrix is omitted in (a) and (c).
These peaks can be indexed as the (211), (220), (321), (400),
(420), and (332) reflections of a cubic structure with Ia3d
symmetry (Table S1, Supporting Information). From the ob-
served d-spacing of (211) reflection, the best fit value for the
cubic lattice parameter is 21.32 nm (Figure 8a). Upon further
heating, 2 displays two reflections with a q-spacing ratio of 1:2
at 70 °C, indicating a lamellar mesophase with a periodic
thickness of 7.47 nm (Figures 7b and 8b).
As compared with the d-spacing of 12.90 nm in the crystalline
lamellar phases, the periodic thickness in the lamellar mesophase
suggests a fundamental change in the molecular organization.
Assuming the length of a stretched octadecyl group to be 2.34
nm, the ratio of the hydrophilic layer thickness (5.13 nm) with
respect to the d-spacing (7.47 nm) is 0.68 which is ap-
proximately identical to the hydrophilic volume fraction of 0.64.
Since molecular volumes are linearly proportional to d-spacing
in lamellar structures, this strongly suggests that the dendron
segments in the lamellar mesophase self-assemble into an
interdigitated monolayer (Figure S1, Supporting Information).
The change in the arrangement of peripheral chains from a
bilayer to an interdigitated monolayer may relieve the confor-
mation of flexible PEO coils in the molten state by enlarging
the cross-sectional area.
For dendron coil 3, with the largest third generation, a cubic
morphology is detected after melting of the peripheries. In
Figure 7c, the SAXS pattern at 60 °C exhibits a considerably
larger number of reflections which can be indexed as the (110),
(200), (210), (211), (220), (310), (321), (400), (420), (422),
(521), and (440), indicative of a Pm3n cubic symmetry (Table
S2, Supporting Information). From the dimension of the (200)
reflection, the cubic lattice parameter can be estimated as 20.49
nm (Figure 8c). Upon further heating, the SAXS pattern changes
into four reflections with q-spacing ratios of 1:ꢀ3:ꢀ4:ꢀ7 which
is characteristic of a 2-D hexagonal columnar structure (Figures
7d and 8d). The lattice constant is determined to be 9.25 nm.
columnar for 3, suggests that the Ia3d and Pm3n cubic phases
are continuous network and discrete micellar morphologies,
respectively. Furthermore, a recent theoretical approach dem-
onstrated that a micellar cubic phase with Pm3n symmetry exists
over a relatively large volume fraction range in dendritic-linear
hybrid block systems.15 On the basis of these results, it should
be noted that the molecular approach proposed in this study,
that is, a simple Variation of dendron generation at a fixed linear
coil, provides a versatile way to engineer lamellar crystalline
and a variety of liquid crystalline phases including micellar,
columnar, lamellar, continuous cubic structures.
Temperature-Dependent Phase Behavior. Besides the rich
phase behavior, the most notable feature of the dendron coils
is the unusual temperature-dependent phase sequence in the melt
states of 2 and 3. On melting of octadecyl peripheries, dendron
coil 2 displays a network cubic mesophase which is transformed
into a lamellar mesophase. Likewise, dendron coil 3 displays a
micellar cubic mesophase, which is transformed into a hexagonal
columnar mesophase. These phase sequences are quite unusual
in comparison with other block copolymers, which show a
completely reverse phase sequence. As temperature increases,
conventional block copolymers with linear-linear architecture
have been known to conform to the following phase sequence;
from lamellar, continuous network cubic, hexagonal columnar
to micellar cubic mesophases in the melt.16 The remarkable
contrast between our dendron coils and other block copolymers
might be originated from their polymeric chain architectures.
In conventional block copolymers with linear-linear components,
for a given microstructure with a curved interface (e. g.,
columnar or micellar), a linear block with a larger volume
fraction (or longer coil length) tends to be located in the outer
(16) (a) Hamley, I. W. The Physics of Block Copolymers; Oxford Univ.
Press: New York, 1998. (b) Hillmyer, M. A.; Bates, F. S.; Almdal,
K.; Mortensen, K.; Ryan, A. J.; Fairclough, J. P. A. Science 1996,
271, 976–978.
(17) (a) Frischknecht, A.; Fredrickson, G. H. Macromolecules 1999, 32,
6831–6836. (b) Pickett, G. T. Macromolecules 2002, 35, 1896–1904.
(c) Grason, G. M. Phys. Rep. 2006, 433, 1–64.
The observed phase sequence as a function of dendron
generation, that is, Ia3d cubic/lamellar for 2 and Pm3n cubic/
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Figure 9. Molecular sections in various morphologies. In the dendron coil system, it is expected that the bulky hydrophobic peripheries are located in the
outer matrix, on the other hand the hydrophilic PEO coil occupies inner core.
Figure 10. SAXS data for (a) 2 and (b) 3 on heating and cooling scans.
matrix, while the other linear block with the smaller volume
fraction occupies the inner core. Through such a location, the
longer coil is able to adopt more stable conformations like a
random coil, which relieves the overall conformational energy.
To this end, the phase diagram of the linear-linear block
copolymer system shows an approximately symmetric shape,
although the asymmetry factor between the blocks distorts the
phase diagram to some extent.
In contrast, the dendron coil molecules are composed of
architecturally distinct dendron and linear segments. From this
structural feature, despite the smaller hydrophobic volume
fractions the bulky peripheries are favored in the outer matrix,
in order to reduce the steric hindrance. In this configuration,
the curved interface relaxes the peripheral octadecyl chains at
the expense of an increased stretching penalty of the PEO
chain.17 By considering the fact that the hydrophobic peripheries
occupy the outer matrix, despite the major hydrophilic volume
fractions of 2 and 3 (i.e., 0.64 and 0.51), it is suggested that the
phase boundaries in the present dendron coil system are
significantly shifted toward larger volume fractions of the
hydrophilic part when compared to conventional linear-linear
block copolymers. As the temperature elevates, however, the
conformational energy of the PEO coil in the inner core would
be increased because the conformational state of the longer PEO
chain is more than that of the octadecyl chain. Therefore, at
higher temperatures the morphology should change to a different
morphology, one which can alleviate the stretching penalty of
the PEO chain. By comparing the interfacial curvature of the
(18) Milner, S. T. Macromolecules 1994, 27, 2333–2335.
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network cubic (micellar cubic) structure with that of the lamellar
(hexagonal columnar) structure, the former would be larger than
the latter. This reduction of the interfacial curvature seems very
surprising when compared to linear-linear block copolymers,
but in our dendron coil system where the phase inversion hardly
occurs, this curvature change is quite plausible. Here, it would
be useful to take into account the molecular sections in the
various morphologies presented by Milner (Figure 9).18 As the
morphology moves from micellar to lamellar structures, the inner
coil conformation becomes more isotropic, which allows the
linear coil to adopt more random coil-like conformations. Thus,
this consideration clearly explains the unique mesophase
transformation upon heating in our dendron coil system.
It is also important to stress the fact that the observed phase
transitions take place in completely reversible fashion on both
heating and cooling scans, as evidenced by the SAXS data
(Figures 10). Frey et al. reported a similar phase transition from
hexagonal columnar to lamellar structures on heating in their
dendritic poly(carbosilane)-b-linear polystyrene copolymers.19
However, the hexagonal columnar structure observed at lower
temperatures was revealed as a metastable phase which trans-
forms into the lamellar structure when annealed. In this regard,
the reversibility of phase transition in our system is quite unique.
1, while the d-spacing of 3 in the k2 is a bit larger that that in
the k1. After the melting of octadecyl peripheries, no mesophase
was observed in dendron coil 1. On the other hand, 2 and 3
were shown to self-assemble into two different kinds of
mesophases as a function of temperature, respectively. As
convinced by SAXS data, upon heating 2 spontaneously
organizes a network cubic mesophase (nc) with Ia3d symmetry,
which is transformed into a lamellar mesophase (lam). In
dendron coil 3, it has been proven that a micellar cubic
mesophase (mc) with Pm3n symmetry turns into a hexagonal
columnar mesophase (col) as temperature increases. Despite the
observed rich phase behavior, the most remarkable feature in
this study is the temperature dependent phases sequence; the
transformations as a function of temperature are quite unusual
in comparison to conventional linear-linear block copolymers
which have shown the reverse phase sequence, for example,
lam-to-nc/col-to-mc. The reversibility of the transformations is
another unique characteristic as compared to a similar transition
observed in linear-hyperbranched block copolymers. Conse-
quently, the molecular design concept proposed in this study
demonstrated that a variety of morphologies and unusual phase
transformations in the bulk crystalline and liquid crystalline
phases can be tuned as a function of dendron generation/or
temperature, and furthermore, may provide a versatile strategy
for nanostructured functional soft materials.
Conclusions
We prepared a series of amphiphilic dendron coils 1-3
consisting of aliphatic polyether dendrons with hydrophobic
octadecyl peripheries and a hydrophilic linear PEO coil. For
the engineering of a variety of crystalline and liquid crystalline
morphologies in the bulk, a molecular design approach, that is,
variation of the dendron generation from first to third at a fixed
PEO coil, was employed. All the dendron coils show two
crystalline phases k1 and k2 which turned out to be bilayered
lamellar morphologies. Interestingly, on transitioning from k1
to k2, the lamellar d-spacing changes in different ways depending
on the dendron coils. The d-spacing decreases in dendron coil
Acknowledgment. This work was supported by the Korea
Science and Engineering Foundation (R01-2006-000-11221-0),
Korea Research Foundation (D00069), and a grant from the
Fundamental R&D Program for Core Technology of Materials
funded by the Ministry of Commerce, Industry and Energy,
Republic of Korea. We acknowledge Pohang Accelerator Labora-
tory (Beamline 10C1), Korea.
Supporting Information Available: Experimental sections;
Tables S1 and S2; Figure S1. This material is available free of
charge via the Internet at http://pubs.acs.org.
(19) Marcos, A. G.; Pusel, T. M.; Thomann, R.; Pakular, T.; Okrasa, L.;
Geppert, S.; Gronski, W.; Frey, H. Macromolecules 2006, 39, 971–
977.
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