Vinyl-terminated liquid-crystalline dendrimers based on dendritic polyols and their siloxane-based elastomers

Article abstract of DOI:10.1002/pola.26373

A new series of side-chain liquid-crystalline dendrimers (LCDs) by grafting vinyl-terminated phenyl benzoate-based promesogens to a novel polypropyleneimine-derived dendritic polyols are reported. Polarized optical microscopy and X-ray diffraction studies

Full text of DOI:10.1002/pola.26373

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Vinyl-Terminated Liquid-Crystalline Dendrimers Based on Dendritic
Polyols and Their Siloxane-Based Elastomers
Yuteng Lin, Jianzhong Sun
State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University,
Hangzhou 310027, China
Correspondence to: J. Sun (E-mail: bigwig@zju.edu.cn)
Received 5 July 2012; accepted 1 September 2012; published online 28 September 2012
DOI: 10.1002/pola.26373
ABSTRACT: A new series of side-chain liquid-crystalline den-
drimers (LCDs) by grafting vinyl-terminated phenyl benzoate-
based promesogens to a novel polypropyleneimine-derived
dendritic polyols are reported. Polarized optical microscopy
and X-ray diffraction studies show that both the compounds
display a smectic-A (SmA) mesophase. The second-genera-
tion dendrimer bearing eight-branched promesogens exhibits
a more stable SmA mesophase with a wide mesomorphic
temperature range. It is demonstrated that ‘‘promoting
groups’’ in the structure of LCD for the enhancement of mes-
omorphic stability are unnecessary in the case of strong ani-
sotropic interactions. In contrast to conventional LCDs, these
two compounds possess reactive vinyl terminals that endow
them with the potential for the preparation of polymeric
materials. For the first time, a type of thermoset elastomers
is explored from LCDs via hydrosilylation crosslinking reac-
tion of vinyl terminals and siloxane crosslinker. Two-dimen-
sional X-ray diffraction study indicates that the lamellar
structures of original dendrimers are reserved in the elasto-
mer networks. Stress–strain curves reveal that these elasto-
mers exhibit excellent elasticity under successive uniaxial
compression. The combination of anisotropic structures of
rigid units and elasticity of flexible networks in this novel
series of elastomers makes them promising candidates for
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2012
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 51:
71–83, 2013
KEYWORDS: dendrimers; liquid crystal; crosslinking; siloxane;
elastomers
INTRODUCTION Dendrimers, as a fascinating class of macro-
molecular family, have intrigued a great deal of interest of
scientists in various areas. In contrast to random hyper-
branched polymers, dendrimers possess a perfectly defined
core–shell molecular architecture consisting of a multivalent
core surrounded by repetitive branching cell units that
attached definite number of functional peripheries.1 These
three components are assembled via divergent method or
convergent method, and both these methods require an iter-
ative sequence of activation and coupling steps.2–4 The hier-
archical assembly synthetic strategies of dendrimers endow
them with precisely controlled molecular size and shape,
monodispersity, absence of chain entanglements, and expo-
nential growth of terminal functional groups. Owing to the
spectacular features, dendrimers are potentially promising
for versatile applications such as unimolecular container, cat-
alyst support, chiral recognition, light-harvesting antenna,
and drug or gene delivery.5–9
different types of liquid-crystalline phases by the incorpora-
tion of anisometric mesogenic groups. Mesophases of these
liquid-crystalline dendrimers (LCDs)10–13 result from the
molecular arrangement at supramolecular level, which is
determined by both the enthalpic gain provided by aniso-
tropic interactions of the mesogenic units and the micro-
phase segregation of two chemically incompatible molecular
regions (flexible dendritic core and terminal mesogenic
units). Percec and Kawasumi14 described the first LCD
derived from incorporating mesogenic units within a hyper-
branched polymer that yielded a nematic phase. With the
introduction of the perfect dendritic architecture, various
mesophases including smectic phase, hexagonal (Colh), and
rectangular (Colr) columnar phase have been observed15–20
in these monodispersed LCDs. Among LCDs, side-chain LCDs
(SC-LCDs) are the most commonly studied. In this approach,
mesogenic or promesogenic units are terminally (end-on)
or laterally (side-on) attached to the periphery of the
preformed dendrimer [polypropyleneimine (PPI), polyamido-
amine (PAMAM), polysiloxane, polycarbosilane, and polycar-
bosilazane]. Terminally attached SC-LCDs exhibit smectic
phase commonly, whereas nematic phases are discovered in
The combination of dendrimers with certain fragments at
the periphery or in the interior provides the possibility to
obtain functional assembles and overall modified macro-
scopic packing. Dendrimers are able to self-organize into
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most cases of side-on SC-LCDs.21,22 Increasing the generation
number or number of terminal alkyl chains linked to the
mesogenic units can give rise to a columnar mesophase due
to self-assembly of disk-like arrangement.23–27 If the branch-
ing cells are anisometric segments other than long flexible
chains, the main-chain LCDs will be obtained. Representative
examples of this group is octopus LCDs. Because of the
hierarchic incorporation of anisotropic moieties in dendritic
branches, the dendrimers favor a regular anisotropic order
inducing by a gain in enthalpy. The octopus LCDs show the
capability of exhibiting Colh mesophase when they bear
more than one terminal chains, analogous to structure–orga-
nization relationship in SC-LCDs.28–30 Moreover, besides the
2D-typed mesophase, molecular engineering of LCDs can
also give birth to some complicated types of 3D-stacking
mesophases such as micellar cubic phase (CubI), tetragonal
P42/mnm phase, and liquid quasicrystal phase.31,32
question, there is still a need to design new LCDs that avoid
the existence of the promoting groups such as imino, amide,
fluorinated, and cyano groups. With this in mind, the intro-
duction of stable ester linkages and alkyl terminal chains
may be good choice. As for the second question, materials
from liquid crystal molecules such as liquid crystal elasto-
mers have been shown to give intriguing properties. Liquid
crystal elastomers, derived from linear liquid crystal mono-
mers, have become an attractive class of liquid crystal mate-
rials. The novel macroscopic feature of combining elasticity
of polymer networks and anisotropic structure of liquid-
crystalline phases offered liquid crystal elastomers the pos-
sibility of application in artificial muscles and active smart
surfaces.41–49 Accordingly, we were interested in transfer-
ring the concept of liquid crystal elastomers to LCDs by pre-
paring elastomers via LCDs owning more prefect anisotropic
structures but not randomly formed liquid-crystalline den-
dritic network.50 To open up the possibility, the terminals of
the LCDs should be modified with reactive groups such as
crosslinkable vinyl group,51 curable epoxide group,52 and
photopolymerizable acrylate group,53 other than the con-
ventional nonreactive groups (saturated alkyl, fluorinated,
and cyano groups). Taking into consideration the concerns
of the two issues above simultaneously, a series of new
LCDs consisting of dendritic part with ester linkages, meso-
genic units, and unsaturated alkyl chains (vinyl terminals)
will be exploited.
Although the formation of mesophases of LCDs was induced
by the anisotropic interactions and microphase segregation,
some ‘‘promoting groups’’ (usually heteroatom-containing
groups) that promote but not induce the anisotropic interac-
tion of liquid crystal units or the phase segregation were
usually incorporated into the backbones of the dendrimers
to enhance the stability of the mesophases. Imino21–27 and
amide28–30,33 groups were commonly seen in the dendritic
part of LCDs. The formation of intramolecular and intermo-
lecular hydrogen bonds between these groups helped to
maintain a stable aggregation of the dendritic structure,
which played a crucial role in the mesogenic behaviors.
Furthermore, heteroatom-containing terminals such as fluori-
nated34–38 and cyano15,16,39,40 groups would promote the
formation of mesophases. The phase-segregation power of
fluorinated terminal chains was confirmed to stabilize the
liquid-crystalline state. As for the cyano groups, their large
dipole moment enable the mesogenic units to preferentially
orient into an antiparallel structure and therefore enhance
the anisotropic interactions.
Therefore, in this work, we first designed a benzoate-type
premesogen with a reactive vinyl terminal. Second, a novel
type of dendritic polyols, G1-(OH)4 and G2-(OH)8, owning
hydroxyl peripheries other than amine peripheries of PPI or
PAMAM, were synthesized according to a two-step proce-
dure. As a consequence of the attachment of these polyols
with vinyl-terminated promesogens using a stable covalent
ester linkage, which was rarely reported in SC-LCDs,54–57
side-chain dendrimers G1-(MU)4 and G2-(MU)8 bearing four
or eight promesogenic units were obtained without incorpo-
ration of any promoting group. The mesogenic behaviors of
these two dendrimers were then investigated by a combina-
tion of analytical techniques such as differential scanning
calorimeter (DSC), polarized optical microscope (POM), and
X-ray diffraction (XRD). Moreover, the reactive vinyl
terminals of this series of LCDs endowed them with the pos-
sibility of further evolution toward polymeric materials. Con-
sidering the characteristics of the vinyl terminals, a classic
technique of siloxane–vinyl hydrosilylation reaction that was
most commonly used in the preparation of liquid crystal
elastomers41–44,46–49 was applied. The crosslinking reaction
of siloxane crosslinker with the terminal vinyl bonds of
G1-(MU)4 or G2-(MU)8 yielded a novel type of thermoset
elastomers, in which the molecular packing of original LCDs
was observed to maintain in the network of elastomers.
During successive uniaxial compression experiments, the
stress–strain curves revealed that these elastomers exhibited
excellent elasticity that indicated the combination of aniso-
tropic structures of rigid units and elasticity of flexible net-
works in this series of elastomers.
As mentioned above, the study of LCDs always focused on
the structure–mesomorphism relationship. LCDs with novel
molecular structures and exotic mesophases come to the
fore successively. Nevertheless, considering the particular
three-component molecular structure (dendritic core, meso-
genic segment, and terminal chains) of LCDs, two fundamen-
tal questions arise:
1. If no chemical group (such as amide and fluorinated
group) that will promote the formation of mesophases
exists in the dendritic core or at the terminals, then can
the dendrimer still display a mesophase? What is the type
of the mesophase and what about its stability?
2. As a type of molecules owning unique anisotropic molecu-
lar packing, can LCDs be used to prepare polymeric mate-
rials such as thermoplastics, thermosets, or elastomers?
Will these materials derived from LCDs present any spe-
cial properties?
Until recently, no detailed study with respect to the men-
tioned issues has been performed. Concerning the first
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EXPERIMENTAL
1.99 (m, 2H), 1.73 (m, 2H), 1.40–1.26 ppm (overlapped
peaks, m, 12 H). 13C NMR (DMSO-d6): d ¼ 166.99, 164.16,
163.73, 154.56, 139.17, 132.48, 131.24, 128.69, 122.53,
120.77, 115.06, 114.96, 68.36, 33.54, 29.31, 29.17, 29.08,
28.87, 28.64, 25.76 ppm. Electrospray ionization-mass spec-
trometry (ESI-MS): m/z: 409.2 [M-Hþ].
Materials
10-Undecen-1-ol, 4-hydroxybenzaldehyde, N, N0-dicyclohexyl-
carbodiimide (DCC), 4-dimethylaminopyridine (DMAP), so-
dium chlorite, and 1,4-diaminobutane were purchased from
Acros and used as received. p-Toluenesulfonyl chloride (TsCl),
methyl 4-hydroxybenzoate, tert-butyl acrylate, LiAlH4, and res-
orcinol (from TCI) were used as received. Polypropyleneimine
tetramine dendrimer (first generation, DAB-Am-4) and
platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane [Pt(dvs)]
complex solution (in xylene, 2%) was purchased from Aldrich.
Tetrakis(dimethylsiloxy)silane (TDS) was purchased from Alfa
Aesar. Pyridine, CH2Cl2, THF, and toluene were purified
according to standard protocols. Resorcinol, tert-butanol, ace-
tone, NaOH, K2CO3, NaH2PO4ꢀ2H2O, MgSO4, Na2SO4, hydro-
chloric acid (37 wt % aqueous solution), MeOH, EtOH, and
Et2O were used as received.
Synthesis of Dendritic Core with Hydroxyl Periphery
Tetra-tert-butyl 3,30,300,3000-(butane-1,4-diylbis(azanetriyl))
tetrapropanoate (6)
Tert-butyl acrylate (6.5 mL, 44.6 mmol) was added dropwise
to a solution of 1,4-diaminobutane (0.758 g, 8.60 mmol) in
MeOH (10 mL) under Ar atmosphere. The reaction mixture
was stirred for 24 h at room temperature. The volatiles of
the mixture were evaporated under vacuum to afford a yel-
low viscous liquid (4.14 g, 80% yield).
1H NMR (CDCl3): d ¼ 2.71 (t, 8H), 2.39–2.32 (overlapped
peaks, m, 12H), 1.44–1.40 ppm (overlapped peaks, m, 40H).
13C NMR (CDCl3): d ¼ 171.98, 80.07, 53.47, 49.14, 33.55,
27.98, 24.96 ppm. ESI-MS: m/z: 601.4 [MþH]þ.
Synthesis of Promesogen
Compound 3 was synthesized via a three-step technique
according to a literature procedure.58
3,30,300,3000-(Butane-1,4-diylbis(azanetriyl))tetrakis
4-Formylphenyl 4-(undec-10-en-1-yloxy)benzoate (4)
Compound 3 (1 g, 3.45 mmol), 4-hydroxybenzaldehyde
(0.421 g, 3.45 mmol), and DMAP (0.005 g, 0.04 mmol) were
dissolved in CH2Cl2 (20 mL) under argon atmosphere. After
addition of DCC (1.06 g, 5.14 mmol), the mixture was stirred
at 25 ꢁC for 48 h under Ar atmosphere. On completion of
the reaction, the reaction mixture was filtered, and then the
filtrate was washed with H2O (15 mL) for three times. The
organic layer was dried over anhydrous Na2SO4. The solvent
was evaporated under vacuum. The resulted yellow solid
was purified by column chromatography (silica gel) with
CH2Cl2 as the eluting solvent to give a white solid (1.01 g,
74% yield).
(propan-1-ol) [G1-(OH)4]
A solution of Compound 6 (3.02 g, 5.03 mmol) in anhydrous
THF (15 mL) was added to a suspension of LiAlH4 (1.07 g,
28.2 mmol) in anhydrous THF (20 mL) in an ice bath under Ar
atmosphere. After stirring in the ice bath, the mixture was
warmed to room temperature and stirred for another 2 h. To
quench the reaction, ice water (1 g), 15% aqueous NaOH (3 g),
and ice water (1 g) were added sequentially to the reaction
mixture. The resulting mixture was dried with anhydrous
MgSO4 and filtered. The filtrate was concentrated under vac-
uum to give a yellow viscous liquid (1.37 g, 85% yield).
1H NMR (CDCl3): d ¼ 4.71 (s, 4H), 3.65 (t, 8H), 2.54 (t, 8H),
2.39 (s, 4H), 1.65 (m, 8H), 1.45 ppm (s, 4H). 13C NMR
(CDCl3): d ¼ 62.07, 53.73, 52.70, 28.43, 24.76 ppm. ESI-MS:
1H NMR (CDCl3): d ¼ 10.03 (s, 1H), 8.16 (d, 2H), 7.99 (d,
2H), 7.40 (d, 2H), 7.01 (d, 2H), 5.78 (m, 1H), 4.96 (m, 2H),
4.06 (t, 2H), 2.07 (m, 2H), 1.84 (m, 2H), 1.49–1.32 ppm
(overlapped peaks, m, 12 H). 13C NMR (CDCl3): d ¼ 190.82,
164.11, 163.79, 155.84, 139.06, 133.81, 132.32, 131.10,
122.49, 120.75, 114.35, 114.05, 68.30, 33.67, 29.36, 29.29,
29.21, 28.97, 28.81, 25.85 ppm.
m/z: 321.4 [MþH]þ, 161.2 [Mþ2H]2þ
.
Di-tert-butyl 8,13-bis(3 -(bis(3 -(tert-butoxy)-3-oxopropyl)
amino)propyl)-4,17-bis(3 -(tert-butoxy)-3-oxopropyl)-
4,8,13,17-tetraazaicosane-1,20-dioate (7)
Compound 7 was synthesized by the same general procedure
as the synthesis of Compound 6. Starting from DAB-Am-4
(0.15 g, 0.474 mmol) in MeOH (3 mL) and tert-butyl acrylate
(0.76 mL, 5.22 mmol), 0.572 g (90% yield) of Compound 7
was obtained as a yellow viscous liquid.
4-((4-(Undec-10-en-1-yloxy)benzoyl)oxy)benzoic acid (5)
To a solution of Compound 4 (0.611 g, 1.55 mmol) and res-
orcinol (0.22 g, 2 mmol) in tert-butanol (30 mL), sodium
chlorite (0.81 g, 8.96 mmol) and NaH2PO4ꢀ2H2O (0.725 g,
4.65 mmol) in H2O (8.5 mL) was added slowly within 20
1H NMR (CDCl3): d ¼ 2.72 (t, 16H), 2.41–2.32 (overlapped
peaks, m, 36H), 1.55 (m, 8H), 1.43–1.37 ppm (overlapped
peaks, m, 76H). 13C NMR (CDCl3): d ¼ 172.07, 80.06, 54.06,
51.90, 51.83, 49.12, 33.49, 28.01, 24.87, 24.70 ppm. ESI-MS:
m/z: 1342.6 [MþH]þ.
ꢁ
min under Ar atmosphere. The mixture was stirred at 30 C
for 48 h. The progress of reaction was monitored by thin
layer chromatography (TLC) (CH2Cl2). After tert-butanol was
evaporated, the mixture was diluted with H2O (50 mL) and
neutralized with aqueous HCl (1 mol/L). The formed precipi-
tate was filtered and washed with plenty of water. Purifica-
tion by column chromatography (silica gel, CH2Cl2) gave the
product as a white solid (0.575 g, 90% yield).
8,13-Bis(3 -(bis(3-hydroxypropyl)amino)propyl)-4,17-bis
(3-hydroxypropyl)-4,8,13,17-tetraazaicosane-1,20-diol
[G2-(OH)8]
A synthetic procedure analogous to the one described in the
synthesis of G1-(OH)4 was used. The reaction of Compound
7 (0.369 g, 0.275 mmol) in THF (5 mL) and LiAlH4 (0.117 g,
1H NMR (DMSO-d6): d ¼ 13.03 (s, 1H), 8.05 (m, 4H), 7.39 (d,
2H), 7.09 (d, 2H), 5.77 (m, 1H), 4.93 (m, 2H), 4.06 (t, 2H),
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3.08 mmol) in THF (10 mL) afforded G2-(OH)8 as a yellow
viscous liquid (0.185 g, 86% yield).
dry toluene. Then, 10 lL of Pt(dvs) (2 wt % in xylene) was
added, and then the tube was sealed with a cap. The reaction
mixture was heated at 50 ꢁC for 2 days. Afterward, the
formed elastomers were carefully removed from the tubes.
The residual solvent was allowed to evaporate slowly at
room temperature to obtain dry elastomers. To evaluate the
formation process of the elastomers under different ratio of
vinyl group to SiAH bond, the amount of TDS added into the
reaction mixture was reduced to 9.8 lL (0.025 mmol) or 4.9
lL (0.0125 mmol).
1H NMR (CDCl3): d ¼ 4.93 (s, 8H), 3.64 (t, 16H), 2.55 (t,
16H), 2.43–2.35 (overlapped peaks, m, 20H), 1.67–1.56
(overlapped peaks, m, 24H), 1.38 ppm (m, 4H). 13C NMR
(CDCl3): d ¼ 62.06, 54.10, 52.73, 52.06, 51.93, 28.59, 24.99,
24.28 ppm. ESI-MS: m/z: 804.1 [MþNa]þ, 781.7 [MþH]þ,
391.3 [Mþ2H]2þ
.
Esterification of Mesogenic Unit with Dendritic Core
Synthesis of G1-(MU)4
Characterization
Compound G1-(OH)4 (0.192 g, 0.6 mmol), Compound 5
(1.18 g, 2.87 mmol), and DMAP (0.0351 g, 0.288 mmol)
were dissolved in CH2Cl2 (50 mL) in an ice bath under Ar
atmosphere. A solution of DCC (0.712 g, 3.45 mmol) in
CH2Cl2 (20 mL) was then added dropwise. The reaction mix-
ture was stirred at room temperature for 2 days. H2O (0.6
mL) was added to the mixture and the precipitate was
filtered. The filtrate was concentrated under vacuum, and
the resulting solid was washed with plenty of hot hexane.
The yellow crude product was further purified by column
chromatography (silica gel, CH2Cl2/MeOH, from 40:1 to
30:1) followed by a recrystallization in acetone to give a
yellow solid (0.454 g, 40% yield).
1H NMR and 13C NMR were recorded on a Bruker DRX-400
(400 MHz) instrument with CDCl3 or DMSO-d6 as the solvent
(with tetramethylsilane as an internal standard). MALDI-TOF
mass spectra were performed using a microflex mass spec-
trometer (Bruker Daltonics) with a-cyano-4-hydroxycinnamic
acid as a matrix. The ESI-MS analyses were measured on a
Bruker Esquire-LC-00075 spectrometer. Thermal transitions
were detected on a TA Instruments 200 DSC at a scanning
rate of 10 ꢁC/min in all cases. First-order transitions were
reported as the maxima and minima of the endothermic and
exothermic peaks. A POM (Nikon LV100 POL) coupled with a
Linkam MDS600 hot stage was used to observe the aniso-
tropic textures and to verify thermal transitions.
1H NMR (CDCl3): d ¼ 8.11 (m, 16H), 7.28 (d, 8H), 6.95
(d, 8H), 5.82 (m, 4H), 4.96 (m, 8H), 4.40 (t, 8H), 4.04 (t, 8H)
2.62 (s, 8H), 2.46 (s, 4H), 2.05 (m, 8H), 1.93 (s, 8H), 1.83 (m,
8H), 1.49–1.32 ppm (overlapped peaks, m, 52H). 13C NMR
(CDCl3): d ¼ 165.71, 164.21, 163.61, 154.68, 139.07, 132.25,
130.97, 127.68, 121.72, 121.00, 114.26, 114.05, 68.24, 63.25,
53.93, 50.32, 33.68, 29.59, 29.37, 29.30, 29.24, 28.99, 28.81,
26.60, 25.87 ppm. MALDI-TOF MS: m/z: 1891.3 [MþH]þ.
The XRD experiments were recorded by synchrotron radiation
XRD at beamline BL14B1, provided by the Shanghai Synchro-
tron Radiation Facility, at a wavelength of 0.68873 Å. BL14B1
is a beamline based on bending magnet, and a Si (111) dou-
ble-crystal monochromator was used to monochromatize the
beam. The size of the focus spot is about 0.5 mm, and the end
station is equipped with a Huber 5021 diffractometer. NaI
scintillation detector was used for data collection. The powder
samples were cast from THF solution and then mounted on
the sample stage. A temperature control unit (Mettler hot
stage FP82HT) in conjunction with the diffractometer was
used to study the structural evolutions as a function of
temperature. Moreover, A MAR345 image detector was used
to record two-dimensional (2D) X-ray patterns of the elasto-
mers. The powder rings were integrated with the FIT2D code.
Synthesis of G2-(MU)8
Compound G2-(MU)8 was synthesized by using a similar
procedure as the synthesis of G1-(MU)4. The reagents used
were G2-(OH)8 (0.156 g, 0.2 mmol), Compound 5 (0.788 g,
1.92 mmol), DMAP (0.0234 g, 0.192 mmol) in CH2Cl2 (25
mL), and DCC (0.475 g, 2.3 mmol) in CH2Cl2 (3 mL). In this
case, longer reaction time (5 days) was necessary for com-
plete esterification of hydroxyl group of G2-(OH)8. Purifica-
tion by column chromatography (silica gel, CH2Cl2/MeOH,
from 30:1 to 16:1) followed by a recrystallization in acetone
afforded G2-(MU)8 as a yellow solid (0.235 g, 30% yield).
Cyclic compression experiments were carried out on a
Zwick/Roell Z020 universal testing machine equipped with a
25-kN load cell. The elastomer was in the form of a cylinder
(ꢂ5 mm diameter and 4 mm in initial thickness). In a typical
compressive cycle, the test began with a compression step
performed at a constant crosshead speed of 0.3 mm/min to
a final strain of 50%, followed by immediate retraction to
zero load and a wait time, ꢂ1 min, until the next cycle of
compression. Stress–strain data were collected during the
compression process.
1H NMR (CDCl3): d ¼ 8.08 (m, 32H), 7.26 (d, 16H), 6.93
(d, 16H), 5.84 (m, 8H), 4.99 (m, 16H), 4.36 (s, 16H), 4.01
(m, 16H), 2.60 (m, 16H), 2.45 (m, 20H), 2.05 (m, 16H), 1.91
(m, 16H), 1.82 (m, 16H), 1.61 (m, 8H), 1.48–1.32 ppm (over-
lapped peaks, m, 100H). 13C NMR (CDCl3): d ¼ 165.66,
164.18, 163.53, 154.60, 139.09, 132.23, 130.96, 127.46,
121.76, 120.86, 114.18, 114.05, 68.13, 63.22, 61.27, 53.07,
52.44, 49.98, 33.70, 29.60, 29.39, 29.31, 29.25, 29.00, 28.79,
26.14, 25.85 ppm. MALDI-TOF MS: m/z: 3922.2 [MþH]þ.
RESULTS AND DISCUSSION
Synthesis of Promesogen, Dendritic Polyols, and
Dendrimers
Preparation of Elastomers
In a 2-mL tube with an inner diameter of 5 mm, G1-(MU)4
(0.0984 g, 0.05 mmol) or G1-(MU)8 (0.098 g, 0.025 mmol)
and TDS (18.6 lL, 0.05 mmol) were dissolved in 0.3 mL of
The synthetic procedure used to obtain promesogen 5 is
outlined in Scheme 1. The benzoate-type mesogenic segment
was accomplished by direct room-temperature DCC-
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SCHEME 1 Synthetic route of the terminally attached mesogenic unit: (i) TsCl, pyridine; (ii) methyl 4-hydroxybenzoate, K2CO3, ace-
tone; (iii) KOH, EtOH/H2O; (iv) DCC, DMAP, CH2Cl2; and (v) sodium chlorite, NaH2PO4, tert-butanol.
promoted esterification of 4 -(undec-10-en-1-yloxy)benzoic
acid (3) and methyl 4-hydroxybenzoate.59 Sequent oxidation
of aldehyde group to afford the corresponding acid was
achieved according to an analogous method described previ-
ously.60 As shown in 1H NMR (Fig. 1), the disappearance of
chemical shift of aldehyde group (d ¼ 10.03 ppm) and
emerging shift of benzoic acid (d ¼ 13.03 ppm) indicated
the complete oxidation. Moreover, characteristic signals of
vinyl group (CH2¼¼CHA) were observed at 5.77 and 4.93
ppm. The integration ratio of all the signals agreed with the
calculated value, which confirmed the exact structure of
Compound 5.
of dendritic core. Every signal in the spectra was assigned to
the corresponding protons of the dendrimers. The ratio of
the quantitative signal integration is in good agreement with
the prediction. MALDI-TOF mass spectrum (Fig. 4) clearly
showed the parent peak at m/z 1891.3 for G1-(MU)4
([MþH]þ, calcd. 1891.4) and 3922.2 for G2-(MU)8 ([MþH]þ,
calcd. 3922.1) that also indicated entire substitution at the
periphery.
Thermal Transition and Mesomorphic Properties
The phase transition of mesogenic monomers (4 and 5) and
mesogen-substituted dendrimers [G1-(MU)4 and G2-(MU)8]
have been studied by DSC, with heating and cooling rates of
10 ꢁC/min. Moreover, the mesomorphic textures were eval-
uated from polarized optical microscopy. The aldehyde-ter-
minated monomer 4 exhibits a narrow nematic phase bellow
70 ꢁC, which is characterized by a schlieren texture. How-
ever, the promesogen 5 with benzoic group owns a more or-
dered and stable smectic-A (SmA) phase between 84 and
206 ꢁC. This is attributed to the intermolecular hydrogen
bond interactions. Thermal transition and thermodynamic
data are summarized in Table 1.
Dendritic polyols were prepared by a two-step synthetic
technique (Scheme 2). First, Micheal addition of tert-butyl
acrylate with 1,4-diaminobutane or PPI (a commercial den-
drimer) gave rise to a series of dendrimers with tert-butyl
ester termination. Sequently, the reduction of the terminated
tert-butyl ester groups with LiAlH4 afforded two generations
of dendrimers with hydroxyl periphery, which were named
as G1-(OH)4 and G2-(OH)8, respectively. 1H NMR spectrum
of G2-(OH)8 [Fig. 2(A)] showed characteristic signal of
hydroxyl periphery at d ¼ 4.93 ppm. The proton signals of
all the methylene groups were assigned as indicated in the
figure. Molecular mass of G2-(OH)8 was determined by ESI-
MS technique, exhibiting three peaks for [MþH]þ, [Mþ2H]2þ,
and [MþNa]þ [Fig. 2(B)]. The formation of G1-(OH)4 was
1
also confirmed by its H NMR and ESI-MS spectra. The spec-
troscopic data are described in detail in the Experimental
section.
The mesogenic functional side-chain dendrimers [G1-(MU)4
and G2-(MU)8] were synthesized by esterification of 4 -((4 -
(undec-10-en-1-yloxy)benzoyl)oxy)benzoic acid (5) with the
terminal hydroxyl groups of the corresponding generation of
dendritic polyols [G1-(OH)4 and G2-(OH)8] (Scheme 3).
These dendritic compounds show much better solubility
than that of promesogen 5 in common solvents such as
dichloromethane, chloroform, and THF. The structures of
both new dendrimers have been confirmed by 1H NMR, 13C
NMR, and MALDI-TOF mass spectrometry. Evidence for the
esterification reactions was provided by the existence of a
1
signal at d ¼ 4.4 ppm in H NMR spectra (Fig. 3), which cor-
responds to the protons of ACH2 group linked to the ester
bond. As shown in 1H NMR spectra, the main difference
between G1-(MU)4 and G2-(MU)8 lies in chemical shift at d
¼ 2.60, 2.45, and 1.61 ppm due to the various architecture
FIGURE 1 1H NMR spectrum of promesogen 5 (upper, DMSO-
d6) and compound 4 (lower, CDCl3).
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SCHEME 2 Synthetic route of dendritic core with hydroxyl periphery: (i) tert-butyl acrylate, MeOH and (ii) LiAlH4, THF.
The first-generation dendrimer, G1-(MU)4, exhibits simple
thermograms in the heating and cooling scan that both con-
sist of two thermal transition peaks [Fig. 5(A)]. In the first
cooling scan, one can distinguish an isotropic liquid–meso-
phase transition at 106 ꢁC and a mesophase–crystallization
transition at 92 ꢁC. The characteristic mesophase was con-
firmed by the optical texture from POM observation. Figure
6(A)ꢁ shows a typical fan-shaped texture of G1-(MU)4 at
100 C, which is characteristic of a lamellar SmA mesophase.
Moreover, a narrow mesomorphic temperature range of
106–92 ꢁC indicates an unstable mesophase of G1-(MU)4.
However, with an analogous molecular architecture, the
PAMAM dendrimer PAMAM[L]4 (first generation) displayed a
stable SmA mesophase due to the stabilization by intramo-
lecular hydrogen bond by promoting groups (amido groups),
which promoted the lamellar packing of mesogens.26
results of polarized optical microscopy. Figure 6(B) shows
the initial formation of a fan-shaped SmA phase at 95 C on
cooling, and the SmA phase maintains at room temperature
ꢁ
In the case of dendrimer G2-(MU)8, a complex melting behav-
ior was observed according to the second heating of DSC scan
[Fig. 5(B)]. On heating, several endotherms and two recrystal-
lization exothermal peaks were visible. The endotherms were
ascribed to the melting of various cryꢁstalline forms. Those
two cold crystallizations (at 29 and 69 C) resulted from the
incomplete ordering of the molecules during cooling. When
the DSC thermogram was recorded at a low cooling rate of
1
ꢁC/min, exotherm at 29 ꢁꢁC disappeared, whereas cold
crystallization remained at 69 C. The obvious cold-crystalline
behaviors indicated that molecular crystallization of G2-
(MU)8 was strongly inhibited by its malleable branch topol-
ogy. This broad melting/recrystallization trajectory during
heating process is commonly for dendrimers and den-
drons.16,61,62 Sequent heating of G2-(MU)8 would lead to a
SmA mesophase characterized by a typical fan-shaped texture.
In the cooling scan of G2-(MU)8, a very simple thermogram
containing only an isotropic transition (Ti ¼ 111 ꢁC) and a
crystalline transition (Tk ¼ 12.6 ꢁC) identifies a wide range
of stability of this mesophase, which is in accordance to the
FIGURE 2 1H NMR spectrum (A) and ESI-MS spectrum (B) of
dendritic core G2-(OH)8 with hydroxyl periphery.
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SCHEME 3 Synthetic route of dendrimers functionalized with mesogenic units: (i) G1-(OH)4, DCC, DMAP and (ii) G2-(OH)8, DCC, DMAP.
(T ¼ 20 ꢁC), which is characterized by the enlarged fan-
The XRD pattern of G1-(MU)4 at 100 ꢁC contains a set of
four equidistant sharp peaks in the small-angel region, with
the intensity of the reflections decreasing with Bragg angles
shaped texture shown in Figure 6(C).
In contrast to G1-(MU)4, increasing mesogenic groups at the
peripheries of dendritic core of G2-(MU)8 enhanced the ani-
sotropic interactions between them. As a consequence, en-
thalpy gain provided by strong anisotropic interactions domi-
nated entropy force that leads to an isotropic distribution,
therefore resulting in the formation of a stable SmA meso-
phase with a wide mesomorphic temperature range. There-
fore, in this series of LCDs without any promoting group,
although driving forces such as hydrogen bond, phase-segre-
gation power, and dipole moments are absent, lamellar mes-
ophases are still displayed. The stronger the anisotropic
interactions between mesogenic units are, the more stable
the mesophase will exhibit.
Molecular Packing of LCDs
As can be seen from the results presented in Table 1 and
Figure 6, G1-(MU)4 and G2-(MU)8 both show a SmA phase.
As SmA phase is typical orientation of elongated molecules,
the phase nature of the molecular packing of the two den-
drimers is identified by wide-angle X-ray diffraction experi-
ments. For this study, the samples all underwent a thermal
treatment consisting of heating to the isotropic state, anneal-
ing at this state for 2 min and cooling consequently.
FIGURE 3 1H NMR spectrum (CDCl3) of side-chain dendrimers
G1-(MU)4 (upper) and G2-(MU)8 (lower).
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FIGURE 5 Second heating (lower curve) and first cooling
(upper curve) DSC traces of dendritic compounds (A) G1-(MU)4
and (B) G2-(MU)8 at a rate of 10 ꢁC/min. Phases are indicated
as follows: K: crystal phases; SmA: smectic-A phase; and I: iso-
tropic phase.
FIGURE 4 MALDI-TOF MS of side-chain dendrimers: (A) G1-
(MU)4 and (B) G2-(MU)8.
[Fig. 7(A)]. The reciprocal spacing (q ¼ 4p sin h/k) ratio of
1:2:3:4 for the four diffractions is characteristic of a well-
developed lamellar morphology. The maxima correspond to
the first-, second-, third-, fourth-order reflections on the la-
mellar planes, respectively. As for G2-(MU)8 at 70 ꢁC, a set
of three equidistant sharp peaks with the reciprocal spacing
ratio of 1:2:3 shown in Figure 7(B) also indicates a layer
packing. These results are consistent with smectic meso-
phase, confirming the POM observations. The layer periodic-
ity (d ¼ 2p/q) of the two dendritic compounds are gathered
in Table 2. The proposed molecular organization model of
the two dendrimers within the smectic mesophase is repre-
sented schematically in Figure 8. The dendritic polyols
spacers are conformationally disordered and occupy the cen-
tral segment of the smectic layers. The mesogenic units nec-
essarily extend up and down from the central dendritic part
in a pseudoparallel mode to allow for the formation of la-
mellar mesophase. As indicated in Table 2, layer thickness
(32.9 Å) of G2-(MU)8 is larger than that of G1-(MU)4 (30.8
Å), which is associated with a preferentially extended confor-
mation of the dendritic core. However, in both cases of G1-
(MU)4 and G2-(MU)8, the layer spacing is much smaller
than the molecular length evaluated from its most extended
conformation (calculated by molecular simulations). Two
extremely disordered parts, the terminal hydrocarbon chains
and the dendritic cores, are responsible for this distinct com-
pression. Dendritic core, the central part of the molecule,
tends to adopt a very curled arrangement, and finally it
contributes mainly to the molecular width rather than the
molecular length.
TABLE 1 Thermal Transitions of Compounds 4, 5, G1-(MU)4,
a
and G2-(MU)8
Thermal Transitions (ꢁC)
Compound
[Corresponding Enthalpy Changes (J/g)]
4
K 56.0 (30.33) N 62.1 (86.17) I
I 63.7 (2.57) N 45.2 (110.3) K
5
K 104.6 (32.41) SmA 210.4 (12.03) I
I 205.7 (11.06) SmA 86.4 (33.19) K
K 97.8 (2.68) SmA 122.2 (65.44) I
I 106.2 (67.09) SmA 92.5 (1.78) K
G1-(MU)4
G2-(MU)8
K1 15.9 (3.01) ꢃK 28.8 (12.73)
K2 63.2 (16.76) ꢃK 68.9 (12.63)
K3 81.5 (15.7) SmA 117.7 (14.45) I
Elastomers from LCDs
I 111.0 (15.82) SmA 12.6 (2.81) K
As shown in Scheme 3, LCDs, G1-(MU)4 and G2-(MU)8, pos-
sess four or eight vinyl groups at their terminals that endow
them with the capability of further functionalization. The
elastomers derived from the two dendrimers were prepared
a
Data from the second heating DSC scan are on the first line, and data
from the first cooling scan are on the second line. Phases are indicated
as follows: K: crystal phases; SmA: smectic-A phase; N: nematic phase;
and I: isotropic phase.
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FIGURE 6 Polarized optical microphotographs of the textures of (A) G1-(MU)4 at 100 ꢁC on cooling, (B) G2-(MU)8 at 95 ꢁC on cool-
ing, and (C) G2-(MU)8 at 20 ꢁC on cooling. Both of the heating and cooling rate is 1 ꢁC/min.
following a one-pot hydrosilylation reaction catalyzed by
Karstedt’s catalyst [Pt(dvs)] and involving the double bonds
of the terminals of the dendrimers and the silane groups of
the crosslinkers (TDS) with four crosslinking points. A rough
2D schematic representation of the formation of such an
elastomer network is shown in Figure 9. The networks con-
sist of rod-like mesogens, flexible dendritic part, and silox-
ane-based crosslinking area.
stoichiometric. Hydrosilylation reaction was carried out in
concentrated toluene solution (0.67 mmol/mL vinyl bonds).
The resulting samples were solid and flawless cylinders with
a diameter of 5 mm and a height of 4 mm [Fig. 10(A, D)].
Both of the samples were proven to be thermoset elastomers
because of their insoluble and infusible characters that indi-
cated highly crosslinked network. Moreover, the ratio of vinyl
bond to SiAH bond had a great impact on the crosslinking
density of the network, which finally affected the formation
of elastomers. A higher ratio of 2:1 would lead to fragile gels
with extremely low strength due to the reduced crosslinking
density of the resulting network [Fig. 10(B, E)]. Furthermore,
in the most weakly crosslinked network (a ratio of 4:1), not
a single piece of gel was found in the final reaction solution.
If the crosslinker with low connectivity such as tetramethyl-
disiloxane was used to prepare the elastomers, only small
pieces of fragile gels or total solution were obtained, no mat-
ter which ratio of vinyl group to SiAH bond and what types
of LCDs [G1-(MU)4 or G2-(MU)8] were applied.
In the preparation of elastomers G1-TDS and G2-TDS, the
ratio between vinyl groups and the hydrogen of silane was
Although it was impossible to identify the phase behaviors
of these thermoset elastomers because of their infusible
feature, a lamellar-like scattering pattern was found in
structural characterization by 2D XRD recorded at room
temperature, as shown in Figure 11(A, B). The corresponding
azimuthally integrated XRD patterns (intensity versus scat-
tering vector magnitude q) are presented in Figure 11(C, D).
For G1-TDS with polydomain, two diffraction arcs located at
low angles indicated ordered lamellar structure within the
elastomer. The two peaks in the small angle region, which
are well fitted by Lorentzian curves, corresponding to the
TABLE 2 X-ray Data for Dendrimers G1-(MU)4 and G2-(MU)8
c
˚
Compound
T (ꢁC)
Phasea
SmA
h k lb
d (A)
G1-(MU)4
100
0 0 1
0 0 2
0 0 3
0 0 4
0 0 1
0 0 2
0 0 3
30.8
15.4
10.3
7.6
G2-(MU)8
70
SmA
32.9
16.5
11.0
a
SmA: smectic-A phase.
h, k, l: Miller indices.
d, layer spacing.
b
c
FIGURE 7 Wide-angle X-ray diffraction diagrams correspond-
ing to G1-(MU)4 at 100 ꢁC (A) and G2-(MU)8 at 70 ꢁC (B).
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FIGURE 10 Photographs demonstrating the formation of
elastomers under different ratio of vinyl group to SiAH bond.
G1-TDS: (A) 1:1, (B) 2:1, and (C) 4:1. G2-TDS: (D) 1:1, (E) 2:1,
and (F) 4:1.
moieties remain unchanged. The characteristic distances
resulting from the fitting of X-ray pattern are summarized in
Table 3. From the 2D-XRD investigation, it is confirmed that
the lamellar structure of dendrimers [G1-(MU)4 and G2-
(MU)8] is actually reserved in the network of elastomers
(G1-TDS and G2-TDS). However, the lamellar spacings of
elastomers are greatly enlarged, when compared with those
of dendrimers (30.8 and 32.9 Å) formed in their SmA
phases. This enlargement can be attributed to the evolution
of packing conformation of the dendritic core. After cross-
linking reactions, the primary very curled arrangement of
dendritic central part was extremely stretched induced by
the traction of the siloxane moieties at the crosslinking
points.
FIGURE 8 Schematic drawing of the proposed molecular pack-
ing of dendrimers G1-(MU)4 (left) and G2-(MU)8 (right).
first- and second-order pseudo-Bragg reflections were associ-
ated with a layer thickness of 41.4 Å [Fig. 11(C)]. The angu-
lar dispersion of the (001) and (002) reflections is due to
the noncomplete orientation of the dendritic molecules dur-
ing the crosslinking process. In the wide-angle region, two
broad diffused peaks partially overlap one another as shown
in Figure 11(C). The diffused peak at 4.5 Å corresponds to
the lateral distance between aromatic segments and alkyl
chains within the layers. Another diffused peak at 8.1 Å is
characteristic of interchain packing of siloxane moieties. In
contrast with the XRD pattern of G1-TDS, G2-TDS exhibits
only one diffraction arc at low angles [Fig. 11(B)], and the
first-order peak (q ¼ 0.12 Åꢃ1) indicates a layer period of
52.2 Å [Fig. 11(D)]. Although the superposition of the broad
peaks at wide angles becomes obvious, the corresponding
distances of the arrangement of alkyl chains and siloxane
To evaluate the mechanical properties and response of the
two novel elastomers, a cyclic uniaxial compression test that
consists of loading (compression) process and unloading (re-
traction) process was performed. For each sample, four com-
pression cycles were performed, and the stress–strain curves
were recorded during the loading process. Figure 12 shows
the stress–strain curves of G1-TDS and G2-TDS under four
FIGURE 9 Schematic representation of the preparation of elastomers from liquid-crystalline dendrimers.
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FIGURE 11 Two-dimensional X-ray diffraction pattern of G1-TDS (A) and G2-TDS (B). Fit of azimuthally integrated X-ray pattern of
G1-TDS (C) and G2-TDS (D) [intensity vs. scattering vector magnitude, q ¼ 4p sin h/k].
successive uniaxial compressions. In the first compression
deformation, the polydomain G1-TDS elastomer has
of 5 mm and a height of 4 mm) within 1 min after removing
of the load. As we can see from Figure 12, the loading curve
closely follows the path of the former loading that indicates
the elasticity of the two elastomers. Although we measured a
full recovery of residual strain after successive compression,
an incomplete recovery of the modulus and a stress-
a
Young’s modulus of E ¼ 2.6 MPa, which is much smaller
than the value for the polydomain G2-TDS elastomer (E ¼
17.0 MPa). Moreover, the stress of G2-TDS is always higher
than that of G1-TDS at the same strain, which means that
more energy is required to deform the less random distribu-
tion of domains of G2-TDS. In the crosslinking network of
G2-TDS (Fig. 9), the mesogens tend to pack closer because
of the dendritic monomer G2-(MU)8 bearing eight mesogens
per molecule. Therefore, a tight lamellar packing and more
ordered polydomain form in G2-TDS network, in contrast to
the loose mesogen packing of G1-TDS, is observed. Both G1-
TDS and G2-TDS exhibit excellent recoverability that they
can nearly recover their original cylindric shapes (a diameter
TABLE 3 Fitted Parameters from Azimuthally Integrated X-ray
Pattern of G1-TDS and G2-TDS
˚
Distance (A) (2p/q)
Diffraction Peak
(001)
G1-TDS
G2-TDS
52.2
–
41.4
(002)
Siloxane moieties
Alkyl chains
8.1
4.5
8.1
FIGURE 12 Stress–strain curves of elastomers under succes-
4.5
sive uniaxial compression.
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REFERENCES AND NOTES
softening phenomenon were observed as mentioned previ-
ously by many elastomers.63 In our case, the stress softening
may arise from a rearrangement of the elastomer network
due to local imbalance in mesogen density and contraction
of network chain ends. In particular, uniaxial compressive
stress–strain of the polydomain G2-TDS revealed three
regions64,65 with different slopes shown in the true stress–
strain curve: (I) d ¼ 0–6%, (II) d ¼ 6–29%, and (III) d ¼
29–50%. The three regions owning different modulus (E)
during deformation of the polydomain elastomer, which
might indicate various deformation mechanisms, can be
explained by the breakage of the random distribution of the
mesogen domains, the rearrangement of those domains, and
the protraction of the polymer backbones, respectively.
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CONCLUSIONS
In summary, we have succeeded in preparing two novel vinyl-
terminated side-chain dendrimers by esterification of promes-
ogens and dendritic polyols. The four-armed dendrimer G1-
(MU)4 and the eight-armed dendrimer G2-(MU)8 are able to
generate a lamellar SmA mesomorphic phase, indicating that
these dendrimers can still display liquid-crystalline phases
without the driving force of promoting groups. Competition
between anisotropic interactions of terminal mesogenic
groups and isotropic distribution of dendritic core is responsi-
ble for the self-organization of these compounds in the liquid-
crystalline state. Furthermore, the terminal functional vinyl
groups of this series of dendrimers make them promising can-
didates for the construction of liquid-crystalline networks. We
innovatively realized the preparation of thermoset elastomers
from LCDs by hydrosilylation reaction of vinyl terminals of
G1-(MU)4 or G2-(MU)8 with siloxane crosslinker. Although no
mesophase was found in the two elastomers (G1-TDS and
G2-TDS), lamellar packing was able to maintain in the cross-
linked network. Moreover, both G1-TDS and G2-TDS exhibited
excellent recoverability after four loading–unloading cycles
that indicated their outstanding elasticity.
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