Liquid crystals derived from hydrogen-bonded supramolecular complexes of pyridinylated hyperbranched polyglycerol and cholesterol-based carboxylic acids

Article abstract of DOI:10.1021/ma047958p

A hyperbranched polyether polyol was reacted with isonicotinoyl chloride hydrochloride for the introduction of the pyridinyl moiety at the external surface. This pyridinylated hyperbranched polymer was subsequently interacted with cholesterol-based carboxylic acids for the formation of the corresponding hydrogen-bonded supramolecular complexes. The materials obtained exhibited smectic A liquid crystalline phases over a broad thermal range from room temperature up to above 170°C. Within this smectic layer the cholesterol moieties are located above and below the hyperbranched scaffold. Increasing the spacer length, located between the carboxylic group and the cholesterol moiety, the temperature range of the liquid crystalline phases of the complexes is lowered.

Full text of DOI:10.1021/ma047958p

Macromolecules 2005, 38, 1705-1710
1705
Liquid Crystals Derived from Hydrogen-Bonded Supramolecular
Complexes of Pyridinylated Hyperbranched Polyglycerol and
Cholesterol-Based Carboxylic Acids
Theodoros Felekis, Leto Tziveleka, Dimitris Tsiourvas,* and
Constantinos M. Paleos
Institute of Physical Chemistry, NCSR “Demokritos”, 153 10, Aghia Paraskevi, Attiki, Greece
Received October 4, 2004; Revised Manuscript Received December 6, 2004
ABSTRACT: A hyperbranched polyether polyol was reacted with isonicotinoyl chloride hydrochloride
for the introduction of the pyridinyl moiety at the external surface. This pyridinylated hyperbranched
polymer was subsequently interacted with cholesterol-based carboxylic acids for the formation of the
corresponding hydrogen-bonded supramolecular complexes. The materials obtained exhibited smectic A
liquid crystalline phases over a broad thermal range from room temperature up to above 170 °C. Within
this smectic layer the cholesterol moieties are located above and below the hyperbranched scaffold.
Increasing the spacer length, located between the carboxylic group and the cholesterol moiety, the
temperature range of the liquid crystalline phases of the complexes is lowered.
Introduction
Scheme 1
The introduction of mesogenic groups at the external
surface of dendrimers1 has been established as a
2
strategy for the preparation of liquid crystals. In most
cases the mesogenic groups have been primarily at-
tached covalently at the surface groups of dendrimers.
The same strategy has been employed for the prepara-
3
tion of hyperbranched liquid crystalline polymers. On
the other hand, noncovalent synthesis for the prepara-
tion of supramolecular complexes based on dendrimers
exhibiting liquid crystalline character has received
4
limited attention.
Extensive work has been conducted during the past
decade for preparing liquid crystals derived from hy-
drogen-bonding interactions between a diversity of
polymer, and the role of the spacers in modifying the
liquid crystalline character of these supramolecular
complexes was investigated. Hydrogen-bonding forma-
tion was established by FT-IR spectroscopy while the
liquid crystalline character of the hydrogen-bonded
complexes was identified with polarized optical micros-
copy and differential scanning calorimetry and estab-
lished by X-ray diffraction.
5
complementary molecules. In a recent paper we have
reported the synthesis and characterization of hydrogen-
6
bonded dendrimeric liquid crystals. However, to our
knowledge at least, analogous hydrogen-bonded su-
pramolecular complexes based on hyperbranched poly-
mers have not yet been reported, and this is one of the
reasons for undertaking this investigation. For prepar-
ing this type of hydrogen-bonded complexes a two-stage
strategy can be followed: Initially, at the external
groups of hyperbranched polymers recognizable moieties
are attached, which interact, at a second stage, through
hydrogen bonding with mesogenic molecules bearing
complementary moieties. The resulting hydrogen-
bonded materials can potentially exhibit liquid crystal-
line character.
Experimental Section
Materials. Hyperbranched polyether polyol (polyglycerol M
n
) 5000, M /M ) 1.5, PG) bearing 68 hydroxy groups according
w
n
to the technical data sheet, was purchased from Hyperpoly-
7
mers GmbH, Germany. The polyglycerol backbone consists
of linear (L), dendritic (D), and terminal (T) units (Scheme 1).
L
13 and L14 describe the two different glycerol-like units, in
which either a primary/secondary or primary/primary ether
linkage is formed, respectively. Isonicotinoyl chloride hydro-
chloride [4-(chlorocarbonyl)pyridinium chloride], p-toluene-
sulfonyl chloride, and DMAP were obtained from Aldrich.
Glutaric anhydride, succinic anhydride, cholesterol, and 1,5-
pentanediol were purchased from Lancaster.
In the present study pyridinyl moieties were intro-
duced at the external surface of a polyglycerol (hyper-
branched polyether polyol, Mn ) 5000, PG) through
esterification of its hydroxy groups with isonicotinoyl
chloride hydrochloride. Pyridinyl groups were subse-
quently interacted, through hydrogen bonding with
succinic acid monocholesteryl ester, Chol-I, pentanedioic
acid monocholesteryl ester, Chol-II, or succinic acid
mono(5-cholesteryloxypentyl) ester, Chol-III (Scheme 1).
In this manner, cholesteryl moieties were noncovalently
attached at the external surface of the hyperbranched
Synthesis of Cholesterol-Based Carboxylic Acids. Suc-
cinic acid monocholesteryl ester, Chol-I, was prepared by a
8
method previously described. Mp ) 178.1 °C.
Pentanedioic acid monocholesteryl ester, Chol-II, was pre-
pared by an analogous method. Specifically, to a stirred
suspension of cholesterol (5.17 mmol, 1 equiv) in 20 mL of
acetone, glutaric anhydride (9.31 mmol, 1.8 equiv) and tri-
ethylamine (1.8 equiv) were added, and the mixture was
refluxed for 3 days under an inert atmosphere. The completion
*
Corresponding author. E-mail: tsiourvas@chem.demokritos.gr.
1
0.1021/ma047958p CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/01/2005

1706 Felekis et al.
Macromolecules, Vol. 38, No. 5, 2005
of the reaction was confirmed by TLC analysis. The solvent
was removed, and the product was redissolved in dichlo-
romethane (20 mL). The resulting solution was washed
Scheme 2
4
sequentially with aqueous 0.5 N HCl, saturated NH Cl, and
water. The organic phase was dried over anhydrous magne-
sium sulfate, and the solvent was evaporated. The residue was
recrystallized twice from ethanol, giving a solid material (yield
7
1%); mp 131-133 °C. The NMR signals were in line with
9 1
those reported in the literature for analogous compounds.
H
NMR (300 MHz, CDCl3): δ ) 5.30 (d, H6a), 4.54 (m, H3a), 2.37
(
t, HOOCCH
HOOCCH CH
75.4 MHz, CDCl
2
), 2.30 (t, CH
), 2.05-0.50 (m, cholesterol skeleton). C NMR
): δ ) 178.8 (HOOC), 172.3 (COO), 139.6
), 74.1 (C ), 56.7 (C14), 56.1 (C17), 50.0 (C ), 42.3
13), 39.7 (C16), 39.5 (C24), 38.1 (C ), 36.9 (C ), 36.6 (C10), 36.2
22), 35.8 (C20), 33.5 (HOOCCH ), 32.9 (CH COO), 31.9 (C ),
), 24.3 (C15), 23.8 (C23),
CH COO), 19.3 (C19),
2
COO), 2.24 (d, H ), 2.00-1.85 (m,
4
13
2
2
Scheme 3
(
(
(
(
3
C
C
C
5
), 122.7 (C
6
3
9
4
1
2
2
7
3
2
1
8 2
1.8 (C ), 28.2 (C12), 28.0 (C25), 27.8 (C
2.8 (C27), 22.6 (C26), 21.0 (C11), 19.9 (CH
2
2
+
+
8.7 (C21), 11.8 (C18). LRMS: [M + Na ] 523, [M + 2Na ] 545.
1
Anal. Calcd for C32
C, 75.47; H, 10.50.
52 4 2 2
H O ‚ / H O: C, 75.40; H, 10.48. Found:
distilled off, yielding succinic acid mono(5-cholesteryloxypen-
1
Succinic acid mono(5-cholesteryloxypentyl) ester, Chol-III,
was prepared in three steps following a method analogous to
the one reported by Ringsdorf et al.10 A solution of cholesterol
tyl) ester, Chol-III (yield 51%); mp 109-111°C. H NMR (300
MHz, CDCl
(t, CH CH
(t, HOOCCH
COOCH CH CH
(75.4 MHz, CDCl
(C ), 121.5 (C ), 79.2 (C
56.7 (C14), 56.1 (C17), 50.2 (C
39.1 (C ), 37.2 (C ), 36.9 (C10), 36.2 (C22), 35.8 (C20), 31.9 (C
31.8 (C ), 29.6 (CH CH O), 29.1, 29.0 (COOCH CH CH
HOOCCH ), 28.3 (HOOCCH CH
23.8 (C23), 22.8 (C27), 22.6 (C26), 22.5 (CH
3
): δ ) 5.31 (d, H6R), 4.11 (t, COOCH
2
CH
), 2.59
4
), 2.09-0.5 (m,
2
), 3.48
2
2
O), 3.15 (m, H3R), 2.65 (t, HOOCCH
CH COO), 2.39-2.10 (m, H
CH CH O, cholesterol skeleton). C NMR
): δ ) 177.2 (HOOC), 172.2 (COO), 141.0
), 67.7 (CH CH O), 64.8 (COOCH CH ),
), 42.3 (C13), 39.8 (C16), 39.5 (C24),
),
2
(
2.58 mmol, 1 equiv) in 40 mL of dichloromethane was treated
with p-toluenesulfonyl chloride (3.62 mmol, 1.4 equiv), DMAP
0.26 mmol, 0.1 equiv), and triethylamine (1.1 mL, 3 equiv) at
room temperature for 2 days under an inert atmosphere. The
reaction mixture was washed with 5% aqueous NaHCO and
water, and subsequently, the organic layer was dried over
anhydrous MgSO . Finally, the solvent was removed, and the
2
2
1
3
2
2
2
2
2
(
3
5
6
3
2
2
2
2
3
9
4
1
7
4
8
2
2
2
2
2
,
solid was subjected to column chromatography (silica gel,
petroleum ether/ethyl acetate 9:1) to give the 3â-p-toluene-
sulfonyloxycholest-5-ene (yield 72%); mp 130-132 °C. The
NMR signals were identical to those reported in the litera-
2
2
2
COO), 28.2 (C25), 24.3 (C15),
2
CH
2
CH
2
O), 21.0 (C11),
+
19.4 (C19), 18.7 (C21), 11.8 (C18). LRMS: [M + Na ] 595, [M +
+
4
2Na ] 617. Anal. Calcd for C36
60 5 3 2
H O ‚ / H O: C, 72.44; H, 10.58.
1
1 1
ture. H NMR (300 MHz, CDCl
3
): δ ) 7.72 (d, 2H, aryl-H),
Found: C, 72.35; H, 10.51.
7
3
.26 (d, 2H, aryl-H), 5.22 (broad s, H6a), 4.25 (m, H3a), 2.40 (s,
Synthesis of Pyrinidylated Polyglycerol Hyperbranced
Polymer (PGPy). Isonicotinoyl chloride hydrochloride (2.0 g,
11.2 mmol) was dissolved in dry pyridine, and to this solution
0.5 g (0.1 mmol, 6.8 mmol with respect to hydroxy groups) of
PG was added (Scheme 2). The reaction mixture was allowed
to react for 4 days at room temperature. Following condensa-
tion of the reaction mixture, the precipitated salt was removed
by filtration. Subsequently, the solvent of the filtrate was
distilled off, and the remaining material was dissolved in dry
dimethylformamide and reprecipitated with water. The so-
obtained solid material was dried over phosphorus pentoxide
in a vacuum at about 45 ° C. The extent of pyridinylation was
13
H, p-CH
3
), 2.44-0.50 (m, cholesterol skeleton). C NMR (75.4
): δ ) 144.4 (p-C), 138.8 (C ), 134.6 (i-C), 129.7
o-C), 127.6 (m-C), 123.5 (C ), 82.4 (C ), 56.6 (C14), 56.1 (C17),
), 42.2 (C13), 39.6 (C16), 39.5 (C24), 38.8 (C ), 36.8 (C10),
), 36.1 (C22), 35.7 (C20), 31.8 (C ), 31.7 (C ), 28.6 (C12),
8.2 (C25), 28.0 (C ), 24.2 (C15), 23.8 (C23), 22.8 (C27), 22.5 (C26),
), 21.0 (C11), 19.1 (C19), 18.7 (C21), 11.8 (C18).
MHz, CDCl
3
5
(
6
3
4
3
2
2
9.9 (C
6.3 (C
9
1
4
7
8
2
1.6 (p-CH
LRMS: [M ] 540.
3
+
Subsequently, to a suspension of 3â-p-toluenesulfonyloxy-
cholest-5-ene (2.77 mmol, 1 equiv) in 25 mL of anhydrous
dioxane, 1,5-pentanediol (27.7 mmol, 10 equiv) was added, and
the mixture was stirred under reflux for 2 days under a dry
1
3
estimated by inverse gated (IG) C NMR.
12
1
nitrogen atmosphere. The solvent was removed under vacuum,
and the residue was dissolved in CHCl (20 mL) and washed
sequentially with water, saturated NaHCO , water, and brine.
The organic phase was dried over anhydrous MgSO . The
H NMR (500 MHz, DMSO-d
CHN), 7.71 (broad s, 2H, CCH), 5.58-4.08 (broad m, CH
CHOOC next to the esterified hydroxy groups), 4.03-3.03
6
): δ ) 8.71 (broad s, 2H,
3
2
OOC,
3
13
4
(broad m, CH
MHz, DMSO-d
2
, CH protons of the polyglycerol). C NMR (62.9
): δ ) 164.2 (OCO), 150.5 (2 CHN), 136.7 (C),
extract was concentrated and subjected to column chromatog-
raphy (silica gel, using petroleum ether/ethyl acetate 9:1 to
6
122.3 (2 CCH), 77.9 (D), 76.4 (L14, T), 72.7 (L14, T), 72.2-67.5
(2D, 2T, L14, L13), 67.4-66.4 (T) 65.1-63.3, 59.7 (T, L13).
Formation of Hydrogen-Bonded Complexes by the
Interaction of PGPy with Cholesterol-Based Acid De-
rivatives. To 1 mmol of pyridinylated PG, dissolved in dry
DMF, 50 mmol of a cholesterol-based acid derivative (Chol-I,
Chol-II, or Chol-III) was added. This ratio corresponds to the
number of all the primary and secondary terminal as well as
of the primary L13 groups (see below). The solvent was distilled
off under reduced pressure, and the remaining material was
extensively dried under vacuum. Subsequently, these materi-
als were heated at temperatures above the melting points of
the corresponding acids and slowly cooled, affording the
PGPy-Chol-N complexes (Scheme 3), where N denotes the
number of the corresponding acid derivative.
1
:1), giving cholest-5-en-3â-oxy-pentan-5-ol as a colorless
1
product (yield 61%); mp 101-103 °C. H NMR (300 MHz,
CDCl CH ), 3.42 (t,
O), 3.08 (m, H3R), 2.42-0.50 (m, HOCH CH CH CH
O, cholesterol skeleton). C NMR (75.4 MHz, CDCl ): δ
), 121.4 (C ), 79.0 (C ), 67.9 (CH CH O), 62.7
), 56.7 (C14), 56.1 (C17), 50.1 (C ), 42.3 (C13), 39.7
16), 39.5 (C24), 39.1 (C ), 37.2 (C ), 36.8 (C10), 36.1 (C22), 35.7
20), 32.4 (HOCH CH ), 31.9 (C ), 31.8 (C ), 29.7 (CH CH O),
), 28.2 (C12), 28.0 (C25), 24.2 (C15), 23.8 (C23), 22.8 (C27),
2.5 (C26), 22.4 (CH CH CH O), 21.0 (C11), 19.3 (C19), 18.7 (C21),
1.8 (C18). LRMS: [M ] 472.
3
): δ ) 5.29 (d, H6R), 3.62 (t, HOCH
CH
2
2
CH
CH
2
2
2
2
2
2
-
1
3
2
3
)
141.0 (C
5
6
3
2
2
(
(
(
HOCH CH
2
2
9
C
C
4
1
2
2
7
8
2
2
2
2
1
8.4 (C
2
2
2
+
2
Finally, to a stirred solution of cholest-5-en-3â-oxy-pentan-
-ol (0.63 mmol, 1 equiv) in 12 mL of dichloromethane, succinic
5
anhydride (1.01 mmol, 1.6 equiv) and triethylamine (1.7 equiv)
were added. The solution was stirred under an inert atmo-
sphere for 4 days. The solution was then diluted in 30 mL of
dichloromethane and extracted sequentially with aqueous 0.5
Characterization. The thermal stability of the hyper-
branched complexes was studied by thermogravimetry em-
ploying a TGA-2050 instrument (TA Instruments) with heating
rates of 5 °C min . Liquid crystal textures were observed
using a Leitz-Wetzlar polarizing microscope equipped with a
Linkam hot stage. Thermotropic polymorphism was investi-
-
1
N HCl, saturated NH
4
Cl, and water. The organic phase was
dried over anhydrous magnesium sulfate, and the solvent was

Macromolecules, Vol. 38, No. 5, 2005
Liquid Crystals 1707
1
3
Figure 1. C NMR spectra (DMSO-d
6
) of the polyglycerol, PG (A), and the pyridinylated polyglycerol, PGPy (B). Carbons in the
terminal, dendritic, linear 1,3, and linear 1,4 units are indicated by T, D, L13, and L14, respectively.
gated by differential scanning calorimetry employing a DSC-
0 calorimeter (TA Instruments) operating under nitrogen at
protons of the pyridinyl moiety. It was found that almost
1
all the primary hydroxy groups of the polymer (T and
L13) were pyridinylated, while the secondary (T and L14)
hydroxy groups were also pyridinylated but to a lesser
extent. This was established by the disappearance of
the signals due to the R- and â-methylene and methine
carbons relative to the two different primary hydroxy
groups at 61.0 (L ), 63.1 (T), 70.5 (T), and 79.6-80.3
-1
heating and cooling rates of 10 °C min . The thermal behavior
was registered during the second heating run, i.e., after first
heating the samples above the melting temperature of the
corresponding hydrogen-bonded complexes. Liquid crystalline
phases of the complexes were also investigated by X-ray
diffraction, using Cu KR radiation from a Rigaku rotating
anode X-ray generator (operating at 50 kV, 100 mA) and an
R-AXIS IV image plate. Powder samples were sealed in
Lindemann capillaries and heated employing an INSTEC hot
stage. FT-IR studies were performed using a Nicolet Magna
1
3
13
ppm (L13) in the C NMR spectra, which have been
7b,c
assigned according to previous reports, along with the
partial disappearance of the respective signals relative
to the two different secondary hydroxy groups at 68.6
(L ), 70.5 (T), 71.6 (T), and 72.8 ppm (L ) (Figure 1).
-
1
spectrometer at a resolution of 4 cm
.
Results and Discussion
14
14
From inverse gated 13C NMR experiments conducted
in PG itself, in which the prerequisites for the reliable
The ester bond formation was also confirmed by the
concurrent appearance of the signals at 63.7, 64.7, 66.7,
67.0, 67.2, 71.9, 72.0, and 76.4 ppm attributed to the
R-methylene and methine carbons relative to the ester
groups. In addition, the carbonyl carbon appeared at
164.2 ppm, while the aromatic carbons of the pyridinyl
group appeared at 122.3, 136.7, and 150.5 ppm.
7a,b
integration of the signals are fulfilled,
the percentage
of the four different structural units of polyglycerol was
estimated. As already mentioned, the polyglycerol back-
bone consists of linear (L), dendritic (D), and terminal
(T) units, while L13 and L14 describe the two different
glycerol-like units (Scheme 1). In accordance with
calculations previously reported for this type of hyper-
Inverse gated (IG) 13C NMR experiments of PGPy
were conducted in order to estimate the percentage of
the esterification. By calculating the integral ratio of
the total area corresponding to all polymer’s carbons to
the pyridinyl aromatic carbons, it was found that ∼85%
of the total hydroxy groups have been esterified. As
discussed above, all the primary T and L13 hydroxy
groups (42.5% of the total) as well as the majority of
the secondary groups were functionalized. It is reason-
able to assume that the most of the secondary T groups
7
b
branched polyglycerols, it was found that in the
polymer used the T units comprise the 34% and the D
units the 27%, while the linear units comprise 39% in
total (11% L13 and 28% L14). Regarding the hydroxy
groups, it was found that the primary and secondary
terminal hydroxy groups (T) comprise the 64% of the
total hydroxy groups of the polymer, while the primary
L13 and the secondary L14 hydroxy groups the 10.5% and
2
5.5%, respectively.
(32% of the total) were also substituted in preference
Pyridinylation of PG was conveniently achieved by
to the less accessible secondary L14 hydroxy groups. It
has to be mentioned that complete functionalization was
not achieved due to steric hindrance, and similar results
have been reported in the case of cyanobiphenyl func-
tionalized hyperbranched polyether polyols.3 Taking
into account the fact that each hyperbranched polymer
is bearing 68 hydroxy groups (according to the technical
esterification employing isonicotinoyl chloride hydro-
chloride under mild experimental conditions. The re-
sulting pyridinylated polyglycerol, PGPy, was obtained
in the form of a thick paste not showing any mesomor-
phism. Ester bond formation was confirmed by the
a,b
1
appearance in the H NMR spectra of characteristic
signals between 4.08 and 5.58 ppm, corresponding to
the R-methylene and methine protons of PG adjacent
to the ester bond and of two broad signals at 7.71 and
data sheet), which was also confirmed with the IG 13
C
NMR experiments, the average number of the esterified
hydroxy groups is approximately 58.
8
.71 ppm attributed to the two different aromatic

1708 Felekis et al.
Macromolecules, Vol. 38, No. 5, 2005
Figure 2. Infrared spectra of (a) succinic acid monocholesteryl
ester, Chol-I, (b) mixture of PGPy with Chol-I resulting after
evaporation of their DMF solution, and (c) of the hydrogen-
bonded complex PGPy-Chol-I obtained after heating the
previous sample at 180 °C, i.e., above the melting temperatures
of the cholesteric acid derivative.
As mentioned in the Experimental Section, hydrogen-
bonded complexes were obtained by the interaction of
the cholesterol acid derivatives and the pyridinylated
hyperbranched polymer PGPy at a 50:1 molar ratio to
secure hydrogen bond formation between the acid and
the more accessible T and L13 pyridine moieties. In this
connection, it should be noted that variations in the
degree of functionalization with mesogenic moieties in
liquid crystalline derivatives of hyperbranched polyether
polyols had no effect on the type of mesophase observ-
Figure 3. Optical textures of PGPy-Chol-I (top) and PGPy-
Chol-II (bottom) in the smectic A phase observed by polarizing
microscopy.
3
a,b
ed.
The complexes were formed by slow evaporation
Table 1. Thermal Transitions of Hydrogen-Bonded PGPy
Complexes with Cholesterol-Based Carboxylic Acids
of their DMF solution under vacuum and subsequent
heating the samples above the melting temperature of
the corresponding acids. The formation of hydrogen-
bonded complexes was established by infrared spectros-
copy. At room temperature, the infrared spectra of the
obtained mixtures do not result from a superposition of
the spectra of the pure components. Specifically, the
carbonyl band of the carboxylic acids, centered between
T /
g
°C
k
T /°C
s
T /°C
i
T /°C
complex
(∆H/J g-1) (∆H/J g-1) (∆H/J g-1)
PGPy-Chol-I
PGPy-Chol-II
155.1 (14.6) 167.9 (2.2) 177.8 (1.35)
10.5
152.5 (1.9)
117.0 (2.5)
PGPy-Chol-III -8.5
the carboxylic acid.14 On the other hand, it has to be
noted that simple evaporation of their DMF solution,
which is usually employed for the formation of similar
low-molecular-weight complexes,5 is not sufficient to
obtain complete complex formation (Figure 2b).
1
708 and 1710 cm-1 for the different cholesterol-based
acid derivatives (Figure 2a), shifted to higher wave-
lengths upon hydrogen bond formation. Thus, in the spec-
tra of the complexes (Figure 2c) only one band is present
,13
-1
centered at 1732 cm which results from the superposi-
The thermotropic liquid crystal polymorphism of the
complexes was investigated by polarizing optical mi-
croscopy and differential scanning calorimetry. The
PGPy-Chol-I complex exhibited a crystalline-crystal-
line transition at about 160 °C, and it was then
transformed to a birefringent fluid at temperatures
above ca. 170 °C, becoming isotropic at temperatures
above 180 °C. On cooling well-developed fan-shaped
textures were observed (Figure 3, top), suggesting the
presence of a smectic A phase. On further lowering the
temperature, crystal formation was shown, however,
with a significant hysteresis of about 50 °C. This
polymorphic behavior was confirmed by DSC (Table 1).
On the other hand, the PGPy-Chol-II and PGPy-Chol-
III complexes, having a longer spacer between the
-1
tion of the ester carbonyl band of the acids at 1737 cm
and that of the PGPy at 1732 cm-1. A second derivative
analysis of this band clearly reveals the presence of four
-
1
bands located at 1719, 1726, 1732, and 1737 cm for
all the PGPy-Chol complexes. The presence of the two
-
1
new bands at 1719 and 1726 cm suggests the forma-
tion of hydrogen bonds with a double minimum-energy
13
potential. Additionally, in the spectra of the pure acids
that are present in the dimeric form, the bands at about
2
650 and 2560 cm-1 which are attributed to OH Fermi
resonances are replaced in the spectra of PGPy-Chol
-
1
complexes by new bands at 2550 and 1960 cm as a
result of intermolecular hydrogen bonding between the
nitrogen of the pyridine ring and the hydroxy group of

Macromolecules, Vol. 38, No. 5, 2005
Liquid Crystals 1709
Figure 5. Schematic representation of the proposed molecular
structure of the hydrogen-bonded complexes in the smectic A
phase. For simplicity, only a portion of the dendritic branches
is shown as straight lines, while pyridine and cholesteryl
moieties are shown as circles and ellipses, respectively.
small-angle equidistant sharp reflections related to a
smectic layering with a lamellar thickness of 60.5 Å at
Figure 4. X-ray diffraction patterns of PGPy-Chol-I in the
crystalline smectic and smectic A phase.
1
70 °C and a diffuse halo in the wide-angle region, con-
sisting of two partially overlapping broad peaks centered
at 5.5 and 4.5 Å representing respectively the mean
lateral distance of cholesterol moieties and of the
branches of the dendritic scaffold that are in a disorder-
ed state. On further heating above the clearing tempera-
ture, the diffraction pattern confirms the presence of
an isotropic melt. On cooling, even with very slow rates,
cholesterol moiety and the carboxylic acid group, exhibit
liquid crystalline phases at room temperature becoming
isotropic at significantly lower temperatures, i.e, at
about 150 and 115 °C for PGPy-Chol-II and PGPy-
Chol-III, respectively. On cooling from the isotropic
phase, they also produce well-developed fan-shaped
textures (Figure 3, bottom). The DSC results were in
line with the polarizing optical microscopy observations,
while they additionally established the presence of a
glassy phase at temperatures below 10.5 °C for PGPy-
Chol-II and -8.5 °C for PGPy-Chol-III (Table 1). It is
interesting to note that even an additional methylene
group in the spacer of PGPy-Chol-II significantly
changes its thermal behavior compared to PGPy-Chol-
I. This can be attributed to the odd-even effect com-
monly encountered in liquid crystalline compounds.
Thermogravimetric experiments showed that the
hydrogen-bonded complexes are all thermally stable up
to their isotropization temperatures. Significant weight
losses were registered about 20 °C above their smectic
to isotropic transition. They are therefore more ther-
mally stable than the analogous poly(ethylene imine)
the smectic A phase can be observed even below 120
-1
°
C. On the other hand, fast cooling (>20 °C min ) pre-
serves the smectic A phase down to room temperature.
In this case, the crystalline smectic phase is formed dur-
ing the subsequent heating run at temperatures above
1
40 °C. It is therefore evident that the presence of the
dendritic part imposes kinetic constraints to the forma-
tion of the thermodynamically stable crystalline phases.
The X-ray patterns of PGPy-Chol-II and PGPy-
Chol-III complexes confirm the presence of smectic A
phases even at room temperature. They contain three
sharp equidistant small-angle peaks corresponding to
smectic layer distances of 64.3 and 70.7 Å, respectively,
and a halo at the wide-angle region indicating that the
pyridinyl-cholesteric acid groups are arranged in a
disordered fashion. The lamellar spacings decrease with
increasing temperature due to the lateral expansion of
the cholesterol and the hyperbranched repeating units,
which is a further proof that no smectic C phases are
6
dendrimeric hydrogen-bonded complexes.
X-ray experiments confirmed the polymorphism ob-
served by polarizing optical microscopy and DSC. The
X-ray diffractograms of PGPy-Chol-I at room temper-
atures contained a number of sharp Bragg reflections
in the small- and wide-angle region, revealing the pres-
ence of a well-developed three-dimensional lattice. The
reflections are different from those registered for Chol-
I, clearly confirming the formation of a pyridine-acid
complex. Additionally, a broad peak present at the wide-
angle region centered at ∼4.5 Å suggests that although
the cholesterol moieties are organized in a three-
dimensional structure, the hyperbranched polymeric
scaffold is, as expected, in a disordered conformation.
On heating above the first transition, the X-ray dif-
fractograms (Figure 4) give evidence for the evolution
of a three-dimensional molecular ordering with a lamel-
lar structure. They contain more than two equidistant
sharp reflections in the small-angle region with a lamel-
lar period of 62.4 Å at 160 °C and a broad peak together
with a number of sharp reflections in the wide-angle
region as a result of interlayer correlations, suggesting
the presence of a crystalline smectic phase. On further
heating above the second thermal transition, in the
smectic A phase, the diffraction pattern contains two
15
formed. From the density of the two complexes (1.07
-1
and 1.06 g cm for PGPy-Chol-II and PGPy-Chol-III,
respectively) and the experimental lamellar spacings,
we can deduce the cross-sectional area of each complex
in the layers. The calculated cross-sectional area is in
2
both cases remarkably the same (880 and 872 Å for
PGPy-Chol-II and PGPy-Chol-III, respectively). Since
1
6
the cross-sectional area of the cholesterol moiety is
2
36.6 Å , it is evident that the area occupied by each
dendritic complex can accommodate ∼24 cholesterol,
corresponding, within experimental error, to half of the
cholesteric acid molecules in each complex. Moreover,
by the known cross-sectional area of the complexes and
the density of PGPy15 (1.22 g cm ), the thickness of
the dendritic part, ∼17 Å, can be deduced. It is therefore
safe to assume that the hyperbranched moiety has the
same dimensions in both cases adopting a rather flat
conformation, which is required for the packing of the
cholesterol groups in separate sublayers above and
below the hyperbranched part of the complexes (Figure
5). It can be argued that the anchoring sites of the
cholesteryl acid derivatives are not all located at the
-1

1710 Felekis et al.
Macromolecules, Vol. 38, No. 5, 2005
external region of the hyperbranched molecule. How-
ever, because of the flexible nature of the polyether
hyperbranched scaffold, the pyridyl moieties segregate
to satisfy the strong tendency of the mesogenic choles-
teric groups to assemble in a lamellar organization.
1750. (i) Guillon, D.; Deschenaux, R. Curr. Opin. Solid State
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3) (a) Sunder, A.; Quincy, M.-F.; M u¨ lhaupt, R.; Frey, H. Angew.
Chem., Int. Ed. 1999, 38, 2928-2930. (b) Quincy, M.-F.;
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Cryst. 1998, 25, 767-769.
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The interaction of pyridinylated polyglycerol hyper-
branced polymer with cholesterol-based acids leads to
the formation of supramolecular hydrogen-bonded com-
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When a short spacer is introduced between the carboxy-
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complex is a crystalline material at room temperature,
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On the other hand, the increase of the spacer length
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temperature.
2
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Acknowledgment. The work was partially sup-
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Program, Action 3.3.1 co-funded by the Greek Ministry
of Development and EU.
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