Macromolecules 2005, 38, 1525-1527
1525
Communications to the Editor
with polystyrene standards to be 11 000 and 33 300 for
5a and 8310 and 20 800 for 5b, respectively.
Smectic Layered Polymer Networks Based on
Side Chain Liquid Crystalline Polymers
Having Thermally Reversible Urea Bonds
Mesomorphic Behaviors. The thermotropic proper-
ties of compounds 3a,b and copolymers 5a and 5b were
investigated by polarizing optical microscopy (POM) and
differential scanning calorimetry (DSC). Compound 3a
showed enantiotropic mesophases at 222 (∆H ) 60 J/g)
∼ 231 °C (∆H ) 52 J/g) on heating and 210 (∆H ) 63
J/g) ∼ 176 °C (∆H ) 11 J/g) on cooling. Compound 3b
with a longer alkyloxy tail, also showed similar ther-
motropic mesophases, at 209 (∆H ) 52 J/g) ∼ 231 °C
(∆H ) 51 J/g) on heating and 198 (∆H ) 54 J/g) ∼ 173
°C (∆H ) 10 J/g) on cooling.
Sang Hyuk Seo, Young-Woon Kim, and
Ji Young Chang*
Hyperstructured Organic Materials Research Center,
School of Materials Science and Engineering,
Seoul National University, Seoul 151-744, Korea
Received November 15, 2004
Revised Manuscript Received January 18, 2005
The POM study showed that copolymers 5a and 5b
had mesophases from 131 to 167 °C and from 125 to
168 °C, respectively, on heating. Phase transitions on
cooling were not observed because the copolymers were
partially cross-linked in the melt state. In the DSC
thermograms, the strong endothermic peaks at 210 °C
for 5a and 200 °C for 5b were attributed to the melting
of the mesogenic pendent groups (3a or 3b) dissociated
from the polymer backbones. The dissociation of the
urea bonds was also confirmed by IR spectroscopy.
Above 130 °C, the characteristic peak for the isocyanato
groups at 2274 cm-1 became stronger, showing that the
urea bonds were cleaved to generate isocyanato groups.
Figures 2(a1) and 2(a2) show the small-angle X-ray
diffractograms of copolymers 5a and 5b, respectively,
which were measured at room temperature after quench-
ing from their LC states (155 °C), respectively. Two
reflection peaks appeared, with d spacings of 35.1 and
17.6 Å (for 5a) and 38.9 and 19.4 Å (for 5b), indicating
that these copolymers had smectic layered structures.
Preparation of Polymer Networks. The copoly-
mers were annealed for 5 h in their smectic phases (155
°C). Following this, they became insoluble in common
organic solvents, including THF, chloroform, and DMSO,
indicating the formation of a network structure. In the
IR spectra of the copolymers, a strong band appeared
Introduction. Layered solids having two-dimen-
sional sheet arrangements, in which the constituents
of the sheets are connected via strong bonds, e.g.,
covalent bonds, and the interactions between the sheets
are relatively weak, are of great interest due to their
having nanosized spaces between the layers.1 Many
layered inorganic or coordination compounds are known
to exist, but layered structures are rarely found in
synthetic organic materials.2
In this communication, we report the synthesis of a
layered polymer network with a void space between the
layers (Figure 1). We prepared a side chain liquid
crystalline polymer showing a smectic phase. Mesogenic
pendants were attached to the polymer backbone via
thermally reversible urea bonds. Thermal annealing of
the polymer in its smectic phase caused the urea bonds
to be cleaved, generating isocyanato groups at the side
chains. The subsequent reaction of the isocyanato
groups in the same layer resulted in the creation of a
polymer network sheet. The reaction between successive
layers was not possible because of the wide layer
spacing, except in the structural defects. The smectic
layered structure was maintained even after the re-
moval of the dissociated mesogenic side groups by
solvent extraction. Various anisotropic materials have
been prepared by liquid crystalline state polymeriza-
tion,3 but to our knowledge, this is the first example of
a layered polymer network with a void space between
successive layers.
Synthesis. Pendent mesogenic compounds (3a,b)
were synthesized by the esterification reaction between
biphenyl derivatives (2a,b) and 4-chlorocarbonylimida-
zole (1) (Scheme 1). The monomers (4a,b) were synthe-
sized by the reaction between compound 3a or 3b and
2-isocyanatoethyl methacrylate (IEM). They were co-
polymerized with MMA by radical initiation to yield
copolymers 5a and 5b. Copolymers 5a and 5b with 65%
MMA units were readily dissolved in common organic
solvents including THF and chloroform. The number-
average molecular weight (Mn) and weight-average
molecular weight (Mw) were determined by GPC in THF
at 3420 cm-1
, corresponding to the N-H stretching
vibration of urea groups, confirming that the polymer
chains in the same layer were cross-linked by the
reaction of the regenerated isocyanato groups. No reac-
tion between successive layers could occur because of
the wide layer spacing, except in the structural defects.
We presume that part of the dissociated isocyanato
groups were converted to amino groups by their reaction
with moisture during the annealing process. The amino
groups would be expected to react with the isocyanato
groups, resulting in cross-linking between the polymer
chains.4 The annealed polymers showed the same dif-
fraction pattern as that of the polymers in the LC states
[Figures 2(b1) and 2(b2)].
The annealed polymers were washed in THF to
remove the released side groups. Interestingly, they still
maintained their smectic layered structures. The X-ray
analysis showed five reflection peaks corresponding to
* Corresponding author: Tel +82-2-880-7190; Fax +82-2-885-
1748; e-mail jichang@snu.ac.kr.
10.1021/ma047645u CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/05/2005
Seo, Sang Hyuk
