SmCP-Networked films obtained by in situ photopolymerization of neat reactive banana-shaped liquid crystals

Article abstract of DOI:10.1021/ja048437k

The synthesis and characterization of reactive banana-shaped compounds have been carried out, and their ability to be photopolymerized in their SmCP mesophase has been assessed. The presence of a SmCP liquid crystalline phase in these compounds has been c

Full text of DOI:10.1021/ja048437k

Published on Web 05/21/2004
SmCP-Networked Films Obtained by in Situ Photopolymerization of Neat
Reactive Banana-Shaped Liquid Crystals
Joaqu´ın Barbera´, Ne´lida Gimeno, Laura Monreal, Rafael Pin˜ol, M. Blanca Ros,* and
Jose´ Luis Serrano
Qu´ımica Orga´nica, Facultad de Ciencias-ICMA, UniVersidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
Received March 18, 2004; E-mail: bros@unizar.es
Table 1. Liquid Crystal Properties of Reactive Banana
Compounds
The combination of liquid crystal order and in situ photo-
polymerization processes1 is widely accepted to be a very versatile
technique for producing materials that are suitable for a variety of
applications. This strategy provides different types of materials that
include gels, side-chain and cross-linked polymers, or anisotropic
networks where a functional response along with thermal and
mechanical stability are required in a well-aligned medium. The
key goal is to incorporate the active structure in an aligned liquid
crystalline medium that is able to transform into a macroscopically
aligned high-molecular weight material.
compound
phase transition (°C, [kJ mol-1])a,b
A
K 83.6 [39.7], SmCP 104.1 [16.01] I
I 102.3 [15.66], SmCP 30.0 SmCPg
SmCPgc 37.0, SmCP 103.4 [16.23] I
K 85.9 [38.33], SmCP 108.6 [19.60] I
I 106.7 [18.11], SmCP 30.0 SmCPg
SmCPgc 35.0, SmCP 107.6 [18.00] I
M
a Data determined by DSC from first scans at a scanning rate of 10 °C/
min. b K, crystalline; SmCP, SmC polar mesophase; SmCPg, glassy SmC
polar phase; I, liquid phase. c Data from a second scan.
Polymerizations in classic liquid crystal phases are reported quite
regularly.1,2 However, among the different types of mesogenic
materials, the new banana-shaped liquid crystals represent a topic
of current interest from both academic and practical points of view.3
Interestingly, some compounds exhibit the unique feature of forming
compounds are clearly tristable, indicating an antiferroelctric
switching (P around 600 nC cm-2) (Supporting Information).
Compounds A and M were subjected to both thermal and radical
photopolymerization experiments to investigate the polymerization
behavior of these reactive banana systems and to ascertain whether
they can be cross-linked with retention of the liquid crystalline
arrangement.9 However, our main interest was focused on the study
of the in situ photopolymerization processes in the SmCP meso-
phase, which is without precedent in the literature. Neither A nor
M absorb above 310 nm, and consequently, the polymerization can
be induced by a photoinitiator absorbing at 365 nm.10 A study of
the photopolymerization of the mixtures was carried out by DSC.
The conversion vs irradiation time (Figure 1) and polymerization
3
polar ordered mesophases (SmCP ) with achiral molecules, thus
providing antiferroelectric, ferroelectric, or nonlinear optical proper-
ties. However, very few attempts at the polymerization of bent
molecules in these mesophases have been reported. Ikeda et al.4
succeded in preparing noncentrosymmetric polar networks by using
blends of a nonmesogenic V-shaped bisacrylate with a nonreactive
bent liquid crystal. And, with similar purposes, Gin et al.5 have
used neat bent-core molecules bearing 1,3-dienes as reactive groups.
Here we report our studies on the in situ photopolymerization
of neat V-shaped bisacrylates (Scheme 1) in which the SmCP order
of their mesophase is also retained in the resulting polymer. These
compounds incorporate an angular core that has a high tendency
to promote the SmCP phase, namely, the 3,4′-disubstituted biphenyl
unit.6
These compounds were prepared following synthetic routes
similar to those reported in the literature.6,7 The thermal properties
of the monomers were determined by polarizing optical microscopy,
differential scanning calorimetry (DSC), and X-ray diffraction. The
results of these studies are collected in Table 1.
Despite the fact that the compounds described here show a
thermotropic SmCP mesophase that vitrifies at around room
temperature on cooling, it is clear that the incorporation of the
reactive end groups dramatically modifies the thermal stability of
the liquid crystal order promoted by these angular structures.8
The SmCP mesophases of A and M were assigned on the basis
of the Schlieren textures observed by microscopy as well as from
the X-ray patterns taken both in the mesophase and the glassy state
that appears on cooling (Table 2). The measured interlayer distance
d is shorter than the molecular length estimated from Dreiding
stereomodels, and the differences between the measured layer
thickness and the molecular length confirm that the molecules are
tilted in the layers. The data shown in Table 2 indicate that there
are no significant differences between either of the two compounds
or between the high-temperature values and the values measured
in the glassy state. Additionally the switching processes of these
Figure 1. Conversion vs time at different temperatures (cooling) for the
photopolymerizable sample prepared with monomer M.
rate vs conversion relationships were evaluated from the photo-
polymerization exotherms of the DSC curves (see Supporting
Information).
The final conversion for both monomers, determined by DSC
was around 75% in the polar mesophase, in the range 95-85 °C.
A faster polymerization rate was observed for the bisacrylate A,
and the Rp/% conversion curves are very similar to those reported
9
7190
J. AM. CHEM. SOC. 2004, 126, 7190-7191
10.1021/ja048437k CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Compounds Studied: A (X ) H) and M (X ) CH3)
Table 2. X-ray Data of the Studied Materials
exhibited by these compounds, can be targeted through these high-
molecular weight materials. The resulting systems have potential
applications as new linear- and nonlinear optical, ferro-, antiferro-,
pyro-, or piezoelectric materials.
measured
T
(°C)
spacings
(Å)
Miller
index
parameters
(Å)
compound
phase
A
94
SmCP
41.3
13.7
001 d ) 41.2
003
001 d ) 40.3
003
001 d ) 42.0
003
001 d ) 41.1
003
Acknowledgment.
Spanish Government-EU (FEDER)
[MAT2003-07806-C02-01] and the DGA (Arago´n-Spain) supported
this work. The authors gratefully acknowledge Dr. B. Villacampa
from ICMA for the SHG measurements.
room temp
98
SmCPg 40.2
13.5
SmCP
M
42.0
14.0
Supporting Information Available: Synthesis, experimental de-
tails, and characterization of monomers and polymerized films. This
material is available free of charge via the Internet at http://pubs.acs.org.
room temp
room temp
SmCPg 41.2
13.7
SmCP
free film-A
45.1
001 d ) 45.5
22.8 (weak) 002
15.3
43.7
003
References
free film-M room temp
SmCP
001 d ) 43.9
(1) (a) Hikmet, R. A. M.; AdV. Mater. 1992, 4, 679-683. (b) Broer, D. J.;
Lub, J.; Mol, G. N. The Wiley Polymer Network Group ReView Series;
Nijenhuis, K. t., Mijs, W. J., Eds.; Wiley: New York, 1998; Vol. 1, pp
361-375. (c) Dierking, I. AdV. Mater. 2000, 12, 167-181. (d) Artal, C.;
Ros, M. B.; Serrano, J. L.; De la Fuente, M. R.; Pe´rez Jubindo, M. A.
Chem. Mater. 2001, 13, 2056-2067. (e) Artal, C.; Ros, M. B.; Serrano,
J. L.; Pereda, N.; Etxebarria, J.; Folcia, C. L.; Ortega, J. Macromolecules
2001, 34, 4244-4255. (f) Stapert, H. R.; del Valle, S.; Verstegen, E. J.
K.; Van der Zande, B. M. I.; Lub, J.; Stallinga, S. AdV. Funct. Mater.
2003, 13, 732-738.
(2) (a) Kitzerow, H. S.; Schmidt, H.; Ranft, A.; Hepke, G.; Hikmet, R. A.
M.; Lub, J. Liq. Cryst. 1993, 14, 911-916. (b) Broer, D. J.; Lub, J.; Mol,
G. N.; Nature 1995, 378, 467-469. (c) Lub, J.; Broer, D. J.; Hikmet, R.
A. M.; Liq. Cryst. 1995, 18, 319-326.
(3) (a) Niori, T.; Sekine, T.; Watanabe, J.; Fukurawa, F.; Takezoe, H. J. Mater.
Chem. 1996, 6, 1231-1233. (b) Pelzl, G.; Diele, S.; Weissflog, W. AdV.
Mater. 1999, 11, 707-724 and references therein. (c) Walba, D. M.;
Ko¨rblova, E.; Shao, R.; Maclennan, J. E.; Link, D. R.; Glases, M. A.;
Clark, N. A. Science 2000, 288, 2181-2184. (d) Anisotropic Organic
MaterialssApproaches to Polar Order; Walba, D. M.; Ko¨rblova, E.; Shao,
R.; Maclennan, J. E.; Link, D. E.; Glaser, M. A.; Clark. N. A.; ACS
Symposium Series 789; American Chemical Society: Washington, DC,
2001; p 281.
22.0 (weak) 002
14.7 003
for bisacrylate and bismethacrylate monomers in calamitic
mesophases.1d
To demonstrate that the banana monomers had cross-linked to
form polar networks, samples of compounds A and M were placed
between two uncoated glass plates and the photopolymerizations
were performed in the SmCP mesophase. Free thin films, mechani-
cally stable at room temperature, were removed from between the
glasses, and different techniques provide evidence that the films
are networks, with a lamellar order at room temperature similar to
the SmCP mesophase exhibited by monomers prior to polymeri-
zation. Their X-ray patterns are consistent with a SmCP order.
Additionally, the second-order maximum (002 reflection), was
visible (albeit with a weak intensity) for the polymers (Table 2).11
To investigate the possibility of obtaining noncentrosymmetric
networks from these neat SmCP photopolymerizable materials, a
sample of monomer M was polymerized under an electric field.12
The optical texture of the film after polymerization showed that it
was still a polydomain, but the polar arrangement could be kept
stable for long time over a wide temperature range from room
temperature to 150 °C and higher. More interestingly, the poly-
merized material showed SHG activity at room temperature and in
the absence of an electric field. The maximum intensity of the SHG
signal from the sample was comparable of that of quartz.13 However,
it must be noted that proper characterization of the dij coefficients
of these materials requires well-aligned samples,14 and more precise
measurements will be performed as part of a future project.
In conclusion, we have shown that the versatility of in situ
photopolymerization can be extended to the new SmCP mesophases
and that, through suitable molecular design, reactive bent molecules
exhibiting these mesophases can be explored. These results open
very interesting ways to design many different materials based on
the singular banana-shaped molecules. Furthermore, attractive
properties such as noncentrosymmetric polar order and helical
arrangements, which have been claimed for some of the mesophases
(4) Keum, C. D.; Kanazawa, A.; Ikeda, T. AdV. Mater. 2001, 13, 321-323.
(5) Sentman, A. C.; Gin, D. L. Angew. Chem., Int. Ed. 2003, 42, 1815-
1819.
(6) Shen, D.; Pegenau, A.; Diele, S.; Wirth, I.; Tschierske, C. J. Am. Chem.
Soc. 2000, 122, 1593-1601.
(7) Rifat, R. A. M.; Lub, J.; Tol, A. J. W. Macromolecules 1995, 28, 3313-
3327.
(8) Nonreactive homologue with terminal chains C14H29O- shows K 85 °C
SmCP 162 °C I.6 A broad SmCP range in a nonreactive V-shaped molecule
seems to ensure the SmCP order for the bisacrylate homologues.4
(9) Indeed, care should be taken when manipulating samples above 130 °C,
even in the presence of a thermal inhibitor.
(10) Photopolymerization of the monomers was carried out using a photo-
DSC experimental setup equipped with a 365 nm lamp and samples
containing 1% (w/w) Irgacure 369 and 200 ppm of the thermal inhibitor
2,6-di-tert-butyl-4-methylphenol. The liquid crystalline properties of the
monomers were not noticeably modified for the blends.
(11) The 10% increase of d in the SmCP order upon cross-linking has been
attributed to a less tilted packing of the bent cores in the network. Using
stereomodels, we deduced tilt angles of ca. 50° for both monomers in the
mesophase and 45-47° in the networks assuming a bent angle of 120°
and all-trans conformations of the chains.
(12) Monomer was placed in a 5 µm ITO cell (from Linkam) and photochemi-
cally polymerized in the SmCP mesophase (at 100 °C) using a DC voltage
(40 V/µm) to ensure a noncentrosymmetric polar order (also called
SmCPF)3c.
(13) Activity of this sample (thickness of 5 µm) was compared at room
temperature with that of a x-cut quartz crystal (1 mm).
(14) Ortega, J.; Gallastegui, J. A.; Folcia, C. L.; Etxebarria, J.; Gimeno, N.;
Ros, M. B. Liq. Cryst. 2004, 31, 579-584.
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