The first blue phase reactive monomers containing a bi-mesogenic core and their side-chain polymers

Article abstract of DOI:10.1039/c5tc00615e

Two novel blue phase (BP) reactive monomers M1 and M2 containing a bi-mesogenic core are first reported by introducing two end-attached acrylic spacers with different lengths. Due to the self-assembly of bi-mesogenic cores with the same side-by-side end-attached acrylate spacers in homopolymers P1 and P2, no blue phase could be induced by homopolymerization of M1 and M2. However, by interrupting the self-assembly of bi-mesogenic cores with different side-by-side spacer lengths of copolymers P12, double-twisted cylinders of BPIII could be further extended by copolymerization of M1 and M2 with different molar ratios. With M1:M2 = 5:5 (molar ratio) in both side-chain copolymers P12(soln:5/5) and P12(photo:5/5) rather than binary mixture M1/M2(5/5), the widest BPIII ranges of 5.3 °C and 3.8 °C could be obtained by solution- and photo-polymerization, respectively.

Full text of DOI:10.1039/c5tc00615e

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Materials Chemistry C
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The first blue phase reactive monomers
containing a bi-mesogenic core and their
side-chain polymers†
Cite this: J. Mater. Chem. C, 2015,
3, 4663
Chong-Lun Wei,a Yen-Ting Lin,b Jin-Huai Chang,a I-Hung Chianga and
Hong-Cheu Lin*a
Two novel blue phase (BP) reactive monomers M1 and M2 containing a bi-mesogenic core are first
reported by introducing two end-attached acrylic spacers with different lengths. Due to the self-assembly
of bi-mesogenic cores with the same side-by-side end-attached acrylate spacers in homopolymers P1
and P2, no blue phase could be induced by homopolymerization of M1 and M2. However, by interrupting
the self-assembly of bi-mesogenic cores with different side-by-side spacer lengths of copolymers P12,
double-twisted cylinders of BPIII could be further extended by copolymerization of M1 and M2 with
different molar ratios. With M1: M2 = 5 : 5 (molar ratio) in both side-chain copolymers P12(soln:5/5) and
P12(photo:5/5) rather than binary mixture M1/M2(5/5), the widest BPIII ranges of 5.3 1C and 3.8 1C could
be obtained by solution- and photo-polymerization, respectively.
Received 5th March 2015,
Accepted 30th March 2015
DOI: 10.1039/c5tc00615e
www.rsc.org/MaterialsC
and extend BP temperature ranges: Yang’s group have shown
that the liquid crystal oligomer could be self-assembled by
Introduction
In the last few decades, various liquid crystal molecules with hydrogen bonds, and it formed a BP complex with a wide
many attractive mesophases have been obtained via special temperature range about 23 1C.10 Azobenzene dimers were also
structural designs, including nematic,1 smectic,2 discotic,3 and used to be doped into BPLCs to control the reflection wave-
blue phases.4–27 However, for liquid crystal scientists, blue length.11 Yoshizawa et al. used the structure of a binaphthyl
phase liquid crystals (BPLCs) have aroused much attention unit to induce a blue phase and observed the BP temperature
as BPLCs have many exotic characteristics in the field of opto- range approximately 30 1C.12 In addition, a novel T-shaped
electronics and photonics,4 such as fast response time (about chiral compound possessing a wide BP temperature range was
sub-milliseconds), no requirement of any alignment layer5 and also developed.13 Photoisomerization was used to stabilize the
no birefringence.6 Nowadays, BP molecular aggregates have liquid crystalline cubic blue phase by Takezoe et al.14 and they
been reported to possess internal helical alignment called the connected a flexible linker between a rod and a cholesterol
‘‘double twisted cylinder (DTC)’’7 and they are classified as mesogenic unit which exhibited a wide range of BPs.15 Oxadiazole-
three types of BPs, i.e., blue phases I, II, and III, where blue based bent-core liquid crystals were introduced to possess a blue
phase I (BPI) is a body-centered cubic structure, blue phase II phase with a wide mesophasic range ca. 30 1C.16 Some researchers
(BPII) is a simple cubic structure and blue phase III (BPIII) has fabricated several biaxial nematic LCs doped with certain amounts
the same symmetry as the isotropic phase.8 Although BPLCs have of chiral dopants, which could enlarge the temperature range of
many benefits, they exhibited a very narrow temperature range BPs.17 Besides, some groups used ZnS nanoparticles18 or combined
(usually ca. 1 K), and were usually observed at high temperatures polymers and nanoparticles together to stabilize the BP.19 Jiangang
between the isotropic phase and the chiral nematic (N*) phase Lu et al. used polyaniline-functionalized graphene nanosheets to
during the cooling process.9 To overcome those disadvantages, stabilize BPLCs and reduce their driving voltages.20 Pivnenko et al.
many experts have tried some methods to stabilize BP structures have reported that dimeric materials could stabilize the blue phase
and enlarge its temperature range ca. 44 1C.21 In the meanwhile, it
is a very common idea and a helpful strategy to use polymeric
structures to stabilize and extend the BP temperature ranges. For
example, Kikuchi et al. have reported to stabilize and extend the
a Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu, Taiwan. E-mail: linhc@mail.nctu.edu.tw
b Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan,
temperature ranges of BPs over 60 1C by photo-polymerization of
Taiwan
reactive monomers within the BP range of blue phase mixtures.22
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5tc00615e
Dierking et al. used short-chain polystyrene to stabilize the BP
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temperature range up to 12 1C.23 On the other hand, Experimental
cyclosiloxane-based side chain co-polymers were reported by
Spectroscopic analysis
Zhang et al., which had a wide BP temperature range.24 Indeed,
1H NMR spectra were recorded on a Bruker Unity 300 MHz
spectrometer using DMSO-d6, CDCl3 and THF-d8 as solvents.
Elemental analyses (EA) were performed on a Heraeus CHN-OS
RAPID elemental analyzer. The molecular weights of polymers
were obtained via gel permeation chromatography (GPC) measure-
ments, which were carried out at 40 1C on a Waters 1515 instru-
ment equipped with three Waters m-Styragel columns (103, 104
and 105 Å) in series and a RI detector (ERMA Inc., ERC-7522). All
GPC data were acquired by using tetrahydrofuran (THF) as the
eluent at a flow rate of 1.0 mL minꢀ1 and polystyrene samples as
the molecular weight standards.
many experts extended the temperature ranges of BPs by simply
using polymer networks to stabilize the BP dislocations.25
As mentioned above, the BPs can be stabilized by doping and
polymerization of reactive monomers in blue phase mixtures,
which contain complicated components, including: nematic
liquid crystals, chiral dopants, reactive monomers, and photo-
initiators. Herein, we report the first type of BP reactive monomer
successfully synthesized by introducing an acrylic end group
into a previously reported BP LC diad. Through this way, the
traditional polymerization technique of multiple components
to stabilize the BP can be simplified to become a simple
component system, i.e., via polymerization of BP LC diads.
Besides, in many studies only cubic BPs (BPI and BPII) can be
locked by polymerization of reactive monomers in the desired
mesophsic range due to the stabilization of dislocation lines in
cubic BPs. Meanwhile, we first introduced BPIII reactive mono-
mers and extended the temperature range of BPIII by solution-
and photo-polymerization. Moreover, BPIII is believed to have
an arbitrary orientation (isotropic symmetry) structure, so BPIII
could be stabilized in the isotropic state. The diad structures of
BPLCs have been reported by our group recently,26 which are
novel asymmetrical liquid crystal diads with a central chiral
Liquid-crystalline and physical properties
The phase transition behaviors of all final asymmetrical diads
were characterized by polarizing optical microscopy (POM)
using a Leica DMLP equipped with a temperature control hot
stage (Mettler Toledo FP82HT). Temperatures and enthalpies of
phase transitions were determined by differential scanning
calorimetry (DSC, model: Perkin Elmer Pyris 7) under N2 at a
heating and cooling rate of 1 1C minꢀ1
.
Preparation of materials
flexible linker to link two different mesogenic cores. As shown All chemicals and solvents were of reagent grade and purchased
in Scheme 1, the first BP reactive monomers (diads M1 and M2 from ACROS, Aldrich, TCI, Fluka, and TEDIA. THF was distilled
with different acrylate spacer lengths) are newly developed to over sodium/benzophenone to keep it anhydrous before use.
possess an odd number asymmetrical chiral linker to form a After distillation over CaH2, DMF was purified by refluxing with
biaxial bent-shaped structure to stabilize BPs. Presently, very calcium hydride and then distilled. Azobisisobutyronitrile
few BP bent-shaped side-chain copolymers have been synthe- (AIBN) was recrystallized from methanol before use. The other
sized so far. Accordingly, both BPIII reactive monomers M1 and chemicals were used without further purification. The mono-
M2 are copolymerized via solution- and photo-polymerization mers of M1 and M2 were synthesized following the already
to produce the first BP bent-shaped side-chain copolymers (P12 published procedure. The reactive monomers M1 and M2 of
with different molar ratios of M1 and M2, see Scheme 1) in appropriate molar ratios were heated to the isotropic state at
this study.
80 1C, and then they were irradiated with a UV light of 1.5 mW cmꢀ2
Scheme 1 Synthetic routes of blue phase (BP) side-chain copolymers P12 by BP reactive monomers M1 and M2 with different acrylate spacer lengths.
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at 365 nm with an exposure time of 20 min. The distance
between the lamp and samples is 15 cm.
Synthesis of (S)-6-((40-cyano-[1,10-biphenyl]-4-yl)oxy)-4-methylhexyl
4-((4-((6-(acryloyloxy)hexyl)oxy)benzoyl)oxy)benzoate, M1. Yield: 78%.
1H NMR (300 MHz, CDCl3): d (ppm) 8.02 (t, J = 8.5 Hz, 4H), 7.81
(s, 4H), 7.68 (d, J = 8.8 Hz, 2H), 7.36 (d, J = 8.7 Hz, 2H), 7.11
(d, J = 9.0 Hz, 2H), 7.04 (d, J = 8.8 Hz, 2H), 6.32 (m, 1H), 6.17
(m, 1H), 5.93 (m, 1H), 4.30 (t, J = 6.3 Hz, 2H), 4.11 (m, 6H), 1.75
(m, 6H), 1.66 (m, 4H), 1.43 (m, 7H), 0.97 (d, J = 6.3 Hz, 3H).
Anal. calcd for C43H45NO8: C, 72.69, H, 6.22, N, 1,82; found: C,
73.38, H, 6.44, N, 1.99%.
Synthesis of (S)-6-((40-cyano-[1,10-biphenyl]-4-yl)oxy)-4-methylhexyl
4-((4-((12-(acryloyloxy)dodecyl)oxy)benzoyl)oxy)benzoate, M2. Yield:
76%. 1H NMR (300 MHz, CDCl3): d (ppm) 8.02 (t, J = 8.1 Hz, 4H),
7.81 (s, 4H), 7.68 (d, J = 8.6 Hz, 2H), 7.35 (d, J = 8.5 Hz, 2H), 7.10
(d, J = 9.0 Hz, 4H), 7.05 (d, J = 8.8 Hz, 2H), 6.31 (m, 1H), 6.16
(m, 1H), 5.92 (m, 1H), 4.29 (t, J = 6.3 Hz, 2H), 4.08 (m, 6H), 1.75
(m, 6H), 1.59 (m, 5H), 1.42 (m, 2H), 1.26 (m, 15H), 0.97
(d, J = 6.0 Hz, 3H). Anal. calcd for C49H57NO8: C, 74.38, H,
7.39, N, 1,69; found: C, 74.69, H, 7.29, N, 1.78%.
All of the polymerization processes were carried out by free
radical polymerization described as follows: to a Schlenk tube,
monomers M1 and M2 were mixed at appropriate molar ratios
(0 : 10, 9 : 1, 3 : 7, 5 : 5, 7 : 3, 1 : 9 and 10 : 0) dissolved in dry
chlorobenzene (0.146 M) and AIBN (0.03 eq.) as an initiator.
The solution was degassed by three freeze–pump–thaw cycles
and then sealed off. The reaction mixture was stirred and
heated at 60 1C for 24 h. After polymerization, the polymer
was precipitated into diethyl ether for homo- and co-polymers.
The precipitated polymers were collected, washed with diethyl
ether, and dried under high vacuum.
Scheme 2 Synthetic routes of blue phase reactive monomers M1 and
M2. Reagents and conditions: (a) HBr (48% in water), toluene, reflux, 18 h;
(b) K2CO3, KI, acetone, reflux, 24 h; (c) KOH, MeOH, reflux, overnight;
(d) acryloyl chloride, 1,4-dioxane, 45 1C, 3 h; (e) chloromethyl ethyl ether,
0 1C, DCM; (f) EDC, DMAP, DMF, r.t., 6 h; (g) PPTS, EtOH, 50 1C; (h) CBr4,
DCM, PPh3, 0 1C, 4 h; (i) O3, MeOH, 0 1C, 30 min, NaBH4, MeOH, 2 h;
(j) K2CO3, KI, acetone, reflux, 18 h; (k) EDC, DMAP, DCM, r.t., 24 h.
P12(soln:5/5) (m/n = 5/5) 1H NMR (300 MHz, CDCl3):
d (ppm): 8.00 (br, 4H), 7.57–7.45 (m, 6H), 7.14 (br, 2H), 6.91
(m, 4H), 4.29–4.00 (m, 8H), 1.78–1.54 (br, 10H), 1.26–1.03 (br, 17H).
Mn: 303, 541 g molꢀ1, PDI: 1.27 Mw/Mn.
Results and discussion
Chemical synthesis
The synthetic procedures of both BPIII reactive monomers M1 and
M2 are summarized in Scheme 2. The final chemical structures of
monomers and polymers were confirmed by 1H NMR spectra, and
the weight average molecular weights (Mw) and polydispersity
indexes (PDI) of polymers (see Table S1 of the ESI†) were determined
by gel permeation chromatography (GPC).
Fig. 1 1H NMR spectra of M1/M2(5/5), P12(soln:5/5), and P12(photo:5/5).
NMR spectral studies
As shown in Fig. 1, the 1H NMR spectra of copolymers show the
disappearance of proton peaks in the region of vinyl acrylate
groups with chemical shifts at 5.5–6.5 ppm (denoted as A, B,
Mesophasic and thermal properties
and C peaks), which indicated that all monomers were reacted The mesophasic properties of these BP reactive monomers
completely by solution- or photo-polymerization and most (M1 and M2), copolymers (P12 with various molar ratios of
sharp peaks became broader. The other related NMR data are M1 and M2 viasolution- and photo-polymerization) and binary
also shown in Fig. S1 of the ESI.†
mixtures (M1/M2 with various molar ratios of M1 and M2) are
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Table 1 Phase transition properties of solution-polymerizationa
Mole ratio (M1/M2)b
Solution-polymerization
Compounds
Feeding
Output
Phase transition temperaturec,d (1C) [enthalpies (J gꢀ1)]
DTBP
P1(soln)
10 : 0
9 : 1
7 : 3
5 : 5
3 : 7
1 : 9
0 : 10
10 : 0
8.4 : 1.6
7 : 3
5 : 5
3 : 7
1.3 : 8.7
0 : 10
Iso 143.7 [0.79] N* 131e G
0
P12(soln:9/1)
P12(soln:7/3)
P12(soln:5/5)
P12(soln:3/7)
P12(soln:1/9)
P2(soln)
Iso 141.6 [0.77] BPIII 140.1e N* 126e G
Iso 139.5 [0.85] BPIII 135.2e N* 114e G
Iso 149.2 [0.71] BPIII 143.9e N* 131e G
Iso 152.5 [1.02] BPIII 149.5e N* 124e G
Iso 152.0 [0.94] BPIII 150.8e N* 139e G
Iso 157.5 [1.03] N* 142e G
1.5
4.3
5.3
3.0
1.2
0
a
b
c
Polymerized at 60.0 1C using AIBN. M1: Iso 73.3 [0.82] BPIII 70.3 N* 41.6 [1.63] Cr; M2: Iso 73.5 [0.88] BPIII 71.1 N* 32.4 [1.70] Cr. Peak
temperatures in the DSC profiles obtained during the first heating and cooling cycles at a rate of 1 1C minꢀ1
phase III; N* = chiral nematic phase; G = glassy state. The transition temperature was observed by POM due to undetectable DSC data.
.
Iso = isotropic state; BPIII = blue
d
e
Table 2 Phase transition properties of photo-polymerizationa
Similar to the previous monomer design, M2 possessing a
longer end-attached acrylate spacer with a carbon number of
twelve revealed a mesophasic range of 2.4 1C for BPIII. Unfortu-
nately, the homopolymer P2(soln) solution polymerized by M2
cannot exhibit BPIII either (see Table 1). The disappearance of BPIII
in homopolymers P1(soln) and P2(soln) might be attributed to the
strong side-by-side self-assembly of the bi-mesogenic cores with the
same lengths of end-attached acrylate spacers in the side-chain
homopolymers.
Photo-polymerization
Phase transition temperatureb,c (1C)
Compounds
[enthalpies (J gꢀ1)]
DTBP
Pl(photo)
Iso 143.0 [0.70] N* 132d
G
0
P12(photo:9/1)
P12(photo:7/3)
P12(photo:5/5)
P12(photo:3/7)
P12(photo:1/9)
P2(photo)
Iso 140.8 [0.92] BPIII 139.4d N* 129d
Iso 134.9 [0.80] BPIII 132.0d N* 110d
Iso 135.5 [0.75] BPIII 131.7d N* 121d
Iso 138.0 [0.85] BPIII 135.9d N* 111d
G
G
G
G
1.4
2.9
3.8
2.1
0.8
0
Iso 139.2 [0.95] BPIII 138.4d N* 131
G
d
Iso 153.7 [0.98] N* 142d
G
In order to reduce the side-by-side self-assembly of the
bi-mesogenic cores in the side-chain homopolymers, copoly-
a
b
Polymerized at 80 1C in the isotropic state. Peak temperatures in the
DSC profiles obtained during the first heating and cooling cycles at a merization of M1 and M2 with different acrylate spacer lengths
c
rate of 1 1C minꢀ1
. Iso = isotropic state; BPIII = blue phase III; N* =
d
might be a good strategy to reduce the side-by-side self-
assembly of the bi-mesogenic cores. Hence, copolymers P12
with various molar ratios of M1 and M2 were synthesized via
solution-polymerization, which was initiated by azobis-isobutyro-
nitrile (AIBN) at 60 1C, to reduce and their phase transition
properties are illustrated in Table 1. Surprisingly, the side-chain
copolymer P12(soln:5/5) with a 5 : 5 molar ratio of M1 and M2
displayed a widest mesophasic range of BPIII (5.3 1C) among all
copolymers P12. This interesting result explained that the different
lengths of end-attached acrylate spacers of M1 and M2 in
copolymer P12(soln:5/5) might disturb the side-by-side self-
assembly of the bi-mesogenic cores so that the bi-mesogenic
cores on the side chains of copolymers can form double-twisted
cylinders more easily. In addition, the other side-chain copoly-
mers P12(soln) possess narrower mesophasic ranges of BPIII,
where the narrowest mesophasic range of BPIII (1.2 1C) was
observed in P12(soln:1/9). Comparably, as shown in Table 2,
photo-polymerization was carried out to acquire homopolymers
P1(photo) and P2(photo) along with copolymers P12(photo)
with various molar ratios of M1 and M2 at isotropic tempera-
ture (80 1C), where the radiation intensity was 1.5 mW cmꢀ2 at
365 nm with an exposure time of 20 min. Similar to solution-
polymerization, no BPIII was observed in P1(photo) and
P2(photo), and the widest and narrowest mesophasic ranges
of BPIII (3.8 and 1.2 1C) were observed in P12(photo:5/5) and
P12(photo:1/9), respectively. In general, all side-chain copolymers
P12 possess broader mesophasic ranges of BPIII via solution-
polymerization than photo-polymerization, which might be due to
the higher molecular weights obtained via solution-polymerization
than photo-polymerization (see Table S1 and Fig. S2 in ESI†).
chiral nematic phase; G = glassy state. The transition temperature was
observed by POM due to undetectable DSC data.
summarized in Tables 1–3. The phase transition of the iso-
tropic phase (Iso)-foggy blue color phase (BPIII)-chiral nematic
phase (N*)-glassy state (G) was determined by polarizing optical
microscopy (POM) and enthalpy changes were determined by
differential scanning calorimetry (DSC). The BP reactive mono-
mer M1 containing a six carbon number end-attached acrylate
spacer possesses a mesophasic BPIII range of 3.0 1C. However,
as shown in Table 1 the BPIII phase of M1 was eliminated by
solution-polymerization of M1 to generate homopolymer
P1(soln), where the mesophasic behaviour of P1(soln) only
demonstrated the isotropic phase (Iso) – chiral nematic phase
(N*) – glassy state (G).
Table 3 Phase transition properties of binary mixturesa,b
Phase transition temperature (1C)
Compounds
[enthalpies (J gꢀ1)]
DTBP
Ml
Iso 73.3 [0.82] BPIII 70.3c N* 41.6 Cr
Iso 74.3 [0.83] BPIII 71.7c N* 38.1 Cr
Iso 74.5 [0.96] BPIII 72.1c N* 35.2 Cr
Iso 74.7 [0.75] BPIII 72.7c N* 38.4 Cr
Iso 73.6 [0.86] BPIII 71.1c N* 35.8 Cr
Iso 73.7 [0.95] BPIII 71.0c N* 34.4 Cr
Iso 73.5 [0.88] BPIII 71.1c N* 32.4 Cr
3.0
2.6
2.4
2.0
2.5
2.7
2.4
M1/M2(9/1)
M1/M2(7/3)
M1/M2(5/5)
M1/M2(3/7)
M1/M2(1/9)
M2
a
Peak temperatures in the DSC profiles obtained upon the first cooling and
second heating cycles at a rate of 1 1C minꢀ1
blue phase III; N* = chiral nematic phase; Cr = crystal. The transition
temperature was observed by POM due to undetectable DSC data.
.
b Iso = isotropic state; BPIII =
c
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In the meantime, the binary mixtures of M1 and M2 (i.e., M1/M2)
with different molar ratios were prepared to compare with their
side-chain copolymers P12(soln) and P12(photo) possessing
analogous compositions, and the phase transition properties
of binary mixtures M1/M2 are illustrated in Table 3.
Though all binary mixtures M1/M2 possessed BPIII, narrower
BPIII ranges were observed in these binary mixtures in contrast
to M1. Thus, the BPIII ranges of M1 and M2 could not be further
extended by the strategy of using binary mixtures, which is
totally different from the copolymerization strategy to enhance
BPIII ranges, e.g., homopolymers P1 and P2 without BPIII vs.
copolymers P12 with BPIII. Particularly, the BPIII range (5.3 1C)
of copolymer P12(soln:5/5) is wider than that (2.0 1C) of binary
mixture M1/M2(5/5) with a molar ratio of 5 : 5. This result might
be explained by the fact that the side-by-side self-assembly of the
bi-mesogenic cores could be disrupted by copolymerization of
two monomers with different acrylate spacer lengths, which did
not occur to the binary mixture of monomers M1 and M2
without the linkage of polymer backbones. All related DSC
curves and BP ranges of binary mixtures, photo- and solution-
polymerization processes are illustrated in Fig. S3 and S4 (ESI†),
respectively, of the ESI.†
Fig. 3 The chirality behaviors of P12(soln:5/5) observed by POM at
146.1 1C. (a) The polarizer was rotated clockwise by a small angle of 101;
(b) The polarizer and analyzer were orthogonal; (c) the polarizer was
rotated counterclockwise by a small angle of 101. (White arrows are the
directions of polarizers and analyzers.)
The photo images of POM and atomic force microscopy
(AFM) are displayed in Fig. 4(a)–(f), respectively. In contrast to
an unclear and random texture of Fig. 4(d) in the isotropic
state, the irregular filamentous structure of BPIII is shown in
Fig. 4(e), in which the filamentary texture is arranged in
random orientations with diameters ca. 2–5 nm and lengths
ca. 0.1–0.5 mm. An expanded AFM image of P12(soln:5/5) in
Optical investigation
The M1/M2(5/5) behavior of BPIII could be verified by the
rotation of a polarizer (see Fig. 2). As the polarizer was rotated
clockwise by a small angle of 101 from the crossed position, the
color was changed from blue to light blue (Fig. 2(a)). Upon
rotating the polarizer counterclockwise by the same angle (101)
from the crossed position, the color was changed from blue to
red (Fig. 2(c)). The POM observations also indicated that the
phase of BPIII has selective reflection colors according to their
pitch lengths, which were examined by changing the cross
angles in the polarizer and analyzer of POM. As shown in
Fig. 3(a)–(c), the similar consequence could be observed in
P12(soln:5/5).
AFM morphology studies
In order to analyze the morphological surface structure of the blue
phase clearly, P12(soln:5/5) with the broadest BPIII mesophasic
range was quenched from different states, including: the isotropic
state, and BPIII and N* phases, through rapid immersion into
liquid N2 to supercool and maintain their frozen textures.
Fig. 2 The chirality behaviors of M1/M2(5/5) observed by POM at 73.5 1C.
(a) The polarizer was rotated clockwise by a small angle of 101; (b) the
polarizer and analyzer were orthogonal; (c) the polarizer was rotated
counterclockwise by a small angle of 101. (White arrows are the directions
of polarizers and analyzers.)
Fig. 4 POM textures of P12(soln:5/5) upon cooling (0.5 1C minꢀ1).
(a) Isotropic state at 155 1C. (b) Blue phase III at 146.1 1C. (c) Chiral nematic
phase at 138.7 1C. AFM images of P12(soln:5/5) was quenched by liquid N2;
(d) isotropic state at 155 1C. (e) Blue phase III at 146.1 1C. (f) Chiral nematic
phase at 138.7 1C. (White scale bar: 1 mm.)
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BPIII is demonstrated in Fig. S5 of the ESI.† Compared with the
highly ordered structures of BPI (body-center cubic structure)
and BPII (simple cubic structure), the structure of BPIII is
predicted to be more symmetric as the isotropic state and to
become amorphous with the short-range order of double
twisted structure alone. As shown in the AFM image of
Fig. 4(e) and Fig. S5 (ESI†), the surface of BPIII exhibited an
irregular array and the cylindrical shaped objects were arranged
in oblique and fractured filaments with random orientations.
They have also been found to exhibit similar morphological
images of spaghetti-like tangles and liquid-like arrangements in
BPIII.27 Upon further cooling, Fig. 4(f) with the disappearance of
BPIII only demonstrates a liquid-like surface in N* due to lack of
the double twisted cylindrical structure.
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Conclusions
In summary, we have developed the first BP reactive monomers
possessing the bi-mesogenic core containing one central chiral
linker. By introducing two end-attached acrylic spacers with
different lengths (i.e., carbon number = 6 and 12 for M1 and
M2, respectively) into the first BP reactive monomers, the blue
phase (BPIII) ranges of copolymers could be further extended
and stabilized by solution- and photo-polymerization of M1 and
M2, with a molar ratio of 5 : 5. Instead of photo-polymerization
of double-twisted cylinders to stabilize the BP structure, this
approach is also the first ever tried solution-polymerization
(instead of photo-polymerization only) of monomers to produce
the BP LCs. However, due to the strong side-by-side self-
assembly of the bi-mesogenic cores with the same lengths of
end-attached acrylate spacers in the side-chain homopolymers
P1 and P2, the homopolymerization of M1 and M2 could not
induce any blue phase. Besides, the morphological surface
structure of the irregular filamentous- and spaghetti-like
tangled BPIII was first observed in copolymer P12(soln:5/5) by
AFM in this report. This research opens up a new avenue for
designing novel reactive monomers and polymers that can
stabilize the blue phase efficiently. Accordingly, this report also
offers blue phase reactive monomers for the first time as
suitable dopant candidates of multi-component LC mixtures
to stabilize BPs for the future display applications.
´
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The financial support of this project is provided by the Ministry
of Science and Technology (MOST) in Taiwan through MOST
103-2113-M-009-018-MY3 and MOST 103-2221-E-009-215-MY3.
20 S. Ni, H. Li, S. Li, J. Zhu, J. Tan, X. Sun, C. P. Chen, G. D. Wu,
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Notes and references
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