Poly[(side-on mesogen)-Alt-(end-on mesogen)]: A compromised molecular arrangement

Article abstract of DOI:10.1021/acs.macromol.9b00607

In recent years, sequence-controlled side-chain liquid crystal polymers (SCLCPs) have gained extensive interest because mesogenic units with different lengths and distributions can form various ordered sequences, which further endow LCP materials with diverse functions. In this manuscript, a side-chain side-on maleimide-containing monomer 2,5-bis-(4-butoxy-benzoyloxy)-benzoic acid 6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexyl ester (Y1801) and a side-chain end-on styrene-containing monomer 4′-[6-(4-vinyl-phenoxy)-hexyloxy]-biphenyl-4-carbonitrile (Y1802) are combined in one single macromolecular chain and orderly polymerized in an alternative sequence to form an alternating copolymer Poly(Y1801-Alt-Y1802). The chemical structure and alternating sequence of Poly(Y1801-Alt-Y1802) are confirmed by GPC and NMR techniques. The combination of DSC, POM, and WAXS data indicates that, although the side-on homopolymer PY1801 and the end-on homopolymer PY1802 both exhibit the nematic phase, their alternating copolymer Poly(Y1801-Alt-Y1802) shows an interdigitated smectic A phase, a compromised molecular arrangement instead. In addition, a strong fluorescence emission of Poly(Y1801-Alt-Y1802) is observed, which might provide this novel alternating-structured liquid crystal polymer with potential applications in luminescent materials and devices.

Full text of DOI:10.1021/acs.macromol.9b00607

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Poly[(side-on mesogen)-alt-(end-on mesogen)]: A Compromised
Molecular Arrangement
Meng Wang,† Wei-Wei Bao,† Wen-Ying Chang,‡ Xu-Man Chen,† Bao-Ping Lin,† Hong Yang,*,†
and Er-Qiang Chen*,‡
†School of Chemistry and Chemical Engineering, Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, Jiangsu Key
Laboratory for Science and Application of Molecular Ferroelectrics, Southeast University, Nanjing, Jiangsu Province 211189, China
‡Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics at the Ministry of
Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
S
* Supporting Information
ABSTRACT: In recent years, sequence-controlled side-chain liquid crystal polymers (SCLCPs) have gained extensive interest
because mesogenic units with different lengths and distributions can form various ordered sequences, which further endow LCP
materials with diverse functions. In this manuscript, a side-chain side-on maleimide-containing monomer 2,5-bis-(4-butoxy-
benzoyloxy)-benzoic acid 6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexyl ester (Y1801) and a side-chain end-on styrene-containing
monomer 4′-[6-(4-vinyl-phenoxy)-hexyloxy]-biphenyl-4-carbonitrile (Y1802) are combined in one single macromolecular
chain and orderly polymerized in an alternative sequence to form an alternating copolymer Poly(Y1801-alt-Y1802). The
chemical structure and alternating sequence of Poly(Y1801-alt-Y1802) are confirmed by GPC and NMR techniques. The
combination of DSC, POM, and WAXS data indicates that, although the side-on homopolymer PY1801 and the end-on
homopolymer PY1802 both exhibit the nematic phase, their alternating copolymer Poly(Y1801-alt-Y1802) shows an
interdigitated smectic A phase, a compromised molecular arrangement instead. In addition, a strong fluorescence emission of
Poly(Y1801-alt-Y1802) is observed, which might provide this novel alternating-structured liquid crystal polymer with potential
applications in luminescent materials and devices.
shown in Figure 1a. The difference of the relative position of
the side groups and polymer backbones between side-on
SCLCPs and end-on SCLCPs could markedly influence their
functional properties.15 Compared with end-on SCLCPs, side-
on SCLCPs with spacers between mesogens and polymer
backbones tend to force the mesogenic directors parallel to the
orientations of polymeric backbones, possess larger backbone
anisotropy, and form the nematic phase predominantly.16−19 A
typical and interesting example was that SCLCPs with chiral
backbones and achiral mesogenic units usually showed the
achiral nematic phase if the side groups were side-on
mesogens.20 Meanwhile, they always presented the cholesteric
phase if the side groups were end-on mesogens21−25 since the
INTRODUCTION
■
Liquid crystal polymers (LCPs) have attracted great scientific
attention since they were first discovered in the middle of the
20th century.1−6 Benefited from their excellent thermal
stability and mechanical property, LCPs have wide applications
in many fields, such as organic photoelectric materials,
engineering plastics, high-modulus fibers, tunable diffraction
gratings, and thermal-isolation materials.7−11 Generally, LCPs
can be divided into two main categories:12−14 (1) main-chain
liquid crystal polymers, which embed the liquid crystal
molecules into the polymer main chains, and (2) side-chain
liquid crystal polymers (SCLCPs), which graft the liquid
crystal molecules as side groups onto the polymer main chains.
Among them, SCLCPs can be divided into two different parts:
side-on SCLCPs and end-on SCLCPs, which depend on
whether the mesogenic units are attached onto the polymer
backbones laterally (side-on) or longitudinally (end-on), as
Received: March 25, 2019
Revised: July 9, 2019
© XXXX American Chemical Society
A
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properties of these monomers and polymers were studied by
nuclear magnetic resonance (NMR) and differential scanning
calorimetry (DSC). Polarized optical microscopy (POM) and
one-dimensional/two-dimensional wide-angle X-ray scattering
(1D/2D-WAXS) were performed to confirm the mesophase
transition behavior and the molecular arrangements of the
alternating SCLCP Poly(Y1801-alt-Y1802). The ultraviolet−
visible (UV−vis) and fluorescence spectra of two monomers
and three polymers were also measured to evaluate their
photophysical properties.
EXPERIMENTAL SECTION
■
General Considerations. N-Methoxycarbonyl maleimide, 6-
amino-1-hexanol, 4-acetoxystyrene, toluene-4-sulfonyl chloride, 4′-
hydroxy-4-biphenyl carbonitrile azodiisobutyronitrile (AIBN), dicy-
clohexylcarbodiimide (DCC), di-tert-butyl peroxide (DTBP), and
dimethylaminopyridine (DMAP) were obtained from Aladdin
(Shanghai) Inc. Dimethylformamide (DMF) and tetrahydrofuran
(THF) were distilled from CaH2 under nitrogen. Other chemical
reagents were used without further purification. 2,5-Bis-(4-butoxy-
benzoyloxy)-benzoic acid (compound 10) was synthesized following
our previous reports.38−40 The detailed synthetic protocols,
instrumentation descriptions, and part of measured data are listed
in the Supporting Information.
Figure 1. (a) Two traditional categories of SCLCPs; (b) alternating
SCLCPs designed in this work.
Synthesis of Monomer Y1801. (1) A mixture of methyl
chloroformate (3.40 mL, 44.0 mmol), maleimide (3.88 g, 40.0 mmol),
N-methylmorpholine (4.82 mL, 44.0 mmol), and ethyl acetate (240
mL) was added into a 500 mL single necked round-bottom flask.
After stirring at 0 °C for 30 min, the precipitate was separated by
filtration. The filtrate was collected and purified by flash column
chromatography (petroleum ether (v)/ethyl acetate (v) = 1/1) to
give the intermediate 3 (6.10 g, yield: 98%) as a white powder. 6-
Amino-1-hexanol (397 mg, 2.56 mmol) and saturated NaHCO3
aqueous solution (20 mL) were slowly added into a 50 mL round-
bottom flask and stirred at 0 °C. After slowly adding compound 3
(300 mg, 2.56 mmol), the above solution was continuously stirred at
0 °C for 2 h. The aqueous layer was adequately extracted with
dichloromethane (25 mL) several times, and the organic solvent was
removed by rotary evaporation to give the crude product. Flash
column chromatography (dichloromethane (v)/ethyl acetate (v) =
3:1) was further used to purify the product to give the intermediate 5
(260 mg, yield: 52%) as a white powder.41 1H NMR (300 MHz,
CDCl3) δ: 6.68 (s, 2H), 3.63 (t, J = 6.5 Hz, 2H), 3.52 (t, J = 7.2 Hz,
2H), 1.65−1.48 (m, 4H), 1.43−1.27 (m, 2H).
end-on mesogenic directors and the main chain orientation
tended to have a tilt angle that might induce helical structures
of the terminally attached mesogens under the twisting force of
chiral backbones.26 Moreover, lightly cross-linked side-on
SCLCPs, also known as side-on side-chain liquid crystal
elastomers (side-on SCLCEs), exhibit faster stimulus-respon-
sive rates and superior shape morphing capabilities in
comparison with end-on SCLCE materials.27,28
In recent years, sequence-controlled SCLCPs have gained
extensive interest because mesogenic units with different
lengths and distributions can form various ordered sequences,
which further endow LCP materials with diverse func-
tions.29−35 For example, Ma and colleagues synthesized four
different sequence-determined SCLCPs that could present
different phase transitions and polarized optical morphologies
by adjusting the specific spacing between adjacent mesogens.36
Chiellini’ group reported three series of alternating end-on
SCLCPs Poly[vinylether-alt-(end-on mesogen)] and realized a
random distribution of mesomorphic groups by appropriately
adjusting the stoichiometry of feed mixtures.37 Inspired by
these pioneering works, we are curious about if side-on
mesogens and end-on mesogens were combined in one single
SCLCP and orderly polymerized in an alternative sequence,
what kind of mesomorphic property the corresponding
alternating SCLCP Poly[(side-on mesogen)-alt-(end-on meso-
gen)] could have? What would be the molecular arrangement
and potential functions of this alternating SCLCP?
In this work, we designed and synthesized a side-chain side-
on maleimide-containing monomer Y1801 and a side-chain
end-on styrene-containing liquid crystal (LC) monomer Y1802
as shown in Figure 1b and further prepared a novel alternating
poly[(side-on mesogen)-alt-(end-on mesogen)] SCLCP Poly-
(Y1801-alt-Y1802) through a classical styrene-maleimide
alternative copolymerization approach, which has the advan-
tages of operational simplicity, mild conditions, and high
polymerization degrees in synthesizing alternative copolymers.
In addition, two homopolymers PY1801 and PY1802 derived
from these two monomers, respectively, were also prepared for
comparison purposes. The structures and thermodynamic
(2) Compound 10 (684 mg, 1.35 mmol), anhydrous dichloro-
methane (25 mL), DMAP (16.5 mg, 0.135 mmol), and compound 5
(266 mg, 1.35 mmol) were carefully added into a 50 mL round-
bottom flask. After slowly adding DCC (279 mg, 1.62 mmol), the
above solution was continuously stirred at 25 °C for 48 h. After
removing the organic solvent, the crude product was further purified
by flash column chromatography (ethyl acetate (v)/petroleum ether
(v) = 1/10) to give the desired monomer Y1801 (633 mg, yield:
1
68%) as a white powder. H NMR (600 MHz, CDCl3) δ: 8.15 (m,
4H), 7.89 (d, J = 2.9 Hz, 1H), 7.45 (m, 1H), 7.25 (s, 1H), 6.98 (m,
4H), 6.65 (s, 2H), 4.14 (t, J = 6.6 Hz, 2H), 4.06 (m, 4H), 3.46 (t, J =
7.2 Hz, 2H), 1.86−1.78 (m, 4H), 1.56−1.46 (m, 8H), 1.25 (m, 2H),
1.16 (m, 2H), 1.00 (m, 6H). 13C NMR (75 MHz, CDCl3) δ: 170.7,
164.8, 164.5, 164.0, 163.6, 148.1, 133.9, 132.3, 127.0, 124.9, 121.4,
121.0, 114.3, 77.4, 76.9, 76.5, 67.9, 65.3, 37.6, 31.1, 29.6, 28.2, 26.2,
25.3, 19.1, 13.7. ESI-MS m/z: 708.28 [M + Na]+.
Synthesis of LC Monomer Y1802. (1) A mixture of anhydrous
ethanol (200 mL) and KOH (22.8 g, 400.7 mmol) was slowly added
into a 1 L round-bottom flask. 4-Acetoxystyrene (22.8 g, 140.7 mmol)
was added into the above solution and stirred at 25 °C for 1 h.
Sodium ethoxide (10.5 g, 154.4 mmol) was slowly added into the
solution and continuously stirred for another 30 min at 80 °C. 6-
Bromohexanol (25.6 g, 140.9 mmol) dissolved in 200 mL of
anhydrous ethanol was then added, and the above solution was
B
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Scheme 1. Synthetic Routes of (a) Monomers Y1801 and Y1802, (b) Homopolymers PY1801 and PY1802, and (c) Alternating
SCLCP Poly(Y1801-alt-Y1802)
maintained at 80 °C for 48 h. The solution was extracted by
dichloromethane, and the organic layer was collected. After removing
the solvent by rotary evaporation, the crude product was purified by
flash column chromatography (ethyl acetate (v)/petroleum ether (v)
= 1/10) to give intermediate 14 as a colorless oil (20.0 g, yield: 70%).
A mixture of p-tosyl chloride (2.30 g, 12.13 mmol), anhydrous
dichloromethane (20 mL), and compound 14 (2.25 g, 11.03 mmol)
was added into a 100 mL round-bottom flask. After dropwise adding
triethylamine (2.2 g, 21.73 mmol) under a N2 atmosphere, the above
solution was further stirred at 25 °C for 24 h. After removing the
solvent by rotary evaporation, the crude product was thus purified by
flash column chromatography (ethyl acetate (v)/petroleum ether (v)
=1/20) to give compound 16 as a white powder (1.86 g, yield: 45%).
1H NMR (600 MHz, CDCl3) δ: 7.79 (d, J = 8.2 Hz, 2H), 7.35−7.31
(m, 4H), 6.84−6.80 (m, 2H), 6.65 (m, 1H), 5.60 (d, J = 17.6 Hz,
1H), 5.12 (d, J = 10.9 Hz, 1H), 4.03 (m, 2H), 3.91 (t, J = 6.4 Hz,
2H), 2.44 (s, 3H).
(2) 4-Hydroxy-4′-cyanobiphenyl (1.06 g, 5.46 mmol), compound
16 (1.86 g, 4.96 mmol), DMF (25 mL), and K2CO3 (0.82 g, 6.00
mmol) were carefully added into a 100 mL round-bottom flask. After
stirring at 80 °C under a nitrogen atmosphere for 48 h, 100 mL of
water was thus added, and the reaction solution was extracted with
dichloromethane three times (50 mL × 3). The resulting white
product was concentrated and purified by flash column chromatog-
raphy (ethyl acetate (v)/petroleum ether (v) = 1/20) to give the
monomer Y1802 as a white powder (1.81 g, yield: 92%).42 1H NMR
(600 MHz, CDCl3) δ: 7.69 (d, J = 8.3 Hz, 2H), 7.64 (d, J = 8.3 Hz,
2H), 7.52 (d, J = 8.6 Hz, 2H), 7.34 (d, J = 8.5 Hz, 2H), 6.99 (d, J =
8.6 Hz, 2H), 6.85 (d, J = 8.6 Hz, 2H), 6.65 (m, 1H), 5.60 (m, 1H),
5.12 (m, 1H), 4.03(t, J = 6.4 Hz, 2H), 3.98 (t, J = 6.4 Hz, 2H), 1.84
(m, 4H), 1.56−1.58 (m, 5H). 13C NMR (75 MHz, CDCl3) δ: 159.7,
158.9, 145.3, 136.2, 132.6, 131.3, 130.3, 128.4, 127.4, 127.1, 119.2,
115.1, 114.5, 111.5, 110.0, 68.0, 29.2, 25.9. ESI-MS m/z: 397.1 [M −
H]−.
C
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Synthesis of Homopolymer PY1801. As shown in Scheme 1b,
Y1801 (100 mg, 0.16 mmol), DMF (0.2 mL), and DTBP (0.2 mg, 1.5
× 10−3 mmol) were slowly added into a 10 mL Schlenk tube. The
above mixture was adequately degassed and exchanged with high-
purity nitrogen via several freeze−thaw cycles and then stirred at 130
°C for 24 h. A large amount of methanol was poured into the above
solution to precipitate the polymer. The gathered product was further
dissolved in dichloromethane and reprecipitated from methanol. After
three cycles, the product was further dried in vacuum for 24 h to give
the homopolymer PY1801 as a brown powder (20 mg, yield: 20%).
1H NMR (600 MHz, CDCl3) δ: 7.95 (m, 4H), 7.71 (m, 1H), 7.26
(m, 1H), 7.06 (m, 1H), 6.78 (m, 4H), 3.87 (m, 8H). 13C NMR (75
MHz, CDCl3) δ: 162.6, 147.2, 131.2, 123.8, 120.4, 113.3, 66.9, 64.7,
30.1, 28.5, 27.2, 24.0, 18.1, 12.8.
Table 1. Molecular Weight Measurements of Polymers
Mn
Mw
a
entry
polymer
(g/mol)
(g/mol)
DP
7
PDI
1
2
3
PY1801
PY1802
Poly(Y1801-alt-Y1802)
4600
136500
104200
7200
290800
221300
1.56
2.13
2.12
342
96
a
The degree of polymerization was estimated based on the GPC data.
calibration using polystyrene standards was performed to
analyzed molecular weight measurements of those polymers.
The homopolymer PY1801 was synthesized by using di-tert-
butyl peroxide (DTBP) as the radical initiator in anhydrous
DMF at 130 °C for 24 h. The number average molecular
weight (Mn) of PY1801 was 4600 g/mol, and the weight
average molecular weight (Mw) was 7200 g/mol, which
indicated a low degree of polymerization (DP) of maleimide-
containing molecule Y1801. Based on the literature, maleimide
is an electron-deficient molecule with a weak electron-donating
ability and could only form oligomers. The styrene-containing
molecule Y1802 was polymerized by using AIBN as the
initiator in anhydrous THF at 60 °C for 24 h to give the
homopolymer PY1802 as a white solid with a yield of 80%.
The Mn and Mw of PY1802 were measured as 136500 and
290800 g/mol, respectively. As a typical electron-rich
molecule, styrene tended to carry out free radical polymer-
ization efficiently under mild reaction conditions to give
homopolymers with high DP and molecular weights. The
styrene-maleimide alternative copolymerization reaction be-
tween Y1801 and Y1802 was carried out using AIBN as the
radical initiator in dry dichloroethane at 70 °C to give the
desired copolymer Poly(Y1801-alt-Y1802) as a white solid
with a yield of 70%. The Mn and Mw of Poly(Y1801-alt-Y1802)
were measured as 104200 and 221300 g/mol. Moreover, from
the GPC chromatogram (Figure S14), only a single sharp peak
of Poly(Y1801-alt-Y1802) was observed, which implied that
side reactions were neglectable during the alternative
copolymerization process.
Synthesis of Homopolymer PY1802. AIBN (0.4 mg, 2.5 × 10−3
mmol), Y1802 (100 mg, 0.25 mmol), and anhydrous THF (0.3 mL)
were carefully added into a 10 mL Schlenk tube. The above mixture
was adequately degassed and exchanged with high-purity nitrogen via
several freeze−thaw cycles and then stirred at 60 °C for 24 h. A large
amount of methanol was poured into the above solution to precipitate
the polymer. The gathered product was further dissolved in
dichloromethane and reprecipitated from methanol. After three
cycles, the product was further dried in vacuum for 24 h to give
the homopolymer PY1802 as a white powder (80 mg, yield: 80%). 1H
NMR (600 MHz, CDCl3) δ: 7.39−7.53 (m, 6H), 6.85 (m, 2H), 6.53
(m, 4H), 3.88 (m, 4H), 1.73 (m, 4H), 1.49 (m, 6H). 13C NMR (75
MHz, CDCl3) δ: 158.7, 144.7, 131.6, 130.3, 127.4, 126.0, 117.6,
114.0, 113.5, 113.0, 110.4, 109.1, 67.0, 28.2, 24.9.
Synthesis of Alternating Copolymer Poly(Y1801-alt-Y1802).
AIBN (0.48 mg, 3.0 × 10−3 mmol), Y1801 (100 mg, 0.16 mmol), 1,2-
dichloroethane (0.4 mL), and Y1802 (58 mg, 0.15 mmol) were
carefully added into a 10 mL Schlenk tube. The above mixture was
adequately degassed and exchanged with high-purity nitrogen via
several freeze−thaw cycles and then stirred at 70 °C for 6 h. A large
amount of methanol was poured into the above solution to precipitate
the polymer. The gathered product was further dissolved in
dichloromethane and reprecipitated from methanol. After three
cycles, the product was further dried in vacuum for 30 h to give
the copolymer Poly(Y1801-alt-Y1802) as a white powder (110 mg,
yield: 70%). 1H NMR (600 MHz, CDCl3) δ: 8.07 (m, 4H), 7.83 (m,
1H), 7.52 (m, 4H), 7.43 (m, 2H), 7.17 (m, 2H), 6.89 (m, 8H), 3.97
(m, 12H). 13C NMR (75 MHz, CDCl3) δ: 163.7, 162.7, 158.6, 147.3,
144.0, 131.5, 131.4, 131.3, 127.2, 125.9, 123.8, 113.9, 113.3, 76.2,
76.0, 30.1, 28.2, 27.2, 24.9, 24.3, 18.1, 12.8.
1H NMR spectra of monomers Y1801 and Y1802 and their
corresponding homopolymers PY1801 and PY1802 are shown
in Figure 2a,b. After the free radical polymerization, the peak at
∼6.65 ppm originally belonging to the olefin protons of the
side-on maleimide group of Y1801 disappeared completely in
RESULTS AND DISCUSSION
■
As shown in Scheme 1a, we designed and synthesized a side-
chain side-on monomer Y1801 and a side-chain end-on
monomer Y1802. Maleimide (1) was first reacted with methyl
chloroformate (2) to give the intermediate 3, which was
further coupled with 6-amino-1-hexanol (4) to provide the
alcohol 5. Meanwhile, 2,5-dihydroxy-benzoic acid (6) was
transformed to 2,5-dihydroxy-benzoic acid benzyl ester (7),
which went through an esterification reaction with 4-butoxy-
benzoic acid (8) and a following reduction reaction with Pd/C
to give the key intermediate acid 10. After a DCC coupling
reaction between compounds 5 and 10, the side-chain side-on
monomer Y1801 was successfully prepared. The synthesis of
the end-on mesogen Y1802 started with a basic hydrolysis of
acetic acid 4-vinyl-phenyl ester (11) and then went through
two etherification reactions with 6-bromo-hexan-1-ol (13) and
4′-hydroxy-biphenyl-4-carbonitrile (17) successively to pro-
vide the desired monomer.
1
the H NMR spectrum of PY1801. Similarly, as presented in
Figure 2b, the peaks at ∼5.03 and ∼5.53 ppm ascribed to the
resonance signals of the three terminal olefin protons of the
1
vinyl group of Y1802 vanished in the H NMR spectrum of
PY1802. The disappearances presented solid evidence for the
successful homopolymerization of Y1801 and Y1802. Fur-
thermore, the spectra of homopolymers PY1801 and PY1802
showed broad peaks, which were typical characteristics of
polymers due to the slower motions of the protons. The
alternating structure of Poly(Y1801-alt-Y1802) was also
1
confirmed by H NMR and 13C NMR techniques. As shown
in Figure 2c, based on the integral ratio of the aromatic
protons (He,Hf ∼8.07 ppm) and the aromatic proton
(Hh,Hi,Hj,Hk ∼6.89 ppm), the molar ratio of Y1801 and
Y1802 could be roughly determine by solving the following
equation: Y1801/Y1802 = (I8.07/4)/(I6.89/8), where I8.07 and
I6.89 represent the integral values of peaks at ∼8.07 and ∼6.89
ppm. It was further confirmed that the molar ratio of two
monomers in Poly(Y1801-alt-Y1802) was approximately 1:1.
Following literature protocols,43,44 the alternating maleimide-
As shown in Scheme 1b,c, the homopolymers PY1801 and
PY1802 and the copolymer Poly(Y1801-alt-Y1802) were
prepared by free radical polymerization, respectively. As listed
in Table 1, gel permeation chromatography (GPC) based on
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Figure 2. 1H NMR spectra of (a) Y1801, PY1801, (b) Y1802, PY1802, and (c) Poly(Y1801-alt-Y1802). 13C NMR spectra of (d) Poly(Y1801-alt-
Y1802), and the expanded regions at (e) 125−155 and (f) 50−70 ppm.
styrene-maleimide structure could be confirmed by 13C NMR
spectra. Two expanded ranges around 50−70 (carbons in the
polymer main chain) and 125−155 ppm (carbons attached to
the main chain) are shown in Figure 2e,f. A main subpeak of
styrene quaternary carbon (C5) was observed at ∼131 ppm
(Figure 2e), which was the most typical characteristic peak of
the alternating maleimide-styrene-maleimide triad based on the
previous work.43 The resonance signals observed in the region
of 50−70 ppm could be ascribed to the carbons (C1, C2, C3,
C4) of the alternating maleimide-styrene-maleimide triad in
the polymer backbone, which presented another evidence for
the successful alternative copolymerization of Y1801 and
Y1802. As a conclusion, the alternating structure of SCLCP
Poly(Y1801-alt-Y1802) has been established through a
classical styrene-maleimide alternative copolymerization ap-
proach.
The mesomorphic behaviors and thermal properties of two
monomers and three polymers were adequately investigated by
DSC and POM. The crystallization of monomer Y1801 was
quite difficult, and no obvious exothermic peak was observed
during the first cooling process (Figure 3a). During the second
heating process, an exothermic process prior to the melting
peak (around 99 °C) was observed. Differently, its
corresponding homopolymer PY1801 showed an enantiotropic
LC phase in the temperature range of 61−87 °C (Figure 3c).
Monomer Y1802 showed complicated phase transitions during
the heating process and has two phase transition peaks in the
cooling process implying that a mesophase possibly existed
between the crystal state and the isotropic state (Figure 3b),
while its corresponding homopolymer PY1802 only exhibited
one phase transition peak around 122 °C (Figure 3d), which
was the LC-to-isotropic transition based on POM observation.
The DSC curve of the alternating copolymer Poly(Y1801-alt-
Y1802) (Figure 3e) presented a glass transition temperature at
68 °C and a clearing point temperature at 129 °C, which was
ascribed to the LC-to-isotropic transition. In good agreement
with DSC results, monomer Y1801 with a crystalline spherulite
texture shown in Figure 4a possessed no mesophases. In
contrast, as indicated in Figure 4b, the POM image of
monomer Y1802 presented the Schlieren texture during the
cooling process, which was the characteristic of the nematic
phase. As shown in Figure 4c−f, PY1801, PY1802, and
Poly(Y1801-alt-Y1802) all showed ambiguous birefringent
textures, which were often observed in polymer samples
because of their high viscosities.
E
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Figure 3. DSC curves of (a) Y1801, (b) Y1802, (c) PY1801, (d)
PY1802, and (e) Poly(Y1801-alt-Y1802) recorded in the first cooling
and second heating processes.
Figure 5. 1D-WAXS patterns of (a) Y1801, (b) Y1802, (c) PY1801,
(d) PY1802 during heating process, and Poly(Y1801-alt-Y1802)
during (e) heating process and (f) cooling process.
typical nematic phase. In agreement with the DSC data, the
1D-WAXS patterns of monomer Y1802 recorded at different
temperatures all presented sharp crystallization peaks during
the heating process, which implied that Y1802 might have at
least two crystal forms on heating. However, during the cooling
from the isotropic state, two diffuse peaks in both small-angle
and wide-angle regions were observed at the temperatures
ranging from 125 to 100 °C, which demonstrated that
monomer Y1802 has a monotropic nematic phase on cooling.
Differently, its homopolymer PY1802 showed one diffuse peak
in the wide-angle region and one diffuse peak in the small-
angle region, which were characteristics of the nematic phase.
The combination of DSC, POM, and 1D-WAXS data indicated
that PY1802 has a nematic phase. As shown in Figure 5e,f, the
1D-WAXS pattern of Poly(Y1801-alt-Y1802) presented two
scattering peaks at ∼30.2 and ∼14.7 Å in the small-angle
region with a reciprocal d-spacing ratio of 1:2 and a diffuse
peak at ∼4.19 Å in the wide-angle region, which indicated that
a smectic layer structure possibly existed in this copolymer
system. As the temperature increased, the sharp peak at small
angles (q = 2.08 nm−1) gradually weakened and disappeared
completely at 160 °C. This peak should be associated with the
lengths of two monomers Y1801 and Y1802. However, the d
spacing (∼30.2 Å) was larger than the fully extended lengths of
two monomers Y1801 (l = 22.0 Å) and Y1802 (l = 25.1 Å),
estimated by Dreiding models, but less than the sum of the
lengths of the two monomers Y1801 and Y1802. Thus, we
considered that the alternating SCLCP Poly(Y1801-alt-Y1802)
might have an interdigitated smectic phase. Meanwhile, it is
noticed that the peak at the small-angle region was slightly
Figure 4. POM images of (a) Y1801 recorded at 50 °C, (b) Y1802
recorded at 110 °C, (c) PY1801 recorded at 60 °C, (d) PY1802
recorded at 120 °C, and (e,f) Poly(Y1801-alt-Y1802) recorded at 70
°C on heating and cooling processes.
To further elucidate the mesomorphic behaviors of these
monomers and polymers, one- dimensional wide-angle X-ray
scattering (1D-WAXS) experiments were performed. As
presented in Figure 5a, the 1D-WAXS pattern of monomer
Y1801 has many sharp crystallization peaks, which once again
denied the possibility of liquid crystallinity. On the contrary, its
corresponding homopolymer PY1801 showed one diffuse peak
in the wide-angle region (Figure 5c), which demonstrated a
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broader than traditional smectic mesogens’ XRD spectra,
which might be derived from the coexistence of end-on and
side-on mesogens. Since Y1801 and Y1802 have different
mesogenic lengths, the alternating polymer Poly(Y1801-alt-
Y1802) might have a compromised molecular arrangement,
which resulted in broad peak width.
The two-dimensional wide-angle X-ray scattering (2D-
WAXS) experiments were further carried out to elucidate the
mesophase of Poly(Y1801-alt-Y1802). The copolymer powder
was mechanically sheared along the Y axis at 150 °C followed
by annealing at 70 °C for 5 h. The 2D-WAXS patterns of a
uniaxial oriented Poly(Y1801-alt-Y1802) sample were re-
corded at 60, 100, 120, 130, and 140 °C with an incident X-
ray beam (Z axis) perpendicular to the sample surfaces. As
illustrated in Figure 6a, during the heating process, a pair of
sharp arcs in the small-angle region was gathered on the
equator, and a pair of crescents in the wide-angle region was
gathered on the meridian. It was indicated that the uniaxial
oriented Poly(Y1801-alt-Y1802) has a smectic mesophase with
layer structures and the mesogens were arranged along the X
axis. A uniform diffuse ring was observed at 140 °C, which
implied that the sample became isotropic. The smectic layer
thickness (∼30.3 Å) was consistent with the datum calculated
from the 1D-WAXS pattern of the untreated sample, which
indicated that mechanical shearing force did not change the
molecular arrangement (Figure 6c). The mesomorphic phase
of Poly(Y1801-alt-Y1802) was identified as interdigitated
smectic A (SmAd) because the quadruple scattering halo,
characteristic of the smectic C phase, was not observed in the
2D-WAXS patterns. This compromised molecular arrangement
was also proven by the POM image of the Poly(Y1801-alt-
Y1802) fiber. The alternating copolymer was heated up to the
isotropic phase, and then a capillary pipette was attached to the
melted polymer and pulled quickly to form the Poly(Y1801-
alt-Y1802) fiber. As shown in Figure 6b, the birefringence was
extinct when the long axis of the Poly(Y1801-alt-Y1802) fiber
was parallel or perpendicular to one polarizer. The trans-
mission was maximized by rotating the fiber by 45°. It
represented that the uniformity of the alignment in the
alternating copolymer Poly(Y1801-alt-Y1802) was qualitatively
excellent. In general, after mechanical shearing, the randomly
oriented main chains of the alternating copolymer Poly-
(Y1801-alt-Y1802) were forced to align along the sheared
direction; meanwhile, the mesogens’ molecular directions were
not only perpendicular to the normal direction of the smectic
layer but also perpendicular to the sheared direction (Figure
6c). This compromised molecular arrangement of Poly(Y1801-
alt-Y1802) should be associated with the styrene-maleimide
alternative structure, which might harden the polymer main
chain, aggravate the steric hindrance, and force the attached
mesogens to form interdigitated structures with ordered
sequences. This hypothesis might explain the interdigitated
smectic A phase of the alternating SCLCP Poly(Y1801-alt-
Y1802).
UV−vis and fluorescence spectra of Y1801, Y1802, PY1801,
PY1802, and Poly(Y1801-alt-Y1802) were further carried out
to study their optical properties. As demonstrated in Figure 7,
the UV−vis spectra of two monomers and three polymers
dispersed in anhydrous THF and DMF with a monomeric
concentration of approximately 1.0 × 10−4 mol/L were
obtained. Here, the concentrations of monomers and polymers
were all calculated based on the moles of the monomeric units.
The two monomers and three polymers all presented two
strong absorption peaks in the UV region centered at around
225 and 280 nm in THF solution. These peaks were associated
with the π−π* electronic transitions of inner carbonyl groups
and phenyl rings. The absorption wavelengths of PY1801,
Poly(Y1801-alt-Y1802), and PY1802 increased (red-shift)
gradually, which were associated with the increased conjugated
structures of phenyl rings. Meanwhile, in DMF solution, the
absorption peaks centered at 225 nm became much weaker.
The main reason was that the vibration effect in polar organic
solvent might lead to the disappearance of the fine structures
and further weaken the absorption intensity. The fluorescence
spectra of two monomers and three polymers dissolved in
THF (Figure 7c) or DMF (Figure 7d) solution with a
monomeric concentration of approximately 1.0 × 10−4 mol/L
were also recorded at 25 °C. The maximum UV absorption
wavelengths of all the compounds were set as fluorescence
excitation wavelengths. A maximum emission wavelength
belonging to Poly(Y1801-alt-Y1802) was found at 435 nm in
THF solution, which was appreciably longer than those of
monomers and homopolymers (around 365 nm in THF).
Figure 6. (a) 2D-WAXS patterns of Poly(Y1801-alt-Y1802) recorded
at 60, 100, 120, 130, and 140 °C during the heating process. (b) POM
images of Poly(Y1801-alt-Y1802) fiber. (c) Schematic illustration of
the proposed molecular arrangement of Poly(Y1801-alt-Y1802).
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Figure 7. UV−vis spectra of polymers and monomers dispersed in (a) THF and (b) DMF with a monomeric concentration of approximately 1.0 ×
10−4 mol/L. Fluorescence spectra of polymers and monomers dispersed in (c) THF and (d) DMF with a monomeric concentration of
approximately 1.0 × 10−4 mol/L.
Based on the literature,45 the π−π interaction between the
phenyl rings and the carbonyl group in the polymer induced
photoluminescence. This conclusion was also applied in the
fluorescence spectra of these monomers and polymers
dissolved in DMF. In addition, compared to the emission
wavelengths in THF, the emission shifting to the long-
wavelength region was observed in DMF. Based on the
previous report,46 the red-shift of emission wavelength in
electron-rich solvents was associated with the formation of
polymer/solvent complexes.
A handheld UV lamp was used to irradiate the polymers
dissolved in DMF with a concentration of approximately 1.0 ×
10−4 mol/L for further study of photophysical properties. As
shown in Figure 8, the emission of the alternating copolymer
Poly(Y1801-alt-Y1802) was much stronger than those of other
two homopolymers. Obviously, these covalent linkages of the
styryl and maleimide units brought a sufficient close distance
for interaction among phenyl rings, carbonyl groups, and
solvents. Besides, PY1801 showed stronger emission phenom-
ena in comparison with PY1802, which might be ascribed to
the rigid conformation of PY1801 in DMF. This rigid
conformation may limit the free rotation of the backbones
along the C−C single bonds, which were beneficial for the
collection of many carbonyl groups and the formation of
polymer/solvent complexes, according to the literature.46
CONCLUSIONS
■
In summary, a new alternating SCLCP Poly(Y1801-alt-Y1802)
comprising a side-on mesogen Y1801 and an end-on mesogen
Y1802 was designed and synthesized via the styrene-maleimide
alternative copolymerization method. Two homopolymers
PY1801 and PY1802 were also prepared, respectively, for
comparison purposes. The alternating molecular structure of
Poly(Y1801-alt-Y1802) was proven by GPC and NMR
experiments. The mesomorphic properties of PY1801,
PY1802, and Poly(Y1801-alt-Y1802) were characterized by
DSC, POM, and WAXS techniques. Both the side-on SCLCP
PY1801 and the end-on SCLCP PY1802 exhibited the nematic
phase, while their alternating copolymer Poly(Y1801-alt-
Y1802) showed an interdigitated SmA phase, a compromised
molecular arrangement instead. In addition, UV−vis and
fluorescence spectra of the alternating SCLCP Poly(Y1801-alt-
Y1802) were measured, and an obvious fluorescence emission
of Poly(Y1801-alt-Y1802) in DMF solution was observed,
which endowed this novel alternating-structured liquid crystal
polymer with potential applications in luminescent materials
and devices.
Figure 8. Photo images of irradiating (a) Y1801, (b) Y1802, (c)
PY1801, (d) PY1802, and (e) Poly(Y1801-alt-Y1802) in DMF with a
concentration of approximately 1.0 × 10−4 mol/L under UV (365
nm) light.
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Elastomers Bearing Polynorbornene Backbone. J. Mater. Chem. C
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ASSOCIATED CONTENT
■
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* Supporting Information
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The Supporting Information is available free of charge on the
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mol.9b00607.
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H. An Entropy-Driven Ring-Opening Metathesis Polymerization
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Instrumentation descriptions, synthetic protocols, NMR
spectra, and GPC chromatogram (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: yangh@seu.edu.cn (H.Y.).
*E-mail: eqchen@pku.edu.cn (E.-Q.C.).
(18) Cotton, J. P.; Hardouin, F. Chain Conformation of Liquid-
Crystalline Polymers Studied by Small-Angle Neutron Scattering.
Prog. Polym. Sci. 1997, 22, 795−828.
ORCID
Hong Yang: 0000-0003-4647-1388
Er-Qiang Chen: 0000-0002-0408-5326
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by Jiangsu Provincial Natural
Science Foundation of China (Nos. BK20170024,
BK20180406), National Natural Science Foundation of
China (Grant No. 21634001), the Fundamental Research
Funds for the Central Universities (No. 2242018K40048), and
the Priority Academic Program Development of Jiangsu
Higher Education Institutions.
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