Triphenylene-based side chain liquid crystalline block copolymers containing a PEG block: Controlled synthesis, microphase structures evolution and their interplay with discotic mesogenic orders

Article abstract of DOI:10.1021/ma400655t

Triphenylene (TP) derivatives are typical and probably the most widely studied discotic liquid crystalline (DLC) materials. Through polymer analogous reactions to attach TP mesogens to the well-synthesized poly(ethylene glycol)-b-poly(2-hydroxyethyl acrylate) (PEG-PHEA) by ATRP, a series of well-defined side chain DLC diblock copolymers PEG-poly(TPm) (m = 6 or 10) with DLC block weight fraction (f_w,DLC) ranging from 37% to 90% have been successfully prepared with narrow molecular weight distribution (PDI ≤ 1.11). An intriguing microphase-separated superstructure evolution and the correlation between overall morphologies and discotic mesogenic orders as a function of f_w,DLC and temperature have been demonstrated by combination of DSC, POM, and variable temperature SAXS/WAXS. Those copolymers with lower DLC contents (f_w,DLC = 37% and 43%) and at lower temperatures formed lamellar structures of variant periods and underwent order-order transitions upon PEG region crystallization at 45 C and different discotic mesophases of N_D or N_col transition at about 25 C. For the copolymer with intermediate f_w,DLC = 62%, a high temperature hexagonal packed cylinder (HPC) structure of amorphous PEG nanocylinders in the matrix of DLC was formed above 35 C, while upon cooling below 35 C it turned into a mixed lamellar structure with PEG region crystallization. The higher f_w,DLC (67% ～ 80%) copolymers exhibited HPC structures with the DLC matrix showing N_col or N_D mesophases. For copolymers with the highest f_w,DLC around 90%, an overall N_D phase was developed in sharp contrast to the ordered columnar phase formed by their corresponding DLC homopolymers, which was quite inspiring and might suggest another pathway of attaining this important nematic discotic phase through introducing a suitable copolymerized block. The better understanding of the interrelation of microstructures and discotic mesogenic orders constitutes the key basis for utilizing such type of organic semiconductor materials and could help to guide the design of complex DLC polymer materials with hierarchical structures for variant applications.

Full text of DOI:10.1021/ma400655t

Article
pubs.acs.org/Macromolecules
Triphenylene-Based Side Chain Liquid Crystalline Block Copolymers
Containing a PEG block: Controlled Synthesis, Microphase Structures
Evolution and Their Interplay with Discotic Mesogenic Orders
Bin Wu,† Bin Mu,† Sai Wang,†,‡ Junfei Duan,† Jianglin Fang,‡ Rongshi Cheng,† and Dongzhong Chen†,*
†Department of Polymer Science and Engineering, Key Laboratory of Mesoscopic Chemistry of Ministry of Education, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China
‡Center for Materials Analysis, Nanjing University, Nanjing 210093, P. R. China
S
* Supporting Information
ABSTRACT: Triphenylene (TP) derivatives are typical and probably the most widely studied discotic liquid crystalline (DLC)
materials. Through polymer analogous reactions to attach TP mesogens to the well-synthesized poly(ethylene glycol)-b-poly(2-
hydroxyethyl acrylate) (PEG−PHEA) by ATRP, a series of well-defined side chain DLC diblock copolymers PEG−poly(TPm)
(m = 6 or 10) with DLC block weight fraction (fw,DLC) ranging from 37% to 90% have been successfully prepared with narrow
molecular weight distribution (PDI ≤ 1.11). An intriguing microphase-separated superstructure evolution and the correlation
between overall morphologies and discotic mesogenic orders as a function of fw,DLC and temperature have been demonstrated by
combination of DSC, POM, and variable temperature SAXS/WAXS. Those copolymers with lower DLC contents (fw,DLC = 37%
and 43%) and at lower temperatures formed lamellar structures of variant periods and underwent order−order transitions upon
PEG region crystallization at 45 °C and different discotic mesophases of ND or Ncol transition at about 25 °C. For the copolymer
with intermediate fw,DLC = 62%, a high temperature hexagonal packed cylinder (HPC) structure of amorphous PEG
nanocylinders in the matrix of DLC was formed above 35 °C, while upon cooling below 35 °C it turned into a mixed lamellar
structure with PEG region crystallization. The higher fw,DLC (67% ∼ 80%) copolymers exhibited HPC structures with the DLC
matrix showing Ncol or ND mesophases. For copolymers with the highest fw,DLC around 90%, an overall ND phase was developed
in sharp contrast to the ordered columnar phase formed by their corresponding DLC homopolymers, which was quite inspiring
and might suggest another pathway of attaining this important nematic discotic phase through introducing a suitable
copolymerized block. The better understanding of the interrelation of microstructures and discotic mesogenic orders constitutes
the key basis for utilizing such type of organic semiconductor materials and could help to guide the design of complex DLC
polymer materials with hierarchical structures for variant applications.
originate from the synthesis difficulties in obtaining suitable
polymerizable discotic monomers and also their LC polymers.
Researches for preparing TP-based discotic liquid crystalline
polymers were pioneered by Ringsdorf and co-workers.24,25
Since then, some TP-based side chain26−36 and main chain
discotic LC polymers37−43 have been synthesized. A few TP-
INTRODUCTION
■
Discotic liquid crystalline (DLC) materials have attracted great
attention due to their capability of self-assembling into well-
ordered supramolecular structures as a new generation of
organic semiconductors with various mesophases and unique
physical properties, which can be applied such as in optical
compensation films, one-dimensional conductors, photovoltaic
solar cells, and anisotropic photoconductors.1−7 Among them,
triphenylene (TP) derivatives are probably the most widely
studied and particularly promising because of their relatively
easy availability, self-assembly ability and high performance.8−13
Liquid crystalline block copolymers (LCBCPs) have been
extensively studied owing to their organization at different
scales to yield structure-within-structure hierarchical structures
and offering an opportunity to investigate the interplay between
the molecular level LC orders and the microphase-separated
domain morphologies.14−18 Poly(ethylene glycol) (PEG) or
poly(ethylene oxide) (PEO) containing LCBCPs have been
demonstrated to be convenience in preparation, exhibit rich
ordered structures and show strong interplay between orders of
different scales.19−23 However, most LCBCPs investigated are
based on rod-like mesogens, there are surprisingly few reports
of LCBCPs bearing discotic mesogens, which may mainly
based discotic LCBCPs have also been reported so far.44−51
A
coil−coil−disk triblock oligomer polyethylene-block-poly-
(ethylene oxide)-block-pentakis(pentyloxy) triphenylene (PE-
b-PEO-b-P5T) was synthesized by Zhu and co-workers, the
discotic TP mesogens sandwiched between amorphous PEO
layers exhibited a nematic columnar (Ncol) to nematic discotic
phase (ND) transition at ca. 23 °C.44 Recently, a TP side chain
discotic block copolymer with amorphous polystyrene (PS) coil
block was prepared by Wendorff and co-workers showing
uniform columnar orientation perpendicular to the lamellar
direction confined by the slit-like nanoconfinement.45
A
triblock LCBCP with a TP-based main chain polymeric liquid
crystal capped at both ends with PEG blocks exhibited
hexagonal packed PEG rods in a matrix of columnar LC and
Received: March 29, 2013
Published: April 5, 2013
© 2013 American Chemical Society
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TCS120). Both small-angle X-ray scattering (SAXS) and wide-angle X-
ray scattering (WAXS) were simultaneously recorded on an imaging-
plate (IP) which extended to high-angle range (the q range covered by
the IP was from 0.06 to 29 nm−1, q = 4π(sin θ)/λ, where the
wavelength λ is 0.1542 nm of Cu Kα radiation and 2θ is the scattering
angle) at 40 kV and 50 mA for 10 min. Typically, the powder sample
was encapsulated with aluminum foil during the measurement and the
obtained X-ray analysis data were processed with the associated
SAXSquant software 3.80 and the aluminum foil background signal
was subtracted.
Synthesis of 11-(3′,6′,7′,10′,11′-Pentahexyloxytriphenylen-2′-
yloxy)undecanoic Acid (4a). A stirred mixture of 2.76 g (3.71
mmol) of 2-hydroxy-3,6,7,10,11-penta(hexyloxy)triphenylene (3a),
1.08 g (4.08 mmol) of 11-bromoundecanoic acid, 0.17 g (1 mmol)
of KI, and 4.33 g (31.4 mmol) of anhydrous potassium carbonate in 30
mL of ethanol was refluxed for 24 h. The cooled solution was decanted
into 350 mL of ice water, the mixture was acidified with diluted
hydrochloric acid, the resulted yellow precipitate was filtered off and
washed with water, then the crude product was purified by silica-gel
column chromatography, using petroleum ether (PE)/ ethyl acetate
(EA) (30:1 v/v) mixture as eluent, 1.59 g TP6C10COOH (4a) was
obtained as a yellow solid. Yield: 46%.
the photocurrent generation of such copolymers was
reported.47−49 TP-based side-chain copolymers of 2,6,7,10,11-
pentapentyloxy-3-(3-acryloylpropyloxy) triphenylene (PPAT)
and tert-butyl acrylate (t-BA) were synthesized through atom
transfer radical polymerization (ATRP) method with a limited
degree of polymerization (DP) around eight of the discotic
segment, and hexagonal columnar discotic mesophases were
observed for the block copolymers.50,51
As mentioned above limited discotic LCBCPs have been
reported in the literature, mainly due to that the synthesis of
discotic LCBCPs with good control on molecular weight and
distribution, as well as chemical structure and composition still
remains a challenge. Whereas the better understanding of the
interrelation between discotic mesogenic orders and overall
morphological structures is especially demanding, which
requires the acquisition of a series of well-defined discotic
LCBCPs. Herein we report the rational design, controllable
synthesis of a series of TP-based discotic LCBCPs with the
length and relative weight fraction of the discotic LC blocks
being varied in a broad range. The preparation is realized
through a two-step synthesis route of preparation of reactive
precursor copolymers by ATRP followed by a polymer
analogous reaction to introduce the side chain TP mesogens,
which allows a systematic study of microphase-separated
structures and their interplay with discotic mesogenic orders
as a function of discotic LC block weight fraction (fw,DLC) and
temperature.
1H NMR (300 MHz, CDCl3), δ (ppm): 7.85 (s, 6H, Ar−H), 4.23
(t, J = 6.3 Hz, 12H, OCH2), 2.36 (t, J = 7.2 Hz, 2H, CH2CH2COOH),
1.95 (m, 12H, OCH2CH2), 1.60−1.40 (m, 44H, CH2), 0.93 (t, J = 6.9
Hz, 15H, CH3CH2). FT-IR (cm−1): 3393, 2925, 2851, 1737, 1617,
1511, 1428, 1386, 1258, 1168. Anal. Calcd (%) for C59H92O8
(928.68): C, 76.25; H, 9.98. Found: C, 76.14; H, 9.87. m/z (ESI):
927.67 (M − H)−.
TP10C10COOH (4b) was prepared by similar procedures and the
characterization results were shown below. 11-(3′,6′,7′,10′,11′-
Pentadecyloxytriphenylen-2′-yloxy)undecanoic acid (4b). Yield 52%.
1H NMR (300 MHz, CDCl3), δ (ppm): 7.84 (s, 6H, Ar−H), 4.23 (t, J
= 6.6 Hz, 12H, OCH2), 2.36 (t, J = 7.5 Hz, 2H, CH2CH2COOH), 1.95
(m, 12H, OCH2CH2), 1.60−1.10 (m, 84H, CH2), 0.89 (t, J = 6.9 Hz,
15H, CH3CH2). FT-IR (cm−1): 3285, 2916, 2847, 1743, 1617, 1511,
1435, 1386, 1258, 1168. Anal. Calcd (%) for C79H132O8 (1208.99): C,
78.42; H, 11.00. Found: C, 78.40; H, 10.92. m/z (ESI) 1207.92 (M -
H)−.
EXPERIMENTAL SECTION
■
Materials. Poly(ethylene glycol) monomethyl ether (Mn,PEG2K
=
2010, Mn,PEG5K = 5640, calculated from high-resolution 1H NMR
measurements) were purchased from Sigma-Aldrich, eliminating trace
water through azeotropic distillation with toluene before use. The
reagents 4-(dimethylamine) pyridine (DMAP), N,N′-dicyclohexylcar-
bodiimide (DCC), N,N,N′,N′,N″-pentamethyldiethylenetriamine
(PMDETA) and 11-bromoundecanoic acid were purchased from
Alfa Aesar and used as received. Dichloromethane (DCM) was dried
over calcium hydride. 2-Hydroxyethyl acrylate (HEA) was purified
according to the literature.52 All other chemical reagents were purified
according to standard procedures before use.
Synthesis of PEG Macroinitiator (5). A solution of PEG2k−OH
(Mn = 2010, 10.73 g, 5.34 mmol) and triethylamine (1.0 g, 10 mmol)
in 60 mL of dry DCM was slightly cooled with an ice water bath. Then
2-bromoisobutyryl bromide (4.3 mL, 35 mmol) was slowly added to
the reaction mixture. The solution was warmed to room temperature
and stirred for 48 h. The mixture was poured into water and extracted
with DCM. The organic extraction was washed successively with 1 M
HCl and 1 M NaOH solution, dried over MgSO4, and the solvent was
removed under reduced pressure. The crude product was dissolved in
a minimum volume of DCM and then precipitated in an excess of cold
diethyl ether. The desired macroinitiator PEG2k-Br was obtained by
simple filtration in white solid of 8.56 g in yield 71%.
1
Measurements. H NMR spectra were obtained from 300 MHz
(Bruker AMX300) instruments with TMS as internal standards.
Fourier transform infrared spectra (FT-IR) were recorded on a
NICOLET NEXUS 870 infrared spectrometer with pressed potassium
bromide pellets. Mass spectra were recorded on a LCQ Fleet ion trap
mass spectrometer by electrospray ionization (ESI). Elemental
analyses (EA) were carried out with an Elementar Vario Micro. Gel-
permeation chromatography (GPC) measurements were performed at
25 °C on a Waters 515 equipped with Wyatt Technology Optilab rEX
differential refractive index and UV detectors. THF was used as the
eluent at a flow rate 1.0 mL min−1, the solvent THF and sample
solutions were filtered over a filter with pore size of 0.45 μm (Nylon,
Millex-HN 13 mm Syringes Filters, Millipore, US). The differential
scanning calorimetry (DSC) thermograms were recorded on a Perkin-
Elmer Pyris I calorimeter equipped with a cooling accessory and under
a nitrogen atmosphere. Typically, about 5 mg of the powdered sample
was encapsulated in a sealed aluminum pan with an identical empty
pan as the reference, the heating and cooling rate was 10 °C min−1.
The phase transitions and liquid crystalline textures were also
investigated and photographed using melt-pressed preparations
sandwiched between two glass plates with a polarized optical
microscope (POM) equipped with a Leitz-350 heating stage and an
associated Nikon (D3100) digital camera. X-ray scattering experiments
were performed with a high-flux small-angle X-ray scattering
instrument (SAXSess mc2, Anton Paar) equipped with Kratky block-
collimation system and a temperature control unit (Anton Paar
1H NMR (300 MHz, CDCl3) δ (ppm): 4.33 (t, J = 4.8 Hz, 2H,
OCH2), 3.65 (b, 176H, OCH2CH2O), 3.38 (s, 3H, CH3O), 1.95 (s,
6H, OCC(CH3)2Br). FT-IR (cm−1): 2889, 1733, 1468, 1345, 1281,
1116, 946, 845.
Synthesis of Block Copolymer PEG−PHEA (6) by ATRP (PEG45−
PHEA15 as an example). 2.0 mL (19.0 mmol) HEA, 1.91 g PEG2k−Br
(0.95 mmol, monomer/initiator molar ratio [M]:[I] = 20:1), 400 μL
PMDETA (1.92 mmol) and 10 mL DMSO were added into a Schlenk
flask with a magnetic stirrer, degassed, and filled with nitrogen. 103 mg
CuCl and 64 mg CuBr2 were added under nitrogen atmosphere ([I]:
[CuCl]:[CuBr2]:[PMDETA] = 1: 1.1: 0.3: 2). Then the flask was
sealed and degassed for at least four times using the freeze−pump−
thaw procedure. After stirring for 30 min at room temperature, the
flask was placed in the preheated 60 °C oil bath to react for 24 h. Then
the reaction solution was cooled to room temperature and passed
through a neutral Al2O3 column with THF as eluent to remove the
catalyst. The yellow filtrate was concentrated under reduced pressure
and precipitated twice into an excess of dry diethyl ether. The polymer
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Scheme 1. Synthesis Route of Carboxyl-Functionalized TP Discotic Mesogens TPmC10COOH
Scheme 2. Schematic Synthesis Route of the Precursor Reactive Polymers through ATRP and the TP-Based Side Chain Discotic
LC Homopolymers and Copolymers by Subsequent Polymer Analogous Reactions
product was collected by filtration and dried under vacuum. Yield 2.75
g (67%).
in 15 mL of dry DCM; 476 mg (0.513 mmol) of TP6C10COOH and 6
mg (0.05 mmol) of DMAP were added under nitrogen atmosphere,
the mixture was cooled with an ice water bath, and 106 mg (0.514
mmol) DCC was added slowly, then the mixture was heated to 45 °C
and reacted with stirring for 7 days. The cooled mixture was filtered to
remove insoluble residue and the solution was concentrated under
reduced pressure and precipitated into an excess of cold methanol.
The crude product was purified through repeated precipitation from
DCM solution into hexane/methanol mixture. Yield: 158 mg (66%).
1H NMR (300 MHz, CDCl3), δ (ppm): 7.77 (b, Ar−H), 4.19 (b,
OCOCH2CH2OCO, ArOCH2), 3.64, (b, OCH2CH2O), 3.37 (s,
1H NMR (300 MHz, CDCl3) δ (ppm): 4.19 (b, OCH2CH2OH),
3.80 (b, CH2CH2OH), 3.65 (b, OCH2CH2O), 3.38 (s, 3H, CH3O),
2.43 (b, OH), 2.20−1.50 (b, CH2CH), 1.20−1.10 (b, O
CC(CH3)2). FT-IR (cm−1): 3421, 2887, 1724, 1454, 1341, 1281,
1243, 1170, 1098.
Synthesis of Discotic Liquid Crystalline Block Copolymer PEG−
poly(TPm) (PEG45−poly(TP6)15 as an example). First, 51.2 mg
(0.0137 mmol) of PEG45−PHEA15 was dried through azeotropic
distillation with toluene to remove traces of water and then dissolved
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Table 1. Molecular Characterization and Segment Composition of the Prepared Copolymers and Comparative Discotic LC
Homopolymers
a
b
c
c
bd
,
sample code
Mn,calcd
Mn,NMR
Mn,GPC
PDI
fw,DLC(%)
overall yield (%)
PEG127−poly(TP6)4
PEG127−poly(TP10)4
PEG127−poly(TP6)10
PEG127−poly(TP10)10
PEG45−poly(TP6)7
PEG45−poly(TP10)7
PEG45−poly(TP6)15
PEG45−poly(TP10)15
poly(TP6)
9740
10 860
15 900
18 700
9190
8920
9830
9400
10 000
10 600
12 800
11 100
11 600
11 900
13 100
17 600
18 500
1.06
1.06
1.06
1.09
1.09
1.08
1.08
1.11
1.11
1.08
36.8
42.6
61.6
66.9
77.1
80.2
88.0
90.4
100
48
52
58
47
51
55
44
56
49
52
14 680
17 040
8770
11 150
17 400
21 600
20 520
26 120
10 130
16 700
21 000
e
e
N/A
N/A
poly(TP10)
100
a
b
c
1
Calculated from the designed polymer composition. Obtained from H NMR signal integral area ratio calculations. Determined by GPC with
d
e
THF as eluent and polystyrene standards. Discotic LC weight fraction. Not applicable for the absence of distinguished reference peaks for
calculation in the H NMR spectra.
1
3H, CH3O), 2.31 (b, OCCH2CH2), 1.90 (b, OCH2CH2), 1.60−
units and relatively readily accessible. The monofunctionalized
triphenylene (3) is the key intermediate for the synthesis of
carboxyl-functionalized discotic mesogens TPmC10COOH (4),
1.20 (b, CH2, CH2CH), 0.88 (b, CH3CH2). FT-IR (cm−1): 2923,
2854, 1738, 1616, 1517, 1435, 1379, 1261, 1166, 1028.
The synthesis procedures and characterization results of precursor
to simplify the synthetic procedure, the intermediate
compounds and other block copolymers and the comparative
compound 3 with variant peripheral chain length of m = 6
(3a) or 10 (3b) were prepared by a two-step reaction (Scheme
homopolymers are provided in the Supporting Information.
1). Herein, etherization of 1,2-dihydroxybenzene with alkyl
bromide gave mixed mono- and disubstituted products, which
RESULTS AND DISCUSSION
Controlled Synthesis of PEG-Containing Discotic LC
■
underwent trimerization in the presence of FeCl3 to form
Diblock Copolymers with Side Chain TP Mesogens.
mixed TP derivatives.35,65 It is worthwhile to mention that
Poly(ethylene glycol) (PEG) is soluble in water and many
theoretically a mixture of TP derivatives with one, two, and
organic solvents which offers convenience for the synthesis and
three hydroxyl groups will be obtained, but under the
purification of PEG-containing block copolymers, furthermore,
optimized reaction condition we adopted of anhydrous FeCl3
the polar hydrophilic PEG segment exhibits high incompati-
and dry dichloromethane at lower temperature, the TP
bility with many other blocks, which provides the driving force
derivatives with two or three hydroxyl groups of higher polarity
for sufficient microphase separation.53−55 Moreover, PEG is an
excellent media for ionic transportation,56,57 so it is also
were partially hindered as indicated by thin layer chromatog-
raphy (TLC), through repeated column chromatography with
expected that the combination of PEG with discotic LC
gradually changed elution polarity of mixed solvents, pure
polymers may lead to novel materials with channels for both
anisotropic ionic and electronic conductivity.58,59
In principle, one can expect that the discotic LC block
copolymers PEG−poly(TPm) may be synthesized straightfor-
single hydroxyl group TP derivative 3 was obtained in yield
around 16−25% with complete characterization by NMR, FT-
IR, EA, and ESI−MS. Similar modified reaction conditions have
also been employed by other groups35,66 and shown higher
wardly through well-established controlled/living radical
polymerization methods, such as ATRP60,61 or RAFT.62−64
selectivity and especially relative ease in separation compared
with the original acid-catalyzed “ferric chloride” route.65,67
Nevertheless, attempts to synthesize the target copolymers
directly adopting ATRP with TP-based (meth)acrylate turned
out to be unsuccessful, in most cases polymer products were
Compound 3 was further converted into the desired product 4
in high yield upon reaction with 11-bromoundecanoic acid. All
separated intermediates and products were characterized and
confirmed by TLC, various spectroscopic methods as well as
elemental analyses (see Experimental Section and Supporting
Information for details).
not obtained or only harvested in low yield without control on
molecular weight and polydispersity. It has also been reported
by Otmakhova et al. that the average DP does not exceed eight
units for the TP-based discotic monomer PPAT by ATRP
method, which may be attributed to the unique aggregation
behavior of TP-based discotic compounds in solution.50,51 This
kind of DP-limiting phenomenon has also been observed in the
conventional radical polymerization of PPAT in solution,30
which implies that chain propagation proceeds well within the
aggregates, but is hindered from one aggregate to another due
to steric interactions.50,51 So in this paper, we adopt an indirect
two-stage synthesis route, a series of TP-based discotic LC
block copolymers PEG−poly(TPm) have been controllably
synthesized via postesterification of the side hydroxyl groups of
well synthesized PEG−PHEA by ATRP with carboxyl-
functionalized TP mesogens through a polymer analogous
reaction (Scheme 2).
The controlled synthesis of precursor block copolymers
PEG−PHEA was realized by employing the well-established
ATRP method initiated by a PEG macroinitiator, which was
produced from poly(ethylene glycol) monomethyl ether
reacted with 2-bromoisobutyryl bromide, to polymerize 2-
hydroxyethyl acrylate (HEA). Instead of polymethacrylate,
HEA was chosen as the monomer in view of that the
polyacrylate backbone of lower Tg is more flexible to be readily
accommodated within the organized mesophase structure.27,28
Our early synthesis attempts resulted in gel-like products
insoluble in any common solvents, which were presumably
ascribed to the strong intermolecular hydrogen bonding and
high polymerization rate of HEA as mentioned in the
literature.68,69 After changing the polymerization conditions,
such as decreasing concentration of the reaction solution,
Hexaalkoxy substituted TP derivatives are chosen as the
discotic mesogens since they are important typical discotic
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adding CuBr2 to reduce the polymerization rate and also
introducing halide exchange,70,71 well-controlled soluble block
copolymer products with designable molecular weight and
narrow polydispersity were successfully attained.
PHEA7 (Figure 1A), the complete disappearance of methylene
(close to hydroxyl) peak g at δ = 3.80 ppm after esterification
and the characteristic proton signal r of TP core at 7.82 ppm
(Figure 1B) confirmed the successful attachment of the side-
chain TP mesogens. The molecular weights of copolymers
PEG−poly(TPm) were calculated from integral area ratio of
protons of poly(ethylene glycol) as shown in peak b and those
of TP aryl protons as shown in peak r (Figure 1B). FT-IR
spectra also revealed the transformation, the characteristic
absorption of hydroxyl groups stretching vibration around 3420
cm−1 in PEG45−PHEA7 completely vanished after esterification
and the CO stretching vibration at 1725 cm−1 shifted to
1739 cm−1 in PEG45−poly(TP10)7 confirming the hydrogen-
bonded carbonyl by hydroxyl groups of HEA completely
transformed into free ester carbonyl groups (Supporting
Information, Figure S1).
The molecular weight (MW) and polydispersity of the series
TP-based discotic LC copolymers were determined by gel-
permeation chromatography (GPC). Monomodal GPC curves
(as shown in Figure S2, Supporting Information) shifting
toward shorter retention times were observed for both PEG2k
and PEG5k series block copolymers with the increasing discotic
LC block indicating a gradual increase in their hydrodynamic
volumes. While it should be noted that although the narrow
molecular weight distribution (PDI ≤ 1.11) and the increasing
tendency of determined molecular weights by GPC, values of
the number-averaged molecular weight (Mn,GPC) increase very
modestly as DP of the discotic block increasing or the
peripheral substituent alkoxys lengthening from 6- to 10-carbon
atom (Table 1), which is presumably due to the large difference
in the hydrodynamic properties between the discotic LC block
copolymers and the calibrating polystyrene standards used for
GPC, in addition, the unique aggregation behaviors of TP
mesogens in solution may also significantly affect their
hydrodynamic volumes.29,35,50,51,72 On the other hand, values
of the number-averaged MW determined from high resolution
1H NMR spectra (Mn,NMR) agreed quite well with the calculated
MW based on the designed polymer composition with 2.8−
9.5% negative deviation, which also further confirmed the high
attachment efficiency of more than 90% side group
esterification with TP mesogens.
The discotic LC block copolymers PEG−poly(TPm) were
prepared through a polymer analogous reaction to attach the
TP derivative mesogens onto the well-synthesized PEG−PHEA
via esterification as outlined in Scheme 2. A 2.5 equiv. molar
excess (relative to hydroxyl group) of TPmC10COOH (4) and
one week of reaction time were applied to ensure high
transformation efficiency of side hydroxyl groups, the mesogen
attachment through polymer analogous reaction can be almost
quantitative under well-chosen reaction conditions.45 Moderate
overall yields of 44−58% have been achieved for the DLC
polymers synthesis as listed in Table 1. The esterification
1
attaching process was monitored by both H NMR (Figure 1)
and FT-IR. Figure 1B shows the 1H NMR spectrum of PEG45−
poly(TP10)7, compared with that of the precursor PEG45−
Thermal Properties, Phase Behaviors, and Self-
Assembled Superstructures in the Bulk State. The
successful preparation of a series of well-defined TP-based
LC block copolymers containing PEG segments with discotic
LC weight fraction (fw,DLC) ranging from 37% to 90% offers an
ideal platform for systematically investigating the phase
behaviors and self-assembled superstructures, and the interplay
of the discotic LC order and microphase structure in such kind
of LCBCPs. First the phase transitions and thermal properties
of the series LCBCPs were examined by differential scanning
calorimetry (DSC) and polarized optical microscopy (POM).
The second heating and first cooling curves of the copolymers
and also the comparative discotic LC homopolymers are shown
in Figure 2, all the samples investigated exhibited quite good
reproducible thermograms in the DSC measurements. In
general, for all the sets of copolymers, the DSC curves exhibit
one or two endothermic peaks in the heating runs and one
exothermic peak with different degree of supercooling in the
cooling runs, some subtle phase changes as disclosed by other
analytical techniques next are not detectable due to their
insensible thermal effects.22,73 In a close inspection, the series of
PEG5k copolymers among them as shown in Figure 2A with
1
Figure 1. H NMR spectra of (A) PEG45−PHEA7 and (B) PEG45−
poly(TP10)7 (in CDCl3).
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Figure 2. DSC thermograms of the series LC block copolymers and their comparative homopolymers and PEG in the first cooling and second
heating runs of (a) PEG127−poly(TP6)4, (b) PEG127−poly(TP10)4, (c) PEG127−poly(TP6)10, (d) PEG127−poly(TP10)10, (e) PEG45−poly(TP6)7,
(f) PEG45−poly(TP10)7, (g) PEG45−poly(TP6)15, (h) PEG45−poly(TP10)15, (i) PEG5k, (j) poly(TP6), and (k) poly(TP10).
Table 2. Summary of Thermal Transition Data and Self-Assembled Superstructures of the Block Copolymers and Their
Comparative Homopolymers Determined by Combination of DSC, POM, variable temperature SAXS/WAXS Analyses
ab
,
T/°C [ΔH (J g−1)]
sample code
heating
Ncol-L 35 ND-L 56.6 [98.8] L 70 I
cooling
c
c
c
PEG127−poly(TP6)4
PEG127−poly(TP10)4
PEG127−poly(TP6)10
PEG127−poly(TP10)10
PEG45−poly(TP6)7
PEG45−poly(TP10)7
PEG45−poly(TP6)15
PEG45−poly(TP10)15
poly(TP6)
I 60 L 37.1 [96.3] ND-L 25c Ncol-L
I 110 L 35.8 [90.1] ND-L 25c Ncol-L
c
c
c
Ncol-L 35 ND-L 56.2 [95.5] L 120 I
c
c
PL/ML 50.8 [50.7] HPC 120 I
I 110 HPC 22.8 [49.4] PL/ML
c
c
HPC1 48.4 [16.5] HPC2 120 I
I 110 HPC2 17.5 [5.4] HPC1
c
c
g −5 HPC1 34.6 [5.5] HPC2 60 I
I 50 HPC2 9.5 [7.8] HPC1
c
c
g 2 HPC1 27.1 [2.0] HPC2 37.0 [3.7] HPC3 70 I
I 60 HPC2 18.0 [11.6] HPC1
g −3 ND 35.5 [3.1] I
g 3 ND 29.5 [1.8], 37.9 [3.8] I
g −5 Colho 40 [12.0] I
I 5.9 [5.6] ND
I 15.7 [10.2] ND
I 11.5 [6.2] Colho
I 28.7 [26.5] Colhd
I 42.4 [177.7] Cr
poly(TP10)
g 0 Colhd1 35.6 [11.4] Colhd2 41.7 [12.1] I
PEG5k
Cr 63.7 [181.8] I
a
The thermal transition peak temperatures and associated enthalpy changes (in square brackets) obtained from DSC measurements of the first
b
cooling and second heating runs at 10 °C/min. Abbreviations: I = isotropic, L = lamellar structure, Ncol-L = columnar nematic phase within lamellar
structure, ND-L = nematic discotic phase within lamellar structure, PL/ML = predominately lamellar structure with modified layer, HPC = hexagonal
packed cylinder structure, Colho = ordered hexagonal columnar phase, Colhd = disordered hexagonal columnar phase, Cr = crystalline, g = glass state.
c
The transition estimated from variable temperature SAXS/WAXS measurements or POM observations.
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fw,DLC in the range of 37−67% containing a longer PEG block
(Mn,PEG = 5640), which will be demonstrated in the following
sections to mainly form variant lamellar structures or
transitional columnar phases, display thermal behaviors largely
comparable to that of PEG (Figure 2C). With fw,DLC increasing,
the melting temperatures (Tm’s) of copolymers decrease
together with a gradually diminished enthalpy changes, and
also a more and more remarkable supercooling as listed in
Table 2. While the PEG2k series copolymers with a shorter
PEG block (Mn,PEG = 2010) and a much higher fw,DLC in the
range of 77−90%, as shown in Figure 2B, behave mostly
comparable with the homopolymers of poly(TP6) and
poly(TP10) (Figure 2D), respectively, which are similar to
rod-like LC block copolymers with the majority LC matrix
phase exhibiting phase transition temperatures comparable to
their corresponding LC homopolymers.16 The homopolymer
poly(TP10) showed two obvious thermal transitions during
heating and also the case in its associated block copolymers,
similar multiple transition behaviors have also been observed in
other discotic liquid crystalline polymers.24,33 The melting
temperatures of the two homopolymers are all below 42 °C,
which agree well with the reported ca. 39−45 °C for TP side
chain poly(meth)acrylates with long flexible spacer.26,29 The
thermal properties and phase behaviors of the series block
copolymers and their comparative homopolymers by combi-
nation of DSC, POM, variable temperature SAXS/WAXS
analyses have been summarized in Table 2.
other higher DLC content copolymers in the PEG5k series as
displayed in Figure 3B for PEG127−poly(TP6)10 as an example.
It is interesting to note that sparkling grainy textures were
observed for PEG2k series copolymers with high fw,DLC of 77−
90% when slowly cooled from their isotropic states and
annealed for a long time (Figure 3C); otherwise usually
homogeneous extinction exhibited while an almost uniform
birefringence appeared after sheared the sample preparations by
slightly moving the cover slide as shown in Figure 3, parts D
and E, respectively, for PEG45−poly(TP6)7 and PEG45−
poly(TP6)15, which can be ascribed to homeotropic orientation
of the discotic mesogens as observed in polysiloxane side chain
discotic polymers.24 A small size pseudo focal-conic fan texture
corresponding to columnar phase was observed for the
comparative homopolymers as shown in Figure 3F for
poly(TP6) after being annealed overnight at 35 °C, which
was in contrast to the usually observed speckled sandy texture28
and manifested the ordered LC structure as supported by SAXS
analysis in the next.
For probing into the microphase-separated structures, mode
of packing, and types of order in the mesophases, simultaneous
SAXS and WAXS were performed in situ using a high-flux
SAXSess X-ray scattering instrument equipped with a temper-
ature control accessory. Typical SAXS and WAXS profiles of
the series copolymers with various fw,DLC on cooling to their
mesophases are shown in Figure 4. The SAXS patterns shown
in Figure 4A and 4C indicate the evolutional microphase
separation structures in tens of nanometer size scale with fw,DLC
increasing, while the WAXS profiles shown in Figure 4, parts B
and D, reveal discotic liquid crystalline order or other
subnanometer molecular level organization. For the series
copolymers containing TP10 discotic core, the evenly spaced
Both the copolymers and homopolymers for comparison
were also studied by POM. Under crossed polarizers obvious
spherulitic crystalline texture was observed for the lowest LC
content copolymer PEG127−poly(TP6)4 with the highest PEG
weight fraction 63% being slowly cooled from its isotropic state
to below the crystallization temperature at 37 °C (Figure 3A),
while poorly defined birefringent textures were observed for all
scattering peaks of up to four orders in the SAXS for PEG127
−
poly(TP10)4 (fw,DLC = 43%) as shown in Figure 4A(a) indicate
a well-defined lamellar structure of period 18.5 nm,
furthermore, two remarkable peaks at q = 13.6 nm−1 (4.6 Å)
and q = 16.5 nm−1 (3.8 Å) in the WAXS as shown in Figure
4B(a) are attributed to the reflections of PEG crystallization,
corresponding to (120) and (032 + 112) reflections of
monoclinic PEG crystals.23,74 For PEG127−poly(TP10)10
(fw,DLC = 67%), a set of peaks with q ratio of 1:√3:2 [indexed
as (100), (110), (200)] are observed as shown in Figure 4A(b),
indicating a well ordered hexagonal columnar organization with
lattice constant a = 18.1 nm. For PEG45−poly(TP10)7 (fw,DLC
= 80%), diminished peaks indexed as (100) and (110) indicate
a less ordered hexagonal columnar structure with lattice
constant a = 11.2 nm. Then to PEG45−poly(TP10)15 with
the highest fw,DLC = 90% investigated in this work, only a weak
broad band exhibits in the SAXS region indicating a very
ambiguous microphase separation47 between discotic LC block
and the less than 10 wt % PEG minority component.
Therefore, with fw,DLC increasing a microstructure evolution
from well-defined lamellar in PEG127−poly(TP10)4 to colum-
nar structures in PEG127−poly(TP10)10 and PEG45−poly-
(TP10)7, then to a quite weakly microphase-separated structure
in PEG45−poly(TP10)15 has been clearly revealed. On the
other hand, the intensity of those peaks in the WAXS resulted
from PEG crystallization remarkably decreases with the
increased discotic LC content suggesting gradually suppressed
PEG crystallization in the block copolymers, which agrees well
with the DSC analyses discussed above. As shown in Figure 4,
parts C and D, the series copolymers containing TP6 discotic
core behave quite similar to the TP10 series, showing a lamellar
Figure 3. Representative POM images under crossed polarizers: (A)
PEG127−poly(TP6)4, 37 °C; (B) PEG127−poly(TP6)10, 30 °C; (C)
PEG45−poly(TP6)7 annealed overnight at 25 °C; (D) PEG45−
poly(TP6)7, after shearing; (E) PEG45−poly(TP6)15, after shearing;
(F) homopolymer poly(TP6), annealed overnight at 35 °C.
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Figure 4. SAXS/WAXS profiles of the TP10 series (A, B) and TP6 series (C, D) copolymers with the discotic LC weight fraction fw,DLC increasing of
(a) PEG127−poly(TP10)4 at 40 °C, (b) PEG127−poly(TP10)10 at 40 °C, (c) PEG45-P(TP10)7 at 30 °C, (d) PEG45−poly(TP10)15 at 30 °C, and (e)
PEG127−poly(TP6)4 at 40 °C, (f) PEG127−poly(TP6)10 at 40 °C, (g) PEG45−poly(TP6)7 at 30 °C, and (h) PEG45−poly(TP6)15 at 30 °C.
structure of lattice spacing 16.5 nm with PEG crystallization in
reversibility and excellent reproducibility during cooling or
heating thermal cycle, while usually about 10 °C supercooling
on the estimated transition temperatures was observed on the
cooling cycle compared to their corresponding heating cycle as
listed in Table 2, which implied that it was not easy to reach the
strictly thermodynamical equilibration state for polymer
materials system. Nevertheless the degree of supercooling
exhibited during variable temperature SAXS/WAXS measure-
ments was quite modest compared to the DSC measurements
(at a rate of 10 °C/min), thus some undetectable transitions by
DSC can be additionally revealed by SAXS/WAXS besides the
insignificant thermal effect. Parts A and B of Figure 5
PEG127−poly(TP6)4 (fw,DLC = 37%) to hexagonal columnar
phase of lattice constant a = 15.4 and 10.2 nm in PEG127
poly(TP6)10 (fw,DLC = 62%) and PEG45−poly(TP6)7 (fw,DLC
−
=
77%), respectively, then to an indistinctive microphase-
separated structure in PEG45−poly(TP6)15 (fw,DLC = 88%).
The peaks due to PEG crystallization completely disappear in
those copolymers with fw,DLC higher than 67%, while there are
two diffuse halos in all WAXS curves as shown in Figure 4,
parts B and D, the one in the wide angle area around q = 14.3
nm−1 (4.4 Å) is attributed to the amorphous packing of the alky
chain, and the one in the low angle region of q = 2.96 nm−1 for
TP10 and 3.69 nm−1 for TP6 copolymer series, respectively,
originates from the discotic TP mesogens.44,50,51
representatively show the SAXS/WAXS patterns of PEG127
−
poly(TP10)4 (fw,DLC = 43%) during cooling from its isotropic
phase. On cooling to around 110 °C, a lamellar structure
started to develop as revealed by the reflections in the low angle
area of Figure 5A with a period of around 14.2 nm. When
further cooling to below 50 °C, an order−order transition
(OOT) occurred to form another lamellar structure with a
remarkably increased period of 18.5 nm at 40 °C, accompanied
by the PEG crystallization as indicated by the two characteristic
Variable-temperature SAXS/WAXS investigation provides
crucial information for exploring both their microphase-
separated structures and mesomorphic orders in detail of the
series block copolymers. The temperature was stepwisely
decreased or increased and equilibrated for additional 3 min
before measurements then the data collection time was 10 min
at each temperature, the SAXS/WAXS profiles exhibit good
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Figure 5. SAXS/WAXS profiles (A, B), the enlarged SAXS patterns in the transition zone (C) and d spacings versus temperature extracted from the
first-order reflections in the SAXS patterns during cooling (D) of PEG127−poly(TP10)4.
peaks in the WAXS patterns as shown in Figure 5B. Very
interesting, at 45 °C an intermediate state with both reflections
of the two lamellar structures coexisting has been clearly
observed in the amplified curves as shown in Figure 5C. The
OOT with remarkably changed lamellar d spacings during
cooling may result from the organization modification of the
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Figure 6. Variable-temperature SAXS (A) and WAXS (B) profiles of PEG127−poly(TP6)10 during cooling.
discotic TP mesogens44 and the formation of PEG crystals.23,73
Figure 5D shows the d spacings extracted from the first-order
reflections in the SAXS patterns of PEG127−poly(TP10)4 as a
function of temperature, two obvious transitions with sharp
step changes can be unambiguously identified. During cooling,
the first remarkable increase of d-spacing from 14.6 to 18.5 nm
at around 40 °C is mainly due to PEG crystallization. The
second transition with a moderate drop from the plateau at
about 18.5 nm to around 17.0 nm below 25 °C is largely
ascribed to the variant organization of TP mesogens. In a closer
inspection of Figure 5A, it is found that on cooling to the
temperature range between 45 and 25 °C, the reflections in
SAXS show more than third-order peaks indicating a well
ordered lamellar structure; upon further cooling below 25 °C
the third-order peaks gradually diminish then almost disappear
which implying a less ordered lamellar structure. Furthermore,
we notice that at every transition point (45 and 25 °C) as
indicated in Figure 5D, there were subtle changes of the low
angle diffuse halo around q = 2.96 nm−1 in Figure 5A,
indicating different kinds of organization of the TP mesogens.
When cooled to 45 °C, the intensity of the diffuse halo became
stronger accompanied by crystallization of PEG segment; when
further cooled to 25 °C, the diffuse halo shifted a little toward
wide angle area, indicating the average distance of TP mesogens
decreased. A columnar structure was presumably formed by the
TP mesogenic block below 25 °C, which induced the moderate
lamellar spacing shrinkage and also slightly disturbing the
lamellar organization by stacking the TP discotic units into
columns. Although the reflection characteristic of π−π stacking
in the WAXS with q values normally ranging from 16 nm−1 (3.9
Å) to 19 nm−1 (3.3 Å)47,75 cannot be recognized in Figure 5A,
which is probably concealed by the nearby strong reflections of
PEG crystals, since peaks of TP π−π stacking have been clearly
observed in copolymer samples of further increased DLC
content with the absence of PEG crystallization as demon-
strated later in the case of PEG127−poly(TP10)10 and others. A
typical nematic phase has a fwhm (full width at half-maximum)
of 0.7 nm−1,44,76 considering the intensity and fwhm of the low
angle diffuse halo below 25 °C as shown in Figure 5A, the
mesophase of the columnar structure is of nematic nature to
form a nematic columnar phase (Ncol). While in the
temperature higher than 25 °C, the TP mesogenic units are
in a nearly parallel orientation to form a nematic discotic phase
(ND) showing even stronger reflections in the SAXS profile
than in the Ncol phase which is believed to be in a microdomain
randomly oriented state. A similar Ncol−ND transition around
23 °C has been demonstrated by Zhu and co-workers in their
TP-based PE-b-PEO-b-P5T triblock oligomer.44 Such kind of
phase transition has been first reported at a much higher
temperature around 165 °C in a side chain LC polyacrylate
bearing large discotic mesogens of pentakis-
(methylphenylethynyl)benzene.77 Very recently, Zhou and co-
workers investigated a kind of mesogen-jacked liquid crystalline
polymer (MJLCP) containing two TP units in the side chain
and found that at around ambient temperature TP mesogens
formed a ND phase in conjunction with the rectangular
columnar phase developed by the rod-like MJLCP as a whole.66
Herein for PEG127−poly(TP10)4, in conjunction with the main
lamellar period change due to PEG crystallization, a subtle ND−
Ncol transition of TP mesogens enriched the supramolecular
organization and phase behavior, which was some unusual
especially the ND phase formation for hexa-ether substituted
TP derivatives as will be discussed later. Similarly, for PEG127
−
poly(TP6)4 (fw,DLC = 37%), upon cooling a lamellar-lamellar
OOT with the lamellar period d increasing from 13.1 to 16.5
nm occurred at 47 °C associated with PEG crystallization (see
Supporting Information, Figure S3).
The temperature dependent SAXS/WAXS patterns of
PEG127−poly(TP6)10 (fw,DLC = 62%) are shown in Figure 6.
Hexagonal packed cylinder (HPC) superstructures can be
clearly identified at higher temperature with q ratio of 1:√3:2
(indexed as (100), (110), (200)) as shown in Figure 6A. The
SAXS pattern below 35 °C such as at 20 °C is tentatively
assigned as a mixed lamellar structure, PEG block is in general
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Figure 7. Variable temperature SAXS (A) and WAXS (B) patterns of PEG127−poly(TP10)10 and WAXS (C) patterns of PEG45−poly(TP10)7
during cooling, the arrows indicate the reflections of π−π stacking of TP mesogens.
in crystalline state as revealed by the WAXS patterns (Figure
6B), while a portion of PEG segments may be still in
amorphous or generate interface frustration due to increased
confinement in this higher DLC content copolymer to form a
modified lamellar (ML) in conjunction with the predominate
lamellar (PL) structure as reported for some rod-like LC block
copolymers.78,79 The SAXS profile of PEG127−poly(TP6)10 at
35 °C exhibits an intermediate state during columnar-lamellar
transition showing some remnant weak peaks of the columnar
structure and one main peak at q = 0.297 nm−1 (21.1 nm),
which may due to the remarkable curvature change from the
columnar to lamellar phase transition. Furthermore, the WAXS
patterns as shown in Figure 6B manifest clearly the PEG
crystallization characteristic at 35 °C and its growth starting
from 40 °C, which is a little lower compared with 45 °C for
organized into a ND phase above 25 °C. Furthermore, below 25
°C, it is interesting to notice that besides the discernible (100),
(110), (200) peaks attributed to the weakly microphase-
separated HPC superstructure, another perceptible set of larger
q’s with ratio of 1:√3:2 [assigned as (01), (11), (02)] were
found to superimpose on and nearby the discotic LC halo both
for PEG45−poly(TP6)7 (see Supporting Information, Figure
S4) and PEG45−poly(TP10)7 (Figure 7C). Thus, TP mesogens
in these copolymers were organized into some sort of more
ordered structure than Ncol, an ill-defined hexagonal columnar
organization (Colh) was formed due to the further increased
DLC content.
For PEG45−poly(TP6)15 and PEG45−poly(TP10)15, with
fw,DLC reaching respectively up to 88% and 90%, which may be
expected to exhibit phase behavior more similar to their
corresponding homopolymers, whereas here the minority PEG
blocks are too few to form a separated microphase, by
contraries they exert a heavy steric disturbance to the
organization of DLC block. They are supposed to form a ND
phase based on their variable temperature SAXS/WAXS
analysis results (Supporting Information, Figure S5 and Figure
S6) and their birefringent performance under POM (Figure
3E). Here in view of the absence of peaks from π−π stacking in
the SAXS/WAXS profiles, the possibility of forming a Ncol
phase is excluded, which was proposed by McKeown and co-
workers for the mesogenic dodecyloxy phthalocyanine (Pc)
derivatives with a single PEG chain of almost the same length as
studied here.80 Therefore, compared with their corresponding
homopolymers, which possess ordered hexagonal columnar
structure Colho as indicated by the POM texture (Figure 3F)
and confirmed by the variable temperature SAXS/WAXS study
for poly(TP6) (Supporting Information, Figure S7), the
LCBCPs of high fw,DLC exhibit notably different phase behavior
due to the strong steric disturbance effect of the introduced
PEG block.
curvature-unchanged lamellar-lamellar transition of PEG127
poly(TP10)4.
−
When fw,DLC reaches up to 67% as in PEG127−poly(TP10)10,
the reflections due to PEG crystallization totally disappear in
WAXS profiles (Figure 7B), herein the minority component
PEG block is no more crystalline and forming amorphous PEG
nanocylinders to construct a less ordered HPC structure
dispersed in the discotic block matrix as indicated by the indices
(100), (110) and (200) (Figure 7A). As shown in Figure 7B,
below 25 °C the changes of the low angle diffuse halo
associated with the discotic TP organization is more
remarkable, and two weak characteristic peaks of d-spacing
4.1 and 3.7 Å, respectively, appear at the wide angle area. The
3.7 Å peak is attributed to the reflection of π−π stacking of TP
mesogens, which is characteristic for discotic columnar
organizations.1−5 The 4.1 Å peak is possibly due to partially
ordered arrangement of the longer ten-methylene spacers and
peripheral alkyl chains in the TP columnar mesophases.
Therefore, upon cooling below 25 °C an OOT from HPC2/
ND to HPC1/Ncol occurs induced by the TP mesophase change
from ND to Ncol.
In general, block copolymers may give a range of
microphase-separated structures such as lamellar, columnar,
bicontinuous, or cubic phases depending on their composition
and temperature,81 for block copolymers with an underlying
discotic molecular architecture the cubic phase is restrained and
columnar structures are more preferred.47 A schematic
presentation of the hierarchical superstructure evolution and
When fw,DLC further increases as in PEG45−poly(TP6)7
(fw,DLC = 77%) and PEG45−poly(TP10)7 (fw,DLC = 80%) as
representatively shown in Figure 4(g) and 4(c), respectively,
still a microphase-separated HPC structure of the amorphous
PEG nanocylinders in the discotic block matrix was generated,
where similar to PEG127−poly(TP10)10, the TP mesogens were
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Figure 8. Schematic presentation of the hierarchical superstructure evolution and phase diagram of the series TP-based discotic LC block
copolymers and their comparative homopolymers in the cooling process, the inset in the upper right is a schematic drawing of the copolymer
structure.
phase diagram for this series copolymers PEG−poly(TPm) as
well as their comparative homopolymers as a function of fw,DLC
and temperature is shown in Figure 8. The copolymers with
low fw,DLC such as PEG127−poly(TP6)4 (fw,DLC = 37%) and
PEG127−poly(TP10)4 (fw,DLC = 43%) generated lamellar
structures, which underwent lamellar−lamellar OOTs due to
PEG crystallization around 45 °C and the variant organization
of TP discotic mesogens into ND or Ncol with the transition
temperature around 25 °C. For an intermediate fw,DLC = 62% in
PEG127−poly(TP6)10, a high temperature phase of HPC
structure was formed above 35 °C, while it turned into a PL/
ML mixed lamellar structure upon cooling below 35 °C with
PEG region crystallization. When fw,DLC increases up to 67% as
in PEG127−poly(TP10)10, the amorphous PEG block of
copolymers formed microphase-separated nanocylinders organ-
izing into HPC structures in the matrix discotic phases, with TP
mesogens in ND or Ncol mesophases above or below 25 °C,
respectively. Similar to PEG127−poly(TP10)10, a further
increase of fw,DLC as in PEG45−poly(TP6)7 (fw,DLC = 77%)
and PEG45−poly(TP10)7 (fw,DLC = 80%), besides the super-
structure change, a little more ordered ill-defined Colh phase
other than Ncol was suggested for their TP mesogens
organization below 25 °C. For the highest DLC content
copolymers as in PEG45−poly(TP6)15 (fw,DLC = 88%) and
PEG45−poly(TP10)15 (fw,DLC = 90%), an overall nematic
discotic phase (ND) was formed which was in a striking
contrast to the Colh phase of their corresponding homopol-
ymers due to the strong steric disturbance effect of the PEG
chain.80 The self-organized microstructures and phase behav-
iors of the copolymers and comparative homopolymers are
summarized in Table 2.
It is noteworthy that the nematic discotic ND mesophases are
not common in discotic molecules, most of them exhibit variant
columnar phases due to a strong π−π interaction among the
aromatic cores, only a very few of them especially multiynes-
extended or benzoate-substituted derivatives based on benzene,
naphthalene, TP, and truxene aromatic cores display ND phases
as recently reviewed.82 Among the less abundant ND phase
DLC materials, polymerizable hexabenzoates of TP have found
practical commercial application in the flourishing liquid-crystal
display industry as the negative birefringence optical
compensation films for broadening the viewing angle.83 The
hexaether-substituted TP derivatives themselves usually exhibit
columnar phases which are attributed to low steric hindrance of
six ether linkages allowing a strong cohesion between cores,
whereas in PEG-containing copolymers studied in this work
Ncol and ND phases are formed presumably mainly due to their
discotic polymers characteristic and the attached PEG block
disrupting the columnar packing of discotic units as also
observed in the TP-based PE-b-PEO-b-P5T triblock
oligomer.44 The ND phase is the least ordered, least viscous
while most symmetric mesophase among the other nematic
phases as pointed out by Kumar,82 the ascription of producing
such nematic discotic phases to the steric hindrance effect
seems reasonable and is reminiscent of some other ND phase
examples based on alkyloxy-substituted TP, such as the dimers
linking two TP discotic moieties via a short rigid spacer formed
ND phases over a wide range of temperature,84 the tricarbonyl
chromium complex of hexanonyloxy-substituted TP displayed a
nematic ND phase between 37 and 58 °C,85 and TP derivatives
possessing oligo(ethylene oxide) tails showed room-temper-
ature ND phases.86 The phase behavior especially the ND phase
formation of this kind of PEG-containing DLC block
copolymers may suggest another pathway of attaining the
rarely observed but quite important nematic discotic phase
through polymerization and introducing a suitable copoly-
merized block.
CONCLUSIONS
■
In conclusion, following the synthesis of reactive precursor
block copolymers PEG−PHEA by ATRP, a series of well-
defined TP side chain discotic liquid crystalline diblock
copolymers PEG−poly(TPm) (m = 6 or 10) with DLC
block weight fraction (fw,DLC) ranging from 37% to 90% have
been successfully prepared through a polymer analogous
reaction. The important intermediate compounds and polymer
products were fully characterized and thus obtained discotic LC
copolymers and their comparative homopolymers were
investigated by combination of DSC, POM and variable
temperature SAXS/WAXS. An intriguing microphase-separated
superstructure evolution in correlation with discotic meso-
phases as a function of fw,DLC and temperature have been
demonstrated. The copolymers with higher DLC contents
exhibited HPC structures of amorphous PEG nanocylinders in
the matrix of discotic liquid crystal showing Ncol or ND
mesophases, and an overall ND phase prevailed in the
copolymers of the highest DLC content around 90% in sharp
contrast to the ordered columnar phase generated by their
corresponding DLC homopolymers. While for copolymers with
lower DLC contents and at lower temperatures lamellar
structures were formed with PEG region crystallization at 45
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ASSOCIATED CONTENT
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S
* Supporting Information
Synthesis and characterization for precursor compounds 1a, 2a,
3a, and 3b and characterization results for DLC block
copolymers and homopolymers, figures of FT-IR spectra
comparison before and after attaching side chain TP mesogens,
GPC traces of block copolymers, variable temperature SAXS/
WAXS profiles of copolymers and homopolymers, and tables of
detailed SAXS and WAXS data and assignments of all
copolymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(D.C.) E-mail: cdz@nju.edu.cn. Telephone: +86-25-
83686621(O). Fax: +86-25-83317761(O).
■
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Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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(28) Boden, N.; Bushby, R. J.; Lu, Z. B. Liq. Cryst. 1998, 25, 47−58.
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This work was supported by the National Natural Science
Foundation of China (NSFC20874044) and Fundamental
Research Plan Project (Natural Science Foundation) of Jiangsu
Province (BK2010244) and also partially by the National
Science Fund for Talent Training in Basic Science (No.
J1103310).
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