Self-assembly of dPS-liquid crystalline diblock copolymer in a nematic liquid crystal solvent

Article abstract of DOI:10.1021/ma300476j

A diblock copolymer dPS(9.1K)-b-LCP(7.2K), the number in parentheses represents the number-averaged molecular weight) with a deuterated polystyrene (dPS) block connected to a side group liquid crystal polymer [poly(6-(4′-cyanobiphenyloxy)hexyl acrylate, LCP] was self-assembled into ～78 A spherical micelles with a dPS core and LCP corona in a small-molecule nematic liquid crystal solvent, 4-pentyl-4′-cyanobiphenyl (5CB). The micellar structures were examined by small angle neutron scattering (SANS) at various block copolymer concentrations and temperatures. The SANS data was analyzed further using a curve fitting method. The critical micelle concentration (CMC) was as low as ～0.0027 wt % and the self-assembled micelles were dissociated reversibly into a unimer state at 33 °C, which is lower than the nematic to isotropic transition temperature (T_ni) of 5CB (36 °C). The entropic penalty imposed on the dPS by the ordered nematic state of the LC solvent caused phase separation of the flexible dPS block to form micelles and vanished above the T_ni of the LC solvent. A low CMC of ～0.0027 wt % suggests that 5CB was a strong selective solvent for dPS-b-LCP in the nematic state. The global orientation of 5CB, produced by a magnetic field (MF, 1.2 T) revealed the structure of the dPS core of the micelle to be prolate (elongated sphere) oriented with the long axis along the applied MF direction.

Full text of DOI:10.1021/ma300476j

Article
pubs.acs.org/Macromolecules
Self-Assembly of dPS-Liquid Crystalline Diblock Copolymer in a
Nematic Liquid Crystal Solvent
Soo-Young Park, * Thangavelu Kavitha, Tahseen Kamal, Waliullah Khan, Taegyu Shin,‡
†
,
†
†
†,§
and Baekseok Seong‡
†
Department of Polymer Science, Kyungpook National University, #1370 Sangyuk-dong, Buk-gu, Daegu 702-701, South Korea
‡
Neutron Science Division, Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu Daejeon, 305-353, South
Korea
§
Department of Chemistry, Abdul Wali Khan University, Mardan, 23200 Khyber Pakhtunkhwa, Pakistan
ABSTRACT: A diblock copolymer dPS(9.1K)-b-LCP(7.2K), the number in
parentheses represents the number-averaged molecular weight) with a
deuterated polystyrene (dPS) block connected to a side group liquid crystal
polymer [poly(6-(4′-cyanobiphenyloxy)hexyl acrylate, LCP] was self-
assembled into ∼78 Å spherical micelles with a dPS core and LCP corona
in a small-molecule nematic liquid crystal solvent, 4-pentyl-4′-cyanobiphenyl
(
5CB). The micellar structures were examined by small angle neutron
scattering (SANS) at various block copolymer concentrations and temper-
atures. The SANS data was analyzed further using a curve fitting method. The
critical micelle concentration (CMC) was as low as ∼0.0027 wt % and the self-
assembled micelles were dissociated reversibly into a unimer state at 33 °C,
which is lower than the nematic to isotropic transition temperature (T ) of
ni
5
CB (36 °C). The entropic penalty imposed on the dPS by the ordered
nematic state of the LC solvent caused phase separation of the flexible dPS block to form micelles and vanished above the T of
ni
the LC solvent. A low CMC of ∼0.0027 wt % suggests that 5CB was a strong selective solvent for dPS-b-LCP in the nematic
state. The global orientation of 5CB, produced by a magnetic field (MF, 1.2 T) revealed the structure of the dPS core of the
micelle to be prolate (elongated sphere) oriented with the long axis along the applied MF direction.
6
,7
INTRODUCTION
selectivity of the solvent, etc. Among them, the selectivity of
■
the solvent can be defined by the Flory interaction parameter
The molecules of liquid crystals (LCs) tend to orient
cooperatively in a preferred direction giving rise to useful
properties, such as high birefringence, excellent dielectric
anisotropy and orientation elasticity, which are forbidden by
symmetry in isotropic liquids. The coupling of order with
fluidity makes LCs particularly intriguing materials because
their orientation−dependent properties can be influenced by
readily accessible fields. For example, LCs has many uses in LC
displays, which rely on the optical properties of certain liquid
crystalline substances in the presence or absence of an electric
field. Block copolymers (BCPs) have also attracted consid-
erable attention over the more traditional low molecular weight
lipids and surfactants in terms of the enhanced stability and
design flexibility, which includes tailoring of the overall
(
χ) between the core-forming block and solvent:
χ ≈ B + RV Ts (δ − δ)
2
p
s
where B represents the entropic effect (close to 0.34), V is the
molar volume of the solvent, and δ and δ are the Hansen
s
p
s
solubility parameters for the core polymer and solvent,
respectively. χ can be controlled by changing the solubility
parameter and solution temperature. The Eisenberg group
screened the effect of solvent selectivity on the micelle
structure, and examined poly(styrene-b-acrylic acid) (PS-b-
PAA) ionic diblock copolymers in water/dioxane or water/
8
−13
dimethylformamide mixtures.
Transitions ranging from
1
spheres, to cylinders, and then to vesicles were observed with
increasing the amounts of water in the mixture and hence
increasing interfacial tension. Lodge et al. also examined the
micelle structures with an asymmetric poly(styrene-b-isoprene)
molecular weight, architecture and composition. BCPs
dissolved in selective solvents (solvents that are good for one
block but bad for another) can associate spontaneously to form
micelles, with the solvent−insoluble block localized in the
(
PS(13K)-b-PI(71K)) in a series of solvent mixtures including
micelle core and solvent−soluble block forming a corona
2
dibutyl phthalate, diethyl phthalate and dimethyl phthalate to
(
shell) around the core because the insoluble block minimizes
contact with the solvent and favors micelle formation. The
morphology of the micelles is dependent on a range of
variables, such as the block copolymer composition and
Received: March 7, 2012
Revised: June 9, 2012
Published: July 18, 2012
3
4,5
concentration, type and concentration of the added ions,
©
2012 American Chemical Society
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adjust the degree of solvent selectivity for styrene, and found
that the predominant micelle shape changed from spherical to
cylindrical, and then to vesicles with increasing solvent
as a solvent at high temperatures. However, the MF alignment
of block copolymers with LC mesogens in a nematic liquid
crystal solvent has to be considered for exploration.
13
selectivity, reflecting the changing interfacial curvature. Our
group also studied the micelle structures of poly(styrene-b-
vinyl4pridine) (PS-b-P4VP) in toluene/ethanol mixtures, which
In this article, a diblock copolymer consisting of a deuterated
PS and a liquid crystal polymer (dPS-b-LCP) was synthesized
using an atom transfer radical polymerization (ATRP) method,
and its micellar structures in 5CB solvent were studied using
the SANS method at various dPS-b-LCP concentrations and
solution temperatures. A 1.2 T MF was applied to the LC
solutions to view the actual structure of the micelle after
aligning the micelle through the induced global orientation of
5CB. The critical micelle concentration (CMC) was measured
and the effect of the nematic to isotropic phase transition
temperature (Tni) on the micelle structure was examined by
heating and cooling the solutions. This study provides
fundamental information on the formation of micelle structures
of flexible chain/LCP diblock copolymers in a nematic LC
solvent.
14−17
covered PS-selective, neutral, and P4VP-selective solvents.
Another way to control the solvent selectivity is to change the
solvent temperature. χ can be controlled by heating (or
cooling) the solutions. An increase in solvent temperature will
decrease χ monotonically for common organic solvents. When
the solvent of choice is thermotropic LC molecules, unlike
conventional solvents, the phase behavior of a dissolved block
copolymer may be ornamented by a discontinuous change in
the solvent quality at the LC phase transitions (e.g., a nematic
1
8,19
to isotropic phase transition).
The nematic phase is the
simplest of LC mesophases and is characterized by molecules
that diffuse freely but tend to retain their orientation in a
preferred direction called the director. This orientational order
can make LCs in the nematic phase a poor solvent for random
coil polymers (e.g., PS) because of the entropic penalty. On the
other hand, the two may become completely miscible upon
EXPERIMENTAL SECTION
Materials. Deuterated styrene (Sigma-Aldrich) was dried over
■
Al
O
and stored at 4 °C. CuBr was purchased from Sigma-Aldrich
2
3
20−22
and purified over acetic acid. Tetrahydrofuran (THF, Aldrich) was
heated with sodium and benzophenone under reflux, and distilled
under nitrogen prior to use. All other chemicals were of analytical
grade and used as received. The ligand (Me [14]aneN ) was made
heating to the isotropic phase.
This random coil polymer
can be connected to a LC polymer (LCP) (in a diblock
copolymer), which has mesogenic moieties similar to a LC
solvent, so that the LCP block is compatible with the LC
solvent in both the nematic and isotropic phases due to the
absence of an entropic penalty, even in the nematic phase.
Therefore, dPS-b-LCP may have a structure with dPS in the
core and a LCP in the corona in the nematic state of the LC
solvent, but a unimer state in its isotropic state. Kornfield et al.
examined a high molecular PS(120 K)-b-SGLCP(101 K)
6
4
according to Scheme 1. The monomer (6-(4′-cyanobiphenyloxy)hexyl
32
acrylate, LC6) was made according to previous work.
Scheme 1. Synthesis of the Ligand Me [14]aneN
6
4
(
SGLCP; side group liquid crystal polymer) in an LC
18
solvent,4-pentyl-4′-cyanobiphenyl (5CB). They reported
that the diblock copolymers had phase-separated into micelles,
and 5CB could switch from a strongly selective solvent to a
good solvent for the SGLCP block when a nematic to isotropic
phase transition occurred. The swelling of a PS-b-SGLCP with
5
CB enhanced the strength and speed of the response to the
electromagnetic stimuli, which makes them promising materials
for liquid crystal displays. The magnetic field (MF) effects on
1
the structures in the solution state have rarely been studied. On
the other hand, MF induced the alignment of lipid domains in
the phospholipid vesicle membranes after reaching the critical
2
3
size. Moreover, the ordering of LC suspensions of helically
24
twisted rods of cellulose microfibrils under an MF have been
reported. A MF (or electric field (EF)) induces the global
orientation of LCs, which can control the orientation of the
micelle of PS-b-LCP if the micelles are anisotropic, such as
prolate and oblate. Kornfield et al. also examined the structures
of the edge-on and side-on SGLCPs in 5CB using small angle
neutron scattering (SANS) under an electrical field, and
reported that the type of attachment of LC side groups
controlled the chain conformation in such a way that the edge-
on and side-on SGLCPs favored oblate and prolate,
Synthesis of Me [14]aneN . The ligand 5,5,7,12,12,14-hexam-
6 4
ethyl-1,4,8,11-tetraazamacrocyclotetradecane (Me
[14]aneN
) was
prepared in three steps according to Scheme 1. In the first step,
,2-diaminoethane dihydrobromide was prepared by adding 1,2-
6
4
3
3
1
diaminoethane (10 mL, 0.15 mol) in methanol (100 mL). The
solution was cooled in an ice bath, and concentrated hydrobromic acid
44%, 34 mL) was added dropwise. The white precipitate was filtered,
(
washed with n-butanol followed by ether, and dried in a vacuum oven
at room temperature. Acetone (100 mL) and 1,2-diaminoethane (3.0
g, 0.05 mol) were added to 1,2-diaminoethane dihydrobromide (10.0
g, 0.05 mol), and the mixture was stirred and heated to 45 °C for 50
min. During this time, a white precipitate of macrocycle dihydro-
bromide was formed. The solution was cooled and the product was
filtered, washed with acetone followed by ether, and finally dried in a
vacuum oven at room temperature for 24 h. In the final step, trans-
2
0,25,26
respectively.
MF alignment of block copolymers has
27−30
been reported in bulk
by mixing LC mesogens which were
selectively sequestrated to one block through hydrogen
bonding. A supramolecular complex system formed via
hydrogen bond association between poly(acrylic acid) chains
and an imidazole-terminated biphenyl mesogen was reported
[
14]-diene dihydrobromide dehydrate (14.0 g) was dissolved in
methanol (180 mL). Subsequently, sodium borohydride (3.0 g) was
added in small portions to this warmed solution. The solution was
heated in a water-bath until effervescence was complete. A 2 M
potassium hydroxide solution was then added until the mixture was
31
by Gopinadhan et al. They reported that LC mesogens could
cocrystallize with the polymer at low temperatures, but function
6
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basic (pH 12), and the solution was cooled to room temperature and
filtered. Approximately 50% methanol was evaporated off, and the
resulting solution was cooled in an ice bath to obtain fine
Me [14]aneN crystals. The product was recrystallized from methanol,
(10.24 mg, 0.037 mmol, ligand) and dPS−Br (9.1K, 127.30 mg, 0.012
mmol) were placed into a Schlenk flask and dried under vacuum.
Anisole (0.72 mL), which had previously been degassed by bubbling
with nitrogen, was then introduced into the flask using a syringe. The
flask was then heated in an oil bath at 80 °C for 64 h. The dPS-b-LCP
obtained was purified by precipitating three times in methanol to
remove the unreacted monomers and dried under vacuum at 40 °C for
6
4
filtered off and dried in a vacuum oven at room temperature for 24 h.
1
Figure 1 shows the NMR spectrum of Me [14]aneN . H NMR
6
4
2
4 h. The yield calculated gravimetrically was 60%.
Characterization. The molar masses and molar mass distributions
were determined by gel permeation chromatography (GPC) with a Pl
GEL Organic GPC Column and RI750F refractive index detector
(Young Lin Instrument Co., Acme 9000, Korea). THF was used as the
eluent with a flow rate of 1.0 mL/min and calibration was carried out
with polystyrene standards (Aldrich Chemical Co., USA). Fourier
Transform Infrared (FT−IR) spectroscopy of dPS-b-LCP was
1
performed using a Jasco FT/IR−620 FT-IR spectrometer. H nuclear
magnetic resonance (NMR) spectroscopy of dPS-b-LCP and
Me [14]aneN was performed at ambient temperature using a 400
6
4
MHz Bruker spectrometer with deuterated chloroform (CDCl ) as a
3
solvent. The SANS measurements were carried out on a HANARO
SANS instrument at the Korea Atomic Energy Research Institute
(KAERI) in Daejeon, Korea. The solution was filled into a quartz cell
with a 2 mm neutron path length. The wavelength (λ) was 6.38 Å with
a full width at half-maximum, Δλ/λ = 0.1. A sample to detector
−1
distance of 1.8 m was used to cover a q range of 0.004−0.04 Å ,
where q = (4π/λ) sin θ/2 is the magnitude of the scattering vector
with θ being the scattering angle. The scattered neutrons were
collected on a two-dimensional (2D) area detector and circularly
averaged. In the case of the MF experiment, the patterns were scanned
at the selected regions (either horizontally or vertically) with an
azimuthal angle span of 30° at the centers of the horizontal and vertical
directions. Sample scattering was corrected for the background, empty
cell scattering and sensitivity of the detector. To explore the effect of
the global orientation of the LC solvent on the micelle structure in the
LC solvent, SANS measurements under a MF of 1.2 T was carried out
at room temperature. MF was applied horizontally in vertical direction
to the neutron beam. Samples were placed in a holder of a specially
designed aluminum stage within a magnet (Walker Scientific Inc.) at a
flux density of 1.2 T.
SANS Data Analysis. The neutron scattering length densities of
the 5CB and LCP block in dPS-b-LCP were similar and dPS exhibited
stronger scattering than those of the LCP and 5CB in the neutron
beam. Therefore, the resulting scattering profiles from the sample
mainly reflect the size and shape of the dPS core. The core of the
micelle was fitted with the form factor of the spheres with a Gaussian
size distribution. The normalized Gaussian distribution is given by the
following equation:
1
Figure 1. H NMR spectrum of Me [14]aneN .
6
4
(
CDCl , 400 MHz, 298 K) peaks were observed at δ = 2.7−2.9 (m,
3
8
H, (CH ) ), 2.58 (td, 2H, (CH) ), 2.3 (m, 4H, (CH ) ), 1.56−1.38
2
4
2
2 2
(
m, 4H, (NH) ), 1.18 (m, 12H, (CH ) ), and 0.99 (d, 6H, (CH ) ).
4 3 4 3 2
Synthesis of a dPS−Br Macroinitiator. CuBr (68.88 mg, 0.48
mmol) was placed in a Schlenk flask and dried under vacuum. 1-
bromoethylbenzene (1-PEBr, initiator) (0.068 mL, 0.48 mmol),
deuterated styrene monomer (7.01 mL, 66.5 mmol), and
N,N,N,N,N-pentamethyl diethylenetriamine (PMDETA, ligand) were
combined in a tube and degassed (by bubbling nitrogen through it for
6
0 min). They were then introduced into the flask with a syringe that
had been purged with dry nitrogen. The flask was then placed in a 110
C oil bath and stirred for 2 h. The resulting polymer solution was
poured into a large volume of methanol. The dPS homopolymer
°
(
dPS−Br) obtained was purified by reprecipitating three times in a
THF/methanol system to remove the unreacted monomers, CuBr and
PMDETA. The polymer was then dried under vacuum at 40 °C for 24
h, giving a yield of 67%. The polymer had a number-average molecular
3
weight (M ) of 9.1 × 10 with a polydispersity index (PDI, M /M ) of
n
w
n
1.09.
1
⎡
1
2σ
2⎤
f(R) =
exp −
(R − Ravg)
2
⎢
⎥
⎦
Synthesis of a dPS-b-LCP. Scheme 2 shows a schematic diagram
of the synthesis of a dPS-b-LCP. In a typical procedure, the monomer
LC6, 350 mg, 1 mmol), CuBr (1.76 mg, 0.013 mmol), Me [14]aneN4
⎣
σ 2π
(1)
(
where Ravg is the mean radius, and σ is the standard deviation from
which the polydispersity (p) can be calculated by σ/Ravg. For a sphere
with radius R, the scattering amplitude F(q,R) is given by the following
equation:
6
Scheme 2. Schematic of the Synthesis of dPS-b-LCP
F(q, R) = 3[sin(qR) − qR sin(qR)]
3
(
qR)
(2)
The total scattering intensity can be expressed as
2
∞
⎛
⎜
4π ⎞
2
6 2
I(q) =
⎟
N Δρ ∫0 f(R)R F (q, R) dR + I
0
BKG
⎝
3 ⎠
(3)
where N is the total number of particles per unit volume with N =
0
0
Φ/⟨V⟩, Φ is the volume fraction, ⟨V⟩ is the mean volume of a particle,
Δρ is the difference in the scattering length density, and I is the
BKG
background due to incoherent scattering from protons. The
parameters of the form factor, Φ, p, scattering length density of the
sphere, scattering length density of the solvent, and incoherent
background were obtained using the curve fitting function of IgorPRO.
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The micelle aggregation number was calculated using the formula, Nagg
=
V /v , where the micelle volume was calculated using V = (4π/3)
m dPS m
3
4
3
R and the volume of the dPS molecule v (1.43886 × 10 Å ) was
derived from the density of dPS of 1.05 g/cm and molecular weight of
dPS
3
dPS of 9100.
RESULTS AND DISCUSSION
Synthesis of dPS-b-LCP. The dPS-b-LCP obtained had an
of 16.3 K with a PDI of 1.21, which means that
■
M
n
Figure 3. (a) Experimental (symbols) and curve-fitted (lines) SANS
patterns of the dPS-b-LCP solutions in 5CB with different
concentrations at 25 °C, and (b) the plot of the intensities at q =
−1
0
.007 Å against the solution concentrations; the numbers in (a) are
the concentration of dPS-b-LCP in wt %.
Table 1. Structural Parameters of the Micelle Obtained from
Curve Fitting of the SANS Data at Various Concentrations
spherical
concentration (wt %)
R (Å)
p (σ/Ravg
)
Nagg
1
Figure 2. (a) FT-IR and (b) H NMR spectra of dPS-b-LCP.
0
0
.1
83.36 ± 0.30
73.62 ± 0.20
83.04 ± 0.42
76.69 ± 0.34
80.11 ± 0.32
78.88 ± 0.73
0.913 ± 0.003
0.903 ± 0.003
0.949 ± 0.006
0.901 ± 0.005
0.882 ± 0.005
0.991 ± 0.012
153
105
167
119
136
130
.08
approximately 7.2 K of the LC6 monomer units (∼21
monomer units) were attached to 9.1 K of the dPS
macroinitiator, even though this molecular weight is relative
to the standard PS. Figure 2a shows the FT−IR spectrum of
dPS-b-LCP. The −C-D stretching bands of dPS were observed
0.07
0.06
0.05
0.04
−
1
at approximately 2942 cm . The −C−H stretching normally
−1
occurs at approximately 3000 to 3100 cm but when H is
substituted with D, the increase in mass from the attached D
successful block copolymerization. Figure 2b shows the NMR
spectrum of dPS-b-LCP. The H NMR spectrum exhibited
1
atom causes absorption at a lower frequency. The peaks at
proton peaks of the main chain backbone of CH −CH in the
2
290, 1750, 1500, 1400, and 1113 cm− correspond to the
1
LCP at 1.68 and 1.11 ppm, which had shifted from the normal
2
34
stretching bands of the CN, CO, C
−O−Caromatic,
vinyl group positions of the monomer due to polymerization.
aliphatic
aromatic CC, and C−O groups of LCP, respectively. The
The spectrum showed signals at 1.26, 1.45, 1.56, 3.74, and 3.85
peak at 827 cm− represents the long aliphatic CH in the
1
ppm, which corresponded to the CH spacer in the LCP block.
2
2
spacer of the LCP. Therefore, the existence of functional
groups in both blocks in the FT-IR spectrum indicates
The doublets at 7.36 and 7.26 ppm and the singlet at 6.68 ppm
were assigned to the protons from the benzene ring of the
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with high intensity decreased significantly with increasing q and
−1
merged at q > ∼ 0.025 Å . The strong SANS scattering
indicated that micelle structures of dPS-b-LCP had formed in
the nematic state of the LC solvent, and detailed structures
were determined by analyzing the SANS curves.
A close inspection of the scattering patterns at various
solution concentrations (cs) revealed a decrease in magnitude
of the scattered intensity at a low q value with decreasing c until
0
.005 wt %. The SANS patterns at c = 0.005, 0.001, and 0.0005
wt % were similar to each other indicating that micelles had not
formed at these low concentrations and the block copolymers
were in a unimer state. Figure 3b shows the intensities of SANS
patterns at q = 0.007 Å− as a function of c. Two lines
representing a constant and increasing intensity were drawn.
The concentration at the crossover point of the two straight
lines, 0.0027 wt %, was the CMC. This low CMC indicates that
1
5
CB is a strong selective solvent for dPS-b-LCP in the nematic
state.
Table 1 lists the radii of the micelle core, R, with p (σ/Ravg)
and micelle aggregation number, N , with respect to c, which
agg
was calculated using a curve fitting method (Figure 3c)
assuming a spherical shape for the micelle core and a Gaussian
normal size distribution. The curve fit was possible until c =
0
.04 wt % because the scattering intensities at c < 0.04 wt %
were too low and scattered to obtain a good fit. The R and Nagg
of all the solutions studied were in the ranges 76−83 Å and
1
05−167, respectively. This independence might be due to the
strong selectivity of 5CB to dPS-b-LCP. On the other hand,
SANS scattering at c < 0.04 wt % might show their dependency
on c, even though they require more resolution to analyze
them. From analysis of chain dimensions, the chain length of
the most extended and random-coil chains of dPS(9.1K) in the
unperturbed state (as the solvent is extremely poor for the dPS
chains) were 109 and 45 Å, respectively, where the chain length
2
2
of the random-coil chains were calculated using r = nl C ,
∞
35
where C∞ = 9.85. The R value of ∼80 Å from SANS was
higher than the length of the random-coil chain and close to
that of the most extended chain so that there is considerable
space between the dPS chains in the core. This space might be
filled with 5CB so that the real aggregation number would be
lower than the values listed in Table 1. The interfacial area
2
between the core and corona of the micelle is 804 nm for a
micelle with an 8 nm radius. The interfacial area occupied by a
2
2
chain (a) would be 6 nm (804 nm /∼136 chains), which is
slightly lower than the value reported for a PS-b-PI spherical
2
13
micelle (13 nm ). This decrease of a as compared to that of
the PS-b-PI micelle might be due to the 5CBs incorporated in
the dPS core, which would reduce Nagg and increase a. The
sizes of the micelle core of PS(13K)-b-PI(71K) in di-n-butyl
phthalate (DBP)/ diethyl phthalate (DEP) and PS(9.1K)-b-
LCP(7.2K) in 5CB were 29 and 8 nm, respectively. The size of
core of the PS(9.1K)-b-LCP(7.2K) micelle in 5CB was smaller
than that of the PS(13K)-b-PI(71K) micelle in DBP/DEP due
to the short chain length. However, even though the short
chain length of PS(9.1K)-b-LCP(7.2K) were considered, the
micelle size of PS(9.1K)-b-LCP(7.2K) micelle in 5CB was
smaller than of PS(13K)-b-PI(71K) in DBP/DEP because the
molecular weight decreased by a factor of 30% but the size
decreased by a factor of 68%. This large decrease in micelle size
might be due to the less selectivity of 5CB than DBP/DEP
toward PS so that 5CB might easily penetrate into the dPS
core.
Figure 4. SANS patterns of a 0.2 wt % dPS-b-LCP solution in 5CB at
different temperatures during (a) heating and (b) cooling and (c) the
plot of their intensities at q = 0.0071 Å .
−
1
biphenyl ring of the LCP. The peak at 2 ppm was assigned to
THF, the dissolving solvent.
To produce micelles in 5CB, dPS-b-LCP and 5CB were
dissolved together in dichloromethane (DCM) followed by
stirring and heating at 40 °C overnight to remove the DCM.
Figure 3 shows the SANS patterns from 0.1, 0.08, 0.07, 0.06,
0
.05, 0.04, 0.02, 0.01, 0.005, 0.001, and 0.0005 wt % dPS-b-LCP
solutions in 5CB at room temperature. The initial scattering
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Figure 5. 2D SANS patterns of a 0.2 wt % dPS-b-LCP solution in 5CB at room temperature (a) without MF and (b) with MF of 1.2 T; (c)
azimuthally averaged 1D scanned data of (a); (d) azimuthally averaged 1D scanned data of part b along the MF direction and perpendicular to it
with an averaged azimuthal angle of 30°; (e) schematic diagram of the micelle structure in 5CB under MF which direction is vertical.
Figure 4 shows the evolution of a SANS patterns of a 0.2 wt
dPS-b-LCP solution in 5CB during heating and cooling over
scattered intensity began to increase with decreasing temper-
ature due to micelle formation.
%
The plot of the intensities at q = 0.0071 Å− against
1
the temperature range between 25 and 43 °C with an increase
or decrease) of 2 °C along with a plot of their intensities at q =
(
0
temperature clearly (Figures 4c) demonstrated the isotropic
.0071 Å− as a function of temperature. The changes in the
1
state of the solution at temperatures higher than the T of 5CB
ni
scattering patterns during heating and cooling were similar,
indicating that the structure formation is reversible. At
temperatures above the Tni of 5CB (36 °C), the scattered
intensity was low and constant, suggesting that the micelle
structure had not formed and the block copolymers were in the
and micelle formation at temperatures lower than the T of
ni
5CB. The scattering angle at q = 0.007 A− has been chosen
because it is close to the lowest angle which could be
measurable; I(q = 0) represents the total scattering so that the
data at low angles might be more reliable and less scattered.
The temperature at which the micelle structures had formed
1
unimer state. At temperatures lower than the T of 5CB, the
ni
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dx.doi.org/10.1021/ma300476j | Macromolecules 2012, 45, 6168−6175

Macromolecules
Tmicelle) was acquired from the intercept of the horizontal line
and the tangent line in the region of the increasing intensity.
The intercepting temperatures for heating and cooling were
Article
(
which was attributed to the end-on type of attachment of the
mesogenic groups in the LCP block.
3
3.2 and 33.1 °C, respectively. These Tmicelles were lower than
AUTHOR INFORMATION
■
*
the T of pure 5CB (∼36 °C). The block copolymers might
play as impurities which could cause the decreased T .
However, we could not find any change of the T . The
lower Tmicelle than that of Tni might be due to the 5CBs
incorporated in the dPS core, which might cause easier
dissociation of the micelles. Similar phenomena were observed
by the Kornfield group for PS-b-SGLCPs with a short PS
block length. The values of the R (and N ) decreased from
ni
Corresponding Author
E-mail: psy@knu.ac.kr.
ni
ni
Notes
The authors declare no competing financial interest.
26
ACKNOWLEDGMENTS
■
agg
This work was supported by the National Research Foundation
of Korea (NRF-2011-0020264, and NRF-2009-0073476).
74.92 ± 10.18 Å (and 132) to 69.83 ± 9.04 Å (and 94) as
temperature increased from 25 to 27 °C. The curve fit was
possible at 25 and 27 °C because the scattering intensities at
higher than 27 °C were too low and scattered to obtain a good
fit. The intensities in Figure 4c also decreased continuously as
temperature increased until Tmicelle. These results indicate that
the selectivity of the 5CB continuously decreased as temper-
ature increased (not a discrete way at Tmicelle) even though the
temperatures were still below Tmicelle. The incorporated 5CB in
the dPS core might play as a mixed solvent and lose the liquid
crystal properties so that the selectivity of the 5CB
continuously decreased leading to the decreased intensity, R
and Nagg as temperature increased.
To examine the anisotropic micellar structure, a 1.2 T MF
was applied to the LC solution at room temperature to obtain
the global orientation of the LC solvent during the SANS
measurements. The spherical micelle would show isotropic
SANS scattering even at the global orientation of the LC, even
though the anisotropic micelle (such as prolate and oblate)
exhibits anisotropic SANS scattering when it is aligned along
the direction of the global orientation of the LC. This
alignment can occur due to the mesogenic groups in the
corona, which anchor 5CB strongly. Figure 5 shows the 2D
patterns and azimuthally averaged 1D scanned data with and
without a MF. The azimuthally averaged 1D scanned data with
the MF were obtained by azimuthal averaging of 30° along the
horizontal and vertical directions. The 2D SANS pattern was
isotropic in the absence of MF (Figure. 5a), indicating
randomly oriented liquid crystal domains. Curve fitting of the
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5
d) with the form factor of a sphere. The resulting values for
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(
CONCLUSION
■
The micelle structures of dPS-b-LCP in the 5CB solvent were
examined as functions of the block copolymer concentration
and temperature using SANS. MF was applied to determine the
anisotropic structure of a micelle through the global orientation
of the LC. Micelles of the dPS-b-LCP formed reversibly in the
nematic state of the 5CB, even though the temperature at
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6
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