Synthesis and characterization of a mesogen-jacketed polyelectrolyte

Article abstract of DOI:10.1021/ma402362w

In an attempt to construct a new kind of rodlike polyelectrolyte, poly[sodium 2,5-bis(4′-sulfophenyl)styrene] (PSBSS) was prepared from its precursor, poly[2,5-bis(4′-neopentylsulfophenyl)styrene] (PBNSS), which was polymerized by atom transfer radical polymerization. Small-angle X-ray scattering (SAXS) results demonstrate that PBNSS exhibits a hexagonal columnar phase and PSBSS exhibits a smectic A phase in bulk. The conformation of PSBSS in the aqueous solution is cylindrical, and the length and the diameter of the cylinder are ca. 25 nm and ca. 2.4 nm, respectively. The persistence length (l_p) of the PSBSS chain in the aqueous solution is 11.50 ± 0.09 nm calculated by fitting the SAXS profile with the modified wormlike chain model. The conformation, the maximum length, and the l_p of the chain are only weakly dependent on the concentration of the added salt. These results indicate that we have successfully obtained a new kind of polyelectrolyte with a highly rigid chain, a high charge density, and a narrow molecular weight distribution, which can serve as a new model macromolecule in studying rodlike polyelectrolytes.

Full text of DOI:10.1021/ma402362w

Article
pubs.acs.org/Macromolecules
Synthesis and Characterization of a Mesogen-Jacketed
Polyelectrolyte
†
‡
†
†
†
†
,†
Wei Qu, Xingqi Zhu, Jiahui Chen, Lin Niu, Dehai Liang, Xinghe Fan, Zhihao Shen,*
and Qifeng Zhou†
†Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
‡Anton Paar China, Shanghai 200233, China
*
S Supporting Information
ABSTRACT: In an attempt to construct a new kind of rodlike
polyelectrolyte, poly[sodium 2,5-bis(4′-sulfophenyl)styrene] (PSBSS) was
prepared from its precursor, poly[2,5-bis(4′-neopentylsulfophenyl)styrene]
(
PBNSS), which was polymerized by atom transfer radical polymerization.
Small-angle X-ray scattering (SAXS) results demonstrate that PBNSS
exhibits a hexagonal columnar phase and PSBSS exhibits a smectic A phase
in bulk. The conformation of PSBSS in the aqueous solution is cylindrical,
and the length and the diameter of the cylinder are ca. 25 nm and ca. 2.4
nm, respectively. The persistence length (l ) of the PSBSS chain in the
p
aqueous solution is 11.50 ± 0.09 nm calculated by fitting the SAXS profile
with the modified wormlike chain model. The conformation, the maximum
length, and the l of the chain are only weakly dependent on the concentration of the added salt. These results indicate that we
p
have successfully obtained a new kind of polyelectrolyte with a highly rigid chain, a high charge density, and a narrow molecular
weight distribution, which can serve as a new model macromolecule in studying rodlike polyelectrolytes.
INTRODUCTION
using rodlike helical polyelectrolyte systems such as DNA or
19,20
■
xanthane.
However, the helical conformation (and thus the
In the past few decades, extensive studies on polyelectrolytes
have been carried out from theory, simulation, and
experimental aspects.
rodlike shape) is lost at very low ionic strengths and at elevated
1
8,21
1
,2
temperatures.
One major limitation of natural polyelec-
However, the understanding of
trolytes is that we can not change the chemical structures, such
as the number of ionic groups per unit length. Therefore, it is
necessary to develop well-defined synthetic rodlike polyelec-
trolytes to study the nature of polyelectrolytes, such as
polyelectrolytes remains controversial, with many issues lacking
satisfactory explanations. Scattering experiments are probably
the most important techniques to study the behaviors of
polyelectrolyte solutions. Static and dynamic light scattering
22
3
−5
6−9
Manning condensation and some conformational problems.
(
SLS and DLS),
small-angle X-ray scattering (SAXS),
3,4,10−13
The first kind of synthetic rodlike polyelectrolyte based on
and small-angle neutron scattering (SANS)
techniques
poly(1,4-phenylenebenzobisoxazoles) and poly(1,4-phenylene-
are commonly used for studying the structures of polyelec-
trolytes like charged spheres, rodlike molecules, and flexible
chains in solutions. Most scattering profiles of polyelectrolyte
solutions without salt show a characteristic maximum q*, which
depends on the polyelectrolyte concentration (c ). The q* of all
types of polyelectrolyte solutions exhibits a very similar
2
3,24
benzobisthiazoles) was reported in the early 1980s.
During
the following decade, two kinds of rodlike polyelectrolytes with
the main chain of poly(p-phenylene) (PPP) were synthesized
25−28
via various efficient precursor routes.
Long alkyl chains
p
were introduced to the side chains to solubilize the main chain
in both kinds of polyelectrolytes. Hydrophilic groups were
linked to the main chain directly by Wegner and co-workers,
who found that the polyelectrolytes could form lyotropic liquid
crystalline phases when the concentration was above a certain
1/3
concentration dependence. In the dilute regime, q* ∝ c
,
p
1
/2 3,6,14−16
while in the semidilute regime, q* ∝ c
.
The
p
relationship in the dilute regime can be understood as the
correlation among centers of polyions, while that in the
semidilute regime is assigned to the nearest-neighbor
27
value, and a similar phenomenon was also found in DNA
29
17
systems. Rehahn and co-workers also synthesized polyelec-
intersegmental correlation.
25,26
trolytes with alkyl-chain spacers.
Aggregated polyelectrolyte
A decrease in the ionic strength may lead to an expansion of
flexible polyelectrolytes due to stronger intramolecular forces
and intermolecular electrostatic interactions. On the other
hand, regardless of the ionic strength of the system, rodlike
polyelectrolytes retain their extended-chain conformation.
Given these points, a number of studies have been performed
chains were in the form of cylindrical micelles in bulk. Ballauff
Received: November 14, 2013
Revised: March 31, 2014
Published: April 11, 2014
18
©
2014 American Chemical Society
2727
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Macromolecules
Article
Scheme 1. Synthetic Route of Monomer and Polymers
et al. discussed the distribution of counterions around the
cylindrical macroion using SAXS and anomalous small-angle X-
ray scattering (ASAXS). However, the persistence length (lp)
SAXS. The results show that the l is 11.50 ± 0.09 nm in the
p
dilute solution. The effects of polymer concentration and added
salt concentration on the structures of PSBSS were also
investigated.
30
of the polyelectrolytes with a PPP main chain characterized by
31
Wegner et al. was 12.6 nm. Because of the long alkyl chains,
the solubility of this kind of polyelectrolyte in the aqueous
solution is poor. The complexity and polydispersity of the
precursors are another problem for polyelectrolytes with a PPP
main chain. Gong and co-workers synthesized and charac-
terized poly(2,2′-disulfonylbenzidine terephthalamide)
EXPERIMENTAL SECTION
■
Materials. 2,5-Dibromotoluene (99%, Acros), N-bromosuccini-
mide (NBS, 99%, Aldrich), triphenylphosphine (PPh , 99%, Acros),
3
bis(pinacolato)diboron (99%, Acros), tetrakis(triphenylphosphine)-
palladium(0) (Pd(PPh ) , 99%, Acros), tetrachloromethane (CCl ,
3
4
4
3
2
(
PBDT), which had a rigid polyamide structure in the main
AR, Beijing Chemical Co.), and aqueous formaldehyde (40%, AR,
Beijing Chemical Co.) were used as received. Tetrahydrofuran (THF,
AR, Beijing Chemical Co.) was refluxed with sodium and distilled out
just before use. Dimethyl sulfoxide (DMSO, AR, Beijing Chemical
Co.) and N,N-dimethylformamide (DMF, AR, Beijing Chemical Co.)
were distilled out from calcium hydride. Methanol was purified by
distillation. All other reagents were used as received from commercial
sources.
chain. There was hydrogen-bonding interaction between the
PBDT chains in addition to the electrostatic interaction.
In our previous study, we have systematically investigated a
special type of liquid crystalline polymers, mesogen-jacketed
3
3,34
liquid crystalline polymers (MJLCPs),
which use a very
short spacer or a single bond between the side-on attached side
chains and the main chain. MJLCPs take an extended-chain
conformation because of the “jacketing” effect introduced by
the steric effect of the bulky side groups. They can be easily
synthesized by radical polymerizations, including living radical
polymerizations such as atom transfer radical polymerization
Measurements. Elemental analysis, thermogravimetric analysis
(TGA), and Fourier-transform infrared spectroscopy (FT-IR) experi-
ments were performed according to the procedures previously
3
6
described. GPC experiments were conducted on a Waters 2410
instrument equipped with a Waters 2410 RI detector, with the Styragel
HR (High-Resolution) columns (4.6 × 300 mm) and with DMF as the
eluent (1.0 mL/min). The calibration curve was obtained with linear
polystyrenes as standards. The elution profiles of the polystyrene
standards and the fitting equation of the calibration curve are shown in
(
ATRP), and the diameters of the rodlike MJLCPs can be
34
tuned by controlling the chemical structures of side chains. It
is a new way to obtain rigid polyelectrolytes by taking
advantage of the “jacketing” effect, leading to mesogen-jacketed
polyelectrolytes (MJPEs) as one new kind of rodlike model
chain molecule. Previously, we synthesized two MJPEs by
1
the Supporting Information. H NMR spectra were obtained using a
Bruker ARX400 MHz with tetramethylsilane (TMS) as the internal
standard at ambient temperature in CDCl and dimethyl-d sulfoxide.
Mass spectra were recorded on a Bruker Apex IV FTMS spectrometer.
SAXS experiments were performed using a SAXSess instrument
Anton Paar) equipped with a Kratky block collimation system. The
X-ray was generated using a Philips PW3080 sealed-tube X-ray
generator with the Cu target. The wavelength was 0.1542 nm. A highly
sensitive SAXS imaging plate which was 264.5 mm away from the
sample was used to collect the signal in vacuum. Samples in bulk were
placed in between aluminum foils which were folded and sandwiched
in a steel sample holder. Scattering data were acquired for a 30 min
exposure. The background scattering from aluminum foils was
acquired and then subtracted from the sample profiles. Solution
samples for SAXS measurements were carefully loaded into a quartz
capillary with a diameter of 1 mm. After background subtraction,
3
6
35
conventional radical polymerization, in which the side-chain
mesogens had a rigid core containing an amide or ester linkage,
with sulfonated groups appended on the two ends of the core.
The two MJPEs exhibit smectic A (SmA) phases in bulk, and
lamellar phases are observed when the two MJPEs complex
with surfactants in the solid state.
(
In this work, we report the design and synthesis of a new
kind of MJPE, poly[sodium 2,5-bis(4′-sulfophenyl)styrene]
(
PSBSS), the precursor of which is polymerized by ATRP. The
side-chain core of PSBSS is terphenyl, which is rigid and
introduces the “jacketing” effect and π−π interaction. The
structures of PSBSS in bulk and in solution were studied by
3
7
desmearing was performed according to Lake’s method. The pair-
2
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1
Figure 1. H NMR spectra of the monomer (curve 1), PBNSS (curve 2), PBSS (curve 3), and PSBSS (curve 4).
distance distribution functions (PDDFs) of scattering curves were
calculated using the generalized indirect Fourier transform (GIFT)
program included in the SAXSess software package.
Synthesis of Monomer and Polymers. The synthetic route of
the monomer and polymers is represented in Scheme 1. The
experimental details are described as follows.
5 °C. After the complete addition of chlorosulfonic acid, the reaction
mixture was stirred for another 45 min at ambient temperature, and
the mixture was poured on to crushed ice. Three portions of CHCl3,
50 mL each, were used to extract the mixture. The organic layers were
3
8,39
combined and dried over anhydrous MgSO . Chloroform was then
4
distilled off. The product was sufficiently pure and was not further
purified.
Synthesis of 2,5-Dibromostyrene. A solution of 2,5-dibromo-
toluene (10.0 g, 40.6 mmol), NBS (7.12 g, 40.6 mmol), and benzoyl
To a mixture of 4-bromobenzene-1-sulfonyl chloride (5.10 g, 20.0
peroxide (BPO, 0.200 g, 0.800 mmol) in CCl (250 mL) was refluxed
mmol) and neopentyl alcohol (1.80 g, 24.0 mmol) in CH Cl (30 mL)
4
2
2
for 8 h. After the solvent was evaporated under reduced pressure, the
at 0 °C, pyridine (3.24 mL, 40.0 mmol) was added dropwise over a
period of 30 min. The reaction mixture was stirred at ambient
residue was suspended in CHCl , and the insoluble solid was removed
3
by filtration. The filtrate was washed with water twice, and the
temperature for 12 h and diluted with Et O. The organic layer was
2
combined aqueous phase was extracted with CHCl . The combined
washed with 0.1% aqueous HCl, water, and brine, dried over MgSO4,
3
organic solution was dried over MgSO and concentrated in vacuo.
and concentrated in vacuo. The crude sulfonate was purified by
recrystallization in ethanol. Yield: 61.0%. H NMR (300 MHz,
4
1
The crude product obtained was mixed with PPh (10.5 g, 40.6 mmol)
3
and acetone (250 mL). The mixture was heated to reflux, and a lot of
precipitates appeared immediately. The reaction was allowed to
continue for 4 h. The solids were collected by filtration and washed
with cold acetone and then dissolved in aqueous formaldehyde (40%,
CDC1 , δ, ppm): 0.92 (s, 9H, −CH ), 3.69 (s, 2H, −OCHH ), 7.64−
3
3
2
7.81 (dd, 4H, Ar).
Synthesis of Neopentyl 4-(4,4,5,5-Tetramethyl-1,3,2-dioxa-
borolan-2-yl)benzenesulfonate (NTDB). To a solution of neo-
pentyl 4-bromobenzenesulfonate (1.00 g, 3.25 mmol) in dioxane (15
2
50 mL). With rapid stirring, 10 wt % NaOH aqueous solution was
added dropwise at ambient temperature. The mixture was stirred for
4 h. When the reaction was completed, three portions of CH Cl , 150
mL), bis(pinacolate)dibron (0.910 g, 3.54 mmol), PdCl (PPh )
2 3 2
2
(0.0300 g, 0.0400 mmol), and KOAc (0.950 g, 9.69 mmol) were
added, and the mixture was stirred at 100 °C for 6 h under nitrogen.
The mixture was diluted with EtOAc and water. The organic layer was
2
2
mL each, were used to extract the mixture. The organic layers were
combined and dried over anhydrous MgSO . The solvent was
4
evaporated under reduced pressure, and the residue was purified by
separated, washed with brine, and dried over anhydrous MgSO . After
4
column chromatography on silica gel with dichloromethane/
evaporation of the solvent, the residue was purified by column
chromatography on silica gel with EtOAc/petroleum ether (1/10, v/v)
as the eluent to give 0.920 g of product as a white solid. Yield: 80.0%.
petroleum ether (1/3, v/v) as the eluent to give 6.40 g of product
1
as a yellow liquid. Yield: 61.0%. H NMR (300 MHz, CDC1 , δ,
3
1
ppm): 5.40−5.43 (d, 1H, =CH ), 5.68−5.74 (d, 1H, =CH ), 6.92−
H NMR (300 MHz, CDC1 , δ, ppm): 0.89 (s, 9H, −CH ), 1.37 (s,
2
2
3
3
7
.02 (q, 2H, =CH), 7.22−7.67 (m, 3H, Ar).
12H, −CH ), 3.66 (s, 2H, −OCHH ), 7.87−7.96 (dd, 4H, Ar).
3
2
Synthesis of Neopentyl 4-Bromobenzenesulfonate. To a
Synthesis of 2,5-Bis(4′-neopentyl sulfophenyl)styrene
(BNSS). To a degassed mixture of 2,5-dibromostyrene (1.10 g, 4.23
mmol), anhydrous K CO (5.84 g, 42.3 mmol), hydroquinone (0.130
mixture of bromobenzene (15.7 g, 100 mmol) in chloroform (50.0
mL) in a 500 mL flask equipped with a dropping funnel, a
thermometer, and a reflux condenser, chlorosulfonic acid (250
mmol) was added dropwise over a period of about 60 min. The
reaction mixture was mechanically stirred at 0 °C in a bath containing
a freezing mixture of ice and salt. Chlorosulfonic acid was added in
such a rate that the temperature of the reaction mixture did not exceed
2
3
g, 1.18 mmol), NTDB (3.30 g, 9.30 mmol), and Pd(PPh ) (0.150 g,
3
4
0.130 mmol), toluene (30.0 mL) and water (10.0 mL) were added
under a continuous stream of argon. The mixture was vigorously
stirred at reflux for 36 h under argon. The organic layer was then
separated and dried over anhydrous MgSO . The solvent was removed
4
2
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under reduced pressure, and the residue was purified by column
chromatography on silica gel with EtOAc/petroleum ether (1/10, v/v)
as the eluent to give 0.950 g of product as a white solid. Yield: 82.0%.
distribution by ATRP. The contour length (L) of PBNSS was
estimated to be about 35 nm. PSBSS has the same
polymerization degree and contour length as PBNSS. TGA
experiment was carried out to examine the thermal stability of
PBNSS (Figure 3). To remove moisture in the sample, it was
1
H NMR (300 MHz, CDC1 , δ, ppm): 0.95−0.98 (m, 18H, −CH ),
3
3
3
5
.78−3.80 (s, 2H, −OCHH ), 3.76−3.78 (s, 2H, −OCHH ), 5.32−
2
2
.35 (d, 1H, CH =), 5.80−5.85 (d, 1H, CH =), 6.62−6.70 (q, 1H,
2
2
CH=), 7.40−7.44 (d, 1H, Ar), 7.56−7.60 (d, 2H, Ar), 7.60−7.64 (q,
1
2
H, Ar), 7.82−7.84 (d, 2H, Ar), 7.86−7.88 (d, 1H, Ar), 7.96−8.00 (d,
13
H, Ar), 8.04−8.00 (d, 2H, Ar). C NMR (100 MHz, CDC1 , δ,
3
ppm): 145.76, 145.70, 139.42, 138.90, 136.75, 135.17, 134.90, 130.67,
30.47, 128.52, 127.82, 127.77, 126.81, 125.34 (p-terphenyl), 116.99
1
(
CHCH ), 79.78, 31.80, 26.05 (neopentyl). Anal. Calcd for
2
C H O S (%): C, 64.72; H, 6.52. Found: C, 64.96; H, 6.58. MS
(
30
36
6 2
•+
EI, m/z): 556.2 (M ).
Polymerization of BNSS through ATRP. In a typical experiment
for polymerization, the monomer BNSS (1.99 g, 3.57 mmol),
N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 12.1 mg,
0.0710 mmol), (1-bromoethyl)benzene (BEB, 3.25 mg, 0.0170 mmol),
CuBr (5.22 mg, 0.0360 mmol), and dry DMF (9.06 g) were charged
into a polymerization tube. After being stirred and degassed by three
freeze−pump−thaw cycles, the tube was sealed under vacuum and
subsequently immerged into an oil bath thermostated at 90 °C for 48
h. It was then quenched in liquid nitrogen and taken out to ambient
condition. The solution was passed through a neutral alumina column
in order to remove copper salt. Finally, the polymer was precipitated
out from a large volume of hexane and dried in vacuum overnight.
Preparation of Poly[sodium 2,5-bis(4′-sulfophenyl)styrene]
Figure 3. TGA curve of PBNSS at a heating rate of 10 °C/min under a
nitrogen atmosphere.
(
PSBSS). PBNSS (100 mg) was converted to acid by thermolysis
first heated at 80 °C for 30 min. At 150 °C, the weight
percentage sharply drops down to 76.7%, close to the
theoretical value of 74.8% when neopentyl groups are removed
from the polymer. This gives the optimized condition for
thermolysis as annealing at 150 °C for 30 min.
under nitrogen at 150 °C for 30 min. The resulting material was
dissolved in 50.0 mL of H O and dialyzed versus 10.0 L of Milli-Q
2
water, which was exchanged every 24 h (membrane pore size 10 000
g/mol). After the conductivity of the water dropped to about 0.1 mS,
dialysis continued for another 12 h. Freeze-drying yielded the desired
product.
FT-IR experiments were carried out to confirm the
completion of the deprotection reaction. As shown in Figure
4
, the vibration of the sulfonate ester groups in PBNSS at 1358
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Monomer and
Polymers. The structures of the monomer, PBNSS, PBSS, and
1
PSBSS were confirmed by H NMR. As shown in Figure 1, the
signals of the vinyl group in the monomer at 5.35, 5.82, and
6.67 ppm in curve 1 completely disappear in curve 2, indicating
the successful polymerization (in curve 2, the signals of benzene
rings are partially shielded by the large neopentyl moieties). In
curves 3 and 4 of Figure 1, the signals of the sulfonate ester
groups in PBNSS disappear, indicating complete deprotection.
As shown in Figure 2, the number-average molecular weight
(
M ), polymerization degree, and polydispersity index (PDI) of
n
PBNSS are 86 300 g/mol (relative to polystyrene), 150, and
.10, respectively, indicating that we have synthesized the
1
polyelectrolyte precursor with a narrow molecular weight
Figure 4. FT-IR spectra of PBNSS, PBSS, and PSBSS.
cm− is not present after thermolysis. Instead, a broad peak is
1
−
1
observed at 3500 cm in PBSS and PSBSS, indicating the
presence of hydroxyl groups either in the polymer structure or
from absorbed water.
Structures of the Polymers in Bulk. SAXS experiments
were carried out to determine the bulk structures of PBNSS
and PSBSS. About 10 mg of PBNSS was dissolved in THF and
cast onto a glass substrate, and then THF was allowed to
evaporate at ambient temperature. For PSBSS, water was used
as the solvent. The samples were directly used for SAXS
experiments without any further treatment.
Figure 2. GPC profile of PBNSS.
2
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Figure 5 shows the SAXS profiles of PBNSS and PSBSS at
ambient temperature. The SAXS profile of PBNSS shows three
Figure 6. SAXS profiles of PSBSS aqueous solutions without salt. The
inset shows the log−log plot of q* versus concentration cp.
Figure 5. SAXS profiles of PBNSS and PSBSS in bulk.
0
.8 wt % is the critical overlapping concentration (c*) of the
p
PSBSS aqueous solution without salt. When the PSBSS
concentration is less than c*, the solution is dilute, and the
polyelectrolyte molecules are well separated. The equivalent
−
1
−1
diffraction peaks with q = 2.97 nm , q = 5.12 nm , and q =
1
2
3
p
−1
5
.83 nm . The scattering vector ratio of the three peaks,
1
/2 1/2
3
q :q :q , is approximately 1:3 :4 , indicating a hexagonal
relationship, n L ≈ 10, where n is the number concentration
1
2
3
P
c
P
columnar (Φ ) liquid crystalline phase with a periodicity of
of polyions and L the polyion contour length, was observed by
c
15,16
H
2
.11 nm (d1 = 2π/q ). The diameter of the cylinders can thus
SAXS.
The theoretical crossover concentration, c*, can be
t
00
1
be calculated to be 2.43 nm, which is close to the calculated
side-chain length (2.38 nm). Therefore, similar to other
MJLCPs with a terphenyl rigid core,
estimated from the equivalent relationship. According to the
GPC result of PBNSS, the degree of polymerization and L is
estimated to be 150 and 35 nm, respectively. Therefore, the
c
4
0,41
PBNSS behaves like
a supramolecular cylinder and forms the columnar LC phase.
The introduction of the sulfonate group at the ends of the
terphenyl group does not change the conformation of the
polymer chain. However, when the sulfonate groups are
ionized, the polymer conformation changes from a cylinder
to a tablet owing to the introduction of electrostatic
result of c* ≈ 2 wt % is obtained, which is comparable with the
t
experimental value of c* ≈ 0.8 wt %. The difference may be
p
caused by the deviations in the estimation and the experimental
measurement.
To further evaluate the polymer shape and dimensions, we
calculated the PDDFs, which represent for homogeneous
particles the value of distances within the particle, by fitting the
35
interactions, consistent with our previous results. This is
38,39
reflected in the SAXS profile of PSBSS, which has two
scattering profiles with the model-free method of GIFT.
−1
−1
diffraction peaks at q = 3.42 nm and q = 6.69 nm with a
Depending on the range and nature of ordering in the system,
the p(r) function can have many peaks. Directly from p(r), a
qualitative classification of the shape and the internal structure
of the particle can be obtained. In addition, several structural
parameters can be determined quantitatively, e.g., the maximum
intraparticle distance rmax, because p(r) drops to zero at r =
1
2
scattering vector ratio of 1:2, revealing a smectic phase with a
periodicity of 1.84 nm. Because the layer spacing is close to the
side-chain length (1.80 nm), PSBSS may have a SmA phase
with the side chains perpendicular to the main chain, similar to
the packing in SmA phases formed by other MJLCPs. To the
best of our knowledge, this is the first report of the liquid
crystalline phase transition upon the introduction of charges.
Structures of PSBSS in Solution. SAXS experiments on
PSBSS solutions with or without added salt were performed to
investigate the structures of PSBSS in solution. Figure 6 shows
the scattering profiles of PSBSS aqueous solutions of various
concentrations, without added salt. The SAXS profiles show a
44
rmax
.
The disappearance of the q* when c < 0.40 wt %
p
indicates that the polyelectrolyte molecules are well separated,
and the total scattering intensity is the sum of the scattering of
individual molecules. The data for the 0.30 wt % PSBSS
solution, shown in Figure 7, has a small overshoot and a nearly
linear tail in the PDDF, which implies that the polyelectrolyte
chains are cylindrical. Because PSBSS does not resemble a
homogeneous particle with a smooth surface, we find that p(r)
maximum of q value (q*) when the concentration c is higher
p
45
than 0.40 wt %, common for polyelectrolyte solutions. With
≠ 0 at r = 0. The crossover of the decay of the PDDF curve
with the abscissa, rmax, reflects the length of the cylinder.
Therefore, the result in Figure 7 indicates that the
conformation of PSBSS in water is cylindrical with a length
of about 25 nm. The PDDF curve shows a pronounced central
maximum and broad side maxima, depending on the size of the
cross section. Oscillations about the linear descent and ripples
in p(r) can occur for cylinders with homogeneous electron
density along the long axis but varying cross section or vice
increasing c , the peak shifts to higher q values, and its intensity
p
is initially increased. This weak interference maximum of q may
be attributed to inter- and intramolecular correlations at short
length scales. In the dilute regime, the electrostatic repulsion
between the polyions induces the formation of a three-
42
dimensional periodic lattice. The q* of the associated
1
/3
correlation peak is predicted to vary as cp . In the semidilute
regime, according to the isotropic model, the chains overlap
without order in the solution, and they constitute a temporary
44
versa. The steric hindrance and electrostatic repulsion
between the side chains of PSBSS make the rotation difficult,
which leads to inhomogeneous cylinders. This impedes the
accurate determination of the diameter of the cylinder.
1
/2 15,16,43
network of mesh, with q* predicted to vary as c
.
The
for
p
1
/2
1/3
inset shows that q* ∝ c
for c > 0.8 wt % while q* ∝ c
p
p
p
c in the range 0.4−0.8 wt %. Therefore, the concentration of
p
2
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Figure 7. PDDF [p(r)] of 0.30 wt % PSBSS aqueous solution without
salt. The inset shows that the GIFT simulated curve (solid line) fits the
raw scattering data (gray dots) well.
Figure 9. Comparison of measured and calculated scattering
intensities. The open circles give the SAXS intensity of 0.30 wt %
PSBSS aqueous solution without salt whereas the solid line displays
the optimal fit by the modified WLC model, eq 1.
The length of the cylinder is large compared to its cross
section. The length contribution is taken into account by
chain, the scattering function is modified by a cross-sectional
scattering factor and is given by
4
6,47
dividing the intensity by q, leading to I (q), which is then
c
Fourier transformed. The resulting p (r) is the cross-sectional
I(q) = I0IWLC(q) exp(−q R 2/2)
2
c
cs
(1)
PDDF. The crossover of the decay of PDDF of the cross-
sectional curve with abscissa reflects the true diameter of the
cylinder, d , shown in Figure 8. The value of d is ca. 2.4 nm,
where R 2 is the mean-square radius of gyration of the cross
cs
c
c
section.
which is close to the calculated length of the laterally attached
side chain.
For L/b > 2, it is suggested to use the following interpolation
4
6,47
expression:
p
1
IWLC(q, L, b) = S (q, L, b) exp[−((qb)/q ) ]
SB
1
p
1
+
S (q, L)(1 − exp[−((qb)/q ) ])
1
loc
(2)
where q and p are empirical constants.
For L/b ≤ 2, the following expression is introduced:
1
1
4
6,47
p
2
IWLC(q, L, b) = SDebye(q, L, b) exp[−((qb)/q ) ]
2
⎛
⎜
⎝
⎞
a1
Lbq2
π
p
+
+
⎟(1 − exp[−((qb)/q ) ])
2
2
Lq⎠
(3)
where
2
2
2
SDebye(q, L, b) = 2[exp(−u) + u − 1]/u , u = q R
g
(4)
Figure 8. PDDF of the cross section [p (r)] of 0.30 wt % PSBSS
⎡
c
⎛
⎜
Lb⎞
3
2nb
3
2nb
⎤
3
4nb
2
aqueous solution without salt. The inset shows that the GIFT
simulated curve (solid line) fits the raw scattering data (gray dots)
well.
⟨R ⟩ =
⎟
⎢1 −
+
−
2
g
3
⎝
⎠
6 ⎣
×
(1 − exp(−2n ))⎥
b
⎦
(5)
To further characterize the bending properties of PSBSS, we
apply the wormlike chain (WLC) model to fit the scattering
curves, which has been extensively used to characterize bending
properties of polymers, e.g., dsDNA fragment. Intermolecular
interactions are coarse-grained in terms of the constraints that
connected segments impose on the orientation of connecting
bonds. The fit in Figure 9 follows the modified WLC
where n (= L/b) is the number of Kuhn segments. Pedersen
and co-workers have optimized the values of the parameters
q , p , p , a , and a by a least-squares fit of eqs 2 and 3.
In this work, we use the following optimized results from the
b
46
1
1
2
1
2
4
6,47
literature:
= 11.7.
q = 5.53, p = 5.33, a = 0.065, p = 3.95, and a
1 1 1 2 2
4
6,47
model.
It deals with the local stiffness of chain segments
The radius R of the polyelectrolyte chain can be calculated
at higher q values and also the Gaussian coiled structure at
lower q values. Hence, the model can be used to fit the data in
the entire q-range, and it also takes into account the excluded
volume effect, IWLC(q,L,b), where L is the chain contour length
and b is the chain Kuhn length. The Kuhn length b is related to
l by 2l = b. To account for the finite thickness of the side
from the cross-sectional PDDF, R = (1/2)d . When cylindrical
c
scatters are considered, R = √2R , which leads to R = 0.85
cs
cs
nm. The value of the contour length estimated from the GPC
result was used as the initial fitting value (L = 35 nm).
Uncertain of the degree of rigidity of the polyelectrolyte
chain, we first fitted the scattering curves by eq 3. However, we
p
p
2
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could not obtain a good fitting. The solid line in Figure 9 is a fit
by eq 2 for this modified model, and the fitting parameters are
shown in the figure. The optimized value of the contour length
were 0.1, 0.3, 0.6, 0.8, 1.0, 1.5, and 2.0 times c were performed,
p
when c = 0.6 wt %, which is 13.0 mM (mmol repeat units in 1
p
L of water). In Figure 11a, the q* shifts toward lower q values
is 38 nm. R values obtained from the fitting results are
g
compared with those obtained from the scattering data in Table
1. It is apparent from the table that the fitting result is
Table 1. Comparison of R Data from SAXS with the Fitting
g
Result Based on the Modified WLC Model
Rg (nm)
Rg (nm) (by fitting,
p
(
measured, Guinier Rg (nm) (measured,
l = 11.50 ± 0.09
sample
analysis)
inverse transform)
nm)
0
.3 wt %
6.44 ± 0.03
6.66 ± 0.02
6.74 ± 0.06
PSBSS
consistent with the measured R , which proves the good
g
reliability of the fitting results. According to the fitting results,
the polyelectrolyte chain is highly rigid, which is determined by
the “jacketed” structure of PSBSS. In addition to the steric
hindrance, there is also electrostatic repulsion interaction
between the neighboring side chains in MJPEs such as
PSBSS. Because the charges are on both ends of the side
chains, the hydrophobic part of the side chains and the main
chain form the hydrophobic core of the cylinder.
In a dilute polyelectrolyte solution, salt added can screen the
electrostatic interaction. Figure 10 shows the SAXS profiles of
Figure 11. SAXS profiles and PDDFs of PSBSS aqueous solutions
with varying ionic strengths at a fixed PSBSS concentration of 0.60 wt
%
.
with increasing ionic strength, which was also observed in some
8
,9
literatures. The peak disappears when c /2c ≥ 0.6. Because c
s
p
p
≤
c*, the polyelectrolyte chain is extended as shown in Figure
1b. The diameters of the cylinders are still 4 nm, the same
p
1
Figure 10. SAXS log−log profiles of PSBSS aqueous solutions with
with that of the chain in the solution without salt. However, the
lengths of the cylinders (rmax values) decrease slightly, and the
tails of the PDDF curves are close to a linear decay, which may
be due to the screening of electrostatic repulsion between the
side chains that reduces chain swelling.
2
5
0.0 mM NaCl. Inset shows the corresponding Kratky plots (I(q)q vs
q).
PSBSS aqueous solutions with 50.0 mM NaCl. The
polyelectrolyte peak disappears when c < 2.00 wt % instead
Figure 12 shows the R values obtained from Guinier analysis
p
g
of c < 0.40 wt % without salt, which is related to the
and the fitting results of l by eq 1. According to the results
p
p
restoration of the osmotic compressibility of polyelectrolyte
with salt added. The conformational variation of chains in
solution can be analyzed from the Kratky plot (I(q)q vs q), as
from the Guinier plots (Figure S4 in Supporting Information),
the R value is almost constant when c /2c ≤ 1. However, R
g
s
p
g
2
abruptly increases when c /2c > 1, which indicates that the
s p
shown in the inset of Figure 10. The data for both the 0.20 and
aggregation of the polyelectrolyte chains occurs because the
hydrophobicity of the chains increases with increasing amount
of added salt. The polyion charge is neutralized by condensed
0
.40 wt % solutions increase monotonically, indicating that the
chain is dispersed in solution. However, for both the 1.00 and
48
2
1
.00 wt % solutions, the curves have a hump with a peak around
.5 nm . The distinct difference in the shape of the curves
or territorially bound counterions, according to Manning.
−1
49
Theory proposed by Kuhn, Kunzle, and Katchalsky suggests
that persistence length scales with the inverse of the square root
of the salt concentration. However, OSF theory shows that the
persistence length scales with the inverse of the salt
indicates the transition from dilute to semidilute regime.
Polyelectrolyte in salt solution starts to behave as a neutral
polymer.
50
In order to explore the added salt effect on the
polyelectrolyte conformation in solution, SAXS experiments
concentration. OSF theory assumes a locally stiff chain for
which electrostatic interactions can modify the bending
properties. The ionic effects on the elasticity of single DNA
on solutions with varied added salt concentrations (c ’s) which
s
2
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ACKNOWLEDGMENTS
■
This research was supported by the National Natural Science
Foundation of China (Grants 21174006 and 20990232).
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