Effects of dendron generation and salt concentration on phase structures of dendritic-linear block copolymers with a semirigid dendron containing PEG tails

Article abstract of DOI:10.1021/ma300654j

We prepared a series of dendritic-linear block copolymers (DLBCPs) bearing a semirigid Percec-type dendron with ionophilic poly(ethylene glycol) (PEG) tails and a polystyrene (PS) linear polymer by nitroxide-mediated living radical polymerization (NMRP).

Full text of DOI:10.1021/ma300654j

Article
pubs.acs.org/Macromolecules
Effects of Dendron Generation and Salt Concentration on Phase
Structures of Dendritic−Linear Block Copolymers with a Semirigid
Dendron Containing PEG Tails
Huanhuan Cai, Guoliang Jiang, Zhihao Shen,* and Xinghe Fan*
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
S
* Supporting Information
ABSTRACT: We prepared a series of dendritic−linear block
copolymers (DLBCPs) bearing a semirigid Percec-type
dendron with ionophilic poly(ethylene glycol) (PEG) tails
and a polystyrene (PS) linear polymer by nitroxide-mediated
living radical polymerization (NMRP). As the DLBCPs are
connected by an ester linkage, through hydrolysis the
molecular weights of the DLBCPs were precisely characterized
by gel permeation chromatography and MALDI-TOF MS.
Differential scanning calorimetry, small-angle X-ray scattering,
and transmission electron microscopy were used to investigate the phase behaviors of the DLBCPs. Results show that the PEG
tails of the semirigid dendron display a cold crystallization peak and a melting peak during the second heating process, while for
the neat DLBCPs, the crystallization of the PEG tails is completely inhibited, and only the glass transition temperature (Tg) of
the PS block is observed. However, Tg of the dendron block can be observed by complexing the DLBCPs with LiCF3SO3,
suggesting that microphase separation occurs in the doped DLBCPs. Comparing the phase behaviors of the DLBCPs having the
same dendron weight fraction (wD = 0.14) with varying dendron generation and salt concentration, we found that the G1 or G2
DLBCP undergoes a phase transition from a hexagonally packed cylinder structure to a lamellar structure with increasing content
of LiCF3SO3. However, the G3 DLBCP only displays a lamellar phase, and the lamellar thickness increases with increasing salt
concentration. The difference can be attributed to the different degree of chain branching, which leads to different interface
curvature.
groups. Combining dendrimer with linear polymers produces
dendritic−linear block copolymers (DLBCPs), which have
been proposed in 1993.5 Many previous publications devote to
the study of bulk and solution properties.6−15 Results have
shown that the branched molecular architecture of DLBCPs
largely shifts the phase boundary in comparison to linear
BCPs.16 Cho et al. have extensively investigated the self-
assembly behavior of amphiphilic dendrons extended with
linear crystallizable PEG chains and their ionic complexes by
changing the dendron generation or the length of the linear
block.17−21 The phase structures are closely related to
crystallization of the PEG segments. The phase behavior of
DLBCPs consisting of a third-generation coil dendron modified
with low molecular weight PEG and a coil linear block
polystyrene (PS) was also explored, and the morphology
changes from lamella (LAM) to a hexagonally packed cylinder
(HEX) structure as the salt content increases.
INTRODUCTION
■
Generally, block copolymers (BCPs) composed of two
chemically different, covalently bonded polymers can self-
assemble into various periodic nanostructures, such as lamella,
cylinder, gyroid, and sphere.1 The morphologies of the coil−
coil diblock BCPs are determined by the volume fraction f of
one block, the Flory−Huggins interaction parameter χ, and the
total degree of polymerization N. We can easily control the
degree of polymerization and vary the composition of the BCPs
chemically. Meanwhile, some methods have been used to
change the interaction parameter χ physically. For example,
selective coordination of ion salts to the ionophilic block
component such as poly(ethylene glycol) (PEG) derivatives
influences the sizes of the microphases and transition behaviors
in ion-doped BCPs which can be candidates as electrolytes in
electrochemical devices.2 In addition, ordered mesoporous
materials with high electrical conductivity have been synthe-
sized from coassembly of metal nanoparticles and block
copolymers.3
We designed a series of DLBCPs (PEG(Gm)-b-PS, where m
is the generation number), in which the periphery of the
semirigid Percec-type dendron blocks was used to introduce
Dendrimer has drawn considerable attention because of its
precise molecular weight (MW), unique three-dimensional
architecture, monodisperse character, and small hydrodynamic
radius.4 In addition, a large number of reactive groups at
periphery of the dendrimer can be modified with functional
Received: March 30, 2012
Revised: July 11, 2012
Published: July 23, 2012
© 2012 American Chemical Society
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different number of ionophilic PEG chains, while the linear PS
block and the semirigid dendron were used to maintain
membrane toughness. The DLBCPs may have applications as
conducting membranes, and chemical structures of the
DLBCPs are shown in Chart 1.
attributed to an increase in χ between the two block
components because of the selective coordination of LiCF3SO3
to the ionophilic PEG component.
EXPERIMENTAL SECTION
■
Materials. Traditional Percec-type benzyl ether dendrons were
synthesized as described in the literature.22 Triethylamine (Et3N),
tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were
distilled before use. CuBr was synthesized from CuSO4, NaBr, and
Na2SO3, purified by washing with acetic acid, followed by washing with
methanol, and then dried under vacuum. PEG with a molecular weight
of 750 g/mol and lithium trifluoromethanesulfonate (LiCF3SO3), both
obtained from Aldrich, were dried under vacuum overnight. All other
reagents were used as received from commercial sources.
Chart 1. Chemical Structures of DLBCPs
LiCF3SO3 Doping. A measured amount of DLBCPs was dissolved
in dry THF, followed by the addition of the appropriate amount of the
salt. The mixture was stirred for 24 h. Then the solvent was removed
under vacuum at 30 °C. The molar ratio of LiCF3SO3 to PEG(Gm)-b-
PS, denoted as r = [Li]/[EO], was varied from 0 to 1.
1
Characterization. H NMR spectra were recorded on a Bruker
300 or 400 MHz spectrometer. 13C NMR spectra were recorded on a
Bruker 400 or 500 MHz spectrometer. Elemental analysis (EA) was
carried out with an Elementar Vario EL instrument. Matrix-assisted
laser desorption ionization time-of-flight mass spectrometry (MALDI-
TOF MS) measurements were performed on a Bruker Autoflex high-
resolution tandem mass spectrometer. Gel permeation chromatog-
raphy (GPC) measurements were performed on a Waters 2410
instrument equipped with a Waters 2410 RI detector and three Waters
μ-Styragel columns (103, 104, and 105 Å), using THF as the eluent at a
We studied the phase transition behaviors of ion-doped
DLBCPs by exploring the influence of salt content and dendron
generation on the bulk phase behaviors of the DLBCPs with
the same dendron volume fraction. Our results show that the
morphologies transform from disordered to hexagonally packed
cylinders or lamellae as the salt content increases, which can be
Scheme 1. Synthetic Routes of Gm-X
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flow rate of 1.0 mL/min at 35 °C. All GPC data were calibrated with
linear polystyrene standards. Thermogravimetric analysis (TGA) was
performed on a TA SDT 2960 instrument at a heating rate of 20 °C/
min in a nitrogen atmosphere. Differential scanning calorimetry
(DSC) examination was carried out on a TA DSC Q100 calorimeter
with a programmed heating procedure in nitrogen. The sample had a
size of 2−5 mg and sealed in an aluminum pan. The temperature and
heat flow scale at different cooling and heating rates were calibrated
using standard materials such as indium. Glass transition temperatures
(Tg’s) were obtained from the second heating scans.
Small-angle X-ray scattering (SAXS) experiments were conducted
on a Bruker NanoStar U SAXS system using a Cu Kα radiation source
(λ = 0.154 nm at 40 kV and 35 mA). SAXS samples were sealed in
aluminum foil while acquiring scattering data under vacuum. One-
dimensional SAXS data are presented as Iq2 vs q, where q is the
azimuthally integrated intensity and q is scattering vecto (q = 4π sin θ/
λ, where 2θ is the scattering angle).
Transmission electron microscopy (TEM) micrographs were
obtained on a JEM-2100 electron microscope at an acceleration
voltage of 200 kV. The solution-cast sample films were stained with
RuO4 vapor for 3 min to enhance contrast. RuO4 reacted with both
blocks of the copolymer, and the PEG part was stained first.
The ionic conductivity of the polymer electrolyte film was measured
using a cell which was assembled by sandwiching the film between two
stainless steel disk electrodes. The polymer electrolyte film was
prepared by solution-casting in humid atmosphere before dried under
vacuum at 100 °C. The ionic conductivity (σ) values were determined
by electrical impedance spectrum measurements (EIS, EG&G
potentiostat/galvanostat model 283), and the samples were thermally
equilibrated at each temperature for 0.5 h before measurement. The
conductivity values can be calculated from the relationship σ = l/
(RbA), where Rb is the bulk electrolyte resistance, l is the thickness,
and A is the area of the film.
mixture was poured into ice/water (400 mL) and extracted with
CH2Cl2. The organic layer was separated, washed with brine (500
mL), dried over anhydrous Na2SO4, and evaporated to give the crude
product, which was purified by silica gel column chromatography using
petroleum ether/CH2Cl2 (1:1, v/v) as the eluent to afford 23.6 g of
1
G1-Br as a white powder. Yield: 89%. H NMR (300 MHz, CDCl3, δ,
ppm): 2.54 (t, J = 2.4 Hz, 2H, CCH), 4.67 (s, 2H, CH2Br,), 4.69 (d,
J = 2.4 Hz, 4H, CH2CH), 6.55 (s, 1H, p-Ar), 6.65(d, J = 2.4 Hz, 2H,
o-Ar).
General Procedure for Hydrolysis. G1-COOH. A procedure
similar to that reported in literature24 was employed. G1-COOMe was
dissolved in a mixture of 60 mL of THF, 10 mL of CH3OH, and 10
mL of distilled water. Addition of sodium hydroxide was followed by
stirring at 65 °C for 18 h. The pH of the mixture was adjusted to 3
using dilute aqueous HCl. The aqueous layer was separated and
extracted with CH2Cl2, washed with brine three times, and dried over
Na2SO4, and the solvent was removed to give the product as a pale
1
yellow powder. Yield: 100%. H NMR (400 MHz, DMSO, δ, ppm):
3.61 (t, J = 1.2 Hz, 2H, CCH), 4.85 (d, J = 1.2 Hz, 4H, CH2CH),
6.85 (s, 1H, p-Ar), 7.16 (d, J = 1.2 Hz, 2H, o-Ar). 13C NMR (100
MHz, DMSO, δ, ppm): 55.81 (2C), 78.61 (2C), 78.90 (2C), 106.97
(1C), 108.40 (2C), 132.91 (1C), 158.22 (2C), 166.77 (1C).
Synthesis of 1-(4′-Chloromethylphenyl)-1-(2″,2″,6″,6″-tetra-
methyl-1-piperidinyloxy)ethyl (TEMPO-Cl). The synthesis of
TEMPO-Cl was carried out according to the method reported in
the literature.25 TEMPO (1.25 g, 8 mmol) and 4-vinylbenzyl chloride
(1.8 g, 11.8 mmol) were dissolved in 10 mL of isopropyl alcohol in an
open flask. The solution was vigorously stirred, and Mn(salen)Cl
catalyst (357 mg, 1 mmol) was added, followed by the addition of
NaBH4 (492 mg, 13 mmol) in small portions for many times. After 24
h, the reaction was carefully quenched at 0 °C by addition of distilled
water, and then the reaction mixture was partitioned between
chloroform and distilled water. The organic layer was then dried
and evaporated to dryness, and the crude product was purified by silica
gel column chromatography using petroleum ether/CH2Cl2 (6:1, v/v)
as the eluent to afford the product (TEMPO-Cl) as a white crystal.
Synthesis of Acetylene-Terminated Dendrons. The synthetic
routes of the dendron intermediates (Gm-X, where m denotes the
generation number and X is the functional group at the focal point)
are depicted in Scheme 1. The experimental details are described
1
Yield: 41.2%. H NMR (300 MHz, CDCl3, δ, ppm): 0.66, 1.02, 1.16,
1
1.28 (s, 12H, CH3), 1.25−1.54 (m, 6H, CH2), 1.45 (d, J = 6.9 Hz, 3H,
CH3), 4.59 (s, 2H, CH2Cl), 4.80 (q, J = 6.6 Hz, 1H, CH), 7.26−7.34
(m, 4H, Ar).
below using G1-X as an example. Other H NMR data of dendron
intermediates are provided in the Supporting Information.
General Procedure for Alkylation. G1-COOMe. Potassium
carbonate (15.1 g, 109 mmol) and 18-crown-6 (0.1 g, 0.4 mmol)
were added to a stirred solution of propargyl bromide (29.7 g, 220
mmol) and methyl 3,5-dihydroxybenzoate (16.8 g, 100 mmol) in
acetone (300 mL). The reaction mixture was heated to reflux under
nitrogen for 24 h, filtered, evaporated to dryness, and partitioned
between water and CH2Cl2. The aqueous layer was then extracted with
dichloromethane (200 mL), and the combined extracts were dried
over Na2SO4, evaporated, and washed with methanol to give the
product as pale yellow crystals. Yield: 63%. 1H NMR (300 MHz,
CDCl3, δ, ppm): 2.54 (t, J = 2.4 Hz, 2H, CCH), 3.91 (s, 3H,
CH3O), 4.72 (d, J = 2.4 Hz, 4H, CH2CCH), 6.82 (s, 1H, p-Ar,),
7.30 (d, J = 2.4 Hz, 2H, o-Ar).
General Procedure for Esterification. G1-TEMPO. The synthesis
was similar to the method reported in the literature.26 Tetrabuty-
lammonium fluoride (TBAF) was added to a solution of G1-COOH
(1.1 mmol) and TEMPO-Cl (1 mmol) in dry DMF. The mixture was
stirred at 25 °C under N2 for 24 h, and it was then poured into
distilled water and extracted with CH2Cl2. The organic layer was
separated, washed with brine (500 mL), dried over anhydrous Na2SO4,
and evaporated to give the crude product, which was purified by silica
gel column chromatography using CH2Cl2 as the eluent to afford a
white powder. Yield: 89%. 1H NMR (400 MHz, CDCl3, δ, ppm): 0.66,
1.02, 1.16, 1.28 (s, 12H, CH3), 1.25−1.54 (m, 6H, CH2), 1.45 (d, J =
8.8 Hz, 3H, CH3), 2.54 (t, J = 3.2 Hz, 2H, CCH), 4.70(d, J = 3.2
Hz, 4H, CH2 CCH), 4.81 (q, J = 8.8 Hz, 1H, CH), 6.80 (s, 1H, p-
Ar), 7.34−7.39 (m, 6H, ArH). 13C NMR (125 MHz, CDCl3, δ, ppm):
17.32 (1C), 20.47 (1C), 23.72 (4C), 40.45 (2C), 56.26 (2C), 59.81
(2C), 66.98 (1C), 76.11 (2C), 78.05 (2C), 82.87 (1C), 107.48 (1C),
109.24 (2C), 126.85 (2C), 127.99 (2C), 132.38 (1C), 134.32 (1C),
146.10 (2C), 158.62 (2C), 165.92 (1C). Anal. Calcd for C31H37NO5:
C 73.93, H 7.41, N 2.78. Found: C 73.88, H 7.38, N 2.82. HRMS: m/z
504.273 89.
G2-TEMPO. This compound was prepared similarly according to the
procedure for the synthesis of G1-TEMPO. 1H NMR (400 MHz,
CDCl3, δ, ppm): 0.66, 1.02, 1.16, 1.28 (s, 12H, CH3), 1.25−1.54 (m,
6H, CH2), 1.45 (d, J = 6.4 Hz, 3H, CH3), 2.53 (t, J = 1.6 Hz, 4H, C
CH), 4.68 (d, J = 1.6 Hz, 8H, CH2 CCH), 4.80 (q, J = 6.8 Hz, 1H,
CH), 5.03 (s, 4H, ArCH2O), 6.58−7.39 (m, 13H, Ar). 13C NMR (100
MHz, CDCl3, δ, ppm): 17.34 (1C), 20.51 (1C), 23.74 (4C), 40.47
(2C), 56.09 (4C), 59.83 (2C), 66.96 (1C), 70.11 (2C), 75.94 (4C),
78.37 (4C), 82.86 (1C), 102.05 (2C), 107.01 (1C), 107.34 (4C),
108.83 (2C), 126.90 (2C), 128.04 (2C), 132.30 (1C), 134.37 (1C),
General Procedure for Reduction. G1-CH2OH. Lithium
aluminum hydride (3.99 g, 105 mmol) was added to a stirred solution
of the ester G1-COOMe (20.6 g, 84.4 mmol) in anhydrous THF (500
mL) in small portions, and the reaction mixture was stirred at ambient
temperature for 2 h. Water was then added slowly to stop the reaction.
The reaction mixture was filtered under vacuum, the solid rinsed with
CH2Cl2, and the filtrate dried with MgSO4. After evaporation of the
solvents, the alcohol was purified by recrystallization from methanol
1
and recovered as white crystals. Yield: 90%. H NMR (300 MHz,
CDCl3, δ, ppm): 2.53 (t, J = 2.4 Hz, 2H, CCH), 4.65 (s, 2H,
CH2OH), 4.67 (d, J = 2.4 Hz, 4H, CH2CCH), 6.54 (s, 1H, p-Ar),
6.63 (d, J = 2.1 Hz, 2H, o-Ar).
General Procedure for Bromination. G1-Br. The synthetic
procedure was similar to that reported in literature.23 Phosphorus
tribromide (27.1 mL, 280 mmol) was added dropwise to a stirred
solution of the alcohol G1-CH2OH (20.56 g, 95 mmol) in anhydrous
CH2Cl2 (150 mL) at 0 °C under argon. Stirring was continued at 0 °C
for 30 min and then at ambient temperature for 2 h. The reaction
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Scheme 2. Synthetic Routes of Dendritic Macroinitiators
139.12 (1C), 146.10 (2C), 159.01 (2C), 159.69 (4C), 166.18 (1C).
Anal. Calcd for C51H53NO9: C 74.34, H 6.48, N 1.70. Found: C 73.93,
H 6.62, N 1.66. HRMS: m/z 824.381 14.
with brine (100 mL) and distilled water and dried with anhydrous
Na2SO4, and then the solvents were evaporated to yield PEG-N3.
Yield: 90%. 1H NMR of PEO-N3 (300 MHz, CDCl3, δ, ppm): 3.38 (s,
3H, CH3O), 3.46 (t, J = 3.3 Hz, 2H, CH3OCH2), 3.54 (t, J = 4.8 Hz,
2H, CH2CH2N3), 3.66 (s, 450H, CH2CH2), 3.82 (t, J = 4.5 Hz, 2H,
CH2CH2N3).
Preparation of Dendritic Macroinitiators. The azide-termi-
nated PEG, the dendritic initiator precursor, CuBr, and N,N,N′,N″,N″-
pentamethyldiethylenetriamine (PMDETA) were added to a polymer-
ization tube with dry DMF as solvent. After three freeze−pump−thaw
cycles, the tube was sealed off under vacuum. Then the resulting
solution was stirred at ambient temperature for 24 h. The tube was
opened, and the reaction mixture was diluted with THF. CuBr was
removed through a short neutral alumina column. DMF solvent was
removed under a reduced pressure, and then the product was dissolved
in 80 mL of CH2Cl2. The mixture was extracted sequentially with brine
and distilled water, dried with anhydrous Na2SO4, and evaporated to
yield the dendritic macroinitiator PEG(G3)-TEMPO.
Synthesis of Block Copolymers. All DLBCPs (PEG(Gm)-b-PS)
were obtained by bulk polymerization. For example, 2.62 g (25 mmol)
of styrene and 0.21 g (0.03 mmol) of PEG(G3)-TEMPO were placed
in a polymerization tube with a magnetic stir bar. After three freeze−
pump−thaw cycles, the tube was sealed off under vacuum.
Polymerization was carried out at 125 °C for 36 h. The tube was
then opened, and the reaction mixture was diluted with THF. The
resultant polymer was precipitated and washed with methanol. To
eliminate the unreacted monomers and residual dendritic macro-
initiator, the precipitate was redissolved in THF and then
reprecipitated and washed with methanol until there were no peaks
corresponding to the monomer and the initiator in the GPC profiles.
After purification, the polymer was dried to a constant weight. Yield:
45%. The molecular weight of PEG(G3)-b-PS DLBCP is 3.41 × 104 g/
mol. By controlling the molar ratio of the dendritic macroinitiator to
the styrene monomer, DLBCPs of varying dendron volume fractions
were synthesized.
G3-TEMPO. This compound was prepared similarly according to the
procedure for the synthesis of G1-TEMPO. 1H NMR (400 MHz,
CDCl3, δ, ppm): 0.68, 1.02, 1.15, 1.45, 1.47 (s, 21H, CH3), 2.51 (t,
8H, CCH), 4.66 (d, J = 1.6 Hz, 16H, CH2CCH), 4.79 (q, J = 6.8
Hz, 1H, CH), 4.99 (2s, 12H, ArCH2O), 6.56−7.39 (m, 25H, Ar). 13
C
NMR (100 MHz, CDCl3, δ, ppm): 17.33 (1C), 20.50 (1C), 23.74
(4C), 40.46 (2C), 56.07 (8C), 59.82 (2C), 66.97 (1C), 69.93 (6C),
75.92 (8C), 78.43 (8C), 82.86 (1C), 101.93 (6C), 106.67 (1C),
106.93 (12C), 108.72 (2C), 126.89 (2C), 128.07 (2C), 132.29 (1C),
134.39 (1C), 138.98 (1C), 146.09 (6C), 158.97 (6C), 159.76 (8C),
166.21 (1C). Anal. Calcd for C91H85NO17: C 74.62, H 5.85, N 0.96.
Found: C 74.56, H 5.95, N 1.02. HRMS: m/z 1464.591 12.
Synthesis of Dendritic Macroinitiators. The synthetic routes
are outlined in Scheme 2, with PEG(G3)-TEMPO as an example.
Preparation of Azide-Terminated PEG (PEG-N3). The synthesis
was performed according to the method reported in the literature.27
Both poly(ethylene glycol) methyl ether (Mn = 750 g/mol; 20.4 g, 27
mmol) and triethylamine (9.64 mL, 66 mmol) were completely
dissolved in CH2Cl2 (500 mL) under a N2 atmosphere. Toluene-4-
sulfonyl chloride (7.9 g, 40 mmol) was added dropwise to the above
solution in an ice/water bath, and then the resulting solution was
stirred at ambient temperature for 24 h. Then the mixture was filtered
under vacuum, and the liquid was dissolved in CH2Cl2, washed with
brine (500 mL) three times, dried over anhydrous Na2SO4, and
1
evaporated to give the monotosylated PEG. Yield: 95%. H NMR of
PEG-Ts (300 MHz, CDCl3, δ, ppm): 2.45 (s, 3H, ArCH3), 3.38 (s,
3H, CH3O), 3.46 (t, J = 3.3 Hz, 2H, CH3OCH2), 3.58 (t, J = 4.8 Hz,
2H, CH3OCH2CH2), 3.64 (s, 450H, CH2CH2), 3.82 (t, J = 4.8 Hz,
2H, CH2CH2OTs), 4.16 (t, J = 4.8 Hz, 2H, CH2CH2OTs), 7.36 (d, J =
8.1 Hz, 2H, o-Ar), 7.81(d, J = 8.4 Hz, 2H, p-Ar). Subsequently, sodium
azide (298.0 mg, 4.58 mmol) was added to a solution of the obtained
PEG monotosylate (406.0 mg, 0.46 mmol) in dry DMF (10 mL)
under a N2 atmosphere, and the reaction mixture was stirred
vigorously at ambient temperature for 24 h. The product was
dissolved in 80 mL of CH2Cl2. The mixture was extracted sequentially
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1
Figure 1. H NMR spectra of PEG(G3)-TEMPO (a) and G3-TEMPO (b) in CDCl3.
Table 1. Molecular Characteristics of All the Polymers
a
b
c
d
e
a
sample
Mn (×104 g/mol)
PDI
Td (°C)
Tg (°C)
wD
wPEG
Mn,PS (×104 g/mol)
PDIPS
PEG(G1)-b-PS
PEG(G2)-b-PS
PEG(G3)-b-PS
1.30
2.07
3.41
1.09
1.18
1.23
328
353
334
75.4
95.8
98.4
0.14
0.18
0.16
0.11
0.13
0.13
1.17
1.83
3.89
1.09
1.18
1.18
a
b
The apparent number-average molecular weight (Mn) and polydispersity index (PDI) were measured by GPC using PS standards. The
c
temperatures at 5% weight loss of the samples under nitrogen (Td’s) were measured by TGA heating experiments at a rate of 20 °C/min. Evaluated
by DSC during the second heating cycle at a rate of 20 °C/min. Mass fraction of the dendritic block. Mass fraction of PEG.
d
e
(Figure S4) and MALDI-TOF MS (Figure S5). Meanwhile, the
RESULTS AND DISCUSSION
■
molecular weight of the linear PS block can be precisely
determined through GPC as all GPC data are calibrated with
polystyrene standards (Figure S6). The PEG weight fractions
and the thermal stability data of the DLBCPs are listed in Table
1. The weight fractions of the dendrons in the DLBCPs are all
about 0.14−0.18.
DSC was used to investigate the thermal transitions of the
dendritic macroinitiators and the neat and doped DLBCPs,
with the results of PEG(G3)-b-PS shown in Figure 2 as an
example. All other results are shown in the Supporting
Information and are summarized in Table 1. DSC scans
(Figure 2a) of the second heating process at a rate of 20 °C/
min show that the dendritic macroinitiator displays a glass
transition at −49.8 °C, a cold crystallization peak at −15.6 °C,
and a melting peak at 27.0 °C. With the consideration of the
composition of the dendritic macroinitiator, these peaks
correspond to transitions of the PEG tail. For the neat
DLBCP, the crystallization peak of PEG disappears completely,
and only a glass transition at 98.4 °C of the PS block is
observed. With the lithium salt added, two distinct transitions
at −28.6 and 99.3 °C are observed, corresponding to the glass
Synthesis and Characterization of Dendritic Macro-
initiators and Polymers. With PEG(G3)-TEMPO as an
example, the dendritic macroinitiators were prepared with the
synthesis of the acetylene-terminated dendrons, followed by
esterification reaction to introduce TEMPO group at the focal
point and the Huisgen’s 1,3-dipolar cycloaddition reaction to
modify the periphery of dendron with ionophilic PEG.28 The
structures of the precursors of the dendritic macroinitiators
1
were confirmed by H NMR, MS, and EA. The disappearance
of the resonance signals of the triplet with δ = 2.51 ppm arising
from the proton of the terminal alkyne moiety verifies the
completion of the coupling reaction, and a new peak at 7.78
ppm corresponding to the triazole ring is observed. Meanwhile,
the ratio of the area of the triazole proton signals to that of the
benzyl group signals in 1H NMR is calculated to be 4:1 (Figure
1).
DLBCPs were prepared through NMRP at 125 °C for 24 h.
Their compositions were determined by GPC and MALDI-
TOF MS. As the two blocks are connected through an ester
group, the DLBCPs can hydrolyze into two blocks. Molecular
weights of the dendritic macroinitiators were measured by GPC
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Figure 2. DSC second-heating thermograms of the dendritic
macroinitiator PEG(G3)-TEMPO and the DLBCP without and with
doped LiCF3SO3 (r = 0.05) (a) and those of the DLBCPs with
different concentrations of the lithium salt (b).
Figure 3. SAXS profiles of the PEG(G1)-b-PS DLBCP with doped
LiCF3SO3 (r = 1) during the heating (a) and cooling (b) processes.
The curves at different temperatures are shifted in the vertical
direction for clarity.
transitions of the doped PEG peripheries and the linear PS
block, respectively. The appearance of Tg of the dendritic block
suggests that adding the lithium salt causes the DLBCPs to
microphase separate. Meanwhile, Tg increases with increasing
salt concentration (Figure 2b). This is attributed to increased
ion-dipole interaction between Li+ and the ether oxygens which
results in decreased segmental mobility and stiffening of the
chain. A similar phenomenon was observed for polyether−
siloxane hybrids doped with LiClO4.29
Effect of Salt Concentration on Polymer Morphology.
Variable-temperature SAXS experiments were performed up to
160 °C, well above the glass transition temperatures of PS and
the dendron, to investigate the morphologies of the samples.
The molar ratio of LiCF3SO3 to EO in PEG(Gm)-b-PS was
varied to observe the effect of lithium salt concentration on
phase behavior. The lithium concentration per ethylene oxide
([Li]/[EO]) was chosen to be 0, 0.02, 0.05, 0.1, and 1. Figure 3
shows the SAXS patterns of the LiCF3SO3-doped PEG(G1)-b-
PS (r = 1) sample obtained during the heating and cooling
processes. Upon heating high-order reflections develop
gradually (Figure 3a), and at 160 °C the SAXS pattern shows
multiple reflections with a q ratio of 1:2:3:4, which is
characteristic of a highly ordered lamellar structure. The
intensities of the high-order reflections decrease after cooling
(Figure 3b).
concentration dependence of phase behavior. For PEG(G1)-
b-PS, where r = 0, no scattering peaks are seen in the SAXS
profile, indicating that the neat polymer is disordered.
Incorporation of small amounts of LiCF3SO3 into disordered
PEG(G1)-b-PS, as shown in Figure 4a for r = 0.02, the SAXS
pattern is essentially identical to the undoped DLBCP,
indicating that the system remains disordered. When r =
0.05, a strong scattering peak is developed, indicating that an
increase in salt concentration leads to microphase separation.
However, higher-order reflections are not detected. Increasing
molar ratio of [Li] to [EO] to r = 0.1, two more peaks appear
in addition to a sharp scattering peak located at q* = 0.426
nm−1 in the SAXS pattern, with a q ratio of 1:31/2:2, which is
characteristic of a two-dimensional HEX structure. From the
primary reflection, the d100 value is determined to be 14.7 nm
(corresponding to an intercolumn distance of 17.0 nm). As the
dendritic block has a weight fraction of 0.14, it forms the core
(with PEG in the center) in the HEX structure and is
encapsulated by the PS block. As r increases further to 1, the
SAXS profile indicates a lamellar morphology with the first-
order peak q value of 0.267 nm−1. The primary peak moves
toward lower q as the amount of LiCF3SO3 increases, as shown
in Figure 4a. TEM experiments were conducted to confirm the
morphologies. Figure 5a,b shows cylinder morphologies viewed
parallel and perpendicular to the cylinder axis. The column-to-
column distance is estimated to be ∼16 nm, consistent with the
SAXS data. Alternating layers are seen when r = 1 for
PEG(G1)-b-PS in Figure 5c.
We also compared the phase behaviors of the DLBCPs with
different salt concentrations at 160 °C, which is far above their
Tg’s. All the SAXS profiles of LiCF3SO3-doped PEG(Gm)-b-PS
at r = 0 to 1 measured at a constant temperature of 160 °C are
displayed in Figure 4. These profiles show lithium salt
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Figure 5. TEM micrographs of lithium-doped PEG(G1)-b-PS when r
= 0.1 (a, b) and 1 (c).
domains (Figure 6a). When r = 0.1 and 1, alternating PEG and
PS layers (Figure 6c,d) are observed.
Figure 4. SAXS profiles of the blends of DLBCPs PEG(G1)-b-PS (a),
PEG(G2)-b-PS (b), and PEG(G3)-b-PS (c) with different contents of
LiCF3SO3. The curves at different salt contents are shifted in the
vertical direction for clarity.
Figure 6. TEM micrographs of lithium-doped PEG(G2)-b-PS when r
= 0.05 (a, b), 0.1 (c), and 1 (d).
Figure 4b shows the SAXS profiles of PEG(G2)-b-PS doped
with lithium salt of similar contents. The results are similar to
those of the G1 DLBCP. Again, the neat polymer is disordered.
When r = 0.02, the scattering peak developed gradually. When r
= 0.05, the SAXS profile exhibits three distinctive peaks located
at q*, 1.73q*, and 2 q*, indicative of a HEX morphology. From
the primary peak, the intercolumn distance is calculated to be
26.8 nm. When r = 0.1 and 1, the SAXS patterns display
multiple reflections with q ratios of 1:2:3 and 1:2:3:4,
respectively, consistent with lamellar structures. TEM experi-
ments were also conducted to further confirm the micro-
structures and micrographs at different lithium concentrations
were obtained. The results are in agreement with the SAXS
data. When r = 0.05, the TEM micrograph shows hexagonal
patterns of dark PEG domains surrounded by bright PS
Figure 4c shows the SAXS profiles of PEG(G3)-b-PS doped
with lithium salt. However, the SAXS patterns are different
from those of the G1 and G2 polymers. The profile of the neat
polymer shows a primary reflection, indicating a microphase-
separated system. Increasing r to 0.02 and up to 1, the G3
DLBCP blends demonstrate lamellar structures with a shift in
the primary peak position, revealing that the lamellar spacing
increases with increasing salt concentration. The phenomenon
was confirmed by TEM results shown in Figure 7.
The effect on the microstructures induced by the addition of
LiCF3SO3 to the DLBCPs can be mainly attributed to changes
in the interaction parameter χ that influences phase behavior.
The results show that adding LiCF3SO3 increases the degree of
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r = 0.05, the G1 DLBCP blend is still disordered but shows a
scattering peak at 0.456 nm−1 and the G2 DLBCP blend shows
a HEX structure, while the G3 DLBCP blend retains a lamellar
structure. As the salt content increases further to r = 0.1, the G1
DLBCP blend forms a HEX structure, while the G2 blend
transforms into a lamellar structure. Meanwhile, the G3 DLBCP
blend is still lamellar, with an increased layer spacing. When r =
1, the blends of all Gm DLBCPs form lamellar structures. The
above discussion indicates that the blends of G1 and G2
DLBCPs both undergo phase transitions from HEX morphol-
ogies to lamellar structures with increasing content of
LiCF3SO3, while the G3 DLBCP blends only form lamellar
structures.
The morphological differences between samples with
different dendron generations at equal dendron weight fractions
are strongly attributed to the difference in the degree of chain
branching. As discussed by Fredrickson, the phase boundaries
are significantly shifted toward lower dendron volume fraction
as the number of generation or the functionality of the branch
points increases.32 Therefore, for the DLBCPs investigated in
this work, the blends of G1 and G2 DLBCPs both undergo
transformations from HEX to LAM, while the G3 DLBCP
blend retains LAM for all salt concentrations. The results agree
well with the theory. We speculate that further reducing the
dendron volume fraction of the G3 DLBCP may cause the
system to form a HEX morphology.
Figure 7. TEM micrographs of lithium-doped PEG(G3)-b-PS when r
= 0.02 (a), 0.05 (b), 0.1 (c), and 1 (d).
segregation between the dendritic and linear blocks, driving the
material toward the strong-segregation regime.30,31
Effect of Dendron Generation on Polymer Morphol-
ogy. Besides the volume fraction, the dendron generation is a
crucial parameter that influences phase behavior, elucidating the
role of shape-induced interface curvature in the microstructure
formation. To simplify the discussion, we assume that f is
constant when the salt concentration is the same for DLBCPs
of different generations with an equal weight fraction.
Conductivity of PEG(G3)-b-PS (r = 1). We measured the
conductivity of lithium-doped PEG(G3)-b-PS with r = 1.
Normal-to-plane conductivity data were obtained at three
different temperatures. The impedance spectra at different
temperatures are shown in Figure 9. The thickness of the film is
300 nm, and the area is 1.8 × 10−4 m2. The conductivity value
−7
−5
is calculated to be 9.1 × 10 S/cm at 30 °C and 1.0 × 10
Figure 8 shows the phase diagram using the SAXS results of
S/cm at 75 °C. As shown in Figure 9b, the ionic conductivity
increases gradually upon heating. This behavior is consistent
with higher ionic and segmental mobilities at higher temper-
atures. Ionic conductivities for polymers at other lithium
concentrations were not measured because of poor film-
forming ability and insufficient amount of samples for the
measurements. However, through subtle structural modifica-
tions in the linear block of the DLBCPs, polymer electrolyte
with good mechanical and film forming properties may be
developed.
lithium salt-doped DLBCPs with [Li]/[EO] in the range 0−1.
CONCLUSIONS
■
In summary, we have prepared a series of dendritic−linear
block copolymers with an equal dendron volume fraction. The
dendritic block is composed of a semirigid Percec-type
dendron, and the periphery of the dendron is ionophilic
PEG. The linear block was synthesized by NMRP. Their
molecular characteristics were determined by NMR, GPC, and
MALDI-TOF MS as the connection point of the two blocks
can be hydrolyzed. The thermal properties and phase behaviors
of LiCF3SO3-doped PEG(Gm)-b-PS DLBCPs as a function of
lithium salt concentration and dendron generation were
studied. Doping the salt drives microphase separation toward
the strong-segregation limit because of the increase in the
effective Flory−Huggins interaction parameter χ between the
two blocks. Studies on salt concentration dependence of phase
behavior show disordered, HEX, and LAM morphologies for
blends of G1 and G2 DLBCPs while G3 DLBCP blends display
lamellar structures throughout the whole salt concentration
range studied. The difference may originate from the different
Figure 8. Phase diagram of DLBCPs depending on dendron
generation and salt concentration.
The y-axis is the generation of the dendritic block. The phase
diagram is composed of three parts. The first part at lower right
is the disordered morphology for neat polymers and blends at
low lithium salt contents. When the lithium salt concentration
increases, microphase separation begins to occur. And the
critical salt content to induce microphase separation decreases
with increasing dendron generation. When r = 0.02, the three
blends of different generations show different self-assembly
behaviors. The blends of G1 and G2 DLBCPs are disordered,
while the G3 DLBCP blend exhibits a lamellar structure. When
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■
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* Supporting Information
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before and after hydrolysis, DSC thermograms of G1 and G2
DLBCPs without and with doped LiCF3SO3 of different
concentrations. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: fanxh@pku.edu.cn (X.F.); zshen@pku.edu.cn (Z.S.).
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was supported by the National Natural Science
Foundation of China (Grants 21134001 and 20974002).
■
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