Phase transformation and self-assembly behavior of supramolecular rod-comb block copolymers

Article abstract of DOI:10.1016/j.polymer.2014.01.056

We investigated the self-assembly behavior of a series of supramolecular rod-comb block copolymer complexes made by the hybridization of rod-coil diblock copolymers of poly (2,5-di(2-ethylhexyloxy)-1,4-phenylene vinylene)-b-poly(2- vinyl pyridine) (PPV-b-

Full text of DOI:10.1016/j.polymer.2014.01.056

Polymer 55 (2014) 2481e2490
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Phase transformation and self-assembly behavior of supramolecular
rodecomb block copolymers
,
Yi-Huan Lee a b, Chun-Jie Chang a, Yi-Lung Yang a, Chi-Ju Chiang b, Yu-Ping Lee b,
,b,
Ching Shen b, Kang-Ting Tsai c, Yi-Fan Chen d, Chi-An Dai a
*
a Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
b Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
c Program of Landscape and Recreation, National Chung Hsing University, Taichung 40227, Taiwan
d Department of Chemical and Materials Engineering, National Central University, Jhongli 32001, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
We investigated the self-assembly behavior of a series of supramolecular rodecomb block copolymer
complexes made by the hybridization of rodecoil diblock copolymers of poly (2,5-di(2-ethylhexyloxy)-
1,4-phenylene vinylene)-b-poly(2-vinyl pyridine) (PPV-b-P2VPf) with different volume fractions, f, of the
P2VP coils and an anionic surfactant, dodecyl benzenesulfonic acid (DBSA), that selectively interacts with
the P2VP to form the side-chain comb teeth. The resulting hybrids show hierarchically ordered structures
at multiple length scales, forming so-called structure-in-structure morphology. Notably, for PPV-b-
P2VP0.56(DBSA)x, the larger-scale structure changes from a lamellar phase, to a broken lamella, and
eventually to a hexagonally packed strip phase with increasing DBSA molar ratio (x) to P2VP monomer
unit. Furthermore, simultaneous SAXS and WAXS measurements showed that the order-disorder tran-
sition temperatures of larger-scale structures in the PPV-P2VP0.56(DBSA) rodecomb systems were higher
than those associated with the pristine PPV-P2VP0.56 polymers. The large-scale structure of PPV-
P2VP0.56(DBSA) exists at temperatures around 210 ꢀC even though the roderod interaction between PPV
blocks disappear at w120 ꢀC, signifying that the formation of the P2VP(DBSA) lamellar mesophase plays
a critical role in forming the large-scale hexagonally packed strip structures.
Received 15 November 2013
Received in revised form
17 January 2014
Accepted 31 January 2014
Available online 14 March 2014
Keywords:
Macroterminator
Rodecomb block copolymer
Hierarchical structure
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, p-conjugated rodecoil block copolymers have received
a great deal of attention in materials science and nanotechnology
[7e15] since they can be used as active materials in organic elec-
tronics such as organic field-effect transistors (OFETs) [16,17] and
Block copolymers have now emerged as a new class of impor-
tant smart materials for both academic study and industrial
application over the past decades owing to their ability to self-
assemble into structures with feature sizes ranging from several
nanometers to hundreds of nanometers [1e6]. Traditional coilecoil
block copolymers can form a variety of ordered morphologies
depending on several parameters such as the volume fraction (f),
organic photovoltaic cells (OPV) [18,19]. The p-conjugated rigid-rod
backbone may induce novel microstructures that are significantly
different from those of conventional random coil-based block co-
polymers. It was found that the phase behavior of a rodecoil system
is governed not only by the aforementioned molecular parameters
FloryeHuggins interaction parameter (
c
), and the degree of poly-
of f,
the rod blocks characterized by the MaiereSaupe interaction
parameter, , as well as by the molecular packing between the rod
and coil blocks [20,21]. Therefore, the interplay between these pa-
rameters maycreate morphologies that are different from that of the
classical coilecoil block copolymer system such as zig-zag, wavy
lamellar, arrow-head [22,23], smectic-A like lamellar [24], hexago-
nal strip [25e27], and puck phases. [27e29].
c, and N, but also by the liquid crystalline interaction between
merization (N) of the copolymers. Typical ordered morphologies for
the coilecoil diblock copolymers include body-centered-cubic
packed spheres (BCC), hexagonally packed cylinders (HPC), bicon-
tinuous gyroids (G), and the lamellar phase (L).
m
* Corresponding author. No. 1, Roosevelt Rd. Sec. 4, Department of Chemical
In addition, a new class of comb-shaped supramolecules con-
sisting of flexible or mesogenic side chains that bind to the
Engineering, National Taiwan University, Taipei 10617, Taiwan. Tel.: þ886
2
33663051; fax: þ886 2 23623040.
E-mail address: polymer@ntu.edu.tw (C.-A. Dai).
http://dx.doi.org/10.1016/j.polymer.2014.01.056
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

2482
Y.-H. Lee et al. / Polymer 55 (2014) 2481e2490
backbone polymer chains via ionic bonding, hydrogen bonding, or
metal-mediated coordination bonding has recently been fabricated
[30e42]. By having one particular block of a diblock copolymer
selectively interacts with surfactants or oligomeric amphiphiles,
complicated hierarchical supramolecular structures of the resulting
hybrid may be observed, showing ordered morphologies at multi-
ple length scales. One advantage of using the diblock copolymer to
form the hierarchical structure is to facilitate the formation of the
comb-shaped architecture via the supramolecular approach in
which the short chain molecules are attached to the copolymer
backbone. The self-assembly behavior of the supramolecular
combecoil complex block copolymers with hierarchical structures
have received significant attention due to its potential for use as a
new approach for fabricating functional materials in electrical,
optical, and other applications [43e50]. In hierarchical combecoil
systems, the larger self-assembled structure that results from the
repulsive interaction between the comb and coil blocks usually has
the domain size in the range of several tens of nanometers, while
the smaller structure is formed through the polar-nonpolar repul-
sion interaction between the backbone and side chains of the comb
block that induces the formation of a lamellar mesophase with a
characteristic length on the order of several nanometers.
ethylhexyloxy)-1,4-phenylene vinylene) (DEH-PPV) chains and on
the formation of the resulting microstructures.
2. Experimental section
2.1. Materials
A macroterminator of aldehyde end-functionalized DEH-PPV
was synthesized by Seigrist polycondensation as shown in Scheme
1 [53,55]. The synthesis routes and characterization of DEH-PPV are
described in the Supporting Information. All reactions were carried
out under purified nitrogen. Ultrahigh purity nitrogen was pur-
chased (purity of 99.9995%) and further purified by passing it
through a column of molecular sieves and a BTS (Fluka) catalytic
oxygen trap. Tetrahydrofuran (Mallinckrodt, 99%, THF) was freshly
distilled after being refluxed with excess sodium metal and ben-
zophenol (Acros, 99%) for use as an indicator for dryness. sec-
Butyllithium (Chemetal, 1.3 M in cyclohexane) was used as an
initiator for the anionic polymerization. 2-Vinyl pyridine (2VP,
Acros, 97%) monomer was purified by stirring the monomer in a
flask with calcium hydride for several days followed by purification
using a freeze-thaw technique with trioctyl aluminum (Aldrich,
25% solution in hexane) as the drying agent before polymerization.
Dodecylbenzene sulfonic acid (DBSA, Fluka, 90%) was dried in
vacuum at 60 ꢀC for 24 h before use.
The self-assembly behavior of the supramolecular comb-shaped
complexes formed by coilecoil diblock copolymers with different
architectures such as linear, heteroarm star, and block-arm star
copolymers has been thoroughly studied [32e35]. In the present
study, we are interested in studying a supramolecular rodecomb
block copolymer complex consisting of an amphiphilic surfactant
2.2. Synthesis of poly(2,5-di(20-ethylhexyloxy)-1,4-phenylene
vinylene)-block-poly(2-vinylpyridine) (PPV-b-P2VP)
and a
between the rod blocks offers a different microphase separation
behavior since the strong interaction of the rod blocks with
p-conjugated rodecoil block copolymer. The pep interaction
The synthesis procedure for the PPV-b-P2VP is shown in
Scheme 2. Anionic polymerization of 2-vinylpyridine was per-
formed in THF. sec-Butyllithium was used to initiate the 2VP to form
poly(2-vinylpyridine) anions at ꢁ78 ꢀC, and an aliquot of poly(2-
vinylpyridine) homopolymer was collected and terminated with
dry methanol to measure its molecular weight and polydispersity.
The anionic polymerization was allowed to react for 3 h. A mac-
roterminator of aldehyde end-functionalized DEH-PPV (1.5 equiv)
was dried under vacuum at room temperature overnight, then
dissolved in THF, and transferred to couple with the living P2VP
anions. The obtained PPV-b-P2VP block copolymers were purified
using either the precipitation or extraction method.
pep
liquid crystalline ordering characteristics often dominates the
phase behavior of rodecoil block copolymer systems, leading to the
formation of a lamellar phase over a wide range of copolymer
composition which results in a highly asymmetric phase diagram
[51e54]. Hence, the liquid crystalline interaction may also have
considerable effects on the phase behavior of the supramolecular
rodecomb complexes compared to that of the coilecomb com-
plexes. This may result in interesting and novel, self-organized
morphologies, in addition to phase transitions.
Therefore, the objective of the current study is to investigate
how the side-chain comb teeth molecules influence the self-
assembled structure and phase transitions of rodecomb block
copolymers. We present a novel rodecomb block copolymer sys-
tem by blending an anionic surfactant, dodecyl benzenesulfonic
acid (DBSA) with a rodecoil block copolymer of poly(2,5-di(2-
ethyl hexyloxy)-1,4-pheny-lenevinylene)-block-poly(2-vinyl pyri-
dine) (PPV-b-P2VP). Two different rodecoil block copolymers of
PPV-b-P2VPf¼0.3 and PPV-b-P2VPf¼0.56, in which f is the volume
fraction of P2VP block in the neat copolymers, were used to study
the effects of copolymer composition, the roderod interaction
between PPV blocks, and the amount of DBSA, on their resulting
morphology. In particular, the effect of temperature on the self-
assembly and phase transformation of the rodecomb block
copolymer was studied. In contrast to the supramolecular comb-
shaped complexes formed by the coilecoil diblock copolymer
systems, different morphological transitions were found in the
series, which are strongly related to their roderod interactions. By
a combination of transmission electron microscopy (TEM) and
simultaneously measured small-angle and wide-angle X-ray
scattering (SAXS and WAXS) measurements on the PPV-b-
P2VP(DBSA)x supramolecular complexes (with x denoting the
average number of DBSA molecules bound with 2-vinyl pyridine
monomer unit), we discuss in detail the effect of DBSA composi-
tion on the roderod interaction between poly(2,5-di(20-
PPV-b-P2VP0.3 1H NMR (400 MHz, CDCl3, ppm):
d 8.20 (d, 1.66H,
Ar eN]CHe), 7.43e7.17 (m, 5.66H), 6.79 (br, 1.66H), 6.30 (br, 1.66H,
eCH3e), 3.88e3.64 (m, 4H, eOCH2e), 2.29e1.27 (m, 22.97H), 0.85
(br, 12H). PDI: 1.12.
PPV-b-P2VP0.56 1H NMR (400 MHz, CDCl3, ppm):
d 8.20 (d, 4.77H,
Ar eN]CHe), 7.44e7.17 (m, 8.77H), 6.79 (br, 4.77H), 6.29 (br,
4.77H), 3.87e3.62 (m, 4H, eOCH2e), 2.31e1.29 (m, 32.30H), 0.87
(br, 12H, eCH3). PDI: 1.12.
2.3. Characterization
The molecular weight distribution of the synthesized block co-
polymers were measured by using a GPC (Waters 2695) equipped
with two Styragel columns (HR3 and HR4E), a refractive index
detector (Waters 2414) and a photodiode array absorbance detector
(Waters 2996). THF was used as the mobile phase at a flow rate of
1 mL/min and monodispersed polystyrene standards (Pressure
Chemicals) were used for system calibration. The synthesized block
copolymers were dissolved in CDCl3 and their 1H NMR spectra
were recorded on a 400 MHz Bruker Avance spectrometer at room
temperature. NMR spectra were reported in ppm. Splitting patterns
were designated as d (doublet), m (multiplet), and br (broad
resonance).

Y.-H. Lee et al. / Polymer 55 (2014) 2481e2490
2483
Scheme 1. Synthetic routes of aldehyde end-functionalized DEH-PPV.
2.4. Sample preparation for rodecomb diblock copolymers
treatment process as the samples for the SAXS/WAXS experiments
were determined by using a JEOL JEM-1230 operated at an accel-
erating voltage of 120 kV. Ultrathin sections (approximately 70 nm)
of PPV-b-P2VP were microtomed at room temperature by using
Leica Ultracut UCT6 machine. The more hygroscopic PPV-b-
P2VP(DBSA)x samples were cryomicrotomed at ꢁ90 ꢀC. Thin sec-
tions were collected onto 200-mesh copper grids coated with
carbon-supporting films. To enhance contrast of the DEH-PPV do-
mains, RuO4 stained the sections for 30 min.
PPV-b-P2VP(DBSA)x rodecomb diblock copolymer hybrid,
where x denotes the molar ratio of DBSA molecules to 2-vinyl
pyridine monomer units, were prepared by dissolving PPV-b-
P2VP and DBSA in dichloromethane (CH2Cl2), a good solvent for
both DEH-PPV and P2VP blocks. The concentration of the neat block
copolymer was maintained at 5wt% to ensure homogeneous solu-
tion formation. The solution was stirred for 24 h and subsequently
evaporated slowly at room temperature. After the solvent was
evaporated, the samples were further dried in a vacuum oven at
40 ꢀC for 1 day to remove any residual solvent.
3. Results and discussion
3.1. Synthesis of PPV-b-P2VP by coupling living P2VP anion chain
with an end-functional macroterminator
2.5. Simultaneous small-/wide-angle X-ray scattering (SAXS/WAXS)
measurement
A series of PPV-b-P2VP block copolymers were synthesized by
coupling the aldehyde end-functionalized DEH-PPV rods of
Mn ¼ 3800 with P2VP living anion chains of different molecular
weights synthesized by using the anionic polymerization. Before
coupling with DEH-PPV, the molecular weight characteristics of
P2VP were measured using gel permeation chromatography (GPC).
The polydispersity index (PDI) of all P2VP samples was less than
1.15. To ensure a complete coupling reaction of the transferred P2VP
living anions with the aldehyde end-functionalized PPVs, an excess
amount of DEH-PPV (1.5 mol equiv) to the P2VP anions was used.
According to the composition of PPV-b-P2VP block copolymer,
unreacted DEH-PPV can be removed by using an extraction method
as described below. Since hexane is a good solvent for DEH-PPV but
a poor one for P2VP, copolymers with coil fractions near or larger
than the symmetric composition can be easily separated from the
copolymer/homopolymer mixture solution by using hexane as the
extraction solvent at the ambient temperature. However, as the
Simultaneous SAXS/WAXS experiments were performed at the
beamline 23A1 of National Synchrotron Radiation Research Center
(NSRRC) in Taiwan. The dried hybrid samples were sealed in
washers of 1.0 mm in thickness and 0.25 mm in diameter, and
KaptonÒ films were used as windows for the X-rays. Before the
scattering experiment, all samples were annealed in the high vac-
uum oven (10ꢁ6 torr) at 120 ꢀC for 24 h. A two-dimensional MAR
CCD with 1024 ꢂ 1024 channels for the SAXS measurements and a
one-dimensional gas-type linear detector for the WAXS measure-
ments were used to collect the scattering patterns. The molecular
characteristics of the complexes are summarized in Table 2.
2.6. Transmission electron microscopy (TEM)
The morphology of neat PPV-b-P2VP copolymers and PPV-b-
P2VP(DBSA)x complexes annealed using the same thermal
Scheme 2. Synthetic routes of PPV-b-P2VP block copolymers.

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Y.-H. Lee et al. / Polymer 55 (2014) 2481e2490
Table 1
Molecular characteristics of PPV-b-P2VP block copolymers.
Block copolymers
Mn (g/mol)
Mn (g/mol)
PDIa
fb
P2VP
Morphology
DEH-PPVa
P2VPa
PPV-b-P2VP0.3
PPV-b-P2VP0.56
3800
3800
1600
4800
1.16
1.14
0.30
0.56
lamella
lamella
a
From GPC.
Volume fraction of P2VP.
b
copolymer with coil fractions significantly lower than the sym-
metric composition was synthesized, the copolymer could not be
separated from the mixture solution at room temperature. Instead,
it could be selectively precipitated in hexane at low temperature
(wꢁ78 ꢀC). After cooling the solution for at least five times for
purification, the PDI and the molecular weight of the PPV-b-P2VP
block copolymer precipitates were evaluated by GPC and 1H NMR,
measurements.
The GPC traces for PPV-b-P2VP0.56 and its precursor homopol-
ymers are shown in Fig.1. The chromatograph of the as-synthesized
PPV-b-P2VP0.56 exhibits two peaks: one for the unreacted DEH-PPV
homopolymer and one for the coupled PPV-b-P2VP0.56 block
copolymer. The number-average molecular weight (Mn ¼ 3800) of
the aldehyde end-functionalized DEH-PPV block was determined
using NMR end-group analysis by the comparison of the aldehyde
proton peak with the eOCH2 peak. Alternately, based on the 1H
NMR spectrum in Fig. 2, the mole fraction of DEH-PPV block in PPV-
b-P2VP0.56 (w17%) was calculated by integrating the area ratio
Fig. 1. The GPC traces of P2VP precursor (blue line), aldehyde end-functionalized DEH-
PPV homopolymer (green line), as-synthesized PPV-b-P2VP0.56 (red dash line), and
purified PPV-b-P2VP0.56 block copolymer (black line). (For interpretation of the ref-
erences to color in this figure legend, the reader is referred to the web version of this
article.)
the first scattering peak values q* increased from 9.1 nm for x ¼ 0, to
15.6 nm for x ¼ 1.0. It is considered that the increase in the domain
spacing is due to the chain-stretching effect upon the introduction
of DBSA complexed with P2VP blocks [34,35]. Therefore, the
domain spacing of the lamellar structure increased with the
bonding fraction of DBSA to P2VP.
Additionally, in the high-q range of the SAXS profiles shown in
Fig. 3a, a new peak located at 2.1 nmꢁ1 emerges (as indicated by a
dotted arrow), and the intensity of the peak increases with
increasing DBSA loading. The presence of the new peak specifies
the formation of a small-scale lamellar mesophase organized by
P2VP(DBSA) comb blocks which improves the alignment between
P2VP chains in their domain, leading to the emergence of the new
peak. Based on the Braggs’ equation, the domain spacing of the
small-scale lamellar structure is found to be equal to 3 nm.
Therefore, the PPV-b-P2VP0.3(DBSA)1.0 rodecomb hybrid exhibits a
structure-within-structure morphology in which DEH-PPV and
P2VP(DBSA) layers were alternatively arranged normal to the
between the eOCH2e signal (
chain of DEH-PPV and the aromatic eNeCHe signal (
d
¼ 3.6e3.9 ppm) of the alkoxy side-
d
¼ 8.0e
8.4 ppm) of P2VP. The volume fraction of P2VP in the synthesized
PPV-b-P2VP sample was calculated to be 0.56. Therefore, the PPV-
b-P2VP block copolymer was denoted as PPV-b-P2VP0.56. The re-
sults of the molecular weight characterization of the copolymers
are listed in Table 1.
3.2. Hierarchical structures of the rodecomb complexes
At first, we report the details on the morphological investigation
of the thermally annealed supramolecular rodecomb complexes by
employing TEM and SAXS. Fig. 3a shows the SAXS profiles for the
PPV-b-P2VP0.3(DBSA)x hybrids. For x ¼ 0, the neat PPV-b-P2VP0.3
block copolymer exhibited a highly ordered lamellar structure with
scattering peaks at integer multiples of the primary scattering peak
(q*). Based on the Braggs’ equation, the long period is estimated to
be 9.1 nm for the neat PPV-b-P2VP0.3 block copolymer. As the
loading ratio increased from x ¼ 0 to x ¼ 0.5, and eventually to
x ¼ 1.0, the presence of a lamellar phase in this hybrid system
remained unchanged as verified by their SAXS profiles with peak
position ratio of 1:2. With increasing DBSA loadings, the long
period of the lamellar structure for the hybrid as determined from
Table 2
Molecular characteristics of PPV-b-P2VP(DBSA)x rodecomb supramolecular
complex.
xa
fP2VPþDBSA
Long
Morphology
b
Rodecomb block
copolymers
period (nm)
PPV-b-P2VP0.3(DBSA)x
PPV-b-P2VP0.56(DBSA)x
0
0.3
9.1
14.6
15.6
14.2
18.0
14.3
lamellae-in-lamellae
lamellae-in-lamellae
lamellae-in-lamellae
lamellae-in-lamellae
lamellae-in-lamellae
strips-in-lamellae
0.5 0.52
1.0 0.64
0
0.5 0.72
1.0 0.81
0.56
a
Number of DBSA molecules per vinyl pyridine repeating unit.
Overall volume fraction of P2VP and DBSA in the copolymers.
b
Fig. 2. 1H NMR spectrum of PPV-b-P2VP0.56 block copolymer in CDCl3.

Y.-H. Lee et al. / Polymer 55 (2014) 2481e2490
2485
located at around 6.5 nmꢁ1 corresponds to the lateral spacing be-
tween the DEH-PPV rods, which gives the molecular diameter of
the DEH-PPV rod of w0.97 nm, with its peak magnitude decreasing
with increasing DBSA fractions. Even though the total volume
fraction of P2VP(DBSA) increased from 0.3 for the neat PPV-b-P2VP
block copolymer to a value of 0.6 for the PPV-b-P2VP0.3(DBSA)1.0
hybrid, the characteristic (110) peak position which represents the
distance between DEH-PPV rods remain unchanged.
However, it can be expected that the chain-stretching effect of
both P2VP and DBSA chains would increase the free energy of the
complex system due to the loss of conformation entropy in the
hybridized P2VP(DBSA) domains. Therefore, this complex system
may undergo morphological transition to reduce its free energy by
increasing defects within the otherwise long-range straight DEH-
PPV lamellae and form a so-called broken lamellar structure to
avoid the excessive chain stretching of P2VP blocks [57]. TEM mi-
crographs of the PPV-b-P2VP0.3(DBSA)x hybrid system can provide
important information about how PPV-b-P2VP and DBSA might be
organized within their nanodomains as a function of different DBSA
fractions. Fig. 4a shows a transmission electron micrograph of the
neat PPV-b-P2VP0.3 stained with RuO4. The DEH-PPV domain
shown as the dark phase in the micrograph exhibits a highly or-
dered and long-range lamellar structure. With increasing DBSA
fraction from x ¼ 0.5 to x ¼ 1.0, their TEM images are shown in
Fig. 4b and c, respectively, illustrating that the long-range order of
lamellae disappears, resulting in broken lamellae. This evolution is
similar to the observation in many previous studies of weakly
segregated rodecoil block copolymer systems with an increase in
their effective coil volume fraction [52,53,57].
Fig. 3. The evolution of (a) SAXS and (b) WAXS patterns as a function of the number of
DBSA molecules per vinyl pyridine repeating unit for the PPV-b-P2VP0.3(DBSA)x
complex.
domain surface. As a result, the way that DEH-PPV, P2VP, as well as
DBSA pack within their domain for the PPV-b-P2VP0.3(DBSA)x
system with x ¼ 0 and x ¼ 1 can be schematically illustrated by
Scheme 3(a) and (b), respectively. The effect of DBSA composition
on the crystalline structure of the hybrid is investigated by using
WAXS measurements. The assignment for the crystalline plane of
each diffraction peak shown in all the WAXS spectra is based on a
previous X-ray study on the DEH-PPV homopolymer by Segalman
et al. [56] We also consistently observed (110), (210), (220), and
The phase behavior of the PPV-b-P2VP(DBSA)x hybrid system is
further studied by changing the neat copolymer composition. For
the P2VP-rich PPV-b-P2VP0.56 block copolymer, the molecular
weight of its P2VP block is increased significantly compared with
that of P2VP block in PPV-b-P2VP0.3, while the molecular weight of
DEH-PPV block remain roughly constant for the two samples. The
corresponding SAXS patterns of PPV-b-P2VP0.56(DBSA)x are shown
in Fig. 5a. Similar to the morphology of the neat PPV-b-P2VP0.3
(310) diffraction planes from DEH-PPV in the neat PPV-b-P2VP0.3
.
As shown in Fig. 3b, the first peak matched with the (110) plane
block copolymer, PPV-b-P2VP0.56 also shows
a large-scale
morphology exhibited as a lamellar structure with SAXS scat-
tering peaks located at integer multiples of the primary scattering
peak (q*). Subsequently, as the DBSA fraction x was increased from
x ¼ 0 to x ¼ 0.5, the PPV-b-P2VP0.56(DBSA)0.5 hybrid still adopts a
self-assembled lamellar structure and the position of the first-order
scattering peak in the SAXS profiles decreasing with increasing
DBSA fractions from x ¼ 0 to x ¼ 0.5, indicating an increase in the
long period of the lamellar phase from 14.2 nm for x ¼ 0, to 18 nm
for x ¼ 0.5.
For the volume fraction typical in the range of 0.2e0.3, con-
ventional coilecoil diblock copolymers often adopt
a self-
assembled hexagonal close packed cylindrical structure (HCP).
However, by treating DBSA as a selective solvent to the P2VP do-
mains to obtain a combined volume fraction of DBSA and P2VP of
0.28, PV-b-P2VP0.56(DBSA)0.5 system still adopts a rather long range
ordered lamellar structure. Therefore, the formation of the lamellar
phase in the PPV-b-P2VP0.56(DBSA)0.5 hybrid indicates that the
roderod interaction between DEH-PPV rods plays a critical role to
prevent the formation of nanostructures with a curved interface.
Furthermore, TEM can be used to provide important information
about how PPV-b-P2VP0.56(DBSA)0.5 might be organized within its
nanostructure. PPV-b-P2VP0.56(DBSA)0.5 stained in the DEH-PPV
domain with RuO4 exhibits only disrupted and short-range dark
lines of the DEH-PPV domain with its morphology shown in Fig. 4e.
This is in great contrast to that of the neat PPV-b-P2VP0.56 block
copolymer shown in Fig. 4d, whereby a long-range ordered
lamellar structure was observed.
Scheme 3. (a) The lamellar morphology formed in the neat PPV-b-P2VP0.3 rodecoil
block copolymer and (b) hierarchical structure formed in PPV-b-P2VP0.3(DBSA)1.0
wherein the lamellar layers of DEH-PPV are embedded in the lamellar mesophase
formed by P2VP(DBSA) comb blocks.

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Y.-H. Lee et al. / Polymer 55 (2014) 2481e2490
Fig. 4. TEM images of PPV-b-P2VP0.3(DBSA)x rodecomb block copolymers with (a) x ¼ 0, (b) x ¼ 0.5, and (c) x ¼ 1.0, and TEM images of PPV-b-P2VP0.56(DBSA)x rodecomb block
copolymers with (d) x ¼ 0, (e) x ¼ 0.5, and (f) x ¼ 1.0. DEH-PPV domains stained with RuO4 exhibit as a dark phase shown in the TEM images.
Moreover, a further phase transformation is observed as the
DBSA fraction was continuously increased. As the DBSA fraction
was increased from x ¼ 0.5 to x ¼ 1.0, the SAXS profile shown in
Fig. 5a for the PPV-b-P2VP0.56(DBSA)1.0 complex exhibits multiple
pffiffiffi
scattering peaks with position at 1 : 3 ratio relative to that of the
first peak located at q* ¼ 0.44 nmꢁ1. This implies an existence of
hexagonally packed microdomains with a shorter-range spatial
order. The nanostructure of the PPV-b-P2VP0.56(DBSA)1.0 hybrid
was also observed by using TEM. The TEM micrograph (Fig. 4f)
showed that the DEH-PPV domains display a strip phase in PPV-b-
P2VP0.56(DBSA)1.0, wherein the aggregates are slightly longer in one
direction than the others.
In addition, analogous to the SAXS profiles of the PPV-b-
P2VP0.3(DBSA) hybrid system, a significant increase in the diffrac-
tion intensity and the sharpening of an additional peak at 2.1 nmꢁ1
(as indicated by a dotted arrow) are also observed, revealing the
formation of a smaller length-scale lamellar mesophase organized
by the P2VP(DBSA) comb phase. The way that DEH-PPV rods adopt
to pack within the PPV-b-P2VP0.56(DBSA)x hybrid nanodomains is
Fig. 5. The evolution of (a) SAXS and (b) WAXS patterns as a function of the number of
DBSA molecules per vinyl pyridine repeating unit for the PPV-b-P2VP0.56(DBSA)x
complex.
also investigated by using WAXS. As shown in Fig. 5b, the first
diffraction peak related to the (110) plane and located at around

Y.-H. Lee et al. / Polymer 55 (2014) 2481e2490
2487
6.5 nmꢁ1 corresponds to the lateral spacing between the DEH-PPV
rods, which is unchanged from that of the previous PPV-b-P2VP0.3
hybrid systems. The peak magnitude for DEH-PPV rod decreases
dramatically with increasing DBSA fractions. Even though the
phase transformation occurred from the broken lamellar phase to
the hexagonally packed strip phase, the characteristic (110) peak
position representing the distance for the roderod interaction re-
mains unchanged. Therefore, based on the combined results of the
TEM, SAXS and WAXS measurements, the hierarchical structure
formed in the PPV-b-P2VP0.56(DBSA)1.0 complex where strip
microdomains of DEH-PPV are embedded in the lamellar meso-
phase formed by the P2VP(DBSA) comb block can be schematically
illustrated in Scheme 4b.
0:9
l
Dhkl
¼
(1)
bhkl cos
q
where Dhkl is the crystallites size along the [hkl] direction, bhkl is
the full width at half maximum of the [hkl] scattering peak, is the
wavelength of the X-rays and 2 is the scattering angle. The strip
length consisting of DEH-PPV rods was estimated to be about
15.2 nm, which is in agreement with the estimate from its TEM
image. Based on an approximate DEH-PPV rod diameter of 1 nm
(w0.97 nm), this corresponds to w15 DEH-PPV rods in each short
strip aggregate.
l
q
Furthermore, the correlation length, D110, of the DEH-PPV-
packed strip can be obtained from the primary scattering peak
(110) by using Scherrer’s equation [58]:
3.3. Phase transformation
Since the anisotropic roderod interaction between
p-conju-
gated DEH-PPV was found to be temperature dependent [51],
simultaneously measured SAXS and WAXS experiments as a func-
tion of temperature were performed to investigate the effect of
temperature on the self-assembling behavior of the solid and liquid
crystalline structures of the PPV-b-P2VP(DBSA) hybrids.
At first, the phase transition of the neat PPV-b-P2VP0.3 was
observed by performing simultaneous SAXS and WAXS experi-
ments as a function of different temperatures. As shown in Fig. 6a of
the SAXS measurement and Fig. 6b of the WAXS measurement,
PPV-b-P2VP0.3 undergoes an ordered lamella-to-disorder transition
and a smectic/isotropic transition simultaneously at w190 ꢀC.
Analogous to the result of the evolution in the SAXS profiles of the
neat PPV-b-P2VP0.3 copolymer with temperature, the characteristic
scattering peaks corresponding to the self-assembled lamellar
phase of the neat PPV-b-P2VP0.56 copolymer also disappears
completely at w190 ꢀC (Fig. 7a), indicating the occurrence of the
phase transition from the lamellar phase to a disordered phase.
Concurrently, the WAXS diffraction peaks of the (110), (210), (220),
and (310) planes associated with the liquid crystalline structure
between DEH-PPV rods also disappear completely at 190 ꢀC
(Fig. 7b), indicating a smectic/isotropic transition. Therefore, it
appears that the presence of the roderod interaction between
DEH-PPV blocks may have the dominant effect on the formation
and the stabilization of the lamellar structure for both neat block
copolymers at temperatures below 190 ꢀC.
Fig. 8a and b show the evolution of SAXS and WAXS patterns,
respectively, as
a function of temperature for the PPV-b-
P2VP0.3(DBSA)1.0 hybrid. Compared with the result of its neat PPV-
b-P2VP0.3 block copolymer, the characteristic scattering peaks
corresponding to the self-assembled lamellar phase of PPV-b-
P2VP0.3(DBSA)1.0 hybrid disappear simultaneously with the aniso-
tropic roderod interaction between p-conjugated DEH-PPV blocks
at 190 ꢀC, as indicated by the complete disappearance of the major
(110) diffraction peak. Note that the diffraction peak at
q ¼ 2.25 nmꢁ1 revealed in Fig. 8a, signifying the ionic/hydrogen
bonding between P2VP and DBSA, remains significantly strong
even at temperatures above 190 ꢀC. In order to pin down the order-
disorder transition temperature (TODT) for the self-assembled
lamellar structure for the hybrids, the inverse of the primary
SAXS scattering peak intensity as a function of the inverse tem-
perature (Iꢁm1 vs Tꢁ1) can be plotted. As shown in Fig. 9a, TODT for the
PPV-b-P2VP0.3(DBSA)1.0 hybrid is estimated to be at around 190 ꢀC,
which is similar to that for the neat PPV-b-P2VP0.3 block copolymer.
Therefore, the presence of roderod interactions between DEH-PPV
chains is found to have dominant effects on both the formation, as
well as the stabilization, of the large-scale lamellar structure of
PPV-b-P2VP0.3(DBSA)1.0 at temperatures around 180 ꢀC. For tem-
peratures above 180 ꢀC, even though the ionic/hydrogen bonding
between P2VP and DBSA still exists, the large-scale lamellar
Scheme 4. (a) Lamellar morphology formed in neat PPV-b-P2VP0.56 rodecoil block
copolymer and hierarchical structure formed in PPV-b-P2VP0.56(DBSA)1.0 wherein strip
microdomains of (b) crystalline and (c) amorphous DEH-PPV are embedded in the
lamellar mesophase formed by P2VP(DBSA) comb block.

2488
Y.-H. Lee et al. / Polymer 55 (2014) 2481e2490
Fig. 8. Simultaneously measured (a) SAXS and (b) WAXS spectra as a function of
temperature for the PPV-b-P2VP0.3(DBSA)1.0 complex.
Fig. 6. Simultaneously measured (a) SAXS and (b) WAXS spectra as a function of
temperature for the PPV-b-P2VP0.3 block copolymer.
structure disappears simultaneously with the roderod interaction
of
p-conjugated DEH-PPV blocks.
Additionally, we analyze the phase behavior of the PPV-b-
P2VP0.56(DBSA)1.0 hybrids as a function of temperature. Fig. 10a and
b show the temperature-dependent SAXS and WAXS patterns of
PPV-b-P2VP0.56(DBSA)1.0 hybrid, respectively. Note that the crys-
talline peak at q ¼ 6.5 nmꢁ1 for the (110) plane for PPV-b-
P2VP0.56(DBSA)1.0 hybrid disappears at a temperature of only
w120 ꢀC, which is also lower than that (180 ꢀC) for the neat PPV-b-
P2VP0.56 block copolymer that exhibited a 1D lamellar ordered
structure. This indicates that the 2D confined strip phase for the
PPV-b-P2VP0.56(DBSA)1.0 hybrid has a significant effect on how its
Fig. 9. Iꢁm1 vs Tꢁ1 plots of (a) PPV-b-P2VP0.3 and PPV-b-P2VP0.3(DBSA)1.0 complexes,
and (b) PPV-b-P2VP0.56 and PPV-b-P2VP0.56(DBSA)1.0 complexes for the determination
of TODT associated with their P2VP(DBSA) comb block.
Fig. 7. Simultaneously measured (a) SAXS and (b) WAXS spectra as a function of
temperature for the PPV-b-P2VP0.56 block copolymer.

Y.-H. Lee et al. / Polymer 55 (2014) 2481e2490
2489
lamellar layer. For PPV-b-P2VP0.56(DBSA)x, the large-scale structure
changes from lamellae, to broken lamellae, and eventually to the
hexagonally packed strip phase as the amount of DBSA was
increased from x ¼ 0 to x ¼ 0.5 and eventually to x ¼ 1.0.
Upon heating, the crystalline phase of DEH-PPV in the PPV-b-
P2VP0.3(DBSA)1.0 hybrid disappeared at temperatures above 190 ꢀC
and the large-scale broken lamellar phase persisted up to w190 ꢀC,
at which point the rodecomb system underwent the order-to-
disorder transition, indicating that the roderod interaction be-
tween DEH-PPV chains plays a key role in the formation and the
stabilization of the broken lamellar structure. On the contrary, for
PPV-b-P2VP0.56(DBSA)1.0
, the roderod interaction disappeared
early at 120 ꢀC while the disordering of block copolymer micro-
domains occurred almost simultaneously at higher temperatures
with that of the P2VP(DBSA) lamellar structure, indicating that the
formation of the P2VP(DBSA) lamellar mesophase plays a critical
role in forming and stabilizing this hexagonally packed strip
structure. The current study may provide new guidelines for the
organization of rod-comb block copolymer systems for future
applications.
Fig. 10. Simultaneously measured (a) SAXS and (b) WAXS spectra as a function of
temperature for the PPV-b-P2VP0.56(DBSA)1.0 complex.
Acknowledgments
The financial support of this work from the National Science
Council of Taiwan is greatly appreciated. The authors would also
like to thank all beamline staff and technical supports at the Na-
tional Synchrotron Radiation Research Center of Taiwan for the X-
ray measurements.
p
-conjugated DEH-PPV blocks pack. Despite the disappearance of
the roderod interaction of -conjugated DEH-PPV blocks, the self-
p
assembled strip phase for PPV-b-P2VP0.56(DBSA)1.0 remains stable
at temperatures higher than 180 ꢀC. From Fig. 9b, the TODT for the
PPV-b-P2VP0.56(DBSA)1.0 hybrid is estimated to be at around 210 ꢀC.
It is also noted that DBSA attached to the P2VP blocks via both ionic
and hydrogen bonding, with the latter being more predominate.
The observed ODT of the strip mesophases may stem from the
dissociation of the weaker hydrogen bonds; the ionically bonded
DBSA remained intact even after the observed ODT has been
reached (Fig. 10a). These strip domains were formed by the
P2VP(DBSA) comb blocks with DBSA ionically bonded to P2VP [35].
Hence, the presence of the smaller-scale P2VP(DBSA) lamellar
mesophase in the PPV-b-P2VP0.56(DBSA)1.0 complex may also have
the governing effect on the formation and stabilization of the
hexagonal strip structure. A schematic for the molecular packing of
PPV-b-P2VP0.56(DBSA)1.0 at high temperatures (<180 ꢀC) is shown
in Scheme 4c. Correspondingly, the current study provides new
insights into rodecomb block copolymer systems for controlling
nanostructures at multiple length scales, as well as providing new
strategies for future applications.
Appendix A. Supplementary data
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.polymer.2014.01.056.
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