Modulating crystallinity of poly(3-hexylthiophene) via microphase separation of poly(3-hexylthiophene)-polyisoprene block copolymers

Article abstract of DOI:10.1021/ma502417w

A series of poly(3-hexylthiophene)-block-polyisoprene (P3HT-b-PI) diblock copolymers (DBCP) and polyisoprene-block-poly(3-hexylthiophene)-block-polyisoprene (PI-b-P3HT-b-PI) triblock copolymers (TBCP) with accurately controlled molecular architecture were synthesized via highly efficient coupling reaction between aldehyde end-functionalized P3HT and living anionic polyisoprene. The self-assembly behaviors, considering morphology and crystallinity, of the thermal annealed bulk samples of these DBCPs and TBCPs containing various PI content were systematically investigated. The DBCPs behaved very differently from most published P3HT BCP systems, showing elongated fibers with preserved crystallinity regardless of the PI fraction. More noteworthy, with PI fraction less than 40 wt %, the DBCPs exhibited parallel straight fibers longer than several micrometers accompanied by concurrent enhanced crystallinity. The unique microstructure of the DBCPs might originate from moderate microphase separation between P3HT and PI as well as high flexibility of PI to conduct the packing of P3HT. The TBCPs, by contrast, exhibited highly curved interdomain boundaries with significant depressed crystallinity, resembling P3HT diblock copolymers in the strong phase segregation regime, as more pronounced entanglement of the two terminal PI segments would restrict the movement of P3HT.

Full text of DOI:10.1021/ma502417w

Article
pubs.acs.org/Macromolecules
Modulating Crystallinity of Poly(3-hexylthiophene) via Microphase
Separation of Poly(3-hexylthiophene)−Polyisoprene Block
Copolymers
Herman Lim,† Chi-Yang Chao,*,† and Wei-Fang Su†,‡
‡
†Department of Materials Science and Engineering and Institute of Polymer Science, National Taiwan University, Taipei City,
Taiwan 10617
S
* Supporting Information
ABSTRACT: A series of poly(3-hexylthiophene)-block-polyisoprene (P3HT-
b-PI) diblock copolymers (DBCP) and polyisoprene-block-poly(3-hexylthio-
phene)-block-polyisoprene (PI-b-P3HT-b-PI) triblock copolymers (TBCP)
with accurately controlled molecular architecture were synthesized via highly
efficient coupling reaction between aldehyde end-functionalized P3HT and
living anionic polyisoprene. The self-assembly behaviors, considering
morphology and crystallinity, of the thermal annealed bulk samples of these
DBCPs and TBCPs containing various PI content were systematically
investigated. The DBCPs behaved very differently from most published
P3HT BCP systems, showing elongated fibers with preserved crystallinity
regardless of the PI fraction. More noteworthy, with PI fraction less than 40 wt
%, the DBCPs exhibited parallel straight fibers longer than several micrometers
accompanied by concurrent enhanced crystallinity. The unique microstructure
of the DBCPs might originate from moderate microphase separation between
P3HT and PI as well as high flexibility of PI to conduct the packing of P3HT. The TBCPs, by contrast, exhibited highly curved
interdomain boundaries with significant depressed crystallinity, resembling P3HT diblock copolymers in the strong phase
segregation regime, as more pronounced entanglement of the two terminal PI segments would restrict the movement of P3HT.
difficulty to move the P3HT crystallites. Efforts using
temperature control10−12 and solubility alternation13 to
promote aggregation of P3HT crystals in solution were also
reported to enhance the degree of crystallization of P3HT, but
the processing parameters were difficult to control practically.
An alternative approach to manipulate the microstructure
and the crystallinity of P3HT is the use of P3HT block
copolymers (BCPs). By connecting one or two coil segment(s)
to the terminal(s) of P3HT via covalent bonding, microphase
separation between P3HT and the coil segment should
substantially affect the assembly of P3HT and thus provide a
good opportunity to achieve desirable morphologies through
suitable processing. When aiming for long P3HT fibers, P3HT-
b-PLLA (poly(3-hexylthiophene)-b-polylactide), for example,
exhibited parallel aligned straight strips with several hundred
nanometers long in the spin-coated thin films.14,15 Never-
theless, microphase separation would interfere with crystal-
lization of P3HT so that most of the P3HT BCPs reported in
the literature exhibited hampered crystallinity. In addition, a
coil segment with relatively high glass transition temperature
(e.g., poly(methyl methacrylate), polystyrene, and poly-
(vinylpyridine)) would also restrict the molecular movement
INTRODUCTION
■
Poly(3-hexylthiophene) (P3HT), the mostly studied con-
jugated polymer, has been investigated widely for its good
potential in the applications of organic electronic devices, such
as solar cells, field emission transistors, light-emitting devices,
and sensors.1−7 The performance of these P3HT based devices
was found to be significantly affected by the optical and
electronic properties of P3HT, which were closely related to its
microstructure and crystallinity governed complicatedly by
crystallization behaviors of P3HT. Owing to the semicrystalline
nature of P3HT, a wide variety of factors, including the intrinsic
properties of P3HT such as the molecular weight and the
regioregularity as well as the processing parameters, would
affect crystallization of P3HT and thus the resulting
morphology of P3HT within the active component.
In most P3HT samples, fast crystallization of P3HT generally
results in randomly oriented short fibers with numerous grain
boundaries to limit the charge mobility. To enhance the charge
mobility, optimizing the microstructure by concurrently
elongating the fibers and increasing the crystallinity of P3HT
was highly desired yet difficult to be achieved. Several
approaches have been reported to improve the crystallinity of
P3HT by promoting chain movements of P3HT via thermal
annealing8 and solvent annealing9 to allow better packing.
Nevertheless, the enhancements in enlarging the crystallite size
and in elongating the fibers of P3HT were limited owing to the
Received: December 17, 2014
Revised: April 15, 2015
Published: May 12, 2015
© 2015 American Chemical Society
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Scheme 1. Syntheses of P3HT-H/CHO and P3HT-CHO/CHO
Scheme 2. Syntheses of P3HT-b-PI and PI-b-P3HT-b-PI by the Coupling Method
to prevent ordered packing of P3HT.16−18 Hence, selecting
suitable coil segment and building the BCP with appropriate
architecture are critical to manipulate self-assembly of P3HT
BCPs toward desirable microstructure with concurrent
enhanced crystallinity.
TBCPs was achieved. The weight fraction of the PI segment in
TBCPs matched that in DBCPs intentionally to investigate the
effect of molecular arrangement. WAXS and DSC were carried
out to evaluate the crystallinity while TEM and SAXS studies
were performed to identify the morphological information. It
was found that a short PI in DBCPs could facilitate elongated
straight strips with enhanced P3HT crystallinity. Nevertheless,
longer PI in DBCPs resulted in stronger microphase separation,
leading to curved fibers with slightly suppressed crystallinity of
P3HT. The crystallinity of P3HT in TBCPs was further
suppressed to form short fibers and cylinders owing to more
pronounced entanglement of two terminal PIs in TBCPs,
which would severely restrict the chain movement of P3HT.
In this work, polyisoprene (PI) was employed for the coil
segment as moderate microphase separation between P3HT
and PI would not overwhelm crystallization of P3HT while
high flexibility of PI could enhance molecular mobility to allow
ordered packing of P3HT. Very limited works related to P3HT-
PI BCPs have been reported which suggested that PI could
potentially modulate the packing of P3HT toward better
organization. For example, our previous work demonstrated
polyisoprene-block-poly(3-hexylthiophene)-block-polyisoprene
(PI-b-P3HT-b-PI) triblock copolymers would enhance π−π
stacking of P3HT in the spin-coated thin film as compared to
the pristine P3HT.19,20 Deribew et al. utilized P3HT-b-PI
diblock copolymers as compatibilizers in P3HT/PCBM bulk
heterojunction solar cells to alter the crystallization of P3HT,
leading to improved photovoltaic performance.21 Hereby, we
aimed to synthesize P3HT-b-PI diblock and PI-b-P3HT-b-PI
triblock copolymers with various compositions and to system-
atically explore the assembly behaviors of these BCPs. Upon
comprehensively understanding interplays between morphol-
ogy and crystallinity, long P3HT fibrils with improved
crystallinity could be achieved by optimizing the molecular
architecture of the P3HT-PI BCPs.
RESULTS AND DISCUSSION
■
Synthesis of P3HT-b-PI Diblock Copolymers and PI-b-
P3HT-b-PI Triblock Copolymers. P3HT-PI diblock and
triblock copolymers with high purity and narrow molecular
weight distribution were synthesized via efficient coupling
reaction between aldehyde end-functionalized P3HT and living
polyisoprene anions. Schemes 1 and 2 illustrate the aldehyde
end-functionalization of P3HT and the synthetic routes for the
block copolymers, respectively. To achieve accurate control on
the molecular architecture of these block copolymers, successful
end-functionalization of regioregular P3HT to afford high
purity of end groups is critical. Highly regioregular P3HT,
generally synthesized via the GRIM method,22−24 possesses
three kinds of end groups: H/Br (a hydrogen at one end and a
bromine at the other end) as the majority alone with H/H
(hydrogens at both ends) and Br/Br (bromines at both ends)
as the minority. P3HT-H/Br is needed for diblock copolymers
while P3HT-H/H is suitable for ABA type triblock copolymers
through further selective end-group modification. In this work,
Well-defined P3HT-b-PI diblock and PI-b-P3HT-b-PI tri-
block copolymers, denoted as DBCPs and TBCPs, respectively,
in this work, were synthesized via efficient coupling reaction
between aldehyde end-functionalized P3HT and living
polyisoprene anions.19 Through this method, accurate control
on molecular weights and compositions of both DBCPs and
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1
Figure 1. H NMR spectra of (a) P3HT-H/Br, (b) P3HT-H/CHO, and (c) P3HT-CHO/CHO in d-chloroform. The insets signify the details of
chemical shifts corresponding to the end-groups.
Figure 2. MALDI-TOF-MS spectra of (a) P3HT-H/CHO and (b) P3HT-CHO/CHO.
P3HT-H/Br was obtained from Yokozawa method instead of
commonly used GRIM method owing to better control on the
end groups.24,25 The key to successful end group control was
the use of 0.95 equiv (instead of 1 equiv) of isopropylmagne-
sium chloride to activate the monomer 2-bromo-3-hexyl-5-
iodothiophene to ensure the absence of unreacted Grignard
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Figure 3. GPC traces (RI signal) of (a) D2 and (b) T1 and the corresponding constituent homopolymers.
Table 1. Molecular Weight, Polydispersity Index (PDI), and Number of Repeat Unit (N) of P3HT-PI BCPs and the
Corresponding Building Blocks
b
P3HT
PI
block copolymers
a
a
a
c
polymer
Mn (PDI)
N
Mn (PDI)
N
Mn (PDI)
PI (wt %)
D1
D2
D3
D4
D5
T1
T2
T3
7500 (1.13)
7500 (1.13)
7500 (1.13)
7500 (1.13)
7500 (1.13)
7500 (1.16)
7500 (1.16)
7500 (1.16)
45
45
45
45
45
45
45
45
2600 (1.08)
5700 (1.06)
9100 (1.07)
12400 (1.06)
18300 (1.07)
3000 (1.08)
6200 (1.07)
11700 (1.06)
38
84
11000 (1.10)
14000 (1.12)
16900 (1.15)
18600 (1.19)
22900 (1.25)
14800 (1.18)
20900 (1.20)
19
38
50
58
68
40
62
71
134
182
269
44
91
172
31900 (1.26)
a
b
c
Obtained from GPC measurements. Mn and N of PI in T1−T3 represent only one of the two terminal PI segments. Calculated from 1H NMR of
the block copolymers.
reagent before the polymerization takes place.26 Benzaldehyde
was then attached to the bromine terminal of P3HT via Suzuki
coupling between the Br terminal of P3HT and 4-
formylphenylboronic acid in high yield to afford mono-end-
functionalized P3HT (P3HT-H/CHO) with high purity. P3HT
with both terminals capped with aldehydes (P3HT-CHO/
CHO) were synthesized according to our previous work.19
The high purity of aldehyde end-functionalized P3HT was
confirmed by 1H NMR and MALDI-TOF-MS studies. Figure 1
the terminal thiophene ring) and 9.99 ppm (5-position of the
terminal thiophene ring)27 as well as the shift of the triplet peak
for Hb′ from 2.61 ppm in P3HT-H/H (1H NMR spectra is
provided in the Supporting Information) to 2.95 ppm
suggested the success in the formylation reactions.
Figures 2a and 2b show the MALDI-TOF-MS spectra of
P3HT-H/CHO and P3HT-CHO/CHO, respectively. In both
spectra, only one set of peaks was observed, and the difference
between two contagious peaks is 166.2 Da, very close to the
molar mass of a 3HT repeating unit. The peak values in Figure
2a were equal to (n × 166.2) + 1 + 106.9 (e.g., 4096.4 Da),
which were identical to the theoretical molecular weights of
P3HT-H/CHO. On the other hand, the peak values in Figure
3b were assigned to (n × 166.2) + 29 + 29 (e.g., 4714.9 Da),
equal to the theoretical formula weights of P3HT-CHO/CHO.
The cleanness of the MALDI-TOF-MS spectra and the good
matches between the measured molecular weights and the
theoretical ones further confirm the high purity of P3HT-H/
CHO and P3HT-CHO/CHO.
1
depicts the H NMR spectra of P3HT-H/Br, P3HT-H/CHO,
and P3HT-CHO/CHO. For P3HT H/Br (Figure 1a), two sets
of triplet peaks assigned to Hb′ and Hb″ of the two terminal
thiophene rings were clearly observed because of two
distinctive chemical environments, which were also different
from Hb in the central thiophene rings. In P3HT-H/CHO
(Figure 1b), the triplet peak at 2.57 ppm corresponding to Hb″
in P3HT-H/Br completely disappeared since Hb″ hereafter
experienced similar chemical environment as Hb in the central
thiophene rings did. The signals appeared at 10.05 ppm in
P3HT-H/CHO further suggested the presence of aldehyde in
P3HT-H/CHO. In Figure 1c for P3HT-CHO/CHO, the
appearance of the aldehyde signals at 10.02 ppm (2-position of
Fast kinetics of polymerization and high reactivity of living
polymeric anions toward aldehyde group are two main
considerations to adopt anionic polymerization to synthesize
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1
Figure 4. H NMR of (a) D2 and (b) T1 in d-chloroform.
polyisoprene segment. The coupling method enables each
building block with accurately controlled molecular weight to
be synthesized individually to produce block copolymers having
designated compositions. In our previous work,19 the coupling
reaction between the terminal aldehyde of P3HT-CHO/CHO
and various polymeric anions has been demonstrated to be
effective and which could successfully afford P3HT triblock
copolymers with different coil segments. In this work, DBCPs
could also be successfully prepared via similar methodology
using mono-end-functionalized P3HT-H/CHO. To ensure the
completeness of the coupling reaction, excess living poly-
isoprene anions (5 equiv) were used and which should simplify
the purification by removing only the excess polyisoprene.
Table 1 summarizes the compositions of all the synthesized
block copolymers as well as the number of repeat unit and the
molecular weight of the corresponding building blocks. DBCPs
and TBCPs were named as DX and TX, respectively. The GPC
traces of D2 and T1 as well as the corresponding P3HT
homopolymers and PI homopolymers are shown in Figure 3 as
representative cases. The monomodal molecular weight
distribution in the GPC trace of D2 and the low polydispersity
(1.12) suggest the success of coupling reaction as well as the
purity of the final product. All synthesized block copolymers
exhibited monomodal molecular weight distribution with low
polydispersity, indicating the coupling reaction was successful
for all the range of molecular weight in this work (see Figure S1
in the Supporting Information for GPC traces of all BCPs).
Furthermore, no aldehyde peak around 10 ppm could be
detected in the 1H NMR spectra of D2 and T1 as illustrated in
Figures 4a and 4b, which further confirmed the completeness of
the coupling reaction between the aldehyde end groups at
P3HT and the living polyisoprene anions.
presented in Figure 5. The bright areas in the TEM images
represent P3HT-rich domains while the dark areas suggest PI-
rich domains. During the thermal annealing process, the
solvent-cast samples were first heated above the melting point
(>230 °C) of P3HT to allow kinetically trapped P3HT
segments to move freely, and the melt samples appeared orange
to ensure complete isotropy. The melts were then cooled down
to 185 °C and annealed for 24 h under vacuum, a temperature
slightly above the crystallization temperature of P3HT and well
above the Tg of PI. The annealed samples were then slowly
cooled to room temperature under vacuum. DBCPs containing
lower weight fraction of PI (<40 wt %), including D1 and D2,
exhibited lamellar morphologies with straight and extended
interdomain boundaries (IDBs) in the TEM images. The
characteristic scattering signals of lamellae were observed in the
SAXS patterns, suggested by the appearance of two peaks with
q1:q2 = 1:2, to further confirm the existence of lamellae. The
thickness of a lamellae was determined by d = 2π/q1 from the
SAXS pattern, suggesting d = 16 nm for D1 and d = 21 nm for
D2, which are close to the periodicity observed from the TEM
images. With increasing weight fraction of PI, D3, D4, and D5
formed shorter fibers with curved IDBs as suggested by the
TEM images. The corresponding SAXS patterns exhibited one
broad peak with relative weak intensity at low q range,
indicating the lack of uniform periodicity in the microstructure.
The morphological disorderliness was found to increase with
increasing PI length. A broad scattering signal at higher q range
was also observed for D3, D4, and D5, which could be
attributed to the P3HT fibers as the corresponding d-spacing
(∼10 nm) was close to the calculated contour length (∼9 nm)
of a fully stretched P3HT. It was noticed that the d-spacing of
this high q signal was almost identical in D3, D4, and D5,
suggesting extensive elongation of PI should trivially affect the
packing of P3HT. For TBCPs, shorter PI in T1 resulted in
lamellar morphology while increasing fraction of PI in T2 and
Morphology of Thermal Annealed Bulk Samples. The
morphologies of the thermal annealed bulk samples of block
copolymers were identified by TEM images and SAXS patterns
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Figure 5. TEM images and SAXS profiles of the thermal annealed bulk samples of P3HT-PI BCPs.
T3 led to short P3HT fibers and cylinders embedded in PI
matrix as suggested by the small circular white spots in the
TEM images. Considering T1 and D2 possessing similar weight
fraction of PI, the thickness of a lamellae in T1 suggested by the
SAXS pattern was 15 nm, obviously smaller than that in D2.
The white strips, representing P3HT domains, in the TEM
image of T1 appeared much thinner than those of D2. The
presence of only one broad peak in the corresponding SAXS
patterns of T2 and T3 also indicates the irregular arrangement
of the microstructure. The above observations indicate PI in
TBCPs would considerably interfere the packing of P3HT, and
further elongation of PI could significantly hamper the
crystallization of P3HT, leading to highly curved domain
boundaries of P3HT. The statements addressing the crystal-
linity will be further confirmed by the crystallinity studies
discussed in the following section.
Crystallinity of P3HT in the BCPs. To understand the
effect of PI on the crystallinity of the P3HT domains, WAXS
measurements were performed to evaluate the crystal structure
and the diffraction patterns are illustrated in Figure 6. The
lattice parameters of a P3HT crystallite and the average
crystallite size retrieved from the WAXS patterns are
summarized in Table S1.
Figure 6. WAXS patterns of the P3HT-PI BCPs and the pristine
P3HT homopolymer.
In all DBCPs, the crystalline structure of P3HT was
preserved as D1−D5 exhibited WAXS patterns similar to that
of the pristine P3HT despite of the attenuation in intensity due
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Figure 7. d(100) and d(020) of DBCPs and TBCPs as well as schematic illustation of the lattice parameters of a P3HT crystal.
to the decreasing P3HT content in the block copolymers. Peaks
of higher order (h00) could also be clearly observed in all
DBCPs but hardly seen in TBCPs. In addition, the (020)
diffraction peak in TBCPs became much broader alone with the
appearance amorphous halo around q = 1.0−1.5. Figure 7
shows the d-spacings along (100) and (020) directions,
denoted as d(100) and d(020), respectively, corresponding to the
lattice parameter a and b in a unit cell of P3HT. d(100) remained
almost unchanged for DBCPs with increasing PI content while
a noticeable enlargement in d(100) was observed for TBCPs as
compared to the pristine P3HT. By contrast, the variation in
d(020) was trivial for both DBCPs and TBCPs. The crystallite
size of P3HT in (100) direction was estimated from Scherer’s
formula based on the full width at half-maximum (fwhm) of the
corresponding peak. Figure 8 depicts the relation of the
of P3HT crystallites. By contrast, the presence of PI in TBCPs
would significantly affect the crystal structure by increasing the
intermolecular distance along the direction of side-chain
interdigitation and depress the crystallinity.
To further investigate the crystallization and melting
behaviors of P3HT within the copolymers as well as the
degree of crystallization (DC), differential scanning calorimetry
(DSC) was studied on the thermal annealed samples, and the
DSC curves of DBCPs and TBCPs are presented in Figures 9a
and 9b, respectively. The temperatures of phase transitions and
DC are also summarized in Table S1. Upon first heating, two
endothermic transitions were found. The first transition
appeared between 33 and 71 °C, denoted as H(I), should
associate with the rearrangement of hexyl side chains of
P3HT.28−30 The second transition located between 130 and
211 °C, labeled as H(II), was assigned to the phase transition
from a two-dimensional crystal to an isotropic liquid.31,32 H(II)
could be further detailed into three subtransitions, which are
denoted as transitions a, b, and c from low to high temperature.
According to the literature, transition a refers to a smectic−
smectic transition and transition b indicates the phase transition
from a two-dimensional smectic-like molecular packing to a
highly mobile nematic phase, while transition c represents the
clearing transition to isotropic phase.32
For H(I), DBCPs exhibited transition temperatures similar
to that of the pristine P3HT while TBCPs’ transitions were
apparently lower, even approaching room temperature. The
significantly lowered H(I) in TBCPs could be attributed to the
suppression in crystallite size and the looser interdigitation of
hexyl side chains. For H(II), whose peak temperatures of the
subtransitions against the weight fraction of PI in the BCPs are
illustrated in Figure 10, the most noteworthy differences
between BCPs and the pristine P3HT are (1) significantly
elevated temperature of transition b in BCPs and (2) the
increasing relative intensity of transition b to transition c, i.e.,
ΔHb/ΔHc. These observations suggested that the presence of
PI might encourage the nematic phase between transitions b
and c, which was unfeasibly achieved by the pristine
P3HT.32−34 Unfortunately, the existence of nematic phase
could not be detected by polarized optical microscopy owing to
the narrow temperature range. The remarkable elevation of the
temperature of transition b further suggests the postponed
melting of the P3HT crystallites, probably due to the increasing
Figure 8. Crystallite size (corresponding to L100) and the degree of
crystallization (DC) against the weight fraction of PI in BCPs.
crystallite size against the PI content in BCPs. Surprisingly, the
crystallite sizes of P3HT in most DBCPs were even larger than
that of the pristine P3HT except D1, whose crystallite size was
comparable to the pristine P3HT. A remarkable 37%
improvement in the crystallite size with respect to the pristine
P3HT was observed for D2, which was larger than 100 nm. For
TBCPs, the crystallite size was apparently smaller than that of
the pristine P3HT and which decreased with increasing PI
content. These observations suggested that the attachment of
PI to P3HT in DBCPs should not alter the intermolecular
stacking of P3HT and which could further enhance the growth
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Figure 9. DSC traces (a) first heating cycle and (b) first cooling cycle of thermal annealed BCPs.
Figure 11. Crystallization temperature (Tc) of P3HT-PI BCPs against
the weight percentage of PI.
content, suggesting the elongation of PI should hamper
crystallization of P3HT in TBCPs but not in DBCPs.
The degree of crystallization (DC), suggesting the fraction of
crystalline domains in the P3HT regions, could be evaluated via
DSC studies. DC was calculated according to eq 1
ΔHm
ΔHm wP3HT
100%
DC =
°
(1)
where ΔHm is the total enthalpy of transition H(II) of a BCP
while ΔHm° corresponds to that of the pristine P3HT with DC
= 100%. wP3HT is the weight fraction of P3HT in a BCP.
It was found that the trends of DC against the PI content in
BCPs (shown in Figure 8) are similar to the variation in
crystallite size. In DBCPs, DC of D1 was very close to the
pristine P3HT while the increment in PI content led to
enhancement of DC in D2, which might associate with the
enlarged crystallite size (from WAXS) and the straight
interdomain boundaries to direct the growth of P3HT crystals.
Nevertheless, the further increment in the weight fraction of PI
would decrease DC slightly in D3−D5 despite the
augmentation in crystallite size, suggesting the number of
crystallites was suppressed which might result from the curved
interdomain boundaries to suppress the formation of P3HT
crystallites. By contrast, DC of P3HT in TBCPs was
remarkably suppressed with the increasing weight fraction of
PI, in good agreement with the decrement in crystallite size,
further confirming the attachment of PI at both terminals of
P3HT should severely restrict the packing of P3HT.
Figure 10. Peak temperatures of subtransitions a, b, and c in H(II)
region during melting for (a) DBCPs and (b) TBCPs.
crystallite size and the interdomain boundaries between P3HT
and PI to restrict the movement of P3HT.
During cooling, two exothermic transitions were observed as
the major one at higher temperature corresponded to
crystallization of P3HT and the minor one at lower
temperature suggested the packing of hexyl side chains. Unlike
the melting behaviors showing three subtransitions, only one
transition was detected for the crystallization and whose peak
temperature was defined as the crystallization temperature (Tc).
As illustrated in Figure 11, Tc of DBCPs was slightly lower than
that of pristine P3HT and the increment of the weight fraction
of PI did not have distinctive effect on Tc. On the other hand,
Tc of TBCPs was significantly lowered with increasing PI
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Figure 12. Temperature-dependent SAXS/WAXS patterns of D1, D2, and T1.
Proposed Model for Self-Assembly and Interplays
between Morphology and Crystallinity. Previous studies
of rod−coil diblock copolymers suggest that the morphology of
these copolymers is governed by four parameters: the Flory−
experimental results. According to the equation of μ and χ, μ
was larger than χ by almost 50 times. Experimentally, lamellar
phase with the orientation of lamellae correlated across several
rods was the only morphology observed for PPV-b-PI with low
N at all volume fraction of PI (f PI), suggesting strong rod−rod
interaction would dominate the assembly of PPV-b-PI in weak
segregation regime.36 When increasing N to increase
segregation strength and geometric asymmetry between PPV
and PI, hexagonally packed phases of rectangular PPV
nanodomains embedded in the PI-rich matrix observed for
the BCP with f PI > 0.87.37 Su and co-workers reported the
phase diagram of poly(diethylhexyloxy-p-phenylenevinylene-
block-methyl methacrylate) (PPV-b-PMMA) block copolymers,
and they observed lamellar structures when f PMMA < 0.54 while
the PPV strips packed into hexagonal lattice as f PMMA > 0.66. In
contrast to PPV-b-PI BCPs, which showed TODT of the
microphase-separated structure was lower than the nematic−
isotropic transition temperature of PPV, the smectic packed
PPV became isotropic at the temperature generally lower than
́
Huggins interaction, the Maier−Saupe interaction, the volume
fraction of the coil segment, and the conformation asymmetry
or scaling dimension mismatch between the rod and the coil
segments. The Flory−Huggins interaction, represented by χN
as χ is the Flory−Huggins parameter and N is the number of
volumetric repeat units, measures the incompatibility between
rod and coil segments. The Maier−Saupe interaction, referred
as μN, generally suggests the strength of rod−rod interaction.
The coil fraction f measures the relative volume of the two
segments. The mismatch in scaling dimension between the rod
and coil is represented in various forms in different studies,
such as coil to rod length ratio ν or competition between rod
and coil segments G = μ/χ.
Because of the difficulty in synthesizing well-defined rod−
coil block copolymers with narrow molecular weight distribu-
tion, systematic studies on the quantitative information on μ
and χ are limited. Segalman and co-workers performed a series
of studies on poly(diethylhexyloxy-p-phenylenevinylene-block-
isoprene) (PPV-b-PI) block copolymers, in which the Maier−
or similar to TODT.
38 Although numerical information on χ was
not available in this study, the microphase segregation strength
between PPV and PMMA was believed to be greater than that
between PPV and PI. Therefore, comparing to PPV-b-PI, lower
coil fraction was required for PPV-b-PMMA to suppress the rod
persistence length to form nonlamellar microstructure as well as
reversible liquid crystalline and microphase phase transitions
was observed.
In P3HT rod−coil block copolymers, studies on identi-
fication of the strengths of rod−rod interaction and rod−coil
incompatibility were even rarer. Unlike PPV showing a
relatively wide temperature range of nematic phase, the
nematic window of P3HT is very narrow and difficult to be
observed experimentally. Lin et al. suggested the Maier−Saupe
parameter of P3HT as μ = 180/T − 0.3 based on low molecular
weight P3HT.39 Accordingly, the rod−rod interaction of P3HT
should be weaker as compared to PPV at the same temperature.
In the same work, the Flory−Huggins parameter between
P3HT and PMMA was also extracted from the structure factor
of the SAXS pattern measured above TODT using random phase
́
Saupe (rod−rod) interaction parameter (μ) of PPV was
extracted using a linear regression fitting to the nematic−
isotropic transition at different molecular weight of pristine
PPV with an assumption of pure rod conformation, and it could
be expressed as μ = 183/T − 0.236.35 The χ value was obtained
by fitting the interfacial segregation of block copolymer to a
homopolymer interface with a self-consistent field theory
(SCFT) simulation, and a temperature-dependent formula was
suggested as χ = 34.8/T − 0.091. The approach adopted in the
study was different from the method using for most coil−coil
BCPs by analyzing the structure factor of SAXS/SANS patterns
measured in the disordered state at the temperatures above the
order−disorder transition temperature (TODT) using the
random phase approximation (RPA) assumption since the
existence of complex aggregated structures in the melting states
would lead to the inconsistency between theoretical and
́
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Scheme 3. Proposed Models for Self-Assembly of P3HT-PI DBCPs and TBCPs at Different PI Fraction
approximation (RPA) assumption of a P3HT-b-PMMA with
fPMMA = 0.49 and which was expressed as χ = 56.370/T +
0.0433. The strength of microphase segregation between P3HT
and PMMA was much stronger than that between PPV and PI
discussed previously and which should be also stronger than
PPV−PMMA segregation.
segregation regime. However, P3HT-b-PI with high PI fraction
in this work did not exhibit any microstructure similar to those
observed in PPV-b-PI and PPV-b-PMMA with high coil
fraction, indicating the self-assembly should be strongly
influenced by factor other than the theoretically suggested
four factors: χ, μ, f, and G.
We believe that crystallization of P3HT should play an
important role in determining the morphology of P3HT BCPs.
The morphologies were also found to closely relate to the
crystallinity of P3HT within the BCPs. In PPV BCPs, PPV has
a relatively wide temperature range of highly mobile nematic
phase to allow PPV segments to organize corresponding to the
rod−rod interaction. By contrast, P3HT tends to crystallize
directly from an isotropic liquid without the privilege to
assemble in the liquid crystalline phase so that crystallization
To evaluate the strength of microphase separation between
P3HT and PI, temperature-dependent SAXS/WAXS studies
were performed on the thermal annealed D1 and D2. In the
SAXS/WAXS patterns provided in Figure 12, the scattering
signal at low q corresponding to microphase separation was lost
at 200 °C during heating, suggesting the order−disorder
transition should occur around 200 °C. On the other hand, the
peak at high q signifying the (100) direction of the P3HT
crystallite totally disappeared between 210 and 230 °C,
indicating complete melting. The melting could be referred as
crystalline−isotropic transition according to polarized optical
microscopy (POM) as optical isotropy was revealed above this
temperature range. Therefore, we might conclude that TODT
was slightly lower than the isotropization temperature in
P3HT-b-PI. The sequence and the temperature differentiation
of ODT and anisotropy−isotropy transitions were similar to
that of PPV-b-PI in moderate segregation regime. On the basis
of the above discussions, we reasonably suggest the microphase
segregation strength of P3HT-b-PI in this work should be
stronger than PPV-b-PI but weaker than PPV-b-PMMA and
P3HT-b-PMMA and which could be categorized into moderate
́
might overwhelm the Maier−Saupe interaction in directing the
morphology. For P3HT BCPs in weak phase segregation
regime, short fibers or strips randomly oriented or packed with
short-range order were generally observed owing to the
dominance of fast crystallization of P3HT. For P3HT BCPs
in strong phase segregation regime, microphase separation
occurred at a temperature much higher than the crystallization
temperature of P3HT, leading to morphologies with highly
curved interdomain boundaries at high coil fraction and
significantly suppressed crystallinity because of confined
crystallization of P3HT. For example, P3HT-b-P2VP (poly(3-
hexylthiophene-block-2-vinylpyridine)) with higher fraction of
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P2VP exhibited blue-shifted optical absorption, decreasing
melting point, and attenuated diffraction peaks in the WAXS
pattern, which indicated a decrease in DC.32 Similar
phenomena was also observed in P3HT-b-PE (poly(3-
hexylthiophene-block-ethylene).40 Nevertheless, P3HT-b-PI
BCPs in this work exhibited very different self-assembly
behaviors in the thermal annealed bulk samples comparing to
the previously reported P3HT BCPs. In DBCPs, only long
fibers with sustainable crystallinity were observed for all
samples with different fraction of PI. In the corresponding
TBCPs with high PI content, short P3HT cylinders without
long-range order and significantly suppressed crystallinity were
revealed. Hereby we proposed the following mechanism mainly
involving crystallization of P3HT and microphase segregation
between P3HT and PI to explain the unique self-assembly of
P3HT-PI BCPs, including the formation of the morphologies
and the interplays between morphology and crystallinity. The
proposed models for self-assembly of DBCPs and TBCPs are
schematically illustrated in Scheme 3 for the ease of
visualization.
During the thermal annealing processes, the samples were
heated above 230 °C to ensure complete isotropy. The melts
were then cooled down to 185 °C and annealed for 24 h under
vacuum, a temperature slightly above the crystallization
temperature of P3HT and over 200 K above the Tg of PI. In
D1 and D2, microphase separation was moderate; therefore,
P3HT would start to crystallize upon subsequent cooling and
which should soon induce microphase separation as the
scattering signals at both low q and high q could be observed
in the temperature-dependent SAXS/WAXS patterns of D1
and D2 at 150 °C (the patterns labeled with C-150 °C). Since
the interfacial areas between P3HT and PI domains tend to be
minimized to decrease energetic penalties, the IDBs would like
to straighten and extend if possible. Fortunately, PI was highly
mobile during the whole cooling step so that the packing of
P3HT could feasibly drag PI segments to accommodate the
most energetic favorable molecular arrangement. When the PI
content was low in DBCPs (D1 and D2), microphase
separation should not be too strong to disturb the stacking of
P3HT; as a consequence, P3HT segments would crystallize
along the direction of IDBs between P3HT and PI domains to
allow the formation of long straight fibers packed in a lamellar
fashion. The straight and extended IDBs, on the other hand,
also provide an encouraging environment for P3HT to
crystallize, leading to enlarged crystallites and enhanced degree
of crystallization. With the elongation of PI in DBCPs, the
increasing microphase segregation strength (χN increased), and
the entanglement of PI might start to perturb crystallization of
P3HT, resulting in curved IDBs. However, long fibers with
relatively good crystallinity could still be observed even at high
PI content (D5), in contrast to short worms or cylinders of
P3HT with significantly suppressed crystallinity exhibited by
most P3HT BCPs with high coil contents. The preservation of
crystallinity might be attributed to the high flexibility of PI since
PI could still move promptly even at low temperatures to allow
the stacking of P3HT, instead of considerably interfering
crystallization of P3HT like other coil segment with high Tg.
In TBCPs, however, the stacking of P3HT segment was
hindered due to more pronounced entanglement of two
terminal PIs to restrict the movements of PI and thus to further
limit the motion of P3HT. Therefore, P3HT in TBCPs
exhibited much lower crystallization temperature and sup-
pressed degree of crystallization as compared to P3HT in the
corresponding DBCPs having similar PI fraction. Comparing
T1 with D2 showing similar lamellar morphologies, the
temperature-dependent SAXS/WAXS pattern of T1 (Figure
12) at 150 °C during cooling (C-150) displayed a stronger
scattering signal at low q attributed to the lamellar micro-
structure and a weaker and broader peak at high q
corresponding to the P3HT crystallites at (100) direction.
During heating, the low q signal disappeared below 200 °C and
the high q peak vanished at 230 °C for D2; while the low q
signal retained until 210 °C and the high q peak became almost
undetectable at 220 °C for T1. The closeness of TODT and the
isotropization temperature in T1 as well as the stronger
scattering signal attributed to microphase separation would
suggest microphase segregation strength is stronger in T1 than
that in D2. Moreover, the high q peak for D2 became narrower
with increasing temperature during heating, suggesting better
organization of P3HT crystallites upon thermal annealing. By
contrast, the fwhm of the (100) peak of T1 remained almost
unchanged during heating, indicating the crystallinity of TCBPs
could not be improved after thermal annealing, which further
confirmed the difficulty for P3HT in TBCPs to move. Because
of the lost of the molecular mobility to accommodate the
favorable morphology for crystallization and of stronger
microphase segregation, TBCPs with higher PI fraction would
exhibit morphologies, such as P3HT cylinders, similar to those
of P3HT diblock copolymers in strong segregation regimes
with low P3HT crystallinity.
EXPERIMENTAL SECTION
■
Materials. Tetrahydrofuran (THF) and toluene were freshly
distilled from CaH2 and sodium/benzophenone under nitrogen.
Solvents for anionic polymerization (toluene and THF) were further
distilled from the corresponding polystyryl anion solution. Isoprene
was degassed and distilled from dibutylmagnesium (Aldrich, 1.0 M
solution in heptane) prior to use. tert-Butylmagnesium chloride (2.0 M
in diethyl ether), isopropylmagnesium chloride (2.0 M), sec-
butyllithium (1.3 M in cyclohexane) N-methylformanilide, POCl3,
Ni(dppp)Cl2, Pd(PPh3)4, 4-formylphenylboronic acid, K2CO3, I2,
NBS, 3-bromohexylthiophene, and 1-bromohexane were purchased
from Aldrich and used as received.
Synthesis of 2-Bromo-3-hexyl-5-iodothiophene. A three-step
reaction was carried out to synthesize 2-bromo-3-hexyl-5-iodothio-
phene from 3-bromothiophene, the monomer of P3HT, including (1)
hexyl side-chain attachment at third position, (2) selective
bromination at second position, and (3) iodination at the fifth
position of the thiophene ring. High purity of monomer is crucial for
producing P3HT with uniform H/Br end-groups. The detailed
procedures of the syntheses are described as follows. Grignard reagent
hexylmagnesium bromide was prepared in distilled ether purified with
sodium benzophenone. 13.42 g (0.552 mol) of magnesium (based on
30 g of 3-bromohexylthiophene) was charged into a 500 mL Rotaflo
flask with a magnetic stir bar, and it was then vacuum-dried for 30 min
and filled with high-purity N2. 300 mL of dried ether was directly
distilled into the flask, and then vigorous stirring and ice bath were
applied. 51.7 mL (0.368 mol) of 1-bromohexane was injected slowly
into the reaction flask under N2 purging. The ice bath was removed
after 15 min stirring, and the dark gray mixture was stirred at room
temperature for another 3 h. Meanwhile, 150 mg (2.76 × 10−4 mol) of
Ni(dppp)Cl2 was charged into another 500 mL Rotaflo flask and
vacuum-dried for 20 min before charging with high-purity N2. 30 g
(0.1853 mol) of 3-bromothiophene was charged into the flask under
N2 purging. The above mixture of 3-hexylmagnesium bromide was
transferred using a double-tip needle, and the mixture was allowed to
react for 24 h. The reaction mixture was poured into MeOH, and the
precipitates were washed with distilled water for several times. Crude
product was dried with anhydrous magnesium sulfate and subjected to
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1
vacuum distillation to afford 3-hexylthiophene. H NMR (400 MHz,
CDCl3) δ, ppm: 7.23 (d, J = 4.89 Hz, 1H), 6.93 (d, J = 6.18 Hz, 2H),
2.62 (t, 2H), 1.62 (m, 2H), 1.31 (m, 6H), 0.88 (t, 3H).
polymerization was performed for 1 day at room temperature, and the
P3HT-H/CHO toluene solution was then transferred into the solution
of PI anions. The resulting mixture was stirred for 3 h at room
temperature to allow the coupling reaction between the terminal
aldehyde of P3HT-H/CHO and anionic PI before being quenched
with 1 mL of anhydrous methanol. Toluene in the reaction mixture
was removed by a rotavapor, and ethyl acetate (100 mL) was then
added. The precipitate was the crude product of BCP, and excess PI
was dissolved in ethyl acetate. The precipitate was collected after
centrifugation and washed with ethyl acetate. The yield was around
70%. For TBCP, the same procedure was adopted on P3HT-CHO/
CHO instead of P3HT-H/CHO.
3-Hexylthiophene (30 g, 0.1782 mol) was charged into 500 mL
reaction flask equipped with a stir bar. 200 mL of THF was freshly
distilled from sodium followed by the addition of N-bromosuccinimide
(33.29 g, 0.1872 mol) at 0 °C. Reaction was carried out for at least 1 h
in an ice bath and terminated with 10 wt % Na2S2O3(aq). Most of
THF was evaporated using a rotavapor before the mixture was
extracted with ether and washed subsequently with 10 wt %
Na2S2O3(aq) and 10 wt % KOH(aq). The ether fraction was collected
and dried with anhydrous magnesium sulfate. Ether was then removed
by a rotavapor, and the residues were subjected to vacuum distillation
(oil bath T = 120 °C, 3 × 10−3 Torr) to afford 2-bromohexylth-
Instruments. The molecular weight of polymers was determined
using a Viscotek gel permeation chromatograph equipped with UV, RI,
1
1
and RALS detector. H NMR spectra were obtained using a Bruker
iophene. H NMR (400 MHz, CDCl3) δ, ppm: 7.18 (d, J = 5.59 Hz,
Advance 400 MHz spectrometer with d-chloroform as solvent.
MALDI-TOF MS spectra were obtained using a Voyager DE-PRO
MALDI-TOF-MS (Applied Biosystems, Austin, TX) equipped with a
N2 laser (337 nm). Cryo-ultramicrotomy was done with a Leica
Ultracut S with cryo-tank. Ultrathin sections (around 100 nm) were
obtained with a Leica Ultracut UCT6 and then deposited onto grid
and subjected to transmission electron microscopy analysis on JEOL
JEM-1400 operating at 80 kV. SAXS and WAXS measurements were
conducted on NSRRC (Hsinchu, Taiwan) beamline 23A, 17A, and
13A. DSC was performed by using a TA Q200 with scanning rate of
10 °C/min.
1H), 6.78 (dd, J = 7.88, 4.23 Hz, 1H), 2.54 (m, 2H), 1.56 (m, 3H),
1.33 (m, 7H), 0.89 (td, J = 15.40, 7.67 Hz, 3H).
2-Bromo-3-hexylthiophene (30 g, 0.1213 mol) was put into a 500
mL reaction flask followed by the addition of 300 mL of
dichloromethane. Iodine (14.63g, 0.057 mol) and iodobenzene
diacetate (19.496 g, 0.060) were added subsequently at 0 °C, and
the mixture was stirred for overnight at room temperature. Reaction
was quenched by addition of 10 wt % Na2S2O3. Dichloromethane was
evaporated to afford the crude product, which was then dissolved in
ether and washed several times with 10 wt % Na2S2O3(aq). By
removing ether and vacuum drying, the purified 2-bromo-3-hexyl-5-
1
iodothiophene was obtained. H NMR (400 MHz, CDCl3) δ, ppm:
CONCLUSION
6.97 (m, 1H), 2.51 (m, 2H), 1.52 (m, 4H), 1.30 (m, 7H), 0.89 (t, J =
6.76 Hz, 3H).
■
In this work, P3HT−polyisoprene (P3HT-PI) diblock and
triblock copolymers having designated molecular architecture
with accurately controlled molecular weight and composition
were successfully synthesized via efficient coupling reaction
between aldehyde end-functionalized P3HT and living anioinic
polysioprene. The self-assembly behaviors of the diblock
copolymers appeared very different from most of the published
P3HT rod−coil block copolymer systems, showing elongated
fibers with preserved P3HT crystallinity. The most noteworthy
finding was D2, the DBCP containing 37 wt % of PI, exhibiting
long straight strips with concurrently enhanced P3HT
crystallinity arranging in a parallel fashion. The unique self-
assembly could be attributed to the high flexibility of PI and the
moderate microphase separation between P3HT and PI to
conduct the crystallization of P3HT. By contrast, the TBCPs
exhibited morphologies of lamellae and short P3HT cylinders
with suppressed P3HT crystallinity since the two terminal PI
segments might undergo more pronounced entanglement to
restrict the chain movement, thus hampering the packing of
P3HT.
Syntheses of Aldehyde End-Functionalized P3HT: P3HT-H/
CHO and P3HT-CHO/CHO. P3HT with bromine at one terminal
thiophene and hydrogen at other terminal thiophene (P3HT-H/Br)
was synthesized using the Yokozawa method as described below. 2-
Bromo-3-hexyl-5-iodothiophene (4 g, 0.0107 mol) was charged into a
500 mL Rotaflo flask equipped with a stir bar and was vacuumed for
30 min. Freshly distilled THF (200 mL) was added under a nitrogen
atmosphere, and the solution was then cooled to 0 °C.
Isopropylmagnesium chloride (205 mL, 0.0102 mol) was injected
via an airtight syringe, and the mixture was allowed to react for 30 min
under an ice bath. The reaction mixture was then poured into a
copious amount of methanol to afford purple precipitates, which was
then collected and washed using Soxhlet extraction with methanol to
produce purified P3HT-H/Br. The molecular weight of the
synthesized P3HT-H/Br was suggested by GPC, and the purity of
the end groups was identified by MALDI-TOF-MS.
P3HT-H/Br (1 g, 0.125 mmol), 4-formylphenylboronic acid
(0.3749 g, 2.5 mmol), and K2CO3 (0.3455 g, 2.5 mmol) were charged
into a reaction flask and vacuum-dried. 250 mL of distilled THF was
then added under nitrogen, and the resulting mixture was stirred until
a clear solution was obtained. Pd(PPh3)4 was added, and then the
mixture was refluxed at 70 °C for 24 h under N2. The Suzuki coupling
reaction was then terminated by adding 2.5 mL of water. The reaction
mixture was poured into methanol to afford the crude product, which
was then subjected to Soxhlet extraction with methanol to afford
purified P3HT-H/CHOP3HT with hydrogen at one terminal and
benzaldehyde at another terminal. The synthesis of P3HT-CHO/
CHO from P3HT-H/Br, P3HT with both terminals being aldehyde,
was performed according to our previous work.19 MALDI-TOF-MS
and 1H NMR studies were performed to evaluate the completeness of
end-functionalization (yield: 97%).
ASSOCIATED CONTENT
* Supporting Information
■
S
Table S1 and Figure S1. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/ma502417w.
AUTHOR INFORMATION
Corresponding Author
*E-mail cychao138@ntu.edu.tw (C.-Y.C.).
■
General Procedure for Syntheses of P3HT-b-PI Diblock
Copolymers (DBCPs) and PI-b-P3HT-b-PI Triblock Copolymers
(TBCPs). P3HT-H/CHO (0.3 g, 0.04 mmol) was charged into a 100
mL Rotaflo flask and vacuum-dried for 2 days before being dissolved in
80 mL of freshly distilled toluene purified with living polystyryl anions.
Polyisoprene in 1,4-addition (5 equiv) was synthesized from anionic
polymerization in toluene at room temperature under the atmosphere
of nitrogen. sec-BuLi (0.165 mL, 0.2 mmol) [1.3 M] in cyclohexane
was injected into freshly distilled toluene and followed by the addition
of purified isoprene (0.88 mL) distilled from dibutylmagnesium. The
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Prof. Jing-Jong Shyue and Academia Sinica
for MALDI-TOF-MS measurements. The financial support
from Ministry of Science and Technology (previously known as
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