Patchy Nanofibers from the Thin Film Self-Assembly of a Conjugated Diblock Copolymer

Article abstract of DOI:10.1002/anie.201700134

An unexpected morphology comprising patchy nanofibers can be accessed from the self-assembly of an all-conjugated, polyselenophene-block-polythiophene copolymer. This morphology consists of very small (<10 nm), polythiophene- and polyselenophene-rich domains and is unprecedented for both conjugated polymers and diblock copolymers in general. We propose that the patchy morphology occurs from the enhanced miscibility of the blocks arising from the longer alkyl chains in comparison to similar block copolymers with shorter alkyl chains, which fully phase separate, as well as the difference in rigidity between the polythiophene and polyselenophene blocks. This work demonstrates a facile way to tune the self-assembly behavior of conjugated block copolymers by modification of the side chain substituents.

Full text of DOI:10.1002/anie.201700134

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International Edition: DOI: 10.1002/anie.201700134
German Edition: DOI: 10.1002/ange.201700134
Nanotechnology
Patchy Nanofibers from the Thin Film Self-Assembly of a Conjugated
Diblock Copolymer
Emily L. Kynaston, Yuan Fang, Joseph G. Manion, Nimrat K. Obhi, Jane Y. Howe,
Dmitrii F. Perepichka,* and Dwight S. Seferos*
Abstract: An unexpected morphology comprising patchy
nanofibers can be accessed from the self-assembly of an all-
conjugated, polyselenophene-block-polythiophene copolymer.
This morphology consists of very small (< 10 nm), polythio-
phene- and polyselenophene-rich domains and is unprece-
dented for both conjugated polymers and diblock copolymers
in general. We propose that the patchy morphology occurs
from the enhanced miscibility of the blocks arising from the
longer alkyl chains in comparison to similar block copolymers
with shorter alkyl chains, which fully phase separate, as well as
the difference in rigidity between the polythiophene and
polyselenophene blocks. This work demonstrates a facile way
to tune the self-assembly behavior of conjugated block
copolymers by modification of the side chain substituents.
such as lamellae.[5–9] Methods to access alternative morphol-
ogies, such as spheres and gyroids, include incorporating
conjugated polymers into crystalline-coil BCPs,[10–12] selecting
conjugated polymers with low crystallinity (such as those with
branched side chains),[12] using high boiling point additives,[11]
or kinetically trapping non-equilibrium morphologies from
a melted sample.[10]
Herein, we demonstrate that a new selenophene–thio-
phene copolymer with long, dodecyl side chains, can self-
assemble into patchy nanofibers, which comprise very small
(< 10 nm), partially phase-separated polythiophene- and
polyselenophene-rich domains. This compartmentalized mor-
phology is unprecedented for both conjugated polymers and
diblock copolymers in general. To date, self-assembled patchy
nanoparticles have been reported from the solution process-
ing of complex polymeric materials such as dendritic and
triblock copolymers[13,14] as well as surface-tethered poly-
mers.[15,16] Such compartmentalized structures have potential
unique applications in nano-templating, organic electronics,
and drug delivery.[17,18]
For this study, low dispersity samples of poly(3-dodecylth-
iophene) (P3DDT50) and poly(3-dodecylselenophene)
(P3DDS50) homopolymers (subscripts denote number aver-
age degree of polymerization or DPn) as well as a seleno-
phene–thiophene copolymer (P3DDS50-b-P3DDT50) were
synthesized from the catalyst-transfer polymerization (CTP)
of their corresponding 2,5-dibrominated monomers (Support-
ing Information, Scheme S1). The selenophene homopolymer
and copolymer were synthesized using an external NiII
initiator to narrow their polydispersity, as polyselenophenes
tend to have higher dispersities than their thiophene counter-
parts. P3DDT has intermediate crystallization kinetics to
poly(3-hexylthiophene) (P3HT), which crystallizes rapidly,
and P3EHT, which crystallizes slowly, so we anticipated that
these materials may exhibit different self-assembly behavior
to previously studied selenophene–thiophene copolymers
comprising P3HT and P3EHT.[8,19] We determined the
number average molecular weight (Mn) of the externally
initiated selenophene homopolymer and the selenophene–
thiophene copolymer by integration of the o-tolyl end-group
proton peaks at 2.48 ppm versus the aromatic proton peaks at
6.98 ppm (thiophene) and 7.11 ppm (selenophene) in the 1H-
NMR spectra (Supporting Information, Table S1 and Figur-
es S4–5). Gel permeation chromatography (GPC) is known to
overestimate the molecular weight of rigid-rod polymers[20]
and, indeed, the GPC-determined molecular weights of the
dodecyl-substituted polymers are ca. 50% higher than those
established from 1H-NMR (Supporting Information, Fig-
ure S6). Assuming the same overestimation, we calculated the
T
he nanophase structure of conjugated polymers heavily
influences their optoelectronic properties.[1–3] For example,
enhanced long-range energy transfer (up to 60 nm) has been
observed in highly ordered annealed poly(phenylene vinyl-
ene) aggregates when compared with disordered aggregates.[2]
The self-assembly of block copolymers (BCPs) can be used to
impart nanoscale features in polymeric thin films.[4] There-
fore, a fundamental understanding of the self-assembly
behavior of conjugated BCPs is key to their application in
devices. However, the range of morphologies accessible from
the self-assembly of conjugated BCPs is limited by crystal-
lization effects, which tend to favor low-curvature structures
[*] Dr. E. L. Kynaston, J. G. Manion, N. K. Obhi, Prof. D. S. Seferos
Department of Chemistry, University of Toronto
80 St. George St. Toronto, ON, M5S 3H6 (Canada)
E-mail: dseferos@chem.utoronto.ca
Y. Fang, Prof. D. F. Perepichka
Department of Chemistry and Center for Self-Assembled Chemical
Structures, McGill University
801 Sherbrooke St. West, Montreal, QC, H3A 0B8 (Canada)
E-mail: dmitrii.perepichka@mcgill.ca
Y. Fang
Department of Chemistry, Division of Molecular Imaging and
Photonics, KU Leuven-University of Leuven
Celestijnenlaan 200F, 3001 Leuven (Belgium)
Dr. J. Y. Howe
Hitachi High-Technologies Canada, Inc.
89 Galaxy Blvd, Suite 14, Toronto, ON, M9W 6A4 (Canada)
and
Department of Materials Science and Engineering
University of Toronto
184 College St. Toronto, ON, M5S 3E4 (Canada)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700134.
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molecular weight of the thiophene homopolymer (which was
not externally initiated) to be 12000 gmolÀ1 (DPn ꢀ 50),
which is consistent with the NiII initiator loading of 2 mol%.
Interestingly, the thiophene homopolymer appears much
smaller (ca. 25%) than the selenophene homopolymer by
GPC despite their structural similarity (Supporting Informa-
tion, Figure S6). This is presumably due to the more rigid
structure of polyselenophenes compared to that of polythio-
phenes,[21] which leads to a larger apparent hydrodynamic
radius (see discussion below). Degree of regioregularity is
known to affect the structural and charge-transport properties
of P3HT[22] and therefore was determined from the 1H-NMR
spectrum of each polymer (Supporting Information, Figur-
es S3–5; Table S1). The regioregularity was similar for both
homopolymers and the selenophene–thiophene copolymer
(ꢁ 96%) despite the different NiII initiators employed for
polymerization. The approximately 1:1 block ratio in the
selenophene–thiophene copolymer was confirmed by inte-
gration of the aromatic proton peaks in the 1H-NMR
spectrum (Supporting Information, Figure S5).
observed at 1078C arises from type-II P3DDS. A single
crystallization transition (Tc = 1328C) was observed on cool-
ing, which indicates that the thiophene and selenophene
blocks crystallize at similar rates in the copolymer. The DSC
profiles of each polymer all have broad peaks on heating and
cooling in the 50–908C range, which were attributed to
melting of interdigitated dodecyl side chains as previously
reported for P3DDT.[19] An annealing temperature of 1808C
was selected for thin film studies, as this was above the
melting temperatures of all three materials. Thermogravi-
metric analysis (TGA) of the selenophene and thiophene
homopolymers indicates that they are thermally robust at the
selected annealing temperatures and only underwent decom-
position (in N2 atmosphere) at 350 and 4308C, respectively
(Supporting Information, Figure S9).
Transmission electron microscopy (TEM) of annealed
thin films of the selenophene homopolymer revealed long
(> 1 mm) fibers that are more than two times wider (number
average width or Wn = 22 Æ 3 nm) than the thiophene homo-
polymer fibers (Wn = 10 Æ 2 nm) (Figure 2 and Supporting
Information, Figures S10–12) despite both polymers having
Differential scanning calorimetry (DSC) indicated that
the thiophene and selenophene segments in the copolymer
crystallize at similar rates. The first DSC cycle of the
thiophene homopolymer showed a broad melt endotherm at
a temperature (Tm) of 1558C on heating and a sharp
crystallization exotherm at a temperature (Tc) of 1168C on
cooling (Supporting Information, Figure S7). Two melt endo-
therms were apparent in the first DSC cycle of the seleno-
phene homopolymer, at 120 and 1738C, which were assigned
the type-II and type-I polymorphs, respectively (Supporting
Information, Figure S8).[23] The type-II polymorph, charac-
terized by having shorter interlayer distances than the type-I
analogue,[23] is converted to the type-I polymorph on anneal-
ing and was not present in the second heating cycle
(Supporting Information, Figure S8). The crystallization exo-
therm for the selenophene homopolymer (Tc = 1368C) was
observed closer to the Tm than that of the thiophene analogue,
suggesting that it crystallizes faster. The first DSC cycle of the
copolymer showed two melting transitions (Tm = 107 and
1618C) (Figure 1). The melt endotherm at 1078C was not
evident in the second cycle suggesting that the transition
Figure 2. Annealed thin films of homopolymers. P3DDT50 imaged by
a) bright-field TEM and b) tapping-mode AFM (height). P3DDS50
imaged by c) bright-field TEM and d) tapping-mode AFM (height).
Scale bars=200 nm.
a similar DPn. The predicted contour lengths of the thiophene
and selenophene homopolymer chains with a DPn of 50 are
around 20 nm based on theoretical and crystallographic data
for poly(3-alkylthiophene)s (P3ATs).[21,24] These findings
suggest that in both samples the polymer chains pack
perpendicularly to the fiber axis, however, the polythiophene
chain folds within the fibers whereas the polyselenophene
chain adopts a fully extended conformation. To test the effect
of the different end-groups on self-assembly behavior, we
synthesized an externally-initiated thiophene homopolymer
of a similar molecular weight and found that the widths of
fibers obtained from the self-assembly of o-tolyl P3DDT50
Figure 1. DSC profile of P3DDS50-b-P3DDT50 (1st cycle) run at
108CminÀ1. The 2nd heating cycle is shown beneath the 1st.
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were within the measurement error of those in the non-
films appeared to comprise individual, side by side fibers
externally initiated homopolymer by TEM (Supporting
Information, Figure S13). Therefore, this difference can be
explained by the known higher rigidity of polyselenophene
compared to polythiophenes.[21,25] Thus, we hypothesized that
a BCP comprising P3DDS and P3DDT might exhibit very
unusual self-assembly behavior. Variations in electron con-
trast can be seen in the TEM images of the thin films,
particularly for the selenophene homopolymer (Figure 2c).
This could be attributed to variations in crystallinity along the
length of the nanofiber axes arising from amorphous regions
corresponding to the dodecyl side chains, which are circa
2.5 nm long (see GIXRD results below). Thin films of the
selenophene–thiophene copolymer were found to comprise
fibers that contain segments that vary in contrast by TEM
(Figure 3a) and differ from the structures commonly
(Figure 3b,d) similar to that of the thiophene homopolymer,
whereas the selenophene homopolymer fibers were inter-
connected. Individual fibers have also been observed in P3AT
BCP thin films in which the blocks have similar solubility and
differ only in alkyl chain length.[26] The selenophene–thio-
phene copolymer fibers are comparable in width to the
selenophene homopolymer fibers. Variations in fiber width
are apparent in both the thiophene homopolymer and
selenophene–thiophene copolymer fibers, possibly owing to
the coexistence of narrower chain-folded and wider chain-
extended regions in these samples. This may not have been
observed in the TEM images, as the narrower chain-folded
regions are likely to consist of thiophene chains (see
discussion later), which is less electron-dense than the
selenophene block.[8] It should be noted that in each sample
the fiber widths observed by AFM are larger than those in the
TEM images by circa 2 nm. This is presumably due to
inclusion of regions with low crystallinity that are not
electron-dense enough to be seen in the TEM images as
well as convolution of the tip curvature (10 nm) in the AFM
measurements.
Scanning tunneling microscopy (STM) studies were con-
ducted to obtain a submolecular-level picture of the patchy
morphology of the selenophene–thiophene copolymer and
the distinct morphologies of the thiophene and selenophene
homopolymers. From high-resolution STM images, individual
polymer chains can be identified. The thiophene homopoly-
mer self-assembled into lamellae composed of both folded
and extended polymer chains, running across their width
(Supporting Information, Figure S18a). Interestingly, some of
these lamellae contain long individual thiophene chains that
span across two lamellae,[27] which can facilitate charge
transport between grains in polythiophene monolayers. On
the other hand, in the selenophene homopolymer monolay-
ers, the polymer chains are almost exclusively chain-extended
(Supporting Information, Figure S18b), consistent with the
previously proposed model for polyselenophene self-assem-
bly perpendicular to the fiber axis. The STM image of the
block copolymer reveals dark and light segments (“patches”)
that consist of 3–5 polymer chains, within the same domain of
crystallized polymer strands. (Figure 3c). The different con-
trast arises from the HOMO of polyselenophene
(À4.57 eV)[28] being closer to the density of states of the
STM tip, compared to polythiophene (À4.60 eV),[28] leading
to a greater tunneling current from the former.[29] To confirm
the attribution of high contrast to the selenophene blocks, we
prepared monolayers of mixtures of both homopolymers. A
30:70 (w/w) polythiophene:polyselenophene monolayer con-
tains a large proportion of darker, chain-folded regions with
several bright, extended polymer chains (Supporting Infor-
mation, Figures S19a–b). The inverse 70:30 polythiophene:-
polyselenophene mixture revealed the presence of bright,
extended chains with fewer darker regions (Supporting
Information, Figures S19c–d). Significantly, this experiment
confirms that thiophene and selenophene polymer chains can
crystallize within the same domain in contrast to similar
materials in which longer-range phase separation is
observed.[26] Recent STM studies of polythiophene BCP
Figure 3. Annealed thin films of P3DDS50-b-P3DDT50 imaged by
a) bright-field TEM. Inset: selenium- (dark) and sulfur-rich areas (light)
are indicated by arrows; b) tapping-mode AFM (height) and d) tap-
ping-mode AFM (phase). Scale bars=200 nm. c) An annealed mono-
layer of P3DDS50-b-P3DDT50 imaged by STM: Iset =246 pA and
Vbias =À618 mV imaged on HOPG. Scale bar=10 nm.
observed in thin films of similar BCPs.[8,12] These fibers have
comparable widths (Wn = 21 Æ 3 nm; Supporting Information,
Figures S12 and S14) to those observed in the selenophene
homopolymer samples, suggesting that extended polyseleno-
phene chains determine the width of the fibers. The areas of
contrast difference along the fiber axes observed by TEM
were larger than those observed for the selenophene homo-
polymer. Statistical analysis on the TEM measurements of the
patch dimensions along the length of the fibers indicated
a number average length (Ln) of 7 Æ 2 nm (Supporting
Information, Figure S15). In previous studies, we have
found that polyselenophenes exhibit greater electron density
than polythiophenes by TEM.[8] We therefore postulate that
the patches could be due to alternating selenophene- and
thiophene-rich domains.
The selenophene–thiophene copolymer thin film struc-
ture observed by atomic force microscopy (AFM) appears
intermediate in width to that of the homopolymers (Fig-
ure 2b,d). By AFM, the selenophene–thiophene copolymer
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(containing blocks with different alkyl chains) monolayers
have shown phase separation which correlates with thin film
self-assembly behavior[26] and hence supports our proposed
model (Figure 4).
Grazing-incidence X-ray diffraction (GIXRD) measure-
ments suggested that the crystallinity of the selenophene–
thiophene copolymer was intermediate to that of the homo-
polymers, reinforcing a model consisting of cocrystallized
thiophene and selenophene domains. The diffraction patterns
of the annealed thin films contain (100) reflections corre-
sponding to interlayer spacings in the plane of the interdigi-
tated alkyl chains[33] of 2.57 and 2.34 nm for the thiophene and
selenophene homopolymers, respectively (Figure 5b). Poly-
selenophenes are known to have shorter interlayer distances
than their polythiophene analogues.[23] The selenophene–
thiophene copolymer had a single (100) reflection of 2.43 nm,
which is intermediate to those of the homopolymers.
In summary, we have synthesized a new selenophene–
thiophene BCP that appears to be the first conjugated
polymer that can form a patchy fiber morphology in thin
films. Investigations into the thin film self-assembly of this
material and its component homopolymers indicate that this
unexpected self-assembly behavior is driven by the rigidity of
polyselenophenes that resist chain folding, even at high
degrees of polymerization, as well as enhanced miscibility of
the dodecyl-substituted polythiophene and polyselenophene
blocks relative to those with shorter alkyl chains, which limits
phase separation. This novel self-assembly behavior for
conjugated polymers may allow access to thin films with
distinct optoelectronic properties to known morphologies.
These findings, coupled with previous work on selenophene–
thiophene BCPs, highlight the rich self-assembly behavior of
these materials, which is proposed to arise from interplay
between miscibility of the blocks and crystallization kinetics.
Several different thin film morphologies can now be accessed
from p–conjugated BCPs simply by tuning the alkyl side-
chain substituents. Future work in this area will focus on
further probing the nature of phase separation in these
materials.
Figure 4. Self-assembly models for polyselenophene-block-polythio-
phene copolymer a) patchy and b) lamellar thin films. The selenophene
blocks are shown in red and the thiophene blocks are blue.
The optical absorption properties of the thin films confirm
a model in which the selenophene and thiophene polymer
chains are partially phase-separated and differ in rigidity. In
the optical absorption spectra of annealed thin films, low-
energy (A0–0) vibronic bands indicative of planarization of the
polymeric chains can be observed around 610 and 690 nm for
the thiophene and selenophene homopolymer, respectively
(Figure 5a).[23,30] The ratio A0–0 to A0–1 (observed at 560 and
Acknowledgements
Figure 5. Crystallinity of thin films. a) Optical absorption spectra and
b) GIXRD patterns of P3DDT50 (dotted line) and P3DDS50 (dashed
line) homopolymers and P3DDS50-b-P3DDT50 (solid line).
The authors wish to thank the Ontario Centre for Character-
ization of Advanced Materials, University of Toronto for
assistance with TEM imaging; the McMaster Analytical X-
ray Diffraction Facility for the GIXRD data; Prof. T. P.
Bender, Department of Chemistry, University of Toronto for
use of the TGA instrument; Prof. C. Allen, Leslie Dan
Faculty of Pharmacy at the University of Toronto for use of
the DSC and Shuyang Ye for preparation of the o-tolylNi-
(dppe)Cl external initiator. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada, The Canadian Foundation for Innovation, the
Ontario Research Fund, and MESRST of Quꢀbec.
630 nm, respectively) is higher for polyselenophene (0.56)
than polythiophene (0.23), which supports the higher rigidity
of the former (Supporting Information, Figure S20).[30,31] The
A0–0 vibronic band, which is associated with planarization of
the polyselenophene block, can be observed at 690 nm in the
absorption spectrum of the copolymer and the high energy
A0–2 vibronic band of the block copolymer at 560 nm overlaps
with the A0–1 band of the polythiophene homopolymer. The
A0–1 vibronic band of the block copolymer at 620 nm is
distinct from those of the homopolymers. These results differ
slightly to fully phase-separated selenophene–thiophene
copolymers with shorter alkyl chains, in which all vibronic
bands can be assigned to the corresponding thiophene and
selenophene homopolymers in the optical absorption spec-
tra.[32] This could be attributed to the lower degree of phase
separation in these samples, which contributes to the unique
self-assembly behavior of this BCP.
Conflict of interest
The authors declare no conflict of interest.
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Manuscript received: January 5, 2017
Revised: January 30, 2017
Final Article published: && &&, &&&&
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Communications
Nanotechnology
Self-assembly required: Patchy nanofib-
ers can be accessed from the self-
E. L. Kynaston, Y. Fang, J. G. Manion,
N. K. Obhi, J. Y. Howe, D. F. Perepichka,*
assembly of an all-conjugated, polysele-
nophene-block-polythiophene copolymer.
This unexpected morphology consists of
sub-10 nm polythiophene- and polysele-
nophene-rich domains and is unprece-
dented for both conjugated polymers and
diblock copolymers in general.
D. S. Seferos*
&&&& — &&&&
Patchy Nanofibers from the Thin Film
Self-Assembly of a Conjugated Diblock
Copolymer
6
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ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!