all-conjugated, rod-rod block copolymers-generation and self-assembly properties

Feature Article

All-Conjugated, Rod-Rod Block Copolymers-Generation and Self-Assembly Properties

Ullrich Scherf,* Sylwia Adamczyk, Andrea Gutacker, Nils Koenen

Based on their rigid-rod structure all-conjugated, rod-rod block copolymers show a preferredtendency to self-assemble into low-curvature vesicular or lamellar nanostructures indepen-dent from their specific chemical structure and composition. This unique and attractivebehaviour is clearly illustrated in a few examples of suchall-conjugated block copolymers. The resulting nanostruc-tured heteromaterials may find applications in electronicdevices or artificial membranes.

Introduction

Coil-coil block copolymers have gained an enormous

interest from synthetic and physical chemists, polymer

physicists, polymer engineers as well as from scientists

from related fields since a couple of decades. Driven by the

development of powerful and reliable synthetic methods

as living anionic or insertion polymerization as well

as controlled radical polymerization, thousands of studies

have been published on the nanostructure formation of

such block copolymers both in solution and the solid state.

The interplay of chemical structure, absolute and relative

block lengths and molecular weight distribution, on one

side, and the resulting nanostructure formation in solution

and the solid state, on the other side, is now well-

understood.[1,2] There has been also an increasing interest

in rod-coil block copolymers that containboth rigid-rod and

coiled building blocks. Also their self-assembly into defined

U. Scherf, S. Adamczyk, A. Gutacker, N. KoenenMacromolecular Chemistry Group and Institute for PolymerTechnology, Bergische Universitat Wuppertal, Gauss-Str. 20,D-42097 Wuppertal, GermanyE-mail: [email protected]

Macromol. Rapid Commun. 2009, 30, 1059–1065

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

nanostructures has been intensively investigated and

reviewed during the last decade.[3,4] Here, as in coil-coil

block copolymers, the occupied volume of the coil block(s)

mainly influence the resulting nanoscaled morphologies,

leading to spherical/cylindrical micelles or vesicular/

lamellar structures. In contrast, studies on rod-rod block

copolymers are currently still in an inferior state. Driven

by the increasing interest in rod-type polymers as

components of synthetic targets with biological (e.g.

polypeptides, DNA) or electronic (conjugated polymers)

function, the research into rod-rod block copolymers have

been strongly intensified during the last 3–4 years.

Combining two (or more) different rigid-rod blocks within

a rod-rod block copolymerwill allow for a controlled spatial

arrangement of these blocks in complex, functional

nanostructures with a high potential in sophisticated

heterostructured materials for applications in artificial

systemswith biological activity (functionalmembranes) or

in electronic devices (bulk heterojunction solar cells,

photodetectors).

Based on the rigid-rod structure of such rod-like building

blocks with their dramatically reduced coiling ability and

the high persistence length of the individual macromole-

cules, generally a low curvature of the aggregates formed

DOI: 10.1002/marc.200900088 1059

U. Scherf, S. Adamczyk, A. Gutacker, N. Koenen

Figure 1. Different architectures formed by block copolymers:Since coiling of the individual blocks is dramatically reduced inrod-like blocks, rod-rod diblock copolymers should preferably self-assemble into low-curvature vesicular or lamellar aggregatesindependent of their individual composition (schematic sketchesafter ref.[5]).

Ullrich Scherf had studied chemistry at the Frie-drich-Schiller-Universitat Jena/GDR and gainedhis Diploma in 1983. In 1988 he received hisPh.D under the guidance of Prof. Dr. H.-H. Horhold.Then he had spent a year at the Institut furTierphysiologie, Friedrich-Schiller-UniversitatJena, working with Prof. Dr. H. Penzlin. He joinedthe Max-Planck-Institut fur PolymerforschungMainz in 1990 and got his Habilitation in 1996.From 2000 to 2002 he has been an AssociateProfessor for Polymer Chemistry at the UniversitatPotsdam. Since September 2002 he is Full Pro-fessor for Macromolecular Chemistry at the Ber-gische Universitat Wuppertal.Nils Koenen was born in Wuppertal (Germany) in1981. He had studied chemistry at the BergischeUniversitat Wuppertal and received his Diplomadegree in 2006. His undergraduate research wasfocused on the synthesis of amphiphilic, chiralpolyarylenes. In 2007 he started his Ph.D. workunder the supervision of Prof. Dr. Ullrich Scherffocusing on the synthesis of amphiphilic, all-con-jugated block copolymers and tactic polyfluor-enes.Andrea Gutacker was born in Wuppertal(Germany) in 1981. She received her diplomadegree in chemistry from the Bergische Universi-tat Wuppertal in 2007 under the supervision ofProf. Ullrich Scherf. During that time she joinedthe group of Prof. Hugh Burrows at CoimbraUniversity (Portugal) for an internship. SinceOctober 2007 she is a Ph.D. student under thesupervision of Prof. Dr. Ullrich Scherf focusing onthe synthesis of all-conjugated, ionic block copo-lymers.Sylwia Adamczyk was born in Lodz (Poland) in1976. She had studied chemistry at the TechnicalUniversity of Lodz/Poland and received herDiploma in 2002. In 2003 she has started aninternship on analytical polymer chemistry undersupervision of Prof. Dr. Ullrich Scherf. Since 2003she is responsible for the AFM lab of the Scherfgroup.

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during their self-assembly will result. Therefore, it is

expected that rod-rod block copolymers preferably assem-

ble into vesicular (dilute solution) or lamellar (concentrated

solution, solid state) nanostructures (Figure 1). This

tendency should be mainly independent of the chemical

structure, the sizeand compositionof theblock copolymers.

Onenice example of vesicle formation innonconjugated,

rod-rod diblock copolymers was reported in 2005 by Kros

et al.[6] They investigated synthesis and nanostructure

formation of a nonconjugated polyisocyanide-block-poly-

benzylglutamate diblock copolymer. The authors could

observe the formation of large vesicles with a diameter



of > 5mm as illustrated in laser scanning confocal and

optical microscopy images (Figure 2). A second example

demonstrating the formation of a lamellar solid-state

morphology in oligo(ether sulphone)-block-oligo(ether

ketone) diblock and triblock molecules was published by

Hayakawa et al.[7] in 2006. Energy-filtering transmission

electron microscopic (EFTEM) images especially of diblock

oligomers showed the formation of lamellar superstruc-

tures with a inter-lamellar spacing of 9.1 nm in full

agreement with a molecular length of 9.2 nm for the fully

elongated (rigid-rod) conformation of the aromatic oligo-

mers.

Targets of this short feature article are now all-

conjugated block copolymers of the rod-rod-type that

contain two (or more) structurally different, conjugated

rod blocks. A review on the synthetic schemes towards all-

conjugated block copolymers has been recently pub-

lished.[8]Afterashortoverviewonthesyntheticapproaches

used in the generation of all-conjugated block copolymers

this short article aims to present examples of the self-

assembly properties of some (well-characterized) all-con-

jugated rod-rod block copolymers both in solution and the

solid state. These initial findings promise a great potential

for a controlled nanostructure design towards tailor-made

organic functional materials for various applications.

All-Conjugated Rod-Rod Block Copolymers-Generation and Self-Assembly Properties

One very remarkable synthetic advantage of the last

two decades is the availability of conjugated polymers of

high purity, low amounts of defects and high regioregu-

larity when applying powerful transition metal-catalysed

or -mediated aryl-aryl coupling methods, e.g. after

Suzuki, StilleorYamamoto.[9–11] Inaddition,novelprotocols

DOI: 10.1002/marc.200900088

All-Conjugated, Rod-Rod Block Copolymers-Generation . . .

Figure 2. Confocal laser scanning (left) and optical (right) microscopy images of a non-conjugated polyisocyanide-block-polybenzylglutamate diblock copolymer of the rod-rodtype (drop casting of a chloroform solution of the diblock copolymer onto glass slides,chemical structure of the diblock copolymer shown in the inset); images after ref.[6]

for an in situ generation of such conjugated polymers with

defined, reactive end groups have been developed.[12]

Moreover, so-called chain-growth polycondensation

schemes towards conjugated polymers have been recently

developed which allow for the synthesis of conjugated

polyarylenes with rather low polydispersity.[13–17] This

synthetic progress has been translated into novel synthetic

schemes towards all-conjugated block copolymers.

The Japanese team of Yokozawa and coworkers,[15–17]

one of the two pioneering groups in the development of

chain-growth (or catalyst-transfer) polycondensation pro-

cesses (realized e.g. for polyphenylenes, polyfluorenes or

N-substituted polypyrroles), has recently adapted their

protocols for the generation of all-conjugated diblock

Scheme 2. Synthesis of a P3HT-b-P3EHT in a chain-growth polycondensation after Zhbis(diphenylphosphino)ethane; i-C3H7, isopropyl; C6H13, n-hexyl; C8H17, 2-ethylhexyl.

Scheme 1. Synthesis of a PPP-b-PPy in a chain-growth polycondensation after Yokoyabis(diphenylphosphino)ethane; C6H13, n-hexyl.



copolymers in a so-called ‘‘grafting from’’

approach. The drastically reduced

amount of chain termination events

allows for a simple, step-by-step poly-

condensation of two (or more) AB-type

monomers in the aryl-aryl cross coupling

sequence (see Scheme 1). Following this

approach the authors have described

the successful generation of a poly(2.5-

dialkoxy-1.4-phenylene)-block-poly(N-

hexyl-2.5-pyrrole) diblock copolymer

(PPy-b-PPP).[18] The resulting all-conju-

gated block copolymer shows a rather

narrow molecular weight distribution

with Mn/Mw of 1.16. The self-assembly

properties of these attractive all-conju-

gated block copolymers have not been

described until now.

A different (‘‘grafting onto’’) approach

towards all-conjugated diblock copolymers based on the

chemical linkage of two pre-formed, mono-functionalized

conjugated blocks have been recently introduced by

Bolognesi et al.[19]

Following the ‘‘grafting from’’ scheme of Yokozawa et al.

another Japanese group has been recently described an all-

conjugated diblock copolymer that is simply based on two

regioregular poly (3-alkylthiophene) blocks with different

3-alkyl substituents (branched 2-ethylhexyl and linear

hexyl, respectively; see Scheme 2).[20] The poly(3-hexylthio-

phene)-block-poly [3-(2-ethylhexyl)thiophene] diblock

copolymer (P3HT-b-P3EHT) diblock copolymer shows the

formationof a lamellar solid statemorphologydrivenbyan

unmixing of the two different poly(3-alkylthiophene)

ang et al.;[20] THF, tetrahydrofuran; dppe, 1,2-

ma et al.,[18] THF, tetrahydrofuran; dppe, 1,2-

www.mrc-journal.de 1061


Figure 3. Formation of lamellar nanostructures in a P3HT-b-P3EHT: P3HT/P3EHT ratio: 83:17; AFM image: phase mode, imagesize: 1� 1mm2, bar length: 250 nm; film preparation by spincoating from a chlorobenzene solution onto a glass slide, anneal-ing temperature: 240 8C; interlamellar distance: 15 nm.

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blocks as a result of the different bulkiness and crystal-

lization ability of the linear and branched C8 alkyl

substituents (Figure 3).

Please note, that the synthesis of a similar poly

(3-hexylthiophene)-block-poly(3-dodecylthiophene) P3HT-

b-P3DDTwas already described in 2005 byMcCullough and

coworkers.[14] A similar approach was also published by

OshimidzuandUeda[21] leading topoly(3-hexylthiophene)-

block-poly(3-phenoxymethylthiophene) diblock copoly-

mers (Scheme 3). Again, these authors could observe the

formation of nanophase-separated lamellar or sheet-like

solid state morphologies.

Wehave studied synthesis and aggregation behaviour of

a series of amphiphilic, all-conjugated rod-rod diblock

copolymers.[22] In addition to the different chemical

structure of the two backbones we have tried to introduce

blocks of rather different polarity. The amphiphilic nature

of the resulting diblock copolymers should further

facilitate their self-assembly. Hereby, we also used a

Scheme 3. Synthesis of a poly(3-hexylthiophene)-b-poly(3-phenoxymetion after Oshimidzu and Ueda[21] THF, tetrahydrofuran; dppe, 1,2-bi



‘‘grafting from’’ approach starting from a monobromo-

endcapped, regioregular poly[3-(1-bromohex-6-yl)thio-

phene] Br-P3BrHT precursor that was generated in a

coupling protocol after McCullough and coworkers.[12]

The macromolecular precursor was further reacted with

aAB-type9.9-dialkylfluorenemonomer [2-bromo-9.9-bis(2-

ethylhexyl)-7-pinacolatoboronate] in a Suzuki-type aryl-

aryl cross couplingunder formationof thenonpolar diblock

copolymer PF2/6-b-P3BrHT. After subsequent purification

(removal of homopolymeric by-products) the bromoalkyl

side groups of the diblock copolymer intermediate were

converted into the amphiphilic target copolymers PF2/6-b-

P3PHT or PF2/6-b-P3AHT, respectively, by Arbusov reaction

with triethylphosphite leading to the phosphonic ester side

groups of PF2/6-b-P3PHT or with several amines (e.g.

trimethylamine or pyridine) leading to the nitrogen-based

cationic side groups of PF2/6-b-P3AHT (Scheme 4).[8,23]

The resulting amphiphilic, all-conjugated diblock copo-

lymers have been investigated for their self-assembly

properties in solution. First, static and dynamic light

scattering experiments (SLS/DLS) demonstrated the rigid-

rod nature of individual, nonaggregated copolymer chains

(Rg/Rh: 3.61). Further on, the scattering data indicate the

presence of a small fraction of spherical, probably vesicular

aggregates (Rg/Rh: ca. 1; diameter: ca. 140nm)already in the

nonspecific solvent THF (as good solvent for both blocks).

Addition of water as a nonsolvent for the hydrophobic

polyfluorene block to a solution of PF2/6-b-P3PHT in THF

leads to the formation of spherical aggregates with an

average diameter of ca. 150nm [as estimated by atomic

force microscopy (AFM)]. Optical spectroscopy (absorption

and photoluminescence spectroscopy) documents a two-

step aggregation process. First step is the aggregation of the

nonpolar polyfluorene blocks for water contents of >30%.

The second step is an additional aggregation of the polar

polythiophene blocks for high water contents >70%. The

second step is accompanied by distinct changes of

absorptionspectraandcolouras resultof thewell-described

solvatochromic properties of regioregular polythiophenes

and by a ca. 30% reduction in the diameter of the spheres.

thylthiophene) diblock copolymer in a chain-growth polycondensa-s(diphenylphosphino)ethane; C6H13, n-hexyl; i-C3H7, isopropyl.

DOI: 10.1002/marc.200900088


Scheme 4. Synthesis of all-conjugated, amphiphilic polythiophene-b-polyfluorenediblock copolymers after Tu et al.[22] and Gutacker et al.;[23] C8H17, 2-ethylhexyl; NRþ

3 ,N(CH3)þ3 or N-alkylpyridyl; Pd[0], Pd(PPh3)4.

Next, we have tried to further characterize the spherical

aggregates by AFM.[24] After spreading a THF solution of

PF2/6-b-P3PHTat theair/water interfaceAFMimagesof the

subsequently transferred thin Langmuir-Blodgett (LB)

layers onto glass slides showed indeed the presence

of spherical vesicles (polymersomes) in coexistence with

lamellar regions (Figure 4). The ratio of vesicular and

Figure 4. AFM images of LB films made from a non-ionic, amphiphilic PF2/6-b-P3PHT diblointerface onto a glass slide at 50 mN/m showing the coexistence of lamellar and vesicchemical structure of the block copolymer is shown in the inset: n: ca. 80; m: ca. 26; AFMright: phase image 3.0� 3.0mm2, zoomed from the left image).



lamellar aggregates depends on the

applied surface pressure. The average

size of the vesicles in the AFM images

was, hereby, in very good agreement

with the light scattering numbers.

The stable, purple-coloured disper-

sions containing the spherical PF2/6-b-

P3PHT vesicles (polymersomes), e.g. in

THF-water or acetone-hexane solvent

mixtures, can be simply spin-coated to

dense and homogeneous layers onto a

suited substrate (glass,mica, silicon). The

spin-coated films show the formation of

terrace-like, layered structures. The indi-

vidual sub-layers are composed of den-

sely packed vesicles. The thickness of the

terraces (ca. 34.5 nm) formed after spin-

coating are a goodmeasure for theheight

of the collapsed (soft) vesicles with their

initial diameter of ca. 120nm (Figure 5). This formation of

such dense, nanostructured films opens up further possi-

bilities to create hetero-structured nanomaterials, e.g. by

spin-coating a second component on top of the primarily

formed layers.

Next, we have investigated the self-assembly properties

of the related, cationic counterparts of PF2/6-b-P3PHT, the

ck copolymer after transfer from the air-waterular nanostructures (diameter: ca. 120–150 nm;: contact mode, left: topography 5.0� 5.0mm2,



Figure 5. Spin-coating of a latex dispersion of vesicles formed from a non-ionic, amphiphilic rod-rod-type PF2/6-b-P3PHT diblock copolymer(chemical structure of the diblock copolymer, see inset) from acetone-hexane (1:1) onto a silicon substrate under formation of terrace-likestructures (the block copolymer vesicles collapse during the spin-coating procedure from an initial diameter of ca. 120 nm to a step height ofca. 34.5 nm.

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ionic copolyelectrolytes PF2/6-b-P3AHT that contain catio-

nic tertiary ammonium groups in the polar P3AHT blocks.

First, the ionic nature of the copolymers leads to an

increased solubility in polar solvents asmethanol orwater.

Again, optical spectroscopy and light scattering experi-

ments indicate the formation of (nano)aggregates. AFM

imagesofdrop-cast layers frommethanolontosiliconslides

again show the formation of vesicular polymersomes.

These results impressively illustrate the former postulated

Figure 6. AFM images of collapsed large vesicles (diameter: 400–1 500 nm) formed by an amphiphilicPF2/6-b-P3AHT diblock copolyelectrolyte (chemical structure shown in the inset) as deposited frommethanol on silicon substrates by drop-casting (tapping mode, left: topography 5.0� 5.0mm2, right:phase image 5.0� 5.0mm2).

tendency of rod-rod block

copolymers towards vesicle

(or lamellae) formation. In

comparison to the polar, non-

ionic block copolymers PF2/

6-b-P3PHT the ionic block

copolymers PF2/6-b-P3AHT

formmuch larger vesicleswith

diameters of 400–1500nm.

Especially the tapping mode

phase images impressively

illustrate the presence of col-

lapsed vesicles. The collapse is

hereby accompanied by some

re-distribution of the organic

material on the silicon sub-

strate (Figure 6).

Further AFM experiments

used higher concentrations of

the ionic diblock copolymer in

methanol. Drop-casting from

methanol onto silicon sub-

strates leads to adenser cover-

age of the substrate accom-



panied by a transition to solid state-like films. As expected,

we could find a transition from a vesicular to a lamellar

morphology with a step height (period) of 35–40nm

corresponding to the average length of the interdigitated

block copolymer chains. The regions with the lamellar,

terrace-like structure show a parallel orientation of the

lamellae with respect to the silicon surface. The detailed

investigation of the bulk structure will be the target of

further studies.

DOI: 10.1002/marc.200900088


Conclusion

All-conjugated rod-rod block copolymers came into the

focus of interest because of their potential to self-assemble

into layered aggregates both in solution and the solid state.

The rigid-rod structure of the individual macromolecules

favours the formation of low-curvature vesicular and

lamellar aggregates independent of the specific chemical

structure and composition. First examples to the self-

assembly of such all-conjugated rod-rod block copolymers

support these expectations.

The block copolymers allow for a simple and reliable

control over the resulting nano-scaled structures and are,

therefore, promising candidates for an application as active

layer in electronic devices (bulk heterojunction-type

organic solar cells, BHJ-OSC) or as functional membranes

(e.g. for sensor applications). Without any doubt, the next

few years will generate a variety of other all-conjugated,

rod-rod block copolymers.

Acknowledgements: The authors would like to thank for thescientific inputs of Guoli Tu, Hongbo Li, Hugh Burrows, RigobertoAdvincula, Reinhard Sigel, Ludwig J. Balk and their coworkers. Theresearch was partially funded by the Volkswagen foundation.

Received: February 9, 2009; Accepted: March 23, 2009; DOI:10.1002/marc.200900088

Keywords: block copolymers; conjugated polymers; nanostruc-tures; self-assembly; vesicles

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all-conjugated, rod-rod block copolymers-generation and self-assembly properties

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