Hierarchical Self-Assembly in Monoaxially electrospun P3HT PCBM nanofibers

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Hierarchical Self-Assembly in MonoaxiallyElectrospun P3HT/PCBM Nanofibersa

Caitlin Dillard, Richa Singhal, Vibha Kalra*

We report monoaxial electrospinning and self-assembly characterization of poly(3-hexylth-iophene)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) nanofibers. Nanofiberswere directly compared to films (as made and annealed) to understand the effects of
extensional forces and high evaporation rateinherent to electrospinning on nanoscale struc-ture. X-ray scattering shows electrospinning canachieve improved crystallinity and reducedlength scale of P3HT and PCBM phases. Due toextensional flow, co-continuous phases formalong the fiber axis, as evident in transmissionelectronmicroscopy. To our knowledge, this is thefirst study to investigate monoaxially-spun, pureP3HT/PCBM nanofibers, a facile processing meth-od with the potential to transform the field ofwearable photovoltaics.
C. Dillard, R. Singhal, Prof. V. KalraDepartment of Chemical and Biological Engineering, DrexelUniversity, 3141, Chestnut Street, Philadelphia, PA 19104, USAE-mail: [email protected]

aSupporting Information is available from theWiley Online Library orfrom the author.

� 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.c

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M), in which efficiencies remain �5% for single cells

6.5% in tandem cells.[2,3] Spin-coating is the typical

1. Introduction

In recent years, the nanoscale morphology of bulk

heterojunction (BHJ) films has been identified as the key

parameter inhibiting organic solar cell (OSC) performance.

The BHJ is comprised of two intimately mixed phases,

electron donor and electron acceptor, that create percola-

tion paths in which electron–hole pairs (exciton) can

separate and transport to electrodes for collection. The

phase domains must be 5–20nm in width while providing

continuous pathways to electrodes.[1] The most widely

researched BHJ system for OSC applications consists of

conjugated electron donating polymer poly(3-hexylthio-

phene) (P3HT), and electron accepting fullerene-derived

molecule [6,6]-phenyl-C61-butyric acid methyl ester

(PCB

and

processingmethod of P3HT:PCBM-based BHJ-OSCs, provid-

ing no control over the nanoscale morphology. Most works

in the past have been conducted solely on films in order to

correlate BHJ morphology or nano-structure to the opto-

electrical properties. Extensive studies on P3HT:PCBM BHJ

films indicate that simpleprocessingvia thermal annealing

can (i) improve crystallinity, which can improve charge

transport and (ii) produce continuous donor/acceptor

pathways, which can increase the probability of charge

collection.[4] Enhanced crystallinity improves charge trans-

port due to increased p–p stacking of the polymer chains,

which is the main route of charge transfer.[5] Thermal

treatment above the glass transition temperature of

P3HT:PCBM (80–100 8C) leads tomobility in the amorphous

phase allowing for increased P3HT crystallization and p–p

interchain stacking as well as PCBM diffusion and

nanoscale aggregation.[6] Lu et al. found that P3HT

crystallinity gradually increased with annealing tempera-

ture between 130 and 170 8C, well below the melting

temperature of P3HT and PCBM (>200 8C).[6,7] Moreover, by

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annealing for less than 5min, they foundmicrometer-sized

PCBMaggregates at 120 8C and PCBM clusters up to 100mm

inlengthat200 8C.Thehighestefficiencyachievedwas4.5%

from annealing a fully fabricated device at 170 8C for

30min. In another work, Ma et al.[2] fabricated P3HT:PCBM

devices reaching 5% power conversion efficiency (PCE)

after annealing. They obtained insightful transmission

electron microscope (TEM) images revealing the develop-

ment of interpenetrating donor/acceptor networks from

annealing at 150 8C – the annealing temperature they

found to be optimal for tuning the nanoscale phase

separation. Although improving BHJ film morphology

has been a focus of most works, enhanced crystallinity

can be achieved by other processing methods, such as

electrospinning. In thiswork, we study the electrospinning

and self-assembly in P3HT/PCBM nanofibers. Electrospin-

ning is a facile fiber fabrication technique that applies a

high voltage to a polymer solution/melt jet.[8–11] The

solution jet undergoes high solvent evaporation rate and

extensional deformation to form nanofibers on the

grounded collector. Electrospinning produces continuous,

cylindrical nanofibers with uniform diameters 50–

500nm.[12] Nanofibers can provide an excellent platform

for the development of a new class of materials – smart

textiles with integrated solar cells. Furthermore, the

extensional flow during electrospinning can create oppor-

tunities to achieve extension-induced crystallinity, which

is expected to improve efficiency, as demonstrated by

Bedford et al.[13] To the best of our knowledge, only two

works have addressed the electrospinning of P3HT/PCBM

nanofibers, but both implemented coaxial electrospinning

that requires more materials, adds an additional step to

purify P3HT/PCBM nanofibers, and also creates the risk of

inefficiently removing the excess material shell.[13,14]

Moreover, both of these studies lack in-depth morphology

studies – even though morphology has been identified

as the most critical parameter for improving electrical

properties and device efficiency.

In this work, we report successful monoaxial electro-

spinning of pure P3HT/PCBM to produce continuous

nanofibers with submicron diameters. We studied the

self-assembly of the P3HT/PCBMsystemusingwide-angle

X-ray scattering (WAXS) and small-angle X-ray scattering

(SAXS). We investigated the local internal nanofiber

structure using transmission electron microscopy (TEM)

of microtomed fiber cross-sections. Finally, we performed

a systematic study of thermal annealing on the self-

assembled nanostructure. We found enhanced crystallin-

ity as a result of electrospinning, as shown in the

narrowing of X-ray scattering peaks from P3HT:PCBM

films to electrospun P3HT/PCBM nanofibers. We believe

this enhancedmorphology and hierarchical self-assembly

could be instrumental in improving charge transport in

OSC devices.

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2. Results and Discussion

Monoaxial electrospinning was used to fabricate P3HT/

PCBMnanofibers. Electrosprayedbeads, beadednanofibers,

and continuous nanofibers were made with varying the

solution concentration of P3HT/PCBM (1:1w/w) in chloro-

form shown in Figure 1. As shown in Figure 1a, the lowest

solution concentration resulted in electrospraying. This

was expected due to lack of entanglements within the

solution both due to the relatively low concentration and

the rigid rod behavior of conjugated P3HT polymer in

solution.[15] Successful electrospinning of continuous

fibers requires viscoelastic properties resulting from chain

entanglements such that the charged polymer jet is stable

enough to withstand the strong elongation forces during

electrospinning.[16] Below the critical concentration, an

axisymmetric instability by lowered surface tension will

create capillary breakup of the solution jet.[17] As the

concentration was increased to 15.8wt% as shown in

Figure 1b, continuous, but beaded nanofibers were

produced with diameters ranging from 200–800nm. The

concentration was substantial enough for successful

spinning, but was still insufficient, resulting in beads. In

Figure 1c, a 19wt% solution was electrospun to produce

fibers with virtually no beads, but with consistently larger

diameters (450 nm–1.5mm). Only a handful of publications

have addressed the electrospinning of P3HT/PCBM nano-

fibers in which coaxial electrospinning was utilized.[13,14]

However, this methodology increases material cost,

requires a more complex setup and scale up, and can also

impede performance due to insufficient shell-removal,

which could act as an insulator. To the best of our

knowledge, there are no works that establish successful

monoaxial electrospinning of continuous P3HT/PCBM

nanofibers.

Conjugated polymers including P3HT exhibit rigid rod

behavior in solution,which results in their inability to form

entanglements and renders them non-electrospinnable

by themselves, even at high molecular weights. A few

publications have electrospun pure P3HT nanofibers many

of which required a coaxial setup.[18–21] In the monoaxial

electrospinning of pure P3HT, Chan et al.[18] note that after

the complete dissolution of P3HT in an appropriate solvent,

if the solution was kept undisturbed for at least 30min

prior to electrospinning, it could form continuous fibers.

This period allowed for sufficient P3HT gelation, which is

described as a two-phase process: a formation phase of

rod-like aggregates followed by a crystallization phase

of physical entanglements and cross-linking of the

aggregates.[18,22] The physical mechanism of chain-chain

entanglement during gelation is required for lower

concentrations (<7wt%). In thework of Lee et al.,[20] higher

concentrations of P3HT were electrospinnable (�11wt%),

but fast crystallization of P3HT at the nozzle rendered the

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Figure 1. SEM images of as made 1:1 (w/w) P3HT/PCBM electrospun nanofibers with increasing concentration in chloroform: a) 14.8%,b) 15.8%, and c) 19.1%.

Hierarchical Self-Assembly in Monoaxially Electrospun. . .

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solution poor for electrospinning, resulting in large ribbon-

like fibers and beaded fibers.

In this work, we found that a gelation period was not

necessary and the P3HT/PCBM solution was successfully

spun instantaneously. This is interesting, as one would

expect the PCBM molecules to further reduce solution

viscosity and therefore electrospinnability. Nevertheless,

the electrospinning of P3HT/PCBM solutions suggests that

PCBMmay be favorable for P3HT chain–chain interactions

and P3HT entanglement.We believe thismay be attributed

to the newly understood miscibility of PCBM in P3HT, in

opposition to previous thinking of total immiscibility.[23,24]

Owing to this compatibility of PCBMmolecules with P3HT

Figure 2. SEM images of 1:0.75 (w/w) P3HT/PCBM electrospun nanofibfabrication annealing times of: b) 5min, c) 30min, and d) 60min. A

Macromol. Mater. Eng. 2014, DO

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chains,we believe they serve as nodes of association for the

P3HT chains resulting in the formation of transient

networks enhancing the material elasticity, similar to

nanoparticles in polymer nanocomposites.[25]

Annealing P3HT:PCBM film-based devices is known to

improve PCE due to increased crystallization of P3HT and

PCBM. However, in a nanofiber-based system, it may

destroy the cylindrical confinement of the fibers. To study

this, we conducted SEM of nanofibers annealed at various

conditions. As shown in Figure 2, annealing in air at 150 8Cfor short times does not exhibit severe deformation. The as

madenanofibers fromFigure1bare reproducedas reference

in Figure 2a. Fibers were imaged (a) as made and after

ers with 17.3wt% concentration in chloroform. a) As made and post-ll annealing was at 150 8C in air.

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annealing at 150 8C for (b) 5min, (c) 30min, and (d) 1 h.

Additional SEM images for 10, 15, and 20min annealing

times have been included in supporting data (Figure S1,

Supporting Information). Highermagnifications show that

the fibers begin to lose cylindricality beyond 5min of

annealing and the fiber surfaces grow spherical protru-

sions, possibly due to PCBM aggregation. We believe the

longer annealing times may cause the fiber structure to

become less stable, due to extended time above the glass

transition temperature, and cannot fully support its

original cylindrical confinement. Within 10min of anneal-

ing, protrusions begin appearing (see Figure S1 in Support-

ing Information). Based on improvements observed in

WAXS and SAXS (i.e., increased crystallinity and enhanced

phase separation, respectively), 5min of annealing shows

significant improvements in the nanoscale structure (to

be discussed later).

As made fibers were microtomed for TEM to understand

the internal nanoscale-phase separation due to electro-

spinning as shown in the Figure 3. A fiber cross-section

(Figure 3a) and longitudinal section (Figure 3b) show the

initial phase separation due to electrospinning. The lighter

and darker regions are P3HT-rich and PCBM-rich regions,

respectively, due to the lower electron density of P3HT

compared to PCBM.[26] In Figure 3, the interpenetrating

networks between P3HT and PCBM regions are approxi-

mately 20–30nm in width and are continuous throughout

the fiber cross-section suggesting the formation of a co-

continuous structure (Figure 3a). The strong elongational

force and geometrical confinement during electrospinning

could induce polymer chains to orient along the fiber

axis.[27] The phase separated pathways seen in Figure 3 are

of similar widths in the longitudinal section, but appear to

Figure 3. TEM images of as made P3HT/PCBM nanofibers. a) Fibercross-section normal to the axis across; and b) longitudinalsection parallel to fiber axis. Scale bars¼ 100nm.




be aligned parallel to the fiber axis. Therefore, we believe

this can be attributed to stretching along the fiber axis

during electrospinning. The lighter contrast around the

outside of the fiber shell could indicate a higher concentra-

tionofP3HTbecause thesurfaceenergy is lower thanthatof

PCBM.[28] This phenomena has been observed in electro-

spinning of block copolymers with different surface

energies.[29] In Figure S2, Supporting Information, TEM of

fibers annealed for 5min reveal similar features compara-

ble to thatof theasmadefibers inFigure3aandb.TEMisnot

sufficient inquantitativelyprobing thephaseseparationon

a nanoscale, therefore X-ray scattering was conducted to

investigate the effect of annealing.

In literature, self-assembly and reordering studies in

P3HT:PCBM films make use of highly surface dependent

characterization techniques, most commonly grazing

incidence X-ray scattering and diffraction (GISAXS/

GIWAXS or GXRD). These techniques are frequently applied

to fully fabricated device in which the active layer was

etched out for assembly studies. It has been found that the

electrode deposition greatly impacts the morphology of

the photoactive layer at the interface, therefore we believe

conventional X-ray scattering techniques on bulk samples

are critical and will complement existing work.[2] Self-

assemblybetweenP3HTandPCBMwascharacterizedusing

small angle X-ray scattering (SAXS) of bulk drop-cast

films and electrospun nanofibers, shown in Figure 4 and 5.

P3HT:PCBM films were made in chloroform to directly

compare our P3HT/PCBM/chloroform nanofibers. Nearly,

all studies on P3HT:PCBM films in literature use chloroben-

zene and 1,2-dichlorobenzene as the solvent due to higher

boiling points and increased solubility of P3HT and PCBM.

In films, a higher boiling point is desired to slow the rate of

solvent evaporation, allowing the materials to approach a

more thermodynamically favorable degree of phase

separation. On the contrary, rapid solvent evaporation is

required in electrospinning for fiber formation. For

consistency, our films were also chloroform-based. As

shown in Figure 4a, (small-angle) X-ray scattering peaks in

P3HT:PCBM chloroform films as made and annealed at

150 8C for 5minwere observed atq�0.213 and0.238nm�1,

corresponding to d-spacings of 29.5 and 26.4 nm, respec-

tively, indicating the characteristic length scale was

reduced during annealing. After further annealing up to

30min, the peak disappears, which could be attributed to

diffusion of PCBM into amorphous P3HT resulting in the

reduced degree of phase separation. During fabrication

processes like electrospinning and solution casting, P3HT

and PCBM phases are often kinetically trapped. Thermal

annealing provides the energy necessary for molecular

reordering to reduce the domain size (i.e., P3HT chains

becomemobile and can arrange to amore crystalline state),

as observed in literature.[24] As shown in Figure 4b, SAXS

peaks in electrospun nanofibers were observed only after

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Figure 4. Small angle X-ray scattering (SAXS) of (a) bulk film and (b) nanofibers to compare as made samples with samples annealed at150 8C for 5 and 30min.


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5min of annealing, q � 0.27 nm�1 (d-spacing of 23.3 nm).

Upon5minof annealing, a characteristic spacingof23.3 nm

was developed, indicating microphase separation was

achieved on a smaller length scalewith respect to annealed

films. Similar trendshavebeenobserved innumerousP3HT:

PCBMfilm-based systems inwhich shorter annealing times

(5–10min) at higher temperatures (140–160 8C) were

optimal to balance improved crystallinity and limited

microphase separation, thus improved optoelectrical prop-

erties.[24,30] Tonote theprominence of peaks at each stage of

annealing, thedata fromFigure4were overlapped fordirect

comparison of film and fibers, as shown in Figure 5.

Wide angle X-ray scattering (WAXS) was performed on

films and nanofibers to determine crystallinity andmolecu-

lar ordering, as shown in Figure 6 and 7. As shown in

Figure6a, theasmadefilmexhibitedpeaksatapproximately

q¼ 3.7, 7.6, and 16.4 nm�1, corresponding to d-spacings of

Figure 5. SAXS of bulk film and fibers (a) as made, (b) 5min annealing




1.7, 0.83, and 0.38nm, respectively. These values are

agreement with those reported in literature on pure P3HT

filmsaswell aspreviousdata inour labandcanbeattributed

to (100), (200), and (010) planes, respectively (the first two

to the lamellar interlayer stacking structure and the last to

the p–p interchain stacking structures).[31–33] The as made

film also exhibits a broader, amorphous peak centered

around q¼ 14 nm�1 (d-spacing of 0.45nm), consistent with

P3HT/PCBM literature (q¼ 12–15 nm�1).[13,33,34] We believe

this can be attributed to the amorphous region in which

diffused, miscible PCBM is dispersed throughout an

amorphous P3HT region.[35] With subsequent annealing

of thefilms, there isa reduction inpeaksharpness indicating

reduced crystallinity. Although annealing is expected to

increase crystallinity, the films show the opposite

trend, which we attribute to drop-casting P3HT:PCBM in

chloroform. Due to the high vapor pressure of chloroform,

, (c) 30min annealing. All annealing treatments were at 150 8C in air.

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Figure 6. Wide angle X-ray scattering (WAXS) of (a) bulk films and (b) fibers to compare as made samples with samples annealed at 150 8Cfor 5 and 30min in air.

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drop-casting the films may be inefficient in minimizing

solidification at the pipette tip while the solution is

deposited on the glass substrate.

For electrospun nanofibers, we observe an increase in

crystallinity during annealing, as shown in Figure 6b. As

made fibers exhibited the same peaks attributable to

the (100), (200), and (010) planes of lamellar interlayer

and p–p interchain stacking structures of P3HT, as well as

the broad amorphous peak around q � 13.9 nm�1. Upon

thermally annealing the nanofibers, the amorphous peak

sharpens at q � 13.5 nm�1 (d-spacing of 0.46 nm), which

could be a result of nanocrystalline PCBM domain growth

or an additional p–p stacking plane. With longer anneal-

ing times above the glass transition temperature, we

expected greater mobility in PCBM to diffuse through

amorphous P3HT. It has been established that PCBM

does not disrupt crystalline P3HT regions, therefore as

PCBM diffuses, the lamellar packing and p–p interchain

stacking structures remain undisturbed by PCBM,

Figure 7. WAXS of bulk films compared to nanofibers (a) as made awas done at 150 8C in air.




hence a sharpening of the peaks. To further distinguish

enhanced crystallinity in fibers over films, the data from

Figure 6 were combined, as shown in Figure 7. Distinct

peaks for fibers, as made and annealed, versus films

indicate that electrospinning enhanced the crystalline

structure in P3HT/PCBM nanofibers due to the strong

extensional deformation during electrospinning.[32]

UV–vis absorption spectra of film solution and fiber

dispersions exhibited a red-shift in the absorption peak

from films to fibers, further supporting that fibers have a

higher degree of crystallinity than films and can be useful

in photoelectronic applications (see Figure S3 in the

Supporting Information).[13,19,36]

3. Conclusion

P3HT/PCBM nanofibers were produced using a single-step

(monoaxial) electrospinningmethodology. The advantages

nd annealing treatment for (b) 5min and (c) 30min. All annealing

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of monoaxial electrospinning include effectively reduced

cost (no excess material), increased simplicity (dual nozzle

systems are more complex for scaling up), as well as pure

fibers (eliminates possibility of residual, insulating, shell-

polymer in fibers). To our knowledge, successfulmonoaxial

electrospinning has not been deemed possible in previous

works owing to the limited chain entanglements of rod-like

P3HT molecules. In our work, we found that when mixed

with PCBM, the solution was indeed electrospinnable

without theneedforacoaxial shell orauxiliarypolymer.We

believe this could be attributed to PCBM molecules or

aggregates possibly serving as nodes for temporary net-

works between P3HT chains to enhance the material

elasticity, similar to nanoparticles in polymer nanocompo-

sites. X-ray scattering and electron microscopy character-

izations were used to correlate the processing effects on

nanoscale structure. Electrospun nanofibers were directly

compared to drop-cast films to understand the effects of

extensional forces and high evaporation rate inherent to

electrospinning on nanoscale structure. The electrospun

nanofibers exhibited increased lamellar packing as well as

increased p–p interchain stacking relative to films.

Annealing electrospun nanofibers at 150 8C for 5min

was foundoptimal based on thewell-balanced advantages:

(i) enhanced crystallinity: the (010) peak at �16.2 nm�1 is

known to be the most important direction of hole

transport in P3HT for devices like OLED/OSCs.[37] WAXS

data provides improved (010) signal at 5min annealing.

(ii) enhanced phase separation: with more pure phases

separated (determined by SAXS) at distances similar to

the exciton diffusion length (�20 nm), we can expect

reduced recombination. Recombination is the largest

factor contributing to the lower efficiencies in OSCs

relative to other PV technologies;[38] (iii) maintained

cylindrical fiber morphology: the fibers maintain their

original structure in the first 5min of annealing while

annealing for 10min or longer begins to compromise the

fiber cylindricality (confirmed by SEM). We believe that

electrospinning could be a useful method to further

develop the field of photovoltaics. In the future, we intend

to implement these electrospun nanofibers in devices to

complete a process–structure–property relationship to

understand how these processing techniques (i.e., electro-

spinning and annealing) impact the opto-electronic

properties in solar cell devices.

4. Experimental

4.1. Materials

Regioregular P3HT (MW 45 000–65 000 gmol�1), PCBM, and

chloroform (>99.8% purity) were purchased from Sigma–Alrich,

Nano-C, and Amresco, respectively. The materials were used as

received.




4.2. Preparation of Nanofibers

P3HT/PCBM were combined in a 1:1 (w/w) ratio in chloroform to

produce solution concentrations between 14.8% (wt) to 20% (wt).

The minimum and maximum concentrations were processed to

demonstrate the range of P3HT/PCBM electrospinnability. All

structural investigations used a solution concentration�17% (wt).

Prepared solutions were sealed in a glass vial and stirred on a

magnetic hot plate for aminimumof 5h at 60 8C. After dissolution,the solutions were transferred to a 100ml gastight microsyringe

with a 20 gauge needle. The syringe was set up in a micropump

(New Era Pump Systems, Inc., N-300). The tip of the needle was

connected toahighvoltagesupply (GammaHighVoltageResearch,

ES-30P) and was placed �12 cm from an aluminum foil grounded

collector where the fibers were collected. The nanofibers were

electrospun at room temperature with relative humidity ranging

from 30 to 40% under an applied electric potential of 6–8kV. The

solution flow rate was held constant during electrospinning at

0.5mlmin�1. The needle tip was wiped cleaned �30–60 s to

prevent dried solution from clogging.

4.3. Preparation of Films

Excess P3HT/PCBM solutions were diluted by addition of chloro-

form (over a factor of 100) and drop-cast on clean glass petri dishes.

Due to rapid chloroform evaporation, the films were immediately

covered by the glass lid in attempt to slow the rate of solvent

evaporation. Multiple thin films were made and removed with a

clean razor blade and placed in Kapton specimen tape for X-ray

scattering. It should be noted that this tape does not exhibit X-ray

scattering peaks that interfere with our signals. All films and

nanofibers were produced at ambient conditions. Annealed

samples were prepared in an incubator (Fisher Scientific, 281A)

at 150 8C in air for 5 or 30min. Any annealed samples were

annealedonglasspetridishes (films)oroncollector foil (fibers)prior

to adhering Kapton specimen tape to prevent any interactions/

reactions from disrupting the sample.

4.4. Characterization

The outer nanofibermorphologywas characterized using scanning

electronmicroscopy (Zeiss Supra 50VP). The internal self-assembly

and structural study was conducted using transmission electron

microscopy (JEOL JEM2100) at an accelerated voltage of 120 kV. In

order to performTEMof the electrospunnanofibers, theywere first

embedded in an epoxy matrix and then microtomed using a Leica

EM UC6 ultramicrotome equipped with a diamond knife into

�80nmthick sections. Self-assemblyof thenanofiberswas studied

with X-ray scattering in small and wide angles at room

temperature using a Rigaku S-MAX3000 pinhole SAXS camera

with a 3-m detector and a Cu K a source. WAXS data was collected

simultaneously with an image plate.

Acknowledgements: The authors would like to thank theNational Science Foundation (Grant number CMMI-1144376) forfinancial support. The authors are grateful to the Centralized

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Research Facility of Drexel University for instrumentationsupport.

Received: June 17, 2014; Revised: September 24, 2014; Publishedonline:DOI: 10.1002/mame.201400214

Keywords: donor-acceptor systems; hierarchical self-assembly;monoaxial electrospinning; nanofibers; transmission electronmicroscopy

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