This is he accep ed manusc ip o he a icle ha appea ed in inal o m in Thin Solid
Films 519: 1882-1885 (2011), which has been published in inal o m a
h ps://doi.o g/10.1016/j. s .2010.10.060. © 2010 Else ie B.V. This manusc ip e sion
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Nanos uc u a ion o sel -assembled poly(s y ene-b-isop ene-b-s y ene) block
copolyme hin ilms in a highly o ien ed py oly ic g aphi e subs a e
Iñaki Zalakain, Jose Angel Ramos, Raquel Fe nandez, Ha i z E xebe ia and Iñaki
Mond agon*
G oup ‘Ma e ials + Technologies’, Escuela Poli écnica, Dep o Chemical &
En i onmen al Enginee ing, Uni e sidad País Vasco/Euskal He iko Unibe si a ea.
Pza. Eu opa 1, 20018 Donos ia-San Sebas ián, Spain
*Co esponding au ho . Tel.: + 34-943017177; Fax: +34-943017200
E-mail add ess: [email p o ec ed] (I. Mond agon)
ABSTRACT
Highly o ien ed py oli ic g aphi e (HOPG) is a use ul subs a e o isualize epi axial
o ma ion due o i s c ys allog aphic s uc u e. The mo phology o a poly(s y ene-b-
isop ene-b-s y ene) block copolyme hin ilm on a HOPG subs a e was in es iga ed by
a omic o ce mic oscopy. Block copolyme domains gene a ed a mo phology wi h
iangula egula i y. This a angemen was induced by he HOPG subs a e s uc u e
due o an de Waals a ac ion be ween he HOPG π-conjuga ed sys em and a oma ic
ing o polys y ene domains. Howe e , inc easing he ilm hickness, he subs a e
e ec on he su ace mo phology dec eased. As a consequence, ilm su aces showed
he coexis ence o di e en s uc u es such as highly aligned cylinde s and pe o a ed
lamellae. When ilm hickness exceeded a h eshold alue, he subs a e did no ha e
e ec in he su ace a angemen s and he su ace showed a simila mo phology o ha
exis ing in bulk.
Keywo ds: Thin ilms, Poly(s y ene-b-isop ene-b-s y ene), Highly o ien ed py oli ic
g aphi e, Epi axy, A omic o ce mic oscopy, Block copolyme
1. INTRODUCTION
The capabili y o block polyme s o sel -assemble in a ange o 10–100 nm makes hem
excellen candida es o applica ions in nano echnology [1], [2], [3], [4], [5], [6], [7].
Block copolyme s a e composed o wo o mo e chemically di e en , and equen ly
immiscible, polyme blocks co alen ly bonded oge he . The modynamic
incompa ibili y be ween blocks gene a es mic ophase sepa a ion maximizing o
minimizing he con ac a ea be ween blocks [8]. The mos ele an pa ame e s in sel -
assembling a e he olume ac ion and molecula weigh o he blocks, and empe a u e
dependen Flo y–Huggins in e ac ion pa ame e [9], [10]. Depending on hese
pa ame e s, he block copolyme can sel -assemble in o a a ie y o mo phologies as
lamella , hexagonally packed cylind ical, body cen e ed cubic sphe ical, gy oidal and
hexagonally pe o a ed laye s, among o he s [1], [2], [3], [4], [5], [6], [7]. The induc ion
o he domains o ien a ion can be achie ed by di e en me hods such as elec ic ields,
di ec ional c ys alliza ion o con ining in physically de ined pa e ns, among o he
me hods [11], [12], [13]. Howe e , ew au ho s ha e epo ed block copolyme domain
o ien a ions by he epi axial o ma ion me hod [14], [15]. The epi axial o ma ion
consis s o mac omolecule alignmen s induced by s ong in e ac ions be ween subs a e
and deposi ed molecules. Thus, he copolyme domains can be o ien ed along he
c ys allog aphic la ice o he subs a e. The a omically la basal su ace o he highly
o ien ed py oly ic g aphi e (HOPG) subs a e makes possible polyme epi axial
o ma ion [16]. Each ca bon a om in g aphi e has a ee elec on due o sp2 hyb idized
o bi als. Thus, he subs a e can in e ac by π-in e ac ions wi h deposi ed
molecules [17]. These a ac ions can induce molecules o ien a ion in wo dimensions
along g aphi e c ys allog aphic axes [18]. The epi axial o ma ion has been ex ensi ely
esea ched in di e en hyd oca bon molecules such as cellulose, alkanes, ca boxylic
acids, hiols, among o he s bu ew imes using block copolyme s [17].
On he o he hand, in block copolyme s mo phology can also be a ec ed by ilm
hickness [6], [7]. Inc easing he ilm hickness, in e ac ions wi h he subs a e canno be
enough o in luence he su ace a angemen . In block copolyme s, he ilm hickness is
ela ed o copolyme domain cha ac e is ic spacing. I he hickness is p opo ional o
he domain cha ac e is ic spacing, he ilm will display a homogeneous su ace.
Howe e , i he ilm is no p opo ional o he domain cha ac e is ic spacing, he su ace
will show islands and holes wi h di e en hickness egions
( e aces) [5], [6], [7], [19], [20]. These egions will keep he domain cha ac e is ic
spacing.
In his wo k, he poly(s y ene-b-isop ene-b-s y ene) (SIS) block copolyme has been
s udied using he HOPG subs a e. The ob ained mo phologies we e s udied by he
a omic o ce mic oscopy (AFM) echnique. The in luence o hickness in he epi axial
o ma ion was analyzed a ying he ilm hickness. The esul s o his s udy e eal a
iangula mic os uc u e in he hinnes ilm. Howe e , inc easing he ilm hickness,
he subs a e has a lowe in luence gi ing a a ie y o a angemen s wi h no sign o
epi axial o ma ion.
2. EXPERIMENTAL SECTION
2.1. SAMPLE PREPARATION
The iblock copolyme used in his s udy was poly(s y ene-b-isop ene-b-s y ene) om
K a on Polyme . The molecula weigh s o polys y ene (PS) and polyisop ene (PI)
blocks a e 17,460 g/mol and 40,740 g/mol, espec i ely. The PS weigh ac ion in he
copolyme is 0.3. The SIS ilms we e p epa ed on eshly clea ed HOPG subs a e. The
subs a e was dipped in o he SIS/ oluene solu ion wi h a a e o 40 mm/min, and
subsequen ly pulled ou o he solu ion a 5 mm/min. Solu ions wi h he ollowing SIS
concen a ions we e employed: 0.5, 1.0, 2.0 and 3.0 w .%. The ea e , he sol en was
e apo a ed o 24 h a oom empe a u e [21]. This SIS copolyme ilm was sel -
assembled a oom empe a u e wi hou any annealing p ocedu es [22], [23].
2.2. ATOMIC FORCE MICROSCOPY IMAGING
The su ace mo phology o he samples was cha ac e ized by AFM. Topog aphic and
phase images we e ob ained wi h a Nanoscope IVa Dimension 3100 AFM ( om Digi al
Ins umen s). Tapping mode in ai was employed using an in eg a ed silicon
ip/can ile e (125 μm in leng h and wi h ca. 300 kHz esonan equency) a a scan a e
o 1.0 Hz and a esonance equency o ~ 300 kHz. The measu emen s we e pe o med
wi h 512 scan lines. Se e al egions we e scanned ob aining simila esul s. Analysis o
AFM images was ca ied ou wi h a WSxM so wa e (Nano ech Elec onica). A 2-
dimensional as Fou ie ans o ma ion (2D-FFT) was pe o med ia he WSxM
so wa e. Fo measu ing ilm hickness he ilms we e sc a ched wi h a b ass wi e
be o e eco ding opog aphic images. Film hickness was measu ed by means o heigh
di e ences om he ilm o a ba e subs a e using he so wa e analysis.
3. RESULTS AND DISCUSSION
AFM was used o analyze he mo phological beha io o SIS block copolyme on
HOPG subs a e. The su aces o ou di e en ilms ob ained om SIS/ oluene
solu ions wi h di e en concen a ion we e examined. Su ace co e age a ied
acco ding o he solu ion concen a ion. As shown below, inc easing he ilm hickness,
he block copolyme shows di e en a angemen s. Fig. 1 shows opog aphical and
AFM phase images o he ilm dip cas om he SIS/ oluene 0.5 w .% solu ion. The
measu ed ilm hickness was < 6 nm. The AFM image o he adso bed SIS copolyme
ilm shows highly aligned polyme segmen s. The image displays no andomly
dis ibu ed domains connec ed be ween hem wi h a iangula egula i y. The molecula
a angemen s adop ed a pa allel alignmen wi h a c oss-angle o 120°. The 2D-FFT
diag am o he AFM image, shown in Fig. 2, con i med his esul . The diag am
displays a hexagonal pa e n whe e h ee componen s c ossed a 120° be ween hem
we e seen, simila o ob ained by Yoko a e al. [24]. This esul sugges s he epi axial
o ma ion o he block copolyme whe e copolyme domains a e aligned along he
HOPG subs a e. To explain his beha io , i is necessa y o conside ha he HOPG
c ys al la ice has a laye ed s uc u e, whe e each g aphene laye is composed by a
plana a angemen o used hexagonal benzene ings, as shown in Fig. 3. In he case o
he uppe g aphene shee ( he shee in con ac wi h he ai ), wo ypes o ca bons do
exis : ca bons in e ac ing wi h ca bons o o he shee s (α-ca bons) and no in e ac ing
ca bons (β-ca bons) [16]. α-Ca bons in e ac wi h adjacen ca bons bu β-ca bons
possess ee elec ons wi h capabili y o in e ac wi h deposi ed molecules. The angle
be ween hose β-ca bons is 120°. Thus, s ong π–π in e ac ions be ween he delocalized
π elec ons o PS phenyl g oups and he β-ca bons o he HOPG subs a e can gene a e
he mac omolecule o ien a ion [17]. This assump ion sugges s ha he domains wi h a
iangula egula i y co espond o polys y ene blocks.
Fig. 4a–c p esen s AFM images o di e en ilms cas om SIS/ oluene solu ions wi h
se e al concen a ions. Fo he 1 w .% solu ion, Fig. 4a, e en i some non-co e ed
egions we e p esen in he ilm, in e es ingly, depending on he hickness o each
egion he su ace exhibi ed wo ypes o cylinde s. The ilm showed well-de ined
cylinde s in sligh ly hicke egions (10–12 nm), whe eas in sligh ly hinne egions (7–
8 nm) he cylinde s had smalle leng h and heigh and we e o ien ed in a common
di ec ion, as indica ed by he a ows. Those esul s sugges ha o a block copolyme
exceeding a ilm hickness, he su ace mo phology does no show epi axial o ma ion,
being independen o he employed subs a e.
Su ace mo phology changed o inc easing SIS concen a ion. Fig. 4b shows he
coexis ence o pe o a ed lamella (S1 egions, 30–32 nm) and long cylinde (S2 egion,
24–26 nm) mo phologies o he ilm cas om he 2.0 w .% solu ion. Pe o a ed
lamella mo phology is gene a ed by a ansi ion be ween wo s able
laye s [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [
21], [22], [23], [24], [25]. This mo phology is caused by PI domain seg ega ion in bo h
in e aces (subs a e and ai ), d i ing i s deple ion in he laye below he su ace.
The e o e, PS would become he mino componen and PI he majo one, hus leading
o he o ma ion o he PS ma ix pe o a ed by PI cylinde s.
Finally, Fig. 4c shows AFM images o he hickes ilm (54–57 nm) cas om he
3 w .% solu ion. In his case, he images show a homogeneous su ace composed by
andomly o ien ed polys y ene cylinde s wi hou sign o o ien a ion. The su ace
mo phology was no a ec ed by he HOPG subs a e, as i was simila o ha ob ained
in bulk (no shown in he p esen wo k). These esul s con i m ha in block copolyme
ilms beyond a hickness h eshold alue, in e ac ions be ween HOPG subs a e and
block copolyme chains a e no able o a ec domain o ien a ion in he ilm su ace.
CONCLUSIONS
The domain alignmen o SIS hin ilms deposi ed in he HOPG subs a e showed clea
dependence on ilm hickness. Fo ilms wi h e y low hickness, he copolyme showed
a mo phology wi h a iangula egula i y. This a angemen was induced by an de
Waals a ac ions be ween ee elec ons o delocalized π o bi als o g aphene β-ca bons
and a oma ic ings o polys y ene domains. Inc easing ilm hickness, domain
o ien a ion on he ilm su ace dec eased only showing some o de along a common
di ec ion. Fo ilm hickness exceeding a h eshold alue, ilm su ace mo phology
displayed he bulk mo phology wi hou signs o in e ac ions be ween subs a e and
block copolyme domains.
AKNOWLEDGMENT
Financial suppo is g a e ully acknowledged om he Basque Coun y Go e nmen in
he ame o G upos Consolidados (IT-365-07), he ETORTEK/inanoGUNE (IE08-225)
p ojec s, and he Spanish Minis y o Educa ion and Science o MAT2009-
06331 p ojec . Technical and human suppo p o ided by SGIke (UPV/EHU, MICINN,
GV/EJ, and ESF) is g a e ully acknowledged.
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