Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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4.2 How to Help Phase Separation 115

molecular - weight homopolymers [55, 56] , which also swell each domain. Exposing

thin ﬁ lms to vapors of a good solvent potentially allows long - range equilibrium

morphologies to be produced without any thermal treatment. However, as genu-

inely neutral solvents are rare, the use of solvents selective for one block may

induce structures far removed from the equilibrium situation [49, 57 – 62] , which

remain frozen after solvent removal and are sometimes difﬁ cult to obtain by

other methods [60] . As a conjecture, a fully selective solvent should lead to a

micellar - like microdomain phase. Once more, Krausch and coworkers were the

ﬁ rst to demonstrate for a polystyrene - block - poly(2 - vinylpyridine) - block - poly( t - butyl

acrylate) (PS - b - P2VP - b - P t BA) triblock copolymer, that the choice of annealing

solvent strongly inﬂ uenced the types of metastable structures observed, and also

investigated the time development of the microdomain structure [60] . More

recently, the morphology evolution has been followed in other BCs, for example

in polystyrene - block - poly(methyl methacrylate) ( PS - b - PMMA ) thin ﬁ lms with dif-

ferent thicknesses exposed to PMMA - selective solvent vapors (Figure 4.2 ) [63, 64] .

Figure 4.2 Atomic force microscopy

topographic images of PS - b - PMMA ( M

PS = 133 000, M

PMMA = 130 000) thin ﬁ lms

cast onto an Si substrate with an SiO

surface

layer exposed to chloroform vapor. (a) After

40 h, the initially featureless structure shows a

disordered microstructure that evolves, at 60 h

(b), into a hexagonally packed nanocylinder

structure. (c) Further treatment, 80 h,

promotes the evolution after 100 h (d) to a

mixed morphology, which completely

develops into stripes having the repeat

spacing ( L

), of 90 nm. For longer annealing

times, the ﬁ lm returns to a ﬂ at surface. It has

also been shown that only when the ﬁ lm

thickness is less than 1/2 L

can the packed

nanocylinder structures form. Reproduced

American Chemical Society.

116 4 Alignment and Ordering of Block Copolymer Morphologies

Other representative examples in controlling the orientation and lateral ordering

of microstructures are poly(ethylene oxide) ( PEO ) - containing block copolymers.

A simple solvent annealing of cylinder - forming PS - b - PEO and PS - b - PMMA - b - PEO

BC thin ﬁ lms can lead to a perpendicular orientation with a high degree of lateral

ordering [65, 66] . In these systems, the relative humidity of the vapor during

solvent annealing was shown to play a fundamental role in achieving such order

[67] , while the addition of small amounts of an alkali halide or metal salts to

cylinder - forming PS - b - PEO, where the salt complexes with the PEO block, was

found to signiﬁ cantly enhance the long - range positional order [68] . In a similar

way, the incorporation of diverse PEO additives into PS - b - PMMA diblock copoly-

mer thin ﬁ lms gave a vertical alignment of the PMMA cylinder, due mainly to

an interaction with the water vapor under the high humidity, solvent - swollen

processing conditions, as a direct consequence of the miscibility of the PEO with

PMMA block [69, 70] .

Finally, an in - depth understanding of the formation of either equilibrium struc-

tures or kinetically trapped morphologies during solvent annealing requires not

only an accurate choice of solvent but also the proper control of experimental

parameters, such as vapor pressure, treatment time, solvent extraction rate and,

in case, relative humidity. In practice, this knowledge facilitates the preparation

of reproducible nanopatterns in thin ﬁ lms, which also, in the case of metastable

assemblies, show a long - term stability, at least for T

s of all the components that

are well above room temperature.

Another plasticizing agent that presents a modest equilibrium sorption and can

be easily removed is supercritical CO

. Although its rapid diffusion in most poly-

mers allows an equally rapid equilibrium distribution in thin ﬁ lms, CO

annealing

has so far been limited to a surprisingly low number of BCs, essentially PS - b -

PMMAs, having molecular weights up to 300 000 g mol

− 1

[71, 72] .

4.3

Orientation by External Fields

Since the ﬁ rst macroscopic alignment of cylindrical domains of an industrial tri-

block copolymer (a PS - b - PB - b - PS; Kraton 102) by extrusion carried out by the Keller

group during the early 1970s [73, 74] , many more mechanical ﬂ ow ﬁ elds have been

proposed to control BC alignment. Following the Keller belief that BC microstruc-

turing and orientation were exciting research subjects, but with very limited appli-

cability (E. Pedemonte, personal communication), and after the shear ﬂ ow

experiments on the same copolymer by Skolios and coworkers [75 – 77] , during the

1990s research interests returned strongly to alignment by relatively weak external

ﬁ elds. Another reason for this was the greater availability of BCs with different

architectures and chemical compositions [5, 18 – 20] . Particularly in recent years,

most investigations moved from bulk materials to thin ﬁ lms, due to their nanote-

chnological potential, with a special focus on the use of electric ﬁ elds. Active

4.3 Orientation by External Fields 117

external ﬁ elds will be discussed later in the chapter, while those that are passive,

such as with interfacial interactions, are considered in the next sections.

4.3.1

Mechanical Flow Fields

Extrusion [73, 74] , compression [78 – 82] , ﬂ ows involving oscillatory shearing [75 –

77, 82 – 91] and other steady shearing [92 – 98] , and techniques that combine differ-

ent ﬂ ow ﬁ elds [99 – 104] have been successfully applied to induce alignment in BCs,

either in a microphase - separated molten state or, to a lesser extent, in gels of

solvent - swollen microdomains [105 – 112] , with the objective of forming single

crystal structures. Large - amplitude oscillatory shear ( LAOS ), which was ﬁ rst pro-

posed at the end of the 1980s to orient commercial triblock copolymers [82, 83] ,

is the most widely employed technique, also due to the easy characterization of

the shear ﬁ eld with respect to other methods. As an example, common parallel

plate - type rheometers enable the modulation of valuable experimental variables to

be carried out, such as shear rate, frequency, strain amplitude, and temperature.

Early studies on the effect of mechanical ﬂ ow ﬁ elds on the orientation of BC

domains have already been summarized in many excellent publications and books,

for example, by Fasolka and Mayes [9] , Hamley [4, 113] , the group working with

Thomas [12, 114] and others [84, 115, 116] , and so need not be reviewed here. Di - ,

tri - , and multiblock copolymers in bulk with different microstructures, such as

lamellar [74, 78, 79, 84 – 91, 96, 98] , cylindrical [80 – 82, 94, 95, 97, 101, 103] , spheri-

cal [92, 93, 97] and gyroid [102] , can be preferentially oriented parallel or, where

applicable, perpendicular or mixed - perpendicular to the ﬂ ow direction. Most

experimental observations have been supported and even anticipated by theory,

with several good theoretical works being published in recent years [117 – 126] .

Further developments concerned the extension of the shearing techniques to thin

ﬁ lms. Albalak and Thomas [99, 100] proposed a novel casting method, termed

roll - casting , in which a BC solution is subjected to a ﬂ ow between two counter -

rotating rolls (Figure 4.3 a). The solution is compressed and, as a result of solvent

evaporation and shearing, the microphase separates into a ﬁ lm with a high degree

of orientation and close to single - crystal characteristics. Only cylinder morphology

from triblock commercial polymers has been oriented in this way [101, 102] , with

a ﬁ lm thickness intrinsically limited by the geometry of the device to hundreds of

nanometers. Thinner ﬁ lms of sphere - and cylinder - forming polystyrene - b -

poly(ethylene propylene) ( PS - b - PEP ) have been shear - aligned by simple ﬂ owing

techniques [93, 94] , and extended to arbitrarily large areas. Films of 30 – 50 nm

spread onto a silicon substrate are covered by an elastomer pad, which is then

slowly pulled forward by a constant force (Figure 4.3 b). The steady shear must be

applied at temperatures between the T

and the temperature of order – disorder

transition, T

ODT

, of the copolymer, thus providing an essentially inﬁ nite orienta-

tional order in all directions without the typical limitations of sheared bulk samples,

related to the presence of multigrains.

118 4 Alignment and Ordering of Block Copolymer Morphologies

Unconventional methods, which to some extent are based on shearing and a

stretching ﬁ eld, such as electrospinning [127 – 130] and orientation by spin coating

[131, 132] , have been proposed in the past few years, but still need to be investi-

gated in depth. Electrospinning in particular [133, 134] seems to offer great potential

for the fabrication of nanoﬁ bers with internal oriented structuring from cylinder -

forming copolymers with at least one partially crystalline block.

4.3.2

Electric and Magnetic Fields

Following the pioneering studies of Amundson et al . on lamellar diblock copoly-

mer thin ﬁ lms [135 – 137] , static electric ﬁ elds have been widely used in copolymer

melts to macroscopically orient lamellar or cylindrical morphologies [48, 138 – 146] ,

Figure 4.3 Schematic of apparatus used to

induce alignment in BC thin ﬁ lms. (a) In the

roll - caster (upper view and section), two

independent rollers – one of stainless steel

and the other of Teﬂ on – are driven at the

same angular velocity by two independent

motors and separated by a micrometer -

controlled gap; (b) In the shear - alignment

set - up, the polydimethylsiloxane ( PDMS ) pad

is pressed with a constant weight against the

ﬁ lm; the support is heated at the annealing

temperature while the displacement of the

pressing block is continuously monitored with

a laser – mirror – optical sensor assembly.

4.3 Orientation by External Fields 119

with most of the investigations being focused on PS - b - PMMA. Although mor-

phologies parallel to the substrate could be obtained using an in - plane ﬁ eld, a

uniaxial orientation along the ﬁ eld and perpendicular to the substrate is preferably

induced between the electrodes compressing the ﬁ lm. As initially proposed by

Russell and coworkers, a convenient procedure consists of the preparation of ﬁ lms

between two Kapton sheets coated on one side with aluminum, with the ﬁ lm

thickness controlled through Kapton spacers. Kapton is a commercially available

polyimide with excellent mechanical properties that allows further manipulation

after alignment of the domains [23] , and also facilitates sample characterization,

such as for microtoming. In order to avoid electric shorting, one of the electrodes

must be placed in contact with the polymer, while the second is inverted with the

Kapton side facing the polymer.

The general statement that the orientation of BC microdomains is possible if

the applied ﬁ eld is high enough for a given difference in the dielectric constant

between blocks, can be expressed by Equation 4.1 [147, 148] :

()

−

εε

(4.1)

where E c is the critical electric ﬁ eld strength, ε

and ε

are the dielectric constants

of blocks, and t is the ﬁ lm thickness, which also takes into account the surface

interactions through the difference between the interfacial energies of each block

with the substrate, Δ γ . In the absence of preferential interactions with the sub-

strate – that is, Δ γ = 0 – the domains do not need any external ﬁ eld to orient normal

to the substrate (see Section 4.5.1 ). From a practical point of view, Equation 4.1

permits the evaluation of the critical parameter for a given set of the different

parameters, eventually imposed by the same experimental design. The thickness

of the region to be oriented is limited to a few millimeters by the electric ﬁ eld

strengths, in the range of approximately 1 to 100 V μ m

− 1

, considered as the upper

limit that prevents dielectric breakdown.

In addition, large - scale topographic structures could be generated by electrohy-

drodynamic ( EHD ) ﬁ lm instability, a technique in which a liquid dielectric inter-

face is destabilized by a perpendicular electric ﬁ eld [149] . The interplay between

EHD structure formation and the structural control over BC microphases allowed

lithographically controlled micrometer - sized features to be obtained, where the

cylindrical or lamellar domains are oriented parallel to the patterned structure axis;

that is, normal to the substrate [144, 150] .

Theoretical studies have been conducted to investigate various aspects of the

dynamics of mesophase formation and orientation, mainly in diblock copolymer

melts [147, 148, 151 – 154] ; here, special mention should be made of the capacitator

analogy proposed by Pereira and Williams for symmetric BCs [147] . The appeal of

this model actually lies in its simplicity which, at a more qualitative level, also

permits justiﬁ cation of the alignment of lamellae perpendicular to the electrodes

on the basis of preferential travel of the electric ﬁ eld lines along easy paths, though

each and every block has the highest polarizability. In the same way, parallel

120 4 Alignment and Ordering of Block Copolymer Morphologies

orientation is unfavorable, as the electric ﬁ eld lines would be forced to cross both

blocks evenly. Tsori et al . have recently proposed that, in these systems, alignment

may also occur by other strong orienting forces, such as by the presence of a

conductivity or mobility contrast [155] , postulating an important role for the ions

remaining in the polymers after synthesis in the reorientation mechanism [153] .

The alignment pathways in thin ﬁ lms of BCs with various morphologies have

been recently followed using in situ small - angle X - ray scattering ( SAXS ) [140, 143]

and scanning force microscopy ( SFM ) [145] . The application of an electric ﬁ eld

(perpendicular to the plane of the ﬁ lm) to the disordered melt of an asymmetric

BC induced the progressive growth of microphase - separated structures oriented

parallel to the ﬁ eld only at temperatures below the order – disorder transition tem-

perature. This occurs up to a stationary, equilibrium state, and also for a copolymer

already having ordered domains parallel to the substrate, either a cylindrical PS -

b - PMMA [140] or a lamellar polystyrene – polyisoprene ( PS - PI ) system [143] , align-

ment parallel to the ﬁ eld could be achieved within minutes. An analysis of the

scattering patterns suggested that, during the reorientation, the initial large grains

are broken up into smaller pieces by the ampliﬁ cation of interfacial ﬂ uctuations,

which are then able to rotate in the ﬁ eld. The ﬁ nal state of the symmetric BCs of

PS and PI consists of many small grains with a high degree of orientation of the

lamellae parallel to the electric ﬁ eld, but with a random orientation in the perpen-

dicular plan. This observation also ﬁ nds some theoretical support in the prediction

by Tsori [148, 153, 154] . The limits imposed by the ﬁ lm thickness and the inter-

facial energy effects, as predicted in Equation 4.1 , have been faced experimentally

by the Russell group for symmetric and asymmetric BC thin ﬁ lms between sub-

strates with modulated interfacial interactions [142, 156, 157] . In the presence of

a preferential interaction of one block with the substrate (see Section 4.5.1 ) such

effects are dominant for ﬁ lm thicknesses up to approximately 10 L

, regardless of

the electric ﬁ eld applied normal to the surface (see, e.g., Figure 4.4 a). For thicker

ﬁ lms, the effects dissipate with distance from the surface and the domains orient

in the direction of the electric ﬁ eld, at least in the interior of the ﬁ lm (Figure 4.4 b),

with a distance of propagation inversely proportional to the strength of the inter-

facial interactions [157] .

All of these melt - based procedures suffer severe limitations because of the melt

viscosity itself, which is directly dependent on the molecular weight and architec-

ture of BCs, and the thickness of the region to be oriented. In addition, the achieve-

ment of high degrees of orientation eventually requires high temperatures and

electric ﬁ eld strengths close to the decomposition conditions. An alternative

approach to circumvent such restrictions consists in the addition of a neutral

solvent with the effect of increasing chain mobility, as already discussed above for

thermal annealing, thus facilitating large - scale domain alignment [158 – 160] . Con-

centrated diblock (PS - b - PI) [158] and triblock [polystyrene - block - poly(hydroxyethyl

methacrylate) - block - poly(methyl methacrylate), PS - b - PHEMA - b - PMMA] [159]

copolymer solutions have been aligned under a dc electric ﬁ eld during solvent

evaporation, leading to highly anisotropic bulk samples (Figure 4.5 ). The orienta-

tion kinetics and mechanisms of alignment – that is, grain orientation and nuclea-

4.3 Orientation by External Fields 121

tion and growth of domains – have been corroborated by simulations [158] based

on dynamic density functional theory ( DFT ) [161] . In summary, several groups

have demonstrated that the application of unidirectional electric ﬁ elds, coupled

with an appropriate choice of materials and experimental conditions, allow an

effective fabrication of patterns with a high degree of orientation and high aspect

Figure 4.4 Cross - sectional transmission

electron microscopy images of ≈ 300 nm

(a) and ≈ 700 nm (b) symmetric PS - b - PMMA

ﬁ lms between PDMS - coated Kapton

electrodes annealed at 170 ° C under

≈ 40 V μ m

− 1

electric ﬁ eld for 6 and 16 h,

respectively. The scale bar represents 100 nm.

Reproduced with permission from Ref. [142] ;

Figure 4.5 (a) 2 - D small - angle X - ray scattering

pattern; (b) Azimuthal intensity distribution

for a ﬁ rst - order reﬂ ection; (c) TEM image of a

bulk sample prepared from a 40 wt% solution

in toluene of symmetric PS - b - PI ( M

= 80 000).

The arrows indicate the direction of the

electric ﬁ eld. Scale bar = 400 nm. Reproduced

American Chemical Society.

122 4 Alignment and Ordering of Block Copolymer Morphologies

ratio, suitable for nanotechnological applications. However, the orientation of

domains is azimuthally degenerate and a further improvement to achieve mor-

phologies with orientation controlled in the three dimensions logically requires

the application of a second, orthogonal, external ﬁ eld. The ﬁ rst sequential combi-

nation toward long - range, 3 - D ordered thin ﬁ lms was shown to control the orienta-

tion of lamellar microdomains through the application of an elongational ﬂ ow

ﬁ eld to obtain an in - plane orientation in ≈ 30 μ m - thick ﬁ lms, and an electric ﬁ eld

normal to the surface [162] . The concurrent application of the orthogonal ﬁ eld

(e.g., a mechanical shearing) is expected to reduce the presence of defects and

grain boundaries in an even more effective fashion.

A further alignment approach is available for materials that exhibit anisotropic

susceptibility due to an anisotropic molecular structure. Magnetic ﬁ eld - induced

orientation has been achieved for liquid crystalline ( LC ) diblock copolymers with

a dielectric diamagnetic isotropy, possibly through the magnetic alignment of LC

mesogens [163 – 165] , and also for BCs with a crystallizable block through an accu-

rate control of the crystallization process [166] . The latter investigation is particu-

larly likely to open a new fruitful research ﬁ eld and, at the same time, pave the

way to an alternative method of relatively wide applicability, which only has the

prerequisite of a high nucleation density during crystallization. Magnetic ﬁ elds

also offer the ability to apply very high ﬁ elds without the risks of electric ﬁ elds,

associated with the danger and limit of electric breakdown. In addition, the orienta-

tion of PEO - based diblock copolymers with a poly(methacrylate) containing an

azobenzene moiety in the side chain as a hydrophobic LC segment was produced

by using a LASER beam, that induced the alignment of photoresponsive azoben-

zenes and transferred the molecular ordering at supramolecular level [167] . Mac-

roscopic parallel array of nanocylinders can be obtained over an arbitrary area, in

principle even on curved surfaces.

4.3.3

Solvent Evaporation and Thermal Gradient

The observation of lamellar and cylindrical microdomains in thin ﬁ lms perpen-

dicular to the surface as a result of solvent evaporation was ﬁ rst reported by Tur-

turro and coworkers [168] , and subsequently investigated in more detail by Kim

and Libera for a similar triblock copolymer [169, 170] . On the basis of these and

other studies on either spun - cast or solution - cast ﬁ lms from solutions in a good

solvent for all the blocks, a reasonable mechanism of orientation could be pro-

posed (Figure 4.6 ). In the ﬁ rst moments after deposition, the T

of the swollen

ﬁ lm is still well below room temperature, thus allowing free chain mobility. With

the decrease in the solvent concentration, the BC undergoes a transition from the

disordered to the ordered state and, as in thin ﬁ lms the diffusion of the solvent

produces a gradient of concentration, the ordering front rapidly propagates from

the air surface to the substrate. The consequent decrease of T

below room tem-

perature, for at least one block, locks in the structures which, due to the high

4.4 Templated Self-Assembly on Nanopatterned Surfaces 123

directionality of the solvent gradient, are highly oriented normal to the surface.

This behavior has been reported so far for ﬁ lms of less than one - half micron thick,

for example, on PS - PB systems [168 – 170] , PS - b - PEO [65, 171, 172] , polystyrene -

block - polyferrocenyldimethylsilane ( PS - b - PFS ) [173] , and PS - b - P4VP [174] , whereas

the applicability of these conditions can be extended to any BCs having the T

one block above room temperature. Commonly, the short - range order of cylindri-

cal microdomains of as - spun ﬁ lms may be signiﬁ cantly enhanced by solvent

annealing. In the case of PS - b - PEO thin ﬁ lms, a better control over domain orienta-

tion was obtained by resorcinol complexation of the PEO cylinder - forming block

[175] . This is possibly due to a lower degree of crystallinity of the PEO/resorcinol

complexes with respect to the pure PEO, that has the effect of enhancing the

mobility of polymers during ﬁ lm formation, and therefore also the orientation of

domains parallel to the solvent gradient.

After the excellent results obtained by the Hashimoto group [176, 177] , the use

of temperature gradient apparatus to produce highly oriented single crystals was

also reported for thin ﬁ lms [178, 179] . As an example, square - millimeter well -

oriented samples of a cylinder - forming BC were obtained, with a density of discli-

nations estimated at 4 × 1 0

− 4

μ m

− 2

(corresponding to an average disclination

separation of 50 μ m) [178] .

4.4

Templated Self - Assembly on Nanopatterned Surfaces

The combination of top - down strategies to fabricate patterns that direct the bot-

tom - up organization of organic or inorganic building blocks is a strategy often

Figure 4.6 Schematic of the solvent evaporation in a diblock

copolymer thin ﬁ lm. The diffusion produces a gradient in the

concentration of the solvent, [S], as a function of depth, d ,

which induces an ordering front from the ﬁ lm surface to the

substrate.

124 4 Alignment and Ordering of Block Copolymer Morphologies

used in the micrometer and, to a minor extent, in the nanometer regime. Also, in

the case of BCs, long - range order and orientation may be induced if self - assembly

is forced to occur into/onto a guide, either topographically or chemically patterned,

or in other 2 - D or 3 - D conﬁ nements. In this section, only the effect of surfaces

with features having periodicity commensurable with those of the BC morpholo-

gies is considered. The study of the control of the registration and orientation of

domains by topographic conﬁ nements (in the form of surface relief or groove

structures) with a length scale much larger than the BC lattice parameters had a

somehow different theoretical and historical development, and is better known as

graphoepitaxy . It will therefore be discussed later as it follows such different

nomenclature.

Rockford and coworkers decorated miscuit silicon wafers with gold to fabricate

striped and essentially ﬂ at surface of nonpolar gold and polar silicon oxide, with

periodicity ranging from 40 to 70 nm [180, 181] . Films of symmetric PS - b - PMMA

solution cast onto such substrates showed, after thermal annealing, lamellar

orientation normal to the plane and parallel to the striping, with the greatest

ordering for patterns commensurable with the bulk lamellar period of the BC.

A mismatch in the length scale of 10% and 25% in thick and ultrathin ﬁ lms,

respectively, appeared to be sufﬁ cient to lose control over orientation. A novel

integrated fabrication strategy that makes use of advanced lithographic techniques

has been tuned, by Nealey and coworkers, to produce perfect periodic domain

ordering [182, 183] . Although Nealey actually discussed epitaxial self - assembly,

we prefer to reserve that term for a different methodology to control spatial and

orientation order, as introduced by De Rosa et al . [184] (for a deﬁ nition of epitaxy,

see Section 4.5.2 ). As schematically reported in Figure 4.7 , in the ﬁ rst essential

step of this procedure a self - assembled monolayer ( SAM ) is precisely patterned

throughout the photoresist using extreme ultraviolet interferometric lithography

[185, 186] . Following the conversion of the topographic pattern to a chemical

pattern and the photoresist removal, a symmetric PS - b - PMMA is spin - coated

onto the chemical pattern and, as the modiﬁ ed regions present polar groups

that preferentially wet the PMMA block, the self - assembly results in lamellae

oriented perpendicular to the substrate. High - quality, defect - free BC ﬁ lms require

pretty much the same period of the chemical pattern and the BC, with com-

mensurability within just a few percent. For values within approximately 10%,

the morphology can still be surface - directed but it is not perfect (Figure 4.8 ).

An extension of this method, through the use of ternary blends of diblock copoly-

mers and the corresponding homopolymers, showed that is possible to pattern

chemical templates with periodicities substantially deviating from those of the

BC [31] . The redistribution of homopolymer macromolecules during thermal

annealing facilitates defect - free assembly of the blend as a whole, to form 50 –

80 nm periodic arrays in addition to pattern structures with different bend

geometries. More recently, the validity of this approach was proven by other

groups, which demonstrated that PS - b - PMMA can self - assemble in a well - aligned,

long - range ordered hexagonal lattice commensurate with chemically prepatterned

templates prepared by electron beam lithography ( EBL ) [187] or into a laterally