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Self-assembly of block copolymer thin films
Self-assembly of block copolymer thin films Block copolymers self-assemble on nanometer length scales, making them ideal for emerging nanotechnologies. Many applications (e.g., templating, membranes) require the use of block copolymers in thin film geometries (~100 nm thickness), where self-assembly is strongly influenced by surface energetics. In this review, we discuss the roles of surface and interfacial effects on self-assembly, with a specific focus on confinement, substrate surface modification, and thermal and solvent annealing conditions. Finally, we comment on novel techniques for manipulating and characterizing thin films, motivating the use of gradient and high-throughput methods for gaining a comprehensive picture of self-assembly to enable advanced nanotechnologies. Julie N. L. Albert and Thomas H. Epps, III* Department of Chemical Engineering, University of Delaware, 150 Academy Street, Newark, Delaware 19716 *E-mail: [email protected] Self-assembling soft materials continue to play an important
chain architecture (e.g., linear, star, etc.)17. More recent literature
role in meeting societal and economic goals for more efficient
has provided a working knowledge of surface energy effects in block
processes, cleaner energy generation, and smaller, hierarchically
copolymer thin films (e.g., on microstructure orientation and phase
structured devices. Block copolymers are a class of self-
transformations)18-20; however, a more comprehensive understanding
assembling material that segregates on nanometer length scales,
of thin film nanostructures is still a significant research focus.
making them ideal for emerging nanotechnologies1, including
investigation of surface and interfacial effects governing the self-
organic optoelectronics15, and anti-reflection coatings16. These
assembly of linear block copolymer thin films. For more general reviews
applications require the use of block copolymers in thin film
on block copolymer thin film phase behavior and related topics, we
geometries (~100 nm thickness), where self-assembly is strongly
refer the reader elsewhere8,15,18,21-23. Herein we highlight confinement
influenced by surface energetics.
and film thickness effects (section 1); discuss the role of substrate
Decades of research have provided a foundation for understanding
24
In this review, we discuss a framework for the experimental
applications in nanotemplating2-10, nanoporous membranes11-14,
surface modifications, including the use of gradient surfaces for high-
block copolymer self-assembly, beginning with the characterization
throughput investigations (section 2); examine the use of thermal and
of bulk morphologies using the Flory-Huggins interaction parameter
solvent annealing conditions to manipulate both free surface energetics
(χ), degree of polymerization (N), block volume fractions (ƒ), and
and block interactions (section 3); and finally comment briefly on
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recent materials development and novel thin film characterization techniques, concluding with an outlook for the future.
(a)
(b)
(c)
Confinement Block copolymer thin films are typically subjected to a significant degree of confinement with film thicknesses comparable to the polymer domain spacing. Because stretching or compression of polymer chains due to confinement is energetically unfavorable, block copolymers selfassemble into morphologies that relieve the stretching/compression or mitigate the entropic penalty with favorable enthalpic interactions at the substrate or free surface, as discussed in later sections24. Two major categories of confinement are defined for substrate-supported block copolymer thin films: “hard” confinement describes a film confined between two rigid interfaces; “soft” confinement refers to a film where one interface (the “free surface”) is in direct contact with the atmosphere18. Systems under soft confinement with effectively unconfined lateral dimensions will be the focus of this review, and we refer the reader to other sources for work involving free-standing films25 and systems subject to hard and/or lateral confinement26,27. Under soft confinement, chain compression/stretching due to incommensurability between the film thickness (t) and polymer domain spacing (L0) is mitigated by the formation of islands or holes or by perpendicular orientation of the morphology (for cylinders and lamellae) depending on surface interactions. Surfaces that are preferential for one block of the copolymer typically lead to island/ hole formation, whereas non-preferential (or neutral) surfaces lead to nanostructure reorientation18,21. The degree of confinement or incommensurability is described by the deviation of the ratio t/L0 from the commensurability condition. Consider a lamellar AB diblock copolymer subject to preferential surface interactions. If surface preferences dictate that block A wets both the substrate and free interfaces (symmetric wetting), then for t/L0 = n (n = 1, 2, 3…), commensurability is achieved, and no island/ hole formation will be observed at the free surface. Similarly, if block A wets the substrate surface and block B wets the free surface or vice versa (anti-symmetric wetting), then the commensurability condition is t/L0 = n+0.518,21. However, for intermediate film thicknesses, incommensurability is described by the deviation of t/L0 from the commensurability condition, and islands/holes will be found at the free surface. Smith and coworkers illustrated the formation of islands/holes for incommensurate film thicknesses as a function of molecular weight of the polymer (Fig. 1)28. For block copolymer microstructures with more complex symmetries, determination of the commensurability condition is more difficult than for lamellae, but the same principle of island/ hole formation on preferential surfaces applies21. Sphere-forming thin films present an additional level of complexity because the spheres can arrange in a hexagonal symmetry in very thin films (up to approximately four layers of spheres) rather than the bulk
Fig. 1 Optical micrographs of gradient thickness lamellar PS-b-PMMA films annealed at 170 °C. Close to the commensurability condition t/L0= 0.5, films appear featureless; with increasing thickness, the morphology in all films progresses from islands to spinodal island/hole structures to holes to featureless at the next commensurate thickness. (a) Mn = 26 kg/mol, L0 ≈ 17-18 nm, (b) Mn = 51 kg/mol, L0 ≈ 27-30 nm, (c) Mn = 104 kg/mol, L0 ≈ 42 nm. Reprinted figure with permission from28. © 2001 by the American Physical Society.
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Self-assembly of block copolymer thin films
body-centered cubic symmetry29,30. Furthermore, the interplay
Typically, the substrate surface field propagates less than 6 × L0
between confinement and surface energy effects (e.g., wetting
from the substrate interface, though the propagation distance is clearly
layer formation and surface reconstruction18,19,31) becomes more
system-dependent20,37,39-42. Free surface effects may become apparent
complicated for non-lamellar-forming block copolymers and may lead
(Fig. 2a) or the film may revert to its bulk morphology with randomly
to phase transformations18,32. In the following sections, we continue
oriented grains some distance away from the substrate surface.
the discussion of confinement effects in the context of surface
In addition to cross-sectional imaging by TEM, the perpendicular
preferentiality.
orientations of lamellae and cylinders near a neutral substrate surface and parallel or mixed nanostructures near the free surface has been
Substrate surface effects
confirmed by neutron reflectivity39,40,43, small angle neutron scattering
Substrate surface energy and surface chemistry are important
(SANS)40,43, successive reactive ion etching (RIE)/scanning electron
parameters for controlling block copolymer thin film self assembly,
microscopy (SEM) imaging40, bottom layer imaging with SEM37, and
both with regards to microstructure
stability19
and microstructure
grazing-incidence small angle X-ray scattering (GISAXS)41,44,45.
orientation (for lamellae and cylinders)18,21. Preferential surfaces are wet by the preferred block, whereas neutral surfaces encourage the presence of all blocks at the
surface18,21.
Herein we highlight
Substrate modification methods Modification of the substrate surface with random copolymers or
these effects on smooth substrates, and we refer the reader to the
self-assembled monolayers (SAMs) are two common approaches
following references describing the influence of surface topology on
to generate neutral or preferential surfaces. The random copolymer
microstructure order and orientation33-35.
methods rely on the statistical composition of the copolymer to tune the substrate surface energy/chemistry. The copolymers can be end-
Substrate field effect
grafted46 or side-grafted45,47 to substrates to form a brush layer or
The substrate surface energy/chemistry effect can be generally viewed
formed as a cross-linked mat covering the substrate48,49. SAM methods
as a field that propagates from the substrate surface into the film36.
for block copolymer studies usually involve chlorosilane chemistry
The propagation distance depends on the relative strength of the field
on silicon oxide surfaces. These methods have employed partial
(i.e., the strength of the interactions between substrate and polymer
monolayers50,51, partial oxidation of hydrophobic monolayers19,28,52,53,
film), the competing interactions from the free surface, and the
and molecules that mimic the block copolymer structure42,54 to
energetics of the equilibrium bulk morphology. In a cross-sectional
manipulate surface energy/chemistry (note: though “SAM” is the
transmission electron microscopy (TEM) image of a thick poly(styrene-
commonly used term, many of these monolayers are not truly self-
b-methyl methacrylate) (PS-b-PMMA) film, Han et al. captured the
assembled).
moderate substrate surface field alignment of perpendicular cylinders ≈200 nm (≈L0 × 6) into the film (Fig. 2a)37. In contrast, Shin et al.
Neutral surfaces
used a strongly preferential surface to align a lamellar poly(styrene-
The application of block copolymer thin films as nanoporous
b-isoprene) (PS-b-PI) film parallel to the substrate surface over a
membranes and templates has motivated interest in identifying neutral
thickness of 40 × L0 (Fig. 2b)38.
surfaces to generate perpendicular lamellae to create line patterns55,
(a)
(b)
Fig. 2 (a) Cross-sectional TEM image of a ≈ 900 nm thick cylinder-forming PS-b-PMMA film on a neutral substrate surface annealed at 230 °C. Scale bar represents 500 nm. Defect line ≈ 200 nm (6 × L0) into the film (arrows) indicates where neutral substrate and neutral free surface field effects intersect. Both fields align cylinders perpendicular to the substrate surface. (b) Cross-sectional TEM image of a ≈ 1400 nm thick lamellar PS-b-PI film on a PS-preferential substrate. (a) Reprinted in part with permission from37 © 2009 American Chemical Society. (b) Reprinted with permission from37 © 2008 American Chemical Society.
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perpendicular cylinders for ring or dot patterns56 or conducting
depend not only on the neutrality of the substrate surface, but also on
channels12,13, and network structures to template nanowires2 or
the film thickness20,37,41,44,45,49, block copolymer composition20,41,44,57,
conducting
pathways19.
For lamellae and cylinders, the perpendicular
assembly windows and the quality of the perpendicular structures
method of surface modification20, and annealing conditions37,58 (see next section). Optimization of substrate surface neutrality has required researchers to consider all of these effects in their studies20. Neutral surface studies have primarily been conducted using PS-b-PMMA films because the PMMA domain is easily removed to create a nanoporous material11; furthermore, an effectively neutral free surface can be simultaneously achieved with appropriate thermal annealing conditions (see next section)37,59. Film thicknesses near L0 have been considered most often due to the loss of orientation or order in thicker films observed by Ham et al. and others (Fig. 3)20,39-41. On substrates modified with random copolymers (PS-r-PMMA), the neutral surface compositions are identified by windows of perpendicular microstructure orientation. As expected, increasing the PS content of the block copolymer shifts the perpendicular window to higher random copolymer PS fraction (Fst)20,41. However, the exact locations and widths of the perpendicular windows also depend on the method of substrate modification (Fig. 4)20. Perpendicular windows are shifted to higher Fst for surfaces modified with side-grafted (PH1) and cross-linked (PG1) random copolymers versus an endgrafted brush (Terminal-OH), due to the polarity of functional groups incorporated into the random copolymer backbones. Unfortunately, the perpendicular phase windows do not quantitatively follow trends in water contact angles of the modified substrates, possibly due to penetration of block copolymer chains into the neutral layer or the polarity of water20. Future work that examines surface energetics in additional detail (e.g., consideration of both polar and dispersive components of the surface energy) may help explain the reported trends. Additionally, free energy considerations suggest that the neutral surface condition for asymmetric block copolymers will not lie in the center of the perpendicular window, and that the narrow width of the perpendicular window for the PS-cylinder-forming polymer may be related to free-surface effects (Fig. 4). A more detailed discussion is provided by Han et al.20
Gradient methods While the phase behavior and methods for creating neutral surfaces for PS-b-PMMA have been fine tuned by more than a decade of study, quickly and easily mapping the phase behavior of new materials poses a significant challenge. Substrate surface energy/chemistry gradients provide an efficient route to probe how a range of surface energies/ chemistries affects morphology. Many gradient fabrication methods exist60, including statistical copolymer brush gradients61 and SAM gradients42,62-66, but few have been applied to understanding block Fig. 3 Scanning force microscopy phase images of different thickness films of cylinder-forming PS-b-PMMA (L0 ≈ 35.4 nm) on a neutral PS-r-PMMA copolymer brush annealed at 170 °C for 24 h. Reprinted with permission from41. © 2008 American Chemical Society.
copolymer thin film phase behavior19,28,42. As one example, Smith et al. combined surface energy and film thickness gradients to map the phase behavior of a model lamellar
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Self-assembly of block copolymer thin films
(a) Terminal-OH
(b) PH1
(c) PG1
(d) Terminal-OH
Fig. 4 (a) – (c) Perpendicular windows for PS-b-PMMA films assembled on substrates modified with end-grafted (Terminal-OH), side-grafted (PH1), and cross-linked (PG1) random copolymers (PS-r-PMMA). (d) SEM images of films on Terminal-OH substrates. Adapted with permission from20. © 2008 American Chemical Society.
PS-b-PMMA block copolymer67. An optical micrograph of the film (Fig. 5) shows the sharp transitions between preferential and nonpreferential wetting of the surface at ≈37 mJ/m2 and ≈40 mJ/m2, as well as between commensurate and incommensurate film thicknesses for symmetric (lower surface energies) and anti-symmetric (higher surface energies) wetting behaviors. More recently, Albert et al. developed a method for generating surface energy gradients that provides versatility in surface chemistry and straightforward adaptability to new systems42. In this method, monolayer gradients are generated in a single step cross-deposition of functionalized monochlorosilanes under dynamic vacuum. Chlorosilane functionalities can be chosen based on the block copolymer of interest, Fig. 5 Optical micrograph of a gradient thickness PS-b-PMMA film cast on a surface energy gradient that is preferential for PS on the left and preferential for PMMA on the right. The cloudy regions indicate island/hole formation. The featureless region centered at ≈38.5 mJ/m2 (yellow box) corresponds to the neutral surface energy window. The other featureless regions correspond to commensurate film thicknesses for symmetric (green boxes) and antisymmetric (blue box) wetting. Reproduced and adapted with permission from67. © Wiley-VCH Verlag GmbH & Co. KGaA.
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and the generation of linear substrate surface gradients can be tuned by the size of the liquid chlorosilane reservoirs within a custom-built insert. The authors located the expected parallel and perpendicular orientations for a cylinder-forming PS-b-PMMA block copolymer film on a substrate surface energy/chemistry gradient containing the appropriate functionalities (Fig. 6)42.
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Fig. 6 Optical micrograph (top) and atomic force microscopy phase images (bottom) of cylinder-forming PS-b-PMMA thin films on pure-component and gradient monolayer substrates. The benzyl-silane is preferential for PS, the methacryl-silane for PMMA. Composition (xm, mole fraction of methacryl-silane) and surface energy (γs) given for each point examined on the gradient. The expected transitions between islands/holes and parallel cylinders on preferential surfaces and featureless optical micrographs and perpendicular cylinders on neutral surfaces were detected. Adapted with permission from42. © 2009 American Chemical Society.
Free surface effects and solvent vapor annealing
lowers the glass transition temperatures (Tg) of the blocks, increasing
Surface preferentiality at the free surface is a similar field effect to
between blocks (χeff) and potentially the relative volume fractions,
that emanating from the substrate surface (Fig. 2a), with preferential
leading to changes in morphology70,72-74,76-81. Fourth, solvent
and neutral interactions affecting both microstructure orientation and
interactions with the substrate surface can lead to the screening of
stability18,36,37.
undesirable surface interactions31,72,75,82 or film dewetting19,83. Finally,
Because the free surface is a “soft” interface, surface preferentiality
chain mobility72,74-77. Third, solvent in the film affects the interactions
swelling and confinement of the film leads to an effective L0 that
is partially governed by the surface tensions of the copolymer blocks,
can be greater77,79,84 or less73,74,77,78,84 than the bulk L0, affecting
with the lower surface tension block preferentially segregating to
commensurability considerations.
the free surface68. However, we note that block surface tension is
In a traditional solvent annealing setup, a reservoir of solvent is
affected by deuteration69 (e.g., for neutron reflectivity or scattering
enclosed in a “bell jar” with the polymer film for a certain period of
studies) and by molecular weight68. Additionally, this surface property
time. The solvent is removed quickly to trap non-equilibrium, but often
can be tuned by thermal annealing. For example, an effectively
well-ordered, structures7,74,75,79,81,83. As discussed later in this section,
neutral free surface can be created in the PS-b-PMMA system
more recent flow setups have provided better control over the solvent
by annealing films between ≈170 °C to 230 °C, where the block
atmosphere and the degree of film swelling31,74,76.
surface tensions are approximately equal37,59. Unfortunately, many
Solvent annealing studies have examined a number of parameters to
copolymers contain blocks with very dissimilar surface tensions or
achieve desired morphologies, including solvent choice14,70,72,78,83,85,86,
blocks that are susceptible to thermal degradation. For these systems,
annealing time7,70,77,78, swollen film thickness31,76, and solvent
solvent vapor annealing provides an attractive alternative to thermal
removal rate14,74,80,85-88. In some cases, the casting history of the film
annealing.
also influences the final annealed morphology or the time required
Solvent vapor exposure during annealing serves multiple purposes.
to obtain well-ordered structures83. Below, we discuss a few specific
First, the vapor establishes surface preferentiality at the free surface
examples detailing how these parameters have affected copolymer
of the film70-74. Second, the vapor swells the film and effectively
morphology.
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Self-assembly of block copolymer thin films
Solvent choice
to fine-tune phase transitions and domain sizes in a poly(styrene-b-
In one example, Xuan et al.70 compared ultrathin (< ½ × L0 thickness)
dimethylsiloxane) (PS-b-PDMS) film with toluene and heptane solvents
lamellar PS-b-PMMA films annealed with acetone (strongly PMMA
(preferential for PS and PDMS, respectively)78. The mixed solvent
selective), chloroform (slightly PMMA selective), toluene (slightly PS
approach has also been employed in annealing75,81 and casting71,82
selective), and carbon disulfide (highly PS selective). Films annealed
of poly(ethylene oxide) (PEO) containing block copolymers at various
with PMMA-selective solvents for 60 h produced hexagonally
relative humidities. In these cases, a solvent mixture consisting of an
packed domains, whereas films annealed with PS-selective solvents
organic solvent and water vapor produced highly ordered and defect-
were mostly flat with small protrusions. The authors suggest that
free perpendicular cylinder motifs (Fig. 7)75,81,82, with high relative
solvent preference for PMMA is needed to overcome the free surface
humidity conditions (> 70 % RH) improving order75,81,82.
preference for the lower surface tension PS block to produce wellordered structures70. The need for a slightly selective solvent to
Annealing time
produce an effectively neutral free surface also was reported by
Annealing with selective solvents or under reduced (non-saturated)
Cavicchi et al.72
solvent vapor conditions may be desirable to obtain a particular
Though the above examples are general, finding a single solvent to
morphology or avoid disorder; however, polymer chain mobility is
create a neutral free surface for each block copolymer system is a time-
reduced in these situations, so longer annealing times may be required
consuming task, and using solvent mixtures in casting80 and annealing78
for self-assembly78. Lengthening annealing times has improved order
is a promising alternative. Mixed solvents were used by Jung and Ross
in some systems (Fig. 7d)7,75,78, but it has not always been successful
(a)
(b)
(c)
(d)
Fig. 7 (a) Atomic force microscopy (AFM) phase image of PS-b-PEO thin film spin-coated from benzene. (b) AFM phase image of PS-b-PEO thin film after annealing 48 h in benzene vapor. (c) Triangulation of the AFM image in (b). Defect-free domains with 6 nearest-neighbors are blue; 5 nearest-neighbor defects are red; 7 nearest-neighbor defects are yellow. (d) Average number of 5 nearest-neighbor defects in a 2 μm × 2 μm area as a function of annealing time. Reproduced with permission from75 © Wiley-VCH Verlag GmbH & Co. KGaA.
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orientation and formation during casting and solvent annealing14,85-87. The main difference between casting from solution and solvent annealing is the initial state of the polymer: in casting from solution the polymer is non-phase separated, while in solvent annealing the polymer is normally phase-separated74. Since an initial study by Kim and Libera87 showing that cylinder orientation could be affected by the rate of solvent evaporation upon casting, interest has grown in understanding morphology development during solvent evaporation in terms of χeff, volume fractions, and changing commensurability conditions80,88. Consistent with Kim and Libera87, Fukunaga et al. found parallel orientations for slow solvent removal rates and perpendicular orientations with fast solvent removal in solventannealed lamellar poly(styrene-b-2-vinylpryidine-b-tert-butyl methacrylate) (PS-b-P2VP-b-PtBMA) films85. However, for solventannealed PS-b-P2VP-b-PtBMA films of different block copolymer Fig. 8 Phase diagram of cylinder-forming PI-b-PLA thin films as a function of swollen film thickness (ts) and solvent concentration in the film (φCHCl3): parallel (), perpendicular (), and mixed () orientations. Reprinted in part with permission from31. © 2007 American Chemical Society.
composition, Elbs et al. observed a series of complex morphologies for different solvent removal rates (Fig. 9)86. Additionally, Zhang et al. suggested that evaporation of residual solvent from films during thermal annealing led to unexpected perpendicular orientation of
for generating long-range order70,77,89. One possible explanation is
cylinders in PS-b-PMMA films on preferential substrates90. Given the
that during a traditional solvent anneal, continual solvent uptake
relatively limited number of studies in this area, the ability of the
forces constant adaptation of the nanostructure to changing interfacial
solvent removal rate to influence copolymer self-assembly merits
interactions and commensurability conditions. Reports of cyclical
further investigation.
changes in morphology89 or transitions leading to disorder70,77 have
Concluding remarks
been noted for several systems.
Demand for nanoscale functional materials has led to advances
Swollen thickness and solvent uptake
in understanding the complex interplay between surface and
In one example of controlled annealing, Cavicchi et al. manipulated the
interfacial energetics, confinement, thermodynamic equilibrium, and
degree of swelling in poly(isoprene-b-lactide) PI-b-PLA films annealed
kinetics on block copolymer thin film self-assembly. Many thin film
in chloroform vapor and mapped the phase behavior as a function of
applications require near-perfect order and external stimuli such as
swollen film thickness (Fig.
8)31.
The authors proposed that at high
electric fields91, shear alignment92, zone-annealing93, and substrate
solvent concentration, the slight preference of chloroform for PLA
patterning4,8,94,95, among others, have been used to promote long-
balances the free surface preference for the lower surface tension PI
range order. Additionally, block copolymer and homopolymer blending
block and mitigates the substrate surface preference for PLA72. In their
has been used to manipulate domain sizes and morphology95-97,
phase diagram, higher solvent concentrations promote perpendicular
while salt-doping or nanoparticle addition has been employed to
ordering, supporting these conclusions31. Complementary work by
impart functionality or affect interfacial energetics98,99. A modified
Zettl et al.76 examined solvent uptake as a function of film thickness
description of thin film free energy proposed by Han et al.20 provides
and solvent vapor partial pressure. Their results suggested that thinner
a framework for the conceptual understanding of block copolymer
films take up more solvent than thicker films for the same solvent
thin film self-assembly: F = Felastic + Fblock + Fsurface + Finterface, where
partial pressure, highlighting the importance of directly measuring
Felastic accounts for chain conformations, including chain stretching
solvent uptake or understanding the relationship between solvent
at the substrate interface (favors chain ends and shorter blocks at the
vapor pressure and solvent uptake. Because the importance of swollen
substrate); Fblock describes block interactions which can be manipulated
film thickness and solvent uptake during annealing is a recent finding,
by surface compatibilization using neutral surfaces or the presence
re-examination of previous solvent vapor annealing studies in this
of solvent molecules, salts, or nanoparticles; Fsurface refers to the free
context may be worthwhile.
surface energy; and Finterface describes enthalpic interactions between substrate and copolymer.
Rate of solvent removal
Even with the above conceptual framework, establishment of a
By controlling solvent evaporation conditions, researchers have
comprehensive understanding regarding block copolymer thin film self-
found another handle to improve order and affect microstructure
assembly that can be readily applied to manipulate the phase behavior
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Self-assembly of block copolymer thin films
(a)
(c)
(b)
(d)
(e)
Fig. 9 SEM images of PS-b-P2VP-b-PtBMA thin films annealed with tetrahydrofuran subjected to different solvent removal rates. PS appears grey, P2VP white, and PtBMA black. (a) Solvent removed in 15 min.; terraced film and poorly ordered “micellar” structure. (b) Solvent removed in 3.5 h; “wagon-wheel” structure typical of bicontinuous gyroid network. (c)-(e) Solvent removed over 4 days; mixture of structures, including core-shell cylinders (c), sphere/cylinder morphology (d), and helix around cylinder morphology (e). Reprinted with permission from86. © 2002 American Chemical Society.
of new functional materials is still necessary. The facile stabilization of
monitored phase changes in-situ during solvent vapor annealing77,
structures such as networks, perpendicular cylinders, and perpendicular
while RSANS experiments have provided information about thin film
lamellae in various thin film systems will be vital for new materials
morphological symmetry and orientation90.
discovery in transport (e.g., ion conduction, organic photovoltaics,
methods provide another framework for gaining a comprehensive
However, the complex interplay between confinement, substrate
picture of block copolymer thin film self-assembly, and these
surface effects, and free surface effects makes selecting the appropriate
methods will be necessary to keep pace with demand for new and
film thicknesses, substrate modifications, and annealing conditions a
better materials. Techniques for employing combinatorial methods
significant challenge in polymeric device design.
at each level of processing are currently under development. One
Further studies of the above-mentioned parameters are facilitated by the development of advanced characterization techniques.
32
Given the large parameter space, high-throughput and gradient
and separation membrane) and nanoscale templating applications.
can now employ gradient monolayers to manipulate substrate surface energy/chemistry42,65-67,104; gradient thickness films (via
Improvements in sample preparation for atomic force microscopy
flow coating67,105) to examine confinement effects67,104,105; and
(AFM) imaging100, AFM cross-sectional imaging101, and TEM
free surface energy/solvent annealing gradients using microfluidic
with combinatorial arrays102 have enabled enhanced real-space
devices to examine soft-confinement annealing (unpublished work)106.
characterization. Additionally, two scattering techniques provide
Rapid morphology characterization is enabled by the scattering tools
versatile methods for probing block copolymer morphologies (surface
described above, and by the technique reported by Roskov et al.
and buried structures): grazing-incidence small-angle X-ray scattering
to generate thin film arrays for TEM characterization102. These
(GISAXS or GSAXS) and rotational small-angle neutron scattering
approaches will become increasingly important as more complex
(RSANS). GISAXS studies have confirmed the presence of complex
block copolymer based thin film systems are examined for advanced
network morphologies2, assessed microstructure orientation103, and
nanotechnologies.
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chain architecture (e.g., linear, star, etc.)17. More recent literature
role in meeting societal and economic goals for more efficient
has provided a working knowledge of surface energy effects in block
processes, cleaner energy generation, and smaller, hierarchically
copolymer thin films (e.g., on microstructure orientation and phase
structured devices. Block copolymers are a class of self-
transformations)18-20; however, a more comprehensive understanding
assembling material that segregates on nanometer length scales,
of thin film nanostructures is still a significant research focus.
making them ideal for emerging nanotechnologies1, including
investigation of surface and interfacial effects governing the self-
organic optoelectronics15, and anti-reflection coatings16. These
assembly of linear block copolymer thin films. For more general reviews
applications require the use of block copolymers in thin film
on block copolymer thin film phase behavior and related topics, we
geometries (~100 nm thickness), where self-assembly is strongly
refer the reader elsewhere8,15,18,21-23. Herein we highlight confinement
influenced by surface energetics.
and film thickness effects (section 1); discuss the role of substrate
Decades of research have provided a foundation for understanding
24
In this review, we discuss a framework for the experimental
applications in nanotemplating2-10, nanoporous membranes11-14,
surface modifications, including the use of gradient surfaces for high-
block copolymer self-assembly, beginning with the characterization
throughput investigations (section 2); examine the use of thermal and
of bulk morphologies using the Flory-Huggins interaction parameter
solvent annealing conditions to manipulate both free surface energetics
(χ), degree of polymerization (N), block volume fractions (ƒ), and
and block interactions (section 3); and finally comment briefly on
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ISSN:1369 7021 © Elsevier Ltd 2010
Self-assembly of block copolymer thin films
REVIEW
recent materials development and novel thin film characterization techniques, concluding with an outlook for the future.
(a)
(b)
(c)
Confinement Block copolymer thin films are typically subjected to a significant degree of confinement with film thicknesses comparable to the polymer domain spacing. Because stretching or compression of polymer chains due to confinement is energetically unfavorable, block copolymers selfassemble into morphologies that relieve the stretching/compression or mitigate the entropic penalty with favorable enthalpic interactions at the substrate or free surface, as discussed in later sections24. Two major categories of confinement are defined for substrate-supported block copolymer thin films: “hard” confinement describes a film confined between two rigid interfaces; “soft” confinement refers to a film where one interface (the “free surface”) is in direct contact with the atmosphere18. Systems under soft confinement with effectively unconfined lateral dimensions will be the focus of this review, and we refer the reader to other sources for work involving free-standing films25 and systems subject to hard and/or lateral confinement26,27. Under soft confinement, chain compression/stretching due to incommensurability between the film thickness (t) and polymer domain spacing (L0) is mitigated by the formation of islands or holes or by perpendicular orientation of the morphology (for cylinders and lamellae) depending on surface interactions. Surfaces that are preferential for one block of the copolymer typically lead to island/ hole formation, whereas non-preferential (or neutral) surfaces lead to nanostructure reorientation18,21. The degree of confinement or incommensurability is described by the deviation of the ratio t/L0 from the commensurability condition. Consider a lamellar AB diblock copolymer subject to preferential surface interactions. If surface preferences dictate that block A wets both the substrate and free interfaces (symmetric wetting), then for t/L0 = n (n = 1, 2, 3…), commensurability is achieved, and no island/ hole formation will be observed at the free surface. Similarly, if block A wets the substrate surface and block B wets the free surface or vice versa (anti-symmetric wetting), then the commensurability condition is t/L0 = n+0.518,21. However, for intermediate film thicknesses, incommensurability is described by the deviation of t/L0 from the commensurability condition, and islands/holes will be found at the free surface. Smith and coworkers illustrated the formation of islands/holes for incommensurate film thicknesses as a function of molecular weight of the polymer (Fig. 1)28. For block copolymer microstructures with more complex symmetries, determination of the commensurability condition is more difficult than for lamellae, but the same principle of island/ hole formation on preferential surfaces applies21. Sphere-forming thin films present an additional level of complexity because the spheres can arrange in a hexagonal symmetry in very thin films (up to approximately four layers of spheres) rather than the bulk
Fig. 1 Optical micrographs of gradient thickness lamellar PS-b-PMMA films annealed at 170 °C. Close to the commensurability condition t/L0= 0.5, films appear featureless; with increasing thickness, the morphology in all films progresses from islands to spinodal island/hole structures to holes to featureless at the next commensurate thickness. (a) Mn = 26 kg/mol, L0 ≈ 17-18 nm, (b) Mn = 51 kg/mol, L0 ≈ 27-30 nm, (c) Mn = 104 kg/mol, L0 ≈ 42 nm. Reprinted figure with permission from28. © 2001 by the American Physical Society.
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Self-assembly of block copolymer thin films
body-centered cubic symmetry29,30. Furthermore, the interplay
Typically, the substrate surface field propagates less than 6 × L0
between confinement and surface energy effects (e.g., wetting
from the substrate interface, though the propagation distance is clearly
layer formation and surface reconstruction18,19,31) becomes more
system-dependent20,37,39-42. Free surface effects may become apparent
complicated for non-lamellar-forming block copolymers and may lead
(Fig. 2a) or the film may revert to its bulk morphology with randomly
to phase transformations18,32. In the following sections, we continue
oriented grains some distance away from the substrate surface.
the discussion of confinement effects in the context of surface
In addition to cross-sectional imaging by TEM, the perpendicular
preferentiality.
orientations of lamellae and cylinders near a neutral substrate surface and parallel or mixed nanostructures near the free surface has been
Substrate surface effects
confirmed by neutron reflectivity39,40,43, small angle neutron scattering
Substrate surface energy and surface chemistry are important
(SANS)40,43, successive reactive ion etching (RIE)/scanning electron
parameters for controlling block copolymer thin film self assembly,
microscopy (SEM) imaging40, bottom layer imaging with SEM37, and
both with regards to microstructure
stability19
and microstructure
grazing-incidence small angle X-ray scattering (GISAXS)41,44,45.
orientation (for lamellae and cylinders)18,21. Preferential surfaces are wet by the preferred block, whereas neutral surfaces encourage the presence of all blocks at the
surface18,21.
Herein we highlight
Substrate modification methods Modification of the substrate surface with random copolymers or
these effects on smooth substrates, and we refer the reader to the
self-assembled monolayers (SAMs) are two common approaches
following references describing the influence of surface topology on
to generate neutral or preferential surfaces. The random copolymer
microstructure order and orientation33-35.
methods rely on the statistical composition of the copolymer to tune the substrate surface energy/chemistry. The copolymers can be end-
Substrate field effect
grafted46 or side-grafted45,47 to substrates to form a brush layer or
The substrate surface energy/chemistry effect can be generally viewed
formed as a cross-linked mat covering the substrate48,49. SAM methods
as a field that propagates from the substrate surface into the film36.
for block copolymer studies usually involve chlorosilane chemistry
The propagation distance depends on the relative strength of the field
on silicon oxide surfaces. These methods have employed partial
(i.e., the strength of the interactions between substrate and polymer
monolayers50,51, partial oxidation of hydrophobic monolayers19,28,52,53,
film), the competing interactions from the free surface, and the
and molecules that mimic the block copolymer structure42,54 to
energetics of the equilibrium bulk morphology. In a cross-sectional
manipulate surface energy/chemistry (note: though “SAM” is the
transmission electron microscopy (TEM) image of a thick poly(styrene-
commonly used term, many of these monolayers are not truly self-
b-methyl methacrylate) (PS-b-PMMA) film, Han et al. captured the
assembled).
moderate substrate surface field alignment of perpendicular cylinders ≈200 nm (≈L0 × 6) into the film (Fig. 2a)37. In contrast, Shin et al.
Neutral surfaces
used a strongly preferential surface to align a lamellar poly(styrene-
The application of block copolymer thin films as nanoporous
b-isoprene) (PS-b-PI) film parallel to the substrate surface over a
membranes and templates has motivated interest in identifying neutral
thickness of 40 × L0 (Fig. 2b)38.
surfaces to generate perpendicular lamellae to create line patterns55,
(a)
(b)
Fig. 2 (a) Cross-sectional TEM image of a ≈ 900 nm thick cylinder-forming PS-b-PMMA film on a neutral substrate surface annealed at 230 °C. Scale bar represents 500 nm. Defect line ≈ 200 nm (6 × L0) into the film (arrows) indicates where neutral substrate and neutral free surface field effects intersect. Both fields align cylinders perpendicular to the substrate surface. (b) Cross-sectional TEM image of a ≈ 1400 nm thick lamellar PS-b-PI film on a PS-preferential substrate. (a) Reprinted in part with permission from37 © 2009 American Chemical Society. (b) Reprinted with permission from37 © 2008 American Chemical Society.
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Self-assembly of block copolymer thin films
REVIEW
perpendicular cylinders for ring or dot patterns56 or conducting
depend not only on the neutrality of the substrate surface, but also on
channels12,13, and network structures to template nanowires2 or
the film thickness20,37,41,44,45,49, block copolymer composition20,41,44,57,
conducting
pathways19.
For lamellae and cylinders, the perpendicular
assembly windows and the quality of the perpendicular structures
method of surface modification20, and annealing conditions37,58 (see next section). Optimization of substrate surface neutrality has required researchers to consider all of these effects in their studies20. Neutral surface studies have primarily been conducted using PS-b-PMMA films because the PMMA domain is easily removed to create a nanoporous material11; furthermore, an effectively neutral free surface can be simultaneously achieved with appropriate thermal annealing conditions (see next section)37,59. Film thicknesses near L0 have been considered most often due to the loss of orientation or order in thicker films observed by Ham et al. and others (Fig. 3)20,39-41. On substrates modified with random copolymers (PS-r-PMMA), the neutral surface compositions are identified by windows of perpendicular microstructure orientation. As expected, increasing the PS content of the block copolymer shifts the perpendicular window to higher random copolymer PS fraction (Fst)20,41. However, the exact locations and widths of the perpendicular windows also depend on the method of substrate modification (Fig. 4)20. Perpendicular windows are shifted to higher Fst for surfaces modified with side-grafted (PH1) and cross-linked (PG1) random copolymers versus an endgrafted brush (Terminal-OH), due to the polarity of functional groups incorporated into the random copolymer backbones. Unfortunately, the perpendicular phase windows do not quantitatively follow trends in water contact angles of the modified substrates, possibly due to penetration of block copolymer chains into the neutral layer or the polarity of water20. Future work that examines surface energetics in additional detail (e.g., consideration of both polar and dispersive components of the surface energy) may help explain the reported trends. Additionally, free energy considerations suggest that the neutral surface condition for asymmetric block copolymers will not lie in the center of the perpendicular window, and that the narrow width of the perpendicular window for the PS-cylinder-forming polymer may be related to free-surface effects (Fig. 4). A more detailed discussion is provided by Han et al.20
Gradient methods While the phase behavior and methods for creating neutral surfaces for PS-b-PMMA have been fine tuned by more than a decade of study, quickly and easily mapping the phase behavior of new materials poses a significant challenge. Substrate surface energy/chemistry gradients provide an efficient route to probe how a range of surface energies/ chemistries affects morphology. Many gradient fabrication methods exist60, including statistical copolymer brush gradients61 and SAM gradients42,62-66, but few have been applied to understanding block Fig. 3 Scanning force microscopy phase images of different thickness films of cylinder-forming PS-b-PMMA (L0 ≈ 35.4 nm) on a neutral PS-r-PMMA copolymer brush annealed at 170 °C for 24 h. Reprinted with permission from41. © 2008 American Chemical Society.
copolymer thin film phase behavior19,28,42. As one example, Smith et al. combined surface energy and film thickness gradients to map the phase behavior of a model lamellar
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Self-assembly of block copolymer thin films
(a) Terminal-OH
(b) PH1
(c) PG1
(d) Terminal-OH
Fig. 4 (a) – (c) Perpendicular windows for PS-b-PMMA films assembled on substrates modified with end-grafted (Terminal-OH), side-grafted (PH1), and cross-linked (PG1) random copolymers (PS-r-PMMA). (d) SEM images of films on Terminal-OH substrates. Adapted with permission from20. © 2008 American Chemical Society.
PS-b-PMMA block copolymer67. An optical micrograph of the film (Fig. 5) shows the sharp transitions between preferential and nonpreferential wetting of the surface at ≈37 mJ/m2 and ≈40 mJ/m2, as well as between commensurate and incommensurate film thicknesses for symmetric (lower surface energies) and anti-symmetric (higher surface energies) wetting behaviors. More recently, Albert et al. developed a method for generating surface energy gradients that provides versatility in surface chemistry and straightforward adaptability to new systems42. In this method, monolayer gradients are generated in a single step cross-deposition of functionalized monochlorosilanes under dynamic vacuum. Chlorosilane functionalities can be chosen based on the block copolymer of interest, Fig. 5 Optical micrograph of a gradient thickness PS-b-PMMA film cast on a surface energy gradient that is preferential for PS on the left and preferential for PMMA on the right. The cloudy regions indicate island/hole formation. The featureless region centered at ≈38.5 mJ/m2 (yellow box) corresponds to the neutral surface energy window. The other featureless regions correspond to commensurate film thicknesses for symmetric (green boxes) and antisymmetric (blue box) wetting. Reproduced and adapted with permission from67. © Wiley-VCH Verlag GmbH & Co. KGaA.
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and the generation of linear substrate surface gradients can be tuned by the size of the liquid chlorosilane reservoirs within a custom-built insert. The authors located the expected parallel and perpendicular orientations for a cylinder-forming PS-b-PMMA block copolymer film on a substrate surface energy/chemistry gradient containing the appropriate functionalities (Fig. 6)42.
Self-assembly of block copolymer thin films
REVIEW
Fig. 6 Optical micrograph (top) and atomic force microscopy phase images (bottom) of cylinder-forming PS-b-PMMA thin films on pure-component and gradient monolayer substrates. The benzyl-silane is preferential for PS, the methacryl-silane for PMMA. Composition (xm, mole fraction of methacryl-silane) and surface energy (γs) given for each point examined on the gradient. The expected transitions between islands/holes and parallel cylinders on preferential surfaces and featureless optical micrographs and perpendicular cylinders on neutral surfaces were detected. Adapted with permission from42. © 2009 American Chemical Society.
Free surface effects and solvent vapor annealing
lowers the glass transition temperatures (Tg) of the blocks, increasing
Surface preferentiality at the free surface is a similar field effect to
between blocks (χeff) and potentially the relative volume fractions,
that emanating from the substrate surface (Fig. 2a), with preferential
leading to changes in morphology70,72-74,76-81. Fourth, solvent
and neutral interactions affecting both microstructure orientation and
interactions with the substrate surface can lead to the screening of
stability18,36,37.
undesirable surface interactions31,72,75,82 or film dewetting19,83. Finally,
Because the free surface is a “soft” interface, surface preferentiality
chain mobility72,74-77. Third, solvent in the film affects the interactions
swelling and confinement of the film leads to an effective L0 that
is partially governed by the surface tensions of the copolymer blocks,
can be greater77,79,84 or less73,74,77,78,84 than the bulk L0, affecting
with the lower surface tension block preferentially segregating to
commensurability considerations.
the free surface68. However, we note that block surface tension is
In a traditional solvent annealing setup, a reservoir of solvent is
affected by deuteration69 (e.g., for neutron reflectivity or scattering
enclosed in a “bell jar” with the polymer film for a certain period of
studies) and by molecular weight68. Additionally, this surface property
time. The solvent is removed quickly to trap non-equilibrium, but often
can be tuned by thermal annealing. For example, an effectively
well-ordered, structures7,74,75,79,81,83. As discussed later in this section,
neutral free surface can be created in the PS-b-PMMA system
more recent flow setups have provided better control over the solvent
by annealing films between ≈170 °C to 230 °C, where the block
atmosphere and the degree of film swelling31,74,76.
surface tensions are approximately equal37,59. Unfortunately, many
Solvent annealing studies have examined a number of parameters to
copolymers contain blocks with very dissimilar surface tensions or
achieve desired morphologies, including solvent choice14,70,72,78,83,85,86,
blocks that are susceptible to thermal degradation. For these systems,
annealing time7,70,77,78, swollen film thickness31,76, and solvent
solvent vapor annealing provides an attractive alternative to thermal
removal rate14,74,80,85-88. In some cases, the casting history of the film
annealing.
also influences the final annealed morphology or the time required
Solvent vapor exposure during annealing serves multiple purposes.
to obtain well-ordered structures83. Below, we discuss a few specific
First, the vapor establishes surface preferentiality at the free surface
examples detailing how these parameters have affected copolymer
of the film70-74. Second, the vapor swells the film and effectively
morphology.
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Self-assembly of block copolymer thin films
Solvent choice
to fine-tune phase transitions and domain sizes in a poly(styrene-b-
In one example, Xuan et al.70 compared ultrathin (< ½ × L0 thickness)
dimethylsiloxane) (PS-b-PDMS) film with toluene and heptane solvents
lamellar PS-b-PMMA films annealed with acetone (strongly PMMA
(preferential for PS and PDMS, respectively)78. The mixed solvent
selective), chloroform (slightly PMMA selective), toluene (slightly PS
approach has also been employed in annealing75,81 and casting71,82
selective), and carbon disulfide (highly PS selective). Films annealed
of poly(ethylene oxide) (PEO) containing block copolymers at various
with PMMA-selective solvents for 60 h produced hexagonally
relative humidities. In these cases, a solvent mixture consisting of an
packed domains, whereas films annealed with PS-selective solvents
organic solvent and water vapor produced highly ordered and defect-
were mostly flat with small protrusions. The authors suggest that
free perpendicular cylinder motifs (Fig. 7)75,81,82, with high relative
solvent preference for PMMA is needed to overcome the free surface
humidity conditions (> 70 % RH) improving order75,81,82.
preference for the lower surface tension PS block to produce wellordered structures70. The need for a slightly selective solvent to
Annealing time
produce an effectively neutral free surface also was reported by
Annealing with selective solvents or under reduced (non-saturated)
Cavicchi et al.72
solvent vapor conditions may be desirable to obtain a particular
Though the above examples are general, finding a single solvent to
morphology or avoid disorder; however, polymer chain mobility is
create a neutral free surface for each block copolymer system is a time-
reduced in these situations, so longer annealing times may be required
consuming task, and using solvent mixtures in casting80 and annealing78
for self-assembly78. Lengthening annealing times has improved order
is a promising alternative. Mixed solvents were used by Jung and Ross
in some systems (Fig. 7d)7,75,78, but it has not always been successful
(a)
(b)
(c)
(d)
Fig. 7 (a) Atomic force microscopy (AFM) phase image of PS-b-PEO thin film spin-coated from benzene. (b) AFM phase image of PS-b-PEO thin film after annealing 48 h in benzene vapor. (c) Triangulation of the AFM image in (b). Defect-free domains with 6 nearest-neighbors are blue; 5 nearest-neighbor defects are red; 7 nearest-neighbor defects are yellow. (d) Average number of 5 nearest-neighbor defects in a 2 μm × 2 μm area as a function of annealing time. Reproduced with permission from75 © Wiley-VCH Verlag GmbH & Co. KGaA.
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orientation and formation during casting and solvent annealing14,85-87. The main difference between casting from solution and solvent annealing is the initial state of the polymer: in casting from solution the polymer is non-phase separated, while in solvent annealing the polymer is normally phase-separated74. Since an initial study by Kim and Libera87 showing that cylinder orientation could be affected by the rate of solvent evaporation upon casting, interest has grown in understanding morphology development during solvent evaporation in terms of χeff, volume fractions, and changing commensurability conditions80,88. Consistent with Kim and Libera87, Fukunaga et al. found parallel orientations for slow solvent removal rates and perpendicular orientations with fast solvent removal in solventannealed lamellar poly(styrene-b-2-vinylpryidine-b-tert-butyl methacrylate) (PS-b-P2VP-b-PtBMA) films85. However, for solventannealed PS-b-P2VP-b-PtBMA films of different block copolymer Fig. 8 Phase diagram of cylinder-forming PI-b-PLA thin films as a function of swollen film thickness (ts) and solvent concentration in the film (φCHCl3): parallel (), perpendicular (), and mixed () orientations. Reprinted in part with permission from31. © 2007 American Chemical Society.
composition, Elbs et al. observed a series of complex morphologies for different solvent removal rates (Fig. 9)86. Additionally, Zhang et al. suggested that evaporation of residual solvent from films during thermal annealing led to unexpected perpendicular orientation of
for generating long-range order70,77,89. One possible explanation is
cylinders in PS-b-PMMA films on preferential substrates90. Given the
that during a traditional solvent anneal, continual solvent uptake
relatively limited number of studies in this area, the ability of the
forces constant adaptation of the nanostructure to changing interfacial
solvent removal rate to influence copolymer self-assembly merits
interactions and commensurability conditions. Reports of cyclical
further investigation.
changes in morphology89 or transitions leading to disorder70,77 have
Concluding remarks
been noted for several systems.
Demand for nanoscale functional materials has led to advances
Swollen thickness and solvent uptake
in understanding the complex interplay between surface and
In one example of controlled annealing, Cavicchi et al. manipulated the
interfacial energetics, confinement, thermodynamic equilibrium, and
degree of swelling in poly(isoprene-b-lactide) PI-b-PLA films annealed
kinetics on block copolymer thin film self-assembly. Many thin film
in chloroform vapor and mapped the phase behavior as a function of
applications require near-perfect order and external stimuli such as
swollen film thickness (Fig.
8)31.
The authors proposed that at high
electric fields91, shear alignment92, zone-annealing93, and substrate
solvent concentration, the slight preference of chloroform for PLA
patterning4,8,94,95, among others, have been used to promote long-
balances the free surface preference for the lower surface tension PI
range order. Additionally, block copolymer and homopolymer blending
block and mitigates the substrate surface preference for PLA72. In their
has been used to manipulate domain sizes and morphology95-97,
phase diagram, higher solvent concentrations promote perpendicular
while salt-doping or nanoparticle addition has been employed to
ordering, supporting these conclusions31. Complementary work by
impart functionality or affect interfacial energetics98,99. A modified
Zettl et al.76 examined solvent uptake as a function of film thickness
description of thin film free energy proposed by Han et al.20 provides
and solvent vapor partial pressure. Their results suggested that thinner
a framework for the conceptual understanding of block copolymer
films take up more solvent than thicker films for the same solvent
thin film self-assembly: F = Felastic + Fblock + Fsurface + Finterface, where
partial pressure, highlighting the importance of directly measuring
Felastic accounts for chain conformations, including chain stretching
solvent uptake or understanding the relationship between solvent
at the substrate interface (favors chain ends and shorter blocks at the
vapor pressure and solvent uptake. Because the importance of swollen
substrate); Fblock describes block interactions which can be manipulated
film thickness and solvent uptake during annealing is a recent finding,
by surface compatibilization using neutral surfaces or the presence
re-examination of previous solvent vapor annealing studies in this
of solvent molecules, salts, or nanoparticles; Fsurface refers to the free
context may be worthwhile.
surface energy; and Finterface describes enthalpic interactions between substrate and copolymer.
Rate of solvent removal
Even with the above conceptual framework, establishment of a
By controlling solvent evaporation conditions, researchers have
comprehensive understanding regarding block copolymer thin film self-
found another handle to improve order and affect microstructure
assembly that can be readily applied to manipulate the phase behavior
JUNE 2010 | VOLUME 13 | NUMBER 6
31
REVIEW
Self-assembly of block copolymer thin films
(a)
(c)
(b)
(d)
(e)
Fig. 9 SEM images of PS-b-P2VP-b-PtBMA thin films annealed with tetrahydrofuran subjected to different solvent removal rates. PS appears grey, P2VP white, and PtBMA black. (a) Solvent removed in 15 min.; terraced film and poorly ordered “micellar” structure. (b) Solvent removed in 3.5 h; “wagon-wheel” structure typical of bicontinuous gyroid network. (c)-(e) Solvent removed over 4 days; mixture of structures, including core-shell cylinders (c), sphere/cylinder morphology (d), and helix around cylinder morphology (e). Reprinted with permission from86. © 2002 American Chemical Society.
of new functional materials is still necessary. The facile stabilization of
monitored phase changes in-situ during solvent vapor annealing77,
structures such as networks, perpendicular cylinders, and perpendicular
while RSANS experiments have provided information about thin film
lamellae in various thin film systems will be vital for new materials
morphological symmetry and orientation90.
discovery in transport (e.g., ion conduction, organic photovoltaics,
methods provide another framework for gaining a comprehensive
However, the complex interplay between confinement, substrate
picture of block copolymer thin film self-assembly, and these
surface effects, and free surface effects makes selecting the appropriate
methods will be necessary to keep pace with demand for new and
film thicknesses, substrate modifications, and annealing conditions a
better materials. Techniques for employing combinatorial methods
significant challenge in polymeric device design.
at each level of processing are currently under development. One
Further studies of the above-mentioned parameters are facilitated by the development of advanced characterization techniques.
32
Given the large parameter space, high-throughput and gradient
and separation membrane) and nanoscale templating applications.
can now employ gradient monolayers to manipulate substrate surface energy/chemistry42,65-67,104; gradient thickness films (via
Improvements in sample preparation for atomic force microscopy
flow coating67,105) to examine confinement effects67,104,105; and
(AFM) imaging100, AFM cross-sectional imaging101, and TEM
free surface energy/solvent annealing gradients using microfluidic
with combinatorial arrays102 have enabled enhanced real-space
devices to examine soft-confinement annealing (unpublished work)106.
characterization. Additionally, two scattering techniques provide
Rapid morphology characterization is enabled by the scattering tools
versatile methods for probing block copolymer morphologies (surface
described above, and by the technique reported by Roskov et al.
and buried structures): grazing-incidence small-angle X-ray scattering
to generate thin film arrays for TEM characterization102. These
(GISAXS or GSAXS) and rotational small-angle neutron scattering
approaches will become increasingly important as more complex
(RSANS). GISAXS studies have confirmed the presence of complex
block copolymer based thin film systems are examined for advanced
network morphologies2, assessed microstructure orientation103, and
nanotechnologies.
JUNE 2010 | VOLUME 13 | NUMBER 6
Self-assembly of block copolymer thin films
REVIEW
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