Phase separation induced ordered patterns in thin polymer blend films

Progress in Polymer Science 37 (2012) 564–594

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Phase separation induced ordered patterns in thin polymer blend films Longjian Xue b , Jilin Zhang a , Yanchun Han a,∗ a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China b Institute for Chemistry, University of Osnabrück, Barbarastraße 7, 49069 Osnabrück, Germany

a r t i c l e

i n f o

Article history: Received 29 March 2011 Received in revised form 27 August 2011 Accepted 31 August 2011 Available online 29 September 2011 Keywords: Polymer thin films Polymer blends Phase separation Pattern formation Heterogeneous substrate Convection Breath figures

a b s t r a c t Strategies for the utilization of phase separation to generate ordered pattern in polymer thin film are reviewed. First, the fundamental theory and factors influencing phase separation in polymer thin film are discussed. That is followed by a discussion of the formation of ordered patterns induced by phase separation in polymer thin films under the influence of a chemical heterogeneous substrate, convection or breath figures. The mechanisms and the conditions for well-ordered structures generated by phase separation are then discussed to show that multi-scaled/multi-component patterns, stimuli-responsive patterns may be developed by controlling the preparation conditions or exposing the sample to different environments more complex structures. Finally, applications of fabricated patterns in pattern generation and reproduction, antireflecting coating, catalysis, bio-chips and optoelectronics are also discussed. © 2011 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phase separation of polymer blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Fundamental theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Phase separation in thin polymer films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ordered pattern generated by phase separation in polymer blend thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Phase separation on patterned substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Experimental investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Numerical simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Phase separation under convection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Convection types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Generation of ordered pattern by phase separation under convection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Phase separation under breath figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Mechanism of breath figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Materials suitable for breath figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Generation of ordered pattern by phase separation under breath figure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Stabilization of porous structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +86 431 85262175; fax: +86 431 85262126. E-mail address: [email protected] (Y. Han). 0079-6700/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.progpolymsci.2011.09.001

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4.

5.

Application of structures generated by phase separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Pattern generation and reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Antireflecting coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Bio-chips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Optoelectronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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588 588 588 589 590 590 590 590 590

Nomenclature z0 a b B c cS d0 D Dp f Fairflow g Gm h Hm k kB kc lsp M Ma Mac Mw,i nc na ns N p Sm r R Ra Rg Rmax RH t T T Tg V

function of velocity of the airflow and surface temperature average statistical segment length variation rate of surface tension with temperature concentration solvent concentration initial size of spinodal structure diameter of droplet diameter of pores volume fraction of one block in block copolymer (F airflow ) force caused by airflow acceleration of gravity change in free energy per segment film thickness enthalpy of mixing thermal diffusivity constant Boltzmann constant constant spinodal wavelength phenomenological parameter characterizing the self-diffusion ability in Eq. (6) Marangoni parameter critical value of Marangoni parameter molecular weights of polymer i refractive index of the coating refractive index of air refractive index of substrate polymerization degree pressure difference entropy of mixing point position characteristic size of phase-separated domains Rayleigh number gyration of an unperturbed chain largest dominant in-plane length relative humidity time temperature temperature difference glass transition temperature volume of sample

˛ (˛ ) ˇ (ˇ )   e m εij (r, t)  w 

v

ς ϕi ϕmax ϕ(r, t) ω

parameter predicting the monolayer/multilayer formation angle of  water droplet with respect to horizontal direction angle of  water droplets/toluene with respect to horizontal direction angle of airflow diverges the normal direction surface tension surface tension of chain end groups surface tension of the main chain part contact energy between i and j segments/components thermal noise periodicity of strip pattern wavelength of the incident light liquid density dynamic viscosity experimentally determined enthalpy coefficient experimentally determined excess entropy coefficient volume fraction of component i in homopolymer mixtures volume fraction for largest dominant inplane length local fraction of component A at point r at time t Flory–Huggins interaction parameter thermal volume expansion coefficient

1. Introduction Thin polymer films are increasingly used in numerous fields, such as electronics, optics, and biotechnology [1–3]. This broad range of applications requires polymer films with a wide spectrum of different surface morphologies, including smooth and uniform surfaces, enhanced-rough surfaces, and special ordered patterns with varying inplane lengths. An ordered pattern is characterized by as a well-arranged substance with high connectivity and symmetry. An economical approach to fabricate patterned surface makes use of surface instability of thin polymer films, such as dewetting [4] and phase separation [5], which are kinds of “bottom-up” methods. In a thin film geometry phase separation is often accompanied with dewetting

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[6]. Dewetting takes place in an unstable or metastable thin polymer film on a non-wetting solid surface when the polymer chains gain enough mobility under thermal or solvent vapor annealing. A typical process of dewetting includes hole generation and growth, joining of holes to form ribbons, and decay of ribbons into droplets [7]. Controlled dewetting can generate ordered structures on a micro/nano-meter scale conveniently and effectively [4]. Most polymer blends of high molecular weight polymers are intrinsically immiscible, and therefore phase separate under appropriate conditions because of the vanishing entropy of mixing. Phase separation occurs when either the undiluted mixture is held above the glass transition temperature Tg of the system or the solvent evaporates from its solution. Phase separation of polymer blends can lead to various morphologies, such as bicontinuous structure, islands, or holes when altering the system characteristics such as the composition, molecular weight and architecture, film thickness, solvent, or changes in the exterior environment, including the substrate, pressure, temperature, and external fields. This offers a means to pattern polymeric materials by controlling the phase separation morphologies in thin polymer films. Compared with dewetting, it is more convenient to achieve ordered patterns composed of multi-components by phase separation. In this review, a brief introduction to the fundamental theory and influencing factors on phase separation in thin polymer blend film are introduced in Section 2. The mechanisms and the conditions for good quality ordered structures generated by phase separation are discussed in Section 3, including: (1) phase separation on patterned substrate; (2) phase separation under convection; (3) phase separation under breath figures. Applications of generated patterns are then briefly summarized in Section 4. 2. Phase separation of polymer blends 2.1. Fundamental theory The change of free energy per segment Gm in a binary mixture of linear homopolymers can be estimated through Gaussian polymer chains on an incompressible (ϕA + ϕB = 1) lattice [8,9]. Gm ϕA 1 − ϕA = ln ϕA + ln(1 − ϕA ) + ϕA (1 − ϕA ) NA NB kB T

(1)

The first two terms (right-hand side) account for the combinatorial entropy of mixing Sm . Because mixing increases the systems randomness, it naturally increases Sm (Sm > 0) and thereby decreases the free energy of mixing. Because of the long chain of polymers and the entanglement of polymer chains, Sm should not be large; and Sm decreases with increasing the polymerization degree, N. The third term represents the enthalpy of mixing Hm and can either increase or decrease Gm depending on the sign of the Flory–Huggins segment–segment interaction parameter , which may be approximated by the following equation: [10] =

1 kB T



εAB −

1 (εAA + εBB ) 2



(2)

where εij is the contact energy between i and j segments (components), T is the temperature, and kB is the Boltzmann constant. Negative values of occur for certain types of specific interactions between A and B, such as hydrogen bonding, that is, A–B segment–segment contacts on average produce a lower system energy than the sum of A–A and B–B contacts. A positive value of represents a net system energy increase upon forming A–B contact pairs. Assuming that there is no volume change or preferential segment orientation during mixing, the mixing of nonpolar polymers, such as polystyrene (PS) and polyisoprene (PI), governed solely by dispersive interactions (van der Waals interactions) is thermodynamically unfavorable, ≥ 0 [11]. In practice, anisotropic monomer structures may lead to nonrandom segment packing which must be absorbed in as an excess entropy of mixing given by = T −1 + ς

(3)

where and ς represent experimentally determined enthalpy and excess entropy coefficients for a particular composition. In general and ς depend on overall volume fraction of a component, ϕ, in homopolymer mixtures (or volume fraction f of one block in block copolymer), N, T, and molecular architecture. If is negative and ς is positive, then a lower critical solution temperature (LCST) may result depending on the magnitude of N and ς . Increasing T would bring the blend of A and B from one phase into two phases. If is positive and ς is negative in Eq. (3), decreasing temperature always increases and an upper critical solution temperature (UCST) results; however, decreasing the temperature would not trigger the happening of phase separation since the polymer chains can hardly move at low temperatures. The phase behavior can then be predicted with Eq. (1) based on the standard criteria for equilibrium, and stability evaluated at constant temperature and pressure by a phase diagram (take a symmetric case, NA = NB , as the example, as shown in Fig. 1). The solid and dash curve represent the solutions of the usual equilibrium and stability equations, respectively: Equilibrium : Stability :

∂Gm (ϕA ) ∂ϕA

∂2 Gm ∂ϕA2

=0

=

∂Gm (ϕ A ) ∂ϕA

(4) (5)

where ϕA and ϕA are the two volume fractions of component A. The region between the equilibrium and stability in the phase diagram is a metastable region, the mechanism of nucleation and growth governs the development of phase separation of the mixture (Fig. 2). Homogeneous mixtures must overcome a free energy barrier in order to nucleate a new phase, which proceeds by diffusion of material from the supersaturated continuum. Once the composition of the supernatant reaches equilibrium, further increases in droplet size may occur by droplet coalescence or Ostwald ripening; the latter refers to the growth of large droplets through the disappearance (“evaporation”) of smaller ones. The second stage of growth may be extremely slow because of the extremely low diffusivity and enormous viscosity of polymers, and may result in unusual particle-size

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distributions. In the thermodynamically unstable state (the region enveloped by the dash line in the phase diagram), mixtures phase separate spontaneously, which is known as spinodal decomposition. It results in a disordered bicontinuous two-phase structure. The initial size d0 of the spinodal structure is controlled by the quench depth s − , where s corresponds to the stability limit (dashed curve in Fig. 1); deeper quench produces better structures. Almost immediately after the bicontinuous pattern begins to form, interfacial tension favors a reduction in surface area by increasing d. However, polymer melts (materials that have been heated) are extremely viscous so that phase-separated homopolymers essentially never reach the equilibrium morphology. 2.2. Phase separation in thin polymer films

Fig. 1. Schematic phase diagram for a symmetric (NA = NB ) binary mixture of linear homopolymers showing the LCST (TLCST ) and UCST (TUCST ).

While for bulk polymers the mechanisms of phase separation are in general well understood, the situation in thin polymer films is quite different from that in bulk and is complicated by the presence of the substrate/film and film/air interfaces, giving rise to complex structures [12]. A thin polymer film may be prepared by spin-coating (a frequent process in industrial practice). After the spinningoff of ca. 90% homogeneous fluid, phase separation occurs for immiscible blends, initiated by the loss of solvent by evaporation [13,14]. The rapid increase in the viscosity of the film captures the phase-separated structure in place, a process that is very sensitive to a variety of parameters including film thickness [8,15–20], substrate (polymer

Fig. 2. Time evolution of structure in phase-separating binary homopolymer mixtures. Nucleation and growth results when a homogeneous mixture is thrust into the metastable region of the phase diagram. Spinodal decomposition occurs when a mixture is placed in a thermodynamically unstable state. Reproduced from [11] with permission. Copyright 1999, Science.

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substrate interactions) [14,21–23], surface tension of polymer [18,24,25], Flory–Huggins parameter [13,25–30], molecular weight of polymer [31–33], component ratio [18,27,34–37], the relevant solvent parameter (polymer solubility) [14,17,38,39] or evaporation speed [40–44], chemical reaction in the blend [45], addition of additives [46] and so on. The structures right after preparation are apparently not in equilibrium [21,22,38,47]. The coarsening of the domains to the equilibrium state takes place on further annealing (thermal or solvent vapor) process [48]. As the film thickness becomes comparable to the decay length of spinodal waves, the growth of phase separation starts from the free surface and the substrate wall and propagates toward the center of film [49,50], in a surfacedirected phase separation [51]. In such a thin film, the temperature for phase separation [52], the Flory–Huggins parameter [53] and breakup mechanism [54] may change. Some blends miscible in the bulk, such as PS/PVME, may undergo phase separation if the film thickness is less than twice the radius of gyration of an unperturbed chain, 2Rg [52,55]. In order to minimize the system energy, the component with lower surface energy is generally segregated to the free surface [14,17,32,56,57], while the component possessing higher affinity with substrate would migrate to the substrate interface [49,58]. This kind of preferential segregation is called a vertical phase separation (Fig. 3a). The preferential segregation creates layered structure [16,59] or gradient distribution [54] in the thickness direction. A tri-layered structure, where a mixed layer is sandwiched between two PVME concentrated layers, is suggested in the PS/PVME films with thickness less than 200 nm. The interfacial width between the layers decreases slightly with increasing total film thickness and increases with annealing time. In a much thicker film (e.g., several microns), the size of PMMA domain decreases from the top surface to the deep inside. On the other hand, a lateral phase separation may develop for a blend of polymers with similar surface tensions on a neutral substrate. The difference of surface energy between chain end ( e ) and main chain ( m ) groups cannot be ignored in case of polymer with a low molecular weight. If  e <  m , the chain end groups are preferentially localized at the surface [60,61]. For example, PMMA with Mn < 144k was preferentially segregated at the air–polymer interface in the PS/PMMA blend, even  PMMA = 41.2 mJ m−2 is larger than that of PS,  PS = 40.2 mJ m−2 [62]. Moreover, the effect becomes more remarkable with a decrease of Mn , due to an increase in the number density of chain end groups. Meanwhile, smaller molecular weight may increase the miscibility of polymer blend resulting in the reducing of microdomain size (even to nanoscale) [33,63]. Reducing the film thickness [64] or the relative concentration of one component [18,35,36] will also reduce the size of the corresponding phase. However, these composition waves normal to the surface will be suppressed in thin blend films with thicknesses below a critical thickness [65,66]. Solvent is another very important parameter determining the phase-separated structure. The different solubilities of polymers in the casting solvent can cause demixing of the polymers as the solvent evaporates, leading to the component of higher solubility being enriched at the film

Fig. 3. (a) Schematic phase separation of thin polymer blend on a homogeneous substrate results in vertical or lateral phase structures. (b) and (c) AFM images of free surface topography formed by the PS/PI blend films spin-cast onto CH3-SAM (b) and COOH-SAM (c) substrates, respectively. Reproduced from [67] with permission. Copyright 2003, American Chemical Society.

surface [14,17,39]. This effect may then result in a vertical phase separation with the higher-surface-tension component on top of the film [17,18]. Lateral phase separation with free surface undulations form on a neutral substrate, for example, with PS/PMMA blends on Au surface [14]. Under evaporation with a selective solvent, at the condition the phase domains of the component with less solubility is solidified, the phase domain with better solubility is still swollen; and the sample surface is essentially flat (Fig. 4b). Further evaporation may collapse the swollen phase to a level that below the vitrified phase, creating relief structures (Fig. 4c–j). For the same kind of solvent (e.g., good solvent for one component), the lower vapor pressure is, the longer the time to reach polymer solidification, and therefore the closer to the thermodynamic equilibrium state of phase separation process is [40–42]. Substrate dependent solvent evaporation can cause the formation of protrusions with either concave or convex topography, as shown in Fig. 3b and c, respectively [67]. Fast evaporation of solvent toluene from a developing film a on self-assembled monolayer (SAM) with COOH end-groups (COOH–SAM) substrate produces a positive pressure-difference p across the surface skin of a PSrich phase domain, resulting in the formation of convex

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Fig. 4. Schematic model describing the formation of the topographic structure during the spin-coating process. Initially, PS and PMMA are dissolved in a common solvent (a). During spin-coating, solvent evaporates and demixing of PS and PMMA sets in (b). At this stage, the sample surface is essentially smooth, apart from a small variation of the liquid surface caused by the different surface tensions of the PS- and PMMA-rich phases. Depending on the relative solubility of the two polymers in their common solvent, one of the phases is more quickly depleted of the solvent and turns solid earlier than the other phase. Subsequent evaporation of the remaining solvents leads to a further collapse of the better soluble phase. Toluene or THF are better solvents for PS, and the PS phase (dark gray) collapses below the average height of the PMMA layer (light gray) (c). For MEK, the PS-rich phase solidifies first, leading to elevated PS islands (d). The shape of the domains is determined by the different surface tensions of the two phases in (b): the phase with the lower surface tension (PS-rich) has a convex curvature in (b), as opposed to a concave surface structure of the PMMA phase. This leads to sharp edges of the PMMA domains in (c) as opposed to a more round topography in (d). AFM images (14 × 14 ␮m) of PS/PMMA mixtures spin-cast from different solvents onto a Au surface: (e)–(g) THF; (h)–(j) MEK. The first column (e, h) contains topographic images of the PS/PMMA samples as cast. In the second column (f, i) the PS-rich phase was removed by immersion in cyclohexane, and the AFM images display the remaining PMMA surface. In the third column (g, j), cross-sections of the topographic images before and after cyclohexane immersion were superimposed to show the vertical PS and PMMA distribution (PS, dark gray; PMMA, light gray). The lines in topographic images (e, f and h, i) indicate the locations where the cross-sections were taken. Reproduced from [14] with permission. Copyright 1997, American Chemical Society.

domain face. In contrast, a smaller pressure difference p is produced by slow evaporation on CH3 –SAM substrate, resulting in the collapse of surface faces. 3. Ordered pattern generated by phase separation in polymer blend thin films The most typical final morphology of a phase-separated film is an isotropic disordered phase structure with a characteristic length scale. Though the pattern has a certain structure, it is far from an ordered structure. A chemically patterned substrate can initiate and guide the phase separation in polymer blend film transferring the designed pattern on the substrate into the polymer film. Wellarranged sea-islands, holes or porous structures may be generated in a polymer film in one step by controlling the convection inside the film or exposing the film to a humid environment during the solidification process. The pattern structure and size can be tuned through several parameters. Even more complex structures like multiscaled/multi-component patterns, patterns with different geometries or stimuli-responsive patterns may be developed by controlling the preparation conditions or exposing the sample to different environments. 3.1. Phase separation on patterned substrate Some impurities and/or some defects always exist on a real surface, and the previous discussion suggests that the substrate exchange or modification will lead to different

final phase arrangements. Thus, spatial patterned substrate on a microscopic or nanoscopic length scale would initiate phase separation and guide the corresponding component to the desired location minimizing the total energy, which therefore transfers the designed pattern on the substrate into the multi-component polymer film. An ideal copy of the substrate pattern can only be achieved under narrowly constrained conditions. 3.1.1. Experimental investigations Phase separation is sensitive to perturbations that break the isotropy of the bulk self-organization process, which makes it possible to achieve a variety of morphologies through the adjustment of the perturbing field. The stronger affinity of one component with a specific area on the substrate will guide the migration of the component to the specific area, reducing the system energy. The match of the size of the phase-separated structure to the pattern periodicity is the key role for the faithful pattern transferring [68–71]. Böltau et al. [72] pioneered the transferring of patterns on a substrate to polymer films via two sets of phase separation in polymer blends, a blend of hydrophobic PS and hydrophilic poly(2-vinylpyridine) (P2VP), and a blend of hydrophobic PS and hydrophilic brominated polystyrene (PSBr). Strip-structured polymer films were achieved after spin-coating the polymer blend solution onto alkanethiol SAM-striped gold substrates which is prepared by microcontact printing technique (Fig. 5). The strong affinity between gold surfaces and hydrophilic

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Fig. 5. AFM images (80 ␮m × 80 ␮m area) of PS/PVP blend spun-cast on a patterned Au/SAM substrate (a) as cast; (b) after the removal of PVP by dissolution in ethanol. (c) The perspective representation visualizes the rectangular cross-section of the 65-nm-high PS stripes in (b). The dotted areas indicate the SAM-covered substrate regions. Reproduced from [72] with permission. Copyright 1998, Nature Publishing Group.

materials binds PVP or PSBr molecules from the mixtures onto the gold surfaces and repels hydrophobic PS molecules away to SAM surfaces. This means the driving force for the pattern formation is the competition between surface and interfacial energy, or in other words,

the substrate/polymer interaction [70,73]. Karim et al. [74] and Cyganik et al. [75] obtained stable and well-aligned stripes in phase-separated deuterated PS/polybutadiene (dPS/PB) and P2VP/dPS (or PSBr) films on microcontact printed (␮CP) alkanethiol patterns with hydrophobic and hydrophilic end groups (–CH3 and –COOH), respectively. While most research focuses on the phase separation on strip-patterned substrate, Han [76] also investigated phase-separated structure in PS/P2VP blends on square- and rectangle-patterned octadecyltrichlorosilane (OTS)/SiOx . Selectively removing P2VP component, squareand rectangle-porous PS films were then developed. On two dimensionally chemically patterned surfaces, pattern replication is expected when both the blend composition ratio matches the surface patterning area ratio, and the pattern periodicity matches the natural phase separation length scale. By varying film thickness and blend composition, ordered morphologies also result for contrary situations, resulting in the formation of a rich variety of hierarchically ordered microstructures [77]. The transferring of pattern from the substrate (with alternating stripes of equal width W and different surface energy) to the phase-domain structure of a thin film of polymer blend depends on a wide variety of parameters. In order to achieve a faithful replication of the pattern on substrate, the inherent dimensional size R of the phase structure should match the pattern periodicity  (strip pattern) [72,74,75,78–83], which is similar to the formation of dewetting pattern directed by substrate [4]. With  < R/2, the film can hardly responds to the existence of the pattern on the substrate, such that only some weak traces of patterns or isotropic morphology are generated, and a marked suppression of phase separation in the blend could be found as compared to that on a laterally homogeneous substrate [84]. If  becomes too large compared to R, then phase separation may occur within each stripe pattern, as for a uniform surface. However, that surface relief with  > R can align locally ordered grains of block copolymers into structures with a long range order [85]. Elongated droplets or bridged strips are generated when R is between /2 and , while normally can the bridged strips be found when R is slightly larger than  [81]. Since R is related to the film thickness, therefore the replication is also controlled by the film thickness. On a heterogeneous Au/SiO surface with a 60 nm stripe period, PS/PMMA blend phase separated as on a neutral surface (Fig. 6a); however, PS was found to locate on the nanometer-scaled Au strips after the selective removing of PMMA component when a nanoscale grooved surface was used (Fig. 6b) [86]. On an asymmetric striped pattern surface composed of 10.3 ␮mwide hexadecanethiol monolayer (HM) and 8.7 ␮m-wide Au, well-aligned PS- and P4VP-rich phases with sharp interfaces formed after the spin-coating since the phase structure R is exactly 8.7 ␮m [79] (Fig. 7a and b). When the strip widths of HM/Au pattern increased to 18.5 ␮m (twice R), the P4VP-rich phases aggregate align along the gold stripes. In the central part of a SAM stripe region, however, small P4VP particles were found to disperse in PS matrix (Fig. 7c and d). Since the substrate/polymer interaction is short ranged, the P4VP molecules on the central part of HM stripe (distance larger than R) do not respond significantly

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Fig. 6. AFM tapping mode height images of PS/PMMA blend films after the selective etching of PMMA rich phase with acetic acid on (a) a heterogeneous Au/SiO surface with a 60 nm stripe pattern; (b) a Au strips-decorated grooved surface as shown in (c). The scale bars in (a) and (b) are 0.5 ␮m. (c) Field emission scanning electron micrograph of a periodic heterogeneous surface. The average stripe period here is 105 nm, with a metallization line width averaging 55 nm. The scale bar shown is 0.6 ␮m. The inset is the schematic diagram of the glancing angle evaporation geometry used. Provided the incidence angle ˛ of the metal atoms is less than the apex angle  of the facets, shadowing will occur and a heterogeneous surface will be produced. Reproduced from [86] with permission. Copyright 1999, American Physical Society.

to the gold substrates and the phase separation occurs as on a homogeneous substrate, resulting in the formation of P4VP droplets. The P4VP at the edges of HM strip will diffuse to the gold stripes forming depletion regions. It is clear that for a film with a certain thickness (and a corresponding phase structure R) there are upper and lower limit on the scale of surface pattern. In other words, for a certain patterned substrate, the film thickness should be restricted to a range thin enough to suppress surface-directed spinodal decomposition, and thick enough to avoid dewetting [74]. The perfect replication of pattern on substrate is then achieved only when the phase structure R matches the pattern periodicity . On the other hand, it means the sizes of ordered polymeric patterns are confined to those of stamps or molds. The molecular weight affects the dynamic of phase separation on a chemically patterned substrate [79]. The asymmetric phase structure on 18.5/18.5 ␮m HM/Au pattern, in which droplets of P4VP aggregate in the center of the HM area and no droplets of PS in the P4VP area, infers that the relatively low viscosity of PS component makes it possible for PS molecules to diffuse to the HM area. When Mw,PS is increased to 129k, P4VP-rich phases formed on the gold stripes, with many defects, (Fig. 7e). On increasing the Mw,PS to 582k, no matter what the blend composition was, no directing effects of patterned-surfaces was observed during spin-coating process even the R matches the pattern periodicity , just like on a homogeneous substrate. The high viscosity of PS component suppresses the diffusion of the P4VP that the further coarsening or aggregating of P4VP into ordered domains on the gold stripe surfaces becomes very difficult during spin-coating. When the diffusion time is prolonged, for example, preparing the film by dip-coating, the ordering of phase structure is improved (Fig. 7f). Budkowsk and coworkers [81] found that the phaseseparated morphologies on a symmetric pattern depend not only on spatial ratio R/, but also on compositional commensuration between blends: In addition to primary  structures, /2-substructures are present when the

selectively adsorbed polymer forms isolated (minority phase) rather than continuous (majority phase) domains. For symmetric blends, pattern replication is affected by the transition from isolated to continuous domains, observed for (thicker) spin-cast films with larger R values. If the compatibility is improved (e.g., adding a third component as mentioned in the preceding), the transition into continuous film structures occurs for smaller R values. Phase separation of blends of conjugated and dielectric macromolecules onto chemically patterned surfaces provides a simple, solution-processing method to fabricate polymer-based circuitries that can be integrated with conventional electronics. For instance, the UCST-type blend [87] of poly(3-alkylthiophenes) (P3AT) and PS will undergo vertical or lateral phase separation depending on the solvent and substrate used [88,89]. High performance and low percolation threshold thin-film transistors (TFT) have been fabricated from such the phase separated structures, benefiting from the improvements of both the crystallinity and the connectivity of P3HT phase in the blend [90]. When the blends are cast on a patterned substrate, the phase separation behavior is not much different from the aforementioned model systems. It should be mentioned that the thiophene groups have strong attractive forces with Au atoms, which may affect the phase diagram [89]. 3.1.2. Numerical simulation Numerical simulation is playing an increasingly important role in illuminating the undergoing mechanism that can be used to guide the experiments for ideal templating. The phase separation during the spin-coating is so complicated that most simulations focus on the phase separation of a thin polymer blend film on strip-patterned substrate under thermal annealing, which will shed some light on how the substrate pattern influences the phase separation. While a number of investigations have studied the factors influencing the resulting pattern and dynamics of phase separation [91–96], that topic is out of the scope of this review; we here only give a brief introduction to the simulation models and the main results for strip-pattern

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Fig. 7. AFM topographical images of the PS/P4VP (50/50, w/w) blend films on the hexadecanethiol monolayer (HM)/Au strip patterned substrates: (a, b) PS-1/P4VP on HM/Au = 10.3 ␮m/8.7 ␮m; (c, d) S-1/P4VP on HM/Au = 18.5 ␮m/18.5 ␮m; PS-2/P4VP (50/50, w/w) on HM/Au = 10.3 ␮m/8.7 ␮m; PS-3/P4VP (50/50, w/w) on HM/Au = 10.3 ␮m/8.7 ␮m. The films in (b, d, and e) had been treated with cyclohexane for 5 min to remove the PS-rich phase. (a)–(e) are prepared by spin-coating and (f) is prepared by dip-coating. Reproduced from [79] with permission. Copyright 2004, Elsevier.

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Fig. 8. (a)–(c) Schematic illustration of pattern replication for polymer blend films on chemically patterned substrate. (b) The checkerboard morphology.

directed phase separation. These reveal details of the phase separation process on the patterned substrate and confirm the experiment results. 3.1.2.1. Theoretical models. Employing the traditional Cahn–Hilliard–Cook (CHC) model augmented by a surface energy term, phase separation of a binary blend near a patterned surface was first investigated in two dimensions (2D) by Karim and co-workers [74] and in three dimensions (3D) by Muthukumar and co-workers [97]. CHC equation describing the dynamics and morphologies evolution in a binary mixture (A and B) can be written as: ∂ϕ ıF{ϕ(r, t)} = M∇ 2 + (r, t) ∂t ıϕ(r, t)

(6)

Here ϕ(r, t) is the local fraction of component A at point r at time t. M is a phenomenological parameter characterizing the self-diffusion ability. (r, t) is the thermal noise. A more complete and detailed kinetic pathways of pattern-directed phase separation in binary polymer mixture can be obtained by coupling the CHC model with the Flory–Huggins-de Gennes (FHdG) model, which better fits the polymer blend [98] The FHdG free energy is given by F(ϕ) (in units of kB T):



F(ϕ) =

dr V

+

ϕ N

ln(ϕ) +

1−ϕ ln(1 − ϕ) + ϕ(1 − ϕ) N

 2 b2 ∇ ϕ  36ϕ(1 − ϕ)



(7)

two phases breaking the original in-phase order structures gradually. 3.1.2.3. Influence of pattern size on replication of substrate pattern. The pattern size (its period) plays a critical role in the faithful replication of substrate pattern. For thin film with thickness of the order of sp the pattern (with period of sp ) will persist throughout the whole film (Fig. 9a), whereas for thicker films the registry of the domain structure will decay some distance away from the surface and the randomly oriented bulk morphology will dominate. As the patter period is too small (e.g., 0.5sp ), bulk phase separation structure dominates the surface structure, just as a very thick film (Fig. 9b). The initial diffusion of material to the corresponding areas of the surface is kinetically favored at short times, but leads to a distribution of material within the film differs from the preferred equilibrium state. Therefore, the larger the surface pattern, the more material needs to be rearranged to get from the transient states to the final equilibrium states. This process is further complicated by the fact that most rearrangement has to occur parallel to the surface (Fig. 9c), where the thinness of the film restricts the flow of the material. Phase separation may happen within a single strip indicating the competition between pattern directed phase separation and bulk phase separation. Meanwhile, the material flow in the plane induces undulations of the pattern edges, and bridges the neighboring strips of same phase. The ratio of polymer blends and the surface pattern symmetry certainly affect the pattern replication and enrich the final structure of pattern directed phase separation [74,97].

The integral is performed over the volume V of the sample. 3.1.2.2. Phase behavior on patterned substrate. No phaseseparated structure can be discerned in the polymer/air interface at the initial time. With increasing time, the stripe structure development in this interface and becomes clear, exhibiting the typical in-phase state where the component in each strip corresponds to the chemical potential in the patterned surface. A fluctuation wave then penetrates into the film, gradually creating checkerboard-like composition fluctuations in thicker films (Fig. 8b) [97,98]. The penetrating speeds of the anisotropic waves induced by the patterned surface vary with the compositions [99]. The closer is the composition to the critical point (ϕ = 0.5), the faster the surface effect propagates into the bulk and the clearer strip the pattern can be seen in the polymer/air interfaces. In the plane, the reduction of interfacial and surface energy causes a pattern-directed lateral phase separation (Fig. 8c). Moreover, the extreme fluctuation of the chemical potential at the edges of stripes leads to the formation of the branched structures at the borders between

3.1.2.4. Influence of pattern contrast on replication of substrate pattern. The final morphology also depends on the results of the competition between the surface and interfacial energy [70,97], that is, to obtain a well-aligned phase separation pattern, substrate/polymer interactions should be carefully adjusted. The intensity and the extent of these oscillations are virtually independent of the interaction strength. At late times the surface-induced fluctuations dissolve for weak surface interactions and bulk phase separation dominates, while the surface-induced fluctuations survive for strong surface interactions [97]. The results of Cyganik et al. [75], however, showed that a large surface energy difference between two types of alternating stripes did not improve the transfer of the substrate pattern. Shou [70] simulated the effects of different substrate pattern widths and surface interaction strengths on the phase morphology. When the strip pattern on the substrate is narrow, a strong surface interaction is able to hold the respective preferred components right on the substrate patterns. As the surface interaction is short-ranged, the phase domains

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Fig. 9. Spinodal decompositions in thin films of thickness: (a) 0.5lsp , (b) 0.5lsp , and (c) lsp on the patterned substrate with pattern periodicity of: (a) lsp , (b) 0.5lsp , and (c) 8lsp , respectively. Reproduced from [97] with permission. Copyright 1999, AIP.

will lose their ordering slightly away from the substrate. Almost perfect ordering was observed when the width of surface pattern is increased and a strong surface interaction strength is chosen.

conditions, the convection brings the solute into the specific locations, such as lower-surface-energy regions, creating ordered structures. Patterns with tunable structure and size, more complex structures, stimuli-responsive properties can be achieved.

3.2. Phase separation under convection During the evaporation of solvent, the uneven evaporation and the undulations of surface tension, density, and component ratio cause convection in the liquid film, thus regulating the dispersion of solute. Under proper

3.2.1. Convection types Convection, a universal phenomenon in the heat and mass transfer in liquids, can be categorized into two types: Rayleigh-Bénard convection [100] and Bénard-Marangoni convection [101]. Usually, when the liquid film thickness is

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Fig. 10. SEM micrographs of a PS/PEG 200 (70/30, w/w) film made from a 90 wt% benzene solution: (a) edge, (b) center. Inset: SEM micrograph magnifying a porous stripe. Reproduced from [107] with permission. Copyright 2008, American Chemical Society.

over 1 mm, Rayleigh-Bénard convection, which originates from the temperature-dependent gradients in the density of the fluid, will dominate. Typical structures of roll-like patterns will be generated, when the Rayleigh number Ra, given by Ra =

gω Th3 , vk

(8)

is over a critical values of 669; g is the acceleration of gravity, ω is the thermal volume expansion coefficient, T is the difference of temperature between the lower plane and the free surface, h is the depth of the layer of fluid, v the dynamic viscosity, and k is the thermal diffusivity. Otherwise (film thickness less than 1 mm), BénardMarangoni convection will be the dominate mode. Different from Rayleigh-Bénard convection, Bénard-Marangoni convection is governed by a balance of the surface-tension driven forces and the dissipation due to the thermal diffusion and the frictional action of viscosity, which can be described by Marangoni parameter Ma, given by Ma =

B Th vk

(9)

where B = −d/dT is the variation rate of surface tension with temperature, and  is liquid density. BénardMarangoni convection will occur if Ma is over a critical

value of Mac = 80. The temperature gradient, which is formed due to the fast evaporation on the free surface of solution, drives the hot fluid to the surface from the bottom, leading to surface tension fluctuations at the free surface of solution, resulting in the formation of high ordered hexagonal arrays. In some case, however, the generated pattern will not always follow these criteria [102]. A hill-and-valley periodic striping pattern is often observed on the surface of spin-cast single or multi-component polymer films, which is considered to be induced by Bénard-Marangoni convection and radial flow during spin-coating [40,103]. Higher surface tension at the hill pulls material up from neighboring regions where the surface tension is lower [104–106]. For the case of PS and PVP polymer blend solutions, due to the surface tension of PS ( PS = 40.2 mJ m−2 ) and PVP ( PVP = 56 mJ m−2 ), PS tended to flow to the hill region and PVP tended to flow to the valleys [104]. A PS and polyethylene glycol (PEG) blend solution investigated by Ohshima [107], however, showed an opposite result that PEG-rich region was found in the hill region and PS was in the valley (Fig. 10). A two-step phase separation process, which consists of a primary phase separation into PEG-rich and PEG-poor phases and a secondary phase separation into solvent-rich and PEG-rich domains within the PEG droplets, was thought to be the reason.

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Fig. 11. Micropattern PS/P2VP surfaces by convection. PS/P2VP mixture ratio is (a) 4/1 (w/w); (b) 7/1 (w/w); (c) 4/1 (w/w) after water treated; (d) 7/1 (w/w) after water treated. Reproduced from [110] with permission. Copyright 2005, American Chemical Society.

3.2.2. Generation of ordered pattern by phase separation under convection The patterns generated by convection in polymer blends are well ordered and size-tunable by controlling the preparation conditions. A variety of controlling parameters provide the opportunity to create more complex structures and to change the structures continuously. The nature of multi-component allows the film to possess different response to various stimulations.

3.2.2.1. Influencing parameters. Many factors, such as temperature, film thickness, substrate, solvent type, concentration, and chemical composition of the polymers

[108] influence the formation, structure, and size of ordered patterns The rate of solvent evaporation plays a key role in the formation of ordered pattern induced by convection. Kumacheva and coworkers [47] found a regular sea-island structure (PMMA islands in a PS sea) in a phase-separated PS/PMMA (9/1, w/w) blend film when the film formed under Bénard-Marangoni convection. They believed that polymer blends would deposit and separate into different phases when their concentrations increased in the solution, and Bénard-Marangoni convection would increase the phase separation of the blends too. Driven by Bénard-Marangoni convection, high surface tension segment (PMMA) moved away from low surface tension region

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and dispersed into continues PS phase to form a regular hexagonal arrayed sea-island structure [101]. They also investigated the effects of different film thickness and different T, and found the regular hexagonal arrayed seaisland structure only formed when the film was under a large T. The rate of solvent evaporation also determines the pattern size [108,109]. For example, in two PS/P2VP/ethylbenzene solutions (mixture ratios of 4:1 and 7:1 by weight), when ethylbenzene evaporation speed was low (the samples were put in a closed vessel with some little holes on the cover), no regular structures were found in either PS/P2VP film. However, when evaporation speed of ethylbenzene increased to 1.0 L/min, both films formed regular honeycomb-like patterns (Fig. 11a and b). The honeycomb-like structures were induced by Bénard-Marangoni convection, which can only be trigged when the temperature difference T between two interfaces (liquid/substrate and liquid/air) is large enough. A higher evaporation speed of the solvent increases T, resulting in a strengthened surface-tension-driven force. Therefore, the diameters of PVP islands and the intervals between the islands increased. Casting a dilute solution of poly(styrene-ran-butadiene) random copolymer in toluene onto a hot plate, the pattern size was found to increase with an increase of evaporation rate in the regime of lower evaporation rate (regime I) and to decrease tremendously with an increase of evaporation rate in the regime of higher evaporation rate (regime II) (Fig. 12f). With slow solvent evaporation the cooling due to evaporation contributes little to the temperature gradient, so that only the convective flows caused by the hot substrate determines the convections adjusting the pattern size. The larger is the evaporation rate, the faster is the cessation of the convection, which results in a thicker film. An increased pattern size then results since the pattern size is determined by the thickness of the film (originating from the concentration of the solution, as shown in Fig. 12a–e). In region II, the convection is enhanced by an additional temperature gradient caused by the fast evaporation that the cessation of convection is delayed, resulting in a decreased pattern size. Only the film composed of polymer with proper molecular weight forms ordered structure. In a study of four blends formed with PS samples (Mw,PS-1 = 11k, Mw,PS-2 = 114k, Mw,PS-3 = 212k, Mw,PS-4 = 582k) blended with PVP at a PS/PVP weight ratio of 5:1, well-defined ordered hexagonal island patterns only formed in blends with PS-2 and PS-3. Moreover, the diameters of the PVP islands and the intervals between the adjacent islands decrease with increasing molecular weight, e.g., the pattern size in PS-2 is larger than in PS-3. The ordered patterns for PS-2 and PS-3 samples demonstrated that the threshold of convection was met, i.e., the viscosity was not high enough to suppress thermal diffusion. By contrast, for PS-4, the viscosity is too large leading to Ma < Mac , and the ordered patterns disappeared. Although the value of Ma increased with the decrease of viscosity due to the decrease of molecular weight (in the case of PS-1), this was counterbalanced by the decrease of the frictional action of viscosity of the system and the increase of diffusibility, making the ordering impossible.

577

Fig. 12. Schematic illustrations for change in side and through views of a casting solution. Reproduced from [109] with permission. Copyright 2002, Elsevier.

Within the appropriate range of molecular weight, the sizes of PVP phase domains and the intervals increased with the decrease of PS molecular weight. Component ratio controls the patterns size. In PS/P2VP system, when the component ratio (w/w) changed from 7/1, 5/1 to 4/1, the diameter of PVP islands increased [108,110]. The island growth is dominated by a diffusive process partly supplemented by coalescence with droplets arriving from the bulk of liquid film. With the increase of PVP component, the quantity of PVP phase coalescence increased. Meanwhile, the viscosity decrease associated

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Fig. 13. AFM topographical images of PS/P2VP (5:1) (w/w)/Triton X-100 with different concentrations of Triton X-100 (whole) polymer concentration is 4 wt% cast from toluene solution on mica flakes under 0.5 L/min air flow speed. The Triton X-100 concentration is: (a) 0 wt%; (b) 0.15 wt%; (c) 0.35 wt%; (d) 0.50 wt%; (e) 1.50 wt% respectively. Reproduced from [111] with permission. Copyright 2007, American Chemical Society.

with the decrease of PS component allows much easier collection of PVP phase. However, the intervals of all the three systems are the same, which infers the frame of convection cell may only relate to the PS component, which did not change when the component ratio changed. 3.2.2.2. More complex and stimuli-responsive structures. In the blends with two components, the classical structures are vertical and lateral phase-separated structures as shown in Fig. 3a. In practice, there is always a trace amount of another component embedded. And the minor component may dewet on top of the major component. These create more complex structures upon further treatment. Although two honeycomb-like porous structures in the PS/P2VP blend with component ratios of 4/1 and 7/1 showed an only difference in size, the distribution of each component on the surface was totally different. In PS/P2VP (w/w = 4/1) blends, hydrophilic P2VP component adhered on the mica substrate firstly, and then hydrophobic PS component formed a honeycomb-like porous structure on top of the P2VP layer, but not covered it completely, i.e., P2VP component in the PS pores still contacted air (schematic model in Fig. 11a). Meanwhile in PS/P2VP (w/w = 7/1) blends, a honeycomb-like porous structure of PS also formed on the top of P2VP layer, but covered it completely (no P2VP in pores). Moreover, there was a very thin P2VP layer on the top of PS phase, but not in the pores (schematic model in Fig. 11b). Due to the lower solubility of PVP in ethylbenzene and the strong interaction between the PVP phase and the mica substrate, the PVP phase tended to deposit on the mica substrate. When the concentration of PVP is low (e.g., PS/P2VP (w/w = 7/1)), the PVP did not form a separate phase before it was driven to the surface, forming a thin layer on the plateau domains of PS. With water placed on its surface a PS/P2VP (w/w = 4/1) film turned from a honeycomb-like porous structure to a sea-island structure (Fig. 11c), which meant the P2VP component swelled and exuded from the pores. However, after the same water treatment, the honeycomb-like porous structure of PS/P2VP (w/w = 7/1) blends did not change, but some mini sea-island structures appeared around the pores (Fig. 11d), which meant no P2VP component in pores but rather a very thin P2VP layer on the surface. This thin P2VP layer formed mini-sea island structure after the film was treated by water.

3.2.2.3. Transformation of ordered structure by adding additive. The morphology study of ternary blend indicated, if the polymer-polymer interactions at the A–C interface are much less favorable than the A–B and B–C interfaces (i.e., in a Flory–Huggins model, the interaction parameter AC is larger than the sum of AB and BC ), a B layer intercalates at the A–C interface to reduce the overall free energy [29,30]. The amount of additive may then change the resulted structure. Different morphologies of PS/P2VP blends can evolve by adding small surfactant molecules, such as Triton® X100 [111]. It was found that with increasing surfactant, the surface morphology switched from the regular sea island structure to an ordered porous structure (Fig. 13a–c). Bénard-Marangoni convection occurs as the solvent evaporates, owing to the increase of the temperature difference between the two interfaces of the solution, which results in the island structure of the P2VP phase. As small amount of Triton® X-100 added to a polymer blend solution distributes at the interface between P2VP and PS domains, while more Triton X-100 remains in the P2VP phase. Owing to its solubility, Triton® X-100 is the last component to deposit from the solution. In the completely dry film, the Triton® X-100 phase formed channels around the P2VP islands as shown in the inset in Fig. 13b. With increasing Triton® X-100 concentration, the weight fraction of Triton® X-100 in the P2VP phase increases. Due to the uneven distribution of Triton® X-100, some P2VP islands collapsed as evaporation of solvent is completed. Further increasing of Triton® X-100 in P2VP phase increases the P2VP phase; therefore the dispersive island-like P2VP phase switch to a continuous phase and dominated the mica substrate due to the strong interaction between P2VP/Triton® X-100 and mica. Meanwhile, the PS layer formed a honeycomb-like porous pattern induced by convection (Fig. 13d). 3.3. Phase separation under breath figures Porous materials have attracted great interest because of their promising use in photonics, carriers of catalyzer, filters, sorbents, and cell culture rooms [43]. The templating approach, including using ordered arrays of microphase separated block copolymer [112,113], colloidal beads [114,115], emulsions [116] and so on, is a promising way to produce porous structures. In these templating

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Fig. 14. A model for the formation of the ordered porous structure by breath figure method in polymer films. The images are color-coded, with blue and orange denoting low and high temperatures, respectively, relative to room temperature. (a) The conditions under which the experiment is performed. (b) Evaporation of the solvent cools the solution surface, thus initiating the nucleation and growth of moisture. (c) Because of the convective currents arising from the evaporation as well as from the airflow across the surface, the water droplets pack into a hexagonal array. (d–f) We hypothesize that the ordered array sinks into the solution, thus leaving the surface of the solution free for the nucleation and growth of moisture to form another ordered array of water droplets. (g) When all of the solvent has evaporated, the film must return to room temperature, thus allowing the water droplets to evaporate as well while leaving behind the scaffold. Reproduced from [125] with permission. Copyright 2001, Science.

methods, an additional step is required to remove the template. Recently, Franc¸ois and coworkers [117–119] used breath figure to pattern honeycomb-like porous polymer films. Since the water droplets serve as the template, no further treatment is necessary to remove the template [120]. The method is in substance a kind of water/oil phase separation. Since the breath figure is a simple, tunable and economic method, a number of investigations have been carried out [121–124]. Based on these, some unusual, even fantastic, structures elliptic such as ordered pores, reversibly stimuli-responsive pattern have been achieved.

3.3.1. Mechanism of breath figures The structures are formed by a templating mechanism: water droplets condense onto the solution surface (breath figures) and act as the vanishing honeycomb template after evaporation. The proposed process of formation is shown in Fig. 14 [125]. The fast evaporation of organic solvent cools down the surface of solution, on which water droplets nucleate and grow. At the early stage of condensation, a few water droplets aggregate together. The droplet diameter D is observed to grow with time according to D ∝ at1/3 , where a is a function of the velocity of the airflow and of the surface temperature [126]. The short ranged

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Fig. 15. AFM images of the regular structures in PS films. (a) Hexagonal hole structures in PS film. FFT pattern is given in the inset of the image indicating a perfect hexagonal arrangement of the holes. (b) Square hole structure in PS film with FFT pattern in the inset. Reproduced from [136] with permission. Copyright 2004, Elsevier.

hard-sphere-like interaction leads to uniformity in the droplet size [127]. The capillary force will bring these droplets into hexagonally arranged aggregates; however, it cannot induce the formation of aggregation with large area. More water droplets condensation on the solution surface causes the formation of more aggregations which will attract each other in a much stronger way than the single water droplets because the attractive force between the aggregations is proportional to the sixth power of their radii [128]. Due to the evaporation of solvent, the convection in the film will bring the water droplets from the hotter areas to the cooler areas, normally causing the water droplets to arrange into hexagonal structures, which provides the most stabile packing mode, as shown in Fig. 15a. However, square arrangement of holes occurs sometimes (Fig. 15b). Karthaus et al. got square arrangement [129] by flowing the solution over an inclined substrate during solvent evaporation and water condensation. Until now there is no clear explanation for this phenomenon. A honeycomb-like porous polymer films results after the complete evaporation of organic solvent and the following evaporation of the water droplets. Though the whole process looks quite clear and the controlling parameters introduced in the following part also shed some light on the mechanism, even though that mechanism is not fully understood and requires further investigation. Pitois and Franc¸ois [119] proposed a “bursting hypothesis” in which condensed droplets are completely encapsulated by a polymer layer first, and then evaporate and burst the top polymer layer leading to the formation of a porous structure when the temperature near the solution surface increases to reach the ambient temperature as most of the solvent evaporates. The “bursting hypothesis” was directly supported by the delayed appearance and movement of surface holes in the system of polyphenylene oxide (PPO)/chloroform solution. The formation of few pores on the surface and much more holes beneath the surface on a concentrated PPO

solution (100 g/L) also suggest the “bursting hypothesis” [130]. 3.3.2. Materials suitable for breath figures In the pioneering work by Franc¸ois [117], selforganized spherical-shaped structures composed of polymers, including PS-polyparaphenylene (PPP) block copolymers, star-like homopolystyrenes and linear PS with a polar terminal group in carbon disulfide were used. They thought the formation of spherical-shaped structures is the determinant element in the formation process. Davis [131] synthesized the star-polystyrene (five arm), which was used in the same way to form such regular porous structures. When adding over 30 wt% linear PS to the starpolystyrene solution, the regular structure was disrupted, ultimately leading to the disappearance of the porous structure if exceeding 70 wt%. Srinivasarao [125] also used polystyrene with one end terminated by a carboxylic acid group. Shimomura et al. [129,132–135] photographed the dynamical movement of the rearrangement of water droplets during honeycomb pattern formation and used different kinds of compounds, including organic-inorganic hybrid materials [129], amphiphilic copolymers [133,134], metalorganic [135], and saccharide-containing polymers [135] for patterning. Later, our researches showed that PS without any polar end group can also form ordered porous film from toluene and CHCl3 solvents [136,137], in which PS did not form aggregates like micelles. Moreover, a regular structure can only be achieved with the proper Mw of PS. When Mw is low, the viscosity of solution is too low to encapsulate the water droplets or prevent their coalescence, resulting in the formation of disordered membranes. While highly viscous polymer solution prevent water droplets sinking into it, resulting in a few holes in PS films. Recently, it was found that rod-like polymers, which are useful in energy and charge transfer, are also suitable to the breath figure method [138–140]. On the other hand, Qiao demonstrated that the end groups of the polymer may affect the morphology of

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Fig. 16. SEM micrographs (a, c, e) taken at a 60◦ tilt of the honeycomb materials made from dendron-functionalized star polymers and corresponding schematic drawing (b, d, f), respectively. (a, b) Acetonide-functionalized G3 star polymer; (c, d) hydroxyl-functionalized G3 star polymer; (e, f) perfluoroalkylfunctionalized G3 star polymer. Insets show end-group structure. Reproduced from [141] with permission. Copyright 2008, Wiley.

the resulting pore structures [141]. While the acetonideG3-functionalized star forms a multilayered honeycomb structure in a similar fashion as reported in most studies (Fig. 16a and b), hydroxyl-G3-functionalized star generates a structure of interconnected pores (Fig. 16c and d). The water droplets may coalesce in the final stages of the film formation because of the increasing of hydroxyl groups and therefore the decreasing surface tension, creating interconnected pores while maintaining some degree of order. The most interesting structure was found in G3-functionalized star polymers with perfluoroalkyl functionality, where a monolayer of highly ordered pores with the depth of the pore being approximately twice the size of the pore opening (Fig. 16e and f). The high hydrophobicity may not enable the precipitating polymer to stably encapsulate the water droplet; instead the water droplet simply sinks into the solution, creating a monolayer of deep pores. Not only polymers (homopolymer, copolymer, dendrimer, biomacromolecule, and amphiphilic polyion complex) [121,142–147], but also inorganic materials, such as carbon nanotube [148–150], ZnO nanorods, silica nanoparticles [151] and metal nanoparticles mixed with

polymers [152–159] can be patterned via this technique. By pyrolysis of UV cross-linked amphiphilic diblock copolymer polystyrene-b-poly(acrylic acid)/functional precursor hybrid films, porous structures composed of carbon nanotubes, silica and ZnO nanorods were achieved (Fig. 17) [150]. Carbon nanotubes modified with macromolecules such as PS can be directly used to form porous structures with the breath figure method [148]. When a water mixable solvent is used, a sol–gel process can be involved in the breath figure method forming the porous silica structures [160]. This method does not require any control of external parameters such as relative humidity or temperature, and can be applied to a wide range of polymer to inorganic compound ratio. In the case of breath figure formation from a PS/chloroform solution mixed with tri-n-octylphosphine oxide (TOPO)-stabilized CdSe nanoparticles, the condensed water droplets serve as templates for the nanoparticle assemblies at the interface (Fig. 18). Without the addition of polymers, Wu [156,157] successfully prepared ordered porous structure with surfactant-encapsulated polyoxometalate complexes and even extended the method to single-molecule magnet [159].

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Fig. 17. SEM image of carbon nanotubes arrays generated by breath figure method: (a) with modified carbon nanotubes, Reproduced from [149] with permission. Copyright 2009, RSC; (b) grown on honeycomb structured ferrous pattern. (c) AFM topographic image of the honeycomb structured silica pattern. (d) SEM image of hydrothermal ZnO nanorod arrays grown on honeycomb structured ZnO pattern. Reproduced from [150] with permission. Copyright 2009, American Chemical Society.

3.3.3. Generation of ordered pattern by phase separation under breath figure The generated structures under breath figure are controlled by a variety of parameters which will affect the condensation and arrangement of water-droplets template. Because of the minimum of surface tension, the water droplets tend to be spherical, however, well-arranged elliptic pores can also be fabricated by simply changing the air flow direction. Moreover, the resulted pattern can exhibit reversible stimuli-response. 3.3.3.1. Influencing parameters. The breath figure method is the combination of nucleation, growing and ordering of water droplet, influenced by parameters like material used, solvent, humidity, etc. Within proper conditions can ordered, size-adjustable porous structures form. Comparison of different solvents indicated that the evaporation speed of solvent plays a critical role in determining the formation and ordering of porous structures [136,161]. Without the air flow across the evaporating surface (e.g., simply depositing the water droplet on chlorinated and aromatic solvents), the water droplet would be encapsulated with a thin polymer film (Fig. 19 top), with coalescence between the droplets (Fig. 19 bottom) [162]. This behavior infers the importance of fast evaporation, which induces the rapid vitrification of polymer on the top surface and convection in the liquid film, on the breath figure method. On the other hand, a too-fast evaporation process of solvent induces a large perturbation of the system in a non-equilibrium state. In this case,

the solvent may completely disappear from the system before water droplets form regular packing [136]. Therefore, only the appropriate evaporation speed, which can be tuned by choosing solvent, using air flow across the film surface, or controlling the temperature or pressure, induces the formation of ordered structure. Within this range, the diameters of the pores are found to increase with slower evaporation. In addition, the solvent should not be miscible with water since the water droplets act as the template on top of the organic solvent. The water droplets can be further stabilized when an amphiphilic copolymer is used [143,158]. Recently, however, some research [129,163] also fabricated breath figure patterns with a water-miscible solvent such as tetrahydrofurane (THF). It appears that the water droplets are stabilized by polymers in organic solvent (water-in-oil emulsions) and serve as the template. Moreover, the solvent will also affect the formation of 3D structures. In the early studies, it was assumed that a solvent less dense than water, such as toluene, is prerequisite for the preparation of a multilayered porous structure. The water droplets sink into the solution to clear the solution surface for the nucleation of new water droplets [125]. Nevertheless, 3D arrays have also been obtained by casting from carbon disulphide and chloroform [117]. On simply changing the experimental temperature from 20 ◦ C to 40 ◦ C, monolayered pores changed to multilayer in 4 wt% PS-2COOH carbon disulphide solution in Bolognesi’s investigation [164]. They proposed a parameter z0 to predict the monolayer/multilayer formation based on interfacial

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Fig. 18. Cross-sectional diagram of nanoparticle assembly at a water droplet-solution interface during the breath-figure formation. Reproduced from [153] with permission. Copyright 2004, Nature Publishing Group.

energy minimization, and it agrees very well with the experiment made by Srinivasarao [125]. z0 is defined as equation (10) with  s and  w the surface tensions of solution and water, respectively, and  w/s is the interfacial tension between water and the solution: z0 =

w − w/s s

(10)

A z0 value between −1 and 1 indicates that the drop is floating at the interface between air and solution. For z0 greater than 1, drops sink below the surface and multilayer structured films can be obtained. For z0 smaller than −1, interactions between the water droplet and the solution are such that the drop does not remain at the interface.

Solution concentration affects the pattern size such that in general, larger the pores can be obtained with lower concentration [164]. Stenzel [165] found an inverse correlation Dp = kc /c between the pore size Dp and the concentration c, with kc a constant dependent on the materials used. For instance, star polymers were less sensitive toward concentration changes in comparison with amphiphiles. When the concentration is too low (extreme condition is pure solvent), the droplets are not stable on the surface, resulting disordered structures. In the opposite extreme, highly concentrated solution will not allow the sinking of droplets, preventing the formation of porous structures. Humidity affects the formation of water-droplet templates since the moisture in air flow is the supplier of water template. In the system of PS in toluene, ordered holes

Fig. 19. (Top) Scheme of water droplet encapsulation with a polymer film. (Bottom) Sequence of snapshots depicting capillary interaction between 10 ml water droplets deposited on a dichloromethane solution of PC (5 wt%). (a) The initial stage of the process, (b) drops come close together, (c) beginning of the coalescence, and (d) coalescence process results in one encapsulated water drop. Reproduced from [162] with permission. Copyright 2007, Wiley.

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Fig. 20. SEM images of the highly ordered elliptic pores with a hexagonal array. The airflow across the solution surface is along the direction at an angle of 15◦ with respect to the normal of the solution surface. The velocity of airflow is (a) 32 m/min, (b) 40 m/min, (c) 48 m/min, and (d) 64 m/min. The aspect ratio of the elliptic pores is (a) 1.50, (b) 1.68, (c) 2.15, and (d) 2.55, respectively. (e) Models of the water droplets as the templates of forming circular pores and elliptic pores. Reproduced from [154] with permission. Copyright 2005, American Chemical Society.

array could be achieved within the range of relative humidity between 46% and 90% relative humidity (RH), within which the hole sizes are found to linearly increase with increasing humidity, below or above this range randomly

distributed holes with broad size distribution were formed. However, under dry condition (RH ∼ 10%), Kim et al. [163] achieved breath figures patterns on a film of cellulose acetate butyrate (CAB), monocarboxylated end-functional

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Fig. 21. Morphology of PS/P2VP (in THF solution) mixture films under different humidities: (a) 10%; (b) 15%; (c) 25%; (d) 30%, (e) 50%; (f) 70%. Reproduced from [168] with permission. Copyright 2005, Elsevier.

polystyrene (PS-mCOOH), and PMMA by spin coating THF solutions added with 1.5 wt% water. Moreover, simply placing the evaporating film above a water bath [166], which is different from the normally used moisture flow across the film surface, facilitated the formation of breath figure patterns. Similarly, Sanchez et al. [167] reported the preparation of hybrid meso-macro structured membranes by controlling the evaporation of water, added for sol–gel processing, in a closed and heated chamber.

been fabricated in highly ordered honeycomb structures of dodecanethiol (C12 H25 SH)-stabilized gold nanoparticles through changing the direction and velocity of airflow in a moist atmosphere (Fig. 20a–d) [154]. The formation mechanism is revealed in terms of the surface and interfacial tension (Fig. 20e). When airflow blows across the solution surface, the water droplet receives an additional force Fairflow originating along the normal of the solution surface, so that the surface tensions at the surface area are given by,

3.3.3.2. From circular to elliptic ordered patter. The pores formed under breath figure tend to be spherical, since the droplets are driven to a spherical shape to minimize surface area. Pores ranging from spherical to elliptical shapes have

sol/air = W/air cos ˛ + W/sol cos ˇ

(11)

W/air sin ˛ = W/sol sin ˇ + Fairflow

(12)

Fig. 22. Reversible switching 3D topographies of a PS/P2VP (w/w = 5/1) film between porous-structure and see-island-structure: treated by water and heating for different times. Reproduced from [172] with permission. Copyright 2005, American Chemical Society.

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Fig. 23. 3D topographies of porous PS/P2VP (w/w = 4/1) film (top left) with reversible responding behavior to different solvent vapors. Reproduced from [172] with permission. Copyright 2005, American Chemical Society.

For airflow blowing across the solution surface at a small angle  with respect to the normal direction, the new equilibrium state of the water-droplet templates can be written as: sol/air + F  sin  = W/air cos ˛ + W/sol cos ˇ 

W/air sin ˛ = W/sol sin ˇ



 + Fairflow

cos 

(13) (14)

To reach the new balance the angles ˛ and ˇ have to decrease, which results in the distortion of spherical water droplets into ellipsoidal shape as shown in Fig. 20e bottom. The larger the shear of F  airflow is, the smaller the angles ˛ and ˇ become and the larger the aspect ratio of the elliptic pores. 3.3.3.3. Reversibly stimuli-responsive ordered pattern generated by phase separation under breath figure. In recent years, increasing attention has been given to ordered micropatterned surface that responds to external stimuli, such as changes in pH, temperature, humidity, ionic strength, stretch, light, or an electrical field. The emphasis of the responding patterned surfaces is on the switchability and/or reversibility; that is, the surface properties or topographies can reversibly change with the presence and absence of external effects. Patterns may be developed in phase-separated polymer blends by the breath figure technique [168–171]. For example, water droplets on a PS/P2VP solution surface worked as template for both the honeycomb-like porous structure and the phase separation of the mixture due to the big difference in wettabilities of the PS and P2VP components [168]. Fig. 21 shows different morphologies formed on PS/P2VP film from THF solution in different humidities. When the external humidity was higher than 30%, a regular honeycomb-like porous structure appeared, and the surface pores were mainly composed of the hydrophilic component P2VP induced by the water droplets. Raczkowska [170] compared the breath

figure structure made from THF solutions of PMMA and PS/PMMA. In PMMA, water added to solution (3 wt% < fH2 O < 20 wt%) has much stronger impact on the ordered structure than that from moisture. And this effect is strengthened in an atmosphere with high RH. In contrast to the pure PMMA, in PS/PMMA the pores form in dry atmosphere, even for the solutions with low water content (fH2 O < 3 wt%). Pore size and pore coverage in the PS/PMMA film are larger than that on pure PMMA film spin-cast at identical conditions, which suggests the preferential accumulation of water in the PMMA phase in the blend, leading to the local increase of pore size. Moreover, the honeycomb-like porous surface is reversibly stimuli-responsive to environment changes [172,173]. Immersed in water, the swollen P2VP could gradually extrude from the pores, and surface morphology switched from a honeycomb-like porous structure to a sea-island structure (Fig. 22). When the film was dried by heating, the swollen P2VP islands collapsed into pores, and surface morphology switched back to honeycomblike porous structure. Furthermore, the film morphology also showed a reversible response to solvent vapors due to the different solubilities for PS and P2VP (Fig. 23). For example, after treated by low-polar solvent vapors, such as CS2 , toluene, and THF, which are selective solvents to PS, the honeycomb-like porous film would switch to sea island structure because of the higher shrinkage of the swollen PS component than that of the swollen P2VP after removing solvent vapor. On the other hand, when the film was treated by high-polar solvent vapors, such as ethanol, MEK, DMF, and chloroform (selective solvents to P2VP), the sea-island film would switch back to the honeycomb-like porous structure as the swollen P2VP component shrinkage exceeded that of the swollen PS on suppression of the solvent vapor. According to the solubility for P2VP and the vapor pressure of solvent, different time is needed for the two groups of solvents to start the morphologies change.

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Fig. 24. AFM images of: (a) demixing PS and PVP at a humidity of 40% in nitrogen and THF as a solvent leads to porous structures in both polymers and (b) after PS has been dissolved. The scale bar is 2 ␮m. Reproduced from [180] with permission. Copyright 2010, American Chemical Society. (c) AFM phase images of internal structure of the holes of polymer film having 10 wt% of PS5F21 -b-PS31 -b-PPEGMA38 after annealing at 80 ◦ C for 2 days. Reproduced from [177] with permission. Copyright 2009, American Chemical Society. (d) SEM micrographs of porous poly(l-lactic acid) structure prepared from PLLA/PEG6000 blend (90/10) and dehydrated 1,4-dioxane solution (7 wt% total polymer concentration) by unidirectional freezing and etching of PEG6000. Reproduced from [182] with permission. Copyright 2009, American Chemical Society. (e) Simulation result Reproduced from [178] with permission. Copyright 2000, American Chemical Society; and (f) phase contrast optical micrographs of pattern formation in phase separation induced by a cross-linking reaction. Reproduced from [179] with permission. Copyright 2005, American Chemical Society.

Moreover, the restructuring behavior of solvent vapor is influenced by the hole depth. When the hole depth is large, there is not enough P2VP to fill the holes, the island-like pattern will not be achieved upon annealing in the vapor of low-polar solvent. Similar response dependence on pore size was also found in a blend of PS and a kind of glycopolymer [173].

3.4. Stabilization of porous structure Though polymeric breath figures provide an easy and low-cost way to organize polymer films into an ordered hexagonal array of pores, such films are, on the other hand, quite difficult to handle and do not exhibit good stability; the porous structure may collapse in a film as exposed

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to high temperature or some solvent vapor. Some post treatment (UV irradiation, thermal annealing, etc.) may significantly improve thermal, solvent, mechanical resistance, and stabilize the porous structures [174–176]. For example, an alternating polyfluorene copolymer (PFOTHP) bearing tetrahydropyranyl groups on its lateral chains became solvent resistant after a thermal treatment. During such a thermal annealing, the pores should be filled with some material stable at high temperature, such as PDMS, and the filler should also be removed easily. The use of PS has the advantage that PS could be effectively cross-linked under irradiation with 254 nm ultraviolet (UV) light. This reaction can be induced by UV radiation and performed in the glassy state at ambient temperature, which avoids possible relaxation of the polymer induced by heat (which would be the case for thermal cross-linking). 4. Application of structures generated by phase separation The formation of ordered polymeric structures is an interesting and intellectually challenging problem with a wide variety of potential applications. The generated pattern can find potential applications in the examples given in the following on pattern replication, antireflecting coating, catalysis, bio-chips, optoelectronics, etc. 4.1. Pattern generation and reproduction One direction of the development of pattern techniques is to fabricate pattern with more complex, multi-scaled and multi-components structures. Phase separation is inherently consistent with multi-component systems. If some components, such as block copolymer or supermolecular system, are involved in the process of phase separation, novel multi-scaled structures with nanometerscale patterns inside microstructures may be generated conveniently in a single step [177]. Alternatively, combining different technologies, such as controlled dewetting together with breath figures, patterned substrate together with breath figures, can also achieve novel structures (Fig. 24) [178–182]. The fabrication of more complicated chemical and topological patterns with polymers still remains a challenge. Functional structures, or even a device such as a solar cell, may be conveniently generated when some functional materials are included as a component of a blend, or are dispersed in one phase of phase-separated structures. For example, phase separation structures were transferred to a photoreactive substrate by spin coating with a thickness of 3–4 nm layer of immiscible polymers onto the substrate, followed by photoirradiation and removal of the unreacted polymer with a solvent [183]. By the use of smart materials, such as the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPA) patterns responsive to stimuli may be fabricated [184]. New techniques and/or new materials will be needed to utilize the phase separation of polymer blends in the development of a pattern size down to the nanometer scale. On the other hand, the microphase separation of block copolymer can develop patterns with dimensional size below 100 nm. However, block copolymers are much

Fig. 25. (a) Schematic representation of the gradient-porosity-ratio film formation process. (b) Transmission spectra of PS porous films with different thicknesses on the glass substrates at normal incidence. Note that both sides of the glass substrates are coated with a porous film. Reproduced from [190] with permission. Copyright 2010, Wiley.

more expensive, and can hardly form a large area of defectfree patterns. Some unique patterns generated by phase separation, which cannot be obtained by other methods, can be used as original molds to transfer a pattern into another material or to produce even more complex patterns associated with technologies such as soft lithography [185–188]. 4.2. Antireflecting coating Porous structures can be fabricated by the methods of convection, breath figure and removing one component of the phase-separated film. Since the pores inside the film will efficiently reduce the effective refractive index of the material, the porous film may be used as antireflection coatings [189,190]. The principle of antireflection is the destructive interference between the reflected light from the air–coating and coating–substrate interfaces [189]. An ideal homogeneous antireflection coating should satisfy the following conditions: the thickness of the coating should be w /4, where w is the wavelength of the incident light; and nc = (na × ns )1/2 , where nc , na , and ns are the refractive indices of the coating, air, and substrate, respectively. Since the domain size of the phase separation are in the order of micro-meter scale and the refractive indices of most polymers are similar to glass (n ≈ 1.5), these large pores will cause light scattering in the film, which is

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Fig. 26. (a) Cross-sectional TEM of a BHJ solar cell with a contrast enhanced image to highlight the continuous domains of P3HT and PC61 BM; Reproduced from [198] with permission. Copyright 2009 American Chemical Society. (b) EQE spectra (%) of indium tin oxide (ITO)/PEDOT:PSS/P3HT:PCBM (1:1)/Al devices treated in the following ways: untreated (black); vapour annealed (green); thermally annealed at 140 ◦ C after (red) and before (olive) the deposition of the Al contact. Reproduced from [197] with permission. Copyright 2008, Nature Publishing Group. (c) EL image of the patterned blend of F8BT/TFB device showing preferential yellow-green luminescence of F8BT from enclosed TFB-rich domains; and (d) EL efficiency-voltage characteristics of fabricated LED expressed in Cd A−1 (left) and Lm W−1 (right). Reproduced from [200] with permission. Copyright 2008, Wiley. (e) Output characteristics and (f) transfer characteristics and gate leakage current (Igs) of field-effect transistor device based on P3HT/PMMA (5:95) without any other dielectric. Inset: Schematic view of the fabricated low-voltage-driven devices Reproduced from [205] with permission. Copyright 2008, Wiley.

harmful for antireflection. Adding some block copolymer to the blend, the domain size reduces remarkably to nanoscales, resulting in an outstanding transmission of 99.7% averaged over the entire visible spectrum [189]. Recently, Han [190] reported broadband (both visible and nearinfrared light) and omnidirectional antireflection coatings by the microphase separation of PS-b-PMMA block copolymer mixed with PMMA homopolymer. Upon the removal of PMMA domain, a gradient distribution of pores in the vertical direction of the entire film is achieved, which is responsible to the excellent antireflection properties (Fig. 25).

4.3. Catalysis The large surface area of the porous film makes it widely used in catalytic processes [191]. The porous material can act as the actual catalyst that reacts with gases/liquid/fluid that passes through the porous material or the internal surface chemistry of the porous material can be modified to attain catalytic properties. The porous material can also act as scaffold for catalyst. Another advantage of using porous structures in catalysis is that the tailored hierarchical porosity can regulate the flow of diverse molecules based on size and shape.

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4.4. Bio-chips

5. Conclusion and outlook

Since most of the sizes of the patterns generated by phase separation are comparable to that of cells (on the order of microns) they can be used as biosensor chips, scaffolds for cell organization and some other biotechniques [192–194]. The sensitivity of these chips has been reported to be 2–3 times higher than that on flat surfaces due to the higher aspect ratio. Porous structures made from polymer blends through breath figure method has hydrophilic inner surface which makes it feasible for site-directed modification. The patterns generated by the breath figure method may be biocompatible if suitable surfactants, e.g., phospholipids, are used to stabilize the water droplets [195]. Wan [196] successfully grafted 2-(2,3,4,6Tetra-O-acetyl-␤-d-glucosyloxy) ethyl methacrylate onto the pores by atom transfer radical polymerization, further specific recognition of the carbohydrate microarrays to lectin (Con A) can lead to an organized microarray of protein.

More and more technological applications as well as fundamental investigations nowadays need patterned polymer structures. It triggers increasing passion to find fast, economic and efficient methods, among which the patterning by phase separation offers this kind of possibility. Controlled phase separation on chemically patterned substrate generating ordered strip pattern has been intensively investigated by computer simulations and experiments. However, 2D or even 3D ordered pattern still lack of investigation. The phase separation under convection and breath figures is an efficient, fast and economic approach to fabricate 2D or 3D structures which are limited in porous structures. Moreover, these methods remain an intriguing problem in both practical and theoretical aspects and need further investigation. The combination with new materials and other techniques like self-assembly of block copolymers etc. provides the way to get novel, smaller and more complex patterns. Another very important issue is to reduce the size of generated pattern. Adding surfactant to the interface of two phases is effective means to reduce the phase domain. The microphase separation of block copolymer is another option to fabricate nanometer scaled structures. The pattern generated by phase separation has been proved possessing significant potential in many applications, especially in optoelectronics since the low-cost renewable energy is a global demand in the 21st century.

4.5. Optoelectronics Combined with some functional materials, functional structures and even some devices can be fabricated (Fig. 26). The use of blends of conjugated, semiconducting polymer (e.g., poly(3-hexylthiophene) and a fullerene (e.g., phenyl-C61-butyric acid methyl ester (PCBM))) has made it possible to fabricate solution processed all-polymer solar cells with high efficiency as high as 7.6% [197–199]. With appropriate choice of polymer molecular weight and blend ratio, the phase-separated structures in the blend film of poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) and poly(9,9-di-noctylfluorene-alt-(1,4-phenylene(4-sec-butylphenyl)imino)-1,4-phenylene) (TFB) on the micrometer-scaled chemically patterned surface results in efficient light-emitting diodes (LEDs) [200]. The use of conjugated polymer blends as active materials has resulted in a new way to tune and optimize the electronic properties of devices because blends of semiconducting and dielectric polymers can combine the optical and electrical properties of semiconductors with the characteristics of dielectric polymers [201–204]. For example, phase separated liquid crystal/polymer blends can be used as microlens array [204]. Semiconductor/dielectric-polymer blends with vertical phase separation have been used to fabricate high performance organic thin film transistors (OTFTs) with improved mobility [205], improved environmental stability [206–208]. Recently, a new method of crystallization-induced phase segregation has been investigated to construct vertically stratified structures in systems of two crystalline components such as poly(3hexylthiophene) (P3HT)/polyethylenes (PE) [209], and P3HT/PEG [210]. Meanwhile, this method offers expanded flexibility for realizing high performance semiconducting architectures with improved mechanical properties and environmental stability. Moreover, if the conjugated polymer can be cross-linked, a solvent-resistant pattern can be achieved [211].

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