Solvent-induced novel morphologies in diblock copolymer blend thin films

We report the morphology and phase behaviors of blend thin films containing two polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers with different blending compositions induced by a selective solvent for the PMMA block, which were studied by transmission electron microscopy (TEM). The neat asymmetric PS-b-PMMA diblock copolymers employed in this study, respectively coded as a1 and a2, have similar molecular weights but different volume fractions of PS block (fPS=0.273 and 0.722). Another symmetric PS-b-PMMA diblock copolymer, coded as s, which has a PS block length similar to that of a1, was also used. For the asymmetric a1/a2 blend thin films, circular multilayered structures were formed. For the asymmetric a1/symmetric s blend thin films, inverted phases with PMMA as the dispersed domains were observed, when the weight fraction of s was less than 50%. The origins of the morphology formation in the blend thin films via solvent treatment are discussed. Combined with the theoretical prediction by Birshtein et al. (Polymer 1992, 33, 2750), we interpret the formation of these special microstructures as due to the packing frustration induced by the difference in block lengths and the preferential interactions between the solvent and PMMA block. Results obtained here suggest that diblock copolymer blend thin films treated with a selective solvent offer an alternative and attractive approach to control the self-organization of polymers.     

Atomic force microscopy investigation of asymmetric diblock copolymer morphologies in thin films

Microphase separation and the resulting morphology of asymmetric diblock copolymers of poly(ε-caprolactone) (PCL) in thin films have been investigated by atomic force microscopy. Copolymers consisted of a short block of PCL (M_n∼2500-4500 g/mole) and a longer second block of poly(methyl methacrylate) (PMMA), poly(styrene) (PS) or poly(cyclohexene oxide) (PCHO). Tendency for microphase separation above the glass transition temperature of the second block (PMMA, PS or PCHO) resulted in a pitted morphology on the surface of the thin films. This tendency was strongest for PMMA and weakest for PCHO. The presence of up to 54% PMMA homopolymer in PCL-PMMA block copolymer did not prevent the formation of such pitted morphology on the surface. The effect of the chemical structure of the second block and the possible orientations of the block copolymer molecules in thin films are discussed.     

Morphologies in solvent-annealed thin films of symmetric diblock copolymer

We have systematically studied the thin film morphologies of symmetric poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer after annealing to solvents with varying selectivity. Upon neutral solvent vapor annealing, terraced morphology is observed without any lateral structures on the surfaces. When using PS-selective solvent annealing, the film exhibits macroscopically flat with a disordered micellar structure. While PMMA-selective solvent annealing leads to the dewetting of the film with fractal-like holes, with highly ordered nanoscale depressions in the region of undewetted films. In addition, when decreasing the swelling degree of the film in the case of PMMA-selective solvent annealing, hills and valleys are observed with the coexistence of highly ordered nanoscale spheres and stripes on the surface, in contrast to the case of higher swelling degree. The differences are explained qualitatively on the basis of polymer-solvent interaction parameters of the different components.     

Surface-induced phase transition of asymmetric diblock copolymer in selective solvents

Surface-induced phase transition of asymmetric diblock copolymer in selective solvents is first theoretically investigated by using the real-space version of self-consistent field theory (SCFT). By varying the distance between two parallel hard surfaces (or the film thickness) W and the block copolymer concentration f(p), several morphologies are predicted and the phase diagram is constructed. Self-assembly morphologies of the diblock copolymer in dilute solution are found to change significantly with different film thickness. In confined systems, stable morphologies found in the bulk solution become unstable due to the loss of polymer conformation entropy. We find that in a very dilute block copolymer solution, phase separation can be induced through polymer depletion as the solution becomes more confined. Our findings provide an interesting starting point for a renewed effort in both experimental and theoretical investigations of confined block copolymer solutions.     

Simulated annealing study of asymmetric diblock copolymer thin films

We report a simulated annealing study of the morphology of asymmetric diblock copolymer thin films confined between two homogeneous and identical surfaces. We have focused on copolymers that form a gyroidal morphology in the bulk. The morphological dependence of the confined films on the film thickness and the surface-polymer interaction has been systematically investigated. From the simulations it is found that much richer morphologies can form for the gyroid-forming asymmetric diblock copolymer thin films, in contrast to the lamella-forming symmetric and cylinder-forming asymmetric diblock copolymer films. Multiple morphological transitions induced by changing the film thickness and polymer-surface interactions are observed.     

Ellipsometry as a probe of crystallization kinetics in thin diblock copolymer films

We present results on the use of ellipsometry as a novel probe for the crystallization kinetics in thin films of a diblock copolymer. Ellipsometry makes use of the change in polarization induced upon the reflection of light from a film-covered substrate to enable the calculation of the refractive index and thickness of the film. The information obtained with these measurements can be compared with information from differential scanning calorimetry, with the additional advantages that small sample volumes and slow cooling rates can be employed and that expansion coefficients can be determined. By studying the temperature dependence of these quantities, we are able to measure the crystallization kinetics within very small volumes (∼10⁻¹⁰ L) of a poly(butadiene-b-ethylene oxide) diblock copolymer. Through a comparison of two different poly (ethylene oxide) block lengths, we demonstrate a reduction in both the crystallization and melting temperatures as the domain volume is reduced. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3448–3452, 2006     

Surface and bulk ordering in thin films of a symmetrical diblock copolymer

Amphiphilic diblock copolymers have the ability to adapt their surface's molecular composition to the hydrophilicity of their environment. In the case of about equal volume fractions of the two polymer blocks, the bulk of these polymers is known to develop a laminar ordering. We report here our investigation of the relationship between bulk ordering and surface morphology/chemical composition in thin films of such an amphiphilic diblock copolymer. Upon annealing in vacuum, the expected lamella ordering in the bulk of the film is observed and we find the morphology of the film surface to be defined by the thickness of the as‐deposited film: If the as‐deposited thickness matches the height of a lamella stack, then the film exhibits a smooth surface. Otherwise, an incomplete lamella forms at the film surface. We show that the coverage of this incomplete layer can be quantified by X‐ray reflectivity. To establish the lamella ordering in the bulk, the film needs to be annealed above the glass temperature of the two blocks. Molecular segregation at the film surface, however, is already occurring at temperatures well below the glass temperature of the two blocks. This indicates that below the glass temperature of the blocks the bulk of the thin film is “frozen,” whereas the polymer chains composing the surface lamella have an increased mobility. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys., 2013 , 51, 1282–1287     

Surface-induced morphologies of ABC star triblock copolymer in spherical cavities

The morphologies and phase diagrams exhibited by symmetric ABC star triblock copolymer nanoparticles are investigated on the basis of real-space self-consistent field theory. The ABC star triblock copolymers were chosen to be tiling-forming with fixed polymer parameter and the spherical boundaries were modeled using the masking technique. We first study a number of examples where the ABC triblock copolymers confined in spherical cavities with neutral surface. Then, two types of spherical cavity distinct preferential surfaces are considered, including both A-block attractive and repulsive preferential surfaces. We aim at the effects due to various spherical cavity diameters and the degree of interactions between the polymer and the spherical surface. A variety of morphologies, such as ring-like structures, concentric sphere, and irregular cylinder, were identified in phase diagrams. The results show that both the degree of interactions and spherical diameters can influence the formation of morphologies so that ring-like structures and other novel structures could be obtained.     

Structural evolution of cylindrical‐phase diblock copolymer thin films

We have measured the time evolution of the self‐assembly process in perpendicular‐oriented cylindrical‐phase diblock copolymer thin films using statistical analysis of high‐resolution scanning electron microscope (SEM) images. Within minutes of annealing above the polymer glass‐transition temperature, microphase separation between polymer blocks results in formation of uniform nanometer‐scale domains whose relative position is initially largely uncorrelated. On further annealing, the cylindrical polymer domains organize into a two‐dimensional hexagonal lattice whose characteristic grain size increases slowly with time (～t^1/4). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1970–1975, 2004     

Guided self-assembly of diblock copolymer thin films on chemically patterned substrates

We study the guided self-assembly of symmetric/asymmetric diblock copolymer (BCP) films on heterogeneous substrates with chemically patterned surface by using a coarse-grained phase-separation model. During the procedure, the free energy employed for the BCP films was modeled by the Ginzburg-Landau free energy with nonlocal interaction, and the flat, chemically patterned surface was considered as a heterogeneous surface with short-range interaction with the BCP molecules. The resulting Cahn-Hilliard equation was solved by means of an efficient semi-implicit Fourier-spectral algorithm. Effects of pattern scale, surface chemical potential, and BCP asymmetry on the self-assembly process were explored in detail and compared with those without chemically patterned substrate surfaces. It was found that the morphology of both symmetric and asymmetric BCP films is strongly influenced by the commensurability between the unconstrained natural period lambda* of the bulk BCP and the artificial pattern period. Simulation shows that patterned surface with period close to lambda* leads to highly ordered morphology after self-assembly for both symmetric and asymmetric BCP films, and it also dramatically accelerates the guided self-assembly process. The present simulation is in a very good agreement with the recent experimental observation in BCP nanolithography. Finally, the present study also expects an innovative nanomanufacturing method to produce highly ordered nanodots based on the guided self-assembly of asymmetric BCP films on chemically patterned substrates.     

Solvent-induced microphase separation in diblock copolymer thin films with reversibly switchable morphology

We have studied the surface morphology of symmetric poly(styrene)-block-poly(methyl methacrylate) diblock copolymer thin films after solvent vapor treatment selective for poly(methyl methacrylate). Highly ordered nanoscale depressions or striped morphologies are obtained by varying the solvent annealing time. The resulting nanostructured films turn out to be sensitive to the surrounding medium, that is, their morphologies and surface properties can be reversibly switchable upon exposure to different block-selective solvents.     

Fabrication of nanoporous templates from diblock copolymer thin films on alkylchlorosilane-neutralized surfaces

The fabrication of nanoporous templates from poly(styrene)-b-poly(methyl methacrylate) diblock copolymer thin films (PS-b-PMMA, volume ratio 70:30) on silicon requires precise control of interfacial energies to achieve a perpendicular orientation of the PMMA cylindrical microdomains relative to the substrate. To provide a simple, rapid, yet tunable approach for surface neutralization, we investigated the self-assembled ordering of PS-b-PMMA diblock copolymer thin films on silicon substrates modified with a partial monolayer of octadecyldimethyl chlorosilane (ODMS), i.e., a layer of ODMS with a grafting density less than the maximum possible monolayer surface coverage. We demonstrate herein the fabrication of nanoporous PS templates from annealed PS-b-PMMA diblock copolymer thin films on these partial ODMS SAMs.     

Preparation of long-range ordered nanostructures in semicrystalline diblock copolymer thin films using micromolding

Long-range ordered nanostructures are prepared in the poly(styrene)-block-poly(ε-caprolactone) diblock copolymer thin films using micromolding. We evaluated the change in crystallinity based on grazing-incidence X-ray diffraction and proved that the crystallinity increased with the decrease of the mold size. This means that ordered nanostructures with atomic length scale order can be adjusted by tuning the mesoscale confinement. The inherent mechanism was the cooperation of geometric confinement, microphase structure and surface-induced ordering of PS-b-PCL in the melt, which paved the way for the subsequent crystal growth. These findings establish a route to promote the cost-effective nanofabrication by combining the mature microfabrication technique with the emerging directed self-assembly of block copolymers.     

A surface-reactive rod-coil diblock copolymer: nano- and micropatterned polymer brushes.

A diblock copolymer consisting of poly(3-(triethoxysilyl)propylisocyanate) (PIC) and polystyrene(PS) was synthesized by anionic polymerization. A polymeric monolayer of the block copolymer was formed on silica substrates by various grafting techniques such as immersion, casting, or contact-printing. The PIC block adheres covalently to Si substrates in an in-plane fashion due to its extended rodlike conformation and reactivity to the silica. The polystyrene blocks aggregate to form mounds on the surface resulting in a new type of nanopatterned polymer brush. The self-limiting adsorption of the rod coils results in a thickness of about 5 nm regardless of the solution concentration and coating method. This particular property allowed microcontact printing directly onto silicon or glass substrates. The resulting surface morphology consisted of nanoscale domains given by the block copolymer and uniform thickness micropatterns transferred from the stamp within the printed area. This study offers a simple new method to prepare a covalent polymeric monolayer with nano- and micropatterns, which can be performed directly onto various silicon-based substrates.     

Mechanism and kinetics of ordering in diblock copolymer thin films on chemically nanopatterned substrates

Lamellae forming diblock copolymer domains can be directed to assemble without defects and in registration with chemically nanopatterned substrates. Initially, thin films of the lamellar poly(styrene-b-methyl methacrylate) block copolymer form hexagonally close-packed styrene domains when annealed on chemical nanopatterned striped surfaces. These styrene domains then coalesce to form linear styrene domains that are not fully registered with the underlying chemical surface pattern. Defects coarsen, until defect-free directed assembly is obtained, by breaking linear styrene domains and reforming new structures until registered lamellae have been formed. At all stages in the process, two factors play an important role in the observed degree of registration of the block copolymer domains as a function of annealing time: the interfacial energy between the blocks of the copolymer and the chemically nanopatterned substrate and the commensurability of the bulk repeat period of the block copolymer and the substrate pattern period. Insight into the time-dependent three-dimensional behavior of the block copolymer structures is gained from single chain in mean field simulations. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3444–3459, 2005     

Order-order and order-disorder transitions in thin films of an amphiphilic liquid crystalline diblock copolymer

In this study, we quantitatively investigated the temperature-dependent phase transition behaviors of thin films of an interesting amphiphilic diblock copolymer, poly(ethylene oxide)-b-poly(11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate) (p(EO)-b-p(MAAZ)) and the resulting morphological structures by using synchrotron grazing incidence X-ray scattering (GIXS) and differential scanning calorimetry. The quantitative GIXS analysis showed that the diblock copolymer in the homogeneous, isotropic melt state undergoes phase-separation near 190 degrees C and then forms a body-centered cubic (BCC) structure of spherical p(EO) domains in the p(MAAZ) matrix, at which point the p(EO) domains and the p(MAAZ) matrix are both in amorphous, liquid states. The BCC structure of spherical p(EO) domains is converted to a hexagonal cylinder structure near 120 degrees C, which is induced by the transformation of the isotropic phase of the p(MAAZ) matrix to the smectic A phase, which is composed of a laterally ordered structure of p(MAAZ) blocks with fully extended side groups. The resulting hexagonal cylinder structure is very stable below 120 degrees C. This microscopic hexagonal cylinder structure is retained as the smectic A phase of the p(MAAZ) matrix undergoes further transitions to smectic C near 104 degrees C and to a smectic X phase near 76 degrees C, while the amorphous, liquid phase of the p(EO) cylinders undergoes crystallization near -15 degrees C. These complicated temperature-dependent disorder-order and order-order phase transitions in the films were found to take place reversibly during the heating run. A face-centered orthorhombic structure of p(EO) domains was also found during the heating run and is an intermediate structure in the hexagonal cylinder structure to BCC structure transformation. We use these structural analysis results to propose molecular structure models at various temperatures for thin films of the diblock polymer.     

Deviations from bulk morphologies in thin films of block copolymer/additive binary blends

Deviations from bulk morphologies in thin films of binary blends of alkyne-functionalized diblock copolymer poly(ethylene oxide)-block-poly(n-butyl methacrylate-random-propargyl methacrylate) (PEO-b-P(nBMA-r-PgMA)) and Rhodamine B azide are reported, where thermal click reaction between the two components leads to microphase separated morphologies. Both in the bulk and in thin films, increasing the azide loading ratio resulted in the transition from a lamellar microdomain morphology to a hexagonally packed cylindrical mircodomain morphology. However, in thin films the lamellae-cylinder transition was observed at a different azide loading ratio, which was determined by film thickness. As a result, significant deviations from the bulk morphology were observed. These results indicate that surface interactions and confined geometry can play an important role in dictating the morphology in thin films of BCP/additive binary blends.     

Supramolecular PS-P4VP diblock copolymer thin films slowly dip-coated from chloroform solutions

This contribution shows the strong influence of using chloroform instead of THF on the characteristics of thin films of supramolecular block copolymers of poly(styrene-b-4-vinyl pyridine) dip-coated in the so-called "capillarity" regime from solutions containing 1-naphthol or 1-naphthoic acid.The small molecule content in the dip-coated films was investigated by infrared spectroscopy and the film morphology by atomic force microscopy.It was found that the small molecule content in the films is constant with dip-coating rate in the range investigated,but it is higher for 1-naphthoic acid than for 1-naphthol.The main morphology observed was in the form of "islands" and "holes",which is typical of flat-on lamellae.These findings are related to hydrogen-bonding between the small molecule and pyridine being conserved in chloroform and to the good solubility of both blocks in this solvent,with differences between the two small molecules related to their differing H-bond strengths.These findings contrast strongly with what was observed previously using THF as a solvent,for which the SM content increases with dip-coating rate and the morphologies are mainly spherical and cylindrical in the same parameter range.     

Comparative analysis of nanostructured diblock copolymer films

Nanostructured polymer films of poly(styrene-block-paramethylstyrene) diblock copolymers P(Sd-b-pMS) on silicon substrates with a native oxide layer are investigated. Resulting from a storage under toluene vapor, a surface structure is installed. The early stages, characterized by the creation of a host structure out of an initially continuous film, are addressed. Grazing incidence small-angle X-ray scattering (GISAXS) experiments were performed as a function of exposure time. Results are compared to modelling of the scattering pattern and other experimental techniques, such as grazing incidence small-angle neutron scattering (GISANS) and atomic force microscopy (AFM) data. Possibilities and limits of the techniques are discussed.