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.     

Microdomain morphology of lamella‐forming diblock copolymer confined in a thin film

We report the self‐consistent field theory (SCFT) of the morphology of lamella‐forming diblock copolymer thin films confined in two horizontal symmetrical/asymmetrical surfaces. The morphological dependences of thin films on the polymer‐surface interactions and confinement, such as film thickness and confinement spatial structure, have been systematically investigated. Mechanisms of the morphological transitions can be understood mainly through the polymer‐surface interactions and confinement entropy, in which the plat confinement surface provides a surface‐induced effect. The confinement is expressed in the form of the ratio D/L₀, here D is film thickness, and L₀ is the period of bulk lamellar‐structure. Much richer morphologies and multiple surface‐induced morphological transitions for the lamella‐forming diblock copolymer thin films are observed, which have not been reported before. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1–10, 2009     

Electric field versus surface alignment in confined films of a diblock copolymer melt

The dynamics of alignment of microstructure in confined films of diblock copolymer melts in the presence of an external electric field was studied numerically. We consider in detail a symmetric diblock copolymer melt, exhibiting a lamellar morphology. The method used is a dynamic mean-field density functional method, derived from the generalized time-dependent Ginzburg-Landau theory. The time evolution of concentration variables and therefore the alignment kinetics of the morphologies are described by a set of stochastic equations of a diffusion form with Gaussian noise. We investigated the effect of an electric field on block copolymers under the assumption that the long-range dipolar interaction induced by the fluctuations of composition pattern is a dominant mechanism of electric-field-induced domain alignment. The interactions with bounding electrode surfaces were taken into account as short-range interactions resulting in an additional term in the free energy of the sample. This term contributes only in the vicinity of the surfaces. The surfaces and the electric field compete with each other and align the microstructure in perpendicular directions. Depending on the ratio between electric field and interfacial interactions, parallel or perpendicular lamellar orientations were observed. The time scale of the electric-field-induced alignment is much larger than the time scale of the surface-induced alignment and microphase separation.     

Thickness-dependent molecular chain and lamellar crystal orientation in ultrathin poly(di-n-hexylsilane) films

The molecular chain and lamellar crystal orientation in ultrathin films (thickness < 100 nm) of poly-(di-n-hexylsilane) (PDHS) on silicon wafer substrates have been investigated by using transmission electronic microscopy, wide-angle X-ray diffraction, atomic force microscopy, and UV absorption spectroscopy. PDHS showed a film thickness-dependent molecular chain and lamellar crystal orientation. Lamellar crystals grew preferentially in flat-on orientation in the monolayer ultrathin films of PDHS, i.e., the silicon backbones were oriented along the surface-normal direction. By contrast, the orientation of lamellar crystals was preferentially edge-on in ultrathin films thicker than ca. 13 nm, i.e., the silicon backbones were oriented parallel to the substrate surface. We interpret the different orientations of molecular chain and lamellar crystal as due to the reduction of the entropy of the polymer chain near the substrate surface and the particularity of the crystallographic (001) plane of flat-on lamellae, respectively. A remarkable influence of the orientations of the silicon backbone on the UV absorption of these PDHS ultrathin films was observed due to the one-dimensional nature of sigma-electrons delocalized along the silicon backbone. With the silicon backbones perpendicular or parallel to the surface of the substrate, the UV absorbance increased or decreased with an increase of the angle between the incident UV beam direction and direction normal to the thin film, respectively.     

Equilibrium behavior of confined triblock copolymer films

Using a two-dimensional self-consistent field calculation, we determine the equilibrium morphology of thin films of ABC triblock copolymers confined between hard, smooth plates. The B segment is chosen to be the central block and all the blocks are incompatible. The chains microphase-segregate into a lamellar phase, with the stripes either perpendicular or parallel to the walls. When all the monomer-surface interactions are identical, the perpendicular orientation has the lowest free energy. When a repulsion is introduced between the surface and the A and C monomers. The surface interactions further stabilize the perpendicular orientation. At strong surface interactions, the morphology of the perpendicular structure is controlled by the overall thickness of the molten layer. In comparing diblocks to triblocks as candidates for forming laterally patterned films, our work indicates that triblocks possess distinct advantages over diblocks. First, no special effort needs to be taken to establish neutral surfaces. Second, the film does not have to be confined between two substrates. Thus, triblocks can be used to fabricate patterned polymer surfaces, which can be used for novel optical or electronic applications.     

Monte Carlo simulations of cylinder-forming ABC triblock terpolymer thin films

We systematically study the cylinder-forming ABC triblock terpolymer thin films using canonical ensemble Monte Carlo simulations. The simulated annealing procedure is applied to the self-assembling process. By judicious choice of the system dimensions, we elaborately investigate the effect of film thickness on the orientation of the cylinders. This confined triblock terpolymer system exhibits different phase behavior under the weak and strong surface fields. In addition, we also investigate the ensemble-averaged chain orientations and relative density profiles.     

SELF ASSEMBLY OF ABC TRIBLOCK COPOLYMER THIN FILMS ON A BRUSH-COATED SUBSTRATE

Self assemblies of ABC triblock copolymer thin films on a densely brush-coated substrate were investigated by using the self-consistent field theory.The middle block B and the coated polymer form one phase and the alternating phase A and phase C occur when the film is very thin either for the neutral or selective hard surface(which is opposite to the brushcoated substrate).The lamellar phase is stable on the hard surface when it is neutral and interestingly,the short block tends to stay on this hard surf...     

Lamellar orientation in thin films of symmetric semicrytalline polystyrene-b-poly(ethylene-co-butene) block copolymers: effects of molar mass, temperature of solvent evaporation, and annealing

Orientation of the lamellar microdomains in thin films of three symmetric polystyrene-b-poly(ethylene-co-butylene) block copolymers (S65E155, S156E358, and S199E452) on mica was investigated via atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). The results show that lamellar orientation in the SxEy block copolymers greatly depends on the molar mass of the block copolymers, the temperature of solvent evaporation, and annealing. The nascent thin film of the low molar mass block copolymer, S65E155, shows a multilayered structure parallel to the mica surface with the PS block at both polymer/mica and polymer/air interfaces, but the high molar mass block copolymers, S156E358 and S199E452, exhibit a structure with lamellar microdomains perpendicular to the mica surface. When the solvent is evaporated at a lower temperature, the crystallization rate is fast and a two-dimensional spherulite structure with the lamellar microdomains perpendicular to the mica surface is observed. Annealing of all the thin films with lamellar microdomains perpendicular to the mica surface leads to morphological transformation into a multilayered structure parallel to the mica surface. In all SxEy thin films on mica, the stems of PE crystals are always perpendicular to the interface between the lamellar PE and PS microdomains. A mechanism is proposed for the formation of different microdomain orientations in the thin films of semicrystalline block copolymers. When the thin film is prepared from a homogeneous solution, microdomains perpendicular to the substrate surface are formed rapidly for strongly segregated block copolymers or at a lower crystallization temperature and kinetically trapped by the strong segregation strength or solidification of crystallization, while for weakly segregated block copolymers or at slower crystallization rate, the orientation of the microdomains is dominated by surface selectivity.     

Lamellae orientation in block copolymer films with ionic complexes

Lamellae orientation in lithium-complexed polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymer films on natively oxidized silicon wafers is investigated as a function of film thickness and percentage of carbonyl groups coordinated with lithium ions using cross-sectional transmission electron microscopy and grazing incidence small-angle X-ray scattering. For films with a lower percentage of ionic complexes, the strong surface interaction of the PMMA blocks with the substrate is not changed significantly and the orientation of the lamellar microdomains depends on the film thickness and is dictated by a coupling of the interfacial interactions and the degree of microphase separation. For films with a higher percentage of ionic complexes, the surface interaction is mediated. Along with the enhanced immiscibility between the two blocks, which drives the self-assembly into a stronger microphase segregation, an orientation of the lamellar microdomains normal to the surface is seen, independent of film thickness. Thus, by tuning the amount of ionic complexes, the orientation of lamellar microdomain can be controlled from a random arrangement to being oriented parallel or perpendicular to the film surface without any surface modification or use of external fields, which opens a simple and general route for the fabrication of nanostructured materials.     

In-plane and out-of-plane defectivity in thin films of lamellar block copolymers

We investigate the ordering of poly(styrene-b-methyl methacrylate) (PS-PMMA) lamellar copolymers (periodicity L₀ = 46 nm) confined between a free surface and brushed poly(styrene-r-methyl methacrylate) silicon substrate. The processing temperature was selected to eliminate wetting layers at the top and bottom interfaces, producing approximately neutral boundaries that stabilize perpendicular domain orientations. The PS-PMMA film thickness (t = 0.5L₀ − 2.5L₀) and brush grafting density (Σ = 0.2–0.6 nm⁻²) were systematically varied to examine their impacts on in-plane and out-of-plane ordering. Samples were characterized with a combination of high-resolution microscopy, X-ray reflectivity, and grazing-incidence small-angle X-ray scattering. In-plane order at the top of the film (quantified through calculation of orientational correlation lengths) improved with tⁿ, where the exponent n increased from 0.75 to 1 as Σ decreased from 0.6 to 0.2 nm⁻². Out-of-plane defects such as tilted domains were detected in all films, and the distribution of domain tilt angles was nearly independent of t and Σ. These studies demonstrate that defectivity in perpendicular lamellar phases is three-dimensional, comprised of in-plane topological defects and out-of-plane domain tilt, with little or no correlation between these two types of disorder. Strong interactions between the block copolymer and underlying substrate may trap both kinds of thermally generated defects. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 339–352     

Effect of Materials Parameters on the Shape of Face-On Lamellae in Semi-Conducting Polymers: Insights From Qualitative Theory

Polymer semiconductors frequently form crystals or mesophases with lamellae, that comprise alternating layers of stacked backbones and side chains. Controlling lamellar orientation in films is essential for obtaining efficient charge carrier transport. Herein, lamellar orientation is investigated in an application-relevant setup: lamellae assembled on a substrate that strongly favors face-on orientation, but exposed to a film surface that promotes orientation along an “easy” direction, other than face on. It is assumed that the face-on order propagates from the substrate, but the lamellae bend to reduce their surface energy. A qualitative free-energy model is developed. The deformation is investigated as a function of film thickness, effective Young modulus, anchoring coefficient, and easy direction at the free surface. The calculations highlight the importance of elastic constants – lamellae can substantially deform already when Young moduli are only an order of magnitude smaller than the values that are reported for crystals. Softer Young moduli are expected when lamellar assembly occurs in a non-solidified mesophase that can be an equilibrium or (more speculatively) a transient state prior to crystallization. The alternative scenario of a two-layered film is also evaluated, where edge-on and face-on grains form, respectively, at the free surface and substrate.     

An investigation of the stable orientations of orthorhombic particles in a thin film and their effect on its critical failure pressure

The effects of shape and contact angle on the behaviour of orthorhombic particles at an interface and in thin films were investigated using Surface Evolver. It is shown that the energetically stable orientations of the particle change with its aspect ratio. Long, wide, flat particles with low contact angles are more stable in flat orientations, i.e. with two faces parallel to the flat film surface. More cubic particles with higher contact angles are more stable in twisted orientations, where the opposite sides of the film can be drawn together at the sharp edges of the particle. The combination of contact angle and orientation has been found to have a large effect on the capillary pressure required to rupture the film. A film containing a particle in a flat orientation will rupture at a capillary pressure up to three times greater than one containing an identical particle in a twisted orientation. Wider, flatter particles with low contact angles stabilise thin liquid films to a greater extent than cubic particles with high contact angles.     

Effects of surface water on gas sorption capacities of gravimetric sensing layers analyzed by molecular descriptors of organic adsorbates

The gas sorption capacities of sputtered carbonaceous films are evaluated with quartz crystal resonators. These films are sensitive to 20 ppm organic vapors and exhibit structure-dependent responses. Films derived from synthetic polymers are hydrophobic, whereas films derived from biomaterials are amphiphilic or hydrophilic. Polyethylene (PE) film has an extremely high sorption capacity for a wide range of vapors. Transient sorption responses are investigated using a humidified carrier by employing carboxylic acid esters, whose aliphatic groups are systematically changed. Small esters with a higher affinity to water induce negative U-shaped responses from amphiphilic films derived from biomaterials. On the other hand, polymeric films exhibit positive exponential response curves. Even if the concentrations are decreased, the response intensities are enhanced with the incremental expansion of carbon chains of aliphatic groups. Only fluoropolymer film shows the opposite tendency. The modeling of quantitative structure property relationships has indicated that the sorption capacities of the PE film to the carboxylic acid esters are fundamentally governed by electrostatic interactions. The intermolecular attractive forces are basically attributable to interactions between the positively polarized sites in esters and the negatively polarized/charged sites in PE film.     

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.     

Morphology Evolution of Block Copolymer Blend Thin Films Induced by Solvent Vapor Annealing

ABSTRACT

Self-assembly of binary block copolymer blends in thin film induced by solvent vapor annealing has been systematically studied. The diblock copolymers polystyrene-b-poly(2-vinylpyridine) with different molecular weights and volume fractions were blended with different molar ratios to cast thin films on silica substrate by spin coating. The films were annealed separately in the vapor of ethanol or toluene over time to induce morphology transformations from spheres, gyroids, and bicontinuous nanostructures, depending on the blending ratio, solvent selectivity, and annealing time, as investigated by atomic force microscopy and X-ray photoelectron spectroscopy. The formation and transformation mechanism of the self-assembly structure are discussed in the context of solvent-copolymer interactions. This study provides new insights into the simple manipulation of self-assembled nanostructures of block copolymer thin films.     

Highly oriented structure formed in a lamella-forming diblock copolymer with high molar mass

Highly oriented films were prepared simply by annealing a lamella-forming block copolymer, poly(ethylene oxide-b-styrene) (PEO-b-PS), with high molar mass under a pressure of 0.2 MPa. The oriented structures were characterized by small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The SAXS measurements showed that the lamellar layers of the block copolymer are highly oriented parallel in the film plane. The WAXD images showed that the c-axis of PEO crystals was oriented normal to the film plane. The Hermans-Stein orientation functions for the lamellar layer and the crystal axis are 0.954 and −0.466, respectively, and are close to the values of perfect orientation. It was considered that the highly oriented structure was formed by the combined effects of shear flow and self-organization of the block copolymer during annealing under stress. The high degree of orientation both for the lamellar layer and crystal planes also suggested that the crystallization in the confined domains results in a high degree of orientation of PEO crystals with respect to the lamellar interface of the block copolymer.     

Measurement of the azimuthal anchoring energy of liquid crystals in contact with oligo(ethylene glycol)-terminated self-assembled monolayers supported on obliquely deposited gold films

We report measurements of the orientations and azimuthal anchoring energies of the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) on polycrystalline gold films that are deposited from a vapor at an oblique angle of incidence and subsequently decorated with organized monolayers of oligomers of ethylene glycol. Whereas the gold films covered with monolayers presenting tetra(ethylene glycol) (EG4) lead to orientations of 5CB that are perpendicular to the plane of incidence of the gold, monolayers presenting tri(ethylene glycol) (EG3) direct 5CB to orient parallel to the plane of incidence of the gold during deposition of the gold film. We also measure the azimuthal anchoring energy of the 5CB to be smaller on the surfaces presenting EG3 (3.2 +/- 0.8 microJ/m2) as compared to EG4 (5.5 +/- 0.9 microJ/m2). These measurements, when combined with other results presented in this paper, are consistent with a physical model in which the orientation and anchoring energies of LCs on these surfaces are influenced by both (i) short-range interactions of 5CB with organized oligomers of ethylene glycol at these surfaces and (ii) long-range interactions of 5CB with the nanometer-scale topography of the obliquely deposited films. For surfaces presenting EG3, these short- and long-range interactions oppose each other, leading to small net values of anchoring energies that we predict are dependent on the level of order in the EG3 SAM. These measurements provide insights into the balance of interactions that control the orientational response of LCs to biological species (proteins, viruses, cells) on these surfaces.     

Evaporation-induced Morphology Pattern of Triblock Copolymer A5B10C5 in Thin Film： a Multibody DPD Simulation Study

We used multibody dissipative particle dynamics method,by which the attractive and repulsive interactions can be effectively considered,to investigate the evaporation-induced morphology patterns of triblock copolymer A5B10C5 in thin film.With changing attractive interactions between solvent vapor and triblock copolymer that represent various selective solvents,lamellar morphology,sandwich lamellar morphology,spherical morphology and disorder morphology patterns of the thin films were obtained for both coil-coil-coil and rod-coil-coil chain architectures,respectively.The order parameter and the film thickness were calculated during the process for characterizing the film properties,and it was found that the rigid A-block of the triblock copolymer hinders the formation of an ordered structure.     

Self-assembly of star ABC triblock copolymer thin films: self-consistent field theory

Microphase separation and morphology of star ABC triblock copolymers confined between two identical parallel walls (symmetric wetting or dewetting) are investigated with self-consistent field theory (SCFT) combined with the "masking" technique to describe the geometric confinement of the films. In particular, we examine the morphology of confined near-symmetric star triblock copolymers under symmetric and asymmetric interactions as a function of the film thickness and the surface field. Under the interplay between the degree of spatial confinement, characterized by the ratio of the film thickness to bulk period, and surface field, the confined star ABC triblock copolymers are found to exhibit a rich phase behavior. In the parameter space we have explored, the thin film morphologies are described by four primary classes including cylinders, perforated lamellae, lamellae, and other complex hybrid structures. Some of them involve novel structures, such as spheres in a continuous matrix and cylinders with alternating helices structure, which are observed to be stable with suitable film thickness and surface field. In particular, complex hybrid network structures in thin films of bulk cylinder-forming star triblock copolymers are found when the natural domain period is not commensurate with the film thickness. Furthermore, a strong surface field is found to be more significant than the spatial confinement on changing the morphology of star triblock copolymers in bulk. These findings provide a guide to designing novel microstructures involving star triblock copolymers via geometric confinement and surface fields.     

Grazing-incidence transmission small angle X-ray scattering from thin films of block copolymers

Thin films of lamellar and cylindrical block copolymers are popular systems for low-cost nanolithography. To be useful as nanoscale templates, the lamellae or cylinders must be oriented perpendicular to the substrate. Domain orientations are usually characterized by microscopy measurements of the film surface, but these techniques cannot detect tilted, bent, or tortuous domains in the film interior. We report a simple method to quantify out-of-plane disorder in thin films of block copolymers based on a variant of grazing-incidence small angle X-ray scattering (GI-SAXS). A typical GI-SAXS experiment illuminates the center of a substrate-supported film at a grazing angle of incidence (near the film/substrate critical angle), and the strong reflected signal is interpreted with the distorted-wave Born approximation. In a new approach, the beam footprint is moved to the far edge of the sample, allowing the acquisition of a transmission pattern. The grazing-incidence transmission data are interpreted with the simple Born approximation, and out-of-plane defects are quantified through analysis of crystal truncation rods and partial Debye-Scherrer rings. Significantly, this study demonstrates that grazing-incidence transmission small angle X-ray scattering can detect and quantify the buried defect structure in thin films of block copolymers. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013