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.     

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.     

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.     

不对称两嵌段共聚物熔体在平板狭缝中微相分离的Monte Carlo模拟

采用改进的键长涨落空穴扩散算法对平板狭缝中不对称两嵌共聚高分子熔体的微相分离进行了Monte Carlo模拟。模拟结果表明：在吸引壁条件下，靠近壁面处将形成平等于壁面的层状相；在弱吸引壁条件下，靠近壁面处将形成垂直于壁面的层状相；不对称共聚物在远离壁面处有丰富的微区形态。从结构因子上分析可知，弱吸引壁条件下不对称共聚物的结构比强吸引壁条件下更接近对称共聚物。     

Depletion-induced surface alignment of asymmetric diblock copolymer in selective solvents

Phase separation of asymmetric diblock copolymer near surfaces in selective solvents is theoretically investigated by using the real-space version of self-consistent field theory (SCFT). Several morphologies are predicted and the phase diagram is constructed by varying the distance between two parallel hard surfaces (or the film thickness) W and the block copolymer concentration f(P). Morphologies of the diblock copolymer in dilute solution are found to change significantly with different film thicknesses. In confined systems, stable morphologies found in the bulk solution become unstable due to the loss of polymer conformation entropy. The vesicle phase region contracts when the repulsive interaction between the blocks is strong (strong segregation regime). The mixture of vesicles, rodlike and spherelike micelles and the mixture of vesicles and sphere-like micelles disappear in contrast to the weakly segregating regime. The walls strongly affect the phase separation of block copolymer in selective solvent, and the depletion layer near the surface contributes much to the micelle formation of the block copolymer. Interestingly, the self-assembled morphologies stay near the walls with the distance on the order of the radius of gyration of the block copolymer. The oscillation of the polymer distribution near the walls allows the surface phase separation to be observed due to the strong repulsion between the blocks A and B.     

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.     

掺杂嵌段共聚物薄膜的相分离研究

采用Monte Carlo方法结合退火方法研究掺杂非对称两嵌段共聚物薄膜的相分离情况.发现随着掺杂极性粒子浓度的增加,嵌段共聚物薄膜体系由层状相逐渐转化成层状和柱状共混相,最后变成分布均匀的柱状相.当掺杂浓度增加到一定程度时,形成了如六边形(6-fold)、七边形(7-fold)和五边形柱状(5-fold)的相结构;六边形柱状相结构的比例随着浓度增加而增加,七边形(7-fold)和五边形柱状(5-fold)相结构的比例随着浓度增加而减小.同时还讨论了两嵌段共聚物大小与掺杂浓度的关系.     

对称长链和近对称短链两嵌段聚合物混合体系相行为研究

利用自洽平均场理论(SCMFT)系统地研究了对称长链和近对称短链两嵌段聚合物混合体系在纳米尺度下的自组装行为.体系中具有较高聚合度的对称长链熔体处于层状相,聚合度较低的近对称短链熔体处于无序相,而其混合体系却随着两种成分的不同比例呈现出有序-无序相转变、有序-有序相转变及有序-无序两相共存等复杂的相行为,计算结果与近期类似体系的实验有着较好的吻合.同时与两种对称的两嵌段聚合物混合体系的计算结果进行了比较,得出这两种体系的异同之处.     

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.     

Control of self‐organized low‐dimensional morphology in poly(styrene‐b‐4vinylpyridine)/polystyrene blend thin films

The surface morphologies of poly(styrene‐b‐4vinylpyridine) (PS‐b‐P4VP) diblock copolymer and homopolystyrene (hPS) binary blend thin films were investigated by atomic force microscopy as a function of total volume fraction of PS (?_PS) in the mixture. It was found that when hPS was added into symmetric PS‐b‐P4VP diblock copolymers, the surface morphology of this diblock copolymer was changed to a certain degree. With ?_PS increasing at first, hPS was solubilized into the corresponding domains of block copolymer and formed cylinders. Moreover, the more solubilized the hPS, the more cylinders exist. However, when the limit was reached, excessive hPS tended to separate from the domains independently instead of solubilizing into the corresponding domains any longer, that is, a macrophase separation occurred. A model describing transitions of these morphologies with an increase in ?_PS is proposed. The effect of composition on the phase morphology of blend films when graphite is used as a substrate is also investigated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3496–3504, 2004     

Microdomain Morphology of Symmetrical Diblock Copolymer Thin Films Confined in a Slit

The microphase separation and morphology of symmetric diblock copolymer thin films confined in a slit with neutral or attractive surfaces were studied by the cell dynamic system method (CDS) and Monte Carlo simulation. The size effect, especially in CDS, was carefully investigated indicating that excessively small sizes in the X‐ and Y‐directions will give incorrect results although periodic boundary conditions are imposed. When the walls are neutral, parallel ordered lamella structure only exists over a short range, while irregular microdomain morphology occurs over the whole region. When directional quenching is applied, or the walls are attractive to one of the blocks, a periodical lamellar structure of alternating A‐rich and B‐rich layers occurs over the whole region of the film. Changing the slit width and the strength of interaction will influence the period and arrangement of lamellae. Agreement between the results from CDS and those from simulation is satisfactory indicating the reliability of the CDS method. Comparisons with corresponding experimental results are also discussed.     

Phase separation induced by shear quenching in polymer blends with a diblock copolymer

The effects of adding A–B diblock copolymer to a polymer blend (A/B) on phase‐separation kinetics and morphology have been investigated in a fixed shallow‐quench condition (ΔT = 1.5 °C) by in situ time‐resolved light scattering and phase‐contrast optical microscopy. A shear‐quench technique was used in this study instead of a conventional temperature‐quench method. Mixtures of nearly monodisperse low relative‐molecular masses of polybutadiene (M_w = 2.8 kg/mol), polystyrene (M_w = 2.6 kg/mol), and a near‐symmetric butadiene–styrene diblock copolymer (M_w = 6.3 kg/mol) as an interfacial modifier were studied. We observed that the addition of the diblock copolymer could either retard or accelerate the phase‐separation kinetics depending on the concentration of the diblock copolymer in the homopolymer blends. In contrast to the conventional temperature quench, we observed complex phase‐separation kinetics in the intermediate and late stages of phase separation by the shear‐quench technique. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 819–830, 2001     

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...     

Simulated annealing study of diblock copolymer thin films confined between two homogeneous surfaces.

Thin films of symmetric diblock copolymers confined between two parallel surfaces have been systematically investigated by means of simulated annealing on a simple cubic lattice. The study was carried out for systems with different film thicknesses and surface-polymer interactions. Very regular equilibrium morphologies of lamella are formed in almost all cases. The dependence of lamellar orientations, total energy, chain-conformation entropy, and free energy of the confined films on the film thickness and the strength of surface-polymer interactions has been studied systematically. The influence of packing frustration on morphology is observed and the mechanisms of lamellar orientations are investigated.     

Monte Carlo simulation on symmetric ABA/AB copolymer blends in confined thin films

The effects of blend composition on morphology, order-disorder transition (ODT), and chain conformation of symmetric ABA/AB copolymer blends confined between two neutral hard walls have been investigated by lattice Monte Carlo simulation. Only lamellar structure is observed in all the simulation morphologies under thermodynamic equilibrium state, which is supported by theoretical prediction. When the composition of AB diblock copolymer (phi) increases, both lamellar spacing and the corresponding ODT temperature increase, which can be attributed to the variation of conformation distribution of the diblock and the triblock copolymer chains. In addition, both diblock and triblock copolymer, chains with bridge conformation extend dramatically in the direction parallel to the surface when the system is in ordered state. Finally, the copolymer chain conformation depends strongly on both the blend composition and the incompatibility parameter chiN.     

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.     

Surface-induced phase transitions in ultrathin films of block copolymers

We study theoretically the lamellar-disorder-lamellar phase transitions of AB diblock and tetrablock copolymers confined in symmetric slitlike pores where the planar surface discriminatingly adsorbs A segments but repels B segments, mimicking the hydrophobic/hydrophilic effects that have been recently utilized for the fabrication of environmentally responsive "smart" materials. The effects of film thickness, polymer volume fraction, and backbone structure on the surface morphology have been investigated using a polymer density-functional theory. The surface-induced phase transition is manifested itself in a discontinuous switch of microdomains or a jump in the surface density dictated by the competition of surface adsorption and self-aggregation of the block copolymers. The surface-induced first-order phase transition is starkly different from the thickness-induced symmetric-asymmetric or horizontal-vertical transitions in thin films of copolymer melts reported earlier.     

Micro- and nanopatterned copper structures using directed self-assembly on templates fabricated from phase-separated mixed Langmuir-Blodgett films

We report a versatile method to confine metal thin films in micro- and nanopatterns using directed self-assembly on the templates fabricated from phase-separated mixed Langmuir-Blodgett (LB) films. The pattern of the mixed LB films can be tuned by adjusting intermolecular interaction between the film-forming molecules in the LB films and by varying the fabrication conditions of the films such as the mixing ratio, subphase temperature, and surface pressure. We use the patterned LB films for templates to confine metal in patterned regions, taking advantage of the difference between the surface free energy of the patterned regions and that of the self-assembled monolayer of the silane coupling agent. Au nanoparticles are confined onto the patterned films as a catalyst for the succeeding Cu electroless deposition. The atomic force microscopic images, Auger electron spectra, and scanning Auger electron maps of a Cu-deposited film show that Cu is selectively deposited on the patterns of phase separation of the original mixed LB films.     

Morphology Tailoring of Thin Film Block Copolymers on Patterned Substrates

It is well known that chemically patterned substrates can direct the assembly of adsorbed layers or thin films of block copolymers. For a cylinder‐forming diblock copolymer on periodically spot‐patterned substrates, the morphology of the block copolymer follows the pattern at the substrate; however, with different periodic spacing and spot size of the pattern, novel morphologies can be created. Specifically, we have demonstrated that new morphologies that are absent in the bulk system can be tailored by judiciously varying the mismatch between the width of the pattern and the periodic spacing of the bulk block copolymer, the top surface affinity, and spot size. New morphologies can thus be achieved, such as honeycomb and ring structures, which do not appear in the bulk system. These results demonstrate a promising strategy for fabrication of new nanostructures from chemically patterned substrates.