A Monte Carlo simulation of morphology and structure for thin films of symmetric triblock copolymer after quenching and annealing

Self-assembly thin films of symmetric triblock copolymer after annealing and quenching were examined by an effective Monte Carlo simulation method. The defects in the ordered lamellae of the thin films after quenching, which were dependent on the initialization of copolymer melts, are removed in the thin films after annealing. The mean-square gyration radius and end-to-end distance of copolymer chains in the thin films after annealing are smaller than those in the thin films after quenching because of the complete relaxation of polymer during annealing. We also find that the density of A block in the region near to the surface is higher than that in the interior of the thin films. As a result, it is different from the thin films of symmetric A(n)B(n) diblock copolymer, in which surface ordering forms before the interior, that ordering phenomena occurs first in the interior region in the thin films of symmetric A(n)B(m)A(n). triblocl copolymer.

The formation of ordered nanoholes in binary, chemically similar, symmetric diblock copolymer blend films

Binary symmetric diblock copolymer blends, that is, low-molecular-weight poly(styrene-block-methyl methacrylate) (PS-b-PMMA) and high-molecular-weight poly(styrene-block-methacrylate) (PS-b-PMA), self-assemble on silicon substrates to form structures with highly ordered nanoholes in thin films. As a result of the chemically similar structure of the PMA and the PMMA block, the PMMA chain penetrates through the large PMA block that absorbs preferentially on the polar silicon substrate. This results in the formation of nanoholes in the PS continuous matrix.

Effects of casting solvents on the formation of inverted phase in block copolymer thin films

Previously, an inverted phase (the minority blocks comprising the continuum phase) was found in solution-cast block copolymer thin films. In this study, the effect of casting solvents on the formation of inverted phase has been studied. Two block copolymers, poly(styrene-b-butadiene) (SB) (M-w = 73 930 Da) and poly(styrene-b-butadiene-b-styrene) (SBS) (M-w = 140 000 Da), with comparable block lengths and equal polystyrene (PS) weight fraction (similar to30 wt %) were used. The copolymer thin films were cast from different solvents, toluene, benzene, cyclohexane, and binary mixtures of benzene and cyclohexane. Toluene and benzene are good solvents for both PS and PB, but have a preferential affinity for PS, while cyclohexane is a good solvent for PB but a Theta solvent for PS (T-Theta = 34.5 degreesC). The differential solvent affinity for PS and PB was estimated in terms of a difference between the polymer-solvent interaction parameter, chi, for each block. Under an extremely slow solvent evaporation rate, the time-dependent phase behavior during such a solution-to-film process was examined by freeze-drying the samples at different stages, corresponding to different copolymer concentrations, rho.

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.

Solvent-induced novel morphologies in diblock copolymer blend thin films

We report the morphology and phase behaviors of blend thin films containing two poly styrene-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 a(1) and a(2), have similar molecular weights but different volume fractions of PS block (f(PS) = 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 a(1), was also used. For the asymmetric a(1)/a(2) blend thin films, circular multilayered structures were formed. For the asymmetric a(1)/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.

Effect of crystallization on the lamellar orientation in thin films of symmetric poly(styrene)-b-poly(L-lactide) diblock copolymer

The effect of crystallization on the lamellar orientation of poly( styrene)-b-poly(L-lactide) (PS-PLLA) semicrystalline diblock copolymer in thin films has been investigated by atomic force microscopy (AFM), transmission electronic microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). In the melt state, microphase separation leads to a symmetric wetting structure with PLLA blocks located at both polymer/substrate and polymer/air interfaces. The lamellar period is equal to the long period L in bulk determined by small-angle X-ray scattering (SAXS). Symmetric wetting structure formed in the melt state provides a model structure to study the crystallization of PLLA monolayer tethered on glassy (T-c < T-g,T-PS) or rubber (T-c > T-g,T-PS) PS substrate. In both cases, it is found that the crystallization of PLLA results in a "sandwich" structure with amorphous PS layer located at both folding surfaces. For T-c <= T-g,T- PS, the crystallization induces a transition of the lamellar orientation from parallel to perpendicular to substrate in between and front of the crystals. In addition, the depletion of materials around the crystals leads to the formation of holes of 1/2 L, leaving the adsorbed monolayer exposure at the bottom of the holes.

Inverted to normal phase transition in solution-cast polystyrene-poly(methyl methacrylate) block copolymer thin films

We have investigated the inverted phase formation and the transition from inverted to normal phase for a cylinder-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer in solution-cast films with thickness about 300 nm during the process of the solution concentrating by slow solvent evaporation. The cast solvent is 1, 1,2,2-tetrachloroethane (Tetra-CE), a good solvent for both blocks but having preferential affinity for the minority PMMA block. During such solution concentrating process, the phase behavior was examined by freeze-drying the samples at different evaporation time, corresponding to at different block copolymer concentrations, phi. As phi increases from similar to 0.1 % (nu/nu), the phase structure evolved from the disordered sphere phase (DS), consisting of random arranged spheres with the majority PS block as I core and the minority PMMA block as a corona, to ordered inverted phases including inverted spheres (IS), inverted cylinders (IC), and inverted hexagonally perforated lamellae (IHPL) with the minority PMMA block comprising the continuum phase, and then to the lamellar (LAM) phase with alternate layers of the two blocks, and finally to the normal cylinder (NC) phase with the majority PS block comprising the continuum phase. The solvent nature and the copolymer solution concentration are shown to be mainly responsible for the inverted phase formation and the phase transition process.

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.

Boundary effect of relief structure on crystallization of diblock copolymer in thin films

In this Letter, crystal growth of a symmetric crystalline-amorphous diblock copolymer, poly(styrene-b-epsilon-caprolactone) (PS-b-PCL), in thin films was investigated by atomic force microscopy (AFM), Relief structures of holes and islands were formed during annealing the film at the molten state, and the in situ observation of subsequent crystal growth at room temperature indicated that the crystals were preferred to occur at the edge of holes or islands and grew into the interior area. It was concluded that the stretched PCL blocks at the edge of relief structures, caused by material transportation or deformation of the interface, could act as nucleation agents during polymer crystallization. The crystal growth rate of individual lamellae varied both from lamellae to lamellae and in time, but the area occupied by crystals increased constantly with time. At 22 degreesC, the growth rate was 1.2 x 10(-2) mum(2)/min with the scan size 2 x 2 mum(2).

Crystallization of weakly segregated poly(styrene-b-epsilon-caprolactone) diblock copolymer in thin films

The surface morphology and crystallization behavior of a weakly segregated symmetric diblock copolymer, poly(styrene-b-6-caprolactone) (PS-b-PCL), in thin films were investigated by optical microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM). When the samples were annealed in the molten state, surface-induced ordering, that is, relief structures with uniform thickness or droplets in the adsorbed monolayer, were observed depending on the annealing temperature. The polar PCL block preferred to wet the surface of a silicon wafer, while the PS block wet the air interface. This asymmetric wetting behavior led to the adsorbed monolayer with a PCL block layer having a thickness of around 4.0 nm. The crystallization of PCL blocks could overwhelm the microphase-separated structure because of the weak segregation. In situ observation of crystal growth indicated that the nucleation process preferred to occur at the edge of the thick parts of the film, that is, the relief structures or droplets. The crystal growth rate was presented by the time dependence of the distance between the tip of crystal clusters and the edge. At 22 and 17 degreesC, the average crystal growth rates were 55 +/- 10 and 18 +/- 4 nm/min, respectively.

Electric field alignment in thin films of cylinder-forming diblock copolymer

We investigate thin films of cylinder-forming diblock copolymer confined between electrically charged parallel plates, using self-consistent-field theory ( SCFT) combined with an exact treatment for linear dielectric materials. Our study focuses on the competition between the surface interactions, which tend to orient cylinder domains parallel to the plates, and the electric field, which favors a perpendicular orientation. The effect of the electric field on the relative stability of the competing morphologies is demonstrated with equilibrium phase diagrams, calculated with the aid of a weak-field approximation. As hoped, modest electric fields are shown to have a significant stabilizing effect on perpendicular cylinders, particularly for thicker films. Our improved SCFT-based treatment removes most of the approximations implemented by previous approaches, thereby managing to resolve outstanding qualitative inconsistencies among different approximation schemes.

Step edges in thin films of lamellar-forming diblock copolymer

Self-consistent field theory (SCFT) is used to study the step edges that occur in thin films of lamellar-forming diblock copolymer, when the surfaces each have an affinity for one of the polymer components. We examine film morphologies consisting of a stack of ν continuous monolayers and one semi-infinite bilayer, the edge of which creates the step. The line tension of each step morphology is evaluated and phase diagrams are constructed showing the conditions under which the various morphologies are stable. The predicted behavior is then compared to experiment. Interestingly, our atomic force microscopy (AFM) images of terraced films reveal a distinct change in the character of the steps with increasing ν, which is qualitatively consistent with our SCFT phase diagrams. Direct quantitative comparisons are not possible because the SCFT is not yet able to probe the large polymer/air surface tensions characteristic of experiment.

Light emitting diodes containing Langmuir-Blodgett films of copolymer of a poly(p-phenylene-vinylene) derivative and poly(octaneoxide)

The properties of Langmuir and Langmuir-Blodgett (LB) films from a block copolymer with polyethylene oxide and phenylene-vinylene moieties are reported. The LB films were successfully transferred onto several types of substrates, with sufficient quality to allow for evaporation of a metallic electrode on top of the LB films to produce polymer light emitting diodes (PLEDs). The photoluminescence and electroluminescence spectra of the LB film and device were similar, featuring an emission at ca. 475 nm, from which we could infer that the emission mechanisms are essentially the same as in poly(p-phenylene) derivatives. Analogously to other PLEDs the current versus voltage characteristics of the LB-based device could be explained with the Arkhipov model according to which charge transport occurs among localized sites. The implications for nanotechnology of the level of control that may be achieved with LB devices will also be discussed.

Light emitting diodes containing Langmuir-Blodgett films of copolymer of a poly(p-phenylene-vinylene) derivative and poly(octaneoxide)

The properties of Langmuir and Langmuir-Blodgett (LB) films from a block copolymer with polyethylene oxide and phenylene-vinylene moieties are reported. The LB films were successfully transferred onto several types of substrates, with sufficient quality to allow for evaporation of a metallic electrode on top of the LB films to produce polymer light emitting diodes (PLEDs). The photoluminescence and electroluminescence spectra of the LB film and device were similar, featuring an emission at ca. 475 nm, from which we could infer that the emission mechanisms are essentially the same as in poly(p-phenylene) derivatives. Analogously to other PLEDs the current versus voltage characteristics of the LB-based device could be explained with the Arkhipov model according to which charge transport occurs among localized sites. The implications for nanotechnology of the level of control that may be achieved with LB devices will also be discussed.

Electrooptic proprieties of layer-by-layer films of a azobenzene containing copolymer

The methacrylic copolymer functionalized with the azo chromophore 4-[N-ethyl-N-(2-hydroxiethyl)]-amino-2′-chloro-4-nitroazobenzene (MMADR13), in its polyelectrolyte form, can be used to fabricate thin films by the layer-by-layer (LbL) technique just if one alternates this anionic polyelectrolyte with a cationic polyelectrolyte such as poly(allylamine hydrochloride) (PAH). Since PAH does not present any particular optical functionality, the main final film feature will came from the side chain DR13 azo-chromophore group due to its large nonlinear optical properties and photoisomerization capabilities. This work reports the electrooptic activity of MMADR13/DR13 LBL films, which arises from the high hiperpolarizability about the azo side chain group.