Biomedical subjects

Yanchun Han

Publications and source records attributed to Yanchun Han.

14 recordsLinked to original sources

PubMed2006

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.

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PubMed2006

Wettability of zinc oxide surfaces with controllable structures.

Zinc oxide (ZnO) surfaces with controllable structures (i.e, microstructure, nanostructure, and micronanobinary structure) have been created by controlling pH at < 4 or > 10.5 in the Zn(gray) + H2O2 reaction. The resulting surface shows superhydrophobicity. It is found that the water contact angle (CA) of the surface with micronanobinary structure is greater than that of nanostructure and that of nanostructure is greater than that of the microstructure. Theoretical analysis is completely in agreement with the experimental results.

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PubMed2006

Optical intensity gradient by colloidal photonic crystals with a graded thickness distribution.

Gradient colloidal crystals with a thickness gradient were prepared by the vertical deposition technique with vertically graded concentration suspensions. The thickness of the gradient colloidal crystal gradually changes at different positions along the specific gradient direction of the crystal. The thickness gradient was determined by the concentration gradient, depending on the initial colloidal concentration and the settling time. The optical transmission intensity at the dip wavelength can be tuned by changing the thickness of the colloidal crystals. The gradient colloidal crystals lead to a gradient of optical intensity at the dip in transmission light. The gradient of optical intensity at the dip increases as the thickness gradient of the colloidal crystal increases.

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PubMed2005

Polymer surfaces with reversibly switchable ordered morphology.

Honeycomb macroporous films fabricated by the "breath figures" method were composed of poly2-vinylpyridine (P2VP) distributed in the holes of polystyrene (PS). The porous films exhibited reversible behavior responding to water and different solvent vapors. When the porous film was treated with water, the honeycomb pattern would change to the hexagonal islandlike pattern. Once heated to remove the water, the honeycomb pattern emerged again. When the porous film was exposed to different solvent vapors, the same reversible process appeared. Carbon disulfide (CS(2)), toluene (TOL), and tetrahydrofuran (THF) solvent vapors induced the honeycomb pattern into the ordered islandlike pattern, and ethanol, chloroform, methyl ethyl ketone (MEK), and dimethylformamide (DMF) solvent vapors made the islandlike pattern come back to the honeycomb pattern. The hygroscopic property of P2VP and the polymer-solvent interaction are the driving force for the reversibly switchable morphology. The appropriate control of the hole depth is very crucial in determining the reversible changes.

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PubMed2005

Honeycomb pattern formation via polystyrene/poly(2-vinylpyridine) phase separation.

The surface morphologies and properties of polystyrene (PS)/poly(2-vinylpyridine) (PVP) blend films cast on the mica substrate from ethylbenzene solution were investigated upon controlling different weight ratios and solvent evaporation rates. A near-honeycomblike surface morphology of the PS/PVP blend film formed under controlling the solvent evaporation rate due to the effect of Marangoni-Benard convection. The results of static water contact angles, X-ray photoelectron spectroscopy, solvent selective etching, and treatment by water illustrated that the near-honeycomblike structures on the surface of PS/PVP blend films were different for different weight ratios of PS and PVP. After treatment with water for several minutes, PVP islands-like structure emerged in the holes of the film for a PS/PVP weight ratio of 4/1, and a quasihexagonal arrangement of alternate big and small PVP droplets emerged on the top layer of the film for a PS/PVP weight ratio of 7/1. The formation mechanisms of different surface structures and their response behaviors to water were discussed.

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PubMed2005

Molecular motions of different scales at thin polystyrene film surface by lateral force microscopy.

Lateral force microscopy (LFM) was used to probe the molecular motions at thin polystyrene film surface. The effect of the applied load on the LFM measurements was investigated by presenting both the LFM results and the surface morphology after several scans over the same area. Depending on the loads, the scanning can be nonperturbative (without alternating the surface morphology) or perturbative (patterning the surface). Temperature-dependent LFM measurements were conducted in order to determine the apparent transitions at the surface. Perturbative scans under high loads (e.g., 150 nN) witnessed that the apparent transitions shifted to low temperatures with an increasing scan rate, while the transitions behaved oppositely under lower loads (1, 10, and 20 nN). The heating effect is suggested to account for the behavior under high loads. According to our results from nonperturbative LFM, the apparent glass transition temperature (T(g)s) is more than 10 K lower than the bulk value. Moreover, rate-dependent LFM measurements were performed under 1 nN in order to detect the surface molecular motions. Time-temperature superposition yields a master curve exhibiting three apparent relaxation peaks. The molecular motions at the surface are discussed on the context of the coupling model.

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PubMed2005

Fabrication of gradient colloidal topography.

A rather simple but yet effective way to self-assemble polystyrene (PS) beads in gradient colloidal crystal topography is proposed. The PS bead concentration, solvent, and substrate have a big effect on the colloidal crystal topography. Whether the gradient-shaped crystals can form or not depends on the Bond number [Bo; the ratio of gravitational potential energy (G) to adhesive energy (E(a)), or gravitational to capillary forces]. When Bo < 1, that is, the capillary force dominates over the gravitational force, the liquid meniscus is stable. The gradient-shaped crystals can form. Otherwise, PS beads form a uniform multilayer structure.

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PubMed2005

Controlling the size of nanostructures in thin films via blending of block copolymers and homopolymers.

The effects of molecular weight and concentration of poly (methyl methacrylate) (PMMA) homopolymer or symmetric short polystyrene-block-poly (methyl methacrylate) (PS-b-PMMA) diblock copolymer on the size of the nanostructures of its blends with symmetric long PS-b-PMMA diblock copolymer have been investigated by atomic force microscopy. By careful controlling of the film thickness, solvent selectivity, and annealing time, PMMA cylindrical microdomains oriented normal to the film surface were obtained in all thin films. With the addition of both low- and high-molecular-weight PMMA homopolymers, the cylindrical domain sizes increased although it was less obvious for the lower molecular weight homopolymer. In contrast to the homopolymer, adding the short chain diblock copolymer resulted in a decrease in the cylindrical domain size, which was ascribed to the reduction of the interfacial tension and increase in the stretching energy.

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PubMed2005

Ordered honeycomb-structured gold nanoparticle films with changeable pore morphology: from circle to ellipse.

Two-dimensionally ordered honeycomb structures have been prepared on dodecanethiol-capped gold nanoparticle films by blowing moist air across the surface of the nanoparticle solution. The pore morphology can be altered from circle to ellipse with tunable aspect ratios by carefully controlling the direction and velocity of airflow. The formation mechanisms of different surface morphologies have been discussed in terms of the surface and interfacial tension.

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PubMed2005

Fabrication gradient surfaces by changing polystyrene microsphere topography.

A rather simple yet effective way to fabricate gradient surfaces by changing polystyrene (PS) microsphere topography in a temperature gradient field is proposed. The wettability of the film has an obvious gradient change from 88.7 to 148.1 degrees . The changes of the PS microsphere topography are believed to be responsible for the changing of the wettability.

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PubMed2004

Formation and photoluminescence of silver nanoparticles stabilized by a two-armed polymer with a crown ether core.

Silver nanoparticles were synthesized by the use of a two-armed polymer with a crown ether core [poly(styrene)]-dibenzo-18-crown-6-[poly(styrene)] based on the flexibility of the polymer chains and the complex effect of crown ether with Ag(+) and Ag. The size of silver nanoparticles could be tailored by controlling the initial concentrations of the polymer and Ag(+), and the molecular weight of the polymer. The emission of silver nanoparticles was blue-shifted, and the intensity of the photoluminescence of silver nanoparticles stabilized by the polymer was significantly increased due to the complex effect between the crown ether embedded in the polymer and the silver nanoparticles.

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PubMed2004

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.

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PubMed2004

Ordered droplet formation by thin polymer film dewetting on a stripe-patterned substrate.

The dewetting process of thin polystyrene (PS) film on flat and stripe-patterned substrates is presented. Different dewetting processes were observed when the thin PS films annealed at above the glass transition temperature on these different kinds of substrates. The final dewetting on the flat substrate led to formation of polygonal liquid droplets, while on the stripe-patterned substrate, the droplets were observed to align at the centers of the stripes. A possible explanation for the dewetting process on the stripe-patterned substrate is proposed.

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PubMed2003

Micropatterning of organic semiconductor microcrystalline materials and OFET fabrication by "hot lift off".

Considerable progress toward the development of electronic devices that rely on organic semiconductors as the active material component has been made in recent years. The key step for realization of the advanced organic electronic, or optical, device is the ability to micropattern different kinds of electronic materials, such as organic semiconductor/conducting materials, over large areas with micrometer-sized resolution. Here we demonstrate a simple and direct method for micropatterning small-molecule microcrystalline films using an epoxy stamp. The "hot lift off" method is highly selective, creating patterns with high resolution and relies on straightforwardly tailoring the adhesive properties between the epoxy and the film. This process is well suited for patterning many types of materials.

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