Myungwoong Kim

Interplay of surface chemical composition and film thickness on graphoepitaxial assembly of asymmetric block copolymers

We study the assembly of PMMA cylinder forming poly(styrene-block-methyl methacrylate) in three different geometries: planar surface, and chemically homogeneous and heterogeneous graphoepitaxy as a function of film thickness and surface chemical composition. On planar surfaces, perpendicular orientation can be achieved by controlling the thickness of the block copolymer (BCP) on surfaces weakly preferential to either block, while on nonpreferential surfaces, perpendicular orientation is independent of thickness within 0.8d0–1.8d0 range. In chemically homogeneous trenches, lateral alignment of domains is effectively controlled by having a chemical composition that is weakly preferential to either block, rather than perfectly nonpreferential wetting condition. By defining these boundary conditions, the nucleation of the domains can be induced primarily from the side walls rather than from the trench bottom. However, in chemically heterogeneous graphoepitaxial geometry, the BCP domains are aligned along the trench walls in both perfectly nonpreferential and weakly preferential conditions. In both the homogeneous and heterogeneous approaches, the thickness is critical for perpendicular orientation on weakly preferential surfaces. These findings affirm that graphoepitaxial assembly behavior of asymmetric BCP is governed by the interplay of surface wetting characteristics and the thickness of BCP.

A single-component inimer containing cross-linkable ultrathin polymer coating for dense polymer brush growth.

We have developed a highly versatile universal approach to grow polymer brushes from a variety of substrates with high grafting density by using a single-component system. We describe a random copolymer which consists of an inimer, p-(2-bromoisobutyloylmethyl)styrene (BiBMS), copolymerized with glycidyl methacrylate (GMA) synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Thermal cross-linking created a mat that was stable during long exposure in organic solvent even with sonication or during Soxhlet extraction. The absolute bromine density was determined via X-ray photoelectron spectroscopy (XPS) to be 1.86 ± 0.12 Br atoms/nm(3). The ratio of experimental density to calculated absolute initiator density suggests that ~25% of the bromine is lost during cross-linking. Surface-initiated ATRP (SI-ATRP) was used to grow PMMA brushes on the substrate with sacrificial initiator in solution. The brushes were characterized by ellipsometry, XPS, and atomic force microscopy (AFM) to determine thickness, composition, and homogeneity. By correlating the molecular weight of polymer grown in solution with the brush layer thickness, a high grafting density of 0.80 ± 0.06 chains/nm(2) was calculated. By synthesizing the copolymer before cross-linking on the substrate, this single-component approach avoids any issues with blend miscibility as might be present for a multicomponent curable mixture, while resulting in high chain density on a range of substrates.

A dual functional layer for block copolymer self-assembly and the growth of nanopatterned polymer brushes.

We present a versatile method for fabricating nanopatterned polymer brushes using a cross-linked thin film made from a random copolymer consisting of an inimer (p-(2-bromoisobutyloylmethyl)styrene), styrene, and glycidyl methacrylate (GMA). The amount of inimer was held constant at 20 or 30% while the relative amount of styrene to GMA was varied to induce perpendicular domain orientation in an overlying P(S-b-MMA) block copolymer (BCP) film for lamellar and cylindrical morphologies. A cylinder forming BCP blend with PMMA homopolymer was assembled to create a perpendicular hexagonal array of cylinders, which allowed access to a nanoporous template without the loss of initiator functionality. Surface-initiated ATRP of 2-hydroxyethyl methacrylate was conducted through the pores to generate a dense array of nanopatterned brushes. Alternatively, gold was deposited into the nanopores, and brushes were grown around the dots after removal of the template. This is the first example of combining the chemistry of nonpreferential surfaces with surface-initiated growth of polymer chains.

Electrospun tungsten trioxide nanofibers decorated with palladium oxide nanoparticles exhibiting enhanced photocatalytic activity

Tungsten trioxide (WO3) based nanofibers have many advantages as photocatalysts due to its band gap which fits with readily accessible light sources. We successfully fabricated novel palladium oxide (PdO) particles decorated WO3 nanofibers by electrospinning combined with chemical deposition processes, leading to improved photocatalytic efficiency for organic dye degradation up to 86.4%. Morphologies, elemental compositions and structural analyses confirmed the successful uniform decoration of PdO particles along WO3 nanofibers. Photodegradation of methylene blue as a model pollutant in water media was performed under UV and visible light in the presence of fabricated nanofibers as a photocatalyst. As a result, improved photocatalytic activity by PdO decoration was observed compared to commercially available WO3 NFs without PdO, attributed to its ability to hold excited electrons and increase surface area of NFs. This fibrous hybrid catalytic materials platform will open up a new and practical route and stimulate further research to improve photocatalytic performance.

Handspinning Enabled Highly Concentrated Carbon Nanotubes with Controlled Orientation in Nanofibers.

The novel method, handspinning (HS), was invented by mimicking commonly observed methods in our daily lives. The use of HS allows us to fabricate carbon nanotube-reinforced nanofibers (CNT-reinforced nanofibers) by addressing three significant challenges: (i) the difficulty of forming nanofibers at high concentrations of CNTs, (ii) aggregation of the CNTs, and (iii) control of the orientation of the CNTs. The handspun nanofibers showed better physical properties than fibers fabricated by conventional methods, such as electrospinning. Handspun nanofibers retain a larger amount of CNTs than electrospun nanofibers, and the CNTs are easily aligned uniaxially. We attributed these improvements provided by the HS process to simple mechanical stretching force, which allows for orienting the nanofillers along with the force direction without agglomeration, leading to increased contact area between the CNTs and the polymer matrix, thereby providing enhanced interactions. HS is a simple and straightforward method as it does not require an electric field, and, hence, any kinds of polymers and solvents can be applicable. Furthermore, it is feasible to retain a large amount of various nanofillers in the fibers to enhance their physical and chemical properties. Therefore, HS provides an effective pathway to create new types of reinforced nanofibers with outstanding properties.

Effect of graphene incorporation in carbon nanofiber decorated with TiO2 for photoanode applications

The photovoltaic performance of dye-sensitized solar cells (DSSCs) using a photoanode fabricated with graphene incorporated carbon nanofibers with a TiO2 layer on their surfaces is reported. The composite nanofibers were prepared through a process consisting of electrospinning and sol–gel process. The distribution of TiO2 particles on the surface of the graphene-incorporated carbon nanofiber was characterized by scanning electron microscopy (SEM), and transmission electron microscopy (TEM). From the microscopic studies, we found that graphene plays a significant role to form TiO2 particles on carbonaceous materials. Further studies on chemical composition using X-ray photoelectron spectroscopy reveal that different oxidation states of Ti in the synthesized titanium oxide were achieved by incorporating graphene in the nanofibers. Furthermore, TiO2 synthetic conditions in the sol–gel process largely affected photovoltaic performance, particularly short circuit current; acidic conditions for the sol–gel process were more effective compared to neutral conditions. The incorporated graphene facilitates conducting charge carriers in TiO2 by coordination with carbon, and increasing the adsorption of dye molecules owing to homogeneous distribution of TiO2 along the nanofibers. This study further highlights the advantages of hybridizing different materials with 1D nanofiber geometry, offering a promising route to improving the resulting efficiency in light harvesting applications.

Transfer of pre-assembled block copolymer thin film to nanopattern unconventional substrates.

In this work, we demonstrate that a preassembled block copolymer (BCP) thin film can be floated, transferred, and utilized to effectively nanopattern unconventional substrates. As target substrates, we chose Cu foil and graphene/Cu foil since they cannot be nanopatterned via conventional processes due to the high surface roughness and susceptibility to harsh processing chemicals and etchants. Perpendicular hexagonal PMMA cylinder arrays in diblock copolymer poly(styrene-block-methyl methacrylate) [P(S-b-MMA)] thin films were preassembled on sacrificial SiO2/Si substrates. The BCP thin film was floated at the air/water interface off of a SiO2/Si substrate and then collected with the target substrate, leading to well-defined nanoporous PS templates on these uneven surfaces. We further show that the nanoporous template can be used for a subtractive process to fabricate nanoperforated graphene on Cu foil in sub-20 nm dimension, and for an additive process to create aluminum oxide nanodot arrays without any polymeric residues or use of harsh chemicals and etchants.

The Chemical Deposition Method for the Decoration of Palladium Particles on Carbon Nanofibers with Rapid Conductivity Changes

Palladium (Pd) metal is well-known for hydrogen sensing material due to its high sensitivity and selectivity toward hydrogen, and is able to detect hydrogen at near room temperature. In this work, palladium-doped carbon nanofibers (Pd/CNFs) were successfully produced in a facile manner via electrospinning. Well-organized and uniformly distributed Pd was observed in microscopic images of the resultant nanofibers. Hydrogen causes an increment in the volume of Pd due to the ability of hydrogen atoms to occupy the octahedral interstitial positions within its face centered cubic lattice structure, resulting in the resistance transition of Pd/CNFs. The resistance variation was around 400%, and it responded rapidly within 1 min, even in 5% hydrogen atmosphere conditions at room temperature. This fibrous hybrid material platform will open a new and practical route and stimulate further researches on the development of hydrogen sensing materials with rapid response, even to low concentrations of hydrogen in an atmosphere.

Enhancement of mechanical properties of polymeric nanofibers by controlling crystallization behavior using a simple freezing/thawing process

We demonstrate the modulation of physical and mechanical properties by controlling crystallinity in cross-linked poly(vinyl alcohol) (PVA) nanofibers using a simple and straightforward freezing/thawing process. PVA chains in the cross-linked network are swollen and rearrange through the freezing/thawing process, resulting in the formation of more hydrogen bonding and hence, a higher degree of crystallization in the nanofibers compared to pristine electrospun nanofibers. Quantitative analyses with X-ray diffraction and FT-IR studies confirm increases of the crystallite diameter from 24.2 A to 28.3 A and the degree of crystallinity from 23.5% to 43.6%, respectively. Also, we found that the increase of crystallinity led to a dramatic enhancement of the mechanical properties: the tensile strength was increased up to ∼165% compared to pristine nanofibers, while the elongation at break was decreased. This straightforward and facile process will enable us to precisely control crystallinity, and also to fine-tune the physical properties of polymeric nanofibers; consequently, the method will broaden the application of polymeric nanofibers.

Readily Functionalizable and Stabilizable Polymeric Particles with Controlled Size and Morphology by Electrospray

Electrospraying is an effective and facile technique for the production of micro- or nanoparticles with tailored sizes, shapes, morphologies, and microstructures. We synthesized functionalizable poly(styrene-random-glycidyl methacrylate) copolymers and used them to fabricate microparticles via the electrospray technique. The sizes and morphologies of the electrosprayed particles are controlled by altering the process parameters (feed rate and applied voltage), and the composition and thermodynamic properties of the polymer (i.e., compatibility of the polymer with the solvent). We further investigated modifying the surfaces of the electrosprayed particles with 3-mercaptopropionic acid by a simple and efficient thiol-epoxy “click” reaction as a proof-of-concept demonstration that desired functionality can be introduced onto the surfaces of these particles; the outcome was confirmed by various spectroscopic techniques. In addition, the epoxides within the particles easily undergo crosslinking reactions, enabling further effective particle stabilization. The results reveal that the structure and properties of the polymer can be used to fine-tune the structural parameters of the electrosprayed particles, such as their sizes and morphologies, which opens up the possibility of imparting a variety of desired chemical functionalities into the structures of stable organic materials via post-electrospray modification processes.