Jay Hoon Park

Confined assembly of asymmetric block-copolymer nanofibers via multiaxial jet electrospinning.

Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene-b-isoprene), PS-b-PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post-fabrication annealing of the fibers to obtain equilibrium self-assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self-assembled structures. Various confined assemblies such as co-existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self-assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self-assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block-copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS-b-PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS-b-PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self-assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high-end applications such as drug delivery.

Cylindrically confined assembly of asymmetrical block copolymers with and without nanoparticles

Our recent experimental study on electrospinning of block copolymer (BCP)–nanoparticle (NP) nanocomposites has revealed the formation of unique self-assembling structures in submicron scale fibers. In this paper, we use coarse-grained molecular dynamics (MD) simulations to investigate the effect of cylindrical confinement on self-assembly of model asymmetrical BCPs with and without NPs with the aim to understand and control our experimentally found structures. First, the effects of the ratio of the cylindrical confinement diameter to the BCP domain spacing, D/L0, the total polymer chain length, and the polymer–wall interactions on the confined assembly were thoroughly investigated. We examined the core assembled structures along the cylinder axis and constructed a phase diagram for asymmetrical BCP. The structures are categorized by three features: the number of layers of domains, radially interconnected domains, and the number of axially perforated domains. Secondly, NPs with selective attraction towards the (i) minor domain (A) and (ii) major domain (B) were incorporated into asymmetric BCPs. We found that swelling of either domain caused by the inclusion of selective NPs yields different morphologies when compared with a pure BCP with the same effective volume ratio. Interestingly, the effect of confinement on nanoparticle placement was prominently seen if nanoparticles were selectively placed into the minor domain that preferentially wets the confining wall. Finally, the predicted BCP–NP structures are validated by those observed in electrospun BCP–NP nanofibers. The current study demonstrates that coarse-grained MD simulation can offer a useful tool to elucidate, predict and tailor self-assembled structures in electrospun BCP–NP nanofibers.

Preparation and Characterization of Amphiphilic Triblock Terpolymer-Based Nanofibers as Antifouling Biomaterials

Antifouling surfaces are critical for the good performance of functional materials in various applications including water filtration, medical implants, and biosensors. In this study, we synthesized amphiphilic triblock terpolymers (tri-BCPs, coded as KB) and fabricated amphiphilic nanofibers by electrospinning of solutions prepared by mixing the KB with poly(lactic acid) (PLA) polymer. The resulting fibers with amphiphilic polymer groups exhibited superior antifouling performance to the fibers without such groups. The adsorption of bovine serum albumin (BSA) on the amphiphilic fibers was about 10-fold less than that on the control surfaces from PLA and PET fibers. With the increase of the KB content in the amphiphilic fibers, the resistance to adsorption of BSA was increased. BSA was released more easily from the surface of the amphiphilic fibers than from the surface of hydrophobic PLA or PET fibers. We have also investigated the structural conformation of KB in fibers before and after annealing by contact angle measurements, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and coarse-grained molecular dynamics (CGMD) simulation to probe the effect of amphiphilic chain conformation on antifouling. The results reveal that the amphiphilic KB was evenly distributed within as-spun hybrid fibers, while migrated toward the core from the fiber surface during thermal treatment, leading to the reduction in antifouling. This suggests that the antifouling effect of the amphiphilic fibers is greatly influenced by the arrangement of amphiphilic groups in the fibers.

Facile Synthesis of Porous Silicon Nanofibers by Magnesium Reduction for Application in Lithium Ion Batteries

We report a facile fabrication of porous silicon nanofibers by a simple three-stage procedure. Polymer/silicon precursor composite nanofibers are first fabricated by electrospinning, a water-based spinning dope, which undergoes subsequent heat treatment and then reduction using magnesium to be converted into porous silicon nanofibers. The porous silicon nanofibers are coated with a graphene by using a plasma-enhanced chemical vapor deposition for use as an anode material of lithium ion batteries. The porous silicon nanofibers can be mass-produced by a simple and solvent-free method, which uses an environmental-friendly polymer solution. The graphene-coated silicon nanofibers show an improved cycling performance of a capacity retention than the pure silicon nanofibers due to the suppression of the volume change and the increase of electric conductivity by the graphene.

Controlling the dispersion and orientation of nanorods in polymer melt under shear: Coarse-grained molecular dynamics simulation study

Incorporation of nanorods (NRs) into a polymer matrix can greatly enhance the material properties, but the aggregation of NRs prevents the full realization of their potential. Using coarse-grained molecular dynamics simulation with the dissipative particle dynamics thermostat, we have systematically examined how key material and processing parameters, such as aspect ratio, particle diameter, rigidity and concentration of NR, polymer chain length, and shear rate can influence the placement and orientation of the self-aggregating NRs in a model polymer melt under shear. When compared with nanoparticles (NPs), the NRs tend to aggregate more severely even under strong shear flow. To improve the dispersion of NRs within the polymer matrix under a given flow condition, we incorporated additional NPs with selective interactions into polymer/NR composites, demonstrating that the current mesoscale simulation study offers insights on how to control the dispersion and orientation of NRs in polymer under shear flow.