Jaseung Koo

Conducting MWNT/poly(vinyl acetate) composite nanofibres by electrospinning

Electrospinning is a relatively simple and versatile method to produce polymer nanofibres and their composites. In this work, functionalized multiwalled carbon nanotubes (f-MWNTs) were used for the fabrication of conducting nanocomposite fibres, in comparison with the composite nanofibres made of unfunctionalized MWNTs (u-MWNTs). Our results showed that the addition of f-MWNTs could improve the dispersion of carbon nanotubes in the polymer solution and therefore result in composite nanofibres with uniform diameters by electrospinning. Alignment of the composite nanofibres was achieved by using a rotating drum as the collector. F-MWNTs were found to align parallel to the axis direction of the nanofibres. DC electrical properties of a single composite fibre were investigated at room temperature as well as cryogenic states (100-300 K). An electrical percolation phenomenon was observed for nanofibres with different mass fractions of MWNTs. It was shown that the conductivity of the material could be significantly improved above the percolation threshold. The conductivity could be of several orders of magnitude higher than the pure PVAc.

Confinement-induced super strong PS/MWNT composite nanofibers

Shear Modulation Force Microscopy (SMFM) together with the Atomic Force Microscopy (AFM) based three-point bending technique were used to measure the mechanical properties of electrospun polymers and polymer nanocomposite fibers. Both techniques showed that the moduli of the fibers increased significantly with decreasing fiber diameter. We attributed this enhancement to the orientation of polymer chains which occurs during the electrospinning process. We then predicted, and confirmed experimentally, that the phenomenon scales with Rg rather than with the absolute fiber diameter and can propagate radially for large distances (~20Rg) into the fiber interior. The inclusion of nanotubes into the fibers further enhanced the orientation by introducing additional surfaces. The additional increase in modulus (more than an order of magnitude) could then be explained by the same model and scaled on a universal curve.

Evaluation of fibrinogen self-assembly: role of its αC region.

Summary.  Background: Exposure of cryptic, functional sites on fibrinogen upon its adsorption to hydrophobic surfaces of biomaterials has been linked to an inflammatory response and fibrosis. Such adsorption also induces ordered fibrinogen aggregation which is poorly understood. Objective: To investigate hydrophobic surface‐induced fibrinogen aggregation. Methods: Contact and lateral force scanning probe microscopy, yielding topography, image dimensions and fiber elastic modulus measurements were used along with transmission and scanning electron microscopy. Fibrinogen aggregation was induced under non‐enzymatic conditions by adsorption on a trioctyl‐surface monolayer (trioctylmethylamine) grafted onto silica clay plates. Results: A more than one molecule thick coating was generated by adsorption on the plate from 100 to 200 μg mL−1 fibrinogen solutions, and three‐dimensional networks formed from 4 mg mL−1 fibrinogen incubated with uncoated or fibrinogen‐coated plates. Fibrils appeared laterally assembled into branching and overlapping fibers whose heights from the surface ranged from approximately 3 to 740 nm. The elastic modulus of fibrinogen fibers was 1.55 MPa. No fibrils formed when fibrinogen lacking αC‐domains was used as a coating or was incubated with intact fibrinogen‐coated plates, or when the latter plates were sequentially incubated with anti‐Aα529–539 mAb and intact fibrinogen. When an anti‐Aα241–476 mAb was used instead, fine, long fibers formed. Similarly, sequential incubations of fibrinogen‐coated plates with recombinant αC‐domain (Aα392–610 fragment) or αC‐connector (Aα221–372 fragment) and fibrinogen resulted in distinctly fine fiber networks. Conclusions: Adsorption‐induced fibrinogen self‐assembly is initiated by a more than one molecule‐thick surface layer and eventuates in three‐dimensional networks whose formation requires fibrinogen with intact αC‐domains.

Morphology Control of Surfactant-Assisted Graphene Oxide Films at the Liquid–Gas Interface

Control of a two-dimensional (2D) structure of assembled graphene oxide (GO) sheets is highly desirable for fundamental research and potential applications of graphene devices. We show that an alkylamine surfactant, i.e., octadecylamine (ODA), Langmuir monolayer can be utilized as a template for adsorbing highly hydrophilic GO sheets in an aqueous subphase at the liquid-gas interface. The densely packed 2-D monolayer of such complex films was obtained on arbitrary substrates by applying Langmuir-Schaefer or Langmuir-Blodgett technique. Morphology control of GO sheets was also achieved upon compression by tuning the amount of spread ODA molecules. We found that ODA surfactant monolayers prevent GO sheets from sliding, resulting in formation of wrinkling rather than overlapping at the liquid-gas interface during the compression. The morphology structures did not change after a graphitization procedure of chemical hydrazine reduction and thermal annealing treatments. Since morphologies of graphene films are closely correlated to the performance of graphene-based materials, the technique employed in this study can provide a route for applications requiring wrinkled graphenes, ranging from nanoelectronic devices to energy storage materials, such as supercapacitors and fuel cell electrodes.

Control of Anti-Thrombogenic Properties: Surface-Induced Self-Assembly of Fibrinogen Fibers

Wound healing is a complex process initiated by the formation of fibrin fibers and endothelialization. Normally, this process is triggered in a wound by thrombin cleavage of fibrinopeptides on fibrinogen molecules, which allows them to self spontaneously-assemble into large fibers that provide the support structure of the clot and promote healing. We have found that the fibrous structures can also form without thrombin on most polymer or metal surfaces, including those commonly used for stents. We show that the relatively hydrophobic E and D regions of the fibrinogen molecule are adsorbed on these surfaces, exposing the αC domains, which in turn results in the formation of large fiber structures that promote endothelial cell adhesion. We show that the entire process can be suppressed when stents or other substrates are coated with polymers that are functionalized to bind the αC domains, leading to the development of potentially nonthrombogenic implant materials.

Long-Term Cyclability of Electrochromic Poly(3-hexyl thiophene) Films Modified by Surfactant-Assisted Graphene Oxide Layers

Organic electrochromic (EC) materials are generally known to be electrochemically unstable during the ion intercalation/deintercalation process. One effective method to stabilize them is incorporating graphene derivatives in the polymer matrix, thereby creating strong interaction between graphene derivatives and polymer. However, previous studies are limited to specific polymers and bulk-blended systems, such as mixing the polymer with graphene derivatives. In this study, we developed a polymer-graphene derivative complex with the chemical assistance of a surfactant (octadecylamine, ODA). Graphene oxide (GO) was introduced as a protective layer on the electrochromic poly(3-hexyl thiophene) (P3HT) films by the Langmuir-Schaefer method. The deposition of the GO-ODA protective layer with high coverage was confirmed by atomic force microscopy and high-resolution X-ray reflectivity. The strong interactions between GO-ODA and P3HT were examined with UV-vis spectrophotometry and X-ray photoelectron spectroscopy. Electrochemical and electrochromic investigations revealed that the GO-ODA layer greatly improved the long-term cyclability of the P3HT film. These findings imply that the GO-ODA complex can significantly stabilize the EC cycling, due to its strong interaction with the P3HT film.

Perpendicular Orientation of Diblock Copolymers Induced by Confinement between Graphene Oxide Sheets

We have studied an orientation structure of self-assembled block copolymers (dPS-b-PMMA) of deuterated polystyrene (dPS) and poly(methyl methacrylate) (PMMA) confined between graphene oxide (GO) surfaces. The results of combination techniques, such as neutron reflectivity, time-of-flight secondary-ion mass spectrometry, grazing-incidence small-angle X-ray scattering, and scanning electron microscopy, show that self-assembled domains of the block copolymers in thin films near the GO sheets are oriented perpendicular to the surface of the GO monolayers, in contrast to the horizontal lamellar structure of the copolymer thin film in the absence of the GO monolayers. This is due to the amphiphilic nature of the GO, which leads to a nonpreferential interaction of both dPS and PMMA blocks. Double-sided confinement with the GO monolayers further extends the ordering behavior of the dPS-b-PMMA thin films. Continuous vertical orientation of the block copolymer thin films is also obtained in the presence of alternating GO layers within thick copolymer films.