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 .

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.

By embedding "dilute" gold nanoparticles in single polystyrene thin films as "markers", we probe the local viscosity of the free surface at temperatures far above the glass transition temperature (T(g)). The technique used was x-ray photon correlation spectroscopy with resonance-enhanced x-ray scattering. The results clearly showed the surface viscosity is about 30% lower than the rest of the film. We found that this reduction is strongly associated with chain entanglements at the free surface rather than the reduction in T(g).

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 lg 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 aC-domains was used as a coating or was incubated with intact fibrinogen-coated plates, or when the latter plates were sequentially incubated with antiAa529-539 mAb and intact fibrinogen. When an anti-Aa241-476 mAb was used instead, fine, long fibers formed. Similarly, sequential incubations of fibrinogen-coated plates with recombinant aC-domain (Aa392-610 fragment) or aC-connector (Aa221-372 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 aC-domains.

No abstract

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.

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.

We have shown that the addition of certain nanoparticles mixtures can enhance flame retardancy to a greater degree than the addition of either of the nanoparticles alone. The effect is particularly efficient in polymer blends, where more variables need to be considered in the flame behavior. We studied PS/ PMMA blends and the respective homopolymers. In this paper we focused on the combination of multiwall carbon nanoutubes (MWCNTs) with clays. We found that the flame retardant (FR) particles segregate to the MWCNTs preferentially, thereby allowing the clays to segregate to the blend interfaces. In this manner both phase stabilization and good dispersion can be achieved. Both long (0.5-40 μm) and short (1-2 μm) MWCNTs were studied and the results indicated that the s-MWCNTs produced the superior flame retardant properties. Addition of all types of nanoparticles, including the standard FR formulations, decreased the time to ignition. On the other hand, the combination of s-MWCNT and clay significantly reduced the heat release rate (HRR) and mass loss rate (MLR) relative to compounds with only one type of nanoparticles. Electron microscopy images of the nanocomposites and the chars showed that after heating the s-MWCNTs were able to diffuse and form a distinct phase in the nanocomposite. Exclusion of the clays allowed the s-MWCNTs to achieve better physical contacts, thereby improving the thermal conductivity. In contrast, the l-MWCNTs were entangled and therefore unable to move. The clays formed a barrier between the tube contacts, resulting in an increase of the specific heat and the HRR and MLR relative to the unfilled or the compound with only clays. We therefore conclude that one must consider the organization of nanoparticles, as well as their chemical nature when designing flame retardant nanocomposites, since specific synergies may be established which can reduce the overall concentration of fillers.