Construction And Properties Of Models Of Thin Fibers

The construction of the model of a nanofiber28 commences with an equilibrated model for a free-standing thin film. The film is continuous along the x and y axes, and it is exposed to a vacuum along the z axis. The periodicity in one of the directions along which the thin films is continuous, say the y direction, is increased by a large amount, so that the parent chains can no longer interact with their images along this direction. After a new equilibration, the model settles down into a thin fiber that is oriented along the x axis, and exposed to a vacuum along the y and z axes.

Cross-sections of the fibers perpendicular to the fiber axis are nearly circular. Models have been constructed with cross-sections that have diameters of 7-8 nm. They are comparable in thickness with some of the nanofibers prepared by the electrospinning technique, which can be as thin as 3 nm (D. H. Reneker, personal communication). If the fiber has a thickness greater than about 4 nm, it recovers bulk density in its interior. Radial density profiles perpendicular to the fiber axis can be fitted to a hyperbolic tangent function, Equation (1). For fibers with diameters in the range 5-8 nm, the correlation lengths, are about 0.6 nm, which is close to the value obtained with the models of the free-standing thin films. The end beads are enriched in the surface region, as was also the case with the free-standing thin films. The anisotropy of the chord vectors, as assessed by the order parameter, S, is also similar to the result obtained with the free-standing thin films.

Surface energies, y, are not easily assessed for the models of the fibers. The excess energy associated with the surface is easily evaluated, but there is an ambiguity in the definition of the surface area. For the free-standing thin films, the surface area for that portion of the film in the periodic box is 2LxLy, which is well defined. In contrast, the surface area of that portion of the fiber in the periodic box is n dL where dyz is the diameter in the direction perpendicular to the fiber axis. This diameter is well-defined only when the density profile normal to the fiber axis is a step function. The actual radial density profile is instead described by Equation (1), with E, close to 0.6 nm. After allowing for the ambiguity in the definition of the surface area for the fiber, it appears that the values of y may be similar in the thin films and in the fibers.

For amorphous fibers composed of C100H202 and with thicknesses of 5-8 nm, the anisotropies of the individual chains, as measured by the principal moments of the radius of gyration tensor, are comparable with those expected for chains in the bulk. However, the radius of gyration tensors tend to be oriented within the fiber, with the nature of the orientation depending on the distance of the center of mass from the axis of the fiber. Chains with their centers of mass close to the fiber axis tend to have the largest component of their radius of gyration tensor aligned with the fiber axis.

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