Civil Engineering Reference
In-Depth Information
Figure 8.16
Pictures of broken i bers during mechanical testing. a) SEM picture taken at er the tensile
test; it is possible to observe that the i ber has a rigid outer casing and a sot inner core. b) POM picture
of a cracked i ber where the broken outer casing exposes the sot core that prevents the i ber from
splitting in two. c) SEM picture of a broken i ber where it is possible to observe that the sot core retracts
at er separation [103]. Reproduced by permission of the Royal Society of Chemistry.
breaking of the rigid outer layer. In this case, the yield stress of the tested i bers corre-
sponds to the mechanical strength of the exterior layer.
h e results obtained from SEM and AFM measurements show that the APC i bers
electrospun from the liquid crystal phase behave like a composite i ber where the phases
present are constituted by the same material.
h e interpretation of the double-layer structure of the electrospun APC i bers is
based on the fact that APC solutions subjected to shear show a transition from choles-
teric to nematic [104]. Calculation of the Reynolds number shows that the l ow inside
the needle is laminar. h e velocity proi le is parabolic and, since the solution becomes
nematic, the director should align parallel to walls; in the center of the l ow, away from
the walls of needle, there is some reorientation due to the l ow and the director makes
an angle with the l ow direction. As the solution exits the needle, a rapid evaporation
of the solvent freezes the orientation of the director in the dif erent areas of the cross-
section of the i ber.
Perhaps the most remarkable result reported on the electrospinning, from the liquid
crystalline phase, for suspended APC i bers is their capability to mimic shapes that
normally are observed in the tendrils of climbing plants. If the electrodes on which they
are suspended are moved closer together, helical structures like helices and spirals can
be observed; these are represented in Figure 8.17.
Comparison of the pictures obtained during SEM observation of APC electrospun
suspended i bers and photographs taken of the tendrils of the climbing plant
Passil ora
edulis
show that the i bers can reproduce even the more complex shapes found on the
tendrils (see Figure 8.18).
