Page 315 - Advances in Textile Biotechnology

296 Advances in textile biotechnology

the collector. Deitzel et al. (2001) reported that there is an optimal range

of electric field strengths for a certain polymer/solvent system, as either too

weak or too strong will lead to the formation of beaded fibres. Increase in

the polymer flow rate results in an increase in fibre diameter and porosity

and, in addition, to bead formation owing to the inability of fibres to dry
completely before reaching the collector, as recently described by Sill and
von Recum (2008). The same report shows that capillary–collector distance

is less significant for the produced fibre size, but rather determines the fi nal
result as electrospinning or electrospraying. Upon an inadequate drying jet
the final result is small droplets instead of fi ne fi bres. The fi bre diameters

of the electrospuns are strongly affected by ambient parameters, such as
temperature and relative humidity. A change in temperature affects two
parameters with two opposing effects on the average diameter: solvent
evaporation rate which increases and viscosity of the polymer solution
which decreases with increase of the temperature. The final result will

depend on which parameter prevails in the system (De Vrieze et al., 2009).
The relative humidity leads to thinner or thicker fibres, only depending on

the chemical nature of the polymer (Tripatanasuwan et al., 2007).
Electrospinning has the potential to be further developed for large-scale

production of nanofibres. Modern electrospinning technology is able to

generate continuous fibres with diameters in the range of nanometers to a
few micrometers (Doshi and Reneker, 1995). Natural chitosan has poor

fibre-forming properties, which makes difficult the preparation of pure

chitosan nanofi bres. Trifluoroacetic acid (TFA) has excellent volatility,

which allows rapid solidification of the TFA–chitosan solution and prepara-

tion of chitosan nanofi bres (Ohkawa et al., 2004). Geng et al. (2005) showed

that pure chitosan nanofibres could also be prepared directly from the
solution in concentrated acetic acid. The increase of acetic acid concentra-
tion allows the strength of the electric field applied to be reduced resulting

in low-diameter, uniform and bead-free nanofibres. However, to reduce

toxicity concerns, chitosan electrospuns are usually prepared from tradi-
tional dilute aqueous acetic acid solvent using fi bre-forming facilitating
additives like poly(ethylene oxide) (PEO) (Bhattarai et al., 2005; Zhang et
al., 2008) or poly(vinyl alcohol) (PVA) (Li and Hsieh, 2006), resulting in
very fi ne nanofibrous structure (Fig. 12.4). Electrospuns of chitosan deriva-

tives such as hexanoyl (Neamnark et al., 2006) and carboxymethyl chitosan
(Du and Hsieh, 2008a) were also reported. Chitin electrospuns can be pre-
pared using hexafluoroisopropyl alcohol as a solvent (Noh et al., 2006).

Electrospinning from such a solution led to 400 nm diameter nanofi bres
(Fig. 12.5).
The versatility of the electrospinning technique is in the formation of

fibres with different morphologies and prepared from different materials.
Therefore, different polymers, blends, mixtures or precursors can be made

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