Civil Engineering Reference
In-Depth Information
material [109] . h e concept of mono-material composites has been evaluated in all-
polymer composites, e.g., all-polyethylene and all-polypropylene, in which the selec-
tive surface melting of the oriented i bers recrystallizes to form a matrix that binds
the i bers together, resulting in a fully recyclable, "self-reinforced" polymeric material
[110]. Because cellulose is known not to melt, the all-cellulose composites have been
created by using dissolved cellulose as a matrix; as a solution or highly swollen i ber
surfaces, followed by precipitation [109]. In all-cellulose composites, the dissolved cel-
lulose is expected to improve the adhesion between the matrix and i bers by the inter-
dif usion of the cellulose molecules across the interface, while maintaining the highly
crystalline cellulose I core of the unaf ected i ber acting as reinforcement in the com-
posite due to its high modulus [109]. Few studies are currently focused on altering the
properties of cellulose-based electrospun i bers via chemical or physical modii cation
to create new biocomposites. Great interest also exists in disintegrating cellulose i bers
into aggregated nanoscale cellulose i brils to exploit the combination of extended-chain
molecules, the high degree of order (crystallinity) and the high cellulose molar mass of
the aggregated cellulose i brils.
Viswanathan
et al.
[99] prepared cellulose-heparin composite i bers from non-
volatile room temperature ionic liquid solvents by electrospinning. h e RTILs are
extracted from the biopolymer i ber at er the i ber formation using a co-solvent.
Micron- to nanometer-sized branched i bers were obtained from 10% (w/w) concen-
tration of polysaccharide biopolymer in RTIL solution. Cellulose-heparin composite
i bers showed anticoagulant activity, demonstrating that the bioactivity of heparin
remained unaf ected even on exposure to a high voltage involved in electrospinning.
A 10% (w/w) solution containing cellulose (MW 5 800 000) in 1-butyl-3-methylimid-
azolium chloride and heparin ((MW 12 500) in 1-ethyl-3-methylimidazolium benzoate
were prepared and subjected to electrospinning and the morphology of the composite
i bers compared with pristine cellulose b ers. h e i bers formed were directly received
in ethanol that can completely dissolve both the RTILs used in the dissolution, but
neither of the polysaccharides are ethanol soluble. Hence, as the i bers formed, the eth-
anol extractively removed the RTIL solvents, af ording pure polysaccharide i bers. It
was found that morphology and diameter distribution of electrospun i bers depend on
solution and/or spinning parameters. h e high viscosity and nonvolatility of the solvent
resulted in micorn-sized and branched i bers. h e average i ber diameter for the cel-
lulose/heparin composite was larger than that for pristine cellulose, mainly due to the
higher viscosity. h e surface roughness of the cellulose/heparin composite i bers was
also much higher than that of the cellulose-only i bers as shown in the (Figure 12.8).
h is dif erence may be due to the phase separation of cellulose and heparin in the elec-
trospinning process, although other phenomena such as the dif erential rate of solvent
removal and skin formation due to dif erences in blend composition or the molecu-
lar weight or i ber diameter might also contribute to the observed roughness of the
composite i bers.
Biological characterization of the cellulose-heparin i bers performed by measuring
the clotting kinetics of human whole blood exposed to these i bers using thromboelas-
tography showed cellulose/heparin composite i bers af ord a prolonged clotting time
in a concentration-dependent fashion. h e observed results indicates that presence of
