Page 253 - Biodegradable Polyesters

9.6 Conclusions 231

material comprised of non-interconnected nanofibrils and thus characterized by
superior mechanical properties as compared with the 3-D networks. Obviously,
this can be realized only if one succeeds in avoiding or replacing the hydrogen
bonding. This could be the case if, from the very beginning, a third blend
component is used, which is the preferred partner for hydrogen bonding with the
water-soluble blend component, as schematically shown below:
A + B → A ⋅ B; A + B + C → A + B ⋅ C

where A is the polymer of interest, B is the water-soluble blend component, and C
is a low molecular-weight water-soluble compound stronger than A in hydrogen
bonding with B.
It should be mentioned that similar approach has been used by Kotek and
coworkers [28–30] for suppression of the intermolecular hydrogen bonding in
polyamides in order to achieve better molecular orientation making it possible to
prepare Nylon filaments with superior mechanical properties.
The other possibility is to use a water-soluble polymer that is not able to form
H-bonds with the main component as a second blend component, regardless
of the fact that both components are basically capable of being involved in
H-bonding. A good example for this case could be the systems comprised
of poly(vinyl pyrrolidon) or poly(ethylene oxide) as the water-soluble blend
component and any polyester (PET, PETG, PLA, PCL, and others). In this way,
through avoiding the formation of H-bonds between the polymer of interest and
the second blend component it would be possible to realize our main goals: (i)
converting condensation polymers into nano-sized materials with nanofibrillar
morphology instead of network type, (ii) further use of water as the only solvent,
and (iii) regeneration of the water-soluble polymer and reusing it for the same
purpose, thus making the approach environmentally friendly and economically
attractive.

9.6
Conclusions

The peculiar properties of nanomaterials arise mainly from their sizes and for this
reason, the search for methods to convert the known materials into nano-sized
ones is of continuously increasing importance. The electrospinning used for
this purpose is a simple and cost-effective method but the final product always
represents a nonwoven textile from nanofibers with a quite limited application
potential. The concept of nanofibrillar composites developed during the last
decade solves the same problem without suffering from the disadvantages of
the electrospinning technique. Starting from a blend of thermodynamically
nonmiscible polymers after extrusion and cold drawing followed by selective
dissolution of the major component of the drawn blend, nanofibrils (with
diameters of 50–250 nm) of the minor component can be separated. These neat
nanofibrils can be used as scaffolds in tissue engineering, micro- and nanofilters