Nanocomposites of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 103

solvent. 35 However, in another procedure, the solvent is immediately evap-
orated after mixing. 32 Upon solvent evaporation, the intercalated polymeric
materials give a nanocomposite that is normally dried in vacuo.
The thermodynamics involved in this process were explained previously. 40
In brief, before the solubilized polymer intercalates the layered nanofillers,
the solvent molecules that occupy the spaces in between have to be
displaced; hence a negative variation in the Gibbs free energy change is
required. The driving force for this process is hypothesized to be due to
entropy gained by desorption of solvent molecules, compensating the
decreased entropy of the confined intercalated polymeric chain. 3,7
In the solution casting method, the polarity of the solvent is critical in
facilitating the intercalation of polymers into the layered nanofillers. 41 A
strongly polar solvent causes excessive swelling of the silicate's hydrophilic
surface, resulting in fracture or cracking of the materials and low polymer
solvation. A highly hydrophobic solvent influences the solvation of the
polymer but leads to poor silicate nanofiller swelling. This warrants the use
of surfactants and/or modifiers in order to compromise the limitations that
arise from these disparities.
Polyhydroxyalkanoates, such as poly-3-hydroxybutyrate, 42 poly-3-hydroxy-
butyrate-co-3-hydroxyvalerate, 15 and poly-3-hydroxybutyrate-co-3-hydroxy-
hexanoate, 11,21 were intercalated using this method. Ten et al. 43 produced
evenly exfoliated nanocomposites of PHBV/cellulose nanowhiskers, with a
well-defined distribution of the nanofillers within the polymer matrix
(Figure 5.2).
Observing the effects of nanofiller loading during Nodax t (PHBHHx)/
layered nanosilicate fabrication in chloroform solution casting, Zhang
et al. 32 found that the addition of nanofillers ( r 5%) increases thermal
properties such as Young's modulus and the tensile strength of the nano-
composites as compared to those of the pure polymer. On the other hand, a

d n 2 r 4 n g | 8

.

PHBHV matrix

nanofillers

Figure 5.2 TEM micrograph showing exfoliated distribution of cellulose nanowhis-
kers in a PHBHV matrix nanocomposite prepared by the solution inter-
calation method.
Reprinted from Ten et al. 43 with permission from Elsevier.