Nanocomposites of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 109

nanocomposite using this method. 52-55 Gorrasi et al. 56 obtained exfoliated
nanocomposites of a poly-6-hydroxyhexanoates matrix through in situ
intercalative ring opening polymerization of caprolactone with modified
montmorillonite in the presence of a dibutyltin dimethoxide catalyst.
Reports on the application of in situ intercalative polymerization for the
production of bacterial PHA nanocomposites, especially those containing a
medium-chain-length poly-3-hydroxyalkanoates matrix, are virtually non-
existent.
The use of toxic organometallic catalysts and initiators in polymerization,
as well as bulk solvents for monomer solutions, are among the industrial
limitations of the in situ intercalative polymerization method, despite its
reported advantage of good nanofiller dispersion in nanocomposite
production. 3

d n 2 r 4 n g | 8

5.3 Characterization Methods for PHA
Nanocomposites
When using layered nanofillers such as layered silicate, poor dispersion of
nanofillers in the matrix results in aggregate (tactoids) nanocomposites.
Interactions between the nanofillers and some of the extended polymer
chains, however, result in an intercalated nanocomposite with an alternate
arrangement of polymer and layered nanofiller (Figure 5.1). Absolute and
homogeneous dispersion of nanofillers in the polymer matrix gives a
nanocomposite with an exfoliated morphological characteristic (Figure 5.1).
Previously, this ordered exfoliation was thought to be induced by steric
interactions. 10,31 However, Zhang et al. 32 suggested that dispersion of the
polymeric side chain into the nanofiller's layered spaces results in the ex-
foliation. The characterization of the state of the nanoparticles' dispersion
allows the interpretation of the preceding morphologies, and the structural
characterization relies heavily on techniques such as X-ray diffraction (XRD),
wide angle X-ray diffraction (WAXD), simultaneous small angle X-ray
scattering (SAXS) and electron microscopy (transmission, TEM or scanning,
SEM). XRD is easy to use and is the most commonly employed method in
establishing the nanocomposites' structural morphology, crystallinity as
well as polymer melt intercalation. 9,33,57 For example, in intercalative poly-
mer nanocomposites, the expansion of interlayer spacing (d-spacing) to
allow polymer intercalation results in a shift of the X-ray diffraction peak
towards lower values of 2y (Figure 5.5). In each case, the d-spacing (Å) of the
silicate layers can be easily calculated from the X-ray diffractogram 2y values
using Bragg's equation (eqn (5.2)),

.

l
2 sin y

d ¼ (5 : 2)

where l is the X-ray wavelength. On the other hand, exfoliated nano-
composites exhibit excessive delamination of the silicate layer resulting in