The natural sodium montmorillonite with a trioctahedral
smectite structure and a cation exchange capacity (CEC) of
110 meq/100 g was used as the dispersed phase to reinforce
the poly(lactic acid) (PLA) matrix. PLA pellets with melt
index of w10 g/10 min were kindly supplied by Wei Mon
Industry Co., LTD (Taipei, Taiwan). The surface of natural
sodium MMT was modified by cationic exchange between
Naþ in layered silicate galleries and n-hexadecyl trimethylammonium
bromide (CTAB) cations in an aqueous solution
with constant mechanical stirring at 60 C for 24 h. The
exchanged MMT was then modified by biocompatible/biodegradable
chitosan in an aqueous solution containing 1 wt%
lactic acid with constant mechanical stirring at 25 C for
24 h in order to further improve the chemical similarity
between the polymer matrix and m-MMT. The PLA/m-
MMT nanocomposites were prepared by using an aqueous
surface-treated m-MMT solution mixed with a methylene
chloride solution and PLA for 24 h. The PLA/m-MMT
nanocomposites were then precipitated with an excess amount
of deionized water, washed with hot water at 80 C for 8 h and
dried at 100 C for 12 h in vacuum.
Powders of pure PLA and 1.5 wt%, 3 wt% and 6 wt% PLA/
m-MMT nanocomposites were sandwiched between two cover
glasses and heated on a hot stage at 200 C. The sample was
pressed into thin film with the thickness in the range of
0.10 mm.
2.2. Characterization
The FTIR spectroscopy was used to characterize the structure
of PLA/m-MMT nanocomposites. The final spectrum is
presented as an average of three spectra recorded at different
regions over the entire range of the sample. X-ray q/2q diffraction
scans of these specimens were obtained using a 3 kW
Rigaku III diffractometer equipped with Ni-filtered CuKa
radiation in the reflection mode. The degree of crystallinity
of PLA/m-MMT nanocomposites was calculated from the integrated
area of X-ray diffraction data, for which we assumed
Gaussian profiles for crystalline and amorphous peaks. Ultrathin
section of the PLA/m-MMT thin film with a thickness of
approximately 50 nm was prepared with an ultramicrotome
equipped with a diamond knife. Transmission electron microscopy
was carried out with a JOEL transmission electron
microscope using an acceleration voltage of 120 keV. Due to
the high electron density difference between clay and polymer
matrix, staining of the samples was not necessary.
Thermal analysis of the samples was performed using
a Perkin Elmer PYRIS Diamond differential scanning calorimeter
(DSC) calibrated using indium and all experiments were
carried out under a nitrogen atmosphere. All specimens were
weighted in the range of 5e6 mg and were heated to 200 C
at a rate of 100 C/min and held for 20 min to remove the
residual crystals, then they were cooled to 20 C at a rate of
10 C/min. The specimens were then heated to 200 C at
a rate of 10 C/min. The crystallization temperature (Tc),
exothermic heat of crystallization (DHc), crystalline melting
temperature (Tm), and heat of fusion of polymer crystalline
(DHm) for PLA and PLA/m-MMT nanocomposites are
recorded. Thermal stability of the samples was conducted using
a Perkin Elmer TGA 7 Series Apparatus with a heating
rate of 10 C/min under nitrogen atmosphere. DMA