AFM is capable of measuring phase as well as height differences
at surfaces [49]. Property differences (e.g., modulus) in the phase
image of AFM were shown on a dimensional scale [50,51]. The
(microtomed) surface of injection molded bars of the 5% unmodified
or HCl-CNC/PLA nanocomposites were examined by AFM and
indications for a heterogeneous morphology were obtained from
the topography and adhesion images (Figs. 10 and 11). The AFM
phase images at high magnification (Fig. 11a0ec0) provide information
that allow estimates of nanofiber average dimensions as
well as of the agglomerates presented by the morphology of the
different nanocomposites. It is assumed that the tapping condition
applied in this study was in the moderate range and that the dark
spots in Figs. 10 and 11 correspond to the stiff cellulose nanofiber
regions while the light regions are rich in the relatively softer PLA
matrix. In particular, the morphological analysis of HCl-CNCs
blended with PLA (Figs. 10a and 11aea0) reflects the expected
problems of poor dispersion between the hydrophilic nanocellulose
and the hydrophobic polyester matrix. The images clearly
show regions attributable to the presence of agglomerates with
sizes for the largest and smallest CNC aggregates ranging from
around 1 mmto a few hundred nm, respectively.When AA-CNCs are
extruded with the PLA, the resulting dimensions of the agglomerates
are reduced and, as expected, the number of agglomerates is
less with rare sighting of fibrils, suggesting some weak interaction
between the AA-CNC nanofibers and the polymeric matrix
(Figs. 10b and 11beb0). However, AFM images of LA-modified CNCbased
nanocomposites show a unique morphology where individual
LA-CNCs are observed that are well distributed through the
matrix along with a low prevalence of smaller aggregates (Figs. 10c
and 11cec0). The increased dispersion of LA-CNCs result in a
network like structure that is observed in Figs. 10c and 11cec0 .
Therefore, AFM analysis confirms the changes in morphology in the
micro-nanoscale of the nanocomposites and provides direct evidence
that, of the CNC's studied herein, LA-CNCs are best dispersed
within the matrix. This is in agreement with its relatively superior
ability to reinforce PLA