3.2.2. Influence of cellulose nanofibers on nanomechanical
properties of PLA matrix
Peak Force QNM-modulus image (scanning area 3 m
×
3 m)
of neat PLA matrix, without any thermal treatment, is shown in
Fig. 3a. From this image, the presence of crystallites is clearly
evidenced in PLA. The matrix presents an organized structure composed
of high modulus (light-colored) areas alternating with low
modulus (dark-colored) areas. High modulus areas can be ascribed
to the growing crystallites consisting of inclined stacks of lamellae.
Henton, Gruber, Lunt, and Randall (2005) have shown that
the crystallization of PLA at temperatures greater than 150 ◦C
results in hexagonal lamellar stacking crystal morphology. Similar
morphological trends were observed in Fig. 3a. At this stage,
the crystallization process is not completed. An increase of the
crystallization yield could be obtained after annealing. From our
knowledge, AFM images illustrating the beginning of the crystallization
process in PLA have not been previously presented.
Peak Force QNM images of PLA composites are presented in
Fig. 3b and c. Many bright colored areas ascribed to cellulose fibers
located close to the sample surface can be seen in the QNM – modulus
images of the PLA composites (Fig. 3b and c). The cellulose
fibers can be detected by QNM technique due to their significantly
higher modulus than that of PLA matrix. Because of the inhomogeneous
distribution of agglomerates in the case of PLA/CNF, some
regions of the PLA are totally free from nanofibers, as can be seen in
the middle of Fig. 3b. In these regions, a more organized structure
is observed similar to the structure of the neat PLA. In PLA composite
containing silane treated nanofibers (Fig. 3c) the lamellar
crystalline structure is less obvious as in the previous case suggesting
a lower crystallinity of PLA matrix. This observation will be
confirmed by the DSC results.