In our opinion, this behavior is a result of the silane treatment which
improves the adhesion to the matrix. The decrease of the mobility
of the PLA chains which are close to the nanofibers could hinder
the crystallization process and so lead to higher values of Tcc.
The endothermic peak corresponding to the fusion of the PLA
crystallites (Tm) can be identified at around 168 ◦C for all the samples.
By considering that the melting temperature depends on the
size and the perfection of the crystalline lamellae (Liu & Donovan,
1995), we could suppose that the crystalline structure of the different
materials obtained after the melting/recrystallization process
are almost the same. The degree of crystallinity, calculated using Eq.
(1), is higher for PLA nanocomposites than for neat PLA (Table 1).
This increase of crystallinity confirms the previous observations
related to the role of cellulose nanofibers as nucleating agents. Similar
observations were made by other authors (Tokoro et al., 2008).
By comparison between the two types of composites, a slight higher
crystallinity yield is observed for the PLA/CNF composites as compared
to the PLA/CNFS. The stronger adhesion between the PLA
matrix and the CNFS due to the silane treatment could hinder the
motion of the PLA chains which are close to the CNFS. This decrease
of mobility could be an explanation of the lower crystalline ratio
and the higher cold crystallization temperature. Our work also supports
the concept of an increase of the polymer chains mobility
when the polymer is confined near a non-interacting interface.
It was postulated by Si, Massa, Dalnoki Veress, Brown, and Jones
(2005) that an increase of the chain mobility near a non-interacting
surface could be attributed to a reduction of the entanglement
effect.