Cellulose Nanocrystals in Polymer Matrices - The Nanoscience and Technology of Renewable Biomaterials - page 286

1000

PVOH, 0 % PAA
PVOH, 10 % PAA
PSf

750

500

250

0

0 5 10 15 20

CNXL content, wt %

Figure 10.11 Water vapor transport rate (WVTR) of various polymer systems filled with
CNXLs.

transport rate (WVTR), around 1500 (g/m 2

dy), which can be explained by the presence

of 10% glycerin as a plasticizer.
The PVOH system was prepared by mixing aqueous solutions of PVOH, and
poly(acrylic acid) (PAA) with aqueous dispersions of CNXLs. A subsequent heat
treatment of 170 ◦ C for 45 minutes was shown to crosslink the polymers and probably
the CNXLs, resulting in greatly reduced swelling and weight loss upon submersion.
The crosslinking mechanism was shown to be ester bond formation between the
hydroxyls of PVOH, and perhaps CNXLs, and the carboxylic acid groups of PAA.
SEM and optical microscope images suggested that the CNXLs were agglomerated at
filler contents of 15% and above in this system.
It is interesting to note that CNXLs in a hydrophilic system (PVOH) reduce the
WVTR up to the agglomeration region while in a hydrophobic matrix, PSf, the WVTR
increases with increasing CNXL content. In the PVOH system, the CNXLs probably
act as barriers, creating a tortuous path for water vapor transport. This is a typical effect
and a standard technique for creating barrier films. The variability of the PSf WVTR at
11% CNXL was attributed to agglomeration. In the agglomerated state there might be
voids which would increase the WVTR.
These data underscore the nanoscale effect in the transport properties of polymer films
and the importance of the surface chemistry and matrix/filler interactions. While these
data are not conclusive, they are highly suggestive. They indicate that close attention
must be given to the chemistry of a particular system. The PSf results in particular
suggest that we may be seeing surface diffusion. Here the water may be adsorbing to