Bacterial Cellulose and Its Polymeric Nanocomposites - The Nanoscience and Technology of Renewable Biomaterials - page 238

in transparency with higher fiber content was rather small with for instance a 3.3%
transmittance loss at 11.7 wt% fiber and a 13.7% transparency loss at 66.1wt% fiber.
While low fiber contents appeared slightly advantageous to maintain transparency, high
fiber content nanocomposites had a lower CTE and therefore were less sensitive to tem-
perature variations. In fact, the incorporation of only 7.4 wt% BC fiber in the resin
contributed to a large reduction in the materials CTE from 86

10 − 6 K − 1
while minimally deteriorating the light transmittance. It thus appeared from this report
that minimal addition of BC fibers allowed maintaining a high transparency while dras-
tically reducing the CTE (15).
Another limitation of BC nanocomposites lies in the hygroscopicity of BC which
imparts poor dimensional stability to the nanocomposites. To circumvent the low
dimensional stability of BC nanocomposites acetylated BC fibers were utilized in BC
nanocomposites in conjunction with acrylic resins (12, 13). At low degree of fiber
acetylation with acetic anhydride (degree of substitution DS up to 0.17) acetylated BC
fibers had similar dimensions than untreated BC fibers but appeared better separated
from each other in a scanning electron microscope (SEM), a likely result of lower inter-
fiber bonding between acetylated fibers (Figure 9.6). As a result of lower interfiber
bonding, acetylated BC sheets had a lower modulus (17.3 GPa) than an untreated BC
sheet (23.1 GPa). As expected acetylation was effective at decreasing the hygroscopy
of the BC/acrylic nanocomposites with two acrylic resins, one based on dimethacrylate
(TCDDMA) and another commercial resin. Nanocomposites with 33% acetylated fiber
content reached similarly low moisture content than the neat acrylic (0.8%) when equili-
brated at 20 ◦ C and 55% relative humidity. Slight changes in optical transparency were
observed in the nanocomposites after acetylation and could go either direction depend-
ing on whether it improved the match in resin and fiber RI (Figure 9.7). Interestingly,
while sheets of acetylated BC had a lower CTE (0.8 ppm/ ◦ K) compared to untreated
BC sheets (3 ppm/ ◦ K), this improvement in CTE was not observed in the corresponding
nanocomposites, at least within this small range of DS. As acetylation also increases the
thermal stability of cellulose, it was no surprise to see that after heat treatment at 200 ◦ C
for various time periods, the acetylated BC nanocomposites did not experience as high
of a loss in optical transparency than the control nanocomposites. The overall positive
effect of acetylation on the properties of the nanocomposites prompted this group to
examine further the impact of a broader DS range from 0.17 to 1.8 on the nanocom-
posite properties (12). Thus 40% fiber content nanocomposites were manufactured with
acetylated fibers and the TCDDMA based resin. With higher DS, wider nanofibers were
observed (Figure 9.8) along with a change in crystalline structure indicating that acety-
lation occurred from the surface to the core of semicrystalline fibers. Also within that
broader DS range, both the nanocomposite transparency and the equilibrium moisture
content (20 ◦ C, 55% RH) passed through an optimum as a function of DS (Figure 9.9).
Maximum transmittance (87.8%) was measured for a DS of 0.74 above which transmit-
tance decreased down to ca. 70% and minimum equilibrium moisture content (EMC)
of the nanocomposite (20 ◦ C and 55% RH) was found (0.5% MC) for a DS

10 − 6

× to 38 ×

0 . 5. At
this substitution level the nanocomposite hygroscopicity was reduced by 1/3 compared
to nanocomposites using untreated BC (MC of 1.5%) but was still higher than that
of the acrylic resins alone (0.35%). Up to a DS of approximately 0.6, the CTE of the
acetylated nanocomposites was similar to that of the control nanocomposite, after which it

=