Bacterial Cellulose and Its Polymeric Nanocomposites - The Nanoscience and Technology of Renewable Biomaterials - page 232

properties of the primary and secondary cell walls of plants and trees. Synthetic polymers
have also been used in biomimetic approaches to develop well dispersed nanocomposites.
In this review, research and developments on BC nanocomposites obtained from these
three manufacturing approaches are successively reviewed, placing a special emphasis
on the nanocomposite morphology and performance. A few studies have used bacterial
cellulose nanocrystals to reinforce polymeric matrices and these advances are reviewed
next. Prospect for the future developments in BC nanocomposites are finally proposed.

9.2 Bacterial Cellulose: Biosynthesis and Basic Physical

and Mechanical Properties

The biosynthesis and properties of BC have been extensively reviewed (1-3) and are
therefore only briefly examined in this review. Special attention is given to the mechan-
ical properties of BC sheets as they constitute a good reference from which to examine
the performance on BC nanocomposites.

9.2.1 Synthesis and Properties of BC

Cellulose is a semi-crystalline high molecular weight homopolymer of β -1,4 linked
anhydroglucose (Figure 9.1). Many living organisms synthesize cellulose affording a
wide range of supramolecular structures, morphologies and properties. Among all cel-
lulosic materials, bacterial cellulose displays the highest mechanical properties. In fact,
it has a tensile strength and Young modulus comparable to Kevlar (4). The outstanding
performance of bacterial cellulose stems from its high purity, high crystallinity (75%)
and ultra-fine network structure (4). Bacterial cellulose also has the highest DPn (up
to 8000), aspect ratio and strength to weight ratio, making it an ideal reinforcement for
natural composites. Besides it can hold ca 100% of its weight of water, defining it as
an hydrogel.
Cellulose produced by bacteria is most commonly of the cellulose I type, although
one bacteria also produces cellulose II (2). In cellulose I the chains are oriented parallel
with a spacing of 0.53 nm between the glucan chains. Native cellulose I has two
suballomorphs, I α and I β . The former exists as a single chain triclinic unit cell while
the latter exists as a two-chain monoclinic unit cell (Figure 9.1). Many organisms
produce cellulose, including plants, eukaryotic bacteria, procariotic organisms and fungi.
However Gluconacetobacter xylinum , a rod shape aerobic gram negative bacteria, is
most commonly used to produce BC nanocomposites.
When cellulose is produced from Acetobacter Xylinum , 12 to 15 cellulose chains are
extruded from the enzymatic terminal complexes into the culture medium as subelemen-
tary fibrils that have a lateral width of 1.5 nm and are amorphous (5). The subelementary
fibrils aggregate and crystallize into 3-6 nm wide microfibrils that comprise cellulose
I α and I β allomorphs (Figure 9.1) (4-6).

9.2.2 Performance of BC Mats

Researchers in Japan first evaluated the mechanical performance of air dried and hot-
pressed bacterial cellulose mats (7-9). Tensile measurements of BC sheets hotpressed