Biomedical Engineering Reference
In-Depth Information
2.3.2
Physical Structure of Cellulose
The physical structure of cellulose refers to the spatial arrangement of different-
scale structural units, including the chain structure and aggregation structure of
the polymer. The chain structure, also known as the primary structure, shows the
geometric arrangement of the atoms or groups in the molecular chain. The short-
range structure is the first-level structure and refers to the chemical structure or
stereochemical structure of one or several structural units in a single-molecule
polymer. Remote structure is the second-level structure and refers to the size
of a single-molecule polymer and a special structure. The aggregation structure,
also called the secondary structure, refers to the inner structure of the whole
polymer, including the crystal structure, noncrystal structure, orientational structure,
and liquid crystal structure. The third-level structure term is used to describe
how molecules in a polymeric aggregate accumulate each other, such as tangly
clew structure and crystal structure formed with ordered folding chains. The
chain structure of the polymer is the main structural hierarchy that reflects many
characteristics of a polymer, such as melting point, density, solubility, viscosity,
adhesion, and so on. The aggregation structure of a polymer is the major factor that
determines the service performance of macromolecular compound products [
9
].
2.3.2.1
Filament Structure
A fibril is a small, stretching unit; these units aggregate and then constitute the
structure of some natural and synthetic fiber materials (such as textile fibers, timbers,
or fibrous protein); they also make long molecular chains gathered into bundles
in one direction. Because the inequality in size of fibrillar aggregation, current
terminologies include elementary fibril, microfilament, macrofilament (also called a
microfilament bundle) [
10
]. Natural cellulose has 10,000 glucose units, and the fibril
contains approximately 60-80 cellulose molecules. Hydrogen bonds are formed
between adjacent molecules. In a certain range of space, hydrogen bonding can be
shown in the X-ray pattern when it reaches a certain number. This space is called
the crystalline region, and the rest is called the amorphous region. Microfilament is
composed of elementary fibrils and is fixed in size. Macrofilament is has more than
one microfilament, and its size varies with the sources or processing conditions of
raw materials.
The structural model proposed by Fengel for the cell wall of timber is the
representative model for the microfilament structure of each layer of the cell wall.
He reported that the elementary fibril with a diameter of 3 nm is the most basic
structure unit; 16 (4
4) elementary fibrils form a fibril with a diameter of 12 nm,
then 4 fibrils form a relatively thick microfibril with a diameter of 25 nm, and
more than one microfibril form macrofilament. Hemicellulose is filled between
the adjacent elementary fibrils; the microfilaments are wrapped with lignin and
hemicellulose. A multilayer of several hemicelluloses is filled between fibrils 12 nm
