Civil Engineering Reference
In-Depth Information
unit to interact with hydroxyl groups on adjacent chains through hydrogen bonding
and van der Waals forces. h e strong intermolecular forces between the chains of cel-
lulose, coupled with the high linearity of the cellulose molecule, account for the crystal-
line nature of cellulosic i bers. h ey can either be natural i bers or artii cial/man-made/
manufactured i bers. h e most common natural cellulosic i bers include cotton, l ax,
hemp, jute, ramie, etc. h e major manmade cellulosic i ber is rayon, a i ber produced
by regeneration of dissolved forms of cellulose [1]. h e natural cellulosic i bers are clas-
sii ed based on the part of the plant they are extracted from. h ey are classii ed as: (i)
the bast or stem i bers, which form the i brous bundles in the inner bark (phloem or
bast) of the plant stems, (ii) the leaf i bers, which run lengthwise through the leaves of
monocotyledonous plants, and (iii) the seed-hair i bers, the source of cotton.
h e most common manufactured/manmade cellulosic i ber is rayon. Chemically, vis-
cose resembles cotton. Rayon is made from purii ed cellulose, primarily from wood pulp,
which is chemically converted into a soluble compound. It is then dissolved and forced
through a spinneret to produce i laments which are chemically solidii ed, resulting in
synthetic i bers of nearly pure cellulose. Because rayon is manufactured from naturally
occurring polymers, it is considered a semi-synthetic i ber. Specii c types of rayon include
viscose, modal and lyocell, each of which dif ers in manufacturing process and properties
of the i nished product. It can imitate the feel and texture of silk, wool, cotton and linen.
h e i bers are easily dyed in a wide range of colors. Rayon fabrics are sot , smooth, cool,
comfortable, and highly absorbent, but they do not insulate body heat, making them ideal
clothing material for use in hot and humid climates. It is strong, absorbent, sot , inex-
pensive, and drapes nicely. However, it shrinks when washed, deteriorates with expo-
sure to light, and is susceptible to mildew.
12.2
Crystalline Structure of Electrospun Cellulose
Due to its linearity and the existing hydroxygroups, the cellulose chains form strong
intra- and intermolecular hydrogen bonds and can be closely packed into a crystal-
line structure. Cellulose is polymorphic, meaning that it can exist in several crystalline
forms. h e crystalline form of native cellulose in wood is cellulose-I, which is actu-
ally a composite of two crystalline forms with dif erent hydrogen bonding patterns:
cellulose-Iα and cellulose-Iβ [2, 3]. Cellulose-Iα has a one-chain triclinic unit cell, and
cellulose-Iβ has a two-chain monoclinic unit cell [4]. h e ratio between the two dif er-
ent cellulose-I crystal forms varies depending on the species; cellulose-I
α
rich speci-
mens are found in the cell walls of some algae and in bacterial cellulose, while in tunicin
and higher plants such as wood and cotton they are dominated by cellulose-I
β
. Both
phases, collectively designated cellulose-I, are similar in that they contain sheets of par-
allel hydrogen-bonded chains that are stacked on top of each other through hydro-
phobic interactions [2, 5, 6] . h e cellulose-I
α
phase is considered to be less stable than
cellulose-I
β
phase because it can be irreversibly converted to cellulose-I
β
by hydrother-
mal treatment [7, 8]. Depending on the application, raw cellulosic material is pretreated
or processed so that cell wall architecture is disrupted and the cellulose crystal structure
is transformed from cellulose-I to another crystal phase, which af ects the ratio between
cellulose-Iα and cellulose-Iβ in i bers. For example, krat pulping is accompanied by the
conversion of cellulose-Iα to cellulose-Iβ [9] and electrospinning converts cellulose-Iα
