Structure
of Synthetic Fibres
A synthetic filament yarn is produced from a polymer made
of long chain molecules. The arrangement of the chain molecules in fibre is
depend on the combination of manufacturing processes. Thus fibres with
different properties can be achieved with same polymer by altering spinning and
drawing parameters or by heat treatment. The relationship between process
conditions and the fibre structure is having importance to the processing point
of view.
Different
theories have been proposed by different researchers for the supermodel
physical structure of synthetic fibre. One is the micelle theory in which
micelle was assumed to be the smallest structural unit. The micelle is the
aggregation of polymer chains. These micelle accounts for the crystallinity of
fibre and oriented w.r.t the fibre axis. The remaining matter is considered
just as amorphous matter. The polymer chains are longer than micelle so they
run through successive crystalline and non-crystalline regions giving fringed
micelle. Thus, crystalline micelles are completely embedded in amorphous matrix
of non-crystalline polymer.
 |
| Fringed Micelle structure |
Hearle
then modified this by assuming that crystalline region is composed continuous
fringed fibrils which diverge from one microfibril to another through
non-crystalline region. These theories are based on two phase
crystalline-amorphous concept. These models were not successful in explaining
some physical and mechanical properties of fibres.
Then
after chain folding model was imposed which suggested that the crystals are
also formed by folding of long chain molecules. This creates imperfection due
to together packing of folded and extended crystalline structures and lead to
formation of microvoids or crystal defect. This crystal defect model for
synthetic fibre structure has been successfully explained a no. of physical and
mechanical observations in which previous models were fail.
For
the melt spun and drawn fibres such as nylon, polyester, polyethylene and
polypropylene the three-phase model of fibre structures has been suggested. The
three phases are the amorphous domain of microfibrils, crystalline domains of
microfibrils and intermicrofibril region. Microfibrils have well defined and
clean endless structure in which polymer chains are arranged to form
crystalline and amorphous regions. These crystalline and amorphous regions are
arranged in regular sequence in the microfibrils. The microfibrils have long
period characteristics that represent the spacing between adjacent crystals.
The length of crystalline region along fibre axis is about two-third of long
periods. The longitudinal dimensions of microfibrils are not well defined. The
crystalline region is made by regular arrangement of polymer chains by chain
folding.
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| Three Phase structural Pet Fiber |
In the
highly extended intermicrofibrilar region the chain molecules are oriented but
crystallinity is absent, also called oriented amorphous regions. These
interfibrillar domains do not contribute fibre strength which is mainly due to
microfibrils. In contrast a scientist considers extended chain interfibrillar
domains are strongest element of structures and have important effect on fibre
strength of PET and Nylon. They consider amorphous domains of microfibrils are
weakest element of fibre structure. The dimensional stability at elevated
temperature is provided by microfibrils. The microfibril have melting point
above the softening temperature of extended interfibrillar region because these
interfibrillar regions have order and density in between crystalline and
amorphous domains of microfibrils.
The
several microfibrils and intermicrofibril regions together presumed as
macrofibrils. These have ribbon like shape about 300-400 °A thick but their
size actually depends on the thermo-mechanical treatment.
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