A study of microstructural changes in synthetic fibres resulting from mechanical deformations
This investigation examines the structure-property relationships of high modulus fibres.
Five fibre classes were chosen for examination. These are p-aromatic copolyamide
(Armos and SVM) and poly-p-aramids (Terlon and Kevlar) obtained from rigid chain
polymers; poly-m-aramids (Phenylon and Nomex) obtained from semi rigid chain
polymers, and aliphatic polyamide (Capron and Nylon) and Polyethylene obtained from
flexible chain polymers.
The thermo-mechanical properties studied include tensile properties, thermal shrinkage,
creep-recovery, stress-relaxation and residual deformation over a range of temperatures.
Results show that mechanical properties are highly related to chain rigidity, orientation
and crystallinity of the fibres. The presence of aromatic rings in polymer chains increase
the polymer rigidity. The higher the intermolecular attractive force and chain rigidity,
the greater the resistance to heat. Study of the creep-recovery properties of polyamide
fibres shows that irrecoverable residual deformation for the rigid chain polymers is
accumulated within a very short initial period of time (15 seconds) when the load is
applied. However for semi-rigid or flexible chain polymer fibres, the residual
deformation is accumulated during the whole creep process. The characteristics of
tensile stress-strain properties and the accumulation of residual deformation are found to
be temperature dependent, especially in the case of Armos and SVM.
The mechanical properties of polyamide fibres are also influenced by moisture which is
associated with intermolecular interaction.
Supplementary studies using FTIR, SEM and DSC were also undertaken. FTIR was
used for preliminary investigation into the intermolecular hydrogen bonding and
associated moisture in fibres. The results support the explanation of the thermomechanical
properties of polyamide fibres. SEM results show the fibre rupture
mechanism related to the fibre structures.