Polybenzobisthiazoles – Critical issues in their performance and properties - Polyimides and Other High Temperature Polymers - page 234

Compressive failure of rigid-rod polymer fibres is always associated with the
formation of kink bands. Kink bands are nucleated in a localized region probably
initiated somewhere near the fibre surface at a critical stress and then propagate
away from the initiation point and continue to grow in toward the centre and the
other side of the fibre. In a certain range of compressive strain, kink band density
is proportional to the applied compressive strain, but at larger compressive strain,
the kink band density tends to saturate [49, 50]. It has been reported that the kink
band angle is closely related to fibre structure and properties [51, 52]. There are
several arguments about the mechanism of kink formation and the compressive
failure. However, many unanswered questions still remain in this field, for many
different deformation processes, such as micro-buckling, shearing and bending,
etc.
4.4.2. Approaches to enhance the compressive properties
The following approaches have been taken to enhance the compressive properties
of rigid-rod polymer fibres.
(1) Changing process conditions
(2) Changing morphological structure
(3) Modifying synthesis routes
(4) Increasing the shear strength by bonding molecular chains more strongly.
Modifications include change of rigid-rod backbones, incorporation of active
groups and diffusion of external agents into the fibre, to form intermolecular
cross-links.
(5) Radiation cross-linking and coating of the fibres as well as infusing reactive
monomers or metals into the fibres [53-59].
One suggested route is to crosslink PBZT fibres based on thermal elimination
or radiation degradation of activated aryl halogen or methyl pendent groups fol-
lowed by combination of aryl/benzyl free radicals leading to ring coupling [19,
28]. In other case a thermally reactive cross-linking fluorine moiety was incorpo-
rated into PBZT [31]. Cross-linking benzobisthiazole rigid-rod copolymers via la-
bile methyl groups, making rigid-rod random copolymers of phenyl benzobisthia-
zole with phenyl benzobisthiazole pendant -p-terphenyl units changes the polymer
packing order. As shown in Table 3 there is no significant change in the compres-
sive properties of cross-linked fibre compared to parent PBZT. The potential ad-
vantage of studying the effect of Lewis acid complexation on intermolecular in-
teractions in order to understand the role of the latter in influencing compressive
properties is clear. It was also observed that the presence of glass within PBZT
sol-gel microcomposite films increased the resistance of the films to compression.
Unfortunately, no significant success has yet been reported. There may be a
good potential for improving the compressive strength of rigid-rod polymer fibres
by coating them with suitable metal or metal compounds [60, 61]. The PBZT fi-
bres immersed in molten aluminium-silicon alloy or vapour deposited with alu-
minium were investigated. It was observed that the molten aluminium formed an