Chemistry Reference
In-Depth Information
weight PPTA in the composites. These ratio values are much smaller than
that of single PPTA chains if the chains are fully extended. A higher degree
of dispersion of PPTA in the matrix should result in stronger reinforcement.
The method used by Helminiak
et al
. (1980) in their attempts to prepare
molecular composites resulted in much thinner microfibrils or molecular
bundles. This method was based on Flory's treatment on ternary systems
consisting of a rod-like polymer, a conventional random coil like polymer
and a solvent (Flory, 1978). According to this treatment there is a critical
concentration C
cr
below which the system is a single isotropic phase con-
sisting of rods randomly dispersed in the coils. If phase separation can be
avoided on solidification by rapid coagulation such that the rod/coil mix-
ture is frozen to overcome the unfavorable thermodynamic driving force for
phase separation, a molecular composite is obtained. On the other hand,
if the concentration is above C
cr
, phase separation occurs resulting in a
rod-rich phase along with a coil-rich phase. The molecular composite is not
possible with the two-phase solution. Thus, in the study of the molecu-
lar composites of the rigid-rod polymer poly-
p
-phenylenebenzobisthiazole
(PBT) and the flexible-coil polymer poly-2,5(6)-benzimidazole (ABPBI),
the work was started off with the determination of C
cr
(Hwang
et al
., 1983).
This was done by slowly titrating the originally biphasic, stir-opalescent
solution of known concentration with solvent until a point was reached
at which the solution became isotropic and stir-opalescence ceased. The
coil polymer ABPBI used in this study had a very high molecular weight
(100,000 g/mol). A high solution viscosity and a high degree of entangle-
ment were achieved to protect the blend from phase separation during
solidification. C
cr
was found to change with the molecular weight of PBT
and the ratio of PBT/ABPBI in methanesulfonic acid solvent containing 2.5
vol-% of chlorosulfonic acid. With the PBT of 41,000 g/mol and the ratio
PBT/ABPBI of 30/70, the critical concentration at room temperature was
found to be 3.05 wt-%. The composite fibers were prepared from a solu-
tion of 2.5 wt-% (which was below C
cr
) by extruding through a single-hole
spinneret into a water coagulating bath at room temperature. The par-
tially coagulated solution was subsequently drawn to a ratio of 3.4 in a
water bath and might be neutralized with NH
4
OH to leach out all the
residual acids. The neutralized samples were further washed with running
deionized water. The bulk specimens were then air dried at ambient con-
ditions. The heat treatment at various temperatures with a further draw
ratio of 1.20 was also carried out for comparison. Thus, while the pure
ABPBI fiber obtained after neutralization with NH
4
OH and heat-treated
