Poly(amic acid)s and their ionic salt solutions: Synthesis, characterization and stability study - Polyimides and Other High Temperature Polymers - page 14

Figure 12. Bulk viscosity as a function of storage time at 25°C for BPDA based polymer systems.
It is known that a poly(amic acid) solution can undergo chemical transforma-
tion upon extended storage. During storage of freshly synthesized PAA at ambi-
ent temperature, in addition to propagation (forward) reaction, depolymerization,
amic acid hydrolysis as well as some imidization reaction can also take place
[18]. Some PAA solutions can also undergo pseudogelation, which could be due
to the physical association of the chains in the solution and polymer-solvent inter-
action [19-20]. If pseudogelation had taken place in our polymers, then bulk vis-
cosity would have increased to well above 100,000 cP. However, this was not the
case for the polymers in our study as viscosity and molecular weight values
drifted to lower values.
We observed that the initial viscosity of [BTDA + o-TDA] PAA and [BPDA +
o-TDA] PAA based PDPAA (with DEEM) increased in the range of about 18 and
>28%, respectively, than using DMEM. This could be attributed to the larger mo-
lecular dimensional volume of DEEM than that of DMEM and ionic bonding of
methacrylate moieties to the carboxylic groups on the PAA. We also noted that
the average rate of bulk viscosity drift of BTDA based Co-PAA system was 2.84
cP/hr, and 1.62 orders of magnitudes higher than that of BPDA based Co-PAA
system. However, the average rate of bulk viscosity drift of both BTDA and
BPDA based Co-PDPAA systems was around 1.10 cP/hr.