Structural characterization of polyimides, 6FDA-PFMB and BPDA-PFMB in dilute solutions

BPDA-PFMB (biphenyl dianhydride-perfluoromethyl benzidine) and 6FDA-PFMB (2.2{dollar}spprime{dollar}-bis(3,4-dicarboxyphenol) hexafluoro propane dianhydride-perfluoromethyl benzidine) are known as semi-rigid rod polymers due to the fact that regimes of structural rigidity are connected by flexible kink joints. The origin of chain stiffness comes from the imide-phenyl coplanarity along the chain backbone. BPDA-PFMB is a typical polyimide which can be solution-spun to produce high performance fibers for composite applications. 6FDA-PFMB has a low dielectric constant, low water absorption, low refractive index, high optical transparency and high solubility in polar solvents. However, the molecular weight and chain dimensions of these materials have not been determined unambiguously.; If the universal calibration of GPC is obeyed, an on-line continuous viscosity detector with a refractometer can be used for the MW determination. Taking into account the fact that universal behaviour for stiff chain polymers is not constant over a wide range of molecular weight, an on-line continuous light scattering detector was used to obtain the molecular weight directly at each interval of elution volume.; Also, steady state flow properties of the dilute polymer solutions of these polymers have been investigated to characterize the global molecular properties. The absolute MWs and intrinsic viscosity values from off-line capillary flow measurements have been used to characterize these chain polymers and then construct a GPC calibration curve.; Size measurements from light scattering measurements show a slow change in the exponent "a" from the Mark-Houwink relation over a range of MW, so that only a very limited GPC universality can apply.