Aromatic poly(amic acids): Fundamental structural studies

Abstract

SynopsisAromatic poly(amic acids) derived from pyromellitic dianhydride and 4,4',-diaminodiphenyl ether were characterized by dilute solution techniques. Number-average molecular weights a,, of 13 samples ranged from 13,000 to 55,000 (DP 31-131). Weight-average molecular weights aw of 21 samples ranged from 9,900 to 266,000. The ratio &fw/&f,, was between 2.2 and 4.8. Heterogeneous polymerization yielded higher molecular weight polymer than homogeneous polymerization. The molecular weight could be varied systematically by control of stoichiometric imbalance. Use of very pure monomers and solvent gave polymers of relatively high number-average molecular weight (-50, 000) and the most probable molecular weight distribution aw/a,, = 2. Impure monomers and/or solvent resulted in lower number-average molecular weight (an E 20,000-30,000) and wider distributions (hTW/a,, = 3-5). The Mark-Houwink relation obtained was [ q ] = 1.85 X 10-*AiwO.*o The exponent is characteristic of moderately extended solvated coils. The unperturbed chain dimensions (T?/M)~'* were 0.848 A., and the steric factor u was 1.24 which is close to the limiting value of unity for an equivalent chain with free internal rotations. The sedimentation constant-molecular weight relation was SO = 2.70 X 10-21i?,~,39. This exponent is consistent with the Mark-Houwink exponent.

Dispersion of nano-carbon filled polyimide composites using self-degradated low molecular poly(amic acid) as impurity-free dispersant

“…As shown in Fig. 2, the intrinsic viscosity of PAA decreases as the milling time increases, slightly at first and then rapidly from 2.61 dL/g to 0.66 dL/g within a milling time of 18 h, corresponding to a large reduction in the viscosity-average molecular weight of PAA from 1.54 Â 10 5 to 2.76 Â 10 4 [18]. It seems that the PAA remains stable within 2 h of ball milling time, and the reduction of viscosity becomes level off if the milling time is higher than 12 h. The significant reduction in the PAA molecular weight should result from the high shearing force and the friction induced temperature rise environment.…”

Preventing gelation in polyisoimide synthesis

“…11 A Varian Unity Inova 500 spectrometer was used to obtain 1 H NMR spectra. Poly(amic acid) solutions were dissolved in deuterated dimethylsulphoxide (DMSO-d 6 ).…”

Viscoelastic properties of polyamic acid solutions—precursors of polyimides

“…The weight average molecular weight of the PAA samples is believed to be of the order of 100,000 to 250,000 (1). Studies of similar aromatic PAA showed M,, in the range 13,000 to 55,000 M , in the range 9,900 to 266,000, and the Mark-Houwink relation given by [q] = 1.85 x 10-4 M,O.W (9,10).…”