Molecular dynamics of polyaniline (PANI), a conducting organic polymer, have been investigated as a function of the physical properties of the polymer, and as a function of several processing treatments. The specific motion studied is the 180° (π) -flips of the para-substituted phenyl rings in PANI. The fraction of the rings that undergo this motion is related to the free volume, which is a function of the chain packing present in the sample. Monitoring the phenyl ring flips has provided information about the extent of order present after processing the polymer under a variety of conditions.
The molecular motion of the phenyl rings was monitored by using the solid-state NMR technique of even-dipolar rotational spin-echo (EDRSE). This technique measures the value of the 1H-13C dipolar coupling of the protonated aromatic carbons of PANI. When the ring is undergoing π-flips, the dipolar coupling is reduced and this narrows the dipolar spectrum. Fits of the data revealed the fraction of dynamic phenyl rings in the sample, and in the as-synthesized PANI powder ~20% of the phenyl rings are undergoing the π-flip molecular motion.
The impact of molecular weight, bulk morphology and sample heating were all investigated. Neither changes in the weight average molecular weight from 50kDa to 300 kDa nor changes in morphology from powder to nanofibers had any impact on the phenyl ring dynamics. However, the effect of heating the polymer on dynamics is highly dependent on the annealing temperature. Annealing at 75°C, a typical temperature for removing solvents during film formation, had no effect on the fraction of phenyl ring flips; however, annealing at ~200°C produced a large increase in the fraction of dynamic rings to 52%. Heating the polymer to this temperature cross-links the polymer through the quinoid ring. The cross-linking increased the free volume of the chain components near the cross-linking sites, which allowed for additional phenyl ring motion.
The dynamics of the polymer after casting films from 0.5-1% polymer solutions were also investigated. The films were cast from both a “poor” solvent, 1-methyl-2-pyrrolidinone (NMP), which promotes the formation of a compact coil conformation in solution, and a “good” solvent, 1,1,1,3,3,3-hexaflouro-2-propanol (HFIP), which favors the formation of a more expanded coil conformation in solution. The absorption spectra of the two polymers solutions are indicative of the different conformations, the spectra from the cast films indicate that the polymer adopts the compact confirmation when cast from either solvent. The EDRSE NMR results show an increase in phenyl ring dynamics in the NMP film (to 32%), when compared to the powder, which results from the solvent interrupting the chain packing during film formation. An even larger fraction of the phenyl rings undergo π-flips in the film cast from HFIP, suggesting greater disorder in the chain packing of this sample, which likely arises from the fast solvent evaporation.
The extent of phenyl ring motions upon doping and de-doping the polymer was dependent on the acid used to dope the polymer. Doping with HBr caused little change to the fraction of dynamic phenyl rings, whereas doping with HF caused a reduction in phenyl rings undergoing flips to 14%. The reduction in phenyl ring motion with HF doping is related to the larger impact on polymer packing from the insertion and removal of the larger hydrated ion.