Probably due to the acid bound to the PAN chain as protonating dopant. PANi acid doped polymer shows higher thermal stability than PAN without doped with acids.

sorry, this is all totally incorrect: 

- undoped PAni (= no acid doping) is extremely stable, > 400 degrees

- doped PAni stability is dependant upon the nature of the acid, i.e. with MSA very low, with HCl low stability, p-toluene sulfonic acid very good, higher Mw sulfonic acids even better, but lower conductivity

Thanks to all for your valuable suggestions. I used HCl as a dopant (as received) which results high degree of protonation and stability also. Another side I diluted HCl dopant resuls in low degree of protonation and stability also. I think molecular weight of HCl is same in both the cases.

Anil, I don't know what is your benchmark regarding "stability". In my experence, HCl doped PAni is quite unstable (I do not remember the max stability temperature, I can search), but it is low.

Definitely, undoped PAni Emeraldine base) is extremely stable.

The molecular weight of PAni does not change with dopants involved, it is anyway quite low. (please do not believe lieterature about Mw determination, it is all wrong because they are using inappropriate methods)

Dear Bernhard Wessling,

Thanks you very much for your cooperation. One thing I want to share with you that their is minor difference in the stability not much. I also found very low polymer degradation at respective temperature. I their any connection with stability ?

The differences in stability between PAni/HCl and other dopants are significant.

I forgot the figures for HCl doped PAni, but max temperature is around 100 degress, I think even lower.

For PAni/pTs, you can process at up to 250 degrees, see here:

https://www.researchgate.net/publication/244490182_Thermal_stability_of_Polyaniline_Tosylate

HCl is the worst dopant you can choose for PAni. It also causes degradation of the polymer chain, it makes covalent Cl bonds at the aromatic rings and all kind of other negative side reactions.

Article Thermal stability of Polyaniline Tosylate

you can also find some evidence for what I said here in this patent application by Ron Elsenbaumer / Allied Signal: US05069820