Hello dear, I think that the main reason is that the ionic liquid interacts with the ionomeric polymer (formation of bonds, may be dipole-dipole interactions, van der waals, hydrogen bond, etc), these interactions require a additional energy for being broken. Therefore the thermal stability is improved.

Explanation of Edwin A. Murillo seems to be reasonable. If we consider the mechanism of thermal decomposition, we can realize that the decomposition of a molecule is the breaking of the covalent bond between the participating atoms. Due to the absorption of thermal energy the vibration of the bond should increase. When the vibration of a bond crosses its elastic limit by absorbing thermal energy, then breaking of the bond should take place, resulting in decomposition. In your case, pristine polymer and IL considered may have different decomposition temperature thresholds. IL used may have higher thermal stability than the polymer. In addition, synergistic effects arisen for the mixing of  IL and polymer may also result in higher decomposition temp of the polymer and IL blends via the interactions mentioned by Edwin. 

Dear  Mohd Saiful Asmal Rani,

Thermal stability of ionic liquids depends on the ionic nature and type of ions present in a particular IL system. Ionic liquids has wide liquid temp. range. The thermal stability of ILs depends strongly on the anion, increasing with their size, [Tf2N]− being one of the most stable, whereas the cation seems to have lower influence on this property. The thermal decomposition is strongly dependent on the salt structure. To improve the thermal stability of ILs, several nanomaterials

including multi-walled carbon nanotubes (MWCNTs) and Au, CuO and Al2O3 nanoparticles can be added into some ILs to prepare IL-based nanofluids (Ionanofluids). 

You can read this paper:

Renewable Energy 63 (2014) 519e523. I hope you will be satisfied with my answer. 

Dear Mohd Saiful Asmal Rani

I know that this is an old discussion, but eventually, this could be useful for other people in the future.

So, I was wondering if the Tgs of your polymer increased after the incorporation of ILs.

I think like Md. Mominul Islam, the main mechanism for polymer decomposition is random scission of covalent bonds. Basically, the bonds absorb thermal energy until their elastic limit. Once this energy limit is overcome the bonds break. If you increase the chain mobility or introduce species that can extend this energy decay pathway you naturally increase the thermal stability. So, when you introduce ILs, the new species present can collide with the polymer chain and help in energy dissipation, since they can assume higher degrees of freedom.