Dear Anand, the activation energy of relaxation processes is determined by steric conditions. My questions are: are you talking about the apparent activation energy of alpha (Tg) or beta (local) or gamma processes? (Perhaps if the polymer is semi-crystalline, it can be a pre-melting peak). Is the bromine substituent attached to the main chain or to a side chain? Is it attached to aliphatic or aromatic moiety? What kind of polymer did you modify? Is it copolymerization or so-called poymer-analog reaction after the synthesis of the polymer chain. Rich data are available on halogen-substituted alkyl methacrylates. You may alos try to look for data on analogous chlorine substituted polymers.

Dear shir, I have synthesized polybenzimidazole (homopolymer) (aromatic) and bromine present in the main chain of the polymer and alpha process.

Dear Anand, PBI is a fairly special polymer with a very dense structure and with relatively symmetric main chain. Probably any substitution would decrease the symmetry and increase the free volume, resulting in lower glass transition temperature and higher mobility. Chapter 2.3 in the following thesis: http://ncl.csircentral.net/739/1/th1706.pdf briefly discusses the effect of substitution on gas permeability in PBI based polymers, which is closely related to the free volume in the polymer. See also

Article Investigation of gas permeation properties of systematically...

Atoher thesis on a related theme:

https://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=3677&context=etd

Could you determine the positon of Br substitution? It is well known e.g. that NH substitution reduces the H-bonding in PBI thus leading to a reduced Tg and probably lower activation energy. Maybe it is worth to study the following article: Article Support Vector Machine-based QSPR for the Prediction of Glas...

https://www.google.ch/patents/US7259230 also discusses substitution effects on PBI thermal properties.

I hope it helps.