Why tanδ of poly (butyl acrylate) and poly (ethyl acrylate) with little double bonds are becoming asymmetric?

Why tanδ of poly (butyl acrylate) and poly (ethyl acrylate) with little double bonds are becoming asymmetric?

Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Polymer - Volume 55, Issue 18, 2 September 2014, Pages 4720-4727

Rui Zhang, Xianru He, Hui Yu,