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

The little amount of internal double bonds were introduced in poly (butyl acrylate) (PBA) and poly (ethyl acrylate) (PEA) through copolymerizing with isoprene (IP). The tanδ of PBA-co-IP (BA-IP copolymer) and PEA-co-IP (EA-IP copolymer) exhibited an asymmetric double-peak structure with a shoulder at higher temperature side and a maximum at lower temperature side, while the tanδ of PBA and PEA generally displayed single-peak structure. The relaxation spectra showed motion units whose relaxation time were larger than 100 s appeared; so the shoulders were preliminarily determined as the slower processes. The chain-ring assumption was used to analyze this phenomenon. The single bonds that are adjunct to internal double bonds were thought as the “rings” and segments that locate between two neighboring single bonds were thought as the “chains”. Because “rings” single bonds had larger rotation ability than general single bonds, the motions of “chains” between two neighboring “rings” would be improved. Thus, PBA-co-IP and PEA-co-IP displayed asymmetric tanδ curves in mechanical spectra.