Experimental evidences for the correlation between chair-to-chair conversion and dynamic fragility in poly(cyclohexyl methacrylate)/hindered phenol blends

Abstract

Polymers with cyclic or ring topologies are typically endowed with unusual molecular dynamics considerably different from their linear or branched counterparts. Here, the primary and secondary dynamics of neat poly(cyclohexyl methacrylate) (PCHMA) and its blends with hindered phenols were systematically investigated by a combination of dielectric and enthalpy relaxation. Notably, neat PCHMA owns a glass transition temperature Tg ∼ 299 K, while the segmental fragility m can be as low as about 42. After introducing hindered phenols capable of forming intermolecular hydrogen bonds, m increases in the same direction as Tg. Surprisingly, the observed increase in segmental fragility exhibits a strong correlation with the suppression of the dielectric γ-relaxation process, which has been previously attributed to chair-to-chair transitions of the cyclohexyl side groups. The concomitant decrease in accessible configurational states is further evidenced by enthalpy relaxation. We suppose that the γ-relaxation may involve less degree of intermolecular freedom, which can be speculated from its increased activation energy that is comparable to that of the Johari–Goldstein secondary process, as well as the broadened γ-peak breadth. These results provide solid experimental evidence that a correlation does exist between the conformational change and fragility and therefore pave a new routine to fabricate strong polymeric glass formers by introducing the ring-containing topology structures.