The chain relaxation behavior of ethylene-propylene-diene rubber (EPDM) was investigated by dynamic mechanical analysis (DMA).The results indicated that there were three tanδ peaks for EPDM at low frequencies

which were attributed to glass transition (α transition)

sub-Rouse mode (α' transition) and Rouse mode (α" transition)

respectively.In addition

compared with the glass transiton

sub-Rouse mode has a much stronger frequency-dependence

showing that the sub-Rouse mode moves to high temperature faster than the α mode with the augment of frequency

which is sensible and consistent with the previous works as the local segmental motions are more intermolecularly coupled than the sub-Rouse mode relaxation in polymers

such as PS

PVAc and PMMA.After comprehensive analyses of frequency-domain spectra of EPDM via figuring out the temperature dependence of relaxation time

relaxation strength

coupling parameter and shape parameter

a dynamic crossover at TB-384 K was found in sub-Rouse (α') mode.What's more

the crossover relaxation time τα'(TB) is calculated as 0.11 s

approximately

which is remarkably close to that of PS

PVAc and PVAc/PEO blends (10(-0.5±0.5) s)

indicating that this crossover arises at a particular point of relaxation time.According to the coupling model

the crossover is suggested to be caused by the strong increase of intermolecular cooperativity below TB.