Dynamic crossover of the sub-Rouse modes in the glass-rubber transition region in poly(n-alkyl methacrylates) with different side chain lengths

►Relaxation behavior of poly(n-alkyl methacrylates) was investigated by the high precision shear mechanical spectroscopy. ► The dynamic crossover of the sub-Rouse modes above glass transition in poly(n-alkyl methacrylates) was observed and compared. ► The crossover relaxation time is essentially independent of side chain length and constant. ► The crossover is supposed to be the distinct variation of intermolecular coupling and cooperativity at TB.