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

The dynamic crossover of the sub-Rouse modes (a transition from one Vogel–Fulcher–Tamman law to another in relaxation map) above glass transition in poly(n-alkyl methacrylates) (PnAMA) was observed and compared for the first time. The α and α′ relaxations become better resolved from each other with increasing side chain length, while the crossover relaxation time is 10−0.5 ± 0.5 s, essentially independent of side chain length of the PnAMA, indicating the universal presence of the sub-Rouse modes in PnAMA. Analysis of the mechanical spectra shows that the crossover originates from change of intermolecular coupling of the sub-Rouse modes when temperature crosses TB.