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 alpha and alpha' 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 T-B. (C) 2012 Elsevier B.V. All rights reserved.