Nature of the Sub-Rouse Modes in the Glass-Rubber Transition Zone of Amorphous Polymers

The shear mechanical compliance of the glass-rubber transition or softening zone of high molecular weight amorphous polymers typically increases from the glassy compliance of similar to 10(-10) cm(2)/dyn up to the rubbery plateau of similar to 10(-6) cm(2)/dyn. The contributions from the local segmental relaxation and the Rouse modes cannot account for the entire range of compliance, leaving three decades of compliance in between their contributions unaccounted for. Although new viscoelastic mechanism called sub-Rouse modes with length scales intermediate between the local segmental relaxation and the Rouse modes have been found to fill the void, so far they have not been found in archetypal polymers including polystyrene (PS) and poly(methyl methacrylate) (PMMA). This calls the generality of the existence of sub-Rouse modes into question. Using high precision shear mechanical spectroscopic tool, we report the observation of the sub-Rouse modes in PS and PMMA. From the various properties of the sub-Rouse modes determined, we find that the sub-Rouse modes are intermolecularly coupled like the local segmental relaxation, albeit to a less degree. The results are isomorphic to those found in poly(vinyl acetate) (PVAc) and PVAc-poly(ethylene oxide) (PVAc-PEO) blends. Thus, the sub-Rouse modes and its properties seem general in amorphous polymers.