DYNAMIC SHEAR MODULUS IN THE SPLITTING REGION OF POLY(ALKYL METHACRYLATES)

A systematic study of the dynamic shear modulus G* = G' + G'' in three poly(alkyl methacrylates) (PEMA, PnPMA, PnBMA) at frequencies between 0.001 and 500 rad/s is presented. As the alpha beta splitting frequencies omega(s) are low, aging effects can be observed in the splitting region. There is a systematic shift of the splitting frequency omega(s) to lower values with increasing length of the alkyl side group. In PnBMA a separate shear alpha appearance is observed about two frequency decades below the local mode beta. This is discussed in terms of the concept of minimal cooperativity. Aging effects are: Shift of the maximum loss frequency omega(alpha) to lower values, peak sharpening of the alpha relaxation, and intensity changes of alpha and beta. These effects are discussed in terms of the sequential aging concept. Aging leads to a pronounced bending of the alpha traces upwards from the equilibrium line in the Arrhenius diagram. These non-equilibrium phenomena are promoted by the small slope m(alpha) = d(log omega(alpha))/dT of the alpha trace in the splitting region.