Anomalous viscoelastic properties of polymers: Experiments and explanations

On this occasion recognizing Kia L. Ngai’s contributions to the understanding of the relaxations of many-body interacting molecular systems I present a brief critical review of current research, which is related to our collaborative efforts on polymer viscoelastic properties, which have extended over a quarter of a century. These research efforts include the observations and explanations of violations to some seemingly established principles held in the realm of the kinetic behavior of glass-forming systems both polymeric and small molecules. Successful time-temperature reduction, which has come to be known as thermorheological simplicity (TS), requires that a single monomeric friction coefficient controls the entire spectrum of the primary or α viscoelastic response. Secondary β, γ, and δ mechanisms are recognized to follow different temperature dependences, which reflect different friction coefficients. Summarized herein are the violations of TS that I and my collaborators have encountered, which in the collaborative efforts with Ngai have been extensively rationalized with Ngai’s Coupling Model. Our results have been confirmed many times by others during the past decade. A recent study published by other workers has suggested an additional universal property. In view of this recent research activity on this important subject, previously unpublished data are presented to test the proposed ‘universal’ behavior.