Rheology of Polymers Near Liquid-Solid Transitions

Polymeric materials near the liquid-solid transition (LST) exhibit a very distinct relaxation pattern. The reference point for analyzing these patterns is the instant of LST at which relaxation becomes self-similar over wide ranges of the relaxation time. The universality of this transition and its consequences have been explored extensively during the past decade. This study will present an overview of rheological implications inherent in liquid-solid transitions of polymers. The LST can be most reliably detected in a dynamic mechanical experiment in which the frequency independence of the loss tangent marks the LST. A wide variety of rheological observations of materials in the vicinity of an LST are discussed with respect to their universality. It is shown that polymer chemistry, molecular weight, stoichiometry, temperature, inhomogeneities, etc. greatly influence the material behavior near the LST. However, the characteristic self-similar relaxation is shown by all investigated materials, independent of the nature of the LST (e.g., both, physically and chemically crosslinking polymers). Several theories predict chemical and rheological properties in the vicinity of an LST. They are briefly discussed and compared with experimental results. A variety of applications for polymers near LST are presented that either already exist or can be envisioned. The self-similar relaxation behavior which results in a power law relaxation spectrum and modulus is not restricted to materials near LST. Different classes of polymers are described that also show power law relaxation behavior. What makes the self-similar relaxation specific for materials at LST is its occurrence at long times with the longest relaxation time diverging to infinity.