A5 Physical Aging

Topic

When this polymer is maintained below its glass transition temperature, it becomes frozen in a non-equilibrium structure; its fictive temperature—the temperature corresponding to that structure were it in equilibrium—remains higher than the polymer's actual temperature. It is precisely this difference between the fictive temperature and the actual temperature that drives a slow structural relaxation toward equilibrium—a process described in greater detail by the Tool-Narayanaswamy mechanism (as outlined in another model within this set)—without any chemical changes occurring in the polymer chains. Consequently, maintaining the polymer below its glass transition temperature for a longer period allows the fictive temperature to move closer to the actual equilibrium temperature.

As structural relaxation progresses during the polymer's storage, the material undergoes progressive densification—an increasingly compact packing of its molecular chains. The closer the fictive temperature moves toward the actual equilibrium temperature through this relaxation, the more the available free volume between the chains decreases, resulting in greater densification. Thus, a longer storage time leads to increased densification. Consequently, in this model—characterized by the structural relaxation and densification present at its onset—the densification itself increases the polymer's modulus and induces brittleness; this occurs because the tighter molecular chain packing resulting from densification restricts chain mobility under an applied load. This physical aging thus contrasts with the degradation described by another model in the set, wherein random chain scission reduces the polymer's molecular weight through chemical bond breakage; in contrast, during this physical aging, no chains break, and it is solely the slow structural relaxation toward equilibrium—occurring without any chemical change—that drives the densification, modulus increase, and embrittlement. The fact that this structural relaxation proceeds without chemical alteration of the polymer chains renders physical aging—unlike chemical degradation—a process that is, in principle, reversible if the polymer is reheated above its glass transition temperature.

ID:4489