Physical aging in the glassy state of a thermosetting system vs. extent of cure

The physical aging behavior of a high‐Tg amine/epoxy thermosetting system has been investigated vs. change of chemical structure induced by cure and vs. aging temperature (Ta) using the Torsional Braid Analysis (TBA) technique. The chemical structure was changed systematically from monomer to highly crosslinked polymer by curing in the equilibrilium state (T > Tg). The aging temperatures ranged from just below the glass transition temperature to deep in the glassy state (Ta > Tg). In the absence of chemical reaction, the physical aging rate at a given temperature, Ta, passes through a minimum with increasing chemical conversion (i.e., change of chemical structure). Analysis of this behavior is simplified by using Tg as an index of measurement of extent of cure. There is a superposition principle for normalizing the physical aging behavior of the thermosetting glasses, which involves a shift of Tg–Ta and a shift of C(Ta) (a function of aging temperature), regardless of chemical structure. Analysis reveals that: (1) this behavior is the consequence of the Tg and Tβ transitions, (2) the segmental mobility (1/τ) is a function of the deviation from equilibrium (as measured by Tg–Tg and the aging time), (3) the segmental mobility, which is involved in the physical aging process in the glassy state, is insensitive to the extreme changes of chemical structure (from monomer, to sol/gel polymer, and to highly crosslinked polymer), and (4) physical aging deep in the glassy state affects both segmental mobility and cohesive energy density.