Aliphatic polyesters as models for relaxation processes in crystalline polymers: 3. Mechanical relaxation in copolymers of adipic acid with 1,6 and 2,5-hexanediols

Abstract

The shear moduli of a series of the title polyesters spanning a crystallinity range of 0–60% have been measured as a function of temperature at ≈1 Hz using a torsion pendulum. The experimental isochronal temperature scans of G′ and G″ are fitted to phenomenological equations. With only minor adjustments, the same relaxation spectrum parameters as found dielectrically for these polymers (relaxation shape, central relaxation times) fit the mechanical data. Thus, for the β (glass—rubber) relaxation in the amorphous fraction the broadness is very sensitive to the presence of the crystal fraction and becomes increasingly broad as the degree of crystallinity increases. In contrast, the γ process dynamic behaviour is insensitive to the presence of and degree of crystallinity. Unrelaxed and relaxed moduli values are determined for the γ and β processes. A composite model approach is used to determine bounds on the amorphous-phase unrelaxed and relaxed γ and β moduli from the bulk specimen values. As was the case dielectrically, the γ process, in addition to being assigned to the amorphous fraction, has a strength that depends on the diol composition also. The relaxed ( γ + β) amorphous-phase rubbery shear modulus is bound reasonably well from application of the composite model to the bulk specimen values and is assigned the value 100 ± 20 MPa at 250 K. It decreases relatively strongly with increasing temperature.