Broadband dielectric investigation of amorphous and semicrystalline L-lactide/meso-lactide copolymers

The dynamics of crystalline and amorphous samples of poly(L-lactide) and three L-lactide/meso-lactide random copolymers were investigated in the frequency domain using broadband dielectric spectroscopy (DRS). Two relaxations, the segmental (α) and subglass (β) processes, were observed in the temperature range between -100 and 105°C. Modification of the characteristics of the α process, i.e., the relaxation strength (Δε), mean relaxation time (τ_m), and shape parameters, as a function of crystal content was investigated on "fully crystallized" samples as well as in real-time crystallization experiments. As expected, the strength of the α relaxation was smaller and its distribution broader for materials with higher degrees of crystallinity. The relaxation strength of the α process of the amorphous samples increased with temperature, while that of the crystalline materials changed very little or in the opposite direction with temperature. This behavior can be explained by the existence of a rigid amorphous component. Changes in τ_m and T₀ (the Vogel temperature) for the copolymers crystallized at various T_c are influenced by crystalline microstructure as well as by crystalline content. A relaxation is also observed in higher temperature (125-155°C) experiments on amorphous and crystalline materials. The evidence at hand suggests that these processes are associated with the normal mode and α_c process, respectively.