Physical aging of inter- and intraspherulitic glassy amorphous structure of poly(ʟ-lactic acid) analyzed by the time-dependence of Young's modulus

Film samples of poly(ʟ-lactic acid) (PLLA) of different crystallinity, ranging from zero to about 70 %, and containing either α- or α′-crystals, were prepared and analyzed regarding the evolution of Young's modulus during long-term annealing at ambient temperature, that is, at about 40 K below the glass transition temperature. Main purpose of the study was the identification of possible different glass relaxation of interspherulitic bulk amorphous phase on one side and intraspherulitic amorphous phase located between spherulitically grown lamellar crystals on the other side. Annealing at room temperature causes a logarithmic increase of Young's modulus, being most pronounced for fully amorphous PLLA but is also measurable in semicrystalline preparations. Data revealed that the aging behaviors of inter- and intraspherulitic amorphous phases are similar, and suggest a negligible effect of, at best, minor amount of immobilized rigid amorphous fraction in interlamellar layers. Due to the relaxation of the amorphous glass, Young's modulus of fully amorphous PLLA may even exceed that of semicrystalline PLLA after long-term aging when containing lower-modulus α′-crystals. Local analysis of the aging characteristics of PLLA spherulites on one side and bulk amorphous structure on the other side by nanoindentation qualitatively confirms the observations obtained on macroscopic films and suggests new routes to assess the mechanical characteristics of structurally inhomogeneous semicrystalline polymers. • Physical aging of PLLA is controlled by the crystallinity. • The increase of Young's modulus during aging is enhanced in fully amorphous PLLA. • Amorphous PLLA located between crystals relaxes similar as bulk amorphous structure. • Nanoindentation allows simultaneous analysis of aging of intra- and interspherulitic sample volumes.