Segmental Dynamics Govern the Cold Crystallization of Poly(lactic acid) in Nanoporous Alumina

Abstract

How segmental mobility influences the crystallization behavior of polymers in confined spaces is still not fully understood. In the present work, a systematic study of the segmental dynamics was carried out by dielectric relaxation spectroscopy (DRS) on racemic poly(DL-lactic acid) (PDLLA) confined in nanoporous alumina. The effect of pore size and thermal history were examined. Two glass transition temperatures (Tg) were observed, and an unusual “hysteresis” of the segmental relaxation time was detected. At lower temperatures, the segmental mobility of PDLLA “speeds up” firstly and “slows down” upon annealing. Both effects indicate the dynamically heterogeneous and nonequilibrium nature of chain segments under confinement. The “equilibrating process” of the relaxation time was monitored by DRS and the characteristic time exhibited Arrhenius behavior. The experimental evidence supports a reversible adsorption/desorption mechanism of chain segments. The enhanced cold crystallization of poly(lactic acid) (PLLA) we reported previously (Macromolecules 2015, 48 (8), 2526-2533) can be explained adequately with the “enhanced mobility” of segments. This was further justified by the shift of the cold crystallization temperature towards high temperatures of infiltrated PLLA after annealing.