Salient Features of Amorphous Fraction Dynamics in Stretched Poly( l -lactic acid)

Abstract

Melt pre-deformation is widely used to tailor the morphology and properties of semicrystalline polymers, yet its influence on the dynamics of the amorphous fractions remains poorly understood. Here, we investigate the evolution of the rigid amorphous fraction (RAF) and mobile amorphous fraction (MAF) in semicrystalline poly(l-lactic acid) (PLLA) upon deformation. Small-angle X-ray scattering (SAXS) shows that melt pre-deformation at different stretching ratios followed by thermal annealing produces semicrystalline PLLA with well-defined lamellar structures, whose long period and average lamellae thickness vary little with melt pre-deformation. In contrast, broadband dielectric spectroscopy (BDS) reveals a pronounced slowdown and broadening of the segmental dynamics, enabling the distinct dynamics of the RAF and MAF to be resolved. Temperature-modulated differential scanning calorimetry (TMDSC) and wide-angle X-ray diffraction (WAXD) confirm that no melting or recrystallization occurs over the temperature range of the BDS measurements, ensuring that the observed dielectric relaxation reflects the intrinsic dynamics of the amorphous fractions. Importantly, the RAF thickness determined by BDS agrees well with that obtained from TMDSC and increases upon cooling and with increasing melt pre-deformation beyond a threshold deformation ratio. Although the RAF relaxes significantly more slowly than the MAF near the glass transition, its apparent glass transition temperature is only ∼4–7 K higher and remains well below the onset of melting and recrystallization. These findings reveal that melt pre-deformation primarily modifies the amorphous polymer dynamics, rather than the lamellar morphology, of semicrystalline PLLA.