Intrinsic Metastability of the α′ Phase and Its Partial Transformation into α Crystals during Isothermal Cold-Crystallization of Poly( l -lactide)

Structural evolution of poly( l -lactide) during cold-crystallization at 80 °C was examined via simultaneous small/wide-angle X-ray scattering (SAXS/WAXS), Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The incipient α′ crystals of loosely packed 10 7 helices, formed at t c = 200 s as detected via both WAXS and FTIR, were hardly identifiable via SAXS or DSC due to its low values of density contrast or latent heat (initially Δ H α′ ≈ 12 J/g, gradually reaching 74 J/g near the end of crystallization, in contrast to Δ H α ≈ 140 J/g), was already large in lateral size (with coherence length Λ 200/110 ≈ 35 nm, increasing to 50 nm during t c = 200–400 s and remained little changed afterward). Once formed, the α′ crystals underwent continuous and persistent perfection for decreasing lattice spacing and partially transformed to smaller α crystals ( t c ≈ 800–1800 s, up to a saturated population ratio of 1:4 between α and α′ phases), followed by slower yet steady increases in SAXS invariant without significant changes in the α and α′ contents for t c > 1800 s. It is concluded that the α′ form is a transient structure of continuously increasing packing density and latent heat toward the α form, reminiscent of the mesomorphic phase as precursors to stable crystals in other polymers. Nevertheless, transformation to α crystals occurred when the lattice parameters of the α′ phase were closer but still finitely different from those of the α phase and that the conversion between the two phases was limited in this isothermal case. Both observations suggest that, in spite of the transient nature of the α′ phase, the final transformation step to the stable α form is still achieved via a first-order route.