2 research outputs found
Mesophase-Mediated Crystallization of Poly(l‑lactide): Deterministic Pathways to Nanostructured Morphology and Superstructure Control
The
effect of the CO<sub>2</sub>-induced mesophase on the isothermal
crystallization of polyÂ(l-lactide) (PLLA) was investigated
by infrared (IR) spectroscopy and microscopy. It was found that the
crystallization rate of PLLA was significantly enhanced by the CO<sub>2</sub>-induced mesophase, showing that the crystallization was completed
even in a short period of 10<sup>0</sup>–10<sup>1</sup> s.
Compared with the directly crystallized samples that showed typical
spherulites with lamellae, the crystallization via CO<sub>2</sub>-induced
mesophase led to nonspherulitic (granular or featureless) morphologies
consisting of nanorods, whereas the polymorphic behavior remained
unaffected by the initial state, resulting in crystallized PLLA containing
identical polymorphs of uniquely different nanostrutured morphologies
and superstructures. The IR imaging results indicated that the formation
of the equilibrium crystal was preceded by the formation of various
metastable intermediate phases, including mesomorphic phase, preordering,
and metastable crystal, all of which continuously evolved with time.
The nucleation process proceeded via a similar pathway. In contrast
to the negligible contribution of mesophase to the nucleation in direct
crystallization, the CO<sub>2</sub>-induced mesophase with extremely
high nucleation density underwent disordering–reorganization
into the preordering, thereby providing a tremendous number of active
nucleation sites for enhancing crystallization and serving as building
blocks for nanorods. Importantly, the present results highlight the
decisive role of mesophase in directing the nanostructure and superstructure
and support a multistep process for the crystallization (including
nucleation and crystal growth) of PLLA, validating the Ostwald step
rule, providing mechanistic insights into the crystallization of polymers
Highly Enhanced Mesophase Formation in Glassy Poly(l‑lactide) at Low Temperatures by Low-Pressure CO<sub>2</sub> That Provides Moderately Increased Molecular Mobility
The mesophase structuring
in melt-quenched polyÂ(l-lactide)
(PLLA) treated in low-pressure CO<sub>2</sub> at 2 MPa and 0–35
°C was investigated by using infrared spectroscopy, differential
scanning calorimetry (DSC), temperature-modulated DSC, and atomic
force microscopy (AFM). It was found that the mesophase formation
in glassy PLLA was significantly enhanced, in particular at lower
temperature (0 °C), which promoted a distinctly faster formation
rate. AFM results revealed that the CO<sub>2</sub>-enhanced mesophase
exhibited nodular morphology with dramatically increased nucleation
density. A framework of multistage model in combination with the moderately
improved molecular mobility exerted by CO<sub>2</sub> was proposed
to explain the main findings. Because of the moderate molecular mobility,
a tremendous number of metastable mesomorphic layers were formed and
stabilized by the accompanying development of the rigid amorphous
fraction (RAF), leading to the immobilization of the remaining mobile
amorphous fraction (MAF). The mesomorphic phases were converted to
more stable crystals via cooperative structural reorganization upon
the devitrification of the RAF, requiring high chain mobility, showing
time and temperature dependence. Consequently, the amorphous PLLA
transiently transformed to the mesophase before transforming into
the crystal during treating at a relatively high temperature (35 °C).
Alternatively, upon heating, the mesophase underwent disordering–reorganization
to form active crystallite, profoundly promoting the cold crystallization
of the surrounding restored MAF, resulting in obviously depressed
cold-crystallization temperatures. The present results have important
implications in understanding and regulation of the crystallization
of polymers