Microstructure and melting behavior of a solution‐cast polylactide stereocomplex: Effect of annealing

Abstract

The effect of annealing on the microstructure and melting behavior of a solution-cast polylactide (PLA) stereocomplex (sc) was systematically investigated by differential scanning calorimetry and small-angle X-ray scattering. A preorder state, an intermediate form between the amorphous and crystalline states, was found in the solution-cast poly(L-lactide)-poly(D-lactide) blend. When the annealing temperature (T a ) was below 220 8C, a part of the preorder state directly formed thicker sc crystallites; these corresponded to the second melting peak, which appeared around 250 8C during the heating process. Although the rest melted and became the amorphous phase, it formed a thinner lamella under the restriction of the unmelted initial sc crystallites during annealing; the melting process of this lamella was parallel to that of the new melting peak, which appeared around 220 8C. The melting of the initial crystal formed as the solvent volatilized corresponded to the range of the first melting temperature around 230 8C. When T a was above 220 8C, the preorder state melted completely, and the initial crystal experienced perfection process. Furthermore, the highest melting temperature of PLA sc (254.1 8C, with a fusion enthalpy of 125.5 J/g) was obtained when T a was 235 8C.

Improved thermal and mechanical performance of ramie fibers reinforced poly(lactic acid) biocomposites via fiber surface modifications and composites thermal annealing

“…Herein, we improved the slow crystallization behavior of PLA polymer via ramie fiber reinforcement and ramie fiber/PLA heat annealing treatment. In a reinforcing system, ramie fibers acted as a nucleating agent for PLA polymer by reducing the movement of PLA molecules during melt crystallization . Figure a–c demonstrates the change in the degree of crystallinity of PLA biocomposites evaluated using XRD tests.…”

Thermal Properties and Structural Evolution of Poly(l-lactide)/Poly(d-lactide) Blends

“…sc-PLA was first reported in 1987 and showed higher heat resistance than homochiral PLA (homo-PLA), including PLLA and PDLA . sc-PLA obtained from an equimolar blend of PLLA and PDLA has a melting temperature ( T m ) of 230–254 °C, − which is approximately 50–75 °C higher than the T m of homo-PLA. The thermal stability is enhanced for the PLLA/PDLA blend compared with those of pure PLLA and PDLA .…”

The crystallization and phase transition behaviors of asymmetric PLLA/PDLA blends: From the amorphous state

“…During the extrusion or melting process, the polymers are melted at first, and cooled to a rapid rate in the mold, which results in amorphous and transparent specimens in most cases for PLA material, these amorphous materials should annealing at different temperatures to satisfy the application requirements at various environments. From the previous reports, the crystallization and phase transition of PLLA/PDLA blends from the amorphous state with equal weight ratio are widely investigated, but crystallization behaviors of the asymmetric PLLA/PDLA blends from amorphous state do not investigate in detail. In this study, the PLLA and PDLA with different molecular weights are synthesized, and the asymmetric PLLA/PDLA blends with various compositions are prepared.…”

“…In the SC, the crystal is produced by alternatively packing 3 1 helices of polymers with opposite configuration . The SC have a higher melting temperature ∼250°C, which is ∼70°C higher than that of neat PLLA or PDLA. In addition, SC is proved to be effective nucleate agent to accelerate the formation of PLA homocrystallites (HC) .…”