Polymorphic Transition in Disordered Poly(l-lactide) Crystals Induced by Annealing at Elevated Temperatures

Abstract

Effects of annealing conditions and molecular weight (MW) on the crystalline phase transition in poly(L-lactide) (PLLA) were studied by wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The disordered crystal (R′-form) of PLLA was found to transform into the R one during annealing process at elevated temperatures. The R′-to-R transition is quite dependent on the annealing period (t a : 0-1440 min) and annealing temperature (T a : 120-160°C). With increasing T a , the polymorphic transition progresses much more rapidly. The R′-to-R transition is mainly involved by the slight rearrangement of the chain conformation (especially related to the side groups) and packing manner in the unit cell to the more energy-favorable state, corresponding to the reduction of unit cell dimension. Besides, it was proposed that the R′-to-R transformation mainly proceeds by the direct solid-solid transition mechanism. Moreover, it was found that MW affects the crystalline phase transition significantly. In the low-MW PLLA sample, the R′-to-R transition becomes much faster, and it can proceed prominently even when annealed at relatively lower temperature.

The effect of two commercial melt strength enhancer additives on the thermal, rheological and morphological properties of polylactide

“…This behavior has been reported as a result of a lamellar rearrangement during crystallization of PLA [44]. The peak at the lower temperature is related to the melting of the disordered α' crystalline form, which can be produced by annealing at lower temperatures, and its recrystallization into the ordered α form, while the second peak corresponds to the melting of the crystal α form [44][45][46]. Cartier et al [47] described the double melting peaks as a result of the difference of crystalline structure that can exist in PLA.…”

Development of novel melt-processable biopolymer nanocomposites based on poly(l-lactic acid) and WS2 inorganic nanotubes

“…However, another possible interpretation in the case of PLLA is due to the phase transition from α′-form crystals to α-crystals and the melting of the astransformed α crystals. 14,[44][45][46] To date no consensus has been reached as for the exact origin of the multiple melting behaviour of PLLA. Fig.…”

Application of Differential Scanning Calorimetry to the Characterization of Biopolymers

“…The presence of filler marginally decreased specific enthalpy values of PLA. The presence of multiple melting peaks in thermograms of annealed samples can be explained by applied annealing that induce other crystal population, namely ´ (initially created with grain like morphology) and  (during annealing created with spherulitic morphology) crystals (Zhang et al, 2008;Pan et al, 2008). Melting temperature for unannealed neat or filled PLA samples were recorded between 134-140°C for the first melting peak and 150-151°C for the second melting peak.…”