Toughening of Poly(L-Lactic Acid) by Annealing: The Effect of Crystal Morphologies and Modifications

Abstract

Poly(L-lactic acid) (PLA) samples prepared by hot pressing were treated by two different processes: process a-quenching of the molten specimens in water, then annealing at high temperatures; process b-direct crystallization at high temperatures from the molten specimens. The crystal modification and morphology of PLA were investigated by differential scanning calorimetry, dynamic mechanical analysis, polarized light microscopy, and wide-angle X-ray diffraction. In the case of process a, the α (disordered α) crystal modification was formed at relatively low annealing temperature (T a < 100 • C), while the ordered α phase was formed in the case of process a at high T a (> 100 • C) and process b. Furthermore, in process a, the nucleation density of spherulites was higher and the radius of the spherulites was much smaller compared with that of the spherulites formed by process b. The effects of crystal modification and morphology on the impact behaviors of PLA were investigated by notched Izod impact testing. The macroscopic fracture toughness was discussed in terms of the microscopic structures. Finally, we suggest an alternative approach for the preparation of high-performance PLA.

Improving Mechanical Properties for Extrusion-Based Additive Manufacturing of Poly(Lactic Acid) by Annealing and Blending with Poly(3-Hydroxybutyrate)

“…For an annealing time of 2 h and an annealing temperature of 100 °C, we observed a notched impact strength that was as high as 15 kJ m −2 bearing in mind that the X c value of PLA—100 °C 0.5 h was triple that of untreated PLA. As PLA crystallized fastest at around 100 °C, a better-defined network comprising crystals and tie molecules likely forms, promoting a larger plastic deformation that can absorb more fracture energy upon impact load [17]. Fragmented, small size crystals (Figure 1a inserts) can significantly improve the impact strength of PLA by reducing the amorphous portion among spherulites, and by having fewer defects within a spherulite [17,18,41].…”

“…The performance of AM-fabricated thermoplastic parts is often unsatisfactory due to intrinsic limitations in the mechanical properties of the starting material. Several methods can be used to increase the performance: (i) reinforcing the starting materials with high strength fibers or fillers [7,10,11,12]; (ii) polymer annealing [13,14,15,16,17]; (iii) blending with a nucleating agent [18,19,20] or polymers, such as polyhydroxyalkanoates (PHAs) [8,21,22,23], which can act as crystallization controllers. Regardless of the approach used to increase the performance, consideration should also be given to controlling the layer thickness and infill density [24,25].…”

“…In general, annealing increases diffusion between distinct particle morphologies, bonding of beads, and crystallinity [10,16,25]. The latter plays a specific and important role in semi-crystalline polymers likely by improving the mechanical and thermal stability of the resulting product [13,17,26,27,28,29]. Notably, most studies illustrate the relevance of annealing for polymers produced by injection or compression molding [13,14,15,16,17]; however, there are relatively few reports concerning annealing for AM fabricated parts, highlighting that further research should be conducted in this area [10,24,25,29].…”

Evaluation of thermal resistance and mechanical properties of injected molded stereocomplex of poly(l‐lactic acid) and poly(d‐lactic acid) with various molecular weights

“…By contrast, the crystallinity degree of injection‐molded PLLA‐1 was calculated according to the following equation: …”

“…By contrast, the crystallinity degree of injection-molded PLLA-1 was calculated according to the following equation [14] :…”