Poly (lactic acid) has been the subject of much attention because of its outstanding performance in terms of high stiffness and strength, biodegradability and thermal processability and is often seen as an alternative to traditional petrochemical plastic [1
]. However, PLA products, under practical processing conditions, are well known to show low crystallinity or are in an amorphous form due to the intrinsic slow crystallization rate, which limits wider applications in sectors, such as automotive and packaging fields [3
]. By and large, commercial PLA products with a high molar mass are usually produced from lactide, the ring-formed dimer of lactic acid, via the ring-opening polymerization (ROP) route [4
]. As an important intermediate for the industrial production of PLA, lactide exists in three different forms, l
-lactide and l
-lactide, due to the chiral nature of lactic acid. Commercial PLA is made of copolymers of poly(l
-lactide) (PLLA) and poly(d
-lactic acid) (PDLLA). The l
-isomer constitutes the main fraction of PLA derived from renewable sources, since the majority of lactic acid from biological sources exists in this form. Gupta and Kumar [5
] reviewed various aspects of PLA synthesis and mentioned the kinds of catalysts in the production of PLA. Specific catalysts can lead to heterotactic PLA, which has been found to show crystallinity. It has been reported [6
] that PLA can crystallize in three forms (α, β and γ), depending on the composition of the optically-active l
- and d
-lactide, and the α phase is more stable and has a melting temperature of 180 °C compared to the β phase, with 175 °C [7
The degree of crystallinity, which has profound effects on the structural, thermal, barrier and mechanical properties, depends on the ratio of d to l enantiomers used. PLA with l-form content greater than 90% tends to crystalline, while those with lower optical purity are amorphous. Furthermore, a suitable selection of the PLA commercial grade with different l-/d- ratios is critical for the requirement of the conversion processing conditions and specific properties.
In processes, such as injection molding, where the orientation is limited with a high cooling speed, it is much more difficult to develop significant crystallinity, and thus, formulation or processing modifications are necessary. Alternatively, nucleating agents can be added to PLA for the promotion of crystallinity via traditional processing, such as injection molding, under a suitable thermal history and cycle time. Basically, the normal nucleating agents reported by some groups are various kinds of inorganic nanoparticles, such as talc, sodium stearate, calcium lactate [8
], montmorillonite (MMT) and carbon nanotubes (CNT) [9
]. It is shown that the crystallization half-time can be required by more than one order of magnitude to less be than 1 min when 1% talc is added [8
]. Another reported potential nucleating agent in the literature is the stereocomplex of PLLA and Poly(d
-lactic acid) (PDLA). Yamane and coworkers [11
] analyzed the crystallization behavior of PLLA with PDLA as a nucleating agent, which formed a large stereocomplex crystalline and effectively increased the number of PLLA spherulites and then the overall crystallization rate. To sum up, the crystallization behavior [12
] of PLA depends on the component isomer, processing temperature, annealing time and molecular weight. In general, crystallinity control of injection-molded PLA can be achieved by optimization of the processing parameters (thermal history) and the formulation of materials (stereochemistry). A better understanding of the crystallization behavior and its effects on the mechanical properties is critical for PLA to extend its application.
However, regardless of the component isomers, both the amorphous and crystalline polylactides show brittle behavior at room and body temperature during application as films, fiber or biomedical materials [13
]. The toughness improvement is a crucial necessity for many consumer applications, such as food package. Numerous approaches, such as plasticization, block copolymerization, blending with tough polymers and rubbers, have been adopted to improve the toughness of brittle PLA bioplastic. Therefore, plasticization of PLA composites with various kinds of low molecular weight compounds to optimize the mechanical properties is an important field of research. For example, polyethylene glycol (PEG), polypropylene glycol (PPG), glycerol and citrate ester are investigated as plasticizers for PLA to lower its glass transition temperature, increase ductility and improve processability [14
]. However, the major drawbacks of those modifications are the consequent decreases in the strength and modulus of the toughened PLA. There is another limitation for wider PLA industrial applications, which is its poor thermal resistance and limited gas barrier properties, especially in the packaging field. Therefore, preparing a PLA-based material having a good stiffness-toughness balance with high bio-based PLA content, which keeps its original biocompatibility and biodegradability, is one of the big challenges.
In this work, the combination of plasticizer and nucleating agents with PLA aims at the synergistic effects of enhanced crystallinity and reduced injection molding cycle time for PLA products. The crystallization behavior of PLA will be investigated by both DSC non-isothermal and isothermal analyses. The nucleating effect of different nanofillers is discussed according to crystallization parameters, such as the half-crystallization time, crystallinity and the cold crystallization temperature of PLA composites. The relationship between crystallinity and mechanical properties was also investigated by dynamic mechanical thermal analysis (DMTA) and tensile measurements.