Polylactide (PLA) is a biodegradable semi-crystalline polymer that has attracted enormous attention over recent years as a biocompatible and environment-friendly material [1,2,3]. It has been approved by the U.S. Food and Drug Administration for biomedical applications and contact with body fluids, e.g., as bioresorbable artificial ligaments or drug delivery systems [4]. PLA is also one of the most important thermoplastic materials for 3D printing [5]. It is easily processable, although its brittleness prevents tensile drawing. The tensile modulus and strength of neat PLA can be increased and the strain at break can be reduced when the polylactide matrix contains a significant amount of crystalline fraction [6]. The crystallinity degree also governs the barrier properties of polylactide [6]. The crystallization behaviour of polylactide has been extensively investigated (details can be found in comprehensive reviews, e.g., [6,7,8]). Two optically active forms (L and D) exist due to the presence of an asymmetric carbon atom in molecules of lactic acid. The commercially available PLA is usually rich in the L-structures, but is not isotactic. D-units that separate longer L-isotactic sequences in statistical copolymers disturb chain order and the morphology of PLA crystals.
In general, the type of crystal structure depends on the crystallization conditions. The most common orthorhombic α’- and α-crystals that have similar 10₃ helical chain conformations and belong to the same crystal system are formed in melt and solution crystallizations. The less ordered α’-form is obtained exclusively at crystallization temperatures below 100 °C, or as an admixture coexisting with the α-form, when PLA is crystallized isothermally at temperatures below 120 °C. The presence of α′ crystals in PLA results in higher elongation at break but also lower modulus and barrier properties than those of samples containing more tightly packed α crystals. A less thermally stable β-form of a frustrated structure (three polylactide chains of 3₁ helical conformation are randomly oriented up and down in a trigonal cell) is obtained by stretching the α-form at high draw ratios in the hot-drawing of melt- or solution-spun fibres. The γ-form was obtained by the epitaxial crystallization of PLA on hexamethylbenzene. In this form two polymer chains of 3₁ helical conformation are oriented antiparallel in the crystal cell. 

2. Applications and Development

Considerable attention was given to the improvement of crystallization kinetics (nucleation and crystal growth) that can be enhanced by nucleators and/or plasticizers. Numerous potential nucleating systems have been examined in the literature, including “green” nucleating agents. PLA-based polymer composites and nanocomposites containing nanoclays [9,10,11], nanosilicas [12,13], carbon nanotubes [14,15], or graphene [16] are well known. Another class of additives for polylactide that can improve its crystallization rate are species that exploit supramolecular phenomena, e.g., hydrogen bonding or host-guest effects, to interact with PLA macromolecules (“soft templating”). Hydrogen bonding is also important for the formation of poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) stereocomplex structures (SC) [17,18,19,20]. The specific C-H···O=C interactions between the paired stereoisomeric PLLA and PDLA chains play a very important role in the formation of SC crystals. It was found that the racemic (3₂/3₁) helical conformation of the pair of macromolecules starts to emerge in the melt of a racemic blend, and the formed structures subsequently act as nucleating sites upon cooling [21]. Thus, the conformation of the polylactide chains may be changed by intermolecular interactions with neighbouring macromolecules. It means that the crystallization of the PLA matrix can also be adjusted by hydrogen bonding species, even before the true crystal nuclei emerge. Hydrogen bonding between polylactide and nucleating agents has been postulated, e.g., for PLA blends with amino acids or poly(amino acids) [22], carbon nanotubes [23], phtalimide [24], bisurea derivatives [25], or d-gluconic acid derivatives [26]. PLA was also modified with macromolecular nucleators, such as linear polysilsesquioxanes [27,28,29] or their cyclosiloxane analogues [30] with side substituents acting as donors/acceptors of hydrogen bonds (-OH···O=C-; -COOH···O=C-; and -C-F···H₃C-) to/from the polyester backbone.

Extremely efficient nucleating systems are based on arylamides and arylhydrazides that may self-organize in the polymer melt. Unfortunately, not all supramolecular nucleators are biodegradable, bioabsorbable, or nontoxic. The potential problems with some of those organic compounds can be a significant concern in the biomedical field. Therefore, polylactide matrices have been also blended with harmless biobased nucleating agents (e.g., orotic acid, humic acids, fulvic acids, nanocellulose, and cyclodextrins). The key feature of those additives is their capability for "soft templating". In this review, the research progress on PLA nanocomposites containing both groups of nucleating agents has been evaluated in the context of their biomedical applicability.

3. Conclusions