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acid to synthesize nano‐amphiphilic chitosan. The PLLA‐modified chitosan (0.5–1.5%) along with PLLA/PDLA (50/50) was melt blended using extrusion followed by injection moulding to form dumbbell sc nanocomposite specimens. Heat treatment (annealing above 160°C) led to the exclusive formation of sc crystals with ~40% crystallinity. Also, cooling the nanocomposite from the melt at 2°C/min increased the crystallinity to ~70% with an exclusive formation of sc crystals. The heat distortion temperature was elevated from 70(sc‐PLA) to 145°C for the sc nanocomposite containing 1.5% filler. In another study, the use of nano‐amphiphilic chitosan (1–3%) in the blends of PLLA/PDLA (50/50) has been demonstrated [20]. The filler was mixed with equal amounts of PLLA and PDLA and mixed by stirring followed by solvent casting into films. The solvent (chloroform) was allowed to evaporate at room temperature for 24 h followed by drying under vacuum at 50°C for 24 h and then annealing at 120°C for 2 h. The sc crystallites were formed in the nanocomposites with a ~56% crystallinity, where the degree of stereocomplexation became higher upon melt cooling as compared with annealing the blend film of PLLA/PDLA (control). The nanocomposite films showed ~56% reduction in the oxygen permeability as compared with the blend film of PLLA/PDLA (control). The addition of the nanofiller also led to an increase in hydrophobicity of the nanocomposite, which was attributed to the increased surface roughness, as well as crystallinity. The viability of fibroblast cells (BHK‐21) on the surface of the nanocomposites have been determined, manifesting the biocompatible behavior of the composite materials.
In another study, the biocomposites of sc‐PLA were prepared by employing cellulose microcrystals (CMC) as a filler (1–10%). The ROP technique was used to develop PDLA‐grafted CMC, which was mixed with PLLA at 50/50 ratio and melt extruded, followed by injection moulding to prepare the biocomposite specimens. The improved dispersion of CMC led to the formation of sc crystallites and suppressed the homo‐crystallite formation. This CMC/sc‐PLA biocomposites resulted in significant improvement of the tensile strength (~96%) as compared with sc‐PLA along with a high storage modulus (~3500 Pa). The enhanced sc formation and the incorporation of CMC reduced the permeability of oxygen and water vapor, suggesting its potential for engineering and packaging applications [81].
The use of nano‐hydroxyapatite (n‐HAP) has drawn enormous attention in the biomedical field because hard bio‐tissues such as human bones and teeth are composed of n‐HAP. In order to exploit sc‐PLA and n‐HAP for biomedical applications, their biocomposites were prepared in a study by Gupta et al.; the n‐HAP was grafted to PDLA via in situ ROP where the OH groups on n‐HAP acted as initiating species. The grafting was confirmed by 13C NMR and thermogravimetric analysis [82]. The grafted PDLA was blended with PLLA to develop sc biocomposites, which gave the exclusive formation of sc crystallites due to the improved dispersion of n‐HAP and extended molecular surface area provided by the PDLA chains. The nanocomposites exhibited improved mechanical properties (~40 MPa in strength, ~132% elongation at break, and ~47% increase in storage modulus). The increase in crystallinity resulted in improved resistance to moisture, as well. The viability of BHK‐21 cells on the nanocomposites revealed their applicability as a biomaterial.
FIGURE 5.5 Applications of the sc‐PLA‐based copolymers and composites.
These findings show that sc‐PLA‐based copolymers and composites are promising for addressing polymer processing and application concerns in the packaging and biomedical domains. The applications of sc‐PLA‐based copolymers and composites are shown in Figure 5.5.
5.7 Advances in Stereocomplex‐PLA
Several processing techniques such as electrospinning [29, 84] and melt spinning [85] have gained enormous recognition for developing sc‐PLA fibers with improved stereocomplexation [86–88). Often, targeted biomedical applications require a controlled hierarchy, which may be possible by selectively modifying the surface of nanofibers [89]. One such method reported by Xie et al. is the combined use of electrospinning and a controlled polymerization technique for designing PLA nanofiber shish kebabs. The sc‐PLA nanofibers produced by electrospinning are used as the shish where the secondary polymer (hc‐PLA or sc‐PLA) is decorated to form a kebab lamella. The soft epitaxy mechanism possibly leads to the formation of shish kebab structures where the sc‐PLA nanofibers (shish) serve as the nucleating sites for kebab lamellae [25]. Such controlled mechanisms are of substantial importance when functionalizing the surface of nanofibers required for intended applications. Furthermore, the functionalization of sc‐PLA using cyclodextrins has facilitated their use as pollutant absorbers and drug carriers [90]. The exclusive formation of sc‐PLA has been achieved during electrospinning along with its functionalization, where the sub‐micrometer dispersion of nanofillers, namely polyhedral oligomeric silsesquioxanes (POSS), has been established. The functionalized sc‐PLA retains the capability of forming pure stereocomplex upon annealing [91]. Nevertheless, efforts have also been made to tune the hydrolytic degradation of sc‐PLA by tailoring the backbone architecture. Stereocomplexation between PLLA and PDLA oligomers has been reported, where the polymer architecture and the end groups altered the hydrolytic degradation rate [92]. Namely, the linear sc architecture having alcoholic end groups exhibited an increased degree of stereocomplexation with higher hydrolytic stability. In contrast, for the polymer with carboxyl chain ends, the degradation was accelerated due to the lower degree of stereocomplexation. The use of sc‐PLA has also been explored in textiles and membranes for oil–water separation. The modification of PLA nonwoven fabric by the formation of sc crystals has been reported by Zhu et al., where the sc crystal phase increased the surface roughness, as well as imparted oleophilicity to the fabric. The modification of the surface by sc‐PLA increased the oil absorption capability by 30–40% [93], which can be repeatedly used for the same purpose. The sc‐PLA, being brittle in nature, is limited to only specific applications. When exploring sc‐PLA for tissue engineering applications, elasticity is often required for the scaffold materials to serve physiological functions [94, 95]. This may be made possible by using toughness modifiers [96, 97]. For example, poly(butylene adipate‐co‐terephthalate) (PBAT) may be regarded as a toughness modifier when loaded into the matrix of PLA to increase elongation and processability [98]. The sc‐PLA/PBAT scaffolds with high porosity have been prepared by Kang et al. by non‐solvent phase separation [99]. The sc‐PLA‐based scaffolds led to a uniform porous structure (as compared to PLLA‐ or PDLA‐based scaffolds) having a wall thickness of ~1 μm, which may be due to the intermolecular forces between the enantiomeric PLA chains. The sc‐PLA/PBAT scaffolds are capable of supporting the adhesion of fibroblast cells, which in turn accounts for its biocompatible nature.
5.8 CONCLUSIONS
Stereocomplexation in PLA has resulted in widespread acceptance accounting to its unique thermal, mechanical, and physical properties. The current chapter has underlined various techniques of achieving improved stereocomplexation in PLA, such as stereoblock formation, copolymerization, and composite formation. These techniques result in the formation of intended materials with customized properties, which have been manifested in the current chapter. Further, insights have been made into the melt‐crystallizability of sc‐PLA in view of improving its industrial applications. An emphasis has also been laid on improving the biocompatibility of sc‐PLA‐based materials for potential biomedical applications. It may be recognized that the bio‐based polymers/copolymers/composites built on sc‐PLA could replace the conventional polymers in multifaceted applications and reduce the human dependence on fossil resources, as well as the carbon dioxide loading on the global sphere.
REFERENCES
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