Poly(lactic acid). Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Химия
Год издания: 0
isbn: 9781119767466
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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.

Schematic illustration of applications of the sc-PLA-based copolymers and composites.

      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.

      1 1. M. Brzeziński, T. Biela, Stereocomplexed polylactides, in: S. Kobayashi, K. Müllen (Eds.), Encyclopedia of Polymeric Nanomaterials, Springer Berlin Heidelberg, Berlin, Heidelberg, 2014. p. 1–10.

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