Poly(lactic acid) - Группа авторов

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system along with an increase in the molecular weight. The block structure was formed by the hetero‐coupling reaction in the minor sc‐domain resulting in complete sc formation upon melt quenching. This method led to the formation of sc‐PLA having HMW suitable for high temperature applications such as car parts and housings of electrical appliances. The SSP of PLA enantiomers using methanesulfonic acid (SO) has been conducted by Kanno et al. to form sb‐PLA [65]. The sb copolymers were obtained with higher crystallinity when the chain length of the PLA was shorter. The structure and yield of the sb‐PLA were affected by the catalyst system used [65]. The tin‐based catalyst was found to induce side reactions during the SSP with a lower yield of sb‐PLA and to reduce the glass transition temperature (T g), whereas SO resulted in the inhibition of side reactions and formation of HMW sb‐PLA with an efficient polymerization process. The molecular weight of the PLA starting materials and the catalyst used were found to be the governing parameters for synthesizing sb‐PLA. Figure 5.4 shows a schematic representation of the methods used for developing sb‐PLA.

Schematic illustration of the methods adopted for developing sb-PLA.

      5.5.1 Stereocomplexation in Random and Alternating Lactic Acid or Lactide‐Based Polymers

PLLA sc‐PLA PGA PHB PCL
T m (°C) 170–190 220–240 225–230 188–197 55–65
T g (°C) 50–65 65–72 40 5 −60
ΔH m (J/g) 93–203 142–155 180–207 146 136
Density (g/cm3) 1.25–1.3 1.21–1.342 1.50–1.69 1.18–1.26 1.1–1.15
Tensile strength (MPa) 120–2260 880 80–980 180–200 8–16
Young’s modulus (GPa) 6.9–9.8 8.6 3.9–1.4 4.9–5.9 0.1–0.4
Elongation at break (%) 12–16 30 30–40 50–70 100–2000

      Furthermore, stereocomplexation involving alternating copolymers comprising lactic acid (LA) units have been studied extensively. For example, blending of enantiomeric alternating copolymer of LA and glycolic acid (GA) having molecular weight of 5000 Da leads to preferential formation of stereocomplex crystallites as reported by Tsuji et al. [68]. They proposed that blending of alternating LA‐based copolymers could be a versatile technique for the development of high‐performance bio‐based biodegradable materials with tunable physical properties and biodegradability. Yet another study reports the incorporation of alanine unit into LA units in random copoly(ester‐amides) with ester and amide linkages leading to the preferential formation of sc crystallites [69]. However, the developed polymers contain low degree of polymerization, and to have them employed in the commercial scale utility, further development is needed.

      5.5.2 sc‐PLA–PCL Copolymers

      Lately, there have been important advances in sc blends of PLA copolymers for biomedical implant applications. In a study reported by us [70], diblock copolymers of PCL‐PLLA and PCL‐PDLA were synthesized by two‐step ROP followed by blending them in a 1 : 1 ratio to achieve sc blends of the enantiomeric diblock copolymers. Using this strategy, it was possible to obtain enantiomeric diblock copolymers having enhanced mechanical properties (28 MPa strength and ~80% elongation at break), which were also processed by the conventional injection moulding technique to develop cancellous bone screws (orthopedic implants). The study reported the thermomechanical stability of the cancellous bone screw at 121°C in comparison to commercial PLA (2003D, NatureWorks LLC). The stereococomplexed diblock copolymer was found to be stable at the sterilization temperature of biomedical devices, unlike the commercial PLA sample. The biocompatibility of the synthesized sc diblock copolymer blend was also ascertained by MTT (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl

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