[57]. The sc‐crystallinity was found to increase when the PLLA/PDLA block ratio approached 50/50, which also increased the storage modulus of HMW sb‐PLA. In another study, Isono et al. applied the combination of sb and star polymer approaches and succeeded in synthesizing stereo‐miktoarm star‐shaped PLAs by a click coupling method [58]. This approach led to the preferential sc formation in the solution cast samples, without any homo‐crystallite formation. The variation in the arm number also led to the variation in T m of the stereo‐miktoarm star polymers, thereby providing a mechanism to tailor the physical properties of sc‐type PLAs. Besides these approaches, the synthesis of cyclic stereoblock copolymers of PLLA and PDLA with head‐to‐head and head‐to‐tail configurations has been carried out by implementing the click chemistry and ring closing metathesis approaches [59]. The orientation of PLLA and PDLA segments in the cyclic structure may be controlled in a facile way by using this approach, along with modulated thermal properties.
5.4.3.3 Polycondensation
The melt polycondensation of low‐molecular‐weight PLLA/PDLA blends has been adopted to synthesize stereo multiblock PLAs having different block lengths. In contrast to the chain extension reaction, the stereo multiblock PLAs synthesized by melt‐polycondensation have higher crystallizability due to the connecting ester groups [60]. Further, stereo multiblock copolymers were synthesized by melt‐polycondensation as reported by Rahaman et al. with a wide range of block lengths [61], which led to the preferential formation of stereocomplex crystallites irrespective of the crystallization temperature and block length. In a study reported by our group, the PLLA and PDLA prepolymers were made by the melt‐polycondensation technique and mixed to develop sb‐PLA consisting of short sequences of D‐ and L‐lactate units where the molecular weight was as high as 100 kDa at the reaction temperature of 180°C [62]. The molecular weight of the homopolymer PLLA or PDLA obtained by SSP was much higher than sc‐PLA, even at a considerably lower reaction temperature. This fact was attributed to the partial chain racemization and difficulty of the elongated chains to crystallize out of the amorphous domain into the solid state. To enhance the crystallization of the elongated chains, it was speculated that the presence of homo‐crystallites may be essential. To further substantiate the hypothesis, non‐equivalent mixtures of PLLA and PDLA were used for SSP [63]. The melt blending of PLLA and PDLA (medium molecular weight) obtained by melt‐polycondensation of L‐ and D‐lactic acids was performed to obtain sb‐copolymers with different compositions and block sequences. The SSP reaction under mild conditions resulted in products with higher yield and larger molecular weight. The molecular weight and the composition of the block sequences were found to govern the thermal properties. The elongated chains of the homo‐sequences were able to crystallize out from the reaction system to allow for the chain extension reaction. Thus, the necessity of the hc domain for developing sb‐PLA with HMW by SSP was determined. Based on this evidence, a new design was proposed where the prepolymers PLLA and PDLA were mixed in the powder state (1 : 1) and subjected to heat treatment to form partial sc‐crystallization at the boundaries, followed by conducting SSP without reaching the melt‐blending state [64]. This method led the elongated hc chains to crystallize out from the reaction 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.
FIGURE 5.4 Schematic representation of the methods adopted for developing sb‐PLA.
5.5 STEREOCOMPLEXATION IN COPOLYMERS
The formation of sc‐PLA has been studied with various copolymers of enantiomeric PLAs with other well‐known bio‐based/biodegradable polymers such as PCL, PEG, and so on due to the complementary properties of the counterparts, which may lead to materials with customized properties. The mechanical and thermal properties of representative bio‐based/biodegradable polymers used for copolymerization are summarized in Table 5.2.
5.5.1 Stereocomplexation in Random and Alternating Lactic Acid or Lactide‐Based Polymers
Stereocomplex crystallites can be formed only by the interaction of enantiomeric PLA chains (PLLA and PDLA), which can be controlled during melt processing. Further, sc‐PLA has a limited ability to reform its crystallinity after being melted (removal of thermal history). To improve the molecular interaction between PLLA and PDLA, Purnama et al. have studied stereocomplexation in the blend of PLLA and random copolymer (PDLCL) of D‐lactide with small amount of caprolactone (CL) [66]. The small amount of CL units in PDLCL act as soft fraction (due to the presence of methylene linkage) in the polymeric system to provide relatively low glass transition and melting temperatures. Further, this soft fraction accelerates the PDLA chain movement to easily interact with PLLA chains, thereby resulting in improved melt stability of sc crystallites. They also reported that the system resulted in the maximum sc crystallinity when the amount of CL was 2.5 unit‐mol%. It was however indicated that the excess use of CL may lead to overactive mobility of PDLA chains, ultimately affecting the stereocomplexation. In an another approach, Jikei et al. attempted to develop a segmented, random multiblock copolymer of PLLA and PCL and blended it with PDLA to improve the stereocomplexation [67]. The soft segment PCL aided in improving the elongation at break (over 400%) of PLLA, whereas preferential formation of stereocomplex crystallites in hard domain (containing PLLA and PDLA) contributed to enhancing the Young’s modulus (over 400 MPa) and ultimately improved the overall toughness of the blend. In particular, the physical crosslinking in hard domain also contributed to enhance the thermal deformation stability of the blend.
TABLE 5.2 Mechanical and Thermal Properties of the Representative Bio‐Based/Bio‐Degradable 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) |
|
