be a subcell of the larger trigonal cell where six helices penetrate one unit cell. The trigonal unit cell parameters may be given as a = b = 14.98 Å, c = 8.7 Å; α = β = 90°, γ = 120°. The crystal was grown from non‐equimolar blends of PLLA and PDLA, which indicated that the co‐crystallization of PLLA and PDLA chains could occur from their asymmetric ratio. However, the proposed model only took 1 : 1 PLLA and PDLA ratio into consideration. Stereocomplex structures with parallel and antiparallel orientation of the molecular chains were studied by molecular simulations by Brizzolara et al. who revealed that the parallel structure (P1) is more stable than the antiparallel structure (P/1). The structures were considered to be triclinic having cell parameters a = 0.912, b = 0.913, c = 0.930 nm, α = β = 110°, γ = 109° for the parallel structure (P1); and a = 0.930, b = 0.940, c = 0.930 nm, α = 111°, β = 112°, γ = 108° for the antiparallel structure (P/1). The growth mechanism of the triangular lamellar crystals in the sc formation was well supported by the molecular simulations [34]. Highly oriented stereocomplex samples were prepared in a study by Sawai et al. who adopted solvent casting technique to prepare the blend films followed by co‐extrusion of the dried polymer blend (draw ratio = 14). The oriented samples showed 20 wide angle X‐ray reflections that were reasonably indexed with a trigonal unit cell as proposed by Cartier et al. with a slight variation of the parameters, which were given as: a = b = 1.50, c = 0.823 nm, α = β = 90 and γ = 120° with R3c space group [35].
These structure models are proposed mainly for the PLLA/PDLA blend having a ratio of 50/50. However, stereocomplexation is also evident in PLLA/PDLA blends having the compositions of 30/70–70/30, for which a new structure model (space group P3) has been proposed by Tashiro et al. [36] on the basis of X‐ray diffraction analysis. According to their model, the co‐existence of PLLA and PDLA chains between the sc crystal lattice is profound for the PLLA/PDLA blend ratios in the range of 30/70–50/50–70/30. Beyond this range, the coexistence of PLLA and PDLA chains is not realizable due to their instability. A statistically disordered packing of PLLA and PDLA chains can be attributed to the P3 space group, unlike the symmetrical R3c model [36, 37]. The unit cell parameters reported by several researchers are tabulated in Table 5.1.
5.4 FORMATION OF STEREOBLOCK PLA
The stereoblock (sb) formation allows for the intermolecular and intramolecular mixing of the neighbouring L‐ and D‐stereosequences, thereby leading to the preferential formation of sc crystallites. This is particularly important when synthesizing sc polymers of HMW. Block copolymerization has received enormous recognition in achieving the desired properties of the resulting materials. The composition of PLLA/PDLA, along with the number of blocks and chain length, can be varied to obtain a variety of diblock and multiblock copolymers with tailored properties. The molecular mixing of the enantiomeric PLLA and PDLA chains in the sb copolymers leads to the improved crystallinity [38–40).
5.4.1 Single‐Step Process
The formation of sc‐PLA usually occurs by blending PLLA/PDLA, which requires synthesis of the individual polymers by the ring‐opening polymerization (ROP) of L‐ and D‐lactides prior to blending the respective enantiomeric PLA chains. The sc‐PLA thus formed exhibits improved thermal and mechanical properties; however, the process requires the formation of enantiopure polymers from the respective enantiopure monomers, which restricts its practical applications. In this regard, the single step preparation of sc‐PLA has been explored from the readily available racemic lactide (rac‐LA, or DL‐lactide, 1 : 1 mixture of L‐ and D‐lactides), which is inexpensive (Figure 5.1). The ROP of rac‐LA usually yields amorphous polymers (atactic or heterotactic) having lower T g and T m than those of the isotactic PLA, i.e., PLLA and PDLA. The random arrangement of L‐ and D‐lactide units in the backbone chain is usually observed upon polymerizing rac‐LA with a limited scope of application. To extend the utilization of rac‐LA precursor, significant efforts have been made in achieving the stereoselective polymerization of LA that causes the resulting stereochemistry in the product [41–43). The direct formation of sc‐PLA from rac‐LA has been achieved by Radano et al. using triethylaluminum‐based catalysts [44]. The formation of sc‐PLA is supported by XRD analysis; however, the lower degree of isotacticity reduces the T m of the resulting polymer to ~191°C as compared with ~230°C of the usual sc. The degree of crystallinity of sc‐PLA is ~42% as determined from the enthalpy of fusion (ΔH fus). In another study, semicrystalline stereoblock copolymers with HMW (~461 kg/mol) have been produced from rac‐LA using chiral oxazolinyl aminophenolate magnesium complexes. The isoselective control of the magnesium complexes is affected by the chirality of the ligand, which may be modified to develop more efficient catalysts [45]. Furthermore, the ROP of rac‐LA by a series of achiral iron complexes has been performed by Marin et al. to yield HMW stereoblock copolymers [46]. The stereoselective catalysts permit the formation of stereoblock copolymers under mild conditions. The catalyst complex also influences the tacticity of the resulting polymers and the stereochemistry. The thermal degradation temperature of the isotactic stereoblock PLA is increased up to ~350°C, which is reasonably dependent on its molecular weight [46]. The degradation temperature correlates more strongly with the molecular weight than with the stereoregularity. The developed stereoblock PLA having high molecular weight may be used for industrial applications.
TABLE 5.1 Unit Cell Parameters Reported for the sc Crystals
| Okihara et al. [32] | Brizzolara et al. [34] | Cartier et al. [33] | Sawai et al. [35] | Tashiro et al. [36] | |
|---|---|---|---|---|---|
| Crystal system | Triclinic | Triclinic | Trigonal | Trigonal | Trigonal |
| Chain conformation | 31 | 31 | 31 and 32 | 31 and 32 | 31 |
| Unit cell parameter | |||||
| a (nm) | 0.916 | 0.912 | 1.498 | 1.50 | 1.494 |
| b (nm) | 0.916 | 0.913 | 1.498 | 1.50 | 1.494 |
| c (nm) | 0.870 | 0.930 | 0.870 | 0.823 | 0.862 |
| α (°) | 109.2 | 110 | 90 | 90 | 90 |
