solution of PLLA with that of PDLA leads to the formation of an irreversible gel due to the formation of sc crystals as crosslinking points. The stereoselective interaction of the optically active PLLA and PDLA enantiomers results in the formation of optically inactive sc crystals consisting of PLLA and PDLA chains in an equimolar ratio [13, 14]. The formation of sc crystals was initially discovered from solution mixing and later from melt blending of both enantiomers. The sc crystals are characterized by the high melting temperature of ~230°C, which is ~50°C higher than that of homo‐crystals of PLLA and PDLA [2]. The enantiomeric PLLA and PDLA chains are packed side by side in a sc crystal lattice, where hydrogen bond between the carbonyl and methyl groups of PDLA and PLLA is responsible for the complexation. The spherulites of sc‐PLA do not have a ring‐band structure at any crystallization temperature, unlike those of PLLA and PDLA homopolymers. Since the development of sc crystals is driven by the diffusion of the macromolecular chains of PLLA and PDLA in the crystallization process, the sc crystallizability is inversely proportional to their molecular weight. Accordingly, an equimolar blending of PLLA and PDLA (1 : 1) with high molecular weight (>100 kg/mol) often leads to the formation of homo‐crystallites (larger extent) along with sc crystallites [15, 16]. Improved miscibility between the PLLA and PDLA chains can enhance the formation of sc in the PLLA/PDLA blend. The mesophase (an ordering of molecules which is intermediate between the crystalline and amorphous states) in sc‐PLA can be observed by annealing the equimolar blends of PLLA/PDLA just above their
T g due to the prevailing weak intermolecular interactions between high‐molecular‐weight (HMW) PLLA and PDLA chains [17]. The stereocomplex mesophase is more prevalent at a lower temperature due to the reduced molecular mobility, while at higher temperature, the formation of hc crystals is enhanced. Furthermore, blending of non‐equimolar PDLA and PLLA results in various fractions of hc and sc crystallites. However, as reported by Woo et al., the non‐equimolar blends of PDLA and 30–50% of low‐molecular‐weight PLLA lead to the formation of sc‐PLA crystals. In such a case, a large amount of hc‐PLA chains may be trapped and dispersed in the spherulites of sc‐PLA crystals, thereby resulting in fluffy lamellae stacking of sc crystals [18].
Lately, attention has been paid to improving the melt crystallizability of sc‐PLA to expand its applications, particularly in industries where melt processing of polymers is employed. The boundary viscosity average molecular weight ( ) for stereocomplexation from the melt is 6 × 103 g/mol, whereas that from the solution casting is 4 × 104 g/mol [19]. However, the ordinary melt crystallization leads to the formation of hc crystals together with sc crystals [20]. Therefore, efforts have been made to achieve exclusive formation of sc crystals from the melt [21, 22]. The use of polyethylene glycol (PEG) as a plasticizer has been reported to enhance the formation of sc crystallites in the HMW blends of PLLA and PDLA. The plasticizer facilitates the interaction between PLLA and PDLA chains by increasing the segmental mobility of the polymer chains, thereby leading to the formation of exclusive stereocomplexation during melt crystallization. The use of cellulose nanocrystals (CNCs) as a nucleating agent has also resulted in the improvement in stereocomplexation from melt. A higher loading of CNCs (~25 wt%) led to an accelerated growth of sc‐PLA as reported by Jiang et al., which further expanded the industrial applications of sc‐PLA [23]. Hence, the use of nucleating agents and plasticizers has become an alternate route to improve stereocomplexation in PLA. Additionally, an aryl amide derivative (TMB‐5) has been adopted as a nucleating agent to promote sc crystallization in equimolar blends of PLLA/PDLA. Exclusive formation of sc crystals from the melt has been reported upon loading 0.5% TMB‐5 in an equimolar PLLA/PDLA blend. However, the T m of sc‐PLA is reduced from 230 to 200°C upon loading TMB‐5 into the matrix of sc‐PLA [24]. The formation of sc‐PLA nanofibers by electrospinning has also been explored by several researchers [25–28]. The HMW blend of PLLA/PDLA has been subjected to electrospinning by Tsuji et al., where the sc crystallization is found to be enhanced with higher voltage. The nanofibers are prepared with dominant sc crystals and negligible amount of hc crystals. The formation and growth of sc crystals is attributed to the high voltage or electrically induced high shearing force used during the electrospinning process [29].
5.3 CRYSTAL STRUCTURE OF sc‐PLA
sc‐PLA often crystallizes in a triclinic or trigonal unit cell with both 31 (or 31 and 32) PLLA and PDLA chains packed side by side [30] unlike the orthorhombic or pseudo‐orthorhombic crystal forms of hc‐PLA [31]. The crystal structure consisting of a triclinic unit cell (P1 symmetry with parallel chain orientation) was proposed in 1991 by Okihara et al., who reported a 31 helical structure of PLLA and PDLA chains having a lamellar thickness of 0.87 nm, where the three enantiomeric chains penetrate one unit cell [32]. The unit cell parameters of a triclinic cell are given as a = b = 9.16 Å, c = 8.7 Å; α = β = 109.2°, γ = 109.8°. The structure was different from that of the trigonal unit cell (R3c or R‐3C group) described by Cartier et al., where the PLLA and PDLA chains have 32 and 31 conformations, respectively [33]. According to the modified trigonal structure, the triclinic cell was assumed to 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
