Читать книгу Poly(lactic acid) - Группа авторов - Страница 109

Enantiomeric PLLA and PDLA are synthesized from L‐ and D-lactides (T _m = 97.5°C) that are derived from L‐ and D‐ lactic acids, respectively. Both PLLA and PDLA are semicrystalline in nature and develop unique morphologies in their block copolymers due to the competition between crystallization and microphase separation, which expands their applications [12]. Intriguingly, mixing of a concentrated 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].