Poly(lactic acid). Группа авторов

Скачать книгу

Mulchandani Yoshiharu Kimura and Vimal Katiyar

5.1 INTRODUCTION

      An intermolecular complex formed by macromolecules having identical chemical composition but different configuration of the repeating units is known as a stereocomplex (sc) [1]. The co‐crystallization of the stereoisomers PLLA and PDLA results in the formation of a well‐known crystal structure stereocomplex PLA (sc‐PLA). The sc‐PLA gained widespread recognition and acceptance ever since it was discovered by Ikada et al. [2] and patented by Murdoch and Loomis [3]. Stereocomplexation in PLA is attributed to the non‐covalent interaction of the enantiomeric macromolecular chains. The sc‐PLA crystals melt at a temperature (T _m) 50°C higher than the homochiral (hc) PLLA or PDLA crystals [4]. Apart from the improved heat resistance, sc‐PLA has better mechanical performance and resistance to hydrolysis [5, 6] than PLLA and PDLA, allowing its exploration in pharmaceutical and biomedical applications. The homopolymers PLLA or PDLA have been utilized for short life applications such as agricultural mulch‐films, disposable trays and bottles, and nonwovens. On the other hand, sc‐PLA can be specifically adopted for high‐performance applications such as structural and engineering plastics [7]. Several studies have also made use of sc‐PLA for drug delivery application due to its improved barrier properties, which prevent the burst release and prolong the release of drugs [8]. The formation of sc crystals may therefore lead to the development of PLA‐based materials with enhanced performance. The molecular weight, optical purity of the enantiomeric PLA, and tacticity are the governing parameters for the sc crystallization; lower optical purity or higher molecular weight likely hinders the sc crystallization [9, 10]. The formation of hc crystals is often more kinetically favorable than that of sc crystals during the crystallization of high‐molecular‐weight PLLA/PDLA blends. However, enhancing the interchain interactions between PLLA and PDLA by covalent or non‐covalent means can improve the formation of sc crystallites [11]. Also, sc crystallizes much faster from the melt as compared with the α‐form. Researchers have been studying the varied crystal morphologies of sc‐PLA and the governing parameters for their formation. A preferential formation of sc can be attained by blending PLLA/PDLA in equimolar (1 : 1) ratio in solution. However, varying the ratios of PLLA/PDLA in the blend give rise to the formation of hc‐PLA (T _m ~ 180°C) along with sc‐PLA (T _m ~ 230°C). In order to improve the sc formation in non‐equimolar blends of PLLA/PDLA, several methodologies have been adopted, some of which are highlighted in this chapter.

5.2 STEREOCOMPLEXATION IN POLY(LACTIC ACID)

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].

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,

Скачать книгу