Thermal stability of poly(l-lactide) α’ crystals and its blocking effect on perfection of α crystals

Abstract

Thermal stability of poly(L-lactide) (PLLA) α’ crystals and its blocking effect on perfection of α crystals was reported in this manuscript. The results showed that, with annealing temperature (T_a) approaching to 150 °C, most α’ crystals transformed into α crystals, and the others with higher thermal stability still existed in those annealed samples. Interestingly, experiencing identical annealing, α crystals with residual α’ crystals displayed the bigger long period but the smaller lamella thickness than that of α crystals directly formed at 130 °C. Further, after the residual α’ crystals were totally melted, during annealing at a higher temperature, both lamella thickness and crystallinity of α crystals increased faster than that of α crystals with residual α’ crystals, even the latter was annealed at a lower temperature. This result is considered to promote understanding of the genesis of the PLLA α՛ crystals and the mechanism of α՛ to α transformation.

Introduction

Poly (L-lactide) (PLLA) has attracted considerable interest in biodegradable and biocompatible material science research [1,2]. As a crystallizable polymer, semi-crystalline structure of PLLA is essential for its application-relevant properties, thus crystallization and melting behaviors of PLLA have been investigated extensively [[3], [4], [5], [6], [7], [8]]. Depending on the preparation conditions, PLLA exhibits different modifications [[9], [10], [11], [12], [13]]. For melt crystallization, when the crystallization temperature (T_c) is higher than 120 °C, the most common modification, α form, would be obtain [10,14,15]. Although there are pronounced difference on cell parameters among different groups [2], the α form consisting of two left-handed antiparallel chains in an orthorhombic is accepted by most researchers.

When the T_c is lower than 120 °C, α՛ crystals would form [[16], [17], [18], [19]]. Many puzzles of the α՛ crystals are still not understood. For the genesis of the α՛ crystals, Cho and Strobl proposed that it belongs to a metastable phase [20]. Lotz believed that it results from a very transient β PLLA phase [21]. Crystal structure of α՛ (δ) form had been proposed by Wasanasuk and Tashiro, the unit cell parameters were reported as a =10.80 Å, b =6.20 Å, c = 28.80 Å, and α = β = γ = 90°, as well the remarkably disordered chain conformation and similar chain packing as regular α form [22]. However, Hu et al. emphasized the α and α՛ crystals contain the same 10₃ helical conformation [23]. Lv et al. proclaimed α′ crystals contain no intrinsic crystalline lattice difference from α crystals, except for their different extents of crystal perfection resulting in the shifting of melting points [24].

During heating or annealing, α՛ crystals will transform into α crystals. This topic has been extensively discussed from a decade ago [[17], [18], [19],21,[25], [26], [27], [28]]. Kawai and co-authors found that the phase transformation from α՛ to α takes place at 150 °C before melting and shows almost no heating rate dependence, they proclaimed this transformation is a solid-solid phase transition [17]. Zhang et al. believed that the α՛ to α transition occurred in the solid state, since the X-Ray fiber pattern kept the high degree of chain orientation during the transition [18]. By determining the effects of the annealing temperature and annealing time on the melting behaviors of α՛ form, Pan et al. proposed a direct solid-solid transition mechanism for the α՛ to α transformation [19]. Besides, based on the temperature-modulated DSC analysis, it was found that the endothermic and exothermic events simultaneously occurred during the α՛ to α transition, which suggested a melting recrystallization mechanism [29,30]. Using a commercial fast-scanning chip-calorimetry - Flash DSC 1, Androsch et al. globally melted α՛ crystals and completely suppressed formation of α at a heating rate beyond 1800 K/min (30 K/s), they suggested a melting recrystallization mechanism for the α՛ to α transition [26]. Soon afterwards, by using Flash DSC 1, Androsch et al. [27] presented melting of the α՛ crystals and the crystallization of the subsequent melt during annealing. Wasanasuk and Tashiro reported that, in the cold crystallization, small domains of the mesophase grew larger to form the domains of the α՛ (δ) form, and the α՛ (δ) form domains are randomly disordered in the relative height along the chain axis [31]. They found during the transition from α՛ (δ) to α form, chain conformational ordering, chain packing regularization and matching of the neighboring domain height occurred simultaneously [22]. Chen et al. examined structural evolution of PLLA during cold-crystallization at 80 °C, they found the α՛ crystals underwent continuous and persistent perfection and partially transformed to smaller α crystals (up to a saturated population ratio of 1:4 between α and α′ phases) [32]. Very recently, Hu et al. collected the WAXS and SAXS simultaneously upon annealing PLLA α′ crystals at 150 °C. They proclaimed that there are three steps during the α′ to α transition process, i.e. α′ form partially melt and recrystallization, solid-solid α′ to α transition and recrystallization [23].

Since the temperature range selected for PLLA processes is favorable for formation of α′ crystals, more details about α′ crystals are essential for the application of the final products. In this manuscript, rather than the genesis of α’ crystals and mechanisms of α’/α transformation, the authors focus on thermal stability of α’ crystals during α’/α transformation. In addition, although results involving coexisting of the two modifications had been reported, interactions between them are not published to date. Here, an impediment effect of residual α’ crystals on perfection of α-crystals, which were transformed from α’ crystals with lower thermal stability, has been reported.