Elsevier

Polymer

Volume 49, Issue 26, 8 December 2008, Pages 5670-5675
Polymer

Unique crystallization behavior of poly(l-lactide)/poly(d-lactide) stereocomplex depending on initial melt states

https://doi.org/10.1016/j.polymer.2008.10.028Get rights and content

Abstract

A unique crystallization behavior of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) stereocomplex was observed when a PLLA/PDLA blend (50/50) was subjected to specific melting conditions. PLLA and PDLA were synthesized by ring opening polymerization of l- or d-lactide using zinc lactate as catalyst. PLLA/PDLA blend was prepared through solution mixing followed by vacuum drying. The blend was melted under various melting conditions and subsequent crystallization behaviors were analyzed by using DSC, XRD, NMR and ESEM. Stereocomplex was exclusively formed from the 50/50 blend of PLLA and PDLA with relatively low molecular weights. Surprisingly, stereocomplex crystallization was distinctly depressed when higher melting temperature and longer melting period were applied, in contrast to homopolymer crystallization. Considering predominant interactions between PLLA and PDLA chains, a novel model of melting process is proposed to illustrate this behavior. It is assumed that PLLA and PDLA chain couples would preserve their interactions (melt memory) when the stereocomplex crystal melts smoothly, thus resulting in a heterogeneous melt which can easily crystallize. The melt could gradually become homogeneous at higher temperature or longer melting time. The strong interactions between PLLA and PDLA chain segments are randomly distributed in a homogeneous melt, thus preventing subsequent stereocomplex crystallization. However, the homogeneous melt can recover its ability to crystallize via dissolution in a solvent.

Introduction

Polylactide (PLA) has received wide attention in the medical and pharmaceutical fields due to its biodegradability, biocompatibility, and good mechanical properties [1], [2]. PLA can be used to fabricate various osteosynthetic devices, drug delivery systems and tissue engineering scaffolds, etc. The degradation characteristics of PLA, which are of major importance for various applications, mainly depend on the crystalline morphology and crystallinity. Lactide exists in three isomeric forms, i.e. l-lactide, d-lactide and meso-lactide, which allows to prepare various PLA homo- and stereo-copolymers with dramatically different properties by adjusting l/d ratios in the monomer feeds.

Since Ikada et al. reported stereocomplex formation between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) [3], many researchers have focused on its significance for PLA-based materials, in particular on the physical properties, thermal properties and hydrolysis resistance [4], [5], [6]. In the meantime, there are a great number of patents dealing with stereocomplex processing technologies because of the potential applications [7], [8]. Stereocomplex crystallites exhibit much higher crystal growth rate and shorter induction period than either PLLA or PDLA [9], [10]. The melting point of the stereocomplex is ca. 50 °C higher than that of pure PLLA or PDLA [3], [11]. Single crystals of the stereocomplex favor a triangular shape with a β-form 31-helice packing of opposite configurations alternating side by side [12], [13]. However, while stereocomplex can be obtained from the melt, the melting conditions and the melt state prior to crystallization have not been investigated, so far. In fact, it was assumed that melt state should not account for the stereocomplex crystallization [9], [10], [11], [12], [13].

Recently, several reports have focused on the relationship between polymer crystallization and heterogeneous melt [14], [15], [16]. Lippits et al. reported that disentangled chain segments of polyethylene (PE) crystallize from heterogeneous melt much faster than entangled chains from homogeneous melt [15]. Entanglements formed during homogenization of heterogeneous melt could retard the crystallization. Comparatively, we can assume that the spatial distribution of interaction regions during homogenization of PLLA/PDLA heterogeneous melt would affect the crystallization process. Indeed, it was found that stereocomplex crystallization was distinctly depressed when higher melting temperature and longer melting period were applied. Since interactions between PLA chain segments of opposite configurations are stronger than those of the same configurations, homogenization of spatial distribution of interaction regions in PLLA/PDLA melt would prevent PLLA and PDLA chain segments from coupling and being involved into crystallization frontier, and thus depress stereocomplex crystallization.

The mechanisms, by which polymer melt of high conformational entropy transform into a semicrystalline state of low entropy, have been previously studied [17]. Several researchers have reported that polymer crystallization from the melt may start from initial transient state (or named as precursors, mesomorphic seeds) [18], [19], [20]. And “melt memory” has been proposed to describe the behavior, i.e. a polymer in melt state might retain a partial memory of its former crystalline structure [21], [22], [23]. Therefore, melt state appears as an important factor in polymer crystallization.

In our previous studies, we have reported the basic crystallization behaviors of PLLA with different molecular weights [24], [25]. In this work, relatively low molecular weight (Mn) PLLA and PDLA (in the order of 104 g/mol) were synthesized since stereocomplex is exclusively formed when the Mn of both PLLA and PDLA is in the order of 103–104 [11]. The relationship between the melt state and stereocomplex crystallization is reported herein.

Section snippets

Materials

l-lactide and d-lactide were purchased from Purac (The Netherlands). Zinc lactate and sodium azide were obtained from Merck. Trizma base, Trizma/HCl and proteinase K in the form of lyophilized powder (30 U/mg) were supplied by Sigma.

Methods

The synthesis of PLLA and PDLA was previously described [24], [25]. PLLA and PDLA were synthesized by ring opening polymerization at 140 °C of l-lactide or d-lactide, using zinc lactate as catalyst and ethylene glycol as co-initiator to control the Mn. The molecular

Results and discussion

Fig. 1 presents the thermal behaviors of the PLLA/PDLA blend. In the first run, only a Tm was detected at 215.9 °C with ΔHm of 64.2 J/g. The melted sample was kept at 230 °C for 1 min, taken out and immediately immersed in liquid nitrogen to be made amorphous. In the second heating process, Tg and Tcc appeared at 53.1 and 95.9 °C, respectively. Tm decreased to 186.8 °C and ΔHm to 26.0 J/g. The same procedure was applied for the third run which exhibits a Tg at 50.9 °C, a very weak cold crystallization

Conclusion

Stereocomplex is exclusively formed from blends of PLLA and PDLA (50/50) with relatively low molecular weights. The crystallization behavior of the PLLA/PDLA blend depends on its initial melt state. When higher temperature and longer period are applied, stereocomplexation is strongly depressed. A model is proposed on the basis of the theory of heterogeneous and homogeneous melts, together with the predominant interactions between PLLA and PDLA chains. This model could explain the unique

Acknowledgements

The authors are indebted to the Project ARCUS 2006 Languedoc-Roussillon/China and to the Shanghai Leading Academic Discipline Project (No. B113) for financial support.

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