Stereocomplex formation between enantiomeric poly(lactic acid)s. XI. Mechanical properties and morphology of solution-cast films

Abstract

Films of 1:1 blend and films non-blended were prepared from poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) with a solution casting method, and the mechanical properties and morphology of the films were investigated using tensile tests, dynamic mechanical relaxation measurements, polarizing optical microscopy, differential scanning calorimetry (DSC) and X-ray diffractometry. The tensile strength, Young’s modulus, and the elongation-at-break of 1:1 blend films were found to be higher than those of non-blended films when their weight-average molecular weight (M_w) was in the range 1×10⁵–1×10⁶. The enthalpy of melting for stereocomplex crystallites in 1:1 blend films was higher than that of homo-crystallites when M_w of polymers was below 2×10⁵, while this relationship was reversed when M_W increased to 1×10⁶. Spherulites formation was suppressed in 1:1 blend films, whereas large-sized spherulites with radii of 100–1000 μm were formed for non-blended PLLA and PDLA films, irrespective of M_w. The mechanical properties of 1:1 blend films superior to those of non-blended films were ascribed to the micro-phase structure difference generated as a result of formation of many stereocomplex crystallites which acted as intermolecular cross-links during solvent evaporation of blend solution. On the contrary, non-blended films had larger-sized spherulites of less contacting area with the surrounding spherulites.

Introduction

In the past two decades, a large number of studies have been performed for poly(lactide)s, poly(lactic acid)s (PLAs) and their copolymers which are hydrolyzable in the human body as well as in natural circumstances [1], [2], [3], [4], [5], [6], [7], [8], [9]. As our first report on the stereocomplexation (racemic crystallization) between enantiomeric poly(l-lactide) or poly(l-lactic acid) (PLLA) and poly(d-lactide) or poly(d-lactic acid) (PDLA) [10], [11], effects of numerous parameters on the stereocomplexation have been intensively investigated [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. These studies revealed that exclusive formation of either stereocomplex crystallites (racemic crystallites) composed of an equimolar amount of l-lactide and d-lactide unit sequences or homo-crystallites composed of either l-lactide or d-lactide unit sequence alone occurs depending on the parameters given below:

• blending ratio of the two isomeric polymers [10], [11], [12], [13], [14], [15], [16], [17];

• molecular weight of the two isomeric polymers [12], [13], [14], [15], [16];

• optical purity of the two isomeric polymers [17], [18], [19];

• temperature and time after blending of the two isomeric polymers in solutions or after melting their blend [12], [15], [16], [17], [19];

• nature of the solvents utilized for polymer blending [13], [14];

• nature of the co-monomer units and length of lactide unit sequences in copolymers [21], [22], [23], [24], [25], [26];

• blending mode of the two isomeric polymers [12], [13], [14], [15], [16], [20].

The most common conditions of polymer blending for exclusive formation of stereocomplex crystallites without formation of homo-crystallites include:

• equimolar blending of d-lactide and l-lactide units [10], [11], [12], [13], [14], [15], [16], [17];

• low molecular weight for both the isomeric polymers [13];

• sufficiently long sequences of both isotactic l-lactide and d-lactide units [17], [18], [19], [23].

The blending mode and nature of solvents also affect strongly stereocomplexation. Moreover, stereocomplexation is found to occur as far as the system contains both l-lactide and d-lactide unit sequences, disregarding whether they are in different molecules or are connected to co-monomer sequences other than lactide units [17], [18], [19], [21], [22], [23], [24], [25], [26], [27]. Examples include stereocomplexation of l-lactide and d-lactide block copolymers [21], [22], [26], [27] between ϵ-caprolactone or ethylene oxide and l- or d-lactide blockcopolymers [22], [25], between l-lactide-rich PLAs and d-lactide-rich PLAs [18], [19], and between glycolide and l- or d-lactide copolymers [23].

l-lactide and d-lactide unit sequences form stereocomplex crystals upon mixing under side by side packing [28], [29], [30], [31]. Okihara et al. [28], [30] found triangular single crystals to be formed by stereocomplexation and Brizzolara et al. [29] proposed a mechanism for their single crystal formation. Similar to the spherulites composed of homo-crystallites of either l-lactide or d-lactide unit sequences, both l-lactide and d-lactide unit sequences could form normal spherulites by stereocomplexation in bulk, from the melt [16], [17], [19] as well as in solution [15], when they were composed of only stereocomplex crystallites. However, spherulite morphology was complicated when the spherulites contained both stereocomplex crystallites and homo-crystallites [16], [19]. Brochu et al. [17] reported epitaxial crystallization of stereocomplex crystallites and homo-crystallites from the melt of blends constituting of PLLA and poly(d-lactide-co-l-lactide) (80/20). We found the equilibrium melting temperature of stereocomplex crystallites to be 279°C and the critical isotactic sequence for stereocomplexation of l-lactide-rich PLA and d-lactide-rich PLA to be 15 isotactic lactate (half of lactide) units [19]. When stereocomplexation was allowed to proceed in concentrated chloroform solution, three-dimensional (3D) gelation occurred as a result of formation of stereocomplex microcrystallites which acted as cross-links. This happened because the critical concentration for stereocomplexation by intermolecular interaction was lower than that of homo-crystallization by intra- and intermolecular interaction. In other words, stereocomplexation took place more readily than homo-crystallization [12], [13]. High-resolution solid-state ¹³CNMR spectroscopy revealed that the stereocomplex precipitated from dilute acetonitrile solution was composed of four regions: rigid stereocomplex crystalline region, disordered stereocomplex crystalline region, trace amounts of homo-crystalline region, and non-crystalline region [32]. Vibrational mobility of PLLA and PDLA in stereocomplex was studied by Kister et al. [33] using Raman and IR spectroscopy. Li and Vert [34], [35] showed that stereocomplexation of statistic copolymers from d- and l-lactides would occur during hydrolysis by predominant scission and preferable removal of the chains having relatively random sequences of d- and l-lactide units, leaving the chains of long isotactic sequences of d- and l-lactide units.

In spite of plenty of information accumulated on PLA stereocomplexation, there has been few reports on mechanical properties of PLA stereocomplexes. Exceptions are the tensile properties of blend fibers from PLLA and PDLA, but they contain both stereocomplex crystallites and homo-crystallites [20]. Improved mechanical properties of PLLA and PDLA blends compared to non-blended materials were reported in a patent article without detailed estimation of molecular characteristics of the polymers and the content of respective crystalline species in the specimens [36]. We found that a 1:1 blend film prepared through solution-casting of PLLA and PDLA having a viscosity-average molecular weight (M_v) of 2–4×10⁴ and containing solely stereocomplex crystallites, exhibited tensile strength higher than that of non-blended PLLA or PDLA film. However, any reason for the higher strength of blend films has not been proposed so far.

The purpose of the present work is to investigate mechanical properties of blended and non-blended films prepared from PLLA and PDLA having a wide range of molecular weight and to find the reason for the difference in mechanical properties between blended and non-blended films. For this purpose blended and non-blended films are prepared by casting the solutions from PLLA and PDLA having a weight-average molecular weight (M_w) from 1.0×10⁴ to 1.0×10⁶, as the critical highest molecular weight below which only stereocomplex crystallites are formed is higher for solution-casting (M_v=4×10⁴) [13] than for melt-crystallization (M_v=6×10³) [16] and melting at high temperatures in the melt-crystallization procedure may lower the polymer molecular weight and change the monomer unit sequences by intermolecular transesterification between PLLA and PDLA. Their mechanical properties and morphology are investigated without any drawing and heat treatment of film specimens using tensile tests, dynamic mechanical relaxation measurements, and polarizing optical microscopy, differential scanning calorimetry (DSC), and X-ray diffractometry.