Improvement of toughness by stereocomplex crystal formation in optically pure polylactides of high molecular weight

Abstract

A solution casting method followed by thermal homogenization was performed for the preparation of 1:1 blends and non-blended films from poly(d-lactide) (PDLA) and poly(l-lactide) (PLLA) of three different molecular weights, and their thermal and mechanical properties were determined via differential scanning calorimetry (DSC) and tensile tests. According to the literature, when Mw is below 1.0×10(5)g/mol only stereocomplex crystallization takes place, and when it is higher, both homocrystallites and stereocomplex crystallites co-exist. In order to promote crystallization as a homocrystal in neat polylactides and to promote the stereoselective crystallization as stereocomplex in the case of non-blended films, and in turn, to achieve different degrees of crystallinity, several thermal treatments of annealing were carried out in this work. Highly stereocomplexed blends were found by the stereospecific thermal treatments. As a consequence, the toughness of 1:1 blends was found significantly enhanced over those of non-blended films, irrespective of molecular weight. For instance, in B2-5050 stereocomplexed blend having poly(l-lactide) and poly(d-lactide) of Mw=1.2×10(5)g/mol, tensile strength increased from 44.0±2.1MPa to 65.1±6.1MPa, and the elongation at break from 10.8±2.5% to 33.1±8.1% with respect to its non-blended poly(l-lactide) counterpart crystallized as homocrystal. This improvement in mechanical properties in stereocomplexed blends is not attributed to the inherent properties of the type of crystal polymorph but to the presence of a higher density of intercrystalline connections through a mobile amorphous phase, i.e. tie chains in the stereocomplexed supramolecular spherulitic entities that provide in the stereocomplexed samples enhanced strength and elongation at break at the same time.

Keywords: Crystallization; Mechanical properties; Polylactide; Stereocomplexation.