Influence of Molecular Weight and Composition of Homopolymers on the Stereocomplex Formation of Polylactides in Gels and Aerogels

Abstract

This work systematically studies the role of polymer chain length and number density of enantiomeric polylactides (PLAs) in the gel state stereocomplex (SC) formation and the corresponding effect on the structure and morphology of aerogels. Poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) with large molecular weight (Mw) differences are blended in the solution state and subjected to thermoreversible gelation. The obtained gels are solvent exchanged with water and freeze-dried to obtain the corresponding aerogels. On gelation, the equimolar blend gels of PLLA100k/PDLA35k and PLLA100k/PDLA15 (1:1) have crystallized predominantly into SC, and this structure remains even after the thermal annealing and cooling of the gel. In contrast, the non-equimolar blends of PLLA100k/PDLA35k (1:3) and PLLA100k/PDLA15k (1:7) crystallize into almost equal fractions of SC and ε-form. This structure on heating transforms to complete SC through the α crystalline phase. But unlike the equimolar gel, pure crystalline SC is not obtained on cooling the annealed gel to room temperature. Instead, a mixture of α and SC phases are obtained due to the independent crystallization of the excess enantiomeric chains. Thus, it has been proved that the composition of PLLA and PDLA in the blend is critical in obtaining crystalline pure SC gels and aerogels.