Stereocomplex formation and hierarchical structural changes during heating of supramolecular gels obtained by polylactide racemic blends

Abstract

Solvents that are capable of forming crystalline complexes with poly(ʟ-lactide) (PLLA) have been known to form thermoreversible gels. However, the gelation behavior of linear PLLA/poly(ᴅ-lactide) (PDLA) blends has not been studied. Herein, the formation of supramolecular gels using PLLA/PDLA blends in N,N-dimethylformamide (DMF) is reported. The structural changes of these gels were investigated in multiple length scales during heating. In blend gels, both homopolymers crystallize into the ε form. On heating, DMF molecules that are present in the crystal lattice of the ε form are expelled out by keeping the lamellar structure intact and the structure transformed to the α form. At 45 °C, stereocomplex (SC) crystals are also formed due to the activated thermal motion of enantiomeric chains in the presence of solvent. The α form became unstable at ∼75 °C and the enantiomeric polymer chains reorganized into the SC. In the case of low molecular weight blend gels, high chain mobility and chain diffusion ability resulted in the more stable SC. A structural model is proposed to explain the phase transition behavior of blend gels. This study not only enhances the fundamental understanding of SC formation but also provides opportunities to exploit the molecular recognition in helical polymers.