Molecular, Crystalline, and Lamellar Length-Scale Changes in the Poly(L‑lactide) (PLLA) during Cyclopentanone (CPO) Desorption in PLLA/CPO Cocrystals

Abstract

Polymer−solvent complexes, poly(L-lactide) (PLLA) with cyclopentanone (CPO), were studied at multiple length scales using differential scanning calorimetry, small-angle neutron scattering, Fourier transform infrared spectroscopy, and temperature-dependent wide- and small-angle X-ray scattering. PLLA crystallizes in the ε form when organic solvents such as CPO are incorporated into the crystal lattice at subambient temperatures. The transformation of this structure into the α form during solvent desorption and the accompanying changes in the lamellar structure were followed by various measurements on PLLA/CPO cocrystals. SANS data suggest that CPO is present stoichiometrically in the crystal lattice and as clusters in the interlamellar amorphous regions in the nominally dried samples. DSC thermogram showed a sharp endotherm during this ε to α transition. Xray fiber diagrams showed that the ε form transforms to the α form over a temperature range (40−55 °C) as the solvent molecules are expelled from the crystalline lattice, while maintaining chain orientation. Infrared spectra showed the splitting of the CH3 symmetric deformation band at 1383 cm−1 into a doublet (1382 and 1386 cm−1) at ε to α transition, indicating the desorption of CPO molecules from the crystal lattice. Changes in the invariant in SAXS data are interpreted as due to the migration of the solvent from the crystalline phase to the amorphous phase during the ε to α transition followed by the evaporation of the solvent from the entire polymer. During this transition, lamellae that are tilted in the presence of CPO in the crystal lattice become perpendicular to the chain axis. In addition, there are changes in long period, lamellar thickness, and amorphous thickness. Continuing the desorption to dryness by further heating results in the removal of the solvent molecules in the amorphous phase of the α form. This is accompanied by increased crystallinity. These studies show that the solvent desorption results in a precise sequence of quantifiable structural changes at multiple length scales