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
