Effects of High Crystallinity on the Molecular Mobility in Poly(lactic acid)-Based Microcapsules

We investigate the molecular mobility in biodegradable poly(lactic acid) (PLA) in the thin shell of spherical hollow semicrystalline microcapsules (MCs), of remarkable crystalline fractions, CFs (>55%). The effect of the solid-state polymerization (SSP) on MCs as a post-encapsulation modification is additionally studied. For comparison to MCs, we study the corresponding precursor linear polylactides, of low molecular weight, M-v similar to 20 kg/mol, and high CF. The emphasis is given on the static and dynamic glass transition and their dependence on crystallinity. To these aims, we employed broad-band dielectric spectroscopy and differential scanning calorimetry, supplemented by X-ray diffraction. Compared with initial untreated semicrystalline PLA with CF similar to 42 wt %, the segmental polymer dynamics (T-g and alpha relaxation) is significantly faster in all treated samples and exhibits strongly suppressed cooperativity, which is shown here for the first time. The thermochemical treatments, hydrolysis and thermal annealing, of pure PLA result in a particular semicrystalline morphology of PLA in MCs and their precursors, severely affecting the glass-transition dynamics. The overall recordings indicate that the mobile amorphous polymer chains, i.e., rather short chains emerging from the crystals, in the micrometric MC shell suffer strong spatial constraints and nanoconfinement by large numbers of nanosized crystals upon all treatments. The additional employment of SSP increases the chains' M-v of only the amorphous polymer part, increases CF by similar to 10%, and, surprisingly, "loosens" the nanoconfinement on the amorphous polymer chains between crystals. This is probably due to a reorganization of the crystalline domains. Overall, the results reveal the potential for manipulation of CF (up to very high amounts approaching similar to 80%) and of semicrystalline morphology. This, subsequently, enables the tuning of barrier properties (permeability), heat transport, and mechanical performance of PLA, being actually desired, as the said PLA MCs are envisaged for use in applications involving active substance-controlled release.