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

Abstract

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 ∼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 ∼42 wt %, the segmental polymer dynamics (T g and α 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 ∼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 ∼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.

Structure–property relationships in renewable composites of poly(lactic acid) reinforced by low amounts of micro- and nano-kraft-lignin

“…This is compatible with the previously mentioned increase of the polymer concentration around the lignins and subsequently the decrease of polymer density away from them. Such a situation has been observed before in PLA-based systems 21,27,30,32 as well as in other polymers. 65…”

“…Regarding PLA, there is already a rich literature on its molecular dynamics, in simple (bulky) 71–74 and complex forms (PNCs, copolymers, under spatial confinement, etc. ), 21,75–79 which sets the origins of the various recorded relaxation processes mainly known.…”

“…, T m ∼ 140–190 °C) temperatures. 16,19–22 More interestingly, the semicrystalline character of PLA provides the ability to tune a series of properties, such as the mechanical performance 23,24 and thermal conductivity, via the easily manipulable crystallinity and semicrystalline morphology (dense/sparse). 25–27 Interestingly, depending on the structure, molecular weight and the thermal treatment, PLA can be transformed from completely amorphous to quite highly crystalline form.…”

“…25–27 Interestingly, depending on the structure, molecular weight and the thermal treatment, PLA can be transformed from completely amorphous to quite highly crystalline form. 20,21,25,28 Except for the PLA's performance within the application, there is a general desire to facilitate its physical degradation, for example in soil – compostability, which in the case of bulk PLA is rather slow. 8,29 Additionally, PLA exhibits high potential for combination with other polymers (copolymers, blends, interpenetrating polymer networks, and stars) 12,22,30–32 as well as with inorganic/organic fillers and nanoparticles.…”