Effects of Small Amounts of Metal Nanoparticles on the Glass Transition, Crystallization, Electrical Conductivity, and Molecular Mobility of Polylactides: Mixing vs. In Situ Polymerization Preparation

The synthesis of two series of poly(lactic acid) (PLA)-based polymer nanocomposites (PNCs)
filled with small amounts (0.5 and 1%) of Ag and Cu nanoparticles (NPs) was performed.
Moreover, two methods for the PNC synthesis were performed, namely, ‘conventional
mixing techniques’ and ‘in situ ring opening polymerization (ROP)’. The latter method was
employed for the first time; moreover, it was found to be more effective in achieving very
good NP dispersion in the polymer matrix as well as the formation of interfacial polymer–
NP interactions. The in situ ROP for PLA/Cu was not productive due to the oxidation of Cu
NPs being faster than the initiation of ROP. The presence of NPs resulted in suppression of
the glass transition temperature, Tg (23–60 ◦C), with the effects being by far stronger in the
case of ROP-based PNCs, e.g., exhibiting Tg decrease by tens of K. Due to that surprising
result, the ROP-based PLA/Ag PNCs exhibited elevated ionic conductivity phenomena
(at room temperature). This can be exploited in specific applications, e.g., mimicking
the facilitated small molecules permeation. The effects of NPs on crystallinity (2–39%)
were found opposite between the two series. Crystallinity was facilitated/suppressed in
the mixing/ROP -based PNCs, respectively. The local and segmental molecular mobility
map was constructed for these systems for the first time. Combining the overall data, a
concluding scenario was employed, that involved the densification of the polymer close
to the NPs’ surface and the free volume increase away from them. Finally, an exceptional
effect was observed in PLA + 0.5% Ag