Poly(lactic acid)−Poly(ethylene oxide) (PLA−PEG) Nanoparticles:  NMR Studies of the Central Solidlike PLA Core and the Liquid PEG Corona

NMR studies on a series of poly(lactic acid)−poly(ethylene oxide) (PLA−PEG) diblock copolymers have been carried out in d 6-acetone and in D2O. Nanoparticles of the PLA−PEG copolymers were obtained using a modified interfacial polymer deposition−solvent evaporation technique (Fessi, H.; Devissaguet, J. P.; Puisieux, F.; Thies, C. French Patent 2 608 988, 1986). In D2O, the PLA block forms a central hydrophobic core, while the PEG block forms an hydrophilic corona layer. In D2O, the hydrophobic core of the nanoparticle is generally not seen, while the PEG corona is observed. Only the PLA methyl protons at the interface between the two regions are observed, and these are seen as a double doublet structure. For nanoparticles with a low molecular weight PLA block length, an additional methyl multiplet signal is seen suggesting that PLA methyl groups are in more than one chemical environment. This is not seen for nanoparticles with a high molecular weight PLA block length indicating more uniform structure in the core interfacial region. As temperature is increased, the core of the latter becomes more liquidlike. Quantitative calibration studies of the PEG corona layer show that most of the PEG layer is seen indicating that it is in the liquid phase and on the surface of the nanoparticle. 13C solid-state NMR spectroscopy studies indicate the presence of a central solidlike core and a more mobile interfacial region at the PLA−PEG interface, while the relaxation rate of the nanoparticle obtained from T1 studies indicates that the PEG corona is a much more mobile environment than the interfacial methyl protons.