D

Figure 2. Different configurations of the PEG chains at the surface of the nanoparticles prepared by using block copolymers: "brush (A), "bunch" (B), "loop" (C), and "star" (D).

to show high surface coverage; the PEG chains were in an extended configuration [51]. JH NMR spectroscopy provided a direct evidence of the presence of PEG chains on nano-spheres made of diblock PLGA-PEG or multiblock PLA-(PEG)3 copolymers [52]. The PEG moieties extended out from the nanospheres' cores into the suspending media and exhibited chain mobility similar to that of PEG in solution.

To determine the amount of PEG located at the nano-spheres' surface, a method was set up to detach, in a controlled manner, the PEG coating. To hydrolyze the ester bridges between PEG and PLA, an alkaline solution was added to a suspension of PEG-PLA nanospheres. First, the PEG corona was detached, and then the PEG chains embedded in the PLA matrix were revealed. Results led to the conclusion that the PEG chains were practically all located at the nanospheres' surface (as in Fig. 2, case "brush").

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