The Depth Profile of Free Volume in Drug Delivery Polymers Studied by Positron Annihilation Spectroscopy

The depth profile of free volume in three drug-delivery polymers: a mixture of poly(N-vinyl-pyrrolidone) (PVP) and poly(ethylene glycol) (PEG), the copolymers of vinyl pyrrolidone with poly(ethylene glycol) diacrylate (PVP-PEGDA) and monomethacrylate (PVP-PEGMMA) was studied using Doppler broadening energy spectra (DBES) of annihilation radiation and positron annihilation lifetime (PAL) as a function of positron incident energy (0-30 keV). Significant variations of free-volume depth profile in terms of S-parameter, ortho-positronium lifetime, and intensity are observed as a result of the hydrogen-bonding replacement of covalent bonds observed near the surface and the bulk.