Estimation of the Poly(ethylene glycol) Chain Length of L-Polylactide-Polyethylene Glycol in Aqueous Dispersions Using Viscoelastic Measurements

Abstract

A range of novel, biodegradable block copolymers have been produced, consisting of polylactidepolyethylene glycol (PLA-PEG)AB block copolymers, as possible drug delivery systems for drug targeting applications. These copolymers form particles in aqueous dispersions, with the PLA in the core and the hydrophilic PEG chains surrounding the core and extending into the media. The viscoelastic properties of an aqueous dispersion of one such copolymer, synthesized from L-polylactide (LPLA-PEG) were investigated to estimate the PEG chain length. This value would give an indication of the steric stability of the particle and hence the ability to avoid uptake by the reticuloendothelial system (RES). A range of volume fractions, O, of the copolymer were studied, with a strain sweep and frequency sweep being performed at each concentration. The results were typical for a viscoelastic system, with the storage modulus, G', becoming greater than the loss modulus, G'', on increasing in the volume fraction, indicating interaction between the PEG chains. The critical volume fraction, O eff (where G'=G''), is the point where overlap of the PEG chains commences. These data, in conjunction with the known hydrodynamic radius of the particles, determined from photon correlation spectroscopy (PCS), gave a PEG chain length estimate of 6.4 nm. It is hoped that the ability to estimate the PEG chain length by this method will lead to greater understanding of the role of PEG in drug targeting and the design of future drug delivery systems