700 J.Rodriguez-Herdndez et al.Prog.Polvm.Sci.30(2005)691-724 2.4.1.Spherical micelle with the so-called core-shell the Since the ly stu ally few selected examples R。 se of our intere in ion mechar ism.In the las en chemistry attention has bee aid to focus on micelles formed from a mphiphilic block mainly motivated by their applications as emulsifiers copolymers in aqueous solution.As reported exten- foam stabilizers or detergents and in biomedicine (as sively by Riess 391.the structure of amphiphilic stabilizing agents in dermatological creams,lotions, block copolymers in aqueous media can be divided etc into three classes depending on the nature of the PEO is a hydrophilic.biocompatible.nontoxic hydrophilic block.There are uncharged blocks such as thermoresponsive polymer,which has been widely poly(ethylene oxide)(PEO) -also referred to as used as the solubilizing block to form the shell in poly(ethylene glycol)(PEG)-positively charged spherical micelles.Hydrophobic blocks include PS. blocks such as quaternized poly(2-or 4-vinylpyr- poly(lactic acid)and polyethers like polypropylene idine).polypeptides such as poly(L-lysine).or nega oxide(PPO)or poly(butylene oxide)(PBO).PEO-b- tively charged ones such as poly(acrylic acid)(PAA and triblo comme ally avallable as poly(styren ate (PSS).or poly(L-glutamic ock)copolymers and as ind quently the extens system Chou ane reviewed o). r drug stic micellization features of these bloc copolymers (a) (c) s obtained from blockcommon structures, such as inverse micelles, bilayers, or cylinders (Fig. 7). Recent reviews analyze in more detail the parameters that afford one or another structure [38]. Since the literature on this topic is abundant and diverse, our discussion is limited to a few selected examples. Because of our interest in biological applications and ‘green’ chemistry we focus on micelles formed from amphiphilic block copolymers in aqueous solution. As reported exten￾sively by Riess [39], the structure of amphiphilic block copolymers in aqueous media can be divided into three classes depending on the nature of the hydrophilic block. There are uncharged blocks such as poly(ethylene oxide) (PEO)—also referred to as poly(ethylene glycol) (PEG)—positively charged blocks such as quaternized poly(2- or 4-vinylpyr￾idine), polypeptides such as poly(L-lysine), or nega￾tively charged ones such as poly(acrylic acid) (PAA), poly(styrene sulfonate) (PSS), or poly(L-glutamic acid) (PGA). As indicated subsequently the charac￾teristics of these systems make them suitable for applications in pharmaceuticals, as vehicles for drug delivery or as separating agents, etc [40]. 2.4.1. Spherical micelles Spherical micelles with the so-called ‘core-shell’ structure have been extensively studied. Formation of spherical micelles via self-assembly of diblock copolymers is directed by an entropically driven association mechanism. In the last decade special attention has been paid to aqueous micellar systems, mainly motivated by their applications as emulsifiers, foam stabilizers or detergents, and in biomedicine (as stabilizing agents in dermatological creams, lotions, etc. PEO is a hydrophilic, biocompatible, nontoxic, thermoresponsive polymer, which has been widely used as the solubilizing block to form the shell in spherical micelles. Hydrophobic blocks include PS, poly(lactic acid) and polyethers like polypropylene oxide (PPO) or poly(butylene oxide) (PBO). PEO-b￾PPO or PEO-b-PBO are commercially available as diblock- (and triblock) copolymers and as a consequence, have been extensively investigated. Chou and Zhou [41] reviewed in detail the characteristic micellization features of these block copolymers. Fig. 7. Examples of structures obtained from block copolymers: (i) direct micelles, (ii) vesicles, and (iii) other morphologies: (iiia) inverse micelles, (iiib) lamellar structures, and (iiic) cylindrical or tubular micelles. 700 J. Rodrı´guez-Herna´ndez et al. / Prog. Polym. Sci. 30 (2005) 691–724