Micelles

Micelles, described in Section 12.4.2, are globular configurations or arrays of molecules containing hydrophobic (water-avoiding) tails that form a cluster on the inside, and hydrophilic (water-seeking) heads that point outward toward the surrounding water solvent. Dendrimers have been made that are equivalent to uni-molecular micelles with an inner structure of (mostly) hydrophobic hydrocarbon chains, and an outer region or periphery with hydrophilic terminal groups. Figure 11.23 sketches the structure of a micetlanoic acid dendrimer, which is a synthetic micelle containing terminal hydrophilic acid groups (—COOH). Inverse dendrimer micelles have also been synthesized that have an interior structure that is water-seeking, and an exterior or peripheral structure that is water-avoiding.

We shall conclude this chapter with an example of a block polymer that is formed into a micelle that modifies a surface. The block copolymer is formed from two

Figure 11.23. Dendritic micelle with an internal structure of hydrophobic hydrocarbon chains depicted as wavy lines, and hydrophilic acid groups —COOH attached around the outer perimeter. [From Archut and Vôgtle (2000), p. 368,]

segments linked to each other. One is a polylactide (PLA) polymer with a terminal polymerizable double bond, and the other is a polyethylene glycol (PEG) polymer with an acetal [CH3CH(OC2H5)2] functional group at its outer or distal end, as shown in the center part of Fig. 11.24. The upper part of the figure shows how the polylactide components form into a substrate matrix with a layer of polyethylene glycol segments comprising a dense polymer block above it of the type illustrated at the bottom of Fig. 11.10. The functional groups that terminate the polyethyene glycol segments are shown reacting with specific cells and proteins. The lower part of the figure shows how dialysis, or membrane separation in water, is used to form spherical micelles from the block copolymer with acetal or aldehyde (—CHO) functional groups on the surface. The micelles are used to modify a surface, as shown.

Electron paramagnetic resonance (EPR) was employed to study the aldehyde (—CHO) groups on a copolymer surface with the aid of the 2,2,6,6-tetramethyl-l-piperidineloxyl (TEMPO) derivative spin-label compound that has the abbreviated formula sketched in Fig. 11.25. The three-line EPR spectrum characteristic of the spin label that is shown in part (a) of the figure indicated the conjugation or attachment of the spin label TEMPO to the aldehyde group at the end of a

Biodegradable surlace

Biodegradable surlace

Functionalization of polylactide surface

Interaction with Tl specific cells and proteins

^^ Ligand molecules: ^^ sugar and oligopeptide

PEG layer inhibits non-specific adsorption of proteins

Functionalization of polylactide surface

, Hydrophilic segment (PEG)

FunctWoup- Hydrophilic segment

Polymerizable/ ^W^ double bond

Dialyse

Acetal in water in water

Acetal

Aldehyde group

Micelle formation

Surface modification

Aldehyde group

Micelle formation

Surface modification

Figure 11.24. Schematic representation of an aggregation of polylactide (PLA)/polyethylene-glycol (PEG) block copolymers (center of figure). The upper illustration shows how these copolymers have formed a PEG layer that inhibits the nonspecific adsorption of proteins. The lower illustration shows how these copolymers form micelles that coat a surface. [From Otsuka et al. (2001).]

Core-polymerized micelle with aldehyde groups on the surface

Figure 11.24. Schematic representation of an aggregation of polylactide (PLA)/polyethylene-glycol (PEG) block copolymers (center of figure). The upper illustration shows how these copolymers have formed a PEG layer that inhibits the nonspecific adsorption of proteins. The lower illustration shows how these copolymers form micelles that coat a surface. [From Otsuka et al. (2001).]

polyethylene glycol (PEG) segment on the surface. When the acetal surface was treated with TEMPO prior to replacing acetal groups with aldehydes, Fig. 11,25b indicates that only a very weak EPR triplet spectrum was observed, probably due to the direct physical adsorption of TEMPO molecules on the surface. Figure 11.25c shows that no EPR spin-label signal appears when the aldehyde surface was treated with a variety of TEMPO that lacked an amino (—NH2) group. The upper part of the figure shows how the TEMPO molecule bonds to the aldehyde (—CHO) group located at the end of die ethylene glycol copolymer segment.

Figure 11.25. Electron paramagnetic resonance spectra (lower right) of a PEG-PLA surface containing acetal or aldehyde groups after interaction with the spin label TEMPO, which produces an EPR triplet spectrum. Spectra are illustrated for (a) an aldehyde surface interacting with 4-amino TEMPO, (b) an Metal surface interacting with 4-amino TEMPO, and (c) an aldehyde surface interacting with TEMPO, which lacks an amino group — NH2. The strong outer calibration lines with a 90 mT spacing on the EPR spectra ere due to the presence of Mn2+ ions. [From Otsuka et al. (2001).]

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