Tuesday, July 1, 2008 - 10:00 AM
Kingsland Room (Sheraton Burlington Hotel and Conference Center)
349
Improved Stability of Reverse Polymeric Micelles Via Core Cross-Linking
Reverse polymeric micelles (RMs), characterized by a hydrophilic core - hydrophobic shell architecture, displayed high loading capacity for small hydrophilic molecules. Despite their interesting properties, multimolecular RMs, as other conventional micelles, may release their cargo prematurely. Cross-linking has been repeatedly shown to be an effective method to increase micelle stability by freezing the assemblies into permanent core-shell structures.
RMs were self-assembled from alkylated star-shaped poly(glycerol methacrylate). First, poly(glycidyl methacrylate) was prepared by ATRP. The polymer was hydrolyzed in water to yield poly(glycerol methacrylate) which was partially esterified using stearoyl chloride. Core cross-linked (CCL) micelles were then produced by an original method based on encapsulation of DVS in the hydrophilic micelle core and subsequent cross-linking of the -OH group via Michael addition (Scheme 1).
Scheme 1. Core-cross-linking reaction
Note: SP represents star-shaped polymer.
The release of the encapsulated dye from RMs and CCL RMs was then evaluated. For this purpose, the organic phase containing Congo red loaded-micelles was isolated following extraction and exposed to an aqueous phase (Figure 1). The amount of dye released was calculated from the optical density of the water phase (lmax = 500 nm). After 1 week, 54.8% of Congo red was released from RMs, whereas only 13.4% of the dye had escaped from CCL RMs over the same time span, confirming that these micelles better retained their cargo. These findings were reproduced with two other hydrophilic dyes (Figure 2). The greater dye retention of CCL RMs can be rationalized in terms of a decreased core porosity and possible affinity of the dyes for the cross-linker's sulfone groups.
Figure 1. Reverse extraction of Congo red.
Figure 2. Release of encapsulated dyes in water from non-cross-linked (A) and CCL (B) RMs dispersed in dichloromethane.