Two novel two-tail surfactants, dicetyldimethylammonium 4-vinyl benzoate (DDVB) and dicetyldimethylammonium 3,5-divinyl benzoate (DDDB), were synthesized by neutralizing the corresponding quaternary ammonium hydroxide with the appropriate benzoic acid. As expected, these surfactants formed both homo and mixedvesicles, which were readily polymerized with a suitable radical photo-initiator. The polymerization process was followed by UV-vis spectroscopy and also reconfirmed by NMR and IR spectroscopy. Polymerization of vesicles prepared from DDVB, unlike the more commonly polymerized vesicles, in which the polymerizable group forms an integral part of the surfactant, leads to the formation of a linear polyelectrolyte chain that is only electrostatically bound to the lipid bilayer. On the other hand, polymerization of DDDB vesicles leads to the formation of a crosslinked shell (or net) that encases the vesicle bilayer. Such counterion crosslinked vesicles were shown to be resistant to destabilization both by lysis as well as in the presence of a fairly high volume fraction of an organic solvent, such as ethanol. However, although the simple polymerized (linearly) vesicles, formed from DDVB, exhibit enhanced stability toward lysis when compared to their unpolymerized counterparts, they are readily destabilized in the presence of ethanol, leading to precipitation. This sharp contrast in the behavior of linearly polymerized and crosslinked systems suggests that crosslinking is essential to arrest conformational reorganization of the polyelectrolyte chains induced by a change in the solvent medium, which in turn leads to precipitation. Such counterion crosslinked vesicular systems also have an added advantage; they may retain the fluidityof the lipid bilayer while at the same time possess enhanced stability.
The diacrylate of 3,5‐dihydroxybenzoic acid was readily prepared under standard interfacial Schotten‐Baumann conditions. This novel crosslinker is rather unique as it contains a carboxylic acid functionality in addition to the two polymerizable acrylate groups. The free‐radical polymerization of this monomer resulted in a highly crosslinked polymer, possessing a very high density of carboxylic acid groups. These acid groups impart a high level of hydrophilicity to the matrix, especially in alkaline medium. An interesting feature of this crosslinked matrix is the facile hydrolytic de‐crosslinking that occurs when it is subjected to aqueous alkali, although it is very stable under neutral and acidic conditions. The kinetics of this de‐crosslinking was readily studied by following the concentration of the degradation products using UV‐Visible spectroscopy. NMR investigations confirm that the de‐crosslinking process is indeed hydrolytic, and that it leads to the formation of poly(acrylic acid) and 3,5‐dihydroxybenzoic acid. In an effort to understand the role of the carboxylic acid groups in the degradation process, other structurally similar crosslinkers that are devoid of the acid groups, namely methyl 3,5‐diacryloyloxybenzoate, 3,5‐diacryloyloxyacetophenone, and 1,3‐diacryloyloxybenzene, were synthesized, and the degradation of the corresponding crosslinked polymers was investigated. It is apparent from these studies that the acid‐containing polymer matrix degraded at significantly faster rates when compared to the other crosslinked systems suggesting the occurrence of a reactive diffusion controlled rate enhancement.
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