Aggregates Formed by Amphoteric Diblock Copolymers in Water

Abstract

The associating behavior of a series of monodisperse poly(2-(dimethylamino)ethyl methacrylate)-b-poly(methacrylic acid) (PDMAEMA-b-PMAA) ampholytic diblock copolymers has been studied in water in the dilute regime as a function of pH and salt concentration. Dynamic light scattering (DLS) has been used to monitor the association behavior and transmission electron microscopy (TEM) to visualize the morphology of the aggregates. At and around the isoelectric point (IEP) of the ampholytic diblocks, strong electrostatic interactions occur and lead to the formation of insoluble complexes between negatively and positively charged blocks. These electrostatic interactions can be screened by the addition of salt, which leads to the partial dissolution of the material, according to Debye screening. For samples containing a major PDMAEMA block, spherical micelles are observed below the IEP. These micelles consist of a PMAA core surrounded by a water-soluble protonated PDMAEMA corona. At and above the IEP, these samples are insoluble. Spherical micelles are also formed below the IEP for samples containing a minor PDMAEMA block. These copolymers are insoluble at and around the IEP, whereas solubility is observed above the IEP with formation of aggregates of remarkable morphologies, including hollow spheres and complex compartmentalized aggregates. This aggregation is most favorable at high pH, and it cannot be accounted for only by electrostatic interaction between oppositely charged blocks. Indeed, short-range hydrophobic interaction between uncharged PDMAEMA blocks seems to play a key role in the association process. This is emphasized by quaternized poly(2-(dimethylamino)ethyl methacrylate)-b-poly(methacrylic acid) ampholytic copolymers which are carrying a permanent positive charge on the aminated monomeric units and do not form these particular aggregates.

Self-Assembly of Stimuli-Responsive Water-Soluble [60]Fullerene End-Capped Ampholytic Block Copolymer

“…This phenomenon is attributed to the formation of charge complexes near this range as suggested by Gohy et al [28]. As pH is increased to above 4.0, some of the MAA units are ionized and they form charge complexes with positively-charged protonated PDMAEMA.…”

“…3. It is evident that between pH 5.4 and 8.8, the copolymer becomes insoluble due to complexation and overall charge neutralization near the isoelectric point (IEP) [28], [29]. This behavior is similar to that of P(MAA-b-DMAEMA) [29] and P(MAA-b-DEAEMA) [9] in solution.…”

“…If electrostatic attraction between positively-charged PDMAEMA and negatively-charged ionized PMAA becomes significant, the formation of electrostatic complexes will produce insoluble precipitates. This will impede the formation of freely-solvated unimers or cause a decrease in the size of aggregates [28]. The stability of unimer size is thus an indication of the stability of the copolymer microstructure.…”

“…In the latter, smaller micelles and large hollow aggregates were formed below and above the IEP respectively at room temperature. The combination of electrostatic and hydrophobic interactions together with the minimization of chain-stretching of PDMAEMA blocks control the formation of these large hollow aggregates [28]. In the C 60 -containing block copolymer, the combination of electrostatic and hydrophobic interactions and the presence of C 60 as an additional hydrophobic group as well as a charge-transfer acceptor produce unimers and aggregates at low pH and at high pH at low temperatures and micelles and aggregates at high pH and at high temperatures V. CONCLUSIONS The well-defined and water-soluble C 60 -containing amphiphilic block copolymer was synthesized by ATRP and its stimuli-responsive self-assembly behaviour was systematically characterized by DLS and TEM.…”

“…Since PMAA and PDMAEMA are proven to be stimuli-responsive and biocompatible [27], studies have been conducted on P(MAA-b-DMAEMA) block copolymers by a few groups. Gohy et al investigated the association behavior of a series of mono-dispersed P(MAA-b-DMAEMA) ampholytic diblock copolymers in water in the dilute regime as a function of pH and concentration [28]. For copolymers containing a major PDMAEMA block, they formed spherical micelles below the isoelectric point (IEP) but are insoluble at and above the IEP.…”