Quantifying and tuning the exchange dynamics of amphiphilic triblock copolymers comprising temperature and/or pH sensitive units

This PhD was part of a collaborative “Dynamic PISA” ANR project which aimed at improving the understanding of morphological transitions (MT) occurring during RAFT-PISA synthesis in dispersion in water, a process where amphiphilic AB diblock copolymers are simultaneously synthesized and self-assembled. Studies reported that self-assemblies obtained by PISA rarely reach the morphologies predicted by thermodynamic considerations. The failure of the self-assembled structures to reorganize and to undergo MT was assumed to be due to a lack of unimer exchange between micelles. The focus of this project was to investigate the kinetic contribution (exchange of unimers) to the MT observed in RAFT-PISA. My PhD aimed at quantifying and tuning the exchange rate of unimers (νuni) by measuring the rheological properties of self-associated amphiphilic triblock copolymers (BAB) in water. This also allowed the design of hydrogels with controllable rheological properties. Clément DEBRIE (PhD student, Paris) synthesized the polymers and investigated the morphologies obtained by PISA. The bibliography highlighted the relevance of 2-methoxyethyl acrylate (MEA) monomer for its use in PISA synthesis in dispersion in water but also to design hydrogels with tunable exchange dynamics. The rheological properties of hydrogels formed by PMEAx-b-PDMAc400-b-PMEAx (PDMAc: poly (dimethylacrylamide)) were measured as a function of x = 50-100-200-300 and temperature (T). These polymers formed transient networks showing an abrupt reorganization above a critical T, Tc. Rheological measurements suggested that this was resulting from a fraction of the elastically active chains with a temporarily faster dynamic. However, its molecular origin could not be explained. The T-sensitivity is opposite to the classical arrhenian behavior, leading to hydrogels with original thermo-thickening properties. We suggest that the moderate hydrophobicity of MEA and the progressive dehydration under heating of PMEA explains this behavior. Together with the colleagues from Paris, we demonstrated that PDMAc-b-PMEA diblock copolymers prepared by PISA yielded different morphologies ranging from spheres to worms and vesicles with increasing PMEA-block length. However, the νuni was too slow to account for the MT observed during PISA. The presence of residual MEA during the polymerization did not enhance unimer exchange. It reduced νuni contrary to what suggests the literature. It was then proven quantitatively for the first time that unimer exchange is not necessary for MT observed in PISA. Still, the monomer played a role as its addition to post-PISA dispersions of PDMAc-b-PMEA resulted in an evolution of the morphologies. Finally, the addition of 5 mol% of Acrylic acid (AA) within PMEA100-b-PDMAc400-b-PMEA100 B-blocks successfully induced controllable dynamics independently with the pH and T. It was attempted to freeze the exchange dynamics by the addition of a multivalent cation, Ca2+, which was expected to act as a physical cross-linker within the micellar cores. The impact of the B-blocks composition profile on the exchange dynamics was also investigated by varying the distribution of AA units within these blocks. However, neither the addition of Ca2+, nor the variation of the composition profile resulted in significant modification of the exchange dynamics. We assumed that this was related to the small proportion of AA units in the B-blocks.