Design of thermo-responsive self-assembly of PEGylated fatty acids: Switching reversibly from tubes or vesicles to micelles at physiological temperature

• PEGylation of self- assemblies fatty acid with various morphologies can be achieved in a simple manner by mixing a low amount of end-capped poly(ethylene glycol) chains with 12-hydroxy stearic acid (12-HSA) molecules. • The concept has been tested with 4 types of 4k end-capped PEG chains, respectively at one end or at both ends with either 12-HSA or stearic acid (SA). • At room temperature, two morphologies are obtained: mixtures of di-functionalized chains by 12-HSA with 12-HSA molecules produce PEGylated vesicles while all other systems make PEGylated multi-lamellar tubes. • All systems transit reversibly into small PEGylated ellipsoidal micelles around 37 °C. • Morphological transition tunes the rheological properties: suspensions are gelled at room temperature and Newtonian liquid above 37 °C. The mixing of end-capped poly(ethylene glycol) (PEG) chains with 12-hydroxy stearic acid (12-HSA) molecules is a simple one-pot strategy to design thermo-responsive PEGylated self-assemblies of fatty acids with various morphology types at room temperature (multi-lamellar tubes or vesicles) that transit reversibly upon heating into small micelles around physiological temperature. 4 types of 4k end-capped poly(ethylene glycol) (PEG) chains, capped respectively at one end or at both ends with either 12-HSA or stearic acid (SA), were mixed with 12-hydroxy stearic acid molecules, at a low constant ratio of end capped fatty acid moieties brought by the chains to that of free 12-HSA molecules. The detailed structure of the self-assemblies of mixtures was obtained using Small Angle Neutron Scattering with contrast variation at both 20 °C and 45 °C, and their temperature-dependent rheological behavior was characterized. For both types of mono-functionalized PEG, the chains insert homogenously in the multi-lamellar tubes formed by 12-HSA molecules. The mixtures of di-functionalized chains by 12-HSA with 12-HSA molecules produce PEGylated vesicles, since the change of packing parameter induced by insertion of the telechelic chains no longer allows the formation of tubes. Conversely, mixtures of di-functionalized chains by SA with 12-HSA molecules enable to keep multi-lamellar tubes, a specific behavior that likely comes from the fact that they only insert by one end within the 12-HSA bilayers. All systems transit reversibly into small PEGylated ellipsoidal micelles. The morphological transitions enable to tune the rheological properties of suspensions, that are gelled at low temperature and turn Newtonian liquid at around 37 °C.