NMR Relaxometry for the Thermal Stability and Phase Transition Mechanism of Flower-like Micelles from Linear and Cyclic Amphiphilic Block Copolymers

Abstract

Linear and cyclic amphiphilic block copolymers consisting of poly(ethylene oxide) (PEO) as the hydrophilic segment and poly(methyl acrylate) or poly(ethyl acrylate) as the hydrophobic segments were synthesized and self-assembled to form flower-like micelles. The micelles from linear (methyl acrylate)12(ethylene oxide)73(methyl acrylate)12 (Mn = 1000-3200-1000, l-MOM) showed a cloud point (Tc) at 46 °C by the transmittance of the micellar solution, whereas that of cyclic (methyl acrylate)30(ethylene oxide)79 (Mn = 2600-3500, c-MO) increased to 72 °C, as previously reported. DLS showed comparable diameters (l-MOM, 14 nm; c-MO, 12 nm) and Tc values (l-MOM, 48 °C; c-MO, 75 °C). For the investigation of the difference in Tc and the phase transition mechanism based on the polymer topology, NMR relaxometry was performed to determine the spin-lattice (T1) and spin-spin (T2) relaxation times. A decrease in T2 of the PEO segment in both l-MOM and c-MO was observed above Tc, suggesting that slow large-scale motions, such as the detachment of a chain end from the core, bridging, and interpenetration of the micelles, were inhibited. T1 of the PEO segment in l-MOM continuously increased in the experimental temperature range, indicating that the segment is hydrated even above its Tc. On the other hand, that of c-MO reached a ceiling above its Tc, likely due to the prevention of the rotation of the PEO main chain bonds caused by dehydration. Similar results were obtained for linear (ethyl acrylate)8(ethylene oxide)79(ethyl acrylate)8 (Mn = 800-3500-800, l-EOE) and its cyclic (ethyl acrylate)15(ethylene oxide)78 (Mn = 1500-3400, c-EO).

Cyclization of PEG and Pluronic Surfactants and the Effects of the Topology on Their Interfacial Activity

“…Inspired by this, cyclic amphiphilic PBA–PEG block copolymers were synthesized by cyclization of linear amphiphilic PBA–PEG–PBA pre-polymers, where PBA and PEG are poly­( n -butyl acrylate) and poly­(ethylene glycol), respectively, to compare the properties of linear and cyclic polymer micelles. , The cloud-point ( T c ) measurement revealed that the cyclic polymer micelles were more resistant to heat and salinity. This is because the chain ends stimulate agglomeration due to the bridging between micelles with an increase in temperature, whereas dehydration was the main cause for the precipitation of the cyclic polymer micelles . In another example, a cyclized PEG homopolymer was found to have an extraordinary stabilizing effect on aqueous dispersions of gold nanoparticles compared to its linear counterpart .…”

Structural and dynamical aspects of PEG/LiClO4 in solvent mixtures via NMR spectroscopy

“…Therefore, an attempt has been made recently to study the dynamics of low molecular weight EO‐based polymer (PEG) in LiClO 4 mixtures . In this context, NMR spectroscopy has proved to be one of the most promising experimental techniques to study the dynamics of the PEO‐based polymers and its mixtures with various salts and molecular solvents . Beside the use of molecular solvents, ionic liquids are also used as liquid plasticizers for the study of PEO/Li + salt mixtures .…”

“…[6] In this context, NMR spectroscopy has proved to be one of the most promising experimental techniques to study the dynamics of the PEO-based polymers and its mixtures with various salts and molecular solvents. [9][10][11][12][13] Beside the use of molecular solvents, ionic liquids are also used as liquid plasticizers for the study of PEO/Li + salt mixtures. [14] Other literature studies have also been reported regarding the dynamical aspects of EO-based polymers and their mixtures with various materials and solvent system.…”

Self‐Organization of Hyperbranched Polyesters Functionalized with Pyrrolo[2,1‐a]isoquinoline End Groups and Their Fluorescent Recognition of Anthracene and Pyrene

“…Self-assembly of hyperbranched polyesters (HBPEs) has attracted widespread interest recently owing to their unique chemical and physical properties; they have been widely used in academic research [1][2][3] and for applications in biology, [4] medicine, [5] pharmacology, [6,7] for their remarkably improved performance. [26][27][28][29] However, most reports on the fabrication of flower-like architectures to date have focused on linear polymers [30,31] and metallic oxides. [32][33][34] For example, Gaillard and co-workers reported flower-like poly(ethylene oxide)-based macromolecular architectures obtained by photocrosslinking of self-assembled block copolymers.…”

“…As 3D flower‐like structured counterparts could provide a larger surface area, extend the dispersion of active sites at different length scales, and shorten the diffusion path compared to that of 2D lamellar materials, 3D flower‐like materials have attracted widespread interest for their remarkably improved performance . However, most reports on the fabrication of flower‐like architectures to date have focused on linear polymers and metallic oxides . For example, Gaillard and co‐workers reported flower‐like poly(ethylene oxide)‐based macromolecular architectures obtained by photocrosslinking of self‐assembled block copolymers .…”