Comparative Thermodynamic Studies of the Micellization of Amphiphilic Block Copolymers before and after Cyclization

The enthalpy and entropy of micellization in water, AHmic and ASmic, respectively, of three linear amphiphilic BAB block copolymers consisting of either poly(methyl acrylate) (Mn similar to 1200 and 700 Da) or poly(ethyl acrylate) (Mn similar to 800 Da) as hydrophobic (B) segments and poly(ethylene oxide) (PEO) as the hydrophilic (A, Mn similar to 3000 Da) segment were determined by isothermal titration calorimetry (ITC). The AHmic and ASmic of the cyclic AB block copolymers obtained by cyclization of the linear triblock copolymers were determined under the same conditions. The AHmic value of the cyclic copolymers was smaller than that of their linear precursors. The ASmic value showed the same trend, but the relative difference between the cyclized and linear copolymers was less pronounced. The hydrodynamic diameter (Dh), critical micelle concentration (CMC), molecular weight (Mw-mic), and second virial coefficient (A2) of the micelles were determined. The Dh value of the cyclic copolymer micelles was smaller than the linear counterpart. On the other hand, the CMC value became larger, whereas the A2 value was comparable or increased by cyclization. Overall, the results suggest that, in the unimer state, the hydrophobic segments of the cyclized copolymers form a tightly coiled structure to minimize contact with water, resulting in the smaller AHmic value. Contrary to the linear copolymer micelles, the cyclic copolymer micelles have no "dangling chains", which may explain the topology-driven slight difference in the ASmic value.