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

The enthalpy and entropy of micellization in water, ΔH_mic and ΔS_mic, respectively, of three linear amphiphilic BAB block copolymers consisting of either poly(methyl acrylate) (M_n ∼ 1200 and 700 Da) or poly(ethyl acrylate) (M_n ∼ 800 Da) as hydrophobic (B) segments and poly(ethylene oxide) (PEO) as the hydrophilic (A, M_n ∼ 3000 Da) segment were determined by isothermal titration calorimetry (ITC). The ΔH_mic and ΔS_mic of the cyclic AB block copolymers obtained by cyclization of the linear triblock copolymers were determined under the same conditions. The ΔH_mic value of the cyclic copolymers was smaller than that of their linear precursors. The ΔS_mic value showed the same trend, but the relative difference between the cyclized and linear copolymers was less pronounced. The hydrodynamic diameter (D_h), critical micelle concentration (CMC), molecular weight (M_w-mic), and second virial coefficient (A₂) of the micelles were determined. The D_h 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 ΔH_mic 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 ΔS_mic value.