Effect of Heating Rate on the Pathway for Vesicle Formation in Salt-Free Aqueous Solutions of Thermosensitive Cationic Diblock Copolymers

Amphiphilic diblock copolymers composed of a thermo-responsive poly(2-isopropyl-2-oxazoline) (PIPOZ) block (M-n = 7000 g/mol) linked to a poly(3-acrylamidopropyl)trimethylammonium chloride) (PAMPTMA) block (M-n, = 3500 or 8200 g/mol) were prepared by cationic ring-opening polymerization (PIPOZ block) and subsequent atom transfer radical polymerization (PAMPTMA block). Salt-free dilute aqueous solutions of the diblock copolymers underwent a reversible phase transition upon heating past 40 degrees C that led to the formation of vesicles, as established by light scattering (LS) and transmission electron microscopy (TEM) studies of solutions heated to 60 degrees C. The pathway toward vesicles varied depending on the heating rate. A slow heating rate (0.2 degrees C/min) led to the formation of small spheres as the temperature reached 40 degrees C; the spheres grew in size as the temperature increased and eventually converted into vesicles by diffusion of some hydrophilic charged blocks toward the center. Upon application of a fast heating rate (0.7 degrees C/min or faster), the copolymers formed rod-like and/or worm-like micelles at 40 degrees C and, at higher temperature, oblate membranes that closed up into vesicles. The kinetic control of the pathway toward vesicles may be attributed to the existence of several metastable states in the system, by analogy with studies of the isothermal self-assembly of diblock copolymers in selective solvents.