(689b) Thermodynamics and Kinetics of Ordered, Stronly Segregated Diblock Copolymers

The formation of ordered states in diblock copolymers is based on the kinetic capability of the system to achieve a thermodynamically equilibrium structure. Strongly segregated, low molar mass (unentangled) diblock copolymers (< 10 kg/mol) eliminate the kinetic limitations of conventional polymer systems and are therefore an ideal platform for understanding equilibrium structures. Although the dynamics in these systems are relatively facile, phase transitions to and from complex particle-forming morphologies, such as a dodecagonal quasicrystal and the Frank-Kasper sigma phase, have been shown to take months. There is motivation, then, to elucidate the underlying dynamics that govern these transitions.

A series of model systems containing compositionally asymmetric hydrocarbon and polylactide blocks were synthesized to probe this strongly segregated regime. Methods such as X-ray photon correlation spectroscopy, small angle X-ray scattering, and rheology were utilized to provide a full physical characterization of the dynamics and phase behavior of these systems. Conformational asymmetry is shown to promote the formation of the Frank-Kasper sigma phase, which often forms through a metastable dodecagonal quasicrystal. Additionally, the Frank-Kasper phase is shown to emerge in blends of two neat diblocks with disparate compositions and degrees of polymerization. These results experimentally validate theoretical predictions, and point towards new controls of complex phase behavior in particle-forming diblock copolymers.