Structure and Dynamics of Compositionally Asymmetric Diblock Copolymers

Abstract

Diblock copolymers are among the simplest amphiphilic molecules, and thus provide a model platform for understanding self-assembly in soft matter. The research presented in this work is broadly focused on the interplay between structure and dynamics in particle-forming diblock copolymer melts, motivated by a recent rise in the number of reports describing complex phase formation in these materials. Analogous complex, low-symmetry structures have been observed in hard materials, such as metals and metal alloys, pointing to the existence of underlying universalities within condensed matter physics. In this work, thermal processing methods commonly employed on hard materials are applied to short, compositionally asymmetric poly(1,4-isoprene)-block-poly(±-lactide) (IL) diblock copolymers. Two disordered IL samples exhibiting characteristic spherical micelle fluctuations above the order-disorder transition (ODT) were quenched in liquid nitrogen and reheated to target temperatures. This processing method lead to the formation of unconventional, low symmetry phases that were not otherwise formed by direct cooling from the disordered state, bearing similarities to metallurgy. However, unlike metals, the ordered states below the ODT imprinted particle densities onto the samples that persisted in the disordered state. This remarkable feature is a manifestation of the fluctuating disordered fluid in self-assembling soft materials. A recent report showed that conformational asymmetry in diblock copolymers, or the difference in space-filling capability between each block, is a key factor in complex phase formation. However, the most important parameter in polymer physics is the length of the polymer chain, which in a diblock copolymer may be represented by the invariant block degree of polymerization N ̅_b. In this work, the role of chain length on complex phase formation is investigated by probing the behavior of asymmetric poly(styrene)-block-poly(1,4-butadiene) (SB; N ̅_b ≈ 80) diblock copolymers. This system was devoid of complex phases, in contrast to previous results for short asymmetric poly(ethylethylene)-block-poly(±-lactide) (EL; N ̅_b ≈ 800) diblock copolymers with approximately the same conformational asymmetry. Differences in phase behavior associated with packing and entanglement theory and resulted in calculation of a universal crossover parameter, N ̅_x ≈ 400. In the case of SB, where N ̅_b > N ̅_x, asymmetry in space-filling capabilities are less important and the system exhibits phase behavior analogous to mean-field predictions. Conversely, the N ̅_b < N ̅_x regime places emphasis on conformational asymmetry and presumably other molecular factors that stabilize complex structures. In a diblock copolymer system, a dynamic constraint is imposed upon complex phase formation as mass (chain) exchange between particles is required to accommodate the multiple discrete particle shapes and sizes comprising these structures. In this work, the dynamics associated with particles below the ODT is investigated using dynamic mechanical spectroscopy (DMS) and X-ray photon correlation spectroscopy (XPCS). In the supercooled liquid prior to ordering, DMS and XPCS measurements conducted on a BCC-forming SB diblock copolymer revealed that particle dynamics are dependent on the ergodicity temperature, above which particle rearrangements are mediated by ergodic chain dynamics and below which non-ergodic ‘frozen’ particle motion becomes dominant. Additionally, a new analytical framework was developed to investigate the time evolution of particle dynamics via XPCS, which uncovered a wealth of dynamic features including time-resolved relaxation time and speed distributions associated with particles in grains.