(4bu) Structure and Dynamics of Block Copolymer Based Soft Materials

Lee, S., University of Minnesota

Block copolymers, made by joining two or more linear polymer blocks, have been studied over a half century, and these compounds are still being intensively explored. We recently discovered an equilibrium phase of packed spheres in poly(1,4-isoprepe-b-DL-lactide) (IL) diblock and poly(styrene-b-1,4-isoprene-b-styrene-b-ethylene oxide) (SISO) tetrablock copolymers, known as Frank-Kasper σ-phase using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) experiments. The σ-phase contains 30 microphase-separated spheres in a large tetragonal unit cell, nucleates and grows from the body-centered cubic (BCC) phase similar to its occurrence in metal alloys. It is also known as a crystal approximant to dodecagonal quasicrystals, indirectly predicted by the Landau theory in diblock copolymer melts.

The mean field treatment on block copolymer thermodynamics near the order-disorder transition has given valuable understandings on this subject, but it assumes an infinitely large degree of polymerization (N) with an infinitesimally small Flory-Huggins interaction parameter (χ). All experimentally accessible block copolymer compounds have N and χ far from these limits, and the fluctuation effect near ODT from finite N has not been fully elucidated. We have investigated the thermodynamics of poly(1,4-isoprene-b-DL-lactide) diblock copolymers near the ODT with small N and relatively large χusing SAXS, dynamic mechanical spectroscopy (DMS), and differential scanning calorimetry (DSC) techniques. These studies provide fresh insights into the role of fluctuations on the state of segregation and ordering kinetics in symmetric and asymmetric diblock copolymers, prompting new questions regarding the application of mean-field and fluctuation corrected theories to block copolymer melts with finite molecular weights.

Block copolymer surfactancy using polyalkyl-based non-ionic surfactants have been investigated over several decades, these macromolecular surfactant compounds form mostly non-equilibrium states in water and/or oil due to the extremely low critical micelle concentrations (CMC's), which are at least several orders lower than the CMC's of traditional non-ionic surfactants. We investigated the non-ergodic effects by examining two ternary mixtures, each containing the same components, poly(1,2-butadiene-b-ethyelene oxide)  (OB), water, and 1,5-cyclooctadiene, but the mixtures were prepared using different protocols. Both systems split into three layers, and the middle layer was evaluated using cryo-scanning electron microscopy (cryo-SEM) and SAXS. Qualitatively different microstructures were found in each system. This difference demonstrates that three-component block copolymer/water/oil mixtures are much more significantly affected by non-equilibrium effects than the analogous block copolymer based two component systems.

Along with above research conclusions, future research plans will be discussed