Ordered Network Phases in Linear Poly(Isoprene-b-Styrene-b-Ethylene Oxide) Triblock Copolymers
Thomas H. Epps, III, Eric W. Cochran,+ Frank S. Bates*
Polymers Division (854); University of California, Santa Barbara+; University of Minnesota*
At mesoscopic length scales (between roughly one nanometer and one micron) interfacial curvature and packing geometry often reflect the self-assembly of molecules endowed with prescribed architectures and directed interactions. Under the appropriate conditions hydrated soaps, lipids, and surfactants form soft, permeable bicontinuous solids, some capable of templating hard mesoporous networks. Block copolymers provide even greater flexibility in manipulating the placement of engineering materials in a plethora of nanoscale configurations. However, aside from linear permutations of two blocks (AB, ABA, ABAB, etc.), there is currently no reliable way to anticipate block copolymer morphologies from theory.
Here we investigate the equilibrium phase behavior of linear poly(isoprene-b-styrene-b-ethylene oxide) (ISO) triblock copolymer melts, with molecular weights that place these materials near the order-disorder transition (ODT). Ordered phase morphologies were characterized using small-angle x-ray scattering (SAXS), transmission electron microscopy (TEM), dynamic mechanical spectroscopy (DMS), and static birefringence measurements. Interpretation of these results was aided by a modeling technique that facilitates resolution of reciprocal (SAXS) and real space (TEM) experimental data, leading to definitive three-dimensional morphological assignments. Three distinct multiply continuous network morphologies were identified across a band of compositions with 0.1 £ fO £ 0.3, situated between conditions leading to two-domain and three-domain lamellae (fO represents the volume fraction of O blocks). Two cubic network phases, Q230 (core-shell double gyroid with space group symmetry) and Q214 (alternating gyroid with I4132 space group symmetry), flank an expansive orthorhombic network phase region, denoted O70 (Fddd space group symmetry), positioned around the isopleth composition fS » fI. The O70 phase is a relatively low symmetry structure that reflects a fascinating compromise between the interfacial tension and area and the chain stretching across three blocks. To our knowledge O70 is the only non-cubic network phase found in self-assembling soft materials. This unusual structure seems to embody a new type of frustrated self-assembly.
These results provide a powerful strategy for designing network phases in linear ABC triblock copolymers by tuning chain stretching, area, and interfacial tension when (where c represents the Flory-Huggins interaction parameter).
Name: Thomas H. Epps, III
Mentor: Michael Fasolka
Division: Polymers (854)
Laboratory: Materials Science and Engineering
Location: Building 224, Room B232
Mail Stop: 8542
Sigma Xi Member: Yes