Microphase Separation of Resilin-like and Elastin-like Diblock Copolypeptides in Concentrated Solutions.

Diblock copolymers are valued for their ability to form thin films with nanoscale features that typically reflect those of their microphase-separated structures in concentrated solution. Here, we show that such self-assembled structures can be easily formed with diblock copolymers composed of thermally responsive polypeptides, such as resilin-like polypeptides (RLP) and elastin-like polypeptides (ELP), by exploiting the inverse temperature transition behavior of ELPs in aqueous media. Specifically, we examine the self-assembly of a series of RLP-b-ELP diblock copolypeptides in concentrated aqueous solution (30 and 50 wt %) by small-angle X-ray scattering (SAXS). By systematically varying RLP block length and temperature (10-45 °C), we observed microphase separation into hexagonally packed cylinders and lamellae. By analyzing the observed order-order transitions (OOT) and order-disorder transitions (ODT), we determined that self-assembly in this system is primarily driven by polymer-solvent interactions. While these thermally responsive polymers showed clear ODTs and OOTs at certain temperatures, temperature only had a weak effect on the spacing of the resulting nanostructures. In contrast, we found that nanostructure spacing was far more sensitive to RLP block length. Finally, we used atomic force microscopy (AFM) to demonstrate that spin casting RLP-b-ELP diblock copolypeptides also produce nanostructured thin films with spacings that correlate with those in concentrated solution.

Self-Assembly of a Midblock-Sulfonated Pentablock Copolymer in Mixed Organic Solvents: A Combined SAXS and SANS Analysis.

Ionic, and specifically sulfonated, block copolymers are continually gaining interest in the soft materials community due to their unique suitability in various ion-exchange applications such as fuel cells, organic photovoltaics, and desalination membranes. One unresolved challenge inherent to these materials is solvent templating, that is, the translation of self-assembled solution structures into nonequilibrium solid film morphologies. Recently, the use of mixed polar/nonpolar organic solvents has been examined in an effort to elucidate and control the solution self-assembly of sulfonated block copolymers. The current study sheds new light on micellar assemblies (i.e., those with the sulfonated blocks comprising the micellar core) of a midblock-sulfonated pentablock copolymer in polar/nonpolar solvent mixtures by combining small-angle X-ray and small-angle neutron scattering. Our scattering data reveal that micelle size depends strongly on overall solvent composition: micelle cores and coronae grow as the fraction of nonpolar solvent is increased. Universal model fits further indicate that an unexpectedly high fraction of the micelle cores is occupied by polar solvent (60-80 vol %) and that partitioning of the polar solvent into micelle cores becomes more pronounced as its overall quantity decreases. This solvent presence in the micelle cores explains the simultaneous core/corona growth, which is otherwise counterintuitive. Our findings provide a potential pathway for the formation of solvent-templated films with more interconnected morphologies due to the greatly solvated micellar cores in solution, thereby enhancing the molecular, ion, and electron-transport properties of the resultant films.

Bicomponent Block Copolymers Derived from One or More Random Copolymers as an Alternative Route to Controllable Phase Behavior.

Block copolymers have been extensively studied due to their ability to spontaneously self-organize into a wide variety of morphologies that are valuable in energy-, medical-, and conservation-related (nano)technologies. While the phase behavior of bicomponent diblock and triblock copolymers is conventionally governed by temperature and individual block masses, it is demonstrated here that their phase behavior can alternatively be controlled through the use of blocks with random monomer sequencing. Block random copolymers (BRCs), i.e., diblock copolymers wherein one or both blocks are a random copolymer comprised of A and B repeat units, have been synthesized, and their phase behavior, expressed in terms of the order-disorder transition (ODT), has been investigated. The results establish that, depending on the block composition contrast and molecular weight, BRCs can microphase-separate. We also report that large variation in incompatibility can be generated at relatively constant molecular weight and temperature with these new soft materials. This sequence-controlled synthetic strategy is extended to thermoplastic elastomeric triblock copolymers differing in chemistry and possessing a random-copolymer midblock.

Effect of Systematic Hydrogenation on the Phase Behavior and Nanostructural Dimensions of Block Copolymers.

Unsaturated polydienes are frequently hydrogenated to yield polyolefins that are more chemically stable. Here, the effects of partial hydrogenation on the phase behavior and nanostructure of polyisoprene-containing block copolymers are investigated. To ensure access to the order-disorder transition temperature (TODT) over a wide temperature range, we examine copolymers with at least one random block. Dynamic rheological and scattering measurements indicate that TODT increases linearly with increasing hydrogenation. Small-angle scattering reveals that the temperature-dependence of the Flory-Huggins parameter changes and the microdomain period increases, while the interfacial thickness decreases. The influence of hydrogenation becomes less pronounced in more constrained multiblock copolymers.