Self-assembly of block copolymers into interesting and useful nanostructures, in both solution and bulk, is a vibrant research arena. While much attention has been paid to characterization and prediction of equilibrium phases, the associated dynamic processes are far from fully understood. Here, we explore what is known and not known about the equilibration of particle phases in the bulk, and spherical micelles in solution. The presumed primary equilibration mechanisms are chain exchange, fusion, and fragmentation. These processes have been extensively studied in surfactants and lipids, where they occur on subsecond time scales. In contrast, increased chain lengths in block copolymers create much larger barriers, and time scales can become prohibitively slow. In practice, equilibration of block copolymers is achievable only in proximity to the critical micelle temperature (in solution) or the order–disorder transition (in the bulk). Detailed theories for these processes in block copolymers are few. In the bulk, the rate of chain exchange can be quantified by tracer diffusion measurements. Often the rate of equilibration, in terms of number density and aggregation number of particles, is much slower than chain exchange, and consequently observed particle phases are often metastable. This is particularly true in regions of the phase diagram where Frank–Kasper phases occur. Chain exchange in solution has been explored quantitatively by time-resolved SANS, but the results are not well captured by theory. Computer simulations, particularly via dissipative particle dynamics, are beginning to shed light on the chain escape mechanism at the molecular level. The rate of fragmentation has been quantified in a few experimental systems, and TEM images support a mechanism akin to the anaphase stage of mitosis in cells, via a thin neck that pinches off to produce two smaller micelles. Direct measurements of micelle fusion are quite rare. Suggestions for future theoretical, computational, and experimental efforts are offered.

In this work, we explore how the chemical reactivity toward an aprotic battery electrolyte changes as a function of lithium salt and silicon surface termination chemistry. The reactions are highly correlated, where one decomposition reaction leads to a subsequent decomposition reaction. The data show that the presence of silicon hydrides (SiH x ) promotes the formation of CO gas, while surface oxides SiO x drive the formation of CO2. The extent and rate of oxidation depend on the surface basicity of the SiO2 surface species. The most acidic surfaces seem to hinder CO2 generation but not the decomposition of the salt. Indeed, the presence of F-containing salts (LiPF6 and LiTFSI) promotes the reactions between carbonate electrolyte and silicon surfaces. Surfaces with high Li content seem to be the most passivating to gassing reactions, pointing to a pathway to stabilize the interfaces during cell formation and assembly.

Polymer/ionic liquid systems are being increasingly explored, yet those exhibiting lower critical solution temperature (LCST) phase behavior remain poorly understood. Poly(benzyl methacrylate) in certain ionic liquids constitute unusual LCST systems, in that the second virial coefficient (A2) in dilute solutions has recently been shown to be positive, indicative of good solvent behavior, even above phase separation temperatures, where A2 < 0 is expected. In this work, we describe the LCST phase behavior of poly(benzyl methacrylate) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide for three different molecular weights (32, 63, and 76 kg/mol) in concentrated solutions (5–40% by weight). Turbidimetry measurements reveal a strong concentration dependence to the phase boundaries, yet the molecular weight is shown to have no influence. The critical compositions of these systems are not accessed, and must therefore lie above 40 wt% polymer, far from the values (ca. 10%) anticipated by Flory-Huggins theory. The proximity of the experimental cloud point to the coexistence curve (binodal) and the thermo-reversibility of the phase transitions, are also confirmed at various heating and cooling rates.

By incorporating a photoactive moiety into a block polymer, the phase behavior can be controlled with light. Previous studies have looked at situating azobenzene, a common photoresponsive molecule, along the backbone of the polymer as a pendant group, and even as part of the solvent rather than as a comonomer. Here, we study the effect of the positioning of pendant azobenzene groups along the polymer backbone on the lower critical disorder–order transition of poly(methyl methacrylate)-block-poly(benzyl methacrylate) (PMMA-b-PBnMA, or MB) in the ionic liquid 1,3-dimethyl imidazolium bis(trifluoromethylsulfonyl)imide. Using small-angle X-ray scattering and ultraviolet (UV)-irradiated small-amplitude oscillatory shear rheology, the placement of azobenzene statistically along the benzyl methacrylate backbone (MBsA) is compared to locating it as a midblock between the PMMA and the PBnMA (MAB) or as an end block after the PBnMA (MBA). Two concentrations of polymer in the ionic solvent were studied, 35 and 50 wt %. At 35 wt %, MBsA microphase-separated at 60 °C, MBA at 100 °C, and followed by MAB at 120 °C, a trend that was repeated at 50 wt %. MBsA was the only polymer to order onto a lattice at 35 wt %, forming hexagonally close-packed spheres. Both MBsA and MBA formed hexagonally packed cylinders at 50 wt %. MBsA consistently ordered onto a lattice over the temperature range of interest, while MBA only did so at 50 wt %, and MAB remained disordered at both concentrations. MBsA was also the only sample of the three to successfully transition reversibly between order and disorder with light. Therefore, adjusting the location of the azobenzene units within the thermo- and photoresponsive polymer solution significantly changes the overall behavior of the solutions and the ability to control that behavior with light and temperature.

Polymers in ionic liquids (ILs) are a fascinating class of materials that exhibit unusual behavior in comparison to more traditional polymer solutions. Previous work characterizing the lower critical solution temperature (LCST) phase behavior of poly(benzyl methacrylate) (PBnMA)/IL mixtures demonstrated that the second virial coefficient is consistently positive, even at temperatures above the observed phase separation boundary, and that the critical composition is shifted strongly toward polymerrich compositions. To better understand these phenomena, smallangle X-ray scattering was utilized along with Ornstein−Zernike analysis to determine the interaction parameter and correlation length of PBnMA in four ILs (one pyrrolidinium-based and three imidazolium-based) as a function of temperature (25−170 °C), concentration (5−30 wt%), and molecular weight (32−76 kDa). The interaction parameter was shown to increase with polymer volume fraction, contrary to the concentration-independent behavior anticipated by Flory-Huggins theory, which clarified the unusual phase diagram of these solutions. The semidilute correlation length of PBnMA in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide was shown to obey a temperature-concentration master curve; however, no such universal behavior was exhibited among the four ILs. Additionally, the concentration dependence of the correlation length was shown to decrease as the solvent quality worsened.