We previously showed that nanoparticles (NPs) could be ordered into structures by using the growth rate of polymer crystals as the control variable. In particular, for slow enough spherulitic growth fronts, the NPs grafted with amorphous polymer chains are selectively moved into the interlamellar, interfibrillar, and interspherulitic zones of a lamellar morphology, specifically going from interlamellar to interspherulitic with progressively decreasing crystal growth rates. Here, we examine the effect of NP polymer grafting density on crystallization kinetics. We find that while crystal nucleation is practically unaffected by the presence of the NPs, spherulitic growth, final crystallinity, and melting point values decrease uniformly as the volume fraction of the crystallizable polymer, poly(ethylene oxide) or PEO, ϕPEO, decreases. A surprising aspect here is that these results are apparently unaffected by variations in the relative amounts of the amorphous polymer graft and silica NPs at constant ϕ, implying that chemical details of the amorphous defect apparently only play a secondary role. We therefore propose that the grafted NPs in this size range only provide geometrical confinement effects which serve to set the crystal growth rates and melting point depressions without causing any changes to crystallization mechanisms.

Membranes are a critical component of redox flow batteries (RFBs), and their major purpose is to keep the redox-active species in the two half cells separate and allow the passage of chargebalancing ions. Despite significant performance enhancements in RFB membranes, further developments are still needed that holistically consider conductivity, selectivity, stability, sustainability, and cost. In this Focus Review, structure−property relationships that have led to advances in membranes for various RFB types (vanadium, zinc, iron, etc.) are analyzed. First, two strategies to increase conductivity are highlighted: tuning membrane microstructure and controlling electrolyte uptake. Next, selectivity improvements through size and/or Donnan exclusion are reviewed. With respect to stability, methods to enhance the mechanical robustness of membranes and factors that affect chemical stability are discussed. Additionally, avenues to reduce battery cost and increase sustainability are explored. Future directions are suggested, which include how more in-depth theoretical studies, microstructure optimization, and enhanced characterization will push the field of RFB membranes forward.

Polyethylene and nanosilica represent the most ubiquitous commodity plastic and nanocomposite filler, respectively. Despite their potential utility, few examples exist in the literature of successfully combining these two materials to form polyethylene nanocomposites. Synthesizing well-defined polyethylene grafted to a surface is a significant challenge in the nanocomposites community. Presented here is a synthetic approach toward polyethylene grafted nanoparticles with controllable graft density and molecular weight of the grafted polymer. The variably grafted nanoparticles were then incorporated into a commercial high density polyethylene matrix. The synthesis, characterization, and challenges in making these materials are discussed.

We systematically examine the role of the self-assembly state of polymer-grafted silica nanoparticles (GNPs) on the spherulitic growth kinetics of a poly(ethylene oxide) (PEO) matrix. The nanoparticles (NPs) are functionalized with either unimodal or bimodal polymer brushes to systematically control their self-assembly within the semicrystalline polymer matrix. In the former case, we employ a poly(methyl methacrylate) (PMMA) brush (miscible with PEO), while the latter has a short dense polystyrene (PS carpet) brush, which is immiscible with the PEO matrix, and a long, sparse PMMA brush. The unimodal GNPs always yield well-dispersed NPs possibly because both the silica NP surface and the PMMA interact favorably with the PEO. The addition of the short PS carpet makes the interaction between the surface and the matrix PEO unfavorable. However, since the NPs also have grafted PMMA chains, they act akin to surfactants and provide access to a range of self-assembled structures. In all cases, the addition of NPs decreases the PEO spherulitic growth rates. Surprisingly, we find that the spatial dispersion of NPs does not change the secondary nucleation activation energy barrier of the PEO crystallization, such that the temperature dependence of spherulite growth kinetics is relatively unaffected by the NPs. Instead, the main effect of the NPs on spherulitic growth kinetics arises from variations of the melt's viscosity. Two apparently "universal" trends are foundbare NPs and large assemblies of GNPs appear to approximately follow the same dependence for the role of additives on polymer viscosity. On the other hand, all self-assembled NP structures which have at least one nanoscale dimension (well-dispersed NPs, one-dimensional strings or two-dimensional sheets) follow another general behavior, but one where the viscosity is much larger. Such unusual viscosity increases have been previously observed in the pioneering experiments of Composto and Winey, but there is currently no available theoretical description that can capture these changes and their effects on polymer crystallization.

We investigate the crystallization-induced ordering of C 18 grafted 14 nm diameter spherical silica nanoparticles (NPs) in a short chain (M w = 4 kDa, Đ M ≈ 2.3) polyethylene and a commercial high-density polyethylene (M w = 152 kDa, Đ M ≈ 3.2) matrix. For slow isothermal crystallization of the low molecular weight matrix, the NPs segregate into the interlamellar regions. This result establishes the generality of our earlier work on poly(ethylene oxide) based materials and suggests that crystallization can be used to control NP dispersion across different polymer classes. The incompatibility between the particles and the matrix in the M w = 152 kDa results in a competition between filler organization and filler agglomeration. The mechanical properties improve due to the addition of NPs and are further enhanced by particle organization, even for the case of the macrophase-separated mixtures in the M w = 152 kDa matrix. In contrast, dielectric behavior is strongly affected by the scale of NP organization, with the lower molecular weight matrix showing more significant increases in permittivity due to the local scale of NP ordering.