Thermodynamics of Block Copolymers with and without Salt

Ion-containing block copolymers are of interest for applications such as electrolytes in rechargeable lithium batteries. The addition of salt to these materials is necessary to make them conductive; however, even small amounts of salt can have significant effects on the phase behavior of these materials and consequently on their ion-transport and mechanical properties. As a result, the effect of salt addition on block copolymer thermodynamics has been the subject of significant interest over the past decade. This feature article describes a comprehensive study of the thermodynamics of block copolymer/salt mixtures over a wide range of molecular weights, compositions, salt concentrations, and temperatures. The Flory–Huggins interaction parameter was determined by fitting small-angle X-ray scattering data of disordered systems to predictions based on the random phase approximation. Experiments on neat block copolymers revealed that the Flory–Huggins parameter is a strong function of chain length. Experiments on block copolymer/salt mixtures revealed a highly nonlinear dependence of the Flory–Huggins parameter on salt concentration. These findings are a significant departure from previous results and indicate the need for improved theories for describing thermodynamic interactions in neat and salt-containing block copolymers

Effect of Chemical Oxidation on the Self-Assembly of Organometallic Block Copolymers

The thermodynamic interactions in poly(styrene-block-ferrocenyldimethylsilane) and poly(isoprene-block-ferrocenyldimethylsilane) copolymers were systematically tuned by oxidation of the ferrocene moieties with silver nitrate. Small-angle X-ray scattering experiments show that oxidizing 8% of the ferrocene moieties lowers the order−disorder transition temperature of the copolymers by as much as 40 °C

Effect of Lithium Polysulfides on the Morphology of Block Copolymer Electrolytes

Lithium polysulfides (Li₂S_<i>x</i>, 1 ≤ <i>x</i> ≤ 8) are produced during the discharge of lithium–sulfur batteries. Lithium–sulfur batteries are of interest due to their high energy density. The morphology of mixtures of polystyrene-<i>b</i>-poly(ethylene oxide) (SEO) copolymers and lithium polysulfides were studied using a combination of X-ray diffraction, small-angle X-ray scattering, differential scanning calorimetry, and ultraviolet–visible spectroscopy. This study is motivated by the possibility of using block copolymers as electrolytes in lithium–sulfur cells. The phase behavior of SEO/Li₂S_<i>x</i> mixtures were found to differ fundamentally from mixtures of SEO and other lithium salts. The morphology of certain SEO/Li₂S_<i>x</i> mixtures obtained below the melting temperature of the poly(ethylene oxide) block has not been previously observed in block copolymer/salt mixtures

Effect of Molecular Weight and Salt Concentration on Ion Transport and the Transference Number in Polymer Electrolytes

Transport of ions in polymer electrolytes is of significant practical interest, however, differences in the transport of anions and cations have not been comprehensively addressed. We present measurements of the electrochemical transport properties of lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) in poly­(ethylene oxide) (PEO) over a wide range of PEO molecular weights and salt concentrations. Individual self-diffusion coefficients of the Li⁺ and TFSI^– ions, <i>D</i>₊ and <i>D</i>_–, were measured using pulsed-field gradient nuclear magnetic resonance both in the dilute limit and at high salt concentrations. Conductivities calculated from the measured <i>D</i>₊ and <i>D</i>_– values based on the Nernst–Einstein equation were in agreement with experimental measurements reported in the literature, indicating that the salt is fully dissociated in these PEO/LiTFSI mixtures. This enables determination of the molecular weight dependence of the cation transference number in both dilute and concentrated solutions. We introduce a new parameter, <i>s</i>, the number of lithium ions per polymer chain, that allows us to account for both the effect of salt concentration and molecular weight on cation and anion diffusion. Expressing cation and anion diffusion coefficients as functions of <i>s</i> results in a collapse of <i>D</i>₊ and <i>D</i>_– onto a single master curve

Synthesis of Well-Defined Polyethylene–Polydimethylsiloxane–Polyethylene Triblock Copolymers by Diimide-Based Hydrogenation of Polybutadiene Blocks

Polyethylene, PE, is a crystalline solid with a relatively high melting temperature, and it exhibits excellent solvent resistance at room temperature. In contrast, polydimethylsiloxane, PDMS, is a rubbery polymer with an ultralow glass transition temperature and poor solvent resistance. PE–PDMS block copolymers have the potential to synergistically combine these disparate properties. In spite of this potential, synthesis of PE–PDMS block copolymers has not been widely explored. We report a facile route for the synthesis of well-defined polyethylene-<i>b</i>-polydimethylsiloxane-<i>b</i>-polyethylene (EDE) triblock copolymers. Poly­(1,4-butadiene)-<i>b</i>-polydimethylsiloxane-<i>b</i>-poly­(1,4-butadiene) (BDB) copolymer precursors were synthesized by anionic polymerization, followed by diimide-based hydrogenation. Under the standard hydrogenation conditions established by the work of Hahn, the siloxane bond undergoes scission resulting into significant degradation of the PDMS block. Our main accomplishment is the discovery of reaction conditions that avoid PDMS degradation. We used mechanistic insight into arrive at the optimal hydrogenation conditions, and we established the efficacy of our approach by successfully synthesizing a wide variety of block copolymers with total molecular weights ranging from 124 to 340 kg/mol and PDMS volume fractions ranging from 0.22 to 0.77

Water Uptake and Proton Conductivity in Porous Block Copolymer Electrolyte Membranes

We demonstrate that the water uptake and conductivity of proton-conducting block copolymer electrolyte membranes can be controlled systematically by the introduction of pores in the conducting domains. We start with a membrane comprising a mixture of homopolymer polystyrene (hPS) and a polystyrene-<i>b</i>-polyethylene-<i>b</i>-polystyrene (SES) copolymer. Rinsing the membranes in tetrahydrofuran and methanol results in the dissolution of hPS, leaving behind a porous membrane. The polystyrene domains in the porous SES membranes are then sulfonated to give a porous membrane with hydrophilic and hydrophobic domains. The porosity is controlled by controlling ϕ_v, the volume fraction of hPS in the blended membrane. The morphology of the membranes before and after sulfonation was studied by scanning transmission electron microscopy (STEM), electron tomography, and resonance soft X-ray scattering (RSoXS). The porous structures before and after sulfonation are qualitatively different. Water uptake of the sulfonated membranes increased with increasing ϕ_v. Proton conductivity is a nonmonotonic function of ϕ_v with a maximum at ϕ_v = 0.1. The introduction of microscopic pores in the conducting domain provides an additional handle for tuning water uptake and ion transport in proton-conducting membranes

Dynamic Heterogeneity of Solvent Motion and Ion Transport in Concentrated Electrolytes

Molecular-level understanding of the cation transference number t+0, an important property that characterizes the transport of working cations, is critical to the bottom-up design of battery electrolytes. We quantify t+0 in a model tetraglyme-based electrolyte using molecular dynamics simulation and the Onsager approach. t+0 exhibits a concentration dependence in three distinct regimes: dilute, intermediate, and concentrated. The cluster approximation uncovers dominant correlations and dynamic heterogeneity in each regime. In the dilute regime, t+0 decreases sharply as increasing numbers of solvent molecules become coordinated with Li+. The crossover to the intermediate regime, t+0 ≈ 0, occurs when all solvent molecules become coordinated, and a plateau is obtained because anions enter the Li+ solvation shell, resulting in ion pairs that do not contribute to t+0. Transference in concentrated electrolytes is dominated by the presence of cations in a variety of negatively charged and solvent-excluded clusters, resulting in t+0 < 0

The Effect of Annealing on the Grain Structure and Ionic Conductivity of Block Copolymer Electrolytes

Block copolymer electrolytes that microphase-separate into rigid non-conducting domains and soft ion-conducting domains are known to exhibit stability against lithium metal anodes. In these systems, order is confined to grains with concomitant defects. When these electrolytes are annealed, the grain size typically increases, which is assumed in the literature to lead to a decrease in the ionic conductivity. In this work, we study the interplay between grain size and ionic conductivity using a block polymer electrolyte composed of a polystyrene (PS) block with a molecular weight of 19 kg/mol and a poly(ethylene oxide) (PEO) block with a molecular weight 20 kg/mol mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt at a variety of salt concentrations. The electrolytes have lamellar morphologies at all salt concentrations. At low salt concentrations, the average grain size before annealing is large and ionic conductivity decreases upon annealing. At high salt concentrations, however, the average grain size before annealing is small and ionic conductivity increases upon annealing

Thermodynamics and Phase Behavior of Poly(ethylene oxide)/Poly(methyl methacrylate)/Salt Blend Electrolytes Studied by Small-Angle Neutron Scattering

We studied the effect of added salt on the thermodynamics of a miscible polymer blend system: poly(ethylene oxide) (PEO) blended with poly(methyl methacrylate) (PMMA). In the absence of salt, PEO/PMMA blends are known to exhibit a negative Flory–Huggins parameter, χ. Not surprisingly, the salt-free PEO/PMMA blends are miscible, regardless of composition. The addition of salt, which in our case was lithium bis(trifluoro­methane­sulfonyl)imide (LiTFSI), induced phase separation in majority-PMMA blends, while majority-PEO blends remained miscible. The effect of added salt was studied at two salt concentrations, r = 0.05 and r = 0.10; r is defined as the molar ratio of lithium ions to ether oxygens (r = [Li]/[EO]). The immiscibility window, which was absent at r = 0, grew upon addition of a small amount of salt (r = 0.05). Further addition of salt to r = 0.10 results in shrinking of the immiscibility window. With small-angle neutron scattering (SANS) profiles from homogeneous blends, we determined χ in both the presence and absence of salt. We measure the composition dependence of this parameter and use it to predict the phase behavior of PEO/PMMA/LiTFSI blends. We find good agreement between theory and experiment

A Solid Lithium Electrolyte via Addition of Lithium Isopropoxide to a Metal–Organic Framework with Open Metal Sites

The uptake of LiOiPr in Mg2(dobdc) (dobdc4– = 1,4-dioxido-2,5-benzenedicarboxylate) followed by soaking in a typical electrolyte solution leads to the new solid lithium electrolyte Mg2(dobdc)·0.35LiOiPr·0.25LiBF4·EC·DEC (EC = ethylene carbonate; DEC = diethyl carbonate). Two-point ac impedance data show a pressed pellet of this material to have a conductivity of 3.1 × 10–4 S/cm at 300 K. In addition, the results from variable-temperature measurements reveal an activation energy of just 0.15 eV, while single-particle data suggest that intraparticle transport dominates conduction