The room temperature morphologies of twelve precise copolymers based on polyethylene (PE) were studied by solid-state 13 C NMR, DSC, and X-ray scattering. These copolymers feature carboxylic acid, phosphonic acid or 1-methylimidazolium bromide pendants on exactly every 9th, 15th or 21st carbon atom along the linear PE chain. The morphologies were categorized by the arrangement of the acid or ionic aggregates into liquid-like, layered, or cubic morphologies. The liquid-like morphology is characterized by an amorphous PE matrix and liquid-like packing of the aggregates, wherein the interaggregate spacing increases with both the PE segment length and the pendant size. The layered morphologies typically have a semicrystalline PE matrix and upon stretching become highly anisotropic. Notably, the orientation of the aggregates and the PE crystallites relative to the stretch direction depends on whether the morphology is dominated by PE crystallization, as found for acrylic acid (AA) and phosphonic acid (PA) copolymers, or by the strong ionic aggregates, as found for the 1-methylimidazolium bromide (ImBr) copolymers. Cubic morphologies in these precise copolymers require geminal substitution, PA pendants, and sufficiently long PE segments to allow the aggregates to order. These precise copolymers provide an unprecedented array of morphologies that enable correlations between chemical structure and nanoscale morphologies.

Designing acid-and ion-containing polymers for optimal proton, ion, or water transport would benefit profoundly from predictive models or theories that relate polymer structures with ionomer morphologies. Recently, atomistic molecular dynamics (MD) simulations were performed to study the morphologies of precise poly-(ethylene-co-acrylic acid) copolymer and ionomer melts.Here, we present the first direct comparisons between scattering profiles, I(q), calculated from these atomistic MD simulations and experimental X-ray data for 11 materials. This set of precise polymers has spacers of exactly 9, 15, or 21 carbons between acid groups and has been partially neutralized with Li, Na, Cs, or Zn. In these polymers, the simulations at 120 °C reveal ionic aggregates with a range of morphologies, from compact, isolated aggregates (type 1) to branched, stringy aggregates (type 2) to branched, stringy aggregates that percolate through the simulation box (type 3). Excellent agreement is found between the simulated and experimental scattering peak positions across all polymer types and aggregate morphologies. The shape of the amorphous halo in the simulated I(q) profile is in excellent agreement with experimental I(q). The modified hard-sphere scattering model fits both the simulation and experimental I(q) data for type 1 aggregate morphologies, and the aggregate sizes and separations are in agreement. Given the stringy structure in types 2 and 3, we develop a scattering model based on cylindrical aggregates. Both the spherical and cylindrical scattering models fit I(q) data from the polymers with type 2 and 3 aggregates equally well, and the extracted aggregate radii and inter-and intra-aggregate spacings are in agreement between simulation and experiment. Furthermore, these dimensions are consistent with real-space analyses of the atomistic MD simulations. By combining simulations and experiments, the ionomer scattering peak can be associated with the average distance between branches of type 2 or 3 aggregates. This direct comparison of X-ray scattering data to the atomistic MD simulations is a substantive step toward providing a comprehensive, predictive model for ionomer morphology, gives substantial support for this atomistic MD model, and provides new credibility to the presence of stringy, branched, and percolated ionic aggregates in precise ionomer melts.

Six perfectly regioregular polyethylene (PE)-based ionomers containing 1-methylimidazolium bromide groups on exactly every 9th, 15th, or 21st carbon (precision ionomers) and two regiorandom analogues have been synthesized and characterized via dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). Because these materials were synthesized by a postpolymerization functionalization route, their number-average molecular weights (M n s) and polydispersity indices (PDIs) could be accurately calculated based on measurements of the preionized polymers; M n s range from 36 to 53 kDa with PDIs all close to 2. Thermal gravimetric analysis (TGA) indicates stability up 250°C, and DSC measurements indicate that crystallinity is a function of the polymer backbone spacer length. T m s range from ∼80 to 106°C, with longer spacer lengths inducing semicrystallinity. DSC measured glass transition temperatures (T g s) range from −1.6 to 26.8°C and appear to be dependent on both spacer length and crystallinity. DMA data loosely mirror the DSC results, but with transitions occurring at lower temperatures that we attribute to differences in the thermal history and/or the different heating ramp rates used. ■ INTRODUCTIONIonomers comprise a class of polymers containing a relatively low concentration of pendant ionic groups. At a global production rate of about 300 million pounds/year, they are of great commercial importance and find use in a range of applications as ion transport membranes, electromechanical devices, thermoplastic elastomers, adhesives, and other uses. 1 Moreover, while polyanions are by far the more common derivatives, recently there has been considerable interest in polycations, which are the focus of this paper, due to their potential applicability in anion exchange membrane fuel cells 2 and mechanical actuators. 3 Ionic polymers are often prepared via polymerization of acryloyl-or vinyl-functionalized ionic liquids or by ionization of electrically neutral polymers. 1−17 Countless studies have been and continue to be conducted in efforts to understand and control ionomer morphology. With some exceptions, such as regularly sequenced polyurethane, 18−20 polysiloxane, 21,22 and poly(ethylene oxide) 23,24 based ionomers, most current synthetic approaches yield a random (or pseudorandom) distribution of ionic groups along a polymer backbone (type A of Figure 1); thus, the impact of perfect regioregularity on ionomer morphology and performance in various applications remains largely unexplored due to a lack of synthetic methodology. 1,25 We recently reported the synthesis of an ionomer and an ionene (in which the ionic group is in the main chain of the polymer) by acyclic diene metathesis polymerization (ADMET) of α,ω-diene-functionalized ionic liquids. 26 This method, for the first time, provided access to a polyolefin-based precision ionomer (type B of Figure 1); an imidazolium hexafluorophosphate group was located on each and every 21st carbon along a linear polyolefin backbone. Synthesis of these ...

The morphologies at elevated temperatures (T > T g , T m ) of 12 precise, polyethylene (PE)-based copolymers with acrylic acid (AA), phosphonic acid (PA), and 1methylimidazolium bromide (ImBr) groups were studied via X-ray scattering. These precise copolymers enable direct comparisons focusing on the length of the spacer between the functional groups and the type of functional group. The polar groups in these materials self-assemble into microphaseseparated aggregates dispersed throughout the nonpolar PE matrix. At high temperatures the PE segments are amorphous, such that the aggregates are distributed in a liquid-like manner in 11 of these precise copolymers. The correlation distances between aggregates increase with the following: carbon spacer length between pendant groups, size and volume fraction of the pendant species, and functional group configuration (single vs geminal substitution). In addition, comparisons are made between precise copolymers and pseudorandom copolymers of the same pendant concentration, wherein the interaggregate distances are much better defined with precise copolymers. Finally, the local packing in copolymers with geminal substitution of PA pendant groups is less compact, which might facilitate ion conduction.

A linear polyethylene precisely functionalized with geminal phosphonic acid pendants on every 21st carbon atom exhibits face-centered cubic (FCC) packing of acid aggregates. X-ray scattering from isotropic films result in higher-order scattering peaks used to determine the lattice parameter at room temperature (a FCC = 4.19 nm) and above the melting temperature of the polyethylene matrix (a FCC = 4.06 nm). Upon stretching the precise acid copolymer, an anisotropic scattering pattern featuring two coexisting crystalline orientations results, both having the ⟨110⟩ direction of the FCC lattice along the stretching direction. This is the first report of cubic ordering of aggregates in an acid copolymer and it is the direct consequence of the molecular precision of the polymer.