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.

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.

This paper presents the first findings on the molecular dynamics of the remarkable new class of linear and precisely functionalized ethylene copolymers. Specifically, we utilize broadband dielectric relaxation spectroscopy to investigate the molecular dynamics of linear polyethylene (PE)-based ionomers containing 1-methylimidazolium bromide (ImBr) pendants on exactly every 9th, 15th, or 21st carbon atom, along with one pseudorandom analogue. We also employed FTIR spectroscopy to provide insight into local ionic interactions and the nature of the ordering of the ethylene spacers between pendants. Prior X-ray scattering experiments revealed that the polar ionic groups in these ionomers self-assemble into microphase-separated aggregates dispersed throughout the nonpolar PE matrix. We focus primarily on the dynamics of the segmental relaxations, which are significantly slowed down compared to linear PE due to ion aggregation. Relaxation times depend on composition, the presence of crystallinity, and microphase-separated morphologies. Segmental relaxation strengths are much lower than predicted by the Onsager theory for mobile isolated dipoles but much higher than linear PE, demonstrating that at least some ImBr pendants participate in the segmental process. Analysis of the relaxation strengths using the Kirkwood g correlation factor demonstrates that ca. 10−40% of the ImBr ion dipoles (depending on copolymer composition and temperature) participate in the segmental motions of the precise ionomers under study, with the remainder immobilized or having net antiparallel arrangements in ion aggregates.

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