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Fibrous soft actuators with high molecular anisotropy are of interest for shape morphing from 1D to 2D and 3D in response to external stimuli with high actuation efficiency. Nevertheless, few have fabricated fibrous actuators with controlled molecular orientations and stiffness. Here, we fabricate filaments from liquid crystal networks (LCNs) with segmental crosslinking density and gradient porosity from a mixture of di-acrylate mesogenic monomers and small-molecule nematic or smectic liquid crystals (LCs) filled in a capillary. During photopolymerization, phase separation between the small-molecule LCs and LCN occurs, making one side of the filament considerably denser than the other side. To direct its folding mode (bending or twisting), we control the alignment of LC molecules within the capillary, either along or perpendicular to the filament long axis. We show that the direction of UV exposure can determine the direction of phase separation, which in turn direct the deformation of the filament after removal of the small-molecule LCs. We find that the vertical alignment of LCs within the filament is essential to efficiently direct bending deformation. By photopatterning the filament with segmental crosslinking density, we can induce a reversible folding/unfolding into 2D and 3D geometries triggered by deswelling/swelling in an organic solvent. Moreover, by taking advantage of the large elastic modulus of LCNs and large contrast of the modulus before and after swelling, we show that the self-folded LCP filament could act as a strong gripper.

Chain-transfer ring-opening metathesis polymerization (CT-ROMP) provided a route to carboxytelechelic polyethylene (PE) with subsequent incorporation into segmented copolymers.

Precise polyamide ionomer produces well-defined nanoscale layers.

We investigated the temperature-dependent phase behavior and interaction parameter of polyethylene-based multiblock copolymers with pendant ionic groups. These step-growth polymers contain short polyester blocks with a single Li+SO3- group strictly alternating with polyethylene blocks of x-carbons (PESxLi, x = 12, 18, 23). At room temperature, these polymers exhibit layered morphologies with semicrystalline polyethylene blocks. Upon heating above the melting point (∼130 °C), PES18Li shows two order-to-order transitions involving Ia3d gyroid and hexagonal morphologies. For PES12Li, an order-to-disorder transition accompanies the melting of the polyethylene blocks. Notably, a Flory-Huggins interaction parameter was determined from the disordered morphologies of PES12Li using mean-field theory: χ(T) = 77.4/T + 2.95 (T in Kelvin) and χ(25 °C) ≈ 3.21. This ultrahigh χ indicates that the polar ionic and nonpolar polyethylene segments are highly incompatible and affords well-ordered morphologies even when the combined length of the alternating blocks is just 18-29 backbone atoms. This combination of ultrahigh χ and short multiblocks produces sub-3-nm domain spacings that facilitate the control of block copolymer self-assembly for various fields of study, including nanopatterning.

We investigated the nanoscale morphologies and ionic conductivities of polyethylene-based multiblock copolymers as single-ion conducting polymer electrolytes. These polymers contain short polar blocks with a single sodium sulfonate group separated by polyethylene blocks of fixed length (PESxNa, x = 10, 12, and 18). At room temperature, these multiblock copolymers exhibit layered ionic aggregates with semicrystalline polyethylene backbones. For PES12Na and PES18Na, the layered ionic aggregate morphologies transition into Ia3d gyroid morphologies upon melting the polyethylene blocks and further transition into hexagonal morphologies at higher temperatures. With a shorter polyethylene block, PES10Na exhibits a layered to hexagonal transition at the melting temperature, without an intermediate gyroid morphology. The phase diagram of these PESxNa polymers is reminiscent of conventional diblock copolymers and identifies the presence of gyroid morphologies at a polar block volume fraction of ∼0.27 to 0.41, which is broad compared to typical diblock copolymers. Temperature-dependent ionic conductivities reveal faster ion transport through bicontinuous gyroids than hexagonal ionic aggregate morphologies and a relationship between conductivity and the characteristic distance between ionic aggregates. This study presents material design strategies for single-ion conducting polymers with a bicontinuous ionic aggregate and toward efficient ion transport.

We measure the center-of-mass diffusion of silica nanoparticles (NPs) in entangled poly(2-vinylpyridine) (P2VP) melts using Rutherford backscattering spectrometry. While these NPs are well within the size regime where enhanced, nonhydrodynamic NP transport is theoretically predicted and has been observed experimentally (2RNP/dtube ≈ 3, where 2RNP is the NP diameter and dtube is the tube diameter), we find that the diffusion of these NPs in P2VP is in fact well-described by the hydrodynamic Stokes–Einstein relation. The effective NP diameter 2Reff is significantly larger than 2RNP and strongly dependent on P2VP molecular weight, consistent with the presence of a bound polymer layer on the NP surface with thickness heff ≈ 1.1Rg. Our results show that the bound polymer layer significantly augments the NP hydrodynamic size in polymer melts with attractive polymer–NP interactions and effectively transitions the mechanism of NP diffusion from the nonhydrodynamic to hydrodynamic regime, particularly at high mole...

Fuel-cell deployable proton exchange membranes (PEMs) are considered to be a promising technology for clean and efficient power generation. However, a fundamental atomistic understanding of the hydronium diffusion process in the PEM environment is an ongoing challenge. In this work, we employ fully atomistic ab initio molecular dynamics to simulate diffusion mechanisms of the hydronium ion in a model PEM. In order to mimic a precise polymer with a layered morphology, as recently introduced by Trigg, et al., Nat. Mater., 2018, 17, 725, a nano-confined environment was created composed of graphane bilayers to which sulfonate end groups (SO3−) are attached, and the space between the bilayers was subsequently filled with water and hydronium ions up to λ values of 3 and 4, where λ denotes the water-to-anion ratio. We find that for the low λ value, the water distribution is not homogeneous, which results in an incomplete second solvation shell for H3O+, fewer water molecules in the vicinity of SO3−, and a higher probability of obtaining a coordination number of ∼1 for the nearest oxygen neighbor to SO3−. These conditions increase the probability that H3O+ will react with SO3− according to the reaction SO3− + H3O+ ↔ SO3H + H2O, which was found to be an essential part of the hydronium diffusion mechanism. This suggests there are optimal hydration conditions that allow the sulfonate end groups to take an active part in the hydronium diffusion mechanism, resulting in high hydronium conductivity. We expect that the results of this study could help guide synthesis and experimental characterization used to design new PEM materials with high hydronium conductivity.

We describe a set of precise single-ion conducting polymers that form self-assembled percolated ionic aggregates in glassy polymer matrices and have decoupled transport of metal cations. These prec...

We use single-particle tracking (SPT) to explore the role of nanoparticles/polymer interactions and polymer molecular weight on nanoparticle (NP) diffusion in unentangled polymer melts. The very di...

The interfacial regions between nanoparticles (NPs) and polymers in polymer nanocomposites (PNCs) underlie enhanced properties, and the temporal stability of these bound polymer layers is necessary...

Nanorod diffusion in polymer melts is faster than predicted by the continuum model (CM). Rutherford backscattering spectrometry is used to measure the concentration profile of titanium dioxide (TiO2) nanorods (L = 43 nm, d = 5 nm) in a polystyrene (PS) matrix having molecular weights (M) from 9 to 2000 kDa. In the entangled regime, the tracer diffusion coefficients (D) of TiO2 decrease as the M–1.4, whereas the CM predicts DCM ∼ M–3.0 using the measured zero-shear viscosity of TiO2(1 vol %): PS(M) blends. By plotting D/DCM versus M/Me, where Me is the entanglement molecular weight, diffusion is enhanced by a factor of 10–103 as M/Me increases. The faster diffusion is attributed to decoupling of nanorod diffusion from polymer relaxations in the surrounding matrix, which is facilitated by the nanorod dimensions (i.e., L greater than and d less than the entanglement mesh size, 8 nm).

Copolymers containing sulfite functionalities precisely placed between run lengths of 8, 14, and 20 methylene units were synthesized via ADMET with weight-average molecular weights up to 40 500 g/mol (PDI = 1.89). No such polymer structures have been observed previously. The primary polymer structures and precise nature were characterized by 1H NMR, 13C NMR, and IR spectroscopy. Thermal degradation temperatures up to 310 °C were observed through TGA, and melting points typical of similar unsaturated ADMET polymers were determined by DSC. X-ray scattering was used to compare the polymers to ADMET polyethylene (PE), and when the polymers have 20 carbons between sulfites, the functional groups self-assemble into layers. Higher carbon content incorporation leads to an increase in crystallinity and thermal stability for these polysulfites.

Herein, we report a polymerized ionic liquid diblock copolymer with high hydroxide conductivity and nanoscale morphology. Surprisingly, the conductivity is not only higher (over an order of magnitude) than its random copolymer analog at the same ion and water content, but also higher than its homopolymer analog, which has a higher ion and water content than the block copolymer. These results should have a significant impact on low-cost (platinum-free), long-lasting, solid-state alkaline fuel cells.

Polymers bearing phosphonic acid groups have been proposed as anhydrous proton-conducting membranes at elevated operating temperatures for applications in fuel cells. However, the synthesis of phosphonated polymers and the control over the nanostructure of such polymers is challenging. Here, we report the straightforward synthesis of phosphonic acid-terminated, long-chain aliphatic materials with precisely 26 and 48 carbon atoms (C26PA2 and C48PA2). These materials combine the structuring ability of monodisperse polyethylenes with the ability of phosphonic acid groups to form strong hydrogen-bonding networks. Anhydride formation is absent so that charge carrier loss by a condensation reaction is avoided even at elevated temperatures. Below the melting temperature (Tm), both materials exhibit a crystalline polyethylene backbone and a layered morphology with planar phosphonic acid aggregates separated by 29 and 55 A for C26PA2 and C48PA2, respectively. Above Tm, the amorphous polyethylene (PE) segments coexist with the layered aggregates. This phenomenon is especially pronounced for the C26PA2 and is identified as a thermotropic smectic liquid crystalline phase. Under these conditions, an extraordinarily high correlation length (940 A) along the layer normal is observed, demonstrating the strength of the hydrogen bond network formed by the phosphonic acid groups. The proton conductivity in both materials in the absence of water reaches 10-4 S/cm at 150 °C. These new precise phosphonic acid-based materials illustrate the importance of controlling the chemistry to form self-assembled nanoscale aggregates that facilitate rapid proton conductivity.

Precisely controlling the position of ionic groups along neutral polymer backbones in ionomers has led to novel nanoscale morphologies and promising properties, although the synthetic routes can be quite demanding. Here, we report a series of Li+-containing ionomers directly synthesized from step-growth polymerization of commercially available diol monomers (x = 6 or 12) and dianhydride monomers followed by neutralization with various amounts of lithium salts. The results are nearly precise linear polymers wherein nonpolar segments have precisely 6 or 12 methylene groups, as determined by the diol. The functional segment has two carboxylic acid pendant groups and 6–8 backbone atoms depending on the dianhydride addition. From X-ray scattering, these segmented ionomers exhibit well-defined nanostructures in contrast to the conventional random ionomers. The nearly precise nature of these segmented ionomers does not compromise the morphological ordering. The temperature-dependent ionic conductivities exhibit ...

Controlling the self-assembled nanoscale ionic aggregates in single-ion conducting polymers is a crucial step toward exceptional transport properties. We report a series of precisely segmented polyethylene-like materials containing sulfonate groups (PES23) with Li

A series of polyethylene-like ionomers with periodic sulfonate groups were synthesized by polyesterification of octatetracontane-1,48-diol and tetrabutylammonium dimethyl sulfosuccinate. Optional cation exchange in aqueous dispersions replaced the NBu4+ counterions for Na+ or Cs+. The defined periodic microstructure of the polymers leads to the formation of layered structures in bulk and to platelet-like self-stabilized nanoparticles in aqueous dispersion, as observed by X-ray scattering and transmission electron microscopy. The bulk polymers exhibit a semicrystalline structure with multiple stacks of ionic layers embedded in the crystallites with a layer-to-layer distance of 50–60 A. Upon melting, the ionic layers in the NBu4+-neutralized polymer transitioned to disordered ionic aggregates, while the layered ionic aggregates in the Cs+- and Na+-neutralized polymers transformed into ionic aggregates with hexagonal symmetry, which is an unprecedented order-to-order transition in microphase-separated ionome...

We analyze the dynamics from microsecond-long, atomistic molecular dynamics (MD) simulations of a series of precise poly(ethylene-co-acrylic acid) ionomers neutralized with lithium, with three diff...

The behavior of polymer melts under planar confinement was investigated using molecular dynamics simulations. Polymers with a range of lengths, from unentangled to highly entangled (N = 25–400), we...

We report the synthesis of ionic telechelic low-molecular-weight polyethylenes (PEs) with precise chain lengths that are derived from plant oils along with their morphologies and temperature-dependent ionic conductivities. Starting from the C48 telechelic dimethyl ester, or the C23 analogue for comparison, different ionic carboxylate end groups (H+, Na+, Cs+, Zn2+) were introduced by microwave-assisted saponification chemistry. Because of the precise length of the polymethylene sequence, these difunctional telechelic PEs crystallize into exceptionally well-ordered nanoscale-layered structures at room temperature. As a consequence of their extended chain crystal nature, the layer thickness is directly encoded by the telechelic molecules methylene chain length. Notably, C21(COONa)2, C46(COONa)2, and C46(COOCs)2 exhibit transitions in the crystalline structure prior to the fully disordered melt state, as evidenced by differential scanning calorimetry and in situ X-ray scattering. The melting transition is ty...

Nanorod diffusion and polymer dynamics in nanocomposites were investigated by means of molecular dynamics simulations. We show that thin nanorods (spherocylinders) can diffuse much faster than that predicted by a Stokes–Einstein continuum model, similar to experiments, in the dilute nanorod regime. In entangled polymer matrices, nanorod diffusion reaches a plateau region. Increase of the nanorod diameter or aspect ratio (AR) slows its diffusion. The nanorod diffusion, at long time scales, displays a non-Gaussian behavior as is depicted by the several Gaussian peaks present in the self-part of the van Hove function. We also show that the unentangled polymer dynamics decreases in the nanocomposite due to the nanorods interfacial area.

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The use of nanoparticle reinforced polymer matrices in continuous fiber composites for infrastructure applications requires a comprehensive understanding of viscoelastic creep. Critical parameters affecting the mechanical reinforcement offered by nanoparticles include nanoparticle size and concentration, as well as the interaction between the nanoparticle surface and polymer matrix. Here, we study the viscoelastic creep of nanocomposite systems comprised of glassy thermoplastic polymers and spherical silica nanoparticles of varying sizes and surface functionalization using a dynamic mechanical analysis (DMA) accelerated testing methodology. Significant differences in the nanoparticle dispersions in these nanocomposites were observed via transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) and are attributed to differences in the polymer–polymer and polymer–particle interaction strengths. The DMA measurements indicate a decrease in compliance at short times with increased nanoparticle loading that is largely independent of nanoparticle dispersion morphology and polymer–particle interaction strength. Conversely, long term creep behavior shows a much stronger dependence on these parameters with the creep onset time increasing by up to three orders of magnitude. For similar nanoparticle loadings, the time to critical deformation in systems with well-distributed, networked nanoparticle morphologies was larger by an order of magnitude compared to systems exhibiting strong nanoparticle aggregation. The networked systems delayed the time to critical deformation by three orders of magnitude over that of neat polymer. The increase in time to critical deformation is also greater in composites with smaller nanoparticles at similar loadings, which we attribute to the development of percolated nanoparticle networks. These results demonstrate the significant effects polymer–particle interactions and dispersion morphologies can have on the long-term creep compliance of thermoplastic nanocomposites.

Polymer segmental dynamics, center-of-mass chain diffusion, and nanoparticle (NP) diffusion are directly measured in a series of polymer nanocomposites (PNC) composed of very small (radius ≈ 0.9 nm...

Multiple computational and experimental techniques are used to understand the nanoscale morphology and water/proton transport properties in a series of sulfonated Diels–Alder poly(phenylene) (SDAPP...

We present a systematic study of segmental dynamics in model attractive polymer nanocomposites comprising poly(2-vinylpyridine) (P2VP) and 26 nm diameter colloidal silica nanoparticles (NPs) using quasi-elastic neutron scattering (QENS). Unlike most dynamic measurements, QENS provides both spatial and temporal information about small length scales (∼1 nm) and fast time scales (∼1 ns) and therefore at temperatures far above the glass transition. We find that on these length and time scales P2VP segmental motion is well-described by classic translational diffusion even under extreme confinement, where the average interparticle spacing is on the order of the Kuhn length. The average segmental diffusion coefficient decreases monotonically with increasing NP concentration by up to a factor of ∼5 at the highest NP concentrations (50 vol %). Interestingly, this reduction in segmental dynamics is very weakly dependent on P2VP molecular weight spanning the unentangled (10 kg/mol) to the highly entangled regimes (1...

Polymer membranes with enhanced charge transport properties would enable solid-state batteries with improved safety, increased lifetime, and decreased cost. However, the ion conductivity of polymers remains low after decades of research. Here, we advocate for an alternative design strategy for ion-conducting polymers, wherein a precisely periodic chemical microstructure leads to backbone crystallization, controlled hairpin chain folding at the position of each functional group, and ordered layers of sub-nanometer thickness that function as pathways for charge transport. This chain-folded layered structure is observed in a linear polyethylene with sulfonic acid groups pendant to precisely every 21st carbon atom, and the membrane exhibits high proton conductivity at high humidity, matching the benchmark membrane Nafion 117. We discuss related instances of ion and proton transport through thin layers in polymers, as well as other examples of controlled polymer folding leading to ordered nanoscale layers. We also propose design rules for achieving the chain-folded layered structure in new polymers. This layered structure has the potential for improved charge transport relative to amorphous polymers, representing a significant advance in ion-conducting polymer membranes.

Ion transport and morphology are studied in cationic polymers based on tris(dialkyl)aminocyclopropenium ions tethered onto polystyrene (PS-TAC) and Cl− counterions. To investigate how molecular structure impacts single-ion transport and physical properties, the substituents on TAC were varied. Modifying the polarity, along with the size and flexibility of the functional groups on TAC significantly changes the glass transition temperature. Ionic conductivity of the PS-TAC with branched functional groups is 1–2 orders of magnitude lower than more compact and bulkier moieties on polymers at their glass transition temperature, Tg. Decreasing the size of the substituents correlates with increasing ionic conductivity in the polymers with non-polar functional groups. However, it is the geometry of the functional groups on the cation (isopropyl, ring, or linear) in PS-TAC that has a much larger effect on conductivity than the size of the group itself. Mimicking the transport properties of ionic liquids using polymers imbued with mechanical stability is essential for the development of robust, non-volatile electrolytes for batteries and fuel cells. Changing a number of variables in this tunable PS-TAC system is a step towards developing soft materials design rules for selecting solid ion supports.

Although many studies have analyzed the behavior of polymers under 1-D thin film and 2-D cylindrical confinement, the knowledge about how confinement geometries with more than one length scale, such as random porous networks, impact polymers is yet to be fully explored. We simulate both entangled and unentangled polymer melts confined in a diamond network geometry with two characteristic length scales, where polymers are likely to inhabit multiple interconnected and neighboring channels. In all the confined systems with entangled polymer chains, the polymer radius of gyration and the number of polymer entanglements per chain are reduced compared to the bulk. We analyze the chain relaxation based on Rouse modes and show separate, competing effects between the local friction near the wall and chain disentanglement. The disentanglement of confined polymers increases diffusivity of entangled polymers along confined channels compared to the bulk after the confined diffusivities are corrected for the tortuosity...

Some types of precise polyethylenes, or linear polyethylenes containing precisely periodic functional groups, exhibit significant strain hardening under tensile deformation. Here we perform in situ X-ray scattering during tensile deformation of two precise polyethylenes: one with pendant carboxylic acid groups every 21st carbon (p21AA) and another with pendant imidazolium bromide groups every 15th carbon (p15ImBr). p21AA exhibits significant strain hardening and its layered structure orients with the layer normal along the tensile axis, while p15ImBr does not exhibit strain hardening and its layer normal orients perpendicular to the tensile axis. Quantitative evaluation of the 2D fiber patterns shows that p21AA undergoes a morphological transition, from a semicrystalline structure to a nanofibrillar structure, while the morphology of p15ImBr reorients and the chain folding structure is preserved. This suggests that fibrillization plays an important role in the strain hardening mechanism.

The morphology evolution during deformation was characterized for a series of lithium neutralized acrylic acid containing linear polyethylenes (PEs). In situ X-ray scattering measurements simultaneously captured the changes in the inter-ionic aggregate peak and amorphous halo during uniaxial elongation. The effects of spacer length between pendant acid groups on the linear PE backbone, fraction of neutralized acid groups, and temperature were examined. This study indicates that the neutralization of the acid-functionalized precise polymers with metal salts significantly increases the melting and glass transition temperatures and improves mechanical properties, while the accessible morphologies remain controlled by the underlying polymer structure. During deformation above the thermal transitions, an anisotropic layered morphology similar to that found in the acid forms with 15 and 21 carbon spacers was observed in the precise ionomers with 9, 15, and 21 carbon spacers. Precise polymers with the highest acid and ionic content corresponding to a 9-carbon spacer have the highest ultimate tensile strength in the melt state due to persistent ionic aggregates. At elevated temperatures strain hardening was observed when significant chain orientation occurred corresponding to an anisotropic layered morphology, indicating some chain mobility through facile aggregate exchange.

Ion transport in polymerized ionic liquids (poly-ILs) occurs via a fundamentally different mechanism than in monomeric ionic liquids, and recently progress has been made toward understanding ion conduction in poly-ILs. To gain insight into the nature of ionic conductivity in ionic polymers, we investigate the physical properties of the trisaminocyclopropenium (TAC) ion, as it is an aromatic carbocation with unique structural and electronic properties. Herein, we characterize the thermal properties, local morphology, and dielectric response of a series of monomeric and polymeric TAC ionic liquids with different counterions. We have found that the extent of a “superionic” mechanism depends on the nature of the ion pair and can result in anomalously high conductivity at the calorimetric Tg. Our results suggest that the molecular volumes of the cationic and anionic species are important parameters that impact ion conductivity in polymerized ionic liquids.

Crystals of precise acid- and ion-containing polyethylenes were prepared from solution. While large pendant groups on polyethylene backbones are typically excluded from the crystalline domain, the precisely-placed acid and ionic functional groups are accommodated into the solution-grown crystals. Polyethylene containing carboxylic acid pendant groups on every 21st carbon atom (p21AA) grows into rectangular shaped crystals with an average thickness of 9 nm, which is 3–4 times the all-trans chain length between the functional groups. This thickness indicates that the carboxylic acid groups are incorporated within the crystals. Electron diffraction images and Raman spectra indicate that p21AA backbones are hexagonally packed with more gauche chain conformations than the polyethylene orthorhombic phase. The precise polyethylene containing geminal carboxylic acid groups placed every 21 carbon atoms (p21gAA) forms irregular-shaped crystals with an average thickness 8 nm. Similar to p21AA, p21gAA incorporates the geminal acid groups into its solution-grown crystals. Surprisingly, the polymer with imidazolium bromide groups on every 21st carbon (p21ImBr) produces remarkably large crystals with thicknesses of 200–900 nm, and widths of 6–12 μm. Finally, we propose a multi-layer stack of adjacent reentry structures that is consistent with the incorporation of large functional groups into the solution-growth crystals of these precise polyethylenes. Interestingly, this stacked structure is distinct from our recent results in melt-crystallized p21AA, where the layers are nearly transverse to the lamellae.

Recently we reported an unusual multilayered structure in a linear polyethylene containing precisely periodic carboxylic acid groups pendant to every 21st carbon atom (p21AA). Within the ordered domains, p21AA executes tight chain folds at the location of each acid group, and the chain-fold surfaces participate in hydrogen bonds with adjacent fold surfaces to form acid-rich layers. Here, we investigate the bulk morphologies of p21AA after isothermal crystallization and, using X-ray scattering and polarized optical microscopy, find p21AA to be semicrystalline. By analyzing X-ray peak widths, creating real-space models of lamellae, and performing in-situ X-ray scattering during tensile deformation, we find that, contrary to the typical structure of polymer crystallites, the polymer stems lie in the plane of the lamellae such that the acid layers are transverse (within 30° of orthogonal) to the crystallite plane. This surprising structure, not reported before to our knowledge, could be useful for designing s...

Polymer diffusion is enhanced when nanorods (NRs) in a polymer nanocomposite are mobile relative to nanocomposites containing immobile NRs. NR mobility is tuned by varying the molecular weight of the matrix polymer and the NR concentration, and diffusion of deuterated polystyrene (dPS) tracers with varying molecular weight is studied using elastic recoil detection. When dPS diffuses faster than the NRs such that the NRs are “immobile” on the time scale of dPS, the tracer diffusion coefficient (D) monotonically decreases as the nanorod concentration increases; we interpret these results as though the nanorods provide additional topological constraints that slow diffusion of dPS. When the tracer diffusion is slow relative to NR diffusion (i.e., “mobile” NRs), diffusion is enhanced relative to the immobile NR case. This enhanced diffusion is captured by a slip-link model with two populations of topological constraints: one fixed population attributed to the PS matrix, and a second population of constraints w...

Given the exceedingly high interfacial area-to-volume ratios in polymer nanocomposites and the ability to manipulate the polymer/nanoparticle interfacial interactions, manipulating the chain dynamics at these interfaces has immense potential for impacting macroscopic properties. There, the polymer center-of-mass tracer diffusion coefficient (D) from attractive (hydroxyl-terminated) and athermal (phenyl-terminated or polymer-grafted) substrates was measured over a range of temperatures and tracer molecular weights using elastic recoil detection. The tracer polymer diffusion slows significantly relative to the bulk when polymers are in direct contact with an attractive substrate and exhibits a weaker molecular weight dependence, D ∼ M–1.4. For polymers without direct contacts on the attractive substrates and for athermal substrates, the diffusion coefficients are similar to the bulk case. The temperature dependence of these diffusion coefficients indicates that the slower diffusion at the interfaces is coup...

Precise control over polymer architecture unlocks the potential for engineered self-assembled crystal structures with useful features on the nanometer length scale. Here we elucidate the structure of the ordered phase of a semicrystalline, functional polyethylene having a precise linear architecture, namely, pendant carboxylic acid groups precisely every 21st backbone carbon atom. By comparing the results of atomistic molecular dynamics simulations with experimental X-ray scattering and Raman spectroscopy data, we find that the polymer chains are folded in a hairpin manner near each carboxylic acid group, giving rise to multiple embedded layers of functional groups that have an interlayer distance of 2.5 nm. This is in contrast to other precise polyethylenes, where the chains are mostly trans within the crystals. Such layers could act as two-dimensional pathways for ionic or molecular transport given an appropriate choice of functional group.

In polymer nanoparticle composites (PNCs) with attractive interactions between nanoparticles (NPs) and polymers, a bound layer of the polymer forms on the NP surface, with significant effects on the macroscopic properties of the PNCs. The adsorption and wetting behaviors of polymer solutions in the presence of a solid surface are critical to the fabrication process of PNCs. In this study, we use both classical density functional theory (cDFT) and molecular dynamics (MD) simulations to study dilute and semi-dilute solutions of short polymer chains near a solid surface. Using cDFT, we calculate the equilibrium properties of polymer solutions near a flat surface while varying the solvent quality, surface-fluid interactions, and the polymer chain lengths to investigate their effects on the polymer adsorption and wetting transitions. Using MD simulations, we simulate polymer solutions near solid surfaces with three different curvatures (a flat surface and NPs with two radii) to study the static conformation of the polymer bound layer near the surface and the dynamic chain adsorption process. We find that the bulk polymer concentration at which the wetting transition in the poor solvent system occurs is not affected by the difference in surface-fluid interactions; however, a threshold value of surface-fluid interaction is needed to observe the wetting transition. We also find that with good solvent, increasing the chain length or the difference in the surface-polymer interaction relative to the surface-solvent interaction increases the surface coverage of polymer segments and independent chains for all surface curvatures. Finally, we demonstrate that the polymer segmental adsorption times are heavily influenced only by the surface-fluid interactions, although polymers desorb more quickly from highly curved surfaces.

The field of polymer nanocomposites has been at the forefront of research in the polymer community for the past few decades. Foundational work published in Macromolecules during this time has emphasized the physics and chemistry of the inclusion of nanofillers; remarkable early developments suggested that these materials would create a revolution in the plastics industry. After 25 years of innovative and groundbreaking research, PNCs have enabled many niche solutions. To complement the extensive literature currently available, we focus this Perspective on four case studies of PNCs applications: (i) filled rubbers, (ii) continuous fiber reinforced thermoset composites, (iii) membranes for gas separations, and (iv) dielectrics for capacitors and insulation. After presenting synthetic developments we discuss the application of polymer nanocomposites to each of these topic areas; successes will be noted, and we will finish each section by highlighting the various technological bottlenecks that need to be over...

Melt state dynamics for a series of strictly linear polyethylenes with precisely spaced associating functional groups were investigated. The periodic pendant acrylic acid groups form hydrogen-bonded acid aggregates within the polyethylene (PE) matrix. The dynamics of these nanoscale heterogeneous morphologies were investigated from picosecond to nanosecond timescales by both quasi-elastic neutron scattering (QENS) measurements and fully atomistic molecular dynamics (MD) simulations. Two dynamic processes were observed. The faster dynamic processes which occur at the picosecond timescales are compositionally insensitive and indicative of spatially restricted local motions. The slower dynamic processes are highly composition dependent and indicate the structural relaxation of the polymer backbone. Higher acid contents, or shorter PE spacers between pendant acid groups, slow the structural relaxation timescale and increase the stretching parameter (β) of the structural relaxation. Additionally, the dynamics ...

We report the morphology evolution under tensile deformation for strictly linear polyethylenes with associating functional groups. In situ X-ray scattering measurements during elongation reveal that periodic acid group placement along the backbone is required to form hierarchical layered morphologies that lead to strain hardening. This phenomenon was observed in both semicrystalline and amorphous precise acid polyethylenes with acrylic, geminal acrylic, and phosphonic acids. Polymers with nonperiodic (pseudorandom) acid placement fail to form layered morphologies and instead retain a liquidlike distribution of acid aggregates. Acid chemistry and acid concentration influence morphological evolution in both periodic and nonperiodic polymers predominately through the modification of Tg and percent crystallinity, which subsequently impact the mechanical properties. Our results indicate that hierarchical acid-rich layered structures, commensurate with improved mechanical properties, form in polymers with stric...