Understanding creep suppression mechanisms in polymer nanocomposites through machine learning.

While recent efforts have shown how local structure plays an essential role in the dynamic heterogeneity of homogeneous glass-forming materials, systems containing interfaces such as thin films or composite materials remain poorly understood. It is known that interfaces perturb the molecular packing nearby, however, numerous studies show the dynamics are modified over a much larger range. Here, we examine the dynamics in polymer nanocomposites (PNCs) using a combination of simulations and experiments and quantitatively separate the role of polymer packing from other effects on the dynamics, as a function of distance from the nanoparticle surfaces. After showing good qualitative agreement between the simulations and experiments in glassy structure and creep compliance, we use a machine-learned structure indicator, softness, to decompose polymer dynamics in our simulated PNCs into structure-dependent and structure-independent processes. With this decomposition, the free energy barrier for polymer rearrangement can be described as a combination of packing-dependent and packing-independent barriers. We find both barriers are higher near nanoparticles and decrease with applied stress, quantitatively demonstrating that the slow interfacial dynamics is not solely due to polymer packing differences, but also the change of structure-dynamics relationships. Finally, we present how this decomposition can be used to accurately predict strain-time creep curves for PNCs from their static configuration, providing additional insights into the effects of polymer-nanoparticle interfaces on creep suppression in PNCs.

pH-Mediated Size-Selective Adsorption of Gold Nanoparticles on Diblock Copolymer Brushes.

Precise control of nanoparticles at interfaces can be achieved by designing stimuli-responsive surfaces that have tunable interactions with nanoparticles. In this study, we demonstrate that a polymer brush can selectively adsorb nanoparticles according to size by tuning the pH of the buffer solution. Specifically, we developed a facile polymer brush preparation method using a symmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer deposited on a grafted polystyrene layer. This method is based on the assembly of a PS-b-P2VP thin film oriented with parallel lamellae that remains after exfoliation of the top PS-b-P2VP layer. We characterized the P2VP brush using X-ray reflectivity and atomic force microscopy. The buffer pH is used to tailor interactions between citrate-coated gold nanoparticles (AuNPs) and the top P2VP block that behaves like a polymer brush. At low pH (∼4.0) the P2VP brushes are strongly stretched and display a high density of attractive sites, whereas at neutral pH (∼6.5) the P2VP brushes are only slightly stretched and have fewer attractive sites. A quartz crystal microbalance with dissipation monitored the adsorption thermodynamics as a function of AuNP diameter (11 and 21 nm) and pH of the buffer. Neutral pH provides limited penetration depth for nanoparticles and promotes size selectivity for 11 nm AuNP adsorption. As a proof of concept, the P2VP brushes were exposed to various mixtures of large and small AuNPs to demonstrate selective capture of the smaller AuNPs. This study shows the potential of creating devices for nanoparticle size separations using pH-sensitive polymer brushes.

Thiol-ene Click Chemistry Incorporates Hydroxyl Functionality on Polycyclooctene to Tune Properties.

Polyolefins compose the majority of plastic waste, but conventional mechanical recycling degrades their properties, thereby reducing their value. We report the functionalization of a model for dehydrogenated polyethylene, polycyclooctene (PCOE), with thiol-ene click chemistry to install pendant hydroxyl ethyl thioethers. Functionalization of PCOE using mercaptoethanol via thiol-ene click chemistry yielded functionalization between 1.4 and 22.9% based on ethylene monomeric units. Reactions were well-controlled by varying the reagent stoichiometry and reaction time. Crystallinity and melting temperature decreased, and glass transition temperature increased with greater functionalization. Contact angle measurements reveal an increase in surface polarity with functionalization. Comparisons with poly(ethylene-co-vinyl alcohol) (EVOH) show comparable surface polarity at similar levels of alcohol functionalization. At 12% functionalization, the ultimate shear stress (USS) of functionalized PCOE in an adhesive configuration is 4.10 ± 0.48 MPa, comparable to EVOH. At >12% functionalization, the failure mode changed from adhesive to mixed adhesive-cohesive, and the USS decreased.

Self-Folding Liquid Crystal Network Filaments Patterned with Vertically Aligned Mesogens.

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.

Double Gyroid Morphologies in Precise Ion-Containing Multiblock Copolymers Synthesized via Step-Growth Polymerization.

The double gyroid structure was first reported in diblock copolymers about 30 years ago, and the complexity of this morphology relative to the other ordered morphologies in block copolymers continues to fascinate the soft matter community. The double gyroid microphase-separated morphology has co-continuous domains of both species, and the minority phase is subdivided into two interpenetrating network structures. In addition to diblock copolymers, this structure has been reported in similar systems including diblock copolymers blended with one or two homopolymers and ABA-type triblock copolymers. Given the narrow composition region over which the double gyroid structure is typically observed (∼3 vol %), anionic polymerization has dominated the synthesis of block copolymers to control their composition and molecular weight. This perspective will highlight recent studies that (1) employ an alternative polymerization method to make block copolymers and (2) report double gyroid structures with lattice parameters below 10 nm. Specifically, step-growth polymerization linked precise polyethylene blocks and short sulfonate-containing blocks to form strictly alternating multiblock copolymers, and these copolymers produce the double gyroid structure over a dramatically wider composition range (>14 vol %). These new (AB) n multiblock copolymers self-assemble into the double gyroid structure by having exceptional control over the polymer architecture and large interaction parameters between the blocks. This perspective proposes criteria for a broader and synthetically more accessible range of polymers that self-assemble into double gyroids and other ordered structures, so that these remarkable structures can be employed to solve a variety of technological challenges.

Enhanced Li-Ion Transport through Selectively Solvated Ionic Layers of Single-Ion Conducting Multiblock Copolymers.

We demonstrate enhanced Li+ transport through the selectively solvated ionic layers of a single-ion conducting polymer. The polymer is a precisely segmented ion-containing multiblock copolymers with well-defined Li+SO3- ionic layers between crystallized linear aliphatic 18-carbon blocks. X-ray scattering reveals that the dimethyl sulfoxide (DMSO) molecules selectively solvate the ionic layers without disrupting the crystallization of the polymer backbone. The amount of DMSO (∼21 wt %) calculated from the increased layer spacing is consistent with thermogravimetric analysis. The ionic conductivity through DMSO-solvated ionic layers is >104 times higher than in the dried state, indicating a significant enhancement of ion transport in the presence of this solvent. Dielectric relaxation spectroscopy (DRS) further elucidates the role of the structural relaxation time (τ) and the number of free Li+ (n) on the ionic conductivity (σ). Specifically, DRS reveals that the solvation of ionic domains with DMSO contributes to both accelerating the structural relaxation and the dissociation of ion pairs. This study is the initial demonstration that selective solvation is a viable design strategy to improve ionic conductivity in nanophase separated, single-ion conducting multiblock copolymers.

Ordered Nanostructures in Thin Films of Precise Ion-Containing Multiblock Copolymers.

We demonstrate that ionic functionality in a multiblock architecture produces highly ordered and sub-3 nm nanostructures in thin films, including bicontinuous double gyroids. At 40 °C, precise ion-containing multiblock copolymers of poly(ethylene-b-lithium sulfosuccinate ester) n (PESxLi, x = 12 or 18) exhibit layered ionic assemblies parallel to the substrate. These ionic layers are separated by crystalline polyethylene blocks with the polymer backbones perpendicular to the substrate. Notably, above the melting temperature (T m) of the polyethylene blocks, layered PES18Li thin films transform into a highly oriented double-gyroid morphology with the (211) plane (d 211 = 2.5 nm) aligned parallel to the substrate. The cubic lattice parameter (a gyr) of the double gyroid is 6.1 nm. Upon heating further above T m, the double-gyroid morphology in PES18Li transitions into hexagonally packed cylinders with cylinders parallel to the substrate. These layered, double-gyroid, and cylinder nanostructures form epitaxially and spontaneously without secondary treatment, such as interfacial layers and solvent vapor annealing. When the film thickness is less than ∼3a gyr, double gyroids and cylinders coexist due to the increased confinement. For PES12Li above T m, the layered ionic assemblies simply transform into disordered morphology. Given the chemical tunability of ion-functionalized multiblock copolymers, this study reveals a versatile pathway to fabricating ordered nanostructures in thin films.

Sub-3-Nanometer Domain Spacings of Ultrahigh-χ Multiblock Copolymers with Pendant Ionic Groups.

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 Ia3̅d 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.

Anhydrous Proton Transport within Phosphonic Acid Layers in Monodisperse Telechelic Polyethylenes.

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 Å 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 Å) 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.

Effect of surface properties and polymer chain length on polymer adsorption in solution.

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.

Conformation and dynamics of ring polymers under symmetric thin film confinement.

Understanding the structure and dynamics of polymers under confinement has been of widespread interest, and one class of polymers that have received comparatively little attention under confinement is that of ring polymers. The properties of non-concatenated ring polymers can also be important in biological fields because ring polymers have been proven to be a good model to study DNA organization in the cell nucleus. From our previous study, linear polymers in a cylindrically confined polymer melt were found to segregate from each other as a result of the strong correlation hole effect that is enhanced by the confining surfaces. By comparison, our subsequent study of linear polymers in confined thin films at similar levels of confinements found only the onset of segregation. In this study, we use molecular dynamics simulation to investigate the chain conformations and dynamics of ring polymers under planar (1D) confinement as a function of film thickness. Our results show that conformations of ring polymers are similar to the linear polymers under planar confinement, except that ring polymers are less compressed in the direction normal to the walls. While we find that the correlation hole effect is enhanced under confinement, it is not as pronounced as the linear polymers under 2D confinement. Finally, we show that chain dynamics far above Tg are primarily affected by the friction from walls based on the monomeric friction coefficient we get from the Rouse mode analysis.

Creep attenuation in glassy polymer nanocomposites with variable polymer-nanoparticle interactions.

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.

Gyroid and Other Ordered Morphologies in Single-Ion Conducting Polymers and Their Impact on Ion Conductivity.

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+, Na+, Cs+, or NBu4+ counterions synthesized from step-growth polymerization. At room temperature, all polymers are semicrystalline with well-defined nanoscale ionic layers separated by 35-38 Å, depending on the cation. In situ X-ray scattering measurements reveal that the layered ionic aggregates in PES23Li, PES23Na, and PES23Cs transform, upon melting the PE blocks, into the Ia3d gyroid morphology. The gyroidal ionic aggregates in PES23Li and PES23Na further evolve into hexagonal symmetry as the temperature increases. These order-to-order transitions in ionic aggregate morphologies were also confirmed by oscillatory shear rheology. The ion transport behavior of these PES23 polymers is strongly dependent on the ionic aggregate morphologies. Specifically, the 3D interconnected gyroid morphology of PES23Li exhibits higher ionic conductivity than the isotropic layered or hexagonal morphologies. This innovative and versatile molecular design of single-ion conducting polymers leads to unprecedented percolated gyroidal ionic aggregate morphologies that provide a continuous pathway for improved ion transport.

Modeling of Entangled Polymer Diffusion in Melts and Nanocomposites: A Review.

This review concerns modeling studies of the fundamental problem of entangled (reptational) homopolymer diffusion in melts and nanocomposite materials in comparison to experiments. In polymer melts, the developed united atom and multibead spring models predict an exponent of the molecular weight dependence to the polymer diffusion very similar to experiments and the tube reptation model. There are rather unexplored parameters that can influence polymer diffusion such as polymer semiflexibility or polydispersity, leading to a different exponent. Models with soft potentials or slip-springs can estimate accurately the tube model predictions in polymer melts enabling us to reach larger length scales and simulate well entangled polymers. However, in polymer nanocomposites, reptational polymer diffusion is more complicated due to nanoparticle fillers size, loading, geometry and polymer-nanoparticle interactions.

The evolution of acidic and ionic aggregates in ionomers during microsecond simulations.

We performed microsecond-long, atomistic molecular dynamics simulations on a series of precise poly(ethylene-co-acrylic acid) ionomers neutralized with lithium, with three different spacer lengths between acid groups on the ionomers and at two temperatures. Ionic aggregates form in these systems with a variety of shapes ranging from isolated aggregates to percolated aggregates. At the lower temperature of 423 K, the ionic aggregate morphologies do not reach a steady-state distribution over the course of the simulations. At the higher temperature of 600 K, the aggregates are sufficiently mobile that they rearrange and reach steady state after hundreds of nanoseconds. For systems that are 100% neutralized with lithium, the ions form percolated aggregates that span the simulation box in three directions, for all three spacer lengths (9, 15, and 21). In the partially neutralized systems, the morphology includes lithium ion aggregates that may also include some unneutralized acid groups, along with a coexisting population of acid group aggregates that form through hydrogen bonding. In the lithium ion aggregates, unneutralized acid groups tend to be found on the ends or sides of the aggregates.

Self-assembled highly ordered acid layers in precisely sulfonated polyethylene produce efficient proton transport.

Recent advances in polymer synthesis have allowed remarkable control over chain microstructure and conformation. Capitalizing on such developments, here we create well-controlled chain folding in sulfonated polyethylene, leading to highly uniform hydrated acid layers of subnanometre thickness with high proton conductivity. The linear polyethylene contains sulfonic acid groups pendant to precisely every twenty-first carbon atom that induce tight chain folds to form the hydrated layers, while the methylene segments crystallize. The proton conductivity is on par with Nafion 117, the benchmark for fuel cell membranes. We demonstrate that well-controlled hairpin chain folding can be utilized for proton conductivity within a crystalline polymer structure, and we project that this structure could be adapted for ion transport. This layered polyethylene-based structure is an innovative and versatile design paradigm for functional polymer membranes, opening doors to efficient and selective transport of other ions and small molecules on appropriate selection of functional groups.

Grafted polymer chains suppress nanoparticle diffusion in athermal polymer melts.

We measure the center-of-mass diffusion of poly(methyl methacrylate) (PMMA)-grafted nanoparticles (NPs) in unentangled to slightly entangled PMMA melts using Rutherford backscattering spectrometry. These grafted NPs diffuse ∼100 times slower than predicted by the Stokes-Einstein relation assuming a viscosity equal to bulk PMMA and a hydrodynamic NP size equal to the NP core diameter, 2Rcore = 4.3 nm. This slow NP diffusion is consistent with an increased effective NP size, 2Reff ≈ 20 nm, nominally independent of the range of grafting density and matrix molecular weights explored in this study. Comparing these experimental results to a modified Daoud-Cotton scaling estimate for the brush thickness as well as dynamic mean field simulations of polymer-grafted NPs in athermal polymer melts, we find that 2Reff is in quantitative agreement with the size of the NP core plus the extended grafted chains. Our results suggest that grafted polymer chains of moderate molecular weight and grafting density may alter the NP diffusion mechanism in polymer melts, primarily by increasing the NP effective size.

Polymer and spherical nanoparticle diffusion in nanocomposites.

Nanoparticle and polymer dynamics in nanocomposites containing spherical nanoparticles were investigated by means of molecular dynamics simulations. We show that the polymer diffusivity decreases with nanoparticle loading due to an increase of the interfacial area created by nanoparticles, in the polymer matrix. We show that small sized nanoparticles can diffuse much faster than that predicted from the Stokes-Einstein relation in the dilute regime. We show that the nanoparticle diffusivity decreases at higher nanoparticle loading due to nanoparticle-polymer interface. Increase of the nanoparticle radius slows the nanoparticle diffusion.

Nanoscale Aggregation in Acid- and Ion-Containing Polymers.

In this review we summarize recent efforts in understanding nano-aggregation in acid- and ion-containing polymer systems. The acid and ionic groups have specific interactions that drive aggregation and alter polymer behavior at the nano-, micro-, and bulk length scales. Advancements in synthetic methods, characterization techniques, and computer simulations have enabled researchers to better understand the morphologies and dynamics, particularly at the nanoscale. This overview of recent advancements in nano-aggregated polymer systems highlights the current understanding of the field and presents promising directions for future investigations and new applications.