Relaxation Studies in PEO/PMMA Blends

All-Polymer Nanocomposite as Salt-Free Solid Electrolyte for Lithium Metal Batteries

Solid polymer electrolytes that combine both a high lithium-ion transference number and mechanical properties at high temperatures are searched for improving the performance of batteries. Here, we show a salt-free all-polymer nanocomposite solid electrolyte for lithium metal batteries that improves the mechanical properties and shows a high lithium-ion transference number. For this purpose, lithium sulfonamide-functionalized poly(methyl methacrylate) nanoparticles (LiNPs) of very small size (20-30 nm) were mixed with poly(ethylene oxide) (PEO). The morphology of all-polymer nanocomposites was first investigated by transmission electron microscopy (TEM), showing a good distribution of nanoparticles (NPs) even at high contents (50 LiNP wt %). The crystallinity of PEO was investigated in detail and decreased with the increasing concentration of LiNPs. The highest ionic conductivity value for the PEO 50 wt % LiNP nanocomposite at 80 °C is 1.1 × 10-5 S cm-1, showing a lithium-ion transference number of 0.68. Using dynamic mechanic thermal analysis (DMTA), it was shown that LiNPs strengthen PEO, and a modulus of ≈108 Pa was obtained at 80 °C for the polymer nanocomposite. The nanocomposite solid electrolyte was stable with respect to lithium in a Li||Li symmetrical cell for 1000 h. In addition, in a full solid-state battery using LiFePO4 as the cathode and lithium metal as the anode, a specific capacity of 150 mAhg-1 with a current density of 0.05 mA cm-2 was achieved.

Polymer architecture effect on rheology and segmental dynamics in poly (methyl methacrylate)-silica nanocomposite melts

Architecturally different polymer chains lead to fundamentally different rheological responses and internal dynamics, which can be utilized to rationalize advanced thermoplastic nanocomposites with tunable mechanical behavior. In this work, three model poly (methyl methacrylate) (PMMA) polymers with linear, bottlebrush, and star architectures with the same total molar mass were investigated in their neat form, and nanocomposites with well-dispersed silica nanoparticles using rheology and broadband dielectric spectroscopy (BDS). The master curves of the dynamic moduli obtained by time-temperature superposition (TTS) over the entire range from the Rouse regime to the terminal flow and a sequence of significantly different relaxation modes were observed for the samples with linear and branch chains. While linear chains form an entangled polymer network, the branched bottlebrush, and star chains show a viscoelastic response with no sign of rubbery entanglement plateau and a weak arm relaxation regime between Rouse and terminal flow, akin to other branched polymers. Moreover, branched chains showed a higher fragility index (m = 3.46 for the bottlebrush and 5.36 for the star) compared to linear chains (m = 3.29) due to dynamical heterogeneities induced by arm relaxation. The addition of nanoparticles affects only the terminal relaxation regime, where the whole chain motion is hindered by the attractive nanoparticles. The dynamics of the polymer segment were investigated by performing broadband dielectric spectroscopy (BDS) at a frequency range from 10-2 Hz to 107 Hz. The results revealed more than 10 times slower segmental relaxation for the star homopolymers and a slowdown in the α-relaxation process for all three architectures in their composite form. The dynamical slowdown in the composites is temperature dependent and more pronounced at low temperatures (leading to approximately equal to 80 times slower dynamics for nanocomposite with bottlebrush PMMA at 150 °C) due to prolonged relaxation of the interfacial polymer compared to the matrix chains. The results from this study have practical applications in fields such as gas separation and polymeric electrolyte membranes, where simultaneous improvement of segmental mobility and mechanical moduli is highly desired.

Lateral diffusion of single poly(ethylene oxide) chains on the surfaces of glassy and molten polymer films

Fluorescence correlation spectroscopy was used to show that the temperature-dependent diffusion coefficient of poly(ethylene oxide) (PEO) adsorbed on polystyrene and different poly(alkyl methacrylate) (PAMA) films in aqueous solution exhibited a maximum close to (but below) the surface glass transition temperature, T_gs, of the film. This elevated diffusion was observed over a small range of temperatures below T_gs for these surfaces, and at other temperatures, the diffusion was similar to that on silicon, although the diffusion coefficient for PEO on polystyrene at temperatures above T_gs did not completely decrease to that on silicon, in contrast to the PAMA surfaces. It is concluded that the enhanced surface mobility of the films near the surface glass transition temperature induces conformational changes in the adsorbed PEO. The origin of this narrow and dramatic increase in diffusion coefficient is not clear, but it is proposed that it is caused by a coupling of a dominant capillary mode in the liquid surface layer with the polymer. Friction force microscopy experiments also demonstrate an unexpected increase in friction at the same temperature as the increase in diffusion coefficient.

Manipulation of Fracture Behavior of Poly(methyl methacrylate) Nanocomposites by Interfacial Design of a Metal-Organic-Framework Nanoparticle Toughener

The interfacial region between nanoparticles and polymer matrix plays a critical role in influencing the mechanical behavior of polymer nanocomposites. In this work, a set of model systems based on poly(methyl methacrylate) (PMMA) matrix containing poly(alkyl glycidyl ether) brushes grafted on 50 nm metal-organic-framework (MOF) nanoparticles were synthesized and investigated. By systematically increasing the polymer brush length and graft density on the MOF nanoparticles, the fracture behavior of PMMA/MOF nanocomposite changes from forming only a few large crazes to generating massive crazing and to undergoing shear banding, which results in significant improvement in fracture toughness. The implication of the present finding for the interfacial design of the nanoparticles for the development of high-performance, multifunctional polymer nanocomposites is discussed.

Spatially and Temporally Resolved Heterogeneities in a Miscible Polymer Blend

Mapping the spatial and temporal heterogeneities in miscible polymer blends is critical for understanding and further improving their material properties. However, a complete picture on the heterogeneous dynamics is often obscured in ensemble measurements. Herein, the spatial and temporal heterogeneities in fully miscible polystyrene/oligostyrene blend films are investigated by monitoring the rotational diffusion of embedded individual probe molecules using defocused wide-field fluorescence microscopy. In the same blend film, three significantly different types of dynamical behaviors (referred to as modes) of the probe molecules can be observed at the same time, namely, immobile, continuously rotating, and intermittently rotating probe molecules. This reveals a prominent spatial heterogeneity in local dynamics at the nanometer scale. In addition to that, temporal heterogeneity is uncovered by the nonexponential characteristic of the rotational autocorrelation functions of single-molecule probes. Moreover, the occurrence probabilities of these different modes strongly depend on the polystyrene: oligostyrene ratios in the blend films. Remarkably, some probe molecules switch between the continuous and intermittent rotational modes at elevated temperature, indicating a possible alteration in local dynamics that is triggered by the dynamic heterogeneity in the blends. Although some of these findings can be discussed by the self-concentration model and the results provided by ensemble averaging techniques (e.g., dielectric spectroscopy), there are implications that go beyond current models of blend dynamics.

Fabrication of PEO-PMMA-LiClO4-Based Solid Polymer Electrolytes Containing Silica Aerogel Particles for All-Solid-State Lithium Batteries

To improve the ionic conductivity and thermal stability of a polyethylene oxide (PEO)-ethylene carbonate (EC)-LiClO4-based solid polymer electrolyte for lithium-ion batteries, polymethyl methacrylate (PMMA) and silica aerogel were incorporated into the PEO matrix. The effects of the PEO:PMMA molar ratio and the amount of silica aerogel on the structure of the PEO-PMMA-LiClO4 solid polymer electrolyte were studied by X-ray diffraction, Fourier-transform infrared spectroscopy and alternating current (AC) impedance measurements. The solid polymer electrolyte with PEO:PMMA = 8:1 and 8 wt% silica aerogel exhibited the highest lithium-ion conductivity (1.35 × 10−4 S∙cm−1 at 30 °C) and good mechanical stability. The enhanced amorphous character and high degree of dissociation of the LiClO4 salt were responsible for the high lithium-ion conductivity observed. Silica aerogels with a high specific surface area and mesoporosity could thus play an important role in the development of solid polymer electrolytes with improved structure and stability.

Unexpected behavior of ultra-thin films of blends of polystyrene/poly(vinyl methyl ether) studied by specific heat spectroscopy

Specific heat spectroscopy (SHS) employing AC nanochip calorimetry was used to investigate the glassy dynamics of ultra-thin films (thicknesses: 10 nm-340 nm) of a polymer blend, which is miscible in the bulk. In detail, a Poly(vinyl methyl ether) (PVME)/Polystyrene (PS) blend with the composition of 25/75 wt. % was studied. The film thickness was controlled by ellipsometry while the film topography was checked by atomic force microscopy. The results are discussed in the framework of the balance between an adsorbed and a free surface layer on the glassy dynamics. By a self-assembling process, a layer with a reduced mobility is irreversibly adsorbed at the polymer/substrate interface. This layer is discussed employing two different scenarios. In the first approach, it is assumed that a PS-rich layer is adsorbed at the substrate. Whereas in the second approach, a PVME-rich layer is suggested to be formed at the SiO₂ substrate. Further, due to the lower surface tension of PVME, with respect to air, a nanometer thick PVME-rich surface layer, with higher molecular mobility, is formed at the polymer/air interface. By measuring the glassy dynamics of the thin films of PVME/PS in dependence on the film thickness, it was shown that down to 30 nm thicknesses, the dynamic T_g of the whole film was strongly influenced by the adsorbed layer yielding a systematic increase in the dynamic T_g with decreasing the film thickness. However, at a thickness of ca. 30 nm, the influence of the mobile surface layer becomes more pronounced. This results in a systematic decrease in T_g with the further decrease of the film thickness, below 30 nm. These results were discussed with respect to thin films of PVME/PS blend with a composition of 50/50 wt. % as well as literature results.

Dielectric relaxation of polymers: segmental dynamics under structural constraints

In this article we review the recent polymer literature where dielectric spectroscopy has been used to investigate the segmental dynamics of polymers under the constraints produced by self-structuring. Specifically, we consider three cases: (i) semicrystalline polymers, (ii) segregated block-copolymers, and (iii) asymmetric miscible polymer blends. In these three situations the characteristics of the dielectric relaxation associated with the polymer segmental dynamics are markedly affected by the constraints imposed by the corresponding structural features. After reviewing in detail each of the polymer systems, the most common aspects are discussed in the context of the use of dielectric relaxation as a sensitive tool for analyzing structural features in nanostructured polymer systems.

Interactions, Structure, and Dynamics of Polymer-Tethered Nanoparticle Blends

We report on the structure, jamming, and dynamics of blends of self-suspended hairy silica nanoparticles grafted with poly(ethylene glycol) (PEG) and poly(methyl methacrylate) (PMMA). We find that favorable enthalpic attraction between tethered PEG and PMMA chains augment previously reported entropic attractions between tethered polymer chains in self-suspended suspensions to enhance particle-particle correlations, increase jamming, and slow down chain dynamics. As with their single-component counterparts, the hairy SiO2-PEG/SiO2-PMMA nanoparticle blends exhibit soft glassy rheological behavior and both the energy dissipated at yielding and the plateau elastic modulus display strong maxima in the symmetric case. A comparison of the small angle X-ray scattering (SAXS) measurements with theoretical analysis from density functional theory (DFT) reveals that the addition of SiO2-PMMA to a self-suspended SiO2-PEG suspension initially leads to a higher degree of stretching of the corona chains, which produces stronger interdigitation of the tethered chains, enhanced jamming, and slower polymer relaxation than observed in the single-component materials. By means of an analysis of the heat of mixing released upon blending tethered and untethered PEG and PMMA chains, we find that the strong enthalpic attraction between the grafted polymer chains enhances entropic attractive forces produced by the space-filling constraint on tethered ligands in self-suspended suspensions to produce entangled-polymer-like physical properties in polymers with molecular weights below the thresholds normally associated with the transition to an entangled state.

New insight into relaxation dynamics of an epoxy/hydroxy functionalized polybutadiene from dielectric and mechanical spectroscopy studies

Dielectric and mechanical spectroscopy methods have been employed to describe the temperature dependencies of the segmental and macromolecular relaxation rates in epoxy/hydroxy functionalized polybutadiene. Dielectric studies on the dynamics of segments of the polymer as well as the mobility of small ions trapped in the system have been carried out both as a function of temperature and pressure under isobaric and isothermal conditions, respectively.

Segmental dynamics in miscible polymer blends: recent results and open questions

In this short review we summarize the outcome of the large amount of effort made during the past decade from both the experimental and the theoretical point of view in order to understand the effect of blending on the segmental dynamics in polymers. Each of the two families of models proposed-one based on thermally activated concentration fluctuations, the other on chain connectivity effects-account for each of the two main experimental observations: the broadening of the component response with respect to that of the homopolymer and the dynamic heterogeneity, respectively. The complementarity of these approaches, their main achievements and failures, are critically revised. We also include recent results on blends of components with very different mobilities. In the neighbourhood of the glass-transition of the slow polymer, the dynamics of the other component seem to be confined within the frozen chains. We suggest possible ingredients and new routes to be considered in order to elaborate more predictive theoretical frameworks for all these phenomena.

The single chain limit of structural relaxation in a polyolefin blend

The influence of composition on component dynamics and relevant static properties in a miscible polymer blend is investigated using molecular dynamics simulation. Emphasis is placed on dynamics in the single chain dilution limit, as this limit isolates the role of inherent component mobility in the polymer's dynamic behavior when placed in a blend. For our systems, a biased local concentration affecting dynamics must arise primarily from chain connectivity, which is quantified by the self-concentration, because concentration fluctuations are minimized due to restraints on chain lengths arising from simulation considerations. The polyolefins simulated [poly(ethylene-propylene) (PEP) and poly(ethylene-butene) (PEB)] have similar structures and glass transition temperatures, and all interactions are dispersive in nature. We find that the dependence of dynamics upon composition differs between the two materials. Specifically, PEB (slower component) is more influenced by the environment than PEP. This is linked to a smaller self-concentration for PEB than PEP. We examine the accuracy of the Lodge-McLeish model (which is based on chain connectivity acting over the Kuhn segment length) in predicting simulation results for effective concentration. The model predicts the simulation results with high accuracy when the model's single parameter, the self-concentration, is calculated from simulation data. However, when utilizing the theoretical prediction of the self-concentration the model is not quantitatively accurate. The ability of the model to link the simulated self-concentration with biased local compositions at the Kuhn segment length provides strong support for the claim that chain connectivity is the leading cause of distinct mobility in polymer blends. Additionally, the direct link between the willingness of a polymer to be influenced by the environment and the value of the self-concentration emphasizes the importance of the chain connectivity. Furthermore, these findings are evidence that the Kuhn segment length is the relevant length scale controlling segmental dynamics.

Collective motion in poly(ethylene oxide)/poly(methylmethacrylate) blends

We present neutron spin echo and structural measurements on a perdeuterated miscible polymer blend: poly(ethylene oxide)[PEO]/poly(methyl methacrylate)[PMMA], characterized by a large difference in component glass transition temperatures and minimal interactions. The measurements cover the q range 0.35 to 1.66 A(-1) and the temperature range Tg -75 to Tg +89 K, where Tg is the blend glass transition. The spectra, obtained directly in the time domain, are very broad with stretching parameters beta approximately 0.30. The relaxation times vary considerably over the spatial range considered however at none of the q values do we see two distinct relaxation times. At small spatial scales relaxations are still detectable at temperatures far below Tg. The temperature dependence of these relaxation times strongly resembles the beta-relaxation process observed in pure PMMA.

Dynamics of poly(ethylene oxide) in a blend with poly(methyl methacrylate): a quasielastic neutron scattering and molecular dynamics simulations study

In this paper, we have addressed the question of the dynamic miscibility in a blend characterized by very different glass-transition temperatures, Tg, for the components: poly(ethylene oxide) and poly(methyl methacrylate) (PEO/PMMA). The combination of quasielastic neutron scattering with isotopic labeling and fully atomistic molecular dynamics simulations has allowed us to selectively investigate the dynamics of the two components in the picosecond-10 nanoseconds scale at temperatures close and above the Tg of the blend. The main focus was on the PEO component, i.e., that of the lowest Tg, but first we have characterized the dynamics of the other component in the blend and of the pure PEO homopolymer as reference. In the region investigated, the dynamics of PMMA in the blend is strongly affected by the alpha-methyl rotation; an additional process detected in the experimental window 65 K above the blend-Tg can be identified as the merged alphabeta process of this component that shows strong deviations from Gaussian behavior. On the other hand, pure PEO displays entropy driven dynamics up to very large momentum transfers. Such kind of motion seems to freeze when the PEO chains are in the blend. There, we have directly observed a very heterogeneous and moreover confined dynamics for the PEO component. The presence of the hardly moving PMMA matrix leads to the creation of little pockets of mobility where PEO can move. The characteristic size of such confined islands of mobility might be estimated to be of approximately 1 nm. These findings are corroborated by the simulation study, which has been an essential support and guide in our data analysis procedure.

Dynamics of a poly(ethylene oxide) tracer in a poly(methyl methacrylate) matrix: Remarkable decoupling of local and global motions

The tracer diffusion coefficient of unentangled poly(ethylene oxide) (PEO, M=1000 gmol) in a matrix of poly(methyl methacrylate) (PMMA, M=10 000 gmol) has been measured over a temperature range from 125 to 220 degrees C with forced Rayleigh scattering. The dynamic viscosities of blends of two different high molecular weight PEO tracers (M=440 000 and 900 000 gmol) in the same PMMA matrix were also measured at temperatures ranging from 160 to 220 degrees C; failure of time-temperature superposition was observed for these systems. The monomeric friction factors for the PEO tracers were extracted from the diffusion coefficients and the rheological relaxation times using the Rouse model. The friction factors determined by diffusion and rheology were in good agreement, even though the molecular weights of the tracers differed by about three orders of magnitude. The PEO monomeric friction factors were compared with literature data for PEO segmental relaxation times measured directly with NMR. The monomeric friction factors of the PEO tracer in the PMMA matrix were found to be from two to six orders of magnitude greater than anticipated based on direct measurements of segmental dynamics. Additionally, the PEO tracer terminal dynamics are a much stronger function of temperature than the corresponding PEO segmental dynamics. These results indicate that the fastest PEO Rouse mode, inferred from diffusion and rheology, is completely separated from the bond reorientation of PEO detected by NMR. This result is unlike other blend systems in which global and local motions have been compared.