Broadband Dielectric Investigation of Amorphous Poly(methyl methacrylate)/Poly(ethylene oxide) Blends

Dielectric Properties in Oriented and Unoriented Membranes Based on Poly(Epichlorohydrin-co-Ethylene Oxide) Copolymers: Part III

The dielectric spectra and conductivity properties of neat poly(epichlorohydrin-co-ethylene oxide)(PECH-co-EO) copolymer and two modified copolymers with a 20% or 40% of dendron 3,4,5-tris[4-(n-dodecan-1-yloxy)benzyloxy] benzoate units were analysed. A process of thermal orientation was applied to the copolymers to fine-tune the molecular motion of the side chains and determine their validity for cation transport materials. The study was conducted using Dielectric Thermal Analysis (DETA). The spectra of the modified unoriented and oriented copolymers consisted of five dielectric relaxations (δ, γ, β, αTg, and αmelting). The analysis of the relaxations processes shows that as the grafting with the dendron units increases, both the lateral and main chains have a greater difficulty moving. The thermal orientation induces in the main chain partial crystallization, including the polyether segments, and modifies the cooperative motion of the main chain associated with the glass transition (αTg). A deep analysis of the electrical loss modulus revealed that the degree of modification only modifies the temperature peak of each relaxation, and this effect is more perceived if the dendron unit content is higher (40%). The thermal orientation process seems equal to the spectra of CP20-O and CP40-O to the point that the degree of modification does not matter. Nevertheless, the fragility index denotes the differences in the molecular motion between both copolymers (40% and 20%) due to the thermal orientation. The study of the electric conductivity showed that the ideal long-range pathways were being altered by neither the thermal orientation process nor the addition of dendrimers. The analysis of the through-plane proton conductivity confirmed that the oriented copolymer with the highest concentration of dendrimers was the best performer and the most suitable copolymer for proton transport materials.

Membranes for Cation Transport Based on Dendronized Poly(epichlorohydrin-co-ethylene oxide). Part 1: The Effect of Dendron Amount and Column Orientation on Copolymer Mobility

Dendronized polyethers give rise to columnar LC structures which can successfully act as cation transport materials. Therefore, we prepared two different materials, based on Poly(epichlorohydrin-co-ethylene oxide) (PECH-co-EO) grafted with methyl 3,4,5-tris[4-(n-dodecan-1-yloxy)benzyloxy] benzoate, containing 20% or 40% modified units, respectively. The obtained polymers were characterized by differential scanning calorimetry (DSC), X-ray diffraction and optical microscopy between crossed polars (POM) and compared to the unmodified PECH-co-EO. In order to reach efficient transport properties, homeotropically oriented membranes were prepared by a fine-tuned thermal annealing treatment and were subsequently investigated by dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA). We found that the presence of the dendrons induces a main chain partial crystallization of the polyether chain and coherently increases the polymer Tg. This effect is more evident in the oriented membranes. As for copolymer orientation upon annealing, the cooling rate and the annealing temperature were the most crucial factors. DMTA and DETA confirmed that grafting with the dendron strongly hinders copolymer motions, but did not show great differences between unoriented and oriented membranes, regardless of the amount of dendrons.

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.

Interchain coupled chain dynamics of poly(ethylene oxide) in blends with poly(methyl methacrylate): coupling model analysis

Quasielastic neutron scattering and molecular dynamics simulation data from poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) blends found that for short times the self-dynamics of PEO chain follows the Rouse model, but at longer times past t(c) = 1-2 ns it becomes slower and departs from the Rouse model in dependences on time, momentum transfer, and temperature. To explain the anomalies, others had proposed the random Rouse model (RRM) in which each monomer has different mobility taken from a broad log-normal distribution. Despite the success of the RRM, Diddens et al. [Eur. Phys. Lett. 95, 56003 (2011)] extracted the distribution of friction coefficients from the MD simulations of a PEO/PMMA blend and found that the distribution is much narrower than expected from the RRM. We propose a simpler alternative explanation of the data by utilizing alone the observed crossover of PEO chain dynamics at t(c). The present problem is just a special case of a general property of relaxation in interacting systems, which is the crossover from independent relaxation to coupled many-body relaxation at some t(c) determined by the interaction potential and intermolecular coupling/constraints. The generality is brought out vividly by pointing out that the crossover also had been observed by neutron scattering from entangled chains relaxation in monodisperse homopolymers, and from the segmental α-relaxation of PEO in blends with PMMA. The properties of all the relaxation processes in connection with the crossover are similar, despite the length scales of the relaxation in these systems are widely different.

Dynamical heterogeneity in binary mixtures of low-molecular-weight glass formers

Homogeneous diethyl phthalate/phenylphthalein-dimethylether (DEP/PDE) mixtures have been investigated by means of broadband dielectric spectroscopy. Contrarily to the widespread view that homogenous binary mixtures should give rise to a single glass transition, the mixture displays two dynamics giving rise to two glass transitions. Such a finding can be rationalized invoking the self-concentration concept that relies on the localized nature of the glass transition phenomenon. In such a way, the analogy with miscible polymer blends, for which this concept has been introduced, is highlighted. A model based on the combination of the Adam-Gibbs (AG) theory of the glass transition and the self-concentration concept resulted to be fully predictive once the only unknown variable, namely, the glass-former specific parameter (alpha) connecting the characteristic length for the glass transition to the configurational entropy, is extracted applying the model itself to DEP/toluene and DEP/PDE solutions highly concentrated, respectively, in DEP and PDE. The alpha parameter obtained in such a way allows the precise determination of the most probable relaxation time even for those DEP/PDE mixtures displaying a strong overlap of the dielectric response. The model incorporating the self-concentration concept to the AG theory also provides the characteristic length scale for the glass transition for both DEP and PDE. Such a length scale was found to be on the order of 1-2 nm. This is comparable to that obtained for other glass formers.

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 Johari-Goldstein beta-relaxation of water

There is a plethora of experimental data on the dynamics of water in mixtures with glycerol, ethylene glycol, ethylene glycol oligomers, poly(ethylene glycol) 400 and 600, propanol, poly(vinyl pyrrolidone), poly(vinyl methylether), and other substances. In spite of the differences in the water contents, the chemical compositions, and the glass transition temperatures Tg of these aqueous mixtures, a faster relaxation originating from the water (called the nu-process) is omnipresent, sharing the following common properties. The relaxation time tau(nu) has Arrhenius temperature dependence at temperatures below Tg of the mixture. The activation energies of tau(nu) all fall within a neighborhood of 50 kJ/mol. At the same temperature where mixtures are all in their glassy states, the values of tau(nu) of several mixtures are comparable. The Arrhenius temperature dependence of tau(nu) does not continue to higher temperatures and instead it crosses over to a stronger temperature dependence at temperatures above Tg. The dielectric relaxation strength of the nu-process, Deltaepsilon(nu)(T), has a stronger temperature dependence above Tg than below, mimicking the change of enthalpy, entropy, and volume when crossing Tg. These general property of the nu-process (except for the magnitude of the activation energy) had been found before in the secondary relaxation of the faster component in several binary nonaqueous mixtures. Other properties of the secondary relaxation in these nonaqueous mixtures have helped to identify it as the Johari-Goldstein (JG) secondary relaxation of the faster component. The similarities in properties lead us to conclude that the nu-processes in water mixtures are the JG secondary relaxations of water. The conclusion is reinforced by the processes behaving similarly to the nu-process found in 6 A thick water layer (two molecular layers) in fully hydrated Na-vermiculite clay, and in water confined in molecular sieves, silica hydrogels, and poly(2-hydroxyethyl methacrylate) hydrogels.

Polymer chain dynamics in a random environment: heterogeneous mobilities

We present a neutron scattering investigation on a miscible blend of two polymers with greatly different glass-transition temperatures Tg. Under such conditions, the nearly frozen high-Tg component imposes a random environment on the mobile chain. The results demand the consideration of a distribution of heterogeneous mobilities in the material and demonstrate that the larger scale dynamics of the fast component is not determined by the average local environment alone. This distribution of mobilities can be mapped quantitatively on the spectrum of local relaxation rates measured at high momentum transfers.

Dynamic crossovers and activated regimes in a narrow distribution poly(n-butyl acrylate): an ESR study

The rotational dynamics of the spin probe cholestane dissolved in a narrow distribution poly(n-butyl acrylate) sample has been investigated via electron spin resonance (ESR) spectroscopy. The measurements were carried out in a wide temperature range: different dynamic regions have been recognized, and the coupling of the probe dynamics to the α and secondary relaxations has been revealed. In particular, the coupling with the structural relaxation is ruled by two fractionary Vogel-Fulcher laws (VF). The crossover from one VF region to the other occurs at the temperature T(C) = 1.17T(g), signalling the onset of the cooperativity in the dynamics and confirming a behaviour previously observed in ESR studies carried out on polymeric glass-formers. Furthermore, in this work we discuss the activated regime at the highest temperatures and show that the activation energy does not depend on the length of the polymer main- and side-chains, while its onset temperature linearly depends on the chain length.