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By means of massive (more than 1.2 · 106 molecules) molecular dynamics simulations at 300 K we have disentangled self- and cross-dipolar contributions to the dielectric relaxation of liquid water that cannot be experimentally resolved. We have demonstrated that cross dipolar correlations are of paramount importance. They amount for almost a 60% of the total dielectric amplitude. The corresponding relaxation function is a one-step Debye-like function with a characteristic time, τcross, of the order of the phenomenological Debye time, τD. In contrast, the relaxation function corresponding to the self-contribution is rather complex and contains a fast decay related to dipolar librations and a second relaxation step that can be well described by two exponentials: a low-amplitude fast process (τ0 = 0.31 ps) and a main slow process (τself = 5.4 ps) that fully randomizes the dipolar orientation. In addition to dipolar relaxation functions, we have also calculated scattering-like magnitudes characterizing translation and rotation of water molecules. Although these processes can be considered as "jump" processes in the short time range, at the time scale of about τD–τcross, at which the cross-dipolar correlations decay to zero, the observed behavior cannot be distinguished from that corresponding to uncoupled Brownian translational and rotational diffusion. We propose that this is the reason why the Debye model, which does not consider intermolecular dipolar interactions, seems to work at time t ≳ τD. [ABSTRACT FROM AUTHOR]

We present an investigation by molecular dynamics (MD)-simulations of the coherent dynamic structure factor, S(Q, t) (Q: momentum transfer), of liquid water at the mesoscale (0.1 Å−1 ≤ Q ≤ Qmax) [Qmax ≈ 2 Å−1: Q-value of the first maximum of the static structure factor, S(Q), of water]. The simulation cell—large enough to address the collective properties at the mesoscale—is validated by direct comparison with recent results on the dynamic structure factor in the frequency domain obtained by neutron spectroscopy with polarization analysis [Arbe et al., Phys. Rev. Res. 2, 022015 (2020)]. We have not only focused on the acoustic excitations but also on the relaxational contributions to S(Q, t). The analysis of the MD-simulation results—including the self- and distinct contributions to the diffusive part of S(Q, t)—nicely explains why the relaxation process hardly depends on Q in the low Q-range (Q ≤ 0.4 Å−1) and how it crosses over to a diffusion-driven process at Q ≈ Qmax. Our simulations also give support to the main assumptions of the model used to fit the experimental data in the above mentioned paper. The application of such a model to the simulation S(Q, t) data delivers (i) results for the relaxation component of S(Q, t) in agreement with those obtained from neutron experiments and (ii) longitudinal and transverse hydrodynamic-like components with similar features than those identified in previous simulations of the longitudinal and transverse current spectra directly. On the other hand, in general, our MD-simulations results of S(Q, t) qualitatively agree with the viscoelastic transition framework habitually used to describe inelastic x-ray scattering results. [ABSTRACT FROM AUTHOR]

The use of an atomic force microscope for studying molecular dynamics through dielectric spectroscopy with spatial resolution in the nanometer scale is a recently developed approach. However, difficulties in the quantitative connection of the obtained data and the material dielectric properties, namely, frequency dependent dielectric permittivity, have limited its application. In this work, we develop a simple electrical model based on physically meaningful parameters to connect the atomic force microscopy (AFM) based dielectric spectroscopy experimental results with the material dielectric properties. We have tested the accuracy of the model and analyzed the relevance of the forces arising from the electrical interaction with the AFM probe cantilever. In this way, by using this model, it is now possible to obtain quantitative information of the local dielectric material properties in a broad frequency range. Furthermore, it is also possible to determine the experimental setup providing the best sensitivity in the detected signal. [ABSTRACT FROM AUTHOR]

2H-nuclear magnetic resonance (NMR) and neutron scattering (NS) on isotopically labelled samples have been combined to investigate the structure and dynamics of polyvinylpyrrolidone (PVP) aqueous solutions (4 water molecules/monomeric unit). Neutron diffraction evidences the nanosegregation of polymer main-chains and water molecules leading to the presence of water clusters. NMR reveals the same characteristic times and spectral shape as those of the slower process observed by broadband dielectric spectroscopy in this system [S. Cerveny et al., J. Chem. Phys. 128, 044901 (2008)]. The temperature dependence of such relaxation time crosses over from a cooperative-like behavior at high temperatures to an Arrhenius behavior at lower temperatures. Below the crossover, NMR features the spectral shape as due to a symmetric distribution of relaxation times and the underlying motions as isotropic. NS results on the structural relaxation of both components-isolated via H/D labeling-show (i) anomalously stretched and non-Gaussian functional forms of the intermediate scattering functions and (ii) a strong dynamic asymmetry between the components that increases with decreasing temperature. Strong heterogeneities associated to the nanosegregated structure and the dynamic asymmetry are invoked to explain the observed anomalies. On the other hand, at short times the atomic displacements are strongly coupled for PVP and water, presumably due to H-bond formation and densification of the sample upon hydration. [ABSTRACT FROM AUTHOR]

Quasielastic neutron scattering, x-ray diffraction measurements, and fully atomistic molecular dynamics simulations have been performed on poly(vinylpyrrolidone) homopolymer above its glass transition temperature. A 'prepeak' appears in the x-ray diffraction pattern that shows the typical features of a first amorphous halo. From an effective description of the experimentally accessed incoherent scattering function of hydrogens in terms of a stretched exponential function, we observe enhanced stretching and a momentum-transfer dependence of the characteristic time different from that usually reported for more simple polymers (main-chain polymers or polymers with small side groups). The comparison with both kinds of experimental results has validated the simulations. The analysis of the simulated structure factor points to a nanosegregation of side groups (SG) and main-chains (MC). The detailed insight provided by the simulations on the atomic trajectories reveals a partial and spatially localized decoupling of MC and SG dynamics at length scales between the average SG-SG distance and the characteristic length of the backbone interchain correlations. Anomalous behavior in correlators calculated for the SG subsystem are found, like e.g., logarithmiclike decays of the density-density correlation function. They might be a consequence of the existing large dynamic asymmetry between SG and MC subsystems. Our results suggest that, as the SGs are spatially extended and chemically different from the backbone, they form transient nanosegregated domains. The dynamics of these domains show similar behavior to that found in other systems displaying large dynamic asymmetry. [ABSTRACT FROM AUTHOR]

By means of computer simulations and solution of the equations of the mode coupling theory (MCT), we investigate the role of the intramolecular barriers on several dynamic aspects of nonentangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question, provide new results, and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However, a progressive discrepancy is revealed as the limit of stiff chains is approached. This disagreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed deviations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and nonmonotonous wavelength dependence of the mode correlators. [ABSTRACT FROM AUTHOR]

In this work we analyze and compare the free volume of a polymer system poly(vinyl methylether) (PVME) at 300 K obtained by the two direct but different approaches: Positron annihilation lifetime spectroscopy (PALS) and computer simulations. The free volume is calculated from the simulated cells of PVME by means of numerical methods based on grid scanning and probing the structure with a probe of a given radius RP. The free-volume structure was found to be percolated for small probes at RP=0.53 Å. As the probe radius increases, the cavity structure breaks into isolated cavities, reaching a maximum of the cavity number at RP=0.78 Å. We further develop methods for a geometrical analysis of the free-volume cavities by considering their shape. The geometrical computations show that the cavities have elongated shape with side-to-length ratio corresponding to approximately 1:0.55 and with an average length of 6 Å. Based on the overlap between the computed cavities and simplified geometrical representations, the best match of the cavity shape is obtained for the approximation to the ellipsoidal shape (overlap on 84.4%). A match with other examined shapes follows the sequence: ellipsoid>cylinder>bar>sphere>cube. Finally, the computed geometrical parameters are used as input parameters into the quantum-mechanical models for the orthopositronium (o-Ps) lifetime in various free-volume hole geometries. Comparison with the experimental data gives support for two ideas about the existence of an o-Ps particle in the polymeric matrix: (i) the positronium cannot localize in a portion of very small cavities; (ii) and in the case of the percolated cavities, several o-Ps particles occupy some subcavities in the same cavity. Additionally, radial distribution functions of the free volume indicate the existence of two kinds of free volume, a structured one, corresponding to interstitial spaces along the polymer chain, and the so-called “bulk free volume,” distributed randomly in the structure. PALS measurements seem to be mainly related with this bulk free volume. The cavities represented by the idealized geometries are visualized in three-dimensional space providing a unique representation on the free-volume structures. [ABSTRACT FROM AUTHOR]

The dynamics of homogenous polychlorinated biphenyl (PCB54)/toluene mixtures have been investigated by means of broadband dielectric spectroscopy. The mixture presents dynamical heterogeneity, alike miscible polymer blends, which is manifested with the presence of two relaxational processes. The relatively slow one has been attributed to the motion of PCB54 in the mixture, whereas the relatively fast one was related to the toluene dynamics in the mixture. These results have been interpreted according to the self-concentration concept, first introduced to describe the dynamics of miscible polymer blends, which relies on the limited size of the cooperative length scale in glass-forming liquids. The self-concentration concept has been incorporated in the Adam–Gibbs theory of the glass transition relating the characteristic relaxation time and the length scale for structural relaxation to the configurational entropy. This allowed the determination of the cooperative length scale of PCB54 and toluene both in mixture and alone through the fitting of a single parameter, namely, that connecting the cooperative length scale to the configurational entropy. This length scale resulted to be in the range of 1–2 nm for all systems. Finally, the out-of-equilibrium dynamics of toluene induced by the selective freezing in of PCB54 has been examined and its relation to the Johari–Goldstein relaxation of pure toluene was critically discussed. [ABSTRACT FROM AUTHOR]

Broadband dielectric spectroscopy and differential scanning calorimetry measurements have been performed to study the molecular dynamics poly (vinyl pyrrolidone) and its water solutions in a wide range of concentrations (0 wt ≤wc≤55 wt %) and in the temperature range from 140 to 500 K. The dry material was completely characterized showing the presence of two relaxations in the supercooled state. The slow one has the characteristics of a Johari-Goldstein-type relaxation. On the other hand, the low temperature water dynamics in the mixtures evidences a prominent loss peak due to the reorientation of water molecules inside the polymer matrix for all the hydration levels. We show that the relaxation times are almost water concentration (wc) independent from wc>20 wt % suggesting that this dynamical process is dominated by water-water interactions. In addition, the temperature dependence of the water relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior during cooling throughout the glass transition range, which has been interpreted as due to the constrains imposed by the rigid polymer matrix on the water molecules dynamics. [ABSTRACT FROM AUTHOR]

We present a computational investigation on the slow dynamics of a mixture of large and small soft spheres. By varying the size disparity at a moderate fixed composition different relaxation scenarios are observed for the small particles. For small disparity density-density correlators exhibit moderate stretching. Only small quantitative differences are observed between dynamic features for large and small particles. On the contrary, large disparity induces a clear time scale separation between the large and small particles. Density-density correlators for the small particles become extremely stretched and display logarithmic relaxation by properly tuning the temperature or the wave vector. Self-correlators decay much faster than density-density correlators. For very large size disparity, a complete separation between self- and collective dynamics is observed for the small particles. Self-correlators decay to zero at temperatures where density-density correlations are frozen. The dynamic picture obtained by varying the size disparity resembles features associated with mode coupling transition lines of the types B and A at, respectively, small and very large size disparities. Both lines might merge, at some intermediate disparity, at a higher-order point, to which logarithmic relaxation would be associated. This picture resembles predictions of a recent mode coupling theory for fluids confined in matrices with interconnected voids [V. Krakoviack, Phys. Rev. Lett. 94, 065703 (2005)]. [ABSTRACT FROM AUTHOR]

We have investigated the dynamics of poly(vinly acetate) using broadband dielectric spectroscopy (DS) covering over 14 decades in frequency up to 20 GHz and high-resolution quasielastic neutron-scattering (QENS) technique. The dielectric results have been interpreted in terms of the phenomenological Kohlrausch–Williams–Watts [G. Williams and D. C. Watts, Trans. Faraday Soc. 66, 80 (1970); F. Alvarez, A. Alegría, and J. Colmenero, Phys. Rev. B 47, 125 (1993)] description. Because of the wide frequency range covered by DS, it provides a precise determination of dynamics over a wide temperature range and it revealed a crossover in polymer dynamics at 387 K through different dielectric parameters, viz., characteristic times, asymmetric shape parameter, and dielectric strength. Moreover, shape parameter is found to be higher in comparison to other typical polymeric systems. The characteristic relaxation times observed by QENS displayed an anomalous dependence of momentum transfer, indicating the possible existence of heterogeneities in the system even at the high temperatures. In addition, spin-lattice relaxation times, T1, were found to be decoupled from dielectric characteristic times. Based on these results, a model was proposed to account for heterogeneities where we consider coexistence of different regions with standard polymeric behavior but with different characteristic times, leading to a distribution of relaxation times. The model is found to account for the anomalous behavior and an inherent shape parameter is found to account for the shape of α relaxation. This model is also found to predict the T variation of T1 characteristic time scales at all temperatures. The origin of the heterogeneous domains is believed to lie in the microstructure of polymer chains. [ABSTRACT FROM AUTHOR]

Controlling the spatial distribution of catalytic sites in metallo-folded single-chain nanoparticles (SCNPs) is a first step toward the rational design of improved catalytic soft nanoobjects. Here an unexplored pathway is reported for tuning the internal structure of metallo-folded SCNPs. Unlike the conventional SCNP synthesis in good solvent (protocol I), the proposed new route (protocol II) is based on the use of amphiphilic random copolymers and transfer, after SCNP formation, from selective to good (nonselective) solvent conditions. The size and morphology of the SCNPs obtained by the two protocols, and the corresponding spatial distribution of the catalytic sites, have been determined by combining results from size exclusion chromatography with triple detection, small-angle X-ray scattering and molecular dynamics (MD) simulations. Remarkably, the use of these protocols allows the tuning of the internal structure of the metallo-folded SCNPs, as supported by MD simulations results. While the conventional protocol I yields a homogeneous distribution of the catalytic sites in the SCNP, these are arranged into clusters in the case of protocol II. [ABSTRACT FROM AUTHOR]

The recent (from 2010 onward) contributions of quasielastic neutron scattering techniques (time of flight, backscattering, and neutron spin echo) to the characterization and understanding of dynamical processes in soft materials based on polymers are analyzed. The selectivity provided by the combination of neutron scattering and isotopic-in particular, proton/deuterium-labeling allows the isolated study of chosen molecular groups and/or components in a system. This opportunity, together with the capability of neutrons to provide space/time resolution at the relevant length scales in soft matter, allows unraveling the nature of the large variety of molecular motions taking place in materials of increasing complexity. As a result, recent relevant works can be found dealing with dynamical process which associated characteristic lengths and nature are as diverse as, for example, phenyl motions in a glassy linear homopolymer like polystyrene and the chain dynamics of a polymer adsorbed on dispersed clay platelets. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013 [ABSTRACT FROM AUTHOR]

Abstract: Among the different dynamical processes that take place in polymers, methyl group rotation is perhaps the simplest one, since all the relevant interactions on the methyl group can be condensed in an effective mean-field one-dimensional potential. Recent experimental neutron scattering results have stimulated a new revival of the interest on methyl group dynamics in glasses and polymer systems. The existence of quantum rotational tunnelling of methyl groups in polymers was expected for a long time but only very recently (1998), these processes have been directly observed by high-resolution neutron scattering techniques. This paper revises and summarizes the work done on this topic over last 10 years by means of neutron scattering. It is shown that the results obtained in many chemically and structurally different polymers can be consistently described in the whole temperature range—from the quantum tunnelling limit to the classical hopping regime—as well as in the librational spectrum, in terms of the Rotation Rate Distribution Model (RRDM), which was first proposed in 1994. This model introduces a distribution of potential barriers for methyl group rotation, which is associated to the disorder present in any structural glass. The molecular and structural origin of the barrier distribution in polymers is discussed on the basis of a huge collection of investigations reported in the literature, including recent fully atomistic molecular dynamics simulations. [Copyright &y& Elsevier]

We present fully atomistic molecular dynamics simulations on polyisobutylene (PIB) in a wide temperature range above the glass transition. The cell is validated by direct comparison of magnitudes computed from the simulation and measured by neutron scattering on protonated samples reported in previous works. Once the reliability of the simulation is assured, we exploit the information in the atomic trajectories to characterize the dynamics of the different kinds of atoms in PIB. All of them, including main-chain carbons, show a crossover from Gaussian to non-Gaussian behavior in the intermediate scattering function that can be described in terms of the anomalous jump diffusion model. The full characterization of the methyl-group hydrogen motions requires accounting for rotational motions. We show that the usually assumed statistically independence of rotational and segmental motions fails in this case. We apply the rotational rate distribution model to correlation functions calculated for the relative positions of methyl-group hydrogens with respect to the carbon atom at which they are linked. The contributions to the vibrational density of states are also discussed. We conclude that methyl-group rotations are coupled with the main-chain dynamics. Finally, we revise in the light of the simulations the hypothesis and conclusions made in previously reported neutron scattering investigations on protonated samples trying to address the origin of the dielectric β -process. [ABSTRACT FROM AUTHOR]

Combining neutron scattering and fully atomistic molecular dynamics simulations allows unraveling structural and dynamical features of polymer melts at different length scales, mainly in the intermolecular and monomeric range. Here we present the methodology developed by us and the results of its application during the last years in a variety of polymers. This methodology is based on two pillars: (i) both techniques cover approximately the same length and time scales and (ii) the classical van Hove formalism allows easily calculating the magnitudes measured by neutron scattering from the simulated atomic trajectories. By direct comparison with experimental results, the simulated cell is validated. Thereafter, the information of the simulations can be exploited, calculating magnitudes that are experimentally inaccessible or extending the parameters range beyond the experimental capabilities. We show how detailed microscopic insight on structural features and dynamical processes of various kinds has been gained in polymeric systems with different degrees of complexity, and how intriguing questions as the collective behavior at intermediate length scales have been faced. [ABSTRACT FROM AUTHOR]

In a recent paper [V. N. Novikov, K. S. Schweizer, and A. P. Sokolov, J. Chem. Phys. 138, 164508 (2013)] a simple analytical ansatz has been proposed to describe the momentum transfer (Q) dependence of the collective relaxation time of glass-forming systems in a wide Q-range covering the region of the first maximum of the static structure factor S(Q) and the so-called intermediate length scale regime. In this work we have generalized this model in order to deal with glass-forming systems where the atomic diffusive processes are sub-linear in nature. This is for instance the case of glass-forming polymers. The generalized expression considers a sub-linear jump-diffusion model and reduces to the expression previously proposed for normal diffusion. The generalized ansatz has been applied to the experimental results of the Q- and temperature-dependence of polyisobutylene (PIB), which were previously published. To reduce the number of free parameters of the model to only one, we have taken advantage of atomistic molecular dynamics simulations of PIB properly validated by neutron scattering results. The model perfectly describes the experimental results capturing both, Q- and temperature-dependences. Moreover, the model also reproduces the experimental Q-dependence of the effective activation energy of the collective relaxation time in the temperature range of observation. This non-trivial result gives additional support to the way the crossover between two different relaxation mechanisms of density fluctuations is formulated in the model. [ABSTRACT FROM AUTHOR]

We present an investigation by combining small-angle neutron scattering (SANS) and coarse-grained molecular dynamics (MD) simulations on the conformational properties of single-chain nanoparticles (SCNPs) in crowded macromolecular solutions. By using linear chains as crowders, SANS shows a crossover from almost unperturbed SCNP conformations in dilute conditions toward a continuous collapse of the macromolecule with increasing crowding. This collapse starts when the total concentration of the solution reaches the value of the overlap concentration of the pure SCNP solutions. MD simulations suggest the generalizability of these experimental findings and extend them to the case when the SCNPs themselves are used as crowders—a situation which in real systems leads to unavoidable formation of aggregates, as shown here by SANS and DLS. Exploiting the simulations, we have calculated the contact probability and the distance between monomers as functions of the contour distance between them; the results suggest that crumpled globular conformations are generally adopted by SCNPs in crowded macromolecular solutions. In the case of linear crowders, the SCNPs show, at fixed monomer concentration, a nonmonotonic dependence of their collapse on the length of the crowders. [ABSTRACT FROM AUTHOR]

The incorporation of high-molecular-weight poly(ethylene oxide) (PEO) in the nanopores of resorcinol-formaldehyde resin nanoparticles (RNPs) leads to the suppression of polymer crystallization, changes in the chain conformation, and a noticeable slowdown of the two dielectric relaxations that reflect the segmental and local PEO dynamics. Both relaxations are significantly slower than those corresponding to bulk PEO. These results are independent of the pore characteristics of the different RNP materials. The segmental relaxation shows a crossover at ca. 220 K in its temperature dependence from non-Arrhenius to Arrhenius-like behavior on cooling. These results suggest the occurrence of limited cooperativity at low temperatures due to the enhancement of long-living hydrogen bonding between PEO and RNP pore walls. [ABSTRACT FROM AUTHOR]

We investigate the collective dynamics of comblike polymers displaying nanosegregation in side groups and main-chain rich domains, where side groups and backbones are dynamically equivalent. Neutron spin-echo experiments on poly(alkylene oxide)s with increasing side-group length have been focused on the decay of inter- and intradomain correlations and are complemented with molecular dynamics simulations on a coarse-grained model. In general, a good qualitative agreement between simulation and experiment is found. The collective times at inter- and intradomain levels converge with decreasing temperature, implying a common glass transition, and separate at high temperatures, revealing an increasing role of connectivity. The features of collective relaxations--regarding stretching, characteristic time, and temperature dependence--are independent of branching. The standard stretching for side-group motions and the closeness of collective and self-motions show that the anomalies exhibited by dynamically asymmetric comblike macromolecules disappear. [ABSTRACT FROM AUTHOR]

We report a new strategy for therapid, efficient synthesis ofsingle-chain polymer nanoparticles (SCNPs) having a nearly globularmorphology in solution, by employing photoactivated radical-mediatedthiol–yne coupling (TYC) reaction as the driving force forchain folding/collapse. Confirmation of SCNP formation was carriedout by means of a combination of complementary experimental techniques.Size exclusion chromatography (SEC), small-angle X-ray scattering(SAXS), and dynamic light scattering (DLS) measurements revealed aconsiderable degree of compaction of the resulting SCNPs. This findingwas confirmed by molecular dynamics (MD) simulations. The analysisof the scattering form factors provided by SAXS revealed a scalingexponent ν ≈ 0.37 for the dependence of the SCNP sizeon its molecular weight. This value is close to that expected forglobular objects, ν = 1/3, and much smaller than the usual observation(ν ≈ 0.5) for SCNPs synthesized with most of the state-of-the-arttechniques, which instead show sparse morphologies. Insight into thephysical origin of this fundamental difference with standard SCNPswas obtained from molecular dynamics simulations. Namely, intrachainbonding mediated by relatively long cross-linkers combined with theuse of bifunctional groups in the SCNP precursor largely increasesthe probability of forming long-range loops which are efficient forglobal chain compaction. [ABSTRACT FROM AUTHOR]

Bymeans of molecular dynamics simulations, we investigate thestructural relaxation in disordered random copolymers and lamellarphases of gradient copolymers, containing chemical species of verydifferent mobilities. Two models have been investigated: a genericbead–spring system and a MARTINI coarse-grained model of apolyester resin. The lamellar phase of the gradient copolymer is formedby domains rich in one species and poor in the other one, which areseparated by broad interfaces. Unlike in strongly segregated blockcopolymers, there is a finite probability of finding monomers of agiven species at any position within the domains rich in the otherspecies. A direct consequence of this feature is that monomers canprobe very different chemical environments, and because of the strongdynamic asymmetry between the two components, their relaxation arecharacterized by an extreme dynamic heterogeneity. This is confirmedby an analysis of dynamic correlators as a function of the distanceto the interface. In the case of random copolymers long-range orderingis not possible, and local microsegregation results in a much weakerdynamic heterogeneity. The former features are consistent with theexperimental observation of narrow glass transitions in random copolymersbut extremely broad ones in lamellar gradient copolymers. [ABSTRACT FROM AUTHOR]

Binary water mixtures usually display a water relaxation (process II) which can be studied by broadband dielectric spectroscopy (BDS) at subzero temperatures. In a large collection of binary water mixtures, a slight increase of the relaxation strength is observed for low water concentration, whereas a faster increase is seen above a critical concentration. The assumption behind this result is that at high water concentration self-associations of water molecules are present in the solutions. In this work, we have studied poly(propylene glycol) water solutions by means of broadband dielectric spectroscopy and Fourier transform infrared spectroscopy (FTIR) using the attenuated total reflectance method (ATR) in the temperature range of 120–300 K. By combining both techniques, we found a critical water concentration xw= 0.20 above which the relaxation strength of the water relaxation (process II) increases more rapidly than at low water concentration indicating the self-association of water molecules. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the physical aging below the glass transition temperature, namely the slow evolution occurring in non-equilibrium glasses, of poly(methyl methacrylate)/silica (PMMA/silica) nanocomposites. To do so we have followed the time evolution of the enthalpy and that of the dielectric strength of PMMA β process during isothermal annealing. The results indicate that physical aging is generally accelerated in all nanocomposites in comparison to pure PMMA, despite the lack of effect of the nanoparticles on PMMA molecular dynamics. Furthermore, the shorter is the interparticle distance, and hence the higher is the area/volume of silica in PMMA, the more pronounced is such acceleration. The acceleration of the physical aging together with the invariance of PMMA dynamics in the nanocomposites in comparison to pure PMMA poses serious questions on the idea that the molecular mobility is the only responsible parameter for the rapidity of physical aging. Thus, an interpretation based on the free volume holes diffusion towards the external surface, in this case represented by the polymer/silica interface, is provided. [ABSTRACT FROM AUTHOR]

Abstract: In this communication, we present first results on the dielectric response of various end-functionalized polystyrenes (PS). The functional polymers have been prepared by using a combination of anionic polymerization techniques and hydrosilation chemistry that gives well-defined polymers where the functional group can be placed with flexibility and high precision. The data show that for rather large functional groups, the overall polymer matrix properties are altered giving rise to a decrease in the glass transition temperature. The trend can be rationalized in terms of free volume effects caused by the bulky functional groups. However, for cyano-functionalized PS, i.e. PS with a CN group at the chain end, the inclusion of the group does not affect the matrix properties. We show that by taking advantage of the strong dipole moment of the CN group, we can obtain a strong dielectric signal that can be used to selectively study the specific dynamics where the group is located. In other words, by appropriately attaching cyano groups at different part of the chains, these can be used as dielectric probes that allow specific contributions to the alpha relaxation. [Copyright &y& Elsevier]