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Using a simple mathematical formalism, we show that temperature dependent nonexponential relaxation found in glass-forming liquids and amorphous polymers, often resulting in a decrease in the stretching exponent when decreasing temperature, can be suitably described assuming the combination of an intrinsic stretched response and the existence of temperature independent heterogeneities. The effect of the latter is incorporated by assuming a Gaussian distribution of Vogel temperatures. Comparison with experimental data of a large number of glass formers showed that this approach is able to quasiquantitatively describes the temperature dependence of the stretching exponent using the width of the distribution as the single fitting parameter. According to this approach, the rapidity of the decrease in the stretching exponent with decreasing temperature depends not only on the magnitude of the standard deviation of Vogel temperatures but also on the value of the intrinsic stretching exponent and on the fragility of the glass former. The latter result is able to rationalize, at least partially, the empirical correlation between the fragility and the stretching exponent at Tg. [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]

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]

The folding/collapse of linear single chains to single-chain nanoparticles (SCNPs) is a field of intense activity. Current synthetic methods allow obtaining from sparse SCNP morphologies in solution - resembling those observed in intrinsically disordered proteins - to compact SCNP conformations, more akin to those displayed by globular enzymes. A useful characterization technique in the field of SCNPs is 'conventional' size-exclusion chromatography (SEC) with differential refractive index (DRI) detection, providing rapid and convincing evidence of SCNP formation through comparison of the SEC traces of the precursor and the SCNPs. Upon SCNP formation, a noticeable shift of the SEC curve toward longer retention times (i.e., lower hydrodynamic sizes) is observed. However, quantitative information embedded in the SEC curve about the actual 'compaction degree' of the SCNPs has not yet been extracted to distinguish between sparse and globular SCNPs. Here, a useful methodology for quantifying the compaction degree of SCNPs by SEC with conventional DRI detector is introduced allowing: i) construction of theoretical SEC curves corresponding to the collapse of a linear precursor having a log-normal molecular weight distribution to either sparse or globular SCNPs and ii) analysis of experimental SEC curves to estimate the actual morphology of synthetic SCNPs in solution. [ABSTRACT FROM AUTHOR]

We review the recent activity aiming to clarify glassy dynamics in nanostructured polymer glasses, in particular thin films, nanocomposites and nanospheres. Special emphasis is devoted to recent results on the out-of-equilibrium dynamics, that is, the way the system leaves equilibrium when cooling, marking the thermal glass transition, or recovers it once in the glassy state – the so-called physical aging. Apart from those systems exhibiting strong interactions at the interface, we show that a huge number of studies probing glassy dynamics in nanostructured glasses finds negative deviations from bulk T g and accelerated physical aging. Analysis of the dependence of the rate of spontaneous fluctuations – namely the linear dynamics – on nanostructuring indicates that there exists a significant component exhibiting bulk-like dynamics. This is the case even in the most extreme case of nanostructuring, that is, semi-isolated polymer chains and freestanding thin film with thickness ∼10 nm. In the latter case this is found at temperatures around the bulk glass transition temperature ( T g ), that is, in the range where deviations of the out-of-equilibrium dynamics are normally observed. All together these results indicate that the linear dynamics alone cannot provide an exhaustive description of the out-of-equilibrium dynamics in nanostructured systems. In this case, purely geometric factors must be included. We discuss recent approaches aiming to capture the phenomenology of glassy dynamics in nanostructured glasses. Special attention is dedicated to the free volume hole diffusion (FVHD) model. [ABSTRACT FROM AUTHOR]

Diffusion in semicrystalline polymers is a complex phenomenon because of the existence of specific interactions (nonpolar or polar), dynamic heterogeneities, and crystalline phases. The diffusion of water in two semicrystalline polyamides (PA6,6 and PA6,10) was investigated in order to determine the diffusion mechanisms and the influence of polymer relaxations on this process. Liquid water diffusion follows a Fickian mechanism in PA6,10 and a non-Fickian or anomalous mechanism in PA6,6. Through a quasi-equilibrium experiment in dynamic vapor sorption, it is shown that this difference results from the dependence of the diffusion coefficients on water concentration. Moreover, the influence of the polymer relaxations was assessed by broadband dielectric spectroscopy. The dielectric characteristic relaxation times of the α relaxation, associated with the glass transition, and of the β relaxation, related to more local dynamics, have been measured. A simple comparison with the time scale of diffusion suggests that diffusion and polyamide α relaxation should not be directly correlated. However, diffusion is correlated to the secondary β relaxation, which encompasses the local chain dynamics of hydrogen-bonded amide groups in the presence of water. A mechanism of diffusion based on the trapping of water molecules between neighboring sorption sites (amide groups) is proposed in these strongly interacting polymers. It is suggested that diffusion is limited by the relaxation time of hydrogen bonds between water molecules and amide groups and the change in conformation of these amide groups present in polyamides. [ABSTRACT FROM AUTHOR]

• ZnO nanorods were fabricated on the surface of the ITO coated PET. • Properties were examined with UV–Vis spectroscopy, TGA, DSC, DMA and DRS. • Changes in transmittance, thermal-mechanical and dielectric properties were observed. ITO coated PET films are promising substrates for flexible, wearable sensors. The sensor functionality can be added by depositing a semiconductor layer on the ITO coating. This study investigates if such functionalization affects the basic thermomechanical properties of the substrate material. ZnO nanorods were grown directly on the ITO/PET surface from pre-made ZnO nanoparticle seeds. The samples were characterized by UV–Vis spectroscopy, scanning electron microscopy, and powder X-ray diffractometry. Their thermomechanical properties such as thermal degradation profile, glass transition temperature, as well as melting and crystallization transitions were studied by thermogravimetric analysis, modulated differential scanning calorimetry, dynamic mechanical analysis, and dielectric relaxation spectroscopy. Our findings indicate that even though the thermomechanical properties are clearly affected by the presence of the ZnO nanorod coating to some extent, the sensor substrate material remains usable because the original characteristics of the ITO/PET substrate do not deteriorate significantly. [Display omitted] [ABSTRACT FROM AUTHOR]

Dynamic heterogeneities in ethylene–vinylacetate (EVA)random copolymers were studied using broadband dielectric spectroscopy(BDS) over a broad frequency and temperature range. BDS data for EVAcopolymers show relatively broad spectra extending over several frequencydecades, which make interpretation of the data complicated. Thus,microscopic dielectric characterizations of the samples were doneusing single pass electrostatic force microscopy (SP-EFM) images.From these experiments two distinct dielectric contributions werefound in the semicrystalline samples. Dielectric spectra of EVA copolymerswere fitted using a model based on the SP-EFM images. According tothis analysis, the broad dielectric responses of semicrystalline EVAcopolymers were constituted from two dielectrically active phases,referred to as the constrained and nonconstrained phases, exhibitingdifferent segmental relaxations. In the constrained phase, vinyl acetate(VA) motions were greatly restricted by crystalline microstructures,whereas in the nonconstrained phase, VA monomers were assumed to becompletely free from crystalline effect. The glass transition temperaturederived from the dynamics of the VA segments in the constrained phasewas found to correspond well with that determined from DSC representingthe overall amorphous phase in the more crystallized state. Moreover,crystallinity of the sample obtained using DSC was found to followa similar variation with temperature to that of the dielectric relaxationstrength of the constrained phase of the copolymers. However, at theearlier stage of crystallization, where crystallinity increases rapidly,the dielectric relaxation remains insensitive to it. [ABSTRACT FROM AUTHOR]

In this study, polymer–graphiteoxide (GO) interactionsare demonstrated to induce thermal instability in both the polymerand GO phase in conditions of maximum polymer uptake, in which thepolymer chains are either intercalated into the GO interlayer or adsorbedon the GO sheets. We investigate the thermal stability of poly(ethyleneoxide) (PEO) intercalated in GO (PEO/GO) in detail, using a combinationof techniques including X-ray diffraction (XRD), thermogravimetry(TGA) and TGA–mass spectroscopy (TGA–MS) in dynamic(nonisothermal) and static (isothermal) modes. Our results show thatintercalated PEO decomposes at 160 °C lower than neat PEO, andthat GO decomposes at 50 °C lower than pristine GO, due to asynergistic instability between GO and intercalated PEO upon heating.Other hydrophilic polymerssuch as poly(vinyl methyl ether)(PVME), poly(vinyl alcohol) (PAA), poly(vinylpyrrolidone) (PVP) andpoly(acrylic acid) (PAA)forming polymer-adsorbed GO structuresare also observed to decompose at noticeably lower temperatures thaneither pristine GO or their own corresponding neat polymers. UnlikePEO/GO intercalation compounds, the excess of PEO phase in PEO–GOcomposites decomposes at temperatures close to that of neat PEO, whichdemonstrates that those polymer chains far from the adsorbing GO surfaceare not significantly affected by the presence of GO sheets. Furthermore,isothermal TGA data for PEO/GO intercalation compounds with PEO chainsfrom 5 to 2135 monomeric units are well described by an autocatalyticmodel similar to that we found for pristine GO in a previous study,which suggests that the overall decomposition of PEO/GO is dominatedby the reduction and exfoliation of GO. However, when PEO is adsorbedon graphene substrate, which is thermally stable at the studied temperatures,the kinetic mechanisms follow a first-order reaction similar to neatPEO, but the decomposition occurs with a considerably lower activationenergy. [ABSTRACT FROM AUTHOR]

The dielectric relaxation of cis-1,4 Polyisoprene [PI] is sensitive not only to the local and segmental dynamics but also to the larger scale chain (end-to-end) fluctuations. We have performed a careful dielectric investigation on linear PI with various molecular weights in the range of 1 to 320 kg/mol. The broadband dielectric spectra of all samples were measured isothermally at the same temperature to avoid utilizing shift factors. For the low and medium molecular weight range, the comparisons were performed at 250 K to access both the segmental relaxation and normal mode peaks inside the available frequency window (1 mHz–10 MHz). In this way, we were able to observe simultaneously the effect of molecular mass on the segmental dynamics—related with the glass transition process—and on the end-to-end relaxation time of PI and thus decouple the direct effect of molecular weight on the normal mode from that due to the effect on the monomeric friction coefficient. The latter effect is significant for low molecular weight ( Mw < 33 kg/mol), i.e., in the range where the crossover from Rouse dynamics to entanglement limited flow occurs. Despite the conductivity contribution at low frequency, careful experiments allowed us to access to the normal mode signal for molecular weights as high as Mw = 320 kg/mol, i.e., into the range of high molecular weights where the pure reptation behavior could be valid, at least for the description of the slowest chain modes. The comparison between the dielectric relaxations of PI samples with medium and high molecular weight was performed at 320 K. We found two crossovers in the molecular weight dependence of the longest relaxation time, the first around a molecular weight of 6.5 ± 0.5 kg/mol corresponding to the end of the Rouse regime and the second around 75 ± 10 kg/mol. Above this latter value, we find a power law compatible with exponent 3 as predicted by the De Gennes theory. [ABSTRACT FROM AUTHOR]

Abstract: A detailed dielectric investigation of the secondary relaxation on a series of bisphenol-A polycarbonate samples with different treatments that influence the local polymer structure is presented. Dielectric spectroscopy measurements were performed in the frequency range 10−2–106 Hz. The structural changes induced by different thermo-mechanical treatments, as well as by polymer crystallization, lead to a reduction of the dielectric losses that is not uniform over the whole frequency range but more pronounced in the high frequency side of the γ-relaxation peak. These effects have been rationalized in terms of the great coupling between the motions of the phenylene rings and those of the dipolar entity of the repeating unit (namely the carbonate group). Furthermore, it is shown that the same approach allows rationalizing the marked differences between the dielectric and mechanical secondary relaxations. [Copyright &y& Elsevier]

Abstract: The α-relaxation time, both at equilibrium and out-of-equilibrium, has been investigated by means of dielectric relaxation in a series of fragile glass formers including polymers and several small molecules. The dielectric experiments combine both, thermally stimulated depolarization (accessible time range 10–106 s) and standard broad-band dielectric relaxation measurements (frequency range 10−2–107 Hz). The analysis of both set of data was based on a stretched exponential response. The experimental temperature dependence so derived for the relaxation times out-of-equilibrium has been found to give account not only of global polarization experiments but also for those thermally stimulated depolarization experiments involving partial polarization. Moreover, the so obtained activation energies in the glassy range have been quantitatively compared with the predictions of the Adam–Gibbs equation. For the small molecule glass formers a good agreement was found, but systematic discrepancies are apparent for polymers. By studying the influence of physical aging on this comparison, we found that after significant annealing also the polymers tends to agree with the predictions of the Adam–Gibbs equation. All these results would support the validity of the Adam–Gibbs equation for describing the temperature dependence of the time scale of the α-relaxation also out-of-equilibrium. However, they would also imply a noticeable qualitative difference between polymers and small molecule systems concerning the configurational entropy behavior. Whereas a significant temperature variation of the configurational entropy would occur for non-annealed polymers some 10K below the glass transition, this temperature dependence would be much weaker for small molecules and well annealed polymers. [Copyright &y& Elsevier]

We investigate the kinetics of enthalpy recovery in stacked glassy polystyrene (PS) films with thickness from 30 to 95 nm over a wide temperature range below the glass transition temperature (Tg). We show that the time evolution toward equilibrium exhibits two mechanisms of recovery, in ways analogous to bulk PS. The fast mechanism, allowing partial enthalpy recovery toward equilibrium, displays Arrhenius temperature dependence with low activation energy, whereas the slow mechanism follows pronounced super-Arrhenius temperature dependence. In comparison to bulk PS, the time scales of the two mechanisms of recovery are considerably shorter and decreasing with the film thickness. Scaling of the equilibration times at various thicknesses indicates that the fast mechanism of recovery is compatible with the free volume holes diffusion model. Conversely, the slow mechanism of recovery appears to be accelerated with decreasing thickness more than predicted by the model and, therefore, its description requires additional ingredients. The implications, from both a fundamental and technological viewpoint, of the ability of thin polymer films to densify in relatively short time scales are discussed. [ABSTRACT FROM AUTHOR]

We report a microscopic observation of the time-dependent dynamic tube dilation process on isofrictional bidisperse melts. By applying neutron spin echo (NSE) and dielectric techniques on blends of long polyisoprene (PI) chains with short PI additives with different topology, we access the dynamics of the tube dilation process on a molecular scale. The time-dependent tube dilation is directly revealed by NSE as an additional time dependence of the dynamic structure factor in the local reptation regime. We identify the characteristic time of tube dilation as the terminal time of the additive. [ABSTRACT FROM AUTHOR]

This work addresses the preparation of new polymer dielectrics working under the dipolar glass polymer (DGP) concept. Herein, we report the synthesis and characterization of four hetero-functionalized polyitaconates bearing sulfones as high dipole moment entities and norbornane or adamantane structures responsible for increasing the glass transition temperature (T g) and thus the range of temperatures where they can work without degrading or showing high energy dissipation. As a result, all obtained polymers exhibited dielectric constants (ε r ') between 5.1 and 6.2 while presenting low loss factors (Tan(δ) < 0.01), reaching the status of high-dielectric polymers with a low dissipative behavior. In addition, it was demonstrated that including bulky structures into their polymer backbone allows for an increase of up to 80 °C in their working temperature ranges, expanding the temperature range where they behave as DGPs in an outstanding manner. A complete structural, thermal and dielectric characterization was carried out in terms of infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), thermogravimetry (TGA), differential scanning calorimetry (DSC) and broad dielectric spectroscopy (BDS). Overall, these materials seem to fulfill the basic requirements to be considered good candidates for dielectric applications such as energy storage, confirming the versatility of polyitaconate-based materials. [Display omitted] • Four new hetero-functionalized polyitaconates were synthesized. • Polyitaconates exhibiting high dielectric constant and low dissipative behavior. • High dipole moment sulfonyl groups allow reaching εr' values as high as 6.2. • Bulky/rigid groups notably expand the working temperature range of these materials. [ABSTRACT FROM AUTHOR]

This work presents the molecular dynamics of both fully amorphous and semicrystalline poly(butylene 2,5-furanoate) (PBF). Broadband dielectric spectroscopy experiments were combined with temperature modulated differential scanning calorimetry measurements. The results showed that the subglass molecular dynamics is characterized by the existence of two dielectric relaxation processes, being the faster one associated to the glycolic subunit, whereas the slower relaxation was assigned to the link in between the ester group and the furan ring. Crystallization affected differently the contribution of these two components. Additionally, crystallization had a stronger effect on the α relaxation process, related to the segmental dynamics of the amorphous phase. In the semicrystalline state, the PBF amorphous phase was described as being composed by different fractions, including a completely rigid one, with distinctly slower mobilities and reduced contributions to the dielectric relaxation, compared to the fully amorphous polymer. [ABSTRACT FROM AUTHOR]

Cyclic and linear, isoregic and aregic, and isotactic and atactic poly(glycidyl phenyl ether) (PGPE) with molecular weights up to Mw = 5.5 kg/mol are synthesized by ring-opening polymerization of glycidyl phenyl ether. Initiation with tetrabutylammonium fluoride leads to isoregic linear polymers with ~95% regular linkages, and initiation with B(C6F5)3 and B(C6F5)3/water leads to aregic cyclic and linear polymers, respectively, with ~50% regular linkages as quantified by 13C NMR. Local, segmental, and chain dynamics in PGPE is investigated by broadband dielectric spectroscopy (10-2-106 Hz). The β-relaxation for linear PGPE is separated into two contributions arising from the motions of side groups and end groups with activation energies of 35.4 and 23.8 kJ/mol, respectively. The β-relaxation process for cyclic PGPE shows the same activation energy as that shown by the side-group contribution in linear PGPE, indicating that topology does not play a key role on the side-group local dynamics. Moreover, cyclic PGPE samples show higher calorimetric and dynamic glass transition temperatures as well as lower dynamic fragility compared to linear chains. Unexpectedly from topological considerations, cyclic PGPE shows low frequency dielectric contributions that can be attributed to short wavelength internal ring motions and that are detectable by dielectric relaxation due to the aregic nature of the rings. [ABSTRACT FROM AUTHOR]

Wehave investigated the dynamical processesα-relaxation,local motions of the side-groups, and methyl group rotationsinpoly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) in the dry stateand in mixtures (at 70 wt% polymer concentration) with tetrahydrofuran(THF) and water, to address the question as to how these polymer motionsare affected by plasticizers interacting in different ways with thepolymer. Differential scanning calorimetry, dielectric spectroscopy,and neutron scattering techniques on labeled samples (with deuteratedsolvents to isolate the signal of the polymer component) have beencombined. The α-relaxation is drastically affected, with similarshifts of the glass-transition temperature for both solvents. Effectsof compositional heterogeneities and reduction of the fragility arealso observed. On the contrary, methyl-group dynamics are unaffectedby the presence of solvent. Regarding side-group local motions (β-relaxation),two kinds of componentsa slow and a fast onecouldbe identified in the dry state. On the basis of the spatial informationprovided by neutron scattering, a model for the geometry of the motionsinvolved in the fast component has been proposed. Adding solvent,this process would remain essentially unaltered, but the populationinvolved in the slower one would be reduced. With THF as solvent,this reduction would be complete, but with water it would be onlypartial. This could be attributed to rather heterogeneous distributionof water molecules in the polymer likely associated with the presenceof water clusters. Such a scenario would also explain the much morepronounced broadening of the glass-transition region observed forthe polymer in the aqueous mixture with respect to that induced byTHF. [ABSTRACT FROM AUTHOR]

Highlights: [•] Supported thin PS films exhibit T g reduction. [•] T g reduction is not accompanied by a change in the segmental dynamics. [•] T g reduction and unchanged segmental dynamics are rationalized by the holes diffusion model. [•] The holes diffusion model captures the phenomenology of glassy dynamics in films. [ABSTRACT FROM AUTHOR]

We investigated the kinetics of enthalpy recovery of several glass-forming polymers at temperatures significantly below the glass transition temperature (Tg) and for aging times up to one year. We find a double-step recovery at relatively low aging temperatures for the longest investigated aging times. The enthalpy recovered after the two-step decay approximately equals that expected by extrapolation from the melt. The two-step enthalpy recovery indicates the presence of two time scales for glass equilibration. The equilibration time of the first recovery step exhibits relatively weak temperature dependence, whereas that of the second step possesses pronounced temperature dependence, compatible with the Vogel- Fulcher-Tammann behavior. These results, while leaving open the question of the divergence of the relaxation time and that of a thermodynamic singularity at a finite temperature, reveal a complex scenario of glassy dynamics. [ABSTRACT FROM AUTHOR]

Macroscopic volume as well as capacitive dilatometry (CD) measurements have been performed on polystyrene/silica nanocomposites during the course of physical aging below the glass transition temperature ( Tg). Our results show that the macroscopic volume recovery during physical aging is not affected by the presence of nanofillers, whereas the CD measurements, delivering also information on the polymer matrix density, show acceleration of the recovery with increasing the silica content. Hence, the main outcome of the present work is that the evolution of macroscopic and matrix densities are markedly different in polymer nanocomposites. We interpret these results invoking an equilibration mechanism based on volume holes diffusion. According to this model, excess free volume migration at the polymer/filler interface only modifies the matrix density, thereby explaining the faster recovery detected by CD measurements in comparison to the macroscopic volume one. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys., 2013 [ABSTRACT FROM AUTHOR]

The aging-time dependence of the segmental relaxation time of polyvinyl acetate) (PVAc) in the glassy state is investigated in the bulk polymer and its nanocomposites with silica (SiO2). These systems present identical segmental dynamics, when this is probed in the equilibrium supercooled liquid by broadband dielectric spectroscopy. An acceleration of the physical aging process of PVAc with SiO2 was detected by monitoring the enthalpy recovery through differential scanning calorimetry. The segmental relaxation time during physical aging, followed by means of BDS, has been shown to increase more rapidly the higher the SiO2 concentration in PVAc is. Thermally stimulated depolarization current experiments show that this is the case over the whole probed glassy state. This means that nanocomposites displaying a relatively slow segmental mobility evolve toward equilibrium more rapidly than the bulk. Furthermore, despite the faster increase in the relaxation time with aging time, so-called self-retardation, the nanocomposites and their bulk counterpart reach the same values of equilibrium relaxation time. These findings not only confirm the assumption of identical equilibrium dynamics even in the aging regime for all nanocomposites and bulk polymers, proposed in previous works, but also highlight the fact that the physical aging rate is not determined solely by the polymer segmental dynamics, the amount of interface being an additional relevant parameter. [ABSTRACT FROM AUTHOR]

Abstract: The physical aging process, namely the spontaneous evolution of the thermodynamic state occurring in glasses, has been monitored in poly(methyl methacrylate) (PMMA)/silica and polystyrene (PS)/silica nanocomposites following the time evolution of the enthalpy by means of differential scanning calorimetry (DSC). The effect on physical aging of the content of silica particles has been investigated in detail varying it over a wide range. Our results indicate accelerated physical aging in the nanocomposites in comparison to the corresponding pure polymer. Furthermore the acceleration is generally more pronounced in nanocomposites presenting a high silica content. This result cannot be attributed to a difference in molecular mobility of the polymers in the nanocomposites in comparison to pure PMMA and PS, since broadband dielectric spectroscopy (BDS) indicates no effect of silica nanoparticles on the polymer segmental dynamics. Moreover, calorimetric measurements reveal a reduction of the heat capacity jump at Tg for the nanocomposites, as well as lower experimentally recoverable enthalpy values. This may result from the faster non-isothermal evolution of the glass state when cooling down the samples from their liquid state. To account for these experimental results the acceleration of physical aging has been rationalized in the framework of the diffusion of free volume holes model as previously proposed. This is able to adequately catch the dependence of the physical aging rate on silica content, determining the area of nanoparticles to the volume of polymer, that is the relevant variable within the diffusion model. [Copyright &y& Elsevier]

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: Treatment of graphite oxide (GO) with organic solvents via sorption from either liquid or gas phase, and subsequent desorption, induces profound changes in the layered GO structure: loss of stacking order, retention of trace amounts of solvents and decreasing decomposition temperature. This study presents new evidences of the effect of organic solvents on the thermal reduction of GO by means of thermogravimetric analysis, X-ray diffraction and X-ray photoelectron spectroscopy. The results reveal a relative higher decrease of the oxygen amounts in solvent-treated GO as compared to untreated GO and the restacking inhibition of the thermally reduced GO sheets upon slow heating. The kinetic experiments evidence changes occurring in the reduction mechanisms of the solvent-treated GO, which support the close link between GO structure and thermal properties. [Copyright &y& Elsevier]

The effect of thermal treatment on the phase separation process of the components of a polymer blend was investigated using electrostatic force microscopy (EFM). EFM technique is an advance on conventional atomic force microscopy, which enables us to measure locally the dielectric properties of the samples under investigation providing compositional information. In this work, we studied the phase separation process of the polymer blend thin films made of polystyrene and poly(vinyl acetate) (PS/PVAc) (75/25 weight fraction). The samples were subjected to different thermal treatments. It was found that at low annealing temperature, PVAc forms many small islands within PS matrix. As the annealing temperature increases, the number of PVAc islands decreases with an increase in the size of the islands. These islands take spherical-like shape when annealed at a temperature well above the glass transition temperatures of both the component polymers. Despite these morphological/topographical changes, EFM images evidence that there is no interdiffusion which was further confirmed by quantitatively measuring the value of the dielectric permittivity across the interphase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1332-1338, 2011 [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]

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]

Abstract: To gain new insight into the molecular motions responsible for the excellent mechanical properties of bisphenol-A polycarbonate (BPA-PC), dielectric relaxation experiments have been performed on the secondary relaxation of BPA-PC uniaxially stretched below the glass transition temperature T g. It has been found that the dielectric γ-relaxation of the cold-drawn samples is reduced significantly, mainly around the loss peak range, being the effect more apparent at higher temperatures. Moreover, this loss reduction is directly related with the level of molecular orientation as characterized by birefringence measurements. The results obtained are rationalized under the consideration of the γ-relaxation of BPA-PC as produced by two distinct molecular motions which would be differently affected by the uniaxial stretching, the fastest one being more hindered by the chain orientation. The analysis of the dielectric loss reduction on the uniaxially stretched samples shows that the fast relaxation component of the γ-relaxation of BPA-PC can be well characterized by a distribution of a Debye activated process with average activation energy of 29.5kJ/mol and standard deviation of about 5.8kJ/mol. These would be characteristic of a single molecular motion involving the carbonate group. [Copyright &y& Elsevier]

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]

The behavior of water dynamics confined in hydrated calcium silicate hydrate (C-S-H) gel has been investigated using broadband dielectric spectroscopy (BDS; 10-2-106 Hz) in the low-temperature range (110-250 K). Different water contents in C-S-H gel were explored (from 6 to 15 wt%) where water remains amorphous for all the studied temperatures. Three relaxation processes were found by BDS (labeled 1 to 3 from the fastest to the slowest), two of them reported here for the first time. We show that a strong change in the dielectric relaxation of C-S-H gel occurs with increasing hydration, especially at a hydration level in which a monolayer of water around the basic units of cement materials is predicted by different structural models. Below this hydration level both processes 2 and 3 have an Arrhenius temperature dependence. However, at higher hydration level, a non-Arrhenius behavior temperature dependence for process 3 over the whole accessible temperature range and, a crossover from low-temperature Arrhenius to high-temperature non-Arrhenius behavior for process 2 are observed. Characteristics of these processes will be discussed in this work. [ABSTRACT FROM AUTHOR]

We have studied the influence of plasticization on the microscopic dynamics of a glass-forming polymer. For this purpose we studied polyvinylchloride (PVC) with and without the commercially used plasticizer dioctylphthalate (DOP). We used dielectric spectroscopy and inelastic neutron scattering employing the neutron spin echo (NSE) technique. For both kinds of spectra the α relaxation could be consistently described by a model involving a distribution of individual relaxations of the Kohlrausch type. In contrast to earlier studies it turned out that an asymmetric distribution is necessary to fit the data at the lower temperatures investigated here. The shape parameters of the distribution (width, skewness) for PVC and PVC/DOP turned out to coincide when the characteristic relaxation times were the same. This means that the plasticizer only induces a remapping of the temperature dependence of the α relaxation. Comparison of NSE spectra S(Q,t)/S(Q) at different scattering vectors Q gave the result that the slowing down at the structure factor peak Qmax is surprisingly small for PVC while it is in the normal range for PVC/DOP. [ABSTRACT FROM AUTHOR]

Cross-linked poly(tetrahydrofuran) (PTHF) networks are useful as organic solvent/oil sorbents with high swelling capacity and mechanically robust solid polymer electrolyte membranes for lithium-ion batteries, as well as solid-state calcium ion batteries. Recently, the control of intra-vs intermolecular interactions has attracted significant interest to modulate the properties of dynamic networks involving non-covalent and dynamic covalent bonds. However, to the best of our knowledge, a comparison of the segmental dynamics in polymeric materials prepared exactly from the same precursor polymer through intra-vs intermolecular covalent bonds is still lacking. In this work, we report the dissimilar dynamic properties of PTHF-based materials prepared through intra-vs intermolecular click chemistry (i.e., films of PTHF-based single-chain nanoparticles (SCNPs) vs cross-linked networks). Particularly we observed remarkable similarities in the segmental relaxation time and glass transition temperature value but evident differences in the dynamic heterogeneities. These differences are attributed to the peculiar morphology of the intra-chain collapsed polymer chains (i.e., SCNPs). [Display omitted] • Segmental dynamics of poly(tetrahydrofuran) films with intra-chain or inter-chain cross-links. • Similar values of segmental relaxation time and glass transition temperature. • Evident dynamic heterogeneity, which is larger after intra-chain cross-linking. • The crumpled morphology of the intra-chain collapsed polymer chains is responsible of the differences. [ABSTRACT FROM AUTHOR]

The supramolecular self-assembly of monocarboxydecyl-terminated dimethylsiloxane oligomer is investigated and characterized by combining different experimental methods. Thermodynamic properties were accessed by temperature-modulated differential scanning calorimetry, and two subambient phase transitions were identified. Fourier transform infrared spectroscopy gave access to the main molecular entities involved in these phase transitions. Diffraction experiments using small-angle X-ray scattering and wide-angle X-ray scattering evidenced the development of two distinct well-ordered sub-10 nm structures, each associated with one of the thermal transitions, without any indication of long-range atomic order. All the results point to the self-assembly of the oligomeric dimers below 230 K, giving rise to supramolecular structures involving new H-bonding interactions. A further structure is developed below 205 K by the assembly of the alkane part of the molecules in an arrangement of hexagonally packed cylinders. Dielectric relaxation experiments evidenced both the pronounced effect of the lowest temperature transition on the H-bond network fluctuations and the very effective segregation of dimethylsiloxane at lower temperatures. Moreover, we found that the structured liquid presents a rubber-like mechanical behavior in the temperature range 150-200 K, where dimethylsiloxane phase remains amorphous, and at lower temperatures a glassy nanostructured material is found. [ABSTRACT FROM AUTHOR]

Poly(butylene succinate-ran-butylene azelate) random copolyesters (PBS-ran PBAz) were studied using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). These copolymers are characterized by being isodimorphic and by displaying a eutectic behavior. Depending on their composition, the PBS-rich phase, the PBAz-rich phase, or both phases can crystallize. The complex development of the amorphous phase, especially at compositions around the eutectic point was demonstrated by BDS results. The comonomer fraction included in the amorphous phase was quantified from the relaxation strength of the dielectric β relaxations, and thus the crystalline fraction can be calculated. The good agreement between the values determined from BDS and the crystallinity obtained from DSC experiments demonstrates that the degree of comonomer exclusion in these random copolymers during crystallization is far larger than comonomer inclusion. However, the glass transition temperatures determined from the dielectric α relaxation behavior are lower in most of the copolymers than those measured by DSC, whereas they are about the same in the homopolymers. This result is probably caused by a significant amount of amorphous phase in the copolymers that is not influenced by crystallinity. Furthermore, the crystallization behavior of these copolymers at different cooling protocols showed that the α relaxation depends not only on the crystalline fraction but also on the details of the crystallization process in these complex multiphasic copolymers. [ABSTRACT FROM AUTHOR]

Polymersconfined at the nanometer scale often exhibit a distinct structuraland dynamical response compared to their bulk counterparts. In thisstudy, we observe that the confinement of poly(ethylene oxide) (PEO)in the nanopores of carbon nanoparticles (CNPs) leads to the suppressionof crystallization and to a significant reduction of the ΔCpat the glass transition.We ask whether these changes are dominated by interfacial interactions(van der Waals type) or by geometrical constraints. For pore diametersbelow 2 nm (micropores following IUPAC nomenclature), we find thatthe larger the pore surface, the higher the amount of PEO intercalatedin the micropores and, consequently, the larger the reduction of theΔCpat the glasstransition (up to 50%). For pore diameters in the range 2–50nm (mesopores), larger pore surfaces lead to a higher amount of PEOadsorbed on the mesopore walls and the smaller the reduction of theΔCpat the glasstransition. Under these conditions of spatial confinement at the nanoscale,PEO chains cannot arrange themselves into large crystalline domains,as evidenced by a negligible degree of crystallization of at most1.8%. High-resolution inelastic neutron scattering data show thatthe PEO chains confined in the pores of CNP adopt a planar zigzagconformation, which is distinctly different from those characteristicof the 7/2 helical structure of the bulk crystal. [ABSTRACT FROM AUTHOR]

We present the preparation of nanostructured conducting PEDOT:PSS thin films by solvent vapor annealing (SVA), using the low boiling point solvent tetrahydrofuran (THF). An Atomic Force Microscopy (AFM) study allowed the observation of distinct nanostructure development as a function of solvent exposure time. Moreover, the nanostructures' physical properties were evaluated by nanomechanical, nanoelectrical, and nanoinfrared (nano-FTIR) measurements. In this way, we were able to differentiate the local response of the developed phases and to identify their chemical nature. The combination of these techniques allowed to demonstrate that exposure to THF is a facile method to effectively and selectively modify the surface nanostructure of PEDOT:PSS, and thereafter its final properties. Moreover, our nanoscale studies provided evidence about the molecular rearrangements that PEDOT:PSS suffers during nanostructure fabrication, a fundamental fact in order to expand the potential applications of this polymer in thermoelectric and optoelectronic devices. [Display omitted] • Solvent vapor treatments modify the nanostructure of solid state PEDOT:PSS. • A polymeric system with conducting and semiconducting alternated microdomains is obtained. • An AFM based study allows understanding the intrinsic physical properties at the nanoscale. • QNM, C-AFM, and SSNOM addressed the nature of the complex different phases. [ABSTRACT FROM AUTHOR]

AAO template is highly recommended to nanostructure polymers and to study polymer properties under confinement. The dynamic properties of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) under confinement using broadband dielectric spectroscopy are investigated in this work and the results compared to those of the bulk. Anodized aluminum oxide (AAO) membranes, having pore diameters from tens to hundreds of nanometers in size, were used to confine PS-b-P4VP. Moreover, the influence of gold nanoparticles (AuNPs) in the copolymer matrix was also studied. The morphology and structure of the bulk copolymer and the copolymer confined in the AAO templates were characterized by transmission electron microscopy, scanning electron microscopy and Small Angle X-Ray Scattering. For PS-b-P4VP in bulk, dielectric relaxation techniques allowed studying selectively the P4VP segmental dynamics within the diblock. At high temperature this copolymer presents a dominant peak (MWS relaxation), most likely originated by the relatively high conductivity combined with the presence of interfaces emerging in the nanostructured samples. Moreover, a pronounced β-relaxation is observed for the copolymer compared with that of pure P4VP. This is likely due to a non-negligible contribution from the α-relaxation of the PS component. The γ-relaxation is markedly different in the copolymer, which is evidenced by a distinct temperature dependence of the resulting relaxation times. When the copolymer is embedded in alumina nanopores with small pore diameters (25 and 35 nm) there are significant changes, where the tendency is going to a faster dynamics when the pore diameter decreases more likely related to the relevance of surface effects. The presence of the AuNPs in the system enhances this effect. These results are in agreement with segregated structures found in the block copolymer by TEM and SAXS. [ABSTRACT FROM AUTHOR]

By using dielectric spectroscopy,we investigate the chain dynamicsof nonentangled polyisoprene (PI) under soft confinement in lamellardomains of block copolymer melts with polydimethylsiloxane (PDMS).The data show a dramatic difference in the end-to-end vector dynamicsof the PI blocks as compared not only with that of the correspondinghomopolymer PI chains but also with respect to previous results forthe same blocks under soft confinement in cylindrical domains. Twocontributions to the dielectric normal mode relaxation are detected.The data are analyzed by means of a model including contributionsfrom internal chain modes (accounting for the fastest component) anda slow component attributed to the junction point dynamics. The contributionof the internal chain modes is modeled according to the analysis ofthe Rouse modes obtained from simulations of a generic bead–springmodel for strongly segregated symmetric diblock copolymers. In thisway it is shown that the internal chain modes of the blocks have timescales close to those expected from the homopolymer chain independentlyof the structural details. In contrast, the contribution attributedto the junction point dynamics depends critically on minor structuraldifferences. We interpret these findings as a result of the presenceof fast moving defects and/or grain boundaries in the lamellar structuresformed by these relatively short, nonentangled diblock copolymers. [ABSTRACT FROM AUTHOR]

Abstract: Filler-polymer interactions in reinforced rubbers are mainly governed by a thin layer of polymer around the filler particles. The dynamics of this layer has long been a matter of intense investigation. Some researchers suggest that this “interacting layer” is immobilized and possesses slower dynamics compared to the bulk, whereas some others consider it equally mobile. In this work, we address this problem by studying the macroscopic characteristics of reinforced rubber using Broadband Dielectric Spectroscopy (BDS) and by measuring its local dielectric behavior using a new AFM based approach: nano-dielectric spectroscopy. This new approach enables us to study the dielectric response of the interacting polymer layer and the bulk dynamics locally, with a spatial resolution of about 20–30 nm. The total amount of interacting polymer within the compounds was also obtained by analyzing the local mechanical properties. Both macro- and nano-measurements show no differences in the polymer local dynamics for the compounds here analyzed. We propose that, for our particular system, the interacting polymer layer shares the same local relaxation dynamics with the bulk rubber due to the presence of a flexible link between the polymer chains and the nanoparticles. [Copyright &y& Elsevier]

Abstract: We have investigated the motion of different molecular groups in glassy polycarbonate below its glass transition temperature. The characterization of methyl group and phenylene ring motions on the one hand, and carbonate group on the other, were carried out by means of neutron scattering and dielectric spectroscopy respectively. The comparison of the results from both the techniques indicate that different motions within polycarbonate may be strongly coupled. [Copyright &y& Elsevier]

The dynamics of n-propanol confined in regular MCM-41matrix with the pore size Dpore = 40 Å, under various matrix conditioning and sample confining conditions, using broadband dielectric spectroscopy (BDS), is reported. First, various drying procedures with the capacitor filling under air or N2 influence the BDS spectra of the empty MCM-41 and the confined n-PrOH/MCM-41 systems, but have a little effect on the maximum relaxation time of the main process. Finally, various filling factors of n-PrOHmedium in the optimally treated MCM-41system lead to unimodal or bimodal spectra interpreted in terms of the two distinct dynamic phases in the confined states. [ABSTRACT FROM AUTHOR]

Access to completely deuterated single‐chain nanoparticles (dSCNPs) has remained an unresolved issue. Herein, the first facile and efficient procedure to produce dSCNPs is reported, which comprises: i) the use of commercially available perdeuterated cyclic ether monomers as starting reagents, ii) a ring‐opening copolymerization process performed in bulk to produce a neat dSCNP precursor, iii) a standard azidation reaction to decorate this precursor with azide moieties, and iv) a facile intramolecular azide photodecomposition step carried out under UV irradiation at high dilution providing with highly valuable, completely deuterated soft nano‐objects from the precursor. dSCNPs are used to investigate by means of neutron‐scattering measurements the form factor (radius of gyration, scaling exponent) of polyethylene oxide (PEO) chains in nanocomposites with different amounts of dSCNPs. Moreover, to illustrate the possibilities offered by the synthetic route disclosed in this communication for potential applications, the significant reduction in viscosity observed in a pure melt of polyether‐based single‐chain nanoparticles when compared to a melt of the corresponding linear polymer chains is shown. [ABSTRACT FROM AUTHOR]