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Broadband dielectric spectroscopy, rheology, and nuclear magnetic resonance spectroscopy are employed to study molecular dynamics in a nanoparticle organic hybrid material (NOHMs) system comprising...

We use broad-band dielectric spectroscopy (BDS) and small-angle X-ray scattering (SAXS) to investigate the impact of architectural asymmetry in the miktoarm star copolymer on the dielectric polymer...

Polymers exhibit deviations from their bulk physical properties in the vicinity of solid interfaces due to changes in configurations, entanglements, and relaxation dynamics at the interfacial regio...

During the course of studying silicon-containing diblock copolymers, it was discovered that poly(3,5-di(trimethylsilyl)styrene)-block-poly(3,4-methylenedioxystyrene) (PDTMSS-b-PMDOS) showed very un...

Molecular dynamics in ultrathin layers is investigated using nanostructured electrodes to perform broadband dielectric spectroscopy measurements, and by atomistic molecular dynamics simulations. Using poly(vinyl acetate) as the model system and taking advantage of access to the distribution of relaxation times in an extended temperature range above the glass transition temperature

Polyurea elastomers are utilized for a myriad of applications ranging from coatings and foams to dielectric materials for capacitors and actuators. However, current synthetic methods for polyureas rely on highly reactive isocyanates, solvents, and catalysts, which collectively pose serious safety considerations. This report details the synthesis and characterization of melt processible, poly(tetramethylene oxide) (PTMO)-based segmented polyurea elastomers utilizing an isocyanate-, solvent-, and catalyst-free approach. Dynamic mechanical analysis and differential scanning calorimetry suggested microphase separation between the hard and soft segments. Tensile analysis revealed high strain at break for all segmented copolymers between 340 and 770%, and tunable modulus between 0.76 and 29.5 MPa. Dielectric spectroscopy revealed that the composition containing 20 wt% hard segment offered the highest permittivity at 10.6 (1 kHz, 300 K) of the segmented copolymers, indicating potential as a dielectric elastomer. National Science Foundation, Division of Materials Research, Polymers ProgramNational Science Foundation (NSF) [DMR-1905597] The authors would like to acknowledge the Huntsman Corporation for generously providing the Jeffamine (R) utilized in this study. The authors also thank Christina Orsino in the Robert B. Moore group for carrying out variable temperature FTIR experiments. Emmanuel U. Mapesa and Joshua Sangoro acknowledge financial support from the National Science Foundation, Division of Materials Research, Polymers Program, through DMR-1905597.

Ion dynamics in a series of imidazolium-based triblock copolymers (triblock co-PILs) are investigated using broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) and compared to their homopolymer counterparts (homo-PILs). Two calorimetric glass transition temperatures (Tg) are observed corresponding to the charged poly(ionic liquid) (PIL) blocks and noncharged polystyrene (PS) blocks. Varying the counterion from Br– to NTf2– decreases the Tg of the charged block by over 50 °C, thereby increasing the room-temperature ionic dc conductivity by over 6 orders of magnitude. Interestingly, for a given anion, varying the volume fraction of the charged block, from ∼0.5 to ∼0.8, has very minimal effect on the dc ionic conductivity, indicating that the choice of counterion is the key factor influencing charge transport in these systems.

The impact of molecular structure on ion dynamics and morphology in ammonium- and imidazolium-based glassy polymerized ionic liquids (polyILs) is investigated using broadband dielectric spectroscopy (BDS), wide-angle X-ray scattering (WAXS), and classical molecular dynamics (MD) simulations. It is shown that ammonium-based polyILs exhibit higher dc ionic conductivity at their respective glass transition temperatures (Tg) compared to imidazolium systems. In addition, the length of the alkyl spacer has a more drastic impact on ionic conductivity at comparable time scales of segmental dynamics for ammonium than imidazolium polyILs. Agreement between the characteristic ion diffusion lengths estimated from the dielectric data and the ion-to-ion correlation lengths from the WAXS and all-atom MD simulations is observed. A recently proposed approach is employed to determine ionic mobility in a broad frequency range spanning 5 orders of magnitude below the Tg of polyILs studied, providing access to a regime of dif...

Broadband dielectric spectroscopy (BDS) is used to probe the chain and segmental dynamics of A2B2 and AB2 miktoarm star copolymers based on polystyrene (PS, A block) and polyisoprene (PI, B block) that display lamellar morphologies as determined using small-angle X-ray scattering (SAXS). While no changes in the distribution of PI segmental relaxation times are observed with variation of the molecular architecture, an unexpected increase in the normalized PI chain relaxation intensity is realized for AB2 miktoarm star copolymers as well as a change in the distribution of chain relaxation rates as compared to A2B2 and AB diblock copolymer systems. This result is attributed to asymmetry in the molecular architecture near the junction point, which affects the osmotic constraint of the tethered PI chains within the interfacial region of the lamellae. The results highlight the importance of macromolecular design on fundamental chain dynamics in phase-separated thermoplastic elastomers.

Broadband dielectric spectroscopy is employed to probe dynamics in low molecular weight poly(cis-1,4-isoprene) (PI) confined in unidirectional silica nanopores with mean pore diameter, D, of 6.5 nm. Three molecular weights of PI (3, 7 and 10 kg/mol) were chosen such that the ratio of D to the polymer radius of gyration, Rg, is varied from 3.4, 2.3 to 1.9, respectively. It is found that the mean segmental relaxation rate remains bulk-like but an additional process arises at lower frequencies with increasing molecular weight (decreasing D/Rg. In contrast, the mean relaxation rates of the end-to-end dipole vector corresponding to chain dynamics are found to be slightly slower than that in the bulk for the systems approaching D/Rg ∼ 2, but faster than the bulk for the polymer with the largest molecular weight. The analysis of the spectral shapes of the chain relaxation suggests that the resulting dynamics of the 10kg/mol PI confined at length-scales close to that of the Rg are due to non-ideal chain conformations under confinement decreasing the chain relaxation times. The understanding of these faster chain dynamics of polymers under extreme geometrical confinement is necessary in designing nanodevices that contain polymeric materials within substrates approaching the molecular scale.

Continuous progress in energy storage and conversion technologies necessitates novel experimental approaches that can provide fundamental insights regarding the impact of reduced dimensions on the functional properties of materials. Here, we demonstrate a nondestructive experimental approach to probe nanoscale ion dynamics in ultrathin films of polymerized 1-vinyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide over a broad frequency range spanning over 6 orders of magnitude by broadband dielectric spectroscopy. The approach involves using an electrode configuration with lithographically patterned silica nanostructures, which allow for an air gap between the confined ion conductor and one of the electrodes. We observe that the characteristic rate of ion dynamics significantly slows down with decreasing film thicknesses above the calorimetric glass transition of the bulk polymer. However, the mean rates remain bulk-like at lower temperatures. These results highlight the increasing influence of the polymer/substrate interactions with decreasing film thickness on ion dynamics.

The effect of 2D confinement on the dynamics of the normal mode (chain mobility) and segmental relaxation in poly(propylene glycol) (PPG) has been studied with the use of broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). It is shown that both processes become faster with increasing degree of confinement. Interestingly, the crossover from VFT to the Arrhenius-like behavior of chain and segmental dynamics, observed in the examined system, is strictly related to the vitrification of the adsorbed polymers. We also report that the mean relaxation times of the normal, τNM, and segmental modes, τα, depend on the thermal history of confined PPG and can be significantly modified using different thermal treatments. It is demonstrated that annealing of the samples below the crossover temperature, Tc, leads to a systematic shift of the segmental relaxation and normal mode toward lower frequencies, resulting in an increase in the glass transition temperature of the spatially restrict...

The influence of swelling on the molecular dynamics of poly(2-vinylpyridine) (P2VP) brushes is measured by broadband dielectric spectroscopy (BDS) in a broad temperature (350–420 K) and spectral (0.1 Hz–1 MHz) range with nanostructured, highly conductive silicon electrodes, separated by 35 nm high insulating silica spacers. A “grafting-to” method is applied to prepare P2VP brushes with a grafting density σ = 0.12 nm–2 and a film thickness d = 7.3 nm as measured by ellipsometry. Swelling of the P2VP brushes is realized with tetrahydrofuran (THF) vapor using a flow cell. In the dry state, the segmental dynamics of the P2VP brushes coincides with the dynamic glass transition of the bulk system while in the swollen state it becomes faster by up to 1–2 decades due to the plasticizing effect of THF.

Dielectric spectroscopy and differential scanning calorimetry (DSC) were applied to study the molecular dynamics and thermal properties of a low-molecular-weight glass-forming liquid, salol (phenyl...

The long lasting highly controversial discussion on glassy dynamics and the glass transition temperature of polymers in nanometric layers and films seems to be converging based on a multitude of recent experiments: (i) Linear response spectroscopies (e.g. alternating current calorimetry (ACC), broadband dielectric spectroscopy (BDS)) measuring in the liquid state do not observe shifts of the mean relaxation rate in dependence on the 1-dimensional confinement down to layer thicknesses of 8 nm (ACC) and 4 nm (BDS); (ii) Frequency dependent photobleaching techniques working essentially below the bulk glass transition temperature find as well a glassy dynamics in thin (freestanding or supported) films of polystyrene (PS) which is primarily bulk-like and does not depend on the thickness of the layer as demonstrated down to 14 nm. (iii) Evidence exists, that close to the bulk T g , a layer of strongly enhanced mobility—and having a temperature dependent thickness—is formed on a free polymer surface. This enormously complicates the interpretation of ellipsometric and fluorescent based experiments, and might be the reason for the widely diverging results. In summary the dynamic glass transition does not show a confinement effect above and below the glass transition temperature in pronounced contrast to the non-equilibrium dynamics in the glassy state, which depend strongly on a variety of parameters including the layer thickness. This decoupling is well described by the free volume hole diffusion (FVHD) model as developed by Cangialosi et al.

Differential scanning calorimetry (DSC), broadband dielectric (BDS), and Fourier transform infrared (FTIR) spectroscopies as well as theoretical computations were applied to investigate inter- and intramolecular interactions between the active pharmaceutical ingredient (API) indomethacin (IMC) and a series of acetylated saccharides. It was found that solid dispersions formed by modified glucose and IMC are the least physically stable of all studied samples. Dielectric measurements showed that this finding is related to neither the global nor local mobility, as the two were fairly similar. On the other hand, combined studies with the use of density functional theory (DFT) and FTIR methods indicated that, in contrast to acetylated glucose, modified disaccharides (maltose and sucrose) interact strongly with indomethacin. As a result, internal H-bonds between IMC molecules become very weak or are eventually broken. Simultaneously, strong H-bonds between the matrix and API are formed. This observation was used to explain the physical stability of the investigated solid dispersions. Finally, solubility measurements revealed that the solubility of IMC can be enhanced by the use of acetylated carbohydrates, although the observed improvement is marginal due to strong interactions.

Polymer Dynamics While free surfaces should allow polymer chains to move faster than in the bulk, the presence of a substrate might slow down the motion if there is an attraction between the two. Tress et al. (p. 1371 ; see the Perspective by Russell ) used dielectric spectroscopy to study “polymer islands” deposited on a substrate from dilute solution, where some islands contained just a few or only one polymer chain. The confinement of the polymer chain to small-surface geometries had virtually no influence on the dynamics of the polymers, aside from the segments in direct contact with the substrate.

Broadband dielectric spectroscopy (BDS) is employed to study the molecular dynamics of hydroxyl- (OH-) and amino- (NH2-) terminated polypropylene glycols (PPGs) of varying molecular weight. Besides the dynamic glass transition (α relaxation), a normal mode process and a secondary γ-relaxation is observed, the latter being assigned to librational fluctuations of the polar COC moiety. Additionally a further process is found and proven in two independent experiments (annealing PPGs at high temperatures and physical aging) to originate from residual H2O impurities in the sample and is therefore not related to the dynamics of PPG. It is occasionally, albeit incorrectly, discussed in the literature as Johari−Goldsten (JG) relaxation. Furthermore, aging in the systems under study is analyzed and found to follow the Nagel−Leheny equation.

Broadband dielectric spectroscopy (BDS), spectroscopic vis-ellipsometry (SE), X-ray reflectometry (XRR), and alternating current (ACC) as well as differential scanning calorimetry (DSC) are combined to study glassy dynamics and the glass transition in nanometric thin (≥5 nm) layers of polystyrene (PS) having widely varying molecular weights (27 500−8 090 000 g/mol). For the dielectric measurements two sample geometries are employed, the common technique using evaporated electrodes and a recently developed approach taking advantage of nanostructures as spacers. All applied methods deliver the concurring result that deviations from glassy dynamics and from the glass transition of the bulk do not exceed margins of ±3 K independent of the layer thickness and the molecular weight of the polymer under study. Our findings are discussed in the context of the highly controversial literature and prove that an appropriate sample preparation is of paramount importance.

Broadband dielectric spectroscopy and positron annihilation lifetime spectroscopy are employed to study the molecular dynamics and effective free volume of 2-ethyl-1-hexanol (2E1H) in the bulk state and when confined in unidirectional nanopores with average diameters of 4, 6, and 8 nm. Enhanced α-relaxations with decreasing pore diameters closer to the calorimetric glass-transition temperature (T(g)) correlate with the increase in the effective free volume. This indicates that the glassy dynamics of 2D constrained 2E1H is mainly controlled by density variation.

The role of anions in charge transport and localized dipolar relaxations in tributyloctylphosphonium ionic liquids is investigated by broadband dielectric spectroscopy and rheology. The dielectric spectra are quantitatively described by a combination of the random barrier model which accounts for ion transport and empirical Havriliak-Negami functions to characterize dipolar relaxations. Two secondary relaxations are observed at temperatures below the calorimetric glass transition temperature, where the primary structural relaxation is essentially frozen at the relevant experimental time scales. The faster process has an anion independent activation energy of 30 kJ/mol and is attributed to libration motion of the phosphonium cation. The slower relaxation is similar to a process previously assigned to a Johari-Goldstein relaxation in imidazolium-based ionic liquids; however, the activation energy is significantly higher in the phosphonium systems. For the charge transport dominated regime, it is observed that variation of the anion results in differences in the dc ionic conductivity and characteristic charge transport rates by ∼2.5 decades. Upon scaling by the calorimetric glass transition temperature, both transport quantities are observed to coincide. From these results, a picture of glass transition assisted hopping emerges as the underlying microscopic mechanism of ion conduction, in agreement with recent results obtained for other classes of aprotic ionic liquids.

Fourier Transform Infrared Spectroscopy and Broadband Dielectric Spectroscopy are combined to trace kinetics of mutarotation in L-fucose. After quenching molten samples down to temperatures between T = 313 K and 328 K, the concentrations of two anomeric species change according to a simple exponential time dependence, as seen by an increase in absorbance of specific IR-vibrations. In contrast, the dielectric spectra reveal a slowing down of the structural (α-) relaxation process according to a stretched exponential time dependence (stretching exponent of 1.5 ± 0.2). The rates of change in the IR absorption for α- and β-fucopyranose are (at T = 313 K) nearly one decade faster than that of the intermolecular interactions as measured by the shift of the α-relaxation. This reflects the fact that the α-relaxation monitors the equilibration at a mesoscopic length scale, resulting from fluctuations in the anomeric composition.

Broadband Dielectric Spectroscopy (BDS)—in combination with a nanostructured electrode arrangement—is employed to study thin layers of poly(cis-1,4-isoprene) (PI). PI is further probed in the 2D confining space of Anodic Aluminum Oxide (AAO) nanopores. Being a type A polymer, PI presents an unrivaled opportunity to investigate two distinct relaxation modes taking place at two different length scales: the segmental motion (corresponding to the dynamic glass transition) which involves structures of about one nanometer in size, and the so-called normal mode which represents the global dynamics of the chain. We report that while the structural relaxation shows no dependence on either layer thickness or molecular weight, the normal mode—actually the fluctuation of the end-to-end vector of the unperturbed chain—is dramatically influenced by confinement: (i) its relaxation strength is layer-thickness-dependent; (ii) for PI having a molecular weight \(M_\mathrm{w}\) comparable to \(M_\mathrm{c}\) (i.e. the critical molecular weight below which Rouse dynamics dominate), the mean spectral position does not shift with layer thickness, (iii) in contrast, when \(M_\mathrm{w} > M_\mathrm{c}\), the relaxation strength and rate of the normal mode respond to the confinement; (iv) it is demonstrated—for the first time—that the concentration of the mother solution from which the thin layers are spin-cast has an impact on the chain dynamics; and (v) the extent by which the normal mode is affected depends on the dimensionality of confinement. Overall, these results show that while the chain dynamics are altered in a manifold of ways (due, for instance, to interactions with the confining surface), the structural relaxation retains most of its bulk-like nature. The latter observation is due to the fact that the length scale underlying the dynamic glass transition is less than a nanometer.

While structure and conformation of condensed (semi-) isolated low molecular weight and polymeric molecules is well explored, nothing is known about their molecular dynamics as measured in a broad spectral range (1 Hz–1 MHz) and at widely varying temperature (200–400 K). This is achieved in this study by employing broadband dielectric spectroscopy (BDS) with nanostructured electrode arrangements, enabling electrode separations down to 35 nm. The approach offers, additionally, to determine in parallel the structure and conformation of the identical sample by scanning force microscopy (AFM). For the case of high molecular weight poly (2-vinylpyridine), it is demonstrated that even condensed (semi-) isolated polymer chains perform glassy dynamics, well comparable to the bulk state with the characteristic Vogel-Fulcher-Tammann temperature dependence; only \({\sim }10{-}15\) % of the mobile segments are weakly slowed down. The results are in full accord with the understanding that the length scale of the dynamic glass transition is \({\sim }2-3\) polymer segments. Complementarily, AFM and infrared (IR) spectroscopy are employed to examine the chain conformation and to analyze the interfacial interactions, respectively. The former yields a condensed coil conformation, the average volume of which agrees within limits of \(\pm \)10 % with that calculated for a single chain assuming bulk density. A fraction of \({\sim }30\) % of the segments is found to be in a 0.4-nm-layer being directly in contact with the solid substrate. From IR investigations it is deduced that only half of them are influenced by the substrate presumably due to Coulombic interactions.

The (dynamic) glass transition of polymers in nanometer thin layers is both a prevailing but as well a highly controversial topic. In the current review the literature for the most studied case of polystyrene (as freestanding films or as deposited and suspended layers) will be discussed. Based on this, the extraordinary impact of sample preparation is immediately evident and outlined in detail. Recent results are presented on nanometric thin (≥5 nm) layers of polystyrene (PS) having widely varying molecular weights and polymethylmethacrylate (PMMA) deposited on different substrates. For the dielectric measurements two sample geometries are employed: the conventional technique using evaporated electrodes and a recently developed approach taking advantage of silica nanostructures as spacers. All applied methods deliver the concurring result that deviations from glassy dynamics and from the glass transition of the bulk never exceed margins of ±3 K independent of the layer thickness, the molecular weight of the polymer under study and the underlying substrate. Novel experiments are described on thin layers of polyisoprene, a type A polymer, having relaxation processes on two different length scales, the segmental and the normal mode. A further exciting perspective is the measurement of the dynamics of isolated polymer coils, for which first results will be presented.

International audience; Spectroscopic vis-ellipsometry and broadband dielectric spectroscopy (BDS) are combined to study the glassy dynamics of thin (>= 10 nm) layers of atactic poly(methyl methacrylate) (PM MA) annealed and measured under identical conditions. In order to unravel a possible effect of the underlying substrate, the interfacial interactions are systematically modified ranging from strong attractive interactions for covalently bonded PM MA brushes with high grafting density and for native silicon oxide (Si/SiO(x)) to weak and strong repulsive interactions as realized by Au-coated and HMDS-treated Si/SiO(x) surfaces, respectively. Down to the thinnest analyzed PM MA layers and independently from the applied substrate, both methods deliver-within the experimental accuracy (+/- 1 K for BDS and +/- 2 K for ellipsometry)-a coinciding result. The glassy dynamics arc not altered due to the one-dimensional confinement in these thin polymer layers. The results are discussed with respect to the highly controversial literature and the impact of the preparative conditions.

The question of whether the glass transition temperature in thin polymer films depends on the film thickness or not has given rise to heated debate for almost two decades now. One of the most puzzling findings is the seemingly universal thickness independence of the dielectric α-relaxation observed for supported films. It is puzzling not only in view of the fact that other techniques or other geometries sometimes showed a significant shift of as a function of film thickness, but more so, because computer simulations for all types of polymer film models revealed changes in the structure and dynamics close to a hard surface or a free surface. Our results suggest to explain this apparent contradiction by the fact that only within 1–2 nm from the wall the density changes are sufficiently large to alter the dynamics. Additionally, the wall desorption kinetics, which introduces a new energy scale (at least for simple van der Waals attraction), is enslaved to the α-relaxation at low temperatures.

Segmental and chain dynamics in nanometric (7–400 nm) layers of poly (cis-1,4-isoprene) (PI) are analyzed by Broadband Dielectric Spectroscopy (BDS) and temperature-modulated AC calorimetry. While for the segmental mode, taking place at the length scale of 2–3 polymer segments and corresponding to the dynamic glass transition, no dependence on the layer thickness and molecular weight is found, the normal mode, reflecting fluctuations of the end-to-end vector of the chain, shows pronounced effects: (i) it strongly varies in its relaxation strength with the layer thickness; (ii) for polymers having a molecular weight Mw comparable to M*, the critical molecular weight marking the onset of reptation dynamics, the mean spectral position does not change with the thickness, (iii) in contrast, polymers with Mw > M* are found to be severely influenced in their relaxation strength and the mean spectral position of the normal mode relaxation, and (iv) it is proven that the concentration of the polymer solution out of which the layers are prepared by spincoating has a hitherto unrecognized impact on the chain dynamics in (one-dimensional) nanometric confinement. These results prove that the dynamic glass transition in thin layers of PI is not influenced by nanometric confinement, while the chain dynamics are altered in a manifold of ways due to interactions with the surface of the underlying substrate.

Fourier Transform Infra Red (FTIR) and Broadband Dielectric Spectroscopy (BDS) are combined to study both the intra- and inter-molecular dynamics of two isomers of glass forming fucose, far below and above the calorimetric glass transition temperature, T(g). It is shown that the various IR-active vibrations exhibit in their spectral position and oscillator strength quite different temperature dependencies, proving their specific signature in the course of densification and glass formation. The coupling between intra- and inter molecular dynamics is exemplified by distinct changes in IR active ring vibrations far above the calorimetric glass transition temperature at about 1.16T(g), where the dynamic glass transition (α relaxation) and the secondary β relaxation merge. For physically annealed samples it is demonstrated that upon aging the different moieties show characteristic features as well, proving the necessity of atomistic descriptions beyond coarse-grained models.