Segmental Dynamics Of Polymers Research Articles

Segmental dynamics of polystyrene near polymer-polymer interfaces.

This study investigated the segmental dynamics of polymers near polymer-polymer interfaces by probing the rotation of polymer-tethered fluorescent molecules using imaging rotational fluorescence correlation microscopy. Multilayered films were utilized to provide spatial selectivity relative to different polymer-polymer interfaces. In the experimental setup, for the overlayer polymer, polystyrene (PS) was employed and a 15 nm-thick probe-containing layer was placed ≈25nm apart from different underlayer polymers with glass transition temperatures (Tg) either lower or higher than that of PS. The underlayer of poly-n-butyl methacrylate had 72K lower Tg than that of PS, whereas polymethyl methacrylate and polysulfone had 22 and 81K higher Tg, respectively, than that of PS. Two key dynamic features of the glass transition, the non-Arrhenius temperature dependence and stretched relaxation, were examined to study the influence of soft and hard confinements on the segmental dynamics of the overlayer polymer near the polymer-polymer interfaces. Although complications exist in the probing location owing to the diffusion of the polymer-tethered probe during the annealing protocol to consolidate the multilayers, the results suggest that either the segmental dynamics of the polymer near the polymer-polymer interface do not change owing to the soft and hard confinements or the interfacial perturbation is very short ranged.

Microscopic mechanism of gas transport in mixed matrix membranes of coordination nanocages

The strong inter-phase attraction is required for the effective and stable integration of inorganics and polymers in mixed matrix membranes (MMMs), which would impact the structures and microscopic dynamics of the two phases and cause unexpected deviation of MMM performance from original design; however, the understanding of its structure-property relationship is still poor. Herein, 2 nm coordination nanocage (CNC) with well-defined, monodispersed structure is complexed with poly(4-vinyl pyridine-r-n-butyl acrylate) (P4VP-co-PnBA) via both coordination and hydrogen bonding to afford model systems. The surface interaction between the two phases is quantified by the thickness of absorbed layers of polymers on CNC surface as well as the segmental dynamics of polymers. In broadband dielectric spectroscopy (BDS) studies, accelerated chain dynamics can be observed in the nanocomposites with loadings of CNC, which is originated from the frustrated packing of polymers absorbed on CNC surface. The co-effect of interfacial layer thickness and chain dynamics leads to the non-monotonic dependence of the MMMs’ gas permeabilities on CNC concentrations. The temperature dependence of their gas permeability and chain dynamics are both explored and the two processes are fully decoupled at temperatures higher than 333 K due to the weakening of multiple supramolecular interaction. Our discoveries provide microscopic and quantitative understanding on structure-property relationship of MMMs, which benefits directly to the rational design of polymer structures for optimized gas separation performance.

Universal features of the manifestation of the relationship between local and segmental dynamics of polymers at terahertz frequencies in IR spectra

Low-frequency IR- spectra of glassy polymers were obtained and analyzed at temperatures from 90 to 400 K. Temperature changes in the IR- spectra in the range from 10 to 140 cm-1 show three universal features: low-temperature, corresponding to the torsional-vibrational motion of monomer units, high-temperature, due to the influence of primary relaxation (glass transition) and intermediate, caused by the activation of conformational mobility in chains at a temperature of β-transition. This versatility extends both to polymers with universal chain-to-chain interactions and to those with hydrogen bonds. Keywords: IR spectra, excess density of vibrational states, bosonic peak, molecular dynamics, α- and β-relaxations.

Utilization of Polymer-Tethered Probes for the Assessment of Segmental Polymer Dynamics near the Glass Transition

Utilization of Polymer-Tethered Probes for the Assessment of Segmental Polymer Dynamics near the Glass Transition

Exponential Water Uptake in Ionomer Membranes Results from Polymer Plasticization

Water content is the most influential factor in the performance of ion-exchange membranes (IEMs), controlling their mechanical rigidity, ionic conductivity, and degradation rates. Membranes absorb water exponentially at high water activity (aw), making that region of the sorption isotherm the most influential on membrane properties. Plasticization of the polymer by water has been proposed to cause this exponential uptake at high aw. However, an integrated microscopic picture of the structure, thermodynamic, and mechanical properties of IEMs as a function of aw has remained elusive. Here, we use large-scale molecular simulations validated with experimental measurements to compute the sorption isotherms, Young’s modulus, polymer dynamics, and structure of IEMs. The simulations unveil that the exponential increase in water uptake (WU) coincides in all cases with the glass to rubber transition of the membrane, as measured through its Young’s modulus and segmental polymer dynamics. Functionalization of the polymer with alkyl groups further contributes to its plasticization, increasing the WU at a given aw and ion-exchange capacity (IEC). The alkyl chains act synergistically with water to plasticize the polymer matrix and allow water penetration in the membrane. The simulations reveal that the width of the water channels depends on the ratio λ of water to ions in the polymer but is independent of its IEC. We conclude that differences in the polymer matrix─not the water channels─are responsible for the distinct uptake response of IEMs to the thermodynamic driving force of water activity.

Conflicting Effects of Extreme Nanoconfinement on the Translational and Segmental Motion of Entangled Polymers

Physically confining polymers into nanoscale pores induces significant changes in their dynamics. Although different results on the effect of confinement on the dynamics of polymers have been reported, changes in the segmental mobility of polymers typically are correlated with changes in their chain mobility due to increased monomeric relaxation times. In this study, we show that translational and segmental dynamics of polymers confined in disordered packings of nanoparticles can exhibit completely opposite behavior. We monitor the capillary rise dynamics of entangled polystyrene (PS) in disordered packings of silica nanoparticles (NPs) of 7 and 27 nm diameter. The effective viscosity of PS in 27 nm $SiO_2$ NP packings, inferred based on the Lucas-Washburn equation, is significantly smaller than the bulk viscosity, and the extent of reduction in the translational motion due to confinement increases with the molecular weight of PS, reaching 4 orders of magnitude reduction for PS with a molecular weight of 4M g/mol. The glass transition temperature of entangled PS in the packings of 27 nm $SiO_2$ NPs, however, increases by 45 K, indicating significant slowdown of segmental motion. Interestingly, confinement of the polymers into packings made of 7 nm $SiO_2$ NPs results in molecular weight-independent effective viscosity. The segmental dynamics of PS in 7 nm $SiO_2$ NP packings are slowed down even further as evidenced by 65 K increase in glass transition temperature. These seemingly disparate effects are explained by the microscopic reptation-like transport controlling the translational motion and the physical confinement affecting the segmental dynamics under extreme nanoconfinement.

Direct Observations of Segmental Dynamics at the Polymer–Substrate Interface Enabled by Localizing Fluorescent Probes with Polymer Brushes

We employed imaging rotational fluorescence correlation microscopy to directly observe the segmental dynamics of the polymer near the polymer–substrate interface using a probe-tethered polymer brush buried in bulk polymer films. The probe-tethered polymers grafted onto the polymer–substrate interface provide spatial selectivity to probe only the segmental dynamics of polymer near the polymer–substrate interface. The perturbation caused by the grafted polymer chains on the segmental dynamics of the polymer film near the interface was minimized by reducing the molecular weight of the polymer brush compared with the critical entanglement molecular weight of the polymer and grafted with a low grafting density (≈0.1 chains/nm²) using a “grafting-to” method, which allows precisely controlled and fully characterized polymer brush structures. In addition, the molecular weight of the brush polymer was further controlled to ≈10 kg/mol with the root-mean-square end-to-end distance of ≈5 nm to probe dynamics within 10 nm of the polymer–substrate interface. Polystyrene and poly(methyl methacrylate) films carrying rather unfavorable and similar surface energies, respectively, with the silane-modified silicon wafer substrates were investigated. The polymer dynamics near the interface characterized by the degrees of non-Arrhenius temperature dependence and nonexponential relaxation were not altered in both systems compared with those of the bulk, suggesting that the polymer dynamics is not sensitive to moderate differences in enthalpic interaction between the polymer and the substrate.

Effects of Chain Length on the Structure and Dynamics of Semidilute Nanoparticle–Polymer Composites

We use molecular dynamics simulations to study how the chain length affects the structure and segmental dynamics of polymer–nanoparticle (NP) composites at semidilute NP concentrations. For NPs having relatively strong interactions with the polymer, we can approximate the system as an ideal NP dispersion, which isolates the effect of direct interactions among the NPs. By varying both the chain length N and NP concentration, we examine regimes where the chain size (i.e., chain radius of gyration Rg) is small compared to the NP separation d (d/Rg > 1), as well as cases where d/Rg < 1, so that chains readily “bridge” between the NPs. We find that the fraction of such bridging chains in our simulations can be expressed as a universal function of d/Rg. Structurally, the polymers slightly elongate near the NP interface and the chains tend to align their longest axis with the NP interface. We show that the effect of NPs on the chain structure is nearly chain length-independent, whereas the effect on chain alignment extends farther from the NP surface as the chain length increases. Chains that bridge between NPs must significantly elongate when the NP separation is large compared to the chain dimensions (d/Rg > 1). Although these bridging chains have a longer relaxation time than nonbridging chains, they do not make a substantial contribution to the overall nanocomposite segmental relaxation time for the conditions studied because the bridging chains represent only a small fraction of the system. Note that relaxation at the scale of chains may differ for bridging chains. When NP separation is comparable or smaller than the chain size (d/Rg < 1), bridging chains are more prevalent, but their properties are more similar to nonbridging chains than when d/Rg > 1. Accordingly, the variation of the computational glass transition temperature Tg with chain length essentially mirrors the trend for the reference pure polymer melt, where Tg increases with chain length and roughly saturates at large polymer mass. Overall, for the conditions studied, bridging chains are found to have a small impact on the segmental dynamics of nanocomposites.

Resolving Segmental Polymer Dynamics in Nanocomposites by Incoherent Neutron Spin-Echo Spectroscopy.

The segmental dynamics of styrene-butadiene nanocomposites with embedded silica nanoparticles (NPs, ca. 20 vol. %) has been studied by broadband dielectric (BDS) and neutron spin-echo spectroscopy (NSE). It is shown by BDS that overlapping contributions only allow us to conclude on a range of distributions of relaxation times in simplified industrial nanocomposites formed with highly polydisperse NPs. For comparison, structurally similar but less aggregated colloidal nanocomposites have a well-defined distribution of relaxation times due to the reduced influence of interfacial polarization processes. This distribution is widened with respect to the neat polymer, without change in the position of the maximum and at most a small slowing down visible in the average time. We then demonstrate that incoherent NSE can be used to resolve small modifications of segmental dynamics of the industrial samples. By carefully choosing the q-vector of the measurement, experiments with fully hydrogenated polymer give access to the self-dynamics of the polymer in the presence of silica on the scale of approximately 1 nm. Our high-resolution measurements show that the segmental motion is slightly but systematically slowed also by the presence of the industrial filler NPs.

Fluctuation suppression in microgels by polymer electrolytes.

Structural details of thermoresponsive, cationically poly(N-iso-propylacrylamide-co-methacrylamido propyl trimethyl ammonium chloride) microgels and the influence of the anionic electrolyte polystyrene sulfonate (PSS) on the internal structure and dynamics of the cationic microgels have been studied with a combination of small angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy. While SANS can yield information on the overall size of the particles and on the typical correlation length inside the particles, studying the segmental polymer dynamics with NSE gives access to more internal details, which only appear due to their effect on the polymer motion. The segmental dynamics of the microgels studied in this paper is to a large extent suppressed by the PSS additive. Possible scenarios of the influence of the polyanions on the microgel structure and dynamics are discussed.

Thermal, nanoindentation and dielectric study of nanocomposites based on poly(propylene furanoate) and various inclusions

In this work, we investigate polymer nanocomposites (PNCs) based on a poly(propylene furanoate) (PPF), a relatively new biobased polymer, envisaged for various application, e.g. exploiting its exceptional gas barrier properties. PPF exhibits extremely slow crystallization and subsequently poor mechanical performance, therefore, targeting at improving its properties, PPF is filled here with five fillers of different geometries, i.e. Carbon Nanotubes (CNT), Halloysite nanotubes, Graphene nanoplatelets, and two Montomorillonite (MMT) clays. Crystallization, glass transition and mechanical performance are investigated employing differential scanning calorimetry (DSC) and mechanical nanoindentation tests supplemented by X-ray diffraction (XRD), on both amorphous and semicrystallized samples. Moreover, we attempt to investigate the structure (filler morphology, semicrystalline morphology) / properties relationship also in combination with recording molecular dynamics via broadband dielectric spectroscopy (BDS). For the fixed filler content of 1 wt%, the majority of the inclusions were found to increase the rate of PPF crystallization at the relatively mild temperature of 100 °C, which is desirable in industrial polymer processing. However the impact of the fillers on the degree of crystallinity is quite low, the latter being expected to correlate with semicrystalline morphology alternations in the composites as compared to the unfilled PPF. The nanoindentation tests showed clear beneficial effects of the various inclusions on the elastic modulus and indentation hardness, being, as expected, more profound in the case of semicrystallized PPF matrix. Additionally, the full-temperature dielectric-calorimetric map was constructed, showing that the effects on local and segmental polymer dynamics (β and α relaxations, respectively) by the PNC composition are insignificant whereas those by crystallization are dominant. Finally, an additional filler-related process (αf relaxation) was recorded in two cases of PNCs, showing non cooperative character and independency from crystallization, the origins of this process being still under investigation.

Chain Flexibility and the Segmental Dynamics of Polymers.

Using molecular dynamics simulations, we examine the dynamics of a family of model polymers with varying chain length and torsional potential barriers. We focus on features of the dynamics of polymers that are seen experimentally but absent in simulations of freely rotating and freely jointed chains. The reduced effect of volume on the segmental dynamics with increasing chain length, a capacity for pressure densification, and the deviation from constant Johari-Goldstein relaxation time at a constant segmental relaxation time all have a common origin, torsional rigidity, and these effects become increasingly apparent for more rigid chains.

Segmental dynamics of polymer by rotational fluorescence correlation microscopy.

The segmental dynamics of polymers is known to be closely related to the glass transition where the glass transition is the single most important parameter in its application. In this study, we designed an efficient and reliable experimental method to study the ensemble segmental dynamics of polymers by probing rotation of fluorescent molecules in the polymer matrix using a home-built microscope setup. The rotational dynamics of fluorescent molecules was analyzed using a fluorescence correlation method that extracts information through orthogonally polarized fluorescence images. From fluorescence intensities, autocorrelation functions (ACFs) were obtained in many areas simultaneously and by averaging several ACFs, well-defined ACF and precise experimental values were obtained from a single measurement movie. The robustness of the method and optimal experimental conditions were investigated by performing experiments with various probe concentrations, frame rates, and measurement lengths. By employing a home-built vacuum chamber, a wide temperature range was achieved, and we demonstrate the versatility and efficiency of imaging rotational FCM (fluorescence correlation microscopy) by probing segmental dynamics of different polymeric systems with glass transition temperature that differ by ≈100 K and with fragility ranging from 49 to 131. The imaging rotational FCM covers dynamics up to 4 orders of magnitude near the glass transition, and it was found that the rapidity of the stretching exponent β variation with temperature correlates with the fragility of polymers.

Conductivity-Relaxation Relations in Nanocomposite Polymer Electrolytes Containing Ionic Liquid.

In this study, we have used nanocomposite polymer electrolytes, consisting of poly(ethylene oxide) (PEO), δ-Al2O3 nanoparticles, and lithium bis(trifluoromethanesolfonyl)imide (LiTFSI) salt (with 4 wt % δ-Al2O3 and PEO:Li ratios of 16:1 and 8:1), and added different amounts of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesolfonyl)imide (BMITFSI). The aim was to elucidate whether the ionic liquid is able to dissociate the Li-ions from the ether oxygens and thereby decouple the ionic conductivity from the segmental polymer dynamics. The results from DSC and dielectric spectroscopy show that the ionic liquid speeds up both the segmental polymer dynamics and the motion of the Li+ ions. However, a close comparison between the structural (α) relaxation process, given by the segmental polymer dynamics, and the ionic conductivity shows that the motion of the Li+ ions decouples from the segmental polymer dynamics at higher concentrations of the ionic liquid (≥20 wt %) and instead becomes more related to the viscosity of the ionic liquid. This decoupling increases with decreasing temperature. In addition to the structural α-relaxation, two more local relaxation processes, denoted β and γ, are observed. The β-relaxation becomes slightly faster at the highest concentration of the ionic liquid (at least for the lower salt concentration), whereas the γ-relaxation is unaffected by the ionic liquid, over the whole concentration range 0-40 wt %.

Decoupling of segmental relaxation from ionic conductivity in [DEMM][TFSI] room temperature ionic liquid incorporated poly(vinylidenefluoride-co-hexafluoropropylene) membranes

Herein, thermophysical and electrical properties of different polymer membranes, prepared by incorporating different amounts of room temperature ionic liquid (IL), N,N-diethyl-N-(2-methacryloylethyl)-N-methylammonium bis(trifluoromethylsulfonyl) imide ([DEMM][TFSI]), in poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer, have been studied. The study reveals that [DEMM][TFSI] IL essentially reduces the crystal growth and promotes electroactive β-phase transformation within the polymer matrix by the interaction between the polymer chains and IL cations. The ionic conductivity of polymeric membranes increases with increasing IL content and attains a maximum value of 4.0×10−4S·cm−1 for highest fraction of ionic liquid (50%) at room temperature (303K). Correlation between ion transport and segmental relaxation in polymer membranes has been analyzed using dielectric spectroscopy. The analysis shows that with increasing faction of the ionic liquid the ionic conductivity gradually decouples from the segmental dynamics of the polymer. This is attributed to the higher diffusivity of relatively free anions at lower temperatures, where the segmental dynamics of polymer significantly slow down.

Interfacial Energy and Glass Temperature of Polymers Confined to Nanoporous Alumina

We report on the effect of interfacial energy on the glass temperature, Tg, of several amorphous polymers with various glass temperatures and polymer/substrate interactions confined within self-ordered nanoporous alumina (AAO). The polymers studied include poly(phenylmethylsiloxane) (PMPS), poly(vinyl acetate) (PVAc), 1,4-polybutadiene (PB), oligostyrene (PS), and poly(dimethylsiloxane) (PDMS). The segmental dynamics and associated Tg’s are studied by means of dielectric spectroscopy. The interfacial energy for the polymer/substrate interface, γSL, is calculated with Young’s equation whereas the AAO membrane surface energy is obtained by measuring contact angles for several reference liquids. We find that interfacial energy plays a significant role in the segmental dynamics of polymers under confinement within AAO. There is a trend for a decreasing glass temperature relative to the bulk with increasing interfacial energy. PDMS exhibits the highest interfacial energy and the highest reduction in glass temp...

Martin Goldstein

Martin Goldstein

Segmental Dynamics of Polymer Melts with Spherical Nanoparticles.

The impact of spherical nanoparticles (NPs) on the segmental dynamics of polymer melts is investigated. The addition of NPs broadens the segmental dynamics with effects of both particle size and loading. Interfacial bound layer thickness is calculated by the difference in magnitude of the segmental dynamics of pure polymer and nanocomposites. These theoretical models suggest that the bound layer thickness in the case of strongly adsorbing polymer matrices may increase with particle size.

Glass Transition and Relaxation Processes of Nanocomposite Polymer Electrolytes

This study focus on the effect of δ-Al(2)O(3) nanofillers on the dc-conductivity, glass transition, and dielectric relaxations in the polymer electrolyte (PEO)(4):LiClO(4). The results show that there are three dielectric relaxation processes, α, β, and γ, in the systems, although the structural α-relaxation is hidden in the strong conductivity contribution and could therefore not be directly observed. However, by comparing an enhanced dc-conductivity, by approximately 2 orders of magnitude with 4 wt % δ-Al(2)O(3) added, with a decrease in calorimetric glass transition temperature, we are able to conclude that the dc-conductivity is directly coupled to the hidden α-relaxation, even in the presence of nanofillers (at least in the case of δ-Al(2)O(3) nanofillers at concentrations up to 4 wt %). This filler induced speeding up of the segmental polymer dynamics, i.e., the α-relaxation, can be explained by the nonattractive nature of the polymer-filler interactions, which enhance the "free volume" and mobility of polymer segments in the vicinity of filler surfaces.

A dielectric relaxation study of nanocomposite polymer electrolytes

(PEO)4:LiClO4 and the effect of δ-Al2O3 nano-fillers on the polymer electrolyte have been investigated by differential scanning calorimetry (DSC) and dielectric relaxation studies. The DSC studies indicated a decrease in the glass transition temperature of (PEO)4:LiClO4 in the presence of fillers. The polymer–salt complex exhibited a conductivity of σD.C.=4.0×10−7Scm−1 at room temperature (298K). In the presence of 4wt% δ-Al2O3 fillers, the ionic conductivity increased by almost two orders of magnitude and exhibited a value of σD.C.=1.5×10−5Scm−1 (298K). By comparing the conductivity data with the calorimetric glass transition it is evident that the enhanced conductivity can be attributed to a speeding up of the segmental polymer dynamics. Hence, the combined data indicate a direct coupling between Li+ ion motions and polymer segmental dynamics, even when 4wt% nano-fillers have been introduced. From the temperature dependence of the conductivity relaxation time, which, thus, is coupled to the structural (α) relaxation time, it is understood that the fragility of the polymer electrolytes decreases with increasing filler concentration. The decrease in Tg and fragility suggests that the interaction between the nano-fillers and polymer is mainly non-attractive in nature. Moreover, the dielectric relaxation studies show that the β-relaxation speeds up with increasing concentration of filler particles. However, this speeding up is probably of less importance for the ionic conductivity. It was also found that the activation energy of the β-relaxation of PEO decreases in the presence of δ-Al2O3 nano-fillers and lithium salt. The strength of the β-relaxation increases also in the presence of salt ions. The γ-relaxation seems to be unaffected by the salt and fillers.