Local Segmental Dynamics Research Articles

Insights into the thermomechanical and interfacial behaviors of polymer‐clay nanocomposites via coarse‐grained molecular dynamics simulations

AbstractPolymer‐clay nanocomposites (PCNs) are commonly applied as multi‐functional structural materials with exceptional thermomechanical properties, while maintaining the characteristics of lightweight and optical clarity. In this study, building upon previously developed coarse‐grained (CG) models for nanoclay and poly (methyl methacrylate) (PMMA), we employ molecular dynamics (MD) simulations to systematically investigate the thermomechanical properties of PCNs when arranged in stacked configurations. Incorporating stacked clay nanofillers into a polymer matrix, we systematically conduct shear and tensile simulations to investigate the influences of variations in weight percentage, system temperature, and nanoclay size on the thermomechanical properties of PCNs at a fundamental level. The weight percentage of nanoclay in nanocomposites proves to have a significant influence on both the shear and Young's modulus (e.g., the addition of 10 Wt% nanoclay leads to an increase of 32.6% in the Young's modulus), with each exhibiting greater mechanical strength in the in‐plane direction compared to the out‐of‐plane direction, and the disparity between these two directions further widens with an increase in the weight percentage of nanoclay. Furthermore, the increase in the size of nanoclay contributes to an overall modulus enhancement in the composite while the growth reaches a saturation point after a certain threshold of about 10 nm. Our simulation results indicate that the overall dynamics of PMMA are suppressed due to the strong interactions between nanoclay and PMMA, where the confinement effect on local segmental dynamics of PMMA decays from the nanoclay‐polymer interface to the polymer matrix. Our findings provide valuable molecular‐level insights into microstructural and dynamical features of PCNs under deformation, emphasizing the pivotal role of clay‐polymer interface in influencing the thermomechanical properties of the composite materials.Highlights CG modeling is performed to explore the thermomechanical behavior of PCN. Effects of nanoclay weight percentage and size on modulus are studied. Interface leads to nanoconfinement effect on Tg and molecular stiffness. Correlations between molecular stiffness and modulus are identified. Simulations show spatial variation of dynamical heterogeneity.

Structure, Dynamics, and Hydrogen Transport in Amorphous Polymers: An Analysis of the Interplay between Free Volume Element Distribution and Local Segmental Dynamics from Molecular Dynamics Simulations

Structure, Dynamics, and Hydrogen Transport in Amorphous Polymers: An Analysis of the Interplay between Free Volume Element Distribution and Local Segmental Dynamics from Molecular Dynamics Simulations

Molecular Complexation of Polymers and Metal Oxide Clusters for Semicrystalline, Thermoplastic Anhydrous Proton Exchange Membranes in Fuel Cells.

With the manipulation of surface charges and loadings, 1 nm super-acidic metal oxide clusters can co-crystallize with poly(ethylene glycol) (PEG) at molecular scale for thermoplastic anhydrous proton exchange membranes (PEMs). The coexistence of crystalline and amorphous regions endows the PEMs with a high Young's modulus and high flexibility, while the noncovalent complex interactions enable facile preparation and (re)processing. Furthermore, the diffusive dynamics of PEG chains is slowed by the confinement effect, while the local segmental dynamics is accelerated due to the transition of the chain conformation from helix to zigzag when confined in the crystalline framework. This greatly facilitates proton transportation in the crystalline region for an excellent anhydrous proton conductivity of 4.5 × 10-3 S cm-1 at 90 °C. The balanced proton conductivity, mechanical strength, and processability of the PEMs contribute to the promising power density of H2/O2 fuel cells assembled with co-crystalline PEMs at high temperatures under dry conditions.

QENS study on local segmental dynamics of polyelectrolytes in complex coacervates

The segmental dynamics of complex coacervates were investigated as a function of salt concentration and a charge type using quasi-elastic neutron scattering (QENS). The coacervates were prepared by mixing two oppositely charged polyether-based polyelectrolyte solutions functionalized for cationic ammonium and guanidinium groups or anionic sulfonate groups. For the QENS time scale, the intrinsic ion pairs were considered effective cross-links, and therefore the dynamics of chain segments between the intrinsic junctions were probed. We observed that as salt concentration increases, both the number of mobile chain segments and their segmental dynamics increase for ammonium-based coacervates where the electrostatic interaction primarily governs the phase behavior. However, the guanidinium-based coacervates show slow and nearly identical segmental relaxation independent of the salt concentration, which is potentially attributed to the non-electrostatic interactions. The combination of the chemical structure of ionic moieties, salt concentration, and temperature plays a significant role to determine polymer chain dynamics mainly affected by the strength of ionic bonding.

Polymer segmental dynamics near the interface of silica particles in the particle/polymer composites

We demonstrate an approach to examine the local segmental dynamics of a polymer near the interface of an inorganic filler by observing the rotational dynamics of the fluorescent probe at the chain ends of polymer brushes grafted onto the surface of the filler particles. Localization of the fluorescent probe was realized by designing and synthesizing fluorophore-tethered polystyrene (PS) brushes anchored on the surface of silica particles of controlled sizes. Fluorophore-functionalized telechelic PS with an azide functionality at the other chain end was achieved via a combination of atom transfer radical polymerization and post-polymerization modification. The azide-bearing PS chains were tethered to alkyne-functionalized particles via copper-catalyzed cycloaddition reaction. The molecular weight of the grafted polymer chains was controlled to be less than the critical entanglement molecular weight, and the chain density was controlled to be low enough so that the observed dynamics was not perturbed by the polymer brush conformation and brush-matrix polymer entanglement. The polymer dynamics near the surface of the particles at low concentrations was closely examined in the bulk film geometry by employing imaging rotational fluorescence correlation microscopy (irFCM). The observed polymer dynamics near the interface were not altered in the inorganic/polymer composite geometry when the surface did not have favorable interaction with the matrix polymer. The presented rational design of the chemical route and examination of local dynamics highlight a feasible approach to construct material systems with high complexities towards a deeper understanding of composite materials.

Activated Transport in Polymer Grafted Nanoparticle Melts

We measure the activation energy for the local segmental dynamics of polymer chains densely grafted to nanoparticles (NPs) using quasielastic neutron scattering. We aim to understand the underpinning physics of the experimentally measured enhanced gas transport in polymer grafted nanoparticle-based membranes relative to the neat polymer (without NPs), especially the permeability maximum, which occurs at intermediate chain lengths. We find that the activation energy goes through a minimum as a function of chain length, while the elementary jump size goes through a maximum around the same chain length. These results, likely, are the dynamic consequence of a structural transition of the grafted polymer brush from “extended” to “interpenetrated” with increasing chain length at fixed grafting density. Evidently, the regimes of different graft chain lengths near this structural transition are associated with lower activation energy, likely due to fluctuation effects, which also lead to enhanced gas transport.

H-Shaped Copolymer of Polyethylene and Poly(ethylene oxide) under Severe Confinement: Phase State and Dynamics.

The self-assembly and the dynamics of an H-shaped copolymer composed of a polyethylene midblock and four poly(ethylene oxide) arms (PE-b-4PEO) are investigated in the bulk and under severe confinement into nanometer-spaced LAPONITE clay particles by means of small- and wide-angle X-ray diffraction (SAXS, WAXS), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), rheology, and dielectric spectroscopy (DS). Because of the H-shaped architecture, the PE midblock is topologically frustrated and thus unable to crystallize. The superstructure formation in the bulk is dictated solely by the PEO arms as inferred by the crystallization/melting temperature relative to the PEO homopolymer. Confinement produced remarkable changes in the interlayer distance and PEO crystallinity but left the local segmental dynamics unaltered. To reconcile all structural, thermodynamic, and dynamic effects, a novel morphological picture is proposed with interest in emulsions. Key parameters that stabilize the final morphology are the severe chain confinement with the associated entropy loss and the presence of interactions (hydrophobic/hydrophilic) between the LAPONITE and the PEO/PE blocks.

Achieving an Optimal Tg Change by Elucidating the Polymer–Nanoparticle Interface: A Molecular Dynamics Simulation Study of the Poly(vinyl alcohol)–Silica Nanocomposite System

The glass transition temperature (Tg) of a polymer-nanoparticle-composite system is strongly dependent on the local segmental dynamics of the polymers. These dynamics, in turn, are dependent on the types of interfacial interactions that exist between the nanoparticle and the polymers. In this work, we investigate the magnitude of Tg-change with respect to the polymer-nanoparticle interactions by performing molecular dynamics simulations for a full-atomistic model of PVA-Silica nanocomposite. Our segmental dynamics analysis and potential of mean force calculations indicate that the strong binding interactions between hydroxylated-silica surface and the poly(vinyl alcohol) (PVA) are able to induce an increase in the Tg of the composite system with respect to the bulk PVA polymer system. While we expect that the Tg of the system will increase with an increasing amount of hydrogen bonds arising from the increasing surface hydroxylation, the trend of increasing Tg reaches a maximum when the surface is about 75...

The α and Johari–Goldstein Relaxations in 1,4-Polybutadiene: Breakdown of Isochronal Superpositioning

Dielectric spectra were measured for 1,4-polybutadiene (PBD) at various temperatures and pressures corresponding to a constant value of the relaxation time, τα, for the local segmental dynamics (α-process). Given the relationship of the Johari–Goldstein secondary relaxation to the α-process, it is of interest to determine whether the frequency separation of the Johari–Goldstein secondary relaxation and the α relaxation remains essentially constant under isochronal conditions. We find for PBD this is not the case; the JG relaxation peak moves systematically to higher frequencies at constant τα with increasing temperature and pressure. We show using molecular dynamics simulations that the behavior of PBD, which differs from that reported previously for molecular liquids, is a consequence of the torsional inflexibility of the polymer backbone. This accentuates the effect of constraints from local intermolecular barriers, with consequent deviation in the response of the JG and α relaxations due to their diffe...

Segmental and secondary dynamics of nanoparticle-grafted oligomers.

The local segmental and secondary dynamics of tetramethylene oxide oligomer grafted to silica nanoparticles (NPs) were investigated as a function of grafting density and molecular weight. Grafting slows the segmental (α) dynamics, but gives rise to faster secondary (β) motions. Interestingly, the magnitude of these effects decreases with the extent of grafting (i.e., surface coverage), as well as with oligomer molecular weight. The disparity in dynamical effects reflects the decoupling of the segmental and more local β dynamics, the former is associated with stronger dynamic correlations that extend over a greater spatial range. This results in greater sensitivity to interactions, including tethering of the chains to the NP surface.

On the relationship between the local segmental dynamics and the tagged monomer dynamics in lamellar phases of diblock copolymers.

In this brief article, we present results from coarse-grained molecular dynamics simulations which probed the relationship between the local segmental dynamics and the tagged monomer dynamics in lamellar phases of diblock copolymers. Our results demonstrate that monomer relaxation times do not provide directly a quantitatively accurate measure of the spatial variations in segmental dynamics. However, a convolution of the monomer density distributions with their corresponding relaxation times is shown to provide an approximate, but accurate, quantitative characterization of the average local segmental dynamics.

Origin of mechanical stress and rising internal energy during fast uniaxial extension of SBR melts

We carry out simultaneous mechanical and IR-thermal-imaging based temperature measurements of SBR melts during uniaxial extension in order to delineate the nature of the observed mechanical responses. Using the first law of thermodynamics, we evaluate the enthalpy change h1 associated with the temperature rise in the extending melt, estimate the heat loss to the surrounding, and conclude that there is an appreciable non-thermal enthalpic buildup h2 = (w − h1 − q) during either adiabatic or isothermal extension. The monotonic increase of h2 with the stretching ratio λ until the onset of inhomogeneous extension or melt rupture reveals that fast melt extension is largely elastic even after yielding in presence of partial chain disentanglement. At high rates, the lock-up of chain entanglement produces such a high level of h2 that is rarely seen in extension of crosslinked rubbers. When melt extension is carried out under isothermal condition, we show that the time-temperature superposition principle (TTS) fails to predict the transient response of a SBR melt at a fixed effective rate involving three temperatures. The failure of the TTS suggests that the terminal chain dynamics show different temperature dependence from the local segmental dynamics that control the transient stress responses.

Morphological, structural, dielectric and electrical properties of PEO–ZnO nanodielectric films

The morphological, structural, dielectric and electrical properties of aqueous solution-cast prepared poly(ethylene oxide)–zinc oxide (PEO–ZnO) nanocomposite films have been investigated as a function of ZnO nanoparticle concentrations up to 5 wt%. Scanning electron microscopy (SEM) images of these films show that the morphology of pristine PEO aggregated spherulites changes into fluffy, voluminous and highly porous with dispersion of ZnO nanoparticles into the PEO matrix. X-ray diffraction (XRD) study confirms that the crystalline phase of PEO greatly reduces at 1 wt% ZnO, and it again increases gradually with further increase of ZnO concentration. The dielectric relaxation spectroscopy (DRS) over the frequency range 20 Hz–1 MHz reveals that the real part of complex dielectric permittivity at audio frequencies decreases non-linearly whereas it remains almost constant at radio frequencies for these polymeric nanocomposites. Dispersion of nanosize ZnO particles into the PEO matrix reduces the values of dielectric permittivity which also exhibits a correlation with the dispersivity of ZnO nanoparticles. The relaxation peaks observed in the dielectric loss tangent and electric modulus spectra reveal that the electrostatic interactions of nanoscale ZnO particles with the ethylene oxide functional dipolar group of PEO monomer units decrease the local chain segmental dynamics of the polymer. Real part of ac conductivity spectra of these films have been analyzed by power law fit over the audio and radio frequency regions, respectively, and the obtained dc conductivity values for these regions differ by more than two orders of magnitude. The temperature dependent relaxation time and dc conductivity values of the nanodielectric material obey the Arrhenius relation of activation energies and confirm a correlation between dc conductivity and PEO chain segmental motion which is exactly identical to the characteristics of solid polymer electrolytes. Results imply that these nanocomposite materials can serve as low permittivity flexible nanodielectric for radio frequency microelectronic devices and also as electrical insulator for audio frequency operating conventional devices in addition to their suitability in preparation of solid polymer electrolytes.

Local and Global Dynamics in Polypropylene Glycol/Silica Composites

The local segmental and global dynamics of a series of polypropylene glycol / silica nanocomposites were studied using rheometry and mechanical and dielectric spectroscopies. The particles cause substantial changes in the rheology, including higher viscosities that become non-Newtonian and the appearance of stress overshoots in the transient shear viscosity. However, no change was observed in the mean relaxation times for either the segmental or normal mode dynamics measured dielectrically. This absence of an effect of the particles is due to masking of the interfacial response by polymer chains remote from the particles. When the unattached polymer was extracted to isolate the interfacial material, very large reductions in the relaxation times were measured. This speeding up of the dynamics is due in part to the reduced density at the interface, presumably a consequence of poorer packing of tethered chains. In addition, binding of the ether oxygens of the polypropylene glycol chains truncates the normal mode, which shifts the corresponding relaxation peak to higher frequencies.

Molecular Modeling and Simulation of Polymer Nanocomposites at Multiple Length Scales

The complexity of intermolecular interactions and confinement in polymer—nanoparticle systems leads to spatial variations in structure and dynamics at both the meso and nanoscale. Molecular simulation holds great promise as a means of predicting these effects and understanding their microscopic origin. In order to shed some light onto local structure and segmental dynamics of atactic polystyrene/silica (PS/SiO2) and atactic polystyrene/fullerene (PS/C60) melt systems, molecular simulations have been conducted using two interconnected levels of representation: 1) A coarse-grained representation. Equilibration of coarse-grained polymer-nanoparticle systems at all length scales is achieved via connectivity-altering Monte Carlo simulations. 2) An atomistic representation. Initial configurations for atomistic molecular dynamics (MD) simulations are obtained by reverse mapping well-equilibrated coarse-grained configurations. The local structure around a silica nanoparticle immersed in the PS matrix, PS segmental, and local dynamics in both composites and mechanical properties and entanglements in PS/SiO2 are studied.

Single Molecule Experiments Reveal the Dynamic Heterogeneity and Exchange Time Scales of Polystyrene near the Glass Transition

In a polymeric material near its glass transition temperature, segmental dynamics of a given spatial region may differ considerably from that of neighboring regions without apparent structural origin, analogous to the supercooled liquid state of low molecular weight glass formers. Given that the supercooled liquid state is a (metastable) equilibrium state, spatial variations in dynamics are expected to average out in time, consistent with ergodicity of the system. By probing the rotations of fluorescent guest molecules, local segmental dynamics of polystyrene was scrutinized molecule by molecule. Two perylene dicarboximide dyes were investigated as potential reporters, and one of these was found to report a substantial proportion of the dynamic heterogeneity of the host polystyrene. Using this probe, we demonstrate that the polystyrene system is ergodic and characterize time scales over which molecules experience changes to their dynamics. We identify a characteristic time scale of exchange much longer th...

ELECTRICAL AND DIELECTRIC PROPERTIES OF RUBBER

ABSTRACT This review describes electrical and dielectric measurements of rubbery polymers. The interest in the electrical properties is primarily due to the strong effect of conductive fillers, the obvious example being carbon black. Conductivity measurements can be used to probe dispersion and the connectivity of filler particles, both of which exert a significant influence on the mechanical behavior. Dielectric relaxation spectra are used to study the dynamics, including the local segmental dynamics and secondary relaxations, and for certain polymers the global chain modes. A recent development in the application of nonlinear dielectric spectroscopy is briefly discussed.

Influence of molecular weight and degree of segregation on local segmental dynamics of ordered block copolymers

Experiments in the context of block copolymer electrolyte materials have observed intriguing dependence of the ionic conductivities upon the polymer molecular weight and the degree of segregation between the blocks. Such results have been partly rationalized by invoking the spatial extent of dynamical inhomogeneities that manifest in ordered phases of block copolymers comprised of a rubbery and a glassy block. Motivated by such observations, we use molecular dynamics simulations to study the extent of spatial inhomogeneities in segmental dynamics of lamellar diblock copolymer systems where the blocks possess different mobilities. We probed the local average relaxation times and the dynamical heterogeneities as a function of distance from the interface. Our results suggest that the relaxation times of rubbery segments are strongly influenced by both the spatial proximity to the interface and the relative mobility of the glassy segments. Scaling of our results indicate that the interfacial width of the ordered phases serves as the length scale underlying the spatial inhomogeneities in segmental dynamics of the fast monomers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016

Segmental dynamics in lamellar phases of tapered copolymers.

Recent experiments have reported that the lamellar phase of salt-doped tapered copolymers exhibit higher ionic conductivity compared to those seen in similar morphologies of diblock copolymers. Such observations were in turn rationalized by invoking the corresponding glass transition temperature of the segregated copolymers. In this work we report the results of coarse-grained molecular dynamics simulations to identify the mechanisms underlying such characteristics. Explicitly, we probe the combined influences of the degree of segregation and the disparity in mobilities of the segments of the two blocks, upon the local relaxation dynamics of tapered copolymers segregated in lamellar phases. Our results show that the local dynamics of tapered copolymers depend on two independent factors, viz., the degree of segregation of such copolymers relative to their order-disorder transition temperature, and the relative mobilities (glass transition temperatures) of the two blocks. In qualitative correspondence with experiments, we find that for appropriate combinations of mobility ratios and degree of segregation, the lamellar phases of tapered copolymers can exhibit faster local segmental dynamics compared to diblock copolymers.

Roles of chain stiffness and segmental rattling in ionomer glass formation

ABSTRACTIonomers–polymers with bound ionic moieties—have received interest for their exceptional properties including high toughness and ion conductivity. One of the primary effects of introduction of covalently bound ions to a polymer is alteration of its glass transition and segmental dynamics. The standard model for these alterations attributes them to a covalent ‘tethering’ effect in which segments near ionic aggregates are immobilized within a range related to the chain persistence length. Here, results from molecular dynamics simulations of glass formation in model ionomers of varying chain stiffness indicate that chain persistence length does not play a central role in ionic‐aggregate‐induced suppression of local segmental dynamics. Instead, these alterations are found to accord with more universal interface‐induced alterations in glass‐forming liquids, consistent with a recent study in which the present authors found a correlation between near‐aggregate mobility suppression and the scale of segmental cooperative rearrangements. In this case, results indicate that shifts in the overall segmental relaxation times of these polymers can be quantitatively rationalized in terms of alterations in the Debye‐Waller factor, reflecting changes in the local picosecond‐timescale segmental rattling. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1458–1469