Dynamics Of Poly Research Articles

Manifestation of quantum statistics in vibrational dynamics of poly(ethylene) crystals

The temperature dependences of the transverse expansion ɛ⊥(T) and the longitudinal contraction ɛ‖(T) (with respect to the axes of chain molecules) in large-sized poly(ethylene) (PE) crystal grains (100×60×60 nm) are measured using x-ray diffraction in the temperature range 5–380 K. The temperature dependence of the elongation of the molecular skeleton ɛC(T) is obtained by Raman spectroscopy. It is found that the dependences ɛ⊥(T), ɛ‖(T), and ɛC(T) exhibit a similar specific nonlinear behavior. Analysis of these dependences indicates that the nonlinearity is associated with the quantum statistics of transverse vibrations. The energies and amplitudes of zero-point (at T=0) transverse (torsional and bending) vibrations and the relevant zero-point components ɛ‖(0) and ɛC(0) are estimated. It is revealed that the zero-point components make a considerable contribution to the dynamics of the PE crystal up to the melting temperature (∼400 K).

Radial aspect of local dynamics in polybutadiene melts as studied by molecular dynamics simulation: To hop or not to hop

The radial local dynamics in poly(1,4-trans-butadiene) melts is studied using molecular dynamics simulations. In this work, the hopping peak is observed in the Van Hove space-time correlation function Gs(r,t) for the first time in polymeric systems. The hopping motion, observed only for methine hydrogen, is also identified in the mean-square displacement 〈Δr2(t)〉 and the intermediate scattering function Fs(k,t) through the relative dynamics of the hopping methine hydrogen and the nonhopping methylene hydrogen. The hopping motion is found to cause an unusual broadening of the dispersion width in the dynamic structure factor Sinc(k,ω). Active free volume is proposed in terms of Gs(r,t) at a short time, which offers a consistency to the relationship between free volume and local dynamics. Fast counterrotation at a pair of CH–CH2 bonds across a CH=CH bond is found responsible for the hopping peak in this polymer, and a new hopping criterion modified for polymeric liquids is proposed.

Heterogeneous structure of poly(vinyl chloride) as the origin of anomalous dynamical behavior

We have investigated the thermal evolution of the structure and the dynamics of poly(vinyl chloride) (PVC) in a wide temperature range. Corroborating earlier findings, small angle neutron scattering revealed the presence of structural heterogeneities. On the other hand, the single chain form factor corresponds to that of Gaussian chains. Gradually with increasing temperature the system becomes homogeneous. A simple description of the heterogeneities in terms of microcrystallites is forwarded. The dynamical behavior of PVC has been investigated combining broadband dielectric spectroscopy (DS) with coherent and incoherent neutron scattering. In a wide temperature range broadband DS facilitated a precise determination of the dynamic response related to the segmental relaxation. Close to the glass transition temperature the line shape strongly deviates from the usual Kohlrausch–Williams–Watts functional form of common glassforming systems. Moreover, the characteristic relaxation time observed by incoherent scattering displays an anomalous dependence on momentum transfer indicating the possible existence of heterogeneities in the sample. Based on the structural and dynamical results, a model is proposed, that considers the coexistence of regions with different dynamical properties leading to a distribution of characteristic relaxation times. The model accounts for the experimental observations, assuming for all regions the same functional form for the α-relaxation. It may be univocally determined from the coherent scattering data at the first static structure peak. The distribution of relaxation times found is compatible with the distribution of only one variable, the glass transition temperature.

Molecular dynamics of poly(ethylene glycol)s studied by optical Kerr effect and Brillouin spectroscopy

The optical Kerr effect and Brillouin scattering study of a series of poly(ethylene glycol)s (PEGs) in the function of temperature and molecular weight (MW) have been performed. The obtained results enabled us to determine some macroscopic and microscopic parameters (such as molecular Kerr constant, hypersonic velocity, adiabatic compressibility), of the compounds investigated. The values of these parameters were found to depend strongly on both temperature and PEG's MW. Analysis of the results indicates on the presence of molecular interactions leading to formation of molecular aggregates. The increase in the MW leads to folding of PEG chains and to formation of statistical coils.

Segmental dynamics in poly(oxymethylene) as studied via combined differential scanning calorimetry/creep rate spectroscopy approach

The peculiarities and kinetics of segmental dynamics in semi-crystalline poly(oxymethylene) (POM) were studied over the temperature range from 130 to 430 K, covering three broad relaxation regions, using DSC and laser-interferometric creep rate spectroscopy (CRS). A number of dynamic anomalies are observed. These included a suppressed glass transition ( T g) with its transformation into a number of segmental relaxations below and above T g, and the pronounced dynamic heterogeneity, with the dispersion of activation energies of segmental motion at elevated temperatures from 80 to 500 kJ mol −1. The formation of double or triple folds was found indicating a predominant contribution of “straightened out” tie chains to the structure of disordered regions in isotropic POM. Discrete high-resolution CRS analysis showed that up to 10 creep rate peaks (kinds of segmental motion) constituted dynamics in POM interlamellar layers. All the anomalies observed could be treated in terms of the concept of common segmental nature of α and β relaxations in flexible-chain polymers; as the breakdown of intermolecular motional cooperativity due to nanoscale confinement effect, and as different constraining influence of crystallites on dynamics in the intercrystalline layers.

Concentration Dependence of Water Dynamics in Poly(Ethylene Oxide)/Water Solutions from Molecular Dynamics Simulations

We have performed molecular dynamics simulations of aqueous solutions of poly(ethylene oxide) (PEO) in order to investigate the influence of the polymer on water dynamics. Simulations were performed on 12 repeat unit CH3-capped PEO chains (530 Da) at 318 K covering a composition range (polymer weight fraction) from 0.17 to 1.0. The simulations employed an ab initio quantum-chemistry-based PEO/water and PEO/PEO force field together with the TIP4P (four-point transferable intermolecular potential) water model. Water translational and rotational diffusion were found to slow monotonically, whereas water−water and water-ether hydrogen bond lifetimes increased with increasing polymer concentration. The slowing of water dynamics in PEO/water solutions was associated with PEO−water interactions that are moderated in concentrated solutions by the formation of water clusters. Water translational motion could be ascribed to a combination of free water (water not involved with PEO hydration) that exhibited bulklike water dynamics and bound (hydrating water) whose motion was strongly correlated with that of the PEO molecule. Water rotational motion was found to be strongly correlated with translation motion and exhibited increasing anisotropy with increasing PEO concentration indicative of preferred rotation of the water molecules around their dipole moment vector. At high PEO concentration, water and ether oxygen atoms exhibited well pronounced subdiffusive behavior occurring on a picosecond time scale that disappears upon dilution. The characteristic length scale for the water subdiffusive behavior associated with water caging correlates well with nearest-neighbor ether oxygen−ether oxygen distance.

Temperature Dependence of Water Dynamics in Poly(Ethylene Oxide)/Water Solutions from Molecular Dynamics Simulations and Quasielastic Neutron Scattering Experiments

The dynamics of water in aqueous solutions of poly(ethylene oxide) (PEO) were studied by performing molecular dynamics (MD) simulations and quasielastic neutron scattering (QNS). The simulations and experiments were carried out on PEO/water solutions for the composition EO:Ow (ether oxygen:water oxygen) = 1:2.3 in the temperature range from 298 to 410 K. To selectively measure the motion of the water, perdeuterated PEO (d-PEO) was used for the QNS experiments. Intermediate scattering functions derived from the MD simulations were found to be in good agreement with those from QNS experiments. The QNS and MD dynamic structure factors were analyzed using the random jump diffusion (RJD) model, a model frequently applied in analysis of water QNS data, yielding rotational and translational diffusion coefficients for water in the d-PEO/H2O solutions. Analysis of MD results indicates that, although the translational self-diffusion coefficient can be extracted from the RJD fits to the data rather accurately, the r...

An Investigation of Poly(dimethylsiloxane) Chain Dynamics and Order in Its Inclusion Compound withγ-Cyclodextrin by Fast-MAS Solid-State NMR Spectroscopy

The dynamics of poly(dimethylsiloxane) in its inclusion compound with γ-cyclodextrin are elucidated using modern fast-MAS solid-state NMR techniques. Measurements of methyl 1H–1H and 1H–13C dipolar coupling constants indicate that the polymer undergoes a uniform motion, rendering all methyl groups equivalent. The dynamics of the Si—C bond is characterized by either a dynamic order parameter of S = 0.72, or, assuming a stably rotating helical structure, an inclination angle of 73° relative to the rotation axis.

Neutron Spin Echo Studies on Poly(Vinyl Alcohol) Gel in a Mixture of Dimethyl Sulfoxide and Water

Dynamics of poly(vinyl alcohol) (PVA) gel have been investigated using a neutron spin echo technique. The intermediate scattering function I(Q,t) of the gel shows a very rapid decay and a non-decaying component in time ranges shorter and longer than

Local Dynamics of Poly(ethylene oxide) Confined in 1nm Slits

ABSTRACTMolecular Dynamics simulations are used to explore the short-time dynamics of nanoscopically confined poly(ethylene oxide). Both bulk and confined systems have been studied using an atomistically detailed force field so as to comparatively illustrate their differences and complement experimental results. Our aim is to elucidate the origins of the counter intuitive distribution of relaxation times for C-H bond reorientation for PEO in severe confinements, as experimentally observed in solid state 2H NMR studies. In contrast with the respective bulk PEO system, where a transition from distinct solid to liquid like dynamics is seen with increasing temperature, for the confined chains there is a coexistence of fast and slow segmental dynamics over a wide temperature range. Our studies have revealed that factors such as local density inhomogeneities, proximity of Li+, and translational motion, synergistically contribute to the generation of fast PEO segmental dynamics in 1nm confinements.

Temperature-dependent exciton dynamics in poly( p-phenylene vinylene) measured by femtosecond transient spectroscopy

Exciton delocalization in the conjugated polymer poly( p-phenylene vinylene) (PPV) is investigated using temperature-dependent spectroscopy. As the temperature is lowered, the steady-state absorption and fluorescence spectra indicate increased electronic conjugation and exciton delocalization. Femtosecond transient absorption measurements at 20 and 290 K over the wavelength range 520–650 nm provide evidence that such delocalization occurs within the first 10 ps after photoexcitation. Comparison of the 20 and 290 K data, along with anisotropy studies, suggests that vibrational cooling plays a significant role in the spectral dynamics observed in PPV on this timescale.

Methyl Group Dynamics in Poly(methyl methacrylate): From Quantum Tunneling to Classical Hopping

We present an extensive neutron scattering study on the ester methyl dynamics of poly(methyl methacrylate) (PMMA) in the temperature range from 1 to 200 K. The crossover from quantum tunneling to classical hopping has been investigated in an energy window from 0.5 μeV to 2 meV. Inelastic measurements of the librational levels have been performed in the millielectronvolt range. The results have been analyzed in terms of a model that considers a Gaussian distribution of potential barriers for methyl group rotation. A 6-fold correction to the main 3-fold term of the potential has been added to get a good description of the librational peak. This distribution of mixed-symmetry barriers gives an excellent description of the spectra in the whole temperature range.

Polymer chain dynamics in dilute solution under Couette flow. II. High molecular weight poly(α-methylstyrene) in good solvent

Dynamics of poly(α-methylstyrene) of high molecular weight Mw=6.85×106 in solution of good solvent was investigated at 25 °C by dynamic light scattering under Couette flow. The applied shear rate was ranging from 0 to 4.5 s−1. The present purpose was to inspect the theoretical way of descriptions of flexible-linear-chain dynamics in dilute solution. At the shear rate below 2.8 s−1, two modes of motions coexisted, i.e., the diffusion motion of the single chain and the intramolecular modes of motions. The decay rate for the diffusion mode increased with increasing the shear rate but that for the internal modes was nearly constant. Whereas, above 2.8–4.5 s−1, the internal modes were suppressed exclusively and only the center-of-mass translational diffusion of the chain was detected. The observed diffusion was found to be the Brownian motion conducted by a quasi-ideal chain, while the internal motions were suffering the Zimm-type of nondraining hydrodynamic interactions. Moreover, the universal ratio Ω/D0q2 under Couette flow was examined closely as a function of qRG because this is a critical measure to inspect how the polymer chain behaves dynamically and how the dynamics can be affected by the hydrodynamic interactions in solution. Here q and RG are the scattering vector and the radius of gyration of the chain, respectively. The obtained result showed that the ratio was located on the theoretical curve for flexible linear chains in Θ state, where the theory was performed under the microscopic descriptions on both chain segments and solvents with full hydrodynamic interactions. This fact confirms strictly that the coupled kinetic equations for chain segments and solvent in the same dynamic level are indispensable to describe rigorously the dynamics of flexible polymers in dilute solution.

Local Chain Dynamics in Poly(ester carbonate)s

Magic-angle spinning 13C{1H} recoupling NMR experiments have been performed on two series of poly(ester carbonate) block copolymers. The first series is based on bisphenol A blocks (B) and the second on tetramethylbisphenol A blocks (T). Three types of saturated-ring ester linkers were used, two of which contained bicyclic rings. The dynamic mechanical γ transitions of the B-series copolymers (but not the T-series copolymers) were shifted to higher temperatures. None of the ester linkers altered significantly the motional averaging at 300 K of 1H−13C dipolar couplings within either the B or T aromatic rings from the averaging observed for the corresponding polycarbonate homopolymers. All the B rings flip faster than 10 kHz at 300 K in the poly(ester carbonate)s, just as they do in polycarbonate itself, and none of the T rings flip, either in tetramethyl polycarbonate homopolymer or in the T-based poly(ester carbonate)s.

Local Chain Dynamics in Poly(fluorocarbonate)s

Magic-angle spinning 13C{1H} and 13C{19F} recoupling NMR experiments have been performed on two polycarbonates with fluoro substituents on every fourth ring in the main chain. The results of these ...

Ultrafast stimulated emission dynamics in poly( p-phenylenevinylene) films

Femtosecond time-resolved studies of photoluminescence (PL) and stimulated emission (SE) have been conducted, for the first time, in neat thin films of poly( p-phenylenevinylene) derivatives. We found that the SE spectral narrowing is accompanied by a dramatic lifetime shortening from ∼250 to ∼3 ps, which enhances energy migration within the inhomogeneously broadened exciton distribution. In addition, the SE is delayed with respect to the PL onset by a few picoseconds depending on excitation intensity and the illumination stripe length. In some cases, a second, delayed SE is formed in the film due to the continuing energy migration into exciton states with superior radiative coupling. A set of coupled rate equations, which take into account the time-dependent exciton generation, propagation effect, optical gain and absorption loss, can simulate correctly all the observed SE dynamics.

Molecular dynamics in grafted layers of poly(γ-benzyl-L-glutamate)

Broadband dielectric spectroscopy (10 -1 Hz - 10 6 Hz) has been employed to study the molecular dynamics of poly-(y-benzyl-L-glutamate) (PBLG) in the bulk and prepared as ultra-thin grafted film. In order to contact the ultra-thin layer, two techniques were applied: (i) the evaporation of aluminium electrodes directly on the sample and (ii) the use of a solid electrode. The latter was prepared by evaporating aluminium on freshly cleaved mica sheets. Both in the bulk and in the grafted layer two relaxation processes are observed: (i) the fluctuation of the helical main chains (chop-stick-motion) and (ii) the dynamic glass transition of the side chains. This second process is found to be slightly slowed down in the grafted layer. In case of evaporated electrodes, the chop-stick-motion could not be observed in the grafted layer.

Using nuclear spin correlation of residual quadrupolar interaction to probe orientation and dynamics of poly(butadiene) networks

A new NMR method, based on nuclear spin correlations of residual dipolar/quadrupolar interactions, demonstrated by Callaghan et. al. (1), is used for the first time to study molecular orientation and dynamics of partially deuterated cis-1,4 poly(butadiene) networks. The polymer networks under deformation and swelling are studied. It is shown that the latter method can probe not only the most dominating orientation and dynamics of the networks but also the sub dynamics of the network due to network inhomogeneities.

Fast dynamics in poly(vinyl chloride) below the glass transition: self and pair correlation functions

In a previous work the incoherent scattering function of poly(vinyl chloride) (PVC) in the microscopic time range was successfully described in the frame of a model which assumes harmonic vibrations coexisting with relaxational contributions controlled by C–C torsional barriers. In the same frame the behaviour of the coherent scattering function well above the glass transition temperature T g could also be interpreted. In this work, we investigate the applicability of such a model to coherent scattering of PVC below and around T g. Within the experimental accuracy, the data can be well described starting from the results independently obtained from the study of the incoherent function. This gives additional support to the ideas involved in the proposed model.

NSE-study of magnetic phase dynamics in poly(vinylalcohol) ferrogel

New magnetic field-sensitive hydrogels have been synthesized using cross-linked poly(vinylalcohol) with attached subdomain particles of Fe 3O 4 (size∼100 Å). The SANS experiments in field ( B∼3 kG) have shown binding to the network of particles. No translational diffusion of Fe 3O 4 particles was observed. In ferrogel dynamics the stretched oscillating modes (frequency∼10 8 Hz) arising from fast Néel's fluctuations of particles magnetic moments dominate