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

Relaxation Dynamics of Biomass-Derived Copolymers With Promising Gas-Barrier Properties

This article presents an experimental study on the relaxation dynamics of a series of random copolymers based on bio-friendly comonomers with interesting gas barrier properties. We analyze the relaxation response in the glassy and ultraviscous regime of poly (trimethylene furanoate/sebacate) random copolymers via dielectric spectroscopy. We report lower values of dynamic fragility [a dimensionless index introduced in 1985 (Angell, Relaxations in Complex Systems, 1985)] in comparison to popular polyesters widely used in industry, such as poly (ethylene terephthalate), suggesting that the amorphous phase of these furanoate-based polyesters adopt an efficient chain packing. This is consistent with their low permeability to gases. We also discuss on different equations (phenomenological and theory-based approaches) for fitting the temperature-evolution of the alpha relaxation time.

Insight into the Structure and Dynamics of Polymers by Neutron Scattering Combined with Atomistic Molecular Dynamics Simulations

Combining neutron scattering and fully atomistic molecular dynamics simulations allows unraveling structural and dynamical features of polymer melts at different length scales, mainly in the intermolecular and monomeric range. Here we present the methodology developed by us and the results of its application during the last years in a variety of polymers. This methodology is based on two pillars: (i) both techniques cover approximately the same length and time scales and (ii) the classical van Hove formalism allows easily calculating the magnitudes measured by neutron scattering from the simulated atomic trajectories. By direct comparison with experimental results, the simulated cell is validated. Thereafter, the information of the simulations can be exploited, calculating magnitudes that are experimentally inaccessible or extending the parameters range beyond the experimental capabilities. We show how detailed microscopic insight on structural features and dynamical processes of various kinds has been gained in polymeric systems with different degrees of complexity, and how intriguing questions as the collective behavior at intermediate length scales have been faced.

Localized translational motions in semicrystalline poly(ethylene terephthalate) studied by incoherent quasielastic neutron scattering

One of the simplest ways to confine polymeric materials is by self-assembling during the crystallization process. The remaining amorphous phase is then constrained by the lamellar crystals. In this manuscript, we aim to shed additional light in the understanding of the amorphous chains dynamics of semicrystalline polymers above the Tg by using incoherent quasielastic neutron scattering QENS in a nanoscopic time scale (10(-9)-10(-10)s) on poly(ethylene terephthalate). The observed dynamics is satisfactorily described by a theoretical model that considers that the proton mobility follows a random jump-diffusion in a restricted environment. We demonstrate that the combination of macroscopic with nanoscopic dynamic tools allows a complete description of the confined dynamics on a paradigmatic semicrystalline polymer like poly(ethylene terephthalate).

Quasielastic neutron scattering study of hydrogen motions in an aqueous poly(vinyl methyl ether) solution

We present a quasielastic neutron scattering (QENS) investigation of the component dynamics in an aqueous Poly(vinyl methyl ether) (PVME) solution (30% water content in weight). In the glassy state, an important shift in the Boson peak of PVME is found upon hydration. At higher temperatures, the diffusive-like motions of the components take place with very different characteristic times, revealing a strong dynamic asymmetry that increases with decreasing T. For both components, we observe stretching of the scattering functions with respect to those in the bulk and non-Gaussian behavior in the whole momentum transfer range investigated. To explain these observations we invoke a distribution of mobilities for both components, probably originated from structural heterogeneities. The diffusive-like motion of PVME in solution takes place faster and apparently in a more continuous way than in bulk. We find that the T-dependence of the characteristic relaxation time of water changes at T ≲ 225 K, near the temperature where a crossover from a low temperature Arrhenius to a high temperature cooperative behavior has been observed by broadband dielectric spectroscopy (BDS) [S. Cerveny, J. Colmenero and A. Alegría, Macromolecules, 38, 7056 (2005)]. This observation might be a signature of the onset of confined dynamics of water due to the freezing of the PVME dynamics, that has been selectively followed by these QENS experiments. On the other hand, revisiting the BDS results on this system we could identify an additional "fast" process that can be attributed to water motions coupled with PVME local relaxations that could strongly affect the QENS results. Both kinds of interpretations, confinement effects due to the increasing dynamic asymmetry and influence of localized motions, could provide alternative scenarios to the invoked "strong-to-fragile" transition.

Structure and texture analysis of PVC foils by neutron diffraction

Crystalline order of molded and then bi-axially stretched foils prepared from atactic PVC resin is investigated by means of wide-angle neutron diffraction (WAND). The observed high-resolution WAND patterns of all samples are dominated by a sharp maximum corresponding to the inter-planar distance 0.52 nm. Two weaker maxima are also resolved at 0.62 and 0.78 nm. Intensities of the peaks vary with deformation ratios of the samples and their diffraction position. Average size of the coherently scattering domains is estimated as approximately 4-8 nm. Based on the experimental data, a novel model of crystalline order of atactic PVC is proposed.

Atomic motions in the alphabeta-merging region of 1,4-polybutadiene: a molecular dynamics simulation study

We present fully atomistic molecular dynamics simulations on 1,4-polybutadiene in a wide temperature range from 200 to 280 K, i.e., in the region where the alpha- and beta-relaxations merge and above. A big computational effort has been performed-especially for the lowest temperatures investigated-to extend the simulation runs to very long times (up to 1 mus for 200 K). The simulated sample has been carefully validated by using previous neutron scattering data on the real sample with similar microstructure. Inspecting the trajectories of the different hydrogens in real space, we have observed a heterogeneous dynamical behavior (each kind of hydrogen moves in a different way) with signatures of combined hopping and diffusive motions in the whole range investigated. The application of a previously proposed model [Colmenero et al., Europhys. Lett. 71, 262 (2005)] is successful and a characterization of the local motions and diffusion is possible. The comparison of our results to those reported in the literature provides a consistent scenario for polybutadiene dynamics and puts into a context the different experimental observations. We also discuss the impact of the hopping processes on the observation and interpretation of experimentally accessible magnitudes and the origin of the deviations from Gaussian behavior in this system.

Plasticizer effect on the dynamics of polyvinylchloride studied by dielectric spectroscopy and quasielastic neutron scattering

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

Spatial regimes in the dynamics of polyolefins: collective motion

Molecular simulation is used to characterize the spatial dependence of collective motion in four saturated hydrocarbon polymers. The observable is the distinct intermediate scattering function, as measured in coherent quasielastic neutron scattering experiments. Ranges of 0.01-1000 ps in time and 2-14 A in spatial scale are covered. In this time range, a two-step relaxation, consisting of a fast exponential decay and a slower stretched decay, is observed for all spatial scales. The relaxation times for the fast process are very similar to those obtained by following self motion, with a small modulation of relaxation times near the peak in the static structure factor which is well described by the narrowing picture suggested by de Gennes. For the slow process, self and collective relaxation times have larger numerical differences and follow different scaling with spatial scale. The modulation of slow relaxation times is larger than that observed for the fast process, but is overestimated by the de Gennes prediction, which only works qualitatively.

A thermodynamic approach to the fragility of glass-forming polymers

We have connected the dynamic fragility, namely, the steepness of the relaxation-time variation upon temperature reduction, to the excess entropy and heat capacity of a large number of glass-forming polymers. The connection was obtained in a natural way from the Adam-Gibbs equation, relating the structural relaxation time to the configurational entropy. We find a clear correlation for a group of polymers. For another group of polymers, for which this correlation does not work, we emphasize the role of relaxation processes unrelated to the alpha process in affecting macroscopic thermodynamic properties. Once the residual excess entropy at the Vogel temperature is removed from the total excess entropy, the correlation between dynamic fragility and thermodynamic properties is reestablished.

Heterogeneous dynamics of poly(vinyl acetate) far above Tg: A combined study by dielectric spectroscopy and quasielastic neutron scattering

We have investigated the dynamics of poly(vinly acetate) using broadband dielectric spectroscopy (DS) covering over 14 decades in frequency up to 20 GHz and high-resolution quasielastic neutron-scattering (QENS) technique. The dielectric results have been interpreted in terms of the phenomenological Kohlrausch-Williams-Watts [G. Williams and D. C. Watts, Trans. Faraday Soc. 66, 80 (1970); F. Alvarez, A. Alegria, and J. Colmenero, Phys. Rev. B 47, 125 (1993)] description. Because of the wide frequency range covered by DS, it provides a precise determination of dynamics over a wide temperature range and it revealed a crossover in polymer dynamics at 387 K through different dielectric parameters, viz., characteristic times, asymmetric shape parameter, and dielectric strength. Moreover, shape parameter is found to be higher in comparison to other typical polymeric systems. The characteristic relaxation times observed by QENS displayed an anomalous dependence of momentum transfer, indicating the possible existence of heterogeneities in the system even at the high temperatures. In addition, spin-lattice relaxation times, T(1), were found to be decoupled from dielectric characteristic times. Based on these results, a model was proposed to account for heterogeneities where we consider coexistence of different regions with standard polymeric behavior but with different characteristic times, leading to a distribution of relaxation times. The model is found to account for the anomalous behavior and an inherent shape parameter is found to account for the shape of alpha relaxation. This model is also found to predict the T variation of T(1) characteristic time scales at all temperatures. The origin of the heterogeneous domains is believed to lie in the microstructure of polymer chains.