Molecular dynamics study of the thermal and the density effects on the local and the large-scale motion of polymer melts: scaling properties and dielectric relaxation

Fractional Coupling of Primary and Johari-Goldstein Relaxations in a Model Polymer

A polymer model exhibiting heterogeneous Johari−Goldstein (JG) secondary relaxation is studied by extensive molecular-dynamics simulations of states with different temperature and pressure. Time−temperature−pressure superposition of the primary (segmental) relaxation is evidenced. The time scales of the primary and the JG relaxations are found to be highly correlated according to a power law. The finding agrees with key predictions of the Coupling Model (CM) accounting for the decay in a correlation function due to the relaxation and diffusion of interacting systems. Nonetheless, the exponent of the power law, even if it is found in the range predicted by CM (0<ξ<1), deviates from the expected one. It is suggested that the deviation could depend on the particular relaxation process involved in the correlation function and the heterogeneity of the JG process.

Mutual Information in Molecular and Macromolecular Systems

The relaxation properties of viscous liquids close to their glass transition (GT) have been widely characterised by the statistical tool of time correlation functions. However, the strong influence of ubiquitous non-linearities calls for new, alternative tools of analysis. In this respect, information theory-based observables and, more specifically, mutual information (MI) are gaining increasing interest. Here, we report on novel, deeper insight provided by MI-based analysis of molecular dynamics simulations of molecular and macromolecular glass-formers on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity (DH) and secondary Johari–Goldstein (JG) relaxation processes. In a model molecular liquid with significant DH, MI reveals two populations of particles organised in clusters having either filamentous or compact globular structures that exhibit different mobility and relaxation properties. In a model polymer melt, MI provides clearer evidence of JG secondary relaxation and sharper insight into its DH. It is found that both DH and MI between the orientation and the displacement of the bonds reach (local) maxima at the time scales of the primary and JG secondary relaxation. This suggests that, in (macro)molecular systems, the mechanistic explanation of both DH and relaxation must involve rotation/translation coupling.

Ion Dynamics of Monomeric Ionic Liquids Polymerized In Situ within Silica Nanopores

Polymerized ionic liquids are a promising class of versatile solid-state electrolytes for applications ranging from electrochemical energy storage to flexible smart materials that remain limited by their relatively low ionic conductivities compared to conventional electrolytes. Here, we show that the in situ polymerization of the vinyl cationic monomer, 1-ethyl-3-vinylimidazolium with the bis(trifluoromethanesulfonyl)imide counteranion, under nanoconfinement within 7.5 ± 1.0 nm diameter nanopores results in a nearly 1000-fold enhancement in the ionic conductivity compared to the material polymerized in bulk. Using insights from broadband dielectric and Raman spectroscopic techniques, we attribute these results to the role of confinement on molecular conformations, ion coordination, and subsequently the ionic conductivity in the polymerized ionic liquid. These results contribute to the understanding of the dynamics of nanoconfined molecules and show that in situ polymerization under nanoscale geometric confinement is a promising path toward enhancing ion conductivity in polymer electrolytes.

Irreversible adsorption of polymer melts and nanoconfinement effects

For almost a decade, growing experimental evidence has revealed a strong correlation between the properties of nanoconfined polymers and the number of chains irreversibly adsorbed onto nonrepulsive interfaces, e.g. the supporting substrate of thin polymer coatings, or nanofillers dispersed in polymer melts. Based on such a correlation, it has already been possible to tailor structural and dynamics properties - such as the glass transition temperature, the crystallization rate, the thermal expansion coefficients, the viscosity and the wettability - of nanomaterials by controlling the adsorption kinetics. This evidence indicates that irreversible adsorption affects nanoconfinement effects. More recently, also the opposite phenomenon was experimentally observed: nanoconfinement alters interfacial interactions and, consequently, also the number of chains adsorbed in equilibrium conditions. In this review we discuss this intriguing interplay between irreversible adsorption and nanoconfinement effects in ultrathin polymer films. After introducing the methods currently used to prepare adsorbed layers and to measure the number of irreversibly adsorbed chains, we analyze the models employed to describe the kinetics of adsorption in polymer melts. We then discuss the structure of adsorbed polymer layers, focusing on the complex macromolecular architecture of interfacial chains and on their thermal expansion; we examine the way in which the structure of the adsorbed layer affects the thermal glass transition temperature, vitrification, and crystallization. By analyzing segmental dynamics of 1D confined systems, we describe experiments to track the changes in density during adsorption. We conclude this review with an analysis of the impact of nanoconfinement on adsorption, and a perspective on future work where we also address the key ideas of irreversibility, equilibration and long-range interactions.

Tea-polyphenol treated skin collagen owns coalesced adaptive-hydration, tensile strength and shape-memory property

Tea-polyphenol, as non-toxic skincare, even a therapeutic agent, was extensively studied from chemical, biological and physiological perspectives. This study reveals physical intelligences of a tea-polyphenol treated skin collagen (TP-treated SC) through a material-approach. Compared to untreated one, the TP-treated SC shows resistance to over-swelling and dehydration damage. There exists an inflection point in stress value of TP-treated SC below extension of 25%. Such promptly transformation from flexibility to stiffness is self-adaptive stretch behavior. Moreover, TP-treated SC owns water responsive shape-memory property. These effects are attributed to polyphenol as plasticizer with chains crosslinked to multi-sites on collagen-fibers as netpoints. The discovery, mechanism and method, which have not been reported before, may help to develop new shape memory device, skincare products, as well as provides insights into the physiological behavior of collagen contained tissue.

Coincident Correlation between Vibrational Dynamics and Primary Relaxation of Polymers with Strong or Weak Johari-Goldstein Relaxation

The correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known.

Vibrational scaling of the heterogeneous dynamics detected by mutual information

The correlations detected by the mutual information in the propensities of a molecular viscous liquid are studied by molecular-dynamics simulations. Dynamic heterogeneity is evidenced and two particle fractions with different mobility and relaxation identified. The two fractions exhibit the scaling of their relaxation in terms of the rattling amplitude of the particle trapped in the cage of the first neighbours 〈u²〉 . The scaling master curve does not differ from the one found for bulk systems, thus confirming identical results previously reported in other systems with strong dynamic heterogeneity as thin molecular films. The excitation of planar and globular structures at short and long times with respect to structural relaxation, respectively, is revealed. Some of the globular structures are different from the ones evidenced in atomic mixtures. States with equal 〈u²〉 are found to have identical time dependence of several quantities, referring to both bulk and the two fractions with heterogeneous dynamics, at least up to the structural relaxation time [Formula: see text].

Dynamical Line-Shifts in High-Field Electron Spin Resonance: Applications to Polymer Physics

High-field high-frequency Electron Paramagnetic Resonance spectroscopy (HF 2 -EPR) is a powerful tool to investigate, with ultra-high angular resolution, the rotational dynamics of complex systems like polymers, viscous fluids and glasses. Usually, information is drawn by detailed numerical analysis of the overall lineshape. Here, we present a simplified analytical model of the line shifts due to the rotational dynamics of the paramagnetic centre. The model captures the basic features of the reorientation process (time scale and size of the angular jump). It is compared with experimental results concerning the reorientation of a paramagnetic guest molecule dissolved in polystyrene. We find that, if the rotational model to describe the reorientation of the radical is consistent, the best-fit parameters yield equally acceptable best-fits of the overall spectrum by numerical simulations and dynamical line shifts by independent analytic expressions.

Molecular probe dynamics reveals suppression of ice-like regions in strongly confined supercooled water

The structure of the hydrogen bond network is a key element for understanding water's thermodynamic and kinetic anomalies. While ambient water is strongly believed to be a uniform, continuous hydrogen-bonded liquid, there is growing consensus that supercooled water is better described in terms of distinct domains with either a low-density ice-like structure or a high-density disordered one. We evidenced two distinct rotational mobilities of probe molecules in interstitial supercooled water of polycrystalline ice [Banerjee D, et al. (2009) ESR evidence for 2 coexisting liquid phases in deeply supercooled bulk water. Proc Natl Acad Sci USA 106: 11448–11453]. Here we show that, by increasing the confinement of interstitial water, the mobility of probe molecules, surprisingly, increases. We argue that loose confinement allows the presence of ice-like regions in supercooled water, whereas a tighter confinement yields the suppression of this ordered fraction and leads to higher fluidity. Compelling evidence of the presence of ice-like regions is provided by the probe orientational entropy barrier which is set, through hydrogen bonding, by the configuration of the surrounding water molecules and yields a direct measure of the configurational entropy of the same. We find that, under loose confinement of supercooled water, the entropy barrier surmounted by the slower probe fraction exceeds that of equilibrium water by the melting entropy of ice, whereas no increase of the barrier is observed under stronger confinement. The lower limit of metastability of supercooled water is discussed.

Idealized glass transitions under pressure: dynamics versus thermodynamics

The interplay of slow dynamics and thermodynamic features of dense liquids is studied by examining how the glass transition changes depending on the presence or absence of Lennard-Jones-like attractions. Quite different thermodynamic behavior leaves the dynamics unchanged, with important consequences for high-pressure experiments on glassy liquids. Numerical results are obtained within mode-coupling theory (MCT), but the qualitative features are argued to hold more generally. A simple square-well model can be used to explain generic features found in experiment.

Structural properties of crystallizable polymer melts: Intrachain and interchain correlation functions

We present results from constant pressure molecular-dynamics simulations for a bead-spring model of a crystallizable polymer melt. Our model has two main features, a chemically realistic intrachain rigidity and a purely repulsive interaction between nonbonded monomers. By means of intrachain and interchain structure factors we explore polymer conformation and melt structure above and below the temperature T{crys}{hom} of homogeneous crystallization. Here, we do not only determine average spatial correlations, but also site-specific correlations which depend on the position of the monomers along the polymer backbone. In the liquid phase above T{crys}{hom} we find that this site dependence can be well-accounted for by known theoretical approximations, the Koyama distribution for the intrachain structure and the polymer reference interaction site model (PRISM) for the interchain structure. This is no longer true in the semicrystalline phase. Below T{crys}{hom} short chains fully extend upon crystallization, whereas sufficiently long chains form chain-folded lamellae which coexist with amorphous regions. The structural features of these polymer crystals lead to violations of premises of the Koyama approximation or PRISM theory so that both theoretical approaches cannot be applied simultaneously. Furthermore, we find a violation of the Hansen-Verlet freezing criterion; our polymer melt crystallizes more easily than a simple liquid. This hints at the importance of the coupling between conformation (backbone rigidity) and density (packing constraints) for polymer crystallization.

Accurate excluded-volume corrections to the single-chain static properties of a melt of unentangled polymers

Actual polymer chains cannot cross themselves and each other. However, the popular Rouse model for unentangled polymers considers the chains as being like 'phantoms'. It is shown that excluded volume effects on single-chain statics may be introduced by analytic corrections to the Rouse results. The final expressions do not depend on free parameters. They exhibit excellent agreement with the molecular-dynamics simulations of polymer melts with chain lengths in the range 3≤M≤30. Preliminary results for entangled polymer melts are presented.

Role of the density in the crossover region of o-terphenyl and poly(vinyl acetate)

The coupling between the reorientation of molecular probes and the density in one low-molar mass glass former [ o -terphenyl (OTP)] and one polymer [poly(vinyl acetate) (PVAc)] is studied in the Goldstein's crossover region where the structural (alpha) and the secondary (beta) relaxations bifurcate. The coupling is found to be strong in OTP and virtually absent in PVAc. The probes sense both the alpha and beta relaxations, and locate their splitting accurately. It is concluded that the density affects the relaxation occurring in the crossover region of OTP but not of PVAc at subnanometer length scales. The findings are compared with recent assessments of the role of the molecular packing close and above the glass transition temperature T(g).