Pressure Effects on the Segmental Dynamics of Hydrogen-Bonded Polymer Blends

Crystallisation Kinetics and Associated Electrical Conductivity Dynamics of Poly(Ethylene Vinyl Acetate) Nanocomposites in the Melt State

Nanocomposite systems comprised of a poly(ethylene vinyl acetate) (EVA) matrix and carbon black (CB) or graphene nanoplatelets (GNPs) were used to investigate conductivity and crystallisation dynamics using a commercially relevant melt-state mixing process. Crystallisation kinetics and morphology, as investigated by DSC and SEM, turn out to depend on the interplay of (i) the interphase interactions between matrix and filler, and (ii) the degree of filler agglomeration. For the GNP-based systems, an almost constant conductivity value was observed for all compositions upon cooling, something not observed for the CB-based compositions. These conductivity changes reflect structural and morphological changes that can be associated with positive and negative thermal expansion coefficients. GNP-based systems were observed to exhibit a percolation threshold of approximately 2.2 vol%, lower than the 4.4 vol% observed for the CB-based systems.

Ultrastable glasses portray similar behaviour to ordinary glasses at high pressure

Pressure experiments provide a unique opportunity to unravel new insights into glass-forming liquids by exploring its effect on the dynamics of viscous liquids and on the evolution of the glass transition temperature. Here we compare the pressure dependence of the onset of devitrification, T_on, between two molecular glasses prepared from the same material but with extremely different ambient-pressure kinetic and thermodynamic stabilities. Our data clearly reveal that, while both glasses exhibit different dT_on/dP values at low pressures, they evolve towards closer calorimetric devitrification temperature and pressure dependence as pressure increases. We tentatively interpret these results from the different densities of the starting materials at room temperature and pressure. Our data shows that at the probed pressures, the relaxation time of the glass into the supercooled liquid is determined by temperature and pressure similarly to the behaviour of liquids, but using stability-dependent parameters.

The Modified VFT law of glass former materials under pressure: Part II: Relation with the equation of state

The dynamical properties of glass formers (GFs) as a function of P, V, and T are reanalyzed in relation with the equations of state (EOS) proposed recently (Eur. Phys. J. E 37, 113 (2014)). The relaxation times τ of the cooperative non-Arrhenius α process and the individual Arrhenius β process are coupled via the Kohlrausch exponent n S(T, P). In the model n S is the sigmoidal logistic function depending on T (and P, and the α relaxation time τ α of GFs above T g verifies the pressure-modified VFT law: log τ α ∼ E β /nsRT, which can be put into a form with separated variables: log τ α ∼ f(T)g(P). From the variation of n S and τ α with T and P the Vogel temperature T 0 (τ α → ∝, n S = 0) and the crossover temperature (also called the merging or splitting temperature) T B (τ α ∼ τ β, n S ∼ 1) are determined. The proposed sm-VFT equation fits with excellent accuracy the experimental data of fragile and strong GFs under pressure. The properties generally observed in organic mineral and metallic GFs are explained: a) The Vogel temperature is independent of P (as suggested by the EOS properties), the crossover is pressure-dependent. b) In crystallizable GFs the T B (P) and Clapeyron curves T m(P) coincide. c) The α and β processes have the same ratio of the activation energies and volume, E*/V* (T- and P-independent), the compensation law is observed, this ratio depends on the anharmonicity Slater-Grüneisen parameter and on the critical pressure P* deduced from the EOS. d) The properties of the Fan Structure of the Tangents (FST) to the isotherms and isobars curves log τ versus P and T and to the isochrones curves P(T). e) The scaling law log τ = f(V (Λ) ) and the relation between Γ and γ. We conclude that these properties should be studied in detail in GFs submitted to negative pressures.

Molecular insights into the damping mechanism of poly(vinyl acetate)/hindered phenol hybrids by a combination of experiment and molecular dynamics simulation

By combining experiment and MD simulation, the relationship between hydrogen bond evolution and damping property variation of PVAc was revealed.

High pressure dynamics of polymer/plasticizer mixtures

Plasticizers are usually added to polymers to give them the desired flexibility and processability by changing the dynamical properties of the polymer chains. It is therefore important to give a quantitative description about how the dynamic behavior of a given polymer is modified by the incorporation of a second component. We analyze in this work, by means of dielectric spectroscopy, the dynamics of poly(vinyl acetate)/diethyl phthalate mixtures, at different concentrations, over a broad range of frequency, pressure, and temperature. The dynamics of these particular mixtures show only one main relaxation process contrarily to what is observed in athermal miscible polymer mixtures. From the dielectric spectra the maximum relaxation time as a function of pressure and temperature was obtained and analyzed. We studied the pressure dependence of the glass transition temperature as well as the fragility of both the neat components and the mixtures at different concentrations (on the rich polymer range). Finally, the experimental data were rationalized within the framework of an Adam-Gibbs (AG) based approach recently developed [G. A. Schwartz et al., J. Chem. Phys. 127, 154907 (2007)]. The model, originally developed for athermal blends, is here modified to take into account the non-negligible interaction between polymer and plasticizer. We found that the temperature-pressure dependence of the alpha-relaxation time is very well described by this AG extended model.

Effect of chain length on fragility and thermodynamic scaling of the local segmental dynamics in poly(methylmethacrylate)

Local segmental relaxation properties of poly(methylmethacrylate) (PMMA) of varying molecular weight are measured by dielectric spectroscopy and analyzed in combination with the equation of state obtained from PVT measurements. Significant variations of glass transition temperature and fragility with molecular weight are observed. In accord with the general properties of glass-forming materials, single molecular weight dependent scaling exponent gamma is sufficient to define the mean segmental relaxation time taualpha and its distribution. This exponent can be connected to the Gruneisen parameter and related thermodynamic quantities, thus demonstrating the interrelationship between dynamics and thermodynamics in PMMA. Changes in the relaxation properties ("dynamic crossover") are observed as a function of both temperature and pressure, with taualpha serving as the control parameter for the crossover. At longer taualpha another change in the dynamics is apparent, associated with a decoupling of the local segmental process from ionic conductivity.

The role of temperature and density on the glass-transition dynamics of glass formers

A correlation between the monomeric volume and the dynamic quantity E*(V)/H*, used to provide a quantitative measure of the role of temperature and density on the dynamics, is demonstrated for a series of polymers and glass-forming liquids. We show that monomeric volume and local packing play a key role in controlling the value of this ratio and thus the dynamics associated with the glass temperature.

Do Theories of the Glass Transition, in which the Structural Relaxation Time Does Not Define the Dispersion of the Structural Relaxation, Need Revision?

Upon decreasing temperature or increasing pressure, a noncrystallizing liquid will vitrify; that is, the structural relaxation time, taualpha, becomes so long that the system cannot attain an equilibrium configuration in the available time. Theories, including the well-known free volume and configurational entropy models, explain the glass transition by invoking a single quantity that governs the structural relaxation time. The dispersion of the structural relaxation (i.e., the structural relaxation function) is either not addressed or is derived as a parallel consequence (or afterthought) and thus is independent of taualpha. In these models the time dependence of the relaxation bears no fundamental relationship to the value of taualpha or other dynamic properties. Such approaches appear to be incompatible with a general experimental fact recently discovered in glass-formers: for a given material at a fixed value of taualpha, the dispersion is constant, independent of thermodynamic conditions (T and P); that is, the shape of the alpha-relaxation function depends only on the relaxation time. If derived independently of taualpha, it is an unlikely result that the dispersion of the structural relaxation would be uniquely defined by taualpha.

Pressure-enhanced dynamic heterogeneity in block copolymers of poly(methyl vinyl ether) and poly(isobutyl vinyl ether)

The static structure factor and associated dynamics have been investigated in a series of block copolymers of poly(methyl vinyl ether) (PMVE) and poly(isobutyl vinyl ether) (PiBVE) using x-ray scattering and dielectric spectroscopy (DS). The origin of the dynamic arrest at the glass temperature (T(g)) of PiBVE has been explored by temperature- and pressure-dependent DS and pressure-volume-temperature measurements. Both temperature and volume are responsible for the segmental dynamics but temperature has a stronger effect. The copolymers display a minimal dynamic asymmetry (Delta T(g) approximately 7 K), nevertheless, are spatially and dynamically heterogeneous. Increasing pressure, unlike temperature, enhances the dynamic heterogeneity. This effect originates from the distinctly different pressure sensitivities of the homopolymers and can be traced back to differences in local packing.