The emergence of cooperative dynamics in polymers as an effect of conformational restrictions: The case of crystallization and an example on heterogeneous confinement (original) (raw)
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Relationship between dynamics and thermodynamics in glass-forming polymers
EPL (Europhysics Letters), 2005
We have tested the validity of the Adam-Gibbs (AG) equation to glass-forming polymers by means of dynamics data from dielectric relaxation and thermodynamic data. The AG equation holds for polymers with simple monomeric chemical structure, whereas it fails for more complex systems possessing secondary relaxation processes with some degree of intra-chain cooperativity. However, we found that the AG equation still works once the contribution to the excess entropy of intra-chain secondary relaxation processes uncoupled to the process is removed. This contribution results to be essentially temperature independent.
Glassy dynamics under nanoscale confinement is currently a topic under intense debate in soft matter physics. The reason is that this kind of studies may deliver important insight on the glassy dynamics in general. Furthermore, from a technological point of view, there exists a rising interest in the understanding of how properties are modified at the nanoscale in comparison to the corresponding bulk system. Within this context, this chapter critically discusses the experimental findings in the field. The vast majority of results concerns thin polymer films. However, other geometries of confinement, such as polymer nanocomposites and nanospheres, are considered as well. Special attention is devoted to the kind of information achieved by a specific technique. Within this context, the ability of dielectric and calorimet-ric techniques is highlighted. Particular attention is devoted to the determination of the different aspects of glassy dynamics in confinement, that is, the equilibrium dynamics in terms of the rate of spontaneous fluctuations as probed by experiments where a perturbation in the linear regime is applied, on the one hand, and the out-of-equilibrium dynamics in terms of thermal glass transition temperature (T g) and the physical aging on the other. In the latter case, the application of a temperature ramp for T g measurements and the recovery of equilibrium in physical aging imply the application of large perturbations, in particular with amplitude well beyond that of spontaneous fluctuations. It is demonstrated how, in view of numerous experimental results, the two aspects are not one-to-one related in confinement. Specifically, the reduction in T g and the acceleration of equilibrium recovery in the aging regime does not imply a concomitant speed-up of the rate of spontaneous fluctuations, which is in several cases found to be unaltered in comparison to the bulk. Finally, a description of suitable frameworks to describe such phenomenology is presented with special attention to the free volume hole diffusion (FVHD) model. This is shown to quantitatively catch the acceleration of physical aging and the T g depression with no need to assume any acceleration on the intrinsic molecular mobility of the glass former.
Journal of Non-Crystalline Solids, 2015
Understanding why the glass transition temperature (T g ) of polymers deviates substantially from the bulk with nanoscale confinement has been a 20-year mystery. Ever since the observation in the mid-1990s that the T g values of amorphous polymer thin films are different from their bulk values, efforts to understand this behavior have intensified, and the topic remains the subject of intense research and debate. This is due to the combined scientific and technological implications of size-dependent glassy properties. Here, we discuss an intriguing aspect of the glassy behavior of confined amorphous polymers. As experimentally assessed, the glass transition is a dynamic event mediated by segmental dynamics. Thus, it seems intuitive to expect that a change in T g due to confinement necessitates a corresponding change in molecular dynamics, and that such change in dynamics may be predicted based on our understanding of the glass transition. The aim of this perspectives article is to examine whether or not segmental dynamics change in accordance with the value of T g for confined polymers based on bulk rules. We highlight past and recent findings that have examined the relationship between T g and segmental dynamics of confined polymers. Within this context, the decoupling between these two aspects of the glass transition in confinement is emphasized. We discuss these results within the framework of our current understanding of the glass transition as well as efforts to resolve this decoupling. Finally, the anomalous decoupling between translational (diffusion) and rotational (segmental) motion taking place in the proximity of attractive interfaces in polymer thin films is discussed.
Dynamics and thermodynamics of polymer glasses
The fate of matter when decreasing the temperature at constant pressure is that of passing from gas to liquid and, subsequently, from liquid to crystal. However, a class of materials can exist in an amorphous phase below the melting temperature. On cooling such materials, a glass is formed; that is, a material with the rigidity of a solid but exhibiting no long-range order. The study of the thermodynamics and dynamics of glass-forming systems is the subject of continuous research. Within the wide variety of glass formers, an important sub-class is represented by glass forming polymers. The presence of chain connectivity and, in some cases, conformational disorder are unfavourable factors from the point of view of crystallization. Furthermore, many of them, such as amorphous thermoplastics, thermosets and rubbers, are widely employed in many applications. In this review, the peculiarities of the thermodynamics and dynamics of glass-forming polymers are discussed, with particular emphasis on those topics currently the subject of debate. In particular, the following aspects will be reviewed in the present work: (i) the connection between the pronounced slowing down of glassy dynamics on cooling towards the glass transition temperature (Tg) and the thermodynamics; and, (ii) the fate of the dynamics and thermodynamics below Tg. Both aspects are reviewed in light of the possible presence of a singularity at a finite temperature with diverging relaxation time and zero configurational entropy. In this context, the specificity of glass-forming polymers is emphasized.
Polymer, 2014
The relaxation dynamics of poly(pentamethylene terephthalate) has been investigated by means of dielectric spectroscopy. The sub-glass dynamics is characterized by the existence of a bimodal β process whose faster and slower components have been assigned to the relaxation of the bond between the ester oxygen and the aliphatic carbon and to the link between the aromatic ring carbon and the ester carbon, respectively. By comparison with other closely related aromatic polyesters it is shown that the faster component strongly depends on the amount of methylene groups while the slower one is not considerably affected by the nature of the glycol subunit. The changes in the process associated to the segmental relaxation during cold crystallization reveal the formation of a rigid amorphous phase fraction. Combination of dielectric experiments with X-ray scattering ones suggests that during cold crystallization PPT crystal lamellae tend to fill the space homogeneously.
The European Physical Journal E, 2009
The effect of confinement in the segmental relaxation of polymers is considered. On the basis of a thermodynamic model we discuss the emerging relevance of the fast degrees of freedom in stimulating the much slower segmental relaxation, as an effect of the constraints at the walls of the amorphous regions. In the case that confinement is due to the presence of crystalline domains, a quasi-poissonian distribution of local constraining conditions is derived as a result of thermodynamic equilibrium. This implies that the average free energy barrier ∆F for conformational rearrangement is of the same order of the dispersion of the barrier heights, δ(∆F), around ∆F. As an example, we apply the results to the analysis of the α-relaxation as observed by dielectric broad band spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized from either an isotropic or an oriented glass. It is found that in the latter case the regions of cooperative rearrangement are significantly larger than in the former.
Polymer, 2002
The aim of this work is to determine the relaxation times of the cooperative conformational rearrangements of the amorphous phase in semi-crystalline poly(ethylene terephthalate) (PET) and compare them with those calculated in amorphous PET. Samples of nearly amorphous polymer were prepared by quenching and samples with different crystallinity fractions were prepared from the amorphous one using cold crystallisation to different temperatures. The differential scanning calorimetry (DSC) thermograms measured on samples rapidly cooled from temperatures immediately above the glass transition show a single glass transition which is much broader in the case of high-crystallinity samples than in the amorphous or low-crystallinity PET. To clarify this behaviour, the samples were subjected to annealing at different temperatures and for different periods prior to the DSC measuring heating scan. The thermograms measured in samples with low crystallinity clearly show the existence of two amorphous phases with different conformational mobility, these are called Phases I and II. Phase I contains polymer chains with a mobility similar to that in the purely amorphous polymer, while Phase II shows a much more restricted mobility, probably corresponding to conformational changes within the intraspherulitic regions. The model simulation allows to determine the temperature dependence of Phase II relaxation times, which are independent from the crystallinity fraction in the sample and around two decades longer than those of the amorphous polymer at the same temperature. q
Cooperativity in plastic crystals
Physical Review E, 2018
A statistical mechanical model previously adopted for the analysis of the α-relaxation in structural glass formers is rederived within a general theoretical framework originally developed for systems approaching the ideal glassy state. The interplay between nonexponentiality and cooperativity is reconsidered in the light of energy landscape concepts. The method is used to estimate the cooperativity in orientationally disordered crystals, either from the analysis of literature data on linear dielectric response or from the enthalpy relaxation function obtained by temperature-modulated calorimetry. Knowledge of the specific heat step due to the freezing of the configurational or conformational modes at the glass transition is needed in order to properly account for the extent to which the relaxing system deviates from equilibrium during the rearrangement processes. A number of plastic crystals have been analyzed, and relatively higher cooperativities are found in the presence of hydrogen bonding interaction.
Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers
Entropy
Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature Tg, whether there is an underlying thermodynamic transition at some finite temperature below Tg, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section disc...
The Journal of Chemical Physics, 2018
The inter-and intra-molecular interactions as they evolve in the course of glassy solidification are studied by broadband dielectric-and Fourier-transform infrared-spectroscopy for oligomeric derivatives of poly(ethylene glycol) derivatives, namely, poly(ethylene glycol) phenyl ether acrylate and poly(ethylene glycol) dibenzoate in the bulk and under confinement in nanoporous silica having mean pore diameters 4, 6, and 8 nm, with native and silanized inner surfaces. Analyzing the spectral positions and the oscillator strengths of specific IR absorption bands and their temperature dependencies enables one to trace the changes in the intra-molecular potentials and to compare it with the dielectrically determined primarily inter-molecular dynamics. Special emphasis is given to the calorimetric glass transition temperature T g and T αβ ≈ 1.25T g , where characteristic changes in conformation appear, and the secondary β-relaxation merges with the dynamic glass transition (α-relaxation). Furthermore, the impact of main chain conformations, inter-and intra-molecular hydrogen bonding, and nanometric confinement on the dynamic glass transition is unraveled.