Structural relaxation and chain dynamics in polymer melts : a computational investigation on the role of the intramolecular barriers

149 p. : il. col.En este trabajo presentamos un estudio sobre el efecto de las barreras intramoleculares en distintos aspectos de los procesos dinámicos en polímeros semiflexibles de longitud corta ("non-entangled"). El trabajo se divide en dos partes. En la primera parte, mediante el uso de la técnica de simulaciones de dinámica molecular, presentamos un estudio sobre el efecto de las barreras en la dinámica desde el regimen de "efecto caja" característico de los sistemas formadores de vidrio, hasta la relajación de los modos de Rouse de la cadena polimérica. Los datos de simulación se comparan con soluciones numéricas de las ecuaciones de la Teoría de Modos Acoplados ("Mode Coupling Theory", MCT). De dicha comparación concluímos que las soluciones de la MCT para la relajación estructural reproducen cualitativamente los resultados del análisis de los datos de simulación para barreras de amplitud baja y moderada. Con respecto a la dinámica de cadena, analizamos la relajación de los grados de libertad internos de las cadenas en el marco del modelo de Rouse. Concluímos que la MCT reproduce semicuantitativamente las desviaciones del modelo de Rouse en presencia de barreras intramoleculares, proporcionando una base microscópica para la interpretación de las desviaciones anómalas observadas en los datos de simulación. En la segunda parte presentamos simulaciones de dinámica molecular de homopolímeros semiflexibles "non-entangled" en una matriz mucho más lenta de homopolímeros flexibles. Estudiamos el efecto de la asimetría dinámica en la relajación de cadena de la componente semiflexible de la mezcla. Observamos que los tiempos de relajación y las amplitudes estáticas de los modos de Rouse siguen las leyes de escala predichas por un modelo Markoviano que extiende el modelo de Rouse introduciendo la contribución de las barreras intramoleculares. Concluímos que los efectos de memoria inducidos por la matriz lenta no son relevantes para polímeros semiflexibles "non-entangled", y que la dinámica puede ser descrita por modelos Markovianos

Dynamic Arrest in Polymer Melts: Competition between Packing and Intramolecular Barriers

We present molecular dynamics simulations of a simple model for polymer melts with intramolecular barriers. We investigate structural relaxation as a function of the barrier strength. Dynamic correlators can be consistently analyzed within the framework of the Mode Coupling Theory (MCT) of the glass transition. Control parameters are tuned in order to induce a competition between general packing effects and polymer-specific intramolecular barriers as mechanisms for dynamic arrest. This competition yields unusually large values of the so-called MCT exponent parameter and rationalize qualitatively different observations for simple bead-spring and realistic polymers. The systematic study of the effect of intramolecular barriers presented here also establishes a fundamental difference between the nature of the glass transition in polymers and in simple glass-formers.Comment: 4 pages, 3 figures, 2 table

Static and dynamic contributions to anomalous chain dynamics in polymer blends

By means of computer simulations, we investigate the relaxation of the Rouse modes in a simple bead-spring model for non-entangled polymer blends. Two different models are used for the fast component, namely fully-flexible and semiflexible chains. The latter are semiflexible in the meaning that static intrachain correlations are strongly non-gaussian at all length scales. The dynamic asymmetry in the blend is strongly enhanced by decreasing temperature, inducing confinement effects on the fast component. The dynamics of the Rouse modes show very different trends for the two models of the fast component. For the fully-flexible case, the relaxation times exhibit a progressive deviation from Rouse scaling on increasing the dynamic asymmetry. This anomalous effect has a dynamic origin. It is not related to particular static features of the Rouse modes, which indeed are identical to those of the fully-flexible homopolymer, and are not modified by the dynamic asymmetry in the blend. On the contrary, in the semiflexible case the relaxation times exhibit approximately the same scaling behaviour as the amplitudes of the modes. This suggests that the origin of the anomalous dynamic scaling for semiflexible chains confined in the blend is esentially of static nature. We discuss implications of these observations for the applicability of theoretical approaches to chain dynamics in polymer blends.Comment: 15 pages (single-column), 6 figure

Technical aspects in dark matter investigations

Some theoretical and experimental aspects regarding the direct dark matter field are mentioned. In particular some arguments, which play a relevant role in the evaluation of model dependent interpretations of experimental results and in comparisons, are shortly addressed.Comment: Proceedings of TAUP 2011 Conferenc

Cluster glasses of ultrasoft particles

We present molecular dynamics (MD) simulations results for dense fluids of ultrasoft, fully-penetrable particles. These are a binary mixture and a polydisperse system of particles interacting via the generalized exponential model, which is known to yield cluster crystal phases for the corresponding monodisperse systems. Because of the dispersity in the particle size, the systems investigated in this work do not crystallize and form disordered cluster phases. The clustering transition appears as a smooth crossover to a regime in which particles are mostly located in clusters, isolated particles being infrequent. The analysis of the internal cluster structure reveals microsegregation of the big and small particles, with a strong homo-coordination in the binary mixture. Upon further lowering the temperature below the clustering transition, the motion of the clusters' centers-of-mass slows down dramatically, giving way to a cluster glass transition. In the cluster glass, the diffusivities remain finite and display an activated temperature dependence, indicating that relaxation in the cluster glass occurs via particle hopping in a nearly arrested matrix of clusters. Finally we discuss the influence of the microscopic dynamics on the transport properties by comparing the MD results with Monte Carlo simulations.Comment: 17 pages, 23 figure

From caging to Rouse dynamics in polymer melts with intramolecular barriers: A critical test of the mode coupling theory

12 páginas, 12 figuras.-- et al.By means of computer simulations and solution of the equations of the mode coupling theory (MCT), we investigate the role of the intramolecular barriers on several dynamic aspects of nonentangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question, provide new results, and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However, a progressive discrepancy is revealed as the limit of stiff chains is approached. This disagreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed deviations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and nonmonotonous wavelength dependence of the mode correlators.We acknowledge financial support from projects FP7-PEOPLE-2007-1-1-ITN (DYNACOP, EU), MAT2007-63681 (Spain), IT-436-07 (GV, Spain), ERC-226207-PATCHYCOLLOIDS (EU), and ITN- 234810-COMPLOIDS (EU).Peer reviewe

Diagnostic Performance of a Novel Noninvasive Workup in the Setting of Dry Eye Disease

Purpose. To evaluate the diagnostic performance of a novel noninvasive automated workup employed for the diagnosis of dry eye disease (DED). Methods. One hundred patients with mild to moderate DED and 100 matched control subjects were enrolled in this cross-sectional study. Ocular surface examinations were carried out by means of IDRA Plus (SBM Sistemi, Turin, Italy), which allows the automated evaluation of noninvasive breakup time (NIBUT), lipid layer thickness (LLT), tear meniscus height (TMH), infrared meibography for the measurement of meibomian gland loss (MGL), and blinking analysis. Continuous variables were compared between patients with DED and controls by using the Mann-Whitney U test. The area under the curve (AUC) of receiver operating characteristic curves was calculated. The correlations between ocular surface parameters were evaluated with Pearson correlation analysis. Results. Patients with DED showed significantly lower values of NIBUT, LLT, and TMH compared to controls (6.9 +/- 2.5 vs 10.4 +/- 2.4 s, P < 0.001; 64.6 +/- 20.3 vs 73.4 +/- 21.9 nm, P = 0.003; 0.231 +/- 0.115 vs 0.289 +/- 0.164, P = 0.012, respectively). Conversely, no significant differences were observed for MGL and blinking analysis (both P > 0.05). NIBUT had the highest diagnostic power (AUC = 0.841, sensitivity = 0.89, and specificity = 0.69), followed by LLT (AUC = 0.621, sensitivity = 0.89, and specificity = 0.55), TMH (AUC = 0.606, sensitivity = 0.57, and specificity = 0.63), blink analysis (AUC = 0.533, sensitivity = 0.48, and specificity = 0.59), and MGL (AUC = 0.531, sensitivity = 0.54, and specificity = 0.48). In patients with DED, NIBUT showed a significant correlation with TMH (R = 0.347, P = 0.002) and blinking analysis (R = 0.356, P < 0.001), while blinking analysis was negatively correlated with MGL (R = -0.315, P = 0.008). Conclusions. The automated noninvasive workup validated in this study may be a useful tool for reaching a noninvasive diagnosis of DED with a good performance, especially for NIBUT