Static Properties of a Simulated Supercooled Polymer Melt: Structure Factors, Monomer Distributions Relative to the Center of Mass, and Triple Correlation Functions

We analyze structural and conformational properties in a simulated bead-spring model of a non-entangled, supercooled polymer melt. We explore the statics of the model via various structure factors, involving not only the monomers, but also the center of mass (CM). We find that the conformation of the chains and the CM-CM structure factor, which is well described by a recently proposed approximation [Krakoviack et al., Europhys. Lett. 58, 53 (2002)], remain essentially unchanged on cooling toward the critical glass transition temperature of mode-coupling theory. Spatial correlations between monomers on different chains, however, depend on temperature, albeit smoothly. This implies that the glassy behavior of our model cannot result from static intra-chain or CM-CM correlations. It must be related to inter-chain correlations at the monomer level. Additionally, we study the dependence of inter-chain correlation functions on the position of the monomer along the chain backbone. We find that this site-dependence can be well accounted for by a theory based on the polymer reference interaction site model (PRISM). We also analyze triple correlations by means of the three-monomer structure factors for the melt and for the chains. These structure factors are compared with the convolution approximation that factorizes them into a product of two-monomer structure factors. For the chains this factorization works very well, indicating that chain connectivity does not introduce special triple correlations in our model. For the melt deviations are more pronounced, particularly at wave vectors close to the maximum of the static structure factor.Comment: REVTeX4, 16 pages, 16 figures, accepted for publication in Physical Review

Structural and conformational dynamics of supercooled polymer melts: Insights from first-principles theory and simulations

We report on quantitative comparisons between simulation results of a bead-spring model and mode-coupling theory calculations for the structural and conformational dynamics of a supercooled, unentangled polymer melt. We find semiquantitative agreement between simulation and theory, except for processes that occur on intermediate length scales between the compressibility plateau and the amorphous halo of the static structure factor. Our results suggest that the onset of slow relaxation in a glass-forming melt can be described in terms of monomer-caging supplemented by chain connectivity. Furthermore, a unified atomistic description of glassy arrest and of conformational fluctuations that (asymptotically) follow the Rouse model, emerges from our theory.Comment: 54 pages, 10 figure

Molecular dynamics simulations of glassy polymers

We review recent results from computer simulation studies of polymer glasses, from chain dynamics around the glass transition temperature Tg to the mechanical behaviour below Tg. These results clearly show that modern computer simulations are able to address and give clear answers to some important issues in the field, in spite of the obvious limitations in terms of length and time scales. In the present review we discuss the cooling rate effects, and dynamic slowing down of different relaxation processes when approaching Tg for both model and chemistry-specific polymer glasses. The impact of geometric confinement on the glass transition is discussed in detail. We also show that computer simulations are very useful tools to study structure and mechanical response of glassy polymers. The influence of large deformations on mechanical behaviour of polymer glasses in general, and strain hardening effect in particular are reviewed. Finally, we suggest some directions for future research, which we believe will be soon within the capabilities of state of the art computer simulations, and correspond to problems of fundamental interest.Comment: To apear in "Soft Matter

Etude de la cristallisation des polymères par simulation numérique

Les polymères semi-cristallins sont des systèmes d'un grand intérêt industriel de par leur très nombreuses applications, et constituent également un riche sujet d'étude du fait de la complexité des structures formées au cours de la cristallisation, ainsi que des mécanismes à l'œuvre lors de celle-ci.Nous avons étudié la cristallisation des polymères par simulation numérique de plusieurs façons différentes : des simulations détaillées à l'échelle atomique nous ont permis de reproduire la structure cristalline de chaînes courtes d'alcanes à basse température, dont la simulation lors d'un réchauffement permet de retrouver une phase transitoire bien caractérisée expérimentalement. Le même modèle a été utilisé pour simuler le processus de cristallisation à partir du fondu au cours d'un refroidissement; ce type de simulations réaliste ne permet pas de reproduire des structures cristallines en un temps de simulation restreint. Afin de pouvoir reproduire efficacement des structures semi-cristallines aux caractéristiques réalistes, nous avons utilisé un autre type de simulations numériques nous permettant de considérer des échelles de longueur et de temps plus importantes : ce modèle coarse-grained a permis d'étudier le phénomène de cristallisation en détail, à l'aide de différents paramètres d'ordre caractérisant le cristal et son évolution au cours du temps.Nous avons aussi procédé à une étude détaillée des facteurs de structure du fondu de polymères à haute température de manière à déterminer quelle est l'influence de la structure du liquide sur la formation du cristal.Ces différentes études permettent une meilleure compréhension de l'influence sur le phénomène de cristallisation des différents paramètres utilisés dans la définition des modèles de simulation numérique.Semi-crystalline polymers are of great interest for industrial purposes, and the complex structures they involve as well as the mechanisms leading to the formation of crystals make their study very challenging.We investigated polymer crystallization by computer simulation via different methods: An atomisticly detailed model was used to reproduce the crystalline structure of short alkanes at low temperature, and continuous heating simulations gave rise to a transient phase that is well characterized in experiments. The same realistic model was used to simulate continuous cooling of the melt, but could not yield crystalline structures in a limited simulation time.In order to reproduce efficiently the characteristic features of semi-crystalline polymers, we used another simulation model which addresses larger length and time scales: This coarse-grained model allowed us to study the crystallization phenomenon in detail with several order parameters to characterize the crystal and its time evolution. The detailed study of the structure factors of the high-temperature melt has also been investigated so as to determine the influence of the liquid phase structure on crystal formation.These different studies yield a better understanding of the influence on crystallization of the various parameters entering the definitions of the simulation models.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Monte Carlo simulation of the glass transition in polymer melts : extended mode-coupling analysis

In this paper, we reanalyse the incoherent intermediate scattering function, determined by a Monte Carlo simulation of a polymer melt, in the framework of the extended mode-coupling theory (MCT). A previous analysis with the idealized MCT showed systematic deviations between theory and simulation at low temperatures, which could be qualitatively attributed to the neglect of hopping processes. If these hopping processes are taken into account quantitatively by the application of the extended MCT, the discrepancies at low temperatures disappear, and a (quite)accurate estimate of the critical temperature becomes passible