Simulated glass-forming polymer melts: Glass transition temperature and elastic constants of the glassy state |
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Authors: | B. Schnell H. Meyer C. Fond J. P. Wittmer J. Baschnagel |
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Affiliation: | (1) CECAM, ENS-Lyon, 46 Allée dItalie, 69007 Lyon, France;(2) Institut Charles Sadron, 6 rue Boussingault, 67083 Strasbourg, France;(3) Institut für Physik, Johannes-Gutenberg Universität, 55099 Mainz, Germany |
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Abstract: | By means of molecular-dynamics simulation we study a flexible and a semiflexible bead-spring model for a polymer melt on cooling through the glass transition. Results for the glass transition temperature T g and for the elastic properties of the glassy state are presented. We find that T g increases with chain length N and is for all N larger for the semiflexible model. The N dependence of T g is compared to experimental results from the literature. Furthermore, we characterize the polymer glass below T g via its elastic properties, i.e., via the Lamé coefficients λ and μ. The Lamé coefficients are determined from the fluctuation formalism which allows to split λ and μ into affine (Born term) and nonaffine (fluctuation term) contributions. We find that the fluctuation term represents a substantial correction to the Born term. Since the Born terms for λ and μ are identical, the fluctuation terms are responsible for the different temperature dependence of the Lamé coefficients. While λ decreases linearly on approaching T g from below, the shear modulus μ displays a much stronger decrease near T g. From the present simulation data it is not possible to decide whether μ takes a finite value at T g, as would be expected from mode-coupling theory, or vanishes continuously, as suggested by recent work from replica theory. |
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