Modelling Surface Properties of Linear Amorphous Polymers
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Modelling Surface Properties of Linear Amorphous Polymers S. Goudeau1, J. Galy1, J.F Gerard1, R. Fulchiron2, J.L Barrat3 1
: LMM (UMR CNRS 5627), INSA Lyon, 20 Av. A Einstein, 69621 Villeurbanne, FRANCE : LMPB, ISTIL - (UMR CNRS 5627), UCB Lyon1, 43 Bd 11 nov. 1918, 69622 Villeurbanne 3 : Dpt. Physique des Matériaux – UCB Lyon1, 43 Bd 11 nov. 1918, 69622 Villeurbanne, FRANCE 2
ABSTRACT Molecular dynamics simulations have been carried out on fully atomistic models of amorphous polymers. Both bulk structures and thin films were simulated for a wide range of thermoplastics such as PS, PMMA, poly(phenylene-ether) (PPE), poly(etherimide) (PEI). The influence of molecular weight and functional end-groups was investigated. The comparison between “bulk” and “surface” models clearly shows the increase of molecular mobility and a strong decrease in density at the vicinity of free surfaces. Simulated surface tensions, computed by different methods, are generally overestimated as compared to the experimental ones. PVT (Pressure-Volume-Temperature) measurements have been conducted, as another method to compute physical properties of amorphous polymers. From such measurements, cohesive energy densities, interaction parameters, and then interfacial properties, were obtained for various thermoplastics and the corresponding incompatible melts. Such a tool could be used for designing polymer surfaces and interfaces in polymer blends. INTRODUCTION Molecular simulations of polymer surfaces have known considerable interest since early 90s, as computer capabilities increased and became sufficient for such applications. Lattice Monte-Carlo simulations, and coarse-grained continuous-space molecular dynamics have first become popular methods for surface modelling, due to their computational efficiency and their tractability for large, multichain systems. Such simulations, in which the polymer chains are considered as necklaces of identical beads, each bead representing one or several monomer units, allowed to investigate chain packing and orientations at interfaces, as well as other conformational properties [1-2]. However, such models apply well for simple hydrocarbon chains, as PE or PP, but cannot be used to predict properties resulting from the specific chemistry of the surface, such as the surface tension of most polymers. As a consequence, fully atomistic simulation, taking into account local specific interactions, is in this case a more convenient technique, which is not so widely used since it is currently limited to short time (10-9s) and length (30-100Å) scales, and requires large computational capabilities. A large part of this work has however been carried out on a current workstation, using the Discover program as implemented through the InsightII molecular modelling software. Since the pioneering work of Mansfield and Theodorou [3], static and molecular dynamics atomistic simulations allowed to calculate surface and interfacial properties of polyolefins [46] , to investigate local interactions between various solvents and po
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