Molecular Dynamics Study of E-Caprolactone Intercalated in Wyoming Sodium Montmorillonite Anouk Gaudel-Siri, †,‡ Patrick Brocorens, † Didier Siri, § Fabrice Gardebien, † Jean-Luc Bre ´das, †,| and Roberto Lazzaroni* ,† Service de Chimie des Mate ´ riaux Nouveaux, Universite ´ de Mons-Hainaut, 20 Place du Parc, B-7000 Mons, Belgium, Laboratoire Re ´ So - UMR 6516, Universite ´ d’Aix-Marseille 3, Faculte ´ St. Je ´ ro ˆ me, Av. Esc. Normandie-Niemen, 13397 Marseille Cedex 20, France, Laboratoire de Chimie The ´ orique et Mode ´ lisation Mole ´ culaire, CNRS-UMR 6517, Universite ´ s d’Aix-Marseille 1 et 3, Faculte ´ St. Je ´ ro ˆ me, Av. Esc. Normandie-Niemen, 13397 Marseille Cedex 20, France, and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 Received March 21, 2003. In Final Form: July 14, 2003 The intercalation process of the ǫ-caprolactone monomer into sodium montmorillonite clay is modeled with a combined molecular mechanics/molecular dynamics approach. This study is aimed at understanding the initial stages of caprolactone polymerization within the channels of the clay, to form highly dispersed nanocomposites. The theoretical method is first validated by modeling dry and hydrated Wyoming sodium montmorillonite; the caprolactone-intercalated clay is then investigated, with particular emphasis on the energetics of the intercalation process and the nature of the interactions building up between the organic molecules and the channel walls and sodium counterions. 1. Introduction Poly(ǫ-caprolactone) (PCL) is a biodegradable aliphatic polyester that is being intensively investigated for use in medical devices and degradable packaging. However, the mechanical properties of the PCL homopolymer are too poor to allow its direct use. To improve the mechanical properties and thermal stability, nanocomposites based on PCL and layered aluminosilicates (montmorillonite- type clay) represent a valuable alternative. Because of the dispersion of nanometer-size clay sheets, nanocom- posites exhibit new and improved properties compared to microcomposites or filled polymers. 1,2 Polymer nanocomposites are usually prepared by melt intercalation; in that procedure, alkali-metal cations of the clay must first be replaced by organic surfactants in order to render the clay channels more organophilic and to allow the intercalation of the polymer. The physico- chemical parameters governing the intercalation process and the influence of the organic counterions on the stability of the nanocomposites have been extensively investigated by model calculations (see refs 3-5 and references therein). Those studies allow for the a priori design of composite materials with a well-defined microstructure. Remarkably, the need for those organophilic counterions can be circumvented if the polymer is directly generated within the channels. It has been shown recently that the ǫ-caprolactone monomers (CL) spontaneously intercalate at ambient temperature between Na-montmorillonite sheets. 6 Subsequent polymerization involving the inter- calated monomers then leads to composites with excellent dispersion of the clay sheets in the matrix. To exploit best this new route to polymer nanocomposites, it is essential to obtain a molecular scale description of the intercalation process for the monomer. Through a molecular modeling approach, this study aims at shedding light on (i) the structure of the intercalated system, that is, the spatial distribution of the CL molecules with respect to the clay walls and the counterions, and (ii) the nature of inter- molecular interactions favoring intercalation. In turn, this will allow an understanding of whether the spontaneous intercalation of the monomer (and the subsequent po- lymerization leading to the nanocomposite) is specific to caprolactone, or this new route could be extended to the preparation of other polymer/Na-montmorillonite nano- composites. Here we focus on the modeling of the CL-intercalated Na-montmorillonite system. Montmorillonite belongs to the family of 2:1 phyllosilicates. Their crystal structure consists of layers of two silica tetrahedral sheets fused to an edge-shared octahedral sheet of aluminum hydroxide. Stacking of the layers leads to a regular van der Waals gap or interlayer. Isomorphic substitutions within the layers (silicon replaced by aluminum in the tetrahedral sheets and aluminum replaced by magnesium in the octahedral sheet) generate negative charges that are counterbalanced by cations residing in the interlayer region. The natural form of Wyoming montmorillonite contains hydrated Na + or K + cations. The layer charge of montmorillonite clays is usually between -0.5 e and -1.2 e per O 20 (OH) 4 formula unit cell, and octahedral substitu- tions are responsible for about 2 / 3 of the total charge. We * To whom correspondence should be addressed. E-mail: Rober- to@averell.umh.ac.be. † Service de Chimie des Mate ´riaux Nouveaux, Universite ´ de Mons-Hainaut. ‡ Laboratoire Re ´So - UMR 6516, Universite ´ d’Aix-Marseille 3. § Laboratoire de Chimie The ´ orique et Mode ´lisation Mole ´culaire, CNRS-UMR 6517, Universite ´s d’Aix-Marseille 1 et 3. | School of Chemistry and Biochemistry, Georgia Institute of Technology. (1) (a) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 8, 1728. (b) Giannelis, E. P. Adv. Mater. 1996, 8, 29. (2) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, R28, 1. (3) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000. (4) Giannelis, E. P.; Krishnamoorti, R.; Manias, E. Adv. Polym. Sci. 1999, 138, 107. (5) Balazs, A. C.; Singh, C.; Zhulina, E.; Lyatskaya, Y. Acc. Chem. Res. 1999, 8, 651. (6) (a) Lepoittevin, B.; Devalckenaere, M.; Pantoustier, N.; Alexandre, M.; Kubies, D.; Calberg, C.; Jero ˆme, R.; Dubois, P. Polymer 2002, 43, 4017. (b) Lepoittevin, B.; Devalckenaere, M.; Alexandre, M.; Pantoustier, N.; Calberg, C.; Jero ˆ me, R.; Dubois, Ph. Macromolecules 2002, 35, 8385. 8287 Langmuir 2003, 19, 8287-8291 10.1021/la034491n CCC: $25.00 © 2003 American Chemical Society Published on Web 08/27/2003