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A method for performing calculations on the lower bound states of van der Waals trimers is developed, which models atom-atom-diatom trimers with basis functions in all five degrees of freedom. Spherical harmonic and distributed Gaussian functions and solutions of one-dimensional adiabatic Hamiltonians are used as basis functions. Arg was examined as a precursor system. No spectroscopy has been performed on Ara, nor is this currently feasible. For the systems considered, most experimental data exists for (^v)HCI = 0 Ar(_2)HCl so this is the main target of the work. Predictions are made for Ar(_2)DCl, for (_v)HCI = 1 Ar(_2)HCl, and for (^v)HF =0,1 Ar(_2)HF ; experiments are currently in progress on some of these systems. The current state of knowledge of the pair potentials of the Ar-Ar, and Ar-HF/CI systems is summarised. Physical models for important three body potential terms are suggested; these arise from dipoles induced on the argon atoms, dispersion effects, orbital deformation and the Ar(_2) overlap-induced field. The parameters in the models come from the literature, where possible, and otherwise from a fit to some ab-initio data points for the Ar(_3) and Ar(_2)HCl trimers (Chalasinski et al.).Calculations on Ar(_s) with various two- and three-body potentials are presented and discussed in the context of earlier work. For Ar(_2)HCl a comparison is made with earlier, approximate, work (Hutson, Halberstadt and Beswick). The possible effects of Hamilto- nian approximations are discussed before addressing the effects of individual three-body components. Two sets of three-body parameters are assessed, and indicate that the physical models used are substantially appropriate, although deficient in detail; agreement with experiment is good, with changes in frequencies of about 1.5cm(^-1) arising from the best three-body model. The most important three-body component is found to be the interaction of the overlap-induced field with the HCI permanent multipoles, with the dispersion effects slightly less important and other terms much less so.

Liées de façon covalente à des chaînes polymères, les nanoparticules POSS (polysilses-quioxanes polyédriques) permettent l’obtention de matériaux nanocomposites hybrides orga-nique/inorganique. Ces nanoparticules présentent deux intérêts majeurs: des dimensions bien définies (cœur inorganique: 0.5 nm), ainsi que leur caractère hybride ( groupements organiques entourant les cages inorganiques). Les nanocomposites polymère/POSS peuvent présenter un renfort important des propriétés mécaniques et de la stabilité thermique de la matrice polymère. Cependant, l’origine moléculaire de ce renfort reste mal comprise. Aussi, afin de mieux comprendre le renfort des propriétés mécaniques de la matrice, nous avons étudié l’effet des particules POSS sur l’organisation à l’état solide et la dynamique des chaînes po-lymères au sein d’une matrice semi-cristalline. Pour cela, nous avons considéré une série de copolymères polyéthylène-POSS, caractérisés par une large gamme de concentration en POSS. Le polyéthylène et le POSS ayant intrinsèquement tendance à cristalliser, les copolymères présentent des organisations à l’état solide complexes que nous avons caractérisés par l’utilisation combinée de la calorimétrie différentielle à balayage (DSC), de la diffraction des rayons X aux grands angles (DRX) et de la résonance magnétique nucléaire en phase solide (RMN). Dans un second volet de cette étude, nous nous sommes intéressés à la dynamique des chaînes de polyéthylène en phase amorphe et à l’évolution de celle-ci avec le taux de charge des nanocomposites. Enfin, nous avons également étudié, de façon sélective, la dynamique des segments de chaînes de polyéthylène situés au voisinage de la charge / Grafting polyhedral oligomeric silsesquioxanes (POSS) to polymer chains offers a novel avenue to prepare hybrid organic/inorganic nanocomposites. The great advantage of such an approach holds in the very well-defined dimensions of the filler particles(inorganic core: 0.5 nm), in contrast with polymer/clay systems. Polymer/POSS nanocomposites display attractive properties such as significant mechanical reinforcement and increased thermal stability. However, the molecular origins of these enhancements are still an open question. The aim of this work is to describe both bulk organization and molecular motions of the polymer chains within these materials: these molecular properties should lead to a deeper knowledge of the enhancement of the polymer-POSS mechanical properties. The systems investigated are polyethylene (PE)-POSS copolymers with various POSS contents. The (semi-)crystalline behaviour of both PE and POSS particles induces complex bulk organization of these hybrid materials. The combined use of NMR, DSC and X-ray scattering experiments enabled to monitor the variation of the solid-state organization with the filler loading. In the second part of this work, the PE chain dynamics was investigated in the amorphous phase and its variation with the filler content was considered. Lastly, selective NMR experiments were used to probe the PE chain segment mobility close to the POSS nanoparticles