| INTERMEDIATE LENGTH N-ALKANES ARE THE PRINCIPAL COMPONENTS OF COMMERCIAL LUBRICANTS; THEREFORE IT IS OF GREAT INTEREST TO UNDERSTAND THE RHEOLOGICAL PROPERTIES OF ALKANE FILMS UNDER SHEAR RATE. ALSO, N-ALKANES ARE USED IN COATINGS, SO THE UNDERSTANDING OF THEIR BEHAVIOR WHEN ADSORBED ON SOLID SURFACES IS IMPORTANT. THIS DISSERTATION CONSIST OF BOTH EXPERIMENTAL AND THEORETICAL STUDIES OF INTERMEDIATE LENGTH N-ALKANE FILMS. IN THE STUDIES OF ALKANE FILMS WE INVESTIGATED THEIR VISCOSITY WHEN SQUEEZED BETWEEN TWO SLIDING SURFACES. IN THE STUDIES OF ADSORBED FILMS, WE INVESTIGATED THE PHASE TRANSITIONS, FRICTION, STRUCTURE, DIFFUSION AND DYNAMICS OF MULTILAYERS AMONG OTHER PROPERTIES. TO STUDY THE BEHAVIOR OF THE N-ALKANE TETRACOSANE ADSORBED ON TWO DIFFERENT SUBSTRATES SUBJECT TO HEATING-COOLING CYCLES, WE HAVE CONDUCTED ELLIPSOMETRY MEASUREMENTS. THIS TECHNIQUE GIVES INFORMATION ABOUT THE PHASE TRANSDITIONS AND ALSO ABOUT RE-ORIENTATIONS OF THE MOLECULES IN THE FILM BY MONITORING THE OPTICAL THICKNESS OF THE SAMPLES. FOR SUBSTRATES, WE USED AU(111) AND SI/SIO2. ELLIPSOMETRIC MEASUREMENTS SHOW QUALITATIVELY THE SAME TEMPERATURE DEPENDENCE OF THE OPTICAL THICKNESS AT 332 K SUGGESTS A PHASE TRANSITION OCCURRING IN THE DOTRIACONTANE FILM BELOW THE BULK MELTING POINT. INTERMEDIATE LENGTH N-ALKANES ORIENT IN TWO DIFFERENT CONFIGURATIONS: A "PARALLEL" AND A "PERPENDICULAR" ORIENTATION [8, 9, 10]. IN THE "PARALLEL" ("PERPENDICULAR") CONFIGURATION THE MOLECULES HAVE THEIR LONG AXIS PARALLEL (PERPENDICULAR) TO THE SUBSTRATE. TO STUDY THE PHASE TRANSITIONS, LATERAL MOBILITY, AND THE STRUCTURE OF A "PERPENDICULAR" LAYER ON TOP OF A "PARALLEL" LAYER OF TETRACOSANE, WE HAVE DONE MOLECULAR DYNAMICS SIMULATIONS AT A CONSTANT SPREADING PRESSURE. WE HAVE ALSO MONITORED THE ROTATION ABOUT THE LONG AXIS OF THE "PERPENDICULAR" MOLECULES SINCE THERE IS EXPERIMENTAL EVIDENCE OF AND INTERMEDIATE PHASE IN WHICH THESE MOLECULES SINCE THERE IS EXPERIMENTAL EVIDENCE OF AN INTERMEDIATE PHASE IN WHICH THESE MOLECULES ROTATE ABOUT HEIR LONG AXIS. THE SIMULATIONS SHOW THAT THE MELTING TRANSITION IN THE "PERPENDICULAR" LAYER ALSO SEEMS TO BE DRIVEN BY THE FORMATION OF GAUCHE DEFECTS IN THE CHAINS. THE ROTATIONAL MOTION AROUND THE LONG AXIS OF THE MOLECULES IN THE "PERPENDICULAR" LAYER SEEMS TO HAVE THE CHARACTER OF A HINDERED ROTATIONAL MOTION, THAT GRADUALLY CHANGES CHARACTER FROM A LIBRATIONAL MOTION TO A FREE ROTATIONAL MOTION, OVER A TEMPERATURE RANGE STARTING AT 150 K UP TO THE MELTING TRANSITION. THERE DOES NOT SEEM TO BE LATERAL TRANSLATIONAL MOBILITY PARALLEL TO THE SUBSTRATE OF THE MOLECULES IN THE "PERPENDICULAR" LAYER CLOSE TO THE MELTING TANSITION. THE "PARALLEL" LAYER OF MOLECULES MELTS BEFORE THE "PERPENDICULAR" LAYER, AND THE LONGITUDINAL MOBILITY OF THE MOLECULES IS MUCH LARGER FOR THE "PERPENDICULAR" LAYER THAN FOR THE MOLECULES IN THE "PARALLEL" LAYER. TO STUDY THE STREUCTURE AND THE PRIOR-TO-MELTING DYNAMICS OF ALKANE MULTILAYERS IN WHICH MOLECULES ARE PARALLEL TO THE SUBSTRATE, WE HAVE PERFORMED MOLECULAR DYNAMICS SIMULATIONS. EXPERIMENTALLY, IT IS IMPOSSIBLE TO GROW A MULTILAYER OF N-ALKANES WITHOUT COMPRESSING THE FILM; IN THIS WAY, A MONOLAYER EXHIBITS A "LOW DENSITY" PHASE, WHILE A MULTILAYER HAS A "HIGH DENSITY" PHASE. WE HAVE THEREFORE SIMULATED A MONOLAYER, A BILAYER, AND A TRILAYER OF TETRACOSANE IN THESE TWO DIFFERENT DENSITY-PHASES. WE FOUND THAT THERE IS NO INTERLAYER MOLECULAR DIFFUSION PRIOR TO MELTING AND THAT THE ADDITION OF ONE LAYER TO THE MONOLAYER INCREASES THE MELTING POINT BY ABOUT 40 K WITH RESPECT TO THE MONOLAYER. WE ALSO FOUND THAT THE ADDITION OF ONE LAYER TO THE BILAYER INCREASES THE MELTING POINT OF THE FIRST TWO LAYERS 10 K WITH RESPECT TO THE BILAYER, WHILE THE TOP LAYER MELTS AT ROUGHLY THE SAME TEMPERATURE AS THE BILAYER. THE SIMULATIONS SHOW THAT, AS TEMPERATURE INCREASES, THE DENSITY OF THE "HIGH-DENSITY-FILMS" DROPS DOWN TO THE DENSITY OF THE "LOW-DENSITY-FILMS" BEFORE THE MELTING. WE STUDIED THE DYNAMICS OF "PARALLEL" MULTILAYERS BY SIMULATIONS OF QUASIELASTIC NEUTRON SCATTERING. TO STUDY "SLOW" AND "FAST" MOTIONS, WE DID SIMULATIONS AT TWO DIFFERENT TIME SCALES: 1-100 PS AND 1-4 NS. THE DYNAMICS OF THE ALKANE MOLECULES IS A SUPERPOSITION OF DIFFERENT TYPES OF MOTIONS: TRANSLATION AND ROTATION OF THE CENTER OF MASS AND INTRAMOLECULAR MOTIONS. ALL THESE MOTIONS CONTRIBUTE TO THE SCATTERING FUNCTION OBTAINED IN THE EXPERIMENTS. MOLECULAR DYNAMICS SIMULATIONS ALLOW US TO IDENTIFY THE CONTRIBUTIONS OF THE DIFFERENT TYPES OF MOTIONS TO THE SCATTERING FUNCTION. IN THE 1-100 PS SCALE, THE SPECTRA SHOW CONTRIBUTIONS FROM TRANSLATIONAL, ROTATIONAL AND INTRAMOLECULAR MOTIONS. IN THE 1-4 NS SCALE, THE ROTATIONAL AND INTRAMOLECULAR MOTIONS CONTRIBUTE TO THE SEPCTRUM OF THE MONOLAYER, WHILE IN THE MULTILAYER SPECTRA WE FOUND CONTRIBUTIONS OF TRANSLATIONAL AS WELL AS ROTATIONAL AND INTRAMOLECULAR MOTIONS. TO CORRELATE THE LATERAL FRICTION OF INTERMEDIATE LENGTH N-ALKANES WITH THEIR MOLECULAR STRUCTURE, WE HAVE PERFORMED NON-EQUILIBRIUM MOLECULAR DYNAMICS SIMULATIONS OF AND AFM TIP THAT SCANS A AFM TIP THAT SCANS A TETRACOSANE SAMPLE ALONG DIFFERENT DIRECTIONS. DIFFERENT TETRACOSANE FILMS WERE STUDIED: A "PARALLEL" MONOLAYER FILM, A "PARALLEL" BILAYER FILM AND A "PERPENDICULAR" MONOLAYER FILM. WE HAVE DEMOSTRATED THAT MOLECULAR DYNAMICS SIMULATIONS MAY BE USED TO DETERMINE FRICTION COEFFICIENTS IN AFM CONTACT-MODE EXPERIMENTS. AFTER EXTRAPLATION TO SMALL VELOCITIES, THE RESULTS COMPARE WELL WITH EXPERIMENTS. IN ADDITION, THEY MAY GIVE STRUCTURAL INFORMATION AND REVEAL DETAILS ABOUT THE ENERGY DISSIPATION MECHANISMS OF THE MOVING TIP. IN THE CASE OF "PARALLEL" LAYERS, A FRICTION ANISOTROPY ASSOCIATED WITH THE LONG AXIS OF THE MOLECULE WAS FOUND. IT WAS OBSERVED THAT THE FRICTION COEFFICIENT IS LARGER WHEN THE TIP SCANS THE SAMPLE IN THE DIRECTION PARALLEL TO THE LONG AXIS OF THE MOLECULES THAN IN THE PERPENDICULAR DIRECTION, PROBABLY BECAUSE THE DENSITY OF METHYLENE GROUPS IS HIGHER ALONG THE DIRECTION OF THE BACKBONES OF THE MOLECULES THAN IN THE PERPENDICULAR DIRECTION. WE FOUND THAT THE FRICTION IS MINIMUM WHEN SCANNING ON "PERPENDICULAR" LAYERS AND MAXIMUM WHEN SCANNING ON A "PARALLEL" BILAYER AND THAT THE FORMATION OF GAUCHE DEFECTS PLAYS A ROLE IN THE ENERGY DISSIPATION PROCESS. WE FOUND A RATIO OF 3-11 BETWEEN THE FRICTION COEFFICIENT OF "PARALLEL" AND "PERPENDICULAR" LAYERS, DEPENDING ON THE SCAN DIRECTION. THIS RESULT COMPARES WELL WITH EXPERIMENTS. WE HAVE ALSO FOUND THAT THE APPLICATION OF AN UPWARD FORCE TO THE TIP REDUCES THE FRICTION COEFFICIENTS BY ABOUT 10-20 PO RCIENTO. THE APPLICATION OF AN UPWARD FORCE IS COMMONLY USED IN AFM CONTACT-MODE MEASUREMENTS TO REDUCE DAMAGE TO THE SOFT FILMS. TO INVESTIGATE THE VISCOSITY OF ALKANES OF THE SAME LENGTH, BUT DIFFERENT ARCHITECTURES WHEN EXPOSED TO SHEAR, WE HAVE PERFORMED NON-EQUILIBRIUM MOLECULAR DYNAMICS SIMULATIONS OF THE N-ALKANE TETRACOSANE AND OF SQUALANE. WE HAVE USED THE REVERSE NON-EQUILIBRIUM ALGORITHM PROPOSED BY F. MULLER-PLATHE[1, 2] IN WHICH A MOMENTUM FLOW IS IMPOSED ARTIFICIALLY BY PERIODIC MOMENTUM EXCHANGES. WE FOUND THAT BOTH MOLECUALR FILMS HAVE ROGHLY THE SAME VISCOSITY AT ROOM TEMPERATURE AT THE NANO-SCALE. |