Interfacial structure and dynamics of complex amphiphiles: I. Langmuir and Langmuir-Blodgett films of amphiphilic bistable rotaxanes. II. Two-dimensional self-assembly of dendritic-linear block copolymers

We have studied two systems of complex amphiphiles at the air-water interface and at solid surfaces. The first study examines a system of amphiphilic rotaxane molecules currently being investigated for molecular electronics device applications. The second study focuses on investigating the properties of self-assembled films of an amphiphilic hybrid dendritic-linear block copolymer.; We have examined a series of amphiphilic bistable rotaxanes with systematic changes in the constitutions of the hydrophilic stopper; both the length of the hydrophilic tails and the endgroups are varied. Langmuir films typically pass from a liquid-expanded (LE) region to a liquid-condensed (LC) region. Film balance and surface rheology experiments have shown that the presence of a cyclophane component and hydroxyl endgroups increases the stability and viscoelasticity of the films. All the rotaxanes form homogeneous Langmuir-Blodgett (LB) films. A proposed model for the molecular conformation of the rotaxanes at the air-water interface takes into account the competing hydrophilicity of the cyclophane. The presence of molecular superstructures has important implications for molecular electronics devices based on amphiphilic rotaxanes.; In the second study, a series of dendritic-linear block copolymers which are comprised of a dendrimer based on 2,2-bis(hydroxymethyl) propionic acid (bis-MPA) attached to linear blocks of poly(epsilon-caprolactone) (PCL) have been investigated at the air-water interface and on solid substrates. The effects of systematic changes in the length of the PCL tail and the number and generation of the dendritic head groups have been examined. Langmuir film balance measurements of these PCL-dendrimer block copolymers indicate stable monolayers with LE and LC phases separated by a transition region. The projected areas of the LE and LC phases scale with the number of repeat units in the PCL chain and the molecular weight of the dendritic head, respectively. Surface rheometry measurements show that the shear moduli of the LC phase increase over four orders of magnitude as the mean molecular area is reduced. We show evidence that the LC phase corresponds to an elastic monolayer. The growth of aggregates at the onset of the LC phase has been observed with Brewster angle microscopy. LB films have been prepared and analyzed with ellipsometry and scanning force microscopy, which showed similar morphologies on the solid substrate as those observed at the air-water interface. We propose that the morphologies are evidence of crystallization. From the measured height of domains, we propose that chain folding is present in the aggregates.