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A spectroscopic study of molecules in restricted geometries
Abstract
The study of molecules in restricted geometries is an important topic in the chemical sciences. Surfaces and ultra-thin films are examples of restricted geometries because they alter the structure of the molecules confined due to the presence of an air interface. The structural changes that occur in these restricted geometries directly influence the fields of adhesion, coatings, lithography, photography, printing, filtration, transport properties, surfactants, sensors, microelectronics, electrodes, biology, biological compatibility and membrane function. The structure of ultra-thin polyurethane films is studied to determine effect of the air-polymer interface on the degree of phase separation and the orientation of the hard segments in both the hard and soft domains. External reflection infrared spectroscopy studies show that as the film thickness is decreased, the degree of phase separation decreased and the hard segments oriented into the plane of the film in both domains. The phase separation process in ultra-thin polyurethane films are also studied. The added free volume of the air-polymer interface allows phase mixing to occur at a much lower temperature than in the bulk. The isothermal phase separation of the ultra-thin films is characterized by a much lower Avrami coefficient than the bulk. A simulation of the phase separation process shows that the decrease in the Avrami coefficients can be attributed to the small film thickness compared to the hard domain size, the hard segment concentration change over the course of the phase separation and the disk like shape of the growing domains. In addition, the overall degree of phase separation observed in the ultra-thin films is kinetically controlled due to the directional grow of the disk like hard domains which is limited not by the amount of isotropic phase, but by the film thickness. Finally, the structures of insoluble monolayers at the air-water interface are studied. The frequency and approximate S to P dichroic ratio are used to characterize the structure of H(CH$\sb2)\sb{18}$OH as a function of area per molecule. F(CF$\sb2)\sb{8}$(CH$\sb2)\sb2$OH and F(CF$\sb2)\sb{10}$(CH$\sb2)\sb2$OH are also studied at the air-water interface. In both cases, the C-H stretching region indicated that the water surface is covered by a monolayer however only the longer fluorinated chain gives consistently useful information in the C-F stretching region. This is attributed to changes in the structure of the shorter chain amphiphiles just after they are spread at the air-water interface that the longer chain molecules do not exhibit.
Subject Area
Polymer chemistry
Recommended Citation
Meuse, Curtis Warren, "A spectroscopic study of molecules in restricted geometries" (1993). Doctoral Dissertations Available from Proquest. AAI9408313.
https://scholarworks.umass.edu/dissertations/AAI9408313