This project focuses on films of alkanes [chemical formula CnH2n+2], flexible chain-like molecules that are of general interest in materials science as prototypes of more complex polymers used in coatings, adhesives, and electronic devices. Alkanes are also the principal constituents of commercial lubricants. A microscopic understanding of the structural and dynamical properties of alkane films could potentially lead to technological advances ranging from more durable polymer coatings to lubricants that reduce wear in automobile engines and in nanoscale devices such as computer hard drives. This project will utilize high-resolution scattering of neutrons and computer simulations as the principal techniques to determine the rates at which flexible alkane molecules change their shape and how these conformational changes affect the molecules' ability to stick to a solid surface. Experiments will be conducted on state-of-the-art neutron spectrometers at the Center for Neutron Research at the National Institute of Standards and Technology and at the newly constructed Spallation Neutron Source at Oak Ridge National Laboratory. The project includes structural studies utilizing Atomic Force Microscopy, synchrotron x-ray scattering at Argonne National Laboratory, and neutron diffraction at the University of Missouri Research Reactor. This research will provide training of undergraduate and graduate students in fundamental aspects of polymer science, preparing them for careers in industry and academia as well as at our expanding national facilities for neutron and x-ray scattering.
Proposal No.: DMR- 0705974
Proposal Title: The Dynamics of Organic Films on Different Time Scales
PI/PD: Haskell Taub
This project focuses on films of alkanes [CnH2n+2], flexible chain-like molecules that are of general interest in materials science as prototypes of more complex polymers used in coatings, adhesives, and electronic devices. Alkanes are also the principal constituents of commercial lubricants. A fundamental issue concerned with films of intermediate-length alkane molecules (15 < n < 50) is the effect that changes in molecular shape (conformation) induced by heating may have on binding of molecules to a solid surface and, at a macroscopic level, the wetting of the fluid film to a surface. These conformational changes are important for understanding such technologically important phenomena as the evaporation of thin lubricant films from the surfaces of magnetic storage disks, the desorption of alkanes from catalytic surfaces, and the lubricating characteristics of alkane films. This project will utilize quasielastic neutron scattering and molecular dynamics simulations as the principal techniques to investigate the molecular conformational changes that occur over a range of time scales (picoseconds to nanoseconds) in alkane films adsorbed on solid surfaces. Experiments will be conducted on state-of-the-art neutron spectrometers at the Center for Neutron Research at the National Institute of Standards and Technology and at the newly constructed Spallation Neutron Source at Oak Ridge National Laboratory. Since interpretation of the dynamical experiments requires knowledge of the film structure, the project includes structural studies utilizing Atomic Force Microscopy, synchrotron x-ray scattering at Argonne National Laboratory, and neutron diffraction at the University of Missouri Research Reactor. This research will provide training of undergraduate and graduate students in fundamental aspects of polymer science, preparing them for careers in industry and academia as well as at our expanding national facilities for neutron and x-ray scattering.
In this project, we conducted a comprehensive set of experiments and computer simulations to investigate the structure, phase transitions, and dynamics of films of alkane molecules. Alkanes [CnH2n+2] are flexible chain-like molecules that are of general interest in materials science as prototypes of more complex polymers used in coatings, adhesives, and electronic devices. Alkane films are also of interest in their own right, since they are the principal constituents of commercial lubricants. A fundamental issue concerned with films of intermediate-length alkanes (15 < n < 50) is the effect that changes in molecular shape or conformation induced by heating may have on the molecule’s ability to adsorb on a surface from the gas phase or to wet to a solid surface from the fluid phase. These interfacial properties are relevant to a number of technologically important phenomena such as the evaporation of thin lubricant films from the surfaces of magnetic storage disks, the desorption of alkanes from catalytic surfaces, and the lubricating characteristics of alkane films. In addition, our studies provide a stepping stone to investigations of molecular motion in bilayer lipid membranes supported on solid surfaces—systems that have been used extensively to model more complex biological membranes that form cell walls. Our principal experimental techniques were elastic neutron diffraction to determine the structure of the alkane films, i.e., the arrangement of the alkane molecules on solid surfaces and quasielastic neutron scattering to probe the diffusive (random) motion of the molecules on a time scale of 10-12 to 10-9 s. This diffusive motion included translational and rotational motion of the alkane molecules as a whole as well as intramolecular motion associated with their conformational changes. The neutron diffraction measurements were conducted at the University of Missouri Research Reactor Facility, while the quasielastic experiments were performed on state-of-the-art neutron spectrometers at the Center for Neutron Research at the National Institute of Standards and Technology and at the newly constructed Spallation Neutron Source at Oak Ridge National Laboratory. In order to conduct a preliminary characterization of the structure of our alkane films for neutron scattering experiments, we used a variety of complementary microscopic structural probes such as Atomic Force Microscopy, synchrotron x-ray scattering, and high-resolution ellipsometry. Furthermore, to interpret the results of our neutron scattering measurements, it was invaluable to have computer simulations of our experiments conducted by our long-time collaborator, Prof. Flemming Hansen of the Technical University of Denmark. To illustrate the type of experiments that we have undertaken, we show in Fig. 1 a three-dimensional image from an Atomic Force Microscope of an alkane nanoparticle supported on single-crystal silicon surface. In this case, the alkane molecules were of intermediate length, containing a backbone of 32 carbon atoms arranged in a zig-zag chain and represented by the chemical formula (n-C32H66 or C32). As shown in the lower panels, the mesa-shaped nanoparticle consists of layers of the rod-shaped C32 molecules oriented with their long axis perpendicular to the silicon surface. These layers are about 4 nm thick, corresponding to the length of the fully extended C32 molecule, and form a particle about 100 nm high. In our experiments, we used a new neutron spectrometer (see Fig. 2) at Oak Ridge National Laboratory along with two spectrometers at the NIST Center for Neutron Research to investigate the motion of molecules in alkane particles of different sizes. Interest in molecular motion in solids dates back to the 1930’s, when Alex Müller found new phases of alkanes just below their melting point and suggested that they might correspond to solids in which the rod-shaped molecules retain their positional order but rotate about their long axis. The structure and associated dynamics of these so-called "rotator" phases has remained an active area of research as alkane chains are the basic building blocks of a wide variety of soft matter, including polymers, liquid crystals, and lipid membranes. Contrary to Müller’s suggestion 80 years ago of a distinct "rotator" phase, our measurements have shown that the alkane molecules actually begin to rotate in the low-temperature alkane crystal; but the number of participating molecules increases abruptly at the transition to the rotator phase. Moreover, a slower motion appears at the transition associated with conformational changes of the molecules. Thus, we were able to develop a detailed picture of the alkane molecular motions necessary for understanding systems ranging from thin lubricant films to the bilayer lipid membranes. The broader impact of our research includes the training of one undergraduate student, four graduate students, and two postdoctoral fellows in state-of-the-art methods for characterizing the structure of thin organic films at the molecular level. Two of the graduate students and the two postdoctoral fellows have gained valuable experience as users of our nation’s facilities for neutron scattering and synchrotron x-ray scattering research.
