46204-AC7
Single Macromolecules as Probes for Pressure and Friction in Fluid Monolayers
1. Research Goals. Flow properties of molecularly thin films are at the foundation of many practical applications such as coatings, microfluidics, and lubrication. The mission of this research program is to develop a molecular-level understanding of flow phenomena in thin polymer films by imaging individual molecules as they flow on a solid substrate. Our goal is to develop a methodology to independently characterize the film pressure and friction at the substrate on length scales below 100 nm. Experimental studies are focused on two sensory properties of flowing macromolecules: (i) flow-induced conformational transitions and (ii) flow-induced fracture of covalent bonds. 2. Human Resources Statement. This award sponsors the research of one graduate student Frank C. Sun, one postdoc Natalia Lebedeva, and Associate Professor Sergei S. Sheiko. Dr. Sun has graduated in May 2007 and accepted job as Staff Scientist with Johnson&Johnson. Dr. Lebedeva continues her postdoctoral work at UNC. Dr. Sheiko is nominated for promotion to Full Professor in 2008-2009 and Humboldt Professorship in 2009-2014. 3. Findings. a) Molecular Pressure Sensors. In Year 1, we explored the conformational transitions of brush-like macromolecules to be used as miniature sensors of the local film pressure. We developed technique for reliable imaging and quantitative analysis of individual molecules during flow. We monitored brush-like macromolecules as they change their shape in response to variations in the film pressure and analyzed the response of molecular dimensions to both molecular architecture and to the interaction with the substrate. After calibration, these molecular sensors were used to gauge both the pressure gradient and the friction coefficient at the substrate. We showed that the friction does not depend on the molecular weight and architecture; however, it exhibits strong dependence on the substrate type and the relative humidity (RH). A decrease in RH from 99% to 95% resulted in four orders of magnitude increase of the friction coefficient. We anticipate the utilization of such miniature sensors for probing flow properties on nanometer length scales. b) Structurally asymmetric mixtures. To sense the film pressure in a melt of linear polymers, a small fraction of brush molecules were added to the melt to be imaged during spreading. The mixing behavior of linear and brush-like macromolecules is significantly different from those of linear chains. Interactions between linear chains are screened leading to ideal conformations obeying random walk statistics. However, application of this model (known as the Flory theorem) for mixtures of macromolecules with complex geometries is ambiguous. In collaboration with theorists (Dobrynin, UConn and Rubinstein, UNC), we conducted experiments and developed a new theory to generalize the Flory theorem for a broader class of polymeric mixtures. Swelling of a brush molecule was shown to be controlled not only by the degree of polymerization (DP) of the surrounding linear chains but also by the DP of the brush's side chains which determines the structural asymmetry of the mixed species. The boundaries of the swelling region were determined and demonstrated excellent agreement with theory. c) Flow-induced scission of covalent bonds. At the end of Year 1, we have focused our studies at the flow-induced fracture branched macromolecules. The pressure gradient associated with the flow leads to the increase of bond tension along the spreading direction. We showed that the tension increase might cause irreversible fracture of flowing macromolecules. To study the kinetics of flow-induced scission of chemical bonds we have prepared brush-like macromolecules with very long backbones up to 3 ?m (collaboration with Matyjaszewski, CMU). We have shown that molecules undergo an avalanche-type degradation once the tension overcome a critical value of about 3 nN. The flow-induced bond scission will be studied as a function of the side-chain length (n=40, 50, 80, 110, 140) on different substrates (mica, graphite, modified Si-wafers) and under various environmental conditions (humidity, solvent vapors). d) Surface-induced amplification of tension in covalent bonds. We proposed a systematic method of designing branched macromolecules capable of building up a high tension (?nN) in their covalent bonds, which can be controlled by changing the interaction with the substrate and the solvent quality. This tension is achieved exclusively due to intramolecular interactions by focusing lower tensions from its numerous branches to a particular section of the designed molecule. In brush-like macromolecules, the side-chain tension is focused to the backbone and its amplification from the picoNewton to nanoNewton range. In collaboration with theorists (Rubinstein from UNC and Panyukov and Zhulina from Russian Academy of Sciences), we established scaling relations between the tension and conformation of the molecular brushes on substrates.
