AmericanChemicalSociety.com

Research Goals

Flow properties of molecularly thin films are at the foundation of many practical applications such as coatings, lithography, microfluidics, and lubrication. The mission of this research program is to develop a molecular-scale understanding of the flow phenomena in thin polymer films by imaging individual molecules as they spread on a solid substrate. Our goal is to characterize the film pressure and friction at the substrate on length scales below 100 nm and to study the effect of the film pressure on the activation of specific chemical bonds within flowing macromolecules. Experimental studies have been focused on two sensory properties of flowing macromolecules: (i) flow-induced conformational transitions and (ii) flow-induced fracture of covalent bonds.

Human Resources Statement

In Yr.1, this grant has sponsored one graduate student Frank C. Sun and one postdoc Natalia Lebedeva. In Yr.2, the grant has been used to support two postdocs Dr. Insun Park and Dr. Alexander Ermochkine. Dr. Sun has graduated in May 2007 and accepted job as Staff Scientist with Johnson&Johnson. Drs. Lebedeva and Park continue their postdoctoral work at UNC. Dr. Ermochkine is currently working as a Senior Scientist at Liquidia Inc.. Dr. Sheiko (PI) has been promoted to Full Professor in 2009.

Findings, Year 1

1. Molecular Pressure Sensors.

In Year 1, we have 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.

2. 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 and imaged during spreading. The application of the Flory theorem for mixtures of macromolecules with complex architectures is ambiguous. Therefore, we conducted experiments to verify the new theory developed by (Dobrynin, UConn and Rubinstein, UNC) 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.

3. Flow-induced scission of covalent bonds.

At the end of Year 1, we have focused our studies on the new film-pressure sensing property, namely flow-induced fracture of branched macromolecules. We have shown that the pressure gradient associated with the flow leads to the increase of bond tension along the spreading direction and eventually causes fracture of flowing macromolecules. It has been 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 on different substrates and under various environmental conditions. From the variation of the rate constants we have extracted both the film pressure and the bond activation parameters, such as activation energy and bond length.

Findings, Year 2

4. Generation of tension in molecular brushes at Substrates. In collaboration with theorists (Rubinstein from UNC and Panyukov and Zhulina from Russian Academy of Sciences), we have studied conformations of molecular brushes molecules at attractive substrates along with the tension developed in their backbones. Maximum tension in the backbone of these molecular brushes adsorbed from a non-solvent onto a strongly attracting substrate is proportional to the spreading parameter and the side chain length, reaching values sufficient to break covalent bonds. The theoretical predictions have been verified by experimental measurements of the bond-scission kinetics on substrate both in static films and during spreading.

5. Surface-induced amplification of tension in covalent bonds.

We have also developed 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 surrounding environment. 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. Adsorption of the branched molecules on a substrate results in further increase in tension as compared to molecules in solution. Branched molecular architectures with a high tension amplification parts (e.g. pom-poms and brushes) can be used as sensors and bond activators in thin films.

6. Molecular Tensile Testing Machines

In order to endow tension with the single-bond selectivity, we have designed a brush-like macromolecule with a disulfide bond in the middle of the all-carbon polymethacrylate backbone. The bond specificity has been demonstrated by monitoring the mid-chain fracture of the disulfide-containing macromolecules during flow. In addition to gauging the film pressure, the designed molecules can be used as a miniature tool for mechanical activation of chemical reactions at specific chemical bonds. The applied force can be finely regulated in a broad range from 10 pN to 10 nN by varying the substrate surface energy. Precise control of tensile forces in the pN-nN range enables application of the developed molecular tool to a wide range of covalent and non-covalent bonds. This new tool may be also used in combination with other activation stimuli, such as light, temperature, and electric field.