62nd Annual Report on Research 2017

As part of our PRF grant, we continued our investigation of the stereoregular poly(methyl methacrylate) (PMMA) interface, examining its response to introduction of water. This was built on our previously reported work (ACS Macro Lett. 4.11 (2015): 1234-1238), wherein we determined the relative amount of various functional groups at the vacuum interface for stereoregular poly(methyl methacrylate) (PMMA) and related it to the surface tension behavior. We observed in this work that the atactic PMMA has the ability to express surface functional groups unhindered by tacticity, and that the helical nature of the isotactic PMMA increases the relative availability of the carbonyl groups, causing an increase in surface tension values.

Upon introduction of water, it was found that the atactic PMMA is the most stable of the three forms to perturbation (Hydrogen Bond Directed Surface Dynamics at Tactic Poly(methyl methacrylate)/Water Interface under review in Soft Matter). This was quantified in terms of the orientation distribution behavior of the carbonyl group. The carbonyl group is the determinant of the hydrogen bond participation, responsible for a minimum of 94% of hydrogen bonds.

The behavior of the atactic PMMA is in line with our previous hypotheses on its ability to tune interfacial response depending on the environment. It is also significant that the isotactic PMMA shows the most change in the carbonyl group orientations. We related this behavior to the hydrogen bond participation and the relaxation of hydrogen bond for the three tactic forms (Table I, below):

Table 1: Hydrogen bond (nm^-2) and relaxation time for bonded water molecules (τ) at 300 K

It should be noted that changes in orientation at 300 K for carbonyl group would be at much larger timescales, and not accessible through all-atom MD. Similarly, the breaking and formation of hydrogen bonds at 560 K is too fast to determine relaxation times from the intermittent hydrogen bond correlation function.

Further work is being carried out to understand how the presence of surfactants would change the interfacial surface expression of various groups. Since interdigitation and favorable hydrogen bonding determine surfactant induced interfacial freezing, the changes in ordering with tacticity would be the ideal control for examining this behavior. Changes in side chain length to analyze acrylate analogs are also planned. We attempted probing the carbonyl group through sum frequency generation (SFG) spectroscopy, but the region was inaccessible for sapphire prisms currently available to us.

Since the water dynamics is critical for mediating interactions at the polymer (or oil)-water interface, we also examined the ordering due to presence of surface charge (Coatings 6.1 (2016): 3.). This would relate to the differences in head group partial charges for various surfactants. For a 20% negatively charged α-sapphire surface, we observed significant changes in ordering leading to a layered structure where the second layer had a sharper peak when measured in terms of cosine of the angle of dipole moment of water. This may be due to a templating effect, with a water overlayer, that has been reported for surface directed growth of ice. Further, the neutral surface shows a quick recovery to bulk around 10 Å, when measured in terms of cosine of the dipole angle, whereas for the charged surface the water molecules do not show a recovery to bulk for even 100 Å. For the charged surface this shows a mixture of behavior at the air interface and at the sapphire interface continued to a significant extent inside the water film.

Our hypotheses are based on the fact that the interfacial ordering shows signatures in the interfacial water dynamics and we have probed it for mineral and polymer surfaces and are continuing to quantify the interfacial water behavior with the introduction of surfactant. As we relate the dynamics of interfacial orientation with the behavior of interfacial water, it would have wide applications in regulating flow and miscibility in a number of applied fields. We summarized the applications for oil & gas industry in a contributed book chapter published by Springer International (Jha, Kshitij C., Vikram Singh, and Mesfin Tsige. "Interfacial Engineering for Oil and Gas Applications: Role of Modeling and Simulation." New Frontiers in Oil and Gas Exploration. Springer International Publishing, 2016. 257-283.)

Impact of the research on the development of human resources:

The projects objective is to establish a molecular-level understanding of the nature of surface freezing at alkane/surfactant-water interface. The students and the postdoc involved in this project are given the opportunity to participate and contribute to the general understanding of the nature of adsorption of small molecules on surfaces and interfaces an important topic by its own right in material science. The graduate student involved works towards his PhD by developing the ability to raise critical questions that should be addressed through simulations. It is thus anticipated that by doing research in the project and in the process of completing his PhD, the graduate student is expected to develop successful professional careers.

Furthermore, the understanding produced during the execution of the project will result in the core content of a graduate level course on computer simulation of polymeric materials that the PI has been teaching every other year for senior undergraduate and graduate students.