Molecular Structure of Poly(methyl methacrylate) Surface II: Effect of Stereoregularity Examined through All-Atom Molecular Dynamics

KC Jha and H Zhu and A Dhinojwala and M Tsige, LANGMUIR, 30, 12775-12785 (2014).

DOI: 10.1021/la5023328

Utilizing all-atom molecular dynamics (MD), we have analyzed the effect of tacticity and temperature on the surface structure of poly(methyl methacrylate) (PMMA) at the polymer-vacuum interface. We quantify these effects primarily through orientation, measured as the tilt with respect to the surface normal, and the surface number densities of the a-methyl, ester-methyl, carbonyl, and backbone methylene groups. Molecular structure on the surface is a complex interplay between orientation and number densities and is challenging to capture through sum frequency generation (SFG) spectroscopy alone. Independent quantification of the number density and orientation of chemical groups through all-atom MD presents a comprehensive model of stereoregular PMMA on the surface. SFG analysis presented in part I of this joint publication measures the orientation of molecules that are in agreement with MD results. We observe the ester-methyl groups as preferentially oriented, irrespective of tacticity, followed by the a-methyl and carbonyl groups. SFG spectroscopy also points to ester-methyl being dominant on the surface. The backbone methylene groups show a very broad angular distribution, centered along the surface plane. The surface number density ratios of ester-methyl to a-methyl groups show syndiotactic PMMA having the lowest value. Isotactic PMMA has the highest ratios of ester- to a-methyl. These subtle trends in the relative angular orientation and number densities that influence the variation of surface structure with tacticity are highlighted in this article. A more planar conformation of the syndiotactic PMMA along the surface (x-y plane) can be visualized through the trajectories from all-atom MD. Results from conformation tensor calculations for chains with any of their segments contributing to the surface validate the visual observation.

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