Molecular Structure of Poly(methyl methacrylate) Surface. I. Combination of Interface-Sensitive Infrared–Visible Sum Frequency Generation, Molecular Dynamics Simulations, and ab Initio Calculations

Abstract

The chemical composition and molecular structure of polymeric surfaces are important in understanding wetting, adhesion, and friction. Here, we combine interfacesensitive sum frequency generation spectroscopy (SFG), allatom molecular dynamics (MD) simulations, and ab initio calculations to understand the composition and the orientation of chemical groups on poly(methyl methacrylate) (PMMA) surface as a function of tacticity and temperature. The SFG spectral features for isotactic and syndiotactic PMMA surfaces are similar, and the dominant peak in the spectra corresponds to the ester-methyl groups. The SFG spectra for solid and melt states are very similar for both syndiotactic and isotactic PMMA. In comparison, the MD simulation results show that both the ester-methyl and the α-methyl groups of syndiotactic-PMMA are ordered and tilted toward the surface normal. For the isotactic-PMMA, the α-methyl groups are less ordered compared to their ester-methyl groups. The backbone methylene groups have a broad angular distribution and are disordered, independent of tacticity and temperature. We have compared the SFG results with theoretical spectra calculated using MD simulations and ab initio calculations. Our analysis shows that the weaker intensity of α-methyl groups in SFG spectra is due to a combination of smaller molecular hyperpolarizability, lower ordering, and lower surface number density. This work highlights the importance of combining SFG spectroscopy with MD simulations and ab initio calculations in understanding polymer surfaces. ■ INTRODUCTIONOne of the simplest and still commonly used technique to characterize surfaces of polymers is the measurement of water contact angle. Although this is a very simple technique to judge the wettability of surfaces, it lacks molecular level sensitivity to identify the chemical composition and molecular structure of surfaces. In addition, the rearrangement of molecules at the surface after exposure to water complicates the interpretation of contact angle results. 1 X-ray photoelectron spectroscopy (XPS), a vacuum-based technique, provides excellent information regarding the elemental composition present at the surfaces. However, to identify different hydrocarbon groups (or bondings) is not trivial and relies on subtle shifts of carbon binding energies in the high-resolution XPS analysis. 2 Attenuated-total-reflectance infrared can be used to pick up molecular fingerprints, but this technique samples a much thicker layer than the surface region that affects wetting, surface energy, and friction. 3 In the past two decades, surface-sensitive infrared−visible sum frequency generation spectroscopy (SFG) has been used to study polymer surfaces, 4−7 polymer/liquid interfaces, 7−11 polymer/metal or metal oxide interfaces, 5,12−15 and polymer/polymer interfaces. 12,16−19 SFG is a second-order nonlinear optical technique, and under electric dipole approximation, the SFG signal is only generated by the ordered molecules at surfaces or interfaces and the contribution from ...

The study of polythiophene/water interfaces by sum-frequency generation spectroscopy and molecular dynamics simulations

“…MD can be employed to elucidate the conformation of the systems either through SFG simulation [67][68][69][70][71] or by energetic analyses. 74,75 In order to simulate the SFG spectra it is required to know (i) the molecular polarizability (which must be obtained from computationally demanding ab initio calculations, and can be different for different molecular groups or even for the same molecular group in a different chemical environment), (ii) the concentration of the molecular groups (thiophene rings, alkyl chains) at the surface and (iii) their orientation distribution (its average and width).…”

“…[67][68][69][70][71] For systems with higher structural and chemical complexity (e.g. For simple systems it is possible to directly simulate the SFG spectra through MD.…”

Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

“…156,157 So far, these novel techniques have not been widely applied to polymer interface studies. Additionally, molecular dynamics simulations, ab initio calculations, and time-dependent density functional theory can assist to predict and interpret the vibrational signatures obtained from SFG-VS. [158][159][160] Future work utilizing these advanced methodologies to interpret polymer interfaces would improve the understanding of molecular behaviors at buried polymer interfaces.…”

On Modulating Interfacial Structure towards Improved Anti-Icing Performance

“…For example, we have explored near-surface concentration as a function of stereoregularity (viz. tacticity) for a common polymer (PMMA) [136][137][138]. In cases where information on buried interfaces is needed for functional design, validated models would provide understanding of the role of various interactions in behaviors such as modulation of hydrogen bond networks and orientation profiles that would be challenging to obtain empirically.…”