Study of surface of amorphous polymer by time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy

Abstract

Surface of spin-cast amorphous poly(methyl methacrylate) lm was studied experimentally using surface sensitive techniques, such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS), and theoretically using molecular dynamics (MD) simulation. The complex ion fragmentation mechanism in ToF-SIMS has always been a barrier hindering our research progress in surface science of polymer. Therefore, static SIMS spectra of structural isomeric polymers, which are poly(2-vinylpyridine) and poly(4-vinylpyridine), were studied in detail using density functional theory. First and foremost, experimental and density functional theory calculation results show that the ionization process of the sputtered fragments from poly(2-vinylpyridine) and poly(4-vinylpyridine), under ToF-SIMS experimental condition relied on the ion stability. Most of the ions from these structural isomers shared the same accurate mass, but had different relative abundance. This could be attributed to the fact that the disparity in the molecular structures can affect the ion stability, if we assume that they shared the same mechanistic pathway of formation. The molecular structures of these ions were assigned and their stability was evaluated based on calculations using the Kohn-Sham density functional theory with Beckes three-parameter Lee-Yang-Parr exchange-correlation functional and a correlation-consistent, polarized, valence, double-zeta basis set for cations and the same basis set with a triple-zeta for anions. The computational results agreed with the experimental observations with exception in the formation of positive ions such as C₆H₇N⁺ (m/z = 93) and C₈H₁₀N⁺ (m/z = 120), that their formation did not necessarily depend on the ion stability. Instead, the transition state chemistry and the matrix effect both played a role. In the negative ion spectra, we found that experimental observation is in line with our computational results without exception. We speculate that whether anions would form from P2VP and P4VP is more dependent on the stability of the ions. Secondly, the surface concentration of different chemical groups of unannealed and annealed spin-cast poly(methyl methacrylate) (PMMA) film was approximated experimentally by angle-dependent XPS with the aid of a model similar to that derived by Akhter et al. (Applied Surface Science 37, 406-418, 1989) and ToF-SIMS. It was found that for both unannealed and annealed spin-cast PMMA film, there is 60-70at.% of α-methyl carbon and methylene carbon at the surface relative to that of the ester group carbon. However, the subtle difference in the atomic concentration of these functional groups at the surfaces of unannealed and annealed PMMA film could not be clearly observed by XPS. Nonetheless, this subtle difference is reflected in the disparity in the normalized intensities of certain ions, such as CH₃⁺ (m/z = 15), C₃H₅⁺ (m/z = 41), C₂H₃O₂⁺ (m/z = 59), C₄H₅O⁺ (m/z = 69), which is larger than the relative standard deviation, in the ToF-SIMS spectra of unannealed and annnealed PMMA film. From the results of MD simulation, it was found that near the surface of PMMA at T > T_g, more methoxy groups are at the surface and they are more tilted towards the surface normal than other chemical groups, such as α-methyl group and methylene group, and there is a higher fraction of gauche conformation at the surface. In summary, the work described in this thesis has elucidated the ion fragmentation mechanism to certain extent and it has given insight on how we should develop a reliable fragmentation model for ToF-SIMS characterization of polymer surface.

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology