THESIS
2016
xiv, 94 pages : illustrations ; 30 cm
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) an...[
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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 C
6H
7N
+ (m/z = 93) and C
8H
10N
+ (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 CH
3+
(m/z = 15), C
3H
5+
(m/z = 41), C
2H
3O
2+
(m/z = 59), C
4H
5O
+ (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.
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