83 research outputs found
Probing Molecular Structures of Poly(dimethylsiloxane) at Buried Interfaces <i>in Situ</i>
Silicone
materials such as poly(dimethylsiloxane) (PDMS) are widely
used in a variety of important applications such as polymer adhesives,
packaging materials for microelectronics, polymer MEMS, microfluidics,
biomedical implants, and marine antifouling coatings. In such applications,
molecular structures of PDMS at buried interfaces will determine interfacial
properties. Therefore, it is important to elucidate PDMS molecular
structures at relevant buried interfaces. In this study, the interfacial
structures of PDMS silicone elastomer in contact with silica and different
polymer materials have been studied using sum frequency generation
(SFG) vibrational spectroscopy. It was found that the PDMS methyl
groups are ordered at the buried poly(ethylene terephthalate) (PET)/PDMS
and fused silica/PDMS interfaces. However, these methyl groups tend
to adopt different orientations at different interfaces. Using the
SFG spectral fitting results, the possible ranges of tilt angles and
twist angles of PDMS methyl groups at the buried PET/PDMS and silica/PDMS
interfaces were determined. At the PET/PDMS interface, the methyl
groups tend to have large tilt angles (>70°) with small twist
angles (<20°). At the silica/PDMS interface, methyl groups
tend to adopt a broad distribution of tilt angles along with large
twist angles. The absolute orientations of the PDMS methyl groups
at the buried interfaces were determined from the interference pattern
of the PDMS SFG signal with the nonresonant signal from a TiO<sub>2</sub> thin film. PDMS methyl groups tend to orient toward the PDMS
bulk rather than the contacting substrates at both the PET/PDMS and
silica/PDMS interfaces. However, at the polystyrene/PDMS and poly(methyl
methacrylate)/PDMS interfaces, PDMS methyl groups orient toward the
hydrophobic polymer substrate surfaces. The different orientations
of PDMS methyl groups at the investigated buried interfaces were correlated
to interfacial polar interactions determined by substrate surface
hydrophobicities
Observing Phthalate Leaching from Plasticized Polymer Films at the Molecular Level
Phthalates, the most widely used
plasticizers in poly(vinyl chloride)
(PVC), have been extensively studied. In this paper, a highly sensitive,
easy, and effective method was developed to examine short-term phthalate
leaching from PVC/phthalate films at the molecular level using sum
frequency generation vibrational spectroscopy (SFG). Combining SFG
and Fourier transform infrared spectroscopy (FTIR), surface and bulk
molecular structures of PVC/phthalate films were also comprehensively
evaluated during the phthalate leaching process under various environments.
The leaching processes of two phthalates, diethyl phthalate (DEP)
and dibutyl phthalate (DBP), from the PVC/phthalate films with various
weight ratios were studied. Oxygen plasma was applied to treat the
PVC/phthalate film surfaces to verify its efficacy on preventing/reducing
phthalate leaching from PVC. Our results show that DBP is more stable
than DEP in PVC/phthalate films. Even so, DBP molecules were still
found to very slowly leach to the environment from PVC at 30 °C,
at a rate much slower than DEP. Also, the bulk DBP content substantially
influences the DBP leaching. Higher DBP bulk concentration yields
less stable DBP molecules in the PVC matrix, allowing molecules to
leach from the polymer film more easily. Additionally, DBP leaching
is very sensitive to temperature changes; higher temperature can strongly
enhance the leaching process. For most cases, the oxygen plasma treatment
can effectively prevent phthalate leaching from PVC films (e.g., for
samples with low bulk concentrations of DBP5 and 30 wt %).
It is also capable of reducing phthalate leaching from high DBP bulk
concentration PVC samples (e.g., 70 wt % DBP in PVC/DBP mixture).
This research develops a highly sensitive method to detect chemicals
at the molecular level as well as provides surface and bulk molecular
structural changes. The method developed here is general and can be
applied to detect small amounts of chemical/biological environmental
contaminants
Multireflection Sum Frequency Generation Vibrational Spectroscopy
We developed a multireflection data
collection method in order
to improve the signal-to-noise ratio (SNR) and sensitivity of sum
frequency generation (SFG) spectroscopy, which we refer to as multireflection
SFG, or MRSFG for short. To achieve MRSFG, a collinear laser beam
propagation geometry was adopted and trapezoidal Dove prisms were
used as sample substrates. An in-depth discussion on the signal and
SNR in MRSFG was performed. We showed experimentally, with “<i>m</i>” total internal reflections in a Dove prism, MRSFG
signal is ∼<i>m</i> times that of conventional SFG;
SNR of the SFG signal-to-background is improved by a factor of ><i>m</i><sup>1/2</sup> and <<i>m</i>. MRSFG also improved
the SFG sensitivity to resolve weak vibrational signals. Surface molecular
structures of adsorbed ethanol molecules, polymer films, and a lipid
monolayer were characterized using both MRSFG and conventional SFG.
Molecular orientation information on lipid molecules with a 9% composition
in a mixed monolayer was measured using MRSFG, which showed a good
agreement with that derived from 100% lipid surface coverage using
conventional SFG. MRSFG can both improve the spectral quality and
detection limit of SFG spectroscopy and is expected to have important
applications in surface science for studying structures of molecules
with a low surface coverage or less ordered molecular moieties
Molecular Level Understanding of Adhesion Mechanisms at the Epoxy/Polymer Interfaces
It is important to understand the buried interfacial
structures
containing epoxy underfills as such structures determine the interfacial
adhesion properties. Weak adhesion or delamination at such interfaces
leads to failure of microelectronic devices. Sum frequency generation
(SFG) vibrational spectroscopy was used to examine buried interfaces
at polymer/model epoxy and polymer/commercial epoxy resins (used as
underfills in flip chip devices) at the molecular level. We investigated
a model epoxy: bisphenol A digylcidyl ether (BADGE) at the interfaces
of poly (ethylene terephthalate) (PET) before and after curing. Furthermore,
small amounts of different silanes including (3-glycidoxypropyl) trimethoxysilane
(γ-GPS), (3-Aminopropyl)trimethoxysilane (ATMS), Octadecyltrimethoxysilane
(OTMS(18C)), and Octyltrimethoxysilane (OTMS(8C)) were mixed with
BADGE. Silane influences on the polymer/epoxy interfacial structures
were studied. SFG was also used to study molecular interfacial structures
between polymers and two commercial epoxy resins. The interfacial
structures probed by SFG were correlated to the adhesion strengths
measured for corresponding interfaces. The results indicated that
a small amount of silane molecules added to epoxy could substantially
change the polymer/epoxy interfacial structure, greatly affecting
the adhesion strength at the interface. It was found that ordered
methyl groups at the interface lead to weak adhesion, and disordered
interfaces lead to strong adhesion
Molecular Ordering of Phenyl Groups at the Buried Polystyrene/Metal Interface
Understanding molecular structures
of buried polymer/metal interfaces
is important for the design and development of polymer adhesives used
in advanced microelectronic devices and polymer anticorrosion coatings
for metals. The buried interfacial molecular structure between polystyrene
(PS) and silver (Ag) was investigated using infrared-visible sum frequency
generation (SFG) vibrational spectroscopy via a “sandwiched”
sample geometry. SFG resonant signals from the phenyl C–H stretching
vibrational modes were detected from the PS/Ag interface, suggesting
that the PS phenyl groups at this buried polymer/metal interface are
ordered. Spectral analysis indicated that the phenyl groups at the
buried PS/Ag interface tilt toward the interface, pointing away from
the Ag side
Molecular Behavior at Buried Epoxy/Poly(ethylene terephthalate) Interface
Epoxies are widely
used as main components in packaging underfills
for microelectronics. Their strong adhesion to different substrate
materials is an important factor for the functioning of electronic
devices. Amines are commonly used cross-linking agents for epoxides.
However, the molecular mechanisms of epoxide–amine mixture
adhesion to substrate materials remain unclear. In this research we
investigated the adhesion mechanism of epoxide–amine mixtures
at poly(ethylene terephthalate) (PET) interfaces using attenuated
total-internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy
and sum frequency generation (SFG) vibrational spectroscopy. Results
show that both epoxide and amine could diffuse into the PET film.
They could also dissolve or modify the PET film at the interphase
region. In the process of epoxy curing on PET, epoxide molecules could
cross-link with the modified PET film, providing strong adhesion.
This hypothesis was further confirmed by adding reactive and nonreactive
silanes to the epoxies and measuring the adhesion strengths of such
mixtures to PET. The reactive silanes could cross-link with the system,
showing good adhesion, while the nonreactive silane prevented sufficient
cross-linking, showing poor adhesion. This research developed an in-depth
insight for molecular behaviors at the epoxy/PET interface which helped
clarify the related adhesion mechanism
Molecular Structural Changes of Plasticized PVC after UV Light Exposure
Plasticized poly(vinyl chloride)
(PVC) materials for industrial,
medical, and household use are often intentionally exposed to UV light,
though its impact on the molecular integrity and toxicity of the surface
and bulk of PVC materials is still not well understood. This paper
investigates the surface and bulk molecular changes of plasticized
PVC films with 25, 10, or 0 wt % bis-2-ethylhexyl phthalate (DEHP)
plasticizer after exposure to short wave (254 nm) or long wave (365
nm) UV light. Surface analytical techniques including sum frequency
generation vibrational spectroscopy (SFG) revealed short wave UV exposure
induced major molecular changes on the plasticized PVC surfaces, resulting
in increased surface hydrophilicity and decreased CH<sub>3</sub> content
with increasing exposure time. Additionally, it was deduced from multiple
techniques that the surface and the bulk of the plastic exposed to
short wave UV contained phthalic monoesters and phthalic acid formed
from multistep radical reactions. In contrast, when exposed to long
wave UV, molecular content and ordering on the surfaces of the plastic
remained relatively unchanged and the introduction of DEHP in plastic
helped protect PVC chains from degradation. Results from this study
demonstrate short wave UV exposure will result in plastic surfaces
containing phthalates and phthalate-related products accessible to
contact by living organisms
Membrane Orientation and Binding Determinants of G Protein-Coupled Receptor Kinase 5 as Assessed by Combined Vibrational Spectroscopic Studies
<div><p>G-protein coupled receptors (GPCRs) are integral membrane proteins involved in a wide variety of biological processes in eukaryotic cells, and are targeted by a large fraction of marketed drugs. GPCR kinases (GRKs) play important roles in feedback regulation of GPCRs, such as of β-adrenergic receptors in the heart, where GRK2 and GRK5 are the major isoforms expressed. Membrane targeting is essential for GRK function in cells. Whereas GRK2 is recruited to the membrane by heterotrimeric Gβγ subunits, the mechanism of membrane binding by GRK5 is not fully understood. It has been proposed that GRK5 is constitutively associated with membranes through elements located at its N-terminus, its C-terminus, or both. The membrane orientation of GRK5 is also a matter of speculation. In this work, we combined sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) to help determine the membrane orientation of GRK5 and a C-terminally truncated mutant (GRK5<sub>1-531</sub>) on membrane lipid bilayers. It was found that GRK5 and GRK5<sub>1-531</sub> adopt a similar orientation on model cell membranes in the presence of PIP<sub>2</sub> that is similar to that predicted for GRK2 in prior studies. Mutation of the N-terminal membrane binding site of GRK5 did not eliminate membrane binding, but prevented observation of this discrete orientation. The C-terminus of GRK5 does not have substantial impact on either membrane binding or orientation in this model system. Thus, the C-terminus of GRK5 may drive membrane binding in cells via interactions with other proteins at the plasma membrane or bind in an unstructured manner to negatively charged membranes.</p> </div
Interaction of Polyethylenimine with Model Cell Membranes Studied by Linear and Nonlinear Spectroscopic Techniques
Polyethylenimine
(PEI) has been widely used as a transfection agent for gene delivery,
but it is cytotoxic and can lead to cell apoptosis. Although several
apoptotic mechanisms have been proposed, a molecular level understanding
of PEI/cell membrane interaction can help develop further insight
into such cytotoxicity. We combined sum frequency generation (SFG)
vibrational spectroscopy and attenuated total-internal reflection
Fourier transform infrared (ATR-FTIR) spectroscopy to study the effect
of PEI on lipid transbilayer movement in supported bilayers (serving
as model cell membranes) as a function of lipid composition, PEI concentration,
and temperature. For both dipalmitoylphosphatidylglycerol (DPPG) and
distearoylphosphatidylcholine (DSPC) bilayers, PEI molecules showed
no significant effect on lipid translocation at room temperature (21
°C).
In contrast, significant lipid translocation was observed near the
physiological temperature (39 °C), indicating the ability of
PEI to induce lipid translocation in both negatively charged and zwitterionic
lipid bilayers, without the assistance of membrane proteins. Furthermore,
results showed that PEI had strong interactions with negatively charged
DPPG and weak interactions with zwitterionic DSPC. Concentration-dependent
studies indicated that the lipid translocation rate had a linear dependence
on the PEI concentration in the subphase. The effects of branched
and linear PEIs were compared in the study, showing that branched
PEI had a greater effect on the lipid translocation rate due to the
higher charge density, which might be a possible indication of higher
toxicity. ATR-FTIR spectroscopy verified that the results observed
in SFG were mainly caused by lipid translocation, not bilayer damage
or removal from the substrate. The combined SFG and ATR-FTIR study
provides a powerful method to examine molecular interactions between
lipid bilayers and polyelectrolytes at a molecular level. The results
can help to develop further understanding on PEI’s cytotoxicity
in biological systems
Molecular Orientation Analysis of Alkyl Methylene Groups from Quantitative Coherent Anti-Stokes Raman Scattering Spectroscopy
Quantitative data analysis in coherent
anti-Stokes Raman scattering
(CARS) spectroscopy is important for extracting molecular structural
information. We developed a method to derive molecular tilt angle
with respect to the surface normal based on quantitative CARS spectral
analysis. We showed that the tilt angle of methylene alkyl chains
on a surface can be directly obtained from the CH<sub>2</sub> symmetric/asymmetric
peak ratio in a CARS spectrum. The lipid alkyl chain tilt angle from
a lipid monolayer was measured to be ∼0° and was verified
by sum frequency generation spectroscopy, which probes the orientations
of the lipid methyl end groups. The tilt angle of a silane monolayer
alkyl chain was derived to be ∼35°, which agrees with
the theoretical prediction. This method is submonolayer sensitive
and can also be used to interpret polarization-dependent signals in
CARS microscopy. It can be applied to elucidate detailed molecular
structure from CARS spectroscopic and microscopic measurements
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