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 TiO₂ 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 DBP5 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>^1/2 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 CH₃ 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_1-531) on membrane lipid bilayers. It was found that GRK5 and GRK5_1-531 adopt a similar orientation on model cell membranes in the presence of PIP₂ 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 CH₂ 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