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

Interchain Overlap Affects Formation of Silk Fibroin Secondary Structures on Hydrophobic Polystyrene Surface Detected via Achiral/Chiral Sum Frequency Generation

Silk fibroin (SF) has been engineered in the biomedical applications on account of its structural robustness, biocompatibility, and biodegradability. However, in situ study is still lacking with respect to the formation of SF secondary structures at the interface. In this paper, by using methanol as an inducing agent, the formation of SF secondary structures at the polystyrene (PS)/SF solution interfaces was detected with achiral and chiral sum frequency generation (SFG) vibrational spectroscopy. SF solutions with two concentrations above and below the critical overlapping concentration (<i>C</i>*) of SF (∼1.8 mg/mL) were chosen, namely, 90 and 1 mg/mL. We found that above <i>C</i>*, before adding methanol to the protein solution, no ordered SF secondary structures could be detected at the PS/SF solution interface; oppositely, after adding methanol to the protein solution, ordered SF secondary structure, for example, antiparallel β-sheet, could be formed at the PS/protein solution interface. Below <i>C</i>*, both before and after adding methanol to the SF solution, ordered SF secondary structure such as antiparallel β-sheet could be formed. Besides, the addition of methanol could induce the formation of an extended helical structure, verified by the achiral and chiral characteristic bands. Because <i>C</i>* represents a critical solution concentration above which the SF chains can interact with each other and below which the SF chains are isolated in the solution, this achiral/chiral SFG study emphasizes the importance of the chain–chain interaction or spatial confinement on the formation of the protein secondary structures, which provides an additional dimension for the future study of interfacial protein folding

Probing Interfacial Aging of Model Adhesion Joints under a Hygrothermal Environment at a Molecular Level

Generally, for adhesive joints, the polar water molecules in humid environments can have a critical effect on the interfacial structures and structural evolution adjacent to the solid substrates. Regarding this, it is still a big challenge to detect and understand the interfacial hygrothermal aging process at the molecular level in real time and in situ. In this study, to trace the interfacial hygrothermal aging process of a classical epoxy formula containing diglycidyl ether of biphenyl A (DGEBA) and 2,2′-(ethylenedioxy) diethylamine (EDDA) with sapphire and fused silica in a typical hygrothermal environment (85 °C and 85% RH), sum frequency generation (SFG) vibrational spectroscopy was used to probe the molecular-level interfacial structural change over the time. The structural evolution dynamics at the buried epoxy/sapphire and epoxy/silica interfaces upon hygrothermal aging were revealed directly in situ. The interfacial delamination during hygrothermal aging was also elucidated from the molecular level. Upon hygrothermal aging, the interfacial CH signals, such as the ones from methyl, methylene, and phenyl groups, decreased significantly and the water OH signals increased substantially, indicating the water molecules had diffused into the interfaces and destroyed the original interactions between the epoxy formula and the substrates. Further analysis indicates that when the integrated signals in the CH range declined to their minimum and leveled off, the interfacial delamination happened. The tensile experiment proved the validity of these spectroscopic experimental results. Our study provides first-hand and molecular-level evidence on a direct correlation between the diffusion of the surrounding water molecules into the interface and the evolution/destruction of the interfacial structures during hygrothermal aging. More importantly, it is proved, SFG can be developed into a powerful tool to noninvasively reveal the local interfacial delamination in real time and in situ under extreme hygrothermal conditions, complemented by the mechanic test

Method to Probe Glass Transition Temperatures of Polymer Thin Films

A new methodology was developed to probe glass transition temperatures (<i>T</i>_gs) of polymer thin films supported on gold (Au) substrates and confined between two solid (silica and silver) surfaces based on the surface plasmon polariton (SFPP) signals generated by sum frequency generation (SFG) spectroscopy. The measured <i>T</i>_gs for polymer (poly­(methyl methacrylate), poly­(benzyl methacrylate) and poly­(ethyl methacrylate)) thin films supported on Au substrates showed similar thickness-dependent trend, that is, the <i>T</i>_g decreased as the thin film thickness decreased due to the free surface effect. However, the measured <i>T</i>_g of the (poly­(methyl methacrylate)) thin films confined between two solid surfaces increased significantly with respect to the bulk value, indicating the strong interfacial effect when the free surface was replaced by a buried interface. This method to measure the <i>T</i>_g can be applied to study different polymer thin films supported on metal surfaces or confined between two solid surfaces with different surface chemistries. More importantly, SFG has the unique selectivity and sensitivity to study surfaces and interfaces, providing the feasibility to develop SFG into a powerful tool to detect surface, interfacial, and bulk <i>T</i>_gs of a polymer thin film simultaneously in the future

Surface Properties of Poly(vinyl alcohol) Films Dominated by Spontaneous Adsorption of Ethanol and Governed by Hydrogen Bonding

The surface structures of poly­(vinyl alcohol) (PVA) films with four different degrees of hydrolysis after immersion in ethanol were investigated using sum frequency generation (SFG) vibrational spectroscopy and contact angle (CA) goniometry. The result showed that the surface chemical structure of the PVA films was strongly dependent on the degree of hydrolysis. The vinyl acetate (VAc) units in the PVA chains resulting from incomplete hydrolysis segregate to the film surface and strongly affect the adsorption behavior of ethanol molecules on their surfaces. The surface hydrophilicity decreased greatly for PVA films with relatively high hydrolysis degrees (i.e., 99% and 97.7%), in which the water contact angle increased by 20°, and increased for PVA with relatively low hydrolysis degrees (95.1% and 84%) after immersion in ethanol. It was found that ethanol molecules adsorb from solution onto a PVA film surface in an ordered and cooperative way governed by hydrogen bonding when the hydrolysis degrees of PVA were higher than 98%. When the hydrolysis degree of PVA was lower than 96%, the surface structure obtained by surface reconstruction dominated after immersion in ethanol, with fewer ethanol molecules adsorbed on the surface, resulting in a decrease of its water contact angle

Structures and Adhesion Properties at Polyethylene/Silica and Polyethylene/Nylon Interfaces

The molecular structures of buried interfaces of maleic anhydride grafted and ungrafted polyethylene films with silica and nylon surfaces were studied in situ using sum-frequency generation (SFG) vibrational spectroscopy. Grafting maleic anhydride to polyethylene altered the molecular structures at buried interfaces, including changing the orientation of polymer methylene groups and resulting in the presence of CO groups at silica interfaces. These molecular level changes are correlated with enhanced adhesion properties, with ordered CO groups and in-plane orientation of the methylene groups associated with higher levels of adhesion. While improved adhesion was observed for grafted polyethylene at the nylon interface, no CO groups were detected at the interface using SFG, for films thermally treated at 185 °C. In this case, either no CO groups are present at the interface or they are disordered; the latter explanation is more likely, considering the observed improvement in adhesion

Self-decontaminating properties of fluorinated copolymers integrated with ciprofloxacin for synergistically inhibiting the growth of <i>Escherichia coli</i>

<div><p>In this paper, copolymers composed of antibacterial monomer containing ciprofloxacin, methyl methacrylate (MMA), and 2-perfluorooctylethyl methacrylate (FMA) were prepared, and the surface properties and antibacterial performance of the copolymers and blends-mixed PMMA were investigated. Surface characterization using dynamic contact angle measurement and X-ray photoelectron spectroscopy showed that anti-adhesive fluorinated moieties and antimicrobial moieties were highly concomitant on the material surface. All the copolymers and blends films exhibited excellent antibacterial properties. It was found that the fluorinated antibacterial copolymers showed significantly enhanced antibacterial efficiency toward <i>Escherichia coli</i> bacterium, and even markedly prevented the formation of biofilm for long term. The PMMA films blended with fluorinated antibacterial polymer also show similar results. In contrast, the common copolymer without fluorinated units cannot effectively resist bacterial adhesion, proliferation, and prevent biofilm formation. The desirable antibacterial polymer prohibiting the biofilm formation performance of copolymer with special push-me/pull-you structure which weaken the interaction among polymer chains resulted in the more easy segregation of ciprofloxacin on surface in real environment by the help of synergistic effect of fluorinated units, potentially enabling the design of new self-decontaminating biomaterials for control biofouling.</p></div

Facile Synthesis of Three-Dimensional PtPdNi Fused Nanoarchitecture as Highly Active and Durable Electrocatalyst for Methanol Oxidation

Electrocatalysts for methanol electrooxidation with high activity and durability are greatly desired for boosting commercialization of direct methanol fuel cells (DMFCs). However, the current methanol electrooxidation catalysts are far from the anticipation, and their application has been limited by the low catalytic activity and fast performance degradation. In this work, facile and rapid synthesis of ternary PtPdNi alloy nanoarchitecture with high performance for methanol electrooxidation is reported. The role of each component has been investigated in detail. The introduction of Pd makes contribution to the improvement of the catalytic activity. Furthermore, Ni plays important roles in improving both catalytic activity and durability. The PtPdNi catalyst shows higher activity than any of its components (Pt, PtPd, and PtNi) and even 3.58 times higher than commercial Pt black (PB) in Pt-based mass activity. Moreover, the PtPdNi nanoarchitecture exhibits extremely high durability compared to PB; 0.6 A/mg_Pt (31% of the initial one) of the catalytic current density is retained even after a 50,000 s endurance test