Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy

This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecular-level understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecular-level information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.     

PROBING HIDDEN POLYMERIC INTERFACES USINGIR–VISIBLE SUM-FREQUENCY GENERATION SPECTROSCOPY

Understanding the molecular-level processes underlying interfacial phenomena is important in the area of adhesion. We briefly introduce IR–visible sum-frequency generation spectroscopy (SFG) using a total-internal-reflection geometry for the study of polymer–air, polymer–solid, and polymer–polymer interfaces. The following examples, predominantly of work done in our lab, illustrating differences in molecular structure and dynamic properties at interfaces are presented: the air- and solid-interface structure of an amorphous polystyrene (PS) and a semicrystalline polymer with side-chain crystallinity, poly(octadecyl acrylate) (PA-18); structure of a polymer–polymer interface between thin films of a semicrystalline polymer with side-chain crystallinity, poly(vinyl-N-octadecylcarbamate- co-vinyl acetate), and an amorphous PS; thermal order-to-disorder transitions of the air and solid interface of PA-18, and the interface of this polymer with PS; and dynamic surface-relaxation studies of a rubbed PS film.     

Polymer molecular behaviors at buried polymer/metal and polymer/polymer interfaces and their relations to adhesion in packaging

Packaging materials are widely used in modern microelectronics. The interfacial structures of packaging materials determine the adhesion properties of these materials. Weak adhesion or delamination at interfaces involving packaging materials can lead to failure of microelectronic devices. Therefore, it is important to investigate the molecular structures of such interfaces. However, it is difficult to study molecular structures of buried interfaces due to the lack of appropriate analytical techniques. Sum frequency generation (SFG) vibrational spectroscopy has recently been used to probe buried solid/solid interfaces to understand molecular structures and behaviors such as the presence, coverage, ordering, orientation, and diffusion of functional groups at buried interfaces and their relations to adhesion in situ in real time. In this review, we describe our recent progress in the development of nondestructive methodology to examine buried polymer/metal interfaces and summarize how the developed methodology has been used to elucidate adhesion mechanisms at buried polymer/metal interfaces using SFG. We also elucidated the molecular interactions between polymers and various model and commercial epoxy materials, and the correlations between such interactions and the interfacial adhesion, providing in-depth understanding on the adhesion mechanisms of polymer adhesives.     

Characterisation of buried glass/cyclo-olefin polymer film interfaces by sum-frequency generation vibrational spectroscopy

Plasma gas-modified cyclo-olefin polymer (COP) surfaces and the interfaces between borosilicate glass and COP films were investigated by sum-frequency generation (SFG) vibrational spectroscopy. Upon exposure to oxygen gas plasma, the SFG signal intensities increased, indicating an improvement in the orientational order at the surface functional groups. In addition, thermal annealing following lamination improved the COP interphase molecular ordering and increased the number density of functional molecules at the interfaces.     

Polymer-Silane Interactions Probed by Sum Frequency Generation Vibrational Spectroscopy

Gaining an understanding of the molecular mechanisms of adhesion to polymeric materials is crucial for the design of better adhesives and adhesion promoters. Silane coupling agents are widely used as polymer-polymer adhesion promoters. However, a molecular level understanding of how silanes enhance adhesion between solid polymeric surfaces is largely unknown. Here we exploit the extreme surface sensitivity of sum frequency generation (SFG) vibrational spectroscopy to probe various interactions at buried interfaces between polymers and silanes in situ. It has been elucidated that silanes can adopt various conformations at the interfaces with different polymers depending on the chemical groups that comprise the silane and the surface-presenting groups on the polymer. Some silanes have been found to diffuse into certain polymers, and SFG has been used to monitor the moving polymer/silane interface and deduce the diffusion coefficient. Hydrogen bonding between polymer surface carbonyl groups and silane amino groups has also been detected. Finally we demonstrate that SFG can probe the buried interface between a polymer and a cured silicone elastomer, and the segregation of silane adhesion promoting molecules to the polymer/elastomer interface can be detected.     

Polymer-Silane Interactions Probed by Sum Frequency Generation Vibrational Spectroscopy

Gaining an understanding of the molecular mechanisms of adhesion to polymeric materials is crucial for the design of better adhesives and adhesion promoters. Silane coupling agents are widely used as polymer-polymer adhesion promoters. However, a molecular level understanding of how silanes enhance adhesion between solid polymeric surfaces is largely unknown. Here we exploit the extreme surface sensitivity of sum frequency generation (SFG) vibrational spectroscopy to probe various interactions at buried interfaces between polymers and silanes in situ. It has been elucidated that silanes can adopt various conformations at the interfaces with different polymers depending on the chemical groups that comprise the silane and the surface-presenting groups on the polymer. Some silanes have been found to diffuse into certain polymers, and SFG has been used to monitor the moving polymer/silane interface and deduce the diffusion coefficient. Hydrogen bonding between polymer surface carbonyl groups and silane amino groups has also been detected. Finally we demonstrate that SFG can probe the buried interface between a polymer and a cured silicone elastomer, and the segregation of silane adhesion promoting molecules to the polymer/elastomer interface can be detected.     

In situ ellipsometry studies on swelling of thin polymer films: A review

The properties of a thin polymer film can be significantly affected by the presence of a penetrant. This can have potential implications for many technological applications, such as protective and functional coatings, sensors, microelectronics, surface modification and membrane separations. In situ ellipsometry is a powerful technique for the characterization of a film in contact with a penetrant. The main advantages of ellipsometry include the very high precision and accuracy of this technique, combined with the fact that it is non-intrusive. Recent advances in the speed and automation of the technique have further expanded its application.This article provides an overview of the research that has been done with in situ UV–vis ellipsometry on penetrant-exposed polymeric films, in the last 15–20 years. The focus is predominantly on films that are not attached covalently to a substrate. Polymer brushes and grafts are therefore excluded. This review addresses a variety of topics, covering instrumental aspects of in situ studies, approaches to data analysis and optical models, reported precision and repeatability, the polymer-penetrant systems that have been studied, the kind of information that has been extracted, and other in situ techniques that have been combined with ellipsometry. Various examples are presented to illustrate different practical approaches, the consequences of the optical properties of the ambient, and the various ways that have been employed to bring polymer films in contact with a penetrant, ranging from simple ex situ-like configurations (i.e., drying studies) to complex high pressure cells. The versatility of in situ ellipsometry is demonstrated by examples of the distinctive phenomena studied, such as film dilation, penetrant diffusion mechanisms, film degradation, electrochemical processes, and the broad variety of polymer-penetrant systems studied (glassy and rubbery polymers, multilayer stacks, etc.). An outlook is given on possible future trends.     

Study of polycarbonate–polystyrene interfaces using scanning transmission electron microscopy spectrum imaging (STEM-SI)

Polymer blends are important for both commercial utility and scientific understanding. The degree of interfacial mixing in polymer blends is important since it influences the blends' mechanical properties. Understanding bulk properties in multiphase polymeric materials requires knowledge of the interfacial properties of the materials. The characterization of the interface, in terms of its width and composition profile, provides insight about the bulk behaviour of the material. Chemical microscopy through electron energy-loss spectroscopy (EELS) in a transmission electron microscope is gaining popularity to characterize narrow polymer–polymer interfaces. In this work, we show how scanning transmission electron microscopy spectrum imaging, a spatially resolved energy-loss spectroscopy, can be employed to calculate the interfacial width in a pair of immiscible polymers, taking a polycarbonate–polystyrene (PC-PS) bilayer as an example. By mapping peaks unique to each of the blend constituents at several points across the interface, we show how the interfacial profile concentrations can be determined. With this method we calculated the interfacial width in the PC-PS bilayer sample to be approximately 32 nm, even utilizing low resolution spectrometers, which are more widely available. Using the technique described with higher resolution EELS instruments having a better signal-to-noise ratio, a higher spatial resolution can be achieved. Using EELS chemical fingerprints of polymers that have been developed earlier, the technique presented here has the potential for effective visualization and morphological measurements of phase-differentiated polymer blends. This paper is an attempt to enable a new user to characterize polymer–polymer interfaces using chemical microscopy. © 2022 Society of Industrial Chemistry.     

Surface and Interface Spectroscopy of High Explosives and Binders: HMX and Estane

Vibrational sum‐frequency generation spectroscopy (SFG) is used to characterize the surfaces of β‐HMX single crystals and Estane polymer binder, as well as the HMX‐Estane interface. SFG is a nonlinear vibrational spectroscopy that selectively probes vibrational transitions at surfaces and interfaces. On the HMX {011} surface, both CH‐ and NO₂‐stretching transitions are observed. Compared to bulk HMX, the surface transitions are blueshifted and the splittings are larger. This effect is explained by surface HMX molecules having partially buried and partially free CH₂ and NO₂ groups. Estane is a diblock copolymer with both soft and hard segments. Comparison of Estane spectra with polymers having only the soft unit and with polymers having predominantly hard units indicate there is a preference for the hard unit on the surface. SFG spectra of the HMX‐Estane interface show smaller splittings of the HMX CH‐stretch transitions than at the HMX‐air interface, because the partially free surface groups are buried in Estane.     

PROBING HIDDEN POLYMERIC INTERFACES USINGIR-VISIBLE SUM-FREQUENCY GENERATION SPECTROSCOPY

Understanding the molecular-level processes underlying interfacial phenomena is important in the area of adhesion. We briefly introduce IR-visible sum-frequency generation spectroscopy (SFG) using a total-internal-reflection geometry for the study of polymer-air, polymer-solid, and polymer-polymer interfaces. The following examples, predominantly of work done in our lab, illustrating differences in molecular structure and dynamic properties at interfaces are presented: the air- and solid-interface structure of an amorphous polystyrene (PS) and a semicrystalline polymer with side-chain crystallinity, poly(octadecyl acrylate) (PA-18); structure of a polymer-polymer interface between thin films of a semicrystalline polymer with side-chain crystallinity, poly(vinyl-N-octadecylcarbamate- co-vinyl acetate), and an amorphous PS; thermal order-to-disorder transitions of the air and solid interface of PA-18, and the interface of this polymer with PS; and dynamic surface-relaxation studies of a rubbed PS film.     

Two antagonistic effects of flow/mixing on reactive polymer blending

Reactive polymer blending is basically a flow/mixing-driven process of interfacial generation, interfacial reaction for copolymer formation, and morphology development. This work shows two antagonistic effects of flow/mixing on this process: while flow/mixing promotes copolymer formation by creating interfaces and enhancing collisions between reactive groups at the interfaces, excessive flow/mixing may pull the in situ formed copolymer out of the interfaces to one of the two polymer components of the blend, especially when the copolymer becomes highly asymmetrical. As such, the copolymer may lose its compatibilization efficiency. The mixing-driven copolymer pull-out from the interfaces is a catastrophic process (less than a minute), despite the high viscosity of the polymer blend. It depends on the molecular architecture of the reactive compatibilizer, polymer blend composition, flow/mixing intensity, and annealing. These findings are obtained using the concept of reactive compatibilizer-tracer and a model reactive polymer blend.     

Macromolecular and supramolecular chirality: a twist in the polymer tales

Significant advances have been made recently in generating chiral polymer surfaces and materials using a range of methods such as block copolymer self‐assembly, layer‐by‐layer assembly and surface functionalization by polymer brushes. This paves the way for novel chiral materials that can harness and tailor chiral interactions for specific functionalities and properties in a range of biomedical and bioanalytical applications. This paper reviews these advances and speculates on the future of chiral surfaces. © 2013 Society of Chemical Industry     

Preparation and Impedance Analysis of Biodegradable Polymer Polyvinyl Alcohol with Amino Acid,Arginine

Biodegradable polymers have an innumerable use in the field of biomedicine, especially in drug delivery system. Polyvinyl alcohol is one of the biodegradable polymer used as a carrier for drug delivery. Amino acids are necessary for maintaining good health for human beings. The present study focuses on the interaction between polyvinyl alcohol and amino acids. An effort is being taken to prepare polymer membrane based on polyvinyl alcohol complexed with different concentration of arginine, a type of amino acids using water as solvent by solution-casting technique. The amphorousity and complex formation between polyvinyl alcohol and Arignine have been confirmed by X-ray diffraction and FTIR spectroscopy, respecticvely. The thermal behavior of PVA–arginine complexes has been analyzed by differential scanning calorimetry. From AC impedance spectroscopy, ion transport mechanism has been investigated in detail. By using Almond and West formulisms, the parameter such as ion hopping frequency ω_p, has been calculated. The polymer membrane 75 Mwt% PVA:25 Mwt% arginine has the highest ionic conductivity as 1.97 × 10^?6 S cm^?1 at ambient temperature.     

Chiral vibrational structures of proteins at interfaces probed by sum frequency generation spectroscopy

We review the recent development of chiral sum frequency generation (SFG) spectroscopy and its applications to study chiral vibrational structures at interfaces. This review summarizes observations of chiral SFG signals from various molecular systems and describes the molecular origins of chiral SFG response. It focuses on the chiral vibrational structures of proteins and presents the chiral SFG spectra of proteins at interfaces in the C-H stretch, amide I, and N-H stretch regions. In particular, a combination of chiral amide I and N-H stretches of the peptide backbone provides highly characteristic vibrational signatures, unique to various secondary structures, which demonstrate the capacity of chiral SFG spectroscopy to distinguish protein secondary structures at interfaces. On the basis of these recent developments, we further discuss the advantages of chiral SFG spectroscopy and its potential application in various fields of science and technology. We conclude that chiral SFG spectroscopy can be a new approach to probe chiral vibrational structures of protein at interfaces, providing structural and dynamic information to study in situ and in real time protein structures and dynamics at interfaces.     

Optimum Polymer - Solid Interface Design for Adhesion Strength: Carboxylation of Polybutadiene and Mixed Silanes Surface Modification of Aluminum Oxide

To understand the optimum design of polymer-solid interfaces for adhesion strength, model polymer-solid interfaces of carboxylated polybutadiene(cPBD) adhered to mixed silane modified Al₂O₃ surfaces were examined. The cPBD, having various ?COOH sticker group concentration φ(X) (0 ～ 10 mol%), was synthesized through high-pressure carboxylation of PBD, while Al₂O₃ surfaces were modified to have various -NH₂ density, φ(Y) (0 ～ 100 mol%), using self-assembly of mixed amine- and methyl-terminated silanes. The coadsorption kinetic model of the two silanes was analyzed through X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and dynamic contact angle (DCA), which gave the capability of controlling the receptor concentration of aluminum oxide surfaces. The polymer surface chain responses after exposure to various media were understood by measuring contact angle changes of various probe liquids. T-peel tests of the model polymer–solid interfaces, as a function of time and sticker and receptor group concentrations showed much longer time dependence than the characteristic time of a bulk polymer chain. Additionally, the classical equation of interface failure was re-examined to see the effects of deformation rate, annealing temperature, and annealing time. A simple scaling analysis of free energy of an adsorbed polymer on a solid surface was extended to predict the adhesion potential of the model polymer–solid interfaces. From the experiments and theory of adhesive vs. cohesive failure, it was found that there existed an optimum product value r* = φ(X)φ(Y)χ of sticker concentration φ(X), receptor concentration φ(Y), and their interaction strength χ, which was approximately 150 cal/mol for this polymer–solid interface. Below or above this optimum product value r*, the fracture energy of polymer-solid interfaces, G _IC, was less than its optimal value, G _lc*.     

Wrinkled interfaces: Taking advantage of surface instabilities to pattern polymer surfaces

The generation of nano-microstructured polymer film surfaces has been a challenge during the last decades. Advances in the fabrication of structured polymer surfaces to obtain micro and nano patterns have been accomplished following two different approaches, i.e., by adapting techniques, such as molding (embossing) or nano/microimprinting or by developing novel techniques including laser ablation, soft lithography or laser scanning among others. Thus, higher resolution capabilities are directly related with technological advances. In contrast to the use of highly sophisticated tools required by the above mentioned techniques, surface instabilities produced by different mechanisms take advantage of the inherent properties of polymers to induce particular surface patterns. Some of the surface instabilities are well known since decades but novel and old known instability mechanisms have been only recently extended their use to pattern polymer surfaces. This recent interest relies on the rich and complex patterns obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate by using traditional patterning techniques.Among the approaches to obtain patterned interfaces by means of surface instabilities the formation of wrinkles is the most explored method and will be the center of this review. The fabrication approaches employed to induce wrinkle formation and the possibilities to fine tune the amplitude and period of the wrinkles, the functionality and their final morphology are thoroughly described. Finally, an overview about the main applications in which buckled interfaces have been already employed or may have an impact in the near future is provided. Their use as templates, as flexible electronics, as supports with controlled wettability and/or adhesion or for biorelated applications are few of the fields in which the unique characteristics of wrinkled interfaces play distinguishing role.     

Effects of interfaces on the thermal degradation of polymer–metal composites

Previous studies by Black and Blomquist on the degradative failure of polymer–metal adhesive bonds have shown that composite failure depends largely on the type of metal substrate employed. In the work reported herein, metal powders of high surface area have been employed to maximize the metal–polymer interface. The composite systems studied consisted for aluminum and iron with polycondensates of bisphenol A–diglycidyl ether, phenol–formaldehyde and poly-2,2′-(m-phenylene)-5,5′-bibenimidazole. The composites were prepared in the absence of air and thermally degraded in a time-of-flight mass spectrometer while the degradation products were continuously monitored from mass 1 to 200. In the polymer and polymer–metal systems investigated, iron accelerated the decomposition of all polymers studied. This was determined by plotting m/e against degradation temperature for the more common mass peaks such as hydrogen and carbon monoxide for the carbon–hydrogen–oxygen-containing polymer and hydrogen cyanide and ammonia for the carbon–hydrogen–nitrogen-containing polymer. This technique offers promise in determining the nature of the interface as well as the effect of the interface on polymer degradation.     

Ultrasound assisted in situ emulsion polymerization for polymer nanocomposite: A review

This review covers an ultrasound assisted synthesis of polymer nanocomposites using in situ emulsion polymerization. First of all, surface modification of core nanoparticles with a coupling agent and surfactant has been employed for the synthesis of core–shell polymer nanocomposites. In addition to application of ultrasound for the synthesis of core–shell polymer nanocomposites, due to its influential efficiency, sonochemistry has been extensively used not only as an aid of dispersion for inorganic nanoparticles and organo-clay, but also acts as an initiator to enhance polymerization rate for synthesis of polymer nanocomposites. In situ emulsion polymerization of hydrophobic monomers, such as methyl methacrylate, butyl acrylate, aniline, vinyl monomers and styrene, using surfactant and water soluble initiator were carried out for a synthesis of core–shell polymer nanocomposite. This technique assists in preparation of stable and finely dispersed polymer nanocomposite with the loading of inorganic particles up to 5 wt.%. Recent developments in the preparation of core–shell polymer nanocomposites using an ultrasound assisted method with their physical characteristics such as morphology, thermal, and rheological properties and their potential engineering applications have been discussed in this review.     

Effect of Lithium Perchlorate on the Optoelectrical and Thermal Properties of Poly(Vinylpyrrolidone)/Nano-Cesium Aluminate Solid Polymer Electrolytes

Solid polymer electrolyte (SPE) of polyvinylpyrrolidone (PVP) with varying amounts, namely, 5, 10, and 15?wt% of lithium perchlorate (LiClO₄) as an electrolyte and 8?wt% cesium aluminum oxide (CsAlO₂) nanoparticle have been fabricated by solution intercalation technique. The optoelectrical behaviors of the SPE films have been evaluated using UV–visible spectroscopy. The UV–visible spectral studies revealed the UV light-absorbing nature of NC films with considerable visible transparency. The chemical structure and morphological behaviors of PVP/8?wt% of CsAlO₂–LiClO₄ SPE films have been established by Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy, respectively. The AC conductivity of the SPEs was evaluated at room temperature by digital LCR meter in the frequency range 100 Hz–5?MHz. The thermal behaviors such as T_g and degradation patterns of the SPEs have been evaluated using differential scanning calorimetric analysis and thermogravimetric analysis, respectively.     

Immobilization of polyethylene oxide surfactants for non-fouling biomaterial surfaces using an argon glow discharge treatment

A non-fouling (protein-resistant) polymer surface is achieved by the covalent immobilization of polyethylene oxide (PEO) surfactants using an inert gas discharge treatment. Treated surfaces have been characterized using electron spectroscopy for chemical analysis (ESCA), static secondary ion mass spectrometry (SSIMS), water contact angle measurement, fibrinogen adsorption, and platelet adhesion. This paper is intended to review our recent work in using this simple surface modification process to obtain wettable polymer surfaces in general, and non-fouling biomaterial surfaces in particular.