Investigation of morphological and electrical properties of the PMMA coating upon exposure to UV irradiation based on AFM studies

The objective of study was to investigate the influence of UV irradiation on morphological changes of a polymeric surface and its electrical properties. In the presented investigation thin poly(methyl methacrylate) (PMMA) film was applied onto iron substrate by solution casting method. UV-C irradiation in range of 200–280 nm was used as a deteriorative factor to induce polymer degradation. Atomic force microscopy (AFM) method was employed to study surface topography of the PMMA coatings before and after exposure to UV-illumination. Photo-induced changes in the polymer surface taking form of microcracks were illustrated by AFM images. In order to support results obtained with AFM method, electrochemical impedance spectroscopy (EIS) measurements were conducted. The authors chose this technique to confirm whether the changes on UV-exposed PMMA surface observed on AFM images could indicate potential sites of the polymer coating long before serious damage could occur. Both methods EIS and AFM were used in order to provide information about durability of PMMA film.     

AFM/FTIR: A New Technique for Materials Characterization

A new type of infrared spectroscopy for obtaining the molecular composition of the surfaces of materials at ultra-high spatial resolution has been developed by combining atomic force microscopy (AFM) with Fourier-transform infrared spectroscopy (FTIR). This new analytical technique involves the use of an AFM to detect the response of a material to the absorption of modulated infrared radiation from an FTIR spectrometer and is referred to as AFM/FTIR spectroscopy. When the technique of AFM/FTIR spectroscopy is completely developed, we plan to use it to probe the molecular structure of interphases in polymer composites and adhesive bonds. Two approaches have been used to measure the response of polymer systems to infrared absorption. The first involves the use of a contact mode AFM probe to measure the thermal expansion of the polymer; the second involves using a scanning thermal microscopy (SThM) probe to measure the polymer's temperature increase. In either case, the output of the probe resembles an interferogram to which a Fourier-transform can be applied to obtain the infrared absorption spectrum. The first approach was used to obtain excellent quality AFM/FTIR spectra from various neat polymer films, including polystyrene, polycarbonate, and a model adhesive system consisting of an epoxy resin cross-linked with dicyandiamide. Excellent spectra were also obtained from polystyrene beads having a diameter of about 2 µm. The second approach, using an SThM probe to determine the temperature increase that accompanies infrared absorption, was also used to obtain interferograms of polymer samples such as polystyrene. However, the interferograms were noisy and the AFM/FTIR spectra obtained from them had a low signal-to-noise ratio. The present results, thus, show that AFM/FTIR spectroscopy is feasible but the spatial resolution of the technique remains to be shown.     

Preparation and tribological properties of polymer film covalently bonded to silicon substrate via an epoxy-terminated self-assembled monolayer

A covalently immobilized polymer film was constructed on silicon substrate by a two-step method. As an anchor interlayer, (3-glycidoxypropyl)trimethoxysilane (GPMS) was self-assembled on hydroxylated silicon substrate to create epoxy-terminated surface, then poly(styrene-b-acrylic acid) (PSAA) was chemically grafted to the epoxy-derivatized substrates. The formation and surface properties of the films were characterized by means of ellipsometry, water contact angle measurement, attenuated total reflectance Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy, and atomic force microscope (AFM). The nano- and micro-tribological properties of the films were evaluated by AFM and ball-on-plate tribometer, respectively. The results show that GPMS–PSAA film exhibits excellent durability and wear resistance, which is attributed to the molecular components of PSAA and the firm bonding between polymer molecules and silicon substrate via epoxysilane molecular glue. The influence of interlayer between polymer and substrate surface on tribological properties of ultrathin polymer film was revealed, which has an important significance upon designing ultrathin lubrication films with excellent tribological properties for micro/nanoelectromechanical systems.     

Probing polymer interdiffusion in carboxylated latices with force modulation atomic force microscopy

Interdiffusion of polymer chains between latex particles is a prerequisite for the development of good mechanical strength and homogeneity in a latex film. This process may be retarded in carboxylated latices if the particles are surrounded by a hard cell wall consisting of ionic groups on the particle surface. The presence of an ionic cell wall can be indirectly detected by atomic force microscopy (AFM) because surfactant migration to the film/air interface is retarded compared with a non-ionic case. In this paper we have used force modulation atomic force microscopy to directly probe the relative polymer density across the film surface during annealing thereby qualitatively monitoring the interdiffusion process. The applicability of this method to study polymer interdiffusion will be discussed.     

Plasma surface modification and characterization of collagen‐based artificial cornea for enhanced epithelialization

Argon plasma treatment enhanced the attachment of epithelial cells to a collagen‐based artificial cornea crosslinked using glutaraldehyde (GA) and glutaraldehyde‐polyethylene oxide dialdehyde (GA‐PEODA) systems. The epithelialization of untreated and treated surfaces was evaluated by the seeding and growth of human corneal epithelial cells. Characterization of polymer surface properties such as surface hydrophilicity and roughness was also made by contact angle measurement and atomic force microscopy, respectively. Contact angle analysis revealed that the surface hydrophilicity significantly increased after the treatment. In addition, AFM characterization showed an increase in surface roughness through argon plasma treatment. Based on the biological and surface analysis, argon plasma treatment displays promising potential for biocompatibility enhancement of collagen‐based artificial corneas. It was also found that the cell attachment to artificial cornea surfaces was influenced by the combined effects of surface chemistry (i.e., surface energy), polymer surface morphology (i.e., surface roughness), and polar interactions between functional groups at the polymer surface and cell membrane proteins. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Determination of the influence of the polymer structure and particle size on the film formation process of polymers by atomic force microscopy

The particle size and morphology of a synthetic polymer latex were shown to influence the film formation behavior. Theoretical models predict that small particles coalesce more easily than large colloids do.The influence of particle size and morphology of differently structured lattices on the film-formation process was investigated by atomic force microscopy (AFM). Sequences of AFM images were acquired over a certain temperature range or at room temperature as a function of time. From the resulting images the average particle diameter of the latex particles in the surface layer was determined as a function of the time or temperature. The resulting curves could be compared to observe differences in the film formation kinetics of the different lattices. These AFM studies confirmed that the film formation behavior is influenced by the particle size and particle morphology, but that the core/shell ratio of core-shell particles has no significant influence on the film formation kinetics.     

Variations on the oxidative induction testing methodology for polymer analysis

We have modified the ASTM method on oxidative induction testing (D-3895-92) into a generalized technique with considerably expanded applications to polymer systems in addition to the polyolefins. Testing parameters we have experimented with success are: much wider temperature ranges, oxygen pressure, combination of oven aging and the differential scanning calorimeter, and both the exothermic and endothermic degradation polymer systems. In terms of non-olefin polymer systems, we have obtained good results from flexible PVC formulations, thermoplastic polyesters, styrene based thermoplastic elastomers, among others. Insights generated from these tests can be used to optimize polymer selection, formulation and processing conditions.     

Elasto-plastic and visco-elastic deformations of a polymer sphere measured using colloid probe and scanning electron microscopy

The adhesive interaction energy between a single 27 μm polystyrene sphere and a flat silica surface has been measured, as a function of applied load on the sphere, using an atomic force microscope (AFM). The pull-off force required to remove the sphere from the surface after application of a given load was found to increase as a function of the applied load. These data are indicative of a plastic or elasto-plastic deformation of the sphere. Simple analyses of these data using established elastic/plastic deformation theories indicate that, at the loads used, the system is most probably undergoing an elasto-plastic deformation. Further evidence for some plastic deformation of the sphere was obtained using scanning electron micrographs of the same sphere after an AFM experiment had been completed. Careful analysis of all these data indicated a significant time dependence of these adhesive interactions due to the viscoelastic nature of the polymer bead in question.     

Force Curve Measurements with the AFM: Application to the In Situ Determination of Grafted Silicon-Wafer Surface Energies

The atomic force microscope (AFM) can be used to perform surface force measurements in the quasi-static mode (cantilever is not oscillating) to investigate nanoscale surface properties. Nevertheless, there is still a lack of literature proposing a complete systematic and rigorous experimental procedure that enables one to obtain reproducible and significant quantitative data. This article focuses on the fundamental experimental difficulties arising when making force curve measurements with the AFM in air. On the basis of this AFM calibration procedure, quantitative assessment values were used to determine, in situ, SAM (or Self Assembled Monolayer)-tip thermodynamic work of adhesion at a local scale, which have been found to be in good agreement with quoted values. Finally, determination of surface energies of functionalised silicon wafers (as received, CH₃, OH functionalised silicon wafers) with the AFM (at a local scale) is also proposed and compared with the values obtained by wettability (at a macroscopic scale). In particular, the effect of the capillary forces is discussed.     

Force Curve Measurements with the AFM: Application to the In Situ Determination of Grafted Silicon-Wafer Surface Energies

The atomic force microscope (AFM) can be used to perform surface force measurements in the quasi-static mode (cantilever is not oscillating) to investigate nanoscale surface properties. Nevertheless, there is still a lack of literature proposing a complete systematic and rigorous experimental procedure that enables one to obtain reproducible and significant quantitative data. This article focuses on the fundamental experimental difficulties arising when making force curve measurements with the AFM in air. On the basis of this AFM calibration procedure, quantitative assessment values were used to determine, in situ, SAM (or Self Assembled Monolayer)-tip thermodynamic work of adhesion at a local scale, which have been found to be in good agreement with quoted values. Finally, determination of surface energies of functionalised silicon wafers (as received, CH₃, OH functionalised silicon wafers) with the AFM (at a local scale) is also proposed and compared with the values obtained by wettability (at a macroscopic scale). In particular, the effect of the capillary forces is discussed.     

Surface modification of PEEK by UV irradiation for direct co-curing with carbon fibre reinforced epoxy prepregs

Adding a layer of thermoplastic resin on the surface of thermoset composites enables welding as a possible joining method for thermoset composites. Adhesion at the thermoset/thermoplastic interface was achieved by direct co-curing of an UV irradiation treated PEEK film with aerospace grade carbon/epoxy prepregs. The effectiveness of UV irradiation for surface modification of PEEK was characterized using Fourier-Transformed InfraRed (FTIR) spectroscopy and contact angle measurement. The adhesion quality at the thermoset/thermoplastic interface was evaluated using a double cantilever beam test and Atomic Force Microscopy (AFM). UV treatment was found to effectively modify the chemical structure of PEEK surface and improve the wettability, which enables the development of a thermoset/thermoplastic interface by direct co-curing.     

Ruthenium staining for morphological assessment and patterns formation in block copolymer films

The selective staining by ruthenium tetroxide (RuO₄) was used in combination with Atomic Force Microscopy and calcination to discriminate and assign microphases at the surfaces of films of complex polymer systems. This paper evaluates this technique on thin films of polystyrene-b-polylactide (PS-b-PLA) and polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) and demonstrates its efficiency on complex thin film of binary blend of PS-b-PMMA and PLA. This method overcomes difficulties in the interpretation of AFM images by assigning PS microphase. In addition we show that this methodology could yield nanostructured inorganic materials with tunable structures such as perforated layers or nanowires that could find potential applications in the fabrication of high specific surface area Ru oxide-based materials.     

AFM Study on the Interactions Across Interfaces Containing Attached Polymer Chains

Summary: Interactions between surfaces with attached polymers are very common in both biological and engineering fields. These types of interactions are critical in processes such as coagulation and flocculation in mineral processing, biological recognition in metabolic processes and stress transference in polymer composites, among others. Although many mechanisms have been proposed to explain phenomena occurring at the interfaces on a molecular level, few experimental procedures can give direct information about them. In this work, interactions occurring at interfaces containing attached polymer chains, such as the ones that are present in polymer composites, were studied by using AFM. In order to identify the effect of the structure of the interface on phenomena such as stress transference and energy dissipation, polymers with different molar mass, areal density and chemical architecture were synthesized and attached to substrates and AFM cantilevers. Force‐distance curves, obtained by AFM, provided some fundamental information about the mechanisms involved when polymers attached to different surfaces interact. Results showed that chains grafted on different surfaces can interact via entanglements and intersegmental bonding. Based upon the application of the AFM modified technique, interfaces containing polymers, such as in polymer composites, can be designed and optimized through the manipulation of its structure to achieve new roles in the performance of systems.

     

How atomic force microscopy has contributed to our understanding of polymer crystallization

Over the past two decades atomic force microscopy (AFM) has become one of the most frequently used tools for studying polymer crystallization. The combination of high resolution, minimal sample preparation and the ability to image non-destructively has allowed visualisation of crystallization, melting and re-ordering processes at a lamellar and sub-lamellar scale, revealing complexities that could only previously be guessed at. Here the insights that AFM has provided into some of the main over-arching questions relating to polymer crystallization are reviewed. The emphasis is on the use of AFM to image growth in real time, and on contributions that have been made to our understanding of polymer crystallization in general, rather than to specific systems.     

A comparative study of different techniques for microstructural characterization of oil extended thermoplastic elastomer blends

This paper gives a relative comparison of different microscopic methods that are presently used to visualize polymer blend morphologies, versus the possibility to visualize the three-dimensional structure of the blends with electron tomography. Oil extended thermoplastic elastomer (TPE) blends based on a high amount of rubber phase and low amount of isotactic polypropylene (PP) were used as samples for this study. Low voltage scanning electron microscopy (LVSEM) and transmission electron microscopy (TEM) proved to be far superior to conventional scanning electron microscopy (SEM) and atomic force microscopy (AFM) for obtaining good quality images of the morphology of these blends. In an attempt to visualize the 3D morphology, electron tomography was carried out on these blends and models of the 3D morphologies were constructed. The usefulness of the different microscopic techniques in providing complementary morphological information and the potential of electron tomography as a new tool for constructing 3D-models of polymer blends are highlighted.     

A review of the research and advances in electromagnetic joining of fiber‐reinforced thermoplastic composites

The demand for polymer composites in structural and nonstructural applications has expanded rapidly due to their lightweight, high strength, and stiffness characteristics. Joining of polymer composite is not an easy task as inadequate joint strength leads to failure of a structure due to stress concentration. The following are the three basic methods available for joining of thermoplastic composites: adhesive joining, mechanical fastening, and fusion bonding. Electromagnetic joining is a class of fusion bonding where electromagnetic force is used for generation of heat. Electromagnetic joining has gained new interest among the research fraternity with the development of thermoplastic composites. This type of joining or welding technique offers many advantages over other joining techniques. This joining technique can be used for assembly as well as repairing of thermoplastic polymer‐based composites parts. The main aim of this article is to review the different electromagnetic joining methods for thermoplastic composites and present the recent developments in this area. The electromagnetic joining methods such as induction welding, microwave welding, and resistance welding have been comprehensively discussed in the context of their applicability for joining of thermoplastic polymer‐based composites. POLYM. ENG. SCI., 59:1965–1985, 2019. © 2019 Society of Plastics Engineers     

In situ atomic force microscopy measurement of the dynamic variation in the elastic modulus of swollen chitosan/gelatin hybrid polymer network gels in media of different pH

The surface morphology and elastic modulus of chitosan/gelatin hybrid polymer network (CS/GLN HPN) gels was investigated by in situ atomic force microscopy (AFM). The surface domains of hydrogel varied from irregular clumps to sponge-like patterns with increasing swelling time. The vertical height and width of the surface domains observed in alkali medium are smaller than in acidic medium. The indentation of the gels caused by the AFM tip increased with time and the elastic modulus of the gels estimated by the Hertz model decreased sharply compared with that of xerogel. In alkali medium, due to the reassociation of hydrogen bondings between networks, the elastic modulus increased slightly. © 1999 Society of Chemical Industry     

Combination of atomic force microscopy and chemical hydrolysis to characterize degradable regions in polymer blends

In this study, we demonstrate the usefulness of chemical‐based method in combination with atomic force microscopy (AFM) to characterize the degradable regions in a wide range of polymer blends. This approach is based on selective hydrolysis of one of the components in a multiple‐phase system, and the ability of AFM to provide nanoscale lateral information about the different phases in the polymer system. Composite films containing different percentage of hydrolyzable polymer were either melt processed or solution casted and then exposed to a hydrolytic acidic environment. Tapping mode AFM was used to analyze the samples before and after hydrolysis. Dramatic topographic changes such as pits were observed on the acid exposed samples, indicating that the degradation was localized and the more susceptible component in the blend was hydrolyzed. Additionally, the progressive hydrolysis of the composites was studied by attenuated total reflection FTIR (ATR‐FTIR) analyses to confirm the AFM results. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 726–733, 2006     

Macrostructure and environment-influenced surface layer in epoxy polymers

Several epoxy polymers were shown to be two-phase systems; roughly spherical floccules arranged in layers in an interstitial fluid resembling the starting materials. The size of the floccules was found to be dependent on the initial rate of cure of a given polymer. The density, hardness, glass transition temperature, etching rate, and dielectric strength were related to the floccule size. The surface layer in the epoxy polymers and in several thermoplastic polymers was found to be different from the bulk material. The properties of the surface layer are dependent on the surface energy of the mold material and on the atmospheric environment. A gradient in properties was found to extend from the polymer surface several hundred microns into the bulk.     

Measuring the Surface and Bulk Modulus of Polished Polymers with AFM and Nanoindentation

Abstract

A new method to determine the elastic modulus of a material using the atomic force microscope (AFM) was proposed by Tang et al. [Nanotechnology 19, 495713 (2008)] and is used in this study. This method models the cantilever and the sample as two springs in a series. The properties of both the spring and cantilever are determined on two reference samples with known mechanical properties and these properties are then used to find the elastic modulus of an unknown sample. The indentation depth achieved with AFM is in the nanometer range (30–130 nm in this study); and hence when this technique is performed on polymers, whose surface structure is different from their bulk structure, AFM gives a measure of the surface elastic modulus. In the present study, after employing AFM to measure the surface modulus of five polymers, traditional depth-sensing nanoindentation, with penetration depths of about 1 μm, was used to determine the elastic modulus in the bulk. The mean values for elastic modulus from the AFM were within 5–50% of the nanoindentation results, suggesting the existence of a surface modulus for polished polymers.