Contact Mechanics of Polymer Interfaces: New Approaches in Testing and Control of Adhesion
The objective of this work presented here has been to study the adhesive properties of various untreated or treated polymeric surfaces, and to compare and study the relationship between the length scale of the adhesion measurements and the adhesion data obtained. The work performed involves adhesion-testing measurements in the range of meso- and nanoscale by a custom-built Adhesion-Testing Device (ATD) constructed by the author, and by Atomic Force Microscopy (AFM), respectively. The results were obtained on various sample systems including surface treated poly(dimethylsiloxane) PDMS by UV/ozone, differently treated dry core-shell latex films, and molecularly thin perfluoropolyether lubricant films deposited on silicon wafers. Chapter One gives a general introduction to polymer adhesion and outlines the scope of the Thesis. Chapter Two was meant to provide the theoretical background necessary to understand the Thesis. First, the definitions of the basic terms used in surface energetics of solids were outlined and explained. In the second part of the chapter the concept and the development of contact mechanics and contact mechanical theories were discussed in detail. In Chapter Three an overview of the adhesion measurement techniques, including nondestructive and destructive methods, are discussed. The major emphasis was placed on the evolution and application of contact mechanical techniques. Chapter Four describes the design and construction of the Adhesion-Testing Device used throughout this work. The main motivation was to build a user-friendly, non-compliant and automated apparatus, which is capable of measuring surface and interfacial properties with high precision and accuracy including measurements at elevated temperatures. In the experiments contact radius, displacement, and force were simultaneously recorded between a semi-spherical lens and a substrate (of interest) in a continuous, or stepwise, manner. From the data obtained, work of adhesion (W) and combined elastic modulus could be determined for the contacting surfaces with the application of the JKR theory of contact mechanics. The practical work of adhesion and the interface energy could also be obtained from the recorded pull-off force. This instrument was shown to be capable of reproducing adhesive parameters of various materials with low error. Experiments performed on poly(dimethylsiloxane) lens and flat substrate system served for validation and calibration of the device. Chapter Five describes the study of the effect of UV/ozone treatment on silicone elastomers, based on poly(dimethylsiloxane) (PDMS). The treated PDMS surfaces were analyzed as a function of time elapsed after stopping the treatments using different standard surface characterization techniques, such as water contact angle measurements, XPS and Atomic Force Microscopy (AFM). However, the primary focus of this study was to apply the Johnson-Kendall-Roberts (JKR) contact mechanics approach to investigate PDMS samples prior to and following UV/ozone surface treatment based on data provided by the ATD device. A gradual formation of a hydrophilic, silica-like surface layer with increasing modulus was observed with increasing UV/ozone exposure. The value of the combined elastic modulus obtained for the PDMS lens and semi-infinite flat surface system showed an increase in full agreement with the formation of a silica-like layer exhibiting a high elastic modulus (compared with untreated PDMS). The work of adhesion observed in JKR experiments exhibited an increasing trend as a function of treatment done in agreement with contact angle data. JKR experiments showed hydrophobic recovery behavior as anticipated from contact angle measurements. Single pull-off force measurements by JKR and numerical analysis of full-approach JKR curves were in quantitative agreement regarding work of adhesion values. In Chapter Six the synthesis and emulsion polymerization of novel 3-(2-pyridyl)propyl methacrylate and 3-(3-pyridyloxy)propyl methacrylate monomers were discussed. The monomers were polymerized onto colloidal polystyrene seeds, resulting in core-shell latex systems (these latex systems served as model coatings in the adhesion measurements discussed in Chapter 7). The dimensions of the cores and the core-shell particles were characterized by dynamic light scattering and scanning electron microscopy. Transmission electron microscopy was used to study the morphology of the core-shell particles. Monodisperse beads with a regular core-shell internal structure were found. The pyridine-functional shells were loaded with HAuCl4 and irradiated with UV light to reduce the salts to metallic gold. FTIR, UV-Vis, TEM and XPS were employed to monitor the metal loading and reduction processes. Core-shell systems, decorated with gold nanoparticles, were obtained. In addition to serving as model colloids for adhesion studies these materials can potentially find use in photonic band gap colloidal crystals due to the refraction index contrast introduced by Au loading. In Chapter Seven an explorative study was presented to investigate the relationships among surface roughness, annealing temperatures, annealing time and work of adhesion using the model latex systems prepared in Chapter 6. It was found that upon heat treatment of the latex samples as a result of the flattening procedure (driven by the surface energy) the surface roughness decreased. The observed change of the surface topography resulted in a decreasing trend of the adherence values measured by ATD. The decreasing trend for W as a function of preannealing temperature was explained by roughness arguments using the particle-multi asperity model for the ATD contact. For essentially single asperity AFM experiments roughness of the compliant surface had an insignificant effect. In the measurements performed at elevated temperatures it was found that the steep slope of the adhesion as a function of test temperature was caused by the combined effect of test temperature (enhanced segmental mobility) and surface roughness. AFM experiments, however, showed increasing adherence with increasing test temperature. This phenomenon was explained by tip penetration into the soft shell layer of the latex. Upon removal of the partially wetted tip a polymer bridge formed, which increased the pull-off force as well as the calculated adherence data with increasing test temperature. Measurements performed on thermally treated latex films by using ATD and AFM suggested that the obtained practical adhesion data can very much depend on the technique and the length scale of the experiments. Comparison of the ATD and AFM measurements revealed structural, behavioural and functional differences of the latex films as a function of the length scale of characterization. Furthermore, these adhesion measurements can be used as complementary methods which can bridge nano- and mesoscale observations by providing more comprehensive information on the surfaces studied. It was also revealed that different energy dissipation process play an important role in these measurements, however, other mechanisms including interlocking, interdiffusion, etc. could contribute to the obtained adhesion data, as well. Considering the measurements performed on latex films, it was concluded that the magnitude of the final surface roughness of a latex film depends on a number of general factors, chief of them include the material of latex (surface energy) and the temperature of film formation. In Chapter Eight (serving as an outlook) a study was presented on molecularly thin perfluoropolyether (PFPE, Fomblin Zdol) films using ATD, AFM, FTIR, contact angle, and ellipsometry measurements in order to explore the application limits of the custom-built ATD device. In the experiments two Fomblin Zdol lubricants exhibiting different molar masses, were either physically or thermally bound to the surface of the silicon substrate. ATD measurements showed that the apparent adhesion strongly depends on annealing and seems independent on relative humidity. The trends of AFM pull-off measurements showed similar results to ATD, though AFM measurements were sensitive to the relative humidity. The values of the combined elastic moduli obtained by ATD correlated with the film thicknesses assessed by ellipsometry. FTIR and contact angle measurements showed molecular conformation changes of the lubricant molecules as a function of thermal treatment. It was also found that the trends of ATD data and contact angle measurements are in qualitative agreement and can be interpreted by multilayer model and changes in surface binding of PFPE molecules. However, the quantitative value of adhesion by ATD is too high, which could not be rationalized by the data presented.