Advanced techniques for nanocharacterization of polymeric coating surfaces

Abstract

Surface properties of a polymeric coating system have a strong influence on its performance and service life. However, the surface of a polymer coating may have different chemical, physical, and mechanical properties from the bulk. In order to monitor the coating property changes with environmental exposures from the early stages of degradation, nondestructive techniques with the ability to characterize surface properties with micro-to nanoscale spatial resolution are required. In this article, atomic force microscopy has been applied to study surface microstructure and morphological changes during degradation in polymer coatings. Additionally, the use of AFM with a controlled tip-sample environment to study nanochemical heterogeneity and the application of nanoindentation to characterize mechanical properties of coatings surfaces are demonstrated. The results obtained from these nanometer characterization techniques will provide a better understanding of the degradation mechanisms and a fundamental basis for predicting the service life of polymer coatings. P olymeric coatings are widely used in buildings, bridges, automobiles, and electronic equipment for both functional and aesthetic purposes. Despite great improvements in coatings technology, problems still exist in the long-term performance of polymeric coatings exposed to environments such as ultraviolet light, humidity, temperature, and other aggressive conditions. Generally, the surface properties of a coating system have a strong influence on its performance and service life. These properties include surface morphology and microstructure, surface chemistry, optical appearance, and surface mechanical properties such as hardness, modulus, and scratch resistance. Application-specific performance requirements often create complicated interactions between these properties that are important to quantify as a function of service conditions. However, the surface of a polymeric coating system may have different chemical, physical, and mechanical properties from the bulk. 1,2 For example, the concentration of low surface-energy materials is often higher at the air surface than in the bulk, 3,4 especially in a multicomponent coating system. Thus, characterization of bulk material properties might not be sufficient for predicting performance. Techniques with sensitivity to the surface chemical, physical, and mechanical properties are required.An additional factor that complicates the prediction of coating performance and service life is that polymer coatings are heterogeneous 5,6 and contain nano-to micrometer scale degradation-susceptible regions. Degradation of a polymer coating is believed to start from these degradation-susceptible regions on the surface and then grow laterally and vertically. In the early stages of degradation, even though obvious chemical changes have been observed, the physical changes of the coating surface could still be small, 7 so that degraded regions such as pits may have dimensions that are on the order of nanometers in depth and perhap...

Multiscale physical characterization of an outdoor-exposed polymeric coating system

“…Similar results have been reported from other AFM and LSCM studies on the weathering of polymeric coatings. 22,24,25 These observations indicate that while higher magnification images are very important for visualizing physical changes occurring in the very early stage of coating degradation 23,26 , the longer wavelength surface roughness may be more directly related to the gloss measurement. More quantitative analysis can be provided by mathematical models.…”

Adhesion between template materials and UV-cured nanoimprint resists

“…A recent study of buried interface morphologies of polymerized organics showed structures not unlike those observed in this work. 17 The mechanism of resist surface texture formation and its importance in adhesive behaviors requires further investigation. …”

Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation

“…1 The chemical modification of polymers resulting from photooxidation would lead to changes in physical and mechanical properties, such as molecular mass, 2 crosslinking density, 3,4 glass transition temperature (T g ), 3,4 modulus and hardness, 5,6 roughness and gloss. 7,8 Extensive studies by NIST 6,[9][10][11][12] on service life prediction of polymeric materials from nanoscale to macroscale via atomic force microscopy, confocal microscopy, nanoindentation and spectroscopy, indicate that degradation of polymeric materials initiates at sub-micrometer degradation-susceptible regions on the surface, and then the degraded regions expand, deepen, and aggregate with longer exposures. The photodegradation mode of polymeric materials appears as a heterogeneous process rather than an ablation.…”