The use of instrumented indentation to characterize the mechanical response of polymeric materials was studied. A model based on contact between a rigid probe and a linear viscoelastic material was used to calculate values for the creep compliance and stress relaxation modulus for two glassy polymeric materials, epoxy and poly(methyl methacrylate), and two poly(dimethyl siloxane) (PDMS) elastomers. Results from bulk rheometry studies were used for comparison with the indentation stress relaxation results. For the two glassy polymers, the use of sharp pyramidal tips produced responses that were considerably more compliant (less stiff) than the rheometry values. Additional study of the deformation remaining in epoxy after indentation creep testing as a function of the creep hold time revealed that a large portion of the creep displacement measured was due to postyield flow. Indentation creep measurements of the epoxy with a rounded conical tip also produced nonlinear responses, but the creep compliance values appeared to approach linear viscoelastic values with decreasing creep force. Responses measured for the unfilled PDMS were mainly linear elastic, with the filled PDMS exhibiting some time-dependent and slight nonlinear responses in both rheometry and indentation measurements.

Dynamic nanoindentation was performed on a cured epoxy, a poly(methyl methacrylate) (PMMA), and two poly(dimethyl siloxane) (PDMS) samples of different crosslink densities. These samples were used to compare dynamic nanoindentation with classical rheological measurements on polymeric samples in the glassy and rubbery plateau regions. Excellent agreement between bulk rheological data and dynamic nanoindentation data was observed for the two glassy materials (epoxy and PMMA) and the less compliant PDMS sample. More divergent results were observed for the more compliant PDMS sample. The theoretical foundation and historical development of the working equations for these two types of instrumentation are presented and discussed. The major difference between nanoindentation and the more classical rheological results is in the treatment of the instrument-sample interface.

This work demonstrates a fabrication technique of high sensitivity flexible strain sensors at room temperature. The grown well-aligned millimeter-long single-walled carbon nanotube (SWCNT) was transferred from the silicon substrate to the pretrenched flexible substrate. The sensor design allows effective adhesion between SWCNT and flexible substrate for SWCNT lengthwise strain and piezoresistivity change. Experimental results show that the sensor achieves a high strain resolution of 0.004%. The measured piezoresistive gauge factor of the flexible sensor is 269. The demonstrated fabrication technique of flexible sensors shows advantage of high sensitivity, high quality, and is suitable for mass production.

No 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...