Interfacial Materials Properties of Polymers: From Mesoscale to Nanoscale Contacts
This thesis is concerned with new approaches to measuring the interfacial mechanical properties of amorphous polymers on the micrometer and nanometer scale. In order to achieve this goal, several innovative instrumental techniques were developed during this work. Polymer adhesion was characterized on the mesoscale with a home-built, highly automated adhesion-testing device. Nanoscale characterization of polymer surfaces was investigated with a customized atomic force microscope. Chapter One gives a general introduction to polymer adhesion and outlines the scope and nature of this thesis. Chapter Two provides an overview of some of the most important concepts relevant to this thesis. Surface energy and polymer mechanical properties are introduced. These properties are then linked through contact mechanics theory. The Johnson-Kendall Roberts (JKR) and Hertizan contact mechanics theories for elastic solids are discussed and compared. Furthermore, some of the limitations of these theories are discussed, with particular emphasis placed on the effects of viscoelasticity. Chapter Three introduces an adhesion-testing device built as a part of this research project. This device is shown to be capable of providing an estimate of the work of adhesion between two solid substrates. The work of adhesion was determined for a polydimethylsiloxane (PDMS) lens and several end-functionalized thiol substrates and was shown to depend on the end-functionality of the monolayer. The adhesive performance measured by this device of two pressure sensitive adhesives is shown to compare favorably with conventional adhesion tests. Lastly, the presence of a capillary neck between the sample substrates is shown to influence the shape of the adhesion curves obtained by this device. Using the instrumentation described in the previous chapter, the dynamic adhesive characteristics of polydimethylsiloxane is further explored in Chapter Four. The adhesion hysteresis of PDMS is shown to be a complicated function of the substrate surface roughness and hydrophobicity, ambient humidity, and the time of interfacial contact. The dynamic adhesive performance of PDMS is postulated to result from the formation of favorable interfacial bonds, both mechanical and physicochemical in nature. From Chapter Five onwards, the focus of the thesis is shifted towards interfacial mechanical properties of polymers on the nanometer scale. In this chapter, a mechanism for the surface modification of glassy polymers is offered. A semiquantitative analysis based on the JKR theory shows that stresses exerted by the AFM tip can easily exceed the critical yield strength of the polymer. A mechanism is offered to explain the nanometer scale deformation of a glassy polymer under repeated scanning by an AFM tip. Chapter Six explores the influence of instrumental parameters on tapping-mode AFM images of phase-separated polystyrene-b-polyisoprene-b-polystyrene thin films. The discussion is begun with a simulation of a dynamic force curves on both of the homopolymers. Because polyisoprene is very compliant at room temperature compared to polystyrene, less damping of the cantilever occurs as the tip penetrates further into the material. Both height and phase-mode contrast are shown to vary locally depending on excitation frequency and averaged damping of the cantilever. The contrast variations are reconciled by considering the local interactions experienced by the tip. The dynamic adhesional character of polydimethylsiloxane is described in Chapter Seven. Two model crosslinked networks were characterized by bulk rheology, which revealed that the network with added free chains was more viscoelastic than the polymer without added free chains. Custom software and hardware were developed to process AFM force curve data in real time. The adhesion hysteresis of AFM force curve experiments is shown to increase on two time scales for the sample with added free chains. At longer times, the increase in adherence is postulated to result from viscoelastic creep. At shorter times, the increase of the adhesion hysteresis is attributed to increased energy dissipation that occurs at the receding crack tip due to viscoelastic effects. Furthermore, the effect of the adherence is shown to increase as a function of the maximum applied load. Finally, the adherence is shown to depend on the molecular weight of the added free chains. No such effects were observed for the sample without added free chains. The results are discussed in terms of the viscoelastic response of the polymer under the applied stress of the AFM tip. Using the instrumentation developed in the previous chapter, the adherence of poly(2-ethyl hexyl methacrylate) was investigated over a range of temperatures and strain rates in Chapter Eight. In AFM force curve experiments, the slope of adherence-indentation curves is shown to scale with the storage modulus, except near the glass transition temperature where a strong peak is observed. The creep compliance of the material could be directly visualized at different temperatures by allowing the tip to interact for a period of time while recording the cantilever deflection. The increase of adherence is reconciled in terms of an increase in the interfacial contact area due to viscoelastic deformation by the AFM tip. In summary, this work has examined the interfacial mechanical properties of polymer interfaces on the micrometer and nanometer scale. On both length scales, the adherence is believed to increase due to an enhancement of the interfacial contact area, the time scale and magnitude of which is determined by the viscoelastic properties of the polymer. Even on the nanometer scale, the interface appeared to behave in accordance with bulk mechanical properties and predictions grounded in contemporary contact mechanics theory of viscoelastic solids. However, truly quantitative confirmation of the theoretical predictions will remain elusive on both length scales until a method is developed in which the true interfacial contact area can be measured.