Nanoindentation of Amorphous and Nanostructured Polymers
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Nanoindentation of Amorphous and Nanostructured Polymers Kyle C. Maner, Matthew R. Begley and Marcel Utz1 Structural and Solid Mechanics Program, Department of Civil Engineering University of Virginia, Charlottesville, VA 22904 1 Department of Physics and Institute for Materials Science University of Connecticut, Storrs, CT 06269 ABSTRACT We present a detailed nanoindentation study of micron-scale thin films of polystyrene (PS), poly(phenylene oxide) (PPO), poly(methyl methacrylate) (PMMA), a metal-centered PMMA-Ruthenium block copolymer, and a PS-poly(ethylene-propylene) (PS-PEP) block copolymer with lamellar morphology. The results show that size-dependence is most readily noticeable for the lamellar PS-PEP film, indicating that the nanoidentation approach has sufficient sensitivity to capture scale dependence on scales in the range of tens of nanometers. The less pronounced scale-dependence (or lack thereof) in the other films is discussed in the context of identifying the physical length-scale of elementary processes of plastic deformation. The results indicate that the upper limit on the size of plastic shear zones in amorphous polymers is approximately 1200-9600 nm3 (i.e. a sphere with a diameter in the range of 20-40 nm). INTRODUCTION: Connections between nanoscale structure and mechanical properties for polymers play important roles in the development of new materials with improved thermomechanical stability. The study of amorphous polymers represents an important step towards identifying molecular characteristics that inhibit unfavorable mechanisms that reduce mechanical performance. In spite of intense research efforts, the mechanism of plastic deformation is still not clear in amorphous polymers. It is now well established that plasticity in amorphous materials in general is mediated by the nucleation of localized relaxation events [1], usually referred to as plastic shear zones (PSZ). In the case of amorphous metals, the size of these localized events is well established through computer simulations to be of the order of a single nearest-neighbor atomic shell. In the case of polymers, by contrast, the size scale of the elementary processes of plasticity is not known. Computer simulations [2] have failed to identify the localized relaxation events due to limitations in simulation cell size. The nature of these events, i.e. the size scale over which they occur and their relationship to molecular arrangements, must be identified before significant progress can be made in tailoring nanoscale structures (e.g. block copolymer morphology) to improve mechanical properties. An obviously related issue is the way in which plasticity events interact with one another when deformation is confined to small volumes. (One possible manifestation of this interaction between mechanisms and scale is size-dependent mechanical properties.) This represents a fundamental question whose answer will play a critical role in establishing the relationship between nanoscale structure (e.g. phase separation with nanoscale domai
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