Atomic Force Microscopy Contact Mode Study on Ultra High Molecular Weight Polyethylene
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Atomic Force Microscopy Contact Mode Study on Ultra High Molecular Weight Polyethylene Yifang Cao† , Jikou Zhou† , Oludele Popoola ‡ , Dal F. Swarts ‡ and Wole Soboyejo† Princeton Institute of Materials Science and Engineering (PRISM) and Department of Mechanical and Aerospace Engineering Princeton University Princeton, NJ 08544 ‡ Zimmer Corporate Warsaw, IN 46581 †
ABSTRACT This paper presents the results of contact mode atomic force microscopy (AFM) study on the nanoscale Young’s modulus and work of adhesion of ultra high molecular weight polyethylene (UHMWPE). Cryoultramicrotomed surfaces of UHMWPE were scanned using the contact mode of AFM. Fibril regions are commonly found on the sample, however, a non-fibril particulate region was also found. AFM force displacement curves were obtained for the sample. The JKR theory and Maugis Dugdale model were used for the analysis. A good fitting between the theories and experimental data was found. The nanoscale Young’s modulus and work of adhesion of UHMWPE extracted from the experimental data were in reasonably good agreement with the values reported in other literatures. I. INTRODUCTION In recent years, there has been significant interest in the measurement of the mechanical properties of polymeric materials that are relevant to biomedical systems [1, 2]. Ultrahigh molecular weight polyethylene (UHMWPE), as the major material used to replace damaged cartilage in joint arthroplasties, has attracted lots of attentions on its mechanical properties [3–8]. In the use of UHMWPE in total joint replacement (TJR), the generated submicrometer wear debris have been observed clinically to cause an inflammatory reaction, which leads to bone resorption (osteolysis) and subsequent device failure [9]. The nanoscale measurements of adhesive characteristics (such as work of adhesion) and mechanical properties (such as elastic modulus) of UHMWPE are crucial for a nanoscale contact analysis, which may provide newer insight to the wear debris generation mechanism than the macroscale contact analysis approach. However, work of adhesion of UHMWPE was usually obtained using the contact angle measurements on bulk materials [6]. Nano scale measurement of the work of adhesion of UHMWPE using AFM contact mode is yet to emerge. Besides, most of nano scale measurements of Young’s modulus for UHMWPE [3, 4, 7, 8] were obtained using the nanoindentation techniques (i.e. OliverPharr method) [10, 11], there are few studies using other tools such as force mode of atomic force microscopy. In this paper, an AFM operated under contact mode was used to capture the indentation force displacement curves and study the nanoscale Young’s modulus and work of adhesion of UHMWPE. II. THEORY Various continuum models have been used to study the effects of contact forces in contact modes of AFM. These include: Hertzian contact mechanics [12]; the Johnson Kendall Roberts (JKR) model [13]; the Derjaguin Muller Toporov (DMT) model [14]; and most recently, the Maugis Dugdale (MD) model [15]. The Hertzian co
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