Ultrahigh Molecular Weight Polyethylene - Springer Link

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(UHMWPE), also known as high-modulus poly- ethylene ... LDPE and. LLDPE refer to low-density polyethylene and ... Cross-linking of the UHMWPE chains is ...
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Ultrahigh Molecular Weight Polyethylene Tauqir A. Sherazi Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad, Pakistan

Ultrahigh molecular weight polyethylene (UHMWPE), also known as high-modulus polyethylene (HMPE), is a subset of the thermoplastic polyethylene. Polyethylene is a polymer formed from ethylene (C2H4), which is a gas having a molecular weight of 28 g/mol. The generic chemical formula for polyethylene is –(C2H4)n–, where n is the degree of polymerization. A schematic of the chemical structures for ethylene and polyethylene is shown in Fig. 1. There are several kinds of polyethylene (LDPE, LLDPE, HDPE, UHMWPE), depending upon the degree of polymerization or molecular weights and chain architectures. LDPE and LLDPE refer to low-density polyethylene and linear low-density polyethylene, respectively. These polyethylenes generally have branched and linear chain architectures, respectively, each with a molecular weight of typically less than 50,000 g/mol. High-density polyethylene (HDPE) is a linear polymer with a molecular weight of up to 200,000 g/mol (Steven 2009). For an ultrahigh molecular weight polyethylene, the molecular chain can consist of as many as # Springer-Verlag Berlin Heidelberg 2014 E. Drioli, L. Giorno (eds.), Encyclopedia of Membranes, DOI 10.1007/978-3-642-40872-4_591-2

200,000 ethylene repeat units. Thus, molecular weight ranges in the millions, usually between two and six million. The longer chain serves to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. This results in a very tough material, with the highest impact strength of any thermoplastic presently made (Stein 1999). UHMWPE is a unique polymer with outstanding physical, chemical, and mechanical properties. Most notable are its chemical inertness, lubricity, impact resistance, and abrasion resistance. Table 1 summarizes the physical and mechanical properties of UHMWPE. Owing to its excellent properties, UHMWPE is widely used as a biomaterial such as in orthopedics as a bearing material in artificial joints. One of the major limiting factors of UHMWPE implant is its wear and damage with time, thus having only a finite lifetime. Cross-linking of the UHMWPE chains is found to be the promising method to dramatic reductions in wear, improving the performance in biomedical applications (Steven 2004). Recently, research has been conducted for the modification of the UHMWPE surface by grafting with different techniques such as radiation, chemical, and plasma grafting. Since UHMWPE is a semicrystalline polymer, its crystalline part normally remained unchanged, while the amorphous part can be grafted. The UHMWPE membrane has been evaluated as a proton exchange membrane for fuel cells (Sherazi et al. 2009, 2010).

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Ultrahigh Molecular Weight Polyethylene

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H Ethylene

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Ultrahigh Molecular Weight Polyethylene, Table 1 Typical average physical properties of ultrahigh molecular weight polyethylene (UHMWPE) (Li and Burstein 1994)

n Polyethylene

Ultrahigh Molecular Weight Polyethylene, Fig. 1 Schematic of the chemical structure of ethylene and polyethylene

References Li S, Burstein AH (1994) Ultra-high molecular weight polyethylene. The material and its use in total joint implants. J Bone Joint Surg Am 76:1080–1090 Sherazi TA, Ahmad S, Kashmiri MA, Kim DS, Guiver MD (2009) Radiation induced grafting of styrene onto ultra-high molecular weight polyethylene powder and subsequent film fabrication for application as polymer electrolyte membrane. II: sulfonation and characterization. J Membr Sci 333:59–67 Sherazi TA, Guiver MD, Kingston D, Ahmad S, Kashmiri MA, Xue X (2010) Radiation-grafted membranes based on polyethylene for direct methanol fuel cells. J Power Sources 195:21–29 Stein HL (1999) Ultra high molecular weight polyethylene (UHMWPE). Guide to engineering plastics

Property Molecular weight (106 g/mol) Melting temperature ( C) Poisson’s ratio Specific gravity Tensile modulus of elasticitya (GPa) Tensile yield strengtha (MPa) Tensile ultimate strengtha (MPa) Tensile ultimate elongationa (%) Impact strength, Izod (J/m of notch; 3.175 mm thick specimen) Degree of crystallinity (%)

UHMWPE 3.5–7.5 132–138 0.46 0.925–0.945 0.5–0.8 21–28 39–48 350–525 >1070 (no break) 39–75

Testing conducted at 23  C

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families: thermoplastic resins. Materials Park: ASM International. pp 167–171 Steven MK (2004) The UHMWPE handbook: ultra-high molecular weight polyethylene in total joint replacement. Elsevier Acadamic Press, London, UK Steven MK (2009) UHMWPE, biomaterials handbook, 2nd edn. Elsevier Acadamic Press, Burlington, USA