The book focusses on the recent technical research accomplishments in the area of polyethylene-based blends, composites and nanocomposites by looking at the various aspects of processing, morphology, properties and applications. In particular, the book details the important developments in areas such as the structure-properties relationship of polyethylene; modification of polyethylene with radiation and ion implantation processes; stabilization of irradiated polyethylene by the introduction of antioxidants; reinforcement of polyethylene through carbon-based materials as additives; characterization of carbon-based polyethylenes composites, polyethylene-based blends with thermoplastic and thermoset; characterization of polyethylene-based thermoplastic and thermoset blends; polyethylene-based blends with natural rubber and synthetic rubber; characterization of polyethylene-based natural rubber and synthetic rubber blends; characterization of polyethylene-based composites.
Visakh P.M. (MSc, MPhil) is working now as a postgraduate student at Tomsk Polytechnic University, Russia. He has edited 11 books for various international publishers as well as having multiple articles and book chapters to his credit. His research interests include: polymer nanocomposites, bio-nanocomposites, rubber based nanocomposites, fire retardant polymers, liquid crystalline polymers and silicon sensors. He has been a visiting researcher in many European universities.
María José Martínez Morlanes gained her PhD in polymer science from the University Zaragoza, Spain where she is now an assistant professor.
Ultra High Molecular Weight Polyethylene (UHMWPE) for Orthopaedic Devices: Structure/Property Relationships
Maurice N Collins1,*, Declan Barron2 and Colin Birkinshaw2
1Stokes Institute, University of Limerick, Ireland
2Department of Civil Engineering and Materials Science, University of Limerick, Ireland
*Corresponding author: Maurice.firstname.lastname@example.org
The following chapter details the structure property relationships in medical-grade polyethylene materials. The chapter is divided into the following sections: the first section is an introductory section on comparing medical grade polyethylenes with more conventional high density grades, and the second section deals with chain structure and alignments. The third section is devoted to describing crystallinity and melting behaviour using classical Avrami and Lauritzen - Hoffman theory. This is expanded to crystal growth regimes. The fourth and fifth sections are dedicated to molecular weight and mechanical performance with particular focus on creep behaviour as this is particularly pertinent for medical device materials. The final section describes radiation induced changes in the microstructure of polyethylene as a result of gamma sterilisation processes. These changes have been linked to wear rates and importantly wear debris has been implicated in joint loosening mechanisms. Latest research on heat treated "stabilised" polyethylenes is discussed and this is expected to influence medical device performance in vivo.
Keywords: Ultra high molecular weight polyethylene, orthopaedics, crystallinity, sterilization, mechanical properties
2.1 Introduction - HDPE and UHMWPE
Karl Ziegler and Erhard Holzkamp together invented high-density polyethylene (HDPE), in 1953. They formulated the material with the use of catalysts and low pressure, this process is one which is applied to make many varieties of polyethylene compounds. It was only two years later that HDPE was brought into the commercial market and was produced as pipe. Due to the success of HDPE in both the private and commercial market, Ziegler was awarded the 1963 Nobel Prize for Chemistry . HDPE is a linear polymer and can contain more than 1000 CH2 groups [CH2 - CH2]n. HDPE is also semi-crystalline, however the amorphous regions are relatively small and it said that HDPE is basically crystalline with uniformly-distributed flaws and imperfections. HDPE has a high density because the linear molecules can pack closely within the crystal.
As HDPE is a linear polymer, it can form a solid with very high percentage crystallinity values, between 60-80%. The reason it can form such highly crystalline solids is due to the zig-zag conformation assumed by its molecular chains. Also like UHMWPE, HDPE is restricted in its level of percentage crystallinity due to its high molecular weight. Occasional short side branches can also inhibit the reorganisation of HDPE to form the lamella crystal structures. The melting behaviour of HDPE is very similar to that of UHMWPE. It too behaves like a glass solid below its glass transition temperature, and increasing the temperature above the Tg will see the material go from an elastic solid to a rubbery, tacky substance, known as the rubbery state. HDPE has a wide melting range which usually begins at around 90°C, but peaks at around 130-137°C. Just as in UHMWPE the lamella structures in HDPE have completely melted at this point and the molecular chains reorganise to form new lamella structures. HDPE will exhibit a flow transition as it has a molecular weight which is rarely above 50,000 g/mol which is below the 500,000 g/mol mark where a material is too entangled to flow .
UHMWPE was not always called so, in fact when UHMWPE was first introduced by Charnley in 1962 it was referred to as HDPE. However as more advances were made in the production of UHMWPE it established its true name, there became a well described difference between the HDPE we know today and the HDPE Charnley introduced. The HDPE that we know today has a molecular weight of approximately 200,000 g/mol, whereas the HDPE Charnley introduced has a molecular weight of approximately 3.1 million g/mol or greater (which is the UHMWPE we know today). In the late 90's the nomenclature for UHMWPE in orthopaedics changed again. The main producer for medical grade UHMWPE - Ticona, declared the four different grades of resin available - GUR 1020, 1050, 1120 and 4150 .
The configuration of the polymer chain has a very prominent influence on the properties of the polymer. Side chains and branching on the carbon-carbon backbone of the polymer chain determine the polymers ability to crystallise and hence the degree of crystallinity, this is called tacticity. UHMWPE does not have as high a degree of crystallinity as HDPE due to its high molar mass, which restricts diffusion. The melting behaviour of UHMWPE is dependent on the thickness and perfection of the crystals in the material, which is a function of the crystallisation temperature. If the crystals are thicker and more perfect, the melt temperature will tend to be higher. In UHMWPE, the glass transition (Tg) occurs around -120°C, and below this temperature UHMWPE behaves like a glass, but as the temperature is increased above this Tg the material becomes more elastic, due to the amorphous regions gaining mobility. When the temperature is increased to approximately 60-90°C, smaller crystallites in the polymer, begin to melt, the melting then peaks at a temperature of 137°C, and this temperature is known as the melting temperature (Tm), for UHMWPE. At the Tm the majority of all the crystalline regions are melted. In the case of most semi crystalline polymers, if the temperature is increased above the Tm the material will undergo a flow transition (Tf) and flow like a liquid. However this will only pertain if the material has a molecular weight less than 500,000 g/mol. Materials with a molecular weight above this have polymer chains which are too entangled and therefore will not flow for e.g. UHMWPE.
In 1998, the nomenclature for UHMWPE was consolidated with availability of four grades for the worldwide orthopaedic market - GUR 4150,1050,1120 and 1020 resins. The first digit of the grade name was originally the loose bulk density of the resin, i.e. the weight measurement of a fixed volume loose, unconsolidated powder; The second digit indicates the presence ('1') or absence ('0') of calcium stearate, while the third digit is correlated to the average molecular weight of the resin. The fourth digit is a Hoechst internal code designation. In 1997, the Technical Polymers business of Hoechst assumed the name Ticona. Hoechst currently supplies 600 to 700 tons of premium grade UHMWPE per year for orthopaedic applications. Hoechst uses the designation GUR for its UHMWPE grades worldwide; the acronym GUR stands for 'Granular'.
2.2 Chemical Structure
Polyethylene (PE) is a linear polymer. The chemical configuration and repeat unit PE is shown in Figure 2.1.
Figure 2.1 Chemical structure of polyethylene.
The conformation of a polymer chain is the three dimensional spatial arrangement of the chain as determined by the rotation about backbone bonds. The conformation and configuration of the polymer molecules have a great influence on the properties of the polymer component. The conformation describes the preferential spatial positions of the atoms in a molecule. It is described by the polarity, flexibility and the regularity of the macromolecule. It is primarily governed by its chemical sequence and configuration. Assuming its chemical sequence and configuration to be regular, the conformation is largely influenced by intermolecular interactions, which define the general chain geometry and by intermolecular interactions in the crystal lattice, which may alter the regularity of conformation of the isolated chain, or confer increased stability to any specific chain conformation (e.g., through hydrogen bonding) [3, 4]. The helix is a typical ordered conformation type for polymers that contain regular chain microstructure. Most helical conformations of vinyl polymers rest on combinations of trans (180°), gauche g (60°); these measurements represent the angle the chain leaves the bond from where it entered. It can be looked at as a one-dimensional crystal. Helix formation is driven by the minimization of the conformational energy, a driving force for crystallisation, discussed in greater detail in the next section. The simplest helix is that of the all-trans polyethylene as illustrated in figure 2.1.
Typically carbon atoms are tetravalent, which means that in a saturated organic compound they are surrounded by four substituents in a symmetric tetrahedral geometry. The tetrahedral geometry sets the bond angle; this angle is maintained between the carbon atoms on the backbone of a polymer molecule, with each individual axis in the carbon backbone is free to rotate.
2.3 Crystallinity and Melting Behaviour
In HDPE, chains fold to form the lamellae and propagate outwards three dimensionally, creating a sphere-like formation. This sphere-like formation is referred to as a spherulite. The spherulites are very small anisotropic spheres (1-5µm) only visible under very high magnification. They form as a result of a complex crystallisation process of macromolecules. UHMWPE cannot form these spherulites due to their very high molecular weight. However spherulites only...