Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 62

Covalent bonding occurs in synthetic and natural polymers, linking atoms in the backbone of the long‐chain molecule, often referred to as a macromolecule. The macromolecules in the synthetic polymer polyethylene, a material used as articulating bearings in hip and knee implants, has the simplest structure, built up of –H₂C–CH₂– repeating units (Figure 2.10). These macromolecules show strong covalent bonds between the carbon atoms in the chain backbone and a tetrahedral arrangement of the covalent bonds at each carbon atom in the chain resulting from sp³ hybridization of the electron orbitals of the carbon atom. The single bond between the carbon atoms in the chain backbone allows one carbon atom to rotate more or less freely, relative to a neighboring carbon atom and this, coupled with weak intermolecular forces between neighboring chains, often lead to a random arrangement of the chains in three dimensions. On the other hand, if the repeating unit has a simple structure as, for example, in polyethylene, some regions of the macromolecule can arrange themselves in an ordered three‐dimensional pattern. When formed under appropriate conditions, such as cooling sufficiently slowly from the molten state, polyethylene is composed of crystalline regions interspersed with amorphous regions, a structure often described as semicrystalline (Chapter 3).

Figure 2.10 Illustration of covalent bonds linking carbon atoms (C) in the chain backbone of the polyethylene molecule and between the carbon atoms and hydrogen (H) atoms.

Other synthetic polymers have a structure that is more complex than polyethylene but similar principles are applicable (Chapter 8). Consequently, most solid polymers produced by conventional methods show low strength and low elastic modulus, become soft or molten at low to moderate temperatures, and are insulating electrically and thermally. On the other hand, alignment of the polymer chains in a certain direction and hydrogen bonding, a strong form of intermolecular bonding (Section 2.5.2), can lead to high strength and high elastic modulus in the direction of alignment as, for example, in fibers of the synthetic polymer nylon.