Basic Technology of Plastics - Introduction to Polymeric Composites with Rice Hulls - page 13

(a negative ion). If it loses an electron, it becomes electropositive (a positive ion). An
ion is an atom that has gained or lost one or more electron.
2.2.2 Molecular Weight (Mass)
The mass of a molecule is determined by adding together the atomic masses of the
various atoms making up the molecule. The atomic mass of hydrogen is 1 and
the atomic mass of oxygen is 16. Thus, the molecular mass of a single molecule of
water (H 2 O) is 1 + 1 + 16 = 18.
The primary factor resulting from a polymerisation reaction is the size of the
macromolecules. After such a reaction, a polymeric material will consist of polymer
chains of various lengths or repeating units. Hence, the MW is determined by the
average or mean MW, which is defined by M = W/N , where W is the weight of
the sample and N the number of molecules in the sample. Methane, for example,
would have a molecular mass (weight) of 12 + (4 × 1.008) = 16.03 g/mol.
The length (molecular mass) of a polymer chain would have a profound effect on the
processing ('mouldability') and properties of the plastic. Increasing the chain length
may increase toughness, melt temperature, melt viscosity, creep and stress resistance.
With higher melt viscosities, it may also increase the difficulty of processing, material
costs, and possible polymer degradation. Because the molecular structure may vary in
shape and length, the molecular mass average is used. It is logical to expect polymers to
contain molecular chains with a wide distribution of lengths (some short, some long)
and the molecular mass average is the product of the average number of repeating
units or mers in the polymer. If all molecules have the same mass, the polymer is
said to have a narrow distribution of molecular mass. Polymers that do not have
sufficient molecular mass (many long molecules) to cause some chain entanglements
are generally weak, have lower melting points, and are readily attacked by appropriate
reactants. The standard molecular mass required for good physical and mechanical
properties is ≈20,000 g/mol.
2.2.3 Polymeric Molecules
Molecules of organic materials are composed of carbon as a base element (most
commonly joined to hydrogen by covalent bonds). These molecules are referred to
as hydrocarbons . As the name suggests, hydrocarbons are composed of hydrogen
and carbon. Because of the ability of the carbon atoms to link up with other atoms
to form chains, rings and other complex molecules, thousands of hydrocarbons are
known to organic chemistry.