AN INTRODUCTION TO PLASTICS - Plastics and Environmental Sustainability - page 78

acrylonitrile-butadiene-styrene (ABS) resin and
styrene-butadiene-styrene (SBS) are copolymers with three and two types
of repeat units, respectively.
PE is the most-used plastic in the world and is made by polymerizing
ethylene gas. Ethylene, used as the raw material, is called the monomer,
and n molecules of it join together to form a PE chain of n repeat units as
shown in Figure 3.1 . As n is typically a very large number, writing out the
full structural formula for the molecule is uninformative and impractical.
Also, n is not even the same for all PE molecules produced in a single
polymerization reaction (aswewillseelater). Therefore, onlyasingle repeat
unit of PE is typically used to denote its structure along with a subscript
n to indicate that it is a polymer (see Fig. 3.1 ). Typically, the term “poly”
is used in front of the parenthesis carrying the name of repeat unit (e.g.,
poly(vinylchloride)(PVC)orpoly(ethyleneterephthalate)(PET)todenoteit
is a polymer). The parenthesis is often dropped in case there is no structural
ambiguity as with PE or polystyrene (PS). 2
Figure 3.1 The polymerization reaction of ethylene yielding polyethylene.
The chemistry of the repeat unit is the key determinant of the physical and
chemical characteristics of the polymer.
Structural formulae of polymers as in Figure 3.1 are incomplete in that
they do not provide any geometric information on the chain molecule.
Macromolecular geometry is better visualized using a simple
three-dimensional (3D) model (or a space-filling model) that takes bond
angles and atomic sizes into account. In such models, it is easy to see that
covalent bonds can rotate, allowing the polymer chain to be very flexible
( Fig. 3.2 ) . The molecules can even bend and fold over themselves behaving
more like a chain of beads rather than a linear rigid rod. This flexibility
allows for profuse entanglement of polymer chains leading to the excellent
mechanical properties of polymers. Images of single molecules of polymers
byatomicforcemicroscopy(AFM)illustratethisflexibility. Figure3.2 shows