Environmental Engineering Reference
In-Depth Information
3.3.3 Copolymers
A copolymer is made by polymerizing a mixture of two or more monomers
and yields a polymer that incorporates the repeat units of all the monomers
used. With a pair of monomers, the ratio of the two repeat units in the
polymer is determined by the ratio of the monomers in the feed. As the
propertiesofthepolymersarehighlydependentonrepeatunitcomposition,
the monomer feed unit ratio is closely controlled during reaction. In some
instances (such as with ethylene copolymerized with heptene or octene),
these can add on branches on the PE chains. This is in fact the case in the
synthesis of LLDPE.
Copolymerization allows a convenient means of modifying properties of
a homopolymer to suit specific applications. For example, PS is a brittle
thermoplastic with poor gas transport properties (important in food
packaging applications). Provided the slight loss in transparency can be
tolerated, a copolymer of butadiene-styrene with large improvement in
both the barrier properties and impact resistance can be used in its place.
This grade of PS called high-impact polystyrene (HIPS) has more desirable
properties including high-impact resistance compared to the
general-purpose polystyrene (GPPS) grade. Acrylonitrile used as a
comonomer also yields a copolymer with similarly improved properties.
Whenever two or more monomers are polymerized together, their
arrangement along the polymer chain also influences the properties of the
polymer. Where the pair of monomers A and B are involved, easiest to
envision is a random distribution of the repeat units A and B along the
chain. However, it is possible to obtain instead an alternating copolymer
with the pair of repeat units A and B arranged alternatively:
(-A-B-A-B-A-B-)
n
. Depending on the monomer ratio and catalysts used
in the reaction, it is also possible to have block copolymers with short
homopolymer sequences or “blocks” of a single repeat unit on the polymer
chain such as (-A-B-A-A-A-A-A-A-B-B-B-A-B-A-B-)
n
. Blocks
often aggregate into domains within the bulk of the polymer to yield various
3D morphologies with consequent interesting and useful properties.
Figure
3.8
illustrates the structure of different copolymers.
