Linear polymer is a polymer in which
the molecules form long chains without branches or cross-linked structures. The
molecular chains of a linear polymer may be intertwined, but the forces tending
to hold the molecules together are physical rather than chemical and thus can be
weakened by energy applied in the form of heat. Such linear polymers are
thermoplastics. The simplest polymer is a linear polymer. A linear polymer is
simply a chain in which all of the monomers exist in a single line. An example
of a linear polymer is Teflon, which is made from tetrafluoroethylene. It is a
single strand of units made from two carbon atoms and four fluorine atoms. When
formed, these linear polymers can create strands of fibers or form a mesh that
can be very strong and hard to break through.
These linear
polymers are well packed and have high magnitude of intermolecular forces of
attraction and therefore have high densities, high tensil (pulling) strength
and high melting points. Some common example of linear polymers are high
density polyethylene nylon, polyester, PVC, PAN etc.
Linear, Branched, and Cross-linked Polymers:
Polyethylene
is called a linear or straight-chain polymer because it consists
of a long string of carbon-carbon bonds. These terms are misleading because the
geometry around each carbon atom is tetrahedral and the chain is neither linear
nor straight, as shown in the figure
As the
polymer chain grows, it folds back on itself in a random fashion to form
structures such as the one shown in the figure below on the left. Straight chains can sometimes fold tightly
enough to make crystal structures (on the right below) even though the
molecules are very long!
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| Neatly packed straight chains can make
crystals! |
 |
Randomly oriented straight chains can
be pretty messy
|
Polymers with branches at irregular intervals
along the polymer chain are called branched polymers (see figure to the
right). These branches make it
difficult forthe polymer molecules to pack in a regular array, and therefore
make the polymer less crystalline and less dense. The amount and type of branching also affects
physical properties such as viscosity and elasticity (see below). Branches often prevent chains from getting
close enough together for intermolecular forces to work effectively. they have low tensile strength,
low density, boiling point and melting points than linear polymers. Some common
examples are low density polythene, glycogen, starch etc. (Amylopectin).
Cross-linked
polymers contain short side chains (cross links) that connect different
polymer chains into a “network” as shown in the figure to the
right. At first, adding
cross-links between polymer chains makes the polymer more elastic (they can stretch and return to their original form.) The links can “pull” the chains back
together when they are stretched! The
vulcanization of rubber, for example, results from the introduction of short
chains of sulfur atoms that link the polymer chains in natural rubber. Cross-linking also decreases the viscosity (the resistance to flow) of
polymers. In order for polymers to flow,
the chains must move past each other and cross-linking prevents this. Elastomers
are elastic polymers created by limited cross-linking. As the number of cross-links increases,
however, the polymer becomes more rigid and cannot stretch as much; the polymer
will become less viscous and less
elastic and might even become brittle. e.g., Bakelite, malamine formaldehyde resin etc.
