There are several factors which determine the physical as well as chemical properties of a polymer, for example, nature of chain packing; type of molecular force etc. These factors influence the crystallinity of polymers, which in turn, along with other factors influence properties as melting point, tensile strength, viscosity, toughness, etc. Based on their structure, i.e., how the monomers are linked to each other we have the following types of polymers:
Branched chain polymers, and
1. Linear Polymers. These are the polymers in which monomeric units are linked together to form long straight chains. The polymeric chains are stacked over one another to give a well packed structure. As a result of close packing, such polymers have high densities, high tensile strength and high melting points. Common examples of such type of polymers are polyethylene, nylons and polyesters.
2. Branched Chain Polymers. In this type of polymers, the monomeric units are linked to constitute, long chains (called the main-chain). There are side chains of different lengths which constitute branches. Branched chain polymers are irregularly packed and thus, they have low density, lower tensile strength and lower melting points as compared to linear polymers. Amylopectin and glycogen are common examples of this type.
3. Cross-linked or Network Polymers. In this type of polymers, the monomeric units are linked together to constitute a three-dimensional network. The links involved are called cross-links. Cross-linked polymers are hard, rigid and brittle because of their network structure. Common examples of this type of polymers are bakelite, melamine formaldehyde resin, etc.
Low Density (LDPE) and High density (HDPE) polyethylene
Polyethylene is a branched chain polymer. This means that the polymer chains do not be in parallel but instead form a tangled mass (Fig. 59.4). Now since the chains cannot be close together, the density of polyethylene is low. It is therefore called low-density polyethylene (or LDPE). LDPE has a lower tensile strength, low melting point (= 130° C)
Linear chains of polyethylene i.e., chains with no branching can be close together and hence results in increase in the density. Such a polymer is called high density polyethylene (HDPE). It was first produced in 1953 in Germany by Karl Ziegler.
As the HDPE chains form ordered structures, therefore it has higher density than LDPE. Its melting point and tensile strength is also higher than LDPE.
High Density Polythene (HDPE)
Low Density Polythene (LDPE)·
nCH2= CH2 → -(CH2-CH2)-n
TiCl4 + Al(C2H5)3
nCH2= CH2 → -(CH2-CH2)-n
Traces of O2
The molecular forces like van der Waals’ forces and hydrogen bonds existing in the monomeric units effect many mechanical properties of polymers such as tensile strength, toughness, elasticity, etc. depend upon. Although these intermolecular forces are present in simple molecules also, but their effect is less significant in them as compared to that in macromolecules. It is because of the fact, that in polymers there is a combined effect of these forces all along the long chains. Obviously, longer the chain, more intense is the effect of intermolecular forces.
On the basis of the magnitude of inter-molecular forces present in the polymers, we have,
Elastomers, Fibers, Thermoplastics, and Thermosetting polymers.
1. Elastomers. These are the polymers in which the polymer chains are held up by weakest attractive forces. They are amorphous polymers having high degree of elasticity. The weak forces permit the polymer to be stretched out .about 10 times their normal length but they return to their original position when the stretching force is withdrawn. In fact,
these polymers consist of randomly coiled molecular chains having few cross-links. When the stress is applied, these randomly coiled chains straighten out and the polymer gets stretched. As soon as the stretching force is released, the polymer regain the original shape because weak forces do not allow the polymer to remain in the’ stretched form.
2. Fibers. These are the polymers which have quite strong interparticle forces such as H-bonds. They have high tensile strength and high modulus. They are thread-like polymers and can be woven into fabrics. Nylon, dacron silk are some common examples of this types of polymers. The H’bonds in nylon-66 have been shown in Fig. 59.5.
3. Thermoplastics. These are the polymers in which interparticle forces of atttaction are in between those of elastomers and fibers. The polymers can be easily moulded into desired shapes by heating and subsequent cooling to room-temperature. There is no cross-linking between the polymer chains. In fact, thermoplastic polymers soften on heating and become fluids but on cooling they become hard. They are capable of undergoing such reversible changes on heating and cooling repeatedly. Common example of thermoplastics are polyethene, polystyrene, polyvinyl chloride, etc.
4. Thermosetting Polymers. These are the polymers which become hard and infusible on heating. They are normally made from semi-fluid substances with low molecular masses, by heating in a mould. Heating results in excessive cross-linking between the chains forming three dimensional network of bonds as a consequence of which a non-fusible and insoluble hard material is produced. Bakelite is a common example of thermosetting polymer. In short, a thermoplastic material can be remelted time and again without change, while a thermosetting material undergoes a permanent change upon melting and thereafter sets to a solid which cannot be remelted.