Effect Of Polymer Structure On Crystalline

Relatively short polymer chains form crystals more readily than long chains, because the long chains tend to be more tangled. High crystallinity generally means a stronger material, but low molecular weight polymers usually are weaker in strength even if they are highly crystalline Low molecular weight polymers have a low degree of chain entanglement, so the polymer chains can slide by each other and cause a break in the material.

There are four variables present in polymer structure that affect crystallinity, and they are:

(1) Stereoregularity  This is a measure of how the mers( (the repeat units in the polymer) are arranged in the polymer backbone chain Isotactic materials have the mers connected together in a single repeated manner, syndiotactic materials have the mers connected together in a regularly repeated but different manner, and atatic materials have the mers connected together in a random manner. Atactic materials are generally amorphous because of the lack of polymer chain order, whereas isotactic and syndiotactic polymers are typically crystalline because of the high degree of polymer chain regularity. This effect can be used to increase the properties of some polymer, such as PP, by encouraging crystallization through the use of stereospecific forms of the material

(2) Pendant( Polar) groups Regular polymers with small pendant groups crystallize more readily than do polymers with large, bulky pendant groups.

Some polymers have polar groups present along the main polymer chain, e. g. the Co group is present in many amorphous polymers. These groups repel each another and prevent the formation of crystalline structures. Equally, some materials, such as the polyamides, form crystals very easily because of the attraction between sections of the main backbone chain

(3) Chain branching The degree of branching on the main backbone chain will affect the ability to crystallize. Highly branched polymers will generally be les crystallinity than linear polymers due to the presence of long branches. LDPE molecules are much branching than HDPE molecules. As a result, the degree of crystallinity of LDPE is much lower than HDPE.

(4) Intermolecular forces Crystallinity is favored by strong interchain forces. The presence of polar and hydrogen bonding groups favors crystallinity because they make possible diple-dipole and hydrrogen bonding intermolecular forces.A polyester, such as poly(ethylene terphalate), contains polar ester groups. Dipole-dipole forces between the polar groups hold the PET molecules in strong crystals.