Question

Question 8.

When a polyethylene wax is chlorinated, chlorine replaces hydrogen at random. Invariably, small amounts of chlorine (10 - 50 mass % CI) cause a lowering of the softening point. However, large amounts (ca. > 70 %) raise the softening point. Rationalise this on the basis of expected morphologies and intermolecular forces.

Answer 1

Chlorination of polymethylene with low concentration (10-50 wt% Cl) causes reduction in the softening temperature as chlorination of some sites on the chain occurs. It becomes a mixture of higher amount of underrivatized polymethylene and in it is dispersed chlorinated polymethyene. The chlorinated polymethylene acts as an impurity in the bulk polymethylene system. Polymethylene originally had certain amount of intermolecular forces existing among the chains which reduces upon chlorination. Lesser amount of heat is required to soften the remaining intermolecular forces existing between pure polumethylene chains. Thus lower amount of heat is required to melt of soften the material.

Upon, large doses of Chlorine (about 70%) treatment, most of the positions are chlorinated on the chain and it becomes more even system. The chlorinated polymethylene would have a higher melting point than the pure polymethylene system. Since the bulk mass is now chlorinated chains, it would require greater energy to soften the system with strong intermolecular forces existing between the chlorinated chains of polymethylene.

OR

Small amounts of Cl replaces small amounts of hydrogen, then polymer has large number of non polar bonds C-H and fewer number of polar bonds C-Cl. So weak London disperson forces are predominant intermocular forces. Hence softening point will be lowered.

Large amounts of (about 70%) of Cl will replace 70 % of H atoms. Then polymer will have 70% polar C-Cl bonds and it has dipole-dipole moilecular attraction forces as predominant inter molecular forces.

Hence softening point will be raised.