LDPE has the lowest average melting temperature (103.6°C) out of the 4 polymers. It is because LDPE is a linear polymer that has a branched structure which is able to expel the applied heat faster through the “branches” and melt faster. HPDE has the second lowest average melting temperature (139.5°C) out of the 4 polymers. Unlike LDPE, HDPE does not have a branched structure and the HPDE polymer chains are able to pack more tightly to one another. Therefore HPDE has a higher density than LDPE and is able to withstand more heat than LDPE.
However HPDE and LDPE has lower average melting temperatures of 139.5°C and 103.6°C respectively than average melting temperature of PP(178.4°C) is due to the fact that the monomer of PP(C3H6) has one more -CH2 than the monomer of PE(C2H4). Hence PP has a bigger molecular size than that of both LDPE and HDPE and more heat energy is required to overcome the intermolecular forces between PP molecules than that of LDPE and HDPE. Nylon has the highest average melting temperature (252.5°C) among the 4 polymers.
The three polymers: LDPE, HDPE and PP are homopolymers while Nylon is a copolymer which is made up of 2 different monomers: diamine and dicarboxylic acid. Since Nylon has the biggest molecular size, more heat energy is needed to overcome the strong intermolecular forces between the Nylon molecules.