Practice Problem 1

Polyethylene can be obtained in two different forms. High-density polyethylene (0.94 g/cm³) is a linear polymer. Low-density polyethylene (0.92 g/cm³) is a branched polymer with short side-chains on 3% of the atoms along the polymer chain. Explain how the structure of these polymers gives rise to the difference in their densities.

Solution

Linear polymers are more regular than branched polymers. Linear polymers can therefore pack more tightly, with less wasted space. As a result, linear polymers are slightly more dense than branched polymers.