This mutation creates a hydrophobic patch on the protein surface that promotes intermolecular aggregation, the molecular event that gives rise to sickle-cell anemia.
This chain scission lowers average molecular weight of the polymer, decreasing the number and strength of intermolecular bonds as well as the degree of entanglement.
These two definitions are equivalent: the boiling point is the temperature at which the increased entropy of the gas phase overcomes the intermolecular forces.