Discussion
The blend of polyethylene and polypropylene is
Answer: B
The blend is immiscible due to entropic constraints.
The free energy of mixing is \(\Delta G_{m} = \Delta H_{m} - T\Delta S_{m}\). For small molecules the combinatorial entropy of mixing is large and drives mixing, but for long chains it is tiny, because each chain contributes only one translational unit however many segments it has (in the Flory-Huggins expression the entropy terms are divided by the chain lengths). Even a small positive enthalpy of mixing between the similar but not identical PE and PP segments then outweighs it, and the polymers separate. The paper attributes this to the entropic constraint.
Being polyolefins with similar solubility parameters is not enough for high polymers, so both 'miscible' options are wrong.
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