 Solution

Symmetric wetting means that both film boundaries prefer the same block: A at both, or B at both. Asymmetric wetting means opposite preferences, such as A at the substrate and B at the free surface. A neutral boundary has approximately equal interfacial free energies against A and B and therefore imposes no first-order preference between them.

These terms compare the substrate–polymer and atmosphere–polymer interfaces, not the volume fractions or molecular weights of the two blocks. A film can have one neutral boundary and one selective boundary; saying only that its top surface is neutral does not make both interfaces neutral. Surface preference competes with chain stretching and the energetic cost of A/B contacts in selecting a block copolymer morphology.