 Solution

For the dry diblock copolymer, the block volume fractionis the volume assigned to the polyisoprene block divided by total polymer volume, not its mass fraction or the volume fraction of polymer in the whole solution. It controls domain curvature and topology. Similar block volumes favor alternating lamellae; a smaller minority block generally favors curved interfaces, cylinders and eventually isolated spherical domains.

The chemical block lengths are fixed. Temperature changes their volumes only through thermal expansion, so $f_{PI}$ is often approximated as temperature independent; unequal expansion coefficients can produce a small change.

Solvent requires a distinction between dry composition and swollen domain composition. The dry $f_{PI}$ remains fixed. If solvent volumes $V_{s,PI}$ and $V_{s,PS}$ partition into the two domains, an effective swollen fraction isA polystyrene-selective solvent therefore lowers $f_{PI}^{\mathrm{eff}}$, whereas a polyisoprene-selective solvent raises it. Equal proportional swelling leaves this effective ratio unchanged.