= Solution
The <Flory interaction parameter> $\chi$ measures the free-energy penalty of unlike block contacts relative to like contacts, in units of $k_BT$. The <degree of polymerization> $N$ counts the reference segments per chain, using the segment-volume convention associated with $\chi$. Their product, the <block copolymer segregation strength> $\chi N$, compares the tendency to avoid unlike contacts with the entropy cost of organizing and stretching connected chains.
At small $\chi N$, the <diblock copolymer> is disordered. Increasing $\chi N$ can cause <microphase separation>: the blocks form distinct domains, but their covalent connection prevents complete macroscopic separation. Domain interfaces cost energy while chain stretching and lost configurations cost <entropy>; their competition selects a finite domain scale.
A common temperature law is $\chi=A/T+B$, so increasing temperature often decreases segregation when $A>0$. This is not a universal sign law for every polymer pair. For an intact, fixed chain, $N$ does not change just because temperature or solvent concentration changes.
Adding a nonselective good solvent usually dilutes unlike-block contacts and reduces an effective segregation strength; it does not chemically shorten the chain. A selective solvent additionally changes domain swelling and solvent-block interactions. One needs a ternary-mixture description, or a clearly defined effective $\chi$, rather than treating the melt's $\chi$ as a universal solvent-concentration function.<Polyisoprene-polystyrene diblock copolymer phase diagram near the order-disorder transition by Ashish K. Khandpur and others>[Definition of the segregation and composition variables and experimental phase behaviour]{ref}
Back to article page
