A diblock copolymer is a single polymer chain with two distinct segments joined end to end — block A and block B, each with different chemical properties. The competition between the blocks' interactions produces rich self-assembly behavior: they can mix, segregate, or form complex intermediate structures depending on temperature, solvent quality, and the relative strength of intra- and inter-block interactions.
Raiola, Locatelli, Marenduzzo, and Orlandini (arXiv 2602.22382, February 2026) add magnetic competition. Each monomer carries an Ising spin. Within each block, spins interact ferromagnetically — they prefer to align. Between blocks, the interaction is antiferromagnetic — neighboring spins from different blocks prefer to anti-align. An external magnetic field adds a third force, pulling all spins toward one orientation regardless of block identity.
The phase diagram has four states. In weak fields at high temperature, both blocks are swollen and magnetically disordered — a conventional coil. As temperature drops, the polymer compacts. If the inter-block antiferromagnetic coupling is strong, the two blocks segregate into separate globules, each internally ordered but magnetically opposite. If the inter-block coupling is moderate, the blocks mix into a single globule with interleaved segments — compact but frustrated.
The surprise is the fourth phase: the tadpole. When the two blocks have different ferromagnetic coupling strengths — one strongly magnetic, the other weakly — the strongly coupled block collapses into a compact, magnetically ordered globule while the weakly coupled block remains extended and disordered. The result is a hybrid structure: a dense head attached to a flexible tail. The same chain, one end frozen, the other free.
The tadpole phase exists only when the block asymmetry is sufficient. Symmetric blocks — equal coupling strengths — never form tadpoles; they either both collapse or both swell. The structural asymmetry of the tadpole is a direct image of the magnetic asymmetry of the blocks. The shape records the forces.
Mean-field predictions and Monte Carlo simulations agree quantitatively on the phase boundaries. The tadpole is not a fluctuation or a crossover — it's a thermodynamic phase with a well-defined stability region.