  Thermoresponsive polymers hold both fundamental and technological importance,
but the essential physics driving their intriguing behavior is not wholly
understood. We introduce a lattice framework that incorporates elements of
Flory-Huggins solution theory and the $q$-state Potts model to study the phase
behavior of polymer solutions and single-chain conformational characteristics.
Importantly, the framework does not employ any temperature- or
composition-dependent parameters. With this minimal Flory-Huggins-Potts
framework, we show that orientation-dependent interactions, specifically
between monomer segments and solvent particles, are alone sufficient to observe
upper critical solution temperatures, miscibility loops, and hourglass-shaped
spinodal curves. Signatures of emergent phase behavior are found in
single-chain Monte Carlo simulations, which display heating- and
cooling-induced coil-globule transitions linked to energy fluctuations. The
model also capably describes a range of experimental systems. This work
provides new insights regarding the microscopic physics that underpin complex
thermoresponsive behavior in polymers.
