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The wide range of applications of poly(N-isopropylacrylamide) (PNIPAM) is based on the temperature dependence of its coil-to-globule transition, which strongly relies on the solvent. Here, we focus on the cononsolvency effect of PNIPAM oligomers in aqueous 1-propanol mixtures that is studied by molecular dynamics simulations of single chains and membrane-like arrangements. The complete phase diagram is sketched from the radius of gyration of the simulated oligomers, and it is compared to that obtained from the hydrodynamic radius of PNIPAM microgels, finding a good agreement. At the water-rich region, the decrease of the lower critical solution temperature (LCST) with increasing cosolvent concentration is independent of the polymer length and concentration. In this region, the radius of gyration of our simulated oligomer is strongly temperature dependent and a coil–globule transition temperature is easily captured. Conversely, at the alcohol-rich region, simulations show a monotonically increasing radius of gyration of the oligomer with alcohol concentration, a radius that is practically independent of temperature. This finding is in line with a polymer phase separation, showing an upper critical solution temperature (UCST) for a small cosolvent concentration window, which depends on the polymer length and concentration. Hence, in this case, polymer–polymer effective interactions are key to phase separation instead of its single chain conformation, contrasting with the coil–globule transition. Indeed, we find a soft-coil like structure for the simulated oligomer around a propanol molar fraction of 0.24, which is close to the mixture composition where the UCST phase transition is detected. Finally, in line with an UCST scenario, our simulated membrane turns unstable at high temperature and alcohol concentration.