To obtain a solution, polyurethane is dissolved in DMF at 25 °С for 24 hours (see work doi: 10.1007/s00289-017-2223-8, 10.1016/j.polymer.2003.08.040). You can use a roller shaker for mixing.
It is important to presoak the polymer in cold DMF for 24-48 hours. Afterwards, just heat the mixture to facilitate dissolution. Solutions with 20% concentrations and above can be quite viscous, though, and even precipitate out or form gels following cooling to room temperature.