Regulation of pore morphologies of PU films and thereof water vapor permeability by varying tetrahydrofuran concentration in binary solvent

The concentration of tetrahydrofuran (THF) was used to regulate the pore morphologies of PU films using a THF/N,N-dimethylformamide (DMF) binary solvent. Macro-void, micro-pore, and potential inter-connected pores are classified according to the cross-sectional FESEM images. Corresponding pore forming mechanisms are proposed. The water vapor permeability (WVP) was linked with total pore areas of different kinds of pores. Mechanical properties as a function of THF concentration was also investigated. Optimized PU coating could be achieved at a 40 wt% THF bearing the tensile stress, strain, toughness, and WVP of 9.2 ± 0.6 MPa, 1063.7 ± 57.1%, 43.6 ± 3.8 MJ m−3, and 1894.3 ± 41.3 g m−2·day−1, respectively. This provides an alternative avenue for the production of PU coating with balanced mechanical property and WVP.