A 1:1 (w/w) mixture of waterborne PUs and deionized water
was poured on a silicon sheet and allowed to dry at room temperature
for 5 days. The films were dried at 80 ◦C for 12 h, and then
kept under vacuum at 50 ◦C until use. For measurements of waterswelling,
a stoichiometric amount of a melamine resin (Aerotex
M-3) was mixed with the waterborne PUs and the same procedure
described above was followed with an additional curing step for
1 min at 160 ◦C. The resulting cross-linked PU films were abbreviated
as PU-0-C, PU-7-C, PU-14-C, and PU-21-C.