A well—defined model hard segment was synthesized with 4

4’-diphenylmethane diisocyanate

dimethylolpropionic acid and 1

4-butanediol by means of fractional steps．The sequence distribution of the model hard segment was measured by131C-NMR and its structure and morphology were characterized by FT—IR

DSC and WAXD．A series of polyester based polyurethaneurea(PUU)aqueous dispersions with this well-defined hard segment were prepared．The physical properties of PUU aqueous dispersions and their films were measured．The experimental results showed that the particle size of the aqueous dispersions decreased with increasing the hard segment content from 1 10 nm to 72 nm．and they wele very stable at ambient temperature(more

than one year)．Furthermore

the films prepared with such PUU aqueous dispersions exhibited the excellent water resistance

i-e. low equilibrium water absorbing capacity(at around 5 wt％)and high contact angle of water on the surface(more than 100°)．