The monolayers of amphiphilic block copolymers PET/PEO with different weight fractions of hard segments were investigated at air-water interface by the Langmuir balance technology.Isotherms of surface pressure and the mean area per hard segment were measured for four samples of the block copolymer with the weight fraction of hard segments in the range of 0.35～0.50.It was found that the molecular limited area of the monolayer was gradually decreasing with increasing the weight fraction of hard segments,and hence the monolayer of the amphiphilic block copolymer became more condensed.The dynamic surface elasticity of the monolayer was measured by using the barrier oscillation method,and it indicated that the dynamic surface elasticity of the monolayer was gradually increased with increasing the surface pressure.Furthermore,for all the samples,the dynamic surface elasticity was almost the same at surface pressure lower than 17～18 mN/m;however,the dynamic surface elasticity of the monolayer for the block copolymer with the weight fraction of hard segments equal to 0.50 was increasing obviously and much greater than those of other copolymers at surface pressures higher than 18 mN/m.
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