2.3. Contact angle measurement
Static water contact angle measurements of the samples were obtained by a contact angle measuring instrument (JGW 360C, Chenghui Testing Machine Co., Ltd, China) at 25 C and 50% relative humidity. The values were obtained when the contact time of water and specimen was 10 s. At least ten measurements on different areas of each film specimen were averaged. Contact angle relaxation of water droplet was measured by sessile drop method. Contact angle at t ¼ 0 s was measured by less than 1 s and estimated from the first droplet image which was collected by the instrument automatically every 0.5 s.