Poly(bis-2,2,2-trifluoroethoxymethyl oxetane): Enhanced Surface Hydrophobicity by Crystallization and Spontaneous Asperity Formation

We report a spontaneous increase in the contact angle (104 degrees --> 136 degrees +/- 4 degrees ) for a semicrystalline polyoxetane with symmetrical CF3CH2OCH2 side chains. Poly(bis-trifluoroethoxymethyl)oxetane, P(B-3FOx), Mn = 21 kDa, was prepared by a modification of conventional cationic ring opening polymerization. At ambient temperature, the polymer is between Tg (-39 degrees C) and Tm (approximately 70 degrees C). Tapping mode atomic force microscopy (TM-AFM) revealed an interesting process-dependent topology. Coatings that were melted and slow-cooled displayed increasing roughness over the course of 4-6 weeks at 25 degrees C. The result was a topology characterized by sharp micrometer-scale ridges and asperities. The heat of fusion increases from an initial value of 21.9 J/g after slow cooling (non-isothermal melt crystallization) to 28 J/g after 6 weeks (non-isothermal melt crystallization plus isothermal melt crystallization). The coating appearance changes from transparent with a slight haze to translucent. The changing topology was accompanied by a 30 degrees increase in the water contact angle, up to 140 degrees , attributed to an asperity-rich surface yielding a discontinuous three-phase contact line and to a change in the proportions of crystalline and amorphous area fractions accompanying crystallization.