Novel low refractive index self‐cross‐linking fluoroterpolymers with very low surface tension and good adhesion to substrates

Abstract

SYNOPSISA novel family of fluoroterpolymers was prepared, containing perfluoroalkylethyl methacrylate (PFAEM) , methacrylic acid (MAA) , and 2-hydroxyethyl methacrylate (2-HEM)residues. The refractive index of the fluoroterpolymers is in the range of 1.3675 5 n I 1.4275. The surface tension of cured thin films falls in the interval from 15 m N / m to as low as 8 mN/m. Upon heating, these polymers undergo thermally initiated self-crosslinking and develop tenacious bonding to substrates. The associated chemical reactions and products were identified by IR spectroscopy and some thermogravimetric scans. The spectra indicate that the reaction products are ester and anhydride groups. In general, the formation of esters occurs more rapidly and requires lower temperatures ( 2 110°C) than does the formation of anhydrides. Prolonged heating, especially a t higher temperatures ( 2 15OoC), favors the formation of anhydrides. When the molar ratio of MAA to 2-HEM is substantially larger than 1.0, the formation of anhydride is favored when the ratio is 1 : 1, the formation of ester is favored; and when the molar ratio is much smaller than 1.0, the rate of formation of both esters and anhydrides is greatly reduced. In the case of the fluoroterpolymers, the formed esters and anhydrides essentially originate exclusively from the MAA and 2-HEM and seem not to involve any attack on the PFAEM residues. Only after very long heating a t the higher temperatures employed, some loss of C-F band intensity was noted. In a copolymer of PFAEM and MAA, a substantial and rapid attack by the acid on the ester group in the PFAEM residues takes place under relatively mild conditions. Such and more drastic conditions appear not to affect at all a homopolymer of PFAEM alone. This suggests that in the fluoroterpolymers the more polar MMA and 2-HEM tend to aggregate close together, so that, when heated, they preferentially react with each other, forming both esters and anhydrides. In the absence of 2-HEM, the MAA may react with itself or with PFAEM, producing only anhydride groups. Both esters and anhydrides may be intermolecular and/or intramolecular, resulting in a cross-linked system in which the cross-link concentration is directly related to the combined amount of ester and anhydride groups.