Polyphosphazenes have a highly flexible backbone comprised of nitrogen and phosphorus atoms with two organic groups on each phosphorus. Materials in this class are inherently thermo-oxidatively stable, soluble, and chemically resistant especially when a variety of fluorinated organic groups are attached to the backbone. Several solution based side group exchange reactions have been examined previously that have profound effects on the surface topology, but may present problems for large scale production. We have utilized a high-throughput process for surface functionalization of polyphosphazenes to yield films with excellent chemical properties that can be tuned by surface treatment with plasma gases.
Films of the poly[bis(2,2,2-trifluoroethoxy)phosphazene] were surface functionalized with plasmas of methane, oxygen, or nitrogen gases. An environmental plasma head was used to treat the spuncast films with free radicals in the treatment gases. These surface treatments were characterized by XPS and water contact angle methods to determine the modified surface structure of the films and the effect on surface properties. More than 90° changes in water contact angle are possible, depending on the plasma used and the density of surface functionalization. Tuning of the water contact angle was also possible from the native 101° static contact angle of poly[bis(2,2,2-trifluoroethoxy)phosphazene] to approximately 70°, 40°, or 4° with appropriate treatment using methane, nitrogen, or oxygen plasmas. Using XPS surface surveys we found that the fluorine concentration on the surface was higher than in the bulk material. Significant changes in the carbon, oxygen, and nitrogen 1s binding energies of the samples were also detected that varied with plasma treatment.
This process allows polyphosphazene polymers or copolymers to be surface functionalized as an alternative to other processes where delamination or differences in thermal or chemical properties would preclude a traditional two polymer composite approach. Printing of controlled patterns with a cover mask are also possible to create a surface with several distinct regions of hydrophobicity or hydrophilicity or of functional groups all of which may be applicable in microfluidics, printing, adhesion, or sensor arrays.