New mixed-substituent fluorophosphazene high polymers and small molecule cyclophosphazene models: Synthesis, characterization, and structure property correlations

This work is an investigation of the influence of bulky fluoroalkoxy side groups on the properties of polyphosphazenes. A new series of mixed-substituent high polymeric poly(fluoroalkoxyphosphazenes) containing trifluoroethoxy and branched fluoroalkoxy side groups was synthesized and characterized by NMR and GPC methods. These polymers contained 19-29 mol % of dibranched hexafluoropropoxy groups or 4 mol % of tribranched tert-perfluorobutoxy groups which serve as regio-irregularities to reduce the macromolecular microcrystallinity. The structure-property correlations of the polymers were then analyzed and interpreted by several techniques: specifically by the thermal behavior by DSC and TGA methods, the crystallinity by wide-angle X-ray diffraction, and the surface hydrophobicity/oleophobicity by contact angle measurements. Ultraviolet cross-linkable elastomers were prepared from the new polymers through the incorporation of 3 mol % of 2-allylphenoxy and photoirradiation. The mechanical properties and the elastomeric deformation-recovery behavior were then monitored by varying the time of ultraviolet irradiation. Side reactions detected during the synthesis of the high polymers, such as side group exchange reactions and α-carbon attack, were analyzed via use of a cyclic trimer model system. The new polyphosphazenes with branched fluorinated side groups expand the scope of phosphazene-related materials and provide an understanding of the effect of different side group geometries on the overall properties of the fluoroelastomers.