Semicrystalline polyphosphazenes: A comparative study of topology, morphology, and contact angles for three fluorous and one aryl polyphosphazene

This report describes the surface morphology and wetting behavior of one aryl and three fluorous polyphosphazenes: poly[bis(2,2,2-trifluoroethoxy) phosphazene], PTFEP, 1, poly[bis(para-fluorophenoxy)phosphazene] 2, poly[bis(meta-trifluoromethylphenoxy)phosphazene] 3, and poly(diphenoxyphosphazene) 4. Thermal analysis confirms high thermal stability for these polyphosphazenes. With phenyl side chains, 2-4 are particularly stable with T_5% >400 °C. Surface morphology via tapping mode atomic force microscopy (TM-AFM) and dynamic contact angles (DMA) are correlated with composition and crystallinity estimated by differential scanning calorimetry (DSC). The resulting structure property relationships define trends in processability, hydrophobicity and oleophobicity. Compared to 1, 3, and 4 poly[bis(para-fluorophenoxy)phosphazene] 2 is noteworthy by virtue of a high melting point (179 °C), high crystallinity (ΔH_f, 30.2 J/g), and a moderate molecular weight. The remaining polyphosphazenes in this group have T_m <100 °C. Heats of fusion (J/g) are in the order 1 (29.4) ≈ 2 (30.2) > 3 (17) >> 4 (4.0). High molecular weights for 3 and 4 (>10³ kDa) would make melt processability problematic. Thus 1 or 2 has a favorable combination of properties, with 2 favored for many applications due to a higher T_m. In summary, for performance and processing, high T_m, and resistance to water and hydrocarbons poly[bis(para-fluorophenoxy)phosphazene], 2 is attractive. This finding suggests other para-substituted phenyl phosphazenes are worth further study.