Structure and surface properties of liquid crystalline fluoroalkyl polyacrylates:: Role of the spacer

The organization of liquid crystalline fluorinated acrylate polymers has been generally described so far in the literature in terms of phase segregation between hydrogenated and fluorinated moieties within the polymer and preferential orientation induced by the backbone. However, little is known on the effects of the chemical composition on the properties of such fluorine-containing polymers. Our study was then focused on the role played by the spacer group located in the side chain between the backbone and the fluorinated segment and in particular, on the properties of poly[2-[[[[2-(perfluoroalkyl)ethyl]sulfonyl]methyl]amino]ethyl]acrylates (pASn). Surface and bulk organization of fluorinated side chains of those polymers were investigated by surface tension, X-ray scattering, and differential scanning calorimetry measurements. Results were compared with those obtained with poly[(perfluoroalkyl)ethyl] acrylates (pAFn) of the same perfluoroalkyl chain lengths. A strong correlation between bulk organization and surface properties of our polymers could be established. Surprisingly, in the perfluorohexyl series, pAS6 with a N-methylsulfonamide spacer group was found to be organized in a crystalline lamellar structure whereas pAF6 was found to be amorphous. This indicates that the introduction of a N-methylsulfonamide spacer group in the side-chain allows the system to crystallize with a shorter -C6F13 fluorinated segment, whereas in the presence of methylene groups, an organization is only present with a -C8F17 segment (pAF8). This was mainly attributed to the strong dipole-dipole interaction between N-methylsulfonamide groups that tends to align the fluorinated segments in a lamellar structure that eventually crystallizes as long as the section of the N-alkylsulfonamide group does not induce steric hindrance.