Self-Organizing Materials with Low Surface Energy: The Synthesis and Solid-State Properties of Semifluorinated Side-Chain Ionenes Jianguo Wang and Christopher K. Ober* Department of Materials Science and Engineering, Cornell University, Bard Hall, Ithaca, New York 14853-1501 Received January 24, 1997; Revised Manuscript Received August 13, 1997 X ABSTRACT: The synthesis of new semifluorinated alkyl side-chain ionenes (SFASI) is reported and involves reaction of semifluorinated 1-bromoalkanes with poly(N,N ′-dimethyl-1,6-hexanediamine) (PDHD) which was prepared by reduction of poly(N,N′-dimethylhexamethyleneadipamide). The quaternization efficiency of the polyamine is 80-85% when carried out using a solvent mixture of DMF/ethanol (1/2 volume ratio) at 65 °C over long reaction times. The structure of SFASI consists of a 10 Å charged layer resulting from quarternary ammonium groups with strong coulombic interactions and a smectic B-like packing of the semifluorinated side-chain layer. The perfluorocarbon segments favor self-organization in a hexagonal array to form a head-to-head bilayer between the charged layers. These ionenes exhibit a smectic to isotropic transition, with the transition temperature dominated by the length of the fluorocarbon side chain. The surface energy of a spin-coated SFASI film, as estimated from the critical surface tension, was as low as 8 dyn/cm at 20 °C. This result indicates that the surface of the as-spun cast film consists largely of CF3 end groups even though hydrophilic ammonium groups are present in the polymer. The surface segregation and orientation of the low surface energy mesogenic semiflorinated side groups were confirmed by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray fine structure (NEXAFS) analysis. Surface reconstruction was observed due to polymer chain defects and surface mobility. Introduction Among the various uses of fluorinated polymers, nonstick-coating materials attract great attention 1-6 because of their extensive used in applications such as peel-off backings for self-stick labels, 7 devices to prevent the accumulation of biological debris, nontoxic antifoul- ing release coatings for ships hulls, 8 stain-resistant fabrics, and self-lubricating surfaces. 9 In general, non- stick-coating materials need a low surface energy, a function of the surface tension (γ), which can be expressed by Macleod’s relation 10 Here γ° and  are constants, independent of tempera- ture, F is the polymer density (which is almost constant for a given polymer), and  is known as Macleod’s exponent and is usually 3.0-4.5 for polymers. The value γ° is, to a close approximation, dependent only on the chemical structure of the surface. It is well- known that the surface tension of substituent groups decreases in the order of CH 2 (36 dyn/cm) > CH 3 (30 dyn/cm) > CF 2 (23 dyn/cm) > CF 3 (15 dyn/cm). 11-14 Among various molecular structures, a closest-packed uniform CF 3 surface was found to possess the lowest surface tension. Therefore, a polymeric material that can maintain a stable, uniform CF 3 surface might be able to play a key role in producing nonadherent materials. The creation of tailored surfaces by the assembly of Langmuir-Blodgett (LB) monolayers using amphiphilic molecules depends on the self-recognition behavior of molecules itself dependent on van der Waals and coulombic forces. Among reported LB films, those of perfluorocarboxylic acid 15-17 exhibit the lowest critical surface energy of 6-8 dyn/cm, as determined from contact angle measurements. Previous work has shown that aliphatic side chain ionenes exhibited excellent self- organization abilities, 18,19 in which alternating alkyl side chain and ionic layers on gold substrates could be prepared by evaporation of a polymer solution. Semi- fluorinated alkane diblocks 20 (SFAB) of even the non- polar perfluoro-n-eicosane 21 have been shown to possess extraordinary self-assembly properties from which an ordered LB monolayer could be prepared at the air- water interface. Owing to the strong amphiphilic and liquid-crystalline character of the semifluorinated alkyl side chains, it is expected that they should possess self- assembly abilities superior to aliphatic groups in side- chain ionenes. Driven by the migration of the low surface energy -CF 3 ends of the semifluorinated chain to the surface, the -CF 3 end groups may be self-oriented at the surface to produce a low surface energy material. Self-assembly also occurs in block copolymers due to the differing thermodynamic interactions between two or more blocks. 22,23 In fact, the semifluorinated alkyl chain is a miniblock oligomer because the perfluorocar- bon segment ((CF 2 ) n , n > 8) is strongly immiscible with its hydrocarbon segments. 24 Formation of low-energy surfaces from fluorinated block copolymers including their segregation behavior kinetics was recently dis- cussed in detail. 25,26 As reported from our recent studies on the semifluorinated side-chain liquid-crytalline block copolymers, 27 the stability of the surface can be en- hanced by the enthalpy penalty to disorder the highly ordered liquid-crystal structure of the semifluorinated compounds. From the standpoint of molecular design, semifluori- nated side-chain ionenes incorporate a number of novel features. The structure is shown in Scheme 1 and consists of a semifluorinated alkyl side chain directly connected to a quaternary ammonium backbone by a covalent bond. The goal of this research was to attempt the construction of a low surface energy material which X Abstract published in Advance ACS Abstracts, October 15, 1997. γ ) γ°F  (1) 7560 Macromolecules 1997, 30, 7560-7567 S0024-9297(97)00090-9 CCC: $14.00 © 1997 American Chemical Society