Surface Modification of Polyethylene Films via Bromination: Reactions of Brominated Polyethylene with Aromatic Thiolate Compounds N. Chanunpanich, †,⊥ A. Ulman,* ,†,⊥ A. Malagon, ‡,⊥ Y. M. Strzhemechny, ‡,⊥ S. A. Schwarz, ‡,⊥ A. Janke, § T. Kratzmueller, § and H. G. Braun § Department of Chemical Engineering and Chemistry, Polytechnic University, Brooklyn, New York 11201, Department of Physics, Queens College of CUNY, Flushing, New York 11367, Institute for Polymer Research, Hohe Strasse 6, D-01069 Dresden, Germany, and The NSF MRSEC for Polymers at Engineered Interfaces Received July 20, 1999. In Final Form: December 15, 1999 Introduction Polyethylene (PE) is one of the most interesting com- modity polymers. However, one problem associated with PE is its low surface energy. For example, the adhesion between PE surfaces and most matrixes is poor, because of their hydrophobicity and smoothness, which prevent chemical as well as mechanical bonding. One possible route to increase PE surface energy is to modify it. Modification on PE surface has been accomplished via a chemical reaction with a contacting solution, 1 corona discharge treatment, 2 plasma or glow discharge treatment, 3 and surface grafting of hydrophilic monomers after photo- irradiation. 1f,4 Unfortunately, acid etching or chemical oxidation and plasma treatments mainly introduce oxygen functionalities, which are in the form of ether, hydroxyl, carbonyl, and carboxyl groups, into the outer few molecular layers of the polymer film. Attempts to introduce other functionalities by plasma treatment utilizing N 2 , 5 NH 3 , 6 CCl 4 , 7 CF 3 Cl, 8 and CHBr 3 9 have been reported. Halogenation has been one route for modification of the PE films, 7,10 resulting in adhesion improvement. 10a,c,e Moreover, halogenated PE surfaces can be further modified to other functionalities using substitution reactions, 10g,1h resulting in new, improved surface proper- ties. Fluorination has been reported on PE and polypro- pylene (PP) films. 10d,e There, the film surface became heavily fluorinated within 1 s of exposure to fluorine gas. Chlorination on PE film has been studied to improve hydrophilicity and was found to occur selectively in the amorphous regions of the PE film. Also, chlorination was found more surface-selective in the presence of light due to a decreased rate of Cl 2 diffusion through a highly chlorinated product. 10f Thus, the reaction took place in the outermost molecular layers (10 Å) more than deeper in the subsurface (40 Å). In addition, several infrared studies have concluded that no CCl 2 moieties exist in chlorine-saturated samples. 11 The kinetics of chlorination, bromination, and iodination of PE were also reported. 10b Bromination took place much slower than chlorination. After 11 days, bromine concen- tration at the PE surface was found only slightly greater than 12%, while after 2 s exposure to chlorine its surface concentration was 6.5%. The rate of iodination on PE film was very slow, and 0.34% iodine was found on the surface after 141 days of exposure to iodine vapor. It has been suggested that the size of the bromine and iodine atoms is larger than that of the chlorine, and surface uptake could be much more facile than subsurface uptake. Moreover, the bond strength of C-halide is 116, 81, 68, and 51 kcal/mol for C-F, C-Cl, C-Br, and C-I, respec- tively. 12 Hence the exothermicity of halogenation reaction also decreases in this order, and the order of reactivity of halogenation is F 2 > Cl 2 > Br 2 > I 2 . Nucleophilic aliphatic substitution reaction on haloal- kanes, on the other hand, depends on the strength of the bond to carbon, the stability of halide anion, its leaving group characteristics, and the nucleophile. 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