The Formation Processes of Functional Groups at Polyolefin Surfaces on Exposure to Oxygen or Ammonia Plasma: A Critical Review

Abstract

Hydroxy (-OH) and primary amino groups (-NH2) are important as anchoring points at polymer surfaces for covalent coupling of molecules used in bio-medicine or components in polymer composites. O2 and NH3 plasmas or the coating of polyolefin surfaces with thin films of plasma polymers from allyl alcohol and allylamine are most often used for generation of OH and NH2 groups. The analysis of polyolefin surfaces exposed to oxygen and noble gas plasma treatments has shown very similar spectra of O-functional groups when the plasma treated samples were post-plasma exposed to air. Therefore, it was not surprising that the variety of O-functional groups formed by the two types of plasmas corresponds well to that of chemical auto-oxidation. This oxidation proceeds via formation of radicals, attachment of molecular oxygen, generation of peroxy radicals, and decomposition of hydroperoxide groups to many types of O-functional groups, among them only a few OH groups are formed. Coating of polyolefins with allyl alcohol plasma polymer leads to a surface with higher concentration of OH groups. It is assumed that many OH groups of the monomer survive the plasma polymerization process and additional OH groups are formed by post-plasma auto-oxidation on exposure to air. Ammonia plasma treatment produces also a low yield in desired NH2 groups at a polyolefin surface, due to unfavorable thermodynamics. Moreover, several side reactions are observed, such as hydrogenation at the topmost surface as well as in subjacent layers, dehydrogenation by UV irradiation, polyene formation, crosslinking and post-plasma extensive auto-oxidation on exposure to air in deeper layers. Allylamine plasma polymerization is characterized by a strong loss in NH2 groups of allylamine and extensive side-reactions, such as dehydrogenation to imine and nitrile groups (-CH2-NH2→CH=NH→C≡N). The auto-oxidation of plasma polymer introduces many O-functional groups. Yield, selectivity, advantages, limitations and drawbacks of the above-mentioned processes for polyolefin surface modification with OH or NH2 groups are discussed in some detail with respect to the mechanism, kinetics and thermodynamics of reactions. In conclusion, introduction of functional groups into the polymer surface is always a compromise between attachment of different types of functional groups and simultaneous degradation and decomposition of the surface region of polymer substrates with weakening of its mechanical strength.