Chemistry and reactivity of carboxylic acid-containing surfaces

Polymer surface modification techniques, such as corona, plasma, and ultraviolet treatments, promote adhesion to polymer surfaces through introduction of specific functional groups that interact with subsequent coating layers. Carboxylic acid groups formed during corona and plasma treatments may play a significant role in adhesion promotion, so such groups have been chosen for model surface chemistry studies. Thin films (<30 nm) of the potassium salt of poly(acrylic acid) (PAA) and poly(vinyl methyl ether-co-maleic acid) (VME-MA) copolymer were analyzed before and after surface protonation by dilute acid solution. The reactive nature of the acid groups was probed using infrared reflection absorbance spectroscopy (IRRAS) to analyze the entire thin film and high-resolution electron energy loss spectroscopy (HREELS) to analyze the top 1-2 nm. The HREELS results show that it is possible to obtain surfaces enriched in carboxylic acid groups by protonation of the potassium salts of PAA and VME-MA. Anhydride formation, followed as a function of temperature, went to completion quickly (5-10 min) at the surface, whereas complete conversion did not occur in the bulk thin film even after 4 h. PAA formed anhydride at 160°C, as compared to 100°C for VME-MA. The large difference between the temperature for anhydride formation in PAA and VME-MA may be a result of the adjacent acid groups available for forming anhydride in VME-MA, or the high Tg of PAA (105°C) relative to that of VME-MA (80°C). IRRAS and HREELS show that PAA and VME-MA form imide functionalities following butylamine exposure and heating at 125°C. The reactive nature of PAA and VME-MA at elevated temperatures suggests that acid-containing surfaces are good candidates for reaction with organic materials containing primary amine groups, an example being gelatin, which is the binder used in photographic emulsions.