Reactivity of carboxylic acid and ester groups in the functionalized interfacial region of "polyethylene carboxylic acid" (PE-CO2H) its derivatives: differentiation of the functional groups into shallow and deep subsets based on a comparison of contact angle and ATR-IR measurements

Abstract

We have differentiated the carboxylic acid groups present in the functionalized interface of “polyethylene carboxylic acid” (PE4O2H, a material prepared from low-density polyethylene film by reaction with aqueous chromic acid) into two subsets: those sufficiently close to the surface of the polymer to influence its wettability by water and those too deep to do so. This differentiation was accomplished by taking advantage of differences in rates of esterification of carboxylic acid groups in different regions of the functionalized interface and of differences in rates of hydrolysis of ester groups derived from them. The subset of functional groups influencing wettability comprises <30% of the total groups present in the functionalized interface and appears to be homogeneous in its chemical reactivity. The remaining groups (-70% of the total) do not directly influence wettability and appear to become less reactive with increasing depth in the polymer. The surface and subsurface carboxylic acid and ester moieties are both less reactive in hydrolysis and formation reactions than are these groups in organic molecules in solution. Reactivities of the interfacial functional groups depend on structure in ways having no analogy in reactions in solution. For example, the rate of base-catalyzed hydrolysis of esters present in the functionalized polymer interface is a strong function of the length of the alcohol component of the ester: n-octyl esters are more than 20000 times less reactive than methyl esters. We have also explored the acidity of the interfacial carboxylic acid groups using ATR-IR spectroscopy and contact angle measurements as probes. Both the local polarity of the environment of individual carboxylic acid groups and charge-charge interactions between carboxylate anions appear to be important in determining acidity. Comparisons of wettability of samples containing different proportions of carboxylic acid, carboxylate anion, and ester groups indicate that wettability is particularly sensitive to low concentrations of carboxylate anion. We hypothesize that this sensitivity reflects a limited ability of the functionalized interfacial region to reconstruct-perhaps only by small-amplitude rotations of its constituent carboxylate ions-in a way that minimizes its free energy by maximizing the number of these hydrophilic moieties in direct contact with the polar, aqueous liquid phase.