In-depth study on aminolysis of poly(ɛ-caprolactone): Back to the fundamentals

Abstract

The aminolysis can effectively introduce primary amine (−NH2) groups onto polyester materials, enabling a variety of subsequent surface biofunctionalization reactions. However, less attention has been paid to the basic knowledge of aminolysis reaction in terms of reaction kinetics and its influences on materials properties. In this study, taking the widely used poly(ɛ-caprolactone) (PCL) as a typical example, the influences of diamines and solvent property on the surface −NH2 density are firstly assessed by using X-ray photoelectron spectroscopy (XPS) and colorimetric analysis. Results show that smaller diamine molecules and nonpolar alcohols could accelerate the reaction. The reaction kinetics with 1,6-hexanediamine is further investigated as a function of temperature, reaction time, and diamine concentration. During the initial stage, the reaction shows a 1st order kinetics with the diamine concentration and has an activation energy of 54.5 kJ/mol. Ionization state of the −NH2 groups on the PCL surface is determined, revealing that the pKa of −NH3+ (<5) is much lower than that of the corresponding diamine molecules in solution. After aminolysis, surface hydrophilicity of PCL membrane is significantly enhanced, while surface elastic modulus and average molecular weight are decreased to some extent, and others such as weight, surface morphology and bulk mechanical strength are not apparently changed. The introduced −NH2 groups are found to be largely lost at 37 °C, but can be mostly maintained at low temperature.