Zeta potential measurements for the characterization of polymer surfaces with varying amide​/amine contents

Zeta potential measurements are widely utilized for colloidal and fiber systems, in which the colloid stability and fiber hydrophobicity / hydrophilicity can be quantified by the isoelectric point IEP and the zeta potential.1-2 However, as of today, only few studies were reported on polymer films employing the ‘streaming potential measurement’. In this presentation, the application of zeta potential measurements in the analysis of polymer surfaces is well elucidated by two examples.
The first example utilizes the streaming potential for the determination of the relative curing degree of amine-cured epoxy resins. With higher curing degrees, more amide groups are formed (coherent with the consumption of amine groups), and the content of functional groups that are capable to be protonated decreases. Correspondingly, the IEPs of the cured epoxy resin surfaces decrease with the cure temperatures, and the surface charging polarity reverses with increasing cure temperatures (Figure 1, left).
The other example to be shown focusses on the application of partially hydrolyzed copolymers of poly(2-nonyl-oxazoline) as contact biocides.3 If these copolymers are used as additive in low quantity in polypropylene compounds, the surface energies of the compounds do not alter (compared to 100% polypropylene). Copolymers with higher degrees of hydrolysis have higher IEPs due to the presence of amine groups, which correlates with the different pKb of amide and amine groups: the compound containing pN100 (unhydrolyzed) has a IEP of 8.7, while the IEPs of samples containing pN50A50 and pN25A75 (50% and 75% hydrolysis respectively) are at pH ≈13.5 (Figure 1, right).