Synthesis and characterization of PAA microgels using multifunctional epoxy cross-linkers with a new cross-linking mechanism via a precipitation polymerization method

Abstract

Polyacrylic acid (PAA) microgels were prepared by a free radical precipitation polymerization method. Initially, epoxy type cross-linkers were used to prepare PAA microgels by the precipitation polymerization method. Different types of multifunctional cross-linkers such as ethylene glycol diglycidyl ether (EGDGE), Triphenylol methane triglycidyl ether (TMTGE) and 4,4′- Methylenebis(N,N-diglycidyl aniline) (MBDGA) were used as bi-, tri- and tetra-functional cross-linkers, respectively. Epoxy type cross-linkers are able to cross-link PAA by reaction between the –COOH groups of PAA and the epoxide groups of the cross-linkers. Some of the epoxy rings do not react with acid groups of PAA chains during the polymerization stage; unreacted epoxy rings can be reacted during the sample drying time, a stage we designated as the ‟post-curing stage”. The effect of cross-linker concentration and its functionality on the behaviors of samples (i.e., swelling capacity, gel content, glass transition temperature and rheological properties) were investigated. A new relationship between average molecular weight of two successive cross-links ( Mc) and epoxy concentration ([epoxy]) was defined. The microgel properties, such as rotational viscosity and rheological properties, were compared with those of microgels prepared by divinyl type cross-linkers. Our results show that microgels synthesized by this novel mechanism have higher rheological properties (at low shear rate, strain and frequency) than those synthesized by conventional methods. These behaviors can be due to the increase in polymer side chains of the polymer network generated by this new cross-linking method.