Abstract
The rheological properties of cross-linked polymer microparticle (CPM) solutions have been studied regarding the influence of the CPM structure. CPM have been synthesized by dispersion polymerization, with the degree of cross-linking varying between 0 and 10% (i.e., with the concentration of diacrylate monomer cross-linking agent ranging from 0 to 10 mol-%). The structure of the CPM has also been changed by using optionally a semi-crystalline stabilizing agent monomer, based on a C18 aliphatic chain. The resulting CPM were either semi-crystalline or totally amorphous with a high Tg. Typical linear acrylic polymers (acrylic LP) have also been synthesized by solution polymerization and compared to the CPM. Dilute solutions in toluene or xylene were used to measure the intrinsic viscosity of polymers with a capillary viscosimeter. More concentrated solutions in the same solvents were used to determine the influence of the shear rate and of the CPM structure and concentration on the solution viscosity with a cone and plate viscosimeter. Because of the intramolecular cross-links, weak interactions took place in dilute solutions between the CPM, resulting in very low viscosity and low dependence on the molar mass, with respect to the acrylic LP. A low molar mass dependence was also found for CPM in bulk, without critical molar mass due to chain entanglement. Concentrated CPM solutions led to shear thinning behavior. It greatly depended on the microparticle structure, especially on the presence of the C18 aliphatic chains. Solutions of cross-linked polymer microparticles without these aliphatic chains can decrease the viscosity by 500 times between low and high shear rates. Thixotropy was observed for CPM solutions with concentrations higher than 20 wt.-%. The recovery time was highly influenced by the CPM concentration, ranging from a few tens of seconds to about two minutes.
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