Benoit Magny

(Meth)acrylic Cross‐Linked Polymer Microparticles: Synthesis by Dispersion Polymerization and Particle Characterization

(Meth)acrylic cross-linked polymer micro-particles have been synthesized by dispersion polymerization in organic media. They were produced by radical copolymerization of mono(meth)acrylate monomers with a certain concentration of a diacrylate as cross-linker, in a mixture of two organic solvents, heptane and propan-2-ol. The reactive surfactant (surfmer) was a low solubility parameter acrylate monomer, based on an aliphatic ester group (generally C 18 ) or an isobornyl cycle, which gave auto-dispersing character to the microparticles. By using glycidyl methacrylate in the monomer mixture, oxirane groups were introduced in the particles. The influence of the major synthesis parameters such as solvent composition, monomer composition and concentration, and initiator concentration has been investigated regarding the size and the molar mass of the cross-linked polymer microparticles. The study was completed with the influence of the nature and the concentration of the surfmer, the concentration of cross-linking agent and the composition of monomers feed. Stable cross-linked microparticles ranging from z-average radius of gyration, R z = 20 nm to R z > 60 nm were obtained by varying the synthesis conditions. The smallest microparticles were prepared with a blend of heptane and propan-2-ol in the 50/50 ratio by weight. Increasing the surfmer concentration or reducing the monomer concentration in the reaction mixture usually led to smaller microparticles. The longer the aliphatic chain of the surfmer, the smaller the microparticles. Minimum sizes were obtained for cross-linking agent concentrations between 5 and 7.5 mol-%, depending on monomers composition. For higher concentrations, macrogelation may occur during the synthesis.

Rheological Properties of (Meth)acrylic Cross-Linked Polymer Microparticles, 1. Comparison with Linear Polymers in Bulk and in Non-Reactive Solvents

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 C 18 aliphatic chain. The resulting CPM were either semi-crystalline or totally amorphous with a high T g . 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 chan entanglement. Comcentrated CPM solutions led to shear thinning behavior. It greatly depended on the microparticle structure, especially on the presence of the C 18 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.

Rheological Properties of (Meth)acrylic Cross‐Linked Polymer Microparticles, 2. Role of Interactions in Non‐Reactive and in Reactive Diluents. Copolymerization Studies

The rheological properties of (meth)acrylic cross-linked polymer microparticle solutions have been studied regarding the influence of physical interactions. Cross-linked polymer microparticles (CPM) were synthesized with different functionalities: epoxy, acrylic, carboxyl, hydroxyl, octadecyl and methyl. The viscosities of either dilute or concentrated solutions in non-reactive solvent (xylene) and in reactive solvents (styrene and isobornyl acrylate monomer) have been measured for each type of CPM. A very weak effect of functionality was noticed on the intrinsic viscosity of CPM, whatever the solvent. Only carboxyl groups had a significant influence by increasing the viscosity because of hydrogen bonding. Greater effects were shown on the rheological properties of concentrated solutions. All solutions exhibited shear-thinning behavior when the solvent was xylene or styrene. Carboxyl functionalized CPM still led to higher viscosity, while octadecyl groups reduced the low shear limiting viscosity. The recovery time was strongly increased for CPM with epoxy and acrylic functionalities when styrene was used instead of xylene, whereas it remained the same for all other CPM solutions. In isobornyl acrylate monomer, only CPM functionalized epoxy and acrylic showed shear-thinning behavior, the other solutions being Newtonian. Finally a copolymerization study has been done on three solutions, containing either non-reactive CPM in reactive diluent, or reactive CPM in non-reactive diluent, or reactive CPM in reactive diluent. This study confirmed the cross-linking mechanism of acrylic systems. First, intramolecular reaction took place within the CPM and small aggregates were formed. Then these aggregates interconnected leading to a viscoelastic solid.