Comparison of Particle Size Techniques to Investigate Secondary Nucleation in HEMA‐Rich Latexes

Abstract

2-Hydroxyethyl methacrylate (HEMA)-rich latex is synthesized via a semibatch monomer-starved emulsion polymerization process. Six emulsion polymerizations, containing 0-40 mol% HEMA in monomer composition, are synthesized and compared. Calculations of mass balances are made in order to predict fi nal particle size and particle size development during the course of polymerization. Average particle sizes obtained by dynamic light scattering (DLS) are compared to ultrasound acoustic attenuation spectroscopy (UAAS) and transmission electron microscopy (TEM). The infl uence of HEMA on polymerization on the onset of secondary nucleation is evaluated using each technique. The DLS, TEM, and UAAS secondary nucleation data are compared to physical latex blends of small and large particles. The blend data are consistent with the secondary nucleation data. Additionally, conductivity and surface tension are monitored. TEM, atomic force microscope (AFM), and UAAS are found to be more appropriate methods than the widely used DLS for particle size and particle size distribution characterization in this system. Increasing HEMA content to 20 mol% results in formation of signifi cant amount of secondary particles via proposed homogeneous nucleation of HEMA-rich oligomers.

Influence of hydrophobic monomers on secondary nucleation of hydroxyl-functionalized latexes

“…Feeding rate was chosen to reach monomer‐starving conditions. In order to control particle size, more extensive calculations were made for the amount of surfactant, as reported previously …”

“…Composition can be found in Table . Seed latex was synthesized batchwise following the procedure described previously …”

“…In thermosetting latexes, a higher proportion of hydrophilic monomers are required to obtain a sufficient crosslink density. Due to the combination of acrylate backbone and crosslink density, the resulting coatings offer excellent thermal, UV, and chemical resistance …”

“…Later those hydroxyl functional latexes can be crosslinked to improve coating performance. A number of crosslinkers can be used including: isocyanate, melamine‐formaldehyde resins or cycloaliphatic epoxides . Unlike carboxylic functionalized monomers, HEMA does not dissociate under emulsion polymerization conditions, therefore its behavior is not pH dependent as is the case with acrylic (AA) or methacrylic acid (MAA) .…”

“…The water solubility and latex swellability of HEMA is driven by hydrogen bonding forces . Additionally, limited water solubility of poly‐HEMA‐rich oligomers was found to induce formation of secondary particles through homogeneous nucleation mechanism . According to the Hansen and Ugelstad theory oligomer agglomeration and new particle population are going to occur when oligomer hydrophilicity will prevent entry in existing primary particles.…”