Cross-linked Polymer Microparticle Characterization

ross-linked polymer microparticles, generally called microgels, have found extensive use in the coatings industry because of their utility in im-

proving the rheological properties of paints as well as the performance of fi lms. Microgels are commonly pre-pared by emulsion polymerization in aqueous media. As is true of all polymers, the properties of microgels are tied closely with their molar mass and size. Trans-mission electron microscopy (TEM), dynamic light scattering (DLS) and gel permeation chromatography (GPC) are now routinely used to measure the size and molar mass of microgel dispersions. Determining the Mw and Rg of microgel dispersions by static laser light scattering is presented herein.

Static laser light scattering methods have been used to determine the Mw and Rg of microgel disper-sions. In this application note, we report the character-ization of a microgel dispersion using a DAWN EOS detector, and a Wyatt Technology Optilab rEX differ-ential Refractive Index detector. The dn/dc value of the samples were determined by the Optilab detector. We measured the microgel sample at different concentra-tions using the DAWN and collected data with WTC’s ASTRA software program. We used ASTRA’s Zimm plot method of calculations for determinig Mw and Rg.

The Zimm plot gives us a weight average molecular weight (Mw) of the microgel (4.935 ± 0.041)×106 g/mol. The Rg of the microgel sample was calculated to be 12.1 nm. We also measured the microgel’s hydrodynamic radius Rh to be (17.2 nm) by dynamic laser light scat-tering.

For a hard sphere of uniform density, the ratio Rg/Rh is 0.775. So the ratio, Rg/Rh, of this sample (0.703) was found to be smaller than that for a solid sphere, which indicates that microgels with low crosslink den-sities have non-uniform polymer segment densities.

This note graciously submitted by Zhibiao Hu, College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072 China.

Figure 1. The Zimm plot of the microgel sample

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Light Scattering for the Masses™

Figure 1. The Zimm plot of the microgel sample