Spectral time moment analysis of microgel deswelling. Effect of the heating rate

Microgel nanoparticles were synthesized in aqueous solutions of neutral polymer hydroxypropylcellulose (HPC) through the self-association of amphiphilic HPC molecules and the subsequent cross linking at room temperature. Dynamic Light Scattering was used to study the transport properties of HPC microgels below and above the volume phase transition. Highly nonexponential, multimodal microgel spectra were observed and successfully analyzed by spectral time moment analysis. This article expands earlier results and focuses on the effect of the heating rate on microgel deswelling. During the fast heating two identified microgel modes with apparent hydrodynamic radii (RH) of 25-30 nm and 400-650 nm collapse into one mode with RH = 100-150 nm. This indicates the shrinkage of microgel size distribution and an apparent decrease in the radius of larger microgels. During the slow heating, however, both microgel-identified modes remain present above Tc. Although equally represented below the transition, the dominance of larger microgels' mode increases almost two fold with rising temperature above 40°C. Moreover, RH for this mode increases from 250-300 nm to about 800-850 nm with a multi-step temperature change from 40 to 42.5°C, indicating the growth (and not shrinkage) of microgels. The second mode is represented by the temperature independent RH, but its contribution goes down from about 50% to less than 10%. © 2008 Wiley Periodicals, Inc.