Spectral time moment analysis of microgel structure and dynamics

Abstract

The structure and dynamics of crosslinked nanoparticles (microgels) made out of hydroxypropylcellulose (HPC) polymer chains were studied using dynamic light scattering spectroscopy. The microgel light scattering spectra were found to be highly nonexponential requiring a spectral time moment analysis in which the spectra were fit to a sum of stretched exponentials. Each term offers three parameters for analysis and represents a single spectral mode. At room temperature microgel spectra reveal three modes. Two faster modes are almost diffusive and correspond to apparent sizes of 25 and 450–650 nm. The slowest mode is independent of scattering angle and is reminiscent of the slow polymer mode observed in identical non‐crosslinked polymer solutions. When solution temperature is varied from 23 to 45°C and back, the microgel undergoes a reversible volume phase transition between 40 and 45°C. According to the time‐moment analysis, above the transition temperature two faster modes collapse into one with apparent hydrodynamic radius of 100–150 nm, while the slow mode remains largely unchanged. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 771–781, 2008

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

“…The procedure (described in detail in ref. 28) yielded the optimal functional form for the field correlation function as a sum of stretched exponentials:…”

“…4, 16, 17 and ref. 28, the effect of temperature increase on microgel deswelling has been described as "significant" by every aforementioned study. What has not been explored in detail (to author's knowledge) is the role of the heating history on the HPC microgel deswelling.…”

“…HPC chains can be chemically cross-linked into stable microgel nanoparticles above the critical temperature T c 4,16 or even at room temperature by effectively lowering T c with salt addition 17,28 consequently lowering the magnitude of steric interactions. In fact, the synthesized microgels undergo a reversible temperature dependent volume-phase transition at the temperature T v which is somewhat higher than T c .…”

“…The approach of ref. 28 revealed that the HPC microgel solutions (obtained under the recipe of ref. 17) yield at least three different contributions to overall size distribution: two particulate modes ("fast" and "intermediate"), which have properties of diffusive ("fast" mode) and almost diffusive ("intermediate" mode) particles, and a third ("slow") mode which has striking similarities with the slow mode of uncrosslinked HPC polymer chains.…”

“…Instead of relying on the cumulant analysis used by Xia et al, 17 ref. 28 employed the spectral time moment analysis. 23 The switch in the analysis approach was motivated by the apparent 13,21,33 ambiguity of the cumulants method in the analysis of highly nonexponential spectra.…”