writer：Wang Liao; Yongjun Zhang; Ying Guan; X.X. Zhu
keywords：PNIPAM; microgel; fractal structure; rheology
source：期刊
specific source：Langmuir
Issue time：2012年
Dispersions of poly(N-isopropylacrylamide) (PNIPAM) microgel thermally gel in the presence of inorganic salts. The in situ-formed hydrogels, with a network of soft
particles, represent a new type of colloidal gels. Here, their fractal  structures  were  determined  by  rheological  measurements,  using  the models  of  both  Shih  et  al.  and Wu and Morbidelli.  According  to  the  definition of Shih et al., the colloidal PNIPAM gels fall into the strong-link regime. Yet the calculated fractal dimension of the floc backbone, x, yielded unrealistic negative values, suggesting this model is inapplicable for the present system. The Wu?Morbidelli model gives physically sounde results. According to this model, the strengths of the inter- and intra floc links are comparable, and the in situ-formed gels are in the transition regime. The fractal dimension, df, of the hydrogel decreases from ～2.5 to ～1.8 when the heating temperature increases from 34 to 40 °C. The df values suggest different aggregation mechanisms at different temperatures, that is, a reaction-limited one accompanied by rearrangement at low temperature, a typical reaction-limited one at the intermediate temperature, and a diffusion-limited one at high temperature. With increasing salt concentration, the df of the hydrogel decreases from ～2.1 to ～1.7, suggesting the aggregation mechanism changes from reaction-limited to diffusion-limited. The effects of both temperature and salt concentration can be explained by the changes in the interactions among the microgel particles. The thermogellable PNIPAM microgel dispersions may serve as a model system for the study of heat-induced gelation of globular proteins.