Abstract
Ultrasoft colloidal particle fluctuates due to its flexibility. Such fluctuation is essential for colloidal structure and dynamics, but is challenging to quantify experimentally. We use dendrimers as a model system to study the fluctuation of ultrasoft colloids. By considering the dynamic polydispersity in the small-angle neutron scattering (SANS) model, and introducing the fluctuation of invasive water into the contrast in SANS, we reveal the fluctuating amplitudes of the size and shape of the dendrimer of generation 6 at finite concentrations. The size fluctuation is suppressed while the shape fluctuation increases as the weight fraction of dendrimers passes 11%. With neutron spin echo data, we suggest that such crossover originates from the competition between the inter- and intra-particle dynamics. Further investigation on lower-generation samples shows a contrary result, which suggests a structural basis for these dynamic phenomena.
Highlights
Ultrasoft colloids, such as dendrimers and star polymers, are featured by extraordinary molecular flexibility [1,2,3,4,5,6]
With neutron spin echo data, we suggest that such a crossover originates from the competition between the interand intraparticle dynamics
Where np is the number density of dendrimer molecules, A denotes the contrast of the scattering length between solute particle and solvent, P(Q) is the average form factor normalized at Q = 0, and S (Q) is the apparent structure factor given by S (Q) = 1 + β(Q)[S(Q) − 1], where β(Q) is the polydispersity factor [46] that incorporates the size and shape fluctuations into the analysis and S(Q) is the interparticle structure factor
Summary
Ultrasoft colloids, such as dendrimers and star polymers, are featured by extraordinary molecular flexibility [1,2,3,4,5,6]. Ultrasoft colloids exhibit significant thermal-activated molecular fluctuations. Many ultrasoft colloidal suspensions of physical and technological importance are with finite concentrations [10,19,36] For these systems, the NSE analysis of the molecular fluctuation is greatly complicated by the difficulty in experimentally determining the collective translational diffusion of particles and its coupling to the intraparticle dynamics [37]. Note that most previous SAS studies of ultrasoft colloids have not explicitly considered the effect of particle size and shape fluctuations [1,38]. In this work we study the size and shape fluctuations of ultrasoft colloids using neutral poly(amidoamine) (PAMAM) dendrimers dissolved in water as the model system. With the NSE results and the investigation into the lower-generation samples, we reveal the dependences of these fluctuations on the molecular structure and dynamics
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