Dendrimer Size Change in Different Solvents

The SANS experiments [51] were performed with solutions of G8 PAMAM dendrimer in D20, methyl-d4, ethyl-d6, and n-butyl-d10 alcohol at a temperature of T = 20.0 °C. PAMAM dendrimers do not dissolve in acetone, but they readily dissolve in methyl alcohol/acetone mixtures over a wide range of composition. Solvents of different composition, were prepared and added to a weighed amount of dried G5 or G8 dendrimer. In a separate set of experiments, the NIST NG7 30 m instrument was used to measure the effects of charging on the dendrimer size. PAMAM G8 dendrimers in D20 were charged by addition of HC1 in the presence of various amounts of NaCl to the charged dendrimers to screen the electrostatic interactions. 

Similar results were observed with mixtures of acetone-d6 and methanol-d4, wherein the Rg of the dendrimers is not significantly affected by the fraction of acetone in the solvent. The same result is found over the whole range of temperature studied (i.e. in the range — 10°C T 50 °C). Only data obtained in the solvent compositions, xs = 0.50 show some consistent decrease of Rg, relative to the other solvent compositions. This may indicate the onset of dendrimer shrinkage in the vicinity of the solubility gap, which is around xs 0.52. However, the difference from the a value of Rg for xs = 0.50 5% of the mean value of Rg for the other solvent compositions. 

Dendritically branched molecules have shapes and interactions that are radically different from traditional linear polymers. SANS, SAXS and TEM have been used to measure the properties of dendrimers and a generic understanding of their properties has emerged. Five technologically important questions on the nature of dendrimers have been posed and generalized observations can be made. 

Dendrimers are nearly spherical in shape when compared to conventional linear polymers [ 17,34], The lower generation dendrimers have internal segment density distributions similar to star molecules, but even the smallest are more compact than stars. As the generation number increases, the sphere-like characteristics increase and the largest dendrimers form a population of spheres with uniform internal density (roughly half dendrimer units-half solvent) [47], sharp interfaces on the outside and a small polydispersity in shape. 

Dendrigraft molecules are star-like in the Oth generation, but have a uniform interior with an exterior transition zone for G3 [17,34], Random hyperbranched molecules have broad distributions both in molecular mass and in shape [17], Scattering from hyperbranched is closer to that of linear polymers than to spheres, indicating that there is a gradually tapering distribution on units from the center to the exterior. 

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