Concentrated dendrimer solutions

Prosa, T.J. Bauer, B.J. Topp, A. Amis, E.J. Scherrenberg, R. Size changes and interpenetration within concentrated dendrimer solutions. Polym. Mater. Sci. Eng. 1998, 79, 307-308. 

Fig. 5. a Total emission intensity, b Linewidth, both as functions of excitation intensity for DCM/dendrimer solution in cuvette. DCM concentration was 4.0 mmol/1. Inset in a shows plot in logarithmic scale at moderate excitation intensity... 

Dendrimer encapsulated Pt nanoparticles (DENs) were prepared via literature methods (1, 11). PtCl42 and dendrimer solutions (20 1 Pt2+ dendrimer molar ratio) were mixed and stirred under N2 at room temperature for 3 days. After reduction with 30 equivalents of BH4 overnight, dialysis of the resulted light brown solution (2 days) yielded Pt2o nanoparticle stock solution. The stock solution was filtered through a fine frit and Pt concentration was determined with Atomic Absorption Spectroscopy (11). Details on catalyst characterization and activity measurements have been published previously (11). 

When dendrimers are exposed to forced intermolecular contact by increasing their concentration in solution, or by placing them on a surface, do they freely pass through one another or do they avoid interpenetration and align themselves Dendrimers have sizes in the range 1-20 nm and would be excellent candidates for nanoscopic structures if such ordering does occur. 

Full generation poly(amidoamine) (PAMAM) dendrimers possessing amino surface groups are easily spread by spin coating on a mica surface to form films. Interdendrimer assembly events appears to influence these operations. Regardless of the generation levels examined, the film uniformity is determined primarily by the concentration of the dendrimer solution. 

Figure 14.5 Dependence of the average distance between dendrimer centers on dendrimer solution concentration, . Interpenetration of neighboring den-drimers occurs when this distance becomes shorter than 2/ H (according to ref. [27])...

Consistent with the Newtonian flow of concentrated PAMAM solutions, it was found that all three types of dendrimers [40, 41, 50] under steady-shear conditions, and both PAMAMs [40] and PPIs [50] under creep [16,50] showed typical viscous behavior at all applied stress levels and testing temperatures. For example, as illustrated in Figure 14.9 [40], all of the first seven generations of PAMAMs showed constant viscosities over the entire ranges of shear rates investigated, and in addition to this, there was no hysteresis between the forward and the reverse stress sweeps in steady shearing, indicating the absence of thixotropy. 

We have synthesized and tested an example boron-chelating polymer based on a commercially available dendrimeric poly(amido amine) (PAMAM). Dendrimeric chelants offer several advantages over polymers typically used in PAUF. Foremost among these is the reduced viscosity of dendrimer solutions as compared to solutions of linear polymers[6]. This allows the use of higher polymer concentrations than previously feasible (though in the present study we worked at polymer concentrations of less than 5% due to the expense of the starting dendrimer). In this study, the dendrimeric chelant also serves as a convenient, monodisperse polymer with which to test the mathematical model for boron speciation which is derived from the work of Wise and Weber[l],... 

Dendrimer-encapstdated catalysts are another area of active research for polymer-supported catalysts. The nanoparticles are stabilized by the dendrimers preventing precipitation and a omeration. Bimetallic nanoparticles with encapsulated metals (dendrimer-encapsulated catalyst DEC) from commercially available fourth-generation PAMAM dendrimers and palladium and platinum metal salts were prepared via reduction by Crooks and co-workers [34], following previous work in this area [35], The simultaneous incorporation of Pt and Pd reflects the concentrations in solution. The bimetallic DECs are more active than the physical mixture of single-metal DEC [35, 36] in the case of the hydrogenahon of allyl alcohol in water, with a maximum TOP of 230 h compared to TOP = 190 h obtained for monometallic palladium nanoparticles (platinum TOP = 50 h ). 

Dendrimers can also be prepared with an inverse relationship between their hydrophobic and hydrophilic constituents, i.e. with a hydrophobic periphery and a hydrophilic interior. They can then behave as reverse micelles and are able to concentrate polar molecules from solutions of nonpolar solvents. The shape of these molecules, when dissolved in a solvent that matches the hydrophobic nature of the periphery, is spherical with chain-ends extended towards the solvent. The interior may then collapse to a minimum volume, so that unfavourable interactions that might result from penetration by solvent molecules are minimized. 

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