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Mobility of poly(amidoamine) dendrimers; a study of NMR relaxation times

A. Donald Meltzer, University of Massachusetts Amherst


The steric nature of the new topology created by the starburst polymer has been studied by $\sp{13}$C and $\sp2$H dynamic nuclear magnetic resonance (NMR) relaxation measurements. For two series of poly(amidoamines), PAMAM, (one OH terminated, the other NH$\sb2$ terminated), $\sp{13}$C correlation times ($\tau$) of the terminal carbons were found to be almost independent of the number of end groups; $\tau$ varied from 1.0 $\times$ 10$\sp{-11}$ to 6.3 $\times$ 10$\sp{-11}$, and no evidence of dense-packing of the end groups was observed. The $\tau$'s of the methylene carbons on the interior of the dendrimers were found to increase with molecular weight, indicative of a progressive increase in local monomer density within the polymer. No significant differences in relaxation parameters of the internal carbons were observed for the NH$\sb2$ terminated PAMAM compared to the OH terminated analogues, in either D$\sb2$O or DMSO-d$\sb6$. Thus, the results reflect topological effects, and are not due to specific solvent or end group behavior. Larger relaxation times were observed for both series when measured in D$\sb2$O. While the differences in polymer behavior in the two solvents indicate that the polymer chains are more flexible in D$\sb2$O than in DMSO-d$\sb6$, intrinsic viscosities were determined to be comparable in the two solvents (0.04-0.10 dl/g). The difference in the NMR behavior is thus attributed to strong H-bonding between the polymer and DMSO, resulting in an increase in the hydrodynamic volume of the mobile unit. The relaxation behavior of the terminal carbon, in D$\sb2$O, differed upon changing the end group. The terminal carbon of the OH terminated PAMAM was observed to be less mobile than the corresponding carbon atom in the NH$\sb2$ terminated PAMAM. $\sp2$H NMR relaxation measurements were used in a more extensive study of the mobility of amine terminated PAMAM chains as a function of molecular weight and positions. The $\tau$'s were found to increase with molecular weight, irrespective of the location of the labelling. In the last generation the $\tau$'s were found to increase as the number of termini increases from 3($\tau$ = 1.7 $\times$ 10$\sp{-12}$s) to 384 ($\tau$ = 2.2 $\times$ 10$\sp{-11}$s), and were smaller than the $\tau$'s observed when the polymers were labelled at interior positions. No significant difference in relaxation parameters was observed when the label was located in the interior repeat units, irrespective of chain length following deuteration. No evidence of radial gradients was observed.

Subject Area

Polymers|Chemistry|Organic chemistry

Recommended Citation

Meltzer, A. Donald, "Mobility of poly(amidoamine) dendrimers; a study of NMR relaxation times" (1990). Doctoral Dissertations Available from Proquest. AAI9022719.