Diffusion NMR study of generation-five PAMAM dendrimer materials

Area of Science:

• Polymer Chemistry
• Materials Science
• Nanotechnology

Background:

• Commercial generation-five poly(amidoamine) dendrimer material (G5c) is a complex mixture.
• Understanding the properties of its structural components is crucial for targeted applications.

Purpose of the Study:

• To isolate and characterize the major structural components of G5c.
• To compare the functional properties of monomeric G5 (G5m) with the heterogeneous G5c material.
• To elucidate the pH-dependent swelling behavior of G5c.

Main Methods:

• Fractionation of G5c into its constituent parts.
• Isolation of monomeric G5 (G5m).
• Diffusion-ordered nuclear magnetic resonance (DOSY-NMR) spectroscopy to determine self-diffusion coefficients and hydrodynamic radii (R(H)).

Main Results:

• Hydrodynamic radius (R(H)) scales with the number of atoms (N) as R(H)∝N(0.35) for G5m and oligomers.
• G5c is a heterogeneous mixture comprising G5m (~65%) and oligomers, with sizes comparable to higher generation dendrimers.
• Isolated G5m (R(H) = 3.1 ± 0.1 nm at pH 7.4) did not exhibit pH-dependent swelling, unlike G5c.
• G5--G5 dimers showed a 44% increase in R(H) upon protonation, indicating oligomers are responsible for G5c's pH response.

Conclusions:

• The pH-dependent swelling of G5c is primarily attributed to the protonation and subsequent hydrodynamic size increase of its oligomeric components.
• The study provides insights into the structural heterogeneity of commercial dendrimer materials and their behavior in solution.
• The findings enable experimental testing of boundary models within the Stokes-Einstein equation for poly(amidoamine) dendrimers.