Structure of poly(propyl ether imine) dendrimer from fully atomistic molecular dynamics simulation and by small angle x-ray scattering

We study the structure of carboxylic acid terminated neutral poly (propylether imine) (PETIM) dendrimer from generation 1 through 6 (G1-G6) in a goodsolvent (water) by fully atomistic molecular dynamics (MD) simulations. Wedetermine as a function of generation such structural properties as: radius ofgyration, shape tensor, asphericity, fractal dimension, monomer densitydistribution, and end-group distribution functions. The sizes obtained from theMD simulations have been validated by Small Angle X-Ray Scattering (SAXS)experiment on dendrimer of generation 2 to 4 (G2-G4). A good agreement betweenthe experimental and theoretical value of radius of gyration has been observed.We find a linear increase in radius of gyration with the generation. Incontrast, Rg scales as ~ N^x with the number of monomers. We find two distinctexponents depending on the generations: x = 0.47 for G1-G3 and x = 0.28 forG3-G6 which reveals their non-space filling nature. In comparison with theamine terminated PAMAM dendrimer, we find Rg of G-th generation PETIM dendrimeris nearly equal to that of (G+1)-th generation of PAMAM dendrimer as observedby Maiti et. al. [Macromolecules,38, 979 2005]. We find substantial backfolding of the outer sub generations into the interior of the dendrimer. Due totheir highly flexible nature of the repeating branch units, the shape of thePETIM dendrimer deviates significantly from the spherical shape and themolecules become more and more spherical as the generation increases. Theinterior of the dendrimer is quite open with internal cavities available foraccommodating guest molecules suggesting using PETIM dendrimer for guest-hostapplications. We also give a quantitative measure of the number of watermolecules present inside the dendrimer.Comment: 33 page