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