Orientational Relaxation of Poly(propylene imine) Dendrimers at Different pH

The dilute solution dynamics of poly(propylene imine) (PPI) dendrimers is investigated at three different solution pH through molecular dynamics (MD) simulations. The dynamics of PPI dendrimers is characterized by both global and local relaxations that occur at different time and length scales. While the global dynamics may be described in terms of rotational diffusion, the local motion may be characterized through orientational relaxation dynamics measured in terms of the time autocorrelation function (ACF), second-order orientational ACF, and the spin-lattice relaxation rate. The global motion of dendrimers decreases with an increase in the size from high pH to low pH with increasing generations of growth. The results reveal that the segments at low pH relax faster than those at high pH, and the local mobility of the segments near the periphery is higher than the core segments. This observation is also evident from the spectral density and spin-lattice relaxation rate. High values of the spectral density at higher frequencies imply higher segmental mobility of the dendrimer at low pH relative to that at high pH. A shift in the maximum of the spin-lattice relaxation rate toward lower frequencies with decreasing generations indicates the dependence of local mobility on the topological distance of the segment from the periphery at all pH conditions.