Polymers Main Page > Out from under the Radar > Detail   Polymers Division - Out from under the Radar   Counterion and Polyelectrolyte Dynamics as viewed by Neutron Spin Echo Spectroscopy   Polyelectrolytes differ in chain dynamics and equilibrium structure from neutral polymers due to the presence of long-ranged electrostatic interactions. This manifests as strong interactions in solutions for both synthetic and biopolymers as observed by aggregation and multimode relaxations. Describing these relaxations requires new experimental methods to test current theories and simulations.   Vivek M. Prabhu and Eric J. Amis   Control of polymer dynamics with designed self assembly, externally supplied fields (e.g., electric, flow, or magnetic), or patterned substrates requires overcoming Brownian motion as well as inter and intra molecular structural correlations. Current examples include small molecule and polymer associations for drug design and biological processes and the docking of polyamides to the minor groove of DNA or motor protein motion along microtubules. These examples rely on intermolecular forces including hydrogen bonding and electrostatics which are balanced by an entropic loss. For biopolymers such as DNA and proteins, the association between counterions and chain is electrostatic in origin. In order to design new technologies with charged polymers the fundamental time scales and length scales of polymer and counterion association need to be quantified.   Charged polymers present a challenge due to the long-range electrostatic interactions and coupled dynamics between small, fast-moving counterions and polymers. The observed multimode dynamics and large scale structure in these systems are influenced by ionic strength, viscosity, and charge valence, but the molecular origin of the dynamics remains to be experimentally ascertained due to the limited number of techniques able to measure the solution behavior at the nanoscale. We have used small angle neutron scattering (SANS) to characterize the equilibrium structure and the pico to nanosecond dynamics by neutron spin echo (NSE) spectroscopy. In particular, NSE provides structural information between 60 nm and 3 nm and time scales between 45 ps and 100 ns.   We have measured the statics and dynamics of labeled counterions and polyelectrolytes through the partial structure factors for a model system composed of poly(styrene sulfonate) (PSS) and counterions including sodium, magnesium, and tetramethylammonium (TMA+). An example of the dynamic structure factor for NaPSS in D2O taken for several values of the scattering wave vector (Q) is shown in Fig. 1.   Figure 1. Normalized intermediate scattering function for NaPSS in D2O at 46gL-1.   The labeled counterion dynamics were measured for a TMA+ PSS system and exhibit a Q dependent diffusion coefficient, shown in Fig. 2. A diffusive slowing down near the static correlation peak is observed signifying the coupled internal polymer relaxations and counterions on the nanoscale.   Figure 2. Inverse effective diffusion coefficient from NSE and scattered intensity from SANS versus Q for TMA-PSS in D2O at 46gL-1.   Contributors and Collaborators D. Bossev and N. Rosov (NCNR, NIST)                     NIST Material Science & Engineering Laboratory - Polymers Division