Ultrafast dynamics in aqueous polyacrylamide solutions

We have investigated the ultrafast dynamics of aqueous polyacrylamide ([-CH₂CH(CONH₂)-]_n, or PAAm) solutions using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The observed aqueous PAAm dynamics are nearly identical for both M_w = 1500 and 10 000. Aqueous propionamide (CH₃CH₂CONH₂, or PrAm) solutions were also studied, because PrAm is an exact model for the PAAm constitutional repeat unit (CRU). The longest time scale dynamics observed for both aqueous PAAm and PrAm solutions occur in the 4-10 ps range. Over the range of concentrations from 0 to 40 wt %, the picosecond reorientation time constants for the aqueous PAAm and PrAm solutions scale linearly with the solution concentration, despite the fact that the solution shear viscosities vary exponentially from 1 to 264 cP. For a given value of solution concentration in weight percent, constant ratios of measured reorientation time constants for PAAm to PrAm are obtained. This ratio of PAAm to PrAm reorientation time constants is equal to the ratio of the volume for the PAAm constitutional repeat unit (-CH₂CHCONH₂-) to the molecular volume of PrAm. For these reasons, we assign the polymer reorientation dynamics to motions of the entire constitutional repeat unit, not only side group motions. Simple molecular dynamics simulations of H[-CH₂CH(CONH₂)-]₇H in a periodic box with 180 water molecules support this assignment. Amide-amide and amide-water hydrogen-bonding interactions lead to strongly oscillatory femtosecond dynamics in the Kerr transients, peaking at 80, 410, and 750 fs.