Kinetic Hydrate Inhibition of Poly(N-isopropylacrylamide) and Poly(N-isopropylmethacrylamide): A Comprehensive Analysis of Amphiphilicity and Hydrogen Bonding Effect

Rapid formation of natural gas clathrate hydrates has been a significant hazard in off-shore natural gas drilling and gas transport. During operation, polymer inhibitors can serve as low-dosage kinetic inhibitors to prevent the formation of gas hydrates. A comprehensive consideration of the multiple factors involved in the actions of kinetic hydrate inhibitors (KHIs) can elucidate the inhibiting behavior of polymers. Here, amphiphilic poly(N-isopropylmethacrylamide) (PNIPMAm) (or poly(N-isopropylacrylamide)) are found to achieve weak to strong inhibition by the variable superimposed effect of the polymer's amide group and hydrophobic effect of the polymer's alkyl groups on the aqueous phase. The interfacial and bulk water mobility and hydrogen bonding energy in the polymer solutions are compared by low-temperature nuclear magnetic resonance (NMR) relaxometry. The mobility of the water is found to be sensitive to the hydrophobic alpha-methyl side group in PNIPMAm and its molecular weight when a guest molecule tetrahydrofuran is present. The water hydrogen bonding energies measured from NMR are also associated with the nonfreezable bound water amount revealed by differential scanning calorimetry. The alpha-methyl side group leads to a significant reduction of the water mobility, an increase in the hydrogen bonding energies of the interfacial water, and a decrease of the hydrate growth rate constants by 2 to 3 orders of magnitude. The retarded reaction kinetics of heterogeneous hydrate growth can be related to the hydrophobic effect of the polymer's alpha-methyl side group and bound water properties (mobility and amount) in the interface.