Plus A Small Molecules Of Poly (n - Isopropyl Acrylamide) Group - Sphere Change The Influence Of The Midline Of The Aqueous Solution

Poly(N-isopropylacrylamide)(PNIPAM) is a well-known thermo-sensitive polymer that exhibits a low critical solution temperature (LCST) at around305K in aqueous solutions. The coil-to-globule transition of PNIPAM can be induced by a small temperature variation (1～2K) accompanied by abrupt conformational changes. The LCST behavior of PNIPAM has been attracting research interests for several decades because of its implication in a number of living phenomena, especially on protein folding and DN A packing. However, the coil-to-globule transition of PNIPAM has been greatly affected by some additives, such as surfactants and inorganic salts. Therefore, the researches on the effects of the additives on the coil-to-globule transition of PNIPAM have gained great realistic significance.In this work, with the applications of laser light scattering (LLS), viscosmetry as well as high-resolution nuclear magnetic resonance (NMR), the effects of the anion surfactant sodium n-dodecyl sulfate (SDS) on the coil-to-globule transition of PNIPAM under various temperatures and surfactant concentrations have been systematically studied. Besides, the effects of eight inorganic salts on the coil-to-globule transition of this polymer have been also explored. Several interesting results have been drawn:(1) SDS can increase LCST of PNIPAM solutions obviously for peculiar interactions with PNIPAM. Results from2D NOESY and pulsed-field gradient diffusion NMR show that a small proportion of SDS (around16%) binds to the PNIPAM chain and forms a polyelectrolyte-like complex via hydrophobic interactions below LCST. The polymer-bound SDS plays an important role in the retardation of the PNIPAM chain collapse and the inhibition of inter-chain aggregation. Interestingly, the polymer-bound SDS gradually dissociates from PNIPAM because of electrostatic repulsion and steric hindrance effects resulting from the coil-to-globule transition. Almost no cross-peak in the2D NOESY NMR spectra above the LCST is observed because of the reduced interactions between PNIPAM and SDS after the polymer chain transforms into a compact globule. The role of SDS during the coil-to-globule transition in the PNIPAM/SDS aqueous solution at various temperatures was illustrated in Figure1.(2) The complex structure of and interaction between PNIPAM and SDS in aqueous solutions below the LCST were investigated further. Results from viscometry and NMR have both indicated that the interactions of PNIPAM and SDS were found to exhibit strong dependences on the SDS concentration. Two critical SDS concentrations for this system were found, i.e.,0.86and7.0mM, within the current measured SDS concentration range. The former was the onset concentration of formation of the PNIPAM and SDS aggregates. The latter was the saturation concentration at which the surfactant molecules could not bound to the polymer chain. At SDS concentrations below0.86mM, the interaction between PNIPAM and SDS was very minimal, resulting in almost no change in the intrinsic viscosity and hydrodynamic size of the polymer. At SDS concentrations below7.0mM but above0.86mM, the polymer-free SDS concentration remained unchanged at around0.86mM near the critical aggregation concentration (CAC). Hence, excess SDS molecules preferred to attach onto the PNIPAM chain until saturation and formed a polyelectrolyte-like complex. Consequently, the intrinsic viscosity and hydrodynamic size of the polymer in solution increased. The amount of polymer-bound SDS and intensities of the cross-peaks in the2D NOESY NMR spectra also increased. At SDS concentrations above7.0mM, the number of polymer-bound SDS reached saturation and no more SDS molecule bound to the polymer chain due to steric hindrance. Subsequently, the amount of polymer-free SDS began to increase. The morphological structures of PNIPAM/SDS at SDS concentrations1.0mM and12mM have shown in Figure2, respectively.(3) Besides SDS, the additives of inorganic salts have also shown great effects on the conformational change of PNIPAM in aqueous solutions. Results from viscometry and LLS indicated that most of tested salts would accelerate the coil-to-globule transition of PNIPAM and reduce the LCST. These effects exhibited strong dependence on the salts concentrations, and the higher salts concentration, the lower of LCST. Furthermore, it was found that the anionic groups of salts played the key roles to affect the coil-to-globule transition behavior of polymer, and followed the order of classic Hofmeister sequence. However, NaI at low concentration region showed opposite effects, which could increased the hydrodynamic size of PNIPAM slightly. The NMR experiments primarily indicated that the effects of the inorganic salts on the conformational change of PNIPAM in aqueous solutions were carried on mainly through interaction between the anionic groups and solvent molecules and destroying the solvation effects of polymer.