Conformation change of an isotactic poly (N-isopropylacrylamide) membrane: Molecular dynamics

Abstract

In this work, isotactic Poly (N-Isopropylacrylamide)-PNIPAM-in neat water and in electrolyte solutions is studied by means of molecular dynamics simulations. This is done for an infinitely diluted oligomer and for an assembly of several PNIPAM chains arranged into a planar membrane configuration with a core-shell morphology. We employed two different force fields, AMBER (assisted model building with energy refinement) and OPLS-AA (all atom - optimized potentials for liquid simulations) in combination with extended simple point charge water. Despite the more water insoluble character of isotactic oligomers, our results support the existence of a coil to globule transition for the isolated 30-mer. This may imply the existence of an oligomer rich phase of coil-like structures in equilibrium with a water rich phase for temperatures close but below the coil to globule transition temperature, T. However, the obtained coil structure is much more compact than that corresponding to the syndiotactic chain. Our estimations of T are (308±5) K and (303±5) K for AMBER and OPLS-AA, respectively. The membrane configuration allows one to include chain-chain interactions, to follow density profiles of water, polymer, and solutes, and accessing the membrane-water interface tension. Results show gradual shrinking and swelling of the membrane by switching temperature above and below T, as well as the increase and decrease of the membrane-water interface tension. Finally, concentration profiles for 1M NaCl and 1M NaI electrolytes are shown, depicting a strong salting-out effect for NaCl and a much lighter effect for NaI, in good qualitative agreement with experiments.

Selective Molecular Transport in Thermoresponsive Polymer Membranes: Role of Nanoscale Hydration and Fluctuations

“…Water content in the collapsed phase. To set up a system of collapsed PNIPAM polymers (20monomer-long atactic chains) above the LCST with the same chemical potential of water in as in bulk water, we first construct a system with two distinct phases [35,61]: a collapsed amorphous polymer phase in one part of the box, forming a membrane, and a polymer-free water reservoir in the other, as shown in Figure 1a. We use a novel, recently introduced, OPLS-based force field [62] for the PNI-PAM polymers, which better captures the thermoresponsiveness than the standard OPLS-AA [58].…”

Transfer Free Energies and Partitioning of Small Molecules in Collapsed PNIPAM Polymers

“…To briefly recap, the PNIPAM chains are composed of 20 monomeric units with atactic stereochemisty (i.e., with random distribution of monomeric enantiomers along the chain). For PNI-PAM polymers we adopt the recent OPLS-based force field by Palivec et al [42] with an ad hoc parametrization of partial charges, which reproduces the thermoresponsive properties much better than the standard OPLS-AA force field, even though the latter one used to be very popular for PNI-PAM simulations [37,[43][44][45][46][47][48]. For water we use the SPC/E water model [49], and the OPLS-AA force field [50,51] for solute molecules.…”

Molecular description of the coil-to-globule transition of Poly(N-isopropylacrylamide) in water/ethanol mixture at low alcohol concentration

“…As a result, in the last years several experimental and numerical studies have been carried out to investigate PNIPAM chains and microgels solution behavior [1,10]. The LCST/VPTT control can be achieved by variations of chemical [11] and stereochemical [12,13,14] composition of the polymer and by variations of the aqueous suspension medium, concerning ionic strength [15,16,17] and cosolute [18,19] or cosolvent addition, the latter being one of the simplest and less expensive ways to decrease the transition temperature [20,21,22]. In particular, the presence of ethanol as water cosolvent has a drastic impact on PNIPAM solution behavior, since this polymer, soluble in ethanol at all temperatures and in water at T ≤ 305 K, is actually insoluble in ethanol/water mixtures having an ethanol mass fraction approximately from 20% to 60%, i.e.…”