Deswelling Mechanisms of Surface-Grafted Poly(NIPAAm) Brush: Molecular Dynamics Simulation Approach

Abstract

Technologies ranging from solvent extraction and drug delivery to tissue engineering are beginning to benefit from the unique ability of “smart polymers” to undergo controllable structural changes in response to external stimuli. The prototype is poly(N-isopropylacrylamide) (P(NIPAAm)) which exhibits an abrupt and reversible hydrophilic to hydrophobic transition above its lower critical solution temperature (LCST) of ∼305 K. We report here molecular dynamics simulations to show the deswelling mechanisms of the hydrated surface-grafted P(NIPAAm) brush at various temperatures such as 275, 290, 320, 345, and 370 K. The deswelling of the P(NIPAAm) brush is clearly observed above the lower critical solution temperature below which the P(NIPAAm) brush is associated with water molecules stably. By simulating the poly(acrylamide) brush as a reference system having the upper critical solution temperature (UCST) behavior with the same conditions employed in the P(NIPAAm) brush simulations, we confirmed that the deswelling of P(NIPAAm) brush does not take place at a given range of temperatures, which validates our simulation procedure. By analyzing the pair correlation functions and the coordination numbers, we found that the dissociation of water from the P(NIPAAm) brush occurs mainly around the isopropyl group of the P(NIPAAm) above the LCST because of its hydrophobicity. We also found that the NH of the amide group in NIPAAm does not actively participate in the hydrogen bonding with water molecules because of the steric hindrance caused by the attached isopropyl group, and thereby the hydrogen bonding interactions between amide groups and water molecules are significantly weakened with increasing temperature, leading to deswelling of the hydrated P(NIPAAm) brush above the LCST through favorable entropic change. These results explain the experimental observations in terms of a simple molecular mechanism for polymer function.

Hairy polymeric nanocapsules with ph-responsive shell and thermoresponsive brushes: Tunable permeability for controlled release of water-soluble drugs

“…Leckband et al investigated that the end‐grafted low molecular weight PNIPAm chian ( M w = 2500, DP = 22) with grafting density of 0.256 chains nm −2 did not collapse above the LCST . The influence of the brush structure parameters, such as M n , σ , and d g , on the temperature‐sensitive swelling behavior of PNIPAm brushes has been investigated . It was confirmed that PNIPAm chain grafted onto a substrate with high graft density can be further stretched in good solvents .…”

Nanophase-Segregated Structures and Transport Properties in Sulfonated Poly(arylene ether sulfone) Multiblock Copolymer Membrane for Proton Exchange Membrane Fuel Cell

“…For the full-atomistic MD simulations, the DREID-ING force field 75 is used as previously used to study Nafion 29 68 69 76-79 and Dendrion 51 52 as well as various molecular systems such as hydrogels, [80][81][82][83][84][85] liquid-liquid and liquid-air interfaces 86 87 and molecular self-assembly. [88][89][90][91][92][93][94][95] The water is described using the F3C force field.…”

Modeling Swelling Behavior of Thermoresponsive Polymer Brush with Lattice Density Functional Theory

“…It is very difficult for molecular simulation to obtain the relationship between the polymer brush conformation change and the bulk phase diagram of polymer solutions. Lee et al 25 reported MD simulations to show the deswelling mechanisms of the hydrated surface-grafted PNIPAM brush at various temperatures. They observed that, at the temperatures above LCST (∼305 K), the water molecules are departed from the brush to form separated water phase, and simultaneously, the brush is collapsed.…”