Computational Modeling of the Temperature-Induced Structural Changes of Tethered Poly(N-isopropylacrylamide) with Self-Consistent Field Theory

Abstract

We modeled the effects of temperature, degree of polymerization, and surface coverage on the equilibrium structure of tethered poly(N-isopropylacrylamide) chains immersed in water. We employed a numerical self-consistent field theory where the experimental phase diagram was used as input to the theory. At low temperatures, the composition profiles are approximately parabolic and extend into the solvent. In contrast, at temperatures above the LCST of the bulk solution, the polymer profiles are collapsed near the surface. The layer thickness and the effective monomer fraction within the layer undergo what appears to be a first-order change at a temperature that depends on surface coverage and chain length. Our results suggest that as a result of the tethering constraint, the phase diagram becomes distorted relative to the bulk polymer solution and exhibits closed loop behavior. As a consequence, we find that the relative magnitude of the layer thickness change at 20 and 40 °C is a nonmonotonic function of surface coverage, with a maximum that shifts to lower surface coverage as the chain length increases in qualitative agreement with experiment.

Modeling lower critical solution temperature behavior of associating polymer brushes with classical density functional theory

“…from 305 K to 297 K. However, we find the LCST changes little with an increase chain length from 100 to 200. In addition, the swelling ratio increases monotonically from about 1.6 to 1.9 with increasing N A from 50 to 200, which is partially consistent with Mendez et al 's SCFT results 27 and Yim et al's experiments. [8][9][10] The partially collapsed structure is not found at N A = 200.…”

“…The effect of grafting density on the LCST is qualitatively consistent with Ishida and Biggs's experiments. 12 The effect of grafting density on the swelling ratio is partially consistent with the viewpoint that intermediate grafting density and high molecular weight gives maximum swelling ratio change with temperature, which was shown by Mendez et al's SCFT results 27 and Yim et al's experiments. [8][9][10] At even higher grafting density ρ g σ 2 = 0.2, due to appearance of partial-collapsed structure, the swelling ratio curve first has a discontinuity at 285 K (from extended to partial-collapsed), and then continuously decreases until a small step at 299 K (from partial-collapsed to extended).…”

“…While there are a large number of experimental works on PNIPAAM brushes, relatively few theoretical [23][24][25][26][27][28] and simulation 29 studies have been conducted to explain the LCST behavior of PNIPAAM brushes. Lee et al 29 performed molecular dynamics simulation to investigate the deswelling mechanism of PNIPAAM brushes in water.…”

“…The model predicted a bilayer structure at the higher grafting density and a single layer profile at lower grafting density. Mendez et al 27 used self-consistent field theory (SCFT) to study the effects of temperature, molecular weight, and grafting density on the equilibrium structure of PNIPAAM brushes. An empirical χ parameter obtained from the bulk phase diagram of PNIPAAM solutions was used as the input of their theory.…”

Density Effects on Collapse, Compression, and Adhesion of Thermoresponsive Polymer Brushes

“…(i) for all densities, α decreases gradually as T increases, displays a stronger temperature sensitivity in the range [30][31][32][33][34][35] • C, and reaches, for T 35 • C, a constant value close to unity.,…”