Geometrical Confinement Modulates the Thermoresponse of a Poly(N-isopropylacrylamide) Brush

Abstract

The structure of grafted-from poly(N-isopropylacrylamide) (PNIPAM) brushes is investigated as a function of confining stress and system temperature using neutron reflectometry (NR), numerical self-consistent field theory, and the reanalysis of colloid probe atomic force microscopy (AFM) data from the literature. For NR experiments, confinement is achieved using a custom-made sample environment, and the corresponding reflectometry data are analyzed using a novel “distribution model.” The NR and AFM experiments probe similar temperature–stress combinations and generally find qualitative agreement, with some variations highlighting path-dependent (isostress vs isothermal, respectively) behavior. All techniques indicate that confinement removes the critical transition point in the thermoresponse of PNIPAM and results in the brush assuming a block-like volume fraction profile with a uniform internal structure. The PNIPAM brushes recover from such treatment, regaining their thermoresponse upon resolvation. Understanding the structure of responsive polymer brushes under confinement is essential, as brush applications are often accompanied by a surface-normal force (i.e., lubrication) and brush properties are dependent on their structure.