Understanding molecular confinement in polymeric nanoporous materials via infrared spectroscopic measurements

Abstract

The existence of molecular confinement in nanoporous polymeric samples is a discussion with notable impact that has been opened in the past decade. This work aims to shed more light on the topic and demonstrate molecular confinement with an additional analytical method. For this purpose, nanoporous polymeric samples were prepared out of PMMA from different commercial grades and foamed via gas dissolution foaming. The molecular confinement in these samples is demonstrated by using Attenuated Total Reflectance (ATR) FTIR spectroscopy in microscopic mode and following the changes in both spectral peak areas and peak shifts with changes in the pore wall thickness. Utilizing its high precision, the shortening of bond lengths due to confinement is evaluated via peak shifts. ATR is compared to previous functional techniques, namely Raman spectroscopy and DSC. The results prove ATR effective in both cases, and reveal that confinement causes changes in the polymeric chains within pore walls with thicknesses of less than $100\mathrm{nm}$. For these cases, ATR and Raman spectroscopy demonstrate that shortening of atomic bond lengths affects molecules' polarizability and causes overall chain immobilization.