Visualizing Phase Transition Behavior of Dilute Stimuli Responsive Polymer Solutions via Mueller Matrix Polarimetry

Abstract

Probing volume phase transition behavior of super-diluted polymer solutions both micro- and macroscopically still persist as an outstanding challenge. In this regard, we have explored 4x4 spectral Mueller matrix measurement and its inverse analysis for excavating the micro-architectural facts about stimuli responsiveness of 'smart' polymers. Phase separation behavior of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) and pH responsive poly(N,N-(dimethylamino)ethyl methacrylate) (PDMAEMA) and their copolymers were analyzed in terms of Mueller matrix derived polarization parameters, namely, depolarization (Δ), diattenuation (d) and linear retardance (δ). The Δ, d and δ parameters provided useful information on both macro- and micro-structural alterations during the phase separation. Additionally, the two step action ((i) breakage of polymer-water hydrogen bonding and (ii) polymer-polymer aggregation) at molecular micro-environment during the cloud point generation were successfully probed via these parameters. It is demonstrated that in comparison to the present techniques available for assessing the hydrophobic-hydrophilic switch over of simple stimuli-responsive polymers, Mueller matrix polarimetry offers an important advantage requiring few hundred times dilute polymer solution (0.01 mg/mL, 1.1 - 1.4 micromolar) at a low-volume format.