S. Ganapathy

LCST in poly(N-isopropylacrylamide) copolymers: high resolution proton NMR investigations

A series of copolymers of N-isopropylacrylamide (NIPA) and acryloyl amino acids were synthesized and conjugated with p-aminobenzamidine (PABA). The values of lower critical solution temperatures (LCSTs) of these polymers estimated by 1 H NMR were in agreement with those estimated by the turbidometric methods. The apparent shifts in the peak positions for the polymer with increasing temperature reported in the past are shown to be artifacts resulting from the deuterium lock experiment. The 1 H- 1 H NOESY experiments were carried out to distinguish the interactions between the protons in: (a) 6-amino caproic acid (6ACA) and NIPA and (b) PABA and NIPA. These explain the variation in the LCST of the polymers based on the contributions of the two monomers to the magnitude of the polymer-polymer interactions. The findings are further supported by proton spin-lattice relaxation time measurements.

Unusual retardation and enhancement in polymer dissolution : role of disengagement dynamics

This work addresses the issue of resolution of some unusual observations in the swelling-dissolving problem in polymers. There are observations showing several fold increase in the rate of dissolution when the residual solvent content is increased only marginally. Additionally, by using an in situ NMR technique to study dissolution, we have found the existence of an intermediate plateau in the dissolution curve indicating an unusual retardation in the rate of dissolution under certain conditions. Interestingly, this plateau disappears on either increasing the molecular weight of the polymer or on doing dissolution under shearing rather than under stagnant conditions. An improved model is proposed to explain these results, which cannot be explained by the existing models. The new feature is the incorporation of a kinetic model to relate the disengagement rate to the swelling rate, through the changing mobility of the disengaging macromolecule at the gel-liquid interface. The present study thus enables a clearer elucidation of the role of disengagement dynamics in the dissolution process.

Proton magnetic resonance imaging in hydrogels: volume phase transition in poly(N-isopropylacrylamide)

Proton Magnetic Resonance (PMR) Imaging has been used to study the volume-phase-transition in a thermoreversible gel. This is demonstrated in the LCST polymer poly(N-isopropylacrylamide), swollen in water. The volume-phase-transition is conveniently monitored through one-dimensional proton images. PMR images, derived from relaxation and diffusion weighted planar spin-echo experiments, provide insights on the hydration state of water and the motional state of polymer, in the fully swollen and collapsed states of the gel.

Self-diffusion of water in thermoreversible gels near volume transition: Model development and PFG NMR investigation*

Pulsed filed gradient spin echo (PFG-SE) NMR technique has been used to measure the self-diffusion coefficient (Dself) of water in a thermoreversible hydrogel, Poly(N-isopropylacrylamide). Dself, was measured as a function of temperature (−5–50°C) and hydration level. A theoretical model to predict Dself was built by using a combination of lattice fluid hydrogen bond theory and free volume theory. The fully swollen gel did not show any discontinuous change in Dself with temperature, Whereas, a partially swollen gel, showing the existence of two kinds of water having diffusivities differing by order of magnitude, exhibits a discontinuous change in Dself. These results were explained within the theoretical framework developed by us. The present work thus enables an elucidation of diffusion phenomenon at the volume transition point for the first time.

1H MAS n.m.r. and two-dimensional nuclear Overhauser enhancement spectroscopy in hydrogels☆

Abstract The combination of 1H magic angle spinning (MAS) n.m.r. and two-dimensional nuclear Overhauser enhancement spectroscopy (NOESY) is shown to be readily applicable to hydrogels. This is demonstrated in the superabsorbing polymer, hydrolysed starch-g-poly(acrylonitrile), which readily responds to hydration. Enhanced molecular mobility of the polymer is directly revealed by polymer-polymer NOESY connectivities. The two-dimensional experiment also reveals the presence of an efficient cross-relaxation mechanism between the water of hydration and the polymer.