Effect of microwave irradiation on the hydroxypropyl methylcellulose powder and its hydrogel studied by Magnetic Resonance Imaging

Proton Magnetic Resonance Imaging (MRI) has been used to study the physical changes of hydroxypropyl methylcellulose (HPMC) hydrogels due to microwave irradiation of the polymer. The proton one-dimensional images, derived from relaxation, spin density and diffusion-weighted spin-echo experiments, provide insights on the dynamics of water and the motional state of the polymer inside the hydrogels. The obtained results indicate that the microwave irradiation causes the breaking of the HPMC polymer–hydrogen bonding network in HPMC powder which influences the dynamics of water and polymer chains within its hydrogels. In order to explain the observed spin–spin relaxation time in hydrogels composed of irradiated polymers, we have to assume that in addition to a fast exchange between the free and bound state of water also a rapid proton exchange between the water protons and the hydrogen of hydroxyl groups of side chains of the monomeric units of HPMC polymer is a major transverse relaxation mechanism.