In situ rheo-NMR investigations of shear-dependent ¹H spin relaxation in polymer solutions

Abstract

The NMR response of a polymeric fluid to the application of shear field has been studied by using specially designed shearing devices conforming to a couette geometry. We show that an air-driven high-resolution liquids sample spinning arrangement, available on any high-resolution liquid-state NMR spectrometer, is easily adapted to set up a couette cell and impart shear rates (γ) of ca. 18-450 s^-1 on polymer solutions of low viscosity. This simple but elegant method has allowed us to conduct in situ rheo-NMR relaxation experiments on poly(acrylamide) solution where we observe large changes in proton spin-lattice relaxation times in the sheared state of the polymer. The enhanced relaxation rates are rationalized on the basis of the release of entanglements due to breaking of intermolecular hydrogen bonds, which decrease the rotational correlation times for the molecular motions. By identifying from the 2-D NOESY experiments that the molecular mobility occurs in the long correlation regime (ω₀τ_c »1), we gather evidence from in situ rheo-NMR measurements that the imposition of shear reduces the motional correlation time τ_c with increasing shear rate. We also find that the restoration of relaxation times, upon cessation of shear, is seen to occur over a much larger time scale (ca. 20 h). This has been interpreted by taking recourse to the postulate of the slow re-formation process in the framework of the energetically cross-linked transient network model (ECTN) proposed recently by our school.