Measurement of the nuclear spin diffusion coefficient in organic glasses doped with paramagnetic centers: Orthoterphenyl and polymeric glasses

Abstract

We have recently developed a method of studying spin diffusion coefficients by doping the materials with paramagnetie centers and measuring the nuclear relaxation in a ti[ted rotating ffame. Using this method, we measure here the spin diffusion eoefficient of orthoterphenyl, a molecular organic glass, and of three polymer glasses: poly(4-vinylpyridine), poly(vinylacetate) and poly(methyl methacrylate). We explore a possible dependence of the measured orthoterphenyl spin diffusion coeffieient on the eleetronic relaxation time and concentration of the paramagnetic centers. We conclude that the experiments can be performed at higher concentrations than previously thought. We also show that our method applies to polymers in the glassy state ir one works at sufficiently small tilt angle, in spite of a short value of T o. We had anticipated that the distribution of proton pairs in these materials precludes the standard dependence of the spin diffusion coefficient on the proton density and free induction decay characteristic decay time. Our results fully confirm such expectation.

Measurement of Spin Diffusion Coefficients in Glassy Polymers: Failure of a Simple Scaling Law

“…ii) The good agreement for D of PVAc with our previous measurement. [22] iii) The only slightly reduced value for PVAcd 3 , compared to PVAc, which stresses the fact that the rapidly rotating methyl groups have a small contribution to the spin diffusion process. The chain to chain diffusion pathway relies then on occasional short distances between protons carried by main chain carbons of two different chains.…”

“…Our method has also been shown to lead to consistent results when working on a glassy supercooled organic liquid, ortho-terphenyl, with paramagnetic dopants of widely different T el and concentrations. [22] It has therefore been applied to some glassy polymers, and large deviations from the scaling law [Equation (1)] have been observed, as shown in Table 1, taken from ref. [22] It is interesting to point that in the case of PMMA-PI block copolymers, the above value for D PMMA would reduce significantly the discrepancy between the NMR domain size determinations and the theoretical values estimated according to the Helfand-Wassermann theory.…”

“…[22] It has therefore been applied to some glassy polymers, and large deviations from the scaling law [Equation (1)] have been observed, as shown in Table 1, taken from ref. [22] It is interesting to point that in the case of PMMA-PI block copolymers, the above value for D PMMA would reduce significantly the discrepancy between the NMR domain size determinations and the theoretical values estimated according to the Helfand-Wassermann theory. [15] Meanwhile, two other methods have been proposed to measure directly D in polymers without any dopants.…”

“…Spin diffusion coefficients and scaling parameters for glassy polymers. [22] Polymer a) From PS-PMMA block-copolymers of known structure. [4] increasing diffusion lengths, and in particular on the fact that ''long range spin diffusion can appear faster because the most efficient paths of magnetization transfer, for instance along polymer backbone with their relatively uninterrupted sequences of protons, permit for relatively fast magnetization exchange''.…”

“…[21,22] Magnetization spin lock at an angle u was obtained by a frequency jump of the radiofrequency field. [32] Its amplitude must therefore stay stable over several hundred milliseconds spin lock durations.…”