CP MAS kinetics in soft matter: Spin diffusion, local disorder and thermal equilibration in poly(2-hydroxyethyl methacrylate)

Laurynas Dagys; Vaidas Klimkevičius; Vytautas Klimavicius; Kęstutis Aidas; Ričardas Makuška; Vytautas Balevicius

doi:10.1016/j.ssnmr.2019.101641

CP MAS kinetics in soft matter: Spin diffusion, local disorder and thermal equilibration in poly(2-hydroxyethyl methacrylate)

Solid State Nucl Magn Reson. 2020 Feb:105:101641. doi: 10.1016/j.ssnmr.2019.101641. Epub 2019 Dec 9.

Authors

Laurynas Dagys¹, Vaidas Klimkevičius², Vytautas Klimavicius³, Kęstutis Aidas⁴, Ričardas Makuška², Vytautas Balevicius⁵

Affiliations

¹ Institute of Chemical Physics, Vilnius University, LT-10257, Vilnius, Lithuania; Department of Chemistry, University of Southampton, SO17 1BJ, Southampton, UK.
² Institute of Chemistry, Vilnius University, LT-03225, Vilnius, Lithuania.
³ Institute of Chemical Physics, Vilnius University, LT-10257, Vilnius, Lithuania; Eduard-Zintl Institute for Inorganic and Physical Chemistry, University of Technology Darmstadt, D-64287, Darmstadt, Germany.
⁴ Institute of Chemical Physics, Vilnius University, LT-10257, Vilnius, Lithuania.
⁵ Institute of Chemical Physics, Vilnius University, LT-10257, Vilnius, Lithuania. Electronic address: vytautas.balevicius@ff.vu.lt.

PMID: 31887667
DOI: 10.1016/j.ssnmr.2019.101641

Abstract

The ¹H-¹³C cross-polarization magic angle spinning kinetics was studied in poly(2-hydroxyethyl methacrylate) (pHEMA), i.e. a soft material with high degrees of internal freedom and molecular disorder, having the purpose to track the influence of increasing local incoherent contributions to the effects of coherent nature in the open quantum spin systems. The experimental CP MAS kinetic curves were analyzed in the frame of the models of isotropic and anisotropic spin diffusion with thermal equilibration. The rates of spin diffusion and spin-lattice relaxation as well as the profiles of distribution of dipolar coupling, the parameters accounting the effective size of spin clusters and the local order parameters were determined. The intensities of the peaks of periodic quasi-equilibrium origin gradually decrease with increasing disorder, i.e. going from most ordered to more disordered sites in the polymer. Assuming that the thermal motion induced by the temperature gradients is much faster than the equilibration driven by spin diffusion due the difference in spin temperatures, it was deduced that the thermal equilibration in pHEMA occurs in the time scale of 10^-4 s. This is one order of magnitude faster than the spectral spin diffusion, which occurs between spins having different resonance frequencies. The thermal equilibration in the case of remote spin clusters was described by the 'stretched exponent' decay. This led to the fractal dimension D_p ≈ 1.65 for both carbon sites (quaternary and carbonyl). The obtained D_p value corresponds to the aggregates, which images are very similar to those for pHEMA and some other related polymer structures are usually conceived.

Keywords: Fractals; Solid-state NMR; Spin diffusion; Thermal equilibration; poly(2-hydroxyethyl methacrylate).