Lithuanian Journal of Physics article / Lietuvos fizikos žurnalo straipsnis

CP MAS KINETICS AND IMPEDANCE SPECTROSCOPY STUDIES OF LOCAL DISORDER IN LOW-DIMENSIONAL H-BONDED PROTON-CONDUCTING MATERIALS
Laurynas Dagys^a,b, Sergejus Balčiūnas^c, Jūras Banys^c, Feliksas Kuliešius^a, Vladimir Chizhik^d, and Vytautas Balevičius^a
^aInstitute of Chemical Physics, Vilnius University, Saulėtekio 3, 10257 Vilnius, Lithuania
^bDepartment of Chemistry, University of Southampton, University Road, SO17 1BJ Southampton, UK
^cInstitute of Applied Electrodynamics and Telecommunications, Vilnius University, Saulėtekio 3, 10257 Vilnius, Lithuania
^dFaculty of Physics, Saint Petersburg State University, Ulianovskaya 1, 198504 Saint Petersburg, Russia
Email: vytautas.balevicius@ff.vu.lt

Received 29 May 2019; revised 3 June 2019; accepted 25 June 2019

The ¹H–¹³C cross-polarization magic angle spinning (CP MAS) kinetics was studied in poly(vinyl phosphonic acid) (pVPA), i.e. material with high degrees of freedom of proton motion along H-bonded chains. It has been shown that the CP kinetic data for the adjacent ¹H–¹³C spin pairs can be described in the frame of the isotropic spin-diffusion approach. The rates of spin diffusion and spin-lattice relaxation as well as the parameters accounting for spin coupling and the effective size of spin clusters have been determined. The local order parameter S ≈ 0.63±0.02, determined as the ratio of the measured dipolar ¹H–¹³C coupling constant and the calculated static dipolar coupling constant, is significantly lower than the values deduced for related sites in other polymers and in series of amino acids. This means that the local disorder of the C–H bonds in pVPA is between those for rather rigid C–H bond configurations having S = 0.8–1.0 and highly disordered –CH₃ groups (S ~ 0.4). This effect can be attributed to the presence of the proton transfer path where proton motion is easy to activate. The activation energy for the proton motion E_a = 59±7 kJ/mol was determined from the impedance spectroscopy data analysing the temperature and frequency dependences of the complex dielectric permittivity of pVPA. The rates of proton spin-lattice relaxation and spin diffusion are of the same order and both run in the time scale of milliseconds.

Keywords: solid state NMR, cross-polarization, impedance spectroscopy, proton conductors, poly(vinyl phosphonic acid)
PACS: 33.25.+k, 82.56.-b

LOKALIOSIOS NETVARKOS ŽEMOS DIMENSIJOS H-RYŠIO PROTONŲ LAIDININKUOSE TYRIMAI CP MAS KINETIKOS IR IMPEDANSO SPEKTROMETRIJOS METODAIS
Laurynas Dagys^a,b, Sergejus Balčiūnas^c, Jūras Banys^c, Feliksas Kuliešius^a, Vladimir Chizhik^d, Vytautas Balevičius^a

^aVilniaus universiteto Cheminės fizikos institutas, Vilnius, Lietuva
^bSoutamptono universiteto Chemijos katedra, Soutamptonas, Jungtinė Karalystė
^cVilniaus universiteto Taikomosios elektrodinamikos ir telekomunikacijų institutas, Vilnius, Lietuva
^dSankt Peterburgo valstybinio universiteto Fizikos fakultetas, Sankt Peterburgas, Rusija

Ištirta ¹H–¹³C CP (kryžminės poliarizacijos) MAS (magiško kampo sukimo) kinetika, vykstanti poli(vinyl fosfoninėje rūgštyje) (pVPA), t. y. medžiagoje, kuriai būdingas didelis protonų judėjimo išilgai vandenilinių ryšių (H-ryšių) grandinės laisvės laipsnių skaičius. Dėl šios savybės pVPA priskiriama medžiagų klasei, vadinamajai protonų laidininkei. Parodyta, kad šioje medžiagoje vykstančios CP MAS kinetikos eksperimentiniai duomenys gali būti aprašyti taikant izotropinės sukinių difuzijos modelį. Nustatytos sukinių difuzijos bei sukinių ir gardelės relaksacijų spartos, taip pat parametrai, nusakantys ¹H ir ¹³C sukinių sąveiką ir efektyviuosius sukinių spiečių matmenis. Lokaliosios tvarkos parametras S ≈ 0,63±0,02, apskaičiuotas remiantis eksperimentiškai išmatuotos dipolinės ¹H–¹³C sąveikos konstantos ir apskaičiuotosios statinės konstantos vertėmis, yra daug mažesnis už S vertes, kurios aptinkamos daugelyje giminingų molekuliniams fragmentams polimerų ir amino rūgščių serijose. Lokalioji C–H jungčių netvarka pVPA užima tarpinę padėtį tarp stingiųjų C–H konfigūracijų, kurioms yra būdingos S vertės, artimos 1,0 (S = 0,8–1,0), ir metilo (–CH₃) grupių, kurių didelė netvarka (S ∼ 0,4) atsiranda dėl lengvai aktyvuojamo šių grupių sukimosi. Galima teigti, kad nemažas C–H jungčių tvarkos parametro sumažėjimas yra nulemtas intensyvaus rūgštinių protonų judėjimo išilgai H-ryšių grandinės. Impedanso spektroskopijos metodu nustatyta šio judesio aktyvavimo energija yra E_a = 59±7 kJ/mol. Sukinių difuzijos bei sukinių ir gardelės relaksacijos spartos tirtajame polimere yra tos pačios eilės, ir šie vyksmai prateka milisekundžių skalėje.

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