A comparative dielectric spectroscopy investigation of beta-cyclodextrin polymer and carboxymethyl beta-cyclodextrin polymer cross-linked by epichlorohydrin

Abstract

The β-cyclodextrin polymer (β-CDP) and the carboxymethyl-β-cyclodextrin polymer (CM-β-CDP) cross-linked by epichlorohydrin were investigated by dielectric spectroscopy over the frequency range 0.1–100 kHz in the temperature ranges 132.6–464.8 K and 132.9–500.9 K, respectively. The β-CDP reveals a reversible order-disorder transformation of some normal hydrogen bonds to flip-flop type at Ttrans = 220.1 K, whereas the CM-β-CDP doesn't show any transformation of hydrogen bonds. The ac-conductivity plot of β-CDP (lnσ vs 1/T) shows a linear part obeying the Arrhenius equation in the range 289.8–268.8 K with Eα = 0.71 eV during cooling and 0.77 eV during heating. A deformed bell-shaped curve with maximum values at Ttrans = 222.7 K during cooling and 223.8 K during heating in the range 132.6–268.8 K, is due to the hydrogen-bonded transformation. At 328.9 < T < 407 K the ac-conductivity presents a bell-shaped curve with maximum value located at 363.1 K, revealing a new order-disorder transformation. The breaking and the restoration of more stable hydrogen bonds are confirmed by the temperature variation of relaxation time, as three successive topical maxima with values 8.9 μs (223.8 K), 8.9 μs (307.6 K) and 11.9 μs (369.7 K). Above 407.0 K, lnσ increases linearly according to Arrhenius law with Eα = 0.26 eV because of the newly-formed hydrogen-bonded chains. In the case of CM-β-CDP the lnσ vs 1/T plot reveals two linear regions where the Arrhenius equation is valid. Τhe first in the range 252.3–288.2 K (Eα = 0.28 eV cooling or Εα = 0.33 eV heating) is due to the protons originated from the β-CD hydroxyl groups and water molecules while the second in the range 316–349 K (Eα = 0.78 eV heating) is due to the carboxymethyl moieties dissociation. The relaxation time τ of CM-β-CDP increases from 7.5 μs (132.9 K) to 9.5 μs (285.4 K) and then rapidly reaches the value 11.8 μs at 293.7 K denoting the no breaking of hydrogen bonds. Cole-Cole plots of both samples are presented at selective temperatures.