Secondary relaxations of orientationally disordered mixed crystals at temperatures lower than the glass transition temperature

Abstract

Low‐molecular weight cyclic alcohols as cycloheptanol (C7H14O, hereinafter referred to as cC7‐ol) and cyclooctanol (C8H16O, cC8‐ol) are prototypical materials displaying OD phases which, under fast cooling give rise to orientational glasses (OG). In addition to the ubiquitous α‐relaxation of canonical glasses, several secondary relaxations appear for the mentioned systems (β and γ for cC8‐ol and β for cC7‐ol). The intramolecular character of these secondary relaxations for these materials as well as their mixed crystals was highlighted at temperatures close but above the glass transition. For lower temperatures the low values of dielectric strength makes difficult to account for the relaxation times obtained from the permittivity losses and, thus in this work we present a data analysis based on the Kramers–Kronig relations which connect the real and imaginary parts of dielectric permittivity and shows up a new method to make evident the existence of such secondary relaxations as well as to avoid phenomenological equations for determining the relaxation time.

Pretransitional Effects of the Isotropic Liquid–Plastic Crystal Transition

“…For plastic crystalline materials when cooling below the I-ODIC freezing/melting temperature the translational ordering appears but the orientational freedom remains. Experimental evidence from previously published results [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ] seems to be clear: there are no pretransitional behavior in the surrounding of , contrary to the discussed above case of LC materials. Note that on cooling ODIC forming materials most often terminate in the orientationally disordered solid glass state.…”

“…Experimental evidences from previously published results [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] seems to be clear: there are no pretransitional behavior in the surrounding of , contrary to the discussed above case of LC materials. Note that on cooling ODIC forming materials most often terminate in the orientationally disordered solid glass state.…”

“…Indeed, strong pretransitional changes of the Kerr effect (KE) [ 4 , 5 , 6 , 10 , 11 ] and Cotton Mouton effect (CME) [ 4 , 5 , 6 , 12 , 13 ] in the isotropic liquid phase of nematic LCs inspired development of the Landau-de Gennes (LdG) model [ 4 , 5 , 6 , 14 , 15 ], representing important cornerstone for theoretical description of LCs [ 3 , 4 , 5 , 6 , 7 ] and soft matter physics in general [ 16 , 17 ]. On the contrary, in ODIC-forming systems the existing evidence indicates negligible weak pretransitional effect [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ], suggesting the behavior found in systems exhibiting classical liquid-crystal discontinuous phase transitions [ 36 , 37 , 38 , 39 , 40 , 41 ]. The only exception is reported for the optical Kerr effect (OKE) pretransitional effect of in p -terphenyl (1993, [ 42 ]), which could be rather linked to the hidden isotropic-nematic (I-N...…”

“…Such behavior is observed in dielectric constant and its strong electric field counterpart, the nonlinear dielectric effect (NDE), both surprisingly not tested so far. Phase behavior was probed in the extreme range of temperatures in cyclooctanol (C 8 OH), one of the most classical ODIC-forming materials [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]. As a reference LC system, we studied a rod-like LC material, n -octyloxycyanobiphenyl (8OCB), which also revealed new aspects of pretransitional effects.…”

“…Phase behavior was probed in the extreme range of temperatures in cyclooctanol (C8OH), one of the most classical ODIC-forming materials. [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] As a reference LC system, we studied a rod-like LC material, noctyloxycyanobiphenyl (8OCB), which also revealed new aspects of pretransitional effects.…”