Thermodynamic, crystallographic, and dielectric study of the nature of glass transitions in cyclo-octanol

Abstract

The stable solid polymorphism of cyclooctanol (C 8 H 16 O, for short C 8-OH) is revealed to be a complex problem and only two stable solid phases, denoted on cooling from the liquid as phases I and II, are found using static ͑thermodynamic and x-ray diffraction͒ as well as dynamic ͑dielectric spectroscopy͒ experimental techniques. Both solid phases are known to exhibit glass transitions if they are cooled down fast enough to prevent transition to ordered crystalline states. Although glass transitions corresponding to both phases had been well documented by means of specific heat measurements, x-ray measurements constitute, as far as we know, the first evidence from the structural point of view. In addition, a great amount of dielectric works devoted to phase I and its glass transition, were published in the past but next to nothing relating to the dielectric properties of phase II and its glass transition. The nature of the disorder of phase II will be discussed.

Influence of internal flexibility on the double glass transition in a series of odd non-symmetric liquid crystal dimers characterised by dielectric measurements

“…Temperature was controlled by means of a System Quatro from Novocontro with stabilization at different temperatures with a precision of the order of 20 mK. Additional details of the experimental technique can be found elsewhere [39,40].…”

Thermodynamic and Kinetic Fragility of Freon 113: The Most Fragile Plastic Crystal

“…Such plastic phases are formed from the liquid and can be supercooled, giving rise to the so-called orientational glasses (OG) or "glassy crystals" [6][7][8][9]. They show typically low fragility, as cyclooctanol (m ¼ 33) [10,11] …”

“…Such plastic phases are formed from the liquid and can be supercooled, giving rise to the so-called orientational glasses (OG) or "glassy crystals" [6][7][8][9]. They show typically low fragility, as cyclooctanol (m ¼ 33) [10,11], cycloheptanol (m ¼ 22) [12], ortho-carborane (m ¼ 20) [13], cyano-adamantane (m ¼ 17) [9,14], adamantanone (m ≈ 16) [9,15], ethanol (m ¼ 48) [9], or mixed molecular crystals NPA 0.7 NPG 0.3 fðCH 2 OHÞCðCH 3 Þ 3 g 0.7 fðCH 2 OHÞ 2 CðCH 3 Þ 2 g 0.3 (m¼30) [16][17][18]. The most fragile OG known to date are the Freon 112 (CCl 2 F-CCl 2 F) with m ¼ 68 [19] and a co-crystal of succinonitrile (60%) and glutaronitrile (40%) with m ¼ 62 [20].…”

α and β relaxation dynamics of a fragile plastic crystal

“…Concerning the dynamics of OD phases, dielectric spectroscopy has been revealed to be a useful tool to understand the complex dynamics of these phases [7,8,9,10,11,12,13].…”

“…But a mesophase can exist between the completely ordered crystalline phase and the translationally and orientationally disordered liquid phase, the so called plastic phase or orientationally disordered (OD) phase [2,3]. In the plastic phase, the centers of mass of the molecules have spatial long range order, forming a lattice which generally has high symmetry (such as cubic, quasi-cubic or rhombohedral [2, 3, 4]), but there is only short-range order with respect to the orientational degrees of freedom [5].As for glass-forming liquids, the typical phenomenology of a glass transition also can be realized in various plastic crystals when the temperature is decreased, but in this case only the orientational degrees of freedom are frozen, yielding the formation of a "glassy crystal"[6], also called OD glass.Concerning the dynamics of OD phases, dielectric spectroscopy has been revealed to be a useful tool to understand the complex dynamics of these phases [7,8,9,10,11,12,13].Based on these works there seem to be some general dynamic features of OD phases and its glasses: They are rather strong (following the definition of Angell [14]) and they follow the Böhmer relation between non-exponentiality of the α relaxation and fragility [15,16].On the contrary, the so called Nagel scaling [17] does not seem to work for these phases [7,11,18]. In addition there is only a weak or no β-relaxation at all and the excess wing, showing up as a second power law at the high frequency flank of the α peak in many canonical glass formers [17,19,20,21], is either absent in plastic crystals [11] or can be ascribed to a weak secondary relaxation [7,9].…”