889. A study of the supercooling of drops of some molecular liquids

Abstract

The extent to which various liquid drops may be supercooled has been observed by examining the appearance of clouds of such drops in a beam of light a t different temperatures. For most of the liquids studied the great majority of the drops did not freeze until a comparatively narrow range of temperature had been reached far below the melting point. In this range where the drops freeze spontaneous homogeneous nucleation is assumed to occur. This is discussed in the light of current theories of nucleation phenomena in which a key role is played by the interfacial tension between solid and liquid. An attempt is made to determine values of this quantity, and correlate them with the corresponding heats of fusion.IN recent years measurements have been made, particularly by Turnbull and his colleagues, on the extent to which liquid metals will supercool in the absence of catalysts (Turnbull and Cech, J . AppZ. Physics, 1950, 21, 804). An essential feature of these experiments was that observations were made on numerous, isolated drops, in the expectation that while some of these might have occluded impurities the majority should not. Experiments on the same lines with molecular liquids with the single exception of water have not been described. In this paper we present some observations on the supercooling of some common organic liquids and of some liquefied gases. An attempt will be made to interpret these on current theories of homogeneous nucleation. EXPERIMENTALThe problem was to find the temperature a t which a large number of isolated drops of liquid froze. One may in principle either examine a cloud of drops simultaneously or else a large number of individual drops. An unsuccessful attempt was made to follow the latter course, by adapting Rumpf and Seigl's method (2. Physik, 1938, I l l , 301), in which individual drops were held stationary and observed in a cold stream of air flowing upwards. It was found that it was not easy to hold a drop stationary, that the drops steadily evaporated, and that freezing could not readily be detected.The method had been used for water, though few experimental details were given (Schaefer, Bull. Auner. Mat. SOC., 1948, 29, 175), and it was reported that the water clouds froze completely over the range -39°f0-10.Consequently, a cloud method was adopted.

Effect of self-nucleation on crystallization and melting behavior of polypropylene and its copolymers

“…9 The material constants for i-PP used in the analysis are listed in Table III. The fold-surface free energy, e , for self-nucleated block propylene-ethylene copolymer and i-PP are listed in Table IV.…”

Crystallization Kinetics of Poly(butylene 2,6-naphthalate) and Its Copolyesters

“…10 Hence, it is considered that CJ and CJe values of the copolyesters are equal to those of the pure PBN homopolymer, respectively. The lateral surface free energy CJ is related with the heat of fusion 11H as the following Thomas-Stavely relation 20 CJ= a!1H(a0b 0 ) 112 (8) where a is an empirical constant and a 0 is the chain stem width. The heat of fusion of copolymer can be considered as the sum of the heat of fusion of homopolymer and the heat of mixing of the pure melt with a noncrystallizable component.…”

Analysis of crystallization kinetics of poly(ether ether ketone) by a nonisothermal method

“…The ratio of G 0 (III)/G 0 (II) for the best fits is about 1.4 ϫ 10 3 . In the past, most assessments of the surface free energies, , have relied on the use of the modified Thomas-Staveley equation, 11 where the value ␣ ϭ 0.1 was assumed to be universal. This expression is…”

“…This microscopic study also showed that there was no morphological change between regimes II and III. Finally, the ␣ value in the modified Thomas-Staveley equation 11 will be discussed.…”