Amorphous second-order transition

Several new thermal analytical techniques are potentially valuable for the study of second-order transitions in the characterization of amorphous solids and for the accurate determination of glass transition temperatures. These modem techniques can detect and characterize glass transitions and other second-order transitions that are not detectable by conventional thermal analytical techniques such as DSC, TGA, or TMA. 

Phase transitions, whether first-order or second-order, are potent sources of instability of solid drugs and can usually be detected and studied by thermal methods of analysis (e.g., DSC, TGA, TMA, ODSC, DMA, DEA). In crystalline solids, typical first-order transitions are polymorphic or desolvation transitions. In amorphous solids, second-order transitions, such as glass transitions, are common. 

The vapour deposition method is widely used to obtain amorphous solids. In this technique, atoms, molecules or ions of the substance (in dilute vapour phase) are deposited on to a substrate maintained at a low temperature. Most vapour-deposited amorphous materials crystallize on heating, but some of them exhibit an intervening second-order transition (akin to the glass transition). Amorphous solid water and methanol show such transitions. The structural features of vapour-deposited amorphous solids are comparable to those of glasses of the same materials prepared by melt-quenching. 

At least two different glass transition temperatures have been reported for PVdF homopolymer. Owing to the large proportion of crystalline structure in this polymer and the rapid crystallization which occurs while heating quenched amorphous samples, it is difficult experimentally to obtain an unambiguous, well-defined second-order transition. Mandel-kem, Martin, and Quinn (16) reported a value below — 40°C based upon an extrapolation of the Tg data for vinylidene fluoride-chlorotri-fluoroethylene copolymers in accordance with the Fox equation (6),... 

A second-order transition is defined as one in which the second derivatives of G and g with respect to temperature exhibit discontinuities at the transition temperature. Although glass-transition of amorphous foods has the properties of a second-order transition, there are no well-defined second-order transitions in foods (Roos, 1998). 

The significance of the amorphous regions in relation to the second order transition is shown clearly in the curves in Fig. 7.11. Orion, the most crystalline of the three fibres shows the least reduction of stiffness with increase of temperature. Dynel, a copolymer, with the least crystallinity exhibits the greatest reduction, and lying between these two is an experimental acrylic fibre specially prepared with an intermediate degree of orientation. Second order transition temperatures are polyethylene tere-phthalate (partly crystalline) 81 "C, nylon 66 (partly crystalline) 47°C, and polyacrylonitrile 81 "C. 

Crank, J. The Mathematics of Diffusion, 2nd Ed., Clarendon Press, Oxford, 1975. Gordon, M. and Taylor, J.S. Ideal copolymers and second order transitions in synthetic rubbers. I. Non-crystalline polymers, /. Appl. Chem., 2, 493,1952. Karel, M., Anglea, S., Buera, M.P., Karmas, R., Levi, G., Roos, Y. et al. Stability related transitions of amorphous foods, 246. pp. 249,1994. 

Polymers can exhibit a number of different conformational changes with each change accompanied by differences in polymer properties. Two major transitions occur at Tg, which is associated with local, segmental chain mobility in the amorphous regions of a polymer, and the melting point (Tjj), which is associated with whole chain mobility. The Tn is called a first-order transition temperature, and Tg is often referred to as a second-order transition temperature. The values for Tjj are usually 33 to 100% greater than for Tg, and Tg values are typically low for elastomers and flexible polymers and nigher for hard amorphous plastics. 

Fluoropolymers are semicrystalline polymers most do not exhibit glass transition in the conventional sense during which all crystalline structures are converted to the amorphous. The glass transitions of fluoroplastics have been described as molecular relaxation (conformational disorder) that takes place in the amorphous phase of the polymer. These temperatures are also called second order transitions their value depends on the technique and the frequency of energy addition to the polymer sample. Table 3.61 presents these temperatures and melting points of perfluorinated and partially fluorinated fluoroplastics. 

The description of the historic Gordon-T ay lor and Wood equations for the glass transition of solutions and copolymers can be found in Gordon M, Taylor IS (1952) Ideal Copolymers and the Second-order Transitions of Synthetic Rubbers. I. Noncrystalline Copolymers. J Appl Chem 2 493-500 Wood LA (1958) Glass Transition Temperatures of Copolymers. 1 Polymer Sci 28 319-330 for the relationship to the volume changes, see Kovacs AJ (1964) Glass Transition in Amorphous Polymers. Phenomenological Study. Fortschr Hochpolym Forsch 3 394-508. 

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