Glass Transitions of Solutions

The upper right curves in Fig. 7.72 display data of solutions of two aliphatic polyesters with polyepichlorohydrin. A large K2 is needed in S to account for the negative deviation of the solution with poly(butylene adipate), PBA, while the solutions with poly(ethylene adipate), PEA, show little redistribution in the contact neighborhood (K2 = 0). The lower right curves demonstrate that solutions of poly(vinyl chloride) and poly(e-caprolactone) can be fitted with all equations. The rather large Kj and K2 must cancel, since Kj = Kj = 0 leads to the Gordon-Taylor 

Plotting the glass transition temperatures of Fig. 7.73 as a function of concentration, yields Fig. 7.74. Only the Gordon-Taylor equation with a fitted constant represents the data. Similarly it is possible to fit with the Schneider equation with its two constants. Two additional equations, not in Fig. 7.69, are compared in Fig. 7.74 to the data one, is the Couchman equation, based on additivity of the products of ACp with the logarithm of T, the other uses a molar additivity of the logarithn of T. All equations without adjustable parameters do not fit the experimental data (o). 

Thermal analysis curves of solutions of polystyrene and poly(a-methylstyrene) at a heating rate of 20 K/min. 

---

The glass transition of solutes that remain amorphous during and after the freezing process can often be seen in the DSC thermogram as a shift in the baseline toward higher heat capacity. This is illustrated in the DSC thermogram of sucrose solution in Fig. 6, in which the glass transition is observed at — 34°C. 

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. 

Various thermal transitions can occur in rapidly cooled solutions, including glass transition, devitrification (ice formation on warming a rapidly-frozen solution) and melting of ice. The relationship between temperature, weight fraction of solids, solubility and glass transition of lactose is shown in Figure 7.16. 

The first data on polymer systems were collected via (laser-) light-scattering techniques [1] and turbidity measurements, further developed by Derham et al. [2,3]. Techniques based on the glass-transition of the polymer-blend constituents were also tested, such as DSC, Dynamic Mechanical Spectroscopy, and Dielectric relaxation [4]. Films made from solutions of... 

Both lactose and sucrose have been shown to crystallize in an amorphous form at moisture contents close to the glass transition temperature (Roos and Karel 1991a,b Roos and Karel 1992). When amorphous lactose is held at constant water content, crystallization releases water to the remaining amorphous material, which depresses the glass transition temperature and accelerates crystallization. These authors have done extensive studies on the glass transition of amorphous carbohydrate solutions (Roos 1993 Roos and Karel 1991d). 

Figure 5.4. A schematic state diagram showing water plasticization at increasing water weight fraction towards glass transition of water at -135°C. Relaxation times decrease rapidly above the glass transition as a result of thermal or water plasticization. Maximally freeze-concentrated solutes show glass transition at Tg and onset of ice melting at TJ. Equilibrium melting is described by the T curve.

For this type of test, the two homopolymers are typically compression molded into 1-2 mm thick sheets. A solution of the block copolymer is then spun-cast on one of the two polymers. The solvent and the homopolymer substrate to receive the block copolymer must be chosen so that the surface of the homopolymer substrate is not dissolved during the spin-casting operation. After a drying step, the two slabs are welded with the block copolymer in between to form an interface. The welding temperature is chosen to be above the respective glass transitions of the two homopolymers and the two slabs are held under a moderate pressure for a time sufficient to allow the local organization of the block copolymer at the interface to reach a metastable equilibrium. 

In all of the polysilylenes studied, the fluorescence from neat thin films on fused silica substrates exhibits a blue shift upon cooling. In cases where our studies have spanned the glass transition of the polymer, no change in behavior is seen (Figure 4). In the polymers which have substantial crystallinity, an abrupt shift in behavior occurs at the crystalline melting point above this temperature the films behave in much the same fashion as the fluid solutions. These phenomena have been extensively studied (9,12,13) and will not be treated here. 

Measurement of the glass transition of frozen solution formulation of the candidate drug is an important preformulation determination, since freeze-drying an amorphous system above this temperature can lead to a decrease in viscous flow of the solute (due to a decrease... 

Figure 6.19 DSC thermogram showing a glass transition of heated frozen drug solution.

A clear-cut Increase of the thermoreversion rate In a given matrix could be observed at temperatures above the glass transition of the polymer. Above Tg the kinetic behavior becomes quite similar to what It Is In solution. Some significant results are given In Table 6. 

---