Solid state amorphous

FIG. 31 Schematic diagram illustrating the transition between a supercooled liquid state (rubber) and an amorphous solid state (glass). The glass transition event is typically caused by a decrease in water content and/or temperature. The reversibility of the transition, as indicated by the dotted arrow, is material dependent (see text for further discussion of the reversibility of the transition). 

Once in the amorphous solid state, undesirable changes in the properties of amorphous ingredients and foods (e.g., stickiness, caking, collapse, loss of crispness) can occur via a reversal in the two events discussed earlier (1) an increase in moisture content (water plasticization) so that the Tg of a material is decreased to below room temperature and (2) an increase in temperature [thermal plasticization (Roos, 2003)] so that the temperature of the material rises above its Tg. In both cases and their combination, the once glassy material is now in a rubbery or liquid state and is undesirable and/or unfit for consumption. 

Bulk crystalline or amorphous solid-state materials whose conductivity is intermediate between metals and insulators and whose resistance decreases with increasing temperature. The valance band of an undoped semiconductor is completely filled, whereas its conduction band is empty. The energy difference between the valence and conduction bands (the band-gap) defines a semiconductor (see Fig. 95). 

The equation for the diffusion coefficient in a gaseous state, Dc, is the starting point for the derivation of an equation for diffusion coefficients in an amorphous solid state (S). The homologous series of n-alkanes is used as a reference class of chemical compounds which asymptotically reach an unlimited molecular chain in the form of poly methylene. The diffusion coefficient, DSji, is derived theoretically for a member of the series with j carbon atoms at infinite dilution in a matrix of a n-alkane with i carbon atoms in the molecule. In addition, the diffusion in polymethylene, which is considered as the reference structure for all polymers is also derived theoretically. 

In 1931 Simon reported that small molecules in their amorphous solid state are not in thermodynamic equilibrium at temperatures below their glass transition u. Such materials are in fact supercooled liquids whose volume, enthalpy, and entropy are greater than they would be in the equilibrium glass. (See Fig. 1). 

Estimate the free enthalpy of polymerisation of 1,3-butadiene to polybutadiene (1 4) when the monomer is in the liquid state and the polymer is in the amorphous solid state. 

The physical state of starch during extrusion can be considered to change from a partially crystalline polymer to a polymer melt which is homogenized by shear. Extrusion may also decrease the molecular size of starch components, which is observed from decreased melt viscosity (Lai and Kokini 1991), and obviously a decreased molecular size results in a decreased glass transition temperature of the extmdate. The dramatic decrease of pressure that occurs as a viscous, plasticized melt exits the die may allow an extremely rapid loss of water, expansion of the melt and cooling to an amorphous solid state. 

Macromolecules in solution, melt, or amorphous solid states do not have regular conformations, except for certain very rigid polymers described in Section 4.6 and certain polyolefin melts mentioned on page 139. The rate and ease of change of conformation in amorphous zones are important in determining solution and melt viscosities, mechanical properties, rates of crystallization, and the effect of temperature on mechanical properties. 

Abdul-Fattah A, Lechuga-Ballesteros D, Kalonia D. Pikal MJ. The impact of drying method and formulation on the physical properties and stability of methionyl human growth hormone in the amorphous solid state. J Pharm Sci 2007 Accepted. 

In amorphous state, solid polymers retain the disorder characteristic for liquids, except that the molecular movement in amorphous solid state is restrained. The movement of one molecule versus the other is absent, and some typical liquid properties such as flow are absent. At low stress, polymers display elastic properties, reverting to a certain extent to the initial shape in a relaxation process. However, they can be irreversibly deformed upon application of appropriate force. The deformation and flow of polymers is very important for practical purposes and is studied by a branch of science known as rheology (see e.g. [1]). The combination of mechanical force and increased temperature are commonly applied for polymer molding for their practical applications. The polymers that can be made to soften and take a desired shape by the application of heat and pressure are known as thermoplasts, and most linear polymers have thermoplastic properties. 

In Example 11-5, freeze crystallization of imipenem. which has lower stability in solution at room temperature, is presented. In this process, the product is rapidly frozen to an amorphous solid state to conserve its chemical purity. The temperature is then raised (still below the fieezing temperature at this stage), and the amoiphous solid converts to a crystalline solid over time. After the completion of the solid-state transition phase, the lyophilization drying cycle is initiated. 

This result may, by a similar argument, be extended to the interpenetration of chains as random coils in the amorphous solid state. These results will be of importance when the rheological properties of the melt through to the developing solid are considered in Chapters 2 and 3. 

Polyethylene terepthalate (PET) differs from PE mainly by a benzene ring, which is incorporated into every monomer unit of the chain molecule. Compared to PE, the chain becomes bulkier, which influences the crystallization behaviour of this polymer. PET can be obtained perfectly amorphous when quenched from the melt. Thus it can be crystallized from the amorphous solid state, in contrast to PE, which is always obtained with a high degree of crystallinity. The degree of crystallinity that is reached in PET depends on many material parameters and crystallization conditions, but more than 60% is unusual. 

It seems appropriate to discuss here the probabUity of interpenetration of polystyrene coils in the model networks. As already mentioned, according to the theoretical considerations of Flory [138] and De Gennes [139], polymeric coils in an amorphous solid state retain unperturbed dimensions. Since the volume fraction of the polymer in an unperturbed coil under -conditions is weU known to be very smaU, only about 2%, the transition from swoUen coils to solid state has to be accompanied by the replacement of aU solvent molecules with fragments of other polymeric molecules. In other words, theoretical notions predict extremely high mutual interpenetration of the polymeric chains in bulk state. Indeed, in order to maintain the coil dimension that is characteristic for a -solution, the coil must accommodate, on removing the solvent, a 50- to 100-fold amount of alien polymeric matter. In the 1970s this problem was discussed in fiiU [149-165], The authors of the tailor-made networks also took part in the discussion. 

The amorphous state is the characteristic of all polymers at temperatures above their melting points (except under special circumstances where liquid crystals may form). If a molten polymer retains its amorphous nature on cooling to the solid state, the process is called vitrification. In the vitrified amorphous state, the polymer resembles a glass. It is characteristic of those polymers in the solid state that, for reasons of structure, exhibit no tendency toward crystallization. The amorphous solid state is characterized by glass transition (Tg), which is described in a later section. We consider below only the behavior of polymer melt. 

The inter-conversion between these two amorphous solid states is achieved by changing the applied pressure through a first-order phase transition. This has been observed experimentally. 

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