Crystallinity stability, amorphous

Block copolymers can contain crystalline or amorphous hard blocks. Examples of crystalline block copolymers are polyurethanes (e.g. B.F. Goodrich s Estane line), polyether esters (e.g. Dupont s Hytrel polymers), polyether amides (e.g. Atofina s Pebax grades). Polyurethanes have enjoyed limited utility due to their relatively low thermal stability use temperatures must be kept below 275°F, due to the reversibility of the urethane linkage. Recently, polyurethanes with stability at 350°F for nearly 100 h have been claimed [2]. Polyether esters and polyether amides have been explored for PSA applications where their heat and plasticizer resistance is a benefit [3]. However, the high price of these materials and their multiblock architecture have limited their use. All of these crystalline block copolymers consist of multiblocks with relatively short, amorphous, polyether or polyester mid-blocks. Consequently they can not be diluted as extensively with tackifiers and diluents as styrenic triblock copolymers. Thereby it is more difficult to obtain strong, yet soft adhesives — the primary goals of adding rubber to hot melts. 

Often the stability of a drug in the solid state depends on its physical state (i.e., crystalline or amorphous [8]). If freeze-drying produces an amorphous solid and the amorphous form is not stable, then freeze-drying will not provide an acceptable product. 

This chapter describes some of the properties of solids that affect transport across phases and membranes, with an emphasis on biological membranes. Four aspects are addressed. They include a comparison of crystalline and amorphous forms of the drug, transitions between phases, polymorphism, and hydration. With respect to transport, the major effect of each of these properties is on the apparent solubility, which then affects dissolution and consequently transport. There is often an opposite effect on the stability of the material. Generally, highly crystalline substances are more stable but have lower free energy, solubility, and dissolution characteristics than less crystalline substances. In some situations, this lower solubility and consequent dissolution rate will result in reduced bioavailability. 

Depending on the processing variables, including the additives, the end product of a lyophilization process may be amorphous or crystalline or of intermediate crystallinity. As explained below the amorphous product has a higher free energy and therefore dissolves faster and tends to be less stable and more hygroscopic than the crystalline product. The choice between the crystalline and amorphous material may depend on whether an improved solubility or improved stability is required. 

M. J. Pikal, D. R. Rigsbee, The Stability of Insulin in Crystalline and Amorphous Solids Observation of Greater Stability for the Amorphous Form , Pharm. Res. 1997, 14, 1379-1387. 

Piacentini et al., 2005). It was resulted that there were three different Ba-containing species amorphous BaO on A1203 surface, amorphous carbonates and crystalline carbonate. Amorphous carbonate showed relatively low thermal stability and possesses high reactivity for NOx storage. 

The use of inorganic ion exchangers to solidify liquid radioactive waste followed by pressure sintering to produce a ceramic waste form appears to be a viable alternative to calcina-tion/vitrification processes. Both the process and waste form are relatively insensitive to changes in the composition of the waste feed. The stability of the ceramic waste form has been shown to be superior to vitrified wastes in leaching studies at elevated temperatures. Further studies on the effects of radiation and associated transmutation and the influence of temperature regimes associated with potential geologic repositories are needed for a more definitive comparison of crystalline and amorphous waste forms. 

In addition to characterizing frozen systems intended to be freeze dried, it is important to characterize the freeze-dried product. This includes determination of the physical state of the dried product that is, crystalline, partially crystalline, or amorphous. It may also include identification of the polymorph of a crystallizing component which exhibits polymorphism and determination of whether the crystal form observed is affected by changes in formulation and processing conditions. For amorphous systems, the glass transition temperature of the amorphous solid, as well as the extent to which Ts changes with residual moisture, may be a critical attribute of the product with regard to both physical and chemical stability. 

The sol-gel process is the name given to a number of processes in which a solution, or sol, undergoes a sol-gel transition. In this broadest sense, the term sol-gel refers to the preparation of inorganic oxides by wet chemical methods, irrespective of final form product—monolith, crystalline, or amorphous (1). Using sol-gel materials for mechanical entrapment of enzymes permitted stabilization of the proteins, tertiary structure owing to the tight gel network (2). Moreover, the easy insertion of substituent groups into... 

The strong points of sPS are its low water absorption, high environmental stress cracking resistance, chemical resistance, high heat resistance, good form stability due the similar densities of its crystalline and amorphous phases and good flow behaviour [31]. Its Achilles heel is, and will probably remain so in... 

Fig. 5 Chemical stability of the crystalline and amorphous forms of cefoxitin. (From Ref. I)...

Rigsbee, D.R. Pikal, M.J. Solid state stability of insulin comparison of crystalline and amorphous forms. Proceedings of the American Association of Pharmaceutical Scientists Annual Meeting, Orlando, FL, U.S.A., 1993. 

Pikal, M.J. Rigsbee, D.R. The stability of insulin in crystalline and amorphous solids Observation of greater stability for the amorphous form. Pharm. Res. 1997, 14 (10), 1379-1387. 

Moisture was known to increase the mobility of the surface groups of protein as measured by solid-state nuclear magnetic resonance spectroscopy The distribution of water between the protein and the excipients in a freeze-dried powder depends on the crystalline or amorphous nature of the excipients. For example, if a protein is formulated with an amorphous excipient and stored in a sealed container, water would distribute according to the water affinity of the protein and excipients.When the amorphous excipient crystallizes (e.g., because of elevated temperatures), it will expel its sorbed water, which may cause stability problems in the protein. ... 

Pharmaceutical solids can generally be described as either crystalline or amorphous (or glassy). In fact, the actual solid phase composition of a pharmaceutical formulation is usually characterized by an intermediate composition composed of both crystalline and amorphous character. In a multicomponent system such as a solid formulation comprising drug and exci-pient(s), certain components or even a single component may be amorphous. Because the amorphous form of a material is always a less stable, higher-energy form than its crystalline counterpart, the distinction between these forms relates to thermodynamic stability of the solid. 

We would like to emphasize that the difference in the structures of LiF and Si02 simulated here, reflects intrinsic ability of substrates to form crystalline and amorphous phases, respectively, and demonstrates the difference in principal properties of solid LiF and Si02- Formation of the SiOz amorphous phase is governed by the nature of the compound, which includes a variety and complexity of the SiOi oligomers and relative stability of the Si-0 bonds. The structure of LiF is naturally crystalline and, thus, an amorphism of LiF may be caused by external forces only, e.g. thermal, pressure, on electric field. 

The stability of the crystalline and amorphous phases at the interface is a matter of kinetics and temperature. At high temperatures, the amorphous region remains randomized. At low temperatures, the structure is effectively frozen in, with only small changes in structure possible over long time periods. It is only at intermediate temperatures, roughly half the randomization temperature T, that the crystalline substrate can effectively serve as a seed for crystallization to extend into the amorphous region on a feasible time scale. 

Biochemistry deals with an enormous number of chemical compounds with widely differing properties. Some are gases, some are liquids and some are solids either crystalline or amorphous. Wide variation in stability is encountered. Obviously no single fractionation technique will be the most effective for the separation of each t q)e. Since the determination of purity requires maximum separating power under conditions of complete stability, a choice of the most effective method must be made. Fortunately, the different methods often supplement each other and wherever possible, more than one method must be applied. Measurement of physical constants with adequate precision always must be done. Agreement of physical constants implies a degree of purity but is not vigorous proof that the substance is pure. 

Solid-state characterization is one of the most important functions of the preformulation group, which is assigned the responsibility of making recommendations for further formulation work on a lead compound. Physical properties have a direct bearing on both physical and chemical stabilities of the lead compound. Much of the later work on formulation will depend on how well the solid state is characterized from the decisions to compress the drug into tablets to the selection of appropriate salt forms. The studies reported in this section, of course, apply to those drugs that are available in solid form, crystalline or amorphous, pure or amalgamated. 

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