Studies of crystalline and amorphous solids are often so closely related that it is natural to treat the two solids as „polymorphs“ of each other. This view is consistent with a definition of polymorphism (all solids that share the same liquid state) (10) and with the „energy landscape model“ of solids (11), which considers crystalline and amorphous states as connected minima on a multidimensional potential energy surface corresponding to different molecular packs and conformations. It`s been a good year since I wrote to you. But if you specify an amorphous address of this kind, what can you expect? Amorphous substances can form intentionally and not during normal pharmaceutical manufacturing processes. The properties of amorphous materials can be used to improve the performance of pharmaceutical dosage forms, but these properties can also lead to adverse effects that must be understood and managed for systems to function as required. An amorphous solid is one in which the constituent particles do not have a regular three-dimensional arrangement. Deposition at low temperatures, where adatoms do not have enough mobility to form a crystal structure (quenching). Amorphous solids are compounds without long-range order and without periodicity (Cusack, 1987). Glasses are a subgroup of amorphous solids that have a pronounced glass transition temperature. Amorphous materials represent one of the greatest challenges for the application of modeling techniques. Experimental methods cannot currently provide precise structures at the atomic level: they provide averaged structures as in the radial distribution functions (RDF) provided by diffraction measurements, or very local information as in experiments with the fine structure of extended X-ray absorption (EXAFS) and nuclear magnetic resonance (NMR); Knowledge of the intermediate order is rare. However, computer modeling techniques have the ability to provide detailed models for amorphous material structures at the short and medium levels. In this chapter, we show how atomistic computer simulation methods can provide unique insights into the structural properties of amorphous bodies.
The range and scope of materials that can currently be studied through modelling techniques are illustrated by recent results, mainly on vitreous materials (defined below), although a brief discussion of other classes of amorphous materials is given towards the end of the chapter. How and what does this amorphous movement bring in this or that direction? In addition to conformational equilibria, configuration equilibria (that between carbohydrate anamers) are expected to have a similar effect on the tendency to crystallization. The effects of these balances on the ability of glass formation have not been well studied. The range of compositions that can be produced in the vitreous state by rapid solidification from melting is wide. Typical compositions are given by the formula T70-90X10-30 (at%). T represents virtually any combination of transition metals, which are naturally given for magnetic applications by Fe, Co and Ni. The letter X refers to metalloid atoms such as Si and B and/or refractory metals such as Nb, Mo, Zr, Hf, etc. These „non-magnetic“ additives are necessary for the formation of glass and the stabilization of the amorphous structure.
Amorphous forms are, by definition, non-crystalline materials that do not have a long-range order. Their structure can be considered similar to that of a frozen liquid, thermal fluctuations being present in a frozen liquid, leaving only a „static“ structural disturbance (1). The degree of crystallinity, according to the USP, depends on the proportion of crystalline material in the mixture, which is called the two-state model. Another way to look at this situation is that crystallinity has a range of 100 percent for perfect crystals (zero entropy) to 0 percent (non-crystalline or amorphous); This is called the OneState model (2). Amorphous solids exist in many products of industrial importance such as polymers, ceramics, metals, optical materials (glasses and fibers), food and pharmaceuticals. Amorphous solids have always been an integral part of pharmaceutical research, but the current interest (3,4,5) has been sparked by two developments: amorphous solids are isotropic. That is, they have uniform properties in all directions. The thermal and electrical conductivities, the coefficient of thermal expansion and the refractive index of an amorphous solid have the same value, regardless of the direction in which the properties are measured. The amorphous solid changes its structure in order to reach a more energetically favorable state thanks to the relaxation mechanism. The following Kohlrausch-Williams-Watts (KWW) equation is used to analyze this process as a function of T and time t [39,40]. Any given crystalline solid can be rendered amorphous by the very rapid cooling of its melting or by the freezing of its steam. This does not allow the particles to organize into a crystalline pattern.
When quartz melts the crystalline form of SiO2 and then cools it quickly, it creates an amorphous solid known as quartz glass or quartz glass. This material has the same SiO2 composition, but does not have the molecular order of quartz. The amorphous shape of metal alloys is achieved when thin layers of molten metal are cooled rapidly. The resulting metallic glasses are strong, flexible and much more resistant to corrosion than crystalline alloys of the same composition. If an amorphous solid is kept at a temperature just below its melting point for long periods of time, the molecules, atoms, or ions that compose them can gradually reorganize into a higher-order crystalline form. Crystals have sharp melting points that are well defined; Amorphous solids do not. Amorphous solids find many applications due to their unique properties. For example, inorganic glasses find applications in construction, household items, laboratories, rubber another amorphous solid is used in the manufacture of tires, inner tubes, shoe soles, etc.
Plastics are widely used in households and industries. In many cases (oxidation reactions initiated by free radicals), the stability of a compound of active ingredient is not significantly influenced by its molecular mobility or by the orientation of molecules; Thus, the amorphous form has a stability comparable to that of the crystalline material. In some cases (insulin), the more orderly structure of the crystalline material can actually increase the likelihood of certain intermolecular contacts and result in lower stability of the crystalline form. „Amorph.“ dictionary Merriam-Webster.com, Merriam-Webster, www.merriam-webster.com/dictionary/amorphous. Retrieved 30 September 2022. It can be obtained in the form of an amorphous substance of light yellow color, which is no different from the appearance of chewing gum. A general cause of a reduced tendency to crystallization in orgànics is conformational flexibility (12). Since conformationally flexible molecules can exist as multiple conformers in a crystallizing medium, the crystallization process must select the „good“ from the „bad“, a difficulty that rigid molecules do not encounter. The effect is enhanced when the crystal conformers correspond to high-energy, low-solution conformists, which means that the act of crystallization requires the medium molecule to undergo a significant conformational change.
The effect is thought to underlie the different crystallization trends of two stereoisomers, e.g. mannitol (mild) and sorbitol (difficult) (13,14). Examples of amorphous solids include glasses, ceramics, gels, polymers, rapidly tempered melts, and thin-film systems deposited on a low-temperature substrate. The study of amorphous materials is a very active area of research. Despite enormous progress in recent years, our understanding of amorphous materials is far from complete. The reason for this is the lack of simplifications associated with periodicity. In the DSC analysis, the peak of recovery that occurs at Tg is integrated to determine the enthalpy of relaxation. However, recovery can begin well below Tg. In such cases, integration should be carried out throughout the glass transition zone [43,44], as the „apparent“ baseline below Tg also includes enthalpy recovery.
Fig. 15.10 shows the effect of annealing on the shape of the DSC curves of ribaviringlas. The peak of recovery is accentuated after the glow, which is explained by the expansion of the cooperative transformation region . Relaxation parameters are significantly influenced by the integration area. For example, the value τβ at 28 °C obtained from the base integration was more than six times higher than that obtained from the peak integration for ribavinglas. The final design of the alloy is largely determined by (1) desired magnetic properties, (2) good formability of the glass, (3) thermal stability and, for nanocrystalline alloys, (4) well-defined crystallization behavior. The material (which often contains silica) is easily cooled from its liquid state when glass is made, but does not solidify when its temperature drops below its melting point. The material is then cooled below the glass transition temperature to become an amorphous solid. Some examples of amorphous solids are glass, rubber, pitch, many plastics, etc.