Polymorphism behavior

Some aspects of the polymorphic behavior of polymers, with particular reference to the structural organization at the molecular level are reviewed. 

After a temptative structure-based classification of different kinds of polymorphism, a description of possible crystallization and interconversion conditions is presented. The influence on the polymorphic behavior of comonomeric units and of a second polymeric component in miscible blends is described for some polymer systems. It is also shown that other characterization techniques, besides diffraction techniques, can be useful in the study of polymorphism in polymers. Finally, some effects of polymorphism on the properties of polymeric materials are discussed. 

In the fourth section the influence of comonomeric units on the polymorphic behavior of some polymers is described. In particular it is shown that comonomeric units can produce changes in the solid-solid transition temperatures as well as variations of the crystalline forms, which can be obtained for a given crystallization procedure. In the same section, some recent studies, showing that the polymorphic behavior of some polymers can be altered in particular miscible blends, are also reviewed. 

Some relevant effects of the polymorphism on the properties of polymeric materials are shown in the final section. In particular, it is shown that, while the occurrence of transitions between polymorphic forms can be detrimental for some systems, a precise knowledge of the polymorphic behavior and of the physical properties of the single forms can be used advantageously to improve the in use properties as well as the processing conditions of some polymeric materials,... 

A polymorphic behavior involving packing of chains having completely different conformations has been found also for isotactic polymers. For instance, isotactic polystyrene, under suitable experimental conditions, can produce crystalline gels in which the chains assume a nearly fully extended conformation [11,12], very close to a truns-planar, rather than the classical conformation of three-fold helix [13]. The two possible conformations proposed for the two crystalline forms of i-PS are shown in Fig. 2. 

Changes in the Polymorphic Behavior of Polymers 4.1 Polymorphism and Comonomeric Units... 

The presence of comonomeric units can alter the polymorphic behavior of a polymer, favoring in particular conditions the obtainment of a given crystalline form with respect to the other ones. 

As usual, this can be due both to thermodynamic and kinetic reasons. In fact, the presence of comonomeric units increases, in general, the energy content of all the crystalline forms, but, since the extent of increase may be different, it may destabilize some chain conformation or some kind of packing more than other ones. On the other hand, the influence of the comonomeric units on the polymorphic behavior of a polymer can be due to a change in the crystallization rates of the various forms. 

For instance, also the homopolymers PVF and PVDF have been described to crystallize in separate crystals in their blends [99] (Though constituted by isomorphous monomeric units which can cocrystallize in the copolymers in the whole range of composition, as seen in Sect. 4.1). Moreover, at least for the studied conditions, the polymorphic behavior of PVDF is not altered by the presence of PVF [99]. 

However, a few cases of miscible blends have been described recently in the literature in which, in particular conditions, the polymorphic behavior of a polymer appears to be altered. 

The polymorphic behavior of PVDF has been found to be changed also in the presence of functionalized ethylene-propylene copolymers, for the case of samples isothermally crstallized from the melt [103]. 

Also the polymorphic behavior of s-PS can be altered by blending, in particular with poly-2,6-dimethyl-l,4-phenylene oxide (PPO), both for the case of crystallization from the melt [104] and for the case of crystallization from the quenched amorphous phase [105]. 

In the identification of different polymorphs in polymers the FTIR technique presents, with respect to the diffraction techniques, the advantage of easier and more rapid measurements. In particular, the high speed of the measurements allows to study the polymorphic behavior under dynamic conditions. As an example let us recall the study of the transition from the a toward the P form of PBT induced by tensile stresses, evaluated by quantitative analysis of the infrared spectra [83],... 

A detailed knowledge of the polymorphic behavior of polymeric materials allows sometime to define better conditions for their processing. 

It is quite interesting that in the case of polypivalolactone PVL (see Table 2) the polymorphic behavior—involving in this case chains with the same 2i... 

Solidification of the particles may not be the final step in the formation process of solid lipid particles. Lipidic materials exhibit rich polymorphism [69,70], which may also occur in the dispersed state. In nanoparticles, the polymorphic behavior of the matrix lipids may, however, differ distinctly from that in the bulk material. Polymorphic transitions are usually accelerated in the nanoparticles compared with the bulk lipids [2,62]. In some cases, polymorphic forms not observable in the corresponding bulk materials were detected in lipid nanoparticles [1,65]. Because polymorphism can affect pharmaceutically relevant properties of the particles, such as the drug incorporation capacity [65], corresponding investigations should also be included in the characterization process. As long as polymorphic or other crystalaging phenomena have not terminated, the particle matrix cannot be regarded as static, and alterations of the particle properties may still occur. 

Eldem T., Speiser P, and Altdorfer H., Polymorphic behavior of sprayed hpid micropellets and its evaluation by differential scanning calorimetry and scaiming electron microscopy, Pharm. Res., 8, 178, 1991. 

The importance of being able to determine the geometrical isomers of TGs in partially hydrogenated fats can be ascribed to their effect on the physical behavior of the fats, such as their polymorphic behavior and melting properties. 

The importance of polymorphism in pharmaceuticals cannot be overemphasized. Some crystal structures contain molecules of water or solvents, known as hydrates or solvates, respectively, and they are also called as pseudopolymorphs. Identifying all relevant polymorphs and solvates at an early stage of development for new chemical entities has become a well-accepted concept in pharmaceutical industry. For poorly soluble compounds, understanding their polymorphic behavior is even more important since solubility, crystal shape, dissolution rate, and bioavailability may vary with the polymorphic form. Conversion of a drug substance to a more thermodynamically stable form in the formulation can signiLcantly increase the development cost or even result in product failure. 

A combination of isothermal and heating DSC scans can be used to study polymorphic behavior in some detail, as described above. 

Many interrelated factors influence the texture of plastic fats. Fatty acid and glyceride composition are basic factors in establishing the properties of a fat. These factors, in turn, are related to solid fat content, crystal size and shape, and polymorphic behavior. Once the crystal network is formed, mechanical treatment and temperature history may influence the texture. 

There are some useful methods to improve the physical stability of a suspension, such as decreasing the salt concentration, addition of additives to regulate the osmolarity, as well as changes in excipient concentrations, unit operations in the process, origin and synthesis of the drug substance, polymorphic behavior of the drug substance crystals, and other particle characteristics. However, methods based on changes of the particle properties and the surfactants used are the most successful [43],... 

In foods containing crystalline lipids, control of polymorphism is important for making products with desirable material properties and shelf stability. Conditions during processing govern the polymorphic behavior of fat. The process of tempering of chocolate nicely demonstrates these principles. 

As fully described below, sPS has been found to be miscible with aPS, PPE, PYME, TMPC and styrene-l,l-diphenylethylene copolymer. Generally the reported investigations deal with the effect of the second component on crystalline features of sPS, such as polymorphic behavior, crystallization kinetics, morphology and growth rate of crystallites. Just one study reports on toughening sPS by adding suitable components. 

With the accumulation of data, there is developing a gradual realization of the generality of polymorphic behavior, but to many chemists polymorphism is still a strange and unusual phenomenon. (Buerger and Bloom 1937)... 

Table 8.1 Collection of some references to polymorphic behavior of pigments ...

Stott, P. E., McCausland, C. W. and Parish, W. W. (1979). Polymorphic behavior and melhng points of certain benzo crown ether compounds. / Heterocycl. Chem., 16, 453-5. [158]... 

Polymorphic Behavior In this section, the polymorphism of the Cig-Cig-Cn, CnCn + iCn, and CnC2Cn TAG Series is discussed in terms of their diversity in occurrence of polymorphic stmctures and their thermodynamic stability and molecular structures. 

Disaturated-2-Unsaturated Mixed Acid Triacyigiycerois In this section, we consider the polymorphic behavior of a series of Sat.Unsat.Sat.TAGs, in which the sn-2 acid moieties are oleic, ricinoleic, and linoleic acids and the even-numbered saturated acids (palmitic, stearic, arachidic and behenic acids) are placed at the sn-l and sn-3 positions. 

POP (l,3-palmitoyl-2-oleoyl-i -glycerol) is a homologous substitution of the stearoyl moiety in SOS with the palmitoyl moiety. It was anticipated that POP might show the same polymorphic behavior as SOS. However, a few differences were observed regarding intermediate forms as explained in the following (Table 4) (58) ... 

The polymorphic nature of the multicomponent TAG systems is related to phase behavior that is affected by molecular interactions among the component TAGs. The fat crystals in a miscible phase may exhibit simple polymorphic properties. By contrast, the immiscile eutectic phase may show complicated polymorphic properties as a superposition of the polymorphic forms of the component TAGs. Furthermore, if the molecular compound is formed by specific TAG components, the polymorphic behavior becomes complicated, as shown for the case of POP-OPO (see Section 5.2). Therefore, knowing the phase behavior of the principal TAG components is a prerequisite for precise understanding of the polymorphism of natural fats. 

The high content of diglycerides (about 6%) in palm oil and the p stabilizing effect of diglycerides probably do not have any significant influence on the polymorphic behavior of canola oil blends with palm oil levels as above. The diglyceride content in canola oil blends is only raised slightly by addition of palm oil in the above levels (46). 

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