Polymorphism in polymers

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals. The complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects. 

The first observation of polymorphism in organic materials is attributed to Friedrich Wohler and Justus von Liebig, when in 1832, they examined a boiling solution of benzam-ide on cooling the benzamide initially crystallized as silky needles, but on standing, these were slowly replaced by rhombic crystals. Present day analysis identifies three polymorphs for benzamide. The least stable one, formed by flash cooling is the orthorhombic form 11. This type is followed by the monoclinic form III (observed by Wohler/Liebig). The most 

Polymorphs have different stabilities and may spontaneously convert from a metastable form (unstable form) to the stable form at a particular temperature or upon stretching. They also exhibit different X-ray crystal and diffraction patterns, different melting temperatures and solubilities. 

Various conditions in the crystallization process are responsible for the development of different polymorphic forms. These conditions include the following  

1) solvent effects (the packing of crystal may be different in polar and nonpolar solvents)  

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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 second section a classification of the different kinds of polymorphism in polymers is made on the basis of idealized structural models and upon consideration of limiting models of the order-disorder phenomena which may occur at the molecular level. The determination of structural models and degree of order can be made appropriately through diffraction experiments. Polymorphism in polymers is, here, discussed only with reference to cases and models, for which long-range positional order is preserved at least in one dimension. 

In the following, some examples of applications of Fourier transform infrared (FTIR) Spectroscopy and of solid-state nuclear magnetic resonance (NMR) to the study of polymorphism in polymers are described. 

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],... 

Reviews on the use of the solid-state NMR in the study of polymorphism in polymers has been presented in chapters of two relatively recent books [111, 112]. However, it is worthy to recall some aspects in the present article. 

Woodward AE. Understanding Polymer Morphology. Cincinnati Hanser-Gardner, 1995. Corradini P, Guerra G. Polymorphism in polymers. Adv Polym Sci 1992 100 183. 

Rastogi S, Kurelec L. Polymorphism in polymers its implications for polymer crystallisation. / Mater Set 20000ctober 35(20) 5121-38. 

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