Molecular aspects crystalline polymers

After a listing of some general definitions relating to crystalline polymers (Section 1), the subject is divided into sections dealing, successively, with local structural arrangements at the scale of a few bond lengths (Section 2), morphological aspects (Section 3), molecular conformation within polymer crystals (Section 4) and, finally, kinetic aspects of crystallization (Section 5). An alphabetical index of terms is provided for the convenience of the reader. 

This survey of the pharmaceutical aspects of polymers and macromolecules emphasises their use in formulation. It stresses the key features of polymers - their molecular weight distribution and their versatility in terms of their morphology, crystallinity, solubility and performance. 

Liquid crystalline polymers exhibit anisotropy in extruded and molded articles as a result of preferential orientation of LCP domains or individual chains. Reference (21 highlights some of the molecular structural features of LCP s that account for their fundamental anisotropy. These include the large aspect ratio of the individual polymer chains and their tendency to form aligned, highly crystalline domains. 

Various methods have been employed to find out about the structure of polymer electrolytes. These include thermal methods such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray methods such as X-ray diffraction and X-ray absorption fine structure (XAFS), solid state NMR methods particularlyusing7LiNMR,andvibrationalspectroscopicmethodssuch as infrared and Raman [27]. The objective of these various studies is to establish the structural identity of the polymer electrolyte at the macroscopic as well as the molecular levels. Thus the points of interest are the crystallinity or the amorphous nature of materials, the glass transition temperatures, the nature and extent of interaction between the added metal ion and the polymer, the formation of ion pairs etc. Ultimately the objective is to understand how the structure (macroscopic and molecular) of the polymer electrolyte is related to its behavior particularly in terms of ionic conductivity. Most of the studies have been carried out, quite understandably, on PEO-metal salt complexes. In comparison, there has been no attention on the structural aspects of the other polymers particularly at the molecular level. 

A complete characterization of liquid crystalline polymers should include at least two aspects the characterization of the molecular structure and that of the condensed state structure. Since the first characterization is nothing more than what is practiced for non-liquid-crystalline polymers, we will restrict the discussion to only a short introduction of methods mostly used in the characterization of the presence and the main types of polymeric liquid crystal phases. The methods include the mostly used polarizing optical microscopy (POM, Section 4.1), differential scanning calorimetry (DSC, Section 4.2) and X-ray diffraction (Section 4.3). The less frequently used methods such as miscibility studies, infrared spectroscopy and NMR spectroscopy will also be discussed briefly (Section 4.4). 

New approaches and techniques have been proposed to characterize peculiar aspects of L.C. phases or amphiphilic-based systems. The director reorientation of a side-chain liquid crystalline polymer was observed under extensional flow using a 4-roll mill placed in the magnet of a NMR spectrometer. A steric obstruction model was proposed to predict charge-induced molecular... 

It is evident from the preceding discussion that many aspects of the deformation of crystalline polymers have yet to be understood on a molecular basis. A great deal of work remains to be done. However, progress has been made by focusing on the independent structural variables that define the crystalline states. 

Many accounts of the dielectric behaviour of partially crystalline polymers are now available (McCrumet al 1967 Ishida, 1969 Hedvig, 1977 Baird, 1973 Wada, 1977 Williams and Crossley, 1978 Sasabe, 1971 Saito, 1964 Hoffman et al., 1966). No attempt can be made in this Section to give a comprehensive account of this and recent work. We shall consider only limited and selected aspects of the dielectric behaviour of polymers of medium-crystallinity and high-crystallinity. This account will be aimed at emphasizing molecular mechanisms for relaxation, realisit of course that, as discussed above in Section II, it is essential to have complementary experimental evidence in order to establish specific relaxation mechanisms. 

Here we consider those aspects of characterisation which fall between measurement of molecular structure and the bulk properties described above. A typical example might include the overall degree of crystallinity in a partially crystalline polymer, which could be determined by thermal analysis, scattering techniques or microscopy. The most appropriate method will of course be determined by the particular system of interest. Another example is taken from the area of polymer blends. In many cases the component materials are immiscible at the molecular level, and a phase-separated structure is formed. The morphology of this structure largely determines the way in which the blend will perform. Again, any of the above techniques could be used. Microscopy, in conjunction with preferential staining of one component, has proved particularly powerful in this area. 

Seguela, R. (2007). On the natural draw ratio of semi-crystalline polymers Review of the mechanical, physical and molecular aspects. Macromol. Mater. Eng. 292, 235-244 Tagawa, T. Ogura, K. (1980), Piled-lamellae structure in polyethylene film and its deformation. J. Polym. Sci. Polym. Phys., 18,5,971-979 Ward, I.M., Sweeney, J. (2004) An Introduction to The Mechanical Properties of Solid Polymers. Wiley, IBSN 047-149626X, New York. 

When we consider the arrangement of molecular chains with respect to each other, there are again two largely separate aspects, those of molecular orientation and crystallinity. In semi-crystalline polymers, this distinction may at times be an artificial one. 

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