Disordered regions, semicrystalline polymer

Figure 8.2 Model of a semicrystalline polymer showing ordered crystalline regions and disordered amorphous regions. Reproduced with permission from S.L. Rosen, Fundamental Principles of Polymeric Materials, 2nd edn., Wiley-Interscience, New York, 1993. ...

Absorbance subtraction can be considered as a spectroscopic separation technique for some problems in polymers. An interesting application in FT-IR difference spectroscopy is the spectral separation of a composite spectrum of a heterophase system. One such example is a semicrystalline polymer which may be viewed as a composite system containing an amorphous and crystalline phase53). In general, the infrared spectrum of each of these phases will be different because in the crystalline phase one particular rotational conformation will predominate whereas in the disordered amorphous regions a different rotamer will dominate. Since the infrared spectrum is sensitive to conformations of the backbone, the spectral contributions will be different if they can be isolated. The total absorbance A, at a frequency v of a semicrystalline polymer may be decomposed into the following contributions... 

A distinction should be made between solvent plasticizers and nonsolvent plasticizers. With an amorphous polymer, any plasticizer is a solvent plasticizer— i.e., under suitable conditions the polymer would eventually dissolve in the plasticizer. With a crystalline or semicrystalline polymer, there are some compounds which enter both the crystalline (ordered) and the amorphous (disordered) regions. These are true plasticizers-sometimes they are called primary plasticizers. If, on the other hand, only the amorphous regions are penetrated, the compound may be considered as a nonsolvent plasticizer, also known as a secondary plasticizer, or softener. Such softeners are used sometimes as diluents for the primary plasticizer. 

Perfectly crystalline polymers are, however, rarely seen in practice and real polymers may instead contain varying proportions of ordered and disordered regions in the sample. These semicrystalline polymers usually exhibit both Tg and T i (not r, ) corresponding to the disordered and ordered regions, respectively, and follow curves similar to E-H-D-A. Tm is lower than and more often represents a melting range, because the semicrystalline polymer contains crystallites of various sizes with many defects which act to depress the melting temperature. 

The state of the polymer before dissolution can significantly affect the enthalpy of solution. The dissolving of a semicrystalline polymer requires an additional amount of heat associated with the disordering of crystalline regions. Consequently, its enthalpy of... 

Morphology. In semicrystalline polymers, chemical reaction is largely confined to the disordered, amorphous regions. Chemical reactions appear to be enhanced by the presence of an interface between crystalline and amorphous phases. 

The initial modulus is determined in the limit of small strain. The initial portion of the force-length curve is usually reversible. The deformation of the disordered interlamellar region is involved and the lamellar structure remains essentially intact. Interpreting the modulus, in terms of the basic structural and molecular parameters that define a semicrystalline polymer, is complex. In this region of very small strain, the primary effect is a rubber-like elastic deformation, whereby chain entanglements and other topological features act as effective cross-links. The total system is constrained by the bounding lamellae and their broad basal planes. 

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