Macromolecules, molecular weight distribution

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Due to the particular mechanism of these reactions, the macromolecules so formed have varying degrees of polymerisation. Even natural polymer compounds except certain proteins are not uniform with respect to their Molecular weight. Because of this reason, any polymer sample has a characteristic Molecular weight distribution (MWD), which can be represented by a distribution function or a distribution curve. 

P. Zhou, G. Q. Chen, H. Hong, F.S. Du, Z.C. Li, F. M. Li, Synthesis of C60-endbonded polymers with designed molecular weights and narrow molecular weight distributions via atom transfer radical polymerization, Macromolecules, vol. 33, pp. 1948-1954, 2000. 

Size exclusion chromatography (SEC) polymer elution profiles yield information regarding the molecular size distributions of polydisperse macromolecules. Polymer molecular weight distribution (MWD) represents an intrinsic property which provides direct correlation with many end-use physical properties and a universal criterion for polymer characterization (1). In order to convert elution profiles or chromatograms into MWD information proper calibration methods are required. SEC molecular weight calibration techniques represent experimental approaches for transformation of polymer elution profiles into MWD information and are dependent upon instrumentation, columns, and the polymer/solvent system under study. 

Iwasaki T, Yoshida J (2005) Free radical polymerization in microreactors. Significant improvement in molecular weight distribution control. Macromolecules 38 1159-1163... 

The fundamental difficulty is that polymeric substances cannot be obtained in a structurally and molecularly uniform state, unlike low-molecular-weight compounds. Thus, macromolecular materials of the same analytical composition may differ not only in their structure and configuration (see Sect. 1.2) but also in molecular size and molecular weight distribution they are polydisperse, i.e., they consist of mixtures of molecules of different size. Hence, it is understandable that the expression identical is not, in practice, applicable to macromolecules. Up to the present time, there is no possibility of preparing macromolecules of absolutely uniform structure and size. It follows, therefore, that physical measurements on polymers can only yield average values. The afore-... 

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